Reductions hydrogen transfer reagents

Durst and Sharma [86] have reported the stereospecific synthesis of 3-spiro-epoxyazetidin-2-ones 55 (Scheme 14). The oxidation of the diastereoisomers of compound 52 with PCC provided a single acetyl compound 3-acetyl-3-benzyloxy-azetidin-2-one 53. Nonchelation controlled L-Selectride reduction of 53 gave the isomerically pure 3-hydroxyethylazetidin-2-one as the sole reduced product, which was further converted to tosylate 54 using NaH/tosylimidazole. The debenzylation-oxirane formation sequence was conveniently performed as a single pot operation with ammonium formate, 5% Pd/C in refluxing methanol as the hydrogen transfer reagent combination. 

In 2005, Rueping et al. reported that chiral phosphoric acids function as an efficient catalyst for the enantioselective reduction of ketimines (Scheme 3.40a 1) [87]. A variety of aryl methyl ketimines were reduced to the corresponding amines in optically active forms using Hantzsch ester as the hydrogenation transfer reagent (HEH) [88]. Subsequently, List and coworkers improved the catalytic efficiency and enantioselectivity by thorough optimization of the substituents (G) that were introduced to the phosphoric acid catalyst (Scheme 3.40a 2) [89]. Almost simulta neously, MacMillan and coworkers successfully developed the enantioselective... 

The competition between insertion and hydrogen transfer is also crucial to the selectivity of the reaction of aluminium alkyls with carbonyl compounds. Aluminium alkyls, like organolithium compounds and Grignard reagents, can add to aldehydes and ketones to form secondary or tertiary alcohols, respectively. If the aluminium alkyl has a j -hydrogen, however, reduction of the carbonyl compound is a common side reaction, and can even become the main reaction [16]. Most authors seem to accept that reduction involves direct j5-hydrogen transfer to ketone. 

It was found that most synthetic processes that are employed to prepare the diimide reagent generate trans-diimide, but ds-diimide undergoes faster hydrogen atom transfer to a double bond than does the trans isomer. It follows that a fast trans-cis isomerization precedes reduction. The transfer of hydrogen atoms takes place in a synchronous process188 via the transition state 19 ... 

Thus, the hydrazide or its azo analogue not only plays a key-role in the catalytic cycle as a hydrogen acceptor and a reductant for the copper catalyst, but it also acts as an acyl transfer reagent generating competitively the undesired mixed carbonate 20. This by-product presumably originates from the inter- or intra-molecular nucleophilic attack of the alcohol on either the copper-hydrazide or azo complexes 13 or 18 respectively, resulting ultimately in the deactivation of the catalyst. To minimize this undesired trans-acylation reaction, steri-cally demanding azo-derivatives were tested (Fig. 7). Whilst di-isopropyl... 

Nitrous Oxide (N20). In contrast to nitric oxide, N20 is not readily reduced by hydrogen atoms. However, it is a good O-atom transfer reagent and spontaneously reacts with metal surfaces at elevated temperatures. Under alkaline conditions with a freshly formed platinum surface (Pt ), N20 induces a vol-tammetric reduction peak (Figure 11.8) 10... 

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