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

Alkyls and aryls
* C0
(O)
Carbonyl insertion
( not observed)
Fig. 7. 3 Migratory insertion of carbon monoxide.
methyl migration
( observed)
inversion at iron, showing that the ethyl group migrates to the site vacated by the
departing carbonyl ligand.
Alkyl~acyl migrations are key steps in many commercial catalytic processes
involving carbon monoxide such as the hydroformylation ofalkenes (p. 38 7) and
the conversion of methanol into acetic acid (p. 3R'i).
7 .1. 7 Some reactions catalysed by palladium complexes
(a) COUPLING VIA BIS-1J 1 -li\'TERI\1EDIATES. Attempts to couple an organometallic
reagent with an organic halide can give unsatisfactory results (R 2 M + R 1 X
-+R 1 R 2 + MX) (p. 88). Mctal-halogcn cxchange am! climination reactions can
lead to a mixture of products. Organocopper compounds are sometimes suitable.
but with the exception of iodides, the reactions are often sluggish and are
accompanied by extensive cross scrambling leading to R 1 R 1 and R 2 R 2 •
One way of overcoming these problems in certain cases is to attach the two
groups R 1 and R 2 to a transition metal centre. The following cycle shows how this
can be done catalytically. The grouping rMJ can be. for example, L,Pd or L 2 Ni (L
= Ph 3 P). The catalyst is conveniently introduced as L 4 Pd, L 4 Ni or L 2 NiCl 2 •
A serious restriction on the scope of these reactions arises from the property of
transition metal alkyls to undergo rapid /3-hydrogen transfer with elimination of
alkene. Alkyl halides (R 1 X) which have a {]-C-H(sp 3 ) functionare therefore
unsuitable. This limitation does not apply quite so rigidly to the group R 2 ,
probably because loss of the coupled product R 1 R 2 from L 2 MR 1 R 2 competes
successfully with loss of alkene. Nevertheless the reactions are most generally
useful when R 1 and R 2 are aryl, alkenyl, alkynyl. benzyl or allyl. Using nickel or
palladium catalysts suitable halides have been coupled with organo derivatives
of, for example, M =Li, Mg, Zn. Cd, Hg, Al. Zr and Sn. Retention of stereo- and
regiochemistry in alkenyl and allyl groups is usually observed.
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