Homogeneous Catalytic Hydrogenation of Polycyclic Arenes

The coordination chemistry of this anionic, cyclometallated precatalyst and its derivatives, along with its stoichiometric reactions with anthracene, have been reported by Halpem. On the basis of these studies, a plausible mechanism can be proposed for the hydrogenation of polycyclic arenes by this catalyst. Although kinetic studies of the complete catalytic cycle have not been reported, Halpem s investigation of the stoichiometric reactions of this system provides strong evidence that the reaction occurs by a series of steps involving soluble complexes. 

Polycyclic arenes are also hydrogenated in the presence of the monohydride catalyst, HCo(CO) (Equahon 15.101). The regioselectivity of this reduction is different from that of the hydrogenation of anthracene catalyzed by the ruthenium complex of Grey and Pez. This difference in regioselectivity results from a different mechanism. Halpem showed that the hydrogenation of polycyclic arenes catalyzed by HCo(CO) occurs by hydrogen atom transfer.  

Several pieces of data suggest that the reaction catalyzed by HCo(CO) occurs by hydrogen atom transfer, rather than by formation of a n-anthracenyl complex. First, added carbon monoxide does not inhibit the rate, nor does it result in carbonylated products. Second, only 9,10-dihydroanthracene products are formed, and they are formed as a mixture of cis and trans isomers. Third, exchange occurs between deuterium in the 9- and 10-positions and H.  

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. 

---