Organic Chemistry of Explosives

28 Synthetic Routes to AliphaticC-Nitro

Feuer and co-workers^209 conducted extensive studies into alkaline nitration with nitrate

esters, exploring the effect of base, time, stoichiometry, concentration, solvent, and temperature

on yields and purity. Reactions are generally successful when the substrateα-proton acidity is in

the 18–25 pKarange. Alkoxide bases derived from simple primary and secondary aliphatic alco-

hols are generally not considered compatible in reactions using alkyl nitrates. Optimum condi-

tions for many of these reactions use potassiumtert-butoxide and amyl nitrate in THF at− 30 ◦C,

although in many cases potassium amide in liquid ammonia at− 33 ◦C works equally well.

Feuer used both reagents to nitrate a number of cyclic ketones (90) to the dipotassium salts

of the correspondingα,α’-dinitrocycloketones (91).^210 Although such salts can be isolated it

is important to note thatα,α’-dinitroketones are intrinsically unstable – direct acidification of

these salts causes decomposition with the evolution of nitrogen oxides. In fact, nitro compounds

derived from the nitration of active methylene groups are often unstable. This is a direct

consequence of having an electron-withdrawing or resonance-stabilizing groupαto a potential

leaving group; both groups being able to stabilize any anion formed on decomposition.

(CH 2 )n

O

(CH 2 )n

O

2 N NO 2 KKO

(CH 2 )n

O

KO 2 N NO 2

O 2 N(CH 2 ) 2 (CH 2 )n(CH 2 ) 2 NO 2

NaBH 4

KOBr

AcOH

Br

C

Br

O 2 NNOCH 2 (CH 2 )nCH 2 2

Br

C

Br

CH 3 (CH 2 ) 4 ONO 2 ,

THF, -30°C

KOtBu

1. Heat

n = 0, 72 %

(3 steps)

n = 1, 78 %

(2 steps)

2. H+

90

93 94

91 92

Figure 1.38

The salts ofα,α’-dinitrocycloketones (91) are readily converted intoα,ω-dinitroalkanes.

Klager^211 developed a method whereby the alkali metal salt of theα,α’-dinitrocycloketone

(91) is treated with alkaline hypobromite to form the corresponding tetrabromide (93) via

a process of ring opening and formal loss of carbon monoxide; selective reduction of the

latter with sodium borohydride yields the correspondingα,ω-dinitroalkane (94). Feuer and

co-workers^212 ,^213 described an improved method whereby the dipotassium salts ofα,α’-

dinitrocycloketones (91) are partially acidified with acetic acid to give the corresponding

mono-potassium salts (92) which undergo spontaneous hydrolytic ring opening and yield

α,ω-dinitroalkanes (94) on further acidification. 1,4-Dinitrobutane (72 %), 1,5-dinitropentane

(78 %) and 1,6-dinitrohexane (75 %) have been synthesized from cyclopentane, cyclohexane,

and cycloheptane respectively, via these routes.^211 −^213

Treatment of acyclic ketones with one equivalent of potassium amide base in liq-

uid ammonia, followed by acidification, yields the α-nitroketone and products resulting

from fragmentation.^214 The same reactions with cyclic ketones, again, only using one

equivalent oftert-butoxide^214 or potassium amide^215 base, generatesα-nitrocycloketones

and ω-nitrocarboxylic esters. α-Nitrocycloketones derived from the nitration of α,α’-

dialkylcycloketones cannot form a stable anion and so cleavage to theω-nitrocarboxylic ester

predominants.^214