- Carbon monoxide and hydrogen (syngas) as a C1-building block for selective catalytic methylation
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A catalytic reaction using syngas (CO/H2) as feedstock for the selective β-methylation of alcohols was developed whereby carbon monoxide acts as a C1 source and hydrogen gas as a reducing agent. The overall transformation occurs through an intricate network of metal-catalyzed and base-mediated reactions. The molecular complex [Mn(CO)2Br[HN(C2H4PiPr2)2]]1comprising earth-abundant manganese acts as the metal component in the catalytic system enabling the generation of formaldehyde from syngas in a synthetically useful reaction. This new syngas conversion opens pathways to install methyl branches at sp3carbon centers utilizing renewable feedstocks and energy for the synthesis of biologically active compounds, fine chemicals, and advanced biofuels.
- Kaithal, Akash,H?lscher, Markus,Leitner, Walter
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p. 976 - 982
(2021/02/06)
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- Highly efficient NHC-iridium-catalyzed β-methylation of alcohols with methanol at low catalyst loadings
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The methylation of alcohols is of great importance since a broad number of bioactive and pharmaceutical alcohols contain methyl groups. Here, a highly efficient β-methylation of primary and secondary alcohols with methanol has been achieved by using bis-N-heterocyclic carbene iridium (bis-NHC-Ir) complexes. Broad substrate scope and up to quantitative yields were achieved at low catalyst loadings with only hydrogen and water as by-products. The protocol was readily extended to the β-alkylation of alcohols with several primary alcohols. Control experiments, along with DFT calculations and crystallographic studies, revealed that the ligand effect is critical to their excellent catalytic performance, shedding light on more challenging Guerbet reactions with simple alcohols. [Figure not available: see fulltext.].
- Lu, Zeye,Zheng, Qingshu,Zeng, Guangkuo,Kuang, Yunyan,Clark, James H.,Tu, Tao
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p. 1361 - 1366
(2021/06/30)
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- Manganese(I)-Catalyzed β-Methylation of Alcohols Using Methanol as C1 Source
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Highly selective β-methylation of alcohols was achieved using an earth-abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] 1 ([HN(C2H4PiPr2)2]=MACHO-iPr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β-methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β-position, opening a pathway to “biohybrid” molecules constructed entirely from non-fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal–ligand cooperation at the Mn-pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C?C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules.
- Kaithal, Akash,van Bonn, Pit,H?lscher, Markus,Leitner, Walter
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supporting information
p. 215 - 220
(2019/12/03)
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- Super electron donor-mediated reductive desulfurization reactions
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The desulfurization of thioacetals and thioethers by a pyridine-derived electron donor is described. This methodology provides efficient access to the reduced products in high yields and does not require the use of transition-metals, elemental alkali-metals, or hydrogen atom donors.
- Nozawa-Kumada, Kanako,Ito, Shungo,Noguchi, Koto,Shigeno, Masanori,Kondo, Yoshinori
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supporting information
p. 12968 - 12971
(2019/11/05)
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- Coordination-Induced Stereocontrol over Carbocations: Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols
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The inherent difficulty in eliciting facial control over carbocations has limited their utility as intermediates in asymmetric catalysis. We have now shown that a docking strategy involving the reversible coordination of a substrate to a chiral transition-metal catalyst can be used to enable highly stereoselective nucleophilic attack on intermediate tertiary carbocations. This approach has been implemented to achieve the first example of enantioselective reductive deoxygenation of tertiary alcohols. This reduction occurs with high enantio- (up to 96% ee) and regioselectivity (up to >50:1 rr) by applying a novel Hantzsch ester analogue as a convenient hydride source. In-depth mechanistic studies support the involvement of a tertiary carbocation that is coordinated to the iridium metal center via the key allene moiety.
- Isomura, Mayuko,Petrone, David A.,Carreira, Erick M.
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supporting information
p. 4738 - 4748
(2019/03/19)
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- Ruthenium(II)-Catalyzed β-Methylation of Alcohols using Methanol as C1 Source
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Selective introduction of methyl branches into the carbon chains of alcohols can be achieved with low loadings of ruthenium precatalyst [RuH(CO)(BH4)(HN(C2H4PPh2)2)] (Ru-MACHO-BH) using methanol both as methylating reagent and as reaction medium. A wide range of structurally divers alcohols was β-methylated with excellent selectivity (>99 %) in fair to high yields (up to 94 %) under standard conditions, and turnover numbers up to 18,000 could be established. The overall reaction rate of the complex catalytic network appears to be governed by interconnection of the individual subcycles through availability of the reactive intermediates. The synthetic procedure opens pathways to important structural motifs following the Green Chemistry principles.
- Kaithal, Akash,Schmitz, Marc,H?lscher, Markus,Leitner, Walter
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p. 5287 - 5291
(2019/05/28)
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- Synthesis of Halomethyl Isoxazoles/Cyclic Nitrones via Cascade Sequence: 1,2-Halogen Radical Shift as a Key Link
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A novel iminoxyl radical-promoted dichotomous regioselective 5-exo-trig cyclization onto vinylic halogen/1,2-halogen radical shift sequence is developed for the synthesis of halomethyl isoxazoles/cyclic nitrones using β-halo-β,?- and ?-halo-?,?-unsaturated ketoximes as the substrates and PhI(OAc)2/TEMPO as the oxidation system. DFT calculations reveal that a halogen-bridged three-membered ring transition state is involved in the 1,2-Cl-/Br-atom shift, while the 1,2-I atom migration can be taken into account with an elimination/readdition mechanism. The migration ability was indicated to be ranked in the following order: I > Br > Cl.
- Chen, Hong-Lei,Wei, Dian,Zhang, Jian-Wu,Li, Cheng-Lin,Yu, Wei,Han, Bing
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supporting information
p. 2906 - 2910
(2018/05/28)
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- Preparation and use of DMF-stabilized iridium nanoclusters as methylation catalysts using methanol as the C1 source
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We report methylations of alcohols and anilines catalyzed by DMF-stabilized Ir nanoclusters using methanol as the C1 source. The DMF-stabilized Ir nanoclusters were prepared in one step and have diameters of 1-1.5 nm. They react in a borrowing-hydrogen reaction and are efficient methylation catalysts (TON up to 310?000).
- Oikawa, Kei,Itoh, Satoshi,Yano, Hiroki,Kawasaki, Hideya,Obora, Yasushi
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p. 1080 - 1083
(2017/02/05)
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- Nickel-Catalyzed Borylation of Aryl and Benzyl 2-Pyridyl Ethers: A Method for Converting a Robust ortho-Directing Group
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The nickel-catalyzed borylation of aryl 2-pyridyl ethers via the loss of a 2-pyridyloxy group is described. This method allows a 2-pyridyloxy group to be used as a convertible directing group in C?H bond functionalization reactions. The nickel catalyst can also borylate arylmethyl 2-pyridyl ethers, in which the stereochemistry at the benzylic position is retained in the case of chiral secondary benzylic substrates. (Figure presented.).
- Tobisu, Mamoru,Zhao, Jiangning,Kinuta, Hirotaka,Furukawa, Takayuki,Igarashi, Takuya,Chatani, Naoto
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supporting information
p. 2417 - 2421
(2016/08/16)
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- Reaction of secondary and tertiary aliphatic halides with aromatic aldehydes mediated by chromium(II): a selective cross-coupling of alkyl and ketyl radicals
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Takai-Utimoto reactions with secondary and tertiary aliphatic halides usually failed according to previous reports. Now, significant improvements could be achieved, and especially secondary aliphatic halides can be coupled to aromatic aldehydes in yields of up to >95%. A variety of processes are competing with the desired one, and thus conditions must be adapted to the nature of the aldehyde as well as the aliphatic halide used, as the outcome of these reactions is strongly affected by the putative radical intermediates.
- Wessjohann, Ludger A.,Schmidt, Gisela,Schrekker, Henri S.
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p. 2134 - 2142
(2008/09/18)
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- Dual role of alkynyl halides in one-step synthesis of alkynyl epoxides
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It was demonstrated that alkynyl halides could serve as a source of Br+ and acetylide ions in the same transformation. This allowed for the efficient one-step preparation of alkynyl epoxides, important organic building blocks, from readily available starting materials. Copyright
- Trofimov, Alexander,Chernyak, Natalia,Gevorgyan, Vladimir
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supporting information; experimental part
p. 13538 - 13539
(2009/02/06)
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- The Barbier-Type Alkylation of Aldehydes with Alkyl Halides in the Presence of Metallic Strontium
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In the presence of metallic strontium, the Barbier-type alkylation of aldehydes with alkyl iodides proceeded smoothly at - 15 °C under an argon atmosphere to afford the corresponding alkylated alcohols in moderate to good yields. The unusual addition reaction of ethyl benzoate with tert-butyl iodide took place to give the p-tert-butylated alkylated product (the 1,6-adduct of the benzoate) in 55% yield. Using strontium isopropoxide, the Meerwein-Ponndorf-Verley-type reduction of the aldehyde proceeded smoothly under reflux conditions in 2-propanol.
- Miyoshi, Norikazu,Kamiura, Koji,Oka, Hiromi,Kita, Akiko,Kuwata, Rika,Ikehara, Daitetsu,Wada, Makoto
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p. 341 - 345
(2007/10/03)
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- Electrochemically driven catalytic Pd(0)/Cr(II) mediated coupling of organic halides with aldehydes. The Nozaki-Hiyama-Kishi reaction
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An electrochemically driven, catalytic, Pd(0)/Cr(II) mediated coupling of organic halides with aldehydes is described. The reaction proceeds under mild conditions and the influence of reaction conditions and the mechanism are briefly discussed.
- Grigg, Ronald,Putnikovic, Bogdan,Urch, Christopher J.
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p. 6307 - 6308
(2007/10/03)
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- Palladium-Catalyzed Isomerization of Aryl-Substituted Epoxides: A Selective Synthesis of Substituted Benzylic Aldehydes and Ketones
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Aryl-substituted epoxides bearing multiple methyl substituants on the epoxide ring isomerize in the presence of 5 mol % Pd(OAc)2/PR3 (R = n-Bu, Ph) to form the corresponding benzylic aldehyde or ketone, with complete regioselectivity for the carbonyl compound formed via cleavage of the benzylic C-O bond. No allylic alcohols or products arising from alkyl migration are observed. Rapid reaction rates and nearly quantitative yields are obtained, even with highly sterically hindered epoxides, using tri-n-butylphosphine as ligand and tert-butyl alcohol as solvent. 2-Aryl-substituted epoxides with two methyl substituents on C3 are completely unreactive, consistent with an oxidative addition/β-hydride elimination mechanism. Catalyst variation studies show that both Pd(OAc)2 and PR3 are essential for optimal activity and that palladium catalysts formed in this manner are superior to other Pd(0) catalysts (e.g., Pd(PPh3)4). The reactivity of catalytic Pd(OAc)2/PR3 toward multiply-substituted epoxides is compared to traditional Lewis acid catalysts; the former is found to be much more selective for isomerization without skeletal rearrangement. A mechanistic rationale involving turnover-limiting SN2-like attack of Pd(0) at the benzylic carbon is proposed.
- Kulasegaram, Sanjitha,Kulawiec, Robert J.
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p. 6547 - 6561
(2007/10/03)
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