Isocyanates, synthesis from acyl azides

Acyl azides may loose N2 on heating and rearrange to isocyanates (Curtius rearrangement), which may be solvolyzed. Some of the possibilities of classical carboxyl conversions are exemplified in the schemes below, which are taken from a triquinacene synthesis (R. Russo, 1971 C. Merder, 1973) and the ergotamine synthesis of A. Hofmann (1963). 

Synthesis of Isocyanates. In a typical procedure a mixture of lauroyl chloride and acetone is added from a dropping funnel with cooling and stirring to a solution of sodium azide in water at a rate such as to maintain a temperature of 10-15°. After the reaction Is complete (cu. 1 hr.), the lower aqueous layer is removed carefully by suction through a glass capillary lube. The upper layer of crude acyl azide is... 

Another vinyl isocyanate was involved in the Curtius-mediated synthesis of fiiranyl heterocycle 63. The acyl azide precursor was obtained from (Z)-alkenyl carboxylic acid 62 using the Weinstock conditions. Upon Curtius rearrangement, the pendant fliran cyclized onto the resulting (Z)-vinyl isocyanate. 

Diphenylphosphoryl azide (DPPA) and triethylamine were formerly employed by Yamada and co-workers in a simple one-pot synthesis of urethanes from carboxylic adds [250]. This procedure involved treatment of a carboxylic acid with triethylamine to produce the triethylammonium carboxylate salt, followed by heating in the presence of DPPA to yield the isocyanate via a Curtius-type rearrangement of the acyl azide. An alcohol was then introduced, functionalizing the isocyanate in situ to yield the urethane. 

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... 

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