Acylcobalt complex

A second interfacial exchange reaction of the o-acylcobalt complex with hydroxide ion leads to the production of the alkanecarboxylate anion, which migrates into the aqueous phase, leaving the cobalt tetracarbonyl anion in the organic phase for subsequent reaction (Scheme 8.2). Optimum yields of the carboxylic acids are obtained with ca. 40 1 ratio of the alkyl halide to dicobalt octacarbonyl. Co(Ph,P)2Cl2 can also be used and has the advantage that the cobalt can be recycled easily [5]. 

The n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

Acylcobaltate complexes, [RCCo(NO)(CO)P(C6H5)3] Li+ (2). These are prepared by reaction of RLi with 1 in THE They are stable at -40° but decompose at 25°. The acyl group is transferred to a, 3-enones, quinones, and allylic halides.1... 

Metal carbonyls Acylcobaltate complexes, 101 Arene(tricarbonyl)chromium complexes, 19... 

Tricarbonyl(naphthalene)chromium, 19 Trimethylsilyl chlorochromate, 327 Cobalt Compounds Acylcobaltate complexes, 101 [Bis(salicylidene-y-iminopropyl)methyl-amine]cobalt(II), 41 Cobalt(II) chloride, 249 Cobalt zeolites, 296 Dicarbonylcyclopentadienylcobalt, 96 Di- x-carbonylhexacarbonyldicobalt,... 

The Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

A second example is the reaction of the acylcobalt complex with hydrogen equivalents and the successive reductive elimination of the aldehyde (step 7). This reaction was studied in great detail and has been controversial [93]. For this very last step of the hydroformylation cycle several pathways may be imagined. The two most plausible pathways are shown in eq. (4). 

Mirbach declared the reaction of the acylcobalt complex with HCo(CO)4 to be the minor pathway, whereas the reaction with H2 dominates the catalytic cycle in the hydroformylation of 1-octene and cyclohexene. On the other hand, Marko found that the hydridocobalt complex reacts 12 times faster with the acylcobalt complex than does hydrogen. Undoubtedly, under industrially applied reaction conditions the hydrogenolysis of the acylcobalt complex is effected exclusively by hydrogen. Yet, the remaining hydridocobalt species is still in question besides HCo(CO)3, the cobalt cluster HCo3(CO)g is discussed [94]. 

An example is the hydroformylation reaction of cyclohexene catalyzed by the unsaturated compound HCo(CO)3 which is formed under reaction conditions from the precursor HCo(CO)4. Following the usual mechanism (see, e. g., [18]), the catalytic cycle is depicted in Scheme 1. Since the oxidative addition of H2 to the acylcobalt complex is the rate-determining step in this case the rate equation follows eq. (2) (cf. Section 2.1.1) ... 

Although the bimolecular process involving reaction of the acylcobalt complex with HCo(CO)4 occurs readily under stoichiometric conditions, the low... 

A further example of skeletal rearrangement is provided by the equilibrium rearrangement of acylcobalt complexes ... 

The major species in the catalytic system with added phosphine is different from that in the system lacking added phosphine. Infrared spectroscopic analyses under steady-state conditions for hydroformylation catalyzed by the phosphine-modified complexes provide no evidence for the accumulation of an acylcobalt complex only phosphine-substituted cobalt carbonyl dimers and hydride complexes are observed.- " ... 

Based on the above experimental observation combined with the assumption of 16-electron reactive intermediates, Heck and Breslow proposed a mechanism for the cobalt-catalyzed olefin hydroformylation, which gives a qualitative explanation of the catalytic aldehyde formation by a sequence of hydrido-, alkyl-, and acylcobalt complexes according to Scheme 1 (99). 

Scheme 15 Alternating copolymerization of imines with carbon monoxide catalyzed by acylcobalt complex.

---