Effects of alkyl size of AlR3 on its reaction with thiophene-2-carbonyl chloride

Abstract

Reactions of AlEt3, Al(n-Oct)3, and AlEt2(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 AlEt3 solution, faster addition of TPCC led to higher ketone yield, and higher temperature caused lower ketone yield. Increasing the size of R in AlR3 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-AlEt2Oct 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]+ [AlR3Cl]− (R′ = thienyl) ion pair is much faster than that of R′COCl·AlR3 donor–acceptor complex, and only the former is able to produce ketone. Though the formation of the R′COCl·AlR3 complex drags behind that of the ion pair, ketone formation is completely depressed when all AlR3 molecules are coordinated by acyl chloride or ketone.

Keywords

• alkylaluminum
• thiophene-2-carbonyl chloride
• ketone
• alkyl size
• mechanism