ChemInform Abstract: Synthesis of 6‐ and 7‐Arylindoles via Palladium‐Catalyzed Cross‐ Coupling of 6‐ and 7‐Bromoindole with Arylboronic Acids.

Abstract

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Preparation of Tyrian Purple (6,6′-Dibromoindigo): Past and Present

“…In this procedure, the starting nitrotoluene 12 is condensed with dimethylformamide dimethyl acetal ( 31 ), usually with an added equivalent of pyrrolidine ( 88 ) (or with tripiperidinomethane alone [ 21 ]), to give the substituted aminostyrene 89 . This is then reduced, affording the desired 6-bromoindole ( 81 ) ( Scheme 41 ) [ 21 , 255 , 256 , 257 , 258 , 259 , 260 , 261 , 262 , 263 , 264 , 265 , 266 ]. While a variety of reducing agents have been used for the second step, buffered aqueous titanous chloride appears to be the most efficient, and overall yields of up to 77% have been reported [ 257 , 263 ].…”

“…While a variety of reducing agents have been used for the second step, buffered aqueous titanous chloride appears to be the most efficient, and overall yields of up to 77% have been reported [ 257 , 263 ]. It is recommended that the reduction step be monitored carefully in order to avoid overreduction to unsubstituted indole, which can make purification of the product difficult [ 260 ]. In an alternative application of Batcho-Leimgruber reaction for the synthesis of 81 , 2,4-dinitrotoluene ( 3 ) is converted to 6-aminoindole by reduction of the respective styrene and subsequently transformed to 81 by a Sandmeyer reaction [ 267 ].…”