N-(3-Benzo[b]thienyl)iminophosphoranes toward the Synthesis of Benzo[b]thieno[3,2-b]pyridines: Reactivity and Alternative Regioselectivity with α,β-unsaturated Ketones and Aldehydes

The novel N-(3-benzo[b]thienyl)iminophosphoranes 1b–d react with α,β-unsaturated aldehydes and ketones 2a–e to give varying mixtures of the regioisomeric benzothieno[3,2-b]pyridines 3a–d and 4a–d as a result of preferential attack of either imino nitrogen or α-thienyl carbon at the enone carbonyl group. The findings indicate that the progressive replacement of phenyl with methyl P-substituent greatly enhances the reactivity of the phosphorane 1 and concomitantly enhances the propensity of the phosphorane itself for addition to the enone by the α-thienyl carbon.