Reactions at a Group VIA Metal Center

Compounds of the Transition Elements 10.3.4. Reactions at a Group VIA Metal Center 10.3.4.1. Reactions of Halogens... 

In Chap. 3, we have examined the addition of an alkane C-H bond to a 16-electron metal center M giving a transient 18-electron metal-alkane intermediate M(RH) that ultimately gives oxidative addition of the C-H bond to M(R)(H). In this organomet-allic reaction, activation of the C-H bond occurs at the less substituted carbon for steric reasons. For instance, a linear alkane yields an w-alkyl-metal species via C-H activation at a terminal methyl group. The selectivity is thus primary > secondary > tertiary. This opens the valuable possibility of generating linear alkyl-functionalized products if such an oxidative addition mechanism can be made catalytic ... 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

The reaction of metal N-alkylcarbamates M(C)2CN 11 R) (M = Na, Mn(II), Co(II) R = Ph, Pr, Cy) with R C(0)C1 (R = Me, Ph) takes place, at ambient temperature, in a more complex way with the formation of isocyanates (RNCO), carboxylic anhydrides (R C(0)0C(0)CR ), amides (RNHC(O)R ) and C02. Amide formation and the evolution of C02 can be due to (i) the decomposition of mixed anhydride RNHC(0)0C(0)R obtained by addition of the acyl chloride to the oxygen atom of the carbamate group or (ii) the direct reaction of acyl chloride at the carbamic nitrogen atom of M(02CNHR) . The mixed anhydride RNHC(O) 0C(0)R might also decompose via another route so as to afford isocyanate and carboxylic acid. However, a different pathway (Scheme 6.6) has been also envisaged for the formation of RNCO and R C(0)0C(0)CR, which excludes any intermediacy of the mixed anhydride [61a], Two acetic acid molecules, bound to the same metal or to different metal centers, would then be dehydrated and acetic... 

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