72 Polynitropolycycloalkanes
1,4-Dinitrocubane (28) has been synthesized by Eaton and co-workers^6 ,^8 via two routes both
starting from cubane-1,4-dicarboxylic acid (25). The first of these routes uses diphenylphos-
phoryl azide in the presence of a base andtert-butyl alcohol to effect direct conversion of
the carboxylic acid (25) to thetert-butylcarbamate (26). Hydrolysis of (26) with mineral acid,
followed by direct oxidation of the diamine (27) withm-CPBA, yields 1,4-dinitrocubane (28).^6
Initial attempts to convert cubane-1,4-dicarboxylic acid (25) to 1,4-diaminocubane (27) via
a Curtius rearrangement of the corresponding diacylazide (29) were abandoned due to the
extremely explosive nature of the latter.^6 However, subsequent experiments showed that treat-
ment of the acid chloride of cubane-1,4-dicarboxylic acid with trimethylsilyl azide allows the
formation of the diisocyanate (30) without prior isolation of the dangerous diacylazide (29)
from solution.^8 Oxidation of the diisocyanate (30) to 1,4-dinitrocubane (28) was achieved
with dimethyldioxirane in wet acetone.^8 Dimethyldioxirane is also reported to oxidize both
the diamine (27) and its hydrochloride salt to 1,4-dinitrocubane (28) in excellent yield.^8
NH 2NH 2 NH 2NO−ONO 2NH 2(^313233)
O ring cleavage
Figure 2.10
1,3-Dinitrocubane is prepared by either of the methods mentioned above.^9 The synthesis of
1,2-dinitrocubane from the oxidation of 1,2-diaminocubane (31) is complicated by cleavage
of the cubane ring – when 1,2-diaminocubane (31) is oxidized, 1-amino-2-nitrocubane (32) is
formed as a halfway intermediate, and the consequence of having an electron-donating group
vicinal to a carbon bearing a electron-withdrawing group is cleavage of the cubane ring.^10 This
is obviously a problem which must be considered when more highly nitrated derivatives of
cubane are synthesized.
34 COCl
ClOC
COCl
ClOC
35
ClOC
COCl
ClOC
COCl
36
ClOC COCl
ClOC
COCl
37
++
35 : 36 : 37 70:8:22
(31 % of 35 after recrystallization)
COCl 2 , hv
COCl
Figure 2.11
Eaton and co-workers also reported the synthesis of 1,3,5-trinitrocubane^11 and 1,3,5,7-
tetranitrocubane (39)^10 ,^11. The required tri- and tetra-substituted cubane precursors were ini-
tially prepared via stepwise substitution of the cubane core using amide functionality to
permitortho-lithiation of adjacent positions.^11 The synthesis of precursors like cubane-1,3,5,7-
tetracarboxylic acid was long and inefficient by this method and required the synthesis of toxic
organomercury intermediates. Bashir-Hashemi^12 ,^13 reported an ingenious route to cubane-
1,3,5,7-tetracarboxylic acid chloride (35) involving photochemical chlorocarbonylation of
cubane carboxylic acid chloride (34) with a mercury lamp and excess oxalyl chloride. Under
optimum conditions this reaction is reported to give a 70:8:22 isomeric mixture of 35:36:37