Revisited the mechanism of the transition metal catalyzed cycloetherification of ω-hydroxy propargylic ester: A DFT study

Computational and Theoretical Chemistry 1114 (2017) 146–152

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Revisited the mechanism of the transition metal catalyzed cycloetherification of x-hydroxy propargylic ester: A DFT study Arpita Chatterjee, Rohini Saha, Dibyajyoti Panja, Sujit Ghosh, Sonjoy Mondal, Animesh Ghosh, Gourab Kanti Das ⇑ Department of Chemistry, Visva-Bharati, Santiniketan 731235, West Bengal, India

a r t i c l e

i n f o

Article history: Received 17 April 2017 Received in revised form 28 May 2017 Accepted 28 May 2017 Available online 30 May 2017

a b s t r a c t DFT calculations on a reported mechanistic pathway of a platinum catalyzed cycloetherification process reveal the requirement of high activation free energy for the progress of the reaction. Parallel calculations on some alternative pathways suggest that a solvent assisted proton transferring mechanism is much more likely to be relevant to explain the formation of the product. Our calculation also supports the experimental finding of the formation of a different product on replacing the platinum salt by a gold one. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Metal mediated pyran and furan ring formation has received much attention in the last one or two decades to the researches in the field of synthetic organic chemistry. These pyran & furan derivatives have several utilities in different biochemical, industrial, medicinal processes. It is found that Pt [1–6], Au [7–14], Ag [15] metal salts are very effective in catalyzing reactions for generating five membered and six membered oxygen heterocycles [16]. Krause et al. [17] reported the effective utilization of Au(III) ion in catalyzing the cyclisation of a-hydroxyl allenes to 2,5dihydrofurans. Widenhoefer et al. [18] had studied Au(I) catalyzed intramolecular allene hydroxylation for the synthesis of furan derivatives. Another report by Wills et al. [19] disclosed an alternative protocol for the effective use of gold metal for this purpose. Most recently Carter et al. [20] reported a stereoselective synthesis of poly substituted pyran ring system under the catalysis of silver salt. Au(I) and Pt(II) catalyzed cycloetherification of x-hydroxy propargylic ester were studied by Brabander et al. [21] and shown that the substrate initially undergoes a [3,3] sigmatropic rearrangement and finally 6-endo-dig cyclisation leading to a cycloether. Depending on the nature of metal ion, various pyran derivatives are generated. In the presence of Zeise’s salt, deacylation takes place through a SN20 type mechanism [22] on allene to form an alkyne derivative of pyran. However an oxacyclic enol acetate results when the Pt salt is replaced by a gold one (Scheme 1).

⇑ Corresponding author. E-mail address: [email protected] (G.K. Das). http://dx.doi.org/10.1016/j.comptc.2017.05.036 2210-271X/Ó 2017 Elsevier B.V. All rights reserved.

While proposing the mechanism of the process the authors assumed the formation of a carboxyl co-ordinated metal complex as an effective intermediate to explain the deacylation reaction. Our interest to this pathway compelled us to validate it by quantum mechanical calculation and our result (shown as pathway-1 in this paper) suggests that the activation energy involved in this pathway is necessarily high. This is probably due to the high energy demand for generation of metal carboxylate co-ordinate bond in unfavourable geometry. It appeared to us that the mechanism, they had shown was most probably not the correct pathway due to such high activation barrier. We started to investigate whether there is any other energetically favourable pathway that leads to the final product by crossing a reasonable barrier of activation process. Our final conclusion revealed that an alternative pathway is more reasonable for explaining the formation of the product. Brabander et al. had simultaneously also studied the Aucatalyzed reaction of that same propargylic substrate and found that the protodeauration process is more favourable that guide the reaction to form a product other than deacylation (Scheme 1). It has been reviewed earlier that protodeauration is one of the most favourable final step in many gold catalyzed reaction of propargylic ester rearrangement [23]. Theoretically it was shown that the protodeauration process is rapid when p-donating groups are present as the ligand of metal ion [23]. In this paper our comparative study on this pathway also reveals that the protodeauration process is more favourable pathway than the deacylation process for Au-catalyzed pyran derivative formation.

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147

Scheme 1. Reported Pt(II) and Au(I) catalyzed cycloetherification of x-hydroxypropargylic ester.

2. Computational methods All the geometry optimizations and energy calculations of the systems were performed by using Gaussian 09 software [24]. Monitoring the progress of calculations and visualization of the final output were done by several in-built script and graphics software like Gauss View, Molden etc. We have chosen hybrid density functional M06-2X as a method for performing DFT calculations [25,26]. 6-31G (d,p) basis set [27–29] was employed for all nonmetal atoms and the LANL2DZ [30,31] basis set (BS1) was employed for Pt and Au atom. This computational method was successfully applied in various mechanistic pathways [32–34]. To find out the geometries of several transition structures on the potential energy surface of the reaction pathway, relaxed scan method was used. Each structure was characterized as corresponding to a saddle point on the energy hyper surface by means of frequency analysis. A transition structure possesses only a single imaginary frequency. Further confirmation of the transition state was done by following the intrinsic reaction coordinate (IRC) pathway. By following the IRC pathway in forward and backward direction, the transition structure is ultimately changed to corresponding reactant and product. Full optimization of these structures and comparison to the structures obtained in the previous and following steps has been done to get the total continuous pathway. Solvent effect was approximated by introducing a polarizable continuum model (PCM) [35], and bulk solvent effects were computed using the gas phase optimized geometries. We used the standard parameters for the solvent tetrahydrofuran (THF) as this solvent was used to carry out the reaction experimentally. To further refine the barriers obtained from the above mentioned basis set of the atoms involved, we carried out single-point energy calculations for the relevant structures on the PES with a larger basis set in tetrahydrofuran solvent at the M06-2X level with quadruple-f valance def2-QZVP basis set on Pt and Au metal and the 6-311+G (d,p) basis set on other atoms [36]. Effective core potentials including scalar relativistic effects were used for the metal atom. We used electronic energies of the species for calculating energy barriers as shown in result and discussion section. All the relative energies on the reaction path were calculated with respect to the energy sum of the free reactant and catalyst. To check the solvent effect on the optimization process of geometries of the stationary points on the most favourable pathways we have further carried out the optimization using PCM model with tetrahydrofuran as

solvent. Geometries and energies of all these stationary points are shown in Electronic Supplementary Information.

3. Result and discussion 3.1. Plausible pathways for Pt-catalyzed cycloetherification reaction For our investigation of the reaction pathways the model structure (R) was selected as the starting material. The plausible mechanistic pathways for cycloetherification process from reactant R under Pt-catalyzed condition are summarized in Scheme 2. The initial step of the process is the co-ordination of metal salt to the reactant R. Previous literature shows [37] that for a fruitful reaction, the coordinate complex should be formed between the triple bond and the metal centre. The resulting complex (A) undergoes a [3,3] sigmatropic shift through a six membered cyclic intermediate (B) to generate metal-bound allene complex (C). The complex (C) then undergoes an intramolecular nucleophilic attack by the terminal alcoholic oxygen to the metal bound allene moiety to form a six membered cyclized intermediate (D). Intermediate D may follow three possible pathways (designated as path-1,2 and 3) to form the deacylated cyclic alkyne (P) as the end product. In pathway-1 a chelate complex between the metal centre and carbonyl oxygen of the transferred acyl group takes place as the first step. The proton transfer from the newly formed pyran ring deacylates the co-ordinated acyl group to form the product. This pathway has the maximum resemblance with the pathway proposed by Brabander. In pathway-2 the protonated oxygen of the newly formed pyran ring transfer its associated proton to the oxygen atom of the acyl group, which facilitates the elimination of the acyl group from the intermediate. Such proton transfer process in pathway-3 occurs through the assistance of an external solvent molecule, THF, to affect the deacylation. A variant of pathway-2 has also been investigated in which the ethylene ligand is not attached to the metal centre. Potential energy surface indicating the transition states and intermediate involved in all the proposed mechanism (Scheme 2) is presented in Fig. 1. The reaction starts by the formation of an allene intermediate (PtI3) through a two steps process in which the first step involves an internal nucleophilic attack of the carboxylic oxygen to the Pt-coordinated triple bond of reactant-metal complex (PtI1) to generate a six membered ring intermediate, PtI2. Formation of metal

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Scheme 2. Overview of the studied mechanistic pathways of Pt(II) catalyzed reaction.

bound allene intermediate, PtI3, takes place when the acyl group migrates by breaking the six membered ring intermediate. The activation energy required in the first and second steps are 12.13 and 10.29 kcal mol1 respsectively. The allene intermediate generated by this process is attached to the Pt-atom as an g1 complex. The intermediate PtI3 then undergoes some conformational changes in its side chain to generate another intermediate PtI3a, suitable for the next step of cyclization reaction. The conformational changes involve the rotation around CAC single bond and are considered to take place by crossing very low energy barrier. Cyclic intermediate PtI4 is generated by the attack of alcoholic oxygen in 6-endo-dig fashion (through PtTS3) with an activation barrier 10.23 kcal mol1. In view to generate the cyclic alkyne as the end product we have made several attempts to transfer the proton from the alcoholic oxygen to the acyl moiety intermediate. As discussed earlier, our first attempt resembles with the mechanistic pathway proposed by Brabander et al. The initial structure (PtI41a) of this pathway is a conformational isomer of PtI4. Though the detailed investigation on the activation barrier for such conformational change has not been performed here, these conformational changes in the transition metal bound allene geometry are quite facile as revealed by previous literature data [38]. With this assumption a deprotonation of PtI41a was performed to generate PtI41b. The deprotonation step is considered to be assisted by the solvent THF. The intermediate then forms a chelate complex with Pt-metal by making an internal coordinate bond to the acyl oxygen (through PtTS41 with an activation barrier of 25.15 kcal mol1). Formation of the co-ordination complex with acyl oxygen results the substitution of one of the chlorine atom from the metal centre. Final detachment of the acyl group from substrate takes place

through PtTS51 (Fig. 2a) to generate the metal bound product complex PtI61. The total activation barrier, as revealed from Fig. 1 and Table 1, for the deacylation process is 157.2 kcal mol1 (or 61.46 kcal mol1 by using higher basis set), indicating the unfavourability of the process. To find out a more reasonable mechanism we have investigated other two pathways. In pathway-2 proton transfer process occurs directly with an activation barrier of 13.87 kcal mol1 and forms the deacylated cyclic alkyne PtI52 through transition state PtTS42 (Fig. 2b). Such a lower activation barrier indicates the favourability of pathway-2 over pathway-1. We also found that another variant of pathway-2 (pathway-2a) occurs in Pt-complex containing no ethylene ligand with an activation energy of 38.62 kcal mol1. Besides pathway-1 and 2, another solvent assisted pathway (pathway-3) has been investigated where the involvement of a solvent molecule is considered to transfer the proton from the oxygen atom of the newly formed six membered ring to the acyl oxygen. The first proton transfer process is found to be barrierless and involve no transition structure. The generated intermediate PtI43 results the deacylated product PtI53 by passing through a transition structure PtTS43 (Fig. 2c) with an activation barrier 21.66 kcal mol1 (19.27 kcal mol1 by using higher basis set) and the relative energy of this transition state (PtTS43) is 35.98 kcal mol1 1 (11.91 kcal mol by using higher basis set) (Table 1). Such process involves the transfer of proton from the protonated THF to the acyl group. Though the proton transferring process in solvent assisted pathway reveals a higher activation barrier the relative energy of all the species involved in this pathway are lower than that for pathway-2. The energy profile diagram (path1, path2 and path3) clearly reveals that the solvent assisted pathway is

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Fig. 1. PES along with the thermodynamic parameters and structures of the stationary points of path 1,2a,2,3 under Pt-catalyzed condition.

PtTS51

(a)

PtTS42

(b)

PtTS43

(c)

Fig. 2. Ball and Stick models of the transition state involved in the deacylation process of Pt(II) catalyzed pathways (a, b and c).

energetically most favourable. To characterise better the geometries in the favourable path (Path-3) we further optimize the gas phase geometries involved in this path by using solvent approximation. We have detected only a small variation of geometries and energies that could hardly affect any conclusion as discussed above.

3.2. Construction of pathways for Au-catalyzed reactions While studying the reaction under the catalysis of Au, we followed the same mechanistic pathways that have already been discussed in the previous section for Pt-catalyzed reaction. The

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Table 1 Comparison of electronic energy barrier of the several segments of different pathways and global activation energy (kcal mol1). Relative energy required (kcal mol1)

Pathway No.

Global activation barriera

Relative energy of the transition state that leads to producta

Allene formation

Nucleophilic attack

Deacylation/ protodeauration

Pt-Catalyzed Path-1 (Fig. 1) Path-2a (Fig. 1) Path-2 (Fig. 1) Path-3 (Fig. 1)

12.13 12.13 12.13 12.13

10.23 10.23 10.23 10.23

157.2 38.62 13.87 21.66

61.46 42.17 12.94 19.27

49.52 30.75 1.52 11.91

Au-Catalyzed Path-4 (Fig. 3) Path-5 (Fig. 3) Path-6 (Fig. 3)

15.68 15.68 15.68

10.39 10.39 10.39

5.51 13.16 0.00

17.37 17.37 17.37

24.48 33.53 42.23

a Figures, as shown in the last two columns, indicate the energy barriers and relative energies calculated with the higher basis set and solvent approximation as described in Computational methods.

process starts by [3,3] sigmatropic migration [39–48] of acetate group of the Au-bound propargylic ester complex (AuI1). The generated allene then undergoes an intramolecular nucleophilic attack by the hydroxyl group, resulting in the formation of 6-membered protonated pyran derivative (AuI5). AuI5 may isomerize by following three pathways (pathway 1,2 and 3). Though pathway-1 and pathway-2 form the deacylated derivative, the last one (pathway-3) involves the protodeauration process. The detailed energy barriers of the three pathways are shown in the energy profile diagram of Fig. 3.

The allene bound Au-complex AuI1 undergoes acyl group migration in the first step and forms the product AuI2 through AuTS1 with an activation barrier 15.68 kcal mol1. In the second step Aucomplex AuI2 forms a six-membered ring intermediate (through AuTS2) and the activation energy needed for this step is 6.27 kcal mol1. The generated Au-bound allene intermediate (AuI3) undergoes some conformation rearrangement in its side chain and transformed to the conformer AuI4a. Complex AuI4a undergoes nucleophilic attack by the alcoholic oxygen to the Au-bound allene moiety and forms 6-membered cyclic intermediate AuI5

Fig. 3. PES along with the thermodynamic parameters and structures of the stationary points of path 1,2,3 under Au-catalyzed condition.

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AuTS51

AuTS52

AuTS53

(a)

(b)

(c)

Fig. 4. Ball and Stick models of the transition state involved in the deacylation process (a and b) and protodeauration (c) of Au(I) catalyzed pathways.

(through AuTS4) with an activation energy 10.39 kcal mol1. In pathway1 the last step involves in the formation of Au-bound deacylated cyclic alkyne as a product through AuTS51 (Fig. 4a) with an activation barrier 5.51 kcal mol1 and the relative energy of this transition state is 29.30 kcal mol1 (24.48 kcal mol1 using higher basis set). In case of pathway-2 Au-complex AuI52 undergoes deacylation in the presence of solvent THF by crossing the activation barrier of 13.16 kcal mol1 and forms the product AuI62 with a relative energy of 40.61 kcal mol1 (33.53 kcal mol1 using higher basis set). Fig. 4b shows the transition structure in which the acyl group orients in a sterically favoured conformation different from the transition structure of the previous one (AuTS51). Hence it is clearly observed that the THF assisted proton transfer process leads to an energetically more favourable pathway. Literature study reveals that protodeauration is also an important possible mode of reaction for Au-catalyzed processes [23]. Pathway-3 reveals that the product AuI73 is formed by protodeauration process from the reactant AuI53 (via intermediate AuI63) and through the transition state AuTS53 (Fig. 4c) with relative energy 50.87 kcal mol1 (42.23 kcal mol1 using higher basis set). We observed that both this pathway and the end product are energetically more favourable and stable. While optimizing the gas phase geometries of the favourable pathway using PCM model we have noted a change in geometry only in the case of the transition structure of protodeauration process where the alteration of the relative orientation of the tetrahydrofuran ring with respect to the rest of the structure has been detected. All the other geometries are little affected by such optimization process. So, it can be concluded that in the case of Au-catalyzed reactions protodeauration process is energetically most favourable pathway.

4. Conclusion In summary, the mechanism of the formation of cycloether from x-hydroxy propargylic ester under Pt or Au-catalytic condition has been followed through several pathways. Energy barriers of different pathways reveal that the formation of a carboxyl coordinated intermediate during the deacylation reaction is highly unfavourable (which was proposed previously in the experimental paper). Our calculation reveals that a solvent assisted proton transferring reaction is energetically more reasonable to explain the deacylation process under Pt-catalyzed condition. Further study on the gold catalyzed reaction suggests that the protodeauration process is more favourable that the deacylation reaction and

explains correctly the experimental result for generation of oxacyclic enol acetates rather than deacylated product.

Acknowledgements We are thankful to the UGC, New Delhi, India for providing financial assistance. We are also thankful to Visva-Bharati for providing us the necessarily infrastructural facility to perform the research work.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2017.05. 036.

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