The Ring Systems in Cross-Coupling Reactions

As reactions in benzo-fused rings are generally similar to simple carbocyclic compounds, our discussion of these is limited. 

Lithiation of A-protected indoles is the main route to 2-boronic acids, which have found wide use, although the usual protecting groups (Boc and arylsulfonyl) increase the ease of deboronation. With Boc-protected derivatives, the indole is less consistent than the pyrrole in coupling reactions. In situ protection of the indole NH as the A-carboxylate can be used to prepare both the 2-boronic acid and 2-tri-n-butylstan-nane for one-pot coupling procedures. 

1-TBS and 1-TIPS indole-3-boronic acids, prepared in situ via halogen exchange, conple weU with a range of heterocyclic snbstrates.  

A synthesis of the natnral product meridianin D demonstrates selectivity in halogen-lithium exchange and substrate selectivity, using a halogenated boronic acid. The presence of the A-tosyl probably controls the halogen exchange selectivity.  

Unprotected 5- and 6-tri-n-butylstannylindoles (and comparable boronic acids) are prepared via lithium-bromine exchange, using the KH then f-butyllithium procedure, and couple cleanly with acid chlorides. 

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