ca01 only detailed ToC 1..24

106 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
Unsymmetrical alkynes can often be cleaved even if one of the substituents is bulky,
giving a 1:1 mixture of the two carbyne products. However, use of an excess of the alkyne
increases the yield of one carbyne product, e.g. 43, by the metathesis procedure described
in Section 2.6.2.3, provided the lower-boiling alkyne is removed. Some of these reactions
afford satisfactory results only in the presence of a stabilizing base, e.g. quinuclidine. [74]
Terminal alkynes react with both tungsten and molybdenum alkoxides, but the CH product
is not isolable unless a stabilizing base is present. [77] The triple bond of selected nitriles
is also cleaved to afford a mixture of the carbyne and the sparingly soluble nitride products.
[78,79]
Scheme 15 Addition of an Alkyne to a Compound Containing
a Metal-Metal Triple Bond [74,79]
X 3W WX 3 + R 1 C CR 1
pentane, Et2O DME or THF
−40 oC to rt
47−91%
WX 3 = W(Ot-Bu) 3, W(CH 2t-Bu)(O-iPr) 2
R 1 = Me, Et, Pr, CH 2NMe 2, CH 2NEt 2, MOM, CH 2OTMS
(Bu t O) 3W W(OBu t ) 3 + R 1 C CR 2
pentane, Et2O
DME or THF
−40 oC to rt
R1 = t-Bu, TMS, Ph,
R2 CH CH2, CH(OEt)2, CO2Me, CH2CO2Me, Ac, St-Bu
= Me, Et
2 W( CR1 )X3 42
W( CR
43
1 )(OBut ) 3 W( CR2 )(OBut +
) 3
Tri-tert-butoxy(ethylidyne)tungsten(VI) (42,X=Ot-Bu; R 1 = Me); Typical Procedure: [74]
But-2-yne (80.1 ìL, 1.02 mmol) was added to a soln of [W 2(Ot-Bu) 6] (0.75 g, 0.93 mmol) in
pentane (30 mL) at –408C. The soln was maintained at –408C for 1 h and allowed to
warm to rt. The orange color faded to light amber, and after 2 h at rt the volatile components
were removed in vacuo, leaving a light brown solid. Larger scale preparations sometimes
yielded a brown oil initially, but the oil always crystallized with time in vacuo or
when seeded. Sublimation of the light brown residue at 258C/10 –6 Torr onto a –788C
probe afforded the pure, white product; yield: 0.59 g (74%). This material is very oxygenand
moisture-sensitive and darkens slightly when removed from the probe in a drybox. It
was generally prepared in pentane or THF and utilized directly for further reactions, assuming
a quantitative yield. 13 C{ 1 H} NMR (benzene-d 6, ä): 254.3 (W”CMe).
2.6.2.3 Method 3:
By Stoichiometric Alkyne Metathesis
A variety of alkoxo–carbyne complexes may be obtained from a preexisting carbyne complex
and an alkyne by a metathesis process (Scheme 16). This exchange takes place via a
metallacyclobutadiene intermediate 44, which has been observed or isolated for the corresponding
aryloxo and fluorinated alkoxo systems. Such intermediates have greater stability
for the tungsten systems. [67] This procedure is synthetically useful for symmetrical,
unsymmetrical, and terminal alkynes. Like the reactions examined in Section 2.6.2.2,
these are also sensitive to steric factors. [67,74] Electronic factors are also important, however,
fluoroalkoxo complexes reacting more readily than the corresponding alkoxo complexes
without fluorine substituents. Terminal acetylenes also react with bulkier carbyne
complexes, but the reaction is often complicated and other products (e.g., deprotonated
metallacyclobutadiene derivatives) may be obtained, limiting the synthetic utility. [67]