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18.2.4.1.12.2 Variation 2: Addition of Iodine Isocyanate

DOI: 10.1055/sos-SD-018-00070

Braverman, S.; Cherkinsky, M.; Birsa, M. L.Science of Synthesis, (200518123.

Iodine isocyanate smoothly adds to double bonds even at room temperature to give β-iodo isocyanates, e.g. 150 from 149 (Scheme 85). The addition is stereospecific and anti: iodine isocyanate adds exclusively trans to open-chain and cyclic alkenes and trans diaxially to steroid alkenes. The reaction occurs via the formation of a three-membered-ring iodonium ion intermediate, which is opened from the backside by the isocyanate ion. The reaction has been applied to mono-, di-, and some trisubstituted alkenes and generally follows Markovnikov's rule.[‌411‌‌415‌] Iodine isocyanate can be prepared from iodine and freshly prepared silver(I) cyanate in diethyl ether. Although iodine reacts only slowly with silver(I) cyanate, if an alkene is present in the mixture then iodine isocyanate generated in situ will react rapidly with the alkene, shifting the equilibrium of the reaction between iodine and silver(I) cyanate toward the iodine isocyanate.[‌415‌] Alternatively, iodine can be allowed to react with a large excess of silver(I) cyanate in an organic solvent, and the precipitated silver(I) iodide can be filtered off. Alkenes can then be added to the preformed solution of iodine isocyanate. Also, thallium(I) cyanate is an acceptable and convenient alternative to the unstable silver(I) cyanate.[‌416‌]

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