Comparison of the Electrophilicities of the Free and the (Tricarbonyl)iron-Coordinated Tropylium Ion

Abstract

The kinetics of the reactions of the tropylium ion (1) and the (tricarbonyl)iron-coordinated tropylium ion (6) with allylsilanes, allylstannanes, and other uncharged nucleophiles were studied photometrically and conductometrically. The second-order rate constants were independent of the counterions, indicating rate-determining carbon−carbon bond formation. The electrophilicity parameters of the (tricarbonyl)iron complexes of the tropylium ion E(6) = −3.81 ± 0.24 and of the dihydrotropylium ion E(22) = −9.88 indicate that the former is 105−106 times more reactive toward nucleophiles. Comparison with the electrophilicity parameter of the free tropylium ion E(1) = −4.62 ± 0.57 shows that coordination by Fe(CO)3 affects its electrophilic reactivity only slightly. Density-functional calculations are used to rationalize the relative reactivities in terms of thermodynamic effects and frontier orbital interactions. On the basis of the linear free enthalpy relationship log k = s(E + N) one can predict that the free and the Fe(CO)3-coordinated tropylium ion react with nucleophiles (N > −1) at room temperature.

Electrophilicity Index

“…The various electrophilicity descriptors reported in this section may be broadly classified into three categories: viz., the kinetic descriptor that measures the rate at which an electrophilic attack takes place, the thermodynamic descriptor which measures the ease of such an attack, and a combination of these two approaches through a linear free energy relationship. Among the kinetic scales, the most important is that of Mayr and co-workers. − They have rank ordered various nucleophiles and electrophiles in terms of their N and E parameters respectively obtained from the associated experimental rate constants. Ritchie's parameters 123,124 or Swain−Scott parameters 142 are similar in spirit.…”

State-Dependent Global and Local Electrophilicity of the Aryl Cations

“…On the grounds of the E parameters and ω adiab indexes for the cation 4 , the aryl cations can be estimated as very strong electrophiles (Table ) and, in overall cases, these should be placed far beyond the multichlorinated benzhydryl cations ( E = 10–14) in the well-known benzhydrylium electrophilicity scale. − ,, It was shown that the singlet state cation 4 reacts at the diffusion-controlled rate ( k ≅ 1.2 × 10 10 M –1 s –1 ) even with weak nucleophiles with no selectivity. In contrast the triplet state cation 4 reacts at the measurable rates (log 3 k NuH /s ≅ 6–12) with several alkenes and alkynes .…”

“…Later on, this concept was extensively explored by Mayr et al − This was resulted in development of an empirical equation (the Mayr–Patz equation), which was further modified by Albini et al for the aryl cations in the form log nobreak0em.25em⁡ k = s ( E + e N ) where N is the relative reactivity, E is the nucleophile-independent electrophilicity parameter, and s and e are the empirical parameters ( e = 0.33 for the aryl cations) . According to this equation the E parameter was established for a wide range of various organic cations. − Pérez has found that the E parameters for aryl diazonium cations are pretty well correlated ( R = 0.9402) with the ω value calculated by the B3LYP/6-31G(d) approach in terms of vertical approximation. To the best of our knowledge, no data are present in the literature regarding calculations of the ω and ω k + indexes for the aryl cations as well as for the high-spin molecules in general.…”

Relationship between local electrophilicity and rate coefficients for the hydrolysis of carbenium ions

“…[5][6][7][8][9][10] They proposed that the rate coefficients for the reactions of carbocations with uncharged nucleophiles obey the linear free energy relationship:…”

“…These authors have clearly emphasized and illustrated the usefulness of the nucleophilicity/ electrophilicity scales to quantitatively discuss reactivity as well as inter molecular selectivity. [5][6][7][8][9][10] From a theoretical point of view, the electrophilicity concept has attracted the attention of several authors. [11][12][13][14][15] Most of the proposed definitions of electrophilicity are framed on reactivity indexes.…”