Hydrogen atom transfer from cobalt hydride

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

A direct hydrogen atom transfer has been shown to occur, on the other hand, from neutral 19-electron transients obtained by one-electron reduction of saturated cationic hydrides. These derivatives apparently have a reduced acidity relative to cationic 17-electron complexes, while the presence of an unpaired electron in an orbital which may have an M-H a component may further contribute to weaken the M-H bond homolytically. An example is provided by complex CpCoH(dppe), which forms as a transient by one-electron reduction of [CpCoH(dppe)]". Cobaltocene is a strong radical scavenger, and has sufficient reducing power to carry our the reduction of the cationic cobalt hydride. Thus, reaction of the Co(III) hydride complex with two equivalents of cobaltocene affords CpCoCCjHg) in 80 % isolated yields, according to the proposed mechanism of Scheme 19 [25]. 

The enthalpy of activation for the turnover-limiting hydrogen-atom transfer step depends upon the affinity of tire anthracene substrate for free radicals. The 9,10-dihydroanthracene is formed as the product because the radical is most stable in these positions, and the mixture of stereoisomers is formed because the process does not involve migratory insertion. This mechanism also explains why simple benzene derivatives are imreactive. The 1,4-hexadienyl radical lies too far uphill of benzene and the cobalt hydride to be a product from a reaction of a catalytic cycle that occurs under mild conditions. 

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