Tungsten complexes, alkyl carbene

An intramolecular alkyl group transfer occurs upon warming up solutions of the ate-complex 180 generated by transmetalation of 1-lithio-l-methoxyethene 56 (equation 76)405,406 Tungsten and chromium carbene complexes 181 and 182, respectively, have been isolated from l-lithio-l,2-dimethoxyethene (equation 11... 

The term alkylidene refers to carbenes, CR2 with alkyl substituents for example, MeCH=ML is an ethylidene complex but alkylidene is sometimes used as a synonym for Schrock carbene in the older literature because the first alkylidenes were of the Schrock type. There are electrophilic Fischer alkylidenes as well as nucleophilic Schrock ones, however, so the terms should be kept separate. For example, [Cp2W(=CH2)Me], and Cp2Ta(=CH2)Me are isoelectronic, but the former is electrophilic and the latter nucleophilic at the carbene carbon the net positive charge on the tungsten complex must stabilize the M(d,) levels and is therefore probably the main reason for the difference. 

Nucleophilic acyl complexes can be 0-alkylated with hard electrophiles to yield the corresponding alkoxy- or (acyloxy)carbene complexes. The first carbene complex ever isolated [61] was prepared by this route the intermediate, anionic acyl complex was generated by addition of phenyllithium to tungsten hexacarbonyl (Figure 2.3). 

Dimethylsulhde can be eliminated from a-(dimethylsulfonium)alkyl complexes simply by heating (Figure 3.15). The resulting, very reactive, electrophilic iron carbene complexes cannot usually be isolated but are generated directly in the presence of a suitable reactant, e.g. an olefin. Cationic nickel [475] and tungsten [476] carbene complexes have been prepared by similar routes. 

For most methoxycarbene complexes a one pot modification of the above method is utilized. This involves direct alkylation of the initially formed lithium acylate carbene complex with trifluoromethanesulfonate or with methyl fluorosulfonate. The method is successfully employed for preparation of chromium140 as well as molybdenum and tungsten monocyclic uation 65), bicyclic (equation 66) and tricyclic carbene complexes... 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Several reviews have been published235"238 concerning complexes with M=CR fragments. An extensive review in this series by Mayr and Hoff-meister6 describes the work carried out on the [MX(CO)2L2(CR)l (M = Cr, Mo, W X = Cl, Br, I R = alkyl, aryl) type complexes, which have been considered to be in oxidation state zero. Hence, this section is restricted to carbene complexes in which the molybdenum and tungsten oxidation state is +2. 

Tungsten carbene complexes contain W=C linkages, and if they also contain alkyl substituents on the carbene carbon,... 

---