Organocobalt i Complexes

3 Cobalt Complexes With t/ -Hydrocarbyl Units 7.01.3.1 Organocobalt(-i) Complexes 

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Co(II) complexes react rapidly with free radicals but, because of the difficulty of handling radicals, most of the evidence is indirect. The radicals and Co(II) complexes can both be prepared in situ by photolysis of organocobalt(III) complexes, i.e.,... 

Other substituted olefins such as acrylonitrile, fumaronitrile, crotono-nitrile, cinnamonitrile, and diethylfumarate also formed adducts with Co (DMG)2 complexes containing py, H2O, or PBuj and, in one case, with [Co (DMG-BF2)2py]. Second-order rate constants were reported for the formation of several Tr-olefin-Co(I) complexes from organocobalt(III) complexes containing, for example, NCCH2CH2- with DMG, DPG, DMG-BF, py, H2O, and PBuj. 

Organocobalt(III) complexes are stable towards oxygen unless they undergo spontaneous fission of the Co—C bond to give Co(II), Co(I), or CoH complexes (see Section VI,B,l,a and D). Reactions with halogens are dealt with in Section VI,C,l,c. 

Jhe reaction mechanism of the various metal complexes clearly have much in common. The recently recognized insertion reaction appears to be a particularly good example of a reaction which is general among the metal compounds. In the following discussion I intend to point out the generality of the insertion reaction with examples from the literature and from our own wrork. The most complete series of substantiated insertion reactions involves the organocobalt carbonyl complexes, and these reactions will form the nucleus of the discussion. 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

The homogeneous catalytic [2+2+2]-cycloaddition of alkynes and nitriles was first discovered by Yamazaki and Wakatsuki [3] using the phosphine-stabilized cobalt(III) complex (Stmcture 5). At the same time, Bdnnemann and co-workers [5] observed the co-cyclization (eq. (2)) at cobalt catalysts prepared in situ, as well as using phosphine-free organocobalt(I) diolefm complexes. 

Both organocobalt(I) and (II) carbonyl complexes can be synthesized from te-trahydrofuran (THF) solutions of bis(pentafluorophenyl)cobalt(II), and this is an... 

Cobalt(II) complexes (such as cob(II)alamin, formerly called B,2,) can be obtained by several methods including controlled potential reduction [14,15], anaerobic photolysis of some organocobalt species [16] (see Section 5.2(b)), partial oxidation of cobalt(I) complexes, in some cases acidification of solutions of cobalt(I) complexes (see below) and chemical reduction. Chemical reduction of cobalt(III) cobalamins to the +2 oxidation state has been achieved with hydrogen over platinum oxide [16], with neutral and acidic solutions of vanadium(III) [17], with chromous acetate at pH 3 [18] and with amalgamated zinc in 0.1 M aqueous perchloric acid [19]. All such cobalt(II) species are probably 5-coordinate, and are low spin d systems containing an unpaired electron and thus displaying an ESR spectrum [20]. 

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