The Historic Camphenyl Cation:  A Detailed Structure Evaluation Including Solvation Energy Calculations

Abstract

The structure of the camphenyl cation 1 has been studied in detail, using both experimental and computational approaches. Like others, we find only one structure on the camphenyl-isobornyl cation PE surface, but this single structure shows some unusual features. These include a very soft PE surface for movement along the C2-C6 axis (a nonbonding distance in a classical description of the cation), and a result of this is that very high computational methods (optimization at MP4 or QCI levels) are required in order to get structural minima that "fit" the experimental data. This PE surface has been probed computationally using fixed C2-C6 distances, and when one also calculates chemical shifts for these "fixed" structures, one sees calculated (13)C NMR chemical shifts for the C2 carbon that are hugely dependent on this fixed distance value, giving near-linear slopes of ca. 25 ppm/0.1 A distance change. Since this distance can vary over at least 0.6 A with relatively small calculated energy changes, there is a total range of ca. 150 ppm involved here. In a second part of this work, and in response to a recent paper in which the historic Meerwein "carbocation intermediate" proposal was rejected, we have calculated solvation energies (SCI-PCM method) for four carbocation systems, including 1. We find carbocation solvation energies (epsilon = 10 "solvent") of 45-53 kcal/mol, and where comparison can be made, the data correlate well with the literature. On the basis of these results, we re-affirm the Meerwein "carbocation" mechanism, but in order to accommodate only a single carbocation intermediate, we offer a description that amounts to a subtle variation of both the nonclassical ion proposal and Meerwein's "two cation" mechanism, namely that the camphenyl cation, 1, as a ground-state structure, can be described as only very weakly interacting in the C2-C6 bridging sense, but that the PE surface along this "bond" is so shallow that an energy input of only 4-6 kcal/mol can produce a bridged "structure". This mechanism explains the preferred formation of exo products in both the camphenyl and isobornyl systems, isotopic exchange of chloride in camphenyl chloride, and it allows for partial racemization of the camphenyl-isobornyl products in the reaction.

Synthesis and Configurational Assignment of Chiral Salicylic Aldehydes: Novel Building Blocks for Asymmetric Catalysis

“…[15] By our current mechanistic understanding, the bridged cation 21 accounts for the product structures observed experimentally. [16] Firstly, attack of a nucleophile at C-2 would give rise to exo-products incorporating the unrearranged (2-camphenyl) skeleton, whereas attack at C-1 would provide access to the isobornyl variants. Clearly, a reversible 1,2-hydrogen shift between positions 6 and 1 of the bridged cation 21 would result in racemization as well.…”

“…The separation of 2.20 g (9.00 mmol) of rac-15 afforded 15 (1.01 g, 4.13 mmol, 46%, Ͼ99.5% ee) and ent-15 (950 mg, 3.89 mmol, 43%, Ͼ99.5% ee). The separation of 2.12 g (8.69 mmol) of rac- 16 …”

“…paration of salicylic aldehyde rac-7 from the phenol rac-15 (vide supra), the phenol ent-16 (226 mg, 925 µmol) was formylated to afford the aldehyde ent-8 (104 mg, 382 µmol, 41%). [α] 589 ϭ Ϫ34.9, [α] 578 ϭ Ϫ37.8, [α] 546 ϭ Ϫ51.1, [α] 436 ϭ Ϫ227.1 (c ϭ 0.50, chloroform).…”

Fluorolactonization of norbornenecarboxylic acids and their methyl esters with F-TEDA-BF4 and XeF2

“…The easy isomerization of norbornyl carbocations [39,40] and a competition of external and intramolecular nucleophiles in the transformations of the carbocation intermediates make the fluorolactonization of 1a and 1b non-selective. Taking into account the inability of fluorine to form three-membered cyclic fluoronium carbocations [41], we postulate the intermediacy of open fluorocarbocations A and B in this reaction (Scheme 3).…”

Solvolysis of 1,1-Dimethyl-4-Alkenyl Chlorides: Evidence for π-Participation

“…Similar trends were recently obtained by Sorensen et al in solvation simulation calculations for the camphenyl and other reference systems. 21 It is well-known that the ion pairs play an important role in such reactions, so the large fraction of elimination products might be rationalized if the basicity of the chloride ion is considered. The chloride can act as a base and eliminate the hydrogen in its vicinity, yielding the elimination instead of the cyclized products.…”