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Study on the mechanism of isomerization of oxaspirohexane catalyzed by Zeise’s Dimer
Molecular Catalysis 452 (2018) 247–259
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Study on the mechanism of isomerization of oxaspirohexane catalyzed by Zeise’s Dimer
T
Sonjoy Mondal, Arpita Chatterjee, Rohini Saha, Animesh Ghosh, Kuheli Chakrabarty, ⁎ Gourab Kanti Das Department of Chemistry, Visva-Bharati, Santiniketan, 731235, West Bengal, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeise’s dimer Oxaspirohexane Rearrangement Catalyst regeneration
Mechanism of the rearrangement of oxaspirohexane to 3-methylenetetrahydrofuran under the catalysis of Zeise’s dimer has been studied using DFT methods. Our results reveal that the catalyst, at the initial stage, undergoes a considerable change in structure under the influence of the substrate and the two metal atoms bind the substrate at different positions. Rearrangement of the substrate molecule then takes place to generate the tetrahydrofuran derivative through the formation of platinacyclobutane intermediate, as revealed in the experimental report. Depending on the variation of substrate structure and environmental condition the diversion of the mechanistic pathway to generate different products can also be explained by the DFT methods. Application of several functionals in calculating the global activation barrier also reveals that M06 functional is the most suitable choice to deal with this type of problem and to predict the global activation barrier.
1. Introduction
it cannot explain several facts observed in the experimental reports. Various mono nuclear platinum complexes were found to be ineffective to bring out the transformation. However, the mechanism, proposed by the authors, shows the involvement of only one platinum atom in the reaction pathway. The authors also observed that the presence of other reagents in reaction medium and the variation of substrate structure divert the pathway to form products other than the tetrahydrofuran derivative (Scheme 3). Specifically the presence of sufficient amount of methanol in the reaction medium leads to an open chain compound, which reveals the involvement of the nucleophilic activity of the methanol in the reaction (Scheme 3(b)). Moreover the dimethyl diphenyl derivative of oxaspirohexane, on similar treatment by Zeise’s dimer, does not result any product of tetrahydrofuran derivative; instead, a fragmentation reaction was observed, yielding dimethyl diphenyl ethane as the sole product (Scheme 3(c)). Another derivative of oxaspirohexane, containing a cyclohexyl ring as the substituent, furnishes an elimination reaction to generate β, γ unsaturated cyclohexene ketone (Scheme 3(d)). Inactivity of the substrates was noted when nitrogen containing substituent is present in the substrate structure (Scheme 3(e)). Formation of such a variety of products on the variation of the substrate structure and the inactivity of some specific oxaspirohexane derivatives clearly reveal that the activation of the substrate molecule by a single metal atom may be an oversimplified mechanism and thus demand a re-investigation to find
Zeise’s salt, the first organometallic compound [1] isolated in the year of 1827, is a platinum metal complex obtained in monomeric and dimeric forms. During the past two or three decades the catalytic activity of many platinum [2] and gold [3] complexes on a variety of substrates has been reported which include the action of Zeise’s salt on several organic substrates [4]. Many of these reports revealed the rearrangement of cyclopropane derivatives. One such recent report has caught our attention for the absolute necessity of the Zeise’s salt as catalyst in its dimeric form [5] (Scheme 1). The substrate in this transformation is an oxaspirohexane derivative where two rings connected to a common carbon atom are nearly perpendicular to each other. Upon treatment with Zeise’s salt at 45 °C in DCM, the substrate changes to a nearly planar tetrahydrofuran derivative. In the proposed mechanistic pathway [5] the authors suggested an initial activation of the cyclopropane ring by forming a coordinate bond to the metal centre (Scheme 2). After binding to the substrate structure the metal atom, on oxidative addition, gets inserted into the cyclopropane ring forming a platina-cyclobutane derivative. Using 13C NMR spectroscopy the authors also confirmed the formation of this fourmembered derivative as an effective intermediate in the reaction pathway. This intermediate, on subsequent rearrangement, transforms to the final product of tetrahydrofuran derivative. Though the proposed pathway gives us a qualitative picture of the mechanism of the reaction, ⁎
Corresponding author. E-mail address: [email protected] (G.K. Das).
https://doi.org/10.1016/j.mcat.2018.04.003 Received 3 January 2018; Received in revised form 16 March 2018; Accepted 3 April 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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Scheme 1. Rearrangement of the oxaspirohexane to 3-methylenetetrahydrofuran.
Scheme 2. Mechanism of the rearrangement of oxaspirohexane proposed by Howell et al.
Scheme 3. Variation of products depending on the variation of the substituents on oxaspirohexane. All reactions are catalyzed by Zeise’s salt.
Bäckvall et al. [6]; they identified the two pathways for activation before insertion of metal atom into the cyclopropane ring. The corner activation, which occurs through the transition state by positioning the metal atom at the corner of the cyclopropane ring was reported to be more favourable than the edge activation process [6]. The later process of edge activation involves the coordination of a specific bond of cyclopropane ring by the metal centre. However these studies were
out a more appropriate mechanism for proper explanation of the formation of different products. In order to understand the mechanism of isomerisation using various catalytic platforms, including Zeise’s catalyst a number of investigations have now been performed using various quantum chemical calculations [6]. Catalysis of platinum complexes in reaction of cyclopropane derivative had previously been studied theoretically by 248
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carried out theoretically on a substrate which contains only the cyclopropane ring without any other coordinating atoms or groups. In the present problem, a little bit complex derivative of cyclopropane system (oxaspirohexane) bearing another effective ligating oxygen should raise several possibilities. The activation of the substrate may be possible by forming coordinate bonds with different parts of the substrate by metal catalyst. Our interest on such substrate structure and the experimental report on variation of products tempted us to search the suitable mechanism involving the planar Zeise’s dimer that could explain all the observations presented in the experimental report [5]. We propose a double activation process of the substrate molecule to explain the reactivity of the substrate and formation of different products in a variety of situations. 2. Computational methods Possible geometries on the designed reaction pathways were optimized using Gaussian 09 software [7]. Construction of trial geometries, monitoring the progress of calculation and visualization of the final output were done by several graphical interface softwares like Gauss View, Molden etc. Primarily we have chosen hybrid density functional M06-2X [8,9] as a method for performing optimization of structures [10,11]. 6–31G (d, p) basis set [12–14] was employed for all non-metal atoms and the LANL2DZ [15,16] basis set was employed for platinum. To find out the geometries of several transition structures on the potential energy surface (PES) of the reaction pathway, relaxed scan method was used. Each structure was characterized as corresponding to a minimum or a saddle point on the energy hyper surface by means of frequency analysis. A transition structure (TS) possesses only a single imaginary frequency while the structure at minimum contains no imaginary frequency at all. Further confirmation of the TS was done by following the intrinsic reaction coordinate (IRC) pathway. We carried out single-point energy calculations for refinement of energies using def2-TZVP basis set for Pt-atom and the 6–311 + G(d,p) basis set for other atoms [17]. Effective core potentials including scalar relativistic effects were used for the metal atom. Solvent effect was considered using PCM model [18]. In all single point energy calculations we used dichloromethane as the solvent. Results obtained using M06-2X functional were again revalidated by optimizing the geometries of some critical points on each of the reaction pathways using two other different functionals (M06 and M06L). Results indicate that the M06-2X functional sometimes overestimates the associated barrier of certain reaction pathways due to giving excessive stresses on the exchange energy. All the relative energies on the reaction path were calculated with respect to the energy sum of the free reactant and catalyst.
Fig. 1. Overview of the possible pathways for activation of oxaspirohexane by Pt-catalyst.
insertion reaction of the Pt metal to form a four membered platinacyclobutane intermediate (1I2). The next step involves the ring expansion process that results a five membered oxygen heterocycle, 3-methylenetetrahydrofuran, as the end product (1I3). In pathway-2 the platinum catalyst activates initially the oxygen atom of the oxaspirohexane by forming a complex (2I1). Next step involves the reorganization of the substrate structure by breaking simultaneously the two rings of the spirane system, resulting the formation of a six membered ring (2I2). While rearranging the atoms, the Pt metal has also been included into the newly formed ring. The final product 2I3 is then generated by contraction of this newly formed ring, excluding again the Pt metal from the ring system. Pathway-3 starts with the substrate activation by two metal centres at site (1) and (2) simultaneously (3I1). After forming a four membered platinacyclobutane ring, 3I2 (by oxidative insertion), cleavage of the OeC bond of oxacyclobutane ring takes place, resulting finally the Pt-bound product 3I3. Potential energy surface associated to the above three pathways are shown in Fig. 2. The formation of the initial complex in all pathways (1I1, 2I1 & 3I1), as apparent from free energy change in Fig. 2, reveals that they are endergonic in nature. In pathway-1 (brown colour), the initial complex (1I1) undergoes an oxidative insertion reaction of Pt- metal by crossing 12.86 kcal mol−1 activation barrier, resulting a four membered platinacyclobutane ring as an intermediate (1I2). The ball & stick models of the intermediates and the transition states (Fig. 3) reveal that the C1eC3 bond of the cyclopropane moiety, that breaks down to make room for the metal atom for expanding the ring to platinacyclobutane, changes from 1.59 Ǻ in 1I1 to 2.26 Ǻ in 1I2 reaching highest energy in 1TS2 with a distance of 1.91 Ǻ. The distance between the metal atom and the ring carbons gets progressively reduced from 2.56 Ǻ (in 1I1) to 2.03 Ǻ in 1I2 through 2.20 Ǻ in the 1TS1. The next step (Fig. 2) involves the expansion of the oxacyclobutane ring of the intermediate 1I2 to a tetrahydrofuran system of 1I3 through 1TS2 with the concomitant collapse of the platinacyclobutane, just generated in the previous step. In
3. Results and discussion 3.1. Rearrangement by single and double activation To find out the possible pathways of the reaction we have selected the oxaspirohexane R (Fig. 1), as the model substrate. Two sites of the substrate that can coordinate with metal centres are the distal bond edge of the cyclopropane moiety (1) and the oxygen lone pair (2). The mechanism, proposed by Howell et al. [5] assumes the starting point as the complex generated by the coordination of a platinum metal with these two sites of the substrate (Scheme 2). However our observation on the geometrical position of these two ligating units reveals that their relative spatial orientations are not suitable to coordinate with a single metal atom. To find out the actual process we have designed three separate pathways by activating these two ligating units separately (pathway-1 and 2) or simultaneously (pathway-3, Fig. 1). The initial step of pathway-1 involves the formation of a coordinate bond (1I1) between the metal centre and distal edge of the cyclopropane ring of the substrate (R), which then undergoes an oxidative 249
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Fig. 2. PES with Gibbs free energy and structures of the stationary points of path-1 (brown colour), 2 (green colour), 3 (violet colour) under single and double Ptcatalyzed conditions. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
membered platina-oxa-cyclohexane ring with the generation of a πbond between the C2-C3 atoms (Fig. 4). Unlike pathway-1, this transformation does not involve any direct activation of the cyclopropane ring by the metal catalyst and thus requires large activation energy (44.72 kcal mol−1, Fig. 2) for rearrangement. The resulting intermediate 2I2 undergoes a further ring contraction process by excluding the platinum metal from the six-membered ring thus forming a tetrahydrofuran derivative through the transition structure 2TS2 (activation barrier 16.8 kcal mol−1). The three atoms, involved in bond breaking and bond making processes in this step (Fig. 4), maintain the distances of C1-O, Pt-O and C1-Pt in 2TS2 as 2.28 Ǻ, 2.01 Ǻ and 2.43 Ǻ respectively. Global activation barrier of this pathway are 48.50, 47.92 and 39.55 kcal mol−1 calculated using M06-2X, M06 and M06L functional respectively. Activating simultaneously the two sites of the substrate in pathway3 (Fig. 2, violet colour) results 3I1 or 3I1a as the possible intermediates. These intermediates differ from each other by the relative conformations of the Pt-complex connected to the oxygen atom (Fig. 5). Oxidative addition of one Pt-atom in intermediate 3I1a leads to the formation of a four membered platinacyclobutane ring (3I3) through 3TS1a. However, 3I1 passes to the platina cyclobutane ring (3I3) via the formation of an additional intermediate 3I2 and the transition structures 3TS1 and 3TS2 (Figs. 2 and 5). The final step of the cleavage of OeC bond of the spirane intermediate takes place through a transition state, 3TS3 with comparatively lower activation energy of 32.98 kcal mol−1 (Fig. 2). At the initial stage, the activated substrate 3I1 maintains a smaller distance between Pt-atom and C1 (2.17Ǻ) relative to that in 1I1 of pathway-1(2.56 Ǻ). However the bond lengths of Pt-C1, Pt-C2 and C1-
this step the migrating oxygen of the cyclobutane system gets detached from the common carbon (C2) of the spirane system while forming a bond with one of the carbon atom attached previously with the metal centre (Fig. 3, 1I2 → 1TS2 → 1I3). The distances between the migrating oxygen and the two carbon atoms that get detached from and attached to this oxygen are respectively 1.53 Ǻ and 1.91 Ǻ in the transition state 1TS2. While forming the C1-O bond, a π-system is generated between C2 and C3 atom, which ultimately forms a co-ordinate bond with the metal centre. The required activation free energy of the rearrangement step is 38.78 kcal mol−1 (Fig. 2), resulting a five membered oxygen containing heterocycle, (1I3), as the end product. The global activation free energies of this pathway are 53.21, 46.93 and 42.12 kcal mol−1 calculated using M06-2X, M06 and M06L functional respectively. In pathway-2 (Fig. 2, green colour) the platinum catalyst activates the oxygen atom of the oxaspirohexane ring initially. As revealed from the structure of 2I1 (Fig. 4), the metal atom activates the substrate by forming a coordinate bond with the oxygen atom of oxabutane ring with a bond distance of 1.91 Ǻ. Such coordination definitely decreases the electron-density around the oxygen atom and facilitates the breaking of the bond between the oxygen and the common carbon (C2) of the spirane substrate. The complex is then rearranged to break the cyclopropane ring (through 2TS1) and forms a six membered ring system (2I2) including platinum and oxygen atoms in the ring in adjacent positions. While progressing through this pathway, the distance between the OeC2 attains 2.16 Ǻ at the transition state 2TS1 (Fig. 4). At the same time the C1-C3 distance of the cyclopropane moiety increases from 1.51 Ǻ (2I1) to 2.04 Ǻ (2TS1) facilitating the fission of this bond and increasing the interaction between the C1 and the metal atom. The process ends up at the geometry of the intermediate 2I2 resulting a six 250
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Fig. 3. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-1.
Comparison of the global activation barriers (Table 1) of all the three pathways clearly reveals that pathway-3 is energetically more favourable than pathway-1 and 2. It is worthy to note that the functional M06L shows the lowest activation barrier (28.86 kcal mol−1). However all the functionals indicate the favourability on pathway-3 occurring through double activation process. The clue that the double activation pathway requires low activation barrier, guides us to study the involvement of dimeric Zeise’s salt in the reaction mechanism.
C2 in the transition structure (3TS1) in the metal insertion process are nearly equal to that in 1TS1 of pathway-1. The intermediate 3I2 and 1I2 also have the similar bond length of C1-Pt and C3-Pt. Rearrangement step, that goes through the 3TS2 and involves the migration of oxygen from the central C2 atom to the C1 atom of the cyclopropane moiety, shows a rather larger distances of the migrating oxygen from both C1 and C2 (2.31 Ǻ and 2.07 Ǻ) relative to the distances found in the mono metal activated transition state 1TS2 in pathway-1 (C1-O and C2-O is 1.91 Ǻ and 1.53 Ǻ). The increased bond length in 3TS2 suggests that the detached oxygen from C2 atom is stabilized by the co-ordination with second Pt-atom. This effect is further reflected by the reduced activation free energy required in the rearrangement for pathway-3. The reduced distance of Pt-C1 in 3TS2 (2.29Ǻ) relative to that distance in 1TS2 (2.63 Ǻ) also suggests that the transition state has been attained in pathway-3 without increasing the Pt-C1 bond length to a longer distance. The product 3I3 shows the two catalytic metal atoms co-ordinated at the opposite faces of the generated tetrahydrofuran derivative. The global activation free energies of this pathway are 46.70 (M06-2X), 36.65 (M06) and 28.86 (M06L) kcal mol−1 calculated using different functionals.
3.2. Involvement of Zeise’s salt and the catalytic cycle After determining the number of platinum atoms involved in the favourable pathway, we turn our attention to the pathway by which the planer Zeise’s dimer can provide two Pt-atoms to the substrate in a perfect orientation to catalyze the reaction. In fact we surmised that the association of a dimeric catalyst and the substrate molecule at the initial stage of the reaction is entropically more favourable process that the association of two monomeric catalysts separately with the substrate molecule. The schematic representation of the result of our search is shown in Fig. 6. The common middle portion of the catalytic cycle 251
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Fig. 4. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-2.
(3I1 → 3I4) is the repetition of pathway-3, discussed in the previous section. Upper portion represents the pathway through which Zeise’s dimer (a) binds the substrate R to form 3I1 and the regeneration of Zeise’s dimer from the complex 3I4. Lower portion shows the transfer of metal centres from the product 3I4 to a reactant molecule R (3I4a → 3I7a). Relevant models of the intermediates and transition states in catalytic cycles are shown in Fig. 7. Interaction of the catalyst ‘a’ with the reactant ‘R’ generates the intermediate 3I0. While forming this intermediate the oxygen atom of the substrate ‘R’ breaks the bridging bond between Pt-Cl and forms a coordinate bond with the metal atom. The remaining portion of the catalyst containing a Pt-metal gets attached to the second bridging chlorine ligand. Several conformational arrangement of the substrate with respect to the attached catalyst have been identified and we have selected the most favourable one in which cyclopropane moiety is in close proximity to the non coordinating Ptatom (3I0, Fig. 7). At this stage the distance between the Pt and the O of the substrate molecule is 2.18 Ǻ and that between the bridging chlorine and another metal centre is 2.50 Ǻ, slightly larger than that in the native Zeise’s dimer. A second internal substitution reaction (3Io to 3I1 through 3TSo) displaces another bridging chlorine atom of the second Pt-metal and allows the second half of the catalyst to form a coordinate bond with the cyclopropane ring (Fig. 6), thus initiate the reaction by edge activation. Such process progressively increases the edge length
(C1-C3) of the cyclopropane ring from 1.52Ǻ in 3Io through 1.57 Ǻ in 3TSo (Fig. 7) to 1.58 Ǻ in 3I1. The formation of 3I1 thus initiates the mechanism of pathway-3 (Fig. 2) details of which have been discussed previously. This leads to the end product 3I4. For the continuation of the catalytic cycle we identified two pathways the first of which recovers the Zeise’s dimer (a) from 3I4 (top portion of Fig. 6) and the second one consists of the transfer of metal centres from 3I4 to reactant R (bottom portion of Fig. 6). While recovering the Zeise’s dimer we observed that the two metal centres that are positioned at the opposite faces of the product (3I4 in Figs. 5 and 6) should come close to each other. A proper scrutiny of the geometry of 3I4 revealed that it would be easier to rotate the Pt-metal centre on the oxygen atom to change its position from one plane to other. 3TS4 (Fig. 7) is the transition structure through which 3I4 passes to 3I5, thus positioning the chlorine ligand of the moving metal atom very close to the Pt-centre, bound to the unsaturated moiety of the product (3I5 in Fig. 7). Getting a chlorine ligand at its close vicinity in intermediate 3I5, the other Pt-atom jumps to join with it by crossing another transition structure 3TS5 (Fig. 7). The resulting intermediate 3I6 (Fig. 7) on optimization without the bound product (P) gives back the Zeise’s dimer. The second pathway that involves the transfer of the catalytic metal centres from product to reactant (lower portion of Fig. 6) starts with the formation of the intermediate 3I4a by combining the spirane R at Pt-
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Fig. 5. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-3.
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centres from product to reactant molecule is energetically more costlier (21.20 kcal mol−1). Thus the regeneration of the Zeise’s dimer (top portion of Fig. 6) should be the favourable process for continuation of the catalytic cycle.
Table 1 Global activation free energy (kcal mol–1) calculated using M06-2X, M06 and M06L. Functional employed
Pathway-1 (Fig. 2)
Pathway-2 (Fig. 2)
Pathway-3 (Fig. 2)
M06-2X M06 M06L
53.21 46.93 42.12
48.50 47.92 39.55
46.70 36.65 28.86
3.3. Reaction in presence of methanol Howell et al. [5] observed that the presence of sufficient amount of methanol in the reaction mixture leads to an open chain compound (II) rather than the tetrahydrofuran derivative (I) (Schemes b and Scheme 4). Such a reaction occurs via the nucleophilic attack of the oxygen of methanol to one carbon atom of cyclopropane moiety of the substrate spirane (Scheme 4). To compare the energy barrier of the formation of tetrahydrofuran derivative (I) and the open chain compound (II), we have explored the potential energy surface associated to the two pathways shown in Fig. 9 (blue and brown colour respectively). The selected starting point for both pathways is the platinacyclobutane derivative (3I2) (shown in Fig. 7), generated through the oxidative addition of platinum complex to the cyclopropane ring. The formation of the tetrahydrofuran derivative has been shown by violet colour and that for the open chain compound has been depicted using brown colour. Presence of methanol leads to the formation of the intermediate MI1. Nucleophilic attack takes place through the transition state MTS1 with an activation barrier of 20.06 kcal mol−1. This leads to the opening of platinacyclobutane ring to form the intermediate MI2. The attached methanol moiety then transfers its proton to the oxygen of oxacyclobutane ring by crossing a low activation barrier (4.86 kcal mol−1, MTS2) to generate the final product MI3. Comparison of the two potential energy surface reveals that the formation of open chain compound by reaction with methanol, is more favourable than the rearrangement process (difference in free
centre bound to the oxygen atom of the product in 3I4. The attachment results a trigonal bipyramidal structure of the co-ordinating metal. Intermediate 3I4b is generated by crossing another transition structure 3TS4a, resulting the complete transfer of one metal centre from the product oxygen atom to the reactant one. After separation of the metal bound spirane system (3I5a/2I1) we observe a transition structure (3TS5a) for the movement of metal centre from oxygen to the distal edge of the cyclopropane moiety in the substrate resulting the intermediate (3I6a/1I1). Recombining again with the previously detached Pa generates another trigonal bipyramidal complex 3I7a that is considered to be finally decomposes into the product P and the metal bound substrate 3I1, ready for the next cycle of the rearrangement process. Detailed PES of the whole process, shown in Fig. 8, clearly reveals that the energy barrier for the binding process of substrate with catalyst and the regeneration of transfer of catalytic centres crosses not more that 22 kcal mol−1. As the diagram reveals, the initial process of binding the Zeise’s salt to the oxaspirohexane substrate (R) occurs by crossing the intermediate 3I0 and the transition structure 3TS0 (barrier 14 kcal mol−1) to form the catalyst substrate complex 3I1. The regeneration of Zeise’s dimer occurs by crossing 7.63 kcal mol−1 free energy barrier while the process of the transfer of catalytic metal
Fig. 6. Change in geometry of Zeise’s dimer catalyst during the rearrangement of oxaspirohexne derivative. 254
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Fig. 7. Ball and Stick models of the stationary points for involving Zeise’s dimer in the catalytic cycle and the regeneration of Zeise’s dimer from product intermediate.
energy is 9.21 kcal mol−1 calculated using M06-2X functional), which fully supports that the double activation process correlates with the experimental fact. Here again we have calculated global barrier using other two functionals (M06 & M06L, Fig. 9). Though the energy values obtained from M06 supports the experimental observation (activation barrier for nucleophilic attack by methanol is low) the M06L method fails to predict the favourability of formation of open chain compound (activation barrier is 34.38 kcal mol−1). This fact reveals that M06L is not suitable for the present system. The anomaly comes from the inclusion of 0% HF exchange energy in the functional and pointed out the necessity of the moderate requirement of this term as given in M06 functional.
fragmentation reaction occurs from the intermediate (3I3). Comparison of these two processes for the unsubstituted and substituted substrate is shown in Fig. 10, (left portion shows the reaction for unstubstituted substrate (R) while the right one shows that for substituted one). Fragmentation to ethylene from unsubstituted oxaspirohexane (R) leads to the formation of 3I4f through the transition state 3TS3f with an activation free energy barrier of 34.74 kcal mol−1 (shown by magenta coloured). However, the rearrangement reaction to 3I4 takes place with an activation free energy of 32.98 kcal mol−1 through the transition state 3TS3 (shown by orange coloured). This fact clearly reveals that substrate, without any substituent, disfavours this fragmentation reaction due to demand of high amount of activation energy (34.74 kcal mol−1). However, presence of two phenyl and two methyl groups in the oxacyclobutane moiety lowers down the energy of the transition state PhTS3f in comparison to the transition state PhTS3 (as shown in the right portion of Fig. 10) associated to the rearrangement reaction (activation barrier for fragmentation reaction is 31.89 kcal mol−1 where as that for rearrangement is 47.56 kcal mol−1). Global barriers calculated using M06 and M06L (shown in Fig. 10 by round bracket and square bracket respectively) also support the favourability of the elimination reaction for highly substituted substrate. The cleavage of CeO bond of the four membered ring to initiate the fragmentation reaction is facilitated mainly by two reasons; increase of the electronegativity of oxygen by coordination of platinum with the spirane oxygen and the presence of two methyl and two phenyl groups that put steric pressure on highly strained substrate. This fact again
3.4. Fragmentation to unsaturated compound While examining the catalytic action on oxaspirohexane derivative bearing several substituents on the oxacyclobutane ring, Howell and coworkers noticed that the formation of tetrahydrofuran derivative is prohibited by another fragmentation reaction yielding ethylene derivative as a sole product (Scheme 3(c)). The fact reveals that there is also another possibility of fragmentation reaction which is only observed in presence of bulky substituent on the four membered ring of the substrate. We compare this fragmentation pathway with that of rearrangement reaction with and without the bulky substituents. We did not repeat the study of the binding and oxidative insertion of metal in cyclopropane ring but only compare the rearrangement step with the 255
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Fig. 8. PES along with the thermodynamic parameters and structures of the stationary points of the pathway of binding Zeise’s salt to oxaspirohexane its further rearrangement to tetrahydrofuran derivative (blue coloured) and the regeneration of catalyst transfer to the reactant. Though the pathway for transfer of catalyst from a product molecule to another reactant one shows a negative barrier the regeneration of catalyst will be more favourable due to the formation of more stable structure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Scheme 4. Formation of open chain compound in presence of methanol.
forms the ketone. To verify the validity of our model for explaining such reactivity we have studied the abstraction of β − hydrogen by the oxygen of oxacyclobutane derivative in another spirane system containing cyclohexane ring substrate model. This pathway is shown in Fig. 11 for the formation of cyclohexene derivative and tetrahydrofuran derivative comparison of the activation barrier shows that the energy required for the fragmentation reaction (26.39 kcal mol−1) is lower than the rearrangement pathway (42.15 kcal mol−1). The high activation barrier for the generation of tetrahydrofuran derivative diverted the pathway to the β − hydrogen abstraction reaction thus clearly correlates with the experimental results and validates our double activation model again. Similar result is also observed using M06 and M06L
suggests the importance of the involvement of two Pt-atoms in the reaction pathway. 3.5. Explanation for other side reaction Howell et al. [5] also pointed out that the substrate containing a cyclohexyl moiety shows a different reactivity that results cyclohexene substituted β, γ unsaturated ketone as the only product (Scheme 3(d)). This process occurs through the abstraction of one hydrogen atom from the β − carbon of cyclohexyl ring system by the oxygen of the oxaspirane ring of the substrate. The resulting hydroxyl group then transfers its hydrogen to one carbon atom of cyclopropane ring and 256
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Fig. 9. PES along with the Gibbs free energy change and structures of the stationary points for the reaction in methanol environment (brown colour). Violet colour indicates the normal pathway for generating a tetrahydrofuran derivative. Values calculated using M06 and M06L functionals are shown in round and square brackets respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. PES along with the thermodynamic parameters and structures of the stationary points of unsubstituted and diphenyldimethyl substituted substrates under Zeise’s dimer catalyzed condition. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional. 257
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Fig. 11. PES along with the thermodynamic parameters and structures of the stationary points of cyclohexyl substituted oxaspirohexane substrate under Zeise’s dimer catalyzed condition for rearrangement and fragmentation reactions. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional.
4. Conclusion In summary, the mechanism of the Zeise’s dimer catalyzed rearrangement of oxaspirohexane substrate to 3-methylene tetrahydrofuran derivative has been followed through several pathways. Energy barriers of different pathways reveal that the involvement of two platinum atoms as catalytic centre is more favourable over the process involving single platinum atom. The Zeise’s dimer provides these two metal atoms, allow it to bind the substrate properly and transform it to the rearranged product. Calculation also reveals that the presence of methanol in the reaction environment leads it preferably to an open chain compound, which is in accordance to the reported result. Our computation also justifies the favourability of fragmentation pathway which is actually taking place when the oxaspirohexane bears certain substituents. Moreover the initial stage of the mechanistic process resembles much with the induced fit model of the enzymatic catalysis on substrate rearrangement reaction [19]. Besides the aspect of mechanism we also performed a comparative study of the effectiveness of several DFT Minnesota functionals in predicting the experimental results. Our results reveal that the most reliable global activation barrier can be obtained using M06 functional. This functional includes a moderate percentage of HF exchange component. Inclusion of larger percentage of HF exchange component, as found in M06-2X, sometimes overestimates the activation energy barrier. Moreover functional like M06L, which contains no HF exchange component, is not suitable for predicting suitably the energy barriers of the pathway in the present reaction involving organometallic compounds.
Fig. 12. Binding of Oxaspirohexane substrates containing nitrogen atom to the Zeise’s dimer.
functional (activation barriers for β-elimination is lower than the rearrangement process as shown by the values in round bracket and square bracket). Experimetal observation [5] also reveals that the Zeise’s dimer remains ineffective to catalyze any reaction when the oxaspirohexane substrate bears any nitrogen containing substituent (e.g.- NHTr, NHBoc) at the adjacent position to the cyclopropane ring. The reason behind this fact is the involvement of two Pt atoms to form coordinate bond with the oxygen as well as nitrogen atoms (Fig. 12) rather than the oxygen and cyclopropane moiety. The later one is essential to initiate the rearrangement reaction (Scheme 3(e)). These observations also indirectly prove the double activation process by Zeise’s dimer as proposed by us. Howell et al. [5] have established their result on the basis of 13Clabelled method. They have demonstrated the presence of the platinacyclobutane ring formation along with an allyl intermediate. From our investigated results we can conclude that our results are also satisfying their conclusions for generating the platinacyclobutane intermediate. In addition to this the involvement of a second Pt-metal has been shown to carry out the total rearrangement reaction.
Acknowledgements We are thankful to the DST, New Delhi, India for providing financial assistance to one of our author (K.C.) in the form of the research project (SR/WOS-A/CS-153/2016). R.S thanks DST-INSPIRE (IF160987), New 258
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Delhi, India for fellowship. A.C. thanks UGC for providing research fellowship. We are also thankful to Visva-Bharati for providing us the necessarily infrastructural facility to perform the research work. Appendix A. Supplementary data
[7]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2018.04.003. References [1] (a) W.C. Zeise, Landmarks in Organo-Transition Metal Chemistry-A Personal View, Helmut Werner Springer link, 2009. [2] (a) B. Adan, G.I. Perez, B. Vaz, O.N. Faza, A.R. de Lera, Mechanistic and sterochemical insights on the Pt-Catalyzed rearrangement of oxiranylpropargylic esters to cyclopentenones, J. Org. Chem. 77 (2012) 8733–8743. [3] (a) D.J. Gorin, B.D. Sherry, F.D. Toste, Ligand effects in homogeneous Au catalysis, Chem. Rev. 108 (2008) 3351–3378; (j) J. Brown, D. Weber, M.R. Gagne, R.A. Widenhoefer, Mechanistic analysis of gold(I)-catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis (gold)-vinyl species and reversible C-O bond formation, J. Am. Chem. Soc 134 (2012) 9134–9137. [4] (a) Literature survey reveals that the catalytic action of Zeise’s dimer is prominent on cyclopropane ring derivative containing oxygen substituent. Some notable references are, P.W. Jennings, L.L. Johnson, Metallacyclobutane complexes of the group eight transition metals: synthesis, characterizations, and chemistry, Chem. Rev 94 (1994) 2241–2290; (b) P.W. Hoberg, Platinum(II)-catalyzed isomerization of alkoxycyclopropanes to alkylated ketones, Organometallics 15 (1996) 3902–3904; (c) K. Ikura, I. Ryu, N. Kambe, N. Sonoda, Room temperature isomerization of siloxycyclopropanes to silyl ethers of 2-methylenealkanols catalyzed by Zeise’s Dimer, J. Am. Chem. Soc. 114 (1992) 1520–1521; (d) M.P. Doyle, D. van Leusen, Rearrangements of oxocyclopropanecarboxylate esters to vinyl ethers. disparate behavior of transition-metal catalysts, J. Org. Chem. 47 (1982) 5326–5339; (e) J. Beyer, R. Madsen, Novel platinum-catalyzed ring-opening of 1,2-cyclopropanated sugars with alcohols. Stereoselective synthesis of 2-C-branched glycosides, J. Am. Chem. Soc 120 (1998) 12137–12138; (f) J. Beyer, P.R. Skaanderup, R. Madsen, Platinum-Catalyzed ring opening of 1,2 cyclopropanated sugars with O-Nucleophiles. Convenient synthesis of 2-C-branched carbohydrates, J. Am. Chem. Soc. 122 (2000) 9575–9583. Another notable reference for the action of Zeise’s dimer is; (h) W. Hiscox, P.W. Jennings, Catalytic hydration of alkynes with Zeise’s Dimer, Organometallics 9 (1990) 1997–1999. [5] C.A. Malapit, S.M. Chitale, M.S. Thakur, R. Taboada, A.R. Howell, Pt-Catalyzed rearrangement of oxaspirohexanes to 3-Methylenetetrahydrofurans: scope and mechanism, J. Org. Chem. 80 (2015) 5196–5209. [6] (a) M.R.A. Blomberg, P.E.M. Siegbahn, J.E. Bäckvall, Theoretical study of the
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Study on the mechanism of isomerization of oxaspirohexane catalyzed by Zeise’s Dimer
T
Sonjoy Mondal, Arpita Chatterjee, Rohini Saha, Animesh Ghosh, Kuheli Chakrabarty, ⁎ Gourab Kanti Das Department of Chemistry, Visva-Bharati, Santiniketan, 731235, West Bengal, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeise’s dimer Oxaspirohexane Rearrangement Catalyst regeneration
Mechanism of the rearrangement of oxaspirohexane to 3-methylenetetrahydrofuran under the catalysis of Zeise’s dimer has been studied using DFT methods. Our results reveal that the catalyst, at the initial stage, undergoes a considerable change in structure under the influence of the substrate and the two metal atoms bind the substrate at different positions. Rearrangement of the substrate molecule then takes place to generate the tetrahydrofuran derivative through the formation of platinacyclobutane intermediate, as revealed in the experimental report. Depending on the variation of substrate structure and environmental condition the diversion of the mechanistic pathway to generate different products can also be explained by the DFT methods. Application of several functionals in calculating the global activation barrier also reveals that M06 functional is the most suitable choice to deal with this type of problem and to predict the global activation barrier.
1. Introduction
it cannot explain several facts observed in the experimental reports. Various mono nuclear platinum complexes were found to be ineffective to bring out the transformation. However, the mechanism, proposed by the authors, shows the involvement of only one platinum atom in the reaction pathway. The authors also observed that the presence of other reagents in reaction medium and the variation of substrate structure divert the pathway to form products other than the tetrahydrofuran derivative (Scheme 3). Specifically the presence of sufficient amount of methanol in the reaction medium leads to an open chain compound, which reveals the involvement of the nucleophilic activity of the methanol in the reaction (Scheme 3(b)). Moreover the dimethyl diphenyl derivative of oxaspirohexane, on similar treatment by Zeise’s dimer, does not result any product of tetrahydrofuran derivative; instead, a fragmentation reaction was observed, yielding dimethyl diphenyl ethane as the sole product (Scheme 3(c)). Another derivative of oxaspirohexane, containing a cyclohexyl ring as the substituent, furnishes an elimination reaction to generate β, γ unsaturated cyclohexene ketone (Scheme 3(d)). Inactivity of the substrates was noted when nitrogen containing substituent is present in the substrate structure (Scheme 3(e)). Formation of such a variety of products on the variation of the substrate structure and the inactivity of some specific oxaspirohexane derivatives clearly reveal that the activation of the substrate molecule by a single metal atom may be an oversimplified mechanism and thus demand a re-investigation to find
Zeise’s salt, the first organometallic compound [1] isolated in the year of 1827, is a platinum metal complex obtained in monomeric and dimeric forms. During the past two or three decades the catalytic activity of many platinum [2] and gold [3] complexes on a variety of substrates has been reported which include the action of Zeise’s salt on several organic substrates [4]. Many of these reports revealed the rearrangement of cyclopropane derivatives. One such recent report has caught our attention for the absolute necessity of the Zeise’s salt as catalyst in its dimeric form [5] (Scheme 1). The substrate in this transformation is an oxaspirohexane derivative where two rings connected to a common carbon atom are nearly perpendicular to each other. Upon treatment with Zeise’s salt at 45 °C in DCM, the substrate changes to a nearly planar tetrahydrofuran derivative. In the proposed mechanistic pathway [5] the authors suggested an initial activation of the cyclopropane ring by forming a coordinate bond to the metal centre (Scheme 2). After binding to the substrate structure the metal atom, on oxidative addition, gets inserted into the cyclopropane ring forming a platina-cyclobutane derivative. Using 13C NMR spectroscopy the authors also confirmed the formation of this fourmembered derivative as an effective intermediate in the reaction pathway. This intermediate, on subsequent rearrangement, transforms to the final product of tetrahydrofuran derivative. Though the proposed pathway gives us a qualitative picture of the mechanism of the reaction, ⁎
Corresponding author. E-mail address: [email protected] (G.K. Das).
https://doi.org/10.1016/j.mcat.2018.04.003 Received 3 January 2018; Received in revised form 16 March 2018; Accepted 3 April 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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Scheme 1. Rearrangement of the oxaspirohexane to 3-methylenetetrahydrofuran.
Scheme 2. Mechanism of the rearrangement of oxaspirohexane proposed by Howell et al.
Scheme 3. Variation of products depending on the variation of the substituents on oxaspirohexane. All reactions are catalyzed by Zeise’s salt.
Bäckvall et al. [6]; they identified the two pathways for activation before insertion of metal atom into the cyclopropane ring. The corner activation, which occurs through the transition state by positioning the metal atom at the corner of the cyclopropane ring was reported to be more favourable than the edge activation process [6]. The later process of edge activation involves the coordination of a specific bond of cyclopropane ring by the metal centre. However these studies were
out a more appropriate mechanism for proper explanation of the formation of different products. In order to understand the mechanism of isomerisation using various catalytic platforms, including Zeise’s catalyst a number of investigations have now been performed using various quantum chemical calculations [6]. Catalysis of platinum complexes in reaction of cyclopropane derivative had previously been studied theoretically by 248
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carried out theoretically on a substrate which contains only the cyclopropane ring without any other coordinating atoms or groups. In the present problem, a little bit complex derivative of cyclopropane system (oxaspirohexane) bearing another effective ligating oxygen should raise several possibilities. The activation of the substrate may be possible by forming coordinate bonds with different parts of the substrate by metal catalyst. Our interest on such substrate structure and the experimental report on variation of products tempted us to search the suitable mechanism involving the planar Zeise’s dimer that could explain all the observations presented in the experimental report [5]. We propose a double activation process of the substrate molecule to explain the reactivity of the substrate and formation of different products in a variety of situations. 2. Computational methods Possible geometries on the designed reaction pathways were optimized using Gaussian 09 software [7]. Construction of trial geometries, monitoring the progress of calculation and visualization of the final output were done by several graphical interface softwares like Gauss View, Molden etc. Primarily we have chosen hybrid density functional M06-2X [8,9] as a method for performing optimization of structures [10,11]. 6–31G (d, p) basis set [12–14] was employed for all non-metal atoms and the LANL2DZ [15,16] basis set was employed for platinum. To find out the geometries of several transition structures on the potential energy surface (PES) of the reaction pathway, relaxed scan method was used. Each structure was characterized as corresponding to a minimum or a saddle point on the energy hyper surface by means of frequency analysis. A transition structure (TS) possesses only a single imaginary frequency while the structure at minimum contains no imaginary frequency at all. Further confirmation of the TS was done by following the intrinsic reaction coordinate (IRC) pathway. We carried out single-point energy calculations for refinement of energies using def2-TZVP basis set for Pt-atom and the 6–311 + G(d,p) basis set for other atoms [17]. Effective core potentials including scalar relativistic effects were used for the metal atom. Solvent effect was considered using PCM model [18]. In all single point energy calculations we used dichloromethane as the solvent. Results obtained using M06-2X functional were again revalidated by optimizing the geometries of some critical points on each of the reaction pathways using two other different functionals (M06 and M06L). Results indicate that the M06-2X functional sometimes overestimates the associated barrier of certain reaction pathways due to giving excessive stresses on the exchange energy. All the relative energies on the reaction path were calculated with respect to the energy sum of the free reactant and catalyst.
Fig. 1. Overview of the possible pathways for activation of oxaspirohexane by Pt-catalyst.
insertion reaction of the Pt metal to form a four membered platinacyclobutane intermediate (1I2). The next step involves the ring expansion process that results a five membered oxygen heterocycle, 3-methylenetetrahydrofuran, as the end product (1I3). In pathway-2 the platinum catalyst activates initially the oxygen atom of the oxaspirohexane by forming a complex (2I1). Next step involves the reorganization of the substrate structure by breaking simultaneously the two rings of the spirane system, resulting the formation of a six membered ring (2I2). While rearranging the atoms, the Pt metal has also been included into the newly formed ring. The final product 2I3 is then generated by contraction of this newly formed ring, excluding again the Pt metal from the ring system. Pathway-3 starts with the substrate activation by two metal centres at site (1) and (2) simultaneously (3I1). After forming a four membered platinacyclobutane ring, 3I2 (by oxidative insertion), cleavage of the OeC bond of oxacyclobutane ring takes place, resulting finally the Pt-bound product 3I3. Potential energy surface associated to the above three pathways are shown in Fig. 2. The formation of the initial complex in all pathways (1I1, 2I1 & 3I1), as apparent from free energy change in Fig. 2, reveals that they are endergonic in nature. In pathway-1 (brown colour), the initial complex (1I1) undergoes an oxidative insertion reaction of Pt- metal by crossing 12.86 kcal mol−1 activation barrier, resulting a four membered platinacyclobutane ring as an intermediate (1I2). The ball & stick models of the intermediates and the transition states (Fig. 3) reveal that the C1eC3 bond of the cyclopropane moiety, that breaks down to make room for the metal atom for expanding the ring to platinacyclobutane, changes from 1.59 Ǻ in 1I1 to 2.26 Ǻ in 1I2 reaching highest energy in 1TS2 with a distance of 1.91 Ǻ. The distance between the metal atom and the ring carbons gets progressively reduced from 2.56 Ǻ (in 1I1) to 2.03 Ǻ in 1I2 through 2.20 Ǻ in the 1TS1. The next step (Fig. 2) involves the expansion of the oxacyclobutane ring of the intermediate 1I2 to a tetrahydrofuran system of 1I3 through 1TS2 with the concomitant collapse of the platinacyclobutane, just generated in the previous step. In
3. Results and discussion 3.1. Rearrangement by single and double activation To find out the possible pathways of the reaction we have selected the oxaspirohexane R (Fig. 1), as the model substrate. Two sites of the substrate that can coordinate with metal centres are the distal bond edge of the cyclopropane moiety (1) and the oxygen lone pair (2). The mechanism, proposed by Howell et al. [5] assumes the starting point as the complex generated by the coordination of a platinum metal with these two sites of the substrate (Scheme 2). However our observation on the geometrical position of these two ligating units reveals that their relative spatial orientations are not suitable to coordinate with a single metal atom. To find out the actual process we have designed three separate pathways by activating these two ligating units separately (pathway-1 and 2) or simultaneously (pathway-3, Fig. 1). The initial step of pathway-1 involves the formation of a coordinate bond (1I1) between the metal centre and distal edge of the cyclopropane ring of the substrate (R), which then undergoes an oxidative 249
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Fig. 2. PES with Gibbs free energy and structures of the stationary points of path-1 (brown colour), 2 (green colour), 3 (violet colour) under single and double Ptcatalyzed conditions. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
membered platina-oxa-cyclohexane ring with the generation of a πbond between the C2-C3 atoms (Fig. 4). Unlike pathway-1, this transformation does not involve any direct activation of the cyclopropane ring by the metal catalyst and thus requires large activation energy (44.72 kcal mol−1, Fig. 2) for rearrangement. The resulting intermediate 2I2 undergoes a further ring contraction process by excluding the platinum metal from the six-membered ring thus forming a tetrahydrofuran derivative through the transition structure 2TS2 (activation barrier 16.8 kcal mol−1). The three atoms, involved in bond breaking and bond making processes in this step (Fig. 4), maintain the distances of C1-O, Pt-O and C1-Pt in 2TS2 as 2.28 Ǻ, 2.01 Ǻ and 2.43 Ǻ respectively. Global activation barrier of this pathway are 48.50, 47.92 and 39.55 kcal mol−1 calculated using M06-2X, M06 and M06L functional respectively. Activating simultaneously the two sites of the substrate in pathway3 (Fig. 2, violet colour) results 3I1 or 3I1a as the possible intermediates. These intermediates differ from each other by the relative conformations of the Pt-complex connected to the oxygen atom (Fig. 5). Oxidative addition of one Pt-atom in intermediate 3I1a leads to the formation of a four membered platinacyclobutane ring (3I3) through 3TS1a. However, 3I1 passes to the platina cyclobutane ring (3I3) via the formation of an additional intermediate 3I2 and the transition structures 3TS1 and 3TS2 (Figs. 2 and 5). The final step of the cleavage of OeC bond of the spirane intermediate takes place through a transition state, 3TS3 with comparatively lower activation energy of 32.98 kcal mol−1 (Fig. 2). At the initial stage, the activated substrate 3I1 maintains a smaller distance between Pt-atom and C1 (2.17Ǻ) relative to that in 1I1 of pathway-1(2.56 Ǻ). However the bond lengths of Pt-C1, Pt-C2 and C1-
this step the migrating oxygen of the cyclobutane system gets detached from the common carbon (C2) of the spirane system while forming a bond with one of the carbon atom attached previously with the metal centre (Fig. 3, 1I2 → 1TS2 → 1I3). The distances between the migrating oxygen and the two carbon atoms that get detached from and attached to this oxygen are respectively 1.53 Ǻ and 1.91 Ǻ in the transition state 1TS2. While forming the C1-O bond, a π-system is generated between C2 and C3 atom, which ultimately forms a co-ordinate bond with the metal centre. The required activation free energy of the rearrangement step is 38.78 kcal mol−1 (Fig. 2), resulting a five membered oxygen containing heterocycle, (1I3), as the end product. The global activation free energies of this pathway are 53.21, 46.93 and 42.12 kcal mol−1 calculated using M06-2X, M06 and M06L functional respectively. In pathway-2 (Fig. 2, green colour) the platinum catalyst activates the oxygen atom of the oxaspirohexane ring initially. As revealed from the structure of 2I1 (Fig. 4), the metal atom activates the substrate by forming a coordinate bond with the oxygen atom of oxabutane ring with a bond distance of 1.91 Ǻ. Such coordination definitely decreases the electron-density around the oxygen atom and facilitates the breaking of the bond between the oxygen and the common carbon (C2) of the spirane substrate. The complex is then rearranged to break the cyclopropane ring (through 2TS1) and forms a six membered ring system (2I2) including platinum and oxygen atoms in the ring in adjacent positions. While progressing through this pathway, the distance between the OeC2 attains 2.16 Ǻ at the transition state 2TS1 (Fig. 4). At the same time the C1-C3 distance of the cyclopropane moiety increases from 1.51 Ǻ (2I1) to 2.04 Ǻ (2TS1) facilitating the fission of this bond and increasing the interaction between the C1 and the metal atom. The process ends up at the geometry of the intermediate 2I2 resulting a six 250
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Fig. 3. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-1.
Comparison of the global activation barriers (Table 1) of all the three pathways clearly reveals that pathway-3 is energetically more favourable than pathway-1 and 2. It is worthy to note that the functional M06L shows the lowest activation barrier (28.86 kcal mol−1). However all the functionals indicate the favourability on pathway-3 occurring through double activation process. The clue that the double activation pathway requires low activation barrier, guides us to study the involvement of dimeric Zeise’s salt in the reaction mechanism.
C2 in the transition structure (3TS1) in the metal insertion process are nearly equal to that in 1TS1 of pathway-1. The intermediate 3I2 and 1I2 also have the similar bond length of C1-Pt and C3-Pt. Rearrangement step, that goes through the 3TS2 and involves the migration of oxygen from the central C2 atom to the C1 atom of the cyclopropane moiety, shows a rather larger distances of the migrating oxygen from both C1 and C2 (2.31 Ǻ and 2.07 Ǻ) relative to the distances found in the mono metal activated transition state 1TS2 in pathway-1 (C1-O and C2-O is 1.91 Ǻ and 1.53 Ǻ). The increased bond length in 3TS2 suggests that the detached oxygen from C2 atom is stabilized by the co-ordination with second Pt-atom. This effect is further reflected by the reduced activation free energy required in the rearrangement for pathway-3. The reduced distance of Pt-C1 in 3TS2 (2.29Ǻ) relative to that distance in 1TS2 (2.63 Ǻ) also suggests that the transition state has been attained in pathway-3 without increasing the Pt-C1 bond length to a longer distance. The product 3I3 shows the two catalytic metal atoms co-ordinated at the opposite faces of the generated tetrahydrofuran derivative. The global activation free energies of this pathway are 46.70 (M06-2X), 36.65 (M06) and 28.86 (M06L) kcal mol−1 calculated using different functionals.
3.2. Involvement of Zeise’s salt and the catalytic cycle After determining the number of platinum atoms involved in the favourable pathway, we turn our attention to the pathway by which the planer Zeise’s dimer can provide two Pt-atoms to the substrate in a perfect orientation to catalyze the reaction. In fact we surmised that the association of a dimeric catalyst and the substrate molecule at the initial stage of the reaction is entropically more favourable process that the association of two monomeric catalysts separately with the substrate molecule. The schematic representation of the result of our search is shown in Fig. 6. The common middle portion of the catalytic cycle 251
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Fig. 4. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-2.
(3I1 → 3I4) is the repetition of pathway-3, discussed in the previous section. Upper portion represents the pathway through which Zeise’s dimer (a) binds the substrate R to form 3I1 and the regeneration of Zeise’s dimer from the complex 3I4. Lower portion shows the transfer of metal centres from the product 3I4 to a reactant molecule R (3I4a → 3I7a). Relevant models of the intermediates and transition states in catalytic cycles are shown in Fig. 7. Interaction of the catalyst ‘a’ with the reactant ‘R’ generates the intermediate 3I0. While forming this intermediate the oxygen atom of the substrate ‘R’ breaks the bridging bond between Pt-Cl and forms a coordinate bond with the metal atom. The remaining portion of the catalyst containing a Pt-metal gets attached to the second bridging chlorine ligand. Several conformational arrangement of the substrate with respect to the attached catalyst have been identified and we have selected the most favourable one in which cyclopropane moiety is in close proximity to the non coordinating Ptatom (3I0, Fig. 7). At this stage the distance between the Pt and the O of the substrate molecule is 2.18 Ǻ and that between the bridging chlorine and another metal centre is 2.50 Ǻ, slightly larger than that in the native Zeise’s dimer. A second internal substitution reaction (3Io to 3I1 through 3TSo) displaces another bridging chlorine atom of the second Pt-metal and allows the second half of the catalyst to form a coordinate bond with the cyclopropane ring (Fig. 6), thus initiate the reaction by edge activation. Such process progressively increases the edge length
(C1-C3) of the cyclopropane ring from 1.52Ǻ in 3Io through 1.57 Ǻ in 3TSo (Fig. 7) to 1.58 Ǻ in 3I1. The formation of 3I1 thus initiates the mechanism of pathway-3 (Fig. 2) details of which have been discussed previously. This leads to the end product 3I4. For the continuation of the catalytic cycle we identified two pathways the first of which recovers the Zeise’s dimer (a) from 3I4 (top portion of Fig. 6) and the second one consists of the transfer of metal centres from 3I4 to reactant R (bottom portion of Fig. 6). While recovering the Zeise’s dimer we observed that the two metal centres that are positioned at the opposite faces of the product (3I4 in Figs. 5 and 6) should come close to each other. A proper scrutiny of the geometry of 3I4 revealed that it would be easier to rotate the Pt-metal centre on the oxygen atom to change its position from one plane to other. 3TS4 (Fig. 7) is the transition structure through which 3I4 passes to 3I5, thus positioning the chlorine ligand of the moving metal atom very close to the Pt-centre, bound to the unsaturated moiety of the product (3I5 in Fig. 7). Getting a chlorine ligand at its close vicinity in intermediate 3I5, the other Pt-atom jumps to join with it by crossing another transition structure 3TS5 (Fig. 7). The resulting intermediate 3I6 (Fig. 7) on optimization without the bound product (P) gives back the Zeise’s dimer. The second pathway that involves the transfer of the catalytic metal centres from product to reactant (lower portion of Fig. 6) starts with the formation of the intermediate 3I4a by combining the spirane R at Pt-
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Fig. 5. Ball and Stick models of the transition states involved in the rearrangement process of Pt(II) catalyzed pathway-3.
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centres from product to reactant molecule is energetically more costlier (21.20 kcal mol−1). Thus the regeneration of the Zeise’s dimer (top portion of Fig. 6) should be the favourable process for continuation of the catalytic cycle.
Table 1 Global activation free energy (kcal mol–1) calculated using M06-2X, M06 and M06L. Functional employed
Pathway-1 (Fig. 2)
Pathway-2 (Fig. 2)
Pathway-3 (Fig. 2)
M06-2X M06 M06L
53.21 46.93 42.12
48.50 47.92 39.55
46.70 36.65 28.86
3.3. Reaction in presence of methanol Howell et al. [5] observed that the presence of sufficient amount of methanol in the reaction mixture leads to an open chain compound (II) rather than the tetrahydrofuran derivative (I) (Schemes b and Scheme 4). Such a reaction occurs via the nucleophilic attack of the oxygen of methanol to one carbon atom of cyclopropane moiety of the substrate spirane (Scheme 4). To compare the energy barrier of the formation of tetrahydrofuran derivative (I) and the open chain compound (II), we have explored the potential energy surface associated to the two pathways shown in Fig. 9 (blue and brown colour respectively). The selected starting point for both pathways is the platinacyclobutane derivative (3I2) (shown in Fig. 7), generated through the oxidative addition of platinum complex to the cyclopropane ring. The formation of the tetrahydrofuran derivative has been shown by violet colour and that for the open chain compound has been depicted using brown colour. Presence of methanol leads to the formation of the intermediate MI1. Nucleophilic attack takes place through the transition state MTS1 with an activation barrier of 20.06 kcal mol−1. This leads to the opening of platinacyclobutane ring to form the intermediate MI2. The attached methanol moiety then transfers its proton to the oxygen of oxacyclobutane ring by crossing a low activation barrier (4.86 kcal mol−1, MTS2) to generate the final product MI3. Comparison of the two potential energy surface reveals that the formation of open chain compound by reaction with methanol, is more favourable than the rearrangement process (difference in free
centre bound to the oxygen atom of the product in 3I4. The attachment results a trigonal bipyramidal structure of the co-ordinating metal. Intermediate 3I4b is generated by crossing another transition structure 3TS4a, resulting the complete transfer of one metal centre from the product oxygen atom to the reactant one. After separation of the metal bound spirane system (3I5a/2I1) we observe a transition structure (3TS5a) for the movement of metal centre from oxygen to the distal edge of the cyclopropane moiety in the substrate resulting the intermediate (3I6a/1I1). Recombining again with the previously detached Pa generates another trigonal bipyramidal complex 3I7a that is considered to be finally decomposes into the product P and the metal bound substrate 3I1, ready for the next cycle of the rearrangement process. Detailed PES of the whole process, shown in Fig. 8, clearly reveals that the energy barrier for the binding process of substrate with catalyst and the regeneration of transfer of catalytic centres crosses not more that 22 kcal mol−1. As the diagram reveals, the initial process of binding the Zeise’s salt to the oxaspirohexane substrate (R) occurs by crossing the intermediate 3I0 and the transition structure 3TS0 (barrier 14 kcal mol−1) to form the catalyst substrate complex 3I1. The regeneration of Zeise’s dimer occurs by crossing 7.63 kcal mol−1 free energy barrier while the process of the transfer of catalytic metal
Fig. 6. Change in geometry of Zeise’s dimer catalyst during the rearrangement of oxaspirohexne derivative. 254
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Fig. 7. Ball and Stick models of the stationary points for involving Zeise’s dimer in the catalytic cycle and the regeneration of Zeise’s dimer from product intermediate.
energy is 9.21 kcal mol−1 calculated using M06-2X functional), which fully supports that the double activation process correlates with the experimental fact. Here again we have calculated global barrier using other two functionals (M06 & M06L, Fig. 9). Though the energy values obtained from M06 supports the experimental observation (activation barrier for nucleophilic attack by methanol is low) the M06L method fails to predict the favourability of formation of open chain compound (activation barrier is 34.38 kcal mol−1). This fact reveals that M06L is not suitable for the present system. The anomaly comes from the inclusion of 0% HF exchange energy in the functional and pointed out the necessity of the moderate requirement of this term as given in M06 functional.
fragmentation reaction occurs from the intermediate (3I3). Comparison of these two processes for the unsubstituted and substituted substrate is shown in Fig. 10, (left portion shows the reaction for unstubstituted substrate (R) while the right one shows that for substituted one). Fragmentation to ethylene from unsubstituted oxaspirohexane (R) leads to the formation of 3I4f through the transition state 3TS3f with an activation free energy barrier of 34.74 kcal mol−1 (shown by magenta coloured). However, the rearrangement reaction to 3I4 takes place with an activation free energy of 32.98 kcal mol−1 through the transition state 3TS3 (shown by orange coloured). This fact clearly reveals that substrate, without any substituent, disfavours this fragmentation reaction due to demand of high amount of activation energy (34.74 kcal mol−1). However, presence of two phenyl and two methyl groups in the oxacyclobutane moiety lowers down the energy of the transition state PhTS3f in comparison to the transition state PhTS3 (as shown in the right portion of Fig. 10) associated to the rearrangement reaction (activation barrier for fragmentation reaction is 31.89 kcal mol−1 where as that for rearrangement is 47.56 kcal mol−1). Global barriers calculated using M06 and M06L (shown in Fig. 10 by round bracket and square bracket respectively) also support the favourability of the elimination reaction for highly substituted substrate. The cleavage of CeO bond of the four membered ring to initiate the fragmentation reaction is facilitated mainly by two reasons; increase of the electronegativity of oxygen by coordination of platinum with the spirane oxygen and the presence of two methyl and two phenyl groups that put steric pressure on highly strained substrate. This fact again
3.4. Fragmentation to unsaturated compound While examining the catalytic action on oxaspirohexane derivative bearing several substituents on the oxacyclobutane ring, Howell and coworkers noticed that the formation of tetrahydrofuran derivative is prohibited by another fragmentation reaction yielding ethylene derivative as a sole product (Scheme 3(c)). The fact reveals that there is also another possibility of fragmentation reaction which is only observed in presence of bulky substituent on the four membered ring of the substrate. We compare this fragmentation pathway with that of rearrangement reaction with and without the bulky substituents. We did not repeat the study of the binding and oxidative insertion of metal in cyclopropane ring but only compare the rearrangement step with the 255
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Fig. 8. PES along with the thermodynamic parameters and structures of the stationary points of the pathway of binding Zeise’s salt to oxaspirohexane its further rearrangement to tetrahydrofuran derivative (blue coloured) and the regeneration of catalyst transfer to the reactant. Though the pathway for transfer of catalyst from a product molecule to another reactant one shows a negative barrier the regeneration of catalyst will be more favourable due to the formation of more stable structure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Scheme 4. Formation of open chain compound in presence of methanol.
forms the ketone. To verify the validity of our model for explaining such reactivity we have studied the abstraction of β − hydrogen by the oxygen of oxacyclobutane derivative in another spirane system containing cyclohexane ring substrate model. This pathway is shown in Fig. 11 for the formation of cyclohexene derivative and tetrahydrofuran derivative comparison of the activation barrier shows that the energy required for the fragmentation reaction (26.39 kcal mol−1) is lower than the rearrangement pathway (42.15 kcal mol−1). The high activation barrier for the generation of tetrahydrofuran derivative diverted the pathway to the β − hydrogen abstraction reaction thus clearly correlates with the experimental results and validates our double activation model again. Similar result is also observed using M06 and M06L
suggests the importance of the involvement of two Pt-atoms in the reaction pathway. 3.5. Explanation for other side reaction Howell et al. [5] also pointed out that the substrate containing a cyclohexyl moiety shows a different reactivity that results cyclohexene substituted β, γ unsaturated ketone as the only product (Scheme 3(d)). This process occurs through the abstraction of one hydrogen atom from the β − carbon of cyclohexyl ring system by the oxygen of the oxaspirane ring of the substrate. The resulting hydroxyl group then transfers its hydrogen to one carbon atom of cyclopropane ring and 256
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Fig. 9. PES along with the Gibbs free energy change and structures of the stationary points for the reaction in methanol environment (brown colour). Violet colour indicates the normal pathway for generating a tetrahydrofuran derivative. Values calculated using M06 and M06L functionals are shown in round and square brackets respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. PES along with the thermodynamic parameters and structures of the stationary points of unsubstituted and diphenyldimethyl substituted substrates under Zeise’s dimer catalyzed condition. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional. 257
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Fig. 11. PES along with the thermodynamic parameters and structures of the stationary points of cyclohexyl substituted oxaspirohexane substrate under Zeise’s dimer catalyzed condition for rearrangement and fragmentation reactions. Energy values not enclosed in bracket are calculated using M06-2X functional. Values indicated in the round bracket are calculated using M06 and that in the square bracket are calculated using M06L functional.
4. Conclusion In summary, the mechanism of the Zeise’s dimer catalyzed rearrangement of oxaspirohexane substrate to 3-methylene tetrahydrofuran derivative has been followed through several pathways. Energy barriers of different pathways reveal that the involvement of two platinum atoms as catalytic centre is more favourable over the process involving single platinum atom. The Zeise’s dimer provides these two metal atoms, allow it to bind the substrate properly and transform it to the rearranged product. Calculation also reveals that the presence of methanol in the reaction environment leads it preferably to an open chain compound, which is in accordance to the reported result. Our computation also justifies the favourability of fragmentation pathway which is actually taking place when the oxaspirohexane bears certain substituents. Moreover the initial stage of the mechanistic process resembles much with the induced fit model of the enzymatic catalysis on substrate rearrangement reaction [19]. Besides the aspect of mechanism we also performed a comparative study of the effectiveness of several DFT Minnesota functionals in predicting the experimental results. Our results reveal that the most reliable global activation barrier can be obtained using M06 functional. This functional includes a moderate percentage of HF exchange component. Inclusion of larger percentage of HF exchange component, as found in M06-2X, sometimes overestimates the activation energy barrier. Moreover functional like M06L, which contains no HF exchange component, is not suitable for predicting suitably the energy barriers of the pathway in the present reaction involving organometallic compounds.
Fig. 12. Binding of Oxaspirohexane substrates containing nitrogen atom to the Zeise’s dimer.
functional (activation barriers for β-elimination is lower than the rearrangement process as shown by the values in round bracket and square bracket). Experimetal observation [5] also reveals that the Zeise’s dimer remains ineffective to catalyze any reaction when the oxaspirohexane substrate bears any nitrogen containing substituent (e.g.- NHTr, NHBoc) at the adjacent position to the cyclopropane ring. The reason behind this fact is the involvement of two Pt atoms to form coordinate bond with the oxygen as well as nitrogen atoms (Fig. 12) rather than the oxygen and cyclopropane moiety. The later one is essential to initiate the rearrangement reaction (Scheme 3(e)). These observations also indirectly prove the double activation process by Zeise’s dimer as proposed by us. Howell et al. [5] have established their result on the basis of 13Clabelled method. They have demonstrated the presence of the platinacyclobutane ring formation along with an allyl intermediate. From our investigated results we can conclude that our results are also satisfying their conclusions for generating the platinacyclobutane intermediate. In addition to this the involvement of a second Pt-metal has been shown to carry out the total rearrangement reaction.
Acknowledgements We are thankful to the DST, New Delhi, India for providing financial assistance to one of our author (K.C.) in the form of the research project (SR/WOS-A/CS-153/2016). R.S thanks DST-INSPIRE (IF160987), New 258
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Delhi, India for fellowship. A.C. thanks UGC for providing research fellowship. We are also thankful to Visva-Bharati for providing us the necessarily infrastructural facility to perform the research work. Appendix A. Supplementary data
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