[75-24-1]  · C3H9Al  · Trimethylaluminum-Dichlorobis(h5-cyclopentadienyl)titanium  · (MW 72.10) (Cl2TiCp2)

[1271-19-8]  · C10H10Cl2Ti  · Trimethylaluminum-Dichlorobis(h5-cyclopentadienyl)titanium  · (MW 248.98)

(carbometallation of alkynes and 1-metalloalkynes; conversion of monosubstituted alkenes to 1,1-disubstituted alkenes; preparation of the Tebbe reagent)

Physical Data: Me3Al: bp 125 °C; d 0.752 g cm-3. Cl2TiCp2: mp 287-289 °C; d 1.600 g cm-3.

Form Supplied in: Me3Al: liquid available neat or as solution in hexanes or toluene. Cl2TiCp2: bright-red crystals. Both are commercially available.

Preparative Methods: this reagent is obtained by mixing the two components.

Handling, Storage, and Precautions: Me3Al is a very pyrophoric compound. It must be handled under an inert atmosphere.

Controlled Carbometallation of Alkynes.

The reaction of diphenylacetylene with a 1:1 mixture of Trimethylaluminum and Dichlorobis(cyclopentadienyl)titanium, in 1,2-dichloroethane at 22 °C, produces, after 12 h, an alkenylmetal species, which after hydrolysis or iodinolysis gives (Z)-1,2-diphenylpropene or (E)-1-iodo-1,2-diphenylpropene, in 84 and 75% yields, respectively (eq 1).1 Both products are >97-98% stereoisomerically pure. The current scope of this reaction is, however, much more limited than the corresponding methylmetallation with the Trimethylaluminum-Dichlorobis(h5-cyclopentadienyl)zirconium system.2 Thus the reaction of 5-decyne produces 6-methyl-4,5-decadiene in 92% yield, rather than the expected 5-methyl-5-decene.1 Presumably the carbometallated product undergoes dehydrometallation (eq 2). Terminal alkynes also react with Me3Al-Cl2TiCp2, but the yields of carbometallated products are low.1 Nevertheless, at least in some cases the Me3Al-Cl2TiCp2 system is superior to the Me3Al-Cl2ZrCp2 system. For example, the reaction of diphenylacetylene shown in eq 1 is not only faster but also higher yielding than the corresponding reaction with Me3Al-Cl2ZrCp2.1

Carbometallation of 1-Metalloalkynes.

The reaction of 1-metalloalkynes, such as 1-octynylzinc chloride, 1-octynyldicyclohexylborane, and 1-octynyldimethylalane, with Me3Al-Cl2TiCp2 proceeds smoothly and gives, after hydrolysis, 2-methyl-1-octene in 75, 81, and 84% yield, respectively (eq 3).1 The reaction is >97-98% regioselective, and the intermediacy of 1,1-dimetalloalkenes is indicated by the formation of >95% pure 1,1-dideuterio-2-methyl-1-octene upon deuterolysis of (1a) and (1c). In the case of 1-octynyltrimethylsilane, the reaction produces an 80% yield of a 70:30 mixture of the desired methylmetallation product (2a) and an allene that can be derived from its regioisomer (2b) via dehydrometallation (eq 4).1 When 1-pentynyldimethylalane is reacted with Me3Al-Cl2TiCp2 (1:1) in CH2Cl2 at 23 °C, a 1,1-dimetalloalkene species (3) is formed in 90-100% yield. The reaction must involve addition of a Me-Ti bond to the alkyne, as the same compound can also be obtained by the reaction of the alkynylalane with preformed Cl(Me)TiCp2.3 Although (3) is formed as a single isomer, which is probably the (E) isomer, it undergoes a slow stereoisomerization to give a 60:40 mixture of (E) and (Z) isomers. The reaction of (3) with cyclohexanone or benzaldehyde gives the corresponding allenes in 83% and 67% yields, respectively (eq 5).3

Reaction with Monosubstituted Alkenes.

Monosubstituted alkenes react with 1-2 equiv of a 1:1 mixture of Me3Al and Cl2TiCp2 to give 2-methyl-1-alkenes in moderate to excellent yields (eq 6).4 The reaction tolerates the presence of ester, hydroxy, and halogens such as Br, provided that these groups are not directly attached to the sp2 carbon center. A speculative but reasonable mechanism of the reaction involves alkylation of Cl2TiCp2 with Me3Al, addition of the Ti-Me bond to an alkene, and b-dehydrotitanation.

Preparation of the Tebbe Reagent.

The reaction of Me3Al with Cl2TiCp2 mixed in a 2:1 ratio in toluene at rt for 2-3 d gives a titanium-aluminum reagent (4), commonly called the Tebbe reagent (see m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium), as reddish orange crystals in 80-90% yields (eq 7).5 This complex may be isolated, stored at low temperatures, and used all under an inert atmosphere. Alternatively, it may be generated in situ and used in the same reaction vessel without purification.6 The formation of (4) from 2 equiv of Me3Al and Cl2TiCp2 is believed to involve the following sequence of reactions. The reaction of Me3Al with Cl2TiCp2 gives Me(Cl)TiCp2 and Me2AlCl. Another molecule of Me3Al reacts with Me(Cl)TiCp2 to induce an a-hydrogen abstraction reaction assisted by Me3Al complexed with Me(Cl)TiCp2, which leads to the formation of (4) along with ClAlMe2 and methane (eq 8).7

The Tebbe reagent reacts with a variety of compounds containing p-bonds, such as alkenes, alkynes, aldehydes, ketones, esters, and amides, as summarized in Scheme 1.8

The reactive species in these reactions is believed to be Cp2Ti=CH2, which can be generated by the interaction of (4) with a Lewis base, such as 4-Dimethylaminopyridine or THF. Alternatively, 1,1-bis(h5-cyclopentadienyl)-3,3-dimethyltitanacyclobutane (5), obtainable by the reaction of (4) with isobutylene (eq 9),9 and even Cp2TiMe210 can serve as sources of Cp2Ti=CH2.

The reaction of diphenylacetylene with (4) to give (6) (eq 10)11 is noteworthy in view of the fact that the reaction of the same alkyne with freshly mixed Me3Al-Cl2TiCp2 gives the methylmetallation product (eq 1).

The methylenation of aldehydes and ketones with (4) appears to be markedly superior to the Wittig reaction in cases where the carbonyl group is hindered.12 The most noteworthy feature of the methylenation with (4) is that esters and amides, which are normally inert to the Wittig reagents, can be methylenated with (4).13

Use of Me3Al-Cl2TiCp2 in the Ziegler-Natta Polymerization.

Polymerization of alkenes with Me3Al-Cl2TiCp2 must have been attempted. However, the literature is virtually devoid of reports on such reactions. On the other hand, various other reagent systems consisting of methylaluminum and titanocene derivatives, such as Me2AlCl-Cl2TiCp2, Me3Al-Me2TiCp2-H2O, and (MeAlO)n-Me2TiCp2, have been employed in polymerizing ethylene and other alkenes.14

1. Van Horn, D. E.; Valente, L. F.; Idacavage, M. J.; Negishi, E. JOM 1978, 156, C20.
2. For a review on Cl2ZrCp2-catalyzed carboalumination, see: Negishi, E. PAC 1981, 53, 2333.
3. Yoshida, T.; Negishi, E. JACS 1981, 103, 1276.
4. Barber, J. J.; Willis, C.; Whitesides, G. M. JOC 1979, 44, 3603.
5. Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. JACS 1978, 100, 3611.
6. Cannizzo, L. F.; Grubbs, R. H. JOC 1985, 50, 2386.
7. Ott, K. C.; deBoer, E. J. M.; Grubbs, R. H. OM 1984, 3, 223.
8. Pine, S. H. OR 1993, 43, 1.
9. (a) Straus, D. A.; Grubbs, R. H. OM 1982, 1, 1658. (b) Brown-Wensley, K. A.; Buchwald, S. L.; Cannizzo, L.; Clawson, L.; Ho, S.; Meinhardt, D.; Stille, J. R.; Straus, D.; Grubbs, R. H. PAC 1983, 55, 1733.
10. Petasis, N. A.; Bzowej, E. I. JACS 1990, 112, 6392.
11. Tebbe, F. N.; Harlow, R. L. JACS 1980, 102, 6149.
12. Pine, S. H.; Shen, G. S.; Hoang, H. S 1991, 165.
13. (a) Pine, S. H.; Zahler, R.; Evans, D. A.; Grubbs, R. H. JACS 1980, 102, 3270. (b) Pine, S. H.; Pettit, R. J.; Geib, G. D.; Cruz, S. G.; Gallego, C. H.; Tijerina, T.; Pine, R. D. JOC 1985, 50, 1212.
14. Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99.

Ei-ichi Negishi & Danièle Choueiry

Purdue University, West Lafayette, IN, USA

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