Norbornanes. Part 20. Inductivity and bridging in 2-norbornyl cations

The solvolysis rates and products of several 1-substituted 2exo- and 2-endo-norbornyl p-toluenesulfonates 7 and 8 , respectively, have been determined. Hydrolyses of these epimeric tosylates yielded rearranged products in varying amounts, except when the substituent was COOCH₃ or CN. The logarithms of the rate constants (log k) for the endo-series 8 correlated linearly with the corresponding inductive constants σ with a reaction constant ρ_I of ?1.24. On the other hand, log k values for the exo-series 7 appear to fit two regression lines, the first line (ρ_I = ?1.90) defined by the tosylates that ionize, with rearrangement, to the tertiary cations 11 , the second (ρ_I = ?1.86) by the tosylates 7 (R = H, COOCH³, and CN) that ionize to an asymmetrically bridged secondary cation 19 . These results confirm the unique participation of C(6) with a ρ_I of ?2.00 in the ionization of 2-exo-nor-bornyl tosylate.     

The generation of substituted classical secondary 2-norbornyl cations

All possible stereoisomers of 5-cyano- and 6-cyano-2-norbornyl tosylate have been synthesized (except the endo, endo isomer) and solvolyzed in 97% hexafluoroisopropanol. The rates and products are most easily interpreted in terms of classical ions. An approximate model of substituent effects allows an estimate of anchimeric assistance in the parent norbornyl ion to be a factor of about 540 at 25° and 18 at 125°.     

Epimeric 2-norbornyl cations in the solvolysis of 6-exo- and 6-endo-substituted 2-exo-norbornyl p-toluenesulfonates

Solvolysis of 6-exo- and 6-endo-substituted 2-exo-norbornyl tosylates $\text{1}$ and $\text{3}$, respectively, yields identical or different product mixtures depending on the inductive effect of the substituent, indicating that induction involves graded σ-participation.     

Solvolysis of 2-aryl-exo-5,6-trimethylene-2-norbornyl p-nitrobenzoates as a reference of 2-aryl-2-norbornyl p-nitrobenzoates solvolysis: Further evidence for the unimportance of σ-participation in the solvolysis of 2-aryl-2-norbornyl derivatives

The rates of solvolysis of 2 - aryl - exo - 5,6 - trimethylene - exo - and endo - 2 - norbornyl p-nitrobenzoates (7 and 8, respectively) with representative substituents [p-CH₃O, p-CH₃, H, m-CF₃, p-CF₃, and 3,5-(CF₃)₂] were determined in 80% aqueous acetone and compared with those of the parent 2-aryl-exo- and endo-2-norbornyl p-nitrobenzoates (5 and 6, respectively). The rate ratios for the endo-p-nitrobenzoates ($\text{6}\text{8}$) are essentially constant and close to unity for these substituents, indicating that the perturbation of the trimethylene bridge toward the C₂-position is virtually negligilbe. The rate ratios for the exo-p-nitrobenzoates ($\text{5}\text{7}$) can also be regarded as invariant over the reactivity range studied. The exo/endo rate ratios ($\text{7}\text{8}$) are 246 (p-CH₃O), 196 (P-CH₃), 129 (H), 80 (m-CF₃), 90 (p-CF₃), and 89 [3,5-(CF₃)₂], being similar to the corresponding $\text{5}\text{6}$ rate ratios. The solvolyses of these p-nitrobenzoates (7 and 8) afford predominantly ( > 97%) exo-alcohols. Since the secondary exo-5,6-trimethylene-2-norbornyl system, with its low exo/endo rate ratio, 11.2, is known to solvolyse without significant σ-participation, the tertiary derivatives should also undergo solvolysis without σ-participation. Consequently, the similarities in the solvolytic behaviors between the two systems (5 vs 7; 6 vs 8) strongly support the previous conclusion that σ-participation is unimportant in the solvolysis of 5.     

Enhancement of neighboring group participation in the 2-norbornyl cation

The solvolysis rates of a series of nitrile-substituted 2-norbornyl tosylates are presented along with a semi-quantitative analysis of the rates.     

Polar effects in the solvolysis of 6-exo-substituted endo-2-norbornyl p-toluenesulfonates

The solvolysis rates of endo-2-norbornyl p-toluenesulfonates $\text{1}$ are affected far less by 6-exo-substituents than are the rates of the corresponding exo-2-norbornyl p-toluenesulfonates $\text{2}$¹.     

Chemical ionization mass spectra of some 2-norbornyl derivatives

Chemical ionization mass spectra of exo- and endo-2-norbornanols and their phenylurethane derivatives have been obtained with several reactant ions. Small differences are noted in the abundances of norbornyl and [M+H]⁺ ions for the phenylurethane derivatives: more norbornyl ions with the exo compounds. Relative rate constants for decomposition of [M+H]⁺ ions, k_exo/k_endo ? 1-2. No evidence was found for s?-participation in the decomposition of these ions. The i-C₄H₁₀ chemical ionization spectrum of endo-2-norbornanol contains a much greater abundance of [M-H]⁺ ions than the i-C₄H₁₀ chemical ionization spectrum of exo-2-norbornanol. This difference presumably results from steric hindrance toward attack of the endo hydrogen.     

Simultaneous bridge-localized and mixed-valence character in diruthenium radical cations featuring diethynylaromatic bridging ligands

A series of bimetallic ruthenium complexes [{Ru(dppe)Cp*}(2)(μ-C≡CArC≡C)] featuring diethynylaromatic bridging ligands (Ar = 1,4-phenylene, 1,4-naphthylene, 9,10-anthrylene) have been prepared and some representative molecular structures determined. A combination of UV-vis-NIR and IR spectroelectrochemical methods and density functional theory (DFT) have been used to demonstrate that one-electron oxidation of compounds [{Ru(dppe)Cp*}(2)(μ-C≡CArC≡C)](HC≡CArC≡CH = 1,4-diethynylbenzene; 1,4-diethynyl-2,5-dimethoxybenzene; 1,4-diethynylnaphthalene; 9,10-diethynylanthracene) yields solutions containing radical cations that exhibit characteristics of both oxidation of the diethynylaromatic portion of the bridge, and a mixed-valence state. The simultaneous population of bridge-oxidized and mixed-valence states is likely related to a number of factors, including orientation of the plane of the aromatic portion of the bridging ligand with respect to the metal d-orbitals of appropriate π-symmetry.     

Exo/emdo soltolysis rate ratios in constrained 2-norbornyl systems

Tetracyclol[6.2.1.0^2,6.0^5,10]undec-3-yl derivatives ($\text{6}$ - $\text{8}$) show typically high tertiary exo/endo solvolysis rate ratios, but the ratios for secondary reactants are suppressed. The rate and product data support the theory of anchimeric assistance in the parent $\text{exo}$-2-norbornyl solvolysis.     

The formation of methyl cations from acetone and 1,2-epoxypropane molecular cations

The technique of photoelectron—photoion coincidence spectroscopy is used to determine the breakdown curves of CH⁺₃ from acetone and 1,2-epoxypropane. The experimental appearance energies of CH⁺₃ from both isomers are found to be identical with the calculated thresholds for the formation of CH⁺₃ + CH₃ + CO, i.e. the secondary fragmentation of C₃H₆O⁺.     

The formation and structure of radical cations

ions generated from a number of different presursors have been studied by high kinetie energy ion—molecule reations. It has been shown that at least four distinct stable species oeeur, of which acetonitrile and methyl isoeyanide retain their original structure. With imidazole or pyrazole as precursors, a mixture of open thain radical cations, not identical to the above species and probably interconvertible via the 1H-azirine radocal cation, is formed. From butrynitrile, pyrrole, crotonitrile, allyl interconvertible via the and cyanocyopropane a fourth species, probably the vinylidenimine ion, is formed.     

Fluorine bridging in the xenon fluoride cations and their salts: The base strengths of XeF2, XeF4 and XeF6

It is a feature of the structures of all the salts of XeF⁺, XeF₃⁺ and XeF₅⁺ that have been studied, that the cation forms one or more fluorine bridges to the anion. The length, direction and number of the fluorine bridges in these salts throw new light on the relative fluoride ion donor abilities of the xenon fluorides XeF₂, XeF₄ and XeF₆. In constrast to what has previously been supposed, it is concluded that the order of fluoride ion donor ability is XeF₂ < XeF₄ < XeF₆.     

Remote inductive effects on secondary and tertiary 2-norbornyl solvolysis rates

A 5-cyano substituent decelerates the solvolysis rate of exo-2-norbornyl brosylate $\text{(1-H)}$ by a factor of 1790. A much smaller deceleration (24–90 fold) is observed for the secondary $\text{endo}$ and for both $\text{exo}$ and $\text{endo}$ tertiary 2-norbornyl derivatives. These results support the occurrence of σ-participation in the solvolysis of $\text{1-H}$.     

On formation of polyatomic mercury cations

This paper analyzes the effect of the increased number of electrons lying on the 6s orbitals of Hg ions on the formation of polyatomic Hg_n cations, their structure, and Hg-Hg distances. The critical role in the formation and structural characterstics of Hg_n cations is played by the degree of expansion of the Hg5d atomic orbitals; when 6s electrons increase in number, this expansion is enhanced by relativistic effects. Formation of Hg_n cations and the observed tendencies are explained in terms of the virial theorem. Translated fromZhurnal Struktumoi Khimii, Vol.40, No. 2, pp. 314–323, March–April, 1999     

Stereoelectronic control of exo/endo rate ratios in the solvolysis of 2-norbornyl p-toluenesulfonates

The variable exo/endo rate ratios observed in the solvolysis of 6-exo-substituted exo- and endo-2-norbornyl p-toluenesulfonates are due to differential transmission of polar effects in the transition state for ionization.     

Halogen bridging in the 2-chloroethyl radical

INDO calculations predict an unsymmetrically bridged structure for the 2-chloroethyl radical with the CCl bond almost perpendicular to the radical plane.