Electrochemical cyclizations of organic halides catalyzed by electrogenerated nickel(I) complexes: towards environmentally friendly methodologies

Electrochimica Acta 242 (2017) 373–381

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Electrochemical cyclizations of organic halides catalyzed by electrogenerated nickel(I) complexes: towards environmentally friendly methodologies Elisabet Duñacha,* , Maria José Medeirosb , Sandra Oliveroa a b

Université Côte d’Azur, CNRS, Institut de Chimie de Nice, UMR 7272, Parc Valrose, 06108 Nice Cedex 2, France Universidade do Minho, Largo do Paço, 4704-553 Braga, Portugal

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 October 2016 Received in revised form 26 April 2017 Accepted 26 April 2017 Available online 2 May 2017

This review concentrates on the intramolecular cyclizations of organic halides catalyzed by nickel complexes using an electrochemical methodology, as an alternative to the classical radical or transitionmetal catalyzed or mediated processes. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Electrosynthesis Cyclization Nickel(I) Organic halide Cyclic voltammetry

1. Introduction Organic electrochemistry, using the electron as a reagent, generally enables cheap and energetically efficient reactions. Of particular interest is the use of the electrosynthesis methodology in redox processes, the electrons avoiding the stoichiometric use of oxidants and reductants. Novel and environmentally more friendly electrochemical procedures will be highlighted, within the search for more selective, cleaner and safer methodologies operating under mild conditions. The selective and catalytic construction of carbon-carbon bonds in organic chemistry for the synthesis of carbocyclic and heterocyclic compounds allows increasing structural complexity and it is still a challenge, in particular in the area of the functionalization of non-activated olefins. Among the several synthetic approaches, radical cyclizations constitute a central methodology for the preparation of a variety of cyclic compounds and of natural products containing heterocyclic rings [1,2]. Radical cyclization procedures may present advantages over other methods, which may require laborious multi-step alternative synthesis.

* Corresponding author. E-mail address: [email protected] (E. Duñach). http://dx.doi.org/10.1016/j.electacta.2017.04.144 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

Generally, radical cyclizations are still accomplished with the aid of tri-n-butyltin hydride, n-Bu3SnH [2–34]. Such syntheses typically involve the use of an excess of n-Bu3SnH in the presence of a radical initiator, usually azobisisobutyronitrile (AIBN). Experimental conditions involve high dilution, in order to minimize the quenching of the initial radical by hydrogen radical abstraction and/or dimerization. To avoid the use of toxic triorganotin hydrides, which are also troublesome to separate from the desired products, considerable effort has been aimed at the development of new methodologies for the generation of reactive radicals. In lieu of n-Bu3SnH, tri-n-butylgermanium hydride (n-Bu3GeH) has also been employed [5]. Although expensive, n-Bu3GeH is less toxic and reacts slightly less rapidly than n-Bu3SnH in promoting reductive intramolecular cyclizations. The use of dialkyl zinc in stoichiometric amounts in radicalmediated reactions has also been described in several coupling reactions [6–8]. Another reductant, tris(trimethylsilyl)silane [(Me3Si)3SiH], is less toxic and easier to remove via conventional work-up procedures than n-Bu3SnH, but remains more costly [9]. It has also been demonstrated that 1-ethylpiperidinium hypophosphite, a low-cost reagent, and AIBN could be used to effect the intramolecular radical cyclizations of alkyl bromides bearing alkene [10] or acetylenic side-chains [11]. A significant advance for processes involving radicals has been the use of solid-phase organic synthesis with supported tin

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hydrides [12,13]. Major advantage of solid-phase syntheses relies on the fact that the radical precursor is attached to a resin and that the grafted-SnH used in the reaction can be removed by simple washing after the radical cyclization is complete; thus, the desired product can be more easily isolated and purified, although the reactions need the use of the supported tin hydride polymer reagent in stoichiometric amounts. Electrosynthesis constitutes an alternative methodology to the use of these stoichiometric reagents. In particular, the electroreduction of organic halides, R-X, has been a well-studied subject [14]. The electroreduction of R-X can either proceed i) through the direct electrochemical reduction of the substrate at the electrode surface or ii) through an indirect process, via the electrode reduction of a catalyst or a mediator with a further electrontransfer or iii) through an organometallic insertion reaction involving the organic halide (Scheme 1). Herein we will concentrate on the electrochemical cyclization processes in which nickel complexes are used as the catalysts or mediators, highlighting the radical reactivity of electrogenerated low-valent nickel species. 2. Nickel complexes as catalysts in electrosynthesis Several LNiII complexes (L = ligand) have been reported as catalyst precursors both for electroxidative and electroreductive organic reactions. In the field of oxidation, the LNiII/LNiIII redox couple, associated with macrocyclic ligands, has been used for the conversion of olefins to epoxides [15]. However, to our knowledge, no electroxidative cyclization reactions have been reported. In the field of electroreduction via organometallic reagents, in addition to LNiII complexes, PdII, CoIII and CoII and FeII complexes have been reported for the activation and further coupling involving carbon-halogen bonds [16–18]. Several reductive coupling reactions have been described with LNiII complexes as precursors: two-electron LNiII/LNi0 or oneelectron LNiII/LNiI processes can be found, mainly depending on the nature of the ligands associated to the metal center. The presence of phosphines or of mono-, di- and triamines, such as tetramethylethylenediamine, pentamethyldiethylenetriamine, or 2,20 -bipyridine as the ligands, favors the reduction of LNiII/LNi0, generally followed by the oxidative addition of the organic halide to the electrogenerated zero-valent nickel complexes [19–22]. In contrast, salen, aza-macrocyclic and porphyrin ligands induce LNiII/LNiI reductions, with a stabilization of the LNiI oxidation state, even in protic media. Among such ligands, cyclam (1,4,8,11-tetraazacyclotetradecane) (A), tmc (1,4,8,11-tetramethylcyclam) (B), Me6-cyclam (5,7,7,12,14,14-hexamethylcyclam) (C) [23], salen (D) and other tetradentate ligands (E, F, G, H) have been reported in electroreductive coupling processes (Scheme 2). Formal electrode potentials for LNiII complexes coordinated by some of these ligands are reported in Table 1. Table 1 The generation of either LNiI or LNi0 intermediate complexes is generally studied by cyclic voltammetry. This important difference of behavior in the electroreduction of LNiII complexes is mainly due to the geometry of the electrogenerated

Scheme 1. Direct and indirect electroreduction of an organic halide.

species: whereas LNi0 adopts a tetrahedral geometry, LNiI is stabilized by a square-planar arrangement [24]. The square-planar arrangement is favored with tetradentate and relatively rigid salen and cyclam-type ligands, whereas bidentate ligands such as 2,20 bipyridine or diamines form more easily tetrahedral-type structures. The electrogeneration of LNi0 or LNiI intermediate complexes strongly influences the reactivity outcome of the organic halides. Thus, LNi0 complexes in the presence of an organic halide R-X may induce an oxidative addition on the carbon-halogen bond to afford [R-NiII(L)X], which in turn presents a relatively low nucleophilictype reactivity (Scheme 3). Carboxylation and addition reactions to carbonyl compounds and other electrophiles have nevertheless been reported [21,22,25]. Alternatively, the reactivity of electrogenerated LNiI with organic halides R-X leads to the initial formation of [RNiIII(L)X] intermediates. These NiIII species can evolve towards the formation of [RNiII(L)] or of R intermediates, in which the R moiety presents a radical-type reactivity (Scheme 3) [26]. Therefore, by the appropriate choice of the ligand, the nickelcatalyzed electrochemical methodology opens the possibilities for the controlled nucleophilic versus radical-type reactivities of the organic moiety of the R-X type substrates. The ease of generation of LNiI complexes in the case of unsaturated R-X derivatives offers the possibility of radical-type intramolecular catalytic electrochemical cyclizations. 3. Electroreduction of organic halides In addition to other synthetic methods, electrochemical radicaltype cyclizations catalyzed by transition-metal complexes have been developed. In the case of cyclizations involving intermediate LNiI species, cyclic voltammetry studies allowed to establish that: - the electrogenerated nickel(I) complexes are readily produced in aprotic and also in protic media by a reversible 1e reduction of the corresponding nickel(II) complexes; - the nickel(I) complexes react with a range of unsaturated organic halides to produce radical intermediates that can undergo cyclization with the nickel(II) complex being reformed; - these electrocatalytic reductions occur at a potential where the organic halides themselves generally do not undergo reduction [27]. These processes are based on EC' mechanism (Scheme 4). Electrogeneration of LNiI from LNiII precursor complexes coordinated with some of the ligands presented in Scheme 2 has been studied by cyclic voltammetry. In particular, cyclic voltammograms for the reduction of [Ni(B)]Br2 at a variety of electrodes (including platinum and glassy carbon) and in different solvent  supporting electrolyte systems show well defined cathodic and anodic peaks for the reversible [Ni(B)]2+/[Ni(B)]+ couple: [Ni(B)]2+ + e Ð [Ni(B)]+ Controlled-potential electrolysis confirmed that the reduction was indeed a one-electron process. To enable a catalytic reaction, the starting LNiII complex must have a reduction potential more positive than that of the substrate, R-X. As shown Table 1, LNiI complexes can be electrogenerated at potentials that are well suited for the catalytic reduction of carbonhalide bonds in a variety of organic halides. Therefore, the LNiII precursor is first reduced at the cathode and the reduced nickel(I) species diffuses into the solution and is able to interact with the substrate R-X, either by its direct reduction (outer sphere

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375

Scheme 2. Examples of macrocyclic ligands of LNiII complexes.

Table 1 Formal electrode potentials for LNiII complexes in several media. NiIIL

Medium (wt %)

-E (V) vs Ag/AgCl

[Ni(A)]Br2 [Ni(B)]Br2 [Ni(B)]Br2 [Ni(B)]Br2 [Ni(B)]Br2 [Ni(D)] [Ni(H)]

DMF/0.1 M TEABF4 EtOH/0.1 M TEABr 1-PrOH/0.1 M TEABr CTAB[a]/C12H26/H2O/1-pentanol (17.5/12.5/35/35) DMF/0.1 M TEABF4 DMF/0.1 M TBABF4 DMF/0.1 M TBABF4

1.36 0.86 0.87 0.95 0.91 1.57 1.77

[a] Cetyltrimethylammonium bromide (CTAB).

+2 e-

LNi0

R-X

RNiII(L)X

LNiI

R-X

RNiIII(L)X

-2 eLNiII +1 e-

-1 e

LNiII + R. + X-

Scheme 3. Electroreduction of LNiII complexes in the presence of organic halides.

mechanism) or by an oxidative addition (inner sphere mechanism) (Scheme 4). The catalytic reaction recycles the LNiII precursor/ mediator back to the electrode. The overall reaction rate and the catalytic efficiency are dependent on the chemical reaction rate of the reduced form of the mediator with the organic substrate and on the number of cycles of the mediator regeneration (turnover). Gosden et al. [28,29] reported the interaction of LNiI species with alkyl halides, indicating that, for a given mediator concentration,

the extent of the catalytic reaction increased when raising the R-Br concentration (excess factor, g = [R-X]/[LNiII]). The rate of the regeneration of the LNiII was highly dependent on the structure of the alkyl halides and also on that of the macrocyclic ligand, as well as on the solution conditions. According to Halcrow and coworkers [30], the process gives rise to R transients and/or to Nialkyl intermediates to produce alkanes, alkenes and dimeric and/or cyclic organic compounds (Scheme 4); the identity and distribution of the observed products varies dramatically with the nature of the organic substrates. It has been reported that [LNiI]+ (L = cyclam-type) complexes transfer one electron to alkyl halides via an inner-sphere mechanism, whereas [LNiI] (L = salen-type) complexes undergo SN2-type nucleophilic substitutions at alkyl halides [30]. In both cases, an intermediate alkyl-nickel may be formed and its further evolution may generate alkyl radicals. The solvent generally used in preparative-scale reductive electrosyntheses are N,N0 -dimethylformamide (DMF) or acetonitrile (ACN). However, DMF or ACN may present some toxicity as

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NiIII(L)X NiIII(L)X X Cyclization

1 e- + H+

-

+1 e LNiII

LNiI -1 e

.

or

.

.

-

Cyclization + LNiII + X-

or H abstraction

.

H elimination + LNiII + XDimerization

Scheme 4. Electrochemical cyclization mechanisms.

solvents [31,32]. In a perspective aimed at cleaner and catalytic syntheses, more recent examples have been concerned with electrosyntheses being performed in ethanol or ethanol/water mixtures [33,34], as well as in microemulsions [35,36]. 3.1. Cyclizations involving unsaturated aryl halides In early studies, the NiII-catalyzed electroreduction of 2allyloxyhalobenzenes has been reported with several ligands [20]. The use of 2,20 -bipyridine (bipy) as the ligand in Ni(bipy)32+, 2 BF4 or Ni(bipy)Br2 precursor complexes led to the formation of LNi0 species. The LNiII complexes were generally used in 10 mol% with respect to the organic substrate. The reactions of aryl halides bearing allyl ether substituents at the side-chain led to the reductive cleavage of the O-allyl moiety, selectively affording halogenated or dehalogenated phenol derivatives, when reacted in one-compartment cells with a Mg anode [20]. In sharp contrast with this reactivity, when the same functionalized aryl halide substrates were reacted with LNiII complexes associated with aza-macrocyclic ligands such as cyclam [37–40], salen [41,42], bis-pyridine-bis-oxazolines [43] (or others), LNiI complexes were formed as intermediates upon one-electron reduction. The activation of the carbon-halogen bond was followed by the intramolecular cyclization on the olefin side-chain to afford dihydrobenzofuran skeletons. Thus, for example, 2-allyloxyphenyl halides led to efficient cyclizations with Ni(cyclam)Br2 (10 mol%) as the catalyst, in one-compartment cell with a Mg anode, either with Cl, Br or I aryl derivatives (Scheme 5). Interestingly, with chiral salen-type ligand H, dihydrobenzofurans were also obtained in good yields. Some asymmetric induction (13% enantiomeric excess) was observed from allyl 2bromophenyl ethers, indicating the close presence of the chiral complex at the stage of the formation of the new C-C bond. Mechanistic studies and a catalytic cycle have been reported [42]. Dihydrobenzopyran structures could also be obtained in this catalytic reductive radical-type cyclization procedure using homoallylic (instead of allylic) ether derivatives. The size of the ring formed was determined by the position and by the

substitution of the olefin, in an anti-Markovnikov-type addition [40,43]. Tandem cyclization-carboxylation reactions have also been reported with Ni(A)(BF4)2 catalysis [44]. In the case of 2methallyloxychloro derivative (Scheme 6), a first cyclization takes place, followed by the reduction of the radical intermediate to the corresponding anion and its reaction with CO2, affording the bicyclic carboxylic acid in 70% yield. In contrast with these results, when LPdII complexes with cyclam-type ligands such as A, B or F were used as the catalyst precursors, a different reductive process occurred selectively, involving the cleavage of the O-C(allyl) bond of the side-chain [45,46]. The preparation of benzothiophene derivatives via the cyclization of o-haloaryl allyl thioethers has also been reported in the presence of Ni(A)Br2 as the catalyst precursor [47]. Analogous Nheterocycles were obtained with Ni(C)(ClO4)2 [48]. The [Ni(A)]+-catalyzed cyclization of aromatic halides bearing acetylenic side-chains has also been explored and benzofuran structures have been obtained as the major compounds in moderate yields (Scheme 7) [40]. 3.2. Cyclizations of aryl halides with epoxides and CO2 Electrogenerated NiI-cyclam has been reported for the efficient preparation of benzolactones in the presence of CO2 in the case of disubstituted epoxides [49]. The reaction was proposed to proceed through the first reductive carboxylation followed by the oxirane ring opening and cyclization (Scheme 8). Alternatively, for terminal epoxides, a cyclic carbonate was obtained, from the direct insertion of CO2 into the epoxide. 3.3. Cyclizations of unsaturated vinyl halides Unsaturated vinyl halides have been reported in a few radicaltype electrochemical cyclizations, in particular with Ni(C)(ClO4)2 as the catalyst precursor, with the possibility to prepare interesting

Cl X

COOH

e-, Ni(A)Br2 (10 mol%)

O X = Cl X = Br X=I

One-compartment Mg anode, C cathode DMF

e-, Ni(A)(BF4)2 (10 mol%)

O O 60% 86% 90%

Scheme 5. Catalytic electroreduction of a-halogenated allyloxy benzenes.

One-compartment Mg anode, C cathode DMF CO2 1atm

O 70%

Scheme 6. Electrochemical tandem cyclisation/carboxylation of unsaturated aryl halides.

E. Duñach et al. / Electrochimica Acta 242 (2017) 373–381

e-, Ni(A)Br2 (10 mol%)

X O

Br

O

One-compartment Mg anode, C cathode DMF

X=I X = Br

e-, Ni(C)(ClO4)2 (50 mol%)

Ph Ph Ph

31% 32%

377

Ph Ph Ph

Two-compartment C cathode DMF

O

O

66% cis/trans 50/50

Scheme 7. Electroreductive cyclization of aromatic halides bearing acetylenic sidechains.

Scheme 9. Electrochemical cyclization of unsaturated vinyl halides.

cyclopropane structures tandem cyclizations [50]. In the examples reported, the LNiII precursor was used at 50 mol% in a twocompartment cell procedure in DMF (Scheme 9). 3.4. Cyclizations of unsaturated benzyl halides The electroreductive intramolecular cyclization of benzyl halide derivatives with unsaturated side-chains has been investigated. The electroreductive cyclization of bromopropargyloxy substrates such as 1 (Scheme 10) catalyzed by electrogenerated [Ni(B)]+ has been reported. The catalytic reduction of 1 led to the formation of the corresponding vinyl substituted tetrahydrofuran 2 as the single product in yields up to 99% under different conditions, both under controlled-potential or constant-current electrolyses with consumable metal anodes (Zn, Mg, Al) [51]. The yields of 2 were not sensibly influenced by the nature of medium. Similar results were found for the other derivatives of 1 [34,36,52] with propargylic side-chain moieties. The selectivity of the reactions in EtOH, EtOH/ H2O mixtures and in microemulsions was similar to that obtained in DMF, although the best yields were obtained in an EtOH/H2O medium (Table 2). Table 2 By cyclic voltammetry, compound 1 presented an irreversible reduction wave at potentials more negative than 1.5 V vs Ag/AgCl, which corresponds to the reductive cleavage of the carbon-bromine bond. Upon addition of 1 to a solution of Ni(B) Br2 in the different media, an increase of the reduction peak intensity of the LNiII/LNiI transition was observed, as the concentration of the substrate increased. The anodic peak for the re-oxidation of the intermediate [Ni(B)]+ complex was no longer observed, due to its chemical consumption. The catalytic cyclization involves a one-electron reduction of the starting halide and the proposed mechanism follows a radicaltype cyclization as shown in Scheme 4, with a final hydrogen abstraction from the electrolytic medium. There is evidence in the literature that DMF [53] as well as EtOH [54] can act as a hydrogenatom donors. The DMF radical formed can be reduced at more

O O

R

e-, + CO2

Br

91%

R=H

O

O

Ni(A)Br2 (10 mol%) KBr/DMF

negative potentials, approximately, 1.95 V vs SCE. [55]; the EtOH radical can preferentially recombine by dimerization or can be reduced at approximately, 1.25 V vs SHE [54]. This can explain the experimental observation that the reduction of unsaturated substrates by LNiI complexes involves the exchange of oneelectron per substrate molecule (see Table 2). These results indicate the feasibility of electrochemical radicaltype cyclizations of benzylic halides bearing unsaturated C-C bonds in their side-chain. This methodology is practical and easy to use for synthetic purposes, needing a simple electrochemical setup and carried out by the use of catalytic amounts of stable Ni(II) starting complexes at room temperature. 3.5. Cyclizations of unsaturated a-bromoesters and a-bromoamides The direct, non-catalyzed electrochemical reduction of a-bromoesters of type 3 at reticulated vitreous carbon or silver cathodes in DMF generally involves a two-electron cleavage of C-Br bond to afford the corresponding anions; these anions can get protonated or react as nucleophiles, but without formation of the corresponding cyclic compounds [56,57]. In contrast, the electroreduction of type 3 bromoesters using electrogenetated [Ni(B)]Br2 as the catalyst precursor, allows the intramolecular coupling with side-chains containing non-activated double or triple bonds, indicating the strong influence of the LNiI complex in the catalytic process (Scheme 11). In particular, the reductive intramolecular cyclization of 3 with Ni(B)Br2 as the catalyst precursor afforded isomeric cyclic ethers 4 and 5 in up to 99% combined yield. Concerning the selectivity for the formation of 5 vs 6 membered-rings, only the cyclization to the five membered-ring ethers was obtained [34,35,58,59,60]. Some comparative results for the cyclization of substrate 3 are summarised in Table 3. Similar efficient reactions could be performed with analogous a-bromoesters containing allyloxy moieties in different media and conditions [34,35,61,62]. Table 3 Cyclizations of 3 have also been performed by conventional methods in the presence of n-Bu3SnH and radical initiators or other reducing agents [63–66] affording the corresponding tetrahydrofuran derivatives in yields up to 89%, but in the presence of stoichiometric amounts of tin hydride or other reducing agents. Regarding the cyclization of related unsaturated a-bromoamides, similar results have been reported. Thus for example, the electrochemical reduction of N-allyl-a-haloamides such as 6 has been reported using several Ni(L)2+complexes (L = A, B [67], G

e-, Ni(A)(BF4)2 (10 mol%) n-Bu4NBF4/DMF R = Me Me OH O 88% O

Scheme 8. Electrosynthesis of benzolactones and cyclic carbonates from epoxidefunctionalised aromatic halides.

Ph

Ph

Br O

O

e-, Ni(B)Br2 MeO

MeO

1

2

Scheme 10. Electrochemical cyclisation of propargyloxy bromoether 1.

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Table 2 Coulometric Data and Product Yields for Catalytic Reduction of 1 (0.04 mmol) by electrogenerated [Ni(B)]+ (10 mol%)[a] in several media. Entry

n[b]

Medium

RBr = 1-[2-bromo-2-phenyl-1-(prop-20 -ynyloxy)ethyl]-4-methoxybenzene (1) 1[d] DMF/0.10 M Et4NBF4 2 EtOH/0.10 M Et4NBr 3 EtOH:H2O/0.10 M Et4NBr 4 MTAB[e]/Tetradecane/H2O/1-pentanol (17.5/12.5/35/35)

Product, yield, %[c] 2 98 79 99 98

1.1 1.0 0.9 1.3

[a] Generally, the time of electrolysis was 0.6–2 h to achieve a complete conversion of starting material at reticulated vitreous carbon (RVC) and at carbon felt (CF) cathodes. [b] Number of electrons per molecule of starting material. [c] % = yield expressed as the percentage of starting material incorporated into each product. [d] Conditions: 4.0 mM (0.060 mmol) of 1. [e] Tetradecyltrimethylammonium bromide (MTAB).

O

O

O

MeO

OEt e-, Ni(B)Br2 Br

MeO

O

MeO

MeO +

MeO

3

EtO

O

MeO

4

EtO

O

5

Scheme 11. Electrochemical intramolecular cyclisation of propargyloxy bromoester 3.

Table 3 Coulometric Data and Product Distributions for Catalytic Reduction of 3 (0.04 mmol) by electrogenerated [Ni(B)]+ (10 mol%)[a] in several media. Entry

n[b]

Medium

RBr = ethyl 2-bromo-3-(30 ,40 -dimethoxyphenyl)-3-propargyloxy-propanoate (3) 1[d] 2 3 4

DMF/0.10 M Et4NBF4 EtOH/0.10 M Et4NBr EtOH:H2O (9:1)/0.10 M Et4NBr CTAB[e]/tetradecane/H2O/1-pentanol (17.5/12.5/35/35)

0.9 1.0 1.0 1.2

Products yield, %[c] 4, 5 (Ratio of isomers 4:5) 94 (77:23) 99 (93:7) 97 (89:11) 96 (89:11)

[a] Generally, the time of electrolysis was 0.6–2 h to achieve a complete conversion of starting material. [b] Number of electrons per molecule of starting material. [c] % = yield expressed as the percentage of starting material incorporated into each product. [d] Conditions: 2.0 mM (0.050 mmol) of 3. [e] Cetyltrimethylammonium bromide (CTAB).

[68,69]) to afford the corresponding g-lactam 7, together with dehalogenated amide 8 (Scheme 12). Constant-current electrolysis of 6 in an one-compartment cell has also been reported in DMF and acetonitrile. In the presence of [Ni(A)]Br2 used in 10  20 mol% in acetonitrile, lactam 7 was formed in 27%, together with the reduced acylic compound 8 in 65% yield [67]. In the presence of [Ni (G)]Br2 used in 10  20 mol% in DMF with 6a (the CH3 group of 6 replaced by H), lactam 7a was formed in 32%, together with the reduced acylic compound 8a in 4% yield [68]). The product distribution was also strongly affected by the ability of the solvent to donate hydrogen atoms. The addition of Ph2PH as hydrogenatom donor (2 equiv.) to the electrolysis of N-allyl-a-haloamide 6a in the presence of [Ni(G)]Br2 increased the yield of lactam 7a to 50%, emphasizing the role of the electrolysis medium to donate hydrogen atoms to the radical intermediates [69]. Yields of 7 from 6 were improved to 98% by using [Ni(B)]Br2 as the catalyst precursor in EtOH [68]. Other N-allyl-a-haloamides derivatives presented a similar behaviour.

H3C

Br

O

N 6

CH3

H3C e-,

Ni(B)Br2 O 7

N

H3C +

O

N

8

Scheme 12. Electrochemical intramolecular cyclisation of N-allyl-a-haloamide 6.

For both families of substrates of type 3 and 6 under different conditions, the reactions under controlled-potential electrolyses with [Ni(B)]Br2, involved the transfer of one-electron per molecule of starting material, with n values near to 1, as shown in Table 3. Illustrated in Fig. 1 are the cyclic voltammograms for the direct reduction of 3 (curve A), for the reversible reduction of [Ni(B)]2+ complex (curve B) and for the catalyzed reduction of 3 (curve C) at a glassy carbon electrode in DMF containing 0.10 M of tetraethylammonium tetrafluoroborate. Curve C shows that upon addition of the a-bromoester 3 to the solution of [Ni(B)]Br2, the cathodic peak current increases significantly. The anodic wave due to the oxidation of [Ni(B)]+ back to [Ni(B)]2+ vanishes, because of the chemical consumption of [Ni(B)]+. Fig. 1 also reveals that the catalytic reduction of 3 takes place at a potential less negative than the potential required for its direct reduction. The ease of electrogeneration of [Ni(B)]+ complex offers the possibility of radical-type intramolecular catalytic electrochemical cyclizations of unsaturated a-bromoester derivatives, in agreement with coulometric data. Similar results were obtained for other a-bromoester and a-bromoamide derivatives in the different media. The mechanism for the Ni(II)-catalyzed cyclization of a-bromoester (and a-bromoamide) derivatives is presented in Scheme 13 for compounds of type 3 (or 6). A first LNiII complex reduction to LNiI complex occurs with its further oxidative addition into the carbon-halogen bond of the

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379

Fig. 1. Cyclic voltammograms recorded with a glassy carbon electrode (area = 0.07 cm2) at 100 mV s1 in DMF containing 0.10 M of tetraethylammonium tetrafluoroborate: (A) 1 mM 3; (B) 2 mM [Ni(B)]Br2; (C) 2 mM [Ni(B)]Br2 and 20 mM 3.

substrate to form an intermediate of type A. The organic moiety in intermediate A has a radical-type character and forms a cyclised intermediate of type B. The further transformation of B with hydrogen radical abstraction from the electrolytic medium (SH = solvent medium) affords the reaction products 4 and 5 and enables the recycling of the LNiII complex. Interestingly, these coupling reactions can be performed using graphite or platinum as the anodes in single-compartment cells, avoiding the use of sacrificial anodes and involving the solvent oxidation at the anode [70]. This investigation provides an example of the feasibility of preparative-scale organic electrosynthesis in “green” solvents in the absence of dissolving metals, in catalytic procedures. Electroreductive intramolecular cyclization of propargyloxy and allyloxy a-bromoesters has also recently been performed in room-temperature ionic liquids (RTILs) [71]. The electroreduction of 3 in N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, [N1112(OH)][NTf2] ionic liquid

e-

Br-, S. [NiII(B)]2+

Isomerization 5

4 [NiI(B)]+ SH

3

NiIII(B)Br +

EtOOC

3.6. Cyclizations of unsaturated aliphatic halides The intramolecular addition of alkyl iodides and bromides to non-activated acetylenic moieties catalyzed by NiII-salen precursors afforded cycloalkenes. Thus, the reduction of 6-iodo- or 6bromo-1-phenyl-1-hexyne, carried out in DMF or acetonitrile containing 0.10 M Et4NClO4 in two-compartment cells with 10 mol % of LNiII complex led to benzylidenecyclopentane in 84-95% yields (Scheme 14) [72]. Mechanistic studies with 6-halogeno-1-phenyl-1-hexyne (or 7halogeno 1-phenyl-1-pentyne) indicated that the NiI-salen species was generated at 1.75 V vs. SCE, a potential where the v-haloalkynes were not electroactive [73–75]. The substrates were reduced catalytically to form radical intermediates, which led to the formation of benzylidenecyclopentane (or the

+

Br(B)NiIII O

catalyzed by electrogenerated [Ni(B)]+ complex led to cyclic compounds 4 and 5 and proceeded in moderate to good yields. The beneficial effect of using protic media such as EtOH and EtOH/H2O mixtures as the preferred media to conduct these reactions relies on their remarkable compatibility with the catalytic system, apart from being environmentally more friendly when compared to DMF. The laboratory-scale of the reported electrochemical procedures is of 0.5–10 mmol, but the synthesis of the corresponding cyclic compounds in larger quantities seems possible, due to the ease of the practical electrosyntheses conditions.

e-, Ni(D) (10 mol%)

EtOOC

Ph

Ar Ar B

O A

X = Br X=I

X Two-compartment C cathode DMF

Ph 84% 95%

II

Scheme 13. Mechanism for the Ni -catalyzed cyclization of a-bromoester (and abromoamide) derivatives.

Scheme 14. Electrochemical reduction of 6-bromo-1-phenyl-1-hexyne.

380

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e-, Ni(B)(ClO4)2 (10 mol%)

e-, Ni(A)(ClO4)2 (10 mol%) Br

One-compartment Zn anode, C cathode DMF

N ( )4 I 55% Trans/cis : 1/0.9

N Two-compartment C cathode DMF

75%

Scheme 16. Electroreductive cyclization of 1-(w-iodoalkyl)pyrroles.

Scheme 15. Reductive cyclisation of (R)-citronellyl bromide.

corresponding cyclohexane) [76,77,75]. The direct electrochemical reduction of the acetylenic halides without catalyst led to reductive dehalogenation without cyclization, emphasising the important role of the LNiII catalyst in the cyclization process [78,79]. Similarly, the catalytic reduction of the aliphatic analogue 1iodo- or 1-bromo-5-decyne by [Ni(D)]+, electrogenerated at a carbon cathode in DMF, led to the formation of pentylidenecyclopentane in yields up to 86% [75]. Cyclic voltammetry and controlled-potential electrolysis were reported and a mechanism involving phenyl-conjugated alkynyl radicals was proposed [80]. Catalytic reductions of (R)-()-citronellyl bromide with [Ni(A) (ClO4)2] in DMF containing 0.10 M Bu4NBF4 led to the expected cyclic compounds, in moderate yields (Scheme 15) [81,82]. Controlled-potential reductions of 6-bromo-1-hexene with [Ni (D]+ led to the formation of methylcyclopentane in up to 84% yield. The cyclizations were also carried out at constant intensity in DMF containing 6 mM Bu4NBF4 at room temperature in singlecompartment cells with consumable Mg or Zn anodes and the cyclic compounds were obtained in yields up to 65%, the main byproducts arising from the reductive dehalogenation of the substrates. The intramolecular electroreductive cyclization of 1(v-iodoalkyl)pyrroles using Ni(B) (ClO4)2 as the catalyst has been performed, giving rise to bicyclic ring systems (Scheme 16) [83]. The electrocatalytic reduction of methyl 1-(4-halobutyl)-2oxocyclopentanecarboxylates or ethyl 1-(3-halopropyl)-2-oxocyclohexanecarboxylates by [Ni(I)(D)]+ electrogenerated at carbon cathodes in DMF has been studied by cyclic voltammetry and controlled-potential electrolysis [84]. The reaction afforded the corresponding ring-expanded products in up to 26% yield, along with the dehalogenated and unsaturated species (Scheme 17). 4. Conclusions and further perspectives The particular advantages of these electrosynthetic methodologies are that these transformations can often be conducted more efficiently with higher selectivities, with decreased pollution and with increased environmentally friendly aspects. In addition, the transition-metal complexes present a large range of possibilities, can easily modulated, and may allow specific reactions to be catalytic. The use of stoichiometric redox reactants can be avoided by the use of the electrochemical methodology, in which the oxidants and reductants are simply removed and replaced by electrons. The new insights into chemical mechanism and the development of adapted techniques, namely indirect electrochemical synthesis with transition-metal complexes as redox catalysts, have become a subject of significant attention. A range of intramolecular electroreductive cyclization reactions catalyzed by nickel(II) complexes have developed in a catalytic manner, generally in high yields and selectivities. All these reactions involved the electrochemical generation of LNiI species, in which the nature of the ligand L, generally an aza-macrocycle, is

O O

Br ( )4

COOMe

-

e , Ni(D) (10 mol%) Two-compartment C cathode DMF

COOMe 24%

Scheme 17. Electrocatalytic reduction with ring expansion.

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