Benzylic positions, deuterium-hydrogen

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

The question is Why are we considering this particular rearrangement and not any other The reason is simple. We must remember that the chromium is not present in the final products, it is replaced by a hydrogen by protonation and reductive elimination. The former position of the metal will be labeled in the final products in the experiments carried out in deuteromethanol. Looking at the structure of labeled compound 6 in Fig. 7.2, only a 1,3-migration of the metallic fragment could explain the presence of a deuterium atom at the benzylic position. 

Conducting Pd/C- HjO labeling reactions in the presence of a small amount of hydrogen gas results in a more active catalyst , as can pretreatment of the catalyst with NaBH . Nevertheless, elevated temperatures are still required (110 C for nucleosides, 110-180 °C for N-heterocycles, > 140 °C for aU but the benzylic positions of aralkylic compounds) . The catalytic hydrogen gas rapidly becomes equilibrated with solvent deuterium . Very dilute tritiated water was used as proof of principle , but this method would not be practical in its current form for the preparation of high specific activity compounds because it requires neat water as solvent/labeling source. 

Under certain conditions surface catalytic deuterations can lead to the exchange of benzylic hydrogens. An example in the steroid field is the exchange of the benzylic hydrogens in estrone methyl ether (42) with deuterium in the presence of palladized charcoal." " According to mass spectrometric analysis, the product (43) contains three deuteriums (83 %), which have been assigned to the 6- and 9a-positions on the basis of NMR evidence." " ... 

Asymmetric transfer hydrogenation of benzaldehyde-l-d with (R,R)-28 and (CH3)3COK in 2-propanol gave (R)-benzyl-l-d alcohol quantitatively in 98% ee (Scheme 41) [114], Introduction of electron-donating and electron-accepting groups at the 4 position had little effect on the enantioselectivity. Catalytic deuteration of benzaldehydes was achieved by use of the same complex (R,R)-28 and a 1 1 mixture of formic acid-2-d and triethylamine to give the S deuter-io alcohols in up to 99% ee (Scheme 42) [114], The dt content in the product alcohol was >99%. Only a stoichiometric amount of deuterium source is required to complete the reaction. 

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