Cobaloxime, hydrogenation

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Several copper, silver, ruthenium, rhodium, and cobalt compounds (e.g., Ru-Cl3 aq, [RuC h(l)ipy) (bipy=2,2 -bipyridine), RhCl3 aq, fotx(dimelbylglyoxima-to)cobalt derivatives (cobaloximes), etc.) have been found to catalyze hydrogenations in aqueous solutions [9]. Although important for the early research into homogeneous catalysis, these catalysts did not gain synthetic significance. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

The dehydration and deamination reactions appear to operate in a parallel fashion and will be considered together. Schrauzer and Silbert propose a base-catalyzed cleavage of the carbon-cobalt bond in the B12-catalyzed diol dehydration reaction as shown in Fig. 17 (81), based on demonstrated lability of the beta hydrogens in alkyl cobaloximes with electronegative groups in this position. 

More successful asymmetric reductions have been based on amine (particularly alkaloid) complexes of bis(dimethylglyoximato) cobalt(II), also known as cobaloxime(II) and represented Co(dmg)2 (compound VII). Cobaloxime-chiral amine complexes have been used to catalyze the hydrogenation of both olefinic and ketonic substrates (Fig. 24). It has been determined that hydroxyamine modifiers, for example, alkaloids such as quinine, quinidine, and cinchonidine, are most effective. The highest optical purity obtained thus far has been 71%, observed for reduction of benzil in benzene solution at 10° using quinine as the... 

FIG. 24. Asymmetric hydrogenations with a quinine complex of cobaloxime(II) [Co (dmg)2 ]. % ee = Percent enantiomeric excess. 

Alkylated cobaloximes yield the corresponding dimeric species of alkyl radicals by photolysis under acidic conditions. But the BDHC complex with a hexyl or benzyl group at its axial site does not yield the corresponding dimeric species by photolysis (dodecane and bibenzyl, respectively). Consequently, the hydrogenation product must be obtained through the formation of a carbanion by heterolytic cleavage of the Co-C bond, followed by its protonation. 

The molecular structure of B after irradiation is shown in Fig. 21. Only the 1-cyanoethyl group was changed after irradiation. The inverted 1-cyanoethyl group takes a different conformation from those observed in the piperidine and pyrrolidine crystals the methyl group is replaced with the cyano group in the alaninate crystal, whereas the methyl group is replaced with the hydrogen atom in the piperidine and pyrrolidine crystals. Such an inversion mode has been found only in a crystal of (l-cyanoethyl)(3-hydroxypyridine)cobaloxime [42]. 

Espenson has found that the rhodoxime behaves very much like the cobaloxime in its association with iron(III) ion to form the bimetallic [Rh(DMG)(DMGFe)(H20)Me]2+. Its formation constant (equation 178) is 68.8 + 4.0 (for cobaloxime, Kf— 15.3 + 0.8912). This unusual cation can be considered to be derived from the structure shown in (63), in which X = Me, Y = H20, and one of the hydrogen-bonded hydrogens is replaced by an Fe3+. The presence of the iron adduct is readily signalled by an intense absorption near 440 nm (s ss 47 000), and a comparison of the kinetic and thermodynamic parameters shows a strong similarity to the previously studied Fe3+ adducts of cobaloxime.913 Even in the presence of forcing concentrations of Fe3+, there was no evidence of incorporation of a second iron(III) ion.912... 

A bimetal redox couple, zinc/cobaloxime, promotes hydropcrfluoroalkylation of electron-deficient alkenes, such as acrylates, acrylonitrile and methyl vinyl ketone, by perfluoroalkyl iodides and bromides, hydrogen replacing iodine or bromine. A typical reaction is the formation of2. ... 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

Cobaloximes with structure 9 have a BF2 bridge rather than the more usual hydrogen atom bridge between the two dioxime moieties. Such a modification leads to better stability of the cobalt11 oximes toward oxidation by oxygen by air. While of little consequence in an academic laboratory, it is impor-... 

It is clear that cobalt catalysts 10—44 are much less active than cobaloximes, generally by 2 orders of magnitude. It is concluded that the hydrogen transfer reaction is not diffusion controlled in their case. This difference in reactivity also suggests that some of the trends found for cobaloximes may not work for other cobalt chelates. Unfortunately, there have been few studies to this end. Most of the values of Cc in Table 4 were calculated having only one or two points on the Mayo dependence. For cases when Cc < 50, it is usually necessary to carry out ad-... 

The active LCo complexes indicated above can be used to test this theory. Porphyrins and phthalocya-nines have an O-shaped system which has a more extended -system than that in cobalamins, but it does not provide a substantial increase in reactivity. It should be noted that the hydrogen bonds of the cobaloxime catalysts are essentially as effective as 7r-bonds in continuing the effects of delocalization around the macrocyclic ring. This effect has been noted elsewhere.142 Catalyst 11 comprises an O-shaped -system. Replacement of one jr-bond with a a-bond in the analogue 13 significantly affects the catalytic properties since both complexes retain their O-shape with -conjugation. Additional replacement of "T-bonds with o-bonds leads to a complete loss of catalytic properties as chelates 13, 20, or 21 indicate. Chelate 22, cannot be a CCT catalyst because of the absence of interaction between the two jr-systems. Chelate 34 is an exception its molecular structure is similar to 21 and 13, but it catalyzes chain transfer with a measurable rate. A possible explanation of this phenomenon will be provided in section 3.7. 

Additional understanding of the role of. T-conjligation is provided by cobaloximes 6—8 and other CCT catalysts with hydrogen bonds which complete the macrocycle (i.e., 32 and 33). The common feature shared by a 7r-bond and these hydrogen-bonded systems is their ability to delocalize their electron density.131132 Resonance isomers with different bonding of the H or BF2 bridges to the oxygen atoms allow the electrons to delocalize around the equatorial plane as shown in Scheme 2. 

Subsequently, Cl was observed in polymerization of MMA with high concentrations of cobaloximes,77 293 phthalocyanines,14 136 294 295 and porphyrins.295 Cl is observed only when the cobaloxime was in the 2+ oxidation state and can catalyze chain transfer.296 Thus, Cl is a phenomenon that is closely connected to CCT. The first explanation involved a hydride (as in eq 31), where P is a polymer obtained from the propagating radical which has been terminated by hydrogen atom donation by Co111—H.161... 

Addition of H+ to any oxygen atom in the ligand interrupts the electronic ring current in cobaloxime, leading to lower effectiveness of the of newly formed cobalt chelate 93 to catalyze hydrogen transfer. A portion of the unreduced starting chelate can also be affected by the released hydrochloric acid to produce 94 and/or 95 (Scheme 7). 

Catalytically active soluble chiral metal complexes containing a relatively cheap metal such as Co give interesting enantioselectivity in the hydrogenation of 1,2-diketones . Bio dimethylglyoximato Co(II) 3 (cobaloxime), associated with a cocatalyst and possessing a chiral amine function such as quinine, quinidine, cinchonidine, ephe-drine, brucine, or 5-( - )-a-methylbenzylamine, forms a chiral system that induces preferentially one enantiomer, as in enzymatic systems ... 

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