Next we investigated the stability of the super silyl enol ether model (5) and the TMS enol ether (6) as well as the cationic species generated after electrophilic reaction of these silyl enol ethers (3 and 4 respectively) to gain insight into the proposed reaction initiation event. Concerning 3 and 4, the CH3CH-O (H-EE) portion is very similar in the two structures while the Si-O bond length and angle of the silyl groups (A(O,Si,C) and A(O,Si,Si)) have some differences (Figure 2). The longer Si-O bond length and a smaller A(O,Si,Si) in 3 implies that the TTMSS group has a smaller interaction with CH3CH-O than the TMS group, that is, the TTMSS group has more cationic property than the TMS group. This arises from the stability of TTMSS-cation species (vide supra). For both cationic species, A(O,Si,X1) (X=C or Si) is larger than the other values of the angle (A(O,Si,X2) and A(O,Si,X3)). The reason for this is the steric repulsion between XH3 in the silyl groups and CH of H-EE. In reference to the stability of the species we calculated with the formulas in Figure 2: the energy difference of the total values (H-EE S1+ + S2’+; S1, S2 = TMS or TTMSS) indicated that 3 was less favorable than 4. This resulting high energy intermediate may help explain the extremely high turnover frequency of this species (>30/sec).7
Roughly speaking, the structure of 6 resembles 4: the dihedral angles of C-C-O-Si in both structures are 180.0 degrees and the values of A(C,O,Si) are almost the same (4; 129.1°, 6; 127.5°). The Si-O distance in 6 (1.694 Å) is smaller than that of 4 (1.881 Å) and A(O,Si,C) of 6 (107.1°~110.5°) is larger than that of 4 (100.3°~104.7°). These results indicate that Si-O bond of 6 has larger covalent character than that of 4. With the same trend, the Si-O bond of 5 has larger covalent character than that of 3 (compare 1.710 Å for 5 vs 1.901 Å for 3). Very notably, the structure for 5 is very unusual. The dihedral angle (D(C,C,O,Si)) is 129.8 degrees (compared to 180° in 6), showing that the C=C bond and Si-O bond are twisted towards each other. In addition, the structure of 5 suggests that one silicon atom has an interaction with C=C double bond. Further comparing 5 and 6, the structures of the CH2=CH-O portion are significantly different. The C-O bond length in 5 is larger than that of 6 (1.380 Å and 1.368 Å respectively). This arises from a lesser extent of conjugation of lone pairs at oxygen with the C=C double bond due to the twist structure. While the Si-O bond length of 5 (1.710 Å) was larger than that of 6 (1.694 Å), the total of angles around the central silyl atom in 5 (S A(O,Si,Si) = 324.0°) were almost same with the value in 6 (S A(O,Si,C) = 324.7°). This is the result of complex interactions in 5. The energy difference of two total values indicated that 5 was more favorable than 6 implying that the TTMSS group will also have a function of stabilizing the EE (Figure 3).
