| 17109538 |
Chakravarty M, Swamy KC: Palladium-catalyzed coupling of allenylphosphonates, phenylallenes, and allenyl esters: remarkable salt effect and routes to novel benzofurans and isocoumarins. J Org Chem. 2006 Nov 24;71(24):9128-38.
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. |
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