SECONDARY AND TERTIARY 2-METHYLBUTYL CATIONS .1. TRIFLUOROACETOLYSIS OF 3-METHYL-2-BUTYL TOSYLATE

Trifluoroacetolysis rates for 3-methyl-2-butyl tosylate (4) and kinetic isotope effects st C(1) (k(H)/k(D) = 1.083 per H atom), C(2) (k(H)/k(D) = 1.10), and C(3) (k(H)/k(D) = 1.32) were determined. The products are 2-methyl-2-butyl trifluoroacetate (5, 98.5%) and 3-methyl-2-butyl trifluoroacetate (6, 1.5%). GC-MS analysis of products from labeled tosylates 4-1-d(3) and 4-2-d showed that 42% of the apparently unrearranged 6 had a methyl group shifted from the original C(3) to the original C(2), whereas 3.6% methyl shift occurred in 5. The results do not substantiate a k(s)-k(Delta) 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 beta 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. 25% 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 9% of 1 is formed from 2 which has undergone methyl shift. Nucleophilic attack an 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 97% 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.