1.3.2- Dioxastannolanes reactions

The enantiomeric allylic stannanes in Equations (111) and (112) show only the H-ene reaction, and proceed with complete stereoselectivity. The former compound gives also some dioxastannolane, and if an electron-donating group is present, this dioxastannolane can be the only product (Equation (113)).2... 

The 1,3,2-dioxastannolanes are important in organic synthesis because they can readily be derived from dialkyltin oxide and 1,2-diols, as in carbohydrates the reaction can be carried out in toluene in a few minutes under microwave irradiation.387 The dioxastannolanes can then be subjected to regioselective reaction with an electrophile such as an acyl chloride (Equation (140)) or sulfonyl chloride, or an isocyanate. The acylation or sulfonation can be carried out with catalytic amounts of the dialkyltin oxide, including the recoverable (C6F13CH2CH2)2Sn0.388... 

Some chiral 1,3,2-dioxastannolanes were used as catalysts in asymmetric Diels-Alder reactions of cyclopentadiene with methyl acrylate <90JCR(S)278>. A-Alkenyl- and -cycloalkenyl 1,3,2-oxaza-stannolanes, generated in situ from chiral amino alcohols, gave optically active 2-substituted aldehydes and ketones in modest to high chemical and optical yields after alkylation with methyl acrylate or acrylonitrile (which is usual for enamines) and subsequent hydrolysis <85CC504,85JOC3863>. 

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). 

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. 

Examples of addition reactions of tributyltin methoxide are given in Table 14-3. Most of these reactions are reversible for example, the alkyl tin carbonate which is formed by the addition of a tin alkoxide to carbon dioxide eliminates C02 on heating to regenerate the tin alkoxide (equation 14-14). Again, the product of the reaction of an acyl halide with a dioxastannolane is involved in an exchange reaction in which the tin alkoxide adds reversibly to the carbonyl group (equation 14-31).41... 

The 1,3,2-dioxastannacycloalkanes can be prepared by the usual routes to organotin alkoxides, but whereas simple alcohols react with diorganotin oxides only to the stage of the distannoxanes (R 0)R2Sn0SnR2(0R ), 1,2-diols react further to give the dioxastannolanes (equation 14-41).61 62 The reactions proceed much more quickly (typically in 4 to 9 min) if the reactants are heated in a microwave oven.63 Similar reactions can be carried out with dialkyltin dihalides64 and, under milder conditions, with dialkyltin dialkoxides.65-67... 

The principal reactions of the dioxastannolanes are illustrated in Scheme 14-3. The fact that the stannolanes exist in solution as dimers or higher oligomers has an important effect on the course of some of these reactions.81... 

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... 

The selectivity can sometimes be rationalised on the basis of the mechanism illustrated in equation 14-45. The less substituted diol moiety, which is more reactive towards electrophiles in the diol, is also more prone to donate to tin in forming the dimer of the dioxastannolane, leaving the unassociated more substituted oxygen centre as the stronger nucleophile. However, the acylation reactions are complicated by the equilibration of the two possible isomers by the reaction 14-46, and by a suitable choice of reaction conditions, regioselectivities of <99% can often be obtained.41,94-96... 

Tosylation gives the corresponding tosylates (equation 14-47).97 Benzylation and allylation by the corresponding bromides is catalysed by quaternary ammonium salts98 and the reactions can be carried out without isolating the dioxastannolane (equations 14-48,98 and 14-4967). 

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... 

Macrocyclic tetraesters with aliphatic substituents (alkane or cycloalkane) have been synthesised analogously using both the 1,3,2-dioxastannolanic and the corresponding stibolanic derivatives as templates [16], The greater reactivity of stibolanic derivatives toward diacyl chlorides reduces considerably the reaction times and makes their use preferable. 

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