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

Arene( tricarbonyl )chromium complexes
C 6 H 5 Cr(C0) 3 group is relatively small, but it is sufficient to produce noticeable
changes in reactivity. Benzyl chloride and its complex (PhCH 2 Cl)Cr(C0) 3 are both
solvolysed by an SNl mechanism in ethanol in which the carbonium ion is an
intermedia te. It has been found that the complex reacts about 10 5 faster than the
free halide. These effects have been exploited in organic synthesis. Complexed
benzyl alcohols can be converted into ethers or amines by reaction with
hexafluorophosphoric acid followed by an alcohol or an amine. The Ritter
reaction, which involves electrophilic addition to a nitrile, is also greatly
accelerated. While an uncomplexed benzyl alcohol requires over a day for
complete reaction, the complex reacts within a few minutes.
~CH 2 0H
H+
~~"'
ROH
-H 2 0 -H+
~CH 2 0R
Cr Cr Cr
(C0) 3 (C0) 3 (C0) 3
Suggested
intermediote
2RNH2
-RNH/
~CH 2 NH 2 R
Cr
(C0) 3
(i) R CN
(ii) H20
~ CH 2 NHC~R ~o
Cr
(C0)3
The Cr( CO) 3 group can finally be removed cleanly by photolysis of a solution of the
product in air.
10.2.4 Steric effects
The steric effect of the bulky Cr( CO) 3 group induces reagents such as nucleophiles
to attack from the side away from the metal centre (exo attack). The reactions are
very stereospecific. Thus reduction of the indanone complex occurs almost
exclusively from above the ring to give a racemic mixture of the indanol
complexes shown.
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