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)CH2 (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.