Hydrogen photoinduced elimination

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

The photochemical studies of transition metal hydride complexes that have appeared in the chemical literature are reviewed, with primary emphasis on studies of iridium and ruthenium that were conducted by our research group. The photochemistry of the molybdenum hydride complexes Mo(tj5-C5H5)2M2] and [MoH4(dppe)2] (dppe = Ph2PCH2CH2PPh2), which eliminate H2 upon photolysis, is discussed in detail. The photoinduced elimination of molecular hydrogen from di-and polyhydride complexes of the transition elements is proposed to be a general reaction pathway. 

Mo(r75-C5H5)2H2] and [MoH dppe ]. Our studies of the di- and trihydride complexes of ruthenium and iridium, described above and published previously (27,35), and those of other workers (discussed at the beginning of this chapter), indicate that photoinduced elimination of molecular hydrogen is a common reaction pathway for di- and polyhydride complexes. To demonstrate the photoreaction s generality and its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of polyhydride complexes of the early transition metals. We focused initially... 

An example of the photoinduced elimination of hydrogen with subsequent ligand coordination to give a coordinately saturated complex as product is found in die conversion of RuH2(CO)(PPh3>3 into Ru(CO)3(PPh3)3 in the presence of CO (Ref. 168) ... 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Parallel to these studies Marks and co-workers have studied the effects of an extensive functionalization of the cyclopentadienyl ligands. It has been observed that substitution of cyclopentadienyl ligands with in-denyl ligands inhibits the photoinduced -hydrogen elimination pro-... 

The photoinduced reductive elimination of hydrogen occurs by a concerted / reaction rather than by a stepwise sequence. This concerted pathway is supported by the experimental observation that photolysis of a solution containing a mixture of IrClH2(PPh3)3 and IrClD2(PPh3)3 gave only H2 and D2, with no formation of the crossover product Several explanations have been offered for the photoin-... 

Organometallic hydride complexes, like those of other transition metal complexes, undergo photochemical reactions. Organometallic hydrides are known that contain either a single M-H bond, or two bonds to hydrogen in complexes of type MH2. A common reaction of the latter type is the photoinduced reductive elimination of hydrogen ... 

For the complexes Cp3ThR (R= Pr or "Bu), (MeC5H4)3Th("Bu), and (indenyl)Th("Bu), UV photolysis in aromatic solvents produces 1 1 mixtures of alkane alkene and Cp3Th, and a photoinduced p-hydrogen elimination mechanism is proposed. In the corresponding... 

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