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. 

SCHEME 6.15 Cobalt-catalyzed asymmetric synthesis of alkylboronates [27]. 

Grigg and co-workers (383) found that chiral cobalt and manganese complexes are capable of inducing enantioselectivity in 1,3-dipolar cycloadditions of azomethine ylides derived from arylidene imines of glycine (Scheme 12.91). This work was published in 1991 and is the first example of a metal-catalyzed asymmetric 1,3-dipolar cycloaddition. The reaction of the azomethine yhde 284a with methyl acrylate 285 required a stoichiometric amount of cobalt and 2 equiv of the chiral ephedrine ligand. Up to 96% ee was obtained for the 1,3-dipolar cycloaddition product 286a. 

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 stereocontrolled synthesis of the /ra j-tetrahydrofuran units in Annonaceae acetogenins that relies on the Sharpless asymmetric dihydroxylation protocol is outlined in Scheme 60 <1999TA2551>. In the first step, the disubstituted double bond of the starting material is dihydroxylated followed by monoprotection as a methoxymethyl ether. Einally, a cobalt-catalyzed oxidation using molecular oxygen as oxidant furnishes the /ra j-tetrahydrofuran. 

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. 

Starting from optically active nitriles, Botteghi and co-workers [32] have applied the cobalt-catalyzed reaction for the prepartion of optically active 2-substituted pyridines (eq. (8)). The chiral center is maintained during the alkyne-nitrile co-cyclization reaction. This reaction has recently been extended to the synthesis of bipyridyl compounds having optically active substituents [33] and provides an access to chiral ligands of potential interest in transition metal-catalyzed asymmetric synthesis. 

The isomerization of allylamines could be more selective compared to their oxygen homologues due to the higher coordination property of nitrogen. This assumption was clearly exemphfied by the discovery of the cobalt-catalyzed isomerization of allylamines to enamines. With the introduction of cationic rhodium complexes of BINAP, the reaction has became one of the most successful asymmetric reactions [32]. 

Diastereoselective asymmetric induction in cobalt-catalyzed cyclopropanation is also reported for the cyclopropanations of (-)-dimenthyl and (-)-dibornyl fumarates with geminal dihalides28. 

Paquin, J.F., Defieber, C., Stephenson, C.R.J., and Carreira, E.M., Asymmetric synthesis of 3,3-diarylpropanals with chiral diene-rhodinm catalysts, J. Am. Chem. Soc. 127 (31), 10850, 2005. Ritter, T., Kvaemo, L., Werder, M., Hanser, H., and Carreira, E.M., Heterocyclic ring scaffolds as small-molecule cholesterol absorption inhibitors, Org. and Biomol. Chem. 3 (19), 3514, 2005. Waser, J., Gonzalez-Gomez, J.C., Nambn, H., Hnber, P., and Carreira, E.M., Cobalt-catalyzed hydro-hydrazination of dienes and enynes access to allyhc and propargylic hydrazides, Org. Lett. 7 (19), 4249, 2005. 

Chang and co-workers developed an intramolecular cobalt-catalyzed cyclotrimerization of both the symmetric and the asymmetric nitrile-diyne... 

Highly stereoselective arylation reactions were reported, as shown in Equation 5.46. With an optically active diamine ligand, a modest asymmetric induction was observed (Equation 5.47). An asymmetric synthesis of a synthetic prostaglandin AH 13 205 was accomplished using the diastereoselective cobalt-catalyzed cycliza-tion/arylation sequence as key step [55]. 

Although there are many reports indicated the potential of cobalt-catalyzed aldol and reductive aldol reactions (Scheme 5) (56-59), little attention has been paid to the catalytic asymmetric aldol reaction so far. However, a chiral rhodium... 

The diphenylphosphoryl azide (DPPA) (38) has been employed as a nitrene source in the presence of a cobalt(II) porphyrin [Co(Por)] system, catalyzing asymmetric olefin aziridination reactions (Scheme 11). iV-phosphorus-substituted aziridines (39) have been formed in these reactions in moderate to high yields and good enantioselectivities. 

The synthesis of chiral racemic atropisomeric pyridines by cobalt-catalyzed [2 + 2 + 2] cycloaddition between diynes and nitriles was reported in 2006 by Hrdina et al. using standard CpCo catalysts [CpCo(CO)2, CpCo(C2H4)2, CpCo(COD)] [34], On the other hand, chiral complexes of type II were used by Gutnov et al. in 2004 [35] and by Hapke et al. in 2010 [36] for the synthesis of enantiomerically enriched atropisomers of 2-arylpyridines (Scheme 1.18). This topic is described in detail in Chapter 9. It is noteworthy that the 2004 paper contains the first examples of asymmetric cobalt-catalyzed [2 - - 2 - - 2] cycloadditions. At that time, it had been preceded by only three articles dealing with asymmetric nickel-catalyzed transformations [37]. Then enantioselective metal-catalyzed [2 -i- 2 - - 2] cycloadditions gained popularity, mostly with iridium- and rhodium-based catalysts, as shown in Chapter 9. 

Subsequently they utilized the cobalt-mediated asymmetric synthesis of [7]helicene-like molecules described above in helicene-based phosphite ligand synthesis. The application of ligand P,S)-2 to rhodium-catalyzed asymmetric hydro-formylation revealed excellent regioselectivity, although the enantioselectivity was low (Scheme 10.14) [15]. For iridium-catalyzed asymmetric allylic amination, the... 

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