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

Abstract

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 (k H /k D = 1.35) measured for 4a in mixtures of [H 12 ]cyclohexane and [D 12 ]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.

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry

“…In general, the high D values of all acylnitrenes can be rationalized in terms of the Oatom not being "willing" to give up its strong C=Obond by donating an electron to form aC = Nb ond and aC ÀOb ond. Boryl- [88] and silylnitrenes [51] also have relatively high D values (1.5-1.6 cm À1 )s imilar to alkylnitrenes,w ith little or no delocalization of spin onto B and Si. A D value of 1.540 cm À1 (j E/hc j = 0.0002 cm -1 )and little evidence of SOC are observed for trimethylsilylnitrene, [89a] and only modest SOC is calculated for silylnitrene [89b] (in contrast to very strong SOC and ah igh D value of 1.64 cm À1 in atriplet silylene,R 2 SiD).…”

Unusual Nitrene Oxidation Product Formation by Metathesis Involving the Dioxygen O−O and Borylnitrene B−N Bonds

“…[12,13] Nitrene 6 and closely relatedb orylnitrenes are very reactive towards sigma bonds (e.g.,C (sp 3 )ÀH, HÀH) after photoexcitation,m ost likely due to the formation of the highly electrophilics inglet nitrene. [14][15][16][17][18] Borylnitrene 6,h owever,a lso shows very high thermal reactivity towardsO 2 in an argon matrixb ecause annealing in the presence of O 2 quickly results in the formation of nitroso oxide 7a (Scheme 1b). [12] The latter was shownt ou ndergo, upon photoexcitation (l = 254 nm), af ormal dyotropic rearrangement [19][20][21] to nitritoborane 9a.N ot only is the thermal reactivity of 6 towards O 2 higher than that of aryl nitrenes, but also the final oxidation product differs because not the nitro-(CatBNO 2 )b ut the nitritoborane( CatBONO) is formed.…”

Encapsulating Inorganic Acetylene, HBNH, Using Flanking Coordinative Interactions

“…Ar elated process has been observed by Paetzold,[1a] who prepared iminoboranes via alkyl-group migration (R 2 N À B(R')-N!R 2 N-B N À R') involving at ransient boranitrene. [24] Cummins and Foxa lso observed nitrogen extrusion from the azidoborate salt nBu [ 25] To gain insight into the mechanism by which IPr·HB = NH·BAr (3). Notably thermolysis of the deutero analogue 3d did not yield any discernable H/D kinetic isotope effect ( Figure S4), [26] suggesting that N 2 loss and formation of at ransient nitrene is the rate-determining step.…”

“…Ar elated process has been observed by Paetzold,[1a] who prepared iminoboranes via alkyl-group migration (R 2 N À B(R')-N!R 2 N-B N À R') involving at ransient boranitrene. [24] Cummins and Foxa lso observed nitrogen extrusion from the azidoborate salt nBu 4 N-[(N 3 )B(C 6 F 5 ) 3 ]inthe presence of (THF)U[N(tBu)Ar] 3 (Ar = 3,5-Me 2 C 6 H 3 )t oy ield the uranium(V) nitride nBu 4 N-[(F 5 C 6 ) 3 B·N=U{N(tBu)Ar}] 3 . [25] To gain insight into the mechanism by which IPr·HB = NH·BAr F 3 (4)i sf ormed, the deuterium-labeled analogue IPr·BD 2 N 3 ·BAr F 3 (3-d)w as synthesized (Scheme 3).…”