3.1 Reaction A
The free energy diagrams for the N–H and C–H bond activation processes of R1 catalyzed by 1cat in reaction A are calculated and presented in Figure 1. The reaction would begin with the N–H bond activation leading to intermediate 1 via transition state TS1 with the free energy barrier of 11.7 kcal/mol. Then the ortho arene C–H bond activation occurs to afford intermediate 2 via transition state TS2 , requiring the barrier of 19.4 kcal/mol. With the releasing of HOAc molecule, a more stable intermediate 3 is generated.
FIGURE 1Free energy profiles for the N–H and C–H bond activation processes in reaction A. The relative free energies and relative enthalpic energies (in parentheses) are given in kcal/mol
The calculated free energy diagrams for the enyne migratory insertion and 1,4-Rh migration steps are given in Figure 2. With the coordination of R2 to 3 , intermediate4 is formed, which is followed by the alkynyl inserting into Rh–C bond to yield intermediate 5 with the barrier of 14.9 kcal/mol (TS3 ). Other infeasible alkynyl and alkenyl insertion transition states are given in Figure S1 in Supporting Information.5 could then isomerize to aη 3intermediate 6 via transition state TS4 with a facile barrier of 5.4 kcal/mol. From 6 , Lam et al proposed the successive acetolysis and 1,4-Rh migration mechanism,[54] but the forbidden high barrier restricts this possibility (see Figure S2 in Supporting Information). Thus, an alternative pathway might be expected.[55] In fact, for the following 1,4-Rh migration process, the H-migration firstly occurs from methyl group to the N center to afford intermediate 7 through transition state TS5 , requiring the barrier of 23.7 kcal/mol. The ensuing H-transferring to C4 atom from N center completes the 1,4-Rh migration by crossing a barrier of 22.7 kcal/mol
FIGURE 2 Free energy profiles for the enyne insertion and 1,4-Rh migration processes in reaction A. The relative free energies and relative enthalpic energies (in parentheses) are given in kcal/mol
The free energy diagrams from 8 to product complex 11are displayed in Figure 3. 8 rearranges to intermediates9 and 10 successively by exothermic of 1.7 and 1.9 kcal/mol, respectively. Finally, C–N reductive elimination occurs to produce the product complex 11 via the transition stateTS8 with the activation barrier of 3.8 kcal/mol. The possible C–O reductive elimination pathway was also considered, but the higher O-coordinated intermediate 10’ compared with the transition state TS8 precludes this possibility (see Figure S3 in Supporting Information).
As shown in Figures. 1-3, the rate-determining step for the annulation of R1 with R2 catalyzed by 1cat in reaction A is the 1,4-Rh migration with an overall barrier of 26.9 kcal/mol (TS5 relative to 6 ), in agreement with the experimental condition.
FIGURE 3 Free energy profiles from 8 to product complex 11 in reaction A. The relative free energies and relative enthalpic energies (in parentheses) are given in kcal/mol