Allyl complexes, transition metal

Takacs JM (1995) Transition metal allyl complexes Telomerization of dienes. In Hegedus LS (ed) Comprehensive organometallic chemistry II, vol 12. Pergamon Press, Oxford... 

Oppolzer, W. Transition Metal Allyl Complexes Intramolecular Alkene and Alkyne Insertions. In Comprehensive Organometallic Chemistry II Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 905-921. 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

The transition-metal allyl complexes are air- and temperature-sensitive solids Cr(allyl)3, m.p. ca. 70° Ni(allyl)2, m.p. ca. 0°. The infrared spectrum of both compounds indicates that the bonding of the allyl group to the metal involves r electrons (the olefinic bond appearing at 1520 and 1493 cm.-1, respectively) they can be identified by their mass spectra. 

Early transition metal allyl complexes have an enormous practical importance as either catalytic precursors or stoichiometric reagents in organic synthesis [103-108]. In the majority of the Group 4 complexes containing the allyl moiety, the metals exhibit the higher oxidation state (+4). Very few of these compounds are available in the literature with a +3 oxidation state, presumably because of their paramagnetic nature (reactivity) and difficulty in their handling. 

Although some Ti(III)-allyl complexes have been fully characterized spectroscopically [ 109,110], well-characterized Zr(III)-allyl compounds in the solid state have not been reported in the literature. On the basis of previous results obtained in our laboratory, it was very attractive and conceptually important to find a route to synthesize simple monomeric Group 4 early transition metal allyl complexes and to compare their catalytic activity to that of the well-characterized heteroallylic octahedral early transition metal compounds. Here we report the synthesis and solid-state X-ray structural characteristics of a Zr(III) bulky bis-allylic complex, and its catalytic activity in the polymerization of a-olefins [111]. 

Allylmagnesium halide and lithio reagents are available from allyl halides by lithiation and Mg reaction techniques. These active reagents combine with binary or complex transition-metal halides to form transition-metal-allyl complexes, including both homoleptic and ligand-substituted tj -allyl complexes with substituted and unsubstituted allyl groups. 

Harrington PJ (1995) Transition metal allyl complexes trimethylene methane complexes. In Abel EW, Stone FGA, Wilkinson G (eds) Comprehensive organometallic chemistry, vol 12, Chap 8.4. Pergamon, Oxford, p 923... 

A wide range of transition metal-allyl complexes are known to react with many types of nucleophiles. In most cases, these reactions occur between cationic allyl complexes and amines or stabilized, anionic carbon nucleophiles. The reaction typically occurs between the nucleophile and the form of the allyl complex, and attack usually occurs at the face of the allyl ligand opposite the metal. However, there are exceptions to these trends. For example, several experiments suggest that unstabilized carbon nucleophiles react first at the metal center, and C-C bond formation occurs between the alkyl and the allyl group by reductive elimination. In addition, a recent study has shown through deuterium labeling that attack of malonate anion on a molybdenum-allyl complex occurs with retention of configuration. ... 

J. M. Takacs, Comp. Organomet. Chem. II, 1995, 12, 785-796. Transition Metal Allyl Complexes Telomer-ization of Dienes. 

General structural aspects of transition metal allyl complexes have been reviewed." A variable-energy photoelectron study of M( 7 -G3H5)2 (M = Ni, Pd, Pt) using He(l), He(ll), and monochromatized synchrotron radiation has been carried out, allowing a more confident assignment of the orbital energies (supported by MS-X, calculations), mainly based on the presence of /ra r-isomers. 

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... 

Finally, the hybridization of the carbon atom also has a marked effect on its willingness to attach to the transition metal. Allyl or benzyl halides undergo oxidative addition faster than aromatic or vinyl halides. The least reactive are alkyl halides which require the use of nickel(O)9 complexes or highly active catalyst systems.10 If we start from an optically active substrate, then the oxidative addition usually proceeds in a stereoselective manner. 

A transition metal cluster complex [Ru3H(CO)n] catalyzes isomerization of allylic alcohols to saturated aldehydes [11]. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

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