Frustrated Lewis‐Pair Neighbors at the Xanthene Framework: Epimerization at Phosphorus and Cooperative Formation of Macrocyclic Adduct Structures. Issue 47 (16th July 2021)

• Record Type: Journal Article
• Authors: Škoch, Karel; Daniliuc, Constantin G.; Kehr, Gerald; Ehlert, Sebastian; Müller, Marcel; Grimme, Stefan; Erker, Gerhard
• Is Part Of: Chemistry. Volume 27:Issue 47(2021)
• Page Start: 12104
• Page End: 12114
• Publication Date: 2021-07-16
• Subjects: boron -- cooperative reactions -- phospha-iminium -- phosphorus -- stereochemistry; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202100835
• Languages: English
• ISSNs: 0947-6539

Abstract

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(C6 F5 )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. Abstract : The meso and rac diastereoisomers of two P‐stereogenic 4, 5‐bis(mesityl, alkynylphosphanyl)dimethylxanthenes underwent slow thermally induced interconversion. The activation barrier of the P‐propynyl derivative was determined experimentally and by DFT calculation. Adding HB(C6 F5 )2 to the tert ‐butylethynyl‐P‐containing system gave intramolecular pairs of geminal frustrated P/B Lewis pairs at the xanthene‐derived backbone. This reaction also proceeded with epimerization of the P‐stereogenic phosphanyl units. The new FLP pairs at the xanthene backbone formed bridged macrocyclic adducts with a variety of bifunctional Lewis base donors.