Isocyanates from benzoyl azide

Azidofor my I phony I isocyanate (2-Isocyanate Benzoyl Azide), OC N-CONj, mw 188.15, N 29.78% Needles (from benz), mp 60° (dec) on careful heating explodes violently on rapid heating or on impact. It can be prepd by heating phthalic acid diazide in benz... 

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>. 

In 1956, Hattori and co-workers estabUshed that aluminum azide adds to alkyl isocyanates or acid chlorides in tetrahydrofuran to afford l-aUcyl-A -tetrazoline-5-ones in excellent yields [ 101 ]. Three years later, Horwitz and coworkers reported on the synthesis of l-aryl-A -tetrazoline-5-ones by reaction of aryl isocyanates with a mixture of sodium azide and aluminum chloride in tetrahydrofiuan at reflux temperature [102]. The in situ produced aluminum azide adds to the N=C-bond of the corresponding isocyanate 122 and yields the 1-substituted A -tetrazoHne-5-one 124. According to this method, different 1-substituted A -tetrazoUne-5-ones 124 were synthesized by reaction of phenyl isocyanate and further 1-p-substituted phenyl isocyanates with aluminum azide. In addition, acyl halides 123, like acetyl chloride and benzoyl chloride, were converted to 1-methyl and 1-phenyl-A -tetrazoline-5-one with aluminum azide under the same conditions (Scheme 28A). It is assumed that in the initial step of the reaction, aluminum azide is able to coordinate to the aryl isocyanate by foiu pathways, forming an aluminum salt 129. The first two possibilities (Scheme 28B 125 and 126) require the separation of an azide ion from the complex, recombination at the electrophihc carbon atom followed... 

In 2012, Grushin s team developed a palladium-catalyzed carbonylative coupling of aryl iodides with sodium azide [103]. This catalytic reaction occurs smoothly at temperatures as low as 25-50 °C and 1 bar to cleanly produce aroyl azides from the corresponding aryl iodides, CO and NaNs (Scheme 4.52). The reaction exhibits high-functional group tolerance and can also be conveniently used for one-pot, two-step procedures furnishing primary benzamides, imino-phosphoranes, isocyanates, and ureas in high yield without isolation of the primary benzoyl azide product. 

The Curtius rearrangement also occurs in the direct photolysis of acylazides, but additional products have been isolated and have been shown to arise from typical nitrene reactions . For example, Horner et have trapped benzoyl nitrene with a variety of scavengers. Similarly to the Wolff rearrangement (vide infra) it has been shown ° that the Curtius rearrangement occius via a singlet intermediate this intermediate can be a nitrene or an excited azide molecule in which a concerted decomposition leads to isocyanate derivatives. This reaction could occur in competition with nitrene formation or, alternatively, originate from a vibrationally-excited ground state azide molecule. 

Alkyl isocyanates, like n-butyl isocyanate, do not react with different alkyl azides and aryl azides respectively. In contrast, aryl isocyanates 131 react with alkyl azides 130 like n-butyl azide or cyclohexyl azide to yield 1-alkyl-4-aryl-A -tetrazoline-5-ones 132, however, aryl isocyanates do not react with aryl azides. The reactions take place within some hours and up to several days at elevated temperatures, ranging from 55 to 130 °C, and are performed in benzene or without solvent (Scheme 29A). The addition of aryl azides to acyl isocyanates, such as benzoyl isocyanate or carboalkoxy isocyanates like chloroacetyl isocyanate and trichloroacetyl isocyanate, was unsuccessfully attempted at different reaction conditions [107]. 

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