Enantiomerization Process Research Articles

Conformational studies by dynamic NMR. 100. Enantiomerization process of stereolabile atropisomers in pyridine-substituted adamantane derivatives.

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine-substituted adamantane derivatives have been determined by dynamic 13C NMR spectroscopy. The trend of these values parallels that anticipated by MM calculations. In at least one case, the computed structure was found to agree with that obtained by single-crystal X-ray diffraction. In addition, it has been possible to achieve a physical separation of a pair of these stereolabile atropisomers at -60 degrees C by means of the enantioselective cryogenic HPLC technique.

Synthesis and stereodynamics of highly constrained 1,8-bis(2,2'-dialkyl-4,4'-diquinolyl)naphthalenes.

The syn and anti isomers of axially chiral 1,8-diquinolylnaphthalenes have been synthesized via Pd-catalyzed Stille coupling of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines. Optimization of the cross-coupling reaction allowed the preparation of highly constrained 1,8-bis(2,2'-dimethyl-4,4'-diquinolyl)naphthalene, 2, and 1,8-bis(2,2'-diisopropyl-4,4'-diquinolyl)naphthalene, 3, in 42% and 41% yield, respectively. Employing Pd(PPh(3))(4) and CuO as the cocatalysts in the coupling reaction of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines proved to be superior over other catalysts such as PdCl(2)(dppf), Pd(2)(dba)(3)/P(t-Bu)(3), and POPd. The C(2)-symmetric anti isomers of 2 and 3 were found to be more stable than the corresponding meso syn isomer. The ratio of the two enantiomeric anti conformers to the syn conformer was determined as 7.9:1 for 2 and 8.6:1 for 3 by NMR and HPLC analysis. The atropisomers of 2 and 3 were found to be stable to rotation about the chiral axis at room temperature and all three stereoisomers of 2 were isolated by semipreparative HPLC on a Chiralpak AD column. The diastereoisomers of 3 were separated via preferential crystallization of the anti isomers from diethyl ether. Slow syn/anti interconversion was observed for both atropisomers at enhanced temperature, and the diastereomerization and enantiomerization processes were monitored by NMR and HPLC. The Gibbs activation energy, DeltaG++, for the isomerization of 2 was determined as 116.0 (112.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 71.0 degrees C. The rotational energy barrier of 3 was determined as 115.2 (111.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 66.2 degrees C.

Chirality of pollutants--effects on metabolism and fate.

In most cases, enantiomers of chiral compounds behave differently in biochemical processes. Therefore, the effects and the environmental fate of the enantiomers of chiral pollutants need to be investigated separately. In this review, the different fates of the enantiomers of chiral phenoxyalkanoic acid herbicides, acetamides, organochlorines, and linear alkylbenzenesulfonates are discussed. The focus lies on biological degradation, which may be enantioselective, in contrast to non-biotic conversions. The data show that it is difficult to predict which enantiomer may be enriched and that accumulation of an enantiomer is dependent on the environmental system, the species, and the organ. Racemization and enantiomerization processes occur and make interpretation of the data even more complex. Enantioselective degradation implies that the enzymes involved in the conversion of such compounds are able to differentiate between the enantiomers. "Enzyme pairs" have evolved which exhibit almost identical overall folding. Only subtle differences in their active site determine their enantioselectivities. At the other extreme, there are examples of non-homologous "enzyme pairs" that have developed through convergent evolution to enantioselectively turn over the enantiomers of a chiral compound. For a better understanding of enantioselective reactions, more detailed studies of enzymes involved in enantioselective degradation need to be performed.

Simulation of elution profiles for two-dimensional dynamic gas chromatographic experiments.

The interconversion of E and Z isomers of acetaldoxime 1 and butyraldoxime 2 have been investigated by comprehensive two-dimensional dynamic gas chromatography (DGCxDGC) and computer simulation. Time-resolved cryogenic modulation is capable of revealing the precise isomeric ratio as a fine structure under the dynamic elution profile, which is characterized in one-dimensional experiments by a plateau formation or peak coalescence caused by interconversion of the isomers during the separation process. The chromatographic theoretical plate model has been extended for the computer simulation of comprehensive two-dimensional dynamic chromatographic experiments. A novel program, ChromWin 2D, based on the new algorithm has been developed for computer simulation to evaluate and predict the elution profiles of DGCxDGC experiments. ChromWin 2D allows the determination of rate constants and barriers of isomerization, epimerization, and enantiomerization processes occurring during chromatographic separations. The Eyring activation parameters of the E/Z and Z/E isomerization barriers in the presence of the stationary phase BP21 (poly(ethylene glycol) terephthalate terminated) were determined by temperature-dependent experiments between 80 and 90 degrees C for 1 and 70 and 130 degrees C for 2. The thermodynamic Gibbs free energy of the E/Z equilibrium of the isomers has been determined from the time-resolved chromatograms by cryogenic modulation. The method described here constitutes a new and important tool for the determination of isomerization barriers, which are of great interest, for example, for the quantitative determination of derivatized aldehydes, such as dinitrophenylhydrazine derivatives, in trace analysis.

DFT studies of structures and enatiomerization mechanisms of bis-chelate complexes of Group 12 elements

The enantiomerization processes of bis-chelate complexes of Group 12 elements have been studied, using the model systems involving acetylacetonate and various O 2N 2-type Schiff bases, by applying both ab initio Hartree-Fock and density functional methods. Both the digonal twist mechanism and bond rupture–formation mechanism have been studied. It was found that the digonal twist pathway including the planar C 2 h and C 2 v transition state (TS) structures is predominant over the bond rupture–formation pathway involving the planar, achiral open-ring TS for the Zn(II) complex with malonaldehyde (i.e. 1,3-propanedionato) anion. The replacement of methyl group by hydrogen did not change the energy barriers of enantiomerization via the digonal twist mechanism for Zn(II), Cd(II) and Hg(II). As expected, the barriers decrease proceeding down the group from Zn to Cd and Hg. Surprisingly, in contrast to the tetrahedral ground-states of Zn(II) and Cd(II) counterparts, the bis(imino-acetylacetonato) Hg(II) and related species possess trans-planar C 2 h ground-state structures, resembling the open-shell Cu(II) and Ni(II) bis-chelate complexes. Solvation effects were also studied using polarizable continuum model for the digonal twist mechanisms and results similar to the gas phase studies were yielded.

Crystal structure and rotational barrier of octakis(bromomethyl)naphthalene.

Octakis(bromomethyl)naphthalene (4) adopts in the crystal a chiral conformation with a helical central naphthalene core and the bromomethyl groups disposed in an alternate up-down "in" arrangement. According to MM3 calculations, this conformation is less stable than the corresponding all alternated "out" form, while B3LYP/LANL2DZ calculations suggest the opposite stability order. The topomerization barrier (16.0 kcal mol(-1)) is ascribed to an enantiomerization process requiring 180 degrees rotation of all the bromomethyl groups and reversal of the helical sense of the naphthalene core.

Stereochemistry of (E)- and (Z)-1,1'-dichlorobifluorenylidenes, substituted overcrowded fullerene fragments.

X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9H-fluoren-9-ylidene (5) are reported. The X-ray molecular structure of (Z)-5 indicated an approximately C2 symmetric conformation with pure twist around C9 = C9' of 40.4 degrees. The fjord regions are somewhat overcrowded: r(C8...C8') = 315.3 pm, r(Cl(1)...Cl(1') = 341.7 pm, r(C(8)...H(8')) = 259.0 pm. The four chlorine atoms of two neighboring molecules of (Z)-5 form a chain. The PM3 calculations showed that the global minimum of 5 is the C2 symmetric twisted conformation t(E)-5, which is 2.4 kJ/mol more stable than its diastereomer C2-t(Z)-5. The corresponding AM1 relative stability is reversed: C2-t(Z)-5 is 1.1 kJ/mol more stable than C2-t(E)-5. The pure twists of t(Z)-5 and t(E)-5 are 37.0 degrees and 37.2 degrees (PM3) and 40.5 degrees and 39.1 degrees (AM1). The corresponding (E) --> (Z) (PM3) and (Z) --> (E) (AM1) energy barriers of diastereomerization are 80.6 kJ/mol (PM3) and 75.8 kJ/mol (AM1). Two anti-folded local minima conformations C2-a(Z)-5 and C(i)-a(E)-5 were found to be 21.2 and 29.5 kJ/mol (PM3) and 25.8 and 35.2 kJ/mol (AM1) less stable than t(E)-5. The syn-folded conformations C(S)-s(Z)-5 and C2-s(E)-5 are transition states for the enantiomerization processes of C2-tz-5 and C2-tE-5, respectively, and lay 79.8 and 94.1 kJ/mol (PM3) and 108.3 and 107.4 kJ/mol (AM1) higher in energy than their corresponding twisted conformations. An alternative pathway for enantiomerization of C2-t(E)-5 via the anti-folded achiral intermediate C(i)-a(E) has a barrier of 56.0 kJ/mol (PM3) and 68.5 (AM1). An alternative pathway for enantiomerization of C2-t(Z)-5 via C2-t(E) and C(i)-a(E) has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.

A [2]catenane and a [2]rotaxane as prototypes of topological and Euclidean molecular "rubber gloves".

A [2]catenane and a [2]rotaxane have been prepared from a C2-symmetric, 2,9-diphenyl-1,10-phenanthroline-based (dpp-based) macrocycle incorporating a 1,5-dioxynaphthalene subunit by means of the transition metal templated technique. In the case of the catenane, this macrocycle is interlocked with a dpp-based macrocycle that is oriented through the location of a p-tolyl substituent in the 4-position of the phenanthroline subunit. In the case of the rotaxane, the C2-symmetric macrocycle is threaded onto an oriented, dumbbell-shaped molecule, based on the same 4-p-tolyl-1,10-phenanthroline subunit, which bears tetraarylmethane stoppers. Both species are chemically achiral molecules, yet they are composed entirely of asymmetric, mirror-image conformations. Conformational enantiomerization processes therefore take place exclusively by chiral pathways, conferring on these molecules the "rubber glove" property. However, while the molecular graph (constitutional formula) of the [2]rotaxane can be deformed into a planar and, hence, rigidly achiral representation, a feature shared by a few other compounds in the literature that have been characterized as "Euclidean rubber gloves", the molecular graph of the [2]catenane cannot be deformed in this way. It therefore has the unique property of being a chemically achiral "topological rubber glove".

Conformational studies by dynamic NMR. 83. Correlated enantiomerization pathways for the stereolabile propeller antipodes of dimesityl substituted ethanol and ethers.

Below -100 degrees C, the NMR spectra of dimesityl derivatives of ethanol and of various ethers reveal how these molecules exist as M and P propeller-like stereolabile enantiomers, owing to the restricted rotation about the Ar-C bond. Single-crystal X-ray diffraction of one such derivative confirmed the existence of a two-blade propeller structure. Computer analysis of the NMR line shape allowed the barriers for the enantiomerization process to be determined. Theoretical modeling (Molecular Mechanics) of the interconversion circuit produced good agreement between the computed and experimental barrier for a correlated dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Introduction of a configurationally stable chiral center allowed two distinct NMR spectra to be detected at appropriate low temperature for two stereolabile diastereoisomers.

Monocyclic zwitterionic lambda(5)Si-silicates with an SiO(2)FC(2) framework: syntheses and structural characterization in the solid state and in solution.

Treatment of the acyclic zwitterionic pentacoordinate silicate F(3)MeSiCH(2)NMe(2)H with 1 molar equiv of Me(3)SiOC(6)H(4)OSiMe(3), Me(3)SiOCH(2)C(O)OSiMe(3), Me(3)SiOC(Ph)=NOSiMe(3), or Me(3)SiOC(O)C(O)OSiMe(3) (solvent CH(3)CN, room temperature) yielded the respective monocyclic zwitterionic pentacoordinate silicates (11a), (12a), (13a), and (14a), along with 2 molar equiv of Me(3)SiF. The derivatives 11b-14b with a 2,2,6,6-tetramethylpiperidinio substituent instead of the dimethylammonio group were prepared analogously, starting from F(3)MeSiCH(2)NR(2)H (NR(2)H = 2,2,6,6-tetramethylpiperidinio). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 11a.1.5CH(3)CN, 12a-14a, and 11b-14b are distorted trigonal bipyramids, the axial positions being occupied by the fluorine atom and one of the two oxygen atoms (12a/12b, carboxylate oxygen atom; 13a/13b, carbon-linked oxygen atom). These results are in agreement with the NMR data ((1)H, (13)C, (19)F, (29)Si) obtained for these compounds in solution. The chiral (C(1) symmetry) zwitterions 11a-14a and 11b-14b exist as pairs of (A)- and (C)-enantiomers in solution. VT (1)H NMR studies with 11b-14b in CH(3)CN in the temperature range 25-85 degrees C gave no indications for an enantiomerization process [(A)/(C)-enantiomerization] at the silicon atom.

Stereochemistry of a spherand-type calixarene.

The stereochemistry of the spherand-type calixarene 2a is analyzed in terms of the configuration of the three 2,2'-dihydroxybiphenyl subunits (R or S) and the disposition of the methylene groups (crown or twist). X-ray crystallography indicates that the neutral 2a and its mono- or dianion (in form of the salts 2a(-) x C(5)H(5)NH(+) and 2a(2)(-) x 2Et(3)NH(+)) exist essentially in the same conformation (RRS/SSR-twist). This asymmetric RRS/SSR-twist form is the lowest energy conformer according to molecular mechanics calculations. Low-temperature (1)H NMR data indicate the presence of a major conformer of C(1) symmetry, in agreement with a RRS/SSR-twist form. The lowest energy topomerization pathway mutually exchanges two pairs of methylene protons and is ascribed to an enantiomerization process involving rotation around an Ar-Ar bond.

Conformational studies by dynamic NMR. 80.(1) cog-wheel effect in the stereolabile helical enantiomers of dimesityl sulfoxide and sulfone.

The (1)H NMR solution spectra of the title compounds display anisochronous lines for the o-methyl substituents below -170 degrees C, due to the existence of two propeller-like M and P conformational enantiomers. The free energies of activation for the interconversion were determined to be 4.5 and 5.0 kcal mol(-)(1), respectively, for dimesityl sulfoxide and dimesityl sulfone. Molecular mechanics calculations indicate that the enantiomerization process occurs via a correlated rotation (cog-wheel effect) entailing a one-ring flip (gear-meshing) pathway. (13)C NMR (CP-MAS) spectra and X-ray diffraction show that these helical enantiomers are stable in the crystalline state.

Conformational studies by dynamic NMR. 79. Dimesityl sulfine revisited: detection of the helical antipodes and determination of their enantiomerization pathways.

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2C=SO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially achiral conformations (Cs symmetry), having the Ph-CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl-CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.

Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: dimesitylketone, dimesitylthioketone, and dimesitylethylene.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.

How to Predict Activation Barriers – Conformational Transformations of Compounds CH3C(CH2PPh2)3–n[CH2P(oTol)2]nMo(CO)3 (n = 1–3): Force Field Calculations versus NMR Data

Tripod metal entities tripodM are sterically congested systems. The conformations adopted by compounds CH3C(CH2PPh2)3–n[CH2P(oTol)2]nMo(CO)3 (n = 1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsive forces acting in these molecules. The steric demand of the o-tolyl groups impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the ΔG‡ values for these conformational enantiomerization processes have been determined as ΔG‡298K = 54.3, 57.9, 65.5 kJ·mol–1 for compounds 1, 2, and 3, respectively. By an exhaustive search on a force field generated hypersurface, activation energies of 53, 57 and 69 kJ·mol–1 have been calculated. Thus, the force field approach correctly reproduces the dependence of the activation energy on the degree of o-tolyl substitution. Moreover, the force field simulation also gives an insight into the individual microsteps of the enantiomerization pathways.

How to Predict Activation Barriers – Conformational Transformations of Compounds CH3C(CH2PPh2)3n[CH2P(oTol)2]nMo(CO)3 (n = 1–3): Force Field Calculations versus NMR Data

Tripod metal entities tripodM are sterically congested systems. The conformations adopted by compounds CH3C(CH2PPh2)3–n[CH2P(oTol)2]nMo(CO)3 (n = 1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsive forces acting in these molecules. The steric demand of the o-tolyl groups impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the ΔG‡ values for these conformational enantiomerization processes have been determined as ΔG‡298K = 54.3, 57.9, 65.5 kJ·mol–1 for compounds 1, 2, and 3, respectively. By an exhaustive search on a force field generated hypersurface, activation energies of 53, 57 and 69 kJ·mol–1 have been calculated. Thus, the force field approach correctly reproduces the dependence of the activation energy on the degree of o-tolyl substitution. Moreover, the force field simulation also gives an insight into the individual microsteps of the enantiomerization pathways.

Formation and structural properties of salicylaldiminato complexes of zirconium and titanium

The metallocene cation [Cp2ZrCH3(THF)+] (2, with BPh4− anion) reacts with N-isopropylsalicylaldimine (1a) by liberation of methane and formation of the pseudotetrahedral [(κO,κN-salicylaldiminato)ZrCp2+] complex 3 that was characterized by X-ray diffraction. Deprotonation of 1a by treatment with n-butyllithium yields the corresponding lithiated reagent 4a, that exhibits a tetrameric structure in the crystal, featuring a cubic Li4O4 core structure. Treatment of 4a with Zr(NMe2)2Cl2(dme) (5) yields the octahedral bis(N-isopropylsalicylaldiminato)Zr(NMe2)2 complex 7a. The X-ray crystal structure analysis of 7a has revealed that the aldiminato oxygen atoms of the κO,κN-chelate ligand are oriented trans to each other at the central metal atom. Treatment of the N-isopropyl, the N-cyclohexyl- and the N-(2,6-diisopropylphenyl)salicylimines (1a,b,d) with the reagent Zr(NMe2)4 (6) directly results in the formation of the respective octahedral (lig)2Zr(NMe2)2 complexes 7a, 7b (also characterized by X-ray diffraction), and 7d in good yields. Treatment of 7b and 7d with Me3Si-Cl replaced the -NMe2 substituents by chloride to yield the corresponding lig2ZrCl2 complexes 8b and 8d, respectively. The X-ray crystal structure analysis of 8d showed an octahedral coordination with the aldiminato nitrogen atoms of the κO,κN-chelate ligands now being trans-oriented at zirconium. Complex 8b undergoes an enantiomerization process that is followed by dynamic NMR spectroscopy (ΔG≠enant. (268 K)≈12 kcal mol−1). Titanium tetrachloride forms a neutral 1:2 adduct with 1d. The NMR analysis and X-ray crystal structure analysis of the resulting bisbetaine-type adduct 9d reveals the formation of an octahedral titanium complex that contains two trans-Ti-O(aryl) bonds. The phenolic hydrogen atoms were transferred to the aldimine nitrogens to form an intramolecular iminium salt. Two equivalents of HCl are removed from 9d by treatment with NEt3 to yield the bis[N-(2,6-diisopropylphenyl)salicylaldiminato]TiCl2 complex 10d (characterized by X-ray diffraction). In a one-pot procedure TiCl4 reacts with N-phenylsalicylaldimine (1c) and triethylamine in a 1:2:2 ratio to directly yield the corresponding (κO,κN-lig)2TiCl2 complex 10c. The X-ray crystal structure analysis of 10c reveals a trans-orientation of ligand oxygen atoms, just like in the adduct intermediate 9. Upon activation with methylalumoxane the complexes 8d, 10c, and 10d form catalysts for the polymerization of ethene.

Formation and Structures of [1,2–Bis(N-tert-butylcarbamoyl)cyclopentadienyl]zirconium Complexes – Coordination Chemistry of a “Fulvenologous” Malonic Amide Anion Ligand System

Treatment of sodium cyclopentadienide with two molar equivalents of tert-butyl isocyanate yields sodium 1,2-bis(N-tert-butylcarbamoyl)cyclopentadienide (6). The [C5H3(1,2-CONHCMe3)2]Na reagent 6 adds to Cp2Zr(CH3)Cl (8) to yield Cp2Zr(CH3)[C5H3(CONHCMe3)2] (9). In 9 the C5H3(CONHCMe3)2 ligand is bonded to zirconium through one of its carboxamido-oxygen atoms (ĸO-coordination). Treatment of 6 with CpZrCl3(THF)2 yields CpZrCl2[C5H3(CONHCMe3)2](THF) 11. In 11 the 1,2-bis(N-tert-butylcarbamoyl)cyclopentadienide moiety serves as a Cs-symmetric chelate ligand, binding to zirconium through both carbamoyl oxygens (ĸ2O,O′-coordination). The same seven-membered metallacyclic structural type is found in the reaction products of 6 with ZrCl4(THF)2 in 1:1 and 2:1 molar ratios. The former yields the distorted octahedral complex ZrCl3[C5H3(CONHCMe3)2](THF) (12), the latter gives the chiral octahedral system ZrCl2[C5H3(CONHCMe3)2]2 (13). In solution, complex 13 undergoes a thermally induced enantiomerization process (Λ Δ interconversion), for which a Gibbs activation energy of ΔG‡enant = 14.0 ± 0.3 kcal mol−1 was determined by dynamic 1H-NMR spectroscopy. The ĸ2O,O′-coordination of the [C5H3(1,2-CONHCMe3]− ligand in the complexes 11, 12, and 13 was secured by X-ray crystal structure analyses.

PROPERTIES AND CLASSIFICATION OF ENANTIOMERIZATION PATHWAYS

Enantiomerization and automerization pathways are analyzed quantitatively using the continuous chirality measures methodology. The analysis gives rise to a number of new concepts and new descriptive tools, including isochirality, chirality maps, phase maps of nearest achirality and non‐handed chirality. These allow one to classify enantiomerization processes into chiral, achiral, isochiral and non‐handed categories.

Internally Phosphine-Stabilized Zirconocene Cations Employing Substituted ((Diarylphosphino)methyl)cyclopentadienyl Ligand Systems

The dimethylzirconocene complex [(Cp−CMe2−PAr2)2ZrMe2], 2a (Ar = p-tolyl), was treated with 1 molar equiv of B(C6F5)3 to yield the salt [(Cp−CMe2−PAr2)2ZrMe+MeB(C6F5)3-], 3a. The X-ray crystal structure analysis of 3a shows that both −PAr2 units are intramolecularly coordinated to zirconium in a close to C2-symmetric arrangement with the [Zr]−CH3 group placed in the central position in the bent metallocene σ-ligand plane. Treatment of 2a with 2 equiv of B(C6F5)3 generates the highly reactive dication system [(Cp−CMe2−PAr2)2Zr2+] (4 with two MeB(C6F5)3- anions). The highly electrophilic cation 4 abstracts chloride from, e.g., dichloromethane solvent to yield [(Cp−CMe2−PAr2)2Zr-Cl+] (5, with MeB(C6F5)3- anion). The same cation (5‘, with ClB(C6F5)3- anion) was obtained from the reaction of [(Cp−CMe2−PAr2)2ZrCl2] (1a) with B(C6F5)3. The C2-symmetric, internally −PAr2-stabilized dication adds acetonitrile or 2,6-dimethylphenyl isocyanide to give the respective C2-symmetric donor-ligand adducts in which the −PAr2 coordination to zirconium is retained. The complexes 3a (ΔG⧧enant (300 K) = 14.0 ± 0.5 kcal/mol) and 5 (ΔG⧧enant (360 K) = 17.5 ± 0.5 kcal/mol) show dynamic NMR spectra due to an intramolecular enantiomerization process proceeding with a rate-determining cleavage of the Zr−P linkages. Complex 5 was also characterized by an X-ray crystal structure analysis.