2.2- Dibutyl-l,3,2-dioxastannolane

Many of the tin heterocycles have been characterized by Mossbauer spectra. Among these the complexes formed by 2,2-dibutyl-l,3,2-dioxastannolane and its methyl-substituted derivatives with pyridine, DMSO, DMF, sulfolane and THE <83JOM(258)7> and fused benzo-1,3,2-dithiastannolanes <85JOM(290)4l> should be mentioned. 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

These compounds are less aggregated in solution. For instance, 2,2-dibutyl-l,3,2-dioxastannolane, a polymer in the solid state, has been shown, by variable temperature Sn NMR spectroscopy, to be a mixture of dimers, trimers, and tetramers in solution, with dimers predominating at room temperature and above. This technique has also indicated that most carbohydrate-derived stannylene acetals are present predominantly as dimers in solution." " " Supporting evidence has been obtained from mass spectral studies and by comparison of solid-state NMR spectra with those of solutions." Some dibutylstannylene acetals derived from cw-diols contain an observable proportion of higher oligomers benzyl 4,6-0-benzylidene-2,3-0-dibutylstannylene-o -D-mannopyranoside is present as a mixture of a dimer and a trimer in chloroform-d at -60 °C but, in the less polar solvent toluene-dg, is mainly present as a tetramer at that temperature. ... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tinatoms. Such complexes are less stable for more substituted stannylene acetals, e.g., those derived from carbohydrates. Addition of nucleophiles to solutions of stannylene acetals in non-polar solvents has been found to markedly increase the rates of reaction with electrophiles and transient 1 1 complexes of this type are... 

The relative abtmdances of cyclic oligomers formed in the polymerization of cyclic stannoxanes such as 2,2-dibutyl-l,3,2-dioxastannolane with diacyl-chlorides, determined by SEC, provided unambigous evidence that the product mixtures are well-behaved eqilibrium distributions, showing that the experimental data are in essential agreement with the n law predicted by the JS theory. ... 

For example, commercially available methyl-AT-trityl-l-serinate undergoes cyclooligomerisation to 1-serine trilactone in the presence of 2,2-dibutyl-l,3,2-dioxastannolane as template in 81% yield [20], compared to 26% of template trimerisation of 1-iV-tritylserine to the desired lactone (Scheme 7-3) [18]. 

Only when a considerable number of large substituents are present do stannylene acetals exist to any extent as the monomer 2,2-dibutyl-4,4,5,5-tetraethyl-l,3,2-dioxastannolane is present almost entirely as the dimer at room temperature in chloroform at 20°C, but the monomer-dimer equilibrium gradually shifts to favor monomer above about 80°C. 2,2-Di-r-butyl-4,4,5,5-tetramethyl-l,3,2-dioxastannolane is a monomer at room temperature.41... 

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