Quantum chemical study on the mechanism of intramolecular cyclization of 2-benzyloxyphenyl trimethylsilyl ketone to give the benzofuran derivatives

• Published in: Journal of physical organic chemistry 2012-05, Vol.25 (5), p.400-408
• Main Authors: Cai, Dinglong, Wang, Mian, Wang, Jianyi, Duan, Wengui
• Format: Article
• Language: English
• Subjects: B3LYP calculation; benzofuran derivatives; siloxycarbene; spin-orbital coupling; stereoselectivity
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1930

Quantum chemical calculations have been performed to explore the mechanism of intramolecular cyclization of 2‐benzyloxyphenyl trimethylsilyl ketone (acylsilane) to give the benzofuran derivatives stereoselectively. This reaction involves a formation of siloxycarbene intermediate and a C–H bond insertion of siloxycarbene. The comparative studies on three possible insertion of siloxycarbene show that the concerted insertion of siloxycarbene into C–H bond (pathway a), which needs overcoming an energy barrier of 45.1 kcal/mol, is the most unlikely pathway, and the stepwise insertion of siloxycarbene without spin multiplicity change (pathway c) is energetically more favorable than the stepwise insertion of siloxycarbene with spin multiplicity change (pathway b). More importantly, this work can provide an insight into the stereoselectivity in this reaction in atomic molecular level. The formation of siloxycarbene is calculated to be endergonic by 22.9 kcal/mol with an energy barrier of 30.2 kcal/mol, being the rate‐determining step of the whole process. Copyright © 2011 John Wiley & Sons, Ltd. Three possible cyclization mechanisms of 2‐benzyloxyphenyl trimethylsilyl ketone are comparatively investigated using quantum chemical method in this work. Our calculations show that 2‐benzyloxyphenyl trimethylsilyl ketone firstly undergoes a 1,2‐Brook rearrangement to give siloxycarbene, and siloxycarbene is then most likely to stepwise insert into the C–H bond without spin multiplicity change to stereoselectively provide the benzofuran derivatives, giving a satisfactory explanation on the experimental findings.