Stannenes

Stannene 1 was found to be an extremely air-sensitive compound, which was identified only by its trapping reactions.99 Replacement of the bis(tri-methylsilyl)methyl groups with the bulky aromatic group 2,4,6-triisopropylphenyl led to the isolation of the stable stannene 2 as its diethyl ether... 

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

A stannaketenimine has been synthesized by the low-temperature reaction of a diarylstannylene with mesityl isocyanide.87 The stannaketenimine is thermally stable, crystals being obtained by sublimation at 40°C/0.01 mmHg. In this case, the stannylene acts as the Lewis acid for formation of the adduct and, along with the stannenes described above, covers the full range of bonding modes available to the stannenes. 

The reaction of an a-bromoalkyltin halide (Scheme 13) with sodium or phenyllithium gives a transient stannene, which self-reacts to give the dimer, or gives an adduct with anthracene, which dissociates into its precursors on heating.326... 

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. 

The formation of a stannene from the coupling of a stannylene and a carbene is illustrated in Scheme 22. The stannene is obtained as yellow crystals, and shows, in solution, S119Sn = 374, 61= 127. It is close to planar at both Sn and C, but twisted through 36°, with rSn=C = 203.2(5) pm. 

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>. 

D. Multiple Bonds to Carbon Germenes, Stannenes and Plumbenes. . . 343... 

While the chemistry of silenes R2Si=CR2 has been well developed, structural information about germenes R2Ge=CR2 and stannenes R2Sn=CR2 is still scarce, and no plumbene R2Pb=CR2 had been isolated to date. 

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 chemistry of stannenes, which are also analogous to ethylene, has been less developed than that of silenes and germenes. In 1987, two stannenes 69 and 70 stabilized by not only steric protection but also by electronic perturbation were synthesized and characterized by X-ray crystallographic analysis by Bemdt and coworkers (Scheme 26)45,62. 

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