Tungsten complexes with alkyls

As described in many reviews, Trost and his co-workers have carried out a pioneering work on the molybdenum-and tungsten-catalyzed allylic alkylation of allylic esters regioselectivity of the reaction is often complementary to the palladium-catalyzed allylic alkylation. The first asymmetric version was disclosed by Pfaltz and Lloyd-Jones in 1995 (Equation (63)). They used a catalytic amount of a novel tungsten complex, prepared from [W(CO)3(MeCN)3] or [W(cycloheptatriene) (COIs] and optically active (diphenylphosphino)phenyloxazolines 57, for the allylic alkylation of 3-aryl-2-propenyl phosphate with dimethyl sodiomalonate to isolate the corresponding branched alkylated compounds as a major isomer with an excellent enantioselectivity (96% ee). Unexpectedly, 3-aryl-2-propenyl carbonates are shown to be unreactive. It is worth noting that an isostructural molybdenum complex does not promote the catalytic alkylation under the same reaction conditions. In contrast, Lloyd-Jones and Lehmann reported the stereocontrolled... 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

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... 

The metal, M, can be Fe, Ru, Os (with n = 4) or Cr (n = 5). Substituents on silicon can vary widely, including aryl, alkyl, alkoxy and Et2N groups. With a slight modification, the same reaction is useful for silylene-tungsten complexes (equation 98). 

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... 

Kinetic studies on the reaction between trans- M(l Mdppe)2 (M = Mo) with alkyl, acyl, or aroyl halides, in the dark, show that the rate of the reaction is independent of the concentration and nature of the alkyl halide, but has a first-order dependence on the concentration of the complex ( bs = 1.3 0.2 x 10 4 sec ). The tungsten reaction requires light. 

Comparison of the second-rate order constants for reactions of the molybdenum and tungsten analogs with a given alkyl halide shows that the tungsten complex reacts six times faster than the molybdenum counterpart (Section VI,E,2). 

For cyclopentadienyl tungsten(II) derivatives barriers of 18-19 kcal/mol characterize alkyne rotation in both the alkyl and acyl cases (67,69,161). Extended Huckel calculations indicate that steric factors play a role in these complexes in addition to the standard dir orbital electronic factors (147). Smaller barriers attend replacement of CO in CpW(CO)-(HC=CH)X complexes with either P(OMe)3 or PMe3. No alkyne rotation... 

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. 

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