Secondary α hydrogen–deuterium kinetic isotope effects in the syn-elimination reaction of 2-phenylethyldimethylamine oxide

Abstract

The secondary a-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMS0-+H20 at 60.OoC. A large secondary a-deuterium isotope effect of 1.158 per a-D was found, which indicates significant C-N bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/ kD), is discussed in terms of the use of secondary a-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylarnmonium salts with ethoxide are considered with respect to the relative transition state structures for syn-and anti-elimination reactions.PETER JAMES SMITH et KENNETH CHARLES WESTAWAY. Can. J. Chem. 65, 2149Chem. 65, (1987.Optrant a 60,OoC, dans un melange a 90 mol% de DMSO-H20, on a mesure l'effet isotopique cinttique secondaire du deuterium en a sur la rCaction thermique de l'oxyde de la phCnyl-2 Cthyl dimtthyl amine. On a observk un effet isotopique secondaire ClevC (1,158) pour le deuterium en a et ce resulta4 indique que, dans 1'Ctat de transition de cette Climination syn concertie, il se produit beaucoup de rupture de la liaison C-N et trks peu de formation de double liaison. On discute de cette valeur normale, mais ClevCe, qui a Ct C observte pour le (kH/ko), en fonction de I'utilisation des effets isotopiques secondaires des deutCrium en a pour la dttermination de la sttrtochimie d'un processus concert6 d'klimination. Divers effets isotopiques qui ont Ct C observCs pour 1'Climination syn de l'oxyde d'amine mentionnee plus haut et d'autres effets rapport& pour I'tlimination and des sels d'aryl-2 Cthyl trimethyl ammonium par 1'Cthylate ont Ct C considkrks en relation avec les structures relatives des Ctats de transition des reactions d'tliminations syn et anti.[Traduit par la revue] Introduction In a recent study (1,2) on the E2 reaction of the 2-phenylethyl halides with tert-butoxide in tert-butyl alcohol it was found that the magnitude of the secondary a-deuterium kinetic isotope effect was dependent on the concentration of the abstracting base when chlorine and fluorine were the leaving groups. The secondary isotope effect, (kH/kD),, was inverse at "low" base concentrations and normal at "high" base concentrations. These and other results were discussed in terms of both syn-and anti-elimination pathways promoted by various associated and dissociated base species. It was suggested that the (kH/kD)a effect may be useful as a criterion for determining the stereochemistry of E2 elimination reactions.At low tert-butoxide concentrations, it was suggested that the reaction of both the chloro and fluoro substrates proceed via a syn-elimination pathway where the loss of the leaving group and the P-hydrogen is assisted by the t-BuOK ion pair. For such a transition state, 1, which is expected to have considerabl...