Synthesis and structures of monocyclic 1,4-diaza-2,3-diborine species

Facile Access to Unprecedented Electron‐Precise Monohydrodiboranes(4), cis‐1,2‐Dihydrodiboranes(4), and a 1,1‐Dihydrodiborane(5)

“…The 1 H NMR spectrum of 4 a displayed a characteristic 2H singlet for the ligand vinylene bridge at 5.23 ppm and a 12H singlet for the two dimethyl amino methyl groups at 2.15 ppm. These data are comparable to those reported for the analogous N‐( 2,4‐dimethylphenyl)‐ substituted species . However, the appearance at room temperature of the dimethylamino methyl protons as a single 12H resonance is surprising given the high B−N bond‐rotation barrier usually observed in dimethylaminoboron species (59–84 kJ mol −1 ) and the crystallographically determined B−N bond lengths of approximately 1.43 Å in 4 a , which suggest multiple bond character (Figure S29 in the Supporting Information).…”

“…The driving force of the reaction is presumably the formation of unreactive dimeric [BH 2 NMe 2 ] 2 and dimethylsulfide (Scheme , bottom). To enforce a cis ‐configuration, bridging N , N′ ‐dimesityl‐1,2‐(diamino)ethene and ‐ethane ligands were chosen to provide electronic and steric stabilization to an endocyclic diborane moiety (Scheme ) …”

Facile Access to Substituted 1,4‐Diaza‐2,3‐Diborinines

“…Compounds 1 b – d are readily identified by their 11 B NMR chemical shifts of δ =33.5 ( 1 b : 11 B{ 1 H}; ω 1/2 =1027 Hz), 34.1 ( 1 c : 11 B{ 1 H}; ω 1/2 =941 Hz), and 36.7 ppm ( 1 d : ω 1/2 =347 Hz) in [D 6 ]benzene solutions, which strongly resemble those found for 1 a ( δ =33.5 ppm) and the 2,4‐xylyl derivative of Sahin ( δ =32 ppm) . In the solid state however, these species feature marked differences with respect to the planarity of the B 2 N 2 C 2 heterocyclic core, which are caused by the nature and the sterics of the groups attached to the backbone nitrogen atoms.…”

“…By contrast, the B 2 N 2 C 2 rings of 1c ( Figure 2) and 1d ( Figure S64) deviate significantlyf rom planarity (N1-C1-C2-N2: 1c 6.7(2)8; 1d 8.8(2)8.N 1-B1-B2-N2: 1c 45.0(2)8; 1b 52.0(1)8), similar to the data reported by Sahin (N1-C10-C9-N2: À5.58.N 1-B1-B2-N2: À41.78). [15] For 1a and 1b,s teric repulsion between the NMe 2 and Mes/Xyl units entails twisting of the aryl groups to adopt an orthogonal arrangement with respectt ot he heterocyclic core structure, which generates sufficient space for the NMe 2 groups in a planar B 2 N 2 C 2 environment. In 1d,r epulsive sterici nteractions between theN Me 2 and the tBu groups within ap lanar geometry would be inevitable, thus the B 2 N 2 C 2 ring displays af lattened chair conformation.…”

“…The first DADBs were isolated in 1963 as benzannulated derivatives by exchange amination of B 2 R 2 (NMe 2 ) 2 with aromatic amines ( A , R=alkyl, Figure ), while it was not until 1997 that a monocyclic oxygen‐bridged DADB dimer was reported as a byproduct ( B , Figure ) . Very recently, a general and more selective route to monomeric DADBs ( C , Figure ) through salt elimination reactions of Li 2 [dab] salts (dab=1,4‐diazabutadiene) with dihalodiboranes(4) has been established independently by Sahin and by our group . So far, reactivity studies on type C DADBs are scarce and limited to the transformation of the NMe 2 ‐substituted derivative 1 a to its dihydrido analog and subsequent ring contraction/expansion reactions .…”