An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small‐Molecule Activation and Rearrangement Chemistry

Abstract A one‐pot synthesis of a CAAC‐stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two‐electron reduction steps from an adduct of CAAC and B2Br4(SMe2)2. Theoretical studies revealed that this diborene has a considerably smaller HOMO–LUMO gap than those of reported NHC‐ and phosphine‐supported diborenes. Complexation of the diborene with [AuCl(PCy3)] afforded two diborene–AuI π complexes, while reaction with DurBH2, P4 and a terminal acetylene led to the cleavage of B−H, P−P, and C−C π bonds, respectively. Thermal rearrangement of the diborene gave an electron‐rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.


Alternative synthesis of compound 1 (compropotionation):
The reaction of VI and V was first examined in THF-d8 at ambient temperature. However, only a small amount (~10%) of 1 was observed by NMR spectroscopy after three days ( Figure S5-S6).
When a THF-d8 solution of VI (20 mg, 0.022 mmol) and V (15.4 mg, 0.026 mmol) mixture was allowed to heat at 60 °C overnight, a green mixture was formed. NMR spectroscopy revealed a full conversion of the starting materials to 1 (85% NMR yield, based on VI) as well as some unidentified species ( Figure S7-S8).

Synthesis of compound 2:
A benzene solution (10 mL) of B2Br4×(SMe2)2 (1.0 g, 2.17 mmol) was slowly added to a CAAC (1.16 g, 4.33 mmol) solution at ambient temptation. The mixture was stirred for 2 h to give a white suspension, then KC8 (0.585 g, 4.33 mmol) was slowly added in small portions. The reaction mixture was allowed to stir overnight, followed by treatment with two equivalents of KC8 (0.585 g, 4.33 mmol) to afford a dark blue mixture. After the reaction was complete, the suspension was filtered, and the residue was washed with benzene (2 x 5 mL). The combined filtrate was dried under vacuum and washed with a small amount of pentane to afford compound 2 as a dark blue crystalline solid (0.7 g, 54%).   Synthesis of compound 2 from 1: KC8 (7 mg, 0.052 mmol) was added to a sealable NMR tube containing a benzene-d6 solution of 1 (17.8 mg, 0.024 mmol). The reaction was monitored by NMR spectroscopy. After two days a blue mixture was afforded, and 1 H and 11 B NMR spectra indicated that full conversion of 1 to 2 was achieved (95 % NMR yield) (Figure S13-S14).

Synthesis of compounds 3 and 4:
[AuCl(PCy3)] (30 mg, 0.058 mmol) was slowly added to a sealable NMR tube containing a benzene-d6 solution of 2 (30 mg, 0.051 mmol). The color of the mixture turned from green via brown to red-brown overnight, and NMR spectroscopy revealed nearly quantitative conversion of 2 to the diborene-Au(I) complexes 3 (63%) and 4 (22%). After the reaction, all volatiles were removed under reduced pressure, the residue was extracted with pentane and concentrated to give an orange red powder of 3. Single crystals of 3 were obtained by slow evaporation of a benzene solution.

Alternative synthesis of 4:
Benzene-d6 (1 mL) was added to a sealable NMR tube containing a mixture of 2 (33 mg, 0.056 mmol) and [AuCl(PCy3)] (11 mg, 0.022 mmol). The reaction mixture was allowed to stand overnight, during which time red crystals were formed on the wall of the NMR tube. The supernatant of the mixture was carefully removed with a pipette, the residue was washed with a small amount of benzene and dried under vacuum, from which compound 4 was obtained as a red solid (17 mg, 70%).

Synthesis of compound 7:
At ambient temperature, white phosphorous (P4) (6 mg, 0.038 mmol) was added to a sealable NMR tube containing a benzene-d6 solution of 2 (30 mg, 0.051 mmol). After two days, a red mixture was obtained, and NMR spectroscopy revealed quantitative formation of a new species. Slowly evaporation of the reaction mixture in a glovebox afforded 7 as red crystals (24 mg, 71%).

Synthesis of compound 8:
(4-methylphenyl)acetylene (TolCCH) (16 mg, 0.140 mmol) was added to a benzene solution of 2 (40 mg, 0.068 mmol) at ambient temperature, and the blue mixture was stirred for 2 d to give a white suspension. After the reaction, all volatiles were removed under reduced pressure and the residue was washed with pentane and dried over vacuum to give 8 as a white solid (34 mg, 61%). Single crystals of 8 were obtained by slow evaporation of a benzene solution at ambient temperature.

Synthesis of compound 9:
A benzene solution of 2 (30 mg, 0.051 mmol) and Hg(CCPh)2 (10 mg, 0.025 mmol) was stirred overnight to give a red brown mixture. After the reaction, the mixture was filtered, and slow evaporation of the benzene mixture gave 9 as orange crystals. However, complex 9 was isolated only in small amounts and could not be characterized by NMR spectroscopy.

Synthesis of compound 10:
A benzene-d6 solution of 2 (42 mg, 0.071 mmol) was heated at 80 °C overnight to give an orange mixture, in which a quantitative conversion of 2 to 10 was manifested by NMR spectroscopy (>95% NMR yield). (No further purification of 10 was performed for further reactivity studies). Orange single crystals of 10 were obtained by slow evaporation of a pentane solution at ambient temperature in a glovebox.

Synthesis of compound 11 and 11':
AgOTf (27 mg, 0.10 mol) was slowly added to a benzene solution of 10 (42 mg, 0.071 mmol) under vigorous stirring to give a red suspension in 10 min. After the reaction, all the volatiles were removed under reduced pressure to give a red solid. The residue was dissolved in dichloromethane and slow evaporation of the solution provided 11 and 11' as colorless crystals (41 mg, 70%) (Despite multiple attempts, we were unable to separate the two isomers by recrystallization).

Crystallographic details
The crystal data of 1-4 and 6-9 were collected on a Bruker D8 Quest diffractometer with a CMOS area detector and multi-layer mirror monochromated MoKa radiation. The crystal data of 10 and 11' were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and multi-layer mirror monochromated MoKa radiation. The crystal data of 11 was collected on a Rigaku XtaLAB Synergy-R diffractometer with a HPA area detector and multi-layer mirror monochromated CuKa radiation. The structure was solved using the intrinsic phasing method, [3] refined with the ShelXL program [4] and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized positions, except those bound to boron, which were refined freely.

Refinement details for 3:
The unit cell contains solvent molecules which have been treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON. The displacement parameters of carbon atoms in the disordered cyclohexyl groups were restrained to the same value with similarity restraint SIMU. The U ii displacement parameters of these atoms were restrained with ISOR keyword to approximate isotropic behavior as well. The distances between atoms C1_6 P1_1 and C1_16 P1_1 were restrained during refinement to the same value. The 1-2 and 1-3 distances of carbon atoms in the disordered cyclohexyl groups were restrained to the same values with SAME. Refinement details for 9: The soft restraint (ISOR) and similarity restraint on displacement amplitudes (SIMU) were employed for the atomic displacement parameters of disordered 'ME' fragments. The bond distances of atoms were restrained to be similar by means of the similarity restraint (SAME). The distances between N11 and C1, as well as the C14 and C4 atoms in disordered 'ME' groups were restrained during refinement to the same value with SADI restraint, respectively. The disordered benzene solvent molecule was treated with SQUEEZE. The following reflections were removed by 'OMIT': 1 -2 3, 1 -1 2, 1 - Refinement details for 11: The displacement parameters of atoms C2 and C7 of residue 2 (CAAC) were constrained to the same value with EADP keyword. The same also applies for atoms C4 and C9 of residue 2 (CAAC) and atoms C6 and C11 of residue 2 (CAAC). The coordinates of atoms C2 and C7 of residue 2 (CAAC) were constrained to the same value. The same also applies for atoms C4 and C9 of residue 2 (CAAC). The atomic displacement parameters of atoms C2 to C11 of residue 2 (CAAC) were restrained with the RIGU keyword in ShelXL input ('enhanced rigid bond' restraint for all bonds in the connectivity list). The same also applies for atoms S1 to F8 of residues 6 and 16 (OTf). The displacement parameters of atoms C2 to C11 of residue 2 (CAAC) were restrained to the same value with similarity restraint SIMU. The same also applies for atoms S1 to F8 of residues 6 and 16 (OTf). The 1-2 and 1-3 distances in residues 6 and 16 (OTf) were restrained to the same values with SAME due to disorder of the counterion.