Addition Reactions of Tris(trimethylsilyl)germyl Radicals to Unsaturated Compounds. An EPR and Product Study

Tris(trimethylsilyl)germyl radicals, (TMS)₃Ge·, thermally or photochemically generated by means of suitable radical initiators, were reacted with a number of unsaturated compounds. The preparative scale reaction of (TMS)₃GeH with a variety of alkynes proceeded stereo- and regioselectively affording exclusively 2-alkenylgermanes in excellent chemical yields. In the analogous reactions with alkenes, no addition products were obtained except in the case of 4-vinylpyridine. This behavior is explained in terms of the reversibility of germyl radical addition to olefines. The reaction with arylalkenes, ketones, quinones, azines, and nitroalkanes was instead carried out in the cavity of an EPR spectrometer. In most cases the corresponding radical adducts were observed; however, quite often the intensity of the spectra was lower than expected and with azines no EPR signals could be detected. These results have been interpreted as an indication that the addition reaction of tris(trimethylsilyl)germyl radicals to multiple bonds is less exothermic than that of trialkylgermyl radicals. With some of the investigated compounds, the addition of (TMS)₃Ge· is thermoneutral or even endothermic and is therefore readily reversible.