Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

104 NITRILE OXIDES
ambient temperature with the formation of thiohydroximic acid derivatives (510).
Competitive experiments with bis(nitrile oxides) shows that for terephthalonitrile
oxide the second addition proceeds more readily than the Þrst, whereas
with anthracene-9,10-bis(carbonitrile oxide) elevated temperatures are needed to
obtain the diadduct. The reaction between prop-2-ene-1-thiol and p-nitrobenzonitrile
oxide affords products resulting from both cycloaddition and 1,3-addition
with the latter predominating. The polysulÞde prepolymer LP-2 has been vulcanized
effectively at ambient temperatures by both, terephthalonitrile oxide and
4,4 ′ -sulfonylbisbenzonitrile dioxide, at CNO to SH ratios of 1.5 and higher.
Unreinforced sealants produced in this manner are Þrm elastomers. The naphthalenebis(carbonitrile
oxides) afford softer products, while anthracene-9,10-bis
(carbonitrile oxide) is ineffective. Formulations involving in situ generation of
nitrile oxide from hydroximoyl chlorides and Ba(OH)2 (formed by action of water
vapor on BaO) have been also carried out.
Sealants obtained by curing polysulÞde liquid polymers with aryl bis(nitrile
oxides) possess structural feature of thiohydroximic acid ester. These materials
exhibit poor thermal stability; when heated at 60 ◦ C they soften within days
and liquefy in 3 weeks. Products obtained with excess nitrile oxide degrade
faster than those produced with equimolar amounts of reagents. Spectroscopic
studies demonstrate that, after an initial rapid addition between nitrile oxide and
thiol, a second slower reaction occurs which consumes additional nitrile oxide.
Thiohydroximic acid derivatives have been shown to react with nitrile oxides
at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol. In
the case of a polysulÞde sealant, the rupture of a C–S bond to form the thiol
involves cleavage of the polymer backbone. Continuation of the process leads to
degradation of the sealant. These observations have been supported by thermal
analysis studies on the polysulÞde sealants and model polymers (511).
It is evident that reactions of unsaturated polymers with bisnitrile oxides lead
to cross-linking. Such a procedure has been patented for curing poly(butadiene),
butadiene–styrene copolymer, as well as some unsaturated polyethers and
polyesters (512–514). Bisnitrile oxides are usually generated in the presence of
unsaturated polymers by dehydrochlorination of hydroximoyl chlorides. Crosslinking
of ethylene–propylene–diene co-polymers with stable bisnitrile oxides
has been studied (515, 516). The rate of the process has been shown to reduce
in record with the sequence 2-chloroterephthalonitrile oxide > terephthalonitrile
oxide > 2,5-dimethylterephthalonitrile oxide > 2,3,5,6-tetramethylterephthalonitrile
oxide > anthracene-9,10-dicarbonitrile oxide (515).
Bis(nitrile oxides) obtained from dialkylbenzenes have been claimed as lowtemperature
rubber vulcanization agents (517). Curing of poly(butadiene-coacrylonitrile)
with 2,4,6-trimethylisophthalodinitrile N-oxide produces rubbery
material of good quality, however, curing of (polybutadiene) was unsuccessful
(518). The solubility of dinitrile oxides and stability of their ketone solutions
has been studied for their application as vulcanizing agents in the production of
rubberized materials (519).