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10.1.1.1.1.1.1.3 2-Halophenols and Alkynes with a Palladium(II) Catalyst

DOI: 10.1055/sos-SD-110-00529

Kwiecień, H.Science of Synthesis Knowledge Updates, (2014466.

Palladium-catalyzed coupling of 2-iodophenols with alkynes in the presence of at least stoichiometric quantities of inorganic bases and a source of chloride ion is an efficient route to 2,3-disubstituted benzo[b]furans 49.[‌103‌‌106‌] Generally, a higher temperature is required and the process appears to be limited to hindered alkylacetylenes or acetylenes bearing aryl, carbonyl, or silyl groups. The higher temperature used causes a problem with the regioselectivity of this process. For example, the annulation of ethyl phenylpropynoate affords a 3:2 mixture of the regioisomers ethyl 3-phenylbenzo[b]furan-2-carboxylate (49, R1 = H; R2 = CO2Et; R3 = Ph) and ethyl 2-phenylbenzo[b]furan-3-carbox­yl­ate (49, R1 = H; R2 = Ph; R3 = CO2Et).[‌103‌] The reaction of 1-(trimethylsilyl)-3-(trimethylsiloxy)prop-1-yne with 2-iodophenol also results in a complex mixture of products from which 3-(hydroxymethyl)-2-(trimethylsilyl)benzo[b]furan can be isolated but only in low yield.[‌104‌] Much better yields of benzo[b]furans are obtained when more stable O-silyl protecting groups are employed. Thus, 1-(tert-butyldimethylsilyl)-3-(tert-butyldimethylsiloxy)prop-1-yne reacts with 2-iodophenol in 3 hours to afford 2-(tert-butyldimethylsilyl)-3-[(tert-butyldimethylsiloxy)methyl]benzo[b]furan (49, R1 = H; R2 = TBDMS; R3 = CH2OTBDMS) in 85% yield (Scheme 26).[‌104‌]

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