Photocatalytic Hydrogenation of Alkenes

The photocatalytic hydrogenation of alkenes and dienes by Group 6 metal carbonyls has been investigated in LNG solvents [15]. Photolysis of trans-[M(C0)4(C2H4)2] (M = Cr, Mo, W) in liquid xenon doped with H2 leads to formation of mer-[M(CO)3(C2H4)2(q -H2)] and ds-[M(CO)4(C2H4)(q -H2)]. The q -H2 complexes for M = Cr and Mo are much less stable than those for M = W. The evidence supported -coordination of H2 rather than oxidative addition to give dihy-... 

This can be illustrated with the photocatalytic hydrogenation of alkenes by Fe(CO)5, which is relatively nonselective and acts as a photocatalyst for... 

Photocatalytic Hydrogenation of Alkenes to Alkanes in Alcoholic Suspensions of Palladium-Loaded Titanium(IV) Oxide Without the Use of Hydrogen Gas... 

Applicability of the photocatalytic hydrogenation of alkenes to alkanes was investigated using various aromatic or aliphatic alkenes, and Table 9.1 shows results of the photocatalytic hydrogenation in methanolic suspensions of Pd-Ti02 particles under deaerated conditions. Hydrogenation reaction proceeded in all cases, and high yield was obtained. 

Imamura K, Okubo Y, Ito T, Tanaka A, Hashimoto K, Kominami H (2014) Photocatalytic hydrogenation of alkenes to alkanes in alcoholic suspensirms of palladium-loaded titartium (IV) oxide without the use of hydrogen gas. RSC Adv 4 19883-19886... 

Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-l-enes over Wilkinson s catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh ] in benzene is quite selective (i.e. was 45 times faster than the cis-aIk-2-ene [15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4 -diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. 

Another direction has been to develop heterogeneous applications of diruthenium paddlewheels [97]. By using multitopic carboxylate ligands, for example, 1,4-benzenedicarboxyate, RUj tetracar-boxylates are assembled into microporous networks. Interesting applications such as the catalytic hydrogenation of alkenes, the oxidation of primary aliphatic alcohols, and photocatalytic hydrogen production from water have been reported [98-101]. 

Anpo, M., Aikawa, N., and Kubokawa, Y. Photocatalytic hydrogenation of alkynes and alkenes with water over Ti02-Pt loading effect on the primary processes. J. Phys. Chem. 1984, 88, 3998. 

There is also evidence for the intermediacy of alkene chromium carbonyl complexes in alkene hydrogenation. An example are the intermediacy of tranj,rrchromium tetracarbonyl in the photocatalytic hydrogenation of tronj,tra/tf-2,4-hexadiene. Substituted chromium tricarbonyl complexes that have ligands that can be replaced by alkenes also act as homogeneous... 

In thermocatalytic process, H2 is dissociatively adsorbed on Pd surface, and thus formed H species (as H-Pd) are incorporated into a C=C double bond. Based on the mechanism of thermocatalytic hydrogenation over Pd catalyst, the mechanism of photocatalytic hydrogenation without the use of H2 can be considered as follows (1) are reduced by photogenerated electrons on Pd surface to give active H species (probably as H-Pd) and (2) this active H species on Pd are inserted into a C=C double bond, resulting in ethylbenzene formation. The active species formed in thermocatalytic and photocatalytic reactions would be essentially same because H2 was evolved after consumption of styrene (Fig. 9.12a). It is apparent that H species photocatalytically formed on Pd are very active for hydrogenation of alkenes. 

Table 9.1 Photocatalytic hydrogenation of various alkenes to alkanes in methanolic suspensions of 0.1 wt% Pd-Ti02 for 30-min photoirradiation (Reproduced from Ref. [41] with permission from the Royal Society of Chemistry)...

A variety of metal carbonyls upon sonication will catalyze the isomerization of 1-alkenes to the internal alkenes (J 8),(27). Initial turnover rates are as high as 100 mol alkene isonierized/mol of precatalyst/h, and represent rate enhancements of 1(P over thermal controls. The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. A variety of terminal alkenes can be sonocatalytically isomerized. Increasing steric hindrance, however, significantly diminishes the observed rates. Alkenes without 6-hydrogens will not serve as substrates. 

Photocatalytic decomposition of water over various metal oxides to H2 and O2 is of current interest, representing an approach to the utilization of solar energy. This catalytic system catalyzes C—C bond fission and, to a lesser extent, the hydrogenation of multiple bonds. Titania220,221 and its binary oxides222 have been studied most in the hydrogenation of simple alkenes (ethylene, propylene and butenes). This photohydrogenation activity is,... 

Examples of photocatalytic pathways involving metal carbonyls would be alkene hydrogenation and hydrosilation (eq. 1) which are effected by a wide variety of metal carbonyls under photochemical excitation. 

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