Theoretical investigation of stannylene acetals and their regioselective reaction mechanisms

This thesis describes the application of high-level theoretical methods to the study of the chemistry of organotin systems. An evaluation of the computational methods for the prediction of organotin geometries, bond dissociation enthalpies, and tin donor-acceptor bond enthalpies is used to establish a reliable method for computations on organotin systems. Various basis sets and effective core potentials are used for tin, and the results are compared to previous experimental and computational studies of similar species. On the basis of computational efficiency and accuracy, the most appropriate method is selected for computations on larger organotin systems, including dialkylstannylene acetals.;Dialkylstannylene acetals are tin-containing species used extensively as intermediates that facilitate high yielding, regioselective, monosubstitution reactions of diols or polyols with electrophiles. Because of their favourable properties, dialkylstannylene acetals have become essential synthetic tools in carbohydrate chemistry and for other molecules containing diols and polyols. Although these tin intermediates are widely employed, the exact mechanisms of their regioselective reactions are still under debate. Factors that have been found to influence regioselectivity include the addition of nucleophiles and the aggregation of stannylene acetal monomers into dimers and higher oligomers.;There is an abundance of experimental studies examining dialkylstannylene acetals and their regioselectivity, however, these tin systems have not been studied extensively by computational methods. This thesis uses theoretical methods to explore the oligomerization of dialkylstannylene acetals and the effects of added nucleophiles in these reactions. This involves examining the geometric and thermodynamic parameters associated with the dimerization of dialkoxydibutylstannanes and 2,2-dialky1-1,3,2- dioxastannolanes. It is shown that steric effects dictate the degree of oligomeric association in these tin intermediates. The effects of added nucleophiles on the monomer-dimer equilibria of these organotin derivatives are also explored by examining monofluoridated dimerization. Based on the thermodynamics, most dialkoxydibutylstannanes prefer to exist as monofluoridated dimers, where the addition of fluoride to dibutyltin dialkoxides increases dimerization. Where comparison can be made, the theoretical findings in this thesis largely agree with experimental results and trends, and provide additional insight into these reactions which are widely used in synthetic carbohydrate chemistry.