Substrate anion cobalt complex

A poly(propylenamine) dendrimer (11, Fig. 6.37) functionalised with poly-(N-isopropylacrylamide) (PIPAAm) (see Section 4.1.2) was used as dendritic host for anionic cobalt(II)-phthalocyanine complexes (a, b) as guests, which are held together by supramolecular (electrostatic and hydrophobic) interactions [57]. These dendritic complexes were investigated as catalysts in the above-mentioned oxidation of thiols, where they show a remarkable temperature dependence the reaction rate suddenly increases above 34°C. One attempted explanation assumes that the dendritic arms undergo phase separation and contraction above the Lower Critical Solubility Temperature (LCST). At this temperature the phthalocyanine complex site is more readily accessible for substrates and the reaction rate is therefore higher. 

Scheme 24 displays an example designed by Giese et al.84 The glycosyl-cobalt complex dissociates and the glycosyl radical adds to styrene. This adduct couples to the cobalt(II) species. The coupling product is not isolated and forms mainly the alkene by a formal dehydrocobaltation . The alkane probably stems from a heterolytic cleavage to a radical anion and a cobalt(III) complex, followed by protonation or a direct protonation of the coupling product because this pathway dominates for electron deficient substrates. 

As cobalt complex-mediated oxygenation of these hindered phenols (11) occurs 30 times faster than base-catalyzed oxygenation 62), the phenolato Co(III) complex (12) evidently possesses special activation for reaction with molecular oxygen. In keeping with the desirability of a localized, soft anionic center in an oxygenase substrate, we can only assume that the metal is able to localize the n-anionic charge into an orbital with more [sp ] character. 

The essential point which is demonstrated by these examples is that, first, the oxygen is not activated by the cobalt complex, but the substrate is, and second, the metal does something more than impart radical character. We maintain that soft anion character is engendered in these substrates. As we will see later, these examples constitute valid precedents for substrate activation . 

A trinuclear cobalt(I) complex, PhCCo3(CO)9, can also catalyse the reduction of nitro compounds in the presence of hydroxide ion at room temperature under a normal pressure of CO [49]. Satisfactory results were obtained under phase transfer conditions. The catalyst and the aromatic nitro compounds were dissolved in benzene under carbon monoxide and an aqueous solution of sodium hydroxide containing cethyltrimethylammonium bromide was added. At a substrate/cat =10 ratio, ca. 60-80 % of amine was obtained in a 18 h reaction. The reaction also proceeded in a homogeneous phase (methanol-water, methanol, dioxane-water) but with lower conversions (less than 45 %). Cobalt complexes such as MeCCo3(CO)9 and MeCo(CO)4 were also active, but less effective. At the end of the reaction, the catalyst was recovered only in part (ca. 15 %). In the organic phase, an IR absorption at 1891 cm, attributable to [Co(CO)4] anion, was observed. Strangely enough, the preformed [Co(CO)4] anion has not been tested as catalyst. The active species was supposed to be the hydride cluster anion reported in Scheme 6. 

The highest selectivity seen in the cobalt Schiff base complex promoted dioxygenation reactions results from the direct nonradical dioxygen incorporation into oxygen-sensitive substrate anions. The substrate anions are produced under neutral conditions by the acid-base reaction between the substrates and hydroxocobalt(III) Schiff base complexes [Co (SB)(OH)] formed in situ. 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

With the same concept, but using the more reactive Ti(III) cationic radical [Cp2TiCl(THF)2] or a cationic salphen aluminum complex in combination with the cobalt anion [Co(CO)4] , Coates et al. succeeded to make the epoxide or aziridine carbonylative ring expansion reaction catalytic (Scheme 60) [149]. For both substrates, it is proposed a nucleophilic attack of the cobalt anion at the least-substituted carbon atom of the three-membered ring, the latter being activated by the Lewis acidic part of the catalyst. Of note, catalysts 106 and 107 used in this reaction are described as ion pairs rather than M-Co bond containing complexes. 

The accepted mechanism for the carbonylation of epoxides is shown in Scheme 17.26, and the basic steps of this cycle are also thought to occur during the carbonylation of aziridines. Alper first proposed a catalytic cycle for the expansion carbonylation of aziridines by [Co(CO)J, and Coates has proposed a similar cycle for epoxide carbonylation catalyzed by complexes containing both Lewis acids and cobalt-carbonyl anions (Scheme 17.26). This mechanism consists of four steps (1) the activation of substrate by coordination to a Lewis acid (2) the S 2 attack on the substrate by [Co(CO)J (3) the insertion of CO into the new cobalt-carbon bond, and the subsequent uptake of CO and (4) ring closing with extrusion of product and regeneration of the catalytic species. 

The radical mechanism involves homolysis of the Co—C bond and the ionic mechanism involves heterolysis of this bond. Since coenzyme Bjj has both Co(II) and Co(I) derivatives, either of these mechanisms is chemically reasonable. The radical pathway provides low-spin Co(II) which is EPR active and therefore easily detectable. However, detection alone does not prove that this is the catalytic pathway. It should be noted that an important function of the protein in these systems is to constrain and protect reactive intermediates and inhibit unwanted side reactions. For example, the substrate radical or anion might combine with the cobalt to give an organocobalt complex as the product of the second step in both reactions (8.13) and (8.14). 

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