Oxidative Coupling Reactions Mediated by MoCl5 Leading to 2,2′-Cyclolignans: The Specific Role of HCl

The scope and performance of oxidative coupling reaction using MoCl5 can be significantly improved by employing Lewis‐acidic additives such as TiCl4, SnCl4, or SiCl4. Since the by‐product, HCl, plays a particular function as an inhibitor for MoCl5 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)