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.
Highlights
Carboranes are carbon–boron molecular clusters, which can be viewed as three-dimensional analogues to benzene
Cage C–H bonds can be deprotonated by strong bases, followed by reactions with electrophiles to give carbon-substituted carboranes[1,2], and cage B–H bonds are subjected to electrophilic substitution reactions, leading to the formation of cage boron-substituted carborane derivatives with the reaction rate B(9,12)–H4B(8,10)–H4 B(4,5,7,11)–H
Encouraged by transition metal-catalysed C–H borylation and application of the resultant boronate esters/boronic acids in C–C/C–O/C–N/C–halogen bond forming process[45,46,47], we initiated a research program to study transition metal-catalysed direct cage B–H borylation of o-carboranes and the results are reported in this study (Fig. 1)
Summary
Carboranes are carbon–boron molecular clusters, which can be viewed as three-dimensional analogues to benzene. The initial reaction of o-carborane (1a) with B2pin[2] ([B(OCMe2CMe2O)]2) in the presence of 3.5 mol% [(cod)IrCl]2 (cod 1⁄4 1,5-cyclooctadiene) in tetrahydrofuran (THF) gave 3-Bpin-1,2-C2B10H11 (2a) in 60% gas chromatography (GC) yield. It was noted that the double bond in 9-vinyl-o-carborane (1k) underwent hydroboration with HBpin, a byproduct of B–H borylation (vide infra), to afford 3k in 91% isolated yield with excellent regioselectivity, probably owing to steric effect of o-carboranyl moiety.
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