Organotitanium compound

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Organotitanium compounds

Organotitanium compounds in organometallic chemistry contain carbon-titanium chemical bonds. Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry and are involved in major industrial processes. [1]

Contents

Brief history

Although the first attempt to prepare an organotitanium compound dates back to 1861, the first example was not reported until 1954. In that year titanocene dichloride was described by Wilkinson and Birmingham. Independently, titanium-based Ziegler–Natta catalysts were described leading to major commercial applications, for which the 1963 Nobel Prize in Chemistry was awarded. This technology underscored the technical significance of organotitanium chemistry.

Properties

The reductive coupling of benzophenone in the McMurry reaction illustrates the oxophilicity of low valent titanium complexes. McMurryBenzophenone.png
The reductive coupling of benzophenone in the McMurry reaction illustrates the oxophilicity of low valent titanium complexes.

The titanium electron configuration ([Ar]3d24s2) vaguely resembles that of carbon and like carbon, the +4 oxidation state dominates. Titanium is however a much larger element than carbon, reflected by the Ti-C bond lengths being about 30% longer, e.g. 210 pm in tetrabenzyltitanium vs a typical C-C bond of 155 pm. Simple tetraalkyltitanium compounds however are not typically isolable, owing to the large size of titanium and the electron-deficient nature of its tetrahedral complexes. More abundant and more useful than the simple tetraalkyl compounds are mixed ligand complexes with alkoxide and cyclopentadienyl coligands. Titanium is capable of forming complexes with high coordination numbers.

In terms of oxidation states, most organotitanium chemistry, in solution at least, focuses on derivatives of titanium in the oxidation states of +3 and +4. Compounds of titanium in the +2 oxidation state are rarer, examples being titanocene dicarbonyl and Ti(CH3)2(dmpe)2. [Ti(CO)6]2− is formally a complex of titanium in the oxidation state of -2. [3] Although Ti(III) is involved in Ziegler–Natta catalysis, the organic derivatives of Ti(III) are uncommon. One example is the dimer [Cp2TiIIICl]2. [4]

Due to the low electronegativity of titanium, Ti-C bonds are polarized toward carbon. Consequently, alkyl ligands in many titanium compounds are nucleophilic. Titanium is characteristically oxophilic, which recommends the use of air-free techniques. On the other hand, high oxophilicity means that titanium alkyls are effective for abstracting or exchanging organyl ligands for oxo groups, as discussed below.

Compounds

Structure of (CH3CH2)TiCl3(dmpe), highlighting an agostic interaction between the methyl group and the Ti(IV) center. MLHGtiag.png
Structure of (CH3CH2)TiCl3(dmpe), highlighting an agostic interaction between the methyl group and the Ti(IV) center.

Alkyl titanium chlorides and alkoxides

Simple alkyl complexes of titanium, e.g. Ti(CH2Ph)4, [6] where Ph is phenyl, are rare. Several mixed alkyl-titanium-halides and alkyl-titanium-alkoxides are utilized in organic synthesis, even if they are not often well characterized. [7] At least from the commercial perspective, the most useful organotitanium compounds are generated by combining titanium(III) chloride and diethylaluminium chloride. As Ziegler–Natta catalysts, such species efficiently catalyze the polymerization of ethene. The process is heterogeneous and no organotitanium intermediates have been well characterized for this process.

Numerous organotitanium reagents are produced by combining titanium tetrachloride, titanium tetraalkoxides, or mixtures thereof with organolithium, organomagnesium, and organozinc compounds. Such compounds find occasional use as stoichiometric reagents in organic synthesis. Methyltitanium trichloride, nominally CH3TiCl3, can be prepared by treating titanium(IV) chloride with dimethylzinc in dichloromethane at −78 °C. It delivers a methyl groups to carbonyl compounds and alkyl halides. "Methyltriisopropoxytitanium" is a related reagent. [8] A dialkyltitanium species is implicated for Ti-promoted cyclopropanations starting from a Grignard reagent and an ester. This reaction is the basis of the Kulinkovich reaction: [9]

Kulinkovich reaction Kulinkovich overview.png
Kulinkovich reaction

"Lombardo's reagent" is used for methylenation. [10] It is functionally related to the Dibromomethane-Zinc-Titanium(IV) Chloride reagent. [11] This chemistry addresses a shortcoming of the Wittig reagent by methylenating enolisable carbonyl groups without loss of stereochemical integrity (Lombardo Methylenation). It can for example also be applied in a conversion of a ketene into an allene: [7] [12]

LombardoReagent.png

Titanocene derivatives

The structure of "titanocene" is not Ti(C5H5)2, but a fulvalene complex GreenTitanoceneWeakM-M.png
The structure of "titanocene" is not Ti(C5H5)2, but a fulvalene complex

Attempted synthesis of "titanocene", i.e. Ti(C5H5)2, produces a fulvalene complex. [13] [15] The titanocene dimer was recognised in the 1970s [15] [16] [17] but not structurally characterised until 1992, [14] and the investigations led to many innovations on cyclopentadienyl complexes of titanium. [13] Only in 1998 was a true titanocene derivative identified, the paramagnetic species (C5(CH3)4Si(CH3)3)2Ti. [18]

In contrast to titanocene itself, titanocene dichloride and to some extent titanocene monochloride have rich and well defined chemistries. [13] Tebbe's reagent, prepared from titanocene dichloride and trimethylaluminium, is used as a methylenation agent (conversion of R2C=O to R2C=CH2).

TebbeRgtEquil.png

Tebbe's reagent adds simple alkenes to give titanocyclobutanes, which can be regarded as stable olefin metathesis intermediates. These compounds are reagents in itself such as 1,1-bis(cyclopentadienyl)-3,3-dimethyltitanocyclobutane, the adduct of Tebbe's reagent with isobutene catalysed with 4-dimethylaminopyridine. [19]

The Petasis reagent, [(e -Cp)2Ti(CH3)2] Cp2TiMe2.png
The Petasis reagent, [(η -Cp)2Ti(CH3)2]

The Petasis reagent or dimethyl titanocene (1990) is prepared from titanocene dichloride and methyllithium in diethyl ether. Compared to Tebbe's reagent it is easier to prepare and easier to handle. It is also a methylenation reagent. [19]

The Nugent-RajanBabu reagent [20] is a one-electron reductant used in synthetic organic chemistry for the generation of alcohols via anti-Markovnikov ring-opening of epoxides, and is generated as a dimer [(η5-Cp)2Ti(μ-Cl)]2 and used in situ from titanocene dichloride. [4] [21] [22] [23]

N-RB equilibrium.jpg

Mono-Cp compounds

Less useful in organic chemistry but still prominent are many derivatives of (cyclopentadienyl)titanium trichloride, (C5H5)TiCl3. This piano-stool complex is obtained by the redistribution reaction of titanocene dichloride and titanium tetrachloride. With an electron count of 12, it is far more electrophilic than the titanocene dichloride with an electron count of 16.

Arene complexes

Titanium tetrachloride reacts with hexamethylbenzene to give [(η6-C6(CH3)6)TiCl3]+ salts. Reduced arene complexes include the oxidation states −1, 0, +1. [24] [25]

Carbonyl complexes

Salts of [Ti(CO)6]2− are known. [26]

Related Research Articles

Metallocene

A metallocene is a compound typically consisting of two cyclopentadienyl anions (C
5H−
5, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.

Titanium tetrachloride Inorganic chemical compound

Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is a volatile liquid. Upon contact with humid air, it forms thick clouds of titanium dioxide and hydrochloric acid, a reaction that was formerly exploited for use in smoke machines. It is sometimes referred to as "tickle" or "tickle 4" due to the phonetic resemblance of its molecular formula to the word.

Titanocene dichloride Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

Tebbes reagent Chemical compound

Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It is a red solid that is pyrophoric in the air, and thus is typically handled with air-free techniques. It was originally synthesized by Fred Tebbe at DuPont Central Research.

Petasis reagent Chemical compound

The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid.

Vanadocene dichloride is an organometallic complex with formula (η5-C5H5)2VCl2 (commonly abbreviated as Cp2VCl2). It is a structural analogue of titanocene dichloride but with vanadium(IV) instead of titanium(IV). This compound has one unpaired electron, hence Cp2VCl2 is paramagnetic. Vanadocene dichloride is a suitable precursor for variety of bis(cyclopentadienyl)vanadium(IV) compounds.

Titanocene dicarbonyl Chemical compound

Dicarbonylbis(cyclopentadienyl)titanium is the chemical compound with the formula (η5-C5H5)2Ti(CO)2, abbreviated Cp2Ti(CO)2. This maroon-coloured, air-sensitive species is soluble in aliphatic and aromatic solvents. It has been used for the deoxygenation of sulfoxides, reductive coupling of aromatic aldehydes and reduction of aldehydes.

Niobocene dichloride is the organometallic compound with the formula (C5H5)2NbCl2, abbreviated Cp2NbCl2. This paramagnetic brown solid is a starting reagent for the synthesis of other organoniobium compounds. The compound adopts a pseudotetrahedral structure with two cyclopentadienyl and two chloride substituents attached to the metal. A variety of similar compounds are known, including Cp2TiCl2.

Group 2 organometallic chemistry

Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organmetallic group 2 compounds are rare and are typically limited to academic interests.

Organozirconium chemistry

Organozirconium compounds are organometallic compounds containing a carbon to zirconium chemical bond. Organozirconium chemistry is the corresponding science exploring properties, structure, and reactivity of these compounds. Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.

Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.

Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. While iron adopts oxidation states from Fe(−II) through to Fe(VII), Fe(IV) is the highest established oxidation state for organoiron species. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

Organovanadium chemistry is the chemistry of organometallic compounds containing a carbon (C) to vanadium (V) chemical bond. Organovanadium compounds find only minor use as reagents in organic synthesis but are significant for polymer chemistry as catalysts.

In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp2TiCl2. Several reagents and much research is based on bent metallocenes.

Titanocene pentasulfide Chemical compound

Titanocene pentasulfide is the organotitanium compound with the formula (C5H5)2TiS5, commonly abbreviated as Cp2TiS5. This metallocene exists as a bright red solid that is soluble in organic solvents. It is of academic interest as a precursor to unusual allotropes of elemental sulfur as well as some related inorganic rings.

Cyclopentadienyliron dicarbonyl dimer Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

Bis(cyclopentadienyl)titanium(III) chloride Chemical compound

Bis(cyclopentadienyl)titanium(III) chloride, also known as the Nugent–RajanBabu reagent, is the organotitanium compound which exists as a dimer with the formula [(C5H5)2TiCl]2. It is an air sensitive green solid. The complex finds specialized use in synthetic organic chemistry as a single electron reductant.

Organoniobium chemistry is the chemistry of compounds containing niobium-carbon (Nb-C) bonds. Compared to the other group 5 transition metal organometallics, the chemistry of organoniobium compounds most closely resembles that of organotantalum compounds. Organoniobium compounds of oxidation states +5, +4, +3, +2, +1, 0, -1, and -3 have been prepared, with the +5 oxidation state being the most common.

Rosenthals reagent

Rosenthal's reagent is a metallocene bis(trimethylsilyl)acetylene complex with zirconium (Cp2Zr) or titanium (Cp2Ti) used as central atom of the metallocene fragment Cp2M. Additional ligands such as pyridine or THF are commonly used as well. With zirconium as central atom and pyridine as ligand, a dark purple to black solid with a melting point of 125–126 °C is obtained. Synthesizing Rosenthal's reagent of a titanocene source yields golden-yellow crystals of the titanocene bis(trimethylsilyl)acetylene complex with a melting point of 81–82 °C. This reagent enables the generation of the themselves unstable titanocene and zirconocene under mild conditions.

In organic chemistry, the Lombardo methylenation is a name reaction that allows for the methylenation of carbonyl compounds with the use of Lombardo's reagent, which is a mix of zinc, dibromomethane, and titanium tetrachloride.

References

  1. ↑ "Organotitanium Reagents in Organic Synthesis (Reactivity and Structure Concepts in Organic Chemistry, Vol 24)" Manfred T. Reetz 1986 ISBN   0-387-15784-0
  2. ↑ Michel Ephritikhine (1998). "A new look at the McMurry reaction". Chem. Commun. (23): 2549–2554. doi:10.1039/a804394i.
  3. ↑ Elschenbroich, C. "Organometallics" (2006) Wiley-VCH: Weinheim. ISBN   978-3-527-29390-2
  4. 1 2 Manzer, L. E.; Mintz, E. A.; Marks, T. J. (1982). Cyclopentadienyl Complexes of Titanium(III) and Vanadium(III). Inorg. Synth. Inorganic Syntheses. Vol. 21. pp. 84–86. doi:10.1002/9780470132524.ch18. ISBN   9780470132524.
  5. ↑ Z. Dawoodi; M. L. H. Green; V. S. B. Mtetwa; K. Prout; A. J. Schultz; J. M. Williams; T. F. Koetzle (1986). "Evidence for Carbon–Hydrogen–Titanium Interactions: Synthesis and Crystal Structures of the Agostic alkyls [TiCl3(Me2PCH2CH2PMe2)R] (R = Et or Me)". J. Chem. Soc., Dalton Trans. (8): 1629. doi:10.1039/dt9860001629.
  6. ↑ Davies, Gwyneth R.; Jarvis, J. A. J.; Kilbourn, B. T. (1971). "The Crystal and Molecular Structures (At –40 °C) of the Tetrabenzyls of Titanium, Hafnium, and Tin". J. Chem. Soc. D (23): 1511–1512. doi:10.1039/C29710001511.
  7. 1 2 Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN   1-891389-53-X
  8. ↑ Imwinkelried, René; Seebach, Dieter (1989). "3'-Nitro-1-Phenylethanol by Addition of Methyltriisopropoxytitanium to m-Nitrobenzaldehyde". Organic Syntheses. 67: 180. doi:10.15227/orgsyn.067.0180.
  9. ↑ Cha, Jin Kun; Kulinkovich, Oleg G. (2012). "The Kulinkovich cyclopropanation of carboxylic acid derivatives". Organic Reactions. 77: 1–159. doi:10.1002/0471264180.or077.01. ISBN   978-0471264187.
  10. ↑ Luciano Lombardo (1987). "Methylenation of Carbonyl Compounds: (+)-3-Methylene-cis-p-menthane". Organic Syntheses. 65: 81. doi:10.15227/orgsyn.065.0081.
  11. ↑ Takai, Kazuhiko; Hotta, Yuji; Oshima, Koichiro; Nozaki, Hitosi (1978-01-01). "Effective methods of carbonyl methylenation using CH2I2-Zn-Me3Al and CH2Br2-Zn-TiCl4 system". Tetrahedron Letters. 19 (27): 2417–2420. doi:10.1016/S0040-4039(01)94789-6. ISSN   0040-4039.
  12. ↑ Marsden, Stephen P; Ducept, Pascal C (2005). "Synthesis of highly substituted allenylsilanes by alkylidenation of silylketenes". Beilstein Journal of Organic Chemistry . 1: 5. doi: 10.1186/1860-5397-1-5 . PMC   1399453 . PMID   16542018.
  13. 1 2 3 4 Mehrotra, R. C.; Singh, A. (2000). "4.3.6 η5-Cyclopentadienyl d-Block Metal Complexes". Organometallic Chemistry: A Unified Approach (2nd ed.). New Delhi: New Age International Publishers. pp. 243–268. ISBN   9788122412581.
  14. 1 2 Troyanov, Sergei I.; Antropiusová, Helena; Mach, Karel (1992). "Direct proof of the molecular structure of dimeric titanocene; The X-ray structure of μ(η5:η5-fulvalene)-di-(μ-hydrido)-bis(η5-cyclopentadienyltitanium)·1.5 benzene". J. Organomet. Chem. 427 (1): 49–55. doi:10.1016/0022-328X(92)83204-U.
  15. 1 2 Wailes, P. C.; Coutts, R. S. P.; Weigold, H. (1974). "Titanocene". Organometallic Chemistry of Titanium, Zirconium, and Hafnium. Organometallic Chemistry. Academic Press. pp. 229–237. ISBN   9780323156479.
  16. ↑ Antropiusová, Helena; Dosedlová, Alena; Hanuš, Vladimir; Karel, Mach (1981). "Preparation of μ-(η5:η5-Fulvalene)-di-μ-hydrido-bis(η5-cyclopentadienyltitanium) by the reduction of Cp2TiCl2 with LiAlH4 in aromatic solvents". Transition Met. Chem. 6 (2): 90–93. doi:10.1007/BF00626113. S2CID   101189483.
  17. ↑ Cuenca, Tomas; Herrmann, Wolfgang A.; Ashworth, Terence V. (1986). "Chemistry of oxophilic transition metals. 2. Novel derivatives of titanocene and zirconocene". Organometallics . 5 (12): 2514–2517. doi:10.1021/om00143a019.
  18. ↑ Chirik, Paul J. (2010). "Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years". Organometallics . 29 (7): 1500–1517. doi:10.1021/om100016p.
  19. 1 2 Hartley, Richard C.; Li, Jianfeng; Main, Calver A.; McKiernan, Gordon J. (2007). "Titanium carbenoid reagents for converting carbonyl groups into alkenes". Tetrahedron . 63 (23): 4825–4864. doi:10.1016/j.tet.2007.03.015.
  20. ↑ Rosales, Antonio; Rodríguez-Garcia, Ignacio; Muñoz-Bascón, Juan; Roldan-Molina, Esther; Padial, Natalia M.; Morales, Laura P.; García-Ocaña, Marta; Oltra, J. Enrique (2015). "The Nugent Reagent: A Formidable Tool in Contemporary Radical and Organometallic Chemistry". Eur. J. Org. Chem. 2015 (21): 4567–4591. doi:10.1002/ejoc.201500292.
    This review article was corrected to refer to the "Nugent–RajanBabu Reagent" rather than the "Nugent Reagent" by:
    Rosales, Antonio; Rodríguez-Garcia, Ignacio; Muñoz-Bascón, Juan; Roldan-Molina, Esther; Padial, Natalia M.; Morales, Laura P.; García-Ocaña, Marta; Oltra, J. Enrique (2015). "The Nugent–RajanBabu Reagent: A Formidable Tool in Contemporary Radical and Organometallic Chemistry". Eur. J. Org. Chem. 2015 (21): 4592. doi:10.1002/ejoc.201500761.
  21. ↑ Handa, Yuichi; Inanaga, Junji (1987). "A highly stereoselective pinacolization of aromatic and α, β-unsaturated aldehydes.dta mediated by titanium(III)-magnesium(II) complex". Tetrahedron Lett. 28 (46): 5717–5718. doi:10.1016/S0040-4039(00)96822-9.
  22. ↑ Nugent, William A.; RajanBabu, T. V. (1988). "Transition-metal-centered radicals in organic synthesis. Titanium(III)-induced cyclization of epoxy olefins". J. Am. Chem. Soc. 110 (25): 8561–8562. doi:10.1021/ja00233a051.
  23. ↑ Jungst, Rudolph; Sekutowski, Dennis; Davis, Jimmy; Luly, Matthew; Stucky, Galen (1977). "Structural and magnetic properties of di-μ-chloro-bis[bis(η5-cyclopentadienyl)titanium(III)] and di-μ-bromo-bis[bis(η5-methylcyclopentadienyl)titanium(III)]". Inorg. Chem. 16 (7): 1645–1655. doi:10.1021/ic50173a015.
  24. ↑ Blackburn, David W.; Britton, Doyle; Ellis, John E. (1992). "A New Approach to Bis(arene)titanium(0) and -titanium(–I) Complexes; Structure of Bis(arene)titanates(1–)". Angewandte Chemie International Edition in English. 31: 1495–1498. doi:10.1002/anie.199214951.
  25. ↑ Calderazzo, Fausto; Ferri, Isabella; Pampaloni, Guido; Englert, Ulli; Green, Malcolm L. H. (1997). "Synthesis of [Ti(η6-1,3,5-C6H3iPr3)2][BAr4] (Ar = C6H5, p-C6H4F, 3,5-C6H3(CF3)2), the First Titanium(I) Derivatives". Organometallics. 16: 3100–3101. doi:10.1021/om970155o.
  26. ↑ Ellis, J. E. (2003). "Metal Carbonyl Anions: from [Fe(CO)4]2− to [Hf(CO)6]2− and Beyond". Organometallics . 22 (17): 3322–3338. doi:10.1021/om030105l.
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