Secondary and Tertiary 2-Methylbutyl Cations. 2. Addition of Trifluoroacetic Acid to Methylbutene

Article abstract of DOI:10.1021/jo00080a026

The reaction of 3-methyl-1-butene (4), 2-methyl-1-butene (5), and 2-methyl-2-butene (6) in trifluoroacetic acid (TFA), neat or buffered with potassium trifluoroacetate, and in 74:26 TFA-MeCN was studied.Alkenes 5 and 6, which form a tertiary carbocation (2) upon hydronation, react 6.6E4 and 5.8E4 times faster than 4 (TFA-MeCN, 25 deg C).The rates in TFA-d gave KIEs of 6.8 (4, 26.5 deg C), ca. 5 (5, -18 deg C), and 3.9 (6, -18 deg C).The esters 3-methyl-2-butyl trifluoroacetate (7) and 2-methyl-2-butyl trifluoroacetate (8) are formed from 4 in the same ratio in TFA and in TFA-d (ca. 53:47), thus providing that 4 reacts entirely by a carbocationic mechanism, with no measurable contribution from a molecular addition.In the presence of p-toluenesulfonic acid the reaction of 4 at 0 deg C forms significant amounts of 3-methyl-2-butyl tosylate (3), and the final ratio of 7:8 was close to that observed in the solvolysis of 3.The different product distributions in solvolyses and alkene hydronations observed before are brought about most probably by the different anions present in the tight ion pairs rather than by differences in the nature of the intermediate carbocations.The reaction in TFA-d showed that the 3-methyl-2-butyl cation (1) undergoes methyl shift to an extent (32percent) smaller than in the solvolysis of the tosylate 3 before being trapped by the solvent.Extensive H/D exchange between the solvent and ester 8 prevented us from ascertaining whether skeletal rearrangement had occured in the latter as well.Formation of the 2-methyl-2-butyl cation (2) from 5 and 6 is reversible, leading to partial isomerization of 5 to 6 in the initial stages of the reaction.Therefore the quoted rate ratios of 5 to 6 to 4 are minimum values.Hydron loss from the ethyl group of 2 is 9.3+/-1.8 faster than hydron loss from a methyl group.In 74:26 TFA-MeCN, cation 2 is trapped by the two solvents in a 12.6:1 ratio at 25 deg C.