In 2003 Zhong1 and MacMillan2 reported the use of nitrosobenzene, PhNO, in the organocatalytic asymmetric α-oxygenation of carbonyl compounds:
which is all nice, except for the slightly counterintuitive fact that PhNO is acting as an electrophile on oxygen rather than on nitrogen.
Logically PhNO should be electrophilic on nitrogen (less electronegative than oxygen), and indeed, a 1994 review on the reactivity of PhNO lists numerous examples where this is the case.3 On top of that, if the enamine were to ad to nitrogen instead of oxygen, the phenyl group could be placed equatorial in the transition state:
Is there an explanation for the regioselectivity here?
Zhong, G. A Facile and Rapid Route to Highly Enantiopure 1,2-Diols by Novel Catalytic Asymmetric α-Aminoxylation of Aldehydes. Angew. Chem. Int. Ed. 2003, 42 (35), 4247–4250. DOI: 10.1002/anie.200352097.
Brown, S. P.; Brochu, M. P.; Sinz, C. J.; MacMillan, D. W. C.The Direct and Enantioselective Organocatalytic α-Oxidation of Aldehydes. J. Am. Chem. Soc. 2003, 125 (36), 10808–10809. DOI: 10.1021/ja037096s.
Zuman, P.; Shah, B. Addition, Reduction, and Oxidation Reactions of Nitrosobenzene. Chem. Rev. 1994, 94 (6), 1621–1641. DOI: 10.1021/cr00030a007.