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1.
The molecular origin of the experimentally observed pronounced difference in the rates of Morita–Baylis–Hillman (MBH) reaction in heterocyclic aldehydes, depending on the position of the formyl group, is investigated herein by using DFT‐based mechanistic studies and free energy computations. These calculations are based on the 1,4‐diazobicyclo[2.2.2]octane (DABCO)‐catalyzed MBH reaction of methyl acrylate with substituted 4‐ and 5‐isoxazolecarbaldehyde, which are slow‐ and fast‐reacting substrates, respectively. As a result of this study, we propose that by tailoring ring substitutions the reactivity of the formyl group for MBH reactions may be enhanced in slow‐reacting heterocyclic aldehydes. This proposition is demonstrated by enhancing the rate of the MBH reaction in 4‐isoxazolecarbaldehyde more than 104‐fold by installing an ester substitution at the C‐3 position. Similarly, the reactivity of the formyl group towards the MBH reaction in substituted 3‐pyrazolecarbaldehyde and pyridinecarbaldehyde is shown to be increased several‐fold by a halo substitution. We also confirm that the reasons for different reactivities of heterocyclic aldehydes and the proposed scheme for improving the reaction rates remains valid for all the three mechanisms proposed for the MBH reaction, namely, Hill–Isaacs, McQuade, and Aggarwal. 相似文献
2.
Competition between Nucleophilic Substitution of Halogen (SNAr) versus Substitution of Hydrogen (SNArH)—A Mass Spectrometry and Computational Study 下载免费PDF全文
Kacper Błaziak Prof. Mieczysław Mąkosza Prof. Witold Danikiewicz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(16):6048-6051
The mechanism of intramolecular gas‐phase reactions of N‐(2‐X‐5‐nitrophenyl)‐N‐methylacetamide carbanions (X=H, F, Cl) has been studied using negative ion electrospray mass spectrometry ((?)ESI‐MS) technique and modelled computationally. It was proven that all three anions form cyclic σH adducts, which undergo elimination of water. In the case of X=F, formation of the σF adduct, leading to SNAr reaction, was a competing process. This is the first proof that also in the gas phase formation of σH adduct proceeds faster than σX adduct and only when X=F, rates of these two processes are comparable. The experimental results are in full agreement with quantum chemical calculations. 相似文献
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Dr. J. Oscar C. Jiménez‐Halla Dr. Marcin Kalek Prof. Jacek Stawinski Prof. Fahmi Himo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(39):12424-12436
The mechanism and sources of selectivity in the palladium‐catalyzed propargylic substitution reaction that involves phosphorus nucleophiles, and which yields predominantly allenylphosphonates and related compounds, have been studied computationally by means of density functional theory. Full free‐energy profiles are computed for both H‐phosphonate and H‐phosphonothioate substrates. The calculations show that the special behavior of H‐phosphonates among other heteroatom nucleophiles is indeed reflected in higher energy barriers for the attack on the central carbon atom of the allenyl/propargyl ligand relative to the ligand‐exchange pathway, which leads to the experimentally observed products. It is argued that, to explain the preference of allenyl‐ versus propargyl‐phosphonate/phosphonothioate formation in reactions that involve H‐phosphonates and H‐phosphonothioates, analysis of the complete free‐energy surfaces is necessary, because the product ratio is determined by different transition states in the respective branches of the catalytic cycle. In addition, these transition states change in going from a H‐phosphonate to a H‐phosphonothioate nucleophile. 相似文献
4.
A Metal‐Free Synthesis of 2‐Alkyl(or Aryl) Thiomethyl‐2‐cyclohexenones from Cyclic Morita–Baylis–Hillman Bromides 下载免费PDF全文
Under mild conditions, an efficient and rapid S‐allylation of thiols with cyclic Morita–Baylis–Hillman (MBH) bromides without the need of a transition‐metal catalyst or an expensive additive is described herein. Treatment of the MBH bromides with various thiols or ethane‐1,2‐dithiol in the presence of Et3N regioselectively affords the corresponding 2‐alkyl(or aryl) thiomethyl‐2‐cyclohexenones or the perhydro benzo[1,4]dithiepinone, respectively, in moderate to good yields (40 – 73%). The reaction is rapid and carried out in THF at room temperature. 相似文献
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Giovanni W. Amarante Mario Benassi Humberto M. S. Milagre Dr. Ataualpa A. C. Braga Dr. Feliu Maseras Dr. Marcos N. Eberlin Dr. Fernando Coelho Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(45):12460-12469
A Morita–Baylis–Hillman (MBH) reaction catalyzed by thiourea was monitored by ESI‐MS(/MS) and key intermediates were intercepted and characterized. These intermediates suggest that thiourea acts as an organocatalyst in all steps of the MBH reaction cycle, including the rate‐limiting proton‐transfer step. DFT calculations, performed for a model MBH reaction between formaldehyde and acrolein with trimethylamine as base and in the presence or the absence of thiourea, suggest that thiourea accelerates MBH reactions by decreasing the transition‐state (TS) energies through bidentate hydrogen bonding throughout the whole catalytic cycle. In the rate‐limiting proton‐transfer step, the thiourea acts not as a proton shuttle, but as a Brønsted acid stabilizing the basic oxygen center that is formed in the TS. 相似文献
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Generation and Application of (Diborylmethyl)zinc(II) Species: Access to Enantioenriched gem‐Diborylalkanes by an Asymmetric Allylic Substitution 下载免费PDF全文
Yeosan Lee Jinyoung Park Prof. Dr. Seung Hwan Cho 《Angewandte Chemie (International ed. in English)》2018,57(39):12930-12934
We report the successful generation of (diborylmethyl)zinc(II) species by transmetallation beteween isolable (diborylmethyl)lithium and zinc(II) halide (X=Br, Cl) and their application in the synthesis of enantioenriched gem‐diborylalkanes bearing a stereogenic center at the β‐position of the diboryl groups by an asymmetric allylic substitution reaction. The reaction has a broad substrate scope, and various enantioenriched gem‐diborylalkanes can be obtained in good yields with excellent enantioselectivity. Further elaboration of the enantioenriched gem‐diborylalkanes provides access to a diverse set of valuable chiral building blocks. 相似文献
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Participation of Alkoxy Groups in Reactions of Acetals: Violation of the Reactivity/Selectivity Principle in a Curtin–Hammett Kinetic Scenario 下载免费PDF全文
Dr. Angie Garcia Jillian R. Sanzone Prof. K. A. Woerpel 《Angewandte Chemie (International ed. in English)》2015,54(41):12087-12090
Nucleophilic substitution reactions of acetals having benzyloxy groups four carbon atoms away can be highly diastereoselective. The selectivity in several cases increased as the reactivity of the nucleophile increased, in violation of the reactivity/selectivity principle. The increase in selectivity with reactivity suggests that multiple conformational isomers of reactive intermediates can give rise to the products. 相似文献
9.
发展了一个高效、高选择性Yb(OTf)3催化的直接取代烯丙醇以及炔丙醇中的羟基的反应。亲核试剂可以是如醇,硫醇,胺以及活性亚甲基化合物等含碳以及杂原子中心的试剂。当前催化体系的有利条件在于广泛可用的开始原料,不需要干燥的溶剂以及添加剂,温和的反应条件,简单的操作以及环境友好,特别对于硫醇类化合物仍具有较好的催化效果。当前的催化剂再回收使用至少10次仍具有较高的催化活性。 相似文献
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Cover Feature: Ion‐Pair SN2 Reaction of OH− and CH3Cl: Activation Strain Analyses of Counterion and Solvent Effects (Chem. Asian J. 9/2018) 下载免费PDF全文
Jalal Z. A. Laloo Dr. Lydia Rhyman Dr. Olatz Larrañaga Prof. Dr. Ponnadurai Ramasami Prof. Dr. F. Matthias Bickelhaupt Dr. Abel de Cózar 《化学:亚洲杂志》2018,13(9):1075-1075
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Evaluating the Thermal Vinylcyclopropane Rearrangement (VCPR) as a Practical Method for the Synthesis of Difluorinated Cyclopentenes: Experimental and Computational Studies of Rearrangement Stereospecificity 下载免费PDF全文
David Orr Prof. Dr. Jonathan M. Percy Dr. Tell Tuttle Dr. Alan R. Kennedy Dr. Zoë A. Harrison 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(44):14305-14316
Vinyl cyclopropane rearrangement (VCPR) has been utilised to synthesise a difluorinated cyclopentene stereospecifically and under mild thermal conditions. Difluorocyclopropanation chemistry afforded ethyl 3‐(1′(2′2′‐difluoro‐3′‐phenyl)cyclopropyl) propenoate as all four stereoisomers ( 18a , 18b , 22a , 22b ) (all racemic). The trans‐E isomer ( 18a ), prepared in 70 % yield over three steps, underwent near quantitative VCPR to difluorocyclopentene 23 (99 %). Rearrangements were monitored by 19F NMR (100–180 °C). While cis/trans cyclopropane stereoisomerisation was facile, favouring trans‐isomers by a modest margin, no E/Z alkene isomerisation was observed even at higher temperatures. Neither cis nor trans Z‐alkenoates underwent VCPR, even up to much higher temperatures (180 °C). The cis‐cyclopropanes underwent [3,3]‐rearrangement to afford benzocycloheptadiene species. The reaction stereospecificity was explored by using electronic structure calculations, and UB3LYP/6‐31G* methodology allowed the energy barriers for cyclopropane stereoisomerisation, diastereoisomeric VCPR and [3,3]‐rearrangement to be ranked in agreement with experiment. 相似文献
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A Mechanistic Investigation of the Kinetic Resolution of Secondary Aromatic Alcohols Using a Ferrocene‐Based Planar Chiral 4‐(Dimethylamino)pyridine Catalyst 下载免费PDF全文
Laura Mesas‐Sánchez Dr. Peter Dinér 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(14):5623-5631
A detailed computational and kinetic analysis of the acetylation of 1‐phenylethanol with acetic anhydride catalyzed by planar chiral 4‐(dimethylamino)pyridine (DMAP) catalyst (?)‐ 1 is presented. The study includes a computational investigation of the potential‐energy surface including the acylation and stereoselective transition states at the DFT level of theory. Experimentally, the kinetic study shows that the reaction proceeds in a first‐order manner in catalyst, whereas both substrates, acetic anhydride and 1‐phenylethanol, show fractional order, which is in accordance with steady‐state conditions. The fractional order depends on an equilibrium between the free catalyst and the acetylated catalyst. 相似文献
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Guillaume Berthon‐Gelloz Dr. Mélanie Marchant Bernd F. Straub Prof. Dr. Istvan E. Marko Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(12):2923-2931
Pd 0 does the trick! Alkenyl silanes are efficiently cyclopropanated by diazoalkanes at low Pd loadings (see scheme). Clear evidence for the involvement of a Pd0 resting state for this reaction is given.
17.
Ryota Sakamoto Dr. Shoko Kume Dr. Manabu Sugimoto Prof. Dr. Hiroshi Nishihara Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(6):1429-1439
Azobenzene derivatives modified with dithiolato‐bipyridine platinum(II) complexes were synthesized, revealing their highly extended photoresponses to the long wavelength region as well as unique photocontrollable tristability. The absorptions of trans‐ 1 and trans‐ 2 with one azobenzene group on the dithiolene and bipyridine ligands, respectively, cover the range from 300 to 700 nm. These absorptions are ascribed, by means of time‐dependent (TD)DFT calculations, to transitions from dithiolene(π) to bipyridine(π*), namely, interligand charge transfer (CT), π–π*, and n–π* transitions of the azobenzene unit, and π–π* transitions of the bipyridine ligand. In addition, only trans‐ 1 shows distinctive electronic bands, assignable to transitions from the dithiolene(π) to azobenzene(π*), defined as intraligand CT. Complex 1 shows photoisomerization behavior opposite to that of azobenzene: trans‐to‐cis and cis‐to‐trans conversions proceed with 405 and 312 nm irradiation, which correspond to excitation with the intraligand CT, and π–π* bands of the azobenzene and bipyridine units, respectively. In contrast, complex 2 shows photoisomerization similar to that of azobenzene: trans‐to‐cis and cis‐to‐trans transformations occur with 365 and 405 nm irradiation, respectively. Irradiation at 578 nm, corresponding to excitation of the interligand CT transitions, results in cis‐to‐trans conversion of both 1 and 2 , which is the longest wavelength ever reported to effect the photoisomerization of the azobenzene group. The absorption and photochromism of 4 , which has azobenzene groups on both the dithiolato and bipyridine ligands, have characteristics quite similar to those of 1 and 2 , which furnishes 4 with photocontrollable tristability in a single molecule using light at 365, 405, and 578 nm. We also clarified that 1 and 2 have high photoisomerization efficiencies, and good thermal stability of the cis forms. Complexes 3 and 5 have almost the identical photoresponse to those of their positional isomers, complexes 2 and 4 . 相似文献
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Barbara Wucher Michael Moser Dr. Stephanie A. Schumacher Frank Rominger Dr. Doris Kunz Priv.‐Doz. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(24):4417-4421
Metal‐to‐metal allyl transfer : Using the first structurally characterized rhodium η1‐allyl complexes it is shown that the σ‐bound allyl substituent can be transferred from the RhIII complex to a RhI complex in a fast equilibrium. This process may account for the decrease in regioselectivity observed in allylic alkylation reactions in which complex 1 is used as a catalyst.
19.
Methane Activation Mediated by a Series of Cerium–Vanadium Bimetallic Oxide Cluster Cations: Tuning Reactivity by Doping 下载免费PDF全文
The reactions of cerium–vanadium cluster cations CexVyOz+ with CH4 are investigated by time‐of‐flight mass spectrometry and density functional theory calculations. (CeO2)m(V2O5)n+ clusters (m=1,2, n=1–5; m=3, n=1–4) with dimensions up to nanosize can abstract one hydrogen atom from CH4. The theoretical study indicates that there are two types of active species in (CeO2)m(V2O5)n+, V[(Ot)2]. and [(Ob)2CeOt]. (Ot and Ob represent terminal and bridging oxygen atoms, respectively); the former is less reactive than the latter. The experimentally observed size‐dependent reactivities can be rationalized by considering the different active species and mechanisms. Interestingly, the reactivity of the (CeO2)m(V2O5)n+ clusters falls between those of (CeO2)2–4+ and (V2O5)1–5+ in terms of C?H bond activation, thus the nature of the active species and the cluster reactivity can be effectively tuned by doping. 相似文献
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Uranium–Carbene–Imido Metalla‐Allenes: Ancillary‐Ligand‐Controlled cis‐/trans‐Isomerisation and Assessment of trans Influence in the R2C=UIV=NR′ Unit (R=Ph2PNSiMe3; R′=CPh3) 下载免费PDF全文
Dr. Erli Lu Dr. Oliver J. Cooper Dr. Floriana Tuna Dr. Ashley J. Wooles Prof. Nikolas Kaltsoyannis Prof. Stephen T. Liddle 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(33):11559-11563
Uranium(IV)–carbene–imido complexes [U(BIPMTMS)(NCPh3)(κ2‐N,N′‐BIPY)] ( 2 ; BIPMTMS=C(PPh2NSiMe3)2; BIPY=2,2‐bipyridine) and [U(BIPMTMS)(NCPh3)(DMAP)2] ( 3 ; DMAP=4‐dimethylamino‐pyridine) that contain unprecedented, discrete R2C=U=NR′ units are reported. These complexes complete the family of E=U=E (E=CR2, NR, O) metalla‐allenes with feasible first‐row hetero‐element combinations. Intriguingly, 2 and 3 contain cis‐ and trans‐C=U=N units, respectively, representing rare examples of controllable cis/trans isomerisation in f‐block chemistry. This work reveals a clear‐cut example of the trans influence in a mid‐valent uranium system, and thus a strong preference for the cis isomer, which is computed in a co‐ligand‐free truncated model—to isolate the electronic trans influence from steric contributions—to be more stable than the trans isomer by approximately 12 kJ mol?1 with an isomerisation barrier of approximately 14 kJ mol?1. 相似文献