Secondary 2-Norbornyl Cation Intermediates Substituted at C(5) and C(7) by Electron-withdrawing Groups. Addition of fluorosulfuric acid to unsaturated norbornane derivatives

Low temperature (?130° to ?110°) addition of exo-norborn-5-en-2-ol ( 7 ) to excess HSO₃F in SO₂CIF yielded a mixture of exo-5-(fluorosulfonyloxy)-exo-2- and endo-2-norbornylhydroxonium ions ( 9+10 ) under kinetic control that was different from the mixture of 9+10 obtained by addition of endo-norborn-5-en-2-ol ( 8 ) to HSO₃F under kinetic control. These mixtures differed from the mixture of 9+10 observed at higher temperature (?80° to ?60°) (thermodynamic control). Addition of 3-nortricyclanol ( 23 ) or exo-2, 3-epoxynorbornane ( 24 ) to HSO₃F at -?120° ± 10° yielded a mixture containing the exo-2-(fluorosulfonyloxy)-anti-7- and syn-7-norbornylhydroxonium ions ( 26+27 ) as major adducts. Qualitative rates of the isomerization of 26+27 to the more stable ions 9+10 and of the isomerization 9 ? 10 were evaluated. The solvolysis of 9+10 in HSO₃F yielded the exo-2, exo-5- and exo-2, endo-5-norbornanediyl bis (fluorosulfates) ( 21+22 ). Norbornadiene and quadricyclane added 2 equivalents of HSO₃F and furnished kinetically a mixture of exo-2, anti-7- and exo-2, syn-7-norbornanediyl bis (fluorosulfates) ( 36+37 ) as major adducts. The latter 36+37 were isomerized into a kinetic mixture of the more stable isomers 21+22 . The rates of these isomerizations were compared. The use of DSO₃F and (exo-2-D)-norborn-5-en-2-ol ( 15 ) confirmed that heterolyses of the fluorosulfates were responsible for the observed isomerization; elimination-addition processes occurred but much more slowly. The results are interpreted in terms of substituted classical and σ-bridged secondary 2-norbornyl cation intermediates. It appears that the electron withdrawing substituents FSO₃ and H₂O⁺ (HO) destabilize the σ-bridged 2-norbornyl cation more at C(5) than C(7). If the σ-bridged ions 5-Z substituted at C(5) by Z = FSO₃ or H₂O⁺ (HO) are transition states in the isomerization of the corresponding classical ions 3-Z, 4-Z , the free enthalpy difference between the ‘non-classical’ σ-bridged ion and the classical ions is not higher than the energy barrier to the quenching of the latter intermediates by FSO in HSO₃F/SO₂CIF.     

Revisiting the Perfluorinated Trityl Cation

Although ultimately not isolable for X-ray structural characterization, the free perfluorinated trityl cation was shown to be observable in neat triflic acid, which represents milder conditions than previous reports of this cation in “magic acid” or oleum. A triflate-bound species could be generated in organic solvents using stoichiometric amounts of triflic acid and was shown to be synthetically viable for hydride abstraction from Et₃SiH. It was demonstrated that the para-position on the -C₆F₅ rings is the primary point of attack for decomposition of the cation.     

The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

Quantum-Chemical Study of Interactions between the Hydroxonium Cation and the Monomethyl Mercury Cation in Aqueous Media

The potential surface of the reaction between the polyhydrated monomethyl mercury cation and the hydroxonium cation is considered. An type reaction, resulting in solvate-separated products (mercury dication and methane molecule), was found to be thermodynamically probable. The thermal balance of the reaction is 132.3 kJ/mole. The activation energy of the limiting stage of the process is 56.7 kJ/mole.     

The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

MINDO/3 Study of the Rearrangement of 1-Methylcyclohexyl Cation to 1,2-Dimethylcyclopentyl Cation

The rearrangement of the 1-methylcyclohexyl cation to the 1,2-dimethylcyclopentyl cation has been studied by MINDO/3 calculations, as an application of the branching mechanism model for cycloalkanes. Possible intermediates and transition states have been characterized by diagonalization of their Hessian matrixes. Two nonequivalent pathways, α and β scissions, are relatively close in energy. The calculated transition states are almost equivalent in energy to those found for cyclohexyl cation. Hence, the energy barriers are higher for the rearrangement of the 1-methyl than for cyclohexyl cation, because the former is less stable than the latter.     

Carbon Participation in the Solvolysis of 6- and 7-Substituted 2-Norbornyl p-Toluenesulfonates. Norbornanes,Part 11

The solvolysis rates and products of several 7-anti-substituted 2-endo-norbornyl p-toluenesulfonates 11 have been determined and compared with those of the previously reported 6-exo-substituted 2-exo-norbornyl p-toluenesulfonates 1. Although the number of bonds between the substituent and the reaction site is the same in the two series, the inductive effect of the substitutents is transmitted far more strongly in the 6-exo-2-exo-series 1 than in the 7-anti-2-endo-series 11 ; i.e. their inductivities differ widely. It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C(2) and that this involves differential bridging strain. The different reactivities of unsubstituted 2-exo- and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect.     

苯分子离子的理论研究

由于 Jahn-Teller效应 [1]具有简并的最高占据分子轨道 ( HOMO)的分子失去一个电子生成阳离子后 ,构型将发生重大变化 .苯分子的构型为 D6 h点群 ,其简并的 HOMO用 e1g表示 ,从简并的 HOMO上失去一个电子后的苯分子离子将具有简并的电子态 2 E1g.根据 Jahn-Teller理论 ,苯分子离子将由 D6 h构型畸变为D2 h构型 ,同时简并的电子态将分裂为 2 B2 g和 2 B1g两个电子态 ,对应于 2个不同的 D2 h结构 .然而 Jahn-Teller理论既未说明 2 B2 g和 2 B1g态中哪个能量更低 ,也未说明 D2 h构型的细节 ,故需用量子化学计算来确定 .苯分子离子…     

Norbornanes. Part 18. Inductive Charge Dispersal in the Solvolyses of 4- and 5-Substituted 2-Norbornyl p-Toluenesulfonates

The solvolysis rates and products of 4- and 5-exo-substituted 2-exo- and 2-endo-norbornyl tosylates 9 and 10 , respectively, are reported. The logarithms of the rate constants (log k) correlate linearly with the inductive constants σ for the substituents. A comparison of the reaction constants p₁ for the 4-, 5-, 6-, and 7-substituted 2-exo- and 2-endo-tosylates 9 , 10 , 1 , and 2 respectively, indicates that inductivity is higher for 2-exo-ionization than for 2-endo-ionization in all series. This observation is attributed to the more favorable alignment of neighbouring C-atoms for dorsal participation in exo-ionization, especially, in the case of C(6).