[1273-09-2]  · C22H20Ti  · Bis(cyclopentadienyl)diphenyltitanium  · (MW 332.28)

(reagent for the preparation of aryl derivatives via thermolysis and trapping with electrophiles;2 catalyst for the hydrogenation of alkenes,3 the polymerization of alkenes,4,5 the polymerization of silanes,6 the hydrosilylation of ketones,7 the silylation of alcohols8 and aldol condensation9)

Alternate Name: diphenyltitanocene.

Physical Data: mp 146-148 °C dec.

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

Form Supplied in: orange-yellow crystals; not available commercially.

Preparative Method: 10,11 a solution of Phenyllithium (2 equiv) in Et2O is added dropwise to a suspension of Dichlorobis(cyclopentadienyl)titanium in Et2O (3-5 mL mmol-1) stirred at rt under N2. After 1 h, the reaction mixture is cooled to -30 °C and quenched with methanol. Evaporation of the solvents upon warming to rt, addition of Et2O, filtration, evaporation, and recrystallization from CH2Cl2/pentane gives 95% of orange-yellow crystals of Cp2TiPh2.

Handling, Storage, and Precautions: sensitive to oxygen and light but can be briefly exposed to air and water; not stable for more than a few days at rt, and decomposes more rapidly in solution; can be stored in the dark in a refrigerator.

Bis(cyclopentadienyl)titanium Derivatives.

The presence of p-bonded cyclopentadienyl substituents on titanium results in a lower Lewis acidity and enhanced thermal stability.1 A variety of bis(cyclopentadienyl)titanium (Cp2Ti, titanocene) complexes can be easily prepared from Cp2TiCl2, which is commercially available and inexpensive. Among the most widely used C-substituted compounds are the dimethyl (see Bis(cyclopentadienyl)dimethyltitanium) and diphenyl derivatives. In general, diaryltitanocenes are less reactive and more thermally stable than the corresponding dimethyl compounds.

Thermolytic Reactions.

Upon heating at high temperatures, diaryltitanocenes undergo extrusion of benzene and formation of a titanocene-benzyne complex, which reacts with alkynes2,12 to form a benzotitanole adduct. Subsequent reaction of this intermediate with electrophiles gives various titanium-free products (eq 1).2 Analogous chemistry occurs with the more synthetically versatile zirconocene complexes,13 while different products are obtained under photochemical conditions.14 The titanocene-benzyne complex can also be trapped with dinitrogen to form anilines in low yields,15,16 as well as with carbon dioxide to form benzoic esters upon methanolysis (eq 2).17

Reactions with Alkenes.

Diaryl titanocenes, like their dimethyl analogs, can serve as catalysts for the hydrogenation of alkenes under thermal3 or photochemical18 conditions. Also, the parent compound, (Cp2TiPh2),4,5 as well as various analogs with other Cp ligands,5,19 are useful catalyst components for the isotactic polymerization of propene (eq 3).5

Reactions with Silanes.

Similarly to Cp2TiMe2 and other related derivatives,20 Cp2TiPh2 catalyzes the dehydrogenative polymerization of hydrosilanes (eq 4).6 It also catalyzes the hydrosilylation of ketones (eq 5)7 and the silylation of alcohols (eq 6) and diols (eq 7).8

Aldol Condensation.

Cp2TiPh2 is a good catalyst for the self-condensation of aliphatic aldehydes and the mixed aldol condensation of acetophenones with aromatic aldehydes (eq 8).9

1. (a) Wailes, P. C.; Coutts, R. S. P.; Weigold, H. Organometallic Chemistry of Titanium, Zirconium and Hafnium; Academic: New York, 1974. (b) Bottrill, M.; Gavens, P. D.; Kelland, J. W.; McMeeking, J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 3, pp 331-432.
2. Masai, H.; Sonogashira, K.; Hagihara, N. BCJ 1968, 41, 750.
3. Cuenca, T.; Flores, J. C.; Royo, P. JOM 1993, 462, 191.
4. Ewen, J. A. JACS 1984, 106, 6355.
5. Erker, G.; Korek, U.; Petrenz, R.; Rheingold, A. L. JOM 1991, 421, 215.
6. Nakano, T.; Nakamura, H.; Nagai, Y. CL 1989, 83.
7. Nakano, T.; Nagai, Y. CL 1988, 481.
8. Nakano, T.; Nagai, Y. Chem. Express 1990, 5, 21.
9. Nakano, T.; Motegi, Y.; Nagai, Y. Chem. Express 1993, 8, 297.
10. Summers, L.; Uloth, R. H.; Holmes, A. JACS 1955, 77, 3604.
11. Tung, H.-S.; Brubaker, C. H., Jr. ICA 1981, 52, 197.
12. Rausch, M. D.; Mintz, E. A. JOM 1980, 190, 65.
13. Buchwald, S. L.; Nielsen, R. B. CRV 1988, 88, 1047.
14. Rausch, M. D.; Boon, W. H.; Mintz, E. A. JOM 1978, 160, 81.
15. Berkovich, E. G.; Shur, V. B.; Vol'pin, M. E.; Lorenz, B.; Rummel, S.; Walren, M. CB 1980, 113, 70.
16. Shur, V. B.; Berkovich, E. G.; Vasiljeva, L. B.; Kudryavtsev, R.; Vol'pin, M. E. JOM 1974, 78, 127.
17. Grigoryan, M. K.; Kolomnikov, I. S.; Berkovich, E. G.; Lysyak, T. V.; Shur, V. B.; Vol'pin, M. E. IZV 1978, 5, 1177.
18. Samuel, E. JOM 1980, 198, C65.
19. Erker, G.; Fritze, C. AG(E) 1992, 31, 199.
20. Corey, J. Y. Adv. Silicon Chem. 1991, 1, 327.

Nicos A. Petasis

University of Southern California, Los Angeles, CA, USA

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