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

Alkylidene and alkylidyne complexes
treatment with CD 3 0- in CD,OD the hydrogen atoms are exchanged for
deuterium; nucleophilic substitution of -OCH, by -OCD, also occurs giving
(OC) 5 CrC(OCD 3 )CD 3 •
Can the carbene ligand be released from these complexes and used in synthesis?
None oftheir reactions has yet been shown to involve free carbenes although the
group C(OR')R is retained on reaction with various reagents or on thermal
decomposition.
se•c
Vinyl- and phenyl-methoxycarbene complexes condense with alkynes to form
benzene or naphthalene derivatives respectively. One carbonyl ligand is also
incorporated into the new ring as a phenolic group. The reaction is a key step in a
recent synthesis of Vitamin E.
>=J
Me
Me
(OC) 5 Cr----\___ + Me-==-vl R ~~~%>
Me
OMe r-
Me
OH
Me
Cr OMe
(COl 3
( + isomer with Me and
long side chain reversed)
s~eps
Vitamin E
In contrast to Fischer carbenes, the alkylidene complexes ofNb and Ta behave as
+ - +
if the polarization is M-CRR'. In this they resemble phosphorus ylids R,P-
CRR' (p. 136) and they react with ketones in a Wittig type reaction.
(Me 3 CCH 2 ) 3 Ta=CHCMe, + RCOR' --> cis and trans RR'C=CHCMe, + [Ta]=O product
7.2.2 Carbyne complexes
Treatmentof(OC) 5 M=C(0Me)R (R = Me, Ph) with Lewis acids (BX 3 , AlX 3 , GaX,;
X= CI. Br, 1) affords carbyne complexes.
235