Cyanoketene cycloaddition reactions

Abstract

Several aspects of (2+2) cycloaddition reactions of tert-butylcyanoketene (TBCK) with different ketenophiles have been studied. Cycloaddition of mono substituted alkenes having tert-butyldimethylsiloxy group attached to non-alkenic carbon has been found to undergo regio- and stereo-selective addition to TBCK to give contrathermodynamic product as the sole adduct. However, addition to styrene and p-methoxy styrene was found to be non-selective and a mixture of adducts were formed in each case. A way has been found to convert the contrathermodynamic adduct to the thermodynamic product. Addition reaction of TBCK with 1,1-disubstituted alkenes having t-butyldimethylsiloxy, or benzyloxy group attached to allylic or homoallylic positions afforded a mixture of cyclobutanones in addition to acyclic product arising out of ene reaction. Evern though the addition reaction of TBCK with phenylacetylene gave cyclobutanone in good yield, the corresponding reaction with aliphatic terminal alkynes, such as, 1-Hexyne, 1-octyne or silyl either of proporgyl alcohol afforded the cycloadduct in very low yield. Cycloaddition of TBCK with cyclic nitrones afforded a complicated mixture of intractable products. The stability of several cyclobutanones in different solvents and temperature are studied. In general 3,3- disbustituted cyclobutanones are found to undergo cycloreversion faster than the 3-substituted cyclobutanones. The rate constants for the cycloreversion of several cyclobutanones were found to be more or less solvent independent. The mechanism of the cycloaddition or the cycloreversion does not seem to have a unified mechanism. Depending on the nature of the ketenophiles, the mechanism seems to vary from concerted-synchronous to concerted asynchronous or to stepwise mechanism involving zwitterion intermediate.