Cluster catalysis mechanistic studies

For Rh4(CO)12 as hydroformylation catalyst, several mechanistic studies including isotope labeling and kinetics have been undertaken. Thus, the deuteroformylation of 2,3,3-trimethylbut-l-ene and of styrene in the presence of Rh4(CO),2 have been studied In the first case, the 3,4,4-trimethyl-pentanal formed was found to be deuterated exclusively in the formyl position (238). In the latter case, the aldehydes formed were deuterated in the formyl and in the corresponding a-position ( > and (Zy PhCH=CHD and PhCH=CD2 were also observed (239). The mechanistic implications of these findings are not entirely clear. However, a kinetic study of the Rh4(CO)12-catalyzed low-pressure cyclohexene hydroformylation provides some evidence in favor of intact cluster catalysis A mechanism proposed on the basis of these findings includes fission of one Rh-Rh bond, whereby the tetranuclear cluster framework remains intact (Scheme 12) (240). 

A full kinetic study of the hydrolysis of c-AMP (248) catalysed by Cu(II) terpyridine (Cutrpy) has revealed that the active species under alkaline conditions is the hydroxide form of the catalyst, (CutrpyOH)+. Although the same species acts as a base catalyst in the transesterification of RNA, here it behaves as a nucleophilic catalyst. Mechanistic studies of the hydrolysis of DNA and RNA by lanthanide ions have revealed that the catalytically active species are dinuclear hydroxo clusters. For example, the mechanism of the Ce(IV) catalysis of DNA hydrolysis (acceleration factor of lO -fold) is considered to involve the facilitated formation of a pentacoordinated intermediate (Scheme 45). However, the Ln(III) catalysis of RNA... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

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