Ab Initio and DFT Studies of the Thermal Rearrangement of Trimethylsilylsilylene

Abstract

A thermal reaction scheme for the rearrangement of trimethylsilylsilylene (Me3Si−S̈i−H) proposed in a previous study [Organometallics 1986, 5, 698] was studied by the MP2 and DFT methods. The reaction pathways were searched by intrinsic reaction coordinate analysis. We report structures and energies of various silicon species. Activation energies for the C−H insertions by the divalent silicon centers in Me2HSiCH2−S̈i−H and MeHSiCH2−S̈i−Me are predicted to be high, 121 and 110 kJ/mol, respectively, in excellent agreement with an experimental value of 121 kJ/mol. However, the energy barrier for the C−H insertion by the divalent silicon center in Me3S−S̈i−H, predicted to be relatively small, 53 kJ/mol, is lower than that for the 1,2-Me shift (78 kJ/mol) in Me3Si−S̈i−H.

Ab initio and DFT studies of the thermal rearrangement of trimethylsilyl(methyl)silylene: Remarkable rearrangements of silicon intermediates

“…In H(L), a hydrogen atom bridges the two silicon atoms involving a three-center/two-electron Si · · · H · · · Si bond, straining the H-Si-C-Si subunit, thereby decreasing the energy while increasing the Gibbs free energy. This nonclassical bonding was also found to occur in our previous study 21 and G(L). It is shown that the H atom is significantly shifted from the carbon atom to the silicon atom.…”

“…In Table 3, we list the relative energies and Gibbs free energies of the various triplet silicon intermediates and transition states. Note that the energy barrier for the C-H insertion leading to 1-methyl-1,3-disilacyclobutane 5 as shown in Scheme 3, calculated with the MP2/aug-cc-pVDZ//B3LYP/aug-cc-pVDZ method in our previous study, 21 is in excellent agreement with the experimental energy barrier. Almost all the calculations indicate that A(L) is lowest in energy.…”

“…4, which are shown to be in accord with the experimental product ratios. Several similarities in the silylsilylene rearrangements in the trimethylsilylsilylene (our previous study) 21 and trimethylsilyl(methyl)silylene (the present study) systems are encountered: (1) A Me-shift converting silylsilylene to disilene commonly occurs with a relatively small barrier about 77 kJ/mol. (2) The silylsilylene to silirane rearrangement (in the present study K(L) to C(L)) is commonly found to occur with relatively a small barrier about 59 kJ/mol.…”

“…(2) The silylsilylene to silirane rearrangement (in the present study K(L) to C(L)) is commonly found to occur with relatively a small barrier about 59 kJ/mol. (3) Only a H-shift in the ring opening of the silirane intermediates is found to occur with barriers 63 (trimethylsilylsilylene system) 21 and 88 (in the present system) kJ/mol. The presence of the Me group in the terminal of trimethylsilyl(methyl)silylene, however, permits the occurrences of a variety of the silylene rearrangements as indicated in the present study.…”

“…These energy barriers are close to those (78 and 105 kJ/mol) for the forward and reverse processes, respectively of the 1,2-Me shift converting trimethylsilylsilylene to trimethyldisilene (process 1) that was recently reported by us. 21 Me 3 Si-Si-H → Me 2 Si=SiHMe (1) Figure 2 presents the reaction pathways initiated by a C-H insertion by the divalent silicon center in K(L). The activation energy for the C-H insertion was also predicted to be low 66 kJ/mol.…”