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

Allyl and diene complexes
~4,neutrat
They are yellow crystalline materials, stable in dry air, but quite readily
hydrolysed back to dienol derivatives. Fe(C0) 3 can be used as a 'protecting group'.
We have seen (p. 264) that an Fe(C0) 3 substituent can be attached under mild
conditions to a diene unit. It can also be readily removed by oxidation, for example
using aqueous iron(III) or cerium(IV) or by trimethylamine oxide in a hydrocarbon
solvent. A diene unit can therefore be 'protected' using Fe(C0) 3 while
reactions are carried out on other parts ofthe ligand molecule. This approach has
been employed in terpene and steroid chemistry.
(d) ATT ACK OF NUCLEOPHILES. Dienetricarbonyliron complexes are not very
reactive towards nucleophiles. Only carbanions stabilized by cyano or ester
groups attack the organic ligand at ali cleanly. Even with these reagents a
mixture of products is often obtained. At - 78°C reversible addition to the 2-
position in (1] 4 -C 4 H 6 )Fe(C0) 3 is favoured, but at ooc terminal addition also occurs.
Substituted butenes are released on protonation of the anionic intermediates
(Fig. 8.7).
The isoelectronic ion (1] 4 -C 4 H 6 )Co(CO): reacts with a wide range of nucleophiles,
the terminal position of the diene being attacked exclusively. The proton
n.m.r. spectrum of the pyridine adduct is consistent with its having antistereochemistry.
By adding a second nucleophile to the neutra! cobalt allyl it is
possible, in some cases, to convert a 1, 3-diene into a 1, 4-bifunctional deriva ti ve
in a regio- and stereoselective fashion.
~ K,ee~H_+ __
R). (~0)3
Fig. 8.7 Nucleophilic attack on butadienetricarbonyliron.
266
R