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■ The Subtle Co-catalytic Intervention of Benzophenone in Radical Cation Mediated Cyclization — An Improved Synthesis of 2-(3’,4’-Dimethoxyphenyl)indoline

Noel Francis Thomas,* Chin-Hui Kee, Azhar Ariffin, Khalijah Awang, Jean-Frédéric Faizal Weber, Chuan-Gee Lim, Mat Ropi Mukhtar, and A. Hamid A. Hadi

*Department of Chemistry, University of Malaya, Pantai Valley, 50603 Kuala Lumpur, Malaysia

Abstract

The addition of benzophenone to a FeCl₃/CH₂Cl₂ mixture in the presence of 3’,4’-dimethoxy-2-acetamidostilbene 17 unexpectedly leads to a dramatic improvement in yield of the indoline 20 from 38% (previously reported by us) to 75.4%, after careful examination of a variety of reaction conditions. A catalytic cycle has been proposed that involves not only stilbene radical cations but also Fe²⁺ promoted benzophenone ketyl radical formation which enhances cyclisation, suppresses dimerization and accounts for the virtually quantitative recovery of the benzophenone.