Tri‐tert‐butylazadiboriridin: Ringerweiterung mit Isonitrilen, α‐Bromlithioalkanen und Aziden

Tri‐tert‐butylazadiboriridine: Ring Prolongation by the Action of Isonitriles, α‐Bromolithioalkanes, and Azides Isonitriles ArNC are 1,1‐boroborated by the B‐B bond of the azadiboriridine (‐BR‐BR‐NR‐) (1; R = tBu) to form the four‐membered ring [‐BR‐C(NAr)‐BR‐NR‐] (2b; Ar = 2,6‐C6H3Me2). Corresponding rings undergo [3 + 3] cyclodimerization of the B‐C=N chain to give the tricyclic products 6a, b when isonitriles RNC with the small ligands R = Me, Et are boroborated by 1. Two molecules of tBuNC open the B‐B bond of 1; the resulting five‐membered ring 7a may add excess tBuNC to yield 7a · 2 CNtBu (8). The results strongly support a mechanistic hypothesis set up for the multistep reaction of 1 and CO. The carbene units, which are present in the α‐bromolithioalkanes [‐C(Br)Li‐CMe2‐CH2‐], LiC(Br)H2, and LiC(Br)Me2, are also 1,1‐boroborated by 1 to give the corresponding four‐membered ring 2c, or by a 2:1 reaction to afford the five‐membered ring 7b, or by a 1:1 reaction and subsequent rearrangement to yield the five‐membered ring 7c. The nitrene units, which are present in the azides PhN3 and PhCH2N3, are also boroborated by 1; the diazadiboretidines 5b, c are formed with evolution of N2. The bis(diazadiboretidinyl)borane RB(N2B2R3)2 (5d) is the corresponding product from the reaction of RB(N3)2 with 1 in the ratio 1:2. The azide Me3SiN3 reacts differently yielding the N‐boryl diazadiboretidine 5e and the iminoborane RB≡N̊. The B‐B bond length of 187.6 pm in 5e indicates a transannular bonding interaction. The products 5d, 5e, and 6a were characterized by a crystal structure analysis.