2-Biphenylyl carbene

Further support for this theory was obtained from 4-pyridyl-(4-biphenylyl)-carbene (286) which, due to the lowered benzenoid LUMO compared to 283, underwent enhanced ring expansion into the benzene ring (Eq. 83).210... 

Nolan reported the use of the 2,6-diisopropylphenyl imidazolium carbene precursor, which contains an unsaturated backbone, for the reaction of aryl chlorides with a variety of amines at 100 °C [165, 166]. This temperature is lower than those conventionally used for reactions of aryl chlorides, but is higher than those used with P(tBu)3 or the 2-biphenylyl di-tert-butylphosphines. Reaction yields were high when 2 mol % palladium was used. Reactions of primary amines occurred in good yield, even when unhindered aryl halides were used. The monoarylamine was obtained in 86 % yield, and only a 5 % yield of the diaryl-amine by-product was isolated. Notably, reactions of both aryl bromides and iodides proceeded at room temperature. 

Until recently use of aryl chlorides for catalytic reactions has been neglected due to low reactivity, despite the fact that chlorides are cheaply and easily available. Remarkable progress has been achieved since 1998 in developing Pd-catalyzed reactions of aryl chlorides. A review of Pd-catalyzed reactions of aryl chlorides has been published by Littke and Fu [1]. Facile Pd-catalyzed reactions of aryl chlorides occur by using electron-rich and bulky ligands and proper bases. P(r-Bu)3, biphenylyl(di-r-butyl)phosphine and heterocyclic carbenes are the most effective ligands. As measures of basicity and bulkiness, the and cone angles of some phosphines are cited ... 

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