Biaryl bond formation

A potentially profitable discovery was that dibromide 74 will undergo an Ullman-type coupling to give the corresponding bis(l,2,4-triazole) 76 in 51% the reaction conditions employed were chosen in order that a trifluoromethyl substituent could be introduced but this reaction did not occur, the biaryl bond formation being preferred (Equation 27) <2006T3301 >. 

Although the bimolecular additions of aryl radicals to aromatic rings are not useful due to a lack of selectivity, the corresponding cyclizations are useful for constructing biaryl bonds. Equation (5) provides a generalized example of the Pschorr reaction.172 This cyclization of a diazonium salt is one of the classical methods for biaryl bond formation, and there are a variety of related reactions.138... 

Intramolecular Biaryl Bond Formation - Exemplified by the Synthesis of Carbazoles... 

Scheme 2. Intramolecular biaryl bond formation in the synthesis of pyranones and illustrative examples (the new bonds formed are indicated).

More complex ellagitannins, such as the natural compounds tellimagrandin I (162) [117], sanguiin H-5 [18], pedunculagin [118], and coriariin A (166) (Scheme 41) [119, 120] have also been synthesized by the lead tetraacetate oxidative coupling procedure, with the same favorable stereoselective (S)-biaryl bond formation. 

A serendipitous use of a chiral auxiliary in atrop-selective biaryl bond formation has recently been published by Kita and co-workers [39, 137]. With diaryl substrate 201, which is related to precursors of the ellagitannins (see Section 14.6.1), PIFA-mediated oxidative coupling did not lead to the expected ellagitannin structure 202 (Scheme 50). Rather surprisingly, this reaction proceeds with intermolecular Ar-Ar bond formation. The chiral glucose framework efficiently transfers stereochemical information, but not in a intramolecular closure. 

The [(OEP)Rh]+ scaffold, while efficient at the metalation of arenes, is ill-suited for catalytic biaryl bond formation. The generation of an intermediate bearing two aryl groups cis to each other as a typical precursor to biaryls through reductive elimination is impossible due to the ligand-imposed geometry at the metal center. Nonetheless, other highly electron-deficient Rh(III) complexes can offer suitable entries into catalytic routes for electrophilic arene arylation. This is especially the case if the electrophilic Rh(III) center can be accessed in situ from the oxidative addition of Ar-X to a Rh(I) complex (Scheme 4). 

Fiirstner reported the iron-catalysed crosscoupling of aryl-halides or aryl-pseudohalides with allgrl-Grignard reagents. Iron-catalysed biaryl bond formation could also be achieved from the coupling of aryl-halides or aryl-pseudohalides with aryl-Grignard reagents. ... 

TABLE 1 Exploration of Biaryl Bond Formation with Bimetallic Oxovanadium Catalysts... 

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