1.3.2- Dioxastannolanes structures

The reaction chemistry of the 1,3,2-dioxastannolanes is vast and has been put to great use in various aspects of organic chemistry. Diorganotin derivatives of carbohydrates incorporate the dioxastannolane ring, and the structures of two such derivatives have already been commented on. The synthetic applications of intermediates of this type have been reviewed elsewhere (214-216) and are not discussed further. Substitution reactions have also proved useful synthetically, for example, in the formation of cyclic tetralactones [Eq. (52)] and urethanes (217-219), a subject which has also been reviewed (220). Two other substitution reactions using electrophilic carbon are shown in Eqs. (53) and (54), which typify several others of the same type (221,222). 

A,A-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), will form complexes with the tin, breaking the polymeric structure of the dioxastannolane but maintaining a coordination number of greater than four at the metal. A m-R2Sn02L structure has been proposed on the basis of 119Sn chemical shifts (—137 to —189 ppm) and Mbssbauer data (A 2.11-2.82 mm/second). The donor ligand is readily lost under reduced pressure (224). 

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... 

Enediolates of Sn(II) are not expected to have this structural ambiguity. We note the facile formation of such a species by reaction of methylglyoxal with activated elemental tin . Assumed monomeric, we note that this metallocycle 15 is formally a 6jr-system analogous to l,3-dioxolen-2-one (vinylene carbonate). Whether it is aromatic like the more classical heterocycle or how it compares with the tetracoordinate 1,3-dioxastannolane (enediolates of tin(IV)) formed from an analogous reaction of biacetyl with dimethylstannylene is not known. 

Crystal structures of four dioxastannolanes76 and one dioxastannanane have been determined. All four of the dioxastannolanes are associated, but the degree of association depends on the steric requirements of the 1,1-dialkyl groups and on the substituents on the methylene groups of the ring. 

Electrophiles such as acyl halides or organic isocyanates react to give the 1-hydroxy-4-carboxylates or -carbamates, or 1,4-bis-carboxylates or -carbamates, depending on the ratio of reagents. Reactions with biselectrophiles such as bis(acyl halides) or bis-isocyanates give cyclic tetracarboxylates or tetracarbamates built up from two dioxastannolane units and two of the biselectrophile (equation 14-44). This may be a consequence of the (principally) dimeric structure of the dioxastannolanes in solution.59-87-88... 

A second apparent consequence of this dimeric structure is that reactions of substituted dioxastannolanes often show a regioselectivity towards electrophiles which is opposite to that shown by the parent diols, as illustrated in Table 14-5.93... 

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