3-Indoleboronic acids

Although the first report of an indoleboronic acid was by Conway and Gribble in 1990, this compound (94) was not employed in Suzuki coupling, but rather it was utilized en route to 3-indolyl triflate 34 as described in section 3.1 [12],... 

In the intervening years, indoleboronic acids substituted at all indole carbon positions have found use in synthesis. For example, Claridge and co-workers employed 94 in a synthesis of isoquinoline 95 under standard Suzuki conditions in high yield [114], Compound 95 was subsequently converted to the new Pd-ligand l-methyl-2-diphenylphosphino-3-(l -iso-quinolyl)indole. 

Martin utilized indoleboronic acids in Pd-catalyzed coupling to great effect, and has improved upon the halogen-metal exchange route to indole-3-boronic acids by adopting a mercuration-boronation protocol as illustrated below for the preparation of 96 and 97 [115,116],... 

Indoleboronic acid 96 was employed by Neel to prepare bis(indolyl)maleimides such as 109 [119]. However, since the standard Suzuki conditions failed (triflate 108 apparently decomposing under the reaction conditions), the use of a phosphine-free Pd catalyst [120] and cesium fluoride [121] was necessary and gave 109 in an acceptable yield of 55%. 

The intermolecular Pd-catalyzed amination of haloindoles [475] and indoleboronic acids [476] and the A -arylation [477] and TV-vinylation of indoles [478] have been described. A method for a tandem carbon-hydrogen functionalization/amide arylation leads to carbazoles, e.g. 410 [479]. 

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