Tungstacyclobutadiene

The compounds (Bu 0)3 W=CR are alkyne metathesis catalysts.280 281 Probable intermediates in these reactions are tungstacyclobutadienes and the latter are seen in the presence of Cl ligands.282 Addition of alkynes to the latter leads to the formation of substituted r/ -cyclopentadiene derivatives... 

Scheme 2. Pioneering experiments by Mortreux et al. (1974) and Schrock et al. (1981). Geometry of the tungstacyclobutadiene 5 in the crystal (OAr = 0-2,6-diisopropylphenyl bond lengths in pm).

Table 2 Comparative computational results for selected tungstacyclobutadiene complexes ...

Cyclopropenyl systems can be formed from metallacyclobutadiene complexes. Here no decomplexation of unsaturated cyclopropanes is observed. The tungstacyclobutadiene complex 12 is converted into a cyclopropenyl complex 13 upon addition of a nitrogen nucleophile. ... 

The motives underlying the original synthesis of 3 were based on the observation that certain tungstacyclobutadienes react with internal acetylenes to give (substituted) cyclopentadienyl complexes.Consequently, alkynyl-substituted organometallics were deemed to be appropriate for use in the syntheses of heterobimetallic complexes. To this end, the alkynyl-substituted (ii -C5Me4CH2CH2C CEt)Rh(CO)2 (4), was synthesized via reaction of [RhCl(CO)2l2 with 3 (Scheme III). Subsequent treatment with... 

McCullough, L.G., Ustemann, M.L, Schrock, R.R., Churchill, M.R., and Ziller, J.W. (1983) Multiple metal-carbon bonds. 34. Why terminal alkynes cannot be metathesized. Preparation and crystal structure of a deproto-nated tungstacyclobutadiene complex, W(eta.5-C5H5)[Cj(CMe3)2]a./. Am. Chem. Soc., 105, 6729-6730. 

Addition of one equivalent of an internal allqtne, trichloro(l,2-dimethoxyethane-0,0 )-(2,2-dimethylpropylidyne)tungsten(VI) affords a stable tungstacyclobutadiene product, but reaction with additional alkyne yields q -cyclopentadienyl products of further alkyne insertion rather than products of allcyne metathesis.l l... 

Further support for this mechanism has also been provided by Schrock et al [37] who have synthesized stable tungstacyclobutadiene complexes. 

---