Discrete Ion Pairs Research Articles

Studies of formal [1,5]-sigmatropic thermal rearrangements of dimethyl 3-alkyl-3-methyl-3H-pyrazole-4,5-dicarboxylates and dimethyl 4-alkyl-5-methyl-4H-pyrazole-3,4-dicarboxylates

Studies of the thermal rearrangements of 3H-pyrazoles, 10a-e, in benzene/toluene, chloroform, and methanol solvents are described. The mechanism for rearrangement appears to be two-step, involving discrete ion-pair intermediates. Large rate enhancements for these rearrangements in methanol and canion trapping by methanol are consistent with the proposed mechanism. Observed rate constants for the rearrangements of 3H-pyrazoles 10c, 10d, and 10e in benzene-d 6 were determined. Insight into the sense of rearrangement by an ion-pair mechanism is also provided by analysis of the 1 H NMR spectra of the product mixtures

Metal complexes as indicators of solvent structures. Part IV. The effect of solvation on the interaction between tris-1,2-diaminoethanecobalt(III) and anions in aqueous solution

Changes in the circular dichroism of (+)tris-1,2-diaminoethanecobalt(III) in the presence of sulphate ion can be attributed to changes in the cybotactic solvation of the complex ion as well as to the formation of discrete ion-pairs.

Reactions of an allylic carbocation with nucleophiles in aqueous solvents

The allylic carbocation formed from 3-(2-X-propan-2-yl)indene (1-X)(X = Cl, OAc, or OH2+), or from 2-acetoxy-1-isopropylideneindan (2-OAc) in aqueous solvents containing ca. 75 vol % water reacts rapidly with nucleophiles. The selectivity is very low: βnucca. 0.07 with alcohols as nucleophiles and βnucca. 0.03 with substituted acetate anions. The nucleophile attacks at both ends of the allylic system but preferentially at the exocyclic carbon atom, giving the thermodynamically more stable product. Azide anion reacts with the carbocation about 50 times as fast as does water. The solvolysis of the allylic isomers (1-OAc) and (2-OAc) is accompanied by intramolecular rearrangement of the acetates as well as hydron abstraction by the leaving acetate anion, yielding the 1,2- and 1,4-elimination products. The elimination product compositions are quite different, which indicates two discrete ion-pair intermediates. It is concluded also that the isomerization proceeds by an ion-pair route, since in less polar solvents it has only been observed along with solvolysis and elimination. The experimental results for the acetates accommodate well a mechanism in which solvolysis, rearrangement, and elimination are connected via two contact ion-pair intermediates.

Solvent and temperature studies of some indenyl‐ and fluorenyllithiums using13C NMR spectroscopy

Abstract13C NMR spectra of indenyl‐ and fluorenyllithium and some of their methyl substituted derivatives have been obtained. The shielding differentials induced by varying the temperature or coordinating ability of the solvent are discussed in terms of an equilibrium between discrete ion pairs or differences in external solvation. An increase of the coordinating ability of the solvent induces shift differentials of similar signs as those given by lowering the temperature. Methyl substituents close to the cation location are found to have only a minor effect on the observed ion pair ratio but seem to influence the preferred cation position in the tight ion pair structure.

The crystal structure of triphenylselenonium isothiocyanate

Three dimensional X-ray crystal structure analysis shows that triphenylselenonium isothiocyanate consists of discrete ion pairs in the solid state with each pair having an Se-N contact (3.197(4)Å) and SeC(NCS) contact (3.260(5)Å) significantly shorter than the respective van der Waals distance. These short distances are accompanied by acute SeNC (83.0(3)°) and SeCN (76.8(3)°) angles, and suggest a weak interaction between the triphenylselenonium cation and the π-electron system of isothiocyanate anion. Ph 3Se(NCS) is neither isomorphous nor isostructural with Ph 3Te(NCS). Refinement of the structure, based on 3126 reflections collected by automatic diffractometry, converged to a conventional R factor of 5.5% and a weighted R factor of 5.2%. Crystal data for Ph 3Se(NCS) are as follows: a = 12.417(4)Å, b = 11.556(5)Å, c = 12.633(3)Å, β = 113.17(2)°, V = 1666Å 3 (23 ± 2°C) and Z = 4; space group P2 1/c.