- Umpolung of B?H Bonds by Metal–Ligand Cooperation with Cyclopentadienone Iridium Complexes
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In contrast to conventional metal–ligand cooperative cleavage of a B?H bond, which provides a B cation on the ligand and an H anion on the metal, we report herein the umpolung of B?H bonds by novel cyclopentadienone iridium complexes. The B?H bonds of 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin) and 1,8-naphthalenediaminatoborane (HBdan) were cleaved to give a B anion on the metal and an H cation on the phenolic oxygen atom of the ligand. Mechanistic investigation by DFT calculations revealed that the alkoxycarbonyl-substituted cyclopentadienone ligand facilitated deprotonation from Ir?H after oxidative addition of the B?H bond to give the umpolung product. The generated boryliridium complex was found to undergo borylation of an allyl halide in the presence of base, thus showing the nucleophilic nature of the boron atom.
- Higashi, Takuya,Kusumoto, Shuhei,Nozaki, Kyoko
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supporting information
p. 2844 - 2848
(2020/12/18)
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- Aliphatic α-Boryl-α-bromoketones: Synthesis and Reactivity
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A protocol for the preparation of α-boryl-α-bromoketones from alkenyl MIDA boronates was developed and applied to functionalized aliphatic derivatives. The reaction sequence included regioselective hydroxybromination of olefin moiety, followed by oxidation of alcohol group with Dess–Martin periodinane. The target trifunctional boronate-containing derivatives were obtained in up to 94 % yield over two steps starting from alkenyl MIDA boronates. In some cases, functional groups present in the substrate participated in the bromohydroxylation step via intramolecular nucleophilic attack at the bromonium cation leading to cyclic products. Additionally, the reactivity of aliphatic α-boryl-α-bromoketones was illustrated by nucleophilic substitution at the α-C atom and heterocyclization reactions.
- Ivon, Yevhen M.,Kuchkovska, Yuliya O.,Voitenko, Zoya V.,Grygorenko, Oleksandr O.
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p. 3367 - 3377
(2020/03/13)
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- Mechanism-based enhancement of scope and enantioselectivity for reactions involving a copper-substituted stereogenic carbon centre
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A rapidly emerging set of catalytic reactions involves intermediates that contain a copper-substituted stereogenic carbon centre. Here, we demonstrate that an intimate understanding of this distinction provides ways for addressing limitations in reaction scope and explaining why unexpected variations in enantioselectivity often occur. By using catalytic enantioselective Cu-boryl addition to alkenes as the model process, we elucidate several key mechanistic principles. We show that higher electrophile concentration can lead to elevated enantioselectivity. This is because diastereoselective Cu-H elimination may be avoided and/or achiral Cu-boryl intermediates can be converted to allyl-B(pin) rather than add to an alkene. We illustrate that lower alkene amounts and/or higher chiral ligand concentration can minimize the deleterious influence of achiral Cu-alkyl species, resulting in improved enantiomeric ratios. Moreover, and surprisingly, we find that enantioselectivities are higher with the less reactive allylphenyl carbonates as chemoselective copper-hydride elimination is faster with an achiral Cu-alkyl species.
- Lee, Jaehee,Radomkit, Suttipol,Torker, Sebastian,Pozo, Juandel,Hoveyda, Amir H.
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- Nickel-Catalyzed Reductive Dicarbofunctionalization of Alkenes
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An intermolecular, three-component reductive dicarbofunctionalization of alkenes is presented here. The combination of Ni catalysis with TDAE as final reductant enables the direct formation of Csp3-Csp3 and Csp3-Csp2 bonds across a variety of π-systems using two different electrophiles that are sequentially activated with exquisite selectivity under mild reaction conditions.
- García-Domínguez, Andrés,Li, Zhaodong,Nevado, Cristina
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supporting information
p. 6835 - 6838
(2017/06/01)
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- Enantioselective syn and anti homocrotylation of aldehydes: Application to the formal synthesis of spongidepsin
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Whereas crotylboration has been a useful method for synthesis of stereochemically complex products, we have shown that homocrotylboration can be achieved with cyclopropanated crotylation reagents, and that the stereoselectivity of the reaction can be predicted by analogous models. This paper presents a full account of this work, including the first examples of asymmetric anti homocrotylation. The scope of this reaction is demonstrated with highly enantioselective homocrotylation of both aliphatic and aromatic aldehydes, as well as double diastereoselection studies. An application of the synthesis of the marine natural product spongidepsin is presented, as well as streamlined syntheses of homocrotylation reagents.
- Lin, Hongkun,Tian, Leiming,Krauss, Isaac J.
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supporting information
p. 13176 - 13182
(2015/10/28)
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- Six-membered N-Heterocyclic Carbene-based catalysts for asymmetric reactions
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The present invention provides a catalyst complex or ligand, and compositions thereof, for use in a variety of organic reactions having high reactivity and enantioselectivity. The catalyst is a N-heterocyclic carbene having three fused rings with first an
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Page/Page column 8; Sheet 9; Sheet 10
(2011/04/14)
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- Stereoconvergent synthesis of chiral allylboronates from an E/Z mixture of allylic aryl ethers using a 6-NHC-Cu(I) catalyst
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We present a 6-NHC-Cu(I) complex that provides α-substituted allylboronates using allylic aryl ether substrates. The method was discovered by comparison of the chemoselectivities exhibited by complexes 1a, 1b, 2, and 3. We observed that 1a preferentially reacts with electron-rich alkenes over electron-deficient alkenes. Development of an asymmetric method revealed that 1b reacts with both the E and Z isomers to provide the same absolute configuration without showing E-Z isomerization. This stereoconvergent reaction occurs with high yields (av 86%), high SN2′ selectivity (>99:1), and high ee (av 94%) and exhibits wide functional-group tolerance using pure E or Z isomer or E/Z alkene mixtures. The stereoconvergent feature enables the use of many different olefination strategies for substrate production, including cross-metathesis. Chiral allylboronates could be purified by silica gel chromatography and stored in the freezer without decomposition.
- Park, Jin Kyoon,Lackey, Hershel H.,Ondrusek, Brian A.,McQuade, D. Tyler
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supporting information; experimental part
p. 2410 - 2413
(2011/04/25)
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