Interception of Deaminatively Generated Benzyl Carbenium Ions by Acetone

Essentially free benzyl carbenium ions were generated via protonation of phenyldiazomethane with benzoic acid in acetone. Interestingly, no proton transfer occurred below -20 degrees C. After protonation and dediazoniation of the diazoalkane at -20 degrees C, the solvent was found to intercept the deaminatively generated carbocations to yield initially the corresponding O-benzyl oxonium ion and benzyl benzoate. The onium ion, however, was labile under the reaction conditions, and decomposed into a cascade of products whose concentrations as a function of time were used to trace the reaction pathway. Thus, the O-benzyl oxonium ion reacted with benzoate ion to yield (2-benzyloxy)isopropyl benzoate; subsequent decomposition of this O-benzyl-O-benzoyl ketal produced 2,2-dibenzyloxypropane (a dibenzyl ketal), 2-benzyloxypropene, and benzyl alcohol. In a related study, benzyl cations were generated via thermolyses of N-benzyl-N-nitroso-O-benzoyl hydroxylamine at 0 and -70 degrees C. The product distributions were found to be temperature-dependent and different from that in the PhCHN(2) + PhCO(2)H case.     

Electronic and steric effects in thermal denitrosation of N-nitrosoamides

N-Alkyl-N-nitrosoamides undergo competitive reactions whose rates are dependent upon the interplay of a number of factors. There already exists a significant body of work delineating the effects of pH on the partitioning of the nitrosoamides along their deaminative (-N(2)) and denitrosative (-"NO(+)") pathways. In this paper, the issue of pH dependence is discussed with particular attention to nitrosoamide decompositions in nonaqueous media. The role of the acidity of the medium in the partitioning of the nitrosoamide between deamination and denitrosation and in the choice of deaminative pathways is revisited. In nonaqueous media under near-neutral conditions, the partitioning's pH dependence is evidently accompanied by a sensitivity to structural features in the nitrosoamide. Thus, diminution of steric crowding around the N-nitroso moiety as well as the presence of strongly electron-withdrawing acyl units (i.e., those derived from strong acids, e.g., tosyl and trifyl) increase the relative yield of amides by encouraging the denitrosative pathway. A mechanism for thermal denitrosation of nitrosoamides under near-neutral conditions is proposed in which rapid protonation at the acyl O rather than slow protonation at the amidic N is the first step in the reaction profile. A rate-limiting, bimolecular reaction between the O-conjugate acid and adventitious nucleophiles at the nitrosyl group then occurs followed by rapid tautomerization to amide.     

Investigating the reaction of N-carbobenzoxy-O-carbobenzoxy- hydroxylamine with dimethyl sulfoxide: formation of S,S-dimethyl-N-[(phenylmethoxy)- carbonyl]sulfoximine

[Reaction: see text]. N-carbobenzoxy-O-carbobenzoxyhydroxylamine (1a) underwent a thermally induced reaction in DMSO in which there is net N-alpha-eliminative oxidation with tandem oxidative incorporation of DMSO to yield S,S-dimethyl-N-[(phenylmethoxy)carbonyl]sulfoximine. Mechanisms for the formation of the sulfoximine are presented as well as the product characterizations, including the X-ray crystal structure.     

Nuclear Magnetic Resonance (NMR) Spectroscopy: A Review and a Look at Its Use as a Probative Tool in Deamination Chemistry

Abstract

This year (2006) represents the 60th anniversary of nuclear magnetic resonance (NMR) spectroscopy (discovered independently by Nobel laureates Edward Purcell and Felix Bloch). It is therefore appropriate and indeed valuable to reflect on how this versatile methodology has developed, expanded, and evolved into a cornerstone of chemical research since 1946. No doubt multiple reviews discussing various aspects of NMR technology will emerge over the course of this year, but the field has grown so exponentially since its inception that it would be impossible for a single review to meaningfully encompass all features of the NMR methodology. This work, therefore, is not meant to provide a comprehensive review of NMR spectroscopy (such an undertaking would prove unwieldy and is inapt in the current context). Instead, it will provide an overview of NMR spectroscopy including the basic principles of NMR (the NMR phenomenon, instrumentation, and spectral interpretation) the historical development of the field, and a few unique applications of the methodology. Finally, illustrations of the utility and application of NMR spectroscopy as a probative tool in the intriguing field of deamination chemistry will be examined. Among the examples highlighted are the elucidation of the mechanism of N‐nitrosoamide conversion to the trans‐diazotate ester, denitrosation under near‐neutral conditions, elucidation of the bond‐forming step of Friedel‐Crafts benzylation, and the identification of novel electronic (π?‐acceptor agostic‐type interaction) and steric (persisteric) effects.     

N-nitrosoamide-mediated N --> O nitroso transfer to alcohols with subsequent denitroxylation

Decomposition of certain N-benzyl-N-nitrosoamides is often accompanied by small amounts of benzaldehyde whose formation was postulated to arise from in situ formation and oxidation of benzyl alcohol. Incubation of excess benzyl alcohol with thermostable N-benzyl-N-nitrosoamides at ambient temperatures in inert solvents generates benzyl nitrite, N-benzyl amides, and benzaldehyde as the major products. Benzyl nitrite formation appears to be linked to N --> O nitroso transfer between the N-benzyl-N-nitrosoamides and benzyl alcohol, which is subject to the previously observed electronic and steric features of the acyl substituent although the former appears to play a much larger role than the latter. Benzaldehyde formation evidently arises from dehydronitrosation (denitroxylation) of the nitrite via O-N bond homolysis and H-abstraction from the resultant benzyloxy radical. Although trans-nitrosation occurs with methanol, 1 degree, 2 degree, and 3 degree alcohols, the reaction is evidently subject to steric effects at both the alpha and beta carbons of the alcohol. Additionally, carbonyl formation only occurs with 2 degrees alcohols and those that can derive resonance-stabilized carbonyls.