Silaheterocycles 37. The combination alkeeinylhalogensilances/lithium-tert-butyl as a components for synthesis of siliaheterocycles with cumulated CCdouble bonds

A series of differently substituted alkeneinylhalogensilanes R2Si(X)-C drop CC(Me)=CH2 (X = Cl: R = Me (2), Cl (3), Ph (4); X = F: R = Me (5), Mes (6) , Tip (7)) is reacted with t-BuLi in solvents of different polarity to give mainly E/Z-isomeric 2.4-bispentenylidene-1,3-disilacyclobutanes (11, 23 an d 31) and E/Z-isomeric 2.4,6-trispentenylidene-1,3.5-trisilacydohexanes (12 and 24) competitively. Obviously the four- and six-membered silacycles are formed stepwise by intermolecular coupling reactions of the lithiated prec ursors R2Si(X)C(Li)=C=C(Me)-t-Bu which preliminary result from the 1,4-addi tion of the lithium organyle to the alkeneinylhalogensilanes. Alternatively , silacumulene formation (R2Si=C=C=C(Me)CH2-t-Bu) might be discussed, but t he isolation of Cl3Si-C(=C=C(Me)CH2-t-Bu)-C-C-C(Me)=CH2 (22) as coupling pr oduct starting from trichlorosilane 3 and t-BuLi and high molecular multist ep coupling products starting from the fluorinated precursors 5 and 7 stron gly favour the the coupling reactions over silacumulene formation. Conseque ntly, all attempts to isolate a stable silacumulene from the silicon steric ally overcrowded starting compounds 6 and 7 failed. (C) 2001 Published by E lsevier Science B.V. All rights reserved.