Substituent Effect Studies on the Thermal -Sigmatropic Hydrogen Shifts of Vinylallenes

Article abstract of DOI:10.1021/ja00258a041

The vinylallenes 13 were synthesized, and the competitive thermal <1,5>-sigmatropic hydrogen shifts of 13 to 14 plus 15 as a function of various allene end groups (13a-i; H, alkyl, and sulfur substituents) were investigated.The presence of the phenylsulfinyl group (13a-e) at the allene terminus was determined not only to exert an accelerating effect on their <1,5> hydrogen shifts but also to effect control of ?-facial geometric stereoselection in these triene syntheses.In the series 13a-e, the bulkier the R group the greater the observed selectivity (3/1 to > 98/2 favoring 15a-e).The kinetic results for 13f-i and 13a (both diastereomers) indicate that their relative rates for <1,5>-shifts parallel the electron-withdrawing nature of the substituent (SO2Ph > SOPh > SPh > H or t-Bu), but only the sulfoxide (phenylsulfinyl) group exerts significant ?-facial selectivity.Kinetic studies of the <1,5>-shifts performed on several selected vinylallene derivatives to determine activation parameters, solvent effects, and kinetic isotope effects (KIE's) reveal results similar to those for classical nonallenic systems.For example, the two diastereomeric sulfoxides of 13a and their isotopically labeled counterparts 21b rearrange with primary deuterium KIE's (k_H/k_D) of 7.5-8.4 at 40 deg C.The temperature-dependent KIE's of unlabeled 13g and labeled 21c were determined over a ca. 50 deg C temperature range (between ca. 40 and 115 deg C).When the data were extrapolated to 25 deg C, a large k_H/k_D of 12.8 could be calculated.This value is similar to the k_H/k_D value of 12.2 reported by Roth and Koenig for the parent cis-1,3-pentadiene, and in fact, there is a striking parallel between the results described here and Roth and Koenig's results over the entire temperature range from room temperature to 200 deg C.