Hydrogenation deuterium scrambling, mechanism

The cationic rhenium complexes [Re(NO)2(PR3)2] (R = Cy, Pr) [50] indeed show great potential for hydrolytic activation of dihydrogen. When [Re(NO)2(PR3)2l is treated with a mixture of H2 and D2 in toluene or chlorobenzene, hydrogen-deuterium scrambling is observed, and HD can be traced in the NMR spectrum of the reaction mixture [51]. The proposed mechanism for this catalytic exchange is illustrated in Scheme 4. 

Fluxional behavior was demonstrated for pentacoordinate alkylhydridosilicates in the gas phase, by the reaction of deuterium-labeled alkylsilicate with carbon dioxide65. The monodeuteriated silicate was first obtained in the flow tube by D- addition to a neutral alkylsilane, followed by H-/D- transfer to CO2 (equation 28). A statistical distribution of HC02 /DCC>2 (3 1 ratio) indicated complete scrambling of the hydrogens and deuterium in the silicate, presumably by a Berry pseudorotation mechanism. 

The proposed mechanism of the deuterium incorporation and the hydrogen scrambling is depicted in Scheme IV-18. 

Hydrogen-containing systems readily lend themselves to isotopic substitution by deuterium and tritium. Scrambling of H2 and D2 to form HD is often facile and can even occur over 18e H2 complexes such as W(CO)3(PR3)2(H2) even in the solid state at 1 atm. The mechanism is enigmatic because there is ostensibly only one labile coordination site. Another remarkable solid-gas reaction is deuterium incorporation into the PMcs ligands of [Fe(D2)H(PMe3>4] (Scheme 10), which may involve agostic interactions where associative (bond metathesis) or dissociative C-H bond activation occurs. "... 

Wilkinson s catalyst undergoes the hydrogenation of alkenes without isotopic scrambling between Hj and and without isotopic scrambling of Dj and protons in the solvent or on the olefin substrate. Under carefully controlled conditions, remarkably clean cis addition of hydrogen (or deuterium) is achieved. This lack of scrambling implies that the mechanism involves a dihydride intermediate in which both hydrides are transferred to the same alkene and that the mechanism involves a migratory insertion in which the metal and the hydride add in a cis fashion across the alkene, as discussed in Chapter 9. 

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