Organocobalt -Complexes

Organocobalt 7r-Complexes.—In molecules of w-cyclo-octenyl- r-cyclo-octa-1,5-dienecobalt (43) the metal atom is sandwiched between the two organic ligands so that the midpoints of the C-C double bonds, the cobalt atom, 

Molecules of bis(w-cyclopentadienylmethoxoborinato)cobalt (45) possess crystallographic symmetry. The six-membered rings are distorted slightly from planarity, the average deviation of atoms from the mean plane being 

---

Sutton, P. W. Dahl, L. F. (1967) "The molecular structure of Co3(CO)g CCH3. A tricyclic organocobalt complex containing a metal-coordinated triply bridging aliphatic carbon atom, J. Am. Chem. Soc. 89,261-268. 

The formulas and names of these organocobalt complexes are usually given in the form [RCo(L4)X], e.g., [vinyl-Co(salen)py]. 

Only one optically active simple organocobalt complex has so far been reported, namely [RCo(DMG)2py], where R is the optically active 1-methylheptyl ligand 55). All cobalt corrinoids, where the ring has been... 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

A series of trivalent organocobalt complexes of the form [RCo(cyclam)(OH2)]+ have been reported, where R = Me, Et, Pr, CH2C1, CH2Br, CH2OMe, and CH2Ph.539 These compounds... 

Organocobalt complexes catalyze the cyelocotrimerization of acetylenes and nitriles, which affords pyridine and benzene derivatives (100). (Cyclo-pentadienyl)cobalt complexes such as CoCp(COD) favor pyridine formation (100), and modification of the Cp ligand has considerable influence on the activity of the catalyst and the chemo- and regioselectivity of the catalytic process (101). 

Thiapyran derivatives can be prepared from preformed organoiron and -cobalt complexes as in Scheme 160 the organocobalt complex (136) can be used as an intermediate in the synthesis of 1,2-dithia cyclopent-4-en-3-thione (See Scheme 122 in Section IV,H.)... 

Saturated ketones and aldehydes have been reduced to alcohols using Co2(CO)8-phosphine (P) mixtures at high H2 pressures, probably via the coordinatively unsaturated species HCo(CO)2P (187). A reassessment of experimental data on hydrogenation of aromatics (A) catalyzed by Co2(CO)8 under hydroformylation conditions has led to the suggestion that free radicals rather than organocobalt complexes are involved (188), e.g.,... 

This reaction leads to an organocobalt complex, RColnX2, which is reduced to the corresponding RConX. Owing to the low stability of the cobalt(I) species, the reaction proceeds in one step (equation 29). 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

---