461-73-4Relevant articles and documents
Secondary and Tertiary 2-Methylbutyl Cations. 2. Addition of Trifluoroacetic Acid to Methylbutene
Farcasiu, Dan,Marino, Gaye,Hsu, Chang S.
, p. 163 - 168 (1994)
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.
Secondary and Tertiary 2-Methylbutyl Cations. 1. Trifluoroacetolysis of 2-Methyl-2-butyl Tosylate
Farcasiu, Dan,Marino, Gaye,Harris, J. Milton,Hovanes, Bruce A.,Hsu, Chang S.
, p. 154 - 162 (2007/10/02)
Trifluoroacetolysis rates for 3-methyl-2-butyl tosylate (4) and kinetic isotope effects at C(1) (kH/kD = 1.083 per H atom), C(2) (kH/kD = 1.10), and C(3) (kH/kD = 1.82) were determined.The products are 2-methyl-2-butyl trifluoroacetate (5, 98.5percent) and 3-methyl-2-butyl trifluoroacetate (6, 1.5percent).GC-MS analysis of products from labeled tosylates 4-1-d3 and 4-2-d showed that 42percent of the apparently unrearranged 6 had a methyl group shifted from the original C(3) to the original C(2), whereas 3.6percent methyl shift occurred in 5.The results do not substantiate a ks-kΔ competition mechanism.Instead, two carbocations, the tertiary 2-methyl-2-butyl (1) and the nominally secondary 3-methyl-2-butyl (2) intervene.The intimate structure of 2 is not established, but a symmetrical, methyl-bridged ion (3) does not agree with the results.A high β isotope effect does not require hydrogen assistance to ionization; ionization concerted with (assisted by) hydrogen migration is unimportant in formation of 1 (and 5) from 4.Instead, the reaction involves reversible formation of an intimate ion pair with subsequent rate-determining H shift (which for 2*OTs(-) is in competition with Me shift and ca. 25percent elimination) followed by solvent capture.Methyl migration in 2 may occur in the solvent-separated ion pair; alternatively, methyl or hydrogen migration is conformationally determined.At least 9percent of 1 is formed from 2 which has undergone methyl shift.Nucleophilic attack on 4 appears important only in strongly nucleophilic media like aqueous ethanol.The claim that nucleophilic solvent assistance is significant in solvolysis of other secondary alkyl substrates in TFA or 97percent hexafluoro-2-propanol is evaluated.Such a conclusion cannot be accepted on the basis of rate correlations alone, (i.e without product studies to support it).The implications of our results for the trifluoroacetolysis of 2-butyl tosylate are briefly discussed.
