Stannenes reactions

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 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>. 

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

The silenes, germenes, and stannenes so produced may be trapped by active reactants They imdergo insertions into a-b simple bonds, react with enes a=b-c-H under ene reactions and form cycloadducts with unsaturated systems a, a=b, a=b=c and a=b-c=d, respectively (Scheme 2) [1,3-11]. 

Two remarkable reactions are shown in equations 21-13 and 21-14. The stannylene 21-1 reacts with the cycloalkyne 21-28 to give the adduct 21-29 [5(119Sn) -536.8] which in solution is in equilibrium with its factors, but can be isolated as a pale yellow solid. It has rSnC 213.6(5) and 213.4(5) pm, and rC=C 134.0 pm, and its structure is probably best described as a 7i-complex on the Dewar-Chatt-Duncanson model.60 The same stannylene reacts with the boranediylborirane 21-30, which behaves as a pseudo-carbene, to give the stannene 21-31 (see Section 21-2).46... 

The formation and reactions of the transient stannene (21-40) are shown in Scheme 21-11. 

Alkylation or arylation of stannous chloride gives stannylenes, RaSn , which normally self react to give linear or cyclic oligomers, but if the R groups are very bulky (t-butylphenyl, or 2,6-bis(2,4,6-tri-isopropylphenyl)phenyl), kinetically stable stannylenes can be obtained (see Chapter 2.8). In the solid state, the stannylenes usually dimerize to give the distannenes, R 2Sn=SnR 2, where R denotes a bulky group. Reaction of a stannylene with a carbene gives a stannene, R 2Sn=CR2. 

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). 

---