Tebbe's reagent is the

organometallic compound Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and ...

with the formula (C₅H₅)₂TiCH₂ClAl(CH₃)₂. It is used in the methylidenation of

carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...

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 A substance is pyrophoric (from , , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolithium compounds and triethylb ...

in the air, and thus is typically handled with

air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less com ...

s. It was originally synthesized by Fred Tebbe at

DuPont Central Research In 1957, the research organization of the Chemicals Department of E. I. du Pont de Nemours and Company was renamed Central Research Department, beginning the history of the premier scientific organization within DuPont and one of the foremost indu ...

. Tebbe's reagent contains two

tetrahedral In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

metal centers linked by a pair of

bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...

s. The titanium has two

cyclopentadienyl Cyclopentadienyl can refer to * Cyclopentadienyl anion, or cyclopentadienide, ** Cyclopentadienyl ligand * Cyclopentadienyl radical, • * Cyclopentadienyl cation, See also * Pentadienyl {{Chemistry index ...

(, or Cp) rings and aluminium has two methyl groups. The titanium and aluminium atoms are linked together by both a

methylene bridge In chemistry, a methylene bridge is part of a molecule with formula . The carbon atom is connected by single bonds to two other distinct atoms in the rest of the molecule. A methylene bridge is often called a methylene group or simply methylene, ...

(-CH₂-) and a chloride atom in a nearly square-planar (Ti–CH₂–Al–Cl) geometry. The Tebbe reagent was the first reported compound where a methylene bridge connects a transition metal (Ti) and a main group metal (Al).Herrmann, W.A., "The Methylene Bridge" ''Advances in Organometallic Chemistry'' 1982, ''20'', 195–197.

Preparation

The Tebbe reagent is synthesized from

titanocene dichloride Titanocene dichloride is the organotitanium compound with the formula (hapticity, ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid t ...

and

trimethylaluminium Trimethylaluminium or TMA is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula (abbreviated as , where Me stands for methyl), as it exists as a dimer. This colorless liquid is pyrophoric. It is an ...

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...

solution.Straus, D. A., "μ-Chlorobis(cyclopentadienyl)(dimethylaluminium)-μ-methylenetitanium": ''Encyclopedia of Reagents for Organic Synthesis.'' John Wiley, London, 2000. ::Cp₂TiCl₂ + 2 Al(CH₃)₃ → CH₄ + Cp₂TiCH₂AlCl(CH₃)₂ + Al(CH₃)₂Cl After about 3 days, the product is obtained after recrystallization to remove Al(CH₃)₂Cl. Although syntheses using the isolated Tebbe reagent give a cleaner product, successful procedures using the reagent "in situ" have been reported. Instead of isolating the Tebbe reagent, the solution is merely cooled in an ice bath or dry ice bath before adding the starting material. An alternative but less convenient synthesis entails the use of dimethyltitanocene (Petasis reagent): ::Cp₂Ti(CH₃)₂ + Al(CH₃)₂Cl → Cp₂TiCH₂AlCl(CH₃)₂ + CH₄ One drawback to this method, aside from requiring Cp₂Ti(CH₃)₂, is the difficulty of separating product from unreacted starting reagent.

Reaction mechanism

Tebbe's reagent itself does not react with carbonyl compounds, but must first be treated with a mild

Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

, such as

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...

, which generates the active

Schrock carbene A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthe ...

. : Also analogous to the Wittig reagent, the reactivity appears to be driven by the high

oxophilicity Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early trans ...

of Ti(IV). The Schrock carbene (1) reacts with carbonyl compounds (2) to give a postulated oxatitanacyclobutane intermediate (3). This cyclic intermediate has never been directly isolated, presumably because it breaks down immediately to the produce the desired

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

(5). :

Scope

The Tebbe reagent is used in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

for carbonyl methylidenation. Pine, S. H. ''Org. React.'' 1993, ''43'', 1. (Review)Beadham, I.; Micklefield, J. ''Curr. Org. Synth.'' 2005, ''2'', 231–250. (Review) This conversion can also be effected using the

Wittig reaction The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide called a Wittig reagent. Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. Most o ...

, although the Tebbe reagent is more efficient especially for sterically encumbered carbonyls. Furthermore, the Tebbe reagent is less basic than the Wittig reagent and does not give the β-elimination products. Methylidenation reactions also occur for

aldehydes In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

as well as

esters In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...

lactones Lactones are cyclic carboxylic esters. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corre ...

and

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

s. The Tebbe reagent converts esters and lactones to enol ethers and amides to enamines. In compounds containing both ketone and ester groups, the ketone selectively reacts in the presence of one equivalent of the Tebbe reagent. : The Tebbe reagent methylidenates carbonyls without racemizing a

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

α carbon. For this reason, the Tebbe reagent has found applications in reactions of sugars where maintenance of

stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...

can be critical. The Tebbe reagent reacts with

acid chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

s to form titanium enolates by replacing Cl⁻. :

Modifications

It is possible to modify Tebbe's reagent through the use of different ligands. This can alter the reactivity of the complex, allowing for a broader range of reactions. For example,

cyclopropanation In organic chemistry, cyclopropanation refers to any chemical process which generates cyclopropane () Ring (chemistry), rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroid insectic ...

can be achieved using a chlorinated analogue. :

References

{{Aluminium compounds Reagents for organic chemistry Titanocenes Organoaluminium compounds Chloro complexes Titanium(IV) compounds

Preparation

Reaction mechanism

Scope

Modifications

See also

Related organotitanium reagents and reactions

Related methylidenation reactions

References