| 1. |
- Afewerki, Samson, et al.
(författare)
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Off-Cycle Catalyst Cooperativity in Amine/Transition Metal Combined Catalysis : Bicyclo[3.2.0]heptanes as Key Species in Co-Catalytic Enantioselective Carbocyclizations
- 2022
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Ingår i: Advanced Synthesis and Catalysis. - : Wiley. - 1615-4150 .- 1615-4169. ; 364:8, s. 1394-1401
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Tidskriftsartikel (refereegranskat)abstract
- The existence of off-cycle catalyst cooperativity in amine/metal combined catalysis is disclosed. The experimental and density functional theory study of the amine/metal co-catalyzed enantioselective Michael/carbocyclization cascade reaction between allenes and α,β-unsaturated aldehydes reveals that the dual catalysts can perform off-cycle cooperativity that gives access to stable bicyclo[3.2.0]heptane species that limits the carbocycle product formation. Insight into this mode of co-catalyst cooperativity sheds new light on the chiral amine/metal co-catalyzed reactions of to date and gives deeper understanding for improved future design of this type of enantioselective reactions.
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| 2. |
- Alam, Rauful, et al.
(författare)
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Stereoselective allylboration of imines and indoles under mild conditions. An in situ E/Z isomerization of imines by allylboroxines
- 2014
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Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 5:7, s. 2732-2738
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Tidskriftsartikel (refereegranskat)abstract
- Direct allylboration of various acyclic and cyclic aldimine, ketimine and indole substrates was performed using allylboronic acids. The reaction proceeds with very high anti-stereoselectivity for both E and Z imines. The allylboroxines formed by dehydration of allylboronic acids have a dual effect: promoting E/Z isomerization of aldimines and triggering the allylation by efficient electron withdrawal from the imine substrate.
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| 3. |
- Bunrit, Anon, et al.
(författare)
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Brønsted Acid-Catalyzed Intramolecular Nucleophilic Substitution of the Hydroxyl Group in Stereogenic Alcohols with Chirality Transfer
- 2015
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 137:14, s. 4646-4649
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Tidskriftsartikel (refereegranskat)abstract
- The hydroxyl group of enantioenriched benzyl, propargyl, allyl, and alkyl alcohols has been intramolecularly displaced by uncharged O-, N-, and S-centered nucleophiles to yield enantioenriched tetrahydrofuran, pyrrolidine, and tetrahydrothiophene derivatives with phosphinic acid catalysis. The five-membered heterocyclic products are generated in good to excellent yields, with high degree of chirality transfer, and water as the only side-product. Racemization experiments show that phosphinic acid does not promote S(N)1 reactivity. Density functional theory calculations corroborate a reaction pathway where the phosphinic acid operates as a bifunctional catalyst in the intramolecular substitution reaction. In this mechanism, the acidic proton of the phosphinic acid protonates the hydroxyl group, enhancing the leaving group ability. Simultaneously, the oxo group of phosphinic acid operates as a base abstracting the nucleophilic proton and thus enhancing the nucleophilicity. This reaction will open up new atom efficient techniques that enable alcohols to be used as nucleofuges in substitution reactions in the future.
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| 4. |
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| 5. |
- Bunrit, Anon, et al.
(författare)
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H3PO2-Catalyzed Intramolecular Stereospecific Substitution of the Hydroxyl Group in Enantioenriched Secondary Alcohols by N-, O-, and S-Centered Nucleophiles to Generate Heterocycles
- 2020
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 10:2, s. 1344-1352
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Tidskriftsartikel (refereegranskat)abstract
- The direct intramolecular stereospecific substitution of the hydroxyl group in enantiomerically enriched secondary benzylic, allylic, propargylic, and alkyl alcohols was successfully accomplished by phosphinic acid catalysis. The hydroxyl group was displaced by O-, S-, and N-centered nucleophiles to provide enantioenriched five-membered tetrahydrofuran, pyrrolidine, and tetrahydrothiophene as well as six-membered tetrahydroquinolines and chromanes in up to a 99% yield and 100% enantiospecificity with water as the only byproduct. Mechanistic studies using both experiments and calculations have been performed for substrates generating 5-membered heterocycles. Rate studies show dependences in a catalyst, an internal nucleophile, and an electrophile, however, independence in an external nucleophile, an electrophile, or water. Kinetic isotope effect studies show an inverse KIE of k(H)/k(D) = 0.79. Furthermore, phosphinic acid does not promote S(N)1 reactivity. Computational studies support a bifunctional role of the phosphinic acid in which activation of both nucleofuge and nucleophile occurs in a bridging S(N)2-type transition state. In this transition state, the acidic hydrogen of phosphinic acid protonates the leaving hydroxyl group simultaneously as the oxo group partially deprotonates the nucleophile. Thereby, phosphinic acid promotes the substitution of the nonderivatized hydroxyl group in enantioenriched secondary alcohols by uncharged nucleophiles with conservation of the chirality from the alcohol to the heterocycle.
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| 6. |
- Bunrit, Anon, 1986-, et al.
(författare)
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H3PO2-Catalyzed Intramolecular Stereospecific Substitution of the Hydroxyl Group in Stereogenic Secondary Alcohols by N-, O-, and S-centered Nucleophiles to Generate Heterocycles
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The direct intramolecular stereospecific substitution of the hydroxyl group in stereogenic secondary alcohols was successfully accomplished by phosphinic acid catalysis. The hydroxyl group was displaced by O-, S-, and N-centered nucleophiles to provide enantioenriched five- and six-membered heterocycles in good to excellent yields and high enantiospecificity with water as the only by product. Mechanistic studies using both experiments and calculations have been performed. Rate order determination shows first-order dependences in catalyst, internal nucleophile, and electrophile concentrations, however, independence on external nucleophile and electrophile. Furthermore, phosphinic acid does not promote SN1 reactivity. Computational studies support a bifunctional role of the phosphinic acid in which activations of both nucleofuge and nucleophile occur in a bridging SN2-type transition state.
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| 7. |
- Bunrit, Anon, et al.
(författare)
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Nucleophilic Substitution of the Hydroxyl Group in Stereogenic Alcohols with Chirality Transfer
- 2016
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Ingår i: Synlett. - : Georg Thieme Verlag KG. - 0936-5214 .- 1437-2096. ; 27:2, s. 173-176
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Tidskriftsartikel (refereegranskat)abstract
- A brief overview of the development of direct substitution of the hydroxyl (OH) group of alcohols in our research group is presented. By applying a BrOnsted acid, an intramolecular substitution of the OH group in stereogenic alcohols with chirality transfer was achieved. Noteworthy, the intramolecular substitution has a wide scope in respect to both the nucleophile and also the nucleofuge. A mechanistic study by both experiments and DFT calculations revealed a unique reaction pathway in which the BrOnsted acid operates in a bifunctional manner to promote an S(N)2-type reaction mechanism.
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| 8. |
- Huang, Genping, et al.
(författare)
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Mechanism and Selectivity of Rhodium-Catalyzed 1:2 Coupling of Aldehydes and Allenes
- 2013
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:20, s. 7647-7659
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Tidskriftsartikel (refereegranskat)abstract
- The rhodium-catalyzed highly regioselective 1:2 coupling of aldehydes and allenes was investigated by means of density functional theory calculations. Full free energy profiles were calculated, and several possible reaction pathways were evaluated. It is shown that the energetically most plausible catalytic cycle is initiated by oxidative coupling of the two allenes, which was found to be the rate-determining step of the overall reaction. Importantly, Rh allyl complexes that are able to adopt both eta(3) and eta(1) configurations were identified as key intermediates present throughout the catalytic cycle with profound implications for the selectivity of the reaction. The calculations reproduced and rationalized the experimentally observed selectivities and provided an explanation for the remarkable alteration in the product distribution when the catalyst precursor is changed from [RhCl(nbd)](2) (nbd = norbornadiene) to complexes containing noncoordinating counterions ([Rh(cod)(2)X]; X = OTf, BF4, PF6; cod = 1,5-cyclooctadiene). It turns out that the overall selectivity of the reaction is controlled by a combination of the inherent selectivities of several of the elementary steps and that both the mechanism and the nature of the selectivity-determining steps change when the catalyst is changed.
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| 9. |
- Huang, Genping, et al.
(författare)
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Mechanism and Stereoselectivity of the BINOL-Catalyzed Allylboration of Skatoles
- 2017
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Ingår i: Organic Letters. - : American Chemical Society (ACS). - 1523-7060 .- 1523-7052. ; 19:21, s. 5904-5907
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations have been performed to investigate the binaphthol-catalyzed allylboration of skatoles. The high stereoselectivity observed for the reaction is reproduced well by the calculations and was found to be mainly a result of steric repulsions in the corresponding Zimmerman-Traxler transition states. The role of the additive MeOH in enhancing the stereoselectivity was also investigated and is suggested to promote the formation of less reactive allylboronic ester intermediates, thereby suppressing the formation of allylboroxine species, which undergo the facile racemic background reaction.
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| 10. |
- Huang, Genping, et al.
(författare)
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Mechanism, reactivity, and selectivity of the iridium-catalyzed C(sp(3))-H borylation of chlorosilanes
- 2015
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 6:3, s. 1735-1746
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Tidskriftsartikel (refereegranskat)abstract
- The iridium-catalyzed C(sp(3))-H borylation of methylchlorosilanes is investigated by means of density functional theory, using the B3LYP and M06 functionals. The calculations establish that the resting state of the catalyst is a seven-coordinate Ir(V) species that has to be converted into an Ir(III)tris(boryl) complex in order to effect the oxidative addition of the C-H bond. This is then followed by a C-B reductive elimination to yield the borylated product, and the catalytic cycle is finally completed by the regeneration of the active catalyst over two facile steps. The two employed functionals give somewhat different conclusions concerning the nature of the rate-determining step, and whether reductive elimination occurs directly or after a prior isomerization of the Ir(V) hydride intermediate complex. The calculations reproduce quite well the experimentally-observed trends in the reactivities of substrates with different substituents. It is demonstrated that the reactivity can be correlated to the Ir-C bond dissociation energies of the corresponding Ir(V) hydride intermediates. The effect of the chlorosilyl group is identified to originate from the alpha-carbanion-stabilizing effect of the silicon, which is further reinforced by the presence of an electron-withdrawing chlorine substituent. Furthermore, the source of selectivity for the borylation of primary over secondary C(sp(3))-H can be explained on a steric basis, by repulsion between the alkyl group and the Ir/ligand moiety. Finally, the difference in the reactivity between C(sp(3))-H and C(sp(2))-H borylation is investigated and rationalized in terms of distortion/interaction analysis.
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