ca01 only detailed ToC 1..24

2.6.1 Metal–Carbene Complexes 91
ents, and is normally considered as a dinegative (=CR 1 R 2 ) 2– ligand for the purpose of formal
oxidation state assignment. Like all other Schrock-type carbene complexes, those of
group 6 metals present marked nucleophilic reactivity and undergo Wittig chemistry
with X=Y molecules, the thermodynamics favoring the M=X and R 2C=Y combination
where X is harder than Y. [8] This reaction, however, does not represent particular advantages
over classical Wittig reagents for organic synthesis, a major use being the metal removal
at the end of organic transformations carried out on carbene complexes (e.g., alkene
metathesis, see Section 2.6.1.5).
Synthesis of Product Subclass 1
2.6.1.1 Method 1:
By Æ-Hydrogen Elimination from Alkyl Complexes
High oxidation state dialkyl complexes may undergo transfer of an Æ-hydrogen atom
from one alkyl ligand to the second one under suitable conditions, with formation of a
carbene product and elimination of alkane. The reaction is favored by an increase of steric
bulk in the metal coordination sphere. This has been achieved in a number of ways, as
outlined in the following variations.
2.6.1.1.1 Variation 1:
Alkylation of Chloride Precursors
The replacement of a halide with a bulky alkyl group is often sufficient to induce the alkane
elimination process. Thus, while the complex tert-butylimidochlorotris(2,2-dimethylpropyl)molybdenum(VI)
(1) does not spontaneously undergo the Æ-hydrogen elimination
process, substitution of the chloride ligand with a fourth 2,2-dimethylpropyl ligand
directly affords the carbene product 2 (Scheme 1). [9] For the analogous tungsten system,
tetraalkylimido intermediates, e.g. phenylimidotetrakis[(trimethylsilyl)methyl]tungsten(VI),
have been isolated, and their slow first-order elimination to the alkylidene product
has been investigated. [10] Other alkane eliminations from stable dialkyl derivatives
may be induced by simple irradiation, [11,12] e.g. the transformation of 3 into 4. [13]
Scheme 1 Æ-Hydrogen Elimination Induced by Alkylation [9,13]
Cl
Mo
CH2But NBut ButH2C ButH2C W
3
1
t-BuCH2Li, benzene
rt, 10 min
75%
toluene, hν (Xe, 340 nm)
26 h
64%
= 4-t-Bu-calix[4]-(O)4
H Pr
W
4
Bu
Mo
tH2C But NBu
H2C
t
CHBut {4-tert-Butylcalix[4]-(O) 4}butylidenetungsten(VI) (4): [13]
A soln of W(cyclo-C 4H 8){4-t-Bu-calix[4]-(O) 4} (8.45 g, 8.77 mmol) in toluene (200 mL) was irradiated
with a Xe lamp (540 W •m –2 at 340 nm) for 26 h. Volatiles were removed in vacuo,
pentane (60 mL) was added to the residue, and pale brown 4 was collected and dried in
vacuo; yield: 5.62 g (64%); 1 H NMR (benzene-d 6, ä): 10.0 [t, 3 J = 7.5 Hz, 1H, WC(Pr)H], 5.47
[m, 2H, WC(H)CH 2CH 2CH 3], 1.69 [m, 2H, WC(H)CH 2CH 2CH 3], 1.15 [t, 3 J = 7.2 Hz, 3H, WC(H)-
2
for references see p 135