Hydrogen transfer reactions catalyzed transition metal complexes

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Hydrido(alkoxo) complexes of late transition metals are postulated as intermediates in the transition metal-catalyzed hydrogenation of ketones (Eq. 6.17), the hydrogenation of CO to MeOH, hydrogen transfer reactions and alcohol homologation. However, the successful isolation of such complexes from the catalytic systems was very rare [32-37]. 

An alternative enzyme/transition metal combination employs transfer hydrogenation catalysts that are capable of racemizing secondary alcohols. The racemization procedure temporarily converts the alcohol into an achiral ketone, which is reduced back to the racemic alcohol. Coupling this racemization procedure to an enzyme-catalyzed acylation reaction affords a dynamic resolution process (Fig. 9-12). Several enzyme/transition metal combinations have been shown to be effective for these reactions, although ruthenium complexes 1-3 appear to be especially effective for the in situ racemization of the alcohol. The product esters are not prone to racemization under the reaction conditions. Early results employing transfer hydrogenation catalysts to effect the racemization of alcohols required the use of added ketone 21, 22. However, it was subsequently shown that added ketone was not required when appropriate transition metal complexes were used as catalysts. Furthermore, the use of 4-chlorophenyl acetate as the acyl donor afforded improved results. 

A classic reduction of ketones is the Meerwein-Pondorf-Verlay reaction, in which a ketone is reduced by an alcohol in an equilibrium process catalyzed by aluminum oxides. A modern version of this overall transformation that occurs by a different mechanism is the "transfer hydrogenation" of ketones and imines using alcohols as reagent. For years, little progress on this reaction had been made with transition metal complexes. However, a breakthrough was made when Noyori discovered that ruthenium complexes of amino sulfonamides (Figure 15.15) and amino alcohols catalyze... 

Isomerization of olefins which takes place as a result of the transfer of hydrogen atoms with concomitant migration of the double bond is catalyzed by transition metal complexes which may react with olefins to form organometallic compounds. Often, it is necessary that cocatalysts be present as sources of hydrogen. Most commonly utilized cocatalysts are acids, water, alcohol, hydrogen, silanes, etc. The formation of hydrido complexes during isomerization reactions is crucial. The following mechanisms of olefin isomerization reactions are known ... 

The mechanism of polymerization by the transition-metal complexes is similar to what is known for conventional Ziegler-Natta catalyzed reactions (Fig. 2.28). Alkylation of the metal center followed by CHs" abstraction leads to a metal-centered cation. Olefin coordination followed by insertion leads to polymer growth. The termination of the polymerization can occur by a p-hydrogen transfer. Clearly the advent of these developments is an indicator that the last word in this area is far from over and the inge-... 

Intermolecular oxidative addition of sp CH bonds are rare [7] and, until recently, were limited to organic molecules with CH bonds activated by electronic effects (reactions 3—5) [12, 13, 14], A particular example is the hydrogen transfer reaction (reaction 4) [13] catalyzed by transition metal complexes. 

In mechanistic studies, monodeuterated alcohols were obtained by using PrOD (Scheme 14). These results indicate that the intermediate for this transfer hydrogenation was not a dihydride complex but rather a monohydride complex, which was generally accepted by analogous transition-metal-catalyzed reactions [55-57]. 

In conclusion, we have found a convenient and practical method for the selective reduction of C=0 bond of a wide spectrum of a-keto-)S, -unsaturated esters with Ru(p-cymene)(TsDPEN) as catalyst. The transition metal catalyzed transfer hydrogenation reaction with good selectivity and high efficiency offers possibilities to provide the optically active a-hydroxy-/l, y-unsaturated esters with chiral catalysts. Table 3.8 gives different substrates that can be reduced with Ru(p-cymene) (TsDPEN) complex in isopropyl alcohol. 

---