Hydrogen generation from hydride species

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

From the kinetics of oxidation of diols by Fc(CN) in aqueous alkali catalysed by R11CI3, it is concluded that oxidation proceeds not by hydride ion transfer from alcohol to Ru(III) but via hydrogen atom transfer, to generate Ru(II) species and an intermediate radical which is further oxidized by more Ru(III).86 Similar conclusions were made from a related study of die oxidation of propan-l-ol under comparable conditions.87... 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

The hydrogenation of aromatics has been a topic of interest since Sabatier s first synthesis of cyclohexane from benzene with metallic nickel. The role of Nb and Ta aryloxides as catalysts for this reaction was mentioned earlier. Another system that has been studied in detail comprises allyl and hydride complexes of cobalt, e.g., (i73-C3H5)Co[P(OMe)3]3. Like the Nb and Ta compounds cobalt gives cyclohexane with cis stereoselectivity. The active species is probably the hydride, generated from (allyl)CoL3 on hydrogenolysis, which reacts with arenes in a stepwise manner. 

A delocalized 0-stannyl radical anion can also be generated from the reaction of an a,/ -unsaturated ketone or aldehyde with tributyltin hydride and radical initiator AIBN [3, 4, 5a, 5b]. Thus, a,/ -unsaturated carbonyl compound 4 (R or R = H or alkyl), can be reacted with wBu SnH under standard free-radical conditions to give allylic O-stannyl ketyl species (5 6), shown in Scheme 2. After hydrogen atom transfer to the -position of 6, a synthetically useful tin(IV) enolate is produced [5b, 5d, 5g. Allylic 0-stannyl ketyls have both one- (radical) and two-electron (anionic) sites for reactivity. These reactions can proceed in a sequential manner - a rapidly-evolving methodology in organic synthesis [2, 5, 8j. If the one-electron reactivity in 6 is used with a radicophile, then the tin enolate or two-electron reactivity can be used in reactions with suitable electrophiles (E ). Note that the carbonyl species. 

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