Effects of Alkyl Size on Reactions of Alr3 and Thiophene-2-Carbonyl Chloride and the Related Mechanism

Reactions of thiophene-2-carbonyl chloride (TPCC) with AlEt3, Al(n-Oct)3 and AlEt2(n-Oct) with Al/TPCC molar ratio lower than 1 were respectively conducted in n-heptane, and the reaction products were identified and quantified by 1H NMR analysis. The reaction produced ketone (ethyl thienyl ketone and/or n-octyl thienyl ketone) as the main product. The ketone yield reached the maximum after very short time (< 1 min) of reaction, and then slightly decreased with further prolonging the reaction. 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 alkylaluminum from ethyl to n-octyl caused marked decrease in ketone yield. Ketone yield of reaction between Al(n-Oct)3 and TPCC was about 1/25 of that between AlEt3 and TPCC. Yield of the ketone produced from Al-Oct was about 1/5 of the ketone from Al-Et in TPCC-AlEt2Oct reaction. The reaction system showed rapid color changes with time, including building-up of deep red color in the first 1 s and quick fading of the color in the subsequent few seconds. Based on the kinetic feature and reaction phenomena, a mechanistic model is proposed, in which 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 R’COCl·AlR3 complex drag behind that of the ion pair, ketone formation is completely depressed when all AlR3 molecules are coordinated by acyl chloride or ketone.