Stannylene-distannene equilibrium

The low-temperature photolysis of hexakis(2,4,6-triisopropylphenyl) cyclotristannane leads quantitatively to the corresponding distannene (Eq. 30).90 The distannene was found to be in equilibrium with the cyclotristannane over a wide range of temperatures. The simplest explanation for this is that the stannylene is formed as an intermediate from either the cyclotristannane or the distannene, but quickly adds to the distannene or dimerizes. This equilibrium can be followed by ll9Sn NMR spectroscopy at elevated temperatures.96... 

The synthesis of doubly bonded tin compounds by the coupling of stannylenes, however useful, is limited by the need for a stable stannylene and often a second divalent species (for example, a carbene or isonitrile). The simplest example of this reaction is the formation of tetrakis[bis(tri-methylsilyl)methyl]distannene from two molecules of the corresponding stannylene,83 with which it is in equilibrium in solution as evidenced by NMR spectroscopy.91... 

Distannenes are in equilibrium with the corresponding stannylenes in solution therefore, reaction with alkynes can lead either to stannacyclo-propene or l,2-distannacyclobut-3-ene derivatives. Reaction of tetra-... 

Most stannylenes, R2Sn , that are two coordinate in solution are in equilibrium with, and separate out as, the distannenes, R2Sn=SnR2. These are the distanna analogs of the alkenes, R2C=CR2, but the nature of the Sn=Sn bond is different from that of the C=C bond, and can be represented by the mutual coordination of the electron pair in the 5sp2 orbital of one stannylene unit into the vacant 5p orbital of the second stannylene unit, as illustrated in Equation (179). 5 The molecules are usually trans-bent (9 = 9 = ca. 40°, 0 = 0°), but are easily distorted, and the... 

The synthesis of distannenes and diplumbenes are generally achieved by dimerization of stannylenes and plumbylenes (Scheme 11). Although most of them exist as divalent species in solution under equilibrium with the corresponding double-bond compounds, the equilibrium occasionally shifted to the corresponding dimer (double-bond compounds) by crystallization. It is essential to choose the proper size of steric protection groups for the synthesis of doubly bonded systems. 

The 3-coordinate stannylenes are notionally sp3 hybridised and tetrahedral (21-17), and the 4-coordinate stannylenes sp3d hybridised and trigonal bipyramidal (21-18), though these ideal structures may be heavily distorted. The 2-coordinated compounds separate from solution as monomers only when the tin is heavily sterically protected (e.g. compounds 21-2, 21-8, and 21-9) in its absence, they solidify as the distannenes (equation 21-6), but bis(2,4,6-triisopropylphenyl)stannylene (21-19) separates out, not as the dimer, but as the cyclic trimer which, in solution, is in equilibrium with the stan-nylene and the distannene.27... 

The syntheses of distannenes are generally achieved by the dimerization of stannylenes (see Section 2.8). As a unique example, distannene 6 is obtained by the thermal or photochemical reaction of tristanna-cyclopropane IS. It was reported that distannene 6 exists in equilibrium between 6 and 18, or stannylene 19 depending on the temperature (Scheme 2.9.3). In addition, the cyclic distannene 17 is obtained as a result of isomerization of tristannaallene 21 (see below). 

Except for 14, the reactivity of acyclic distannenes reflecting the Sn=Sn double-bond character has not been investigated because of their equilibrium in solution, and these distannenes react as the stannylene in many cases (see Section 2.8). Although some [2-1-2]-cycloaddition reactions were reported (Scheme 2.9.4),these cycloadducts could be formed either by [2-1-2]-cycloaddition or via the insertion of the second stannylene unit into the initially formed three-membered ring compound. 

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