Alkene–Carbene Isomerization induced by Borane: Access to an Asymmetrical Diborene

Abstract

A 2,3-dihydro-1H-1,2-azaborole derivative 2 was converted to a cyclic (alkyl) (amino)carbene (cAAC) via 1,2-hydrogen migration triggered by boranes to afford cAAC-borane adducts. This procedure allowed us to develop an asymmetrical diborene cAAC·(Br)B═B(Br)·IDip 6, which was isolated and fully characterized. The B NMR spectrum, X-ray diffraction analysis and computational studies indicate that π-electrons on the central B moiety in 6 are unequivalently distributed, and thus polarized. A complete scission of the B═B double bond in 6 was achieved by the treatment with an isonitrile, which led to the formation of a base-stabilized B,N-containing methylenecyclopropane 7.

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

“…More recently the group of Kinjo synthesized and structurally characterized the first mono‐adduct of B 2 Br 4 using a particularly π‐acidic CAAC ligand displaying an endocyclic boron atom, CAAC B (Scheme c) . B 2 Br 4 (CAAC B ) reacted with IDip (Scheme c) and PMe 3 to yield the first unsymmetrical bis‐adducts of B 2 Br 4 , which were reduced to the corresponding diborene (Scheme c) and, in the presence of additional PMe 3 , a geminal bis(PMe 3 )‐stabilized neutral allenic diborene, respectively …”

Increasing the Reactivity of Diborenes: Derivatization of NHC‐Supported Dithienyldiborenes with Electron‐Donor Groups

“…[11]À [13] While all hitherto mentioned diborenes present a trans-arrangement of their respective anionic and neutral ligands,t he use of chelating bis(phosphine) ligandse nabled the synthesis of the first cis-1,2-diaryldiborenes ( Figure 1C). [12] Most recently,t he group of Kinjo succeeded in isolating the first unsymmetrical diborene, supported on one side by ap articularly p-acidicc AAC ligand and on the other by I Dip (Figure 1D), [14] as well as au nique allenic diborene, stabilized by two PMe 3 ligands on the terminal boron. [15] With such al imited number of combinations of anionic substituents and neutral donor ligandsc apable of successfully stabilizing diborenes, there is still little known about their reactivity.L ike their alkener elatives, [LBR] 2 diborenes form p-adducts with late transition metals of groups 11 and 12.…”

CO2 Binding and Splitting by Boron–Boron Multiple Bonds

“…Stirring a purple solution of 4 at room temperature and under an atmosphere of CO2 resulted in a color change to orange, from which yellow crystals were obtained after workup. This compound, 5 (Figure 3), was found to have two broad 11 B NMR signals (δB 43.2, -27.1), the former being more broad than the latter, both significantly upfield from that of precursor 4 (δB 80). The 13 C NMR spectrum of 5 showed low-field signals at δC 172.0, 210.4 and 218.2, the former corresponding to a standard carboxyl carbon nucleus of an ester group.…”

“…The apparently facile reaction of CO2 with B-B multiply bound species is attributed to the high reactivity of the latter, which is able to overcome the lack of polarity in the bond and effect the initial cycloaddition step. Dibromodiborenes (L(Br)B=B(Br)L), very few of which exist in the literature, [10,11] were chosen as candidates for CO2 binding due to their sterically unhindered B=B bonds and thus presumed high reactivity. Upon treatment with one atmosphere of CO2 at room temperature, after 7 min the 11 B NMR spectroscopic signal of diborene 1 [10] (dB 20) was found to have completely disappeared, replaced by two broad signals (dB ca.…”