Asymmetric Induction in the Diels-Alder Reaction Catalyzed by Chiral Metallocene Triflate Complexes: Dramatic Effect of Solvent Polarity

Jaquith, James B.; Guan, Jinying; Wang, Shaotian; Collins, Scott

doi:10.1021/om00003a001

Cited by 64 publications

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“…The solution was filtered, and the solid was washed with toluene (1 mL) and dry hexane (3 × 1 mL) to yield 343 mg of bright yellow crystals (77% yield). Lit …”

Section: Methodsmentioning

confidence: 99%

Diels−Alder Reactions of Oxazolidinone Dienophiles Catalyzed by Zirconocene Bis(triflate) Catalysts: Mechanism for Asymmetric Induction

et al. 1998

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The Diels−Alder cycloaddition reaction between enoyl−oxazolidinones 3 and dienes is efficiently catalyzed by zirconocene bis(triflate) catalysts (4 and 8) in CH2Cl2 or nitroalkane solvents. The use of chiral catalyst [S]-8 led to significant asymmetric induction in the adducts derived from dienophiles 3 and cyclopentadiene, but only in nitroalkane solution at low temperatures and at lower catalyst loadings. The binding of acryloyloxazolidinone 3a to 8 was studied in detail by 1H, 13C, and 19F NMR spectroscopy in both CD2Cl2 and nitroalkane solvent. These studies reveal that two isomeric, five-coordinate, monotriflate complexes (9a and 9b) are formed from 3a and 8 in both solvents. The minor isomer (9a) has the carbamate CO coordinated to the metal at a central site, while in the major isomer (9b) it is coordinated to a lateral site. In nitromethane solvent, the ratio of 9a:9b ≈ 1:2.6 at −30 °C, while in CD2Cl2 the ratio is ∼1:6.7. These studies, along with the sense of asymmetric induction observed, suggest that it is the minor isomer 9a that reacts most rapidly and selectively with dienes under catalytic conditions. The binding of 3a to 8 is strongly favored in nitromethane solvent with K eq = 33.3 ± 1.5 at −30 °C. Thermodynamic parameters for substrate binding were derived from VT 1H NMR spectroscopic studies (ΔH o = −9.8 ± 1.0 kcal mol-1; ΔS o = −33 ± 3 cal mol-1 K-1). Complexes 9a and 9b interconvert predominantly by an intramolecular process (as revealed by VT NMR studies at different concentrations of 3a and 8 as well as 2D-EXSY spectra), while free triflate ion (formed via complexation of 3a to 8) undergoes rapid, associative exchange with bound triflate in residual complex 8 (as independently revealed by VT 19F NMR studies involving 8 and [nBu4N][OTf]). The rates of both of these processes were studied by VT 19F NMR spectroscopy. The activation parameters for triflate exchange involving 8 are ΔH ⧧ = 2.9 ± 0.3 kcal mol-1 and ΔS ⧧ = −26 ± 3 cal mol-1 K-1 while the barrier to interconversion between 9a and 9b is 13.8 ± 0.7 kcal mol-1 at −30 °C with ΔH ⧧ = 17.2 ± 0.9 kcal mol-1 and ΔS ⧧ = 13.9± 0.7 cal mol-1 K-1. Diels−Alder reactions of 3a with, e.g., cyclopentadiene in nitroalkane or dichloromethane solvent are very rapid, even at low catalyst loadings and low temperatures, and rates of reaction cannot be conveniently monitored by 1H NMR spectroscopy. However, the enantioselectivity was shown to vary with conversion, indicating that non-Curtin−Hammett conditions apply.

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“…The solution was filtered, and the solid was washed with toluene (1 mL) and dry hexane (3 × 1 mL) to yield 343 mg of bright yellow crystals (77% yield). Lit …”

Section: Methodsmentioning

confidence: 99%

Diels−Alder Reactions of Oxazolidinone Dienophiles Catalyzed by Zirconocene Bis(triflate) Catalysts: Mechanism for Asymmetric Induction

et al. 1998

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“…Group 4 complexes containing one or two cyclopentadienyl (Cp) ligands are versatile reagents which find use in catalytic and stoichiometric reactions. Compounds of this type are effective catalysts for the polymerization of olefins, the asymmetric cyclopolymerization of dienes, Diels−Alder cycloaddition reactions, enantioselective carbomagnesations, Pauson−Khand type cyclocarbonylations, and the hydrogenation and hydrosilation 7 of olefins, among others . New preparative methods for Cp−group 4 metal complexes therefore make an impact in diverse areas of chemical synthesis.…”

Section: Introductionmentioning

confidence: 99%

Fulvene to Cyclopentadienyl Conversion with Homoleptic Complexes of Zirconium and Hafnium

1999

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The reaction of 6,6-dimethylfulvene with M(CH2Ph)4 (M = Zr, Hf) in benzene gives [η5-C5H4(CMe2CH2Ph)]M(CH2Ph)3 (1, M = Zr; 2, M = Hf) without any observable byproducts. A similar reaction for M = Ti is not observed. The single-crystal X-ray study of 1 shows a three-legged piano-stool geometry with an η2-bound benzyl ligand. A second equivalent of 6,6-dimethylfulvene does not react with either 1 or 2. The bulkier 6,6-diphenyfulvene only reacts cleanly with the more Lewis acidic Hf(CH2Ph)4 to give [η5-C5H4(CPh2CH2Ph)]Hf(CH2Ph)3 (3). Using the tetraamido complexes M(NMe2)4 and 6,6-dimethylfulvene, one obtains dimethylamine and [η5-C5H4(CMeCH2)]M(NMe2)3 (4 for M = Zr) in good yield. These products are formally derived from the deprotonation of a fulvene methyl group and subsequent coordination of the resulting 2-propenylcyclopentadienyl fragment. Reaction of 4 and 6,6-dimethylfulvene affords the bent metallocene [η5-C5H4(CMeCH2)]2Zr(NMe2)2 (5). Excess 8,8-dimethylbenzofulvene and M(NMe2)4 provides exclusively the product with only one coordinated indene.

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“…An enantiopure sample of the ansa-titanocene [(EBTHI)Ti(OTf) 2 ] ( Fig. 4 ) [ 79 – 81 ] indeed catalyzed the slow fluorination of 1 (14% conversion after 18 h, 25% after 36 h, 30 d to completion) or 2 (85% conversion after 18 h), but gave racemic products 1 - F and 2 - F .…”

Section: Resultsmentioning

confidence: 99%

Development of the titanium–TADDOLate-catalyzed asymmetric fluorination of β-ketoesters

Hintermann¹,

Perseghini²,

Togni

2011

Beilstein J. Org. Chem.

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SummaryTitanium-based Lewis acids catalyze the α-fluorination of β-ketoesters by electrophilic N–F-fluorinating reagents. Asymmetric catalysis with TADDOLato–titanium(IV) dichloride (TADDOL = α,α,α',α'-tetraaryl-(1,3-dioxolane-4,5-diyl)-dimethanol) Lewis acids produces enantiomerically enriched α-fluorinated β-ketoesters in up to 91% enantiomeric excess, with either F–TEDA (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)) in acetonitrile solution or NFSI (N-fluorobenzenesulfonimide) in dichloromethane solution as fluorinating reagents. The effects of various reaction parameters and of the TADDOL ligand structure on the catalytic activity and enantioselectivity were investigated. The absolute configuration of several fluorination products was assigned through correlation. Evidence for ionization of the catalyst complex by chloride dissociation, followed by generation of titanium β-ketoenolates as key reaction intermediates, was obtained. Based on the experimental findings, a general mechanistic sketch and a steric model of induction are proposed.

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Asymmetric Induction in the Diels-Alder Reaction Catalyzed by Chiral Metallocene Triflate Complexes: Dramatic Effect of Solvent Polarity

Cited by 64 publications

References 0 publications

Diels−Alder Reactions of Oxazolidinone Dienophiles Catalyzed by Zirconocene Bis(triflate) Catalysts: Mechanism for Asymmetric Induction

Diels−Alder Reactions of Oxazolidinone Dienophiles Catalyzed by Zirconocene Bis(triflate) Catalysts: Mechanism for Asymmetric Induction

Fulvene to Cyclopentadienyl Conversion with Homoleptic Complexes of Zirconium and Hafnium

Development of the titanium–TADDOLate-catalyzed asymmetric fluorination of β-ketoesters

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