The Protonation of Cubane Revisited

Abstract

Protonation of saturated hydrocarbons is the simplest and practically most important electrophilic reaction.[1] Small alkanes like methane, ethane, and isobutane [2] form welldefined protonated species, and their experimental proton affinities (PAs) are excellently reproduced computationally. [3][4][5] However, the current data on the protonations of strained hydrocarbons are rather controversial, and facile rearrangements are thought to account for the difficulties encountered in measuring the PAs.[6] Cyclopropane (strain energy(E str ) = 27.2 kcal mol À1 ) is an exception because the number of energetic downhill paths is very limited and the barrier for the ring opening of corner-protonated C 3 H 7 + to the 2-propyl cation is high. The experimental PA of cyclopropane (179.4 kcal mol À1 [7] ) is well reproduced computationally (179.3 kcal mol À1 at CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVDZ for corner-protonated C 3 H 7 + ).[8] In contrast to cyclopropane which displays secondary CÀH bonds only, a surprisingly small range of PAs is characteristic for a number of hydrocarbons with tertiary C À H bonds. These hydrocarbons differ considerably in their strain energies, for example, dodecahedrane (PA = 201.7 kcal mol À1 , [9] E str = 65.4 kcal mol), adamantane (PA = 175.7 kcal mol À1 , [11] E str = 6.3 kcal mol À1 [12] ), and cubane [13] (PA = ca 200 kcal mol À1 , [7,9] E str = 161 kcal mol À1 [14] ). For these systems the question remains: How much does the strain energy affect the proton affinity of a saturated hydrocarbon and to what extent should isomerizations be considered? The experimental PA of cubane (1) measured by ion-cyclotron resonance spectroscopy [9] is surprisingly low and was recently questioned by Koppel et al. [15] on the basis that 1 rearranges rapidly to cuneane so that the PA is attributed to the energy change of the entire protonation/rearrangement/deprotonation process. However, cuneane is also highly strained, so where one does stop? Are there any other rearrangement paths for protonated cubane? Herein we address both computationally and experimentally the question to what extent the cubane cage is able to survive electrophilic attack [16] and what the most favorable paths for the reaction of 1 with a proton are.Despite enormous strain, cubane [15,17,18] is kinetically quite stable because breaking just one CÀC bond causes only minor structural changes and hence only little relief of strain. However, cleavage of a second CÀC bond is highly exothermic and followed by rapid rearrangements. For instance, thermolysis [16,19] and single-electron oxidation [20] of 1 leads to cyclooctatetraene. In contrast, mild radical reagents [21] lead to substitution products with conservation of the cubane cage. [22] Electrophilic cubane conversions are limited to reactions of soft electrophiles like metal ions, [18] and a number of fascinating rearrangements (e.g., to syn-tricyclooctadiene (2) with Pd 2+ and to cuneane (3) with Ag + and Li + ) are preparatively useful. [18,23] The attack of a proton on cubane i...

A rearrangement of 1,1-linked four-membered rings to bicyclic bridgehead five-membered rings: A phosphorus turn

“…2,3-Diphospha-1,4butadienes are known and quite reactive in cyclization processes. 13 [1,5]-Cycloaddition of less sterical hindered C-H atom of a vinyl group in 8 to phosphorus atom of conjugated phosphaalkene group in 8 will afford the five-membered bicyclic system with a P-P bridgehead (9). [1,5]-Additions with high degree of stereo-and regiocontrol were described particularly in conjugated systems.…”

“…[1][2][3][4] The relief of strain energy in hydrocarbons, such as cubane, also involves the rearrangement of four-membered rings to five-membered rings (Scheme 1A). [5][6][7][8] There is one report on the rearrangement of 1,1-N,N-linked four-membered rings to the bicyclic five-membered nitrogen heterocycles (Scheme 1B). 9 Analogous rearrangement of phosphorus heterocycles is unknown so far (Scheme 1C).…”

Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles – An Interpretation by Triadic Analysis

“…However, their calculated GB value differed by about 60 kcal mol -1 from that measured experimentally, 37 suggesting that the latter value, determined by FT-ICR mass spectrometry, does not correspond to the reversible protonation of cubane and that the proposed cation is not the final protonation product. This discrepancy was later clarified by Fokin, Schreiner and coworkers 38 who suggested 1,8-dihydropentalene as the final cubane protonation product along the most favourable protonation/isomerization/deprotonation downhill path at the CCSD(T)/cc-pVDZ//MP2/cc-pVDZ level, in that way tying in with experimental thermodynamics. As an even more typical example, one should mention the original paradigmatic superbasic proton sponge, namely Alder's 1,8-bis(dimethylamino)naphthalene, DMAN.…”

Carbon‐rich Compounds: Computational Considerations

“…As the strain energy of 7 is 155-161 kcal mol À1 [118] the reason for the instability of 38-(OMe) 8 must lie in its substitution pattern that allows stabilization of otherwise kinetically higher lying transition structures or intermediates along the decomposition pathways.…”