Replacement with azides

Double-base propellants containing azide polymers are termed nitro-azide polymer propellants. DEP used as a plasticizer of double-base propellants is replaced with azide polymers in order to increase energy density. The compatibility of GAP prepolymer with NG serves to suitably desensitize the mechanical sensitivity of NG and gives superior mechanical properties in the formulation of rocket propellant grains. 

Besides short ELPS, longer ELPs have also been conjugated to synthetic polymers. In one approach, Cu(I)-catalyzed azide-alkyne cycloaddition click chemistry was applied. For this purpose, ELPs were functionalized with azides or alkynes via incorporation of azidohomoalanine and homopropargyl glycine, respectively, using residue-specific replacement of methionine in ELP via bacterial expression [133]. More recently, an alternative way to site-selectively introduce azides into ELPs was developed. Here, an aqueous diazotransfer reaction was performed directly onto ELP[V5L2G3-90] using imidazole-1-sulfonyl azide [134]. 

While it was felt that some of the individual issues above could be addressed using the same synthetic sequence (e.g., alternate catalysts for the reduction step) it seemed unlikely that all the above would be solvable, especially as efforts to replace sodium azide with other nucleophiles had failed. Based on this assessment the team felt it would be necessary to evaluate a fundamentally new approach to taranabant and, in particular, to look for a method for installation of the chiral centers without the intermediacy of an alcohol. 

Prior to our original report7 on this method, acceptable and general preparative routes to a-iodocycloalkenones had not been described. Treatment of a p-substituted cycloalkenone with trimethylsilyl azide and a mixture of iodine and pyridine sequentially in dichloromethane has now been reported as a method for the preparation of p-substituted-a-iodocycloalkenones.8 The combination of iodine and pyridinium dichromate has also been reported to provide a-iodoenones from enones as well as from ethynyl carbinols.9 10 Some successes have also been achieved with enones and iodine azide (IN3)11 and iodine/ceric ammonium nitrate.12-14 The submitters first variant5 of the present procedure used carbon tetrachloride as a solvent. In this procedure this solvent has been replaced with the more benign diethyl ether. 

Reactions of aliphatic nitro compounds with nucleophiles have been reviewed442-444. The oxidative reaction of nitronate anions, e.g. 410, with thiocyanate anions to yield thiocyanates 411 proceeds by a radical radical-anion chain mechanism SrnI (equation 133). Analogous replacements by azide, benzenesulphinate and 4-chlorobenzenethiolate have been reported445. 

A variety of ie.so-epoxidcs could be selectively ring-opened this way with e.e. s as high as 97% [28], The azides can be converted to 1,2-amino alcohols, which are very desirable synthetic intermediates. Surprisingly, the mechanism of the ARO (asymmetric ring-opening) was more complicated than expected [29], First, it turned out that the chloride ion in Cr-salen was replaced by azide. Secondly, water was needed and HN3 rather than Me3SiN3 was the reactant nucleophile. Thirdly, the reaction rate was found to be second order in catalyst concentration, minus one in epoxide (cyclopentene oxide), and zero order in HN3 [30],... 

The burning rates of a nitro-azide propellant composed of NC, NG, and GAP are shown in Fig. 6.19. For comparison, the burning rates of a double-base propellant composed of NC, NG, and DEP are shown in Fig. 6.20. The chemical compositions of both propellants are shown in Table 6.6. The adiabatic flame temperature is increased from 2560 K to 2960 K and the specific impulse is increased from 237 s to 253 s when 12.5% of DEP is replaced with the same amount of GAP. 

Diphenyl phosphorszldate can be replaced with diethyl phosphorazidate in the above procedure. Use of other azides such as p-toluenesulfonyl azide, p-methoxybenzyloxycarbonyl azide, diphenylphosphinic azide, or diphenylthio-phosphinic azide is less satisfactory. No reaction occurs when trimethylsilyl azide, dimethylthiophosphinic azide, or alkaline azides are used, while decomposition of formed trimethylsilyldiazomethane seems to occur when... 

In the study of a nucleophile such as azide, it is possible to replace perchlorate with azide even to 1M concentration without an appreciable change in the amount of water or the activity of water in the system but by the time one has reached M pyridine, 78 grams of pyridine, the medium has changed rather substantially, and the reverse path involving water may be substantially effected. 

Piperidines are obtained from 5-amino-5-deoxy sugars by cycliza-tion with C-l their preparation therefore follows the general methods of preparation of such amino sugars. The 5-O-p-tolylsulfonyl- or 5-0-(methylsulfonyl)-pentoses constitute excellent starting-materials, as these sulfonyloxy groups can be replaced by azide, and the azides can be reduced to the 5-amino derivatives, which are capable of cyclization.240-251 The formation of piperidine 178 from 5-O-p-tolyI-sulfonyl-L-arabinose diethyl dithioacetal240 (176) and 177, and of 181 from benzyl 2,3-0-isopropylidene-5-0-(methylsulfonyl)-a-D-lyxofuranoside250 (179) and 180, are examples of this reaction sequence. 

When this reaction was carried out in 0.10 M hydroxide and 1.0 M azide, both tbe hydroxo (90.3%) and azido (9.7%) complexes were formed. Keep in mind that without base, azide substitution would not be observed within the experimental time period. Furthermore, tbe rate of hydrolysis is not much different when azide is replaced with perchlorate or acetate. It becomes clear that these anions are spectators of the five-coordinate activated complex and are not involved in the loss of X . 

There have been more equilibrium measurements for reactions of carbocations with azide than halide ions. Regrettably, there is little thermodynamic data on which to base estimates of relative values of pARz and pAR using counterparts of Equations (17) and (18) with N3 replacing Cl. Nevertheless, a number of comparisons in water or TFE-H20 mixtures have been made87,106,226,230 and Ritchie and Virtanen have reported measurements in methanol.195 The measurements recorded below are for TFE-H20 and show that whereas pA" 1 is typically 4 log units more positive than pA R. pA Rz is eight units more negative. The difference should be less in water, perhaps by 2 log units, but it is clear that azide ion has about a 1010-fold greater equilibrium affinity for carbocations than does chloride (or bromide) ion. 

The hydroxyl group must be replaced by azide with inversion of configuration. First, however, a leaving group must be introduced, and it must be introduced in such a way that the configuration at the stereogenic center is not altered. The best way to do this is to convert (R)-sec-butyl alcohol to its corresponding p-toluenesulfonate ester. 

Cryptate 72, in which the aryl spacer of 71 is replaced with a furanyl unit, acts a colorimetric sensor for anions. UV-vis titrations in aqueous solution gave log K values for the 1 1 halide/receptor adducts of 3.98 for chloride, 3.01 for bromide and 2.39 for iodide. X-ray diffraction studies confirm that bromide is held between the two copper atoms. Under the same conditions 72 also interacts strongly with azide (log K=4.7) and thiocyanate (log X=4.28) anions. This receptor is interesting because of its lack of selectivity compared to 71. The complex appears to be able to expand and contract its bite length in order to accommodate anions of various sizes. 

An ester group at C-3 or C-5 is not different in function from any ester in aliphatic or aromatic compounds. The ester can be converted into an amide or a hydrazide, or it may be hydrolyzed. The hydrazide can be converted into an acid azide and rearranged to an isocyanate which in turn will form a carbamate or can be hydrolyzed to an amine. However, the amine group can be replaced with chlorine by diazotization in hydrochloric acid, not an ordinary pattern of behavior (76AHC(20)65) although similar reactions do occur in the 1,2,4-triazole ring system. 

One solution for primary amines is to replace ammonia with azide ion N3. This is a linear tri-atomic species, nucleophilic at both ends—a little rod of electrons able to insert itself into almost any electrophilic site. It is available as the water-soluble sodium salt NaN3. 

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