Titanium Carbene Research Articles

Titanium Carbene Complexes Stabilized by Alkali Metal Amides

Facile α-H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN-type macrocyclic anionic ligand Me3 TACD [(Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH2 group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes.

ChemInform Abstract: Diversity Oriented Synthesis of Conjugate Dienes and Alkenylcyclopropanes Utilizing Silyl Group‐Substituted Titanium Carbene Complexes as Bimetallic Synthetic Reagents.

AbstractThe silyl compounds prepared by this method undergo cross‐coupling reactions with retention of configuration.

Phosphorus-stabilized titanium carbene complexes: synthesis, reactivity and DFT studies.

The synthesis of two novel titanium carbene complexes from the bis(thiophosphinoyl)methanediide geminal dianion 1 (SCS(2-) ) is described. Dianion 1 reacts cleanly with 0.5 equivalents of [TiCl4 (thf)2 ] to afford the bis-carbene complex [(SCS)2 Ti] (2) in 86 % yield. The mono-carbene complex [(SCS)TiCl2 (thf)] (3) can also be obtained by using an excess of [TiCl4 (thf)2 ]. The structures of 2 and 3 are confirmed by X-ray crystallography. A strong nucleophilic reactivity towards various electrophiles (ketones and aldehydes) is observed. The reaction of 3 with N,N'-dicyclohexylcarbodiimide (DCC) and phenyl isocyanate leads to the formation of two novel diphosphinoketenimines 8 a and 8 b. The bis-titanium guanidinate complex 9 is trapped as the by-product of the reaction with DCC. The X-ray crystal structures of 8 a and 9 are presented. The mechanism of the reaction between complex 3 and DCC is rationalized by DFT studies.

Diversity oriented synthesis of conjugate dienes and alkenylcyclopropanes utilizing silyl group-substituted titanium carbene complexes as bimetallic synthetic reagents

Diversity oriented synthesis of conjugate dienes and alkenylcyclopropanes by the sequential reactions using silyl group-substituted titanium carbene complexes was studied. Dienylsilanes were obtained by the olefination of carbonyl compounds with γ-silylvinylcarbene complexes. Cyclopropanation of 1-alkenes with the vinylcarbene complexes produced (E)-(β-silylvinyl)cyclopropanes with high stereoselectivity. The reaction of β-(trialkylsilyl)carbene complexes with zinc alkoxides of homopropargyl alcohols produced 6-silyl-3,5-hexadien-1-ols with high regio- and stereoselectivity. The (E)-alkenylsilanes thus obtained were transformed into a range of unsaturated compounds by the palladium-catalyzed or copper(I)-promoted cross-coupling with organic halides with retention of configuration.

Preparation of silyl polyenol ethers by carbonyl olefination of silyl esters

Silyl enol ethers were produced by the carbonyl olefination of silyl esters with titanium carbene complexes generated by the desulfurizative titanation of thioacetals. The regioselective preparation of silyl dienol and trienol ethers has been achieved by using unsaturated silyl esters and thioacetals.

ChemInform Abstract: Titanium Carbene Complexes as Useful Tools in Organic Synthesis

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Titanium carbene complexes as useful tools in organic synthesis

Various titanium carbene complexes are prepared by the reductive titanation of thioacetals, gem-dihalides, and related organosulfur and organohalogen compounds with the titanocene(II) reagent Cp(2)Ti[P(OEt)(3)](2). Alkylidene-, heteroatom-substituted methylidene-, 2-alkenylidene-, 2-alkynylidene-, and vinylidene-titanocenes thus formed are highly reactive toward organic compounds bearing a multiple bond and are employed for a variety of organic transformations such as carbonyl olefination and olefin metathesis.

Titanium Carbene Mediated Reactions

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Development of a Novel Method for the Preparation of Dithioacetal in the Presence of Titanium(IV) Chloride/Zinc in Dimethoxymethane.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Development of a Novel Method for the Preparation of Dithioacetal in the Presence of Titanium(IV) Chloride/Zinc in Dimethoxymethane

Organosulfur compounds are important intermediates in organic synthesis. Since the pioneering report by Corey and Seebach, sulfur-stabilized carbanion has played an important role in organic synthesis. Recently, a new reaction method was developed that consists of a reductive lithiation of alkyl or arylthioesters and ring closing metathesis of titanium carbene complex prepared from thioacetals. In general, bis(methylthio)methane is used for the synthesis of ketene thioacetal by the reaction with aldehyde or ketone, and bis(phenylthio)methane is used for the synthesis of alkenes, ketones, allyic alchols and 1,3-dienes. In these reactions, only a few reagents, such as bis(methylthio)methane and bis(phenylthio)methane, are used presumably because the reagents are commercially available and can be prepared under limited conditions. In spite of the importance of dithioacetals or sulfur-stabilized carbanions in organic synthetic chemistry, only a few synthetic methods were known. A general method for the preparation of bis(alkyl or arylthio)methanes is shown in Eq. 1. Sodium thioalkoxide or thiophenoxide reacts with diiodomethane in an appropriate solvent.

Organic Syntheses Utilizing Titanium Carbene Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Organic Syntheses Utilizing Titanium Carbene Complexes

Abstract Synthetic reactions utilizing titanium carbene complexes prepared by the desulfurizative titanation of thioacetals and related organosulfur compounds with the titanocene(II) species [Cp2Ti{P(OEt)3}2] are described. The Wittig type olefination of carbonyl compounds including carboxylic acid derivatives with the titanium carbene complexes affords a variety of olefinic compounds. The reaction is successfully applied to the intramolecular carbonyl olefination of carboxylic acid derivatives, giving various carbo- and hetero-cycles. The organotitanium species generated by the treatment of gem-dihalides and certain alkyl halides with the titanocene(II) species are also useful for the transformation of carbonyl compounds into highly substituted olefins or into olefins bearing a bulky substituent. The alkylidenetitanocenes generated from thioacetals react with olefins in both inter- and intramolecular fashions to give olefin metathesis products. The ring-closing olefin metathesis of the carbene complexes formed from thioacetals having a terminal olefin provides a variety of unsaturated heterocyclic compounds. On the other hand, cyclopropanation proceeds when terminal olefins are treated with alkenyl- and alkynyl-carbene complexes. Cyclopropanes are also obtained by treatment of gem-dihalides having a terminal carbon–carbon double bond. The carbene complexes are also highly reactive toward triple bonds. Their reactions with nitriles afford the acylation products after hydrolysis of intermediary titanium vinylimido complexes. The reactions of the carbene complexes with internal alkynes via the formation of titanacyclobutene intermediates afford conjugated dienes with high stereoselectivity, whereas metathesis polymerization proceeds to give trans-polyacetylene when the carbene complexes are treated with excess acetylene. The carbene complexes also react with t-alkyl halides and group 14 metal hydrides.

The Ti(IV) Complex of a Benzannulated N-Heterocyclic Carbene

The yellow titanium(IV) carbene complex [TiCl4(1)], 2 (1 = N,N’-bis(2,2-dimethylpropyl)benzimidazol-2-ylidene) was synthesized by reaction of TiCl4 and the benzannulated carbene 1 in toluene. The X-ray structure analysis revealed a mononuclear complex with a titanium(IV) center coordinated in a distorted trigonal-bipyramidal fashion by the carbene ligand in an equatorial position and four chloro ligands

Formation of Pyrrolidines by the Titanocene(II)‐Promoted Intramolecular Reaction of N‐[3,3‐Bis(phenylthio)propyl]anilides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Formation of pyrrolidines by the titanocene(II)-promoted intramolecular reaction of N-[3,3-bis(phenylthio)propyl]anilides

The reaction of anilides with the titanium carbene complexes generated by the desulfurization of thioacetals with the titanocene(II) species Cp 2Ti[P(OEt) 3] 2 produced the corresponding enamines. Unusual formation of pyrrolidines was observed when N-[3,3-bis(phenylthio)propyl]anilides were treated with the titanocene(II) reagent.

Preparation of ( Z )‐Alk‐2‐ene‐1,5‐diols by the Titanocene(II)‐Promoted Cyclization of Thioacetals Possessing a Terminal Carbon−Carbon Double Bond

Titanocene(II)-promoted ring-closing metathesis of the titanium carbene complexes prepared from [2,2-bis(phenylthio)ethyl](but-3-enyloxy)dimethylsilanes or dimethyl(prop-2-enyl)silyl ethers of 3,3-bis(phenythio)propanols gave seven-membered cyclic unsaturated silyl ethers. Oxidative cleavage of a silicon−carbon bond of the cyclic silyl ethers resulted in olefinic diols, with high Z stereoselectivity.

Ring-Closing Metathesis of Titanium–Carbene Complexes Prepared from Thioacetals Having a Carbon–Carbon Double Bond

The ring-closing metathesis proceeded to give cycloalkenes in good yields when diphenyl thioacetals having a carbon–carbon double bond were treated with the low-valent titanium species Cp2Ti[P(OEt)3]2 in THF at room temperature and then at reflux. This methodology is successfully applied to the preparation of cyclic ethers and sulfides.

Conversion of meso-alkenes to chiral alkenes via titanocene-catalyzed ring-opening/ring-closing olefin metathesis

The preparation of 7- anti-(3-methyl-3-butenyl)norbornene 11 is described. This meso-symmetrical diene underwent ring-opening/ring-closing olefin metathesis catalyzed by bis(cyclopentadienyl)titanacyclobutane 4 to give (1R ∗, 6R ∗, 7S ∗) -7-ethenyl-3-methylbicyclo[4.3.0]non-2-ene as a single diastereomer. The less-substituted 7- anti-(3-butenyl)norbornene 1 did not react selectively at the norbornene double bond. Related 7-ester-substituted norbornenes underwent stoichiometric ring-opening/ring-closing olefin metathesis to give single diastereomers of bicyclic ketones after acidic work-up. Attempts to form a chiral Tebbe-type complex from ansa-{2,2′-bis[8-(bicyclo[4.3.0]-nona-1(6),7-dienyl)]-1,1′-binaphthyl}titanium dichloride 18 or a reactive chiral titanium carbene intermediate from its dimethyl derivative 20 were unsuccessful.

Mechanistic Study of Ester Olefinations Using Dimethyltitanocene

Olefination of esters using dimethyltitanocene is proposed to proceed via the titanium carbene, Cp2TiCH2, on the basis of the following evidence: (1) No H/D exchange or 13C scrambling occurs in the reactions of ethyl acetate or dodecyl acetate using either labeled ester or labeled dimethyltitanocene, (2) kinetics are zero order in ester and first order in Cp2TiMe2, (3) the reactions of ethyl acetate and dodecyl acetate with Cp2Ti(CD3)2 produce substantial kinetic isotope effects of 9−10, (4) esters of differing electronic and steric environments have similar reactivity, and (5) there is regiochemical preference for the least stable exo-methylene product. These findings are inconsistent with the methyl addition mechanism proposed in the literature.

Heterocycles as ligands 21 1. Stable titanium carbene complexes

The titanium carbene complexes 2 are obtained by the reaction of the imidazol-2-ylidenes 1 with TiCl 4 in good yields. Careful hydrolysis of 2c gives the dinuclear complex 3c in which two complex fragments of pentacoordinated titanium are connected by a near linear oxygen bridge.