691-75-8Relevant articles and documents
FRAGMENTATION PROCESSES OF METHYLBUTYL TRIFLUOROACETATES STUDIED BY TANDEM QUADRUPOLE MASS SPECTROMETRY AND ISOTOPE LABELING
Hsu, Chang S.,Farcasiu, Dan
, p. 737 - 747 (1989)
The fragmentation patterns of 3-methyl-2-butyl trifluoroacetate and 2-methyl-2-butyl trifluoroacetate were investigated by GC/MS/MS, with electron impact and collision-induced dissociation, on regular and isotope-labeled (deuterium and 18O) esters.The atoms found in various fragments could be traced back to specific positions in the parent molecules.In this way, molecular rearrangements potentially occuring during the formation of esters by trifluoroacetolysis of 3-methyl-2-butyl p-toluenesulfonate or trifluoroacetic acid addition to various 2-methylbutenes could be shown.Rearrangements also occured during the fragmentation, particularly during the expulsion of the small fragments CO, C2H4 and F2CO.For the decompositions of oxygen-containing ions these rearrangements were highly specific.By contrast, alkyl cations lead to fragments that are fully scramled (statistical label distribution).Alkene radical cations ((CnH2n)(+.)) fragment to daughter ions that are extensively, but less than statistically scramled.Hydrogen scrambling may also occur in fluoroalkyl cation fragments.
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.
