Synthesis 2017; 49(15): 3495-3504
DOI: 10.1055/s-0036-1590504
paper
© Georg Thieme Verlag Stuttgart · New York

Nucleophilicities and Lewis Basicities of Sterically Hindered Pyridines

Elsa Follet
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13, Haus F, 81377 München, Germany   Email: guillaume.berionni@cup.uni-muenchen.de
,
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13, Haus F, 81377 München, Germany   Email: guillaume.berionni@cup.uni-muenchen.de
,
Sami Lakhdar
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13, Haus F, 81377 München, Germany   Email: guillaume.berionni@cup.uni-muenchen.de
,
Armin R. Ofial
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13, Haus F, 81377 München, Germany   Email: guillaume.berionni@cup.uni-muenchen.de
,
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13, Haus F, 81377 München, Germany   Email: guillaume.berionni@cup.uni-muenchen.de
› Author Affiliations
This work has been funded by the Deutsche Forschungsgemeinschaft (SFB 749, projects B1 and C6).
Further Information

Publication History

Received: 05 May 2017

Accepted: 07 May 2017

Publication Date:
12 June 2017 (eFirst)

Dedicated to Professor Herbert Mayr on the occasion of his 70th birthday

Abstract

The structures of the covalent Lewis adducts and/or frustrated Lewis pairs derived from 2- and 2,6-substituted pyridines with diaryl (Ar2CH+) and with the more bulky triaryl (Ar3C+) carbenium ions were analyzed by UV-vis and NMR spectroscopy. Thermodynamics (equilibrium constants) and kinetics (rate constants) of the associations of the carbon-centered Lewis acids Ar2CH+ with a series of sterically hindered pyridines were investigated and used for the determination of the Lewis basicities and nucleophilicities, on the basis of the Mayr electrophilicity/nucleophilicity and Lewis acidity/basicity linear free energy relationships. In addition, methyl and benzhydryl cation affinities were computed to elucidate the respective steric and electronic contributions of the substituents to the nitrogen atom Lewis basicity. The influence of the size of the reference carbenium ion on the magnitude of the repulsion induced by the pyridine substituents (Me, tBu in 2- or 2,6-positions) was also analyzed. Cumulated steric repulsion was found to decrease the reactivity of the nitrogen atom by up to 10 orders of magnitude.

Supporting Information

 
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