PEYOTE
(Hive Bee) 10-03-01 14:50 No 219978 |
The Heck Reaction | Bookmark | ||||||
The Heck Reaction (Advanced Organic Chemistry: Part B, Carey & Sundberg; 4th Ed. Kuwer Academic/Plenum Press; pp. 503-507) An important and useful reaction with organpalladium chemicals, with the oxidation of carbonyl compounds from alkenes by PdCl2 (Wacker reaction), is the Heck reaction. Globally: Ar-X + CH2=CH-Y == Pd(0)(*) ==> Ar-CH=CH-Y + HX (*) = in catalytic amounts with: X = Br, I Y = an electron-withdrawing group The reaction works also with alkenyl halides (CR2=CR-X). As catalyst Pd(OAc)2 or PdCl2 are used: they develops Pd(0) in situ. Usually a phosphinic ligand is used to stabilize Pd(0). More used ligands are: PPh3, o-tolylphosphine (preferred), diphosphines, also phosphites are effective. Mechanism (with bromobenzene and a terminal alkene): Ph-Br + Pd(PPh3)2 -> Ph-Pd(PPh3)2-Br == CH2=CH-Y ==> Ph-(Pd(PPh3)2...CH2=CH-Y)-Br -> Ph-CH2CH(Y)-Pd(PPh3)2-Br -> Ph-CH=CH-Y + Pd(PPh3)2 + HBr Some modifications: 1) use of Ag salts activating the halide; 2) use of NaHCO3 or Na2CO3 with a PTC for milder conditions of reaction; 3) use of R4N+ accelerating the rxn; Ar-Cl arent very reactive, but in some conditions they can be used. The reaction goes more effectively if I is used instead of Br. If I and Br are attached in the benzene ring, the substitution is toward the I. If Y is an electron-withdrawing group, then a beta regioisomer is formed, if it is a donor, a mixture of alfa and beta regioisomers will be formed. References: [1] JACS, 96 1974 1133; Acc. Chem. Res., 12 1977 146; Org. React., 27 1982 345 [2] JOC, 43 1978 3898; JOC 46 1981 1061; JOC 46 1981 1067 [3] Synlett, 1998 792 [4] JOC, 52 1987 4130; JACS, 110 1988 2328 [5] J. Chem. Soc. Chem. Commun., 1984 1287; Tetrahedron Lett., 26 1985 2687; Synthesis, 1987 70; Tetrahedron Lett., 28 1987 5291 [6] Angew. Chem. Int. Ed. Engl., 33 1994 2379; J. Heterocycl. Chem. 31 1994 631; Tetrahedron Lett., 52 1996 10113 [7] Angew. Chem. Int. Ed. Engl., 37 1998 481 [8] JOC, 55 1990 3654; JOC 57 1992 3558; JOC, 57 1992 1481 Some examples: JOC: 44 1979 4078; 45 1980 1549; 46 1981 1067; 52 1987 4130; 58 1993 832; 59 1994 6142 Synlett: 1996 903 Tetrahedron Lett.: 29 1988 905; 34 1994 4103; 52 1996 12465 http://www.orgsyn.org/orgsyn/prep.asp?pr |
||||||||