Azides protonation

The aminopyridinium route has been employed in flash photolysis studies of aryl as well as diarylnitrenium ions. Several examples of nitrenium ion species, along with their absorption maxima and some trapping rate constants are given in Table 13.7. To the extent the data are comparable, there is good agreement with the behavior of nitrenium ions generated by the azide route. For example, the 4-biphenylyl systems from the azide protonation and /-aminopyridinium routes both give absorption maxima at 460 nm and live for several microseconds in water. Likewise, the 4-methoxyphenyl systems show maxima at 300 nm (from azide) and 320 nm (from aminopyridinium ion). The discrepancy in this case can be attributed to the A -methyl substituent, present in the aminopyridinium route, but absent in the azide experiment. 

Figure 5. H-NMR spectra indicating the resonances of the terminal benzyl bromide protons (left) and the terminal benzyl azide protons (right).

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

The influence of boron-bonded ligands on the kinetics and mechanistic pathways of hydrolysis of amine boranes has been examined (37,38). The stoichiometry of trimetbyl amine azidoborane [61652-29-7] hydrolysis in acidic solution is given in equation 10. It is suggested that protonation occurs at the azide ligand enabling its departure as the relatively labile HN species. 

Nitrogen forms more than 20 binaiy compounds with hydrogen of which ammonia (NH3, p. 420), hydrazine (N2H4, p. 427) and hydrogen azide (N3H, p. 432) are by far the most important. Hydroxylamine, NH2(OH), is closely related in structure and properties to both ammonia, NH2(H), and hydrazine, NH2(NH2) and it will be convenient to discuss this compound in the present section also (p. 431). Several protonated cationic species such as NH4+, N2H5+, etc, and deprotonated anionic species such as NH2 , N2H3 , etc. also exist but ammonium hydride, NH5, is unknown. Among... 

The azide ion is a weak base and accepts a proton to form its conjugate acid, hydrazoic acid, HN3. Hydrazoic acid is a weak acid similar in strength to acetic acid. 

There is evidence that the mechanism with carboxylic acids is similar to that of 18-14, except that it is the protonated azide that undergoes the rearrangement ... 

A gas ehromatographic analysis on the produet by the submitter, using an 0.3 x 80 cm. column packed with 10% silicone rubber (SE-30) supported on acid-washed, 60-80 mesh Chromasorb P at 80°, exhibited a single peak. The retention times of di-ter(-butyl malonate, di-fert-butyl diazomalonate, and p-toluenesulfonyl azide were 2, 6, and 9 minutes, respectively. The purity of the product obtained by the checkers was estimated from proton magnetic resonance spectra to be ca. 94%, the remainder being di-tert-butyl malonate. 

In addition to the Beckmann reaction, the Schmidt rearrangement is used to generate M-alkylated lactams, too. Alkyl azides 231 react with the cyclic ketones (and aldehydes) in the presence of proton or Lewis acids. On running the inter-molecular reactions, in most cases symmetric ketals 230 have been converted... 

Substitution of a protonated imidazole group by an azide anion permits an azido-carboxylic ester to be obtained in good yield [137]... 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

A study of the kinetics and products of the thermolysis of a series of diaryl-phosphinic azides has been reported.119 Diethyl 1-diazomethylphosphonates undergo an aldol-type reaction with aldehydes to give l-diazo-2-hydroxyalkylphosphonates (152).120 Acidification of the diazophosphonates (153) possessing a chiral phosphorus centre yields mixtures of diastereoisomers (154) and epimers at C. For given R1 and R2, the reaction becomes increasingly stereoselective for X= OAc < Cl < OTs. It may be argued that protonation of (153) will yield a mixture of diastereoisomeric di-... 

In the presence of alcohols, the corresponding ethers are formed and added nucleophiles such as chloride ion40 or azide ion41 lead to the chloro- and azido-amine products, respectively. Rate constants are independent of the concentration of added nucleophile. Labelled 180 from the solvent is incorporated in the product42. All the evidence points to a reaction mechanism where water is lost from the O-protonated reactant to give a nitrenium ion-iminium ion intermediate which is rapidly trapped by a nucleophile (H2O in this case) to give the final product. This is shown in Scheme 7. Protonation at N- is likely to be more extensive, but there is no pathway to products from the N-protonated intermediate. 

---