Department of Chemistry, The City College and The City University of New York, 160 Convent Avenue, New York, New York 10031-9198, United States.
J Org Chem. 2011 Oct 7;76(19):7842-8. doi: 10.1021/jo201192c. Epub 2011 Aug 31.
Facile reduction of alkylamino-, anilino-, and pyridyl-N-oxides can be achieved via the use of diboron reagents, predominantly bis(pinacolato)- and in some cases bis(catecholato)diboron [(pinB)(2) and (catB)(2), respectively]. Reductions occur upon simply mixing the amine N-oxide and the diboron reagent in a suitable solvent, at a suitable temperature. Extremely fast reductions of alkylamino- and anilino-N-oxides occur, whereas pyridyl-N-oxides undergo slower reduction. The reaction is tolerant of a variety of functionalities such as hydroxyl, thiol, and cyano groups, as well as halogens. Notably, a sensitive nucleoside N-oxide has also been reduced efficiently. The different rates with which alkylamino- and pyridyl-N-oxides are reduced has been used to perform stepwise reduction of the N,N'-dioxide of (S)-(-)-nicotine. Because it was observed that (pinB)(2) was unaffected by the water of hydration in amine oxides, the feasibility of using water as solvent was evaluated. These reactions also proceeded exceptionally well, giving high product yields. In constrast to the reactions with (pinB)(2), triethylborane reduced alkylamino-N-oxides, but pyridine N-oxide did not undergo efficient reduction even at elevated temperature. Finally, the mechanism of the reductive process by (pinB)(2) has been probed by (1)H and (11)B NMR.
通过使用二硼试剂，主要是双(频哪醇酯基)-和在某些情况下双(邻苯二酚基)二硼[(pinB)(2)和(catB)(2)]，可以轻松还原烷基氨基-、苯胺基-和吡啶基-N-氧化物。只需将胺 N-氧化物和二硼试剂在合适的溶剂中在合适的温度下混合，就会发生还原反应。烷基氨基-和苯胺基-N-氧化物发生极快的还原，而吡啶基-N-氧化物则进行较慢的还原。该反应对各种官能团如羟基、硫醇基和氰基以及卤素具有耐受性。值得注意的是，敏感的核苷 N-氧化物也被有效地还原。烷基氨基-和吡啶基-N-氧化物还原速度的不同已被用于逐步还原（S）-(-)-尼古丁的 N,N'-二氧化物。由于观察到(pinB)(2)不受胺氧化物中结合水的影响，因此评估了使用水作为溶剂的可行性。这些反应也进行得非常出色，得到了高的产物收率。与(pinB)(2)的反应相反，三乙基硼烷还原烷基氨基-N-氧化物，但吡啶 N-氧化物即使在高温下也没有进行有效的还原。最后，通过（1）H和（11）B NMR 探测了(pinB)(2)的还原过程的机理。
