Stannylenes formation

In favourable cases, stannylene formation can be conducted in refluxing methanol as solvent with no special provision being made for the removal of water. In the example shown in Scheme 4.164, the crude stannylene derivative obtained on removal of the methanol was treated with benzyl bromide and cesium fluoride as the activator in DMF at room temperature to give the benzyl ether in 81% yield.307... 

Acid-catalysed alkylation of an alcohol with O-alkyl trichloroacetimidate prepared from allyl alcohol and trichloroacetonitrile is readily accomplished Scheme 4.233]440 as previously discussed for the preparation of benzyl and tert-butyl ethers.311 However, these conditions are not compatible with many of the protecting groups employed in oligosaccharide synthesis. For such cases, two methods for 0-allylation under essentially neutral conditions have been devised. The first method takes advantage of the mild conditions and regioselectivity of stannylene alkylations (see section 4.3.3). The method is illustrated by the selective O-allylation of o-lactal, which began with stannylene formation on an 0.8 mole scale [Scheme 4.234].441... 

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... 

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. 

As previously pointed out in Chapter 2, monomeric stannylene can be in equilibrium with oligomeric species which are formed by tin-tin or tin-substituent inter-molecular interactions. The tendency for the formation of the oligomers increases the more the molecules approach one another. Thus, when passing from the vapor to the liquid phase and finally to the solid state, the molecules usually exhibit quite different structures. In Table 13 examples of the corresponding structural changes are given. 

This outstanding behavior of bis(pentamethylcyclopendienyl)stannylene has been explained by the energetically favorable formation of the ionic compound 79 which contains the 6-membered cluster C5Sn 174>. The structure of the boron tetrafluoride compound is illustrated in Fig. 15 the tin atom in the cation is located at the apex of a pentagonal pyramide 173). 

The molecules I, II and III of Scheme 3 can be obtained, depending on the molarities of the reactants 176) or the nature of the substituent R 177). When R is tert-butyl, thermolysis of the adduct from tert-butylamine and stannylene leads to a mixture of I and II these compounds can be isolated in the molarities indicated in Scheme 3. In the case R = tert-butyl, compound III is not formed directly. It can however be synthesized by thermolysis of I or II at elevated temperatures. On the other hand, if R is dimethylamino, the reaction leads directly to compound III without formation of I or II177). 

Decomposition with the elimination of arsenic or phosphorus ylide and formation of silylene (germylene, stannylene) Me2E14, which represents the process inverse to the synthesis of these betaines. 

The formation of betaines in the reaction of silylenes (germylenes, stannylenes) Me2E14 (E14 = Si, Ge, Sn) with ylides H2C=E15Me3 (E15 = P, As) under gas-phase conditions occurs without a barrier as a strongly exothermic process. The thermal effects of the reactions are presented in Table XVII. 

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 other important type of reaction leading to the formation of two bonds between a metal and four-atom fragment is addition of carbenoids, i.e. germylenes and stannylenes, to a-dicarbonyl and related compounds. The process can be classified as redox (4 + 1) cycloaddition since the dicarbonyl fragment is reduced while the carbenoid is oxidized. Formally the simplest example of such reactions is the interaction of methylglyoxal with activated tin metal to give a cyclic stannylene, Sn(II) enediolate (see Scheme 34) <83CL1825>. 

The very inexpensive material sold undo- the name dibutyltin oxide is an amorphous, insoluble, polymeric powder with the composition BujSnO. A suspension of this powder in an appropriate organic solvent in the presence of a 1,2- or 1,3-diol becomes clear more or less quickly because of the formation of a stannylene. These derivatives are soluble in most solvents, polar or nonpolar. Equation (1) indicates the stoichiometry of the process ... 

In this tetrol, a single secondary alcohol is oxidized with 88% yield thanks to the formation of the most stable cyclic stannylene intermediate by the regioselective reaction of BU2S11O with one of the 1,2-diol moieties in the molecule. 

The Pd-catalysed reaction is possible with a stannylene on condition that it is carried out at low temperature, in order to avoid the competing uncatalysed formation of a polymeric product, and in the presence of a labile complex as catalyst (equation 6). The stannole 23... 

The reaction between styrene oxide and stannaneselone 194 (generated from sterically crowded stannylene 193) leads to the formation of products 195 and 196 in 46% total yield. Compound 195 exists as a mixture of cis- and trans-isomers, which can be separated using chromatography (Scheme 9) <1995JOM43>. 

A kinetically stabilized stannanetellone, synthesized by the reaction of stannylene with ( -Bu)3P=Te, reacts with DMAD resulting in formation of the corresponding [2+2] cycloadduct 87 in 13% yield (Scheme 26) <2006OM3552>. 

It was found that treatment of the corresponding dibromostannane with lithium naphthalenide afforded stannylene <1995OM3620>. The reaction of the latter with an excess of carbon disulfide resulted in the formation of the unsymmetrical ethene 24 (Scheme 53). Although the details of the mechanism are not clear, it is considered that the reaction proceeds via the formation of dithiocarbene 142 followed by dimerization to give a product that is thermally unstable and easily loses carbon disulfide. Thermolysis of 24 afforded symmetrically substituted ethene 143, in a quantitative yield via extrusion of carbon disulfide. 

Regioselective acylations can also be achieved by the stannylene procedure but, in general, selectivities are not as high as for alkylations. For example, treatment of methyl P-D-galactoside with dibutyltin oxide followed by reaction with benzoyl chloride results in the formation of a mixture of methyl 6-O-benzoyl-P-D-galactoside (53%) and methyl 3,6-O-di-benzoyl- P-D-galactoside (21 %).40... 

TABLE 4. Binding energies (kcalmol-1) for H2E—AH adduct formation of silylene, germylene and stannylene with various AH3 and AH2 units3... 

SCHEME 2. Probable mechanism for the formation of stannylene 22 from stannylene 26 and sily-lene 25... 

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