Density functional theory study of adamantanediyl dications C₁₀H₁₄²⁺andprotio-adamantyl dications C₁₀H₁₆²⁺

Abstract

Structures of the isomeric adamantanediyl dications C10H14 2؉ and protio-1-and protio-2-adamantyl dications C10H16 2؉ were investigated by using the density functional theory (DFT) method at the B3LYP͞6 -31G** level. Four structures, 1 b-e, were found to be minima on the potential energy surface of C10H14 2؉ . The 1,3-adamantanediyl dication 1b with two bridgehead tertiary carbocationic centers was found to be the most stable structure. On the potential energy surface of C10H16 2؉ (protonated adamantly cation), five structures, 2 b-f, were found to be minima. Each of the structure contains a two-electron, three-center bond. The COC protonated 1-adamantyl dication, 2f, was characterized as the most stable structure. 13 C NMR chemical shifts of the structures were also calculated by using gaugeincluding atomic orbital-density functional theory and gauge-including atomic orbital-self-consistent field methods.A damantane, C 10 H 16 i, has been used extensively as a model compound to investigate electrophilic reactions of saturated hydrocarbons. The unique structural features of adamantane are ideal for a systematic study of its cations, including both their structure and chemical reactivity. The tight interlocking of cyclohexane rings into the rigid, relatively strain free-chair conformation in adamantane makes it stable against deprotonation, prohibiting the easy formation of double bonds and back-side (nucleophilic or electrophilic) attack (1).Of particular interest is the bridgehead 1-adamantyl cation, which has been prepared and characterized under long-lived stable ion conditions (2). The bridgehead 1-adamantyl cation is stabilized by COC hyperconjugation. X-ray crystal structural studies of the 3,5,7-trimethyl-1-adamantyl cation demonstrates the COC hyperconjugative effect elegantly (3). A series of 2,6-disubstituted 2,6-adamantanediyl dications ii (Scheme 1) (R ϭ C 6 H 5 , c-C 3 H 5 ) were also prepared in superacid media by Prakash et al. (4). The dications were stable only with stabilizing groups such as phenyl and cyclopropyl. Attempts to generate the unsubstituted secondary dication ii (R ϭ H) were unsuccessful (4). The previously attempted preparation of the 1,3-adamantanediyl dication in superacid solutions were also unsuccessful (3).We now report density functional theory (DFT) studies (5) of the possible isomeric adamantanediyl dications C 10 H 14 2ϩ and protio-adamantyl dications C 10 H 16 2ϩ , hitherto not yet observed as persistent long-lived species. Superelectrophilic (6) COH or COC protonation of the adamantyl cation is also possible and can lead to extremely reactive protio-adamantyl dications. Previously, we have been able to show by hydrogen͞deuterium exchange experiments and theoretical calculations that the tertbutyl (7) as well as isopropyl (8) cations can undergo COH protonation in superacids to form the activated highly electrondeficient gitonic carbenium-carbonium dications protio-tertbutyl and protio-iso-propyl dications, respectively (Scheme 2). ‡

Why Benchmark-Quality Computations Are Needed To Reproduce 1-Adamantyl Cation NMR Chemical Shifts Accurately

“…The MP2 computation performs best overall; but the average 13 C NMR error is still 6 ppm. Reference reports similar results for the C 1 carbocationic center employing HF-SCF and B3LYP calculations with much smaller Pople split-valence basis sets. The HF-SCF results are the worst (note the C 1 and C 3 deviations), especially when the poor 1 H performance also is considered.…”

“…Exceptionally, the first 13 C chemical shift computations (at the IGLO level) deviated by 37 ppm from experiment for C 1 , the carbocation center . Subsequent computations using other quantum-chemical procedures, even those including rather elaborate second-order Møller-Plesset perturbation theory (MP2), , also failed to reproduce the experimental NMR data for the 1-adamantyl cation satisfactorily. , We have now employed further methodological developments to revisit this problem and have identified the difficulties. A careful analysis of possible error sources has been carried out.…”

Adamantane Annulation to Arenes: A Strategy for Property Modulation of Aromatic π-Systems

“…1.54 Å). These changes would be not only due to the cationic character generated on C1 but also owing to the hyperconjugation between the four bonds on adamantane and carbocation on C1 or C3. − From the distribution of the LUMO as well, the positive charge of 2w seems to be delocalized on the perylene scaffold as well as on the pertinent four C–C bonds (Figure d). In addition to the kinetic protection and inductive effect of the adamantane scaffold, this outstandingly rigidified, thus effective hyperconjugation possibly increases the stability of 2w , which strongly supports that the annulation of adamantane is a powerful strategy for generating stable cationic arenes.…”

Interconversion between Planar-Triangle, Trigonal-Pyramid and Tetrahedral Configurations of Boron (B(OH)<sub>3</sub> -B(OH)<sub>4</sub><sup>-</sup> ), Carbon (CH<sub>3</sub><sup>+</sup> -CH<sub>3</sub>X) and for the Group 15 Elements as Nitrogen (NH<sub>3</sub>-NH<sub>4</sub><sup

“…Geometry calculations have been performed by Rasul et al for this cation [17]. The bond distance of the planar C1 cation with the surrounding carbons has a value of 1.458 Å.…”