Aryl with arylbismuth reagents,

Ester derivatives of a-amino acids underwent A-arylation with arylbismuth reagents under mild conditions338-341. The arylation was performed with both the free amino esters and with the protonated amino ester salts, although most of the latter reacted in only moderate yields. High yields of A,A-diphenylated products were obtained with 1.2 equivalents of triarylbismuth diacetate A,A-diphenylated products were also obtained by a two-step process (equation 77)339. 

The number of S- and N-arylations with arylbismuth reagents is relatively limited. The reaction of thiols with pentavalent organobismuth reagents leads mostly to oxidation products and, in some instances, to 5-phenylation. In the case of the nitrogen derivatives, the most efficient arylation by arylbismuth reagents are reactions which are copper-catalysed, and these will be reviewed in a later part of this chapter (see section 6.7). 

A variety of pentavalent substituted triarylbismuth derivatives are known and electron-withdrawing or electron-donating substituted phenyl groups can be introduced by arylation with these reagents. The electronic properties of the substituents of the arylbismuth do not influence significantly the overall yield of arylated products (arylation of 2-naphthol 84% with the 4-nitrophenyl derivative, 76% with the unsubstituted phenyl derivative and 69% with 4-methoxyphenyl derivative). 

As a synthetic method, the reaction of aryllead triacetates with phenols is useful for the production of two structural types the 6-aryl-2,4-cyclohexadienones and the sterically hindered 2,6-diarylphenols substituted with alkyl groups on the C-3 to C-5 positions. The steric compression in the putative aryloxylead intermediate seems to play a decisive role in the success of the ligand coupling step. This is particularly true for the arylation of hindered phenols with the more hindered aryllead reagents, which takes place at room temperature. In contrast to the reactions of phenols with arylbismuth reagents (chapter 6), 0-arylation products are exceptional and generally obtained in very small yields. 

The reaction of arylation of 1,3-dicarbonyl derivatives was also used in a new approach towards the synthesis of isoflavanones and 3-aryl-4-hydroxycoumarins. Phenylation of activated chroman-4-ones to isoflavanones was obtained in moderate to good yields. Selective monophenylation was performed in high yield in a low-temperature reaction (- 23 C) of the activated chroman-4-one with pentaphenylbismuth and mixtures of mono- and di-aryl derivatives were obtained in modest yields with other types of arylbismuth reagents. 

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