Addition reactions of diarylcarbenium ions to 2-methyl-1-pentene: kinetic method and reaction mechanism

Abstract

observed percent I8O incorporation the scrambling rates were calculated by standard methodsi6.Two control experiments in each solvent were performed in order to show that the observed I8O scrambling is not the result of ( I ) chemical workup or ( 2 ) external ion return.In the first experiment 25 mg of the labeled ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of 2,6-lutidinium brosylate, and the reaction mixture was worked up immediately in the same manner.In the second experiment 50 mg of the unlabeled sulfonate ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of "0 enriched (60.7%) 2,6-lutidinium brosylate and after solvolysis for 1 half-life the solution was worked up in the same manner.In the I3C NMR spectrum of the isolated ester from both experiments, no "0 was observed to be present at the a-carbon.Oxygen Scrambling Studies.2. 4-Methyl-3-homoadamntyl Heptafluorobutyrate.A 5.3 mM solution of the ether I80 enriched (50.70%) 4-methyl-3-homoadamantyl heptafluorobutyrate in 80E (100 mg/50 mL) containing a I . 1 equiv of 2,6-lutidine was reacted at 25 OC for 9.5 1 h.The reaction flask was then placed in a 0 "C bath, and the workup was the same as that used in the I8O studies of 2s.The composition of the product mixture was analyzed by IH NMR (300 MHz) in an analogous fashion to that performed in the product studies of the tertiary ester above.In the spectrum the additional methyl doublet of the unreacted tertiary ester occurs at 0.93 ppm.The percentage I80 incorporation at the a-carbon of the two esters and of the solvolysis products was determined from the natural abundance 125-MHz "C spectrum recorded on a Bruker 500-MHz Fourier transform spectrometer with the conditions for data acquisition being similar to those in the I8O studies of 2a.No "0 incorporation was observed at the a-carbon of the alcohol and ether solvolysis products.In the unreacted tertiary ester and the rearranged secondary ester the percent present was 43.68% and 41.56% respectively, and their recorded spectra are shown in Figure 3.The proportion of unreacted tertiary ester that was equilibrated is 27.7% [(50.7 -43.68)/23.35X 1001 while that for the secondary ester was 36.0%.The calculation of the rate of I80 equilibration (9.47 X IOd S-I) for the tertiary ester as well as the ratio of return of the originally bonded oxygen relative to the carbonyl oxygen (6.6:l) which occurs in the formation of the secondary ester from unscrambled tertiary ester is given in detail in the Supplementary Material.)$Solvent Preparation.UV and Conductance Kinetic Procedures.The procedures were the same as those which have been previously report-