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 (3), pinacol (4a),
1,2-diaminoethane (4b,c), 1,2-ethanedithiol
(4d), and 1,2,4,5-tetrahydroxybenzene as well as acyclic
dialkoxy species (5) were synthesized and,
in the case of 4c (N,N′-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 (4a) to vanishingly small, depending on the substitution of
the azidoborane. For a number of borylnitrenes the zero-field splitting
parameter D was measured in organic glasses at 4
K. The small primary kinetic isotope effect (kH/kD = 1.35) measured for 4a 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, 4a, 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.