109221-64-9Relevant academic research and scientific papers
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 (1994)
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.
