共查询到20条相似文献,搜索用时 31 毫秒
1.
K.K. Hii TimothyD.W. Claridge JohnM. Brown Andrew Smith RobertJ. Deeth 《Helvetica chimica acta》2001,84(10):3043-3056
Reactive intermediates in the asymmetric Heck reaction between aryl electrophiles and 2,3‐dihydrofuran have been identified by NMR and mass spectrometry, with BINAP or the achiral diphosphanes dppp and dppf as ligands. The major cationic species observed is an alkylpalladium produced by addition of PdAr to the alkene followed by two further PdH‐mediated isomerisation steps. This last species has been characterised at −60° by 1H‐, 13C‐, and 31P‐NMR, including HMQC techniques. The regiochemistry of PdAr and PdH addition to the reactant are opposite as defined by the reaction with (2‐2H1)‐2,3‐dihydrofuran. DFT Calculations on the reaction pathway between [CH2(PH2)]PdPh+ and 2,3‐dihydrofuran reveal several structurally interesting intermediates involving agostic β‐H‐atom, ipso‐Ph or reactant O‐atom bonded to the Pd‐atom, and elucidate the isomerisation pathway. 相似文献
2.
Mónica Trincado Dr. Hansjörg Grützmacher Prof. Dr. Francesco Vizza Dr. Claudio Bianchini Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(9):2751-2757
The combination of the d8 RhI diolefin amide [Rh(trop2N)(PPh3)] (trop2N=bis(5‐H‐dibenzo[a,d]cyclohepten‐5‐yl)amide) and a palladium heterogeneous catalyst results in the formation of a superior catalyst system for the dehydrogenative coupling of alcohols. The overall process represents a mild and direct method for the synthesis of aromatic and heteroaromatic carboxylic acids for which inactivated olefins can be used as hydrogen acceptors. Allyl alcohols are also applicable to this coupling reaction and provide the corresponding saturated aliphatic carboxylic acids. This transformation has been found to be very efficient in the presence of silica‐supported palladium nanoparticles. The dehydrogenation of benzyl alcohol by the rhodium amide, [Rh]N, follows the well established mechanism of metal–ligand bifunctional catalysis. The resulting amino hydride complex, [RhH]NH, transfers a H2 molecule to the Pd nanoparticles, which, in turn, deliver hydrogen to the inactivated alkene. Thus a domino catalytic reaction is developed which promotes the reaction R‐CH2‐OH+NaOH+2 alkene→R‐COONa+2 alkane. 相似文献
3.
Dr. Indranil Chatterjee Dr. Zheng‐Wang Qu Prof. Dr. Stefan Grimme Prof. Dr. Martin Oestreich 《Angewandte Chemie (International ed. in English)》2015,54(41):12158-12162
A transition‐metal‐free transfer hydrogenation of 1,1‐disubstituted alkenes with cyclohexa‐1,4‐dienes as the formal source of dihydrogen is reported. The process is initiated by B(C6F5)3‐mediated hydride abstraction from the dihydrogen surrogate, forming a Brønsted acidic Wheland complex and [HB(C6F5)3]?. A sequence of proton and hydride transfers onto the alkene substrate then yields the alkane. Although several carbenium ion intermediates are involved, competing reaction channels, such as dihydrogen release and cationic dimerization of reactants, are largely suppressed by the use of a cyclohexa‐1,4‐diene with methyl groups at the C1 and C5 as well as at the C3 position, the site of hydride abstraction. The alkene concentration is another crucial factor. The various reaction pathways were computationally analyzed, leading to a mechanistic picture that is in full agreement with the experimental observations. 相似文献
4.
Iron Dicarbonyl Complexes Featuring Bipyridine‐Based PNN Pincer Ligands with Short Interpyridine CC Bond Lengths: Innocent or Non‐Innocent Ligand? 下载免费PDF全文
Dr. Thomas Zell Dr. Petr Milko Kathlyn L. Fillman Dr. Yael Diskin‐Posner Dr. Tatyana Bendikov Dr. Mark A. Iron Dr. Gregory Leitus Yehoshoa Ben‐David Prof. Michael L. Neidig Prof. David Milstein 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(15):4403-4413
5.
Zhou Tang Dr. Edwin Otten Prof. Dr. Joost N. H. Reek Dr. Ir. Jarl Ivar van der Vlugt Prof. Dr. Bas de Bruin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(36):12683-12693
Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNNH2 in combination with RhI. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment. 相似文献
6.
Dr. Yann Gloaguen Christophe Rebreyend Dr. Martin Lutz Pravin Kumar Dr. Martina Huber Dr. Ir. Jarl Ivar van der Vlugt Prof. Dr. Sven Schneider Prof. Dr. Bas de Bruin 《Angewandte Chemie (International ed. in English)》2014,53(26):6814-6818
Photochemical activation of [(PNNH)Rh(N3)] (PNNH=6‐di‐(tert‐butyl)phosphinomethyl‐2,2′‐bipyridine) complex 2 produced the paramagnetic (S=1/2), [(PNN)Rh?N.‐Rh(PNN)] complex 3 (PNN?=methylene‐deprotonated PNNH), which could be crystallographically characterized. Spectroscopic investigation of 3 indicates a predominant nitridyl radical (.N2?) character, which was confirmed computationally. Complex 3 reacts selectively with CO, producing two equivalents of [(PNN)RhI(CO)] complex 4 , presumably by nitridyl radical N,N‐coupling. 相似文献
7.
Summary: A first route to the isomerisation of erythro‐ and threo‐2,3‐disubstituted succinyl units in alternating alkene‐maleic acid copolymers is presented. The isomerisation reactions of the disodium salt of ethene‐maleic acid copolymer were carried out in aqueous solution at 180–240 °C in an autoclave. It is demonstrated that the erythro‐content can be changed from 17% up to 67%. The dissociation of methine C H bonds with the intermediate formation of a sp2 hybridised carbon is proposed as the mechanism.
8.
The silver(I)‐catalyzed synthesis picture of axially chiral allenes based on propargylamines has been outlined using density functional theory (DFT) method for the first time. Our calculations find that, the coordination of silver(I) into triple bond of propargylamines at anti‐position of nitrogen shows a stronger activation on the triple bond than that at cis‐position, which is favorable for the subsequent hydrogen transfer. The NBO charge analysis for the hydrogen transfer affirms the experimental speculation that this step is a hydride transfer process. The energy barrier of the anti‐periplanar elimination of vinyl‐silver is 26.9 kJ·mol?1 lower than that of the syn‐periplanar elimination, supporting that (?)‐allene is the main product of this reaction. In a word, the most possible route for this reaction is that the silver(I) coordinates into the triple bond of propargylamines at anti‐position of nitrogen, then the formed silver(I) complex undergoes a hydride transfer to give a vinyl‐silver, finally the vinyl‐silver goes through an anti‐periplanar elimination to give (?)‐allene. The hydride transfer with the energy barrier of 44.8 kJ·mol?1 is the rate‐limiting step in whole catalytic process. This work provides insight into why this reaction has a very high enantioselectivity. 相似文献
9.
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
10.
An ion‐neutral complex (INC)‐mediated hydride transfer reaction was observed in the fragmentation of protonated N‐benzylpiperidines and protonated N‐benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC6H4CH2]+ (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N‐benzylpiperidines, strong electron‐donating substituents favored the formation of [RC6H4CH2]+, whereas strong electron‐withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
11.
Catalina Ferrer Dr. Jordi Benet‐Buchholz Dr. Antoni Riera Prof. Xavier Verdaguer Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(28):8340-8346
An alkyne tetracarbonyl dicobalt complex with a chelated phosphine–alkene ligand, in which the phosphorus atom and the alkene from the ligand are attached to the same cobalt atom has been prepared, isolated, and characterized by X‐ray crystallography. The complex serves as a mechanistic model for an intermediate of the Pauson–Khand (PK) reaction. Although the alkene fragment is located in an equatorial coordination site with an appropriate orientation, and, therefore, should undergo insertion, it failed to give the PK product upon either thermal or N‐methylmorpholine N‐oxide activation. However, a phosphine–alkene complex that contains a terminal alkene readily provided the corresponding PK product. We attribute this change in reactivity to the different ability of each olefin to undergo 1,2‐insertion. These results provide further insights into the factors that govern a crucial step in the PK reaction, the olefin insertion. 相似文献
12.
Jacob C. Timmerman Bradley D. Robertson Prof. Ross A. Widenhoefer 《Angewandte Chemie (International ed. in English)》2015,54(7):2251-2254
The cationic gold phosphine complex [{PCy2(o‐biphenyl)}Au(NCMe)]+SbF6? (Cy=cyclohexyl) catalyzes the intermolecular, anti‐Markovnikov hydroamination reaction of monosubstituted and cis‐ and trans‐disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin‐2‐ones and other nucleophiles. This reaction forms 1‐cyclopropyl alkylamine derivatives in high yield and with high regio‐ and diastereoselectivity. NMR spectroscopic analysis of gold π‐ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction. 相似文献
13.
Steffen Brand Holger Elsen Jens Langer Samuel Grams Sjoerd Harder 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(43):15642-15649
The low‐valent ß‐diketiminate complex (DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(DIPPBDI)CaH]2 at 20 °C led to (DIPPBDI)Al(Ph)H (DIPPBDI=CH[C(CH3)N‐DIPP]2, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C?H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (DIPPBDI)Al(m‐tolyl)H with I2 gave m‐tolyl iodide, H2 and (DIPPBDI)AlI2 which was recycled to (DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPPBDI)Al(H)M(DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (DIPPBDI)Al(H)Ca(DIPPBDI). This is in equilibrium with the hydride bridged complex (DIPPBDI)Al(μ‐H)Ca(DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C?H bond activation. 相似文献
14.
Callum R. Woof Derek J. Durand Dr. Natalie Fey Dr. Emma Richards Dr. Ruth L. Webster 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(19):5972-5977
Iron-catalyzed isomerization of alkenes is reported using an iron(II) β-diketiminate pre-catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H3N⋅BH3). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre-catalyst activation step that gives access to an η2-coordinated alkene FeI complex, followed by oxidative addition of the alkene to give an FeIII intermediate, which then undergoes reductive elimination to allow release of the isomerization product. 相似文献
15.
Dr. Rigoberto Barrios‐Francisco Tania Benítez‐Páez Dr. Marcos Flores‐Alamo Dr. Alma Arévalo Prof. Dr. Juventino J. García 《化学:亚洲杂志》2011,6(3):842-849
The current work describes the synthesis and full characterization of zerovalent nickel complexes of the type [(dippe)Ni(η2‐C,C‐Fn‐alkyne)] (dippe=1,2‐bis(di‐isopropylphosphino‐ethane), Fn‐alkyne=fluorinated aromatic alkyne, n=1, 3, 5; 3a , 3b , 3c ) and [{(dippe)Ni}2(μ2‐C,C‐Fn‐alkyne)] ( 4 ). Reactions with complexes 3a , 3b , 3c , and water as the hydrogen source, yield selective semihydrogenation of the bound alkyne to the corresponding alkene, accompanied by partial hydrodefluorination of the aromatic ring. Different alkynes were tested; on using the alkyne with five fluorine atoms over the aromatic ring, partial defluorination was achieved under the mildest reaction conditions, followed in reactivity by the alkyne with three fluorine atoms. The alkyne with only one fluorine atom was barely defluorinated. The use of triethylsilane as a sacrificial hydride source resulted in an overall increase in reactivity towards defluorination. 相似文献
16.
Sheng Feng Prof. Fanyang Mo Dr. Ying Xia Zhenxing Liu Zhen Liu Prof. Yan Zhang Prof. Jianbo Wang 《Angewandte Chemie (International ed. in English)》2016,55(49):15401-15405
The rhodium(I)‐catalyzed C?C bond activation reaction of siloxyvinylcyclopropanes with diazoesters demonstrates a novel mode of C?C bond cleavage of siloxyvinvylcyclopanes. The alkene products were obtained as single E‐configured isomers in good yields. A σ,η3‐allyl rhodium complex, which has been previously proposed as the key intermediate in rhodium(I)‐catalyzed cycloaddition of vinylcyclopropanes, has been isolated and characterized by X‐ray crystallography. 相似文献
17.
Heiko Bauer Katharina Thum Mercedes Alonso Christian Fischer Sjoerd Harder 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(13):4292-4297
The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′2 catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe3)2] using 1,4‐cyclohexadiene (1,4‐CHD) as a H source. Reaction of 1,4‐CHD with AeN′′2 gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1‐hexene) needed a higher temperature (120 °C) but proceeded without double‐bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4‐CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4‐CHD followed by H transfer. Convenient access to larger quantities of BaN′′2, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation. 相似文献
18.
Xianghui Shi Guorui Qin Yang Wang Lanxiao Zhao Zhizhou Liu Jianhua Cheng 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(13):4400-4404
Hydrogenolysis of the half‐sandwich penta‐arylcyclopentadienyl‐supported heavy alkaline‐earth‐metal alkyl complexes (CpAr)Ae[CH(SiMe3)2](S) (CpAr=C5Ar5, Ar=3,5‐iPr2‐C6H3; S=THF or DABCO) in hexane afforded the calcium, strontium, and barium metal–hydride complexes as the same dimers [(CpAr)Ae(μ‐H)(S)]2 (Ae=Ca, S=THF, 2‐Ca ; Ae=Sr, Ba, S=DABCO, 4‐Ae ), which were characterized by NMR spectroscopy and single‐crystal X‐ray analysis. 2‐Ca , 4‐Sr , and 4‐Ba catalyzed alkene hydrogenation under mild conditions (30 °C, 6 atm, 5 mol % cat.), with the activity increasing with the metal size. A variety of activated alkenes including tri‐ and tetra‐substituted olefins, semi‐activated alkene (Me3SiCH=CH2), and unactivated terminal alkene (1‐hexene) were evaluated. 相似文献
19.
Concurrent Stabilization of π‐Donor and π‐Acceptor Ligands in Aromatized and Dearomatized Pincer [(PNN)Re(CO)(O)2] Complexes 下载免费PDF全文
Michael G. Mazzotta Dr. Kothanda Rama Pichaandi Dr. Phillip E. Fanwick Prof. Dr. Mahdi M. Abu‐Omar 《Angewandte Chemie (International ed. in English)》2014,53(32):8320-8322
Aromatized cationic [(PNN)Re(π acid)(O)2]+ ( 1 ) and dearomatized neutral [(PNN*)Re(π acid)(O)2] ( 2 ) complexes (where π acid=CO ( a ), tBuNC ( b ), or (2,6‐Me2)PhNC ( c )), possessing both π‐donor and π‐acceptor ligands, have been synthesized and fully characterized. Reaction of [(PNN)Re(O)2]+ ( 4 ) with lithiumhexamethyldisilazide (LiHMDS) yield the dearomatized [(PNN*)Re(O)2] ( 3 ). Complexes 1 and 2 are prepared from the reaction of 4 and 3 , respectively, with CO or isocyanides. Single‐crystal X‐ray structures of 1 a and 1 b show the expected trans‐dioxo structure, in which the oxo ligands occupy the axial positions and the π‐acidic ligand occupies the equatorial plane in an overall octahedral geometry about the rhenium(V) center. DFT studies revealed the stability of complexes 1 and 2 arises from a π‐backbonding interaction between the dxy orbital of rhenium, the π orbital of the oxo ligands, and the π* orbital of CO/isocyanide. 相似文献
20.
Computational Insight to Improve the Thermal Isomerisation Performance of Overcrowded Alkene‐Based Molecular Motors through Structural Redesign 下载免费PDF全文
Synthetic overcrowded alkene‐based molecular motors achieve 360° unidirectional rotary motion of one motor half (rotator) relative to the other (stator) through sequential photochemical and thermal isomerisation steps. In order to facilitate and expand the use of these motors for various applications, it is important to investigate ways to increase the rates and efficiencies of the reactions governing the rotary motion. Here, we use computational methods to explore whether the thermal isomerisation performance of some of the fastest available motors of this type can be further improved by reducing the sizes of the motor halves. Presenting three new redesigned motors that combine an indanylidene rotator with a cyclohexadiene, pyran or thiopyran stator, we first use multiconfigurational quantum chemical methods to verify that the photoisomerisations of these motors sustain unidirectional rotary motion. Then, by performing density functional calculations, we identify both stepwise and concerted mechanisms for the thermal isomerisations of the motors and show that the rate‐determining free‐energy barriers of these processes are up to 25 kJ mol?1 smaller than those of the original motors. Furthermore, the thermal isomerisations of the redesigned motors proceed in fewer steps. Altogether, the results suggest that the redesigned motors are useful templates for improving the thermal isomerisation performance of existing overcrowded alkene‐based motors. 相似文献