Reactivity, stannenes

The first stannenes were obtained as reactive intermediates that were identified by their reaction products.570,571 For example, Me2Sn=C(SiMe3)2 (Scheme 20) was prepared by the 1,2-elimination of LiBr, or by a retro-Diels-Alder reaction, and was characterized by cyclodimerization, by ene reactions with alkenes, and by cycloaddition with 1,4-dienes.572... 

The chemical behavior of phosphastannenes, possessing the Sn=P double bond, is still poorly understood. Phosphastannenes are less reactive in comparison with stannenes. However, reaction of phosphastannene 112 with ketones and aldehydes, including benzaldehyde, afforded the expected [2+2] cycloadducts, which were not obtained in a pure crystalline state, but identified by NMR spectroscopy as cmde products (Equation 11) <1999CHE965>. 

In contrast to the extensively studied doubly bonded systems of silicon and germanium, the chemistry of the corresponding tin compounds, i.e. distannenes and stannenes, has not been fully disclosed yet probably due to the much higher reactivity and instability of such low-coordinated organotin compounds. In the following sections we briefly describe the synthesis and properties of stable distannenes and stannenes. 

The formation of n-complexes of silenes, as well as of germenes or stannenes is of central importance to the reactivity of the unsaturated systems. I will only offer one example out of many to illustrate the facts. As we found out, our stable silaneimine (Bu2Si=NSirBu3 takes up carbon monoxide under formation of supersilyl isocyanide as well as cyclosiloxanes (Scheme 7). 

Recent years have seen important developments in the chemistry of tin(II) compounds and compounds with multiple bonds to tin (Scheme 1-3). The cyclopentadi-enyltin(II) compounds, which are formed from CpM and SnCL, are pentahapto monomers. When R is a simple alkyl or aryl group, the stannylenes R2Sn(II) are known only as short-lived reactive intermediates, but when the organic group is bulky [e.g. bis(trimethylsilyl)methyl or 2,4,6-trisubstituted aryl], as indicated by R in Scheme 1-3, the monomeric stannylenes, R 2Sn , have been isolated, and have provided routes to the stannenes (R 2Sn=CR.2) and distannenes (R 2Sn=SnR 2), and other compounds with a multiple bond to tin. 

The reactivity of heteroatom-substituted stannenes has not been fully reported so far. On the other hand, those of tetraarylstannenes 26 and 27 and transient stannenes 33 and 34 have been widely investigated (Scheme 2.9.10). These compounds readily undergo 1,2-addition with various protic reagents and iodomethane. Their reactions with multiple-bond compounds such as ketones, butadienes, and azides result in the formation of the corresponding [2- -n]-cycloadducts (n = 2, 3,4). 

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