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Superbasic Alkyl‐Substituted Bisphosphazene Proton Sponges: Synthesis, Structural Features, Thermodynamic and Kinetic Basicity, Nucleophilicity and Coordination Chemistry
Author(s) -
Kögel Julius F.,
Xie Xiulan,
Baal Eduard,
Gesevičius Donatas,
Oelkers Benjamin,
Kovačević Borislav,
Sundermeyer Jörg
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201402226
Subject(s) - chemistry , protonation , superbase , nucleophile , alkyl , iodide , proton nmr , isopropyl , proton affinity , pyridine , coordination complex , stereochemistry , medicinal chemistry , organic chemistry , ion , metal , catalysis
Herein we describe an easily accessible class of superbasic proton sponges based on the 1,8‐bisphosphazenylnaphthalene (PN) proton pincer motif and P‐alkyl substituents ranging from methyl (TMPN) to n ‐butyl (TBPN), isopropyl (TiPrPN) and cyclopentyl (TcyPPN). These neutral bases with a p K BH + value (MeCN) of ∼30 were accessible via a Kirsanov condensation using commercially available 1,8‐diaminonaphthalene, and in case of TMPN and TBPN, simple one‐pot procedures starting from trisalkylphosphanes can be performed. Furthermore, the known pyrrolidinyl‐substituted superbase TPPN previously synthesized via a Staudinger reaction could also be prepared by the Kirsanov strategy allowing its preparation in a larger scale. The four alkyl‐substituted proton sponges were structurally characterized in their protonated form; molecular XRD structures were also obtained for unprotonated TiPrPN and TcyPPN. Moreover, we present a detailed description of spectroscopic features of chelating bisphosphazenes including TPPN and its hyperbasic homologue P 2 ‐TPPN on which we reported recently. The four alkyl‐substituted superbases were investigated with respect to their basic features by computational means and by NMR titration experiments revealing unexpectedly high experimental p K BH + values in acetonitrile between 29.3 for TMPN and 30.9 for TBPN. Besides their thermodynamic basicity, we exemplarily studied the kinetic basicity of TMPN and TPPN by means of NMR‐spectroscopic methods. Furthermore, the competing nucleophilic versus basic properties were examined by reacting the proton sponges with ethyl iodide. Insight into the coordination chemistry of chelating superbases was provided by reacting TMPN with trimethylaluminum and trimethylgallium to give cationic complexes of Group XIII metal alkyls that were structurally characterized.