1,3,5-Triphosphabenzenes: Synthesis, reactivity and theory

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Review

1,3,5-Triphosphabenzenes: Synthesis, reactivity and theory Rosalyn L. Falconer, Christopher A. Russell ∗ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK

Contents 1.

2. 3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Scope of this review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1. Note on nomenclature and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. ␩1 -Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. ␩6 -Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Ring contraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Extrusion of a C-unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Extrusion of a P-unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. 1,4-Addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. Small molecule addition(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Dihydrogen and related reduction reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3. With alkynes and alkyne analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4. With alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5. Masked 1,4-addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.6. Other addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and opportunities for further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 19 December 2014 Received in revised form 11 February 2015 Accepted 11 February 2015 Available online xxx Keywords: Phosphorus Triphosphabenzenes Phosphinines Low-coordinate

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

a b s t r a c t In this review, the synthesis, reactivity and theoretical studies on 1,3,5-triphosphorus analogues of benzene are discussed including all the literature up to September 2014. Whereas the structure, spectroscopic studies and theory all point towards a significant degree of aromaticity for 1,3,5-triphosphabenzenes, the chemistry that has thus far been demonstrated is largely distinct from its carbocyclic counterparts. This can be ascribed to the substituents that have thus far been employed in 1,3,5-triphosphabenzene synthesis. Hence the discussion of triphosphabenzene reactivity is broken down into four sections describing ␩1 -coordination, ␩6 -coordination, 1,4-addition and ring-contraction reactions. Furthermore, the section on 1,4-addition reactions contains a subsection which highlights the reactions of 1,3,5triphosphabenzenes with small molecules (principally dihydrogen, alkynes and alkenes), a facet of reactivity that has greater links to the chemistry of transition metals than traditional aromatic chemistry. © 2015 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +44 0117 928 7599; fax: +44 0117 929 0509. E-mail address: [email protected] (C.A. Russell). http://dx.doi.org/10.1016/j.ccr.2015.02.010 0010-8545/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction

1.2. Scope of this review

1.1. Background

In this review, we wish to give a comprehensive summary of the science of 1,3,5-triphosphabenzenes up to September 2014. This updates the topic from a 2003 review on the same topic by Heydt [5]. We will generally restrict ourselves only to a strict interpretation of the title compounds, although we will consider the energetics of the various structural isomers of P3 (CR)3 . This is often seen as a niche main group topic, but we believe it has taken on a wider relevance by the recent observation that some aspects of the chemistry of a 1,3,5-triphosphabenzene has been shown to resemble those properties traditionally associated with the transition elements, in particular the binding and activation of small molecules. This industrially and academically important topic underpins the application of compounds of the transition elements in catalysis, and the unexpected link between the chemistry of this 1,3,5-triphosphabenzene and transition metals may herald an increased level of interest in these fascinating compounds. Given the relatively limited number of papers on this topic, we aim to offer more than just a superficial glance at each topic. Thus, this review is organised into different methods of preparing 1,3,5triphosphabenzenes, details of the characterisation and properties, reactivity and theoretical studies relating to the molecules. On the subject of reactivity, we have further subdivided the topic according to four distinct modes of reactivity which we have identified. Inevitably there may be some overlap between these different sections and we have attempted to highlight this where necessary.

The quest for heavy element analogues of multiply bonded carbon species has been one of the defining goals of modern synthetic inorganic chemistry. The early years of these adventures were shaped by the challenge of overcoming the so-called double bond rule, which held that multiply bonded species of the heavy p-block elements could not be formed. Despite this unpromising landscape, chemists attempted to synthesise such species, but the experimental successes gave products of limited stability. In many ways these observations reinforced the double bond rule, although they challenged the assertion that such species are unobtainable. Careful analysis of the common decomposition products showed that the species were readily attacked by small atmospheric molecules (such as O2 and H2 O). Furthermore under strict anaerobic conditions, a common decomposition pathway was through oligomerisation of the multiple bond, forming products linked by heavy element to heavy element single bonds. Thus this led to the breakthrough which was the realisation that the use of large, sterically encumbered groups may prevent attack of small molecules, and, more importantly, prevent the decomposition to form oligomers that would be disfavoured on account of the increase in steric clashes that would result from such a reaction. Using the steric bulk strategy has allowed many elaborate and beautiful species to be identified, and the similarities and differences in the structures compared to their hydrocarbon analogues has allowed the refinement of bonding theory. The vast number of products that have been formed allows us to conclude that the double bond rule has been laid to rest. Despite the widespread appeal of many of the products, there is little doubt that some species have a greater appeal to the chemical psyche than others, often reflecting the importance of the parallel all-carbon species in the realm of organic chemistry. In the field of carbon phosphorus multiple bonds, to which this review belongs, we would suggest that phosphorus containing analogues of alkenes, alkynes and arenes are of special significance, both in terms of the chronology of their discovery and their importance in shaping the subject. We shall briefly recap on some key points in the timeline. The parent HC P, was first proposed as reactive intermediate by Albers in 1950 [1]; over a decade later, Gier pyrolised PH3 gas using rotating arc graphite electrodes, and from the “witches brew” generated, he was able to both chromatograph and characterise HC P by IR spectroscopy [2]. However, the compound was reported to be extremely pyrophoric, decomposing above −124 ◦ C. Thus it was suggested that although such multiple bonds may be synthesised, they only have a fleeting existence. Interestingly, as an addendum to this story, Mathey and Le Floch were later able to synthesise this compound via a route which did not produce highly reactive by-products; under these conditions HC P proved to be far more stable than was initially believed [3]. Despite the reported lack of stability, this observation served to inspire further research in the field which culminated in the 1981 report of Becker and co-workers of the preparation of the first kinetically stabilised phosphaalkyne, t BuC P [4]. Nowadays a vast number of phosphaalkynes are known. Surprisingly, many of these phosphaalkynes showed relatively high kinetic stability, e.g., for t BuC P, oligomerisation reactions to form a mixture of products do not initiate until the temperature reaches above 180 ◦ C. Importantly, the specificity of these oligomerisation reactions could be improved by using transition metal mediated reactions. Initially, both dimers and tetramers were identified, and recently trimers, pentamers and even hexameric cages have been identified. The specific focus of this report are the trimers and in particular the species which are isolobal to benzene, where the question of heteroaromatic stability is pivotal.

1.2.1. Note on nomenclature and abbreviations In this review we will refer to phosphorus substituted analogues of benzene as phosphabenzenes rather than the IUPAC approved phosphinines as we feel the former is more evocative of the character of the molecules we wish to portray. The majority of the reactions that have been described have tertbutyl substituents attached to the carbon atoms, a feature that is encoded from the synthesis starting from the gram-scale-accessible tert-butylphosphaalkyne. This combination of ring carbon with tert-butyl substituent is so commonplace that we will use a solid circle (i.e., •) to denote the moiety in figures, tables, etc. However, in instances where we wish to emphasise the coordination environment about a C-centre, we will drop the •-notation and spell out the t Bu substituent explicitly. On occasions where reactivity has been determined using other 1,3,5-triphosphabenzenes, we will seek to emphasise this distinction. We will use bold-type numbers to denote compounds referred to in the text; we have chosen not to assign compound numbers to the various triphosphabenzenes in the report as we feel that this adds to the emphasis on the topic. In several instances, we have chosen to use the same compound number for species which have been reported with a number of different substituents; whereas we acknowledge that this is scientifically poor practice, we have done to limit the vast number of compound numbers that can be developed in a such a review and to place the emphasis on compound type rather than on each distinct molecule. 2. Synthesis 1,3,5-Triphosphabenzenes have been observed as products from a range of reactions. Several higher oligomers (mainly tetramers) were identified as products in the thermolysis of phosphaalkynes. In order to improve specificity, transition metal mediated cyclotrimerisation of phosphaalkynes have been sought. The first report from the group of Cowley described the isolation of a 1,3,5-triphosphabenzene molybdenum complex formed by reaction of tert-butylphosphaalkyne and with [Mo(CO)3 (␩6 cycloheptatriene)] [6]; however, it should be noted that Binger and

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Scheme 1. Cyclotrimerisation reactions of tert-butylphosphaalkyne at Hf centres.

co-workers have cast doubt on the repeatability of this procedure [7]. Following this initial report, Binger and co-workers described some hafnium mediated reactions which led to phosphaalkyne oligomers (Scheme 1) [7]. The cyclooctatetraene hafnium butadiene complexes 1 and 4 react with t BuC P in a number of ways dependent on the substitution pattern on the COT ring and on the precise conditions employed. In addition to forming dimers and tetramers, a species formulated as a tris(phosphaalkyne) HfCOT (5) complex was observed, in addition to a cyclic trimer (2) where the Hf(COT) fragment binds across 1,4-sites of what we would now recognise as a 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene unit. The metal may be liberated from these complexes by reaction with C2 Cl6 , forming a triphospha-Dewar-benzene (3) from the latter, and 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene from the former. The latter reaction is proposed to go through the bicyclic adduct 6, which upon passing down a denatured silica column gives the 1,3,5-triphosphaarene in reasonable yield. Despite this experiment providing conclusive evidence that triphosphorus analogues of benzene could be prepared as discrete, stable entities, this discovery did not lead to a surge in activity in probing the chemistry of these heterocycles. This was presumably on account of the substantial synthetic hurdles that remained to access useful quantities of the reagent. However, it was not long before a more convenient approach to triphosphabenzenes was reported that remains the mainstay for researchers involved in the field. Preuss and co-workers described vanadium mediated reactions of phosphaalkynes [8]; the most active vanadium species employed was tert-butylimido vanadium trichloride. When this vanadium compound was reacted as its DME adduct with phosphaalkynes, a tert-butylimido unit is incorporated in a P4 (Ct Bu)2 framework in an azatetraphosphaquadricyclane cage (7) [8,9]. More interestingly, when Lewis base free t BuNVCl3 is employed, free 1,3,5-triphosphabenzenes are readily obtained (Scheme 2) [8]. The authors describe this as a catalytic reaction [10], largely on account of being able to form and detect the initial addition product (8, Scheme 3) both before and after a cycle, although note that a 1:4 stoichiometric reaction affords the highest yields. The reaction is reported to work for a variety of phosphaalkynes, all of which contain a tertiary carbon centre bound to the phosphaalkyne unit. A catalytic cycle was proffered [10] (Scheme 3) involving the following steps: (a) [2+2] cycloaddition of the

Scheme 2. Reactions of tert-butylphosphaalkyne at vanadium(V) centres.

imidovanadium unit to the phosphaalkyne; (b) sequential insertion of three phosphaalkyne units into the V C bond; (c) elimination of the 1,3,5-triphosphabenzene. Direct experimental evidence for the chemical composition of complexes 8 (with a variety of organic groups) was obtained, although the connectivity of the product was assigned solely by spectroscopic techniques. Furthermore, whether this is formed from a true [2+2] cycloaddition (which, of course, are thermally forbidden by Woodward–Hoffmann rules [11]) or whether an alternative mode of addition operates is unclear. With this caveat apart, the proposed catalytic cycle uses steps that have clear precedence in corresponding organometallic chemistry, but little direct experimental evidence or computational techniques have been employed to obtain a deeper understanding into this important reaction. The development of triphosphabenzene chemistry will depend to a degree on developing a detailed understanding of the similarities and differences between the mechanism of phosphaorganic and organometallic chemistry. 3. Characterisation All of the 1,3,5-triphosphabenzenes thus far characterised contain tertiary carbon centres on each of the ring carbon atoms. Hence

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Fig. 1. The molecular structure of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene as determined by X-ray crystallography (CCDC 101646). The inset shows the planar nature of the core atoms. Phosphorus atoms are coloured purple and carbon atoms are grey. Hydrogen atoms have been omitted for clarity.

Scheme 3. Proposed mechanism for the cyclotrimerisation of t BuC P. Table 1 31 P and 13 C NMR chemical shifts for the ring atoms of 1,3,5-triphosphabenzenes.

R

t

Bu

1-Ad

CMe2 Et

1-Me-cyclohexyl

1-Me-cyclopentyl

31

232 212

238 212

239 209

234 212

243 212

P NMR 13 C NMR

it is no surprise that their physical and spectroscopic signatures are also similar. All are yellow solids, with the exception of the tertpentyl derivative that forms as a yellow oil. These are only slightly reactive towards air and moisture but are best stored under an inert atmosphere in the absence of light. The NMR chemical shifts of the ring phosphorus and carbon atoms [10] are typical of values for these elements in aromatic systems and are given in Table 1. In probing the mechanism of the vanadium(V) reaction used to prepare the 1,3,5-triphosphabenzenes, Regitz and co-workers were able to isolate the product (of compounds 8) from the 1:1 reaction of a phosphaalkyne and Cl3 V Nt Bu. Subsequent reaction of 8 with an excess of a different phosphaalkyne leads to unsymmetrical 1,3,5-triphosphabenzenes; these were prepared with either two adamantyl groups and one tert-butyl group or vice versa. Neither product could be isolated pure, but the breaking of the symmetry of the C3 P3 unit led to a more complicated spectroscopic signature for these species which made them readily identifiable [10]. Gleiter and co-workers performed a single crystal X-ray diffraction study on 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (Fig. 1) [12]. The C3 P3 core was shown to be planar with P C bonds of equal ˚ a value in between those length with an average value of 1.727 A,

classically associated with P C single and P C double bonds. The bond angle at the ring carbon atoms is more acute compared to the corresponding angle at phosphorus (av. C P C 109.3◦ ; av. P C P 130.7◦ ), giving the species a core which does not have the “perfect” hexagonal symmetry of the parent benzene. In the same paper, the photoelectron spectroscopy spectrum of 2,4,6-tri-tert-butyl1,3,5-triphosphabenzene is reported and assigned to a series of the higher occupied molecular orbitals by comparing experiment and theory. Importantly, the HOMO–LUMO gap in the triphosphaarene (ca. 2.8 eV) is significantly smaller compared to that of benzene (ca. 5.1 eV). Photoelectron spectroscopy, in conjunction with DFT calculations have also been used to probe the electronic structure of a series of M(CO)3 (M = Cr, Mo, W) complexes of 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene; the data were compared to the Mo(CO)3 complex of 1,3,5-tri-tert-butylbenzene [13]. This showed the triphosphabenzene complexes possessed a higher first ionisation energy than the carbocyclic analogues as a result of greater metal to ligand back donation into the relatively low lying LUMO of the triphosphabenzene. These important bonding conclusions were corroborated by analysis of M–CO and C–O distances in the single crystal X-ray structures of [Mo(CO)3 L] (L = 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene; 2,4,6-tri-tert-butyl-benzene). Although 1,3,5-triphosphabenzenes are diamagnetic species, the accessibility of the low lying LUMO means they are readily reduced. The resulting anion (with a potassium counteraction) has been probed by EPR spectroscopy giving a single species with a characteristic AB2 31 P hyperfine structure, suggesting single occupancy of one of the degenerate 2e orbitals, with a static Jahn–Teller distortion causing loss of C3 symmetry and the differentiation of the A and B2 31 P nuclei [14]. The generation and reactivity of such anions is discussed in greater detail in Section 4.4. 4. Reactivity Given the pronounced aromaticity in 1,3,5-triphosphabenzenes and thus the link to the corresponding carbocyclic species, it is noteworthy that the reactivity of 1,3,5-triphosphabenzenes is rather different. In particular, no analogue of electrophilic aromatic substitution is known, a fact that is readily explained by the substituents on the known triphosphabenzenes (tertiary carbon centres on carbon and a lone pair of electrons on phosphorus) rather than for any underlying difference in reactivity. We have approached the description of triphosphabenzene reactivity by trying to compartmentalise reactions into a number of different classes. Of these classes, we note that only ␩6 -coordination has ready precedent in organic chemistry; 1,4-addition, although known under photochemical conditions for aromatic systems, is commonly seen for triphosphaarenes; ␩1 -coordination and ring contraction reactions are without any precedent for the corresponding organic systems.

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Scheme 4. First example of ␩1 -coordination of triphosphabenzene.

Cl

P P

P Cl

Pt

Cl PMe3

9

3 H2 O - HCl

tBu

Pt

PMe3

OH Bu P H t Bu H P OO H 11 H

P

t

Scheme 5. Further reaction of 9 with water.

4.1. 1 -Coordination The inclusion of the three phosphorus atoms in the arene, each of which possess a lone pair of electrons, offers the intriguing possibility of binding through both the arene system or directly to one or more of the phosphorus centres. This in turn raises the additional possibility of both modes occurring simultaneously with the metals communicating through the 1,3,5-triphosphabenzene ring. The first example of a ␩1 -coordinated 1,3,5-triphosphabenzene complex was reported by Nixon et al. [15]. This involves direct reaction of [PtCl2 (PR3 )]2 (R3 = Me3 , Et3 , Me2 Ph, MePh2 ) with 2,4,6tri-tert-butyl-1,3,5-triphosphabenzene, as shown in Scheme 4. The complexes were characterised by NMR spectroscopy, identifying the trans isomer as the product formed and noting that only the reaction with PR3 = PMe3 goes to completion. For compound 9 (R = Me), the value for 1 J(Pt-P) for the 1,3,5-triphosphabenzene (2418 Hz) compared to the PMe3 ligand (2901 Hz) indicates the s-character of the 1,3,5-triphosphabenzene phosphorus atom is lower than that of the PMe3 ligand. Coordination of more than one platinum fragment to the triphosphabenzene was not observed. Comparison to the analogous phosphabenzene complex trans[PtCl2 (PMe3 )(PC5 H2 t Bu3 )] (1 J(Pt-P) = 2548 Hz) led to the conclusion that the higher the number of phosphorus atoms in the ligand ring, the lower the s-character of the P-lone pair. Prolonged storage of the product led to isomerisation from trans- to ciscomplexes (9–10) in the case of some of the Pt complexes. The rate of isomerisation is slow, implying that the kinetic trans-product is formed first, followed by relaxation to the thermodynamic cis-product. The ␩1 -coordination of the 1,3,5-triphosphabenzene was found to increase the reactivity of the ring, as has been shown with other phosphaarenes [15]. In this case, attempts to recrystallize 9 in air resulted in reaction with water, giving the product 11 (Scheme 5). Characterisation by X-ray crystallography was carried out, providing evidence for an interesting hydrogen-bonded dimeric structure of the product. Reaction between 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene and other metal halides {e.g., MCl4 (M = Ti or Zr), MCl5 (M = Nb or Ta), CuX (X = Cl or I)} were attempted, but were unsuccessful. Direct comparisons between the coordination chemistry of 2,4,6-tri-tert-butylphosphabenzene and 2,4,6-tri-tert-butyl1,3,5-triphosphabenzene were drawn in a paper by Nixon et al. [16]. Although analogous compounds are formed upon reacting [PtCl2 (PEt3 )]2 with the 1,3,5-triphosphabenzene and monophosphabenzene, attempts to coordinate to palladium

(di-␮-chloro-bis[(R)-dimethyl(1-ethyl-␣-naphthyl)-aminatoC2 ,N] palladium) and gold (AuCl(tht), tht = tetrahydrothiophene) centres did not result in the same outcome. 2,4,6-tri-tertbutylphosphabenzene readily coordinated with both Pd and Au fragments (Scheme 6), however, no reaction with 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene was observed under analogous conditions. X-ray structures of these compounds were reported, although compound 12 was found to be in equilibrium with the starting material, therefore a mixture of starting material and product crystals were formed even in the presence of excess phosphabenzene. All complexes were also characterised by NMR spectroscopy. Further attempts to coordinate 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene with a gold centre were more successful. Russell et al. utilised a cationic gold fragment [LAu]+ (L = Pt Bu2 (o-biphenyl), NHC) along with a relatively weakly coordinating [SbF6 ]− anion to form ␩1 -2,4,6-tri-tert-butyl-1,3,5triphosphabenzene coordinated to linear gold centres (Fig. 2, top; 14 and 15) [17]. In the X-ray crystal structure of compound 14, it is possible to see that the coordinated 1,3,5-triphosphabenzene ring is non-planar. In order to assess the effect of this, both DFT and NICS-1 (nucleus independent chemical shift) were used. The latter is the measure of absolute chemical shielding of a ghost atom placed 1 A˚ above the centre of the ring and is used as an indication of degree of aromaticity of that system. Fig. 2 summarises the values calculated. The DFT studies reproduced the solid-state structures of 14 and 15, whilst the NICS-1 values showed that despite the deformation in the 1,3,5-triphosphabenzene in 14, aromaticity is maintained in the ring. Although not viewed as classical coordination complexes, ␩1 -coordination chemistry has also been observed though protonation, methylation and silylation reactions of 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene. These products (Scheme 8) were realised using reagents which feature weakly coordinating perhalogenated carborane anions [19]. These were selected since they are strong sources of the desired electrophiles (H+ , Me+ and [R3 Si]+ ), however, are chemically easy to handle and have low nucleophilicity, therefore are relatively mild and allow for isolation of products before they decompose (Scheme 7). The X-ray crystal structure for the protonated 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene (16) was obtained, showing direct binding of the proton in a ␴-fashion to one of the P-centres; this perturbs the regular bond lengths in the ring, consistent with partial oxidation of the protonated P-centre. NMR spectroscopy was also used to characterise the protonated, methylated (17) and silylated (18) products. The analogous reactions were attempted with monophosphabenzene PC5 H2 t Bu3 , which displayed the same reactivity as 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene. 4.2. 6 -Coordination Whereas ␩1 -coordination requires a lone pair of electrons at a heteroatom centre, the corresponding ␩6 -coordination can occur in a similar fashion to that long known for arenes. Computational and experimental studies have given an insight into the strength of 1,3,5-triphosphabenzene binding relative to

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Scheme 6. Reactivity of a monophosphabenzene towards Pd and Au moieties; the analogous reactions for the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene were unsuccessful.

benzene [13]. Of particular note is the increased strength of ␲backbonding to 1,3,5-triphosphabenzene due to the lower energy LUMO compared to benzene, as well as greater steric protection from the tert-butyl groups. The first example of ␩6 -bound 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene was reported by Cloke and Nixon et al. using metal vapour techniques whereby scandium metal was reacted with tert-butylphosphaalkyne [20]. This features a highly unusual scandium (I) triple decker species, [{(␩5 -P3 C2 t Bu2 )Sc}2 (␮-␩6 :␩6 P3 C3 t Bu3 )] (19, Fig. 3), where a planar 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene is coordinated on each side by a scandium

bearing a 1,3-diphospholide anion. The species was characterised primarily by X-ray crystallography. The low electron count of this complex (22 valence electrons) is of interest, as is the planarity of the central 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring, contrasting with some other triple decker examples that exhibit a puckered central ring [21,22]. The first mononuclear ␩6 -2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene bound complexes were reported by Binger et al. and involve coordination to a variety of metal centres [23]. These were principally synthesised by displacement of other ␩6 -bound aromatic ligands such as benzene and naphthalene from the metal

Fig. 2. Top – ␩1 -Coordination of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene to a cationic gold fragment. Bottom – NICS-1 values for various aromatic compounds and 1,3,5triphosphabenzene complexes. For computational simplicity, 2,4,6-trimethyl-1,3,5-triphosphabenzene was used instead of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene, PH3 instead of P(t Bu)2 (o-biphenyl) and 1,3-bis(2,5-dimethylphenyl)imidazole-2-ylidene in the place of the NHC. We note that other computations have highlighted the importance of using tertiary-carbon centres in calculations [18] rather than simplified primary carbon centres – this work is discussed in more detail in Section 5. The use of primary carbon substituents in the above computations was not found to affect the qualitative conclusions.

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Scheme 7. Protonation, methylation and silylation of 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene.

Scheme 8. Synthesis of ␩6 -2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene complexes.

P

Sc P

P Sc

19 Fig. 3. The scandium triple-decker complex 19.

centre, as shown in Scheme 8. Such displacement reactions give an indication of the binding strength of the 1,3,5-triphosphabenzene ring. For example, 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene displaces toluene from Mo(CO)3 and W(CO)3 moieties, and therefore it was concluded that it bonds more strongly than toluene. In the examples reported, no primary ␩1 -interactions are observed with the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring, which was suggested may be due to the bulky nature of the tert-butyl groups on the ring. As shown in Scheme 8, the synthesis of 23 gives a very poor yield, but an equilibrium reaction has been ruled out since distillation

7

of the by-product benzene from the product does not help drive the reaction to completion. Instead, 80% of the starting material and 16% of product are isolated from the reaction mixture upon work-up. In all of these complexes 20–23, analysis by 31 P{1 H} NMR spectroscopy indicates that there is ␩6 -coordination as there is only one signal for the 1,3,5-triphosphabenzene ring. This shows that all P atoms are in identical environments and the peak is shifted significantly upfield relative to the free ligand due to metal coordination. Compound 22 was also shown to be active in the catalysis of the hydrogenation of 1-hexene, although significantly slower than the analogous ␩6 -benzene complex under comparable conditions. Subsequently, Tate et al. reported the X-ray structure of [Mo(CO)3 (␩6 -P3 C3 t Bu3 )], 20, along with an alternative synthesis for the complex [24]. This involves the displacement of labile EtCN ligands from [Mo(CO)3 (EtCN)3 ], as shown in Scheme 9. The essentially identical P C bond lengths and planarity of the 1,3,5-triphosphabenzene ring corroborates that aromaticity is maintained upon coordination. This observation contrasts with that of the analogous complex [Cr(CO)3 (␩6 -C6 H6 )], where alternating long/short C C bond distances are observed. It is possible for a 2,4,6-tri-tert-1,3,5-triphosphabenzene to act as both an ␩6 - and ␩1 -donor simultaneously, as demonstrated by compound 24 [24]. Here, the 1,3,5-triphosphabenzene ring donates 8 electrons in total, coordinating ␩6 - to Mo and ␩1 - to a separate platinum centre. In theory, it is possible for further ␩1 -coordination to occur from the other phosphorus atoms, however, further ligation was not observed when excess platinum was added. In the same report, the synthesis of ␭5 -phosphorus atoms within a 1,3,5-triphosphabenzene framework was reported along with their coordination to metal centres [24]. Further characteristics of these ␭5 -1,3,5-triphosphabenzene complexes were reported in the form of both experimental and computational studies, however, further discussion is outside the defined scope of this review. Ruthenium complex 25 was reported by Francis et al. in a displacement reaction involving 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene and [Cp*Ru(NCMe)3 ][PF6 ] [25]. Surprisingly, the corresponding reaction between 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene and the analogous Fe compound led to no reaction. The further reactivity of compound 25 was also investigated, as detailed in Scheme 10. Reaction with MeLi involved nucleophilic attack on a 1,3,5-triphosphabenzene phosphorus atom to give compound 27, likely due to the polarisation of P␦+ C␦− bonds, to give the first example of a ␩5 -triphosphacyclohexadienyl complex. Reaction of 25 with LiAlH4 was expected to also react on P, however, the product contains a saturated C centre, likely due to reaction on P and subsequent hydride migration reaction. Complexes 26 and 27 were also characterised by NMR spectroscopy and X-ray crystallography studies. The first group 9 complex of a triphosphabenzene, 28, was prepared as shown in Scheme 11 [25]. The weakly coordinating anion [BArF 4 ]− was necessary in order to encourage binding of the 1,3,5-triphosphabenzene unit in preference to the anion. The complex remains intact in the presence of large excesses of other ligands such as benzene, PPh3 , CO or 2,2 -bipyridine. This contrasts with the analogous [Rh(COD)(arene)]+ complexes, where arene ligands are rapidly displaced, which is likely due to the higher degree of ␲-backbonding and steric protection provided by the bulky nature of the 1,3,5-triphosphabenzene unit. In a similar way to complex 25, the phosphorus atoms in complex 28 are activated compared to the free ligand. For example, 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene itself is resistant to reaction with water, however, compound 28 reacts readily at 0 ◦ C, as shown in Scheme 11. Compound 28 was unreactive with respect to oxidative addition of H2 , MeI or bis(catecholato)diborane, and therefore was deemed to be not useful catalytically.

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Scheme 9. Alternative synthesis of 20 and further coordination to a platinum centre.

Scheme 10. Synthesis and reactivity of 25.

P P

½[Rh(COD)Cl] P

Na[BAr f4]

2

P P P Rh

[BAr f4]

OR P

P H 2 O or EtOH

P

[BAr f4 ] H

Rh

R = H or Et

28

29

Scheme 11. Synthesis and reactivity of Rh-complex 28.

A further reaction of interest was formulated in response to the reaction shown at the top of Scheme 12, where reaction of an iron complex with a mixture of alkynes and phosphaalkynes led to formation of [Fe(␩6 -1,3-diphosphabenzene)(␩4 -1,3-diphosphete)] (30) [26]. Thus Eggers et al. probed a similar reaction using only the phosphaalkyne (Scheme 12) [14]. However, the reaction did not deliver the anticipated iron triphosphabenzene/1,3-diphosphete product, delivering instead a toluene/1,3-diphosphete iron complex (31) and pentaphosphaferrocene (32). DFT calculations suggested that a low energy pathway may be responsible for the rearrangement of expected to observed product. This includes distortion of the 1,3,5-triphosphabenzene ring from planarity, bearing much similarity to 1,4-addition, ring contraction and hydrogenation reactions that will be discussed in more detail below. 4.3. Ring contraction 4.3.1. Extrusion of a C-unit 2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene can also exhibit some unusual reactivity for which there is no analogue in organic

chemistry. Nixon et al. reported (Scheme 13) the contraction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring to extrude an atom when reacted with a N-heterocyclic carbene (1,3,4,5tetramethylimidazol-2-ylidene) [27]. The structure of 33 was confirmed by mass spectroscopy and 31 P, 1 H and 13 C NMR spectroscopy. The product was also coordinated to molybdenum to afford [Mo(CO)3 (␩5 -P3 C2 t Bu2 )Ct Bu(carbene)], which allowed for crystallisation and structure determination by single crystal X-ray diffraction. A number of structural features point to the triphosphole ring of 33 as being highly delocalised system; for example, the P C bond distances within the ring are almost identical and the C C bond outside the triphosphole ring is very short. The complex was also characterised by NMR spectroscopy and mass spectrometry. Computational studies were undertaken in order to gain a deeper insight into the mechanism of the ring extrusion [27]. These calculations indicated that the ring deforms with one carbon atom moving out of the plane of the ring. Simultaneous build-up of positive charge on the carbon atom of the diaminocarbene and negative charge over the PCPCP ring also occurs, in an analogous way to that of an ylide. The next stage involves ring closure via

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Scheme 12. Reactions of phosphaalkynes/acetylenes and phosphaalkynes at low-valent iron centres.

Scheme 13. Extrusion of a C-unit from a triphosphabenzene by reaction with a N-heterocyclic carbene.

P P bond formation, followed by a P C bond breaking to form the product. The reaction pathway shows that the Gibbs free energies of the transition states are not significantly higher than the intermediates, therefore the reaction should proceed readily. The atom extrusion reaction was found to be reversible with regard to the reformation of 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene in the presence of a platinum centre (Scheme 14) [27]. When compound 33 is left at room temperature for 4 days in the presence of [PtCl2 (PR3 )2 ] (PR3 = PMe3 or PMe2 Ph), a mixture of platinum complexes are formed, as well as 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene (observed by NMR spectroscopy). Calculations were also carried out on the mechanism of this system, which reveals that the first step involves losing the carbene to the metal complex. This is then followed by rearrangement of the remaining compound such that the carbon atom becomes re-incorporated into the ring to reform 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene.

4.3.2. Extrusion of a P-unit Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with a lithium amide also causes a ring contraction to give both a 1,3diphospholide anion (34− )and a phosphorus–carbon cage species (35) as the products (Scheme 15) [28]. In contrast to the previous example, here it is a phosphorus atom that is extruded with the aid of the amide to form an aminophosphinidine fragment, PN(Ph)SiMe3 . The 1,3-diphospholide anion was isolated as [Li(TMEDA)(␩5 -P2 C3 t Bu3 )] (Li(TMEDA)34). Both products were characterised by mass spectrometry, single crystal X-ray diffraction and 31 P, 13 C, 7 Li and 1 H NMR spectroscopy. The X-ray crystal structure of compound Li(TMEDA)34 reveals the planar nature of the anionic P2 C3 t Bu3 ring, which is ioncontacted in an ␩5 -fashion to a Li(TMEDA) unit. This evidence, along with the P C bond lengths within the ring, are indicative of the aromatic character of the diphospholide ring. The cage structure 35 is a tricyclic organophosphorus compound containing a combination of five-, four- and three-membered rings. The full atom connectivities were verified by the single crystal X-ray diffraction study, however, there was found to be significant disorder within the structure, leading to a high R factor {R1 (I > 2(I) 9.4%)}. Therefore, although useful in determining the topological connectivity, detailed comments of the bond dimensions are of limited value. In an attempt to remove this disorder, 35 was coordinated to a PtII centre, however, although it was possible to analyse this structure by NMR spectroscopy, crystals suitable for X-ray analysis were not obtained. In order to help elucidate the mechanism of the reaction, detailed computational studies were undertaken [28]. This initially

Scheme 14. Reformation of 1,3,5-tri-tert-butyl-1,3,5-triphosphabenzene from 33. The intermediate is proposed from computational investigations on the P3 C3 H3 simplified analogue.

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Scheme 15. Ring-contraction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene by reaction with a lithium amide.

Scheme 16. Ring-contraction of a triphosphabenzene by reaction with a sodium amide.

involved using the model system NH2 − + 2P3 C3 H3 , but further calculations were also carried out on a system that included substituents. First, the formation of 34− was investigated. The first step involves nucleophilic attack by the amide anion on either the carbon or phosphorus atom, which echoes intermediates for the synthesis of compound 33 described above. Attack at the phosphorus atom is marginally favoured, though nucleophilic attack at either atom leads to the same intermediates, namely deformation of the ring such that the P atom moves out of the plane and the ring contracts to form a new C C bond. Again, this is analogous to the reaction with the carbene described above. Although it would be possible for the amide to attack the ring in a 1,4-addition, an energy minimum for this species was not found. The overall energy profile for the reaction shows that each barrier is relatively low and therefore explains why the reaction can proceed easily at room temperature. The reaction profile was also assessed with the synthetically relevant substituents. The inclusion of these extra atoms alters the relative energies of the intermediates, however, does not affect the overall mechanism. The next part of the reaction involves attack of either the carbon or phosphorus atom in the second 1,3,5-triphosphabenzene molecule by the phosphinidene. It is found by computation that both 1,2- and 1,4-addition products are possible intermediates in the reaction mechanism, followed by ring extrusion to form the cage product. In order to probe the mechanism further, the reaction was also carried out with Na[NPh(SiMe3 )] (Scheme 16) [28]. This reaction also proceeded at room temperature and was monitored by 31 P NMR spectroscopy, revealing a long lived intermediate, which corresponded to the 1,4-addition product of the phosphinidene complex to the 1,3,5-triphosphabenzene. The reaction was complete after 3 days at room temperature to give 35 and the sodium salt Na34. Another reaction involving phosphorus atom extrusion from 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene to form a diphospholide anion has been reported by Cloke and Nixon et al. from the simple reaction between 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene and a potassium mirror in THF to form [K(THF)(␩5 -P2 C3 t Bu3 )] [29]. The product was confirmed by single crystal X-ray diffraction and contains an interesting chain-like structure of alternating K(THF) cations and P2 C3 t Bu3 anions. The potassium bonds in ␩5 -fashion to the anion ring, which itself displays significant electron delocalisation as depicted by short P C bonds and a planar structure. The P3− anion was also identified amongst the other products, by further reaction with Me3 SiCl and identification of the product (P(SiMe3 )2 anion) by NMR spectroscopy.

2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene reacts with alkyllithium reagents to form salts, [Li(THF)n ][P3 (R)C3 t Bu3 ] (R = n Bu, Me), where one phosphorus atom is attacked by the nucleophilic alkyl group to leave an anionic species, Li(THF)2 36 (Scheme 17) [30–32]. Additional reactivity of this compound towards alkyl halides in discussed in the section on 1,4-addition reactions. Of direct relevance to this section is the reaction that occurs when the compound is heated to 80 ◦ C for 3 h, whereby the anion rearranges into a 1,3-diphospholide ion and can be subsequently reacted with an alkyl halide to yield a 1,3-diphosphole (37). This has been achieved with a range of primary alkyl halides, however, reactions with secondary and tertiary alkyl halides were not possible, likely due to the steric influence of the tert-butyl groups. Reaction of 2,4,6-tri-tert-butyl1,3,5-triphosphabenzene with 2-pyridyllithium, proceeds at room temperature to yield the corresponding 1,3-diphosphole. Reaction with diphenylchlorophosphine was also tested and yielded a product with direct P-P bond. Reactivity of the 1,3-phospholide anion with ␣,␤-dihalogens was also investigated. 1,3,5-Tri-tert-pentyl-2,4,6-triphosphabenzene was found to have analogous reactivity when reacted with n BuLi and subsequently MeI to give the corresponding product in 39% yield. The fate of the eliminated phosphorus atom is likely to be in phosphinidene-derived oligomeric by-products, as indicated by 31 P{1 H} NMR spectroscopy studies. Gallium reagents can also effect the contraction of the ring [33]. For example, reaction of 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene with the anionic gallium heterocycle 38− , gives a diphospholide anion via phosphorus atom abstraction (Scheme 18). The fate of the extruded P atom is unknown, however, it is likely to be an insoluble gallium phosphide. In a similar way, metal coordinated 1,3,5-triphosphabenzene rings can contract with expulsion of a phosphorus atom. This can be seen in the reaction between triphosphacyclohexadienyl anion [MeP3 C3 t Bu3 ]− and FeCl2 (Scheme 19) where the products are a tetraphosphaferrocene (39) and an unusual cage complex (40) [25]. The eliminated PMe units aggregate to form (PMe)n cyclophosphanes. 4.4. 1,4-Addition 1,4-Addition to the 1,3,5-triphosphabenzene ring is also possible, with the use of organolithium and Grignard reagents (Scheme 20) [34]. Initial addition of a Grignard reagent across 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is postulated to be synchronous, due to the regio- and stereoelectivity of the

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R

1

1

P

R

P

P R1

R2 P

1

R2Li -78 °C to 25 °C THF

R

P

R1 P

R1 [Li(THF)2]+

R1 80 °C, 3 h THF -[PR2]

R1

P

R3Hal THF, 25 °C 10 min -LiHal

P R1

11

R1

R1

P

P R3 R1

[Li(THF)2]+

Li(THF)236

Li(THF)234

37

R1

t

Bu

t

Bu

t

Bu

t

Bu

t

Bu

CMe2Et

R3

Me

n

n

Bn

PPh2

Me

Hal

I

I

Br

Br

Cl

I

32

60

48

Not determined

39

Yield of final 63 step (%)

Pr

Bu

Scheme 17. Anion formation and ring-contraction reactions of 1,3,5-triphosphabenzenes.

Scheme 18. Ring contraction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene using gallium heterocycle 38− .

reaction, however, the product was isolated as a salt. The transition state for this process is postulated to be 41, where the 1,3,5-triphosphabenzene ring has distorted into a boat conformation, which allows for concerted addition. The organic R group

Me P

P Me P P

P 36-

adds to the phosphorus atom, therefore the negative charge is delocalised over the rest of the ring in the anionic product. This type of charge separated product has not been previously described. X-ray crystal diffraction of the product 42 (R = Bn) revealed the Mg cation is stabilised by THF molecules and the 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene ring is planar, except for the newly functionalised P atom, which lies out of the plane, corroborating the delocalisation of charge over this section of the ring. Protonation of these organomagnesium compounds led to the formation of 1,4-triphosphacyclohexadienes (43, for R = Me or Ph) [34]. When Grignard reagents with bulky substituents (such as R = i Pr) are used, however, a triphospha-Dewar-benzene (44) is formed, where the H and R groups have added in adjacent positions.

P

P

Fe

½ FeCl2 P

+

Fe

P P Me

P

P

P

P

Me

P Fe

P

P

Me

P 39

40

Scheme 19. Reaction of 36− with FeCl2 .

Scheme 20. Reactivity of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with Grignard reagents and subsequent protonation of the products.

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P P

RLi P

THF

R R'

R P

[Li(thf)n]+

P

P

P R'X

P

-LiX

45

Li(THF)n36 I2 THF -LiI R

I

Et3O+ BF4R = Me Me

P

R = Me 46

Me

Et P

P P

P

P

P 1 47

+

:

P P

P Et 1 48

Scheme 21. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with organolithium reagents and further reaction of the products with alkyl halides, iodine and triethyloxonium tetrafluoroborate.

Further elucidation of the mechanism has not yet been achieved. X-ray crystallography was used to confirm the structures and stereochemistry of the products, including the boat conformation of the protonated product 43. Similar reactivity can be achieved with organolithium reagents (Scheme 21). Reaction with n-butyllithium and methyllithium formed salts Li36 where the lithium cation is stabilised by either TMEDA or a coordinating solvent such as THF [30,31]. Alkylation of the anions to form 1␭5 ,3,5-triphosphabenzenes (45) is possible by further reaction with alkyl halides. Primary alkyl halides provide the highest yields and no reaction with t BuBr was observed, likely due to the high steric bulk provided by this alkyl group. NMR spectroscopic data confirmed the products and also indicated that the aromaticity of the ring is maintained in the products. If iodine is used in the place of an alkyl halide, halogenation of the anionic fragment is achieved giving compound 46. Similarly, if a Meerwein salt is used to trap the reactive species, a 1:1 mixture of 1␭5 ,3,5triphosphabenzene (47) and 1,4-addition product (48) are formed. Comparison of the alkylation with the protonation reaction to form 44 (Scheme 20) by 31 P NMR spectroscopy provides mechanistic insight into the protonation reaction. The protonation of anion 42− was followed by 31 P NMR spectroscopy, revealing an intermediate with significant similarities to alkylated products 45. This implies that the proton attacks the substituted phosphorus atom to form a ␭5 centre, then undergoes a 1,2-H shift to form the triphospha-Dewar-benzene product 44. Calculations corroborate this proposed mechanism, since they show that the negative charge of the anion is mostly centred on the alkylated phosphorus atom. This P atom is not blocked completely by the bulky t Bu substituents so is available for subsequent attack by a proton. [1+4] cycloadditions have also been observed to occur 1,4-across the 1,3,5-triphosphabenzene ring. For example, silylene Si[(NCH2 t Bu)2 C6 H4 -1,2] reacts with 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene at room temperature to yield 49 as shown in Scheme 22 [35]. This was characterised using NMR spectroscopy and X-ray analysis. Attempts to mimic this chemistry with heavier group 14 analogues resulted in no reaction. In a similar way, Russell et al. used latent sources of [PX2 ]+ and [AsX2 ]+ to form cage structures upon reaction with 2,4,6-tritert-butyl-1,3,5-triphosphabenzene (Scheme 23) [36]. The reagents chosen were made by reaction of EX3 (E = P, As; X = Cl, Br) with the Lewis acid GaCl3 . Thus, reaction with AsCl3 /GaCl3 or AsBr3 /GaCl3 yielded open-book-shaped cages (50) where halogen atoms have migrated from the arsenic to the P atoms of the P3 C3 t Bu3 core. This

Scheme 22. [1+4] cycloaddition with a silylene.

is postulated to be due to an initial 1,4-addition leading to an unstable AsV centre that sheds halide anions to return to AsIII . Conversely, reaction with PCl3 /GaCl3 leads to an apparent chelotropic addition product 51, where the P unit adds to the 1- and 4-positions of the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring in a similar fashion to the silylene described in Scheme 22. PMeCl2 /GaCl3 also reacted to form an analogous product. The reaction does not proceed if GaCl3 is not present, suggesting the reaction of 2,4,6tri-tert-butyl-1,3,5-triphosphabenzene with an EX2 + electrophile proceeds in a ␲ 6s + ␻ 0s process, although initial formation of an ␩1 -P cation would also be possible. Confirmation of the mechanism for the formation of these products has not yet been achieved. It was observed that when PCl3 addition product is dissolved in THF at room temperature, PCl3 and 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene are reformed. Lammertsma et al. investigated the reaction of 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene with a complexed phosphinidene, [PhPW(CO)5 ], which was formed in situ [37,38]. When this phosphinidene was reacted with substituted benzenes, the 1,4-addition product was formed, therefore similar reactivity was expected with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene [39,40]. The products, however, do not resemble compounds formed from direct 1,2or 1,4-addition to the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring, therefore required further characterisation by NMR spectroscopy and X-ray analysis to elucidate the structures (Scheme 24) [37]. Compound 52 is suggested to be formed by initial 1,2-addition of [PhPW(CO)5 ] to one P C bond to form a three membered ring. This gives rise to both syn- and anti-isomers as products, however, the further reaction of the syn-isomer was reasoned as more feasible and was further investigated by DFT calculations. This led to the use of computational techniques to optimise of the geometries

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Scheme 23. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with sources of [PX2 ]+ and [AsX2 ]+ .

[M]

P

[M]

[PhPW(CO)5]

P P

P

H P

P P

[M] = W(CO)5

[M]

P P P

[M] P

P P 52

H P

carbon atom in the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring, however the electrophilic complexed phosphinidene is more likely to attack phosphorus.

Ph P

P P P

Ph P

53a

Scheme 24. Reaction of a complexed phosphinidene with 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene.

for the simplified 1,2-addition, transition structure and the product (W(CO)5 was omitted and t Bu groups replaced by H atoms). The reaction is therefore suggested to proceed via non-ionic, unimolecular electrophilic aromatic substitution, where a proton is abstracted from the phosphinidene by a 1,3,5-triphosphabenzene carbon atom and replaced by a 1,3,5-triphosphabenzene phosphorus atom to form the caged product. No other intermediates were found along the reaction pathway, suggesting a concerted process. Compound 53a is suggested to be formed from the original syn-1,2-addition product described above, followed by a 1,3sigmatropic shift. This then leads to a rearrangement into the quadricyclane structure observed. MO theory was used to calculate the relative energies of this effective 1,4-addition product and compound 53a (again, the simpler parent structures were used for calculations), which showed the quadricyclane structure to be more stable than the norbornadiene isomer [37]. This contrasts with hydrocarbon chemistry, where the norbornadiene form is heavily favoured due to the ring strain present, and nicely reinforces the concept that phosphorus can accommodate smaller bond angles than carbon. [MePW(CO)5 ] was also used in an analogous reaction and formed only the quadricyclane product (Scheme 25) [37]. This is formed in an 8:1 equilibrium ratio with the 1,4-addition product, as determined by 31 P NMR spectroscopy. Heating the reaction gave the same ratios, therefore suggesting a thermal equilibrium is present between these two products. The two possible mechanisms for this transformation are a Woodward–Hoffmann thermally forbidden [2+2] cycloaddition or a stepwise dimerization of two phosphaalkene bonds. When exposed to sunlight and heated, full conversion to 53b is observed, following the photochemically allowed [2+2] addition. The difference in reactivity between silylenes, carbenes and the coordinated phosphinidene is postulated to be due to the difference in nucleophilicity of the reagent [37]. Silylenes and carbenes are strongly nucleophilic, therefore are likely to initially attack a

4.4.1. Small molecule addition(s) In this section we shall consider reaction of 1,3,5triphosphabenzenes with classical small molecules. It is striking that the net chemical reactivity shows similarities to chemistries that are normally associated with the transition elements, which is a topical area that has itself been recently reviewed [41]. Whereas such reactivity has precedent in heavy homonuclear p-block multiple bonded chemistry, it is very surprising for it to be observed for P/C multiple bonds and especially where those bonds are part of an aromatic system. 4.4.2. Dihydrogen and related reduction reactions A recent publication by Stephan et al. sheds light on the mechanistic reasoning behind the previous 1,4-addition reactions described [42]. The report describes the uncatalyzed, direct hydrogenation of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene under mild conditions. The triphosphabenzene uses synergic acceptor and donor orbitals to mimic a transition metal complex and activate a molecule of hydrogen in a reversible manner. The reaction continues via an irreversible rearrangement to give the final, diastereotopic products. The reaction was followed by NMR spectroscopy, which identified the initial 1,4-addition of hydrogen as a key intermediate. This was confirmed by use of para-hydrogen, which additionally provided evidence that both hydrogen atoms add from the same H2 molecule and, importantly, showed that the reaction is reversible. Finally, computational techniques were used to model possible transition states in order to probe the full mechanism of the addition (Fig. 4) [42]. The key step involves distortion of the planar 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene ring into a boat conformation so that H2 can approach from above the ring, aligned with the 1,4axis of the ring. This emphasises the flexibility of the ring, whilst still maintaining aromaticity, as was suggested by NICS values described above (see ␩1 -section) [17]. This also helps describe some of the above 1,4-addition reactions, where addition across the ring is favoured over 1,2-addition. Indeed, distortion into a boat conformation enhances the zwitterionic character of the 2,4,6tri-tert-butyl-1,3,5-triphosphabenzene ring, increasing donor and acceptor ability important in synchronous reactions. Isomerisation between possible conformations of the intermediates can occur, before the irreversible hydride shift reveals the final products 54a and 54b (Scheme 26). The product distribution depends on the relative proportions of two of the conformers, one of which leads to the minor product and one leads to the major product. The isomeric product structures were verified using NMR spectroscopy and X-ray diffraction analysis. These calculations explain much of the reactivity described by Jones et al. in 2004 when they attempted to reduce 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene with a variety of hydride sources (Scheme 27) [33]. Upon reaction with 3 equivalents of LiAlH4 ,

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Scheme 25. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with [MePW(CO)5 ].

a triphosphabicyclo[3.1.0]hexanediyl aluminate (56) was formed. The structure was determined using NMR spectroscopy and Xray crystal analysis. The reaction is too fast to monitor by NMR spectroscopy, however, Scheme 30 shows the proposed mechanism going via intermediate 55. This echoes the hydrogenation reaction described above significantly, with 1,4-addition followed by rearrangement into a bicyclic product. The final step involves hydroalumination of the last P C double bond. Acid hydrolysis and basic work-up provided the free bicyclic product 57 as a mixture of isomers. Analogous reaction with LiGaH4 provided the gallium analogue of 56. Reactions with neutral element hydride reagents (Scheme 28) were also attempted, such as [AlH3 (NMe3 )] [33]. This provided a mixture of products 58–60 along with unreacted 2,4,6-tritert-butyl-1,3,5-triphosphabenzene. It is clear that 58 is closely

related to the intermediate 55 and that 59 and 60 are formed in the subsequent hydroalumination reactions of this compound. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with three equivalents of [GaH3 (quin)] (quin = quinuclidine) yielded only the gallium analogue of 58, reflecting the less hydridic nature of gallium hydrides compared to aluminium. Acid quenching and basic work-up provided the free phosphacycle 54b. Me3 SnH was also reacted with 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene (Scheme 29) in an attempt to reduce the species, which resulted in compound 61 [33]. Again, the analogies to the hydrogenation reaction of 2,4,6-tri-tert-butyl1,3,5-triphosphabenzene are clear in this reaction. Remarkably, upon quenching and after work-up, compound 63 was formed, suggesting that under some conditions, it is possible to reverse

Fig. 4. Scheme showing pathway of significant located stationary points (DFT) in the reaction of a 1,3,5-triphosphabenzene and H2 . Species labelled A represent the various conformers of 1,3,5-triphosphacyclohexa-1,4-dienes.

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Scheme 26. Hydrogenation of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene.

the rearrangement of 1,4-hydrogenated 2,4,6-tri-tert-butyl-1,3,5triphosphabenzene. There is no spectroscopic evidence for 62, therefore it was suggested that it is present as a minor species in equilibrium with 61. In order to crystallise 63, it was coordinated to W(CO)5 , allowing for full determination of the conformation and structure of 64 [33]. ‘GaI’ is known as a good reducing agent within the remits of organic chemistry, therefore it was also used in an attempt to reduce 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (Scheme 30) [43,44]. Instead of the desired reduction, the reaction is proposed to proceed via a 1,4-addition of GaI to give 65, followed by reaction with a further equivalent of GaI in a disproportionation reaction. Proton abstraction from the solvent (toluene) completes the reaction to give the observed product 66.

4.4.3. With alkynes and alkyne analogues As a particular class of 1,4-additions, 1,3,5-triphosphabenzenes also displays reactivity not often seen by arenes with respect to cycloaddition reactions. Whereas benzene and phosphabenzenes require photoactivation and/or high temperatures, 1,3,5-triphosphabenzenes were found to readily react with a range of alkynes in [4+2] cycloadditions under mild conditions (Scheme 31) [45]. The homo Diels–Alder products are not visible by 31 P{1 H} NMR spectroscopic monitoring of the reaction, nor when the reaction is carried out in a 1:1 ratio of alkyne to the 1,3,5-triphosphabenzene indicating that they are quickly consumed to form the final products. The regioselectivity for the reaction is likely to be determined by steric effects, since altering the polarity of the alkyne does not influence the product. Disubstituted alkynes do not react, which also highlights the importance

Scheme 27. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with anionic element hydrides.

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Scheme 28. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with neutral element hydrides [AlH3 (NMe3 )] and [GaH3 (quin)] (quin = quinuclidine).

Scheme 29. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with Me3 SnH.

Scheme 30. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with ‘GaI’.

of steric factors. The exception is cyclooctyne, where the change in reactivity is attributed to the ring strain present in the triple bond. With monosubstituted alkynes, the electronic influence of the substituent can affect the reactivity, for example, electron withdrawing substituents decrease necessary reaction times and temperatures. The reactions were also carried out with different substituents on the 1,3,5-triphosphabenzene ring. In the reaction with methylpropynoate, changing from tert-butyl to tert-pentyl gave little differences in yield (71% and 60%, respectively), but altering the substituent to methyl-cyclohexyl or 1-adamantyl significantly decreased the yield (21% and 3%, respectively).

The products were analysed by microanalysis, mass spectroscopy and NMR spectroscopy. Final confirmation of all product structures was achieved through single crystal X-ray analysis. The reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with tert-butylacetylene was also probed [45], however, this occurs only at temperatures above 100 ◦ C and gives significantly different product to the reactions described above. 2,4,6-Tritert-butyl-1,3,5-triphosphabenzene reacts with 2 equivalents of tert-butylacetylene to afford a tricyclic product, which was characterised by mass spectrometry, elemental analysis, NMR spectroscopy and X-ray crystallography (Scheme 32). Although

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Scheme 31. Cycloaddition reactions between 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene and alkynes. Similar reactivity, but with different yields, are observed for alternative 1,3,5-triphosphabenzenes.

Scheme 32. Cycloaddition reaction between 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene and tert-butylacetylene.

attempts were made, intermediates have not been detected, therefore a plausible mechanism has not yet been determined [45]. Similar reactivity can be observed with phosphaalkynes: [4+2] cycloadditions to form tetraphosphabarrelenes (71) occur readily under mild conditions (Scheme 33) [46]. Contrary to the reaction with alkynes, only one phosphaalkyne adds to the

Scheme 33. [4+2] Cycloaddition between 1,3,5-triphosphabenzenes and phosphaalkynes.

2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene unit. Reaction with di(isopropylamino)phosphaalkyne, however, leads to an alternative product: a tetraphosphasemibullvalene derivative 72. The structure was deduced using NMR spectroscopy and X-ray crystal analysis. It was reasoned that the increased nucleophilicity of the (diisopropylamino)phosphaalkyne P atom explains the difference in reactivity from phosphaalkynes. This, in turn, arises from increased ␲-donor ability of the R2 N substituent. The more nucleophilic P atom can therefore attack a carbon atom on the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene ring, followed by ring closure to form the tricyclic unit. Further nucleophilic attack and valence isomerisation complete the mechanism. A [2+2] cycloaddition mechanism was also considered, but it was suggested that this is less probable owing to the diminished olefinic character of the aminophosphaalkyne and the fact that high level calculations found the semibullvalene to be more stable than the cuneane form. 4.4.4. With alkenes Alkenes can also react in similar [4+2] cycloaddition reactions with 1,3,5-triphosphabenzenes (Scheme 34) [47]. These were attempted with a range of substrates: acceptor-substituted alkenes reacted under mild conditions, however, electron poor or styrene substituted alkenes required a higher excess of alkene and higher temperatures to react. The regiochemistry of the reaction with terminal alkenes arises from electronic influence, therefore it is unsurprising that reaction with more hindered substrates leads to incomplete regiocontrol as the steric and electronic factors compete in product formation. Cyclic alkenes were also used, including maleic anhydride and norbornene, to give analogous products. The reactions with ethyl maleate (Scheme 35) involves initial reaction to give the cis-[4+2] addition product 74a, followed by isomerisation to the trans-product 74b, likely due to the reversible nature of the Diels–Alder reaction. Cyclopropene reacts with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzenes even at very low temperatures (Scheme 36) and requires

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Scheme 34. Cycloaddition reactions of 1,3,5-triphosphabenzenes with alkenes. For R = H, R = t Bu, t Pen, 1-methylcyclohexyl, 1-adamantyl. For all other R , R = t Bu.

EtO2C

P P

EtO2C CO2Et EtO C 2 P

P P

P 74a

isomerisation

CO2Et

EtO2C P P

P 74b

Scheme 35. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with diethyl maleonate followed by isomerisation.

Scheme 36. Cycloaddition reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with cyclopropene.

is preferred (as shown in Scheme 38), though experimental data to corroborate this has not been reported.

Scheme 37. The regioselective reaction of cyclopentadiene with 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene.

2 alkene equivalents for completion of the reaction [47]. This is due to the initially formed product reacting with a further cyclopropene unit to form an unusual cage structure 75. 1,3,5-Triphosphabenzenes with different substituents were also tested in analogous reactions with norbornadiene and maleic anhydride [47]. Tri(tert-pentyl)- and tri(methylcyclohexyl)-1,3,5triphosphabenzene derivatives react in a similar way to the tri-tert-butyl derivative, however, the tri-adamantyl species forms cycloadducts in much lower yield, likely owing to the increased substituent steric bulk. The products from these reactions with alkenes were determined by elemental analysis, mass spectrometry, NMR spectroscopic data and, where possible, X-ray analysis. Reactions were also attempted with cyclic dienes: cyclopentadiene and norbornadiene. The reaction with cyclopentadiene proceeds regioselectively in a 1:1 [4+2] cycloaddition across the ring in a 1,4-manner (Scheme 37). For the reaction with norbornadiene, both mono- (77) and bisadducts (78) are possible (Scheme 38). These were not separated, and though they are present in a 3:1 ratio (as observed by 1 H NMR spectroscopy), the structure of the major isomer has not been deduced. It was found by computation that exo annulation

4.4.5. Masked 1,4-addition reactions The iron complex [(1,5-cyclooctadiene)(P3 C3 t Bu3 )Fe] (79) was originally thought to be an ␩6 -complex, however, subsequent investigation led to the thesis that it is of the structure [Fe(␩4 -cyclooctadiene)(␩6 -2,4,6-tri-tert-butyl-1,3,5-triphosphacyclohexa-2,5-diene-1,4-diyl)] [14]. This unusual structure was deduced using DFT calculations for geometry optimisation. This contrasts with the analogous Ru complex 22 [23], and is likely due to the ease of oxidation of iron to the +2 oxidation state, compared to ruthenium, where neutral metal and planar ligand are maintained. This is also aided by the excellent ␲-acceptor ability and low reduction potential of the 1,3,5-triphosphabenzene ring. The geometry is corroborated by NMR studies of 79, which finds the ligand to be an AB2 system. The same system is found for the [1,3,5-triphosphabenzene]− species (discussed above), indicating the preferred geometry for the reduced triphosphabenzene ring involves the distortion away from planarity such that the 1- and 4-positions of the ring move towards each other (Scheme 39). Further reaction of 79 with [Cr(CO)5 (THF)] has been reported giving a complicated product involving the threefold reduction of the 1,3,5-triphosphabenzene ring, coordination of the chromium centre in an ␩1 -fashion as well as rearrangement of the cyclooctadiene ligand to form 80. 4.4.6. Other addition reactions In addition to the examples of 1,4-additions described above there are a number of reactions where addition occurs in a less specific fashion. Thus 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene reacts readily with lithium alkoxides (Scheme 40) to form 2,4,6-tritert-butyl-1,3,5-alkoxy-1,3,5-triphosphacyclohexanes (81) [48],

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Scheme 38. Cycloaddition of norbornadiene with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene.

Scheme 39. Top – synthesis of compound 79 and reaction with [(CO)5 Cr(THF)]. Bottom – structure of 22 compared to the computed structure of 79 .

Scheme 40. Reaction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene with lithium alkoxides.

which can be seen as addition of the lithium alkoxide to each nominal P C double bond. Lithium alkoxides in the presence of their respective alcohols have been previously shown to react with isolated phosphaalkene units, however, this example shows they are also able to react with P C bonds within an aromatic system

[49]. The addition is very regio- and stereoselective to form a highly distorted chair structure. The rate-determining step is suggested to be initial 1,2- or 1,4-addition of the first alcohol to the ring, thereby removing aromaticity from the ring. This then readily undergoes addition to the remaining, and now more reactive, P C bonds to

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Scheme 41. Reactions of 1,3,5-triphosphabenzenes with nitrile oxides.

form the product, explaining the lack of observable intermediates when the reaction is monitored by NMR spectroscopy. It is possible to oxidise these species by using either an excess of sulfur in the presence of triethylamine, or bis(trimethylsilyl)peroxide, which furnishes each P atom with a S or O centre, respectively (82 and 83). In a similar way, 81 (when R = Et) can be put under reflux with sulfonyl chloride to provide 84 [33]. Nitrile oxides also react with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (Scheme 41). When the nitrile oxides substituent is small, such as phenyl, a tris-adduct is formed (85). When the substituent is larger, such as mesityl, the triphosphabenzene contracts and extrudes a phosphorus atom with acyl substituent. A nitrile is also eliminated [50]. This difference in reactivity is assumed to arise after the formation of the monocycloadduct, since addition of the second nitrile oxide can occur from the top or bottom face. Addition to the 5Re/6Si-side (the opposite side to the original addition) results in steric clash between the R1 and R2 groups, therefore favours ring contraction to form 86 eliminates of a nitrile. Conversely, attack of the 5Si/6Re face avoids the steric clash and leads to the product 85. The products were confirmed by NMR spectroscopy, IR spectroscopy and X-ray crystallographic analysis. Heating 85a in toluene at 100 ◦ C for 14 days resulted in quantitative conversion to the product 87 in a [2+2+2] cycloreversion reaction [50].

5. Theoretical studies Computations are now routinely used in many aspects of chemistry, so the papers referred to in this section refer to those where calculations form the whole part or the major part of the publication. Their application to triphosphabenzene is principally in attempting to quantify aromaticity of these molecules and in assessing the stability of the aromatic cyclic form against other geometries which are not such a significant feature of the chemistry of arenes.

In this spirit, we start the story with the report from Hofmann, Schleyer and Regitz [51] of an evaluation of the stability of HC P trimers derived from the isolobal replacement of the CH units for P atoms in benzene, prismane, Dewar benzene, benzvalene and biscyclopropenyl – a spiro structure of formulation P3 (CH)3 which cannot be broken down into distinct HCP units was also computed but was found to be only marginally more stable than free HCP. Of course, in each of these structural forms, the introduction of three P centres leads to a range of isomers for each structural form. A line drawing of each isomer (save the spiro structure mentioned above) and its relative computed energy is shown in Fig. 5. The computed thermodynamic stability order for the triphosphorus substituted species was the benzenes > benzvalenes > Dewar benzenes > prismanes > biscyclopropenyls. Furthermore, the different isomers within each category have only a small effect on the overall thermodynamic stability. We refer the reader to the primary text [51] for a detailed examination of the stability of each isomer, but will focus this discussion upon the stability and aromaticity of the three possible isomers of triphosphabenzene itself, viz. the 1,2,3-, the 1,2,4- and the 1,3,5-isomers. Of these, only the 1,2,4- and 1,3,5-isomers have been observed experimentally, but the calculations suggest that in fact the 1,2,3-isomer in the most thermodynamically stable, being 6.7 and 8.9 kcal mol−1 more stable than the 1,2,4- and 1,3,5-isomers, respectively. Computed bond lengths in all three isomers were characteristic of aromaticity and the NICS values, although increased (i.e., less negative and thus less aromatic) compared to benzene, were characteristic of aromaticity. Nyulászi has carried out a similar set of calculations but with the important difference of using the experimentally relevant t Bu groups on the C atoms rather than simplified CH units [18]. This was found to have a profound effect on stability (Fig. 6) with the 1,2,3- and 1,2,4-isomers adopting non-planar geometries so as to minimise steric repulsion between adjacent t Bu groups. Furthermore, this resulted in the 1,3,5-isomer being the most stable (14.1 kcal mol−1 compared to the 1,2,4-isomer and 29.5 kcal mol−1

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Fig. 5. Line drawing and relative energies (kcal mol−1 ) of isomers of P3 (CH)3 calculated at the HF/6-31G* level. Note the zero point is defined relative to the lowest energy form within each valence isomer – comparisons of energies between columns is not intended.

compared to the 1,2,3-isomer). He makes the interesting point that the destabililation energy of 45–50 kcal mol−1 from the interaction of adjacent t Bu groups in such a system more than outweighs the 20 kcal mol−1 aromatic stabilisation of 1,3,5-triphosphabenzene. The reactivity of 1,3,5-triphosphabenzene with respect to reduction to form anions, including a comparison with its all organic, mono-phospha and pyridyl analogues, has been examined by computational techniques in combination with electron transmission spectroscopy [52]. This found that the increasing replacement of CH units by P centres leads to significant stabilisation of the anionic states, in direct contrast to the donor abilities which are largely unaffected by these changes. Thus, the experiments predicted that, although in general a strong link between the chemistries of benzene and the phospha-substituted derivatives would be expected, the behaviour towards reduction should be

markedly different. This has, of course, been verified by experiment (see Section 4 above). The binding of a variety of heteroaromatics, (mono-, di- and trisubstituted; N- and P-substituted; analogues of cyclopentadiene and benzene) to a range of s-block metals was probed computationally by Sastry and Vijay [53]. Marked differences were observed between the N- and P-systems, with azoles and azines preferring ␴-binding whereas phospholes and phosphabenzenes showing no real preference between ␴- and ␲-binding modes. Theoretical techniques have also been used to probe potential reactivity. In this way Bachrach and Magdalinos examined the Diels–Alder reaction between 1,3,5-triphosphabenzene and phosphaacetylene to yield tetraphosphabarrelene and compared the results to the all-carbon equivalent (i.e., benzene + acetylene) [54]. Both reactions proceed via similar pathways, being synchronous

Fig. 6. Line drawing and relative energies (kcal mol−1 ) of isomers of P3 (Ct Bu)3 calculated at the B3LYP/6-31+G* level.

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and concerted; the main differences were revealed in the energy profiles of the reactions, with the phosphorus reaction being less exothermic and having a significantly decreased activation energy than the all-carbon reaction, consistent with the higher aromatic stabilisation in the benzene compared to triphosphabenzene. 6. Conclusions and opportunities for further research 1,3,5-Triphosphabenzenes show an extensive chemistry that shows both similarities and distinctions from that of the parent carbocyclic species. The discovery that some of the chemistry shows parallels to that traditionally associated with transition metals is very surprising and may instigate further investigations into these fascinating molecules. However, it will be clear on reading this text that the range of 1,3,5-triphosphabenzenes is relatively limited; the vast majority of the chemistry described has been with tert-butyl substituents on each carbon centre; of the limited range of chemistry carried out with other substituents, all involve tertiary carbon centres attached to the C atom. This limited range of substrates is a reflection of the limitations of the vanadium mediated route used to access the species, so further developments may need to seek alternative methods of preparation. Any method would need to be accessible to a range of synthetic chemists and allow the preparation of the substrates on a suitable scale. One can envisage the possibility of alternative transition metal mediated routes which display a greater functional group tolerance, or perhaps by preparing a triphosphabenzene bearing reactive functional groups at the C centres. Furthermore, the ability to access unsymmetrical 1,3,5triphosphabenzenes in synthetically useful quantities would be an important development. Further investigations are required to elucidate the mechanisms of the reactions reported in this text; commonly mechanisms have been constructed using steps analogous to those which have precedent in organic chemistry. Whereas this is without doubt an excellent starting point, detailed experimental and theoretical studies are required to establish the validity of this approach. Lastly, it goes without saying that further reactivity studies are required to draw out the comparisons to the chemistries of both arenes and transition metals. The latter is a nascent area of contemporary importance, and establishing that the similarities and differences in the chemistries of 1,3,5-triphosphabenzenes and “transition metals” will be key in reinvigorating this topic. Acknowledgements RLF thanks the Bristol Chemical Synthesis Centre for Doctoral Training, funded by EPSRC (EP/G036764/1), and the University of Bristol, for a PhD studentship. References [1] [2] [3] [4] [5] [6] [7]

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