Theoretical Studies of Carbocations in Ion Pairs. 4.¹The Interconversion of the 1-Propyl Cation and Protonated Cyclopropane

Abstract

The structure of the 1-propyl cation in the ion pair with the model anion trihydrofluoroborate, proven in earlier work to be appropriate for such studies, was investigated by ab initio calculations at the level previously reported to give the definitive structure of carbocations. In previous work, it was shown that the carbocation structure does not change with the nature of the anion. The cation structure is determined, however, by the distance between the cation and anion, d, and their relative orientation. At infinite interionic distance the only stable chemical species (energy minimum) is the protonated cyclopropane, 1. As the ions move toward each other, the cyclopropane bond opposite to the anion becomes progressively longer and eventually it breaks up in the contact ion pair. Three domains of cation stability are identified as a function of d: at long distances, ion 1 is the only energy minimum; at intermediate distances 1 and the 1-propyl cation 2 are both energy minima; at short distances, ion 2 is the only energy minimum. Thus, ionization of 1-propyl precursors forms the tight ion pair of 2 as the first intermediate. Isomers 1 and 2 differ in both the C1−C2−C3 angle and the conformation of the C2−C3 bond; the transition structure for their interconversion has been determined by calculations. At the MP4(FC)/6-311G**//MP2/6-311G** level, the two isomers have the same energy content for d = 2.40 Å, but correction for the zero-point energies obtained from the vibrational frequencies calculated at the MP2/6-311G** level reduces the energy of 2 relative to 1, thus requiring a slight upward correction in the value of d for equal stability of isomers. The interconversion of 1 and 2 is observed for a position of the anion essentially in the same plane as the three carbon atoms. Movement of the anion above the same plane results in hydrogen shift with the formation of the 2-propyl cation, 3. Some literature results in which primary carbocations could intervene as intermediates are discussed. In particular, the data on carbon and hydrogen scrambling in 3 in superacid solution are better accounted for by the results of calculations for ion pairs, with both 1 and 2 as intermediates, than by the results of calculations for isolated ions.

The Protonation of Gaseous Cyclopropane

“…[30] Alternatively, the presence of a polar molecule (A) may affect the relative stability of isomeric C 3 H 7 ions within [A´C 3 H 7 ] complexes. [31,32] Finally, neither the CF nor the FC pathway are sensitive to a temperature change from the usual 30 8C up to 120 8C. It appears then that even bare cC 3 H 7 ions do not undergo any thermal isomerization process in the observation time window of 10 À8 s. Assuming a pre-exponential factor of ca.…”

On the structure and dynamics of secondary n-alkyl cations

“…Note also that ten of the 11 low-energy longarm-clinal forms are also closed forms, thus demonstrating a general prevalence for closed forms on the PES in agreement with past studies. [9][10][11][12][13][14][15][16][17][18][19][20] Curiously, the lowest-energy form, with two clinal longarms, is not fully closed but open-clinal on both sides.…”

“…Fărcaşiu was concerned for many years about the role of ion-pairs in the formation of alkyl ions in superacidic molecular liquids and suggested that the nearby presence of an anion "partner" could be causing open-form secondary ions as intermediates. For instance, he performed restricted optimizations of the structures of several small alkyl ions in ion-pair complexes with the approaching and weakly nucleophilic H 3 BF − ion 13,14,41,42 and found that open structures become lower in energy than closed ones on the PES once the anion approaches within ϳ3 Å of the cation. In contrast, one of us has done unrestricted optimizations of complexes of closedstructure alkyl ions with NH 3 , 17 H 2 O, 17 20 and various small zeolite fragment ions, 19,20 and no structure-opening was seen unless the alkyl ion became covalently bonded to the nucleophile.…”

“…PCP + structures have some CCC bond angles of Ͻ90°and are commonly seen as the lowest-energy PES minima in computational studies. [9][10][11][12][13][14][15][16][17][18][19][20] Fărcaşiu and co-workers [12][13][14][15] referred to them as "bridged" structures, but we prefer calling them "closed" structures, first to avoid confusion with H-bridged structures and second to recognize their dynamical relationship with classical "open" structures. We appear to be the first to point out that closed structures are rather rare in DFT simulations at catalytic ͑ϳ800 K͒ temperatures.…”

Unusual 1,2‐Acyl Rearrangement Accompanying Ring Expansion Reaction of Alkaloid Cotarnine under the Action of α‐Haloketones: a New Route to 5‐Acyl‐1,2‐dihydro‐3‐benzazepine Moiety

“…A good illustration of these considerations is the simplest cationic structure MeCOCH 2 CH 2 + ( 14 ) which could be considered as σ‐adduct of acetyl cation with such a very weak π‐electron‐donor as ethylene molecule. In contrast to the carbocations 8 , 9 , 12 , 13 [as well as to the σ‐type adduct of methyl cation and ethylene (i. e. n‐propyl carbocation)], this structure is not represented by any energy minimum and undergoes barrierless decomposition, giving rather weak, symmetric (C S ) π‐complex MeCO + …C 2 H 4 ( 15 ). The latter is an energy minimum on the potential energy surface and stabilized by 2 equivalent and rather long (2.75 Å) π‐bonds Ac−C (see Supporting Information for details).…”