Direct Synthesis of Metallacycloalkanones via the Cyclic Hydroboration of Dialkenyl Derivatives of Silicon, Germanium, and Tin

The hydroboration of a number of representative dialkenylsilanes was examined to determine the regiochemistry of the addition process. For compounds containing a vinylsilane moiety, the relative amount of products containing the boron atom in a β position with respect to silicon was invariably greater on using thexylborane as opposed to borane-methyl sulfide complex (BMS). Compounds which contain an allylsilane group undergo hydroboration to give exclusively a terminally placed boron adduct for that portion of the molecule. Diisopropenyl and bis(a-styryl) derivatives of silicon, germanium, and tin were directly converted to the corresponding novel metallacyclohexanones by using Pelter's yanoborate procedure. In several cases, the DCME reaction could also be used to prepare these metallacyclohexanones. The cyanoborate methodology was used to prepare silacycloheptan-4-one and silacyclooctan-5-one systems in an improved manner. Novel disilacycloalkanones and germacyclooctanone systems were also prepared. All metallacycloalkanones were isolated isomerically pure from the simple sequence. Spectroscopic data for reaction intermediates and products are also reported and discussed.