Bis(tri‐<i>tert</i>‐butylphosphine)palladium(0)‐Catalyzed Iodine–Fluorine Exchange at <i>closo</i>‐Carboranes

Ishita, Keisuke; Khalil, Ahmed; Tiwari, Rohit; Gallucci, Judith C.; Tjarks, Werner

doi:10.1002/ejic.201800486

Cited by 4 publications

(3 citation statements)

References 28 publications

(38 reference statements)

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“…While the formation of OH-mCB was observed previously by us and others 26,50 and is likely due to the presence of adventitious moisture in the system, the formation of F-mCB by crosscoupling of nucleophilic fluoride is rare. 27 Neither of these byproducts are surprising given the larger thermodynamic driving force for the formation of B−O and B−F bonds relative to C−O and C−F bonds. 51 This contrasts with Suzuki− Miyaura cross-coupling reactions, where off-cycle C−O and C−F bond formation is rarely observed even under aqueous reaction conditions.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Off-Cycle Processes in Pd-Catalyzed Cross-Coupling of Carboranes

Dziedzic

Axtell

Rheingold

et al. 2019

Org. Process Res. Dev.

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Off-cycle processes in a catalytic reaction can dramatically influence the outcome of the chemical transformation and affect its yield, selectivity, rate, and product distribution. While the generation of off-cycle intermediates can complicate reaction coordinate analyses or hamper catalytic efficiency, the generation of such species may also open new routes to unique chemical products. We recently reported the Pd-mediated functionalization of carboranes with a range of ON N-, and C-based nucleophiles. By utilizing a Pd-based catalytic system supported by a biarylphosphine ligand developed by Buchwald and coworkers, we discovered an off-cycle isomerization process ("cage-walking") that generates four regioisomeric products from a single halogenated boron cluster isomer. Here we describe how several off-cycle processes affect the regioisomer yield and distribution during Pd-catalyzed tandem cage-walking/cross-coupling. In particular, tuning the transmetalation step in the catalytic cycle allowed us to incorporate the cage-walking process into Pd-catalyzed cross-coupling of sterically unencumbered substrates, including cyanide. This work demonstrates the feasibility of using tandem cage-walking/cross-coupling as a unique low-temperature method for producing regioisomers of monosubstituted carboranes.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Off-Cycle Processes in Pd-Catalyzed Cross-Coupling of Carboranes

Dziedzic

Axtell

Rheingold

et al. 2019

Org. Process Res. Dev.

View full text Add to dashboard Cite

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“…This difference in B-X bond strength allowed radiolabelling of carboranes with 76 Br and 19 F because the resulting B-bromo- and B-fluoro-carboranes do not undergo oxidative addition in the presence of commonly used Pd catalysts. 35,36 A survey of the catalyst systems used in cross-coupling of carboranes reveals that the majority of Pd-catalyzed reactions use a small set of phosphine ligands, mainly triphenylphosphine (PPh 3 ).…”

Section: Introductionmentioning

confidence: 99%

“…4. 34–36 The lack of reactivity in B-bromo and B-chloro polyhedral boranes in cross-coupling reactions was attributed to the stronger B-X bonds, compared to aryl C-X bonds, which prevented oxidative addition of B-bromocarboranes. This difference in B-X bond strength allowed radiolabelling of carboranes with 76 Br and 19 F because the resulting B-bromo- and B-fluoro-carboranes do not undergo oxidative addition in the presence of commonly used Pd catalysts.…”

Section: Introductionmentioning

confidence: 99%

Metal-catalyzed cross-coupling chemistry with polyhedral boranes

2019

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Over the past several decades, metal-catalyzed cross-coupling has emerged as a very powerful strategy to functionalize carbon-based molecules. More recently, some of the cross-coupling methodologies have been adapted to inorganic compounds including boron-rich clusters. The development of this chemistry relies on the ability to synthesize halogenated boron-rich clusters which can serve as electrophilic cross-coupling partners with nucleophilic substrates in the presence of a metal catalyst. While the cross-coupling chemistry with boron-clusters is conceptually reminiscent to that of its hydrocarbon counterparts, several key aspects including the spheroidal bulk of clusters, and the distinct nature of boron-halogen / boron-heteroatom bonds make this chemistry unique. The utility of metal-catalyzed cross-coupling can be extended to several classes of polyhedral boranes including neutral and anionic carboranes, metallaboranes, and carbon-free boranes. Importantly, cross-coupling enables a suite of boron-heteroatom (C, N, O, P, S) couplings to prepare boron cluster-based systems that can be used for ligand design, medicinal chemistry, and materials applications.

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