Abstract
Several bis(dimethylamino)‐substituted 1,4‐diaza‐2,3‐diborinines (DADBs) were synthesized with variable substituents at the backbone nitrogen atoms. By reaction with HCl or BX3 (X=Br, I), these species were successfully converted into their synthetically more useful halide congeners. The high versatility of the generated B−X bonds in further functionalization reactions at the boron centers was demonstrated by means of salt elimination (MeLi) and commutation (NMe2 DADBs) reactions, thus making the DADB system a general structural motif in diborane(4) chemistry. A total of 18 DADB derivatives were characterized in the solid state by X‐ray diffraction, revealing a strong dependence of the heterocyclic bonding parameters from the exocyclic substitution pattern at boron. According to our experiments towards the realization of a Dipp‐substituted, sterically encumbered DADB, the mechanism of DADB formation proceeds via a transient four‐membered azadiboretidine intermediate that subsequently undergoes ring expansion to afford the six‐membered DADB heterocycle.
Highlights
A recent review article by Marder and Westcott entitled diboranes(4) as synthetic workhorses.[1]
We have significantly expanded the scope of known 1,4-diaza-2,3-diborinines (DADBs), a class of cyclicdiborane(4) molecules neglected for a long time
We have shown that the synthetic approach to enter DADB chemistry, the synthesis of doubly NMe2-substituted DADBs 1 via one-pot salt metathesis reactions of Li2[dab] with B2Cl2(NMe2)2, provides the possibility to simple vary the steric demand of the DADB backbone
Summary
The synthesis of B2(NMe2) by Brotherton in 1960[9] is generally recognized as the commencement of diborane(4) chemistry, and it has been established as a versatile and easy to handle reagent that is readily transformed into other diborane(4) species.[4,10] One strategy to significantly modify the structural appearance and properties of (amino)diboranes is the incorporation of the B-N units into heterocyclic ring systems to generate for example 1,4-diaza-2,3-diborinines (DADB, Figure 1).[11] These cyclic (amino)diboranes(4) can be considered as B,N isosteres of benzene, in which two of the C= C bonds have been replaced by isoelectronic and isostructural. The B2N2C2 rings of 1 c (Figure 2) and 1 d (Figure S64) deviate signif-
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