Metal allyl complexes

3-cyclohexadiene bound to Co. Repeated oxidative addition of hydrogen followed by hydride transfer transforms the carbocycle into cyclohexene and finally cyclohexane. If a terminal olefin like 1-hexene is present in the reaction medium, it will compete with cyclohexene in the last step of the cycle, so that the final product contains varying amounts of cyclohexene and cyclohexane in the absence of added olefins, the only reaction product is the fully saturated hydrocarbon. 

By contrast, some ruthenium hydride clusters reported by Siiss-Eink are thought to hydrogenate arenes through r -bondcd intermediates, as represented in Fig. 

The proposed mechanisms, however, have not yet been clearly established and more detailed studies under catalytic conditions [46-47] would be welcome. These ruthenium clusters have also been shown to be active for arene hydrogenation in a biphasic medium using ionic liquids as the solvent, where the catalysts is retained, allowing its easy recovery and recycling [48]. 

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Alkali Metal Derivatives of Metal Carbonyls, 2, 1S7 Alkyl and Aryl Derivatives of Transition Metals, 7, 1S7 Alkyl cobalt and Acylcobalt Tetracarbonyls, 4, 243 Allyl Metal Complexes, 2, 32S... 

Comparison of IR and ED data for the C3H5 radical with IR and X-ray data of ir-allyl metal complexes shows that the formation of such complexes... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Catalytic Asymmetric Alkylation of n-Allyl Metal Complexes... 

Bis-allyl-metal complexes exist in three basic types of structural isomers bis-TT-ally 1-, tr, o-bis-allyl- and bis-o-allyl-moieties. Taking this into consideration and applying out principle results in the analogies shown in Scheme 2.5-1. These analogies are only complete if one con-... 

Allylic substitution using hard nucleophiles proceeds through a different mechanism. Instead of attacking the allyl group of the 71 allyl-metal complex, hard nucleophiles attack the metal first and the product is subsequently formed by reductive elimination. Nickel(O) complexes have often been used for this purpose. Reports of good enantioselectivities in this type of reaction are limited. 

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Sachtler [270] notes that the 7r-allyl complex can be attached to a metal ion or to an oxygen anion but doubts that a 7r-allyl metal complex can be stable at the high temperatures normally used. He draws attention to the fact that, in the case of aromatic oxidations, benzoates, maleinates, etc. are observed spectroscopically, indicating that a carbon—metal bond is not formed. 

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