Carbonyl catalyzed hydrogenation, probes

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]. 

The mechanistic details of these laccase/mediator catalyzed aerobic oxidations are still a matter of conjecture (51-54). However, experiments with a probe alcohol point towards one-electron oxidation of the mediator by the oxidized (cupric) form of the laccase followed by reaction of the oxidized mediator with the substrate, either via electron transfer (ET), e.g., with ABTS, or via hydrogen atom transfer (HAT), e.g., with N-hydroxy compounds which form N-oxy radicals (55). TEMPO and its derivatives form a unique case one-electron oxidation of TEMPO affords the oxoammonium cation which oxidizes the alcohol via a heterolytic pathway (Fig. 6), giving the carbonyl product and the hydroxylamine. The Tl copper center in fungal laccases has a redox potential of ca. 0.8 V vs. NHE. Consequently, fungal laccases can easily oxidize TEMPO to the corresponding oxoammonium cation, since the oxidation potential of the latter, which was first measured by Golubev and co-workers (55,57), is 0.75 V. This was confirmed by EPR measurements, which showed that laccase is reduced in the presence of TEMPO One equivalent of laccase could oxidize at least three equivalents of TEMPO within a few minutes under anaerobic conditions (58). 

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