INFLUENCE OF STRAIN ON CHEMICAL-REACTIVITY - RELATIVE REACTIVITY OF TORSIONALLY STRAINED DOUBLE-BONDS IN 1,3-DIPOLAR CYCLOADDITIONS

The addition of picryl azide to a series of mono- and bicyclic olefins including trans-cycloalkenes and bridgehead alkenes is reported. Secondary reaction products were identified, and their mechanism of formation was explained by an analysis relating the transition state energies to the conformation of the reaction intermediates. Bridgehead alkenes showed reversed regiochemistry of cycloaddition when compared with model alkenes. It is proposed that the reversal of regiochemistry is caused by the out of plane bending of the bridgehead double bond. The kinetics of the cycloaddition reveals significant increases in reaction rate for the picryl azide addition to strained olefins. A good correlation was found in the plot of log k(rel) vs DELTA-SE, where DELTA-SE values were calculated from differences of MM-2 steric energy between olefins and a model of the corresponding triazoline cycloadduct. This finding indicates that the strain relief in the transition state is a major factor affecting the reactivity of alkyl-substituted mono- and bicyclic olefins. The HOMO energy change did not affect the reactivity significantly in the picryl azide addition to alkyl-substituted olefins as evidenced by the absence of correlation in log k(rel) vs ionization potential. Up to 60% of the steric energy difference (DELTA-SE) is found to be relieved in the transition state from the slope of the log k(rel) vs DELTA-SE plots. By comparison of the reaction rates for the MCPBA epoxidation and the picryl azide addition, reactivity patterns were found to be similar among the olefins studied except for norbornene. An attempt was made to explain the norbornene anomaly by calling attention to the torsional interactions that develop between forming bonds and existing carbon-carbon bonds in the methylene bridge. An important outcome of this research is the quantification of the effect of strain on chemical reactivity and selectivity. The olefin strain changes both the rate of reaction and regiochemistry of the cycloaddition.