Effects of alkyl size of AlR ₃ on its reaction with thiophene-2-carbonyl chloride

Abstract Reactions of AlEt 3 , Al( n -Oct) 3 , and AlEt 2 ( n -Oct) with thiophene-2-carbonyl chloride (TPCC) at Al/TPCC (molar ratio) <1 were respectively studied. The reaction produced ketone (ethyl thienyl ketone and/or n -octyl thienyl ketone) as the main product. The ketone yield reached the maximum after a very short reaction time and then slightly decreased. When TPCC solution was injected into AlEt 3 solution, faster addition of TPCC led to higher ketone yield, and higher temperature caused lower ketone yield. Increasing the size of R in AlR 3 from ethyl to n -octyl caused a marked decrease in ketone yield. The yield of the ketone produced from Al−Oct was about 1/5 of the ketone from Al-Et in the TPCC-AlEt 2 Oct reaction. The reaction system showed rapid color changes with time in the first few seconds. Based on the kinetic feature and reaction phenomena, a mechanistic model is proposed, in which the formation of [R′CO] + [AlR 3 Cl] − (R′ = thienyl) ion pair is much faster than that of R′COCl·AlR 3 donor–acceptor complex, and only the former is able to produce ketone. Though the formation of the R′COCl·AlR 3 complex drags behind that of the ion pair, ketone formation is completely depressed when all AlR 3 molecules are coordinated by acyl chloride or ketone.