“…Dauben has shown that the reaction of 4 with Ph 3 C + leads to tritylation (→ 5 ) and not to the formation of 6 . Access to the tropylium complex 6 has been reported, however, to be possible by acid treatment of 9a , 9b , or the corresponding thioethers (eq 4). …”
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.
“…Dauben has shown that the reaction of 4 with Ph 3 C + leads to tritylation (→ 5 ) and not to the formation of 6 . Access to the tropylium complex 6 has been reported, however, to be possible by acid treatment of 9a , 9b , or the corresponding thioethers (eq 4). …”
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.
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