Hydroformylation cobalt catalyzed asymmetric

No substantial progress has been made in the field of cobalt-catalyzed asymmetric hydroformylation since our last review on this subject1S). Besides (+)-N-(l-phenyl-ethyljsalicylaldimine, which was originally used as asymmetric ligand6, a chiral catalyst formed in situ from HCo(CO)4 and (—)-DIOP has been employed 16) (Table 1). With the latter catalytic system, optical yields of 2.7% and 1.2% have been obtained in the case of (Z)-2-butene and of bicyclo[2,2,2]oct-2-ene, respectively. 

Only a few examples of cobalt-catalyzed asymmetric hydroformyl ations are reported21-1,1 134 using (R )-Sal as a chiral ligand. Low inductions of up to 1.9% ee are observed21-131. Higher optical yields of the opposite configuration are obtained, if the reaction is carried out in the presence of ethyl orthoformate45-134. 

The above results were reviewed in 1974 (5). Since then the main advances in the field have been the achievement of asymmetric hydro-carbalkoxylation (see Scheme I, X = -OR) using palladium catalysts in the presence of (-)DIOP (6), the use of other diphosphines as asymmetric ligands in hydroformylation by rhodium (7), and the achievement of the platinum-catalyzed asymmetric hydroformylation (8, 9). Further work in the field of asymmetric hydroformylation with rhodium catalysts has been directed mainly towards improving optical yields using different asymmetric ligands (10), while only very few efforts were devoted to asymmetric hydroformylation catalyzed by cobalt or other metals (11, 12) and it will be discussed in a modified form in this chapter. 

A number of metals catalyze the hydroformylation reaction, of which rhodium is by far the most active, Rh >> Co > Ir, Ru > Os > Pt. Platinum and ruthenium are mainly of academic interest, although L2PtCl(SnCl3) complexes with chiral ligands find use in asymmetric alkene hydroformylations.59 In most cases, and certainly in industrial processes, cobalt has now been replaced by rhodium. 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

Hydroformylation, also known as the oxoprocess, was first discovered in 1938 when it was found that in the presence of a cobalt catalyst, ethylene could be converted into propanal when treated under high pressures of CO and H2. Since then, many transition metal complexes have been found to catalyze hydroformylation, with cobalt, platinum, and rhodium catalysts being the most commonly used. However, cobalt catalysts have not featured prominently in asymmetric syntheses and are not discussed here, whereas platinum catalysts have been superseded in recent years. 

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