Stereochemistry of the Reversible Cyclization of ω-Formylalkyl Radicals

Article abstract of DOI:10.1021/jo00044a034

The reactions of 6-bromo-4-(1',1'-dimethylethyl)hexanal (4), 5-bromo-3-(1',1'-dimethylethyl)pentanal (11), and 5-bromo-4-(1',1'-dimethylethyl)pentanal (17) with tributylstannane have been investigated in detail.The major products are the debrominated aldehydes and cycloalkanols which arise from the cyclization of the intermediate ω-formylalkyl radicals.The stereochemical outcome of the cyclization of these radicals is dependent on the stannane concentration.At high concentrations of stannane the cyclization is essentially irreversible with the cycloalkoxy radicalsbeing trapped before β-scission can occur.Under these conditions the relative amount of cis and trans cycloalkanols formed are equal to the ratio of the rate constants for the two modes of cyclization; both 4 and 17 show a small preference for trans cyclization, but 11 gives equal amounts of the two diastereomers.When the stannane concentration is lowered, the lifetime of the cycloalkoxy radicals increases allowing β-scission to occur.Thus, the cis and trans cycloalkoxy radicals approach a thermodynamic equilibrium which is reflected in the relative yields of the cis and trans cycloalkanols.