Regeneration, dihydrides

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

A plausible mechanism involves the reaction of the dihydride precursor with t-butylethylene to the 14-e complex [Ir(C6H3-2,6 CH2P-f-Bu2 2)]> which undergoes the oxidative-addition reaction of the alcohol to afford a hydride alkoxide complex. Further /i-hydride ehmination gives the alde-hyde/ketone and regenerates the dihydride active species [55]. In the particular case of 2,5-hexanediol as the substrate, the product is the cycHc ketone 3-methyl-2-cyclopenten-l-one. The formation of this ketone involves the oxidation of both OH groups to 2,5-hexanedione followed by an internal aldol reaction and further oxidation as in the final step of a Robinson annotation reaction [56]. 

The polymer-supported organotin dihydride 70 was shown to be an efficient reducing agent for aldehydes and ketones, but substantial loss of activity was observed after regeneration. More recently, various polymer-supported butyltin reagents (71, 72) were studied as reagents for the acetylation of sucrose158. 

The present hydrogen transfer reaction is extended to the aerobic oxidation of alcohols. Thus, the oxidation of alcohols can be carried out with a catalytic amount of hydrogen acceptor under an O2 atmosphere by a multistep electron-transfer process. As shown in Scheme 3.4, the ruthenium dihydrides formed during the hydrogen transfer can be regenerated by a multistep electron-transfer process including hydroquinone, ruthenium complex, and molecular oxygen. 

The next step is the oxidative addition of hydrogen, converting the square planar diastereomers of line 2 into the octahedral dihydrides of line 3 [93]. In the present system this reaction is the rate-determining step. The fast step following is the insertion of the coordinated olefin into one of the Rh-H bonds, giving rise to the two diastereomeric (T-alkyl complexes of line 4. By reductive elimination they generate the enantiomeric forms of the product, regenerating the catalytically active square planar species, which reenters the catalytic cycle. 

Scheme 1, which shows reaction pathways available for hydrogenation of alkenes using dihydride catalysts , has been developed largely from studies on Rh catalysts. The steps define the hydride route, and Kg,k2 the unsaturated route via oxidative addition of H2 to the metal-alkene complex. The common key dihydride-alkene intermediate 1 gives the saturate product wifii regeneration of catalyst M via two successive hydrogen atom transfer steps k. The and equilibria are usually established... 

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