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An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

ABSTRACT: A new NMR-based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The ¹⁹ F?NMR chemical shifts and the ¹ J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well-established Gutmann-Beckett method.

SUBMITTER: Kunzler S

PROVIDER: S-EPMC6899571 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

Künzler Sandra S Rathjen Saskia S Merk Anastasia A Schmidtmann Marc M Müller Thomas T

Chemistry (Weinheim an der Bergstrasse, Germany) 20191031 66

A new NMR-based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The <sup>19</sup> F NMR chemical shifts and the <sup>1</sup> J(CF) coupling constants of these nitrilium ions vary in a pre ...[more]

PMID: 31469201

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Project description:Alkyl-B(C6F5)2 boranes are markedly ?-CH-acidic. Using DFT we have calculated the pKa-values of a series of examples. Typically, (C6F5)2B-CH3 [pKa (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this ?-CH-B(C6F5)2 acidity is in most cases not sufficient to allow for internal proton transfer in vicinal phosphane/borane frustrated Lewis pairs (FLPs). An exception is the slightly endergonic (+0.3 kcal mol-1) tautomerization of the in situ generated indane derived 1,3-P/B FLP 6 to its zwitterionic borata-alkene/phosphonium isomer 7, which was successfully trapped by Piers' borane [HB(C6F5)2] to yield the stable product 8. The pronounced ?-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane E-H2C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C(Me)CHCHB(C6F5)2 undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR2 (R: phenyl, mesityl, or t-butyl) reagents. ?-CHB(C6F5)2 carbanion (i.e. borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.

Dataset Information

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

Publications

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

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