Metallacycles higher

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

However, we have demonstrated the formation of a metallacycle [(dipy)(Cl)Rh-(0-CH2-CHPh) or (dipy)(Cl)Rh-(0-CHPh-CH2)] from styrene and dioxygen. These intermediates could give rise to both styrene oxide and the carbonate. The higher reaction rate when starting from styrene and dioxygen with respect to the epoxide can be, thus, justified. High temperature (> 353 K) often cause decomposition of the catalyst. Two mutually free cis positions are necessary for the formation of the metallacycle, that interacts with carbon dioxide and yields the carbonate so, in the presence of Rh(diphos)2Cl and Rh(dipy)2Cl, no conversion at all into the carbonate has been observed, either starting from styrene or from styrene oxide. In the latter case, only a minor isomerization into acetophenone and phenylacetaldehyde has been observed. 

IUPAC nomenclature is generally followed . The cyclic structures are called heterometallacycles and dimetallacycles. When the ring is composed of a metal, a nonmetal, and a carbon, the rings are numbered in that order. When two metals are present, the higher atomic number metal takes precedence. Metallacyclic three-rings which contain a double bond may possess cyclopropenyl cation-like aromaticity and such structures have been proposed <84AG(E)89>. 

P methylene-carbon of the adamantyl NHC substituent, leading to product 52, which contains a new five-membered metallacycle. While adamantyl-chelated catalyst 52 demonstrated significantly lower activity at room temperature than 49 or 50, appreciable conversion could be obtained at higher temperatures. Catalyst 52 was tested in CM of allylbenzene and CDAB and, for the first time, high Z-selecfivity was achieved in the CM of allylbenzene and CDAB, where the desired product was formed in up to 90% Z-selectivity (corresponding to an ElZ ratio of 0.12). 

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