A new life for nitrosocarbonyls in pericyclic reactions

Abstract

Cu(I/II) salts or Rose Bengal, used in a catalytic amounts, allow for the mild and easy oxidation of hydroxamic acids to nitrosocarbonyls, using air as final oxidant. These new mild oxidative protocols open new vistas in the field of nitroso Diels-Alder (NDA) and ene reactions (ER) of these fugitive intermediates.

Ene Reactions of Nitrosocarbonyl Intermediates with Trisubstituted Cycloalkenes: “Cis Effect” and Steric and Conformational Factors Drive the Selectivity

“…Nitrosocarbonyls 1 (RCONO) are highly reactive intermediates discovered by Kirby and traditionally generated by periodate oxidation of hydroxamic acids 2 . Alternative methods for their generation have been recently proposed and reviewed, where the hydroxamic acids (R-CONHOH) can be easily oxidized with air or oxygen and Cu­(II) metal catalyst in the presence of a 2-ethyl-2-oxazoline ligand or with a photoredox catalyst such as the inexpensive and common Rose Bengal . Moreover and in particular, the mild oxidation of nitrile oxides 3 with tertiary amine N -oxides easily leads to a variety of aromatic and aliphatic nitrosocarbonyls .…”

Generation and Trapping of Nitrosocarbonyl Intermediates

“…In terms of precursors, we have seen the hydroxamic acids represent the most common starting material, while periodate salts are the most used oxidant for the generation of NCs. Whereas the complexity of the application increased, new solutions were developed, in order to make the use of NCs possible and easy. , For example, the new aerobic generation method proposed by Read de Alaniz and Whiting, that makes the use of NCs suitable even on substrates with other delicate functional groups, is remarkable . For the same reasons, it is worth mentioning the photochemical generation by cleavage of the 1,2,4-oxadiazole-4-oxides, representing the mildest entry to NCs as well as the oxidation of hydroxamic acid using Rose Bengal as photosensitizer.…”

“…389,390 For example, the new aerobic generation method proposed by Read de Alaniz and Whiting, that makes the use of NCs suitable even on substrates with other delicate functional groups, is remarkable. 391 For the same reasons, it is worth mentioning the photochemical generation by cleavage of the 1,2,4-oxadiazole-4-oxides, representing the mildest entry to NCs as well as the oxidation of hydroxamic acid using Rose Bengal as photosensitizer. In recent years, an increasing number of papers have been published dealing with the synthesis of optically pure products through the use of these fleeting intermediates, in the presence of chiral catalysts, further widening their field of applications and getting good results in terms of enantio-and diastereoselection.…”

Photocatalyzed Generation of Nitrosocarbonyl Intermediates Under Solar Light Irradiation

“…When required, oxidative or aerobic conditions can be adopted as well. [6] Among these protocols, different research groups have explored the aptitude of transition metals [7] with hydrogen peroxide, metal oxides [8] and peroxides with TEMPO, [9] to convert hydroxamic acids or, more in general, hydroxylamine derivatives into nitroso reactive species. At any rate, the multitude of activation modes showcases the need of these intermediates in synthesis, where excellent chemical yields and mild reaction conditions are often required to prepare complex molecules.…”