Hydrogermylation radical reactions

A range of addition reactions of (TMS)3GeH with alkynes, alkenes, ketones, azines, and quinones has been studied using EPR. In addition, synthetic studies of hydrogermylation of alkynes have shown that the reaction proceeds regio- and stereo-selectively, whereas reactions with alkenes do not take place (presumably owing to the reversibility of the germyl radical addition) (Scheme 29). 

In fact, additions of tributylgermyl radical and tributyltin radical to activated alkenes occur at about the same rate (see refs, 38 and 101). This addition reaction is probably more readily reversible in the case of tin (because a weaker bond is formed) and therefore hydrostannylation is a less serious problem than hydrogermylation. Thus, very reactive precursors (preferably iodides) are required as precursors if germanium hydride is used with an electron deficient alkene but this is not because the germanium radical is less reactive towards halides than the tin radical. 

The nature of the unsaturated compounds dictate the reaction pathway to a great extent, but it still remains unclear why a radical mechanism predominates in some cases and why a radical hydrogermylation may lead to a radical double germylation. 

Tin hydrides are used extensively for organic functionalization of tin via hydrostan-nation ° . These reactions resemble the corresponding hydrogermylation reactions rather than hydrosilation. Although many hydrostannation reactions occur in the absence of catalysts, reflecting the lower Sn-H bond energy, they are most commonly carried out under free radical catalysis. Some typical hydrogermylation and hydrostannation reactions are illustrated in Table 2 . 

Predominant retention of configuration was also found in the thermal radical hydrogermylation reaction shown in (eq. [5]) (22,23). 

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