Surprisingly great difference in reactivity depending upon the ring size: solvolysis and molecular structure study of some N-trimethylsilylated cyclic ureas

The molecular structure of N-trimethylsilylated five- (1a–c), six- (2a,b) and seven-membered (3) cyclic ureas was studied by single crystal X-ray diffraction method and quantum chemical calculations. Solvolysis rate constants of compounds in n-octanol/tetrahydrofuran mixtures were determined by gas chromatography. An alternative sampling method eliminating the effect of the hot injector was also developed in the rate measurement of 2a. The half-lives of the five-membered rings (1a,b) were found to be at least three orders of magnitude higher than those of the six- (2a,b) and seven- (3) membered ones. Relationship between the reactivity and the extent of the pseudo-pentacoordination around the silicon centre in reactants was found. The results are in correlation to our simple static (structure-based) predictive model established previously.