Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: new substituents and alternative substrates

Abstract

A series of new boranes capable of forming intramolecular N → B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations. A dimethylborane (BMe₂), haloborane derivatives (BBr₂, BF₂, BI₂) and mixed cyano/isocyano-borane (B(CN)(NC)) have been prepared by different techniques. Furthermore, 2′-alkynyl-substituted 2-phenylpyridines bearing terminal tert-butyl- and trimethylsilyl-groups are introduced as a new class of substrates for hydroboration. Successful hydroboration with either 9H-borabicyclo[3.3.1]-nonane (9H-BBN), dimesitylborane (Mes₂B-H), or Piers’ borane ((C₆F₅)₂B-H, BPF-H) furnished new π-extended boranes capable of forming intramolecular six- or seven-membered N → B-heterocycles (tBuBBN, SiBPF), and, in the case of Mes₂BH, formation of a sterically crowded styrylborane (SiBMes₂) incapable of intramolecular N → B-coordination was observed. All the boranes listed above except BMe₂ have been structurally characterized, and a study of their electrochemical properties showed that the systematic variation of the substituents on boron allows for the incremental variation of the electron affinity of the phenylpyridine-model system over a total range of >0.7 eV between alkylboranes (BMe₂, BBN) and B(CN)(NC). B(CN)(NC) shows the strongest N → B-bond (≈175 kJ mol⁻¹), and highest electron-affinity observed so far, and is the first example of a borane bearing an isocyano-substituent on boron.