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1.
Artigo em Inglês | MEDLINE | ID: mdl-32358911

RESUMO

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin-state ordered phase of a spin crossover complex. Single crystal X-ray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5-diBr-sal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc → Pc and is thermodynamically continuous, while the second, Pc → P1 at 85 K, is discontinuous and related to spin crossover and spin-state ordering. Stress-induced domain wall mobility was detected as softening of the phonon modes at the Pc → P1 transition.

2.
Anticancer Drugs ; 2020 Apr 08.
Artigo em Inglês | MEDLINE | ID: mdl-32282370

RESUMO

The anticancer drug candidates 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene gold(I) dimethylamino dithiocarbamate and 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl-1-thiolate derivative exhibited nanomolar in-vitro activity against prostate cancer cells advanced prostate cancer (PC3) and micromolar inhibition of mammalian thioredoxin reductase. Encouraging maximum tolerable dose experiments led to human prostate cancer subcutaneous xenograft experiments; 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene gold(I) dimethylamino dithiocarbamate and 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl-1-thiolate derivative were applied twelve times at two doses in groups of n = 5 PC3 to tumor-bearing NMRI:nu/nu mice. 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene gold(I) dimethylamino dithiocarbamate and 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl-1-thiolate derivative at the dose of 10 and 20 mg/kg showed good tolerability, while no significant body weight loss was seen in both groups. In particular, for the drug 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene gold(I) dimethylamino dithiocarbamate the tumor growth inhibition suggested to be dose dependent, reflected by the respective optimal T/C values of 0.45 at the dose of 10 mg/kg and of 0.31 at the dose of 20 mg/kg. By contrast, the 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl-1-thiolate derivative treated groups showed no indication for dose-dependent antitumoral activity, as reflected by the optimal T/C values of 0.44 for the 10 mg/kg and for the 20 mg/kg treated mice. Immunohistochemical experiments involving Ki67 staining of tumor tissue showed that both compounds reduced PC3 cell proliferation against the difficult to treat advanced human prostate tumors derived from PC3.

3.
Artigo em Inglês | MEDLINE | ID: mdl-32208375

RESUMO

Structural, magnetic and electromechanical changes resulting from spin crossover between the spin quintet and spin triplet forms of a mononuclear Mn3+ complex embedded in six lattices with different charge balancing counterions are reported. Isostructural ClO4- and BF4- salts (1) and (2) each have two unique Mn3+ sites which follow different thermal evolution pathways resulting in a crossover from the spin quintet form at room temperature to a 1:1 spin triplet:quintet ratio below 150 K. The PF6- (3) and NO3- (4) salts which each have one unique Mn3+ site show a complete conversion from spin quintet to spin triplet over the same temperature range. A complete two step spin crossover is observed in the CF3SO3- lattice (5) with a 1:1 ratio of spin quintet and spin triplet forms at intermediate temperature, while the BPh4- lattice (6) stabilizes the spin triplet form over most of the temperature range with gradual and incomplete spin state switching above 250 K. An electromechanical piezoresponse was detected in NO3- complex 4 despite crystallization in a centrosymmetric space group. The role of defomations associated with strain-induced spin triplet-spin quintet switching in breaking the local symmetry are discussed and computational analysis is used to estimate the energy gap between the two spin states.

4.
Acta Crystallogr E Crystallogr Commun ; 75(Pt 11): 1657-1663, 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31709086

RESUMO

The reactions of N-heterocyclic carbene CuI and AgI halides with potassium thio- or seleno-cyanate gave unexpected products. The attempted substitution reaction of bromido-(1,3-dibenzyl-4,5-di-phenyl-imidazol-2-yl-idene)silver (NHC*-Ag-Br) with KSCN yielded bis-[bis-(1,3-dibenzyl-4,5-di-phenyl-imidazol-2-yl-idene)silver(I)] tris-(thio-cyanato)-argentate(I) diethyl ether disolvate, [Ag(C29H24N2)2][Ag(NCS)3]·2C4H10O or [NHC*2Ag]2[Ag(SCN)3]·2Et2O, (1), while reaction with KSeCN led to bis-(µ-1,3-dibenzyl-4,5-diphenyl-2-seleno-imidazole-κ2 Se:Se)bis-[bromido-(1,3-dibenzyl-4,5-diphenyl-2-seleno-imid-azole-κSe)silver(I)] di-chloro-methane hexa-solvate, [Ag2Br2(C29H24N2Se)4]·6CH2Cl2 or (NHC*Se)4Ag2Br2·6CH2Cl2, (2), via oxidation of the NHC* fragment to 2-seleno-imidazole. This oxidation was observed again in the reaction of NHC*-Cu-Br with KSeCN, yielding catena-poly[[[(1,3-dibenzyl-4,5-diphenyl-2-seleno-imidazole-κSe)copper(I)]-µ-cyanido-κ2 C:N] aceto-nitrile monosolvate], {[Cu(CN)(C29H24N2Se)]·C2H3N} n or NHC*Se-CuCN·CH3CN, (3). Compound (1) represents an organic/inorganic salt with AgI in a linear coordination in each of the two cations and in a trigonal coordination in the anion, accompanied by diethyl ether solvent mol-ecules. The tri-blade boomerang-shaped complex anion [Ag(SCN)3]2- present in (1) is characterized by X-ray diffraction for the first time. Compound (2) comprises an isolated centrosymmetric mol-ecule with AgI in a distorted tetra-hedral BrSe3 coordination, together with di-chloro-methane solvent mol-ecules. Compound (3) exhibits a linear polymeric 1 ∞[Cu-C≡N-Cu-] chain structure with a seleno-imidazole moiety additionally coordinating to each CuI atom, and completed by aceto-nitrile solvent mol-ecules. Electron densities associated with an additional ether solvent mol-ecule in (1) and two additional di-chloro-methane solvent mol-ecules in (2) were removed with the SQUEEZE procedure [Spek (2015 ▸). Acta Cryst. C71, 9-18] in PLATON.

5.
Dalton Trans ; 48(41): 15679-15686, 2019 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-31512699

RESUMO

We present the synthesis, magnetic and photophysical properties of four mononuclear LnIII complexes in two isostructural lattices containing GdIII and ErIII. A heptadentate Schiff base ligand and acetate versus trifluoroacetate were used to synthesise complexes 1-4, among which the two ErIII complexes 2 and 4 exhibit field-induced SIM behaviour with almost similar Ueff values (31.6 K for 2 and 32.7 K for 4). Ab initio calculations show the structure of the low-lying energy states and highlight that there is already significant tunnelling in the ground doublet state, but the application of a weak magnetic field of 0.05 T is sufficient for ac magnetic measurements to suppress tunnelling in the ground state. The calculated main magnetic axes (gZ) of the ground Kramers doublets show small differences between the two ErIII compounds 2 and 4 due to their different ligand fields.

6.
Chem Sci ; 10(26): 6531-6538, 2019 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-31341606

RESUMO

Phenones with elongated chains are shown to be excellent substrates for ligand-promoted asymmetric Grignard synthesis of tertiary alcohols. In turn this enables the simple, short and highly enantioselective (up to 96% ee) preparation of chiral 2,2-disubstituted THFs and THPs. Thus, asymmetric addition of Grignard reagents to γ-chlorobutyrophenones and δ-chlorovalerophenones takes place in the presence of a chiral diaminocyclohexyl-derived tridentate ligand and subsequent base-promoted intramolecular cyclisation occurs with complete retention of asymmetry. As examples of the methodology, we report the shortest syntheses of gossonorol, γ-ethyl-γ-phenylbutyrolactone and δ-methyl-δ-tolylvalerolactone, the joint-shortest and flexible synthesis of boivinianin A and the shortest formal syntheses of boivinianin B and yingzhaosu C.

7.
Dalton Trans ; 48(41): 15560-15566, 2019 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-31342029

RESUMO

Structural, magnetic and spectroscopic data of four complex salts, [Mn(napsal2323)]NTf2, 1,[Mn(napsal2323)]ClO4, 2, [Mn(napsal2323)]BF4, 3 and [Mn(napsal2323)]NO3, 4, of the [Mn(napsal2323)]+ complex cation indicate that the Mn3+ ion is stabilized in the rare S = 1 spin triplet form in this ligand sphere. Zero-field splitting values of D = +19.6 cm-1 and |E| = 2.02 cm-1 for complex 1 were obtained by High Field Electron Paramagnetic Resonance (HFEPR) measurements conducted over a range of frequencies. Structural and magnetic data also indicate that co-crystallization of complexes 2 and 3 with 0.5 equivalents of ethanol yields the high spin S = 2 forms of the perchlorate and tetrafluoroborate solvates [Mn(napsal2323)]ClO4·0.5(C2H5OH), 2·0.5EtOH and [Mn(napsal2323)]BF4·0.5(C2H5OH), 3·0.5EtOH.

8.
Acta Crystallogr E Crystallogr Commun ; 74(Pt 12): 1804-1807, 2018 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-30574378

RESUMO

We present here three compounds consisting of pyridinium or morpholinium hydrogen oxalates, each displaying different hydrogen-bonding motifs, resulting in chains for 4-(di-methyl-amino)-pyridinium hydrogen oxalate 0.22-hydrate, C7H11N2 +·C2HO4 -·0.22H2O (1), dimers for 4-tert-butyl-pyridinium hydrogen oxalate, C9H14N+·C2HO4 - (2), and chains for morpholin-ium hydrogen oxalate, C4H10NO+·C2HO4 - (3).

9.
Inorg Chem ; 57(16): 9880-9891, 2018 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-30080034

RESUMO

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH3, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

10.
Molecules ; 23(8)2018 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-30110951

RESUMO

Ten novel N-heterocyclic carbene gold(I) complexes derived from lepidiline A (1,3-dibenzyl-4,5-dimethylimidazolium chloride) are reported here with full characterisation and biological testing. (1,3-Dibenzyl-4,5-diphenylimidazol-2-ylidene)gold(I) chloride (NHC*-AuCl) (1) was modified by substituting the chloride for the following: cyanide (2), dithiocarbamates (3⁻5), p-mercaptobenzoate derivatives (12⁻14) and N-acetyl-l-cysteine derivatives (15⁻17). All complexes were synthesised in good yields of 57⁻78%. Complexes 2, 12, 13, and 14 were further characterised by X-ray crystallography. Initial evaluation of the biological activity was conducted on all ten complexes against the multidrug resistant MCF-7topo breast cancer, HCT-116wt, and p53 knockout mutant HCT-116-/- colon carcinoma cell lines. Across the three cell lines tested, mainly single-digit micromolar IC50 values were observed. Nanomolar activity was exhibited on the MCF-7topo cell line with 3 displaying an IC50 of 0.28 µM ± 0.03 µM. Complexes incorporating a Au⁻S bond resulted in higher cytotoxic activity when compared to complexes 1 and 2. Theoretical calculations, carried out at the MN15/6⁻311++G(2df,p) computational level, show that NHC* is the more favourable ligand for Au(I)-Cl when compared to PPh3.


Assuntos
Ouro , Compostos Heterocíclicos/síntese química , Compostos Heterocíclicos/farmacologia , Metano/análogos & derivados , Antineoplásicos/síntese química , Antineoplásicos/química , Antineoplásicos/farmacologia , Linhagem Celular Tumoral , Sobrevivência Celular/efeitos dos fármacos , Ouro/química , Compostos Heterocíclicos/química , Humanos , Espectroscopia de Ressonância Magnética , Metano/química , Modelos Moleculares , Estrutura Molecular , Relação Estrutura-Atividade , Difração de Raios X
11.
Dalton Trans ; 47(1): 74-82, 2017 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-29200225

RESUMO

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]n+ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H2) or etherification (elimination of H2O).

12.
Org Biomol Chem ; 15(38): 8166-8178, 2017 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-28920985

RESUMO

The highly enantioselective synthesis of sterically hindered α-allyl-α-aryl oxindoles possessing an all-carbon quaternary stereocenter at the oxindole 3-position has been developed. The key step in the synthetic route employed was a novel one-pot, two-step synthesis of α-aryl-ß-amido allyl ester substituted oxindoles in good yields of 41-75% (13 examples) by interception of an unstable allyl ester intermediate through reaction with aryllead triacetate reagents. Pd-Catalyzed decarboxylative asymmetric allylic alkylation (DAAA) was optimized with 2,4,6-trimethoxyphenyl as the aryl-containing substrate. A screen of chiral P,N- and P,P-based ligands showed that the ANDEN-phenyl Trost ligand was the most effective, affording the corresponding α-allyl-α-aryl oxindole product in 96% yield and 99% ee. A substrate scope of a further 12 α-aryl-ß-amido allyl ester substituted oxindoles showed that products containing bulky di-ortho-methoxy substituted arenes and naphthyl groups were formed in very high ee's (94-98%), whereas those lacking this substitution pattern were formed in more moderate levels of enantioselectivities (56-63% ee). Surprisingly, the 2,6-dimethylphenyl-substituted substrate afforded the O-allylated product in contrast to the expected C-allylated product. A crystal structure was obtained of the 2,4,6-trimethoxyphenyl-substituted α-allyl-α-aryl oxindole product which enabled us to identify the absolute stereochemistry of the quaternary stereocenter formed. A plausible explanation to rationalise the sense of enantioselection observed in these DAAA transformations is also proposed.

13.
Chemistry ; 23(37): 8901-8911, 2017 Jul 03.
Artigo em Inglês | MEDLINE | ID: mdl-28383127

RESUMO

Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such Npyr ⋅⋅⋅H-N interactions are demonstrated by X-ray diffraction analysis, 1 H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand-metal cooperative substrate activation.

14.
Chemistry ; 23(20): 4794-4802, 2017 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-28164404

RESUMO

P-Alkoxyphosphonium (AP) chlorides were generated by reacting P-chlorophosphonium chlorides with alcohols. Their well-known spontaneous Arbuzov-type collapse leading to phosphine oxides was studied and its rate found to be dependent on a number of factors in an unexpected fashion: it is inversely proportional to the initial concentration and it shows strong dependence on the acidity of the media but is not very sensitive to the presence of base. To explain these observations, we evoke a self-inhibition model with the formation of the less nucleophilic hydrodichloride anion HCl2 in solution. Detailed analysis of the kinetic data yields the association constant (K=3×102 m-1 ) of the putative HCl2 species in chloroform. Experimental observations for the collapse of highly enriched diastereomeric alkoxyphosphonium (DAP) chlorides are fully analogous to the achiral AP also implying the involvement of HCl2 anions. Moreover, crystallisation of a highly enriched DAP salt derived from (-)-menthol furnished, for the first time, crystals of individual (RP )-DAP hydrodichloride as confirmed by X-ray diffractometry. Importantly, the P-configuration and detailed conformation of the DAP moiety is in good agreement with DFT-level computational results. The thermal collapse of (RP )-DAP⋅HCl2 proceeds with complete retention of the P-configuration furnishing the phosphine oxide of exceptional enantiomeric purity.

15.
Chemistry ; 23(10): 2332-2339, 2017 Feb 16.
Artigo em Inglês | MEDLINE | ID: mdl-27779793

RESUMO

The energy barriers in our recently discovered Walden-type inversion of chlorophosphonium salts are similar to those for Cope rearrangements of caged cyclic hydrocarbons. Therefore, we have designed a molecular system that integrates the two processes, thereby producing the first embodiment of a chemical species that can undergo two entirely different and independent stereomutation mechanisms at the same nominal asymmetric center. Thus, the energy barrier to the rearrangement of 9-phenyl-9-phosphabarbaralane oxide, which is easily prepared by a new high-yielding synthesis, was found to be roughly 11 kcal mol-1 . This value is in contrast to the parent barbaralane (7.3 kcal mol-1 ) but in good agreement with our computational results for the rearrangement barriers. Crucially, in the corresponding chlorophosphonium derivative, two stereomutations occur simultaneously: a fast Cope rearrangement (barrier ≈12 kcal mol-1 ) and a slow Walden-type inversion of the phosphorus center (barrier ≈21 kcal mol-1 ). The computational model also revealed a relationship between the Cope rearrangement barrier and the bridgehead distance. The phenomenon of two independent and geometrically orthogonal stereomutations at a single asymmetric center provided important general insights into reaction pathway bifurcation, microscopic reversibility, and dynamic stereochemistry. This first example of coexisting alternative mechanisms that involve covalent bonds may encourage the design of new types of dynamic molecular structures.

16.
Acta Crystallogr C Struct Chem ; 72(Pt 11): 857-860, 2016 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-27811426

RESUMO

Gold monochloride and monobromide can be transformed into monomeric complexes by ligands such as CO, PPh3 or Me2S, and such ligand-stabilized gold monochloride compounds have been investigated as catalysts, luminescent materials and anticancer drugs, especially when coordinated to a lipophilic benzyl-substituted N-heterocyclic carbene (NHC) ligand. The triclinic structures of NHC-Au-Cl {chlorido(1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene)gold, [AuCl(C29H24N2)]} and NHC-Au-Br {bromido(1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene)gold, [AuBr(C29H24N2)]}, determined by X-ray crystallography at 100 K, have one and four molecules, respectively, in their asymmetric units. The chloride compound shows an almost linear C-Au-Cl fragment [179.76 (8)°], with an Au-C distance of 1.976 (3) Šand an Au-Cl distance of 2.3013 (6) Å, while the bromide compound shows surprisingly large geometry deviations, from 1.969 (12) to 2.016 (10) Šfor the Au-C distance and from 2.4279 (14) to 2.4796 (12) Šfor the Au-Br distance, in the four independent molecules.

17.
J Org Chem ; 81(22): 11394-11396, 2016 11 18.
Artigo em Inglês | MEDLINE | ID: mdl-27779394

RESUMO

The benzylation of alcohols with the commonly used combination of benzyl bromide and sodium hydride in DMF can lead to the formation of an amine side product, N,N'-dimethyl-1-phenyl-1-(o-tolyl)methanamine. This amine coeluted with benzylated galactal during column chromatography and was found to be a catalyst poison in thiourea-catalyzed glycosylations of galactals. It may also be problematic for other base-sensitive reactions involving benzylated substrates. Solutions to this problem are described.

18.
Dalton Trans ; 45(37): 14591-602, 2016 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-27363515

RESUMO

A series of novel 1,2,3-triazolylidene gold(i) chloride complexes have been synthesised and fully characterised. Silver-free methodologies for chloride ion abstraction of these complexes were evaluated for their potential as Au-based catalyst precursors. Using simple potassium salts or MeOTf as chloride scavengers produced metal complexes that catalyse both the regioselective synthesis of oxazolines and the C-H activation of benzene or styrene for carbene transfer from ethyl diazoacetate. These results indicate that Ag-free activation of 1,2,3-triazolylidene gold(i) chloride complexes is feasible for the generation of catalytically active Au triazolylidene species. However, silver-mediated activation imparts substantially higher catalytic activity in oxazoline synthesis.

19.
Org Lett ; 18(15): 3578-81, 2016 08 05.
Artigo em Inglês | MEDLINE | ID: mdl-27399930

RESUMO

The influence of anomeric configuration upon thioglycoside donors remains relatively unexplored. Utilizing methodology developed for the stereoselective and high-yielding synthesis of α-glycosyl thiols, a series of α-thioglycosides were synthesized, and their reactivity was compared to that of their ß-counterparts. The highly selective activation observed for anomeric pairs containing a 2-O-acyl moiety and additional findings are reported. Application of a pair of "superarmed" thioglycosides to a one-pot oligosaccharide system is also described, in which selectivity is a result of configuration-based orthogonal activation.

20.
Angew Chem Int Ed Engl ; 55(37): 11115-9, 2016 09 05.
Artigo em Inglês | MEDLINE | ID: mdl-27418323

RESUMO

Readily available chiral diol scaffolds are useful as sources of chirality for asymmetric synthesis, however, few such scaffolds are readily available in enantiopure form. Reported herein is a cheap and modular synthesis of a novel family of chiral ferrocenyl diols in excellent yields with excellent enantio- and diastereoselectivity (>99 % ee and 99 % de). These diols possess not only planar and central chirality, but also axial chirality around the central iron atom. Characterization of these diols by X-ray crystallography revealed intra- and intermolecular hydrogen-bond networks depending on substitution at the carbinol positions. The potential of these diols as catalysts was subsequently demonstrated in an asymmetric hetero-Diels-Alder reaction which provided cycloadducts in up to 84 % yield with ee values ranging from -92 to +72 %.

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