Theoretical studies of carbocations in ion pairs. Part 7. Formation of ionpairs from 2-fluoropropane and Lewis acids [1-3]

High level ab initio MO calculations have identified two complexes of boran e with 2-fluoropropane. In the first, the boron atom is above the small C1, C2,C3 angle (in complex); in the other, the boron atom is outside that angl e (out complex). The ionization to form the 2-propyl cation and the trihydr ofluoroborate ion A was followed by lengthening the C-F bond, d, in each co mplex and reoptimizing all other geometrical parameters. It was concluded t hat ionization occurred at d = 1.8-2.0 A. The out ionization pathway was of lower energy (1 - 2 kcal/mol at MP2(FC)/6-31 G**) at al values of d. A thi rd pathway (top), in which the F-B bond was held perpendicular to the C1,C2 ,C3 plane, rather than allowed to tilt, was 8-9 kcal/mol higher in energy t hroughout. Ionization to the tight ion pair did not require a large energy expenditure (19.0 kcal/mol over the isolated reactants at d = 2.00 Angstrom , 30.3 kcal/mol at d = 2.20 Angstrom). A medium of dielectric constant 8.93 , in SCRF(IPCM)-MP2(FC)/6-31G**//MP2/6-31G** calculations, reduced the ioni zation energy to 14.2 kcal/mol at d = 2.20 Angstrom. The dielectric medium also reorders the relative energy of the three orientations to out < top (1 .0 kcal/mol) < in (4.5 kcal/mol), at d = 2.20 Angstrom. Changes in bond len gths and angles show hyperconjugative assistance to the ionization by the m ethyl hydrogens anti to the fluorine along all pathways. As the ions separa te, this effect decreases and. the interaction with the anion of one of the hydrogens initially syn to the fluorine becomes important. Along the in pa thway, rotation of one of the methyl groups occurred along the in pathway a t 2.20 Angstrom and elimination at 2.30 Angstrom; along the top pathway, it occurred at 2.30 and 2.40 Angstrom, respectively. Along the out pathway, m ethyl group rotation brought a hydrogen in each close to the anion at d = 2 .50 Angstrom and elimination followed at 2.70 Angstrom. The top orientation had not eliminated at distances about 2.6 Angstrom and higher in our previ ous work. Both the in and out forms were optimized without elimination at d = 2.80 Angstrom with tetrafluoroborate (C) as anion. The calculations pred ict features of solvolyses, such as transfer of a nucleophile from the back side of the anion (retentive solvolysis), elimination to olefin within the ion pair, and recombination at the backside of the anion (oxygen scrambling faster than racemization of tosylates and carboxylates). (C) 2001 Elsevier Science B.V. All rights reserved.