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

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)] 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 EtO(2)CCH=C=CR(2) R = H (2c), Me (2d)]. While 1b and 1c couple with different stereochemical outcomes using PhI in the presence of Pd(OAc)(2)/PPh(3)/K(2)CO(3) to give phenyl-substituted 1,3-butadienes, 1a does not undergo coupling but isomerizes to the acetylene (OCH(2)CMe(2)CH(2)O)P(O)CCMe (7). In the reaction of 1c with PhI, use of K(2)CO(3) affords the butadiene (Z)-(OCH(2)CMe(2)CH(2)O)P(O)CH=C(Ph)-C(Me)=CH(2) (12); in contrast, the use of Ag(2)CO(3) leads to the allene (OCH(2)CMe(2)CH(2)O)P(O)C(Ph)=C=CMe(2) (20), showing that these bases differ very significantly in their roles. The reaction of 1a with PhI or PhB(OH)2 in (t)he presence of Pd(OAc)2/CsF/DMF leads mainly to (E)-(OCH(2)CMe(2)CH(2)O)P(O)CH=C(Me)Ph (21) and (OCH(2)CMe(2)CH(2)O)P(O)CH2-C(Ph)=CH(2) (22) and is thus a net 1,2-addition of Ph-H. Compound 1b reacts with iodophenol in the presence of Pd(OAc)(2)/PPh(3)/K(2)CO(3) 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.