_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Alkyls and aryls
(b) THE HECK REACTION. Organopalladium species also add across
carbon-carbon double bonds; the stereochemistry of the addition is cis. If there is
a P-hydrogen in the resulting palladium complex which can, by rotation, move
into a syn position relative to palladium, loss of [Pd]-H occurs and a substituted
alkene is generated.
R' R' ) H
R' R [Pd)
"'==/ + [Pd]-R--+ 1 {Pd]-R- / ---
R'
• R'
R'
R" r
}' ·>l -[Pd]H
~.
R
PhHgOAo+ Pd(OAol,+ V'- Ph~H
Ph>--vH
(Pdl
Ph Ph
~ Me
H
[Pd]
As described above, the initial [Pd]-R complex can be formed by the transfer ofR
from another organometallic RM. Compounds of mercury, boron or tin have
often been used. Alternatively an aryl, heteroaromatic or vinyl halide is added to
'Pd1 2 ', generated in situ from Pd1 4 or by reduction of L 2 PdX 2 • In the second
method a base such as a tertiary amine is also required to react with the hydrogen
halide which is produced.
Pd cot/
Et 3 N/10o•c
The Heck reaction involves electrophilic attack by palladium on an alkene. In
contrast to mercuration (p. 6 3 ). however. cis addition of [Pdj-R to the double
bond occurs. The palladium becomes attached, as expected, to the more negative
carbon atom ofthe alkene. Steric effects are also significant. The less hindered the
olefin, the faster the rate. In the absence of a strong electronic effect the new
carbon-carbon bond is formed at the less sterically congested centre.
Allyl alcohols couple with aryl halides to give aldehydes or ketones.
ArX +
Pd cot/
Et 3 N
229