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)CHCCRR‘ [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 PhCHCCR2 [R = H (2a), Me (2b)] and allenyl esters EtO2CCHCCR2 [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)CHC(Ph)−C(Me)CH2 (12); in contrast, the use of Ag2CO3 leads to the allene (OCH2CMe2CH2O)P(O)C(Ph)CCMe2 (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)CHC(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.