Chiral Titanocene Research Articles

A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation.

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2, a suitable model for catCl2, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.

Titanocene catalysts with modifiable C2-symmetric chiral ligands

The synthesis of three chiral titanocene dichlorides based on a readily-accessible C2-symmetric tetrahydropentalenyl ligand is described. Two of the complexes are unsymmetrical containing also Cp and Cp∗ ligands, and one is the symmetric complex with two chiral tetrahydropentalenyls. The redox properties of the new titanocenes are investigated by cyclic voltammetry and the catalytic performance in asymmetric reductive ketone-nitrile cyclizations and cross-couplings is evaluated. Moderate to good yields and up to 33% ee are observed, which in case of the cross-coupling reactions exceeds the result obtained with (R,R)-ebthi-TiCl2.

Ni-Catalyzed Enantioconvergent Coupling of Epoxides with Alkenylboronic Acids: Construction of Oxindoles Bearing Quaternary Carbons

We have developed a nickel- nickel/bisphosphine-catalyzed stereoconvergent cross-coupling reaction of epoxides with alkenylboronic acids. Racemic spiroepoxyoxindoles were converted to chiral homoal...

Enantioselective cross-coupling of meso-epoxides with aryl halides.

The first enantioselective cross-electrophile coupling of aryl bromides with meso-epoxides to form trans-β-arylcycloalkanols is presented. The reaction is catalyzed by a combination of (bpy)NiCl2 and a chiral titanocene under reducing conditions. Yields range from 57 to 99% with 78–95% enantiomeric excess. The 30 examples include a variety of functional groups (ether, ester, ketone, nitrile, ketal, trifluoromethyl, sulfonamide, sulfonate ester), both aryl and vinyl halides, and five- to seven-membered rings. The intermediacy of a carbon radical is strongly suggested by the conversion of cyclooctene monoxide to an aryl [3.3.0]bicyclooctanol.

ChemInform Abstract: Ti‐Catalyzed Straightforward Synthesis of Exocyclic Allenes.

Exocyclic allenes constitute useful building blocks in organic synthesis and have recently been identified as key intermediates in the synthesis of natural products. Here the first general method for the most straightforward synthesis of exocyclic allenes reported to date is presented. This method is based on the Barbier-type cyclization of propargyl halides catalyzed by titanium; a safe, abundant, and ecofriendly metal. The reaction proceeds under mild conditions compatible with different functional groups and provides good yields of five-, six-, and seven-membered carbocycles and nitrogen-containing heterocycles bearing an exocyclic allene group. Experimental evidence supporting the proposed reaction mechanism is also provided. Moreover, this procedure can be carried out in an enantioselective manner by using chiral titanocene(III) catalysts. The utility of this method has been proved in the synthesis of the natural alkaloid stemoamide.

Ti‐Catalyzed Straightforward Synthesis of Exocyclic Allenes

Exocyclic allenes constitute useful building blocks in organic synthesis and have recently been identified as key intermediates in the synthesis of natural products. Here the first general method for the most straightforward synthesis of exocyclic allenes reported to date is presented. This method is based on the Barbier-type cyclization of propargyl halides catalyzed by titanium; a safe, abundant, and ecofriendly metal. The reaction proceeds under mild conditions compatible with different functional groups and provides good yields of five-, six-, and seven-membered carbocycles and nitrogen-containing heterocycles bearing an exocyclic allene group. Experimental evidence supporting the proposed reaction mechanism is also provided. Moreover, this procedure can be carried out in an enantioselective manner by using chiral titanocene(III) catalysts. The utility of this method has been proved in the synthesis of the natural alkaloid stemoamide.

ChemInform Abstract: Efficient Kinetic Resolution in the Asymmetric Hydrosilylation of Imines of 3-Substituted Indanones and 4-Substituted Tetralones.

Kinetic resolution of the N-methyl imines of 3-substituted indanones and 4-substituted tetralones could be accomplished by hydrosilylation with a chiral titanocene catalyst. N-Methyl imines of 4-substituted tetralones were resolved to yield, after hydrolysis of the unreacted starting materials, ketones with high ee's and the amine products with high diastereomeric and enantiomeric purity. The utility of this process was demonstrated in the synthesis of sertraline.

Stereocontrolled Coupling between Aldehydes and Conjugated Alkenals Mediated by TiIII/H2O

In the presence of water, titanocene(III) complexes promote a stereoselective C-C bond-forming reaction that provides gamma-lactols by radical coupling between aldehydes and conjugated alkenals. The method is useful for both intermolecular reactions and cyclizations. The relative stereochemistry of the products can be predicted with confidence with the aid of model Ti-coordinated intermediates. The procedure can be carried out enantioselectively using chiral titanocene catalysts. [reaction: see text].

Unprecedented Barbier-type reactions catalysed by titanocene(iii)

Selective Barbier-type allylations, benzylations and propargylations of aldehydes and ketones can be carried out under extremely mild conditions employing titanocene(III) complexes as catalysts. In this way, chiral titanocene catalysts provided yields ranging from 50-80% of optically active products.

Highly Enantioselective Hydrogenation of Acyclic Imines Catalyzed by Ir-f-Binaphane Complexes This work was supported by a NIH grant and a Dreyfus Teaching-Scholar Award. We acknowledge a generous loan of precious metals from Johnson Mathey Inc. and a gift of chiral GC columns from Supelco.

Highly Enantioselective Hydrogenation of Acyclic Imines Catalyzed by Ir-f-Binaphane Complexes This work was supported by a NIH grant and a Dreyfus Teaching-Scholar Award. We acknowledge a generous loan of precious metals from Johnson Mathey Inc. and a gift of chiral GC columns from Supelco.

Efficient kinetic resolution in the asymmetric hydrosilylation of imines of 3-substituted indanones and 4-substituted tetralones.

Kinetic resolution of the N-methyl imines of 3-substituted indanones and 4-substituted tetralones could be accomplished by hydrosilylation with a chiral titanocene catalyst. N-Methyl imines of 4-substituted tetralones were resolved to yield, after hydrolysis of the unreacted starting materials, ketones with high ee's and the amine products with high diastereomeric and enantiomeric purity. The utility of this process was demonstrated in the synthesis of sertraline.

Titanocene-Catalyzed Asymmetric Ketone Hydrosilylation: The Effect of Catalyst Activation Protocol and Additives on the Reaction Rate and Enantioselectivity

The efficient asymmetric hydrosilylation of ketones with a chiral titanocene catalyst has been realized. In this procedure, (R,R)-ethylenebis(tetrahydroindenyl) titanium difluoride (1) was used as the precatalyst, and alcohol additives were employed. Aromatic and α,β-unsaturated ketones were reduced to the corresponding alcohols with a high level of enantioselectivity.

Synthesis of chiral amines catalyzed homogeneously by metal complexes

This review describes developments in catalytic asymmetric hydrogenation of prochiral imines. The homogeneous systems were initially dominated by ones based on Rh complexes containing chiral, chelating diphosphine ligands, although related Ru- and Ir-based systems are becoming more prominent; a very recent, extremely effective hydrogen transfer system (from formic acid), based on Ru catalysts containing chiral 1,2-diamine ligands, is especially significant. A fundamentally different type involving an early transition-metal catalyst (a chiral titanocene) has been reported. Enantiomeric excess (e.e.) values in the range of 90–100% have now been achieved with certain substrates. Emphasis is given to some Rh and Ru catalysts developed in the Department of Chemistry, The University of British Columbia (UBC). Factors discussed include: dependence of conversions, rates and e.e. values on substrate and catalyst type; kinetic dependences; and mechanistic insights, especially possible roles of intermediate metal-hydride and -imine species.

Asymmetric Hydrogenation of Unfunctionalized Trisubstituted Olefins with a Chiral Titanocene Catalyst J. Am. Chem. Soc. 1993, 115, 12569−12570

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTAsymmetric Hydrogenation of Unfunctionalized Trisubstituted Olefins with a Chiral Titanocene Catalyst J. Am. Chem. Soc. 1993, 115, 12569−12570Richard D. Broene and Stephen L. BuchwaldCite this: J. Am. Chem. Soc. 1996, 118, 39, 9458Publication Date (Web):October 2, 1996Publication History Published online2 October 1996Published inissue 1 January 1996https://doi.org/10.1021/ja965416kCopyright © 1996 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1124Altmetric-Citations3LEARN 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 (65 KB) Get e-AlertsSUBJECTS:Catalysts,Elimination reactions,Hydrogenation,Materials,Sandwich compounds Get e-Alerts

ChemInform Abstract: Catalytic Asymmetric Hydrogenation of Imines with a Chiral Titanocene Catalyst: Kinetic and Mechanistic Investigations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Enantioselective Hydrosilylation of Ketones with a Chiral Titanocene Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Kinetic Resolution of Racemic Disubstituted 1‐Pyrrolines via Asymmetric Reduction with a Chiral Titanocene Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Catalytic Asymmetric Hydrogenation of Imines with a Chiral Titanocene Catalyst: Scope and Limitations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Homogeneous catalysis: use of chiral titanocene complexes for asymmetric catalytic Diels–Alder reactions

A stable, chiral diaquo titanocene complex is an excellent catalyst for certain Diels–Alder reactions giving good enantioselectivity which indicates that catalysis occurs by metal rather than by proton activation of the dienophile.

ChemInform Abstract: Asymmetric Hydrogenation of Enamines with a Chiral Titanocene Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.