Cobalt porphyrins features

The catalytic chain-transfer (CCT) process displays all of the features characteristic of typical, uncatalyzed chain transfer other than taking place at a rate competitive with chain propagation. Thus, the rate of polymerization at low conversions is independent of the concentration of the cobalt porphyrin (Figure 1) while the molecular weight, Mn, decreases linearly by over 2 orders of magnitude with increasing concentration of cobalt catalyst (Figure 2). As expected for a typical polymerization, the rate of polymerization increases linearly with the square root of the concentration of the azo initiator and no polymerization occurs in the absence of the initiator. 

Steady-state, free-radical methods of LCoD generation were developed.197 The methods are versatile and work for LCo like cobalt porphyrins that are not readily reduced by borohydrides. The use of tribu-tyltin hydride has also been reported.235 The initial approach employed AIBN-cfo. Using this deuterated radical source, cis addition of the resulting LCoD was demonstrated to be the predominant mode of reaction for maleic anhydride and other cyclic olefins such as cyclohexene and 2,5-dihydrofuran. Selectivity depended upon temperature, and this important feature will be discussed below. Unfortunately, AIBN has a limited thermal operating window of 50—70 °C. Lower or higher temperatures would require the nontrivial synthesis of different deuterated azo initiators. To circumvent this problem, a second steady-state free-radical approach was developed. 

Interestingly, cobalt porphyrin catalysts tend to prevent carbene dimerization reactions, and allow cyclopropanation reactions with electron-deficient alkenes. This feature illustrates the more nucleophilic behavior of the carbenoid species formed as compared to typical electrophilic Fischer carbenes. The enhanced nucleophilic character of the carbene reduces its tendency to dimerize and allows reactions with more electron-deficient olefins. 

In CCT a metalloradical reversibly abstracts H from the chain-carrying radical and starts a new chain. Early work on CCT during radical polymerizations employed cobalt porphyrins during the polymerization of methyl methacrylate, and was carried out in the USSR (Smirnov, Marchenko in 1975 Enikolopyan in 1977). Gridnev discovered in Moscow in 1979 that cobaloximes were effective CCT catalysts, then moved to the US in 1992 (Wayland laboratory. University of Pennsylvania) and joined DuPont in 1994. The basic features of CCT have been described in a series of patents (at first Russian, then largely DuPont) that appeared in the 1980s [71], and in a comprehensive review that appeared in 2001 [72]. The mechanism in Scheme 1.8 has become generally accepted, and CCT has been successfully applied to other monomers (styrene, methacrylonitrile) and comonomers. 

If our postulates are correct the most interesting feature of P-450 is the manner in which the protein has adjusted the coordination geometry of the iron and then provided near-neighbour reactive groups to take advantage of the activation generated by the curious coordination. Vallee and Williams (68) have observed this situation in zinc, copper and iron enzymes and referred to it as an entatic state of the protein. It is also apparent that some such adjustment of the coordination of cobalt occurs in the vitamin B12 dependent enzymes. As a final example we have looked at the absorption spectra of chlorophyll for its spectrum is in many respects very like that of a metal-porphyrin. This last note is intended to stress the features of chlorophyll chemistry which parallel those of P-450. 

With the name corrinoid we will refer to a class of compounds which have a molecular skeleton similar to that of the cobalt complex present in Vitamin B12 of which the main characteristics are the direct link between two pyrroles and the fully saturated p positions. These major features afford a macrocycle which is more contracted than a porphyrin and without an aromatic n system. 

Pertinent to this discussion are recent studies on porphyrin- r-oation radicals derived from magnesium and cobaltic octaethyl porphyrins (171, 172), and zinc and magnesium tetraphenyl porphyrins (J71). In all cases, the optical spectra of such radicals share features also found in Compound I (Fig. 7). These are (a) a decrease of w-w transitions associated with the Soret band and (b) the appearance of bands between 600 and 700 nm. The chief objection to the proposal implicating a free radical moiety stems from the absence of a distinct EPR signature for Compound... 

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... 

Cobalt macrocychc, or pseudo-macrocyclic, complexes have proven among the most widespread cobalt-based proton reduction catalysts. For example, Fisher and Eisenberg demonstrated in 1980 that some cobalt tetraazamacrocyclic complexes are active in both CO2 and reduction [75]. Similarly, porphyrins have been extensively investigated. Nocera and coworkers showed that cobalt(II) hangman porphyrins can catalyze proton reduction with less overpotential and weaker acids than their standard porphyrin cousins (Fig. 13d) [85, 86]. Both features are thought to be a result of the enhanced proton donation by the carboxylic acid of the hangman substituent. Bren and coworkers showed that the biologically derived cobalt-substituted microperoxidase-11 is stable with a turnover number of 25,000, but the catalytic rate is relatively low at 6.7 s [87]. 

A study of labeled compounds derived from ethylcobalt(m) oetaethylporphyrin and other porphyrins indicated that the NMR resonances of the carbon atoms bound to cobalt were quite broad and were upheld shifted. The anomalous features of the NMR spectra were ascribed to the quadrupolar cobalt nucleus, and to the paramagnetic contact shifts that arose from thermal population ground state instead of agostic bond. ... 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

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