Acyl cobalt complex

I.3.4.2.5.2. Other Transition-Metal-Acyl Complexes 1.3.42.5.2.1. Chiral Cobalt-Acyl Complexes... 

The procedures used to prepare and resolve these chiral cobalt-acyl complexes are as follows ... 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

The cyclic cobalt-acyl complex 1 undergoes a-proton abstraction from the least-hindered face opposite the phosphane ligand upon treatment with lithium hexamethyldisilazide at 0 °C to generate the chiral enolate species 283. Treatment of 2 with primary iodoalkanes diastereoselec-tively produces the alkylated cobaltocycles 3 also via attack of the reagent on the face opposite the bulky phosphane. 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

Insertion of CO into RCo(CO)4 to give acyl complexes is facile (see Mechanisms of Reaction of Organometallic Complexes). This reaction proceeds at 1 atm of CO at ambient temperature. The alkyl-acyl equilibrium lies far towards the acyl complex. The activation energy for the process has been calculated by MO methods to be about 85 kJmol (see Molecular Orbital Theory). Thus, RCo(CO)4 complexes can only be obtained under conditions of low CO pressure, which in turn opens the way for CO dissociative decomposition. Cobalt acyl complexes can be derivatized in several ways to form various products (Scheme 5). 

Reaction 6a may proceed directly without intermediates as an hydrogenolysis reaction as has been found experimentally for ruthenium analogues [32] and as has been found to be a likely pathway for cobalt acyl complexes on the basis of ab initio calculations [33]. Alternatively, reaction 6a may proceed via an oxidative... 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Because of structural similarities to the vitamin B,2 coenzyme, cobalt(III) complexes of the type RCo(L4) or RCo(L2)2 [L4 = bis(salicylaldehyde)-ethylenediiminate and L2 = dimethylglyoximate, inter alia] have been actively investigated 40, 76, 77, 125). Corresponding acyl complexes have been synthesized 40, 76). However, neither the CO insertion into the Co—-R linkage nor the decarbonylation of the Co—COR moiety has been achieved (77, 125). A probable reason for this was presented in Section II. 

Before addition of the benzyl halide, the only carbonyl adsorption peak is found at 1900 cm, indicative of the cobalt tetracarbonyl anion. After addition, this band immediately disappears and peaks at 2000 cm l are observed. These most likely represent the corresponding acyl complex. Reaction with methoxide yields the product and regenerates the cobalt anion. In the absence of sufficient methoxide, the reaction requires attack by the much... 

ATR-HP IR spectroscopy has also been used to follow the cobalt-catalysed carbonylation of epoxides to give lactones or polyesters [46]. Addition of excess propylene oxide to [HCo(CO)4] (generated in situ by protonation of [Co(CO)4] ) under 20 bar CO was found to give an acyl complex, [Co(C(0)CH2CH(OH)Me)(CO)4]. Depending... 

The overall response to the reaction variables is very similar in the carbonylation and reductive carbonylation reactions. This may indicate similar catalysts and reaction mechanisms. In the carbonylation reaction Co(CO) " was identified by its characteristic CO stretching frequency ( v(CO) r 1890 cm" as the dominant species present in high pressure infrared experiments carried out at 170 °C and 5000 psig. Similar results were obtained in the reductive carbonylation of methanol. It is known that Co(CO) " rapidly reacts with CH I to yield CH C(0)Co(C0) (J9) however, in the carbonylation and reductive carbonylation reactions acyl-cobalt complexes are not observed by infrared under catalytic conditions. This indicates that once formed, the acyl complex rapidly reacts as outlined by Equations 7 and 8. 

The mechanism of the reaction of the alcohol (or water) with the acyl complex to produce ester (or acid) and regenerate the cobalt hydride complex is not known. Because the reaction of the analogous manganese complex with alcohols is known to proceed through a hemiacetal-like complex, this mechanism has been written for the carboxylation reaction (equation 42). 

Reduction by nucleoi ilic attack of the original metal hydride has been own to occur under stoichiometric reaction conditions, for instance in the reduction of acyl cobalt complexes with CoH(CO) . Kinetic considerations, Imwever, make it unlikely that such a sequence plays a major role in the hydroformylaiion process. In fact, the rate-determining step, corresponding to the hydrogenolysis of the acyl complex, is slowed by an increase of carbon... 

Many industrially important catalytic processes use alkyl and acyl cobalt carbonyl complexes, which can be synthesized from (2 ). Acyl complexes produced after formation of... 

The rate equation 6.12 was derived without use of any short-cuts. With rules to be shown in Section 7.3.1, algebra could have been reduced. Specifically, the steps following the irreversible formation of the cobalt acyl (X5) could have been omitted (Rule 7.12) and the pairs of steps from X, to X3 and from X3 to X5 been consolidated each into a single step (Rule 7.24). This would have reduced the mathematics of conversion of the x-complex to the aldehyde to that of a pathway with only two instead of six steps. 

The alkyl complex reacts with carbon monoxide to apparently insert CO into the cobalt-alkyl bond (insertion reactions will be discussed later in this chapter) to give an acyl complex (containing a —C( = 0)R ligand) ... 

In reductive acylation and dimerization, the cathode is often superior to dissolving metal or radical anions reductants. So a, j6-unsaturated ketones or esters can be acylated in high yield to 1,4-dicarbonyl compounds at the mercury cathode [39], but the corresponding reaction with sodium in tetrahydrofuran (THE) fails [40]. On the other hand, reductive acylation of double bonds becomes possible in high yield, when vitamin Bj2 is used as mediator [41]. Here cobalt-alkyl complexes play a decisive role as intermediates. 

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