C–H Bond Amination by Photochemically Generated Transient Borylnitrenes at Room Temperature: A Combined Experimental and Theoretical Investigation of the Insertion Mechanism and Influence of Substituents

A number of azidoboranes having substitution patterns that are derived from catechol (<b>3</b>), pinacol (<b>4a</b>), 1,2-diaminoethane (<b>4b</b>,<b>c</b>), 1,2-ethanedithiol (<b>4d</b>), and 1,2,4,5-tetrahydroxybenzene as well as acyclic dialkoxy species (<b>5</b><b></b>) were synthesized and, in the case of <b>4c</b> (<i>N</i>,<i>N</i>′-ditosyl-2-azido-1,3,2-diazaborolane), also structurally characterized. The azidoboranes were photolyzed in cyclohexane solvent in order to investigate the tendency of the generated borylnitrenes to undergo intermolecular C–H insertion reactions. The yields of intermolecular insertion products ranged from very good (<b>4a</b>) to vanishingly small, depending on the substitution of the azidoborane. For a number of borylnitrenes the zero-field splitting parameter <i>D</i> was measured in organic glasses at 4 K. The small primary kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 1.35) measured for <b>4a</b> in mixtures of [H₁₂]­cyclohexane and [D₁₂]­cyclohexane suggests that the insertion reaction is concerted and involves the singlet state of the borylnitrene. Computations at the CBS-QB3 and CCSD­(T)/TZ2P levels of theory show that the relative energies of singlet and triplet states of a wide variety of borylnitrenes and even their nature as minima or saddle points depend strongly on the substituents. Photolysis of the most reactive azidoborane, <b>4a</b>, in methane in a flow reactor at atmospheric pressure produces an intermolecular insertion product in low yields, in agreement with the expectation of intersystem crossing to the less reactive triplet state of the borylnitrene