Stereochemical aspects of sulfur dioxide insertion into cyclohex-2-enylstannanes

Sulfur dioxide insertion into a range of (4-alkylcyclohex-2-enyl)-, (5-alkylcyclohex-2-enyl)-, and (6-alkylcyclohex-2-enyl)stannanes has been studied for the solvents chloroform and methanol, and the reaction proceeds γ-regiospecifically (with allylic rearrangement) to provide the O-sulfinate. For chloroform solvent, syn stereospecificity was observed (and hence sulfur dioxide insertion is less sensitive to steric factors than trifluoroacetolysis) whereas for methanol solvent (despite γ-regiospecificity) insertion is not stereospecific, possibly reflecting the importance of methanol coordination at tin, reducing the necessity for intramolecular coordination and the consequential operation of the γ-syn stereocourse. Comparisons of the results for acidolysis and sulfur dioxide insertion of cyclohex-2-enylstannanes demonstrate that SE′ reactions lack a unique, stereoelectronically enforced stereochemical outcome and that factors such as the nature of the electrophile, solvent, steric factors, etc. may operate to render γ-syn and γ-anti stereocourses of comparable energies. © 1988 American Chemical Society