Tungstenacyclobutadiene complexes

W(CCMe3)(dme)Cl3 reacts with one equivalent of 2-butyne to give a violet complex whose 13C NMR data are consistent with 1t being a tungstenacyclobutadiene complex (equation 14) (36). In particular, two signals are found at 268 and 263 ppm (cT7 335 for the neopentyl 1 dyne a-carbon atom in W(CCMe3)(dme)C13) and a third... 

R.R. Schrock We believe so, but the t-but oxide ligand seems to be a unique alkoxide. We have prepared tungstenacyclobutadiene complexes containing other alkoxide ligands which are stable toward decomposition to an alkylidyne complex. 

Scheme 16. Formation of tungsten perethyl-Cp complexes (and modifications) from a tungstenacyclobutadiene complex and alkynes (37,81-84). The generalized reaction indicates the formation of different isomers (more than two are possible) when three or more different substituents R are present. The WfC BuXdmeJQj complex also reacts with alkynes to give bulky Cp derivatives (82) (dme = dimethoxyethane). Molecule drawings are schematic.

C(r-Bu)] and 127.6 (2CMe) ppm, indicating fast rotation about the C3-M axis. The pyridine complex [(pyr)2Cl3W(>f-C3Me2( -Bu)] was similarly prepared by addition of two equivalents of pyridine to the trichloro tungstenacyclobutadiene complex. ... 

Metallabenzenes have been invoked as possible intermediates in several other reaction types. Schrock, " for example, proposed tungstenabenzenes as possible intermediates in certain alkyne metathesis reactions that proceed by associative mechanisms. Shown in Scheme 32 is a proposed sequence for the metathesis of 3-heptyne to 3-hejQTie and 4-octyne using a tungstenacyclobutadiene complex as catalyst. The postulated metallabenzenes are formed by alkyne insertion into the metal carbon bonds of the metallacyclobutadienes. Of course, it is also possible to envisage a catalytic cycle based on Dewar metallabenzene intermediates. 

When R = 2,6-Me2C H3—, tungstenacyclobutadiene complexes in which the metal atom adopts an approximately trigonal bipyramidal geometry, have been isolated from the reaction with 3-heptyne. X-ray crystallography shows that the four membered WC3 ring is essentially planar. On account of the low coordination number (five) and low electron count (12), vacant sites are available at the metal centre for attachment of alkyne molecules. These properties are consistent with the high activity of these catalysts. 

The preparation and study of metallacycles has been a subject of active investigation for organometallic chemists. We have just seen one example where metallacycle formation is a key step in a catalytic process and there are several others most notably, olefin metathesis. The metal acts as a geometrical and electronic template in these reactions. For unsaturated metallacycles there are interesting questions concerning delocalization [29]. Certain metal carbynes can react with acetylene to give metallacyclobutadienes as intermediates [30]. One such example of an insoluble molecule is the tungstenacyclobutadiene complex, 18.36 [31]. The compound is quite stable and not very reactive (in contrast to cyclobutadienes... 

It probably also destablizes the tungstencyclobutadiene complex relative to the alkylldyne complex so that the actual concentration of a tungstenacyclobutadiene tri-t-butoxide complex is small. Interestingly, one t-butox1de ligand can be added in the equatorial position but if addition of a second 1s attempted, only tri-t-buto yalkylidyne complexes result (equation 16). 

CH3OC//2CH2OCH3), S 1.28 (s, 9H, CC(CH3)3). The complex Cl3(dme)W= CC(CH3)3 is an important intermediate for preparing olefin and acetylene metathesis catalysts.3,4 It is also a useful starting material for preparing certain tungstenacyclobutadiene, /3-cyclopropenyl, and 5-cyclopentadienyl... 

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