Metal-allyl complexes Preparation

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The general interligand C—C coupling reaction shown in Eq. (8) for (metalla-/3-diketonato)BF2 compounds also occurs directly with metalla-/3-diketonate anions, thereby precluding the need to prepare the neutral difluoroboron complexes (53). As a one-pot synthesis, metal carbonyl acetyl compounds can be converted to neutral 77-allyl complexes [Eq. (12)]. 

Similar reactions can be utilized to prepare allyl complexes of platinum and palladium. In this case, the product can exist in two isomers as described earlier. Analogous reactions can be used to prepare the tris allyl complexes of several metals. 

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

Palladium forms the largest series of ir-allylic complexes of any metal, and many of these complexes are readily prepared. The majority are chloro-bridged complexes of the type [PdCl(all)]2, where all = an allylic radical. 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

Up to now, the most used metal complexes to prepare catalysts by anchoring/grafting have been orga-nometallics, and especially, metal allyls it appears possible to obtain versatile catalysts containing metal ions with controlled dispersion, various coordination numbers, various ligands, and various but uniform oxidation states, in contrast with impregnated samples. 

Palladium forms a more extensive series of 7r-allyl complexes than any other transition metal. In many cases these molecules are thermally stable, air-stable crystalline sohds. In addition to stoichiometric 7r-allyl compounds, many palladium 7r-allyls are prepared under catalytic conditions as intermediates in palladium-catalyzed organic synthetic reactions. 

The allylmagnesium halide in ether is slowly added to a suspension or solution of the metal halide at 0°C or lower. Addition at low T improves yields and is essential for the synthesis of thermally unstable, homoleptic allyl complexes. The complexes are isolated by evaporation of the reaction mixture and extraction into an aromatic or alkane solvent. Purification is effected by crystallization or sublimation. When stable allyl compounds are prepared, an aqueous wash of the reaction mixture can be used to remove Mg salts and xs RMgX or -Li reagent the crude products can then be isolated from the resultant organic layer. 

Homoleptic allyl complexes and their derivatives are prepared by reacting allylmag-nesium chloride with metal halides . 

A general synthetic method for allyl complexes using allylmercuric halides (Table 6 ) is available. The reactions generally proceed at RT and give moderate to good yields. Several classes of allyl metals have been prepared in this way with a variety of substituents on the allyl fragment. 

Several new types of allylic complexes have recently been prepared. Atkinson and Smith prepared Tr-allylic complexes from 2,2,4-trimethy-pent-3-en-l-ol of the type [(CHg—CHMeCH)CMe2CHaOH]PdCl 2, in which the alcohol function is not coordinated to the Pd(II) (10). In contrast, the Rh(III) and Pt(II) complexes are olefin complexes, with the alcohol function complexed to the metal. 

Few metal-metal-bonded metal carbonyl complexes contain three-electron donor group IVB ligands. Dimers containing allyl ligands are known [e.g., (C3H5NiCl)2], but these do not contain metal-metal bonds. These are prepared in high yield M = Pd will... 

Anchoring of metal complexes through interaction with surface hydroxyl groups of inorganic supports continues to be of interest. Studies with catalysts prepared with allyl, carbonyl, chloride, and ethoxy ligands have been reported. Kuznetsov and co-workers conclude that the precursors of metathesis-active centres of surface metal complexes, prepared by anchoring allyl and ethoxy compounds of Mo, W, and Re to silica, are co-ordinatively unsaturated metal ions with oxidation number +4. Metathesis activity of the surface species depends on the ligand environment of the metal ion. 

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