Palladium-catalyzed coupling of allenylphosphonates, phenylallenes, and allenyl esters: Remarkable salt effect and routes to novel benzofurans and isocoumarins

Coupling reactions of allenylphosphonates ( OCH2CMe2CH2O) P( O) CH=C=CRR' [ R, R' = H ( 1a), R = H, R' = Me ( 1b), R = R' = Me ( 1c)] with aryl iodides, iodophenol, and iodobenzoic acid in the presence of palladium( II) acetate are investigated and compared with those of phenylallenes PhCH=C=CR2 [ R = H ( 2a), Me ( 2b)] and allenyl esters EtO2CCH=C=CR2 [ R = H ( 2c), Me ( 2d)]. While 1b and 1c couple with different stereochemical outcomes using PhI in the presence of Pd( OAc)(2)/PPh3/K2CO3 to give phenyl-substituted 1,3-butadienes, 1a does not undergo coupling but isomerizes to the acetylene ( OCH2CMe2CH2O) P( O) C CMe ( 7). In the reaction of 1c with PhI, use of K2CO3 affords the butadiene ( Z)-( OCH2CMe2CH2O) P( O) CH=C( Ph)-C( Me) dCH(2) ( 12); in contrast, the use of Ag2CO3 leads to the allene ( OCH2CMe2CH2O) P( O) C( Ph)=C=CMe2 ( 20), showing that these bases differ very significantly in their roles. The reaction of 1a with PhI or PhB( OH)(2) in the presence of Pd( OAc)(2)/CsF/DMF leads mainly to ( E)-( OCH2CMe2CH2O) P( O) CH=C( Me) Ph ( 21) and ( OCH2CMe2CH2O) P( O) CH2-C( Ph)=CH2 ( 22) and is thus a net 1,2-addition of Ph-H. Compound 1b reacts with iodophenol in the presence of Pd( OAc)(2)/PPh3/K2CO3 to give a benzofuran that has a structure different from that obtained by using 1c under similar conditions. Treatment of 1a with iodophenol/Pd( OAc)(2)/CsF/DMF also gives a benzofuran whose structure is different from that obtained by using 2a under similar conditions. In the reaction with 2-iodobenzoic acid, 1a and 2c afford one type of isocoumarin, while 1b, c and 2a, b give a second type of isocoumarin. The structures of key compounds are established by X-ray crystallography. Utility of the phosphonate products in the Horner-Wadsworth-Emmons reaction is demonstrated.