Metallocarbenes

Synthesis of aziridines by treatment of carbenes with imines was reported by Jacobsen [56]. A metallocarbene 104 derived from ethyl diazoacetate and copper fluorophosphate was treated with N-arylaldimines to form aziridines with reasonable diastereoselectivities (>10 1 in favor of cis) but with low enantioselectivities (about 44% ee). This was shown to result from a competitive achiral reaction path-... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

The initiation step for metallocarbyne catalyst precursors is not well-understood. But, in the case of Re, a well-defined metallocarbene, the initiation products have been detected 0.7 equiv. of a 3/1 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene (Fig. 2) [81,82]. 

The stability and reactivity of metallocarbenes depends on the degree of back donation from the metal to the carbene. If this is small, the metallocarbenes are highly reactive and electrophilic in character. If back bonding is substantial, the carbon will be less electrophilic, and the reactions are more likely to involve the metal. 

An NMR and structural study characterized the intermediates generated from diimine catalysts on reaction with diazodiphenylmethane.193 The dominant species in solution is dinuclear, but a monomeric metallocarbene species can be detected. 

Muroni et al. have reported a tandem metallocarbenoid/ylide/[l,2]-Stevens rearrangement <2004TA2609>. The azoniaspirocycle ylide 5 is formed by trapping of the metallocarbene precursor (Equation 14). Heating the ylide in toluene then gave indolizidinone 55 as a single diastereoisomer. 

Carbonyl insertions into metallocarbenes have previously been observed for several different metals, including iron48 (see Section VI,C) and manganese.49 Indeed, carbonyl insertions into chrominum and tungsten diphenyl-carbenes have been shown to be viable processes.50 Most importantly, Wulff has isolated51 an 774-vinylketenecobalt (I) complex from the reaction between a cobalt carbene and an acetylene, a transformation that necessitates such a carbonyl insertion (see Section V,B). 

Hydrocracking, 30 48-52 behavior, thermal, 29 269 catalytic, 26 383 deethylation, 30 50 demethylation, 30 50 metallocarbene formation, 30 51-52 of f -decane, 35 332-333 primary coal liquids, 40 57 procedure, 40 66-67 product distribution, 30 49 reactions, over perovskites, 36 311 suppression by sulfur, 31 229 zeolite-supported catalysts, 39 181-188... 

The mechanistic similarity between olefin and alkane metathesis, even if intuitive, has needed time to become obvious in the laboratory for various reasons olefin formation starting from alkanes is thermodynamically disfavored, especially at low temperature, and, as a consequence, the proposal of olefinic intermediates was not obvious. Several facts lead to the metallocarbenic mechanistic assertion ... 

Early work in this area has involved the investigation of the olefin metathesis activity of [W( Bu)(CH 2Bu)J supported on silica or other oxide supports [65, 66]. While highly active, these systems do not contain a metallocarbene [40, 67]. For instance, the silica-supported system has been characterized as [ iOW( Bu)(CH 2Bu)J, and therefore it is very likely that the propagating metallocarbene is generated in situ even if it is not clear how it is formed. [ iOMo( Bu)(CH 2Bu)J also displays similar reactivity towards olefins, but is more sensitive to functional groups [67, 68]. For tantalum, despite the presence of a weU-defined metallocarbene in [( iO)Ta(=CH Bu)(CH 2Bu)J, this system is... 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

In all of the examples cited in Section 1.2.2.3.2.3.1, the diazo compounds are arranged such that none has a 3-hydride available. It could be expected that if a simple a-diazo ketone with fi-C — H bonds were exposed to the rhodium catalyst, metallocarbene formation would proceed as usual, but that /S-hydride elimination would compete with the desired 1,5-insertion. Such a /3-hydride elimination could, in fact, be viewed as a 1,2-insertion, i.e., 1 to 2. 

As discussed above, accumulated data demonstrate that the catalyst, the substrate structure, and other competing metallocarbene pathways significantly affect the ylide formation and the subsequent rearrangement process. West and co-workers have recently studied selectivity in rearrangement via five- or six-membered oxonium ylides by... 

Coperet, C. Lefebvre, F. Basset, J.-M. Well-Defined Metallocarbenes and Metallocarbynes Supported on Oxide Supports Prepared via Surface Organometallic Chemistry. In Handbook of Metathesis, Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2003 Vol. 1, pp 190-205. 

Most cyclic monomers of interest in the field of composites happen to be heterocyclic in nature. Polymerizability of some monomers is summarized in Table 1.1. Ring opening polymerizations invariably follow ionic mechanisms, although a few are known to proceed via the free radical route and some via metathesis involving metallocarbene intermediates. 

At present the accepted mechanism is a nonconcerted, nonpairwise process involving metallocarbenes that reversibly react with alkenes to yield metallacyclo-butane intermediates 48,64,68... 

Further support came from the isolation and use of metallocarbene complexes7,14,30,48,49 and metallacyclobutanes54,70 as catalysts in olefin metathesis. 

It is generally accepted that ring-opening polymerization of cycloalkenes proceeds via metallocarbenes. Although less extensively studied, metathesis of alkynes... 

In another model, the stereoselectivity is predetermined by steric interactions between the metal and/or its ligands and alkene substituents arising during the olefin coordination to the metallocarbene.91 These steric factors govern the orientation of alkene approaching the metallocarbene in a way to minimize alkyl-ligand repulsion. Compounds 5 and 6 depict favored cis-cis and trans-trans orientations, respectively. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

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