Nitrenes generation from azides

This indicates that the prebaking temperature higher than the melting point of the azide decomposes the azide (50%) and it totally decomposes upto 100 mJ/cm2 irradiation. It is possible that subsequent reactions of the nitrene, generated from the azide thermolysis and photolysis, with the styrene resin could be responsible for solubility modulation of this type resist (16). 

The typical reactions of nitrenes generated from aliphatic azide precursors are shown below and include isomerization to imines (a), intramolecular insertion into C—H bonds (b) has been questioned (see 1. c. n> p. 57) and intermolecular hydrogen abstraction (c). 

The preparation of an indole (nitrene generated from an azide - the Hemetsberger-Knittel synthesis) and of carbazole ° (nitrene generated by deoxygenation of a nitro group) illustrate the power of the method. 

The most general preparations are based on the reaction of the heteroaromatic bases with O-substituted hydroxylamines. An alternative but less efficient method is the reaction of heteroaromatic bases with nitrenes generated from acyl and sulfonyl azides. 

Ketenimines also react as dipolarophiles in their reactions with ethoxycarbonyl nitrene, generated from the corresponding azide at 100 °C to give 109 . 

Also nitrenes, generated from the allenyl azides 99, undergo the [2+1] cycloaddition reaction to give the annelated indole derivatives 100. ... 

In remembrance of the isolation of carbene 78 (Section m.C) it seems acceptable that 141, formed from 140 by a [l,2]-migration of the substituent, functions as a second intermediate. Carbene 141 can subsequently fragment into a nitrene 143 and carbon monoxide or open to an isocyanate 144, which alternatively may also be generated from an acid azide via acylnitrene 142. In comparison to calculated reaction pathways 75 78 and 82 81 (Schemes... 

Nitrenes can be generated from many precursors such as azides, isocyanates, ylides, heterocycles, and nitro compounds.236,237 Amongst these, azides are the most convenient precursors since they are easily prepared and can be decomposed by heat, light or a suitable catalyst. Despite considerable endeavors, no one has yet provided a synthetically viable method to use azides as sources of nitrenes.237 The breakthrough of nitrene chemistry was the recognization of the value of A-arenesulfonyl iminoiodinanes (ArS02N=IPh) as nitrene precursors by Breslow and Mansuy. - They reported inter- and intramolecular C-H insertions by tosylimino phenyl-iodinane (TsN=IPh) in the presence of Mn(m) or Fe(m) porphyrins or [Rh2(OAc)4]. Subsequently, Muller... 

JV-Alkoxycarbonyl- and iV-arenesulfonyl-imines can be prepared by the reaction between pyridines and nitrenes, the latter being generated from the corresponding azides (72JOC2022, 64TL1733). Thermolysis of pyridinium iV-acylimines gives isocyanates and the parent heterocycle <79JCS(P1)446). 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

Sidewall functionalization of SWCNTs was achieved via the addition of reactive alkyloxycarbonyl nitrenes obtained from alkoxycarbonyl azides. The driving force for this reaction is the thermally-induced N2 extrusion. The nitrenes generated attack nanotube sidewalls in a [2+l]-cycloaddition forming an aziridine ring at the tubes sidewalls (Scheme 1.19). 

Aroyl nitrenes can be generated from their corresponding azides by three methods ... 

The intramolecular insertion reaction of arylnitrenes proceeds with retention of configuration at carbon. For example, heating the (S)-aryl azide (11 X = Na) in the vapor phase gives 2-ethyl-2-methylin-doline in 50- % yield in ca. 100% optical purity.The optical purity of the product is lower if the azide is heated in solution, or if the nitrene is generated from the corresponding nitro compoimd (11 X = NO2) with triethyl phosphite. ... 

The addition of alkoxycarbonyl nitrenes, generated photochemically from azides, to substituted dihydropyrans and tri-O-acetyl-D-glycal57"59 in alcoholic solution gave the products of alcoholysis 30 and 31-33 of the intermediate aziridines by a one-pot procedure59. The attack of the nitrene takes place mainly on the less hindered face of the double bond, but complete control of the diastereoselectivity was not accomplished even the ring opening of the aziridine can follow both anti and syn stereochemistry. 

The decrease in solubility upon exposure in this type of resist was first ascribed to the formation of a secondary amine generated from nitrene insertion into C-H bonds of the polymer (see Scheme 3.6) (88). However, gel permeation chromatographic analyses revealed that the molecular weight of poly(p-vinylphenol) increased upon irradiation in the presence of the azide. Hydrogen abstraction from the polymer by nitrene and subsequent polymer... 

JV-cyano]- and [7V-(methylsulfonyl)alkoxy-carbimidoyl]nitrenes, generated in situ from the corresponding azides by 300 nm UV light, convert a variety of alkenes cleanly and stereospecifically to the corresponding aziridines <89JOC3945>. 

Photochemical and Radical Reactions Most photoreactions in the field of nanotube chemistry serve to the generation of reactive intermediates that attack the nanotube afterward. The functionalizing step itself is rarely photochemical. Examples of such a preparatory step prior to functionalization include the conversion of azides into nitrenes or the radical generation from acyl peroxides, iodoalkanes, etc. In the photochemical reaction of nanotubes with osmium tetroxide, on the other hand, the essential step occurs only under irradiation. 

The chemistry of short-lived intermediates is difficult to study because usually the species cannot be directly observed under the reaction conditions. Much of the knowledge is therefore obtained by deduction from product studies. In the present review we are dealing mainly with intramolecular gas-phase reactions, and the carbenes and nitrenes in question are most frequently generated from diazo-compounds and azides (or in heterocyclic systems valence isomers thereof, triazo-loazines and tetrazoloazines). It is known that diazo-compounds and azides deld carbenes and nitrenes, respectively, by low-temperature photolysis i>2). However, the cycloheptatrienylidenes and azepinylidenes invoked in many reaction mechanisms have not yet been observed by any spectroscopic means, and their existence is deduced exclusively from their chemistry. 

Nitrenes generated photochemically or thermally from arenesulfonyl azides and alkoxycarbonyl azides add to the nitrogen atom of heteroaromatic bases to give the corresponding N-(substituted imines).20,21,50,64-69 However, the yields are usually low and the scope is severely limited. 

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