Pd-catalyzed reactions of allenylphosphonates and related allenes with functionalized 2-iodophenols, 2-iodobenzoic acid, and 2-iodobenzyl alcohol leading to functionalized benzofurans, isocoumarins, and benzopyrans

Abstract

Regioselective, palladium-catalyzed coupling reactions of allenylphosphonates (OCH2CMe2CH2O)P(O)CH=C= CRR′ [R, R′ = H (la), R = H, R′ = Me (lb), R = R′= Me (1c)] and phenyl alienes PhCH=C=CR2 [R = H (2a), Me (2b)] with functionalized iodophenols (in PEG-400), 2-iodobenzoic acid, and 2-iodobenzyl alcohol are investigated. Benzofurans with free aldehyde functionalities are formed in high yields (1H/31P NMR) in reactions by using functionalized iodophenols, essentially as single isomers. The synthetic potential of these products possessing an aldehyde functionality is demonstrated by isolating a compound with the skeleton of Obovaten and many other 2,3,5,7-tetrasubstltuted benzofurans, From the reaction of 2-lodophenol and PdII(OAc)2/PAr3, isolation and structural characterization of the (hydroxy)aryl phosphane oxides (Ar)2P(O) (C6H4-2-OH) (Ar = Ph, 4-MeO--C6H4) that suggests the P-aryl to Pd-aryl bond exchange, is described. An interesting structural problem related to the formation/crystallization of benzofurans Is also highlighted. The reaction of alienes with iodobenzyl alcohol or iodobenzoic acid led to benzopyrans or isocoumarins (isochromanones), respectively, as single isomers in good to excellent yields. The reaction of 3-methylbuta-1,2- dienyl acetate with 2-iodobenzoic acid led to a novel acetic acid elimination product along with the expected isocoumarin. The structures of key compounds are confirmed by X-ray crystallography. These results establish that in the formation of benzofurans or benzopyrans, [[β,γ] attack on the aliene is preferred except in the case of PhC=C=CH2, where [β,α] attack is observed. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.