Organotitanium compound

Organotitanium compounds in organometallic chemistry contain carbon to 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][2]

Contents

Brief history

Although the first attempt at an organotitanium compound dates back to 1861, it took until 1953 for the first such compound to arrive. In that year titanium phenyl-tri(isopropoxide) was prepared by Herman and Nelson from titanium isopropoxide, phenyllithium and titanium(IV) chloride. Titanocene dichloride was invented in 1954, and the first methyltitanium compounds were introduced in 1959. Ziegler-Natta catalysts utilizing titanium-based catalysts soon followed as a major commercial application for which the Nobel Prize in Chemistry of 1963 was awarded.

Properties

The titanium electron configuration ([Ar]3d24s2) resembles that of carbon and like carbon the +4 oxidation state dominates and like carbon compounds, those of titanium have a tetrahedral molecular geometry. Thus, the boiling points of TiCl4 and CCl4 are very similar. Titanium is however a much larger element than carbon, reflected by the Ti-C bond lengthes 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 stable, owing to the large size of titanium and the electron-deficient nature of its tetrahdral complexes. More abundant and more useful than the simple tetraalkyl compounds are organic derivatives 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 Ti(IV). Ti(II) compounds are rarer, examples being titanocene dicarbonyl and Ti(CH3)2(dmpe)2. [Ti(CO)6]2- is formally a complex of Ti(-II).[3] Although Ti(III) is involved in Ziegler-Natta catalysis, the organic derivatives of Ti(III) are not common.

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 presents challenges to handling these compounds, which require air-free techniques. On the other hand, high oxophilicity means that titanium alklyls are effective for abstracting or exchanging organyl ligands for oxo groups, as discussed below.

Chemistry

Organotitanium compounds are important reagents in organic chemistry.[4] Some reagents include the following.

Titanocene derivatives

A particularly rich area of organotitanium chemistry are derivatives of titanocene dichloride. Early work on "titanocene" itself eventually revealed that this species was a fulvalene complex, and this discovery led to many innovations on cyclopentadienyl complexes of titanium.

Tebbe's reagent (1978) is prepared from titanocene dichloride and trimethylaluminium. It is used as a methylenation agent for carbonyl compounds (converstion of R2C=O to R2C=CH2). It is an alternative for Wittig reagents when the carbonyl group is sterically challenged or when it easily forms the enol.[4] 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.[9]

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.[9]

See also

CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo
↓
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac Th Pa CU Np Pu Am Cm Bk Cf Es Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown / not assessed

References

  1. ^ "Encyclopedia of Reagents for Organic Synthesis", L.A. Paquette, Ed.: J. Wiley and Sons: Sussex, England, 1996
  2. ^ "Organotitanium Reagents in Organic Synthesis (Reactivity and Structure Concepts in Organic Chemistry, Vol 24)" Manfred T. Reetz 1986 ISBN 0-387-15784-0
  3. ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
  4. ^ a b c Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 189138953X
  5. ^ Organic Syntheses, Coll. Vol. 8, p.495 (1993); Vol. 67, p.180 (1989) Link.
  6. ^ Organic Syntheses, Coll. Vol. 8, p.386 (1993); Vol. 65, p.81 (1987) Link.
  7. ^ Takai, K.; Hotta, Y.; Oshima, K.; Nozaki, H. Tetrahedron Lett. 1978, 2417-2420.
  8. ^ "Synthesis of highly substituted allenylsilanes by alkylidenation of silylketenes" Stephen P Marsden and Pascal C Ducept Beilstein Journal of Organic Chemistry 2005, 1:5 doi:10.1186/1860-5397-1-5
  9. ^ a b "Titanium carbenoid reagents for converting carbonyl groups into alkenes" Hartley, R. C.; Li, J.; Main, C. A.; McKiernan, G. J. Tetrahedron 2007, 63, 4825-4864 (Review) doi:10.1016/j.tet.2007.03.015.