B−B Cleavage and Ring‐Expansion of a 1,4,2,3‐Diazadiborinine with N‐Heterocyclic Carbenes

Abstract

A 1,4,2,3‐diazadiborinine derivative was found to form Lewis adducts with strong two‐electron donors such as N‐heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N‐embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction, were identified as B,N‐heterocycles with fused 1,5,2,4‐diazadiborepine and 1,4,2‐diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B−H, C−N, and B−B bond activation steps are responsible for these “intercalation” reactions between the 1,4,2,3‐diazadiborinine and NHCs.

Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled anti-selective hydroalkynylation, cycloaddition and CC triple bond scission

“…§ 7 Tn -Fc and 7 Fu -Fc can be viewed as resulting from the cycloaddition of FcC^CH to the diborene, yielding the intermediate 1,2-dihydro-1,2-diborete I2 (Scheme 5), followed by ring expansion of one NHC ligand through the addition of the B-B bond of I2 to the carbene carbon atom and insertion of one boron atom into the C carbene -N bond. This type of NHC ring expansion has previously been observed for a number of NHCstabilised diboranes, [50][51][52] and is probably favoured in this case by the release of ring strain upon expansion of the B 2 C 2 to the B 2 C 3 ring. The 11 B NMR resonances of 7 Tn -Fc appear at À4.6 and À11.5 ppm, those of 7 Fu -Fc at À6.0 and À12.7 ppm.…”

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

“…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 . In this contribution, we will demonstrate that this route is easily expanded to include other backbones, and more importantly that the NMe 2 group is very well suited for functionalization reactions without affecting the B 2 N 2 C 2 heterocyclic core structure.…”

“…[16] So far,r eactivity studies on type C DADBs are scarce and limited to the transformation of the NMe 2 -substituted derivative 1a to its dihydrido analog and subsequent ring contraction/expansion reactions. [16,17] In this contribution,w ew ill demonstrate that this route is easily expanded to include other backbones, and more importantly that the NMe 2 group is very well suited for functionalization reactions without affecting the B 2 N 2 C 2 heterocyclic core structure. Thus, awide range of DADBs becomes accessible with high potential for furthera pplications.…”

Diborane(4) Azides: Surprisingly Stable Sources of Transient Iminoboranes

“…These iminoboranes are rare examples of borylated iminoboranes and the first examples of borylimino(amino)boranes. Indeed, while borepines are not uncommon, seven‐membered rings bearing the NBNBN sequence have, to our knowledge, never been reported, although related seven‐membered boron‐nitrogen‐containing heterocycles have been reported in the context of ring‐expansion reactions of boroles and in carbene‐diazadiborinine adducts . Our seven‐membered rings are also closely related to a six‐membered cyclic iminoborane reported by Bettinger et al., which likewise displays a behavior typical of iminoboranes (oligomerization, cycloaddition, etc.)…”