Hydrazone Hydride abstraction

Ort/io-substituted benzaldehyde complexes have been prepared in high enantiomeric purity (97% ee), and in a one-pot sequence, from an optically pure hydrazone derivative, readily available from -q -benzaldehyde chromium tricarbonyl and SAMP [(S)-l-amino-2-(methoxymethyl)pyrrolidine]. The novelty derives from the combined use of a diastereoselective orthoaddition reaction of an organolithium nucleophile and a hydride abstraction with a triphenylmethyl cation. The subsequent acid hydrolysis serves to remove the hydrazone group, thus liberating the aldehyde functionality (Scheme 6.13). 

According to a detailed mechanistic study, the first step is the abstraction of the relatively acidic hydrazone proton (93- 97). This is followed by hydride attack on the trigonal carbon of the C=N bond, mainly from the a-side at C-3, together with the concomitant loss of the tosylate anion (97 -> 98). Expulsion of nitrogen from the resulting intermediate (98) yields a fairly insoluble anion-metal complex (99) which upon decomposition with water provides the methylene derivative (100). 

A mechanism which is consistent with the various experimental results for olefin formation involves the initial abstraction of the hydrazone proton (103->106) In this case, however, expulsion of the tosylate anion is associated with the abstraction of a second hydrogen from C-16 instead of hydride attack on the C=N bond (compare 97 98 and 106 107). Ex-... 

A mechanism which is consistent with the various experimental results for olefin formation involves the initial abstraction of the hydrazone proton (103 - 106)82 In this case, however, expulsion of the tosylate anion is associated with the abstraction of a second hydrogen from C-16 instead of hydride attack on the C=N bond (compare 97 - 98 and 106 - 107). Expulsion of nitrogen from the resulting intermediate (107) yields an anion (108) which is most probably stabilized in the form of a metal complex and can be readily decomposed by water to give an olefin (109). This implies that 17-d1-androst-16-ene (104) can be prepared by using deuterium oxide as the sole deuterated reagent.82... 

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