TERT-BUTYLCYANOKETENE-SUBSTITUTED STYRENE CYCLOADDITIONS - A KINETIC-STUDY

Rate constants for the cycloaddition of tert-butylcyanoketene (TBCK) 1 with various substituted styrenes at different temperatures have been determined by H-1 NMR spectroscopy. The addition reaction afforded contra-thermodynamic adducts 3 stereoselectively; a way has been found to convert adducts 3 into thermodynamic cyclobutanones 4. The activation parameters of cycloadditions along with the Hammett reaction constant rho and the solvent effect on cycloreversion of 3 indicate a concerted asynchronous mechanism involving a transition state with some degree of charge separation. An approximate difference in the energy of activation between the favourable and unfavourable mode of TBCK-p-methoxystyrene addition has been determined to be 10.6 kJ mol(-1).