Heterocycles Containing Tin-Group 16 Element Bonds

The work on stannacycloalkanes and -cycloalkenes up to 1972 has been reviewed (9), and again, though in less detail, in 1982 (10). Most of the early studies concern the formation of five- and six-membered rings, and claims for smaller systems in particular should be treated with caution. More recent efforts have been directed to the synthesis of both strained and expanded rings. The synthetic methodology, however, remains dominated by the use of difunctional carbanion sources (Grignard and lithium reagents) or the hydrostannation reactions of tin dihydrides. 

1-Stannacyclopropanes are unknown, though the homocyclic 1,2,3-tristannacyclopropane has been synthesized (see Section II,D). The small- 

Rings of different sizes are readily distinguished by NMR methods, as both the 119Sn chemical shift and V(ll9Sn-C— H) values (to both endo-and exocyclic substituents) are sensitive to the bond angles at tin (15,16). Typically, 6 and 7 have 8 9Sn values of 0 and — 74 ppm, respectively (12). Further NMR data can be found in the compendia of data now available (17-19). 

In general, the endocyclic Sn—C bonds are more reactive than the exocyclic ones toward both electrophiles and nucleophiles, though as might be expected the situation is more competitive when the exocyclic bond is Sn—Ph (20). Ring expansion reactions provide a convenient route to oxa- and thiastannolanes [Eq. (3)] (21), whereas thermolysis in metha- 

3-Distannacyclobutane (8), stabilized by bulky Me3Si substitution in the 2,4-positions, has been synthesized and characterized in part by its ring cleavage reactions with nucleophiles [Eq. (4)] (24,25). Surprisingly, given 

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