Stannenes, preparation

Stannenes, R2Sn=CR2, are usually prepared either by coupling of a stable stannylene with a stable carbene, or by an elimination reaction (Equation (185)). 

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... 

Persistency can be conferred on the stannenes by steric protection to hinder self-reaction. For example, bis(2,4,6-triisopropylphenyl)(fluorenylidene)stannene can be prepared as the ether complex (6n9Sn = 288, <513C = 133.9, UV Amax = 542 nm), as violet crystals, by dehydrofluorination as shown in Scheme 21.573-576 At room temperature, it slowly converts into the head-to-tail dimer. Protic reagents and methyl iodide add across the double bond, and the bond is reduced by lithium aluminum hydride. 

Scheme 2. Preparation, stabilization, and reactions of silenes, germenes, and stannenes...

Two 1,3-distannacyclobutanes have been prepared, both by dimerisation of the corresponding stannenes as shown in equations 10-1934 and 10-20.35 36 The 4-membered ring in 10-11 is planar with rSnC 228.8-229.3 pm. 

A few stannenes, R2Sn=CR 2, have been prepared, but not enough yet to allow confident interpretation of the nature of the bonding. 

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