Organic Chemistry of Explosives

104 Synthetic Routes to Nitrate Esters

Trimethylsilyl ethers and other simplen-alkylsilyl ethers give high yields of nitrate ester

products and reactions are clean. Nitrate esters prepared in this way include: 2-ethylhexanol

nitrate (92 %), 2-octanol nitrate (88 %), 1-decanol nitrate (87 %) and 1,6-hexanediol dini-

trate (83 %). Yields are notably lower for compounds with silyl groups containing secondary

(TIPS) and tertiary (TBDMS) alkyl groups. This in no way detracts value from the method;

trimethylsilyl chloride, the reagent most likely to be used to synthesize these substrates on

an industrial scale, is the cheapest and most readily available of the chlorosilanes. Under the

conditions of nitrodesilylation, the nitrate ester is obtained as a solution in methylene chloride

and so this method must be regarded as safer than conventionalO-nitration with mixed acid.

This reaction is suitable for the synthesis of products containing acid-sensitive functionality

such as the oxetane (48), an energetic polymer precursor, provided that anhydrous conditions

are maintained; the low yield in this reaction is due to the neopentyl-type steric hindrance at

the reacting centres and not due to ring cleavage. The reaction of dinitrogen pentoxide with

cyclic silyl ethers is more complex and depends on the nature of the substrate and the reaction

stoichiometry.

3.6 Additions to alkenes

3.6.1 Nitric acid and its mixtures

C

H 3 C

H 3 C

(CH 3 ) 3 CONO 2

50

HNO 3 , CH 2 Cl 2

-20 °C, 48 %

49

CH 2

Figure 3.29

Solutions of nitric acid in chlorinated solvents can add to some alkenes to give nitrate esters.

Some tertiary nitrate esters can be prepared in this way; isobutylene (49) reacts with fuming

nitric acid of 98.6 % concentration in methylene chloride to givetert-butyl nitrate (50).^91

However, the products obtained depend on both the substrate and the reaction conditions;

β-nitro-nitrate esters,vic-dinitrate esters,β-nitroalcohols and nitroalkenes have been reported

as products with other alkenes.^92 Oxidation products like carboxylic acids are also common,

especially at elevated temperatures and in the presence of oxygen.^93 The reaction of alkenes

with fuming nitric acid is an important route to unsaturated nitrosteroids, which assumedly arise

from the dehydration ofβ-nitroalcohols or the elimination of nitric acid fromβ-nitro-nitrate

esters.^94

Mixed acid has been reported to react with some alkenes to giveβ-nitro-nitrate esters

amongst other products.^91

Solutions of acetyl nitrate at subambient temperature can react with alkenes to yield a mix-

ture of nitro and nitrate ester products. Cyclohexene forms a mixture of 2-nitrocyclohexanol

nitrate, 2-nitrocyclohexanol acetate, 2-nitrocyclohexene and 3-nitrocyclohexene.^95 This il-

lustrates one of the problems of allylic and homoallylic alcohol O-nitration with this

reagent.