And Chlorodimethylbutyl Cations

Study of the ﬂuoro- and chlorodimethylbutyl cations

George A. Olah1, G. K. Surya Prakash, and Golam Rasul

Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1661

Contributed by George A. Olah, April 3, 2013 (sent for review March 19, 2013) A comparative study of the 2,3-dimethyl-3-fluoro-2-butyl cation In following up our previous experimental studies (6) and in- and its chloro analog was carried out by the ab initio/GIAO-CCSD terest in the question of fluoronium ions, our present calcula- (T) (gauge invariant atomic orbital-coupled cluster with single, tional study based on ab initio/GIAO-CCSD(T) methods show double, and perturbative triple excitation) method. The struc- that there is no evidence for a 1,2-fluorine shift, and conse- tures and 13C NMR chemical shifts of the cations were calcu- quently involvement of bridged tetramethylethylenefluoronium lated at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. Bridged ion ii either as an intermediate or a transition state in the equili- fluoronium ion 1, carbenium ion 2, and fluorocarbenium ion 3 bration of the 2,3-dimethyl-3-fluoro-2-butyl cation i in superacid were found to be minima on the potential energy surface. Bridged solutions. fluoronium ion 1, although a minimum on the potential energy surface, is 12.8 kcal/mol less stable than the open chain fluorobutyl Calculations cation 3. In contrast to the fluorinated ion, bridged chloronium ion Geometry optimizations and frequency calculations were carried 5 was found to be the lowest energy minimum being 10.6 kcal/mol out with the Gaussian 09 program (8). The geometry optimiza- more stable than ion 6 and 7.4 kcal/mol more stable than ion 7. tions were performed at the MP2/6-31G** level. Vibrational frequencies at the MP2/6-31G**//MP2/6-31G** level were used carbocations | computational studies to characterize stationary points as minima [number of imaginary frequency (NIMAG) = 0] or transition state (NIMAG = 1). The alonium ions are an important class of onium ions (1). The MP2/6-31G** geometries were further optimized at the higher Hrole of alkene halonium ion as reaction intermediates in MP2/cc-pVTZ level. Calculated energies are given in Table 1. electrophilic addition of halogens to olefins is well recognized NMR chemical shifts were calculated by the gauge-invariant – (1). A wide variety of chloronium, bromonium, and iodonium atomic orbitals (GIAO) method (9 11) using MP2/cc-pVTZ geometries. GIAO-CCSD(T), GIAO-MP2, and GIAO-SCF cal- ions have been prepared as stable long-lived ions (1). Several – studies on the fluorination of olefins have led to suggestion that culations using tzp/dz basis set (12 14) have been performed with the ACES II program (15). The 13C NMR chemical shifts were only trivalent persistent fluorocarbenium ions are involved as in- computed using tetramethylsilane (calculated absolute shift, i.e., termediates (2). There was some suggestion (3) of the formation σ = fl (C), tzp/dz 193.9 (GIAO-SCF), 199.6 (GIAO-MP2), 197.9 of bridged uoronium ion in the condensed state. Evidence of [GIAO-CCSD(T)]) as a reference. asymmetricalfluoronium ion as low-energy transition state was found in the condensed state when 1,8-bis(diphenylmethylium) Results and Discussion − naphthalenediyl dication was treated with Me3SiFe2 (4). A Si- 2,3-Dimethyl-3-fluoro-2-butyl Cation. Three structures, bridged flu- fl containing ring system involving uoronium ion, analogous to oronium ion 1, fluorocarbenium ion 2,andfluorocarbenium ion 3, naphthalenediyl dication system, has also been reported in the + were found to be energy minima for Me2CCFMe2 at the MP2/6- condensed state (5). 31G** level of theory (Fig. 1). The structures were further opti- In an attempt to observe the tetramethylethylenefluoronium mized at the higher MP2/cc-pVTZ level. Fluorine bridged structure ii fl ion in solution, Olah and Bollinger (6) ionized 2,3-di uoro-2, 1 was found to be C2 symmetrical at the MP2/cc-pVTZ level − 1 3-dimethylbutane in SbF5/SO2 at 90 °C. The H NMR spectrum with the C-F length of 1.610 Å. Carbenium structure 2 seems to be CHEMISTRY of the acid solution at −90 °C showed a deshielded doublet at δ stabilized by C-H and C-C hyperconjugations. However, fluo- 3.10 (J = 11.0 Hz), indicating the formation of cationic species rocarbenium structure 3 is stabilized by C-C hyperconjugation as wherein all of the methyl groups were equivalent with a long- well as by fluorine back donation as the C-F bond length was range proton fluorine coupling. In the 19F NMR spectrum, the found to be considerably shortened (1.282 Å) compared with observed fluorine signal was deshielded by 31 ppm from the that of structure 2. difluoro progenitor. On the basis of these data, it was suggested At the MP2/cc-pVTZ//MP2/cc-pVTZ + zero-point vibrational (6) that the ion obtained was the 2,3-dimethyl-3-fluoro-2-butyl energy (ZPE) level 3 was found to be more stable than the cation (a fluorodimethylisopropylcarbenium ion) i, wherein the structure 2. We have located a transition structure, 4 (Fig. 1), at 2 methyl groups become equivalent either by a fast intramolecular the MP2/cc-pVTZ level for the interconversion of the ions and 3 4 fluorine exchange or by methyl group shifts. The formation of a through intramolecular methyl migration. Structure lies only 2 long-lived tetramethylethylenefluoronium ion ii was ruled out on 2.1 kcal/mol higher in energy than structure . The interconversion between 2 and 3 through transition state 3, therefore, is the basis of comparison with model compounds. If equilibration 1 of ion i occurred through rapid intramolecular fluorine exchange, facile. Fluoronium ion , although a minimum on the potential energy surface, is 12.8 kcal/mol less stable than the structure 3 one could invoke ii only as a noncontributing high-lying inter- (Table 1). This clearly rules out a path involving equilibration of mediate or a transition state (Scheme 1, path i). If the exchange the ion 2 through intramolecular fluorine migration through in- involves rapid 1,2-methyl migrations, then the 3,3-dimethyl-2- 1 1–4 fl fl tert iii termediate . Relative energies of the structures were plot- uoro-2-butyl cation [(1- uoro-1- -butyl)ethyl cation] should ted in Fig. 2. be involved. These two possibilities, however, could not be dis- tinguished at the time on the basis of 1Hand19FNMRstudies(6). + Later studies of the equilibrating Me2C CFMe2 by Olah et al. 13 Author contributions: G.A.O., G.K.S.P., and G.R. designed research; G.R. performed re- (7) using C NMR spectroscopy showed that equilibration search; G.A.O., G.K.S.P., and G.R. analyzed data; and G.A.O., G.K.S.P., and G.R. wrote occurs through rapid intramolecular methyl migration rather the paper. than intramolecular 1,2-fluorine exchange, thus ruling out the The authors declare no conflict of interest. involvement of ii in the equilibration process (Scheme 1). 1To whom correspondence should be addressed. E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1306252110 PNAS | May 21, 2013 | vol. 110 | no. 21 | 8427–8430 Downloaded by guest on September 30, 2021 Fig. 1. MP2/cc-pVTZ optimized structures of 1–4.

as indicated by our present calculations, an equilibrium mixture (undergoing rapid methyl exchange on the NMR timescale as shown in Scheme 1) involving ions 2 and 3 mightbestrepresenttheex- perimentally observed 2,3-dimethyl-3-fluoro-2-butyl cation (Scheme 2). Such an equilibrium was not considered previously (7). This also rules out a path, wherein the equilibration of ion 2 takes place through intramolecular fluorine migration involving intermediate 1. Scheme 1. 2,3-Dimethyl-3-chloro-2-butyl Cation. Similar to fluoro cations, three related chloro cations, bridged chloronium ion 5,car- 13 6 7 The C NMR chemical shifts of 1–3 were calculated by the benium ion , and chlorocarbenium ion , were calculated and GIAO-coupled cluster method at the GIAO-CCSD(T)/tzp/dz also found to be as minima at the MP2/cc-pVTZ level (Fig. 2). level using MP2/cc-pVTZ geometry (Table 2). For comparison, However, unlike fluoronium ion 1, chloronium ion 5 was the 13C NMR chemical shifts of the ions were also computed at found to be the lowest energy minimum, being 9.0 kcal/mol the GIAO-MP2/tzp/dz and GIAO-SCF/tzp/dz levels (Table 2). more stable than 6 and 7.4 kcal/mol more stable than 7.This The calculated 13C NMR spectrum of 2 shows absorption at + δ13C 346.8 (C2 carbon or C ), 108.6 (C3), and 25.5–45.3 (methyl + carbons). The computed value of the C2 (C )carbonofδ13C, 346.8 ppm, greatly deviates from the reported experimental value by 22.5 ppm (7). The calculated δ13C of the C3 carbon of 115.2 also deviates from the experimental value by 6.6 ppm. The ion 2 was observed by Olah et al. (7) by ionizing 2,3-difluoro-2, 13 3-dimethylbutane SbF5/SO2 at −90 °C and studied in detail by C NMR spectroscopy. As the ions (2 and 3) are very close energetically

Table 1. Total energies (-au), ZPE, and relative energies Relative energy, No. MP2/6-31G** ZPE MP2/cc-pVTZ kcal/mol

1 334.37824 102.4 334.72751 12.8 2 334.39062 101.6 334.74251 2.3 3 334.39566 103.0 334.74887 0.0 4 334.38638 102.2 334.74064 4.4 5 694.74311 102.4 694.74311 0.0 6 694.72670 101.1 694.72670 9.0 7 694.73108 102.2 694.73108 7.4 8 694.72536 102.4 694.72536 11.1

Zero-point vibrational energies (ZPE) at MP2/6-31G**//MP2/6-31G** scaled by a factor of 0.96; relative energy at MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level. au, atomic unit. Fig. 2. MP2/cc-pVTZ optimized structures of 5–8.

8428 | www.pnas.org/cgi/doi/10.1073/pnas.1306252110 Olah et al. Downloaded by guest on September 30, 2021 Table 2. Calculated and experimental 13C NMR chemical shifts No. Atom GIAO-SCF GIAO-MP2 GIAO-CCSD(T) Experimental value

1* C1,C6 21.1 24.3 23.5 C2,C3 156.6 162.2 159.4 C4,C5 21.9 25.2 24.3 Total 399.0 423.5 492.2 2* C1 38.7 46.0 45.2 36.3 C2 349.3 351.2 346.8 324.3 C3 96.7 112.7 108.6 115.2 C4 21.0 26.2 25.5 36.3 C5 38.6 46.1 45.3 36.3 C6 22.6 28.9 28.3 36.3 Total 545.8 584.9 574.2 584.7 3* C1 20.3 24.6 23.5 C2 51.2 64.2 62.6 C3 281.3 278.6 276.9 C4 27.7 31.4 31.0 C5 18.6 22.1 21.0 C6 31.1 41.3 40.0 Total 399.1 420.8 415.0 Fig. 3. Potential energy surfaces of fluorocations and chlorocations. 5* C1,C4,C5,C6 22.5 27.2 28.0 C2,C3 126.4 130.0 151.7 Total 342.8 368.8 415.4 6* C1 37.0 46.8 possible as the total number of electrons involved in chloro ions fl C2 331.1 346.9 are considerably more than those of structurally similar uoro 13 C3 73.3 90.4 ions. Calculated δ C of C1 and methyl carbons of the chloro- C4 24.2 31.2 nium ion 5 are 130.0 and 27.2, respectively (Table 2), compared C5 39.3 49.4 with the reported experimental values of 151.7 and 28.0, re- C6 29.2 40.0 spectively (16). Again, as the ions (5, 6, and 7) are energetically Total 510.7 573.6 different by less than 10 kcal/mol, an equilibrium mixture (un- 7* C1 21.5 28.8 dergoing fast chlorine exchange on the NMR timescale) in- C2 59.0 77.2 volving ions 5, 6, and 7 might best represent the structure of the C3 328.6 332.0 2,3-dimethyl-3-chloro-2-butyl cation (Scheme 3). C4 40.4 48.9 C5 22.8 29.8 Conclusion C6 33.1 47.4 The structures and 13C NMR chemical shifts of the 2,3-dimethyl- Total 472.3 516.6 3-fluoro-2-butyl cation and its chloro analogs were calculated fl Calculated 13C NMR chemical shifts were referenced to tetramethylsilane. using the ab initio/GIAO-CCSD(T) method. Bridged uoronium For numbering schemes, please see Figs. 1 and 2. ion 1, although a minimum on the potential energy surface, is *Experimental values were taken from refs. 7 and 16. considerably less stable than the open chain fluoro carbenium ion 2 and fluorocarbenium ion 3. However, unlike the fluoro system, bridged chloronium structure 5 was found to be the CHEMISTRY indicates that in the structurally similar carbenium ions intra- lowest energy minimum for 2,3-dimethyl-3-chloro-2-butyl cation. molecular stabilization by chlorine atom is much better than by For the fluoro system, the calculated 13C NMR chemical shifts the fluorine atom. Transition state structure 8 for conversion of of neither of the structure 2 and 3 agree with the experimentally 6 to 7 also was located. However, transition structure 8 lies only reported values. As the ions (2 and 3) are very close energet- 2.1 kcal/mol above that of ion 6. The interconversion between ically, an equilibrium mixture (undergoing rapid methyl ex- 6 and 7 through intramolecular methyl migration involving tran- change on the NMR timescale as shown in Scheme 1) involving sition state 8, therefore, is also very facile. Relative energies of ions 2 and 3 might best represent the structure of 2,3-dimethyl- the structures 5–8 are plotted in Fig. 3 along with fluoro ions. 3-fluoro-2-butyl cation. In contrast with fluoronium ion 1,bridged However, no transition state for conversion of 5 to 6 could chloronium structure 5 was found to be the global minimum, be located. being 9.0 kcal/mol more stable than 6 and 7.4 kcal/mol more We have also computed the 13C chemical shifts of chloro ions stable than 7. 5–7 at the GIAO-MP2/tzp/dz level using MP2/cc-pVTZ geometries, and these are shown in Table 2. More accurate 13C chemical shift Materials and Methods calculations of 5–7 at the GIAO- CCSD(T)/tzp/dz level were not All calculations were performed with the Gaussian 09 program series (8). The geometry optimizations were performed at the MP2/cc-pVTZ level. Final energies were calculated at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE

Scheme 2. Scheme 3.

Olah et al. PNAS | May 21, 2013 | vol. 110 | no. 21 | 8429 Downloaded by guest on September 30, 2021 level. NMR chemical shifts were calculated by the GIAO method (9–11) ACKNOWLEDGMENTS. Support of our work by Loker Hydrocarbon Research using the ACES II program (15). Institute is gratefully acknowledged.

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