Enantioselective Rhodium(III)-Catalyzed Markovnikov Hydroboration of Unactivated Terminal Alkenes

Smith, James R.; Collins, Beatrice S. L.; Hesse, M.; Graham, M. A.; Myers, Eddie L.; Aggarwal, Varinder K.

doi:10.1021/jacs.7b05149

Cited by 116 publications

(71 citation statements)

References 46 publications

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“…We felt that the introduction of an additional substituent on the ligand backbone would push the four 1-phenethyl substituents on the aryl groups of the NHC closer to the reactive center by ab uttressing effect, and thus improve the level of enantiocontrol. Accordingly,al igand based on the acenaphthoimidazolylidene framework, [16] (R,R,R,R)-ANIPE (L2), was synthesized with the expectation that the 1phenethyl substituents would exert am ore effective steric influence and that the stronger donor properties of the ligand would allow al igated copper boryl complex to react efficiently with a-olefins.W en ote that during preparation the manuscript, Aggarwal and co-workers [17] reported an elegant rhodium(III)-catalyzed asymmetric protocol by diverting Nishiyamasd iboration [18] to protoboration using an ovel preactivated boron reagent. While this method represents ab reakthrough in stereoselective alkene protoboration, we believe that the enantiomeric excesses observed for a-olefin substrates (72-90 % ee)l eaves room for further improvement.…”

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confidence: 99%

Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

et al. 2018

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Reported is a highly enantioselective copper-catalyzed Markovnikov protoboration of unactivated terminal alkenes. A variety of simple and abundant feedstock α-olefins bearing a diverse array of functional groups and heterocyclic substituents can be used as substrates, and the reaction proceeds under mild reaction conditions at ambient temperature to provide expedient access to enantioenriched alkylboronic esters in good regioselectivity and with excellent enantiocontrol. Critical to the success of the protocol was the development and application of a novel, sterically hindered N-heterocyclic carbene, (R,R,R,R)-ANIPE, as the ligand for copper.

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Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

et al. 2018

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“…Despite these results, Markovnikov hydroboration of unactivated terminal alkenes that can be obtained from petrochemical feedstocks or synthesized in a straightforward manner remain elusive. Very recently, the research groups of Ito 9,10 , Montgomery 11 , Aggarwal 12 , and Shi 13 independently realized Cu-or Rh-catalyzed Markovnikov hydroboration of unactivated alkenes based on the addition of metal-boron intermediates to alkenes (Scheme 1c, i). Although these efforts provide elegant methods for the synthesis of branched alkyl borates, the scope of the reactions with respect to alkenes are generally limited to monosubstituted alkenes, and they require either expensive and toxic phosphine ligands or carbene ligands.…”

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confidence: 99%

Ligand-Free Iron-Catalyzed Regiodivergent Hydroboration of Unactivated Terminal Alkenes

Su¹,

Qiao²,

Zheng³

et al. 2020

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The control of regioselectivities has been recognized as the elementary issue for alkene hydroboration. Despite considerable progress, the specificity of alkene substrates or the adjustment of ligands were necessary for specific regioselectivities, which restrict the universality and practicability. Herein, we report a ligand-free iron-catalyzed regiodivergent hydroboration of unactivated terminal alkenes that obtains both Markovnikov and anti-Markovnikov hydroboration products in excellent regioselectivities. Notably, solvents and bases were shown to be crucial factors influencing the regioselectivities and further studies suggested the iron-boron alkoxide ate complex is the key intermediate that determines the unusual Markovnikov regioselectivity. Terminal alkenes with diverse structures (mono-substituted and 1,1-disubstituted, open-chain and exocyclic) underwent the transformation smoothly. The reaction does not require the addition of auxiliary ligands and it can be performed on a gram scale, thus providing an efficient and sustainable method for the synthesis of primary, secondary, and tertiary alkyl borates.

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“…[14] However,aninitial trial using L1 as the supporting ligand for the copper-catalyzed protoboration of 4-phenyl-1-butene with B 2 pin 2 and methanol afforded the desired product in low yield and poor enantioselectivity (31 %y ield, 9% ee). Accordingly,al igand based on the acenaphthoimidazolylidene framework, [16] (R,R,R,R)-ANIPE (L2), was synthesized with the expectation that the 1phenethyl substituents would exert am ore effective steric influence and that the stronger donor properties of the ligand would allow al igated copper boryl complex to react efficiently with a-olefins.W en ote that during preparation the manuscript, Aggarwal and co-workers [17] reported an elegant rhodium(III)-catalyzed asymmetric protocol by diverting Nishiyamasd iboration [18] to protoboration using an ovel preactivated boron reagent. We felt that the introduction of an additional substituent on the ligand backbone would push the four 1-phenethyl substituents on the aryl groups of the NHC closer to the reactive center by ab uttressing effect, and thus improve the level of enantiocontrol.…”

mentioning

confidence: 99%

Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

et al. 2018

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Reported is ah ighly enantioselective copper-catalyzed Markovnikovp rotoboration of unactivated terminal alkenes.Avariety of simple and abundant feedstock a-olefins bearing ad iverse arrayo ff unctional groups and heterocyclic substituents can be used as substrates,a nd the reaction proceeds under mild reaction conditions at ambient temperature to provideexpedient access to enantioenriched alkylboronic esters in good regioselectivity and with excellent enantiocontrol. Critical to the success of the protocol was the development and application of an ovel, sterically hindered N-heterocyclic carbene,( R,R,R,R)-ANIPE, as the ligand for copper.The asymmetric hydro-and protoboration of alkenes has long been recognized as an important stereoselective transformation in chemical synthesis. [1] Thep reparative value of this transformation stems from its conversion of widely available alkenes into synthetically versatile enantioenriched alkyl boronates under generally mild reaction conditions.The C À Bb onds in the resulting organoboron compounds are readily transformed into ab road array of valuable products with high enantiospecificity,including those containing CÀO, CÀN, CÀC, and CÀX(X= F, Cl, Br, I) bonds. [2] However,over the past several decades and since Brownsf irst asymmetric hydroboration, [1a] and despite the considerable progress made in the development of stoichiometric [1b] and metal-catalyzed processes, [1c-e] the substrate scope of this transformation remains largely limited to styrenes, [3] alkenes bearing directing groups, [4] and electronically biased or activated alkenes. [5][6][7] Theapplication of asymmetric hydro-and protoboration to simple unactivated a-olefins abundantly found in petrochemical feedstocks has proven to be al ongstanding challenge,and even astoichiometric process remains elusive. Indeed, this class of substrates continues to pose substantial challenges for asymmetric catalysis as aw hole,w ith only ah andful of reactions developed, so far,t hat provide satisfactory levels of enantiocontrol. [8] We have recently become interested in the development of ac opper-catalyzed asymmetric protoboration of a-olefins (Figure 1a). [9] As illustrated in the proposed catalytic cycle (Figure 1b), ar egio-and enantioselective borylcupration of the terminal alkene B by the copper boryl A would afford aprimary alkylcopper intermediate D via the p-complex Ca. Abulky ligand would facilitate the selective formation of Ca by avoiding the steric interaction between the ligand and the alkene substituent as shown in Cb.Subsequent protonation of the transient alkylcopper species D with an alcohol as the proton donor would furnish the chiral protoboration product E and release the copper alkoxide F.As-metathesis reaction between F and the diboron reagent G would regenerate A and close the catalytic cycle.T he regio-and enantiocontrol of the olefin borylcupration step is the key event that determines the regio-and stereochemical outcome of the transformation. However,d espite recent advances in copper-catalyz...

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Enantioselective Rhodium(III)-Catalyzed Markovnikov Hydroboration of Unactivated Terminal Alkenes

Cited by 116 publications

References 46 publications

Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

Ligand-Free Iron-Catalyzed Regiodivergent Hydroboration of Unactivated Terminal Alkenes

Copper‐Catalyzed Enantioselective Markovnikov Protoboration of α‐Olefins Enabled by a Buttressed N‐Heterocyclic Carbene Ligand

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