[1271-66-5]  · C12H16Ti  · Bis(cyclopentadienyl)dimethyltitanium  · (MW 208.14)

(reagent of low basicity and acidity, useful for the methylenation of carbonyl compounds;2 methylenates highly enolizable ketones,2 esters2 and lactones,2 including aldonolactones3 and acid-labile substrates;4 homologs of the reagent perform carbonyl alkylidenations;5-7 reacts with alkynes8,9 and nitriles8-10 and can initiate the ring-opening metathesis polymerization of strained cyclic alkenes;11 it is a useful precursor for Ziegler-Natta polymerization catalysts,12 a cocatalyst for alkene metathesis13 and a catalyst for alkene hydrogenation14 and the dehydrogenative coupling of silanes15,16)

Alternate Name: dimethyltitanocene.

Physical Data: mp >90 °C dec.

Solubility: sol most aprotic organic solvents, e.g. Et2O, THF, CH2Cl2, toluene, hexanes.

Form Supplied in: orange crystals; not available commercially.

Analysis of Reagent Purity: 1H NMR (250 MHz, C6D6) d 0.04 (s, 6H), 5.69 (s, 10H).

Preparative Method: 17 a solution of Methyllithium (2.1 equiv) in Et2O is added dropwise to a suspension of Dichlorobis(cyclopentadienyl)titanium in Et2O (3-5 mL mmol-1) cooled in an ice bath. After 1 h the reaction mixture is quenched with ice water, the ether layer is separated, dried over MgSO4, filtered and evaporated. The resulting bright orange crystals of the reagent, obtained in 95% yield, are dissolved in THF or toluene and stored in the dark, in the refrigerator or freezer.

Handling, Storage, and Precautions: sensitive to oxygen and light, but can be briefly exposed to air and water. In the solid state the reagent is not stable for more than a few hours at rt, but it can be stored for months in the dark as a THF or toluene solution. Although it is stable in solution at rt, the reagent is best stored in a refrigerator.

Methylenation of Carbonyl Compounds.2

Although the Wittig reagent18 CH2=PPh3 (see Methylenetriphenylphosphorane) and several other P-, Si-, or S-stabilized methylene anions19 are commonly used for the methylenation of aldehydes and ketones, these reagents have several synthetic limitations due to their inherent basicity. Wittig-type alkenations generally fail with readily enolizable carbonyls or substrates that undergo a facile nucleophilic addition or elimination. Furthermore, sterically hindered substrates are often methylenated in low yields, while the alkenation of esters and lactones is usually not possible with these methods. Several Ti-based methylenation procedures1c have been developed that avoid most of the problems encountered with Wittig-type processes. The combination of Dibromomethane-Zinc-Titanium(IV) Chloride,20 particularly Lombardo's reagent,21 is useful for the methylenation of sterically hindered and highly enolizable aldehydes and ketones. The similar reagent CH2I2-Zn-TiCl422 is also used for this purpose, while two other variations, CH2I2-Zn-Ti(O-i-Pr)4 and CH2I2-Zn-Me3Al, allow the chemoselective methylenation of aldehydes in the presence of ketones.23 Another widely used reagent is the Tebbe reagent (see m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium),1b,1c,24,25 which is useful for the methylenation of a variety of carbonyls, including aldehydes, enolizable ketones, esters and lactones. This reagent, however, is not suitable for highly acid-labile substrates and requires stringent inert atmosphere techniques due to its high sensitivity to air and water.

Dimethyltitanocene is an experimentally convenient alternative to the Tebbe reagent.2 It is easier to prepare and handle and is suitable for the methylenation of aldehydes, ketones, esters, and lactones, including highly enolizable and acid-labile substrates. Heating a solution of a carbonyl compound and Cp2TiMe2 (1-3 equiv) in the dark at 60-80 °C, in a sealed flask or under argon, results in a clean and efficient methylenation. Unlike the reactions of the Tebbe reagent, this procedure does not require aqueous workup. The product can be isolated by precipitation of the yellow titanium oxide byproduct, filtration and purification by chromatography or distillation. Although the yields for the methylenation of aldehydes are moderate (eq 1),2 the reaction works well for alkyl and aryl ketones (eq 2),2 as well as for cyclic ketones (eq 3).26

Aldehydes (eq 4)27 and ketones (eq 5)2 are methylenated faster than esters or lactones. Due to its suppressed basicity and similarity to the Tebbe reagent28 and Takai's procedure,22 Cp2TiMe2 also methylenates highly enolizable ketones (eq 6).2

Esters (eq 7)2 and lactones (eq 8),29 including aldonolactones (eq 9),3 are efficiently methylenated with this reagent. Since the resulting enol ether products undergo facile acid-catalyzed hydrolysis, their isolation and purification should be done under basic conditions. Highly acid-sensitive substrates, such as spiroketal lactones (eq 10)4 which decompose with the Tebbe reagent, can be readily methylenated with Cp2TiMe2.

Alkylidenations of Carbonyl Compounds.

While homologs of the Tebbe reagent are difficult to prepare, the corresponding dialkyltitanocene derivatives are readily prepared from Cp2TiCl2 and alkyllithium or Grignard reagents. With the exception of compounds that undergo facile b-hydride elimination, dialkyltitanocenes are adequately stable for isolation in a pure form. Thermolysis of such derivatives in the presence of aldehydes, ketones, esters, lactones, or amides, results in carbonyl alkylidenation. Thus dibenzyltitanocene gives benzylidenation products (eq 11),5 bis(trimethylsilylmethyl)titanocene forms vinylsilanes (eq 12),6 while bis(cyclopropyl)titanocene forms cyclopropylidenes (eq 13).7

Formation of Titanium Enolates with (C5Me5)2TiMe2.

The pentamethyl-Cp derivative (C5Me5)2TiMe2 behaves differently from Cp2TiMe2. Upon thermolysis it forms a reactive titanium methylidene species which undergoes insertion into one of the Cp methyl groups.30 Rather than carrying out methylenations, (C5Me5)2TiMe2 reacts with ketones to form titanium enolates with a high degree of regio- and stereocontrol (eq 14).31 It also forms similar enolates from epoxides (eq 15).32

Reactions with Alkynes.

Although addition-elimination pathways are also possible,2,8,9 it is likely that the thermolysis of Cp2TiMe2 generates the highly reactive intermediate Cp2Ti=CH2, which can be trapped by various substrates. In the case of alkynes, clean quantitative conversions to the corresponding titanacyclobutenes take place.8,9 Titanacyclobutenes33 can also be prepared with the Tebbe reagent, and they react with aldehydes, ketones, nitriles, and other electrophiles to give a variety of titanium-free products (eq 16).8 Interestingly, the reaction of Cp2TiMe2 with alkynes under photolytic conditions forms titanacyclopentadienyl derivatives,34 presumably via a titanocene (Cp2Ti) intermediate.

Reactions with Nitriles.

Thermolysis of Cp2TiMe2 in the presence of nitriles (2 equiv) generates 1,3-diazatitanacycles,8-10 which can be hydrolyzed to 4-amino-1-azadienes (eq 17).10

Ring-Opening Metathesis Polymerization.

Norbornene readily undergoes ROMP with Cp2TiMe2 and other dialkyltitanocenes (eq 18).11

Alkene Metathesis.

Among other related systems, the combination of Cp2TiMe2 with WOCl4 shows the best catalytic activity for alkene metathesis (eq 19).13

Ziegler-Natta Polymerization of Alkenes.

In combination with Al compounds or other acids,35 dialkyltitanocenes generate cationic titanocene complexes12 which catalyze the Ziegler-Natta polymerization of alkenes (eq 20).36-38

Hydrogenation of Alkenes.

A variety of dicyclopentadienyl Ti derivatives, including Cp2TiMe2, can catalyze the hydrogenation of alkenes, presumably via a Ti-H species (eq 21).14

Reactions with Silanes.

In the presence of catalytic amounts of Cp2TiMe215 or other similar derivatives,16,39 hydrosilanes undergo dehydrogenative coupling to form oligomers and polymers (eq 22).15 Alkene hydrogenation, isomerization, or hydrosilylation occurs when an alkene is present,40,41 while in ammonia a different type of dehydrocoupling takes place (eq 23).42

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Nicos A. Petasis

University of Southern California, Los Angeles, CA, USA

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