Electron‐Deficient Borinic Acid Polymers: Synthesis, Supramolecular Assembly, and Examination as Catalysts in Amide Bond Formation

Abstract

The Lewis acidic character of borinic‐acid‐functionalized polymers suggests broad potential applications in supramolecular materials, chemo‐ and biosensors, as well as supported catalysts. Two highly electron‐deficient borinic acid copolymers (3 a and 3 b) with variable steric hindrance at the boron center were prepared by reaction of aryldibromoboranes ArBBr2 (2, Ar=2,4‐Cl2Ph, 3,5‐Cl2Ph) with a 10 % stannylated polystyrene random copolymer, followed by conversion to the desired PS‐B(Ar)OH functionalities. The supramolecular assembly of these polymers through Lewis acid–Lewis base interactions and reversible covalent B−O−B bond formation was investigated. Exposure of a polymer solution of 3 a to pyridine triggered spontaneous gelation, whereas 3 b only gelled upon addition of molecular sieves to favor formation of boroxane crosslinks. The crosslinking process was readily reversed by addition of small amounts of water or wet solvent. The dynamic processes were studied in detail by variable‐temperature (VT) NMR by using molecular model compounds. The polymers and their corresponding model compounds were also examined as catalysts in the amide bond formation reaction between phenylacetic acid and benzylamine. The 3,5‐dichlorophenyl borinic acid derivatives proved to be the more effective catalysts. Mechanistic studies suggested that the borane Lewis acid‐catalyzed coupling involves initial acid‐induced protodeboronation to release the dichlorophenyl boronic acid as the active catalyst.

Recent developments in catalytic amide bond formation

“…A observation that was further substantiated in a report on solid supported borinic acid amidation, wherein the authors observed by 11 B NMR that the catalytically active species was indeed the corresponding boronic acid. [65] Sheppard and coworkers then developed a tris-(trifluoroethyl) borate catalyst that is capable of catalyzing amidation of unprotected amino acids. [66][67][68] This reagent can also be used in greener solvents such as tert-butyl acetate.…”

Conjugated Boron Porous Polymers Having Strong p−π* Conjugation for Amine Sensing and Absorption

“…Our interest in B-based building blocks stems from the empty p orbital of the B atom, enabling diverse applications in the areas of materials science and organic catalysis. For example, Yamaguchi and co-workers have reported that B containing materials show selective sensing properties toward the fluoride anion. − Liu and co-workers have shown that B–N containing units enhance the electron-accepting and electron-transporting characters of smaller molecules and polymers. , Jakle and co-workers revealed that tuning the chemical structure of B-moieties can improve the mechanical properties and catalytic performance of the polymeric materials. − Although they have been shown to exhibit diverse functionalities, B containing small molecules and polymers generally require blocks having large protecting groups to stabilize the B-centers against water and oxygen, which seriously limits enriching structure design and diversifying functionality of the B containing materials (Chart ). Furthermore, the large steric groups can also create twisted aromatic groups (Ars) at the B-centers that will diminish the electronic communication between the B-centers (Chart ).…”

Recent Advances in the Synthesis of Borinic Acid Derivatives

“…As seen in the previous section, the transmetalation of arylstannanes is a powerful way of controlling the regioselectivity of the borylation of BX 3 . Within the framework of borinic acid synthesis, the method was extended to the reaction with ArBBr 2 ( Scheme 43 ) [ 90 , 91 , 92 ]. Several electron-deficient borinic acids 134a–b or boron quinolate complexes 135a–c were obtained in modest to good yields.…”