Isocyanates acylnitrenes

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. 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

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>. 

The Curtius degradation of acyl azides (Figure 14.43) expels molecular nitrogen and at the same time leads to the [1,2]-rearrangement of the substituent that is attached to the carboxyl carbon. It is the simultaneous occurrence of these two events that prevents the formation of an energetically unacceptably disfavored acylnitrene intermediate. The rearranged product is an isocyanate. 

Pyrolysis of the cyclic carbonate at 275 C and atmospheric pressure results in expulsion of carbon dioxide, and the resulting acylnitrenes rearrange to give an almost quantitative yield ofperfluoroalkyl isocyanates 4. Trifluoromethyl, heptafluoropropyl and perfluoroheptyl isocyanates can be prepared by this method. 

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). 

The photolysis of aminimides produces acylnitrenes, which have usually been trapped in intermolecular reactions.151,159 Again, isocyanates often accompany the reactions. The thermolysis of 2,4,6-triphenylpyridine N-acylimines at 170-250°C affords isocyanates in good yields.160... 

The yields of the isocyanates produced upon photolysis of benzoyl azide (R=Ph, 31b) and its pam-methoxy, para-chloro and meta- uoro derivatives were found to be in the range of 40-50% in both inert solvents and in solvents that intercept acylnitrenes. Similar results were obtained for 2-naphthoyl azide (35). Irradiation of 35 in cyclohexane at room temperature produces isocyanate (36, -45%), A-cyclohexyl-2-naphthamide - the product of 2-naphthoylnitrene (37) insertion (-50%), and a trace (<1%) of 2-naphthamide (38). Therefore, it was concluded that carbonylnitrenes (R-CO-N) do not rearrange to isocyanates (R-N=C=0) at a rate that is competitive with their capture by trapping agents. ... 

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