Aminimides cyclic

Dialkylamino isocyanates are known transient intermediates and, in the absence of other reagents, have been shown to dimerize. When the cyclic aminimide (c) was heated at 130°C under vacuum (0.3 mm) for 3 h, (d) was obtained in 89% yield. At higher temperature (d) was formed but rearranged to (e) as depicted in scheme 146. 

In a preferred example, when UDMH in anhydrous THF was added to a solution of 1-chloroethyl chloroformate in THF at 10-20°C, the intermediate carbazate (a) was formed. After filtration to remove the UDMH hydrochloride, we accomplished a cyclisation to (b) just by refluxing the THF solution for 1 h.. The resulting hygroscopic salt (b) precipited immediately in 71 % overall yield. In solid form, (b) is stable undefinitely at room temperature. The cyclic aminimide (c) was then easily prepared from (b) by treatment with a resin supported base in high yield. The three steps of the synthesis are depicted in scheme 145. 

Scheme MS New cyclic aminimide from UDMH and ACBCI.

Table 3-25 Preparation of UDMH derivatives through reaction of cyclic carbal-koxy aminimide (c) with a nucleophile Nu-H.

The trimerization of isocyanates to form isocyanurate rings is particularly important in producing iire-resistant rigid foams, and a large volume of work has appeared on this topic. Much of it has been done by Kresta and co-workers, who have discussed the general mechanism, the co-catalytic effect of urethane groups, and the use of cyclic sulphonium zwitterions and aminimides as catalysts. Model systems have been used for these studies, but attention has also been paid to the kinetics and mechanism of commercial polyisocyanurate systems. ... 

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