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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
Author(s) -
Pavan M. Phani,
Chakravarty Manab,
Kumara Swamy K. C.
Publication year - 2009
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200900865
Subject(s) - chemistry , isocoumarins , benzopyrans , aryl , aldehyde , regioselectivity , alcohol , allene , medicinal chemistry , catalysis , stereochemistry , organic chemistry , alkyl
Regioselective, palladium‐catalyzed coupling reactions of allenylphosphonates (OCH 2 CMe 2 CH 2 O)P(O)CH=C=CRR′ [R, R′ = H ( 1a ), R = H, R′ = Me ( 1b ), R = R′ = Me ( 1c )] and phenyl allenes PhCH=C=CR 2 [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 ( 1 H/ 31 P 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‐tetrasubstituted benzofurans. From the reaction of 2‐iodophenol and Pd II (OAc) 2 /PAr 3 , isolation and structural characterization of the (hydroxy)aryl phosphane oxides (Ar) 2 P(O)(C 6 H 4 ‐2‐OH) (Ar = Ph, 4‐MeOC 6 H 4 ) 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 allenes 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 allene is preferred except in the case of PhC=C=CH 2 , where [β,α] attack is observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)