Titanium-alkyne complexes

Similar to Cp2TiCl2, T OPr1) as a less expensive precursor, can also be utilized for the synthesis of titanium-alkyne complexes.13 The reactivity of the (Pr10)2Ti-alkyne complexes toward a variety of substrates has been investigated.14 In the case of unsymmetrical alkynes as a starting material, the less hindered carbon of the resultant... 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

Treatment of the thermally stable, formally divalent bis(alkoxide)titanium alkyne complexes, (Pr 0)2Ti( 7Z-RC=CR) (R = Et, Pr, p-Me-C(,H4),222 with aryl iodides in the presence of Ni(COD)2 (COD = 1,5-cyclooctadiene) affords the corresponding cross-coupled products.223 Other catalysts such as Ni(acac)2 (acac = acetylacetonate), Pd(OAc)2, and Pd(PPh3)4 promote the reaction but are significantly less active. As anticipated, aryl iodides undergo cross-coupling more effectively than the corresponding bromides, chlorides, and triflates. 

These titanium compounds can be described as an alkene 7r-complex or a metallacyclopropane, which is of practical importance. According to several computational studies, it has been concluded that the alkene titanium complexes are best represented as titanacyclopropane derivatives. The synthesis of titanium-alkyne complexes Ti(Me3SiC=CG6H13)(OR)2 from reaction between l-(trimethylsilyl)oct-l-yne with achiral or chiral alkoxo titanium compounds Ti(OR)4 has been described (Scheme 95).184 A series of organotitanium compounds (Scheme 96) are obtained by metathesis reactions.41... 

Titanium-alkyne complexes Ti(Me3SiC=CC6Hi3)(OR)2, as well as the chiral complex derived from chloro-tris[(—)-menthoxo]titanium/2MgClPr1 and alkynes, react with carbonyl compounds to afford optically active allylic alcohols in up to 38% ee (Scheme 127).184 Introduction of two different electrophiles at each of the acetylenic terminal carbon atoms was possible in a regio- and stereoselective manner.45 Similarly, the titanacyclopentene compounds react with imines, metalloimines, or hydrazones under mild conditions to afford allylic amines or their derivatives in good to excellent yields (Scheme 128).258... 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

On protonation, the latter complex would produce the stereodefined conjugated diene as a single regioisomer. On the basis of this proposal, the authors checked the importance of the metallated alkoxide in a related experiment, showing that the reaction of an internal alkyne bearing a tert-butyldimethylsilyl (TBS)-protected hydroxy group with a preformed titanium alkyne complex led exclusively to a complex mixture of products [125]. 

The formation of a bis(guanidinate)-supported titanium imido complex has been achieved in different ways, two of which are illustrated in Scheme 90. The product is an effective catalyst for the hydroamination of alkynes (cf. Section V.B). It also undergoes clean exchange reactions with other aromatic amines to afford new imide complexes such as [Me2NC(NPr )2]2Ti = NC6F5. ... 

The guanidinate-supported titanium imido complex [Me2NC(NPr02l2Ti = NAr (Ar = 2,6-Me2C6H3) (cf. Section IILB.2) was reported to be an effective catalyst for the hydroamination of alkynes. The catalytic activity of bulky amidinato bis(alkyl) complexes of scandium and yttrium (cf. Section III.B.l) in the intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentenylamine has been investigated and compared to the activity of the corresponding cationic mono(alkyl) derivatives. 

Based on the established mechanism for titanium-catalyzed hydroamination, the authors propose a reversible reaction between a titanium imide complex and the alkyne to form metalloazacyclobutene 86, which in turn undergoes 1,1-insertion of the isonitrile into the Ti-C bond. The generated five-membered ring iminoacyl-amido complex 87 with the new C-C bond is protonated by the primary amine to afford the desired three-component coupling product, with regeneration of the catalytic imidotitanium species. Very recently, titanium-catalyzed carbon-carbon bond-forming reactions have been reviewed.122... 

Although terminal acetylenes themselves do not form stable titanium—acetylene complexes upon reaction with 1, the reaction with terminal alkynes having a keto group at the 5- or y-position induces an intramolecular cyclization, apparently via the above titanium-acetylene complex to afford the four- and five-membered cycloalkanols, as shown in Eq. 9.6 [28]. 

Reaction of Titanium Carbene Complexes with Alkynes... 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

The species shown in Schemes 8.1 and 8.2 do not contain vacant coordination sites suitable for binding weakly donating ligands such as alkenes. Even in Breslow s zwitterionic intermediate (Scheme 8.1) the nature of the metal-ethene interaction is unclear alkenes do not bind to the LUMO of 16-electron complexes CP2MCI2 (M = Ti, Zr, Hf) or their alkyl derivatives. The isolation by Eisch in 1985 of a cationic titanium vinyl complex [Cp2TiC(Ph)=C(Me)SiMe3], apparently formed by insertion of an alkyne into a putative [Cp2TiMe] intermediate [29], raised the... 

Jeong and co-workers utilized a cobalt-alkyne complex to enhance enantioselectivity of the addition of bis (homoallyl)zinc to propargyl aldehydes 68 by the exaggeration of steric environment. The reaction provided optically enriched propargyl alcohol 69 in the presence of a chiral ligand and titanium tetra(isopropoxide) in excess. Adduct 69 was subjected to PKR to yield optically enriched bicyclic compounds 70 (Equation (39)). ... 

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

Catalytic asymmetric vinylation of ketones has been achieved. Vinylzinc reagents have been generated by hydrozirconation of terminal alkynes which are then transmet- allated with zinc.199 A titanium(IV) complex of a tims-cyclohexane-bis(sulfonamide) provides chiral catalysis it also facilitates dienylation of ketones, with ees also >90% in this case. 

Ti(Cp)2(CO)2l is a catalyst for the hydrogenation of phenylacetylene to ethylbenzene, while alkyl-substituted terminal alkynes are reduced to alkenes. Electron rich titanium(II) complexes, [Cp2Ti(PhC OPh)(PMe3)], [(MeCp)2Ti(PhC=CPh)(PMe3)] and [CpCp Ti(PhCsCPh)] are also catalyst precursors for the hydrogenation of alkynes to alkanes at 20 C under atmospheric pressure of hydrogen. "... 

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