Stannyl special

Substitution of the dimethylsilyl group by bis(tert-butyl)-stannyl does not change the structure in solution, e.g. 33 is found to be monomeric. A very interesting dimer is 26. In contrast to the centrosymmetrical dimer of 1 (C-Symmetry), 26 has a twofold axis (C2, see Fig. 9). This special structure may be due to intramolecular Lewis acid-base interactions between the boron and nitrogen atoms 39). Nevertheless,... 

The reduction of a-ketobenzoates by BusSnH constitutes a special case, probably involving addition of the stannyl radical to the ester carbonyl. Reductions of isomerically pure (97) and (98) gave the same 4 1 ratio of the epimeric products (99) and (100). 

Carbostannylation sometimes proceeds with no activators, but the applicability of such reactions is severely limited to a few types of special substrates. The development of activation methods in the mid 1990s made carbostannylation a synthetically significant tool. Three major activators are now available. Thus, radical initiators, Lewis acids, and late transition metals were found to be effective as activators by Hosomi, Yamamoto, and Shirakawa-Hiyama, respectively. These activators are required only in a catalytic amount to promote the reaction. Scheme 5.7.2 summarizes the mechanism of the activation of carbon-tin bonds by these activators. The cleavage of carbon-tin bonds by a radical initiator gives a stannyl radical, which adds to an unsaturated bond. The resulting alkenyl or alkyl radical abstracts an R... 

Unlike hydrosilation discussed in the preceding subsection, which requires catalysis in essentially all cases, hydrostannation can be observed typically at or above 50 °C under thermal conditions. Under such conditions, however, the reaction tends to be capricious and unpredictable. Thus, for example, the reaction of terminal alkynes with HSnMes, HSnBus, and other triorganylstannanes tends to produce mixtures of a-, cis-f -, and trans-j8-stannyl-substimted alkenes except for some special cases, such as those shown in Scheme 19.W7H49]... 

Many of the applications of group IV element NMR are relatively straightforward from a spectroscopic point of view and are therefore not discussed here. Worthy of special note, however, are Si studies of aqueous silicates and silicones, molten silicates, and silyl transition metal derivatives the use of Si/ H 2D correlation spectra to make assignments and the substantial and growing body of body of Si MAS and/or CP work on solid materials such as zeolites, other silicate minerals, polysilanes, and surface-immobilized materials. The potential of solid-state NMR for Sn and Pb has yet to be fully realised, although solid trimethyl stannyl hydroxide and tributyl stannyl fluoride have been studied. 

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