C versus O Protonation in Zincate Anions: A Simple Gas‐Phase Model for the Surprising Kinetic Stability of Organometallics

Abstract

For better understanding the intrinsic reactivity of organozinc reagents, we have examined the protolysis of the isolated zincate ions Et3Zn−, Et2Zn(OH)−, and Et2Zn(OH)2Li− by 2,2,2‐trifluoroethanol in the gas phase. The protonation of the hydroxy groups and the release of water proceed much more efficiently than the protonation of the ethyl groups and the liberation of ethane. Quantum‐chemical computations and statistical‐rate theory calculations fully reproduce the experimental findings and attribute the lower reactivity of the more basic ethyl moiety to higher intrinsic barriers, which override the thermodynamic preference for its protonation. Thus, our minimalistic gas‐phase model provides evidence for the intrinsically low reactivity of organozinc reagents toward proton donors and helps to explain their remarkable kinetic stability against moisture and even protic media.

The Fe-MAN Challenge: Ferrates–Microkinetic Assessment of Numerical Quantum Chemistry

“…Obviously, the value of any benchmarking study depends on the quality of the benchmark data, i.e., the rate constants k exp of gas-phase ion-molecule reactions measured in a QIT in the present case. This methodology is well established and known to be quite reliable. ,,, However, for the explicit purpose of benchmarking, it is worth reviewing possible sources of errors in the experiments.…”

“…We determined the experimental bimolecular rate constants k exp of the gas-phase protolysis reactions of the trisarylferrate anions FePh 3 – and FeMes 3 – (Mes = mesityl) by the proton donors 2,2,2-trifluoroethanol (R F3 OH) and 2,2-difluoroethanol (R F2 OH) at T = (310 ± 20) K , following the previously described methodology. , …”

“…This instrument permits the selection of ions of a given mass-to-charge ( m / z ) ratio and thereby excludes any interference by dynamic equilibria. Some of us (RR and KK) have recently demonstrated the success of this approach for the gas-phase protonation of organozincate anions. , The measured bimolecular rate constants could be reproduced by theoretical calculations within a factor of 8 and thus served as a valuable benchmark for the latter. In contrast to zinc, the iron present in the complexes probed here features an open shell of d electrons and therefore poses a significantly greater challenge to theoretical calculations.…”

Recent Advancements in the Utilization of s-Block Organometallic Reagents in Organic Synthesis with Sustainable Solvents

“…These encompass (i) intricate dynamic equilibria and changes in the aggregation states of organometallic species, (ii) the presence of robust H-bonded networks, (iii) the formation of highly efficient "ate" complexes exhibiting enhanced nucleophilicity, and (iv) catalytic effects occurring at the interface between organic compounds and the aforementioned green solvents (water and DESs) [21][22][23][24][25][26][27]. In this line, a recent computational investigation carried out by Koszinowski and Rahrt, aimed at addressing the experimental observation of how organozinc reagents survive instantaneous protonolysis in protic media, also disclosed the significance of achieving favorable kinetics despite less favorable thermodynamics [104].…”

Introducing Water and Deep Eutectic Solvents in Organosodium Chemistry: Chemoselective Nucleophilic Functionalizations in Air

“…Detailed quantum chemical computations may be surely of help in future supporting studies. Recent investigations performed by Koszinowski, for example, on the gas‐phase reactivity of ethylzincate ions toward protonolysis have disclosed that, despite their higher basicity, the zinc‐bound ethyl groups of these complexes are protonated significantly less efficiently than their hydroxy counterparts, because of the higher intrinsic barrier associated with the proton transfer [17r] …”