Cobalt hydridic nature

As shown in Figure 3, a positive p-value (+0.92) was observed in the hydrogenation of substituted benzaldehydes, giving strong support to the postulation by Heil and Marko that the rate determining step in the formation of alcohol (Mechanism 2) is the hydride addition step. It is therefore suggested that coordination of amine to rhodium increases the hydridic character of the Rh-H bond, much the same as is postulated in cobalt-tributylphosphine complexation (20). The differing effect of amine on rhodium (promoter) and on cobalt (inhibitor) is attributed to the more hydridic nature of a Rh-H bond as compared with the very protonic HCo(CO)4. Addition of amines to HCo(CO)4 results in formation of inactive species similar to I. 

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

Co(OAc)2 in the presence of sodium hydride and a sodium alkoxide has been used to catalyze the carbonylation of aryl bromides, giving mixtures of carboxylic acids and esters, again at normal pressure. When amines were present, amides were formed. Unfortunately, nothing is known about the nature of the cobalt complexes involved. 

For the purposes of this review, it is important to recognize the dual nature of cobalt macrocycle complexes. Both species, LCoinH and LCo1, can add to double and triple bonds forming alkyl and alkenyl cobalt chelates but the products are different. The Co111 hydride reaction occurs in a Markovnikov addition while LCo1 provide anti-Markovnikov products. It is believed that eqs 21 and 22 explain the difference. 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

Proving the individual steps of the catalytic cycle in rhodium-catalyzed hy-droformylation is much less elaborate than in the cobalt case. Therefore, the nature of complexes involved in the catalytic cycle has been deduced mostly from the kinetics of the aldehyde formation and from spectroscopy of the reaction solutions. In situ infrared and NMR spectroscopy revealed so far only the main resting states of the catalyst being the pentacoordinate hydride- and the pen-tacoordinate acylrhodium complexes. The concentration of the postulated active intermediates in equilibrium with the resting states is obviously too low for direct observation. The main support of the involvement of 16-electron complexes is the negative effect of carbon monoxide and phosphine concentration on the rate of aldehyde formation. 

The microstructures are influenced primarily by the nature of the alkylaluminum compound. With triethylaluminum the portion of trans-, 4 double bonds reaches a relatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl) aluminum hydride yield cis-, A contents as high as 99% [190]. Similarly, high cis-1,4 portions are obtained in the polymerization of 1,3-butadiene with j -allyluranium complexes. The osmometric measured mole mass ranges from 50 to 150 000, the molecular mass distribution between 3 and 7. The extremely high temperature-induced crystallization rate of uranium polybutadiene in comparison with titanium or cobalt polybutadiene corresponds to a greater tendency toward expansion-induced erystallization. A technical application, however, is in conflict with the costly removal of weakly radioactive catalyst residues from the products [132],... 

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