Disilanyl groups

Ito and co-workers have also used the Pd(OAc)2/t-alkyl isocyanide catalyst to affect the double silylation of carbon-carbon multiple bonds in an intramolecular system to yield silacarbocycles.59 Alkenes or alkynes that are tethered to a disilanyl group through a carbon chain, an ether linkage, or an amine functionality undergo intramolecular addition of the disilane moiety to the multiple bond. Activation of the disilane by the presence of electron-withdrawing groups on silicon is not necessary for the reaction to... 

The intriguing combination of an a-silyl and a /1-silyl substituent in the disilanyl group (—SiR2SiR3) prompted Shimizu, Tsuno and coworkers to study the effect of a /3-Si—Si bond on the stability of carbocations86. They studied the solvolysis of the benzylic bromides 213-215 and found that the /i-SiMe3 substituent in 213 increases the solvolysis rate by a factor of 1.07 x 105 compared with the f-butyl-substituted reference compound 214 (Table 7). 

TABLE 7. Kinetic effect of the disilanyl group in solvolysis... 

From the relative solvolysis rates of 213 and 215, Shimizu and coworkers calculated a stabilization by 7 kcalmoD1 of the benzyl cation by the /1-Si—Si bond compared with a /1-C—Si bond. Due to the antagonistic effects of the a- and the /)-silyl groups, the net effect of the disilanyl group is relatively small86. Thus, 213 is only 12 times as reactive as a-methylbenzyl bromide 219, X = Br (Table 7 in 30% acetone the relative rate is decreased to 0.66). This might be compared with the rate acceleration of 1.05 x 105 by... 

The overall kinetic effect of the disilanyl group is, however, small due to the different solvolysis mechanisms. 224 solvolyses only 3.48 times faster than 226 in TFE at 100 °C. For the k process Shimizu and coworkers calculated the relative rates for 224 225 226 to be 1.60 1.0 10.7. They concluded that these small silicon effects reflect the concerted nature of the k solvolysis involving transition states with a highly delocalized charge distribution88. 

The choice of silicon as the bridging atom results primarily from the ease with which these compounds can be prepared (vide infra). Nevertheless, certain silicon-containing bridging units have been chosen for electronic reasons for example, it is well known that a disilanyl group qualitatively behaves like an ethylene unit. 

Another six-membered cycle with two isolated Si-Si linkages was formed by Schmidbaur554) while investigating the stability of ylids with disilanyl groups. They found a high tendency to the formation of cyclic ylids ... 

Brook, A. G., Bassindale, A. R. Molecular rearrangements of organosilicon compounds (Academic Press, New York, 1980) 149-227. Suginome, M., Matsunaga, S.-i., Ito, Y. Disilanyl group as a synthetic equivalent of the hydroxyl group. Synlett 1995, 941-942. 

Suginome, M., Matsunaga, S.-i., Ito, Y. Disilanyl group as a synthetic equivalent of the hydroxyl group. Synlett 1995, 941-942. 

As in the case of bis-silylation of acetylenes, intramolecular bis-silylation of alkenes is more effectively performed compared to the intermolecular reactions. The isonitrile-palladium catalyst provides 5-exo cyclization products stereospeciflcally from (Z)- and ( )-alkenes tethered to disilanyl groups by ether linkage (Scheme... 

---