Primary alkylidene complex

An essential feature of these (and other such) d primary alkylidene complexes M=CHR is their existence in two stereoisomeric syn and anti forms (below). In the syn isomer, the carbene substituent points towards the imido ligand, and in the anti isomer, the carbene substituent points away from the imido nitrogen. The two isomers are in an equilibrium that brings insight into the intimate mechanism of olefin metathesis. It is the syn isomer that is usually observed in the solid state and predominates in solution, and the rate of rotation between these two forms varies from slow (10" s ) to fast (100 s ) depending on the imido and alkoxy substituents ... 

The value of -NMR and 13C-NMR spectroscopy in characterizing transition metal carbene complexes was noted in Section III,B,2. The carbene carbon resonance is invariably found at low field (200-400 ppm) in the 13C-NMR spectrum, while protons attached to Ca in 18-electron primary and secondary carbene complexes also resonate at low fields. NMR data for some Ru, Os, and Ir alkylidene complexes and related compounds are given in Table V. 

The primary reason for attempting to synthesize imido alkylidene complexes, e.g., W(NR)(CH-f-Bu)(0-f-Bu)2, was the belief that the appropriate imido ligand will block bimolecular decomposition reactions more effectively than an oxo... 

The utility of carbene complex s)mthesis via olefin metathesis is restricted by the need to begin with a reactive alkylidene complex prepared by a different route, and except in special circumstances (see below), modification of the primary route provides easier access. 

For well-defined silica-supported catalysts (Y = OSi=) deactivation occurs via degraftrng, is first-order in ethene, and does not involve bimolecular decomposition pathways [42]. The formation of by-products, mainly 1-butene as a primary product and alkanes in the case of propene metathesis, when X = alkyl ligand, has also been observed [42]. Of note, the silica-supported, alkyl-alkylidene complexes deactivate faster and form larger amounts of by-products than the pyrrolyl-contarning complexes [23, 32]. 

Complex 169 is very susceptible to electrophilic attack, as shown in Scheme 32. The protonation of 169 with PyHCl gave back 166. In this reaction, the assistance of one of the oxygens as the primary site of the protonation cannot be excluded. The alkylation with MeOTf, unlike in the case of 161 (see Scheme 29),22 occurs at the alkylidene carbon as well, forming the 2,3-dimethyl-2-butene-W derivative 167, which was obtained also by the direct synthesis given in Scheme 31. 

When Group VI carbonyl complexes were reacted with alane or Cp2NbH3 reduction of CO to ethylene was noted.- - Ethylene was the primary product of the Cp2NbH3 reduction although it subsequently was hydrogenated to ethane. Masters and co-workers suggested that ethylene was formed through an alkylidene dimerization as shown below. 

---