ca01 only detailed ToC 1..24

94 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
extract was filtered through a bed of Celite and the toluene removed from the filtrate in
vacuo to give the product as yellow flakes. The product should be checked by NMR for
contamination by anilinium trifluoromethanesulfonate, which is slightly soluble in toluene;
yield: 5.9 g (65%); 1 H NMR (benzene-d 6, ä): 14.29 (s, CHt-Bu); 13 C NMR (benzene-d 6, ä):
331.9 (d, Mo=C, 1 J CH = 121 Hz).
2.6.1.1.4 Variation 4:
Deprotonation with an External Base
Deprotonation of alkyl ligands at the Æ-position is another potentially general method for
forming carbene complexes from alkyl compounds that are either electronically unsaturated
or possess labile ligands. However, this method is little documented for group 6
metals as well as for metals of other groups. The neutral dialkyl compound 11 reacts
with a range of lithium reagents to yield the dimeric, anionic alkyl–alkylidene complex
12, as shown in Scheme 4. [26] Methylidene product 13 is only stable at low temperature,
decomposing upon warming in a nonselective manner to yield alkylidene-bridged dinuclear
products. [27]
Scheme 4 Deprotonation of Alkyl Complexes [26,27]
Mo
ON CH 2TMS
CH 2TMS
11
[W(Me) 4Cp ∗ ] + PF 6 − + Et3N
+
LiHMDS
THF
−100 oC to rt
62%
CH2Cl2
< −40 o C
TMS
W(Me)3Cp ∗ (
13
THF
Li
Mo NO ON Mo
CH2TMS Li TMSH2C THF THF
12
TMS
Bis{nitrosyl(ç 5 -pentamethylcyclopentadienyl)[(trimethylsilyl)methyl][(trimethylsilyl)methylidene]molybdenum(II)}
(Dilithium)tris(tetrahydrofuran) (12): [26]
[Mo(CH 2TMS) 2Cp*(NO)] (200 mg, 0.46 mmol) and LiHMDS (90 mg, 0.46 mmol) were intimately
mixed and cooled to –1008C in a small flask. THF was slowly poured down the
sides of the flask and allowed to freeze onto the solid mixture. Over the course of 4 h, a
color change from purple to red occurred; the final mixture was taken to dryness in vacuo.
The remaining red solid was extracted into pentane (2 mL), and the extracts were filtered
through Celite. Slow evaporation of the pentane filtrate resulted in the deposition
of pale red crystals, which were recrystallized (pentane) to obtain pale yellow crystals of
12; yield: 143 mg (62%).
2.6.1.2 Method 2:
By Stoichiometric Alkene Metathesis
The addition of an alkene to a carbene complex may lead to a metathesis reaction with
exchange of the carbene ligand with one of the two halves of the alkene. The reaction proceeds
via formation of an alkene–carbene complex, which rearranges via a metallacyclobutane
intermediate. [28] This reaction is synthetically useful when a terminal alkene is
used and vacuum evaporation of the more volatile alkene product (typically 3,3-dimethylbut-1-ene)
is possible to displace the equilibrium. [22] An excess of the alkene reagent is also
used to ensure a favorable equilibrium position (see Scheme 5). The properties of the an-
CH 2)