Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

Phosphinophosphoranes: Mixed-Valent Phosphorus Compounds with Ambiphilic Properties

Herein, we present a simple synthesis of mixed-valent phosphinophosphoranes bearing three- and five-coordinate phosphorus centers. Compounds with phosphorus-phosphorus bonds were synthesized via a reaction of lithium phosphides RR'PLi with cat₂PCl (cat = catecholate), whereas derivatives with methylene-linked phosphorus centers were obtained via a reaction of phosphanylmethanides RR'CH₂Li with cat₂PCl. The presence of accessible lone-pair electrons on the P-phosphanyl atom of phosphinophosphoranes during the reaction of the title compounds with H₃B·SMe₂, where phosphinophosphorane-borane adducts were formed quantitatively, was confirmed. Furthermore, the Lewis basic and Lewis acidic properties of the phosphinophosphoranes in reactions with phenyl isothiocyanate were tested. Depending on the structure of the starting phosphinophosphorane, phosphinophosphorylation of PhNCS or formation of a five-membered zwitterionic adduct was observed. The structures of the isolated compounds were unambiguously determined by heteronuclear nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. Moreover, by applying density functional theory calculations, we compared the Lewis basicity and nucleophilicity of diversified trivalent P-centers.

Automated iterative Csp3-C bond formation

Fully automated synthetic chemistry would substantially change the field by providing broad on-demand access to small molecules. However, the reactions that can be run autonomously are still limited. Automating the stereospecific assembly of Csp3-C bonds would expand access to many important types of functional organic molecules1. Previously, methyliminodiacetic acid (MIDA) boronates were used to orchestrate the formation of Csp2-Csp2 bonds and were effective building blocks for automating the synthesis of many small molecules2, but they are incompatible with stereospecific Csp3-Csp2 and Csp3-Csp3 bond-forming reactions3-10. Here we report that hyperconjugative and steric tuning provide a new class of tetramethyl N-methyliminodiacetic acid (TIDA) boronates that are stable to these conditions. Charge density analysis11-13 revealed that redistribution of electron density increases covalency of the N-B bond and thereby attenuates its hydrolysis. Complementary steric shielding of carbonyl π-faces decreases reactivity towards nucleophilic reagents. The unique features of the iminodiacetic acid cage2, which are essential for generalized automated synthesis, are retained by TIDA boronates. This enabled Csp3 boronate building blocks to be assembled using automated synthesis, including the preparation of natural products through automated stereospecific Csp3-Csp2 and Csp3-Csp3 bond formation. These findings will enable increasingly complex Csp3-rich small molecules to be accessed via automated assembly.

Synthesis and structure of a phosphinoboronic ester in a fused bicyclic framework

The first phosphinoboronic ester bearing a fused bicyclic framework was synthesised by either deprotonation and hydride abstraction or Rh-catalysed dehydrogenation of a hydrophosphineboronic ester. The phosphinoboronic ester reacted as a Lewis acid with KF/18-crown-6, pyridine and DMAP to give the corresponding adducts. Furthermore, its crystal structure shows a remarkably short P-B bond in comparison with other P-B bonded derivatives in spite of the trigonal pyramidal geometry of the phosphorus. Consistent with the phosphorus pyramidality, the π-type donor-acceptor interaction of the P-B bond is small as revealed by the DFT calculations. The P-B bond shared within the fused six-membered rings has to shorten because of the geometrical requirement and high s-character of the boron.