Derivatives of Aza‐ nido ‐tetraborane and Diaza‐ arachno ‐pentaborane from Tri‐ tert ‐butylazadiboriridine

The Lewis acids RBH 2 are added to the basis B‐B bond of tri‐ tert ‐butylazadiboriridine, NB 2 t Bu 3 ( 1a ), to give 5‐alkyl‐1,2,3‐tri‐ tert ‐butyl‐1‐aza‐ nido ‐tetraboranes of type 3 (two endo ‐H atoms at B5) or type 4 (one H atom in a B2–B5 bridging position, one exo ‐H atom at B5), either as a mixture of 3 and 4 (3, 4b–d : R Me, t Bu, CMe 2 i Pr) or with the isomer 4 in high excess ( 4e, f : R Ph, s Bu). The reaction of dialkylboranes R 2 BH with 1a yields 5,5‐dialkyl‐1,2,3‐tri‐ tert ‐butyl‐1‐aza‐ nido ‐tetraboranes of type 4 (one H atom in a B2‐B5 bridging position; 4g, h : R 2 Et 2 , cycloocta‐1,5‐diyl). In boiling THF, trialkylboranes BR 3 transform 1a into NB 2 t Bu 2 R which, expectedly, dimerizes spontaneously to give a dialkyltetra‐ tert ‐butyl‐2,5‐diaza‐ nido ‐hexaborane of type 2 (2a, b : R Me, Et). The products of the addition of aminoboranes H 2 BNRR' to 1a are identified as alkyl derivatives of 2,5‐diaza‐ arachno ‐pentaborane, N 2 B 3 t H 2 t Bu 3 RR' ( 5a–d , R/R' = H/Pr, H/ t Bu, Me/Me, Et/Et). The aminoborane H 2 B N i Pr 2 with a sterically more demanding amino group hydroborates the B‐B bond of 1a to give the acyclic product i Pr 2 NBH‐B( t Bu)‐N( t Bu)B( t Bu)H ( 6 ). In contrast to the B atoms, the N atoms in the electron‐deficient cluster skeletons of 2– 5 need not take part in (3c,2e) bonds of the s̀ type. The molecular structures of 4c, 5d , and 6 are confirmed by X‐ray structural analyses.