Synthesis of polyheterocyclic 1,1-diboryltriazenes by γ-nitrogen insertion of azides into activated B–B single bonds

Abstract

The γ-nitrogen insertion of arylazides into the B–B bond of electron-rich cyclic μ-hydridodiboranes yields unsymmetrical polyheterocyclic 1,1-diboryltriazenes, which may undergo further NHC ring expansion/fusion and thermally induced loss of N2.

Activation of Nitrogen-Rich Substrates by Low-Valent, Redox-Active Aluminum Species

“…A remarkable feature of 5 is the presence of azido groups PhCH 2 –NN–N 2– , despite of the fact that azido are susceptible to release of dinitrogen . Within the azido moiety, the nitrogen–nitrogen distances (N(3)–N(4) 1.323(3) and N(4)–N(5) 1.284(4) Å) are intermediate between single- and double-bond lengths, showing some electron delocalization across the fragment, similar to those found in other boron/aluminum-containing γ-azide insertion products. ,− The initially linear N 3 moiety becomes bent (N(3)–N(4)–N(5): 116.7(3)°), which would be expected for sp 2 nitrogen center, and the N(4)N(5) double bond has a trans -configuration (dihedral angle N(3)–N(4)–N(5)–C(29): 174.9°). It is noted that 5 is thermally unstable and readily undergoes nitrogen loss and quantitative formation of diimido complex 4b in toluene at 50 °C, as revealed by X-ray diffraction.…”

“…While organic azides are commonly subject to N 2 elimination to generate the nitrene :NR intermediates, which then form metal imido complexes, ,,− ,, the insertion of azides into M–E bonds at the γ-nitrogen position is rare, with examples focusing mainly on boron. − ,− Certain steric and electronic factors favor the insertion of aryl azides into B–E bonds (E = B, C, N, and P) at the (less sterically hindered) γ-nitrogen. − ,− In contrast, one-electron reductants such as Al 2 (CH­(SiMe 3 ) 2 ) 4 were found to react with insertion of one Me 3 SiN 3 unit into the Al–Al bond . The reaction of NacNacAl­(η 2 -C 2 H 2 ) with N 3 Ar* (Ar* = 2,6-Ar′ 2 C 6 H 3 ; Ar′ = 2,4,6-Me 3 C 6 H 2 ) results in an end-on azide insertion to yield an aluminaazacyclobutene NacNacAl­(CHCHN­(NNAr*)) .…”

1,2-Dialkynyldiboranes(4): B–B versus CC bond reactivity

“…The decomposition of 6 Me may be linked to the propensity of certain NHC-stabilised hydroboranes to undergo NHC ring-opening and -expansion reactions. 22,54–57…”

“…Since azides are known both to insert as nitrene moieties into B-B bonds [22][23][24][25][26] and undergo copper-catalysed click reactions with alkynes, [27][28][29][30][31] the reaction of 1 TMS with arylazides was studied. Independent of the stoichiometry, the reaction temperature, the electronic effect (+I, −I, +M, −M) of the N-aryl substituent or the addition of a typical Huisgen cycloaddition copper catalyst (CuOTf, CuI, [(MeCN) 4 Cu]OTf, [(MeCN) 4 Cu] BF 4 , ((EtO) 3 P)CuI), the bis(aminoboryl)arylamine product 2 TMS -Ar (Ar = Ph, pTol (4-MeC 6 H 4 ), Ph CF3 (4-CF 3 C 6 H 4 ), Ph OMe (4-OMeC 6 H 4 ), Ph CN (4-CNC 6 H 4 )) was obtained as a colourless solid in good to excellent isolated yield (63-89%, Scheme 2).…”

“…The decomposition of 6 Me may be linked to the propensity of certain NHC-stabilised hydroboranes to undergo NHC ring-opening and -expansion reactions. 22,[54][55][56][57] The solid-state structure of 6 H (Fig. 4) shows two different conformers, (A) and (B), present in the asymmetric unit in a ca.…”