O-Halogenophenols

The reaction between copper(I) acetylides and o-halogenophenols or -anilines provides a general and convenient route to 2-substituted benzofurans or indoles (Scheme 71g) (66JOC4071). 

Heating o-alkynylphenols (36, X = 0), which are available from o-halogenophenols and alkynes, without or with palladium catalysts, yields benzofurans (37, X = 0). 

In addition to the palladium catalysts mentioned, other transitional metal catalysts were explored and applied in this transformation as well. In 1995, Dunach and Olivero studied the nickel-catalyzed electrochemical reductive deprotection of allyl ethers. Among the various substrates, allyl-o-halogenophenols were tested as well. 2-Chlorophenol was produced from allyl-o-chlorophenol phenol was produced from allyl-o-bromophenol while 3-methyl-2,3-dihydrobenzofuran (33%) was produced from allyl-o-iodophenol together with phenol (52%). 

Cobalt catalysts were studied by Oshima and co-workers in cyclization reactions in 2002. By using the cobalt complex [CoCl2(dppb)] as the catalyst and assisted by Me3SiCH2MgCl, the intramolecular Heck-type cyclization reactions of 6-halo-l-hexenes proceeded via a radical process to provide the corresponding tetrahydrofurans in good yields. Allyl-o-halogenophenols were tested under identical conditions and gave the desired dihy-drobenzofurans in low to excellent yields (Scheme 2.41). 

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