Theoretical Studies of Carbocations in Ion Pairs. 8. Search for Anchimeric Assistance in the Ionization of 2-Butyl Cation Precursors

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2008-04, Vol.112 (13), p.2955-2961
• Main Authors: Fǎrcaşiu, Dan, Leu, Rodica
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0750553

The structures of carbocations formed in the ionization of 2-butyl precursors were investigated by high level ab initio MO calculations on the reaction of 2-butyl fluoride (1) with borane, which gives a C4H9 + cation paired with trihydrofluoroborate (FBH3 -, A). Two conformations of the “open,” secondary cation (2) in the ion pair resulted from two conformations of 1, with F gauche and trans to C4 (2-g and 2-t, respectively). No anchimeric assistance by hydrogen (in 1-g) or methyl (in 1-t) was evidenced. In fact, attempts at optimizing the geometry of the H-bridged (3) and methyl-bridged (6) cations at short interionic distances (d) led to the corresponding conformations of 2. Upon ion separation, proton transfer from 2 to the anion occurred at intermediate interionic distances, consonant with experimental observations in trifluoroacetic acid. Elimination was prevented by addition of a lithium cation to the ion pair, i.e., running computations on triple ions (2.A.Li + ). Cation 6 became an energy minimum beyond d = 2.5 Å and 3 beyond 2.8 Å. Cation 2-g was still the most stable isomer at d = 3.2 Å, which was greater than the interionic distance in the crystals of the isomeric tert-butyl cation salts (3−3.1 Å). Thus, spectral determinations of 2-butyl cations in the solid state should be interpreted with 2-g as the main component of the ion mixture. When the ions became separated (d ≥ 4 Å), only the bridged ions were energy minima. In this process, bridging did not occur opposite to the leaving group to assist the ionization, but on the same side with it, being controlled by the electrostatic interaction with the anion, as it departed from the vicinity of the cation. Such behavior was also noted in the ionization of the 3-methyl-2-butyl homolog.