Hydrazoic acid protonated

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. 

The seemingly complex imidazolone (78-3) is in fact obtained in a single step by reaction of the amino-ester (78-1) with the iminoether (78-2) derived from capro-nitrile. The relatively acidic proton on the heterocyclic ring is next removed by reaction with sodium hydride. This anion is then alkylated with the same biphenyl-methyl bromide (77-2) that was used to prepare losartan to afford (78-4). The nitrile group is in this case converted to the tetrazole by means of tributyltin azide, a reagent that involves milder conditions than the traditional acidic medium used to generate hydrazoic acid. Thus, treatment of (78-4) with the tin reagent affords irbesartan (75-5) [82]. 

Thus in the reaction of aldehydes with hydrazoic acid, nitriles are generated from the unprotonated form (B) and the formanilides from the protonated form (BH+) of the aldehyde. A relationship between the ratio of the concentrations of the nitrile [N] and formanilide [F] and the pKa value is given in equation 16. 

A surface-mediated (SiC>2 or AI2O3) addition of hydrazoic acid, generated in situ from Me3SiN3 and CF3SO3H, to 1-methylcyclohexene and 1,2-dimethylcyclohexene has been reported69. The reaction obeys the Markovnikov rule and is therefore believed to proceed via the initial protonation of the double bond to generate a carbocation. This mechanism is also supported by the observed non-stereospecificity69. 

Other Substituted Diazonium Ions. A series of aminodiazonium ions have been prepared under superacidic conditions [Eq. (4.141)]. Schmidt495 described the preparation and IR spectra of protonated hydrazoic acid 212 and methylazide as their hexachloroantimonate salts. Olah and co-workers496 have carried out a comprehensive study on aminodiazonium ions (protonated azides) by H, 13C, and 15N NMR spectroscopy. Even the electrophilic aminating ability of aromatics of 212 has been explored.496 The tetrachloroaluminate salt of 212 has also been prepared496... 

However, reaction of acyclic dienamines with hydrazoic acid gives a mixture of products derived by 1,2-, 1,4- and 3,4 + 1,2-addition of HN3 to the diene system. In this case C-protonation is followed immediately by addition of the strongly nucleophilic azide anion, so that equilibrium of the C-protonated enamines cannot occur3c. Treatment of the morpholine dienamine of isophorone with trichloroacetic acid in boiling benzene resulted in decarboxylation and the 1,4-addition of a proton and the trichloromethyl anion. Basic hydrolysis of the adduct gave dienoic acid 54 (Scheme 4). 

The nature of the nucleophile is critical when considering the Mitsunobu reaction as a means of introducing amine functionalities. Because of the proton transfer components illustrated in the reaction mechanism, only acidic nitrogens can be introduced. The most common nitrogen nucleophiles include phthahmide, hydrazoic acid and zinc azide [62]. Once placed, phthahmides are easily converted to amines utihzing hydrazine [6]. Moreover, azides are easily reduced to amines under numerous conditions [32]. Schemes 6.32 and 6.33 illustrate the application of this chemistry to nucleosides [63] and pyranosides [64], respectively. 

Although hydrazoic acid is a weak acid and is probably unable to protonate the hetero atom of the activating groups listed, hydrogen bonding may occur and this would result in mild catalysis of the Ad process. This is illustrated for a, -unsaturated carbonyl compounds equation (73). The examples in Table 5 include reactions in... 

Unstable azido compounds which have been obtained by addition of hydrazoic acid to enamines such as 181 are also thought to arise from an Adf processAlthough catalysis of an Adj reaction by protonation of the heterocyclic nitrogen atom in 181 is possible, and in fact the salt (182) was isolated from the reaction, the position of the azido group in the products (183-185) indicates that an electrophilic addition on the unprotonated enamine is the rate-limiting step. Conjugation of the nitrogen lone-pair with the diene chain could provide sufficient activation for such an addition. 

Ketene also appears to react with hydrazoic acid according to the general mechanism outlined in equation (88). Carbamoyl azides are produced and it is thought that the reaction proceeds by initial protonation and subsequent attack by azide ion to afford an acyl azide further reaction of the derived isocyanate would then lead to the observed product (equation 90). 

Cleavage of the N—N bond of the azido group in triarylmethyl azides does not occur readily in strong, concentrated acids. Protonation of the a-nitrogen atom results in elimination of hydrazoic acid and formation of the triarylmethyl carbonium ions . For example, when 9-azido-9-phenylxanthene (16) was dissolved in sulphuric acid. 

In the Schmidt rearrangement, the treatment of a carboxylic acid with hydrogen azide (hydrazoic acid) also gives the amine, via the isocyanate, when catalysed by an acid, such as sulphuric acid. The first step is the same as the AAC1 mechanism to form the acylium cation, and so is favoured by hindered substrates. The protonated azide undergoes the rearrangement reaction. Illustrate this mechanism as well. 

Misiti et al. found that in coned, sulfuric acid at 0° hydrazoic acid reacts with some alkylated quinones in an entirely different manner to give 2,5Af-2,5-azepindiones such as (5). A solution of the quinone (I) in coned, sulfuric acid is treated at 0° with 1 equivalent of sodium azide, added in portions. When evolution of nitrogen ceases the mixture is poured into ice and water and the precipitate is crystallized from aqueous ethanol. Yields are in the range 75-85%. NMR data for (5) show that the NH proton is directly coupled to the vinylic proton and hence that these groups are adjacent. 

Glaser and Choy (1991) calculated the energy change of protonation of diazomethane, nitrous oxide and hydrazoic acid at RHF/6-31G and MP2/6-31G levels (945, 585, and 792 kJ mol respectively). Consideration of vibrational zero-point energies reduces these values to 907, 560, and 757 kJ mol respectively. The value of diazomethane (907) corresponds reasonably well to Beauchamp s experimental value mentioned earlier (p. 142, 867 kJ mol ). Later Horan and Glaser (1994) were able to get an even better value (883). It is discussed in Section 5.3. 

Hydrazoic acid (HN3) is another member of the set of 16 valence electron 1,3 dipoles. As with both NNO and HCNO, HNNN has a hypervalent central nitrogen atom that is both doubly and triply bound to its neighbouring atoms. The calculated properties of the neutral, protonated and metal ion complexed hydrazoic acid are tabulated in Tables 2, 3 and 33-36. The photochemistry and detonation properties of HN3 have been of particular interest. 

The properties of protonated hydrazoic acid have recently been reviewed As in the... 

The reaction is catalysed by protons (Aq dependence) and Lewis acids, and is strictly intramolecular by the criteria of section III.B. Substituent effects are similar to those in the Hofinann and Lossen processes and a similar concerted mechanism is likely. Despite the forcing conditions, no Wagner-Meerwein shifts in suitably chosen migrating groups have been detected by C-tracer studies . Although 79 could directly form 82, the route shown is necessary to accommodate (i) the ineffectiveness of alkyl azides as substitutes for hydrazoic acid, and (ii) the formation of tetrazoles by 1,3-addition of 81 to azide ion, when the latter is in excess - . 

We see from Table 13.1 that most uncharged acids have the acidic proton bonded to oxygen, a halogen, or sulfur. Occasionally an acid may have H bonded to N as in hydrazoic acid (H—N=N=N), or to C, as in hydrogen cyanide (H—C=N). In most molecules an H atom bonded to C, N, P, Si, or B is not acidic. When writing the molecular formula of a compound with acidic and nonacidic H atoms, the acidic H atoms are shown first (see Appendix II). 

Here HY is a proton donor and Z is a proton acceptor— in most cases these roles are performed by the amine and/or the solvent. All these reactions have a negative activation energy, and all the experimental observations can be accommodated by the mechanism shown in the Scheme. The same carbene complex, Cr(CO)s C(OMe)Ph, reacts with aminoethanol in two ways, to produce (14) and (15). Both are formed by attack at the carbene-C (14) arises from attack by HN=CHMe (aminoethanol minus the elements of water), and (15) from attack by NHg (aminoethanol minus the elements of acetaldehyde). The compound Cr(CO)6 acetoxy-(2-furyl)carbene reacts with hydrazoic acid to give Cr(CO)5(2-furonitrile). 

The reaction of benzene with sodium azide in the presence of concentrated sulfuric acid or aluminum chloride catalyst was first explored by Schmidt, giving aniline in low yield (Eq. 2.8) [96]. Later, Kovacic investigated the mechanism of Lewis Acids catalyzed aromatic amination with hydrazoic acid [97]. Aminodiazonium ion could also be used for the direct amination of aromatics. Olah and coworkers investigated the protonation of hydrazoic acid and alkyl azides with superacid, such as FSOsH/SbFs, HF/SbFs, or HF/BF3, finding the formation of stable aminodia-zomium ions by H, NMR spectroscopic studies [98]. The aminodiazo-... 

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