Interconversion Of Diastereomers Research Articles

Biphenyl Bis(amino alcohol) Oxalamide Gelators: Complex Gelation Involving Coupled Equilibria, Central‐to‐Axial Chirality Transfer, Dia­stereoisomer Interconversion, and Self‐Sorting

AbstractChiral gelators 3 and 4, with two valinol‐ or leucinol‐oxalamido arms attached to the 2,2′‐positions of the proatropisomeric biphenyl group, were prepared, and their gels were studied. Compound (R,R)‐3 in the solution and gel states forms a mixture of major [(R,aR,R)‐3] and minor [(R,aS,R)‐3] diastereomers due to central‐to‐axial chirality transfer. 1H NMR studies of its toluene gel provide evidence of diastereomer interconversion and self‐sorting, which results in exclusive incorporation of (R,aR,R)‐3 into the gel network. Gels formed in the 10–3 M concentration range show an irregular Tg/concentration dependence, which is in contrast to those formed in the 10–2 M concentration range. The peculiar properties of the former gels may be explained by kinetic effects due to the presence of coupled equilibria comprising diastereomer interconversion and (R,aR,R)‐3 self‐assembly where the rate of gelation becomes dependent on the rate of formation of the gelling (R,aR,R)‐3 from the nongelling (R,aS,R)‐3.

Diastereomer Interconversion via Enolization: A Case Study.

The three-component reaction of indole, isobutyraldehyde, and methyl acetoacetate affords methyl 2-(acetyl)-3-(1H-indol-3-yl)-4-methylpentanoate as a single diastereomer. To investigate the origin of the observed diastereoselectivity, the thermodynamics and kinetics of interconversion of diastereomers 1 and 2 in solution were studied by a combination of (1)H nuclear magnetic resonance (NMR) spectroscopy, high-performance liquid chromatography (HPLC), mass spectrometry, and deuteration experiments. The results indicate that interconversion is both acid- and base-catalyzed, and that the alpha carbon is the only stereolabile center in the molecule. The evidence points to an enolization mechanism for the interconversion process. The selective precipitation of 1 in the presence of the equilibrium 1⇆2 eventually results in the exclusive formation of 1 (crystallization-induced asymmetric transformation).

A stimuli responsive system of self-assembled anion-binding Fe4L68+cages

A new cationic Fe4L6 cage molecule was synthesised from 4,4′-diaminobiphenyl, 2-formylpyridine and iron(II). The cage exists as a system of interconverting diastereomers in solution. The system adapts to the addition of anionic guest molecules, expressing a new combination of diastereomers that synergistically bind the guest molecules. Not only do the cage diastereomers interconvert, the volume of the individual cages adapts physically through the rotation of bonds, providing a tailored binding pocket for the guest lined with hydrogen-bond donors. A model for the resulting complex network of species was developed that allowed the system to be fully described. The anion binding constants and the kinetics of both diastereomer interconversion and guest exchange were measured.

Chiral-at-Metal Compounds [CpFe(Prophos)L] (L = Cl, I, CN), [CpFe(Prophos)CO]X (X = I, PF6), and [IndFe(Prophos)CO

The compounds [CpFe(Prophos)Cl] and [CpFe(Prophos)I] were prepared in photochemical reactions of [CpFe(CO)2Cl] and [CpFe(CO)2I] with (R)-Prophos. They consist of pairs of RFe,RC and SFe,RC diastereomers which only differ in the configuration at the metal atom. The diastereomerically pure compounds (SFe,RC)-[CpFe(Prophos)Cl] and (RFe,RC)-[CpFe(Prophos)I], which have the same relative configurations, were isolated. They epimerize via change of the Fe configuration and approach the equilibria (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)Cl] = 5/95 and (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)I] = 95/5 in first-order reactions with half-lives of 43 min at 20 °C and 50 min at 50 °C in C6D6, respectively. The reaction of (RFe,RC)-/(SFe,RC)-[CpFe(Prophos)I] = 95/5 with KCN afforded the cyano complex [CpFe(Prophos)CN] in the diastereomer ratio RFe,RC/SFe,RC = 50/50. Both diastereomers (RFe,RC)- and (SFe,RC)-[CpFe(Prophos)CN] could be isolated diastereomerically pure. The compounds (RFe,RC)- and (SFe,RC)-[CpFe(Prophos)CN] are configurationally stable at the metal center. There is no diastereomer interconversion, not even at higher temperatures. The carbonyl complexes [CpFe(Prophos)CO]I, [CpFe(Prophos)CO]PF6, and [IndFe(Prophos)CO]I were prepared in thermal reactions of [CpFe(CO)2I] and [IndFe(CO2)I] with (R)-Prophos or in an autoclave reaction of [CpFe(Prophos)I]/NH4PF6 with CO under pressure. All the carbonyl complexes are configurationally stable at the metal center. Seven diastereomers were characterized by X-ray crystallography. Including the two diastereomers (RFe,RC)-[CpRu(Prophos)Br] and (RFe,RC)-[CpRu(Prophos)I], a conformational analysis of the M-Prophos chelate ring was carried out, resulting in characteristic differences between major and minor diastereomers.

Reduction of π-Bound Nitriles to π-Bound Imines in a Tungsten(II) Bis(acetylacetonate) Coordination Sphere

Addition of MeOTf (OTf = CF3SO3) to complexes of the type W(CO)(acac)2(η2-N≡CR) (acac = acetylacetonate) [R = Ph (1a), Me (1b)] yields cationic iminoacyl triflate salts of the type [W(CO)(acac)2(η2-MeN═CR)][OTf]. Complexes 2a and 2b undergo nucleophilic attack at the iminoacyl carbon with Na[HB(OMe)3] or MeMgBr to form neutral η2-imine complexes of the type W(CO)(acac)2[η2-MeN═C(Nuc)R] [Nuc = H (3a, 3b), Me (4)]. Hydride addition to 2b results in W(CO)(acac)2(η2-MeN═CHMe), 3b, a complex that exhibits interconversion of diastereomers at ambient temperature on the NMR time scale. X-ray structures of cationic iminoacyl complex 2a and neutral imine complex 4 confirm η2-C═N bonding of both the iminoacyl and imine ligands.

Stereoselectivity in a Chiral Ruthenium Ethylene Hydride Complex: Evidence of an Agostic Intermediate

Treatment of Ru(η6:η1-Me2NC6H4C6H4PCy2)Cl2 with MeLi resulted in the formation of Ru(η6:η1-Me2NC6H4C6H4PCy2)Me2, which reacted with Ph3CPF6 to form the cationic olefin hydride complex [Ru(η6:η1-Me2NC6H4C6H4PCy2)(H2CCH2)(H)]PF6. The new complexes were characterized by spectroscopy and by X-ray crystallographic analysis. The diastereomeric olefin hydride complexes were observed to exhibit two fluxional processes: a facile olefin rotation and another process that results in the exchange of the hydride and olefin protons. As the rate of diastereomer interconversion is much slower than that of olefin insertion, our studies suggest that the latter exchange takes place through an agostic species which undergoes dynamic methyl rotation.

A Study of [Co2(alkyne)(binap)(CO)4] Complexes (BINAP=(1,1′‐Binaphthalene)‐2,2′‐diylbis(diphenylphosphine))

Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.

Dialkyl Effect on Enantioselectivity: π-Stacking as a Structural Feature in P,N Complexes of Palladium(II)

A phosphino,oxazoline P,N-bidentate ligand, 4, containing 3,5-di-tert-butylphenyl groups has been prepared. In the Heck arylation of dihydrofuran, 4 is shown to afford higher ee's than either 2 or 3, the unsubstituted and m-dimethylphenyl analogues, respectively. Several Pd(0) complexes of 4 are reported. The exchange dynamics of Pd(4)(dba) are shown to involve an interconversion of diastereomers via an intramolecular process. The X-ray structure for PdCl2(4), 8, was determined by X-ray diffraction methods. Comparison of data with PdCl2(2), 9, and PdCl2(3), 10, suggests that differing amounts of π−π stacking influence the structures of these relatively simple Pd complexes, with 9 and 10 revealing the strongest π−π interactions. An estimation of the van der Waals energies involved in the interaction supports a ca. 4 kcal/mol stabilization via π−π stacking.

Methane Loss from Cationic μ-Methyl Dimers Formed via Trityl Borate Activation of Monocyclopentadienyl Ketimide Complexes Cp[(tBu)2CN]Ti(CH3)2 (Cp = C5H5, C5Me5, C5Me4SiMe3)

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe2 (L = tBu2CN; Cp = C5H5, 1a; C5Me5, 1b; C5Me4SiMe3, 1c) with the trityl borate activator [Ph3C]+[B(C6F5)4]- are described. Formation of μ-methyl dimers of formula {[Cp(L)TiMe]2(μ-Me)}+[B(C6F5)4]- as a 1:1 mixture of rac and meso diastereomers is observed when 0.5 equiv of [Ph3C]+[B(C6F5)4]- is employed (−25 °C, C6D5Br; Cp = C5H5, rac/meso 2a; C5Me5, rac/meso 2b; C5Me4SiMe3, rac/meso 2c). Dynamic NMR and crossover experiments suggest that the dimers 2 are relatively nonlabile with respect to dissociation, intramolecular methyl group exchange, or diastereomer interconversion. Dimers 2 are observed to undergo methane loss in solution at room temperature, affording the new dimeric compounds 3a−c, {[Cp(L)Ti]2(μ-CH2)(μ-CH3)}+[B(C6F5)4]-. For the less sterically demanding C5H5 ligand, 3a is formed as a mixture of rac/meso diastereomers (7:3), but for the bulkier C5Me5 and C5Me4SiMe3 ligands, the rac isomers of 3b and 3c are formed exclusively. In contrast to μ-methyl dimers 2, in which rac/meso interconversion is not observed, the diastereomers of 3 do undergo interchange, as determined by EXSY spectroscopy, and thus the rac/meso ratios observed are thermodynamic.

NMR study on the coordination of dibenzylideneacetone to chiral palladium(0) units. Fluxional behaviour including an intramolecular double bond exchange†

New derivatives with the general formula Pd(L–L)(η2-dba) (dba = dibenzylideneacetone), L–L = 1-diphenylphosphino-2,1′-(1-dimethylaminopropanediyl)ferrocene, PAPF, 1; 2-[1-(dimethylamino)ethyl]-1-(diphenylphosphino)ferrocene, PPFA, 2; N,N-dimethyl-1-[1′,2-bis(diphenylphosphino)ferrocenyl]ethylamine, BPPFA, 3; 1-diphenylphosphino-2,1′-(1-dicyclohexylphosphinopropanediyl)ferrocene, PPCyPF, 4 were synthesized from Pd2(dba)3·CHCl3 and the appropriate ferrocenyl ligand. When an excess of PPFA was used, the complex Pd(PPFA)3 with P-coordinated PPFA ligands was formed. The dba unit preferentially adopts an s-cis,trans conformation with the s-trans alkene coordinated to the Pd(L–L) fragment. Several intramolecular dynamic processes were identified: at room temperature a fast rotation of dba about the alkene–Pd bond, a Pd–N bond rupture process for complex 2 and, at elevated temperatures, an intramolecular diastereomer interconversion involving an alkene face exchange accompanied by an interchange between the coordinated and non-coordinated dba double bonds. The crystal structure of complex 2 was determined by X-ray diffraction.

Pyridine Adducts of Chiral Arene Ruthenium Compounds. Diastereoselective Synthesis and Characterization

The compounds [CyRu(η2-Ph2PCH(Me)Ph2PO)L][SbF6]2, where Cy = p-CH3C6H4CH(CH3)2; L = pyridine, 2; 3,5-lutidine, 3; isoquinoline, 4, were synthesized by chloride abstraction from [CyRu(η2-Ph2PCH(Me)Ph2PO)Cl][SbF6], 1, followed by treatment of the intermediate Lewis acid [CyRu(η2-Ph2PCH(Me)Ph2PO)][SbF6]2 with the appropriate ligand. Characterization by elemental analysis, 1H, 31P{1H}, VT NMR spectroscopy, and IR spectroscopy indicated that 2−4 were obtained as a mixture of exo and endo diastereomers (de = 14−60%). The endo diastereomer of compound 2 was also characterized by X-ray crystallography which revealed the (RRu*,RC*) relative configuration of the two stereogenic centers within the molecule. In contrast to analogues with other neutral donors, 31P{1H} NMR spectroscopy experiments demonstrated that 2−4 were stable to diastereomer interconversion in solution. This suggests that epimerization of the ruthenium center in an intermediate exo-[CyRu(η2-Ph2PCH(Me)Ph2PO)][SbF6]2 occurred by a path unavailable to 2−4 by inversion of the Ru(η2-Ph2PCH(Me)Ph2PO) ring.

Diastereoselectivity in Chiral Osmium Complexes of a Bidentate Bisphosphine Monoxide Ligand

The chelation of [(η6-Cy)OsL] fragments (Cy = cymene; L = monodentate ligand) by bisphosphine monoxide ligands generates chiral-at-metal complexes. If the bisphosphine monoxide backbone contains a chiral center, diastereomeric products are formed. Osmium complexes of this type prepared using rac-Ph2PCH(Me)P(O)Ph2 epimerize to form a preferred isomer in which the methyl group is distal to L. This has been observed for the analogous ruthenium complexes, but the interconversion of diastereomers occurs at a greatly reduced rate. The slower interconversion allows spectroscopic characterization of the thermodynamically less favored isomers, as the corresponding ruthenium species are too short-lived for such investigations.

Oxidative Addition of Dihydrogen to [Ir(bisphosphine)(1,5-cyclooctadiene)]BF4 Complexes: Kinetic and Thermodynamic Selectivity

The activation of dihydrogen by cationic diphosphine complexes of Rh is the rate-determining and enantiodetermining step in the catalytic asymmetric hydrogenation of prochiral enamides. The addition of H2 to [Ir(bisphosphine)(COD)]+ (COD = 1,5-cyclooctadiene) complexes is examined herein as a model for stereocontrol and dynamic processes related to catalytic hydrogenation. The diastereoselectivity of H2 oxidative addition to form diastereomeric [Ir(chiral bisphosphine)(H)2(COD)]+ complexes at −80 °C is kinetically controlled and varies substantially with the structure of the diphosphine. In one instance (chiral bisphosphine = CHIRAPHOS), the kinetic and thermodynamic selectivities of H2 addition are inverted; i.e., the dominant kinetic product is thermodynamically less stable than the minor kinetic product. For the [IrH2(Me-DuPhos)(COD)]+ system, quantitative analysis of the 2D NOE data using two-dimensional conformer population analysis (2DCPA) establishes the absolute stereochemistry and the three-dimensional structures of the predominant conformers. Kinetic analysis of the interconversion of the [IrH2(Me-DuPhos)(COD)]+ diastereomers, and of their exchange with D2, reveals a complex pathway for isotope scrambling and diastereomer interconversion that does not involve reductive elimination of H2 to form [Ir(Me-DuPhos)(COD)]+. During the isotope scrambling, exquisite selectivity is seen for exchange in only two ligand sites, one on the Me-DuPHOS ligand and one on the COD ligand.

Stereochemical Investigations of the Mechanism of C−H Bond Activation. Diastereomeric and Isotopic Scrambling in (Hydrido)alkyliridium Complexes

The diastereomeric complexes (RS),(SR)-((2,2-dimethylcyclopropyl)(Cp{sup *})(PMe{sub 3})IrH) (2a) and (RR),(SS)-((2,2-dimethylcyclopropyl)(Cp{sup *})(PMe{sub 3})IrH) (2b) and their {alpha}-deuterated analogues (2a-{alpha}{sub d{sub 1}}, 2b-{alpha}{sub d{sub 1}}) were synthesized in racemic form and separated by low-temperature ({minus}80 C) column chromatography. Thermolysis (140 C) of diastereomerically pure 2a or 2b in C{sub 6}D{sub 6} results in its interconversion to the other diastereomer. Thermolysis of the deuterium-labeled analogues 2a-{alpha}{sub d{sub 1}} and 2b-{alpha}{sub d{sub 1}} results additionally in scrambling of deuterium from the {alpha}-position of the dimethylcylopropyl ring to the metal hydride position. Diastereomer interconversion and isotopic scrambling occur at similar rates, which are faster than the rate observed for the reductive elimination of dimethylcyclopropane and subsequent oxidative addition of C{sub 6}D{sub 6}. Quantitative analysis of these rate data is reported. The similarity of these rates is discussed in terms of a common intermediate mechanism involving a metal alkane (or {sigma}-alkane) complex. This mechanism is used as a basis for comparison of the rearrangement processes in the current iridium system and the previously reported analogous rhodium system.

Facile .alpha./.beta. diastereomerism in organocobalt corrinoids. Studies of the interconversion of diastereomers by thermolysis, photolysis, and cobalt-to-cobalt alkyl group transfer

Nonalkylated cobinamides in the Co III , Co II , and Co I oxidation states have been shown to catalyze the interconversion of α- and β-methylcobinamide (CH 3 Cbi) by direct cobalt-to-cobalt methyl group transfer. In every case, the final product composition, starting from either diastereomer, is ca. 95% β-CH 3 Cbi and 5% α-CH 3 Cbi, the same product distribution obtained by reductive alkylation of combinamide by CH 3 I

A variable-temperature 13C NMR study of (η5-C5H5)2Mo2(CO)4 complexes of chiral alkynes derived from a terpene or a hormonal steroid: interconversion of diastereomers via exchange of terminal and semi-bridging carbonyls

The reaction of (C5H5)2Mo2(CO)4 with the steroidal hormone 11β-(4-dimethylaminophenyl)-17β-hydroxy-17α-(1-propynyl)-estra-4,9-dien-3-one (RU 38486) or with propynylborneol yields in each case a chiral cluster that exhibits two cyclopentadienyl resonances in both the 1H and 13C NMR regimes. At low temperature the interconversion of diastereomers arising from the different orientations of the semi-bridging carbonyl ligand can be slowed on the NMR time scale. The barrier to diastereomer interchange has been determined for the terpenoid cluster and the mechanism of this process is discussed.

Nuclear magnetic resonance studies of chirality in triarylmethyl cations. Mechanism of enantiomer and diastereomer interconversion

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTNuclear magnetic resonance studies of chirality in triarylmethyl cations. Mechanism of enantiomer and diastereomer interconversionJ. W. Rakshys Jr., S. V. McKinley, and H. H. FreedmanCite this: J. Am. Chem. Soc. 1971, 93, 24, 6522–6529Publication Date (Print):December 1, 1971Publication History Published online1 May 2002Published inissue 1 December 1971https://doi.org/10.1021/ja00753a032RIGHTS & PERMISSIONSArticle Views84Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (969 KB) Get e-Alerts Get e-Alerts

Nuclear magnetic resonance studies of chirality in triarylmethyl cations. Mechanism of enantiomer and diastereomer interconversion

Nuclear magnetic resonance studies of chirality in triarylmethyl cations. Mechanism of enantiomer and diastereomer interconversion