Nitrenes acylnitrenes

Intermolecular addition of photochemically generated nitrenes and in particular acylnitrenes to alkenes provides a useful and widely applied route to aziridines.385 An analogous intramolecular photoreaction is thought to be involved in the conversion of the o-azidophenylethylfuran 461 into the pyrrolo[l,2-a]quinoline 462 as outlined in Scheme 13,386 and intramolecular addition to an azo group has been observed in the 8-azido-1-arylazonaphthalenes 463.387... 

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... 

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

However, Lwowski and co-workers demonstrated that photolysis of 7 in cyclohexane, cyclohexene- or 2-methylbutane does lead to the formation of adducts. Therefore, the acylnitrene 8 is a trappable intermediate. The thermal Curtius rearrangement does not involve free nitrenes and it must be a concerted process. 

Pyrolysis or photolysis of methanesulfonyl azide in 2-methyl butane (RH) produces formal C H insertion adducts attributed to reactions of the thermally generated nitrene. The selectivity of this nitrene for 3°/2°/l° C—H bonds was 6/2.3/1.0 Sulfonyinitrenes, like acylnitrenes, insert into the OH bonds of alcohols. 

An interesting development of this azepine synthesis is the intramolecular addition of acylnitrenes to arenes, as exemplified in Scheme 27 for the nitrene (231) generated from benzyl azidoformate by spray pyrolysis (80CC1161). 

Benzoyl nitrene 247 (R = Ph) generated by photolysis of benzoyl azide in the presence of carbonyl compounds affords 1,4,2-dioxazolines 248-251 (Scheme 34). Moderate yields are obtained upon irradiation at 254 nm, while better yields are obtained upon irradiation at 365 nm in the presence of a sensitizer, such as Michler s ketone, which diminishes the photo-Curtius side reaction of acylnitrene to isocyanate. With ethoxycarbonylnitrene 247 (R=—OEt) and acetone, compounds 251 and 252 are obtained the latter is possibly formed from acetone and the dipolar intermediate 252 <1995T7181>. In another report on reactions of chiral aroylnitrene 254, the obtained dioxazolines 255 do not show any diastereoselectivity, although the presence of a chiral auxiliary in the ortfe-position ensures its vicinity to the reaction center (Scheme 35) <2001S1125>. 

Acylnitrenes are formed by photolysis of acyl azides and react intramolec-ularly to form lactams.149 154 Evidence was adduced that it is a singlet nitrene which cyclizes to the lactam 108 on photolysis of the sulfilimine 107.155 Thermolysis gave only mesityl isocyanate (109).155 Isocyanates often,156 but not always157 accompany photolyses of acyl azides but generally this is thought to be a nonnitrene process (see below, however). 

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