ca01 only detailed ToC 1..24

2.6.1 Metal–Carbene Complexes 95
cillary alkoxide ligands can influence whether this reaction results in metathesis or the
formation of stable tungstacyclobutane derivatives (see Section 2.6.5). [17,29]
Scheme 5 Stoichiometric Alkene Metathesis [8,17,22]
M( CHBu t )(L) n + H 2C CR 1 R 2 M( CR 1 R 2 )(L) n + H 2C CHBu t
M(L) n R1 R2 Conditions Yield (%) of
[M(=CR1R2 )(L) n]
Ref
[WBr2(OCH2t-Bu) 2] (CH2) 4 CH2Cl2,rt,4h 87 [22]
[Mo(=NC6H3-2,6-iPr2){OCMe(CF3) 2} 2] H TMS pentane,
–308C, 2.5 h
74
[8]
[W(OR3 ) 2(=NC6H3-2,6-iPr2)] a H Si(OMe) 3 pentane, rt,
30 min to 2.5 h
50–78
[17]
a R 3 = 2,6-iPr2C 6H 3,CMe 2(CF 3), CMe(CF 3) 2.
Dibromo(cyclopentylidene)bis(2,2-dimethylpropan-1-olato)tungsten(VI);
Typical Procedure: [30]
Methylenecyclopentane (0.36 mL, 3.42 mmol) was added to an orange soln of [WBr 2(=CHt-
Bu)(OCH 2t-Bu) 2] (0.127 g, 0.215 mmol) in CH 2Cl 2 (10 mL). After 4 h, the volatiles of the red
mixture were removed in vacuo, and the orange residue was washed twice with pentane
to give the product as an orange powder; yield: 110 mg (87%); 13 C NMR (CD 2Cl 2, ä, gated
decoupled): 47.1 (t, 1 J CH = 134 Hz, W=CCH 2), 28.2 (t, 1 J CH = 131 Hz, W=CCH 2CH 2); the W=C
signal could not be observed.
2.6.1.3 Method 3:
By Carbene Transfer
The carbene ligand may be either directly added to the metal center or exchanged for two
X groups. The first strategy requires preparation of the metal in a reactive, reduced form.
For the preparation of compound 14 (Scheme 6), this is achieved by sodium reduction of
the dichloro precursor complex. [31] The phosphorane carbene transfer agent is readily accessible
with a variety of carbene functionalities and the triphenylphosphine byproduct
does not coordinate with the metal center, allowing its easy removal by scavenging with
copper(I) chloride. For the tungsten(II) system 15 (L= tertiary phosphine), even ketones
and imines are sufficiently good carbene transfer reagents, providing alkylideneoxo and
alkylideneimido tungsten(VI) products 16, respectively, in remarkable four-electron oxidative
addition reactions. [32] For ketones different than cyclopentanone, stable and unreactive
bis(ç 2 -ketone) adducts are obtained by use of an excess of the ketone, but the
use of one equivalent still leads to the alkylideneoxo product.
The exchange strategy involves the use of reactive tantalum alkylidene complexes,
resulting in the replacement of alkoxo ligands. This method, which is also accompanied
by the scrambling of other ligands, applies equally well to the synthesis of the oxo and
imido tungsten products 17, [33] but is limited by the availability of the tantalum alkylidene
reagent and has not been reported for molybdenum.
for references see p 135