Stannylenes cycloadditions

The other important type of reaction leading to the formation of two bonds between a metal and four-atom fragment is addition of carbenoids, i.e. germylenes and stannylenes, to a-dicarbonyl and related compounds. The process can be classified as redox (4 + 1) cycloaddition since the dicarbonyl fragment is reduced while the carbenoid is oxidized. Formally the simplest example of such reactions is the interaction of methylglyoxal with activated tin metal to give a cyclic stannylene, Sn(II) enediolate (see Scheme 34) <83CL1825>. 

Stannoles bearing dialkylboryl groups in the 3-position (145) react with phosphaalkynes R4C=P (R4 = t-Bu, CH2Bu-t) by [4 + 2] cycloaddition (isomeric adducts were not detected) and elimination of stannylene to give phosphabenzenes (146 and 147) in high yield205. 

Stannanethiones 106, generated from kinetically stabilized stannylenes, were trapped with phenyl isothiocyanate to afford 1,3,2-dithiastannetanes 107 (Scheme 29) <19960M4531, 2004JA15572>. Treatment of the corresponding stannanethione with an excess of carbon disulfide resulted in a [2+2] cycloaddition of the C=S bond across the Sn=S bond to yield l,3,2-dithiastannetane-4-thione 23 <19960M4531>. 

C-Unsubstituted or 3,4-disubstituted stannoles can be obtained from a [2+2+1] cycloaddition reaction of two alkynes and one stannylene unit in the presence of a Pd° catalyst. The key intermediate in the catalytic cycle is a strained 3,4-pallada(II)stan-na(IV)cyclobutene (Scheme 21.8).121... 

Stannoles can also be formed by a 2 + 2 + 1 cycloaddition between two acetylene units and a stannylene,28 and an analogous reaction of a cobalt-bonded stannylene gives an q4-bonded stannole.29... 

Except for 14, the reactivity of acyclic distannenes reflecting the Sn=Sn double-bond character has not been investigated because of their equilibrium in solution, and these distannenes react as the stannylene in many cases (see Section 2.8). Although some [2-1-2]-cycloaddition reactions were reported (Scheme 2.9.4),these cycloadducts could be formed either by [2-1-2]-cycloaddition or via the insertion of the second stannylene unit into the initially formed three-membered ring compound. 

Generation of 1-stannaallene 35 was suggested as an intermediate in the synthesis of the first stable distannirane 36, by Escudie et al. (Scheme 2.9.12). Because of the probable lability of the Sn—C double bond, 35 could behave as a stannylene-vinylidene carbene complex, as observed in the related 1-stannaketenimine 31, which behaves as a stannylene-isocyanide complex. Therefore, the generation of the final product of this reaction, 36, should be most likely interpreted in terms of the [2+ l]-cycloaddition of 35 with the stannylene 37, which should be generated in an equilibrium amount. 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

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