A Mild Radical Procedure for the Reduction of <i>B</i>-Alkylcatecholboranes to Alkanes

Pozzi, Davide; Scanlan, Eoin M.; Renaud, Philippe

doi:10.1021/ja055691j

Cited by 110 publications

(46 citation statements)

References 17 publications

(31 reference statements)

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“…A mild and efficient radical-mediated reduction of organoboranes has been developed (Scheme 20) [33]. An in situ-generated B-methoxycatecholborane-methanol complex acts as a reducing agent.…”

Section: Boron-alcohol Complexes As Reducing Agentsmentioning

confidence: 99%

Boron: A key element in radical reactions

Renaud

Beauseigneur

Brecht‐Forster

et al. 2007

Pure and Applied Chemistry

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Boron derivatives are becoming key reagents in radical chemistry. Here, we describe reactions where an organoboron derivative is used as a radical initiator, a chain-transfer reagent, and a radical precursor. For instance, B-alkylcatecholboranes, easily prepared by hydroboration of alkenes, represent a very efficient source of primary, secondary, and tertiary alkyl radicals. Their very high sensitivity toward oxygen-and heteroatom-centered radicals makes them particularly attractive for the development of radical chain processes such as conjugate addition, allylation, alkenylation, and alkynylation. Boron derivatives have also been used to develop an attractive new procedure for the reduction of radicals with alcohols and water. The selected examples presented here demonstrate that boron-containing reagents can efficiently replace tin derivatives in a wide range of radical reactions.

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Section: Boron-alcohol Complexes As Reducing Agentsmentioning

confidence: 99%

Boron: A key element in radical reactions

Renaud

Beauseigneur

Brecht‐Forster

et al. 2007

Pure and Applied Chemistry

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“…While this phenomenon has been observed for many different metals, bond types, and ligand platforms, the exploitation of homolytic bond weakening as a mechanism of substrate activation in catalysis has not been extensively explored. 41,42 We envisioned that this bond-weakening strategy might prove to be a general elementary step that could enable the ‘soft homolysis’ of normally strong bonds using comparably weak H-atom acceptors via PCET (Figure 12b). The synthetic value of such processes would stem from their ability to furnish metallated intermediates from simple E-H and C-H bond precursors in the absence of an identifiable Brønsted base.…”

Section: Introductionmentioning

confidence: 99%

Synthetic Applications of Proton-Coupled Electron Transfer

2016

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Conspectus Redox events in which an electron and proton are exchanged in a concerted elementary step are commonly referred to as proton-coupled electron transfers (PCETs). PCETs are known to operate in numerous important biological redox processes, as well as recent inorganic technologies for small molecule activation. These studies suggest that PCET catalysis might also function as a general mode of substrate activation in organic synthesis. Over the past three years, our group has worked to advance this hypothesis and to demonstrate the synthetic utility of PCET through the development of novel catalytic radical chemistries. The central aim of these efforts has been to demonstrate the ability of PCET to homolytically activate a wide variety of common organic functional groups that are energetically inaccessible using known molecular H-atom transfer catalysts. To do so, we made use of a simple formalism first introduced by Mayer and coworkers that allowed us to predict the thermodynamic capacity of any oxidant/base or reductant/acid pair to formally add or remove H• from a given substrate. With this insight, we were able to rationally select catalyst combinations thermodynamically competent to homolyze the extraordinarily strong E-H σ-bonds found in many common protic functional groups (BDFEs >100 kcal/mol) or to form unusually weak bonds to hydrogen via the reductive action of common organic π-systems (BDFEs <35 kcal/mol). These ideas were reduced to practice through the development of new catalyst systems for reductive PCET activations of ketones and oxidative PCET activation of amide N-H bonds to directly furnish reactive ketyl and amidyl radicals, respectively. In both systems the reaction outcomes were found to be successfully predicted using the effective bond strength formalism, suggesting that these simple thermochemical considerations can provide useful and actionable insights into PCET reaction design. The ability of PCET catalysis to control enantioselectivity in free radical processes has also been established. Specifically, multisite PCET requires the formation of a pre-equilibrium hydrogen-bond between the substrate and a proton donor/acceptor prior to charge transfer. We recognized that these H-bond interfaces persist following the PCET event, resulting in the formation of non-covalent complexes of the nascent radical intermediates. When chiral proton donors/acceptors are employed, this association can provide a basis for asymmetric induction in subsequent bond-forming steps. We discuss our efforts to capitalize on this understanding via the development of a catalytic protocol for enantioselective aza-pinacol cyclizations. Lastly, we highlight an alternative PCET mechanism that exploits the ability of redox-active metals to homolytically weaken the bonds in coordinated ligands, enabling nominally strong bonds (BDFEs ~100 kcal) to be abstracted by weak H-atom acceptors with concomitant oxidation of the metal center. This ‘soft homolysis’ mechanism enables the generation of metallated intermediates...

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“…The reduction is a two-step process involving hydroboration followed by radical reduction mediated by methanol and the slow introduction of oxygen (Scheme 48). 156 The methodology reduces a range of alkenes including those that give rise to primary, secondary and tertiary alkylboranes. Interestingly, a series of labelling experiments suggest that methanol is the source of the hydrogen atom that reduces the alkyl radical with zwitterion 168 as the active reagent.…”

Section: Miscellaneous Reducing Agentsmentioning

confidence: 99%