Author Topic: The Heck Reaction  (Read 864 times)

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The Heck Reaction
« on: October 03, 2001, 02:50:00 PM »
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?prep=cv7p0361

and

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0361