Oxidative Coupling Reactions Mediated by MoCl 5 Leading to 2,2′‐Cyclolignans: The Specific Role of HCl

The scope and performance of oxidative coupling reaction using MoCl 5 can be significantly improved by employing Lewis‐acidic additives such as TiCl 4 , SnCl 4 , or SiCl 4 . Since the by‐product, HCl, plays a particular function as an inhibitor for MoCl 5 by forming chloro complexes, exploiting typical chloride scavengers, like silver salts or molecular sieves, is also successful. A variety of differently substituted 1,4‐diarylbutanes 4a − g , which were easily synthesized by a modular approach, were subjected to the dehydrodimerization reaction. For the oxidative coupling reaction, a para ‐substitution pattern — with a donor group on the aryl system — is pivotal for allowing access to the 2,2′‐cyclolignans 5a , e − g . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)