The Motherwell synthesis of biaryls

n-Bu3SnH 0.7 eq. AIBN slow addition / 15 h benzene / reflux 

Moreover, the powerful directing effect was observed with ortAo-methoxycarbonyl group which favours the ipso-attack [3]. Further improvements have been introduced by using the diazonium salts as parent substrates, similarily to the Pschorr reaction [4]. 

The Motherwell method for synthesis of biaryls is apparently an important and perspective approach to the synthesis of ort/zo-substituted biaryl structures, still, further achievements with different models concerning substituent effects, as well as more efficient methodology are necessary. 

Related tin hydride-mediated free-radical cyclizations of iodo- or bromo-derivatives such as 428 have been widely used in the synthesis of naturally occuring products. 

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The Motherwell synthesis of biaryls Preparation of 2N-methyl-4%6-dimethyl-I-aminobiphenyl (537) I4j... 

Motherwell s group developed an original approach to the synthesis of biaryls based on the intramolecular free-radical zjo o-substitution reaction. This includes the reaction of ortho-io o (or bromo) phenols, A -methylanilines or 7V-methylbenzamides with various arylsulfonyl chlorides to give the respective sulfonates XXX, Ai-methyl sulfonamides XXXI, or Y-acylsulfonamides XXXII. The latter are reacted with tri- -butyltin hydride in the presence of azobisisobutyronitrile (AIBN) in refluxing benzene to produce spirocyclic intermediate XXXIII, which, upon extrusion of sulfur dioxide, afforded the ort/jo-hydroxy- XXXIV, methyamino- XXXV, or iV-methyl carboxamido-substituted biphenyls XXXVI in moderate to good yields [1-4], Scheme 1. 

Later, Motherwell developed a new biaryl synthesis using 1,4- as well as 1,5-aryl migrations from sulfur to aryl radicals [101], Easily available arenesulfonates as well as arenesulfonamides were used as starting materials in these reactions 72). As in the Speckamp studies with primary alkyl radicals, the aryl radicals 71 were shown to also react by ortho attack to afford the side product 73. However, by judicious choice of the migrating aryl group, it is possible to completely suppress the ortho attack [102]. 

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