Steric crowding, effects stability

We now focus on conformational preference within the cis and trans isomers, respectively. If steric effects play the major role in dictating the preferred conformation in the trans isomer, one would expect that the relative order of stability will be Tss > Tse > Tee- However, the results of ab initio calculations shown above predict that the order of stability of the trans isomers is Tee > Tse > Tss in the calculations utilizing the STO—3G basis set and Tee > Tss > Tse in the 4—31G computations. An additional electronic factor which can account for the greater stability of the more sterically crowded Tee isomer will be discussed in a subsequent section. 

Small unsubstituted silaethylenes, such as (1), (2) and (3) are stable only in low temperature matrices. On warming, head-to-tail dimerization usually occurs. Steric crowding, however, has a dramatic stabilizing effect, and the sterically hindered silaethylenes, (5) and (6), may be isolated as stable crystalline solids. Nevertheless, the Si==C double bond even in these compounds is very reactive, undergoing aerobic oxidation and many other reactions with both singly and multiply bonded reagents (Scheme 1).22-26... 

Cyclic voltammetry, kinetic studies, and DFT calculations using a BP functional and the TZVP basis set showed that the major pathway of the non-regiospeciflc zinc-reduced titanocene-mediated ring opening of epoxides was initiated by a titanium dimer-epoxide compound that reacted in a rate-determining electron transfer mechanism 25 The calculations showed that the transition state is early so the stereoselectivity is determined by steric effects rather than by the stability of intermediate radicals. This was confirmed by studies with more sterically crowded catalysts. 

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