Fast transfer hydrogenation using a highly active orthometalated heterocyclic carbene ruthenium catalyst

The free carbene 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene reacts with trans,cis-RuHCl(PPh₃)₂(ampy) (ampy = 2-(aminomethyl)pyridine) affording an orthometalated N-heterocyclic carbene complex characterized by an X-ray diffraction study. This compound in presence of NaOH shows very high catalytic activity for the transfer hydrogenation of several ketones to alcohols using 2-propanol as hydrogen source, affording TOF values up to 120,000 h⁻¹ (at 50% conversion).     

Function of an N-heterocyclic carbene ligand based on concept of chiral mimetic

Function of a new N-heterocyclic carbene ligand based on the concept of a chiral mimetic is described. With (4R,5R)-4,5-diphenyl-1,3-dialkyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborates as N-heterocyclic carbene precursors, Rh-catalyzed enantioselective arylation (up to 27% ee) of aromatic aldehydes with arylboronic acids and Pd-catalyzed enantioselective intramolecular alpha-arylation (up to 66% ee) of N-(2-bromoaryl)-N-alkyl-2-arylpropanamides are investigated.     

Ruthenium complexes with chelating carboxylate-NHC ligands as efficient catalysts for CH arylation in water

Ruthenium(ii) complexes with chelating N-heterocyclic carbene (NHC) ligands were studied in the arylation of phenyl group in 2-phenylpyridine and 1-phenylpyrazole with aryl chlorides in water. Complexes with NHC-ligands containing a hemilabile coordinating N-carboxylatomethyl group enable fast and selective ortho-CH-diarylation in the absence of carboxylate-assisting additives.     

NHC Pdii complexes for the solvent-free telomerisation of isoprene with methanol

A comparative study of N-heterocyclic carbene Pd complexes in the head-to-head isoprene telomerization with methanol revealed significant impact of ligand structure as well as axial group structure on the catalyst activity. Some N,N'-diaryl substituted imidazol-2-ylidene, imidazolidin-2-ylidene and expanded-ring tetrahydropyrimidin-2-ylidene and tetrahydrodiazepin-2-ylidene based ligands were tested to explore the fundamental correlations between structure (ring carbene size along with the substituent sterical and electronic properties) and catalytic activity.     

N-heterocyclic carbenes: III. N-heterocyclic carbene ligands based on abietane in Suzuki-Miyaura reaction

By reactions of N-alkyl- and N-arylimidazoles with methyl 12-bromoacetyldehydroabietate a series of unsymmetrically substituted chiral imidazolium bromides with the abietane fragment was synthesized. The salts obtained were suggested as new N-heterocyclic carbene ligands in the Suzuki-Miyaura reaction.     

Synthesis,biological activities and 3D-QSAR studies of (R)-2-phenyl-4,5-dihydrothiazole-4-carboxamide derivatives containing a sulfur ether moiety

A series of (R)-2-phenyl-4,5-dihydrothiazole-4-carboxamide derivatives containing a sulfur ether moiety were designed, synthesized, and tested for their antifungal activities against several phytopathogenic fungi. The established CoMSIA model could predict the antifungal activity.     

Axially chiral C2-symmetric N-heterocyclic carbene (NHC) palladium complex-catalyzed asymmetric α-hydroxylation of β-keto esters

Axially chiral C₂-symmetric N-heterocyclic carbene (NHC) palladium diaquo complexes 1a and 1b, derived from (R)-BINAM, are effective catalysts for the enantioselective α-hydroxylation of β-keto esters using oxaziridine 2a as the oxidant to give the corresponding products in high yields along with moderate enantioselectivities.     

Synthesis and crystal structures of palladium(II) complexes of bis-(<Emphasis Type="Italic">N</Emphasis>-heterocyclic carbenes) on a calix[4]arene platform

A series of calix[4]arene supported N-heterocyclic carbene palladium complexes were successfully prepared and characterised. Their X-ray crystal structures were obtained and are discussed. Notably, the dimeric compound b[5-(3-N-4,5-diphenylimidazol-2-yliden-1-yl)-25,26,27,28-tetrapropyloxy calix[4]arene] palladium(II) dibromide (cone) showed pseudo-polymorphism.     

Synthesis of Platinum(II) N-Heterocyclic Carbenes Based on Adenosine

Organometallic derivatization of nucleosides is a highly promising strategy for the improvement of the therapeutic profile of nucleosides. Herein, a methodology for the synthesis of metalated adenosine with a deprotected ribose moiety is described. Platinum(II) N-heterocyclic carbene complexes based on adenosine were synthesized, namely N-heterocyclic carbenes bearing a protected and unprotected ribose ring. Reaction of the 8-bromo-2′,3′,5′-tri-O-acetyladenosine with Pt(PPh₃)₄ by C8−Br oxidative addition yielded complex 1, with a Pt^II centre bonded to C-8 and an unprotonated N7. Complex 1 reacted at N7 with HBF₄ or methyl iodide, yielding protic carbene 2 or methyl carbene 3, respectively. Deprotection of 1 to yield 4 was achieved with NH₄OH. Deprotected compound 4 reacted at N7 with HCl solutions to yield protic NHC 5 or with methyl iodide yielding methyl carbene 6. Protic N-heterocyclic carbene 5 is not stable in DMSO solutions leading to the formation of compound 7, in which a bromide was replaced by chloride. The cis-influence of complexes 1–7 was examined by ³¹P{¹H} and ¹⁹⁵Pt NMR. Complexes 2, 3, 5, 6 and 7 induce a decrease of ¹J_Pt,P of more than 300 Hz, as result of the higher cis-influence of the N-heterocyclic carbene when compared to the azolato ligand in 1 and 4.     

Regioselectivity in nickel(0) catalyzed cycloadditions of carbon dioxide with diynes

The regioselectivity of Ni(0)-catalyzed cycloadditions of CO₂ (1 atm) with various asymmetrical diynes to afford pyrones was explored. The use of 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) provided high regioselectivity when one terminal substituent on the diyne was a methyl group and the other was medium or large in size (R_L=i-Pr, t-Bu, or TMS). In contrast, the use of a relatively small N-heterocyclic carbene, 1,3-dimesitylimidazol-2-ylidene (IMes), afforded high selectivity only when R_L was large (TMS). X-ray crystal analysis of the major isomer indicated that the relatively large R_L group was in the 3-position of the pyrone.     

Chiral N-heterocyclic carbenes as asymmetric acylation catalysts

Chiral N-heterocyclic carbenes are generated from C₂-symmetric 1,3-bis(1-arylethyl)imidazolium salts and potassium tert-butoxide. These C₂-symmetric imidazolidenyl carbenes catalyze enantioselective acylation of racemic secondary alcohols. The asymmetric acylation of 1-(1-naphthyl)ethanol was achieved in up to 68% ee of the acylated product, using (R,R)-1,3-bis[(1-naphthyl)ethyl]imidazolium tetrafluoroborate as a precursor of the chiral N-heterocyclic carbene and vinyl propionate as the acyl donor.     

Efficient chirality switching in the asymmetric addition of indole to N-tosylarylimines in the presence of axially chiral cyclometalated bidentate N-heterocyclic carbene palladium(II) complexes

Efficient dual stereocontrol can be achieved by using axially chiral cyclometalated bidentate N-heterocyclic carbene palladium(II) complexes for the addition of indole to N-tosylarylimines simply by the adjustment of the R group on the benzene rings of the NHC–Pd(II) complexes.     

Synthesis of (5R*)- and (5S*)-5,7,11-trimethyl-3a-phenyl-4,5-dihydro-3aH-1,4-methano[1,3]oxazolo[3,2-a]quinolin-2-ones

Heating of N-benzoyl-N-[2-(1-methylbut-2-en-1-yl)-4-methylphenyl]glycine with ethyl chloroformate or acetic anhydride gave (5R*) and (5S*) isomers of (1R*,3aS*,4S*,11S*)-5,7,11-trimethyl-3a-phenyl-4,5-dihydro-3aH-1,4-methano[1,3]oxazolo[3,2-a]quinolin-2-one.     

A pyridine promoted ‘weak base route’ to (NHC)2NiCl2 complexes with bulky N,N&#x27;-diaryl carbene ligands

A simple one-step synthesis of (NHC)₂NiCl₂ complexes containing bulky N-heterocyclic carbene (NHC) ligands relies on the C–H metallation of N,N'-diaryl-substituted azolium salts, NHC-precursors, with NiCl₂Py₂ in the presence of Cs₂CO₃ and pyridine. This procedure allows one to implement all synthetic manipulations in air.     

Enantioselective synthesis of (1S,2S)-1,2-di-tert-butyl and (1R,2R)-1,2-di-(1-adamantyl)ethylenediamines

An enantioselective synthesis of sterically congested 1,2-di-tert-butyl and 1,2-di-(1-adamantyl)ethylenediamines has been developed. Thus, diastereomerically pure trans-1-apocamphanecarbonyl-4,5-dimethoxy-2-imidazolidinones 6 and 7 were successfully prepared by optical resolution of (±)-trans-4,5-dimethoxy-2-imidazolidinone using apocamphanecarbonyl chloride (MAC-Cl) followed by stereospecific and stepwise substitution of the dimethoxyl groups using tert-butyl or 1-adamantyl cuprates to provide (4S,5S)-4,5-di-tert-butyl and (4R,5R)-4,5-di-(1-adamantyl)-2-imidazolidinones 12 and 15, respectively. Furthermore, N-acetyl 4,5-di-tert-butyl and 4,5-di-(1-adamantyl)-2-imidazolidinones 16a,b were enantioselectively deacetylated using a catalytic oxazaborolidine system to provide enantiopure 1-p-tolylsulfonyl-4,5-di-tert-butyl-2-imidazolidinones 12 and 19 and 1-p-tolylsulfonyl-4,5-di-(1-adamantyl)-2-imidazolidinones 18 and 20, respectively. Finally, N-p-tolylsulfonyl-2-imidazolidinones 12 and 15 were treated with 30 equiv of Ba(OH)₂·8H₂O to achieve ring cleavage and to provide (1S,2S)-1,2-di-tert-butylethylenediamine 3 and (1R,2R)-1,2-di-(1-adamantyl)ethylenediamine 4.     

Cyanosilylation of aldehydes catalyzed by N-heterocyclic carbenes

N-Heterocyclic carbenes produced in situ from salts of imidazolium, benzimidazolium, pyrido[1,2-c]imidazolium, imidazolinium, thiazolium, and triazolium catalyze the addition of trimethylsilylcyanide to aldehydes to yield cyanohydrin trimethylsilyl ethers. The use of C₂-symmetric imidazolidenyl carbene derived from (R,R)-1,3-bis[(1-naphthyl)ethyl]imidazolium chloride led to enantioselective cyanosilylation.     

Design of nickel chelates of tetradentate N-heterocyclic carbenes with subdued cytotoxicity

A series of nickel complexes, 1b-3b, exhibiting subdued cytotoxicity have been designed with the intent of their use as agents for developing resistance to nickel toxicity. Indeed, the nickel complexes, 1b-3b, display less cytotoxic activity towards two commonly occurring human cancer cell lines namely, HeLa cells (16-64%) and MCF-7 cells (70-90%) in culture as compared to the maximum inhibition by NiCl₂ · 6H₂O under analogous conditions at three different concentrations (1 μM, 5 μM and 20 μM). Similarly, the suppression of cytotoxicity through chelation of the metal ion can also be seen in normal cells as was evident from a significant reduction in cytotoxicity (9-41%) for a non-tumorigenic CHO cell line in case of a representative complex 3b. The reduction in carcinogenic activity in the complexes relative to nickel(II) ion from NiCl₂ · 6H₂O is brought about by successful chelation of the metal center by a class of specially designed new tetradentate N/O-functionalized N-heterocyclic carbene ligands. The two strongly σ-donating carbene moieties coupled with two negatively charged amido moieties present in the N-heterocyclic carbene ligands facilitate complete chelation of the metal center and thereby significantly reduce the cytotoxic effects of the metal.     

New tripodal N-heterocyclic carbene chelators for small molecule activation

In an effort to develop new tripodal N-heterocyclic carbene (NHC) ligands for small molecule activation, two new classes of tripodal NHC ligands TIME^R and TIMEN^R have been synthesized. The carbon-anchored tris(carbene) ligand system TIME^R (R = Me, t-Bu) forms bi- or polynuclear metal complexes. While the methyl derivative exclusively forms trinuclear 3:2 complexes [(TIME^Me)₂M₃]³⁺ with group 11 metal ions, the tert-butyl derivative yields a dinuclear 2:2 complex [(TIME^t-Bu)₂Cu₂]²⁺ with copper(I). The latter complex shows both “normal” and “abnormal” carbene binding modes and accordingly, is best formulated as a bis(carbene)alkenyl complex. The nitrogen-anchored tris(carbene) ligands TIMEN^R (R = alkyl, aryl) bind to a variety of first-row transition metal ions in 1:1 stoichiometry, affording monomeric complexes with a protected reactivity cavity at the coordinated metal center. Complexes of TIMEN^R with Cu(I)/(II), Ni(0)/(I), and Co(I)/(II)/(III) have been synthesized. The cobalt(I) complexes with the aryl-substituted TIMEN^R (R = mesityl, xylyl) ligands show great potential for small molecule activation. These complexes activate for instance dioxygen to form cobalt(III) peroxo complexes that, upon reaction with electrophilic organic substrates, transfer an oxygen atom. The cobalt(I) complexes are also precursors for terminal cobalt(III) imido complexes. These imido complexes were found to undergo unprecedented intra-molecular imido insertion reactions to form cobalt(II) imine species. The molecular and electronic structures of some representative metal NHC complexes as well as the nature of the metal–carbene bond of these metal NHC complexes was elucidated by X-ray and DFT computational methods and are discussed briefly. In contrast to the common assumption that NHCs are pure σ-donors, our studies revealed non-negligible and even significant π-backbonding in electron-rich metal NHC complexes.     

Synthesis and structures of 4,5-dimethyl-1,3-bis(pyridin-2-ylmethyl)-1H-imidazolium chloride and 1,1′-bis(pyridin-2-ylmethyl)-2,2′-bis(4,5-dimethylimidazole)

4,5-Dimethyl-1,3-bis(pyridin-2-ylmethyl)-1H-imidazolium chloride (1) was synthesized and characterized by IR and NMR spectroscopy and X-ray diffraction. An attempt to prepare the free tridentate N-heterocyclic carbene pincer ligand by the reaction of 1 with KN(SiMe₃)₂ resulted in the formation of 1,1′-bis(pyridin-2-ylmethyl)-2,2′-bis(4,5-dimethylimidazole) as a product of dimerization of the target carbene followed by the rearrangement accompanied by the elimination of dipyridylethane.