Hydrides, metal photochemical substitution

Opposite regioselectivity to that of regular metal hydride reductions has been observed in the NaBH4 reductions of oxiranes in the presence of triethylamine under photochemical conditions (hydride attacks the more highly substituted carbon) <92CC1133>. Diborane (B2H6) likewise tends to reduce oxiranes at the sterically more hindered oxirane carbon the mechanism and stereochemistry of the diborane reduction in connection with aliphatic oxiranes has been studied <82H(l8)28l>. 

Photochemical enzyme-free methods have been reported for the reduction of NAD(P) [419-421]. The Rh(bpy)3 + derivatives have been successfully used as reduction catalysts, enabling electrochemical reduction [421-423]. The better stability of the bis-(tridentate) Kgand Rh(terpy)2 " allowed a mechanistic study of the photoinduced reduction of NAD+, which seems to proceed by a hydride-transfer route according to the dependence of NAD+ reduction on pH [424]. The intermediate Rh hydride complex was identified by H NMR spectroscopy [422]. A similar conclusion is drawn when the process is studied at modified electrodes with elec-tropolymerized pyrrole-substituted Rh (111) complexes in which hydrogenation involves the formation of an electrogenerated hydride complex, followed by the insertion of NAD" " in the metal-hydride bond [425,... 

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