Borylative Radical Cyclizations of Benzo[3,4]cyclodec‐3‐ene‐1,5‐diynes and N‐Heterocyclic Carbene‐Boranes

Watanabe, Takashi; Hirose, Daisuke; Curran, Dennis P.; Taniguchi, Tsuyoshi

doi:10.1002/chem.201700689

Cited by 78 publications

(34 citation statements)

References 51 publications

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“…Elegant work from Fensterbank, Lacôte, Malacria, and Curran has shown that N‐heterocyclic carbene (NHC)–boryl radicals can be generated by hydrogen atom abstraction from NHC−BH 3 complexes, which are powerful intermediates useful in building a wide range of value‐added boron compounds. For instance, Wang has developed radical borylation/cyclization cascade of 1,6‐enynes with NHC−BH 3 , and Curran and Taniguchi reported the Sato–Myers cyclization triggered by NHC–boryl radicals . Consistent with our continuing interest in development of atom‐economic and redox‐neutral transformations through hydrogen atom transfer (HAT)‐based photocatalysis, we envisioned that a photo‐mediated catalytic HAT process with NHC−BH 3 might deliver the boryl radical in a mild pathway, which could react with polyfluoroarenes and alkenes to introduce synthetically valuable fluorinated organic boranes (Scheme c).…”

Section: Introductionmentioning

confidence: 88%

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

et al. 2020

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Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value‐added fluorinated organic compounds. Recent progress has been mainly focused on the transition‐metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B−H activation of N‐heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom‐economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem‐difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition‐metal free, and the regioselectivity obtained is complementary to the reported transition‐metal‐catalysis in many cases.

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Section: Introductionmentioning

confidence: 88%

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

et al. 2020

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“…Thereaction of the 10membered diyne 13 a with NHC-borane 17 (5 equiv) in the presence of di-tert-butyl hyponitrite (0.4 equiv) at 100 8 8Cd id not provide aborepin but instead gave 7,8,9tetrahydrobenzo[a]azulene 18 in 59 %y ield. [15] This product forms by an interrupted single hydroboration reaction, wherein the interrupting reaction is the transannular cyclization of the initial alkenyl radical.…”

Section: Experimental Studiesmentioning

confidence: 99%

The Thermal Rearrangement of an NHC‐Ligated 3‐Benzoborepin to an NHC‐Boranorcaradiene

et al. 2019

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An N‐heterocyclic‐carbene‐ligated 3‐benzoborepin with a bridged structure has been synthesized by double radical trans‐hydroboration of benzo[3,4]cycloundec‐3‐ene‐1,5‐diyne with an N‐heterocyclic carbene borane. The thermal reaction of the NHC‐ligated borepin at 150 °C gives an isolable NHC‐boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π‐electrocyclic reaction. This is followed by 1,5‐boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open‐shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

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“…Recently, we disclosed radical borylation/cyclization reactions of strained cyclic diynes with NHC‐boranes . Likewise, Wang and co‐workers reported radical borylation/cyclization reactions of enynes with NHC‐boranes and thiols as polarity‐reversal reagents .…”

Section: Figurementioning

confidence: 98%

Radical trans‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes

et al. 2018

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Hydroboration of internal alkynes with N-heterocyclic carbene boranes (NHC-boranes) occurs to provide stable NHC (E)-alkenylboranes upon thermolysis in the presence of di-tert-butyl peroxide.T he Eisomer results from an unusual trans-hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by ar adical-chain reaction involving addition of an NHC-boryl radical to an alkyne to give a b-NHC-borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC-borane provides the product and returns the starting NHC-boryl radical. Experiments suggest that the observed trans-selectivity results from kinetic control in the hydrogen-transfer reaction. Figure 1. Methods for trans-hydroboration of alkynes.

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Borylative Radical Cyclizations of Benzo[3,4]cyclodec‐3‐ene‐1,5‐diynes and N‐Heterocyclic Carbene‐Boranes

Cited by 78 publications

References 51 publications

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

The Thermal Rearrangement of an NHC‐Ligated 3‐Benzoborepin to an NHC‐Boranorcaradiene

Radical trans‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes

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