I-Silaallenes

Compound 59a underwent intermolecular reactions characteristic of silenes (Scheme 19). Water added instantly across the Si=C double bond of the I -silaallene is expected to give vinylhydroxysilane 65 in 71% yield, and methanol was added... 

Several groups have reported ab initio calculations of C2H4Si isomers some of the results are listed in Table I. The most stable structure is ethynylsilane. Relative to this molecule, I -silapropadiene is less stable by about 25-30 kcal moF (Ref. 12(f) places the energy of the parent silaallene ca. 55 kcal moP above ethynylsilane) and 2-silapropadiene is even more unstable, lying ca. 50 kcal moF above ethynylsilane. This is eonsistent with the fact that 1-heteroallenes have been isolated, but 2-heteroallenes are still unknown. 

Silaallene 73"" was synthesized in an manner analogous to that of 70. Compound 73 was stable at room temperature over I month, but in the presence of any protic source (i.e., water, methanol), it underwent a rearrangement different than that observed for 70, inserting into a methyl C—H bond (74) on the octamethylfluorenyl moiety rather than into one of the groups on silicon (Eq. (6)). It is believed that the favored mode of rearrangement for these groups is that of silaallene 73, but 70... 

Since that time, a few additional silenes or silaallenes have been prepared by this method references are given in Table I. 

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

The polarity of the Si=C bond in l-sUaaUenes is reduced compared with that in silenes. Thus, for the parent l-sUaaUene H2Si=C=CH2 604 the calculated Mullikan atomic charges on Si and on the central C are -I-0.17 and —0.10 , respectively, compared to 0.46 and —0.65 in H2Si=CH2 25. This is expected to reduce the reactivity of 1-silaallenes... 

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