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. 2017 Mar 16:8:14827.
doi: 10.1038/ncomms14827.

Iridium-catalysed regioselective borylation of carboranes via direct B-H activation

Ruofei Cheng  1 , Zaozao Qiu  1 , Zuowei Xie  1   2
Affiliations

Iridium-catalysed regioselective borylation of carboranes via direct B-H activation

Ruofei Cheng et al. Nat Commun. .

Abstract

Carboranes are carbon-boron molecular clusters, which can be viewed as three-dimensional analogues to benzene. They are finding many applications in medicine, materials and organometallic chemistry. On the other hand, their exceptional thermal and chemical stabilities, as well as 3D structures, make them very difficult to be functionalized, in particular the regioselective functionalization of BH vertex among ten similar B-H bonds. Here we report a very efficient iridium-catalysed borylation of cage B(3,6)-H bonds of o-carboranes with excellent yields and regioselectivity using bis(pinacolato)diboron (B2pin2) as a reagent. Selective cage B(4)-H borylation has also been achieved by introducing a bulky TBDMS (tert-butyldimethylsilyl) group to one cage carbon vertex. The resultant 3,6-(Bpin)2-o-carboranes are useful synthons for the synthesis of a wide variety of B(3,6)-difunctionalized o-carboranes bearing cage B-X (X=O, N, C, I and Br) bonds.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Functionalization of B(3,6)-H bonds in o-carboranes (Bpin=B(OCMe2CMe2O), B2pin2=pinB-Bpin).
(a) Known methods for B(3) and B(3,6) functionalization. (b) This work: Iridium-catalysed regioselective borylation of carboranes via direct B−H activation. (c) Numbering system of o-carborane.
Figure 2
Figure 2. Synthesis of 4-Bpin-o-carboranes.
(a) Ir-catalysed regioselective B(4)−H borylation in o-carboranes by introducing a bulky substituent at the cage C position. (b) Molecular structures of 7c and 8a.
Figure 3
Figure 3. Chemical transformations of 3a.
Reaction conditions: (a) PhBr (3 equiv.), Pd(PPh3)4 (20 mol%), Cs2CO3 (3 equiv.), cyclohexane, 150 °C (bath), 8 h. (b) Allyl chloride (6 equiv.), Pd(dba)2 (20 mol%), Cs2CO3 (3 equiv.), toluene, room temperature, 24 h. (c) PhX (3 equiv.), Pd(PPh3)4 (10 mol%), tBuOK (3 equiv.), THF, 80 °C, 24 h. (d) Cu(OAc)2 (6 equiv.), KF (6 equiv.), CH3CN, 80 °C, 12 h, under 1 atm of O2. (e) MeONHLi, THF, 80 °C, 8 h. (f) TMSN3 (2.4 equiv.), CuCl (2.1 equiv.), KF (2.4 equiv.), THF, 60 °C, 24 h. (g) Diethyl acetylenedicarboxylate (2.4 equiv.), toluene, 95 °C. (h) 1) DEA (diethanolamine, 2.5 equiv.), Et2O, room temperature, 18 h, 2) HCl aq. (0.5 M, excess).
Figure 4
Figure 4. Mechanistic investigations.
Control experiments. (a) Stoichiometric reaction of [(cod)IrCl]2 with 2-MePy. (b) Stoichiometric reaction of [(cod)IrCl]2 with 2-MePy and o-carborane (1a). (c) Stoichiometric reaction of [(cod)IrCl]2 with 2-MePy and B2pin2. (d) Stoichiometric reaction of [(cod)IrCl]2 with 2-MePy and HBpin. (e) Standard catalytic borylation reaction of 1a monitored by 1H and 11B NMR. (f) (η6-MesH)Ir(Bpin)3 catalysed borylation reaction of 1a.
Figure 5
Figure 5. Proposed reaction mechanism.
The ligand on iridium has been omitted for clarity.
See this image and copyright information in PMC

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