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 PdCl
2 (Wacker reaction), is the Heck reaction.
Globally:
Ar-X + CH
2=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 (CR
2=CR-X). As catalyst Pd(OAc)
2 or PdCl
2 are used: they develops Pd(0)
in situ. Usually a phosphinic ligand is used to stabilize Pd(0). More used ligands are: PPh
3, o-tolylphosphine (preferred), diphosphines, also phosphites are effective.
Mechanism (with bromobenzene and a terminal alkene):
Ph-Br + Pd(PPh
3)
2 -> Ph-Pd(PPh
3)
2-Br == CH
2=CH-Y ==> Ph-(Pd(PPh
3)
2...CH
2=CH-Y)-Br -> Ph-CH
2CH(Y)-Pd(PPh
3)
2-Br -> Ph-CH=CH-Y + Pd(PPh
3)
2 + HBr
Some modifications:
1) use of Ag salts activating the halide;
2) use of NaHCO
3 or Na
2CO
3 with a PTC for milder conditions of reaction;
3) use of R
4N
+ 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?prep=cv7p0361
and
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0361