- Palladium(II) Weak-Link Approach Complexes Bearing Hemilabile N-Heterocyclic Carbene-Thioether Ligands
-
A new class of homoligated palladium(II) weak-link approach (WLA) complexes bearing hemilabile N-heterocyclic carbene (NHC)-thioether ligands is reported that, unlike previous tweezer-like WLA complexes, expand and contract in a linear fashion when switching between configurational states. These complexes can be chemically switched between a trans open state and a trans closed state via the addition or subsequent extraction of Cl-. These bis(NHC) complexes also display unusual isomerization behavior. For example, an NMR spectroscopic investigation into the solution-state configuration of the open complex reveals the presence of interconverting syn,trans and anti,trans isomers, and a kinetic study shows that the barrier is large enough to isolate, store, and study the anti,trans isomer at room temperature. Notably, the linker length between the NHC and thioether moieties can be tailored with additional -CH2- groups, which affords considerable control over the geometric changes imposed by switching. Therefore, this class of complexes may be useful in the construction of allosterically regulated supramolecular assemblies and materials.
- Liu, Yuan,Kean, Zachary S.,D'Aquino, Andrea I.,Manraj, Yashin D.,Mendez-Arroyo, Jose,Mirkin, Chad. A.
-
-
Read Online
- Steric effect of NHC ligands in Pd(II)–NHC-catalyzed non-directed C–H acetoxylation of simple arenes
-
Although there has been a lot of progress in oxidative arene C–H functionalization reactions catalyzed by Pd(II/IV) system, the non-directed, site-selective functionalization of arene molecules is still challenging. It has been established that ligands play a pivotal role in controlling rate- as well as selectivity-determining step in a catalytic cycle involving well-defined metal-ligand bonding. N-heterocyclic carbene (NHC) ligands have had a tremendous contribution in the recent extraordinary success of achieving high reactivity and excellent selectivity in many catalytic processes including cross-coupling and olefin-metathesis reactions. However, the immense potential of these NHC ligands in improving site-selectivity of non-directed catalytic C–H functionalization reactions of simple arenes is yet to be realized, where overriding the electronic bias on deciding selectivity is a burdensome task. The presented work demonstrated an initiative step in this regard. Herein, a series of well-defined discrete [Pd(NHCR′R)(py)I2] complexes with systematically varied degree of spatial congestion at the Pd centre, exerted through the R and R’ substituents on the NHC ligand, were explored in controlling the activity as well as the site-selectivity of non-directed acetoxylation of representative monosubstituted and disubstituted simple arenes (such as toluene, iodobenzene and bromobenzene, naphthalene and 1,2-dichlorobenzene). The resulting best yields were found to be 75% for toluene and 65% for bromobenzene with [Pd(NHCMePh)(py)I2], 75% for iodobenzene and 79% for naphthalene with [Pd(NHCMeMe)(py)I2], and 41% for 1,2-dichlorobenzene with [Pd(NHCCyCy)(py)I2]. Most importantly, with increasing the bulkiness of the NHC ligand in the complexes, the selectivity of the distal C-acetoxylated products in comparison to the proximal ones, was enhanced to a great extent in all cases. Considering the vast library of NHC ligands, this study underscores the future opportunity to develop more strategies to improve the activity and the crucial site-selectivity of C–H functionalization reactions in simple as well as complex organic molecules.
- Mandal, Tanmoy,Yadav, Sudha,Choudhury, Joyanta
-
-
- Mechanistic investigations of the asymmetric hydrosilylation of ketimines with trichlorosilane reveals a dual activation model and an organocatalyst with enhanced efficiency
-
Structural probes used to help elucidate mechanistic information of the organocatalyzed asymmetric ketimine hydrosilylation have revealed a new catalyst with unprecedented catalytic activity, maintaining adequate performance at 0.01 mol% loading. A new ‘dual activation’ model has been proposed that relies on the presence of both a Lewis basic and Br?nsted acidic site within the catalyst architecture.
- Li,Reeder,Torri,Adams,Jones
-
p. 2422 - 2435
(2017/03/20)
-
- N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: Dynamics and catalysis
-
The novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NBD = norbornadiene, NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me (3a), Bz (3b), Tr (3c), tBu (3d)], containing on one nitrogen the benzyl substituent and on the other increasing bulky alkyl substituents were prepared. All the complexes display restricted rotation around the metal-carbene bond and yield conformational enantiomers. The stereodynamics and racemization barriers about the Rh-carbene have been determined by means of NMR spectroscopy for 3a-c, whereas for the bulkiest 3d only the lower limit (91 kJ mol-1) could be calculated. Whilst the racemization barriers obtained by DFT calculations for 3a,b and 3d matched the experimental values, in the case of 3c the latter (62.3 kJ mol-1) was much smaller with respect to the calculated one (101.7 kJ mol-1). The lower experimental barrier has been attributed to a dissociative pathway that produces a solvated ionic pair in the transition state. The catalytic activity of the neutral rhodium(i) complexes 3a and 3d in the hydrosilylation with HSiMe2Ph of the terminal alkynes PhC≡CH, TolC≡CH, nBuC≡CH, Et3SiC≡CH, and (CPh2OH)C≡CH has been investigated, and compared with the amide-functionalized [RhCl(NBD){1-(2-NHBoc-ethyl)-3-Me-imidazolin-2-ylidene}] (4) and with [RhCl(NBD){1-butyl-3-Me-imidazolin-2-ylidene}] (5).
- Cassani, Maria Cristina,Brucka, Marta Anna,Femoni, Cristina,Mancinelli, Michele,Mazzanti, Andrea,Mazzoni, Rita,Solinas, Gavino
-
supporting information
p. 1768 - 1779
(2014/05/06)
-
- Synthesis, structure, and reactions of 1-fert-butyl-2-diphenylphosphino- imidazole
-
Metalation reactions were studied of a sterically demanding imidazole derivative, namely, 1-ferf-butylimidazole (1),with different metalation reagents and subsequent reaction with diphenylchlorophosphane. The reaction product, 1-ferf-butyl-2-diphenylphosphino-imidazole (2), was subjected to oxidation and complexation reactions to yield the corresponding products Ph 2(Imi)P-E (E = O (3),S (4),Se (5),W(CO)5 (8)) and in the case of borane-THF the N-BH3 coordination product 10 was obtained. The analytical data of the new compounds are discussed, including X-ray diffraction studies of 3-5.
- Sauerbrey, Susanne,Majhi, Paresh Kumar,Daniels, Joerg,Schnakenburg, Gregor,Bmndle, Gerhard Markus,Scherer, Katharina,Streubel, Rainer
-
experimental part
p. 793 - 799
(2011/04/22)
-
- Origins of stereoselectivity in optically pure phenylethaniminopyridine tris-chelates M(NN′)3n+ (M = Mn, Fe, Co, Ni and Zn)
-
One-pot reactions of 2-pyridinecarboxaldehyde, chiral phenylethanamines and Fe(ii) give single diastereomer fac diimine complexes at thermodynamic equilibrium so that no chiral separations are required (d.r. > 200:1). The origins of this stereoselectivity are partly steric and partly a result of the presence of three sets of inter-ligand parallel-offset π-stacking interactions. Mn(ii), Co(ii), Co(iii), Ni(ii) and Zn(ii) give similar fac structures, alongside the imidazole analogues for Fe(ii). While most of the complexes are paramagnetic, the series of molecular structures allows us to assess the influence of the π-stacking present, and there is a strong correlation between this and the M-N bond length. Fe(ii) is close to optimal. For the larger Zn(ii) ion, very weak π-stacking leads to poorer measured stereoselectivity (NMR) but this is improved with increased solvent polarity. The mechanism of stereoselection is further investigated via DFT calculations, chiroptical spectroscopy and the use of synthetic probes.
- Howson, Suzanne E.,Allan, Laura E. N.,Chmel, Nikola P.,Clarkson, Guy J.,Deeth, Robert J.,Faulkner, Alan D.,Simpson, Daniel H.,Scott, Peter
-
experimental part
p. 10416 - 10433
(2011/11/13)
-
- Investigation of ligand steric effects on a highly cis-selective Rh(i) cyclopropanation catalyst
-
Four new Rh(i) complexes bearing chelating imine-functionalized N-heterocyclic carbene ligands have been synthesized and characterized. The catalytic activity of these new Rh(i) complexes has been tested in the cyclopropanation reaction between ethyl diazoacetate and styrene. One of the new complexes, having ethyl groups on the ligand N-aryl ring, exhibited a reactivity and a cis-diastereoselectivity that were comparable to our previously reported Rh(i) cyclopropanation catalyst of this type, and a higher yield and cis-diastereoselectivity were obtained at lower catalyst loadings and higher temperatures. The other new Rh(i) complexes were found to be inferior to the previously reported Rh(i) cyclopropanation catalyst. The catalytic study gave important information about the effect that changing the steric requirements of the substituents at the ligand system has on the efficiency and cis-diastereoselectivity of the complexes as cyclopropanation catalysts.
- Rosenberg, Marianne Lenes,Langseth, Eirin,Krivokapic, Alexander,Gupta, Nalinava Sen,Tilset, Mats
-
scheme or table
p. 2306 - 2313
(2011/12/03)
-
- USE OF PT-AND PD-BIS-AND TETRA-CARBON COMPLEXES WITH BRIDGED CARBON LIGANDS IN OLEDS
-
The use of Pt- and Pd-bis- and tetracarbene complexes with bridged carbene ligands in organic light-emitting diodes, organic light-emitting diodes comprising at least one aforementioned Pt- or Pd-carbene complex, at least one transition metal-carbene comp
- -
-
Page/Page column 8
(2010/01/31)
-
- Synthesis and cytotoxic activities of novel phenacylimidazolium bromides
-
A series of novel phenacylimidazolium derivatives, bearing an aryl or alkyl substituent at position-1 and a phenacyl substituent at position-3 of the imidazole ring, has been prepared and evaluated in vitro against a panel of human tumor cell lines. Phena
- Yang, Xiao-Dong,Zeng, Xiang-Hui,Zhang, Yan-Li,Qing, Chen,Song, Wen-Jian,Li, Liang,Zhang, Hong-Bin
-
scheme or table
p. 1892 - 1895
(2009/11/30)
-
- An acidity scale of 1,3-dialkylimidazolium salts in dimethyl sulfoxide solution
-
(Chemical Equation Presented) Equilibrium acidities of 16 1,3-dialkylimidazolium-type ionic liquid (IL) molecules (1-16) were systematically measured by the overlapping indicator method at 25°C in dimethyl sulfoxide (DMSO) solution. The pKa values were observed to range from 23.4 for IL 12 to 19.7 for IL 6 (Tables 1 and 2), responding mainly to structural variations on the cation moiety. Excellent agreement between the spectrophotometrically determined pKa and that derived from NMR titration for 1,3,4,5-tetramethylimidazolium bis(trifluoromethanesulfonyl)imide (12) and the close match of the obtained pK values with the reported data in literature provide credence to the acidity measurements of the present work. The substituent effects at the imidazolium ring and the effects of counterions on the acidities of ionic liquids are discussed.
- Chu, Yuan,Deng, Hui,Cheng, Jin-Pei
-
p. 7790 - 7793
(2008/02/13)
-
- Optically active iridium imidazol-2-ylidene-oxazoline complexes: Preparation and use in asymmetric hydrogenation of arylalkenes
-
This work explores the potential of iridium complexes of the N-heterocyclic carbene oxazoline ligands 1 in asymmetric hydrogenations of arylalkenes. The accessible carbene precursors, imidazolium salts 2, and robust iridium complexes 5 facilitated a disco
- Perry, Marc C.,Cui, Xiuhua,Powell, Mark T.,Hou, Duen-Ren,Reibenspies, Joseph H.,Burgess, Kevin
-
p. 113 - 123
(2007/10/03)
-
- Hydrogen bonding in complexes of carboxylic acids with 1-alkylimidazoles: Steric and isotopic effects on low barrier hydrogen bonding
-
The nature of hydrogen bonding within intermolecular complexes of carboxylic acids and 1-methylimidazole (1-MeIm), 1-n-butylimidazole (1-BuIm). and 1-tert-butylimidazole (1-t-BuIm) in chloroform was characterized by Fourier transform infrared spectroscopy
- Cassidy, Constance S.,Reinhardt, Laurie A.,Cleland, W. Wallace,Frey, Perry A.
-
p. 635 - 641
(2007/10/03)
-
- Synthesis of 1-alkylimidazoles
-
A mechanistic investigation of the synthesis of imidazole and 1- alkylimidazoles has led to an improved synthesis starting from glyoxal, formaldehyde and alkylammonium chlorides.
- Gridnev,Mihaltseva
-
p. 1547 - 1555
(2007/10/02)
-
- Quaternary salts of 2-[(hydroxyimino)methyl]imidazole. 3. Synthesis and evaluation of (alkenyloxy)-, (alkynyloxy)-, and (aralkyloxy)methyl quaternarized 2[(hydroxyimino)methyl]-1-alkylimidazolium halides as reactivators and therapy for soman intoxication
-
A series of structurally related monosubstituted 1-[(alkenyloxy)methyl]-, 1-[(alkynyloxy)methyl]-, and 1-[(aralkyloxy)methyl]-2-[(hydroxyimino)methyl]-3-methylimidazolium halides were prepared and evaluated. All new compounds were characterized with respect to (hydroxyimino)methyl acid dissocation constant, nucleophilicity, and octanol-buffer partition coefficient. The alkynyloxy-substituted compounds were also evaluated in vitro with respect to reversible inhibition of human erythrocyte (RBC) acetylcholinesterase (AChE) and kinetics of reactivation of human AChE inhibited by ethyl p-nitrophenyl methylphosphonate (EPMP). In vivo evaluation in mice revealed that coadministration of alkynyloxy-substituted imidazolium compounds with atropine sulfate provided significant protection against a 2 x LD50 challenge of GD. For the alkynyloxy-substituted imidazolium drugs there is a direct relationship between in vitro and in vivo activity: the most potent in vivo compounds against GD proved to be potent in vitro reactivators against EPMP-inhibited human AChE. These results differ from the observations made on the sterically hindered imidazolium compounds (see previous article) and suggest that several antidotal mechanisms of protective action may be applicable for the imidazolium aldoxime family of therapeutics. The ability of the alkynyloxy substituents to provide life-saving protection against GD intoxication was not transferable to the pyridinium or triazolium heteroaromatic ring systems.
- Bedford,Harris III,Howd,Goff,Koolpe,Petesch,Koplovitz,Sultan,Musallam
-
p. 504 - 516
(2007/10/02)
-
- 2,5-Dilithiation of N-Protected Imidazoles. Syntheses of 2,5-Disubstituted Derivatives of 1-Methoxymethyl-, 1-Triphenylmethyl-, and 1-(N,N-Dimethylsulphonamido)-imidazole
-
The conditions previously established for the dilithiation of 1-methylimidazole are shown to be applicable to 1-methoxymethyl- and 1-triphenylmethyl-imidazole allowing good-yielding syntheses of 1,2,5-trisubstituted imidazole derivatives.The suitability of the 1-substituted (and of other groups) for the N-protection of imidazoles in dilithiation experiments is discussed and the use of the N,N-dimethylsulphamoyl protecting group is proposed. 1-Sulphamoylimidazole undergoes mono- and 2,5-di-lithiation quantitatively at low temperatures and in short reaction times.The results of work-up of the 2,5-dilithio intermediate with 1 mol equiv. of iodomethane or dimethyl sulphate indicate selectivity in favour of reaction at the 5-position.
- Chadwick, Derek J.,Ngochindo, Raphael I.
-
p. 481 - 486
(2007/10/02)
-