SYNTHETIC AND KINETIC-STUDIES OF THE INTRAMOLECULAR DIELS-ALDER REACTIONS OF CYCLOALKENYLALLENYLPHOSPHINE OXIDES

Kinetic investigations reveal that diphenylphosphinoyl (diphenylphosphine oxide) substituted cycloalkenylallenes 13b-d undergo intramolecular Diels-Alder (IMDA) cyclizations at room temperature to afford adducts with gem-dialkyl effect accelerations of 4.6 (gem-dimethyl), 21.1 (gem-diethyl), and 27.8 (gem-dipropyl), respectively, relative to 13a. Arrhenius data reveals ΔG*'s of between 22.1 and 23.9 kcal/mol for vinylallenes 13a-c. Vinylallenes 24 and 25 revealed mono-tert-butyl acceleration effects of 70.5 and 205, respectively, relative to the parent 13a. Cycloalkenyl ring size studies showed that vinylallenes 13b and 33a-c had cyclization rates within a factor of 10 of one another. Tether length studies revealed that the three carbon tethered allene 43b exhibits an 850-fold decrease in cyclization rate versus the two carbon tethered allene 13b. A comparison of the rate of IMDA cyclization of the three carbon tethered 43a and 43b revealed a gem-dimethyl effect of 2.6, quite similar in magnitude to the 4.6 value observed for the two carbon tethered derivatives 13a and 13b. An X-ray crystallographic analysis of 34a firmly established the stereochemical course of the reaction, including the anti relationship of the bridgehead hydrogens in the resulting polycycles. © 1990, American Chemical Society. All rights reserved.