Acyl-transition metal species

Acyl transition metal species such as Collman s reagent and the Corey-Hegedus nickel carbonyl system, although also formally acyl anions, have not found wide use in synthesis, possibly... 

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

If the acylpalladium species can undergo further CO insertion to give a-ketoacylpalla-dium species and the complex should be attacked by a nucleophile, the double carbonylation process might be realized. However, the CO insertion into the acyl-transition metal bond seems to be a thermodynamically unfavorable process and the double CO insertion process is considered not operative for Pd-catalyzed double carbonylation of aryl halides. ... 

The mechanism of this catalytic dihaptoacyl isomerization is proposed to involve initial insertion of the acyl carbon atom into the Th-H bond, followed by -hydride elimination. This process is illustrated in eq. (11). There is precedent in recent transition metal chemistry for the formation of stable MOC(R)HM species analogous to 13. from MH and M (r -cOR) precursors (61). 

Direct evidence for a mechanism involving a single mode of chain growth as depicted in Scheme 1 came from our studies on the copolymerization reaction in alcoholic solvents35). The mechanism, as outlined in Scheme 1, involves the formation of Pd-acyl species as intermediates at every other step in the propagation sequence. Since the formation of esters through the reaction of transition metal-acyls with alcohols is well-precedented, it should be possible to intercept the propagation sequence if the polymerization was carried out in the presence of alcohols, Eq. (13). 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

The method of combined enzyme- and transition metal-catalyzed reactions widely applied to the DKR of secondary alcohols has also been applied to the DKR of a-hydroxy acid esters rac-1. The principle is based on the enantioselective acylation catalyzed by Pseudomonas species lipase (PS-C from Amano Ltd) using p-Cl-phenyl acetate as an acyl donor in cyclohexane combined with in situ racemization of the non-acylated enantiomer catalyzed by ruthenium compounds [7]. Under these conditions, various a-hydroxy esters of type 1 were deracemized in moderate to good yields and high enantioselectivity (Scheme 13.2). 

The range of reactions which are possible at the ring nitrogens of imidazole and benzimidazole has been outlined (Section 4.02.1.3). As they are the most basic of the azoles, imidazoles are particularly prone to such reactions as protonation, alkylation and acylation, and they readily become involved in complex formation with transition metals. Benzimidazoles, too, are reactive, but less so than the non-condensed species. 

Reaction of [Mn(R)(CO)j] with neutral nucleophiles is by far the most widely studied type of reaction for [Mn(R)(CO)s] compounds. The reaction usually involves addition of the neutral neucleophile, L, and is accompanied by CO insertion/alkyl migration to form an acyl species [Eq. (29)]. L is usually a tertiary phosphine (PR3), an alkylated amine (RNH2), or free carbon monoxide. Besides being a carbon-carbon bond forming reaction of fundamental importance, alkyl migration reactions of transition metal alkyl species have direct relevance to catalysis, especially for the 0X0 or hydroformylation process (2), the Monsanto acetic acid synthesis (2), and the synthesis of ethylene glycol (94). 

The highly electronegative metal ion affords the appropriate empty orbitals for the stabilization of the complex anion in the transition state. Both late and early transition metal alkyls are prone to this reaction, but its occurrence had to be particularly, invoked in the case of the early transition metals. Many similar reactions, such as the reaction of metal alkyls with other FIX compounds, could be described as if they followed this pathway, but the use of the term a-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. Two reactions have been depicted in Fig. 4.34. There are, of course, borderline cases when the reacting hydrocarbon is acidic, as in the case of 1-alkynes, a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also... 

The palladium-catalyzed acylation of siloxycyclopropane furnishes a 1,4-di-carbonyl compound. A C-C bond of the three-membered ring is cleaved by an electrophilic attack of a palladium(II) species [103]. An analogous electrophilic ring opening of siloxycyclopropane was induced by various so-called ligand free transition metals such as Ag+ and Cu2+ [104]. 

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