Chloro(cyclopentadienyl)dimethyltitanium1

[99279-72-8]  · C7H11ClTi  · Chloro(cyclopentadienyl)dimethyltitanium  · (MW 178.52)

(heteroannulation agent of alkynylamines;2 catalyst for metathetic ring-opening of cyclic alkene;3 thermally stable methylating agent4)

Physical Data: orange crystals; generally prepared in situ.

Analysis of Reagent Purity: 1H NMR (CDCl3) d 6.48 (Cp) and 1.39 (Me).

Preparative Methods: to a solution of Trichloro(cyclopentadienyl)titanium (13.7 mmol) in toluene (50 mL) is added slowly (about 10 min) a 2.6M solution of methylmagnesium chloride (1.7 equiv, 23.9 mmol) in THF at rt. The resulting mixture is stirred for 10 min and the precipitate allowed to settle for 1 h. After the solvent is removed in vacuo at 0 °C, the remaining CpTiMe2Cl is distilled out to a cold trap at -78 °C by warming the flask to room temperature. Yield of CpTiMe2Cl as orange crystals is 42% based on CpTiCl3. The product is sometimes contaminated with a trace amount of CpTiMe3. This contaminant can be converted to CpTiMe2Cl by adding the appropriate amount of CpTiCl3 in solution and stirring until equilibration.4 For the sake of convenience, it is highly recommended that CpTiMe2Cl be generated in situ by treating CpTiCl3 with 2 equiv of Methyllithium in THF at -50 °C and stirring the mixture at -50 °C to 0 °C for 1 h.3

Handling, Storage, and Precautions: the isolated solid is moisture sensitive and should be stored under an inert atmosphere in a refrigerator.

Heteroannulation.

This reagent generated in situ can be used for the heteroannulation of alkynylamine derivatives through formation of metal-imido complexes (CpTiCl=NR) to give heterocyclic compounds. These reactions are the first examples of intramolecular [2 + 2] cycloaddition involving Group 4 metal-imido complexes (eq 1).2 It is worth noting that the generation of CpMX=NR complexes from the precursors CpTiMe2X and H2NR proceeds rapidly at <=25 °C. Treatment of the alkynylamine with CpTiMe2Cl gives the putative azametalletine intermediate with concomitant evolution of methane. Similar intramolecular [2 + 2] reaction can be effected by using catalytic amounts of CpTiCl32a,b and has been applied to the synthesis of an alkaloid.2b The intermediate azametalletines can serve as conventional organometallics in electrophilic substitution reactions leading to selective carbon or nitrogen functionalization. Thus the direct trapping of the intermediate with isobutyronitrile followed by protonation with aqueous HCl gives the vinylogous amidine (eq 1).

Metathesis Catalyst.

For the ring-opening metathesis polymerization (ROMP) of cyclic alkenes, CpTiMe2Cl is an effective initiator (eq 2).3b It is also found that CpTiMe2Cl initiates the polymerization at a lower concentration and lower temperature compared to Bis(cyclopentadienyl)dimethyltitanium (dimethyltitanocene). Furthermore, the use of THF as solvent has no effect on this polymerization initiator while with other zirconium catalysts (Cp2ZrMe2, etc.) initiation activities are inhibited by THF. This substituent-dependent solvent effect is attributed to changes in the relative contributions of nucleophilic and neutral resonance structures. For this polymerization it is presumed to involve the in situ generation of free or complexed titanium alkylidene species.

Alkylation.

Reactions of CpTiMe2Cl (which possesses significantly greater thermal stability) with carbonyl compounds proceed similarly to other alkyltitanium reagents (eq 3). That is, in the reaction of a 1:1 ratio of CpTiMe2Cl and acetophenone, the methyl addition product along with a small amount of the gem-dimethylated product, t-butylbenzene, is obtained.4

Related Reagents.

Bis(cyclopentadienyl)bis(2,6-dimethylphenylamino)zirconium; Bis(cyclopentadienyl)dimethyltitanium; Trichloro(cyclopentadienyl)titanium.


1. For a review of related titanium complexes, see Bottrill, M.; Gavens, P. D.; McMeeking, J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 3, pp 281-329.
2. (a) McGrane, P. L.; Jensen, M.; Livinghouse, T. JACS 1992, 114, 5459. (b) McGrane, P. L.; Livinghouse, T. JOC 1992, 57, 1323.
3. (a) Giannini, U.; Cesca, S. TL 1960, 19. (b) Petasis, N. A.; Fu, D.-K. JACS 1993, 115, 7208.
4. Erskine, G. J.; Hunter, B. K.; McCowan, J. D. TL 1985, 26, 1371.

Takeo Taguchi & Yuji Hanzawa

Tokoyo College of Pharmacy, Japan



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