Rac Isomers Research Articles

Synthesis, Structure, and Kinetics and Stereochemistry of Base-Catalyzed Hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, Bis(pentaamine) Complexes Devoid of Deprotonatable NH Centers

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.

Photochemical Intramolecular [2+2] Cycloaddition of Bridged Bis-azulenyl Zirconocenes

Photochemical reaction of a rac/meso mixture of bridged bis-azulenyl zirconocenes was investigated. Irradiation of the rac/meso mixture of dichlorodimethylsilylenebis(2-methyl-4-phenyl-4H-azulenyl)zirconium resulted in a rapid conversion of only the meso isomer to give a novel bridged metallocene with a bis-azulenyl ligand cross-linked by a cyclobutylene ring, which was produced by intramolecular [2+2] cycloaddition, and the rac isomer was stable toward photoirradiation. The structure of the cyclobutylene-bridged metallocene derived from meso-dichlorodimethylsilylenebis(2-methyl-4-(2-fluoro-4-biphenylyl)-4H-azulenyl)zirconium was determined by X-ray crystallographic analysis. The structure shows a bond formation between two azulenyl rings at the 7- and 8-position, respectively, to form a four-membered ring. The solubility of cyclobutylene-bridged metallocenes in solvent was found to be much higher than those of rac/meso isomers. After photoirradiation of a rac/meso mixture, rac isomers were easily isolated.

Zwitterionic Phosphorus Ylide Adducts of Boron‐Bridged ansa‐Zirconocene Complexes as Precatalysts for Olefin Polymerization

AbstractMethylenetriphenylphosphorane was coordinated to the boron of phenylborylidene‐bridged bis(cyclopentadienyl)‐ and bis(2‐Me,4‐Ph‐indenyl)zirconium dichloride to form zwitterionic ansa‐zirconocene complexes. The cyclopentadienyl complex 2, when activated with MAO, exhibits remarkable polymerization activity that is more than an order of magnitude greater than that of a related amidoborylidene‐bridged complex as well as the commercially important precatalyst [Cp′−SiMe2−N−tBu]TiCl2 (Cp′ = C5Me4). The zwitterionic bis(2‐Me,4‐Ph‐indenyl)zirconium dichloride species is isolated as a mixture of rac and meso isomers 4−6. The propylene polymerization efficiency of the isomer mixture activated by MAO is greater than that of a related silylene bridged system at 70 and 85 °C; however, the activity of the zwitterionic system decreased with further increase in temperature. The high level of isotactic triads (63.6%) and pentads (52.4%) in the polypropylene formed by 4−6 indicates that the rac isomer is the most active precatalyst of the mixture. The molecular structures of 2 and the rac bis‐indenylzirconium isomer 4 and one of the meso isomers 5 are described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Ethylene/1‐hexene copolymerization with tetramethyldisiloxane‐bridged bis(indenyl) metallocenes

AbstractEthylene/1‐hexene copolymerizations with disiloxane‐bridged metallocenes, rac‐ and meso‐1,1,3,3‐tetramethyldisiloxanediyl‐bis(1‐indenyl)zirconium dichloride (rac‐1, meso‐1) activated by modified methylaluminoxane were performed to investigate the influence of conformational dynamics on comonomer selectivity. Although 1H NOESY (nuclear Overhauser and exchange spectroscopy) analysis indicated that the most stable conformation for the meso isomer in solution was that in which both indenes project over the metal coordination site, this isomer showed higher 1‐hexene selectivity in copolymerization (re = 140 ± 30, rh = 0.024 ± 0.004) than the rac isomer with only one indene over the coordination site (re = 240 ± 20, rh = 0.005 ± 0.001). The meso isomer showed high 1‐hexene selectivity, a high product of reactivity ratios (rerh = 3.3 ± 0.5) and produced copolymers that could be separated into fractions with different ethylene content suggesting that the active species exhibited multisite behavior and populated conformations with different comonomer selectivities during the copolymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3323–3331, 2004

Syntheses and structures of highly hindered N-functionalised alkyl and amido group 12 complexes MR 2 (M=Zn, Cd, and Hg), [MRCl] 2 (M=Zn and Hg)

The reaction of MCl 2 (M=Zn, Cd and Hg) with Li(2-(C(SiMe 3) 2)(6-Me–C 5H 3N)), [Li(MeR)], affords mononuclear metal(II) alkyls. These, in the solid state, show intramolecular M–N interactions in their four-membered chelate rings, which progressively weaken down the series, the C–M–C angles becoming more open (〈M–N〉= 2.30(2), 2.52(2), 2.913(4) Å, C–M–C=160.7(2), 168.5(2), 180(-)°). Alkylmercury chloride complexes have been prepared by the reaction of HgCl 2 with [Li(MeR)], Li(2-(C(SiMe 3) 2)(C 5H 4N)), [Li(HR)], or Li(2-(NSiMe 3)(6-Me–C 5H 3N)), [Li(mpsa)], or from the redistribution reaction of the dialkyl (or amido) mercury complex with HgCl 2. The alkylmercury chloride complexes are dimeric or polymeric in nature with near-linear 2-coordination; (Hg–Cl=2.329(5), 2.318(4), 2.272(8) Å, Cl–Hg–C(N)=175.0(4), 174.7(4), 175.6(5)°). Cd(2-(CH(SiMe 3))(6-Me-C 5H 3N)) 2(tmen), prepared from the reaction of Li(2-(CH(SiMe 3))(6-Me–C 5H 3N)) and CdCl 2 in the presence of tmen, has also been structurally authenticated as the rac isomer with the ipso protons directed towards the tmen.

Synthesis, structures and rac/meso isomerization behaviour of bisplanar chiral bis(phosphino-η 5-indenyl)iron(II) complexes

Syntheses of the phosphinoindenes 1-(diphenylphosphino)-3-methylindene ( 1b), 3-(diphenylphosphino)-2-methylindene ( 1c), 1-(diphenylphosphino)-2,3-dimethylindene ( 1d), 4,7-dimethyl-3-(diphenylphosphino)indene ( 1e), 1-(diphenylphosphino)-3,4,7-trimethylindene ( 1f) and 3-(diisopropylphosphino)indene ( 1i) were carried out by treatment of the appropriate indenide with the appropriate chlorophosphine. The silylphosphinoindene 3-(diphenylphosphino)-1-(trimethylsilyl)indene ( 1h) was prepared by treatment of the indenide of 3-(diphenylphosphino)indene ( 1a) with trimethylsilylchloride. These indenes, in addition to 1a, were then used, after deprotonation with BuLi, to prepare the corresponding indenyl ferrocenes, 2a– 2e, 2h and 2i, by treatment with ferrous chloride in a 2:1 ratio. These compounds were characterized by 1H, 13C, and 31P NMR spectroscopy, as well as by mass spectrometry, except for the highly-sensitive diisopropylphosphine 2i that could only be characterized by 31P NMR spectroscopy. All of these ferrocene complexes are bisplanar chiral systems that can potentially form rac and meso isomers. In all cases both isomers were observed but for 2b and 2h only one could be isolated. The rac isomers of complexes 2a, 2b, 2d, and 2e, as well as the meso isomer of 2e, were studied by X-ray crystallography. Only complexes 2a and 2i were observed to undergo rac/meso isomerization processes at ambient temperature in THF solvent. We were unable to prepare the sterically congested hexamethylferrocene 2f. Generally, it was found that increasing substitution on the indenyl ring increases the reactivity and sensitivity of the ferrocene.

Synthesis and characterization of bis(di-2-pyridylmethanamine)ruthenium(II).

The complex Ru(dipa)(2)(2+) (dipa = di-2-pyridylmethanamine) has been prepared, yielding approximately a statistical ratio of the meso and rac isomers. The electronic spectra of both isomers show pyridyl pi --> pi transitions in the UV region and MLCT bands in the visible region. The solvent dependence of the spectra provides evidence of hydrogen bond formation between the solvent and the NH(2) site on the ligand. The electrochemical properties of the two isomers are identical; each undergoes a reversible one-electron oxidation in acetonitrile (E(1/2) = 0.933 V vs Ag/AgCl) and in aqueous solution below pH 3 (E(1/2) = 0.786 V vs Ag/AgCl). In aqueous solution above pH 3, one-electron oxidation of the ruthenium center is followed by deprotonation of the ligand NH(2) site yielding a reactive amidoruthenium(III) species. The ruthenium-bound dipa ligand possesses structural constraints that prevent the usual oxidative dehydrogenation reaction, which would yield exclusively the corresponding imine. Instead the amidoruthenium(III) intermediate finds alternative reaction routes leading to multiple products.

Mechanistic Studies on a Facile Ring-Flipping Process in Planar Chiral Ferrocenes under Ambient and High Pressure and Its Relevance to Asymmetric Catalysis

The bis-planar chiral ferrocenyldiphosphine bis(1-(diphenylphosphino)-η5-indenyl)iron(II) is observed to undergo an isomerization from the meso isomer to the rac isomer in THF solvent at ambient temperature. This process requires ring flipping of one of the indenyl ligands. The isomerization is slowed by the addition of the noncoordinating solvent chloroform and is accelerated by addition of salts such as LiCl and LiClO4. Rate and activation parameters have been determined for the spontaneous isomerization: kobs = 1.6 × 10-5 s-1 at 23 °C, ΔH⧧ = 58 ± 4 kJ mol-1, ΔS⧧ = −140 ± 15 J mol-1 K-1, and ΔV⧧ = −12.9 ± 0.8 cm3 mol-1. Labeling of the ferrocene with deuterium in the 3- and 3‘-positions followed by isomerization showed no incorporation of deuterium into the 2- or 2‘-positions, thus ruling out 1,2-hydrogen-shift mechanisms. Attempted crossover experiments with dideuterated and nondeuterated ferrocenes gave no monodeuterated products of crossover, thus ruling out ligand dissociative mechanisms. The propo...

Syntheses of linear-type S-bridged trinuclear complexes composed of heavy d 6 metal ions and 2-aminoethanethiolate: comparative characterization by X-ray diffraction, spectroscopy and electrochemistry

The S-bridged trinuclear complexes composed of heavy d 6 metal ions, [Rh III{M(aet) 3} 2] 3+ (M=Ir III( 1), Rh III( 2); aet = 2-aminoethanethiolate), have been prepared by the reactions of fac( S)-[M(aet) 3] with RhCl 3 · 3H 2O. The complexes were separated into meso ( 1a, 2a) and rac ( 1b, 2b) isomers by SP-Sephadex C-25 column chromatography. 1b and 2b were optically resolved by the column chromatographic method and characterized by CD spectroscopy. Crystal structures of 1a, 1b and 2a were determined by X-ray diffraction, and it was found that they consist of linear-type trinuclear structures. The central Rh(III) ion in the present complexes has d 6 electronic configuration with the non-degenerated A-type cubic field term, and showed long Rh⋯M distances, acute S–M–S angles and obtuse Rh–S–M angles. These are in contrast with the complexes having the degenerated T-type cubic field term such as [M ′{M(aet) 3} 2] n+ (M ′=V III, Mo IV and Re III, M=Ir III, Rh III, n=3 or 4). All the isomers have been comparatively characterized and discussed in solid state and the solution for spectrochemical and electrochemical properties.

Rac- and meso-3,3'-Bis(3,5-dimethylphenyl)-3H,3'H-2,2'-biindenylidene-1,1'(2H,2'H)-dione.

In the rac isomer of the title compound, C(34)H(28)O(2), the two C-Ph(dimethylphenyl) bond axes make an angle of 58.7 (1) degrees. There is no short contact between the two 3,5-dimethylphenyl rings, although the dihedral angle between them is 4.93 (7) degrees. The meso isomer has a center of symmetry at the middle of the C=C bond, and the two C-Ph(dimethylphenyl) bond axes are antiparallel to one another.

Extended chains via hydrogen bond linkages of dinuclear copper(II) and cadmium(II) complexes with a new flexible disulfoxide ligand

A new flexible disulfoxide ligand 1,6-bis(benzylsulfinyl)hexane ( L), which is a mixture of the meso and rac isomers, was treated with Cu II or Cd II nitrate and obtained dimeric complex [Cu 2( L) 3(H 2O) 2(NO 3) 4] 2 or [Cd 2(L) 3(H 2O) 2(NO 3) 4] 3. In the reacting system the crystals of meso isomer 1 of L together with 2 or 3 were obtained. 2 and 3 have similar molecular structures. In the neutral dimer, three ligands present two kinds of coordination models: monodentate and bis-monodentate. The neutral dimeric units in 2 and 3 are linked by hydrogen bonds to yield a chain structure. Crystal structures of all three compounds were determined by single-crystal X-ray diffraction methods. Crystal data for 1: monoclinic, space group Cc, a=41.95(2), b=5.132(2), c=8.660(4) Å, β=94.898(9)°, V=1857.7(15) Å 3, Z=4, final refinement ( I>2 σ( I)): R1=0.0659, w R2=0.1415. Crystal data for 2: triclinic, space group P–1, a=9.242(4), b=9.539(4), c=21.042(9) Å, α=83.888(9), β=87.971(8), γ=74.177(9)°, V=1774.6(13) Å 3, Z=2, final refinement ( I>2 σ( I)): R1=0.0577, w R2=0.0954. Crystal data for 3: triclinic, space group P–1, a=9.203(4), b=9.831(3), c=20.860(7) Å, α=84.313(6), β=86.432(7), γ=74.188(6)°, V=1805.9(11) Å 3, Z=2, final refinement ( I>2 σ( I)): R1=0.0548, w R2=0.1192.

Isospecific bulk polymerization of propylene with the use of ansa-metallocenes as a mixture of rac and meso isomers

The effects of various pathways of the formation of catalytic systems based on the rac-Me2SiInd2ZrCl2 metallocene on the activity and properties of polypropylene prepared by the bulk polymerization of propylene were studied in detail. It was found that the conditions of formation of the catalytic system affect not only its activity and the character of kinetic curves but also the molecular weight of the synthesized polymer. Propylene polymerization was studied with the use of a number of bisindenyl derivatives of zirconium, which were mixtures of rac and meso forms that differ in the nature of substituents in the Si bridge or the indenyl ligand. The conclusion was drawn that, with the use of metallocenes as a mixture of rac and meso forms, high-molecular-weight isotactic polypropylene can be prepared with high stereoregularity at a very high rate; thereby, the stage of separation of a pure rac isomer can be excluded in the synthesis of the catalyst.

Facile meso to rac Isomerization of the Bis-Planar Chiral Ferrocenyldiphosphine Bis(1-(diphenylphosphino)-η5-indenyl)iron(II)

The meso isomer of the ferrocenyldiphosphine bis(1-(diphenylphosphino)-η5-indenyl)iron(II) isomerizes to the rac isomer overnight in tetrahydrofuran at room temperature. The isomerization is accelerated by salts but does not occur in chloroform solvent.

Assigning structures to diastereomeric aliphatic α,α'-dibromo ketones

X-ray crystal structures are reported for two symmetrical aliphatic α,α'-dibromo ketones: a meso diastereomer of 3,5-dibromo–2,2,6,6-tetramethylheptan-4-one, and a rac isomer of 4,6-dibromo–2,2,3,3,7,7,8,8-octamethylnonan-5-one. Using these secure assignments and a previously confirmed structure for the diastereomers of 2,4-dibromopentan-3-one, we show in this study that gas-liquid chromatography (GLC) retention times (meso > rac) can be used to confidently assign the diastereomers for a range of symmetrical and unsymmetrical aliphatic α,α'-dibromo ketones. 1H NMR chemical shifts for the >CHBr hydrogen(s) can also be corroboratively used for assignment purposes (δracH > δmesoH).Key words: α,α'-dibromo ketones, X-ray crystal structures, GLC retention times, isomer assignment

Stereoisomers of Mn(III) complexes of ethylenebis[( o-hydroxyphenyl)glycine

We have prepared the Mn(III) complexes rac-Na[Mn(EHPG)]·3H 2O ( 1) and rac, meso-Na[Mn(EHPG)]·H 2O ( 2), where H 4EHPG is ethylenebis[( o-hydroxyphenyl)glycine], and determined their X-ray crystal structures. Complex 1 contains N( S, S)C( R, R) configurations at the N and C stereogenic centres, whilst in the unit cell of complex 2 there are two independent molecules, 2a ( meso) and 2b ( rac), with N( R, R)C( S, R) and N( R, R)C( S, S) configurations, respectively. Enantiomers of each complex are also present. The Mn(III) centres have Jahn–Teller-distorted octahedral geometry. The rac isomer has two long axial MnO(carboxylate) bonds (2.162–2.202 Å) and the equatorial plane contains two short MnN bonds (2.012–2.063 Å) trans to short MnO(phenolate) bonds (1.865–1.901 Å). The meso isomer has long axial MnN (2.194 Å) and MnO(carboxylate) (2.152 Å) bonds, and shorter equatorial MnN (2.005 Å) trans to MnO(phenolate) (1.901 Å) and MnO(carboxylate) (1.988 Å) trans to O(phenolate) (1.897 Å) bonds.

Stereochemical features of the disulfoxides 1,3-bis( n-propylsulfinyl)propane (BPSP) and 1,2-bis(methylsulfinyl)ethane (BMSE), and their copper(II) complexes. Crystal and molecular structure of meso-BPSP, [ trans-Cu( meso-BPSP) 2(H 2O) 2](ClO 4) 2, trans-Cu( meso-BMSE) 2(ClO 4) 2, and trans-Cu( rac-BMSE) 2(ClO 4) 2

Diastereomeric meso and rac forms of 1,3-bis( n-propylsulfinyl)propane (BPSP) and 1,2-bis(methylsulfinyl)ethane (BMSE) have been separated and treated with Cu(ClO 4) 2 to obtain [ trans-Cu( meso-BPSP) 2(H 2O) 2](ClO 4) 2 ( 2) trans-Cu( meso-BMSE) 2(ClO 4) 2 ( 3) and trans-Cu( rac-BMSE) 2(ClO 4) 2 ( 4). The configurational assignment to BPSP has been performed by single crystal X-ray analysis, showing that the α (m.p. 139.0–139.9 °C) and β (m.p. 107.0–107.6 °C) forms correspond to the meso and rac isomers, respectively. Enthalpies of fusion, determined by differential scanning calorimetry techniques, are 34.3 ( meso-BMSE), 22.2 ( rac-BMSE), 40.6 ( meso-BPSP), and 35.1 kJ mol −1 ( rac-BPSP). Single crystals, acceptable for X-ray diffraction studies, were grown by the hanging-drop method of vapor diffusion and micro-seeding techniques. The crystal structure of meso-BPSP ( 1) is characterized by an anti conformation of the propane bridge. The two sulfinyl bonds (av. SO bond distance, 1.487(4) Å) are nearly parallel. In fact, the S→O vectors form an angle of 2.3° and the pseudo-torsion angle OS⋯SO is of 2.2°. Crystals belong to the C2/ c space group, with a=32.285(11), b=5.315(5), c=15.232(7) Å, β=109.98(3)°. The structure was refined to R=0.092. Crystal data for the copper complexes are space group P2/ n, a=13.914(7), b=7.834(4), c=14.805(11) Å, β=96.61(5)° for 2; space group P2 1/ c, a=7.348(7), b=17.892(4), c=8.620(2) Å, β=113.06(2)° for 3; space group P2 1/ a, a=8.343(3), b=12.337(6), c=9.390(3) Å, β=97.35(3)° for 4. The structures were refined to R=0.066, 0.062, and 0.097 for 2, 3, and 4, respectively. The crystal structures of the meso Cu(II) complexes 2 and 3 are characterized by linear chains of copper atoms, bridged by two disulfoxide molecules acting as bis-monodentate O-ligands. In the rac complex 4, BMSE acts again as a μ 2-O,O ligand, but forms layers with a net structure containing four copper atoms per mesh, and each layer contains sulfur atoms with both R and S chirality. In the copper complexes the mean SO bond length is of 1.520(3) Å, confirming the already observed increase in comparison to free sulfoxides.

Synthesis and characterization of meso-[Ru(NO)Cl(dioxocyclam)] and the 1H NMR comparison with [M II(dioxocyclam)] complexes (M II=Ni II and Pd II) (dioxocyclam=1,4,8,11-tetraazacyclotetradecane-5,7-dione)

trans-[Ru(NO)X(dioxocyclam)], X=Cl − and OH −, dioxocyclam=(1,4,8,11-tetraazacyclotetradecane-5,7-dione) have been prepared and characterized by NMR, IR and ESI-MS techniques. The trans-[Ru(NO)Cl(dioxocyclam)] shows the nitrosyl stretch at 1846 versus 1875 cm −1 in the cyclam analogue, indicative of strong π-donation from the deprotonated dioxocyclam ligand to the Ru II center and, in turn, to the NO + group. Upon coordination, the dioxocyclam ligand no longer undergoes facile H/D exchange of the NH and C-6 protons. The methylene protons exist in symmetric patterns indicative of the meso isomer with both C-12 and C-14 methylene carbons projecting below the RuN 4 plane toward the side of the axial Cl − for the major product. A lesser amount of the rac isomer, wherein the C-12 and C-14 methylene carbons reside on opposite sides of the RuN 4 coordination plane, is detected. mmff94 calculations determined that the meso form is more stable than rac by 3.33 kcal mol −1. The Ru II–N(amide) bonds were calculated as having normal lengths (2.07 Å), but the calculated Ru II–N(amine) bonds are elongated to 2.35 and 2.36 Å. (It is known that molecular mechanics can overestimate bond lengths for second and third row metal centers by 0.10–0.15 Å). Even after correcting for the over-calculated bond distance factor, it is seen that the amine–ruthenium distances are longer, and hence the bonds are weaker, than for ‘normal’ Ru II–amines. The presence of the axial Cl − is established by the ESI-MS ion fragments at m/ z=394 for {d 2-[Ru(NO)Cl(dioxocyclam)]H} +. Ions for {[Ru(NO)(H 2O)(dioxocyclam)]} + ( m/ z=377) and {[Ru(NO)(dioxocyclam)]} + ( m/ z=359), the latter from loss of HCl or H 2O from the 394 and 377 ions, are detected. 1H and 13C NMR data for meso-[Ru(NO)Cl(dioxocyclam)] are compared to the meso-[Pd II(dioxocyclam)] complex, and to the Ni II derivative that was synthesized at lower temperature than in previous literature reports. The meso-[Ni II(dioxocylam)] complex was identified previously. In the present work, it is shown that below 50°C the product for the Ni II system is rac-[Ni II(dioxocyclam)]. The assignments for the {Ru(NO)} 3+, Ni II and Pd II dioxocyclam complexes as having the dominant isomeric forms as meso for {Ru(NO)} 3+ and Pd II, and rac for Ni II are supported by the H–H COSY spectrum for {RuNO)} 3+, H–H and C–H COSY spectra for Ni II and the H–H COSY spectrum for Pd II derivatives.

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.

Hypervalent Silicon via Intramolecular Coordination in a Four-Membered Ring in Complexes of Substituted Pyridyl Ligands

Reaction of SiXnCl4-n (X = H, Me) with “Li(C(SiMe3)2(2-C5H4N))” or “Li(C(SiMe3)2(6-Me-2-C5H3N))” yields hypervalent five-coordinate compounds, established in the solid state for SiRHCl2 (5 and 7), which show strong intramolecular Si−N interactions in the four-membered chelate ring (NSiC2) that persist in solution (Si−N = 2.066(9) and 2.072(4) A, X = H). Complexes based on HCPh2(2-C5H4N) form four-coordinate species, whereas complexes based on HCPh(SiMe3)(2-C5H4N) afford either four- or five-coordinate complexes; the rac isomers of Si(CH(SiMe3)(2-C5H4N))2MenCl2-n (26 and 27), formed stereospecifically, are four-coordinate.

Diastereoselective Ce(OiPr)(3)-Catalyzed Pinacol Couplings of Aldehydes.

Aliphatic aldehydes as well as aromatic aldehydes can be converted by means of cerium-catalyzed pinacol coupling [Eq. (1)]. The coupling reaction products can be isolated with good to very good diastereoselectivities in favor of rac isomers (rac:meso>88:12). R=alkyl, aryl.