Frustrated Lewis-Pair Neighbors at the Xanthene Framework: Epimerization at Phosphorus and Cooperative Formation of Macrocyclic Adduct Structures |
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Authors: | Dr. Karel Škoch Dr. Constantin G. Daniliuc Dr. Gerald Kehr Sebastian Ehlert Marcel Müller Prof. Dr. Stefan Grimme Prof. Dr. Gerhard Erker |
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Affiliation: | 1. Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraβe 40, 48149 Münster, Germany;2. Mulliken Center for Theoretical Chemistry Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstraβe 4, 53115 Bonn, Germany |
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Abstract: | Attachment of a pair of P-stereogenic mesityl(alkynyl)phosphanyl groups at the 4- and 5-positions of a 9,9-dimethylxanthene framework gave mixtures of the respective rac- and meso-bisphosphanyl diastereoisomers. They slowly epimerized in a thermally induced reaction with Gibbs activation barriers of about 25 kcal mol−1 at room temperature (measured and DFT calculated). The reaction of the meso-mesityl(tert-butylethynyl)phosphanyl derivative with two molar equivalents of Piers′ borane [HB(C6F5)2] led to the formation of the alkylidene-bridged geminal bisphosphane/borane-frustrated Lewis pair system. The compound was obtained enriched (>85 %) in the rac diastereoisomer. With a variety of bifunctional donor substrates, the rac-bis-P/B FLP formed macrocyclic compounds. They were all formally derived from meso-configurated diastereoisomers of the bisphosphanylxanthene backbone. |
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Keywords: | boron cooperative reactions phospha-iminium phosphorus stereochemistry |
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