3.1 Reaction A
The free energy diagrams for the ligand exchange andβ -carbon elimination in reaction A are calculated and shown in Figure 1. At the entrance of the reaction,R firstly coordinates with cat and Ag2CO3 to yield the intermediate1A by exoergic of 32.2 kcal/mol. The inaccessible direct ring-opening of R with the assistance of Ag2CO3is given in Figure S1 in Supporting Information (TS1-P2-1A ). Then the ligand exchange takes place to generate an alkoxide intermediate 2Avia transition state TS1A with a facile barrier of 0.2 kcal/mol. In the following step,2A undergoes the β -carbon elimination to form an alkyl-rhodium homoenolate 3A by overcoming the barrier of 20.5 kcal/mol (TS2A ). The possible ring-opening processes from 2A and 3A , respectively, caused by the protonation, were also calculated and precluded due to the high barriers (TS1-P2-2Aand TS1-P2-3A in Figure S1 in Supporting Information).
FIGURE 1 Free energy diagrams for the ligand exchange andβ -carbon elimination steps in reaction A. The relative free energies and relative enthalpic energies (in parentheses) are given in kcal/mol
FIGURE 2 Free energy diagrams for the second ligand exchange, second β -carbon elimination, and reductive elimination steps in reaction A. The relative free energies and relative enthalpic energies (in parentheses) are given in kcal/mol
With the coordination of another molecule of R ,3A transforms into intermediates4-cisA and4-tranA , respectively, according to the relative position of two moieties of R (Figure 2). The subsequent ligand exchange then occurs to result into intermediates5-cisA and5-tranA , respectively. The unfavorableβ -hydride elimination from 3A was excluded (TS3-P1’-1A in Figure S2 in Supporting Information). In the next step, another β -carbon elimination from 5-cisA and5-tranA takes place to bring about intermediates 6-cisA and6-tranA , through transition statesTS4-cisA andTS4-tranA , respectively. The corresponding barriers are 19.0 and 22.1 kcal/mol. Finally, the C−C reductive elimination occurs to produce the product-coordinated complexes7-cisA and7-tranA , by overcoming the barriers of 47.7 and 28.1 kcal/mol (TS5-cisA andTS5-tranA ), respectively. Another infeasibleβ -hydride elimination from 6-tranA was also considered and put into Figure S2 in Supporting Information (TS5-P1’-1A ). Ravikumar et al also proposed an alternative pathway (path b as shown in Scheme 2), but this possibility is precluded due the high barrier (Figure S3 in Supporting Information).
As suggested in Figures 1 and 2, the rate-determining step for reaction A is the C−C reductive elimination and the overall barrier is 28.1 kcal/mol. The cis - or tran -selectivity is controlled by the competition betweenTS5-cisAand TS5-tranA . The lowerTS5-tranA indicates that thetran -product is the main product in reaction A. The steric effect could account for the regioselectivity. As shown the structures ofTS5-cisA andTS5-tranA in Figure 3, the strong repulsion of Rh···C1 interaction at 2.64 Å in TS5-cisAdestabilizes it and restricts the subsequent process.
FIGURE 3 The optimized structures of transition states ofTS5-tranA andTS5-cisA . The bond distances are given in Å
FIGURE 4 NBO charges (e ) in the optimized structures of transition states TS3-P1-1’A and