Phosphoryl nitrenes

The types of reaction open to a nitrene are broadly similar to those undergone by carbenes, but the reactivity of nitrenes is considerably lower. Nitrenes are less indiscriminate than carbenes in their reactions with primary, secondary, and tertiary carbon-hydrogen bonds, for instance, and nitrenes are also somewhat electrophilic, preferring an 0-H over a C-H bond. By far the most frequently used nitrene source is an aryl azide (see Table IV). The derived aryl nitrenes are much less reactive than a-keto, a-sulfonyl, or a-phosphoryl nitrenes, but the use of these acyl nitrenes is ruled out by the high chemical reactivity of the precursor species acyl azides, sulfonyl azides, and phosphoryl azides. 

Phosphoric-carboxylic anhydride, see Carboxylic-phosphoric anhydride Phosphorus tribromide, 39 Phosphorylase b, 241, 293, 310-318 activation, 293 Phosphorylation cd nucleosides, 324,325 oxidative, uncoupling, 83 Phosphoryl azide, 78 Phosphorylazide diesters, 87 of-Phosphoryl nitrenes, 78... 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

Aryldioxophosphoranes such as aryl- and alkyl-iminooxophosphoranes number among the short-lived metaphosphonates. The former are best obtained by fragmentation of cyclic phosphinic esters, and the latter by rearrangement of aryl- and alkyl-substituted phosphoryl azides and nitrenes, respectively. This reaction is reminiscent of the phosphorylcarbene/methyleneoxophosphorane rearrangement discussed in Section 2. 

Photolysis of bis(dimethylamino)phosphoryl azide 2071401 represents an entirely different entry to a metaphosphorimidate. If the reaction is performed in cyclohexane, it gives only 7 % of the amide 209 which can be rationalized as the insertion product of the intermediate nitrene 208 into a CH bond of cyclohexane. The major product component is a polymer. The assumption that it is polymeric aminometa-phosphorimidate 212 is substantiated indirectly by the nature of the principal product of photolysis of 207 in methanol. A 1,2-shift of a NMe2 moiety which... 

McCulla, R. D., Gohar, G. A., Hadad, C. M., Platz, M. S., Computational Study of the Curtius like Rearrangements of Phosphoryl, Phosphinyl, and Phosphinoyl Azides and Their Corresponding Nitrenes, J. Org. Chem. 2007, 72, 9426 9438. 

The highly reactive nitrenes derived from phosphoryl azides do not readily undergo rearrangement. They also appear to have a low selectivity with respect to —H insertion reactions. Thus, irradiation of diethylphosphoryl azide (143 R = Et) in cyclohexane, for example, led to formation of diethyl cyclohexyl-phosphoramidate (144 R = Et) in 88% yield.83 A lower yield (67%) of insertion product, diphenyl cyclophosphoramidate (144 R = Ph), was obtained on... 

General. Diphenyl phosphorazidate is a readily available, nonexplosive, and relatively stable azide widely used as a reagent in peptide synthesis, " and as a versatile reagent in a wide array of organic transformations. DPPA has been successfully utilized in the synthesis of a-amino acids and o-aryl carboxylic acids direct preparation of thiol esters from carboxylic acids and thiols the stereospecific preparation of alkyl azides and the phosphorylation of alcohols and amines The application of DPPA in a modified Curtius reaction permits a simple one-step conversion of carboxylic acids to urethanes under mild reaction conditions. DPPA acts as a nitrene source, and can undergo 1,3-dipolar cycloaddition reactions. The Curtius degradation of carboxylic acids in the presence of f-butanol gives the Boc-protected amine directly (eq 1). 

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