Theoretical studies of methane elimination from C 4H 9+ and H 2 elimination from C 3H 7+

Abstract

Experimental evidence indicates that CH 4 elimination from C 4H 9 + forms CH 3C +CH 2, suggesting a 1,2-process. This reaction releases appreciable translational energy, and is formally Woodward–Hoffmann forbidden. However, according to its appearance energy it takes place at its thermochemical threshold. We investigated the reaction by theory because it might be an unprecedented concerted 1,2-alkane elimination. We found transition states for two pathways for CH 4 elimination from C 4H 9 + isomers. The lower energy one involves migration of an H from a methyl to the central carbon of the tert-butyl cation followed by insertion of that H into a CCH 3 bond to form CH 3C +CH 2+CH 4; the other is a 1,1-elimination from protonated methylcyclopropane to produce CH 2CHCH 2 ++CH 4. H 2 elimination from C 3H 7 + was also characterized to determine how that reaction and CH 4 elimination from C 4H 9 + may be related. In accord with earlier studies, we found H 2 elimination from C 3H 7 + by a 1,1-elimination through a 1-propyl-like stage in which an H migrates from C1 to C2 to C3, joining an H from C3 in the latter part of the last step. Also in accord with previous studies, we found a 1,1-H 2 elimination from a protonated cyclopropane. The two pathways for H 2 elimination from C 3H 7 + are quite similar to the corresponding two for CH 4 elimination from C 4H 9 +. One type of elimination involves H-migration from methyl to an adjacent positively charged site, further migration and insertion into a bond, and finally dissociation. The other pathway in both instances involves a 1,1-elimination from a pentavalent carbon in a protonated cyclopropane. These findings extend the scope of evidence that most formal 1,2-eliminations have transition states much like ones for 1,1-eliminations and add an alkane elimination to the 1,2-H 2 eliminations that essentially go through 1,1-transition states. CH 4 elimination from C 4H 9 + also bypasses the Woodward–Hoffmann barrier that opposes a direct 1,2-elimination.