Bis(cyclopentadienyl)methylzirconium Tetraphenylborate-Tetrahydrofuran1

[100909-50-0]  · C39H41BOZr  · Bis(cyclopentadienyl)methylzirconium Tetraphenylborate-Tetrahydrofuran  · (MW 627.78)

(alkene polymerization agent;2 can insert carbon monoxide, nitriles, and alkynes;3 can function as a Lewis acid4)

Solubility: sol CH2Cl2 and 1,2-dichloroethane; reacts with protic solvents.

Analysis of Reagent Purity: 1H NMR (CD2Cl2) d 7.6-6.7 (m, 20H, BPh4-), 6.31 (s, 10H), 3.44 (m, 4H, THF), 1.80 (m, 4H, THF), 0.74 (s, 3H).

Preparative Methods: treatment of Cp2ZrMe2 with 1 equiv of AgBPh4 in MeCN followed by recrystallization from THF gives Cp2ZrMe(THF)+BPh4-.3a Alternatively, it can be obtained directly by treating Cp2ZrMe2 with [HNBu3][BPh4] or [Cp´2Fe][BPh4] (Cp´ = C5H4Me) in THF.5

Purification: recrystallization from hot THF gives a relatively stable pale yellow crystalline solid.

Handling, Storage, and Precautions: the dry solid is highly sensitive to moisture and oxygen and should be stored in a dry box; a coordinating solvent, such as MeCN, causes ligand exchange.

Ligand.

Although complexes having ligands other than THF are known,3a most descriptions focus on the Lewis acidic THF complex due to there being abundant examples of its use in organic synthesis. The THF-free species Cp2ZrMe+, generated as a transient intermediate in CH2Cl2 solution, decomposes, principally to Cp2MeCl.2a The THF ligand is labile and generally undergoes rapid exchange with nitriles, small phosphines, pyridine, and other heterocycles. In general, the cationic Cp2ZrMe(THF)+ complex is more reactive than neutral Cp2ZrMeCl or Cp2ZrMe2 complexes as a result of the increased unsaturation and charge.

Insertion Reaction.

The high insertion reactivity of this complex has been noted and representative insertions are summarized in eqs 1-4.

The Zr-CH3 bond of this complex in THF can be cleaved by H2 to yield the insoluble hydride Cp2Zr(H)(THF)+BPh4-.6 The reaction of the cationic methyl complex with stoichiometric amounts of a-picoline yields the h2-pyridyl complex, Cp2Zr(h2-N,C-picolyl)(THF)+, via initial ligand substitution followed by rapid C-H activation/CH4 elimination (t1/2 6 min, 23 °C, CH2Cl2) (eq 5).7

Interestingly, use of pyridine itself causes a much slower reaction (hours, 50 °C, CH2Cl2). The acceleration of C-H activation by the a-methyl group of a-picoline is considered to be due to steric crowding which enhances the overlap of the ortho C-H bond with the Zr LUMO.8 The C-H activation of a variety of N-heterocyclic compounds9 can be achieved in CH2Cl2 as shown in eqs 6-8. These metallacycles react with alkenes and alkynes to give stable five- or six-membered metallacycles which can be converted to a-substituted heterocycles upon hydrolysis (eqs 9-11).7a,9,10 The formation of these metallacycles was suggested to be both kinetically and thermodynamically favored.10a More importantly, alkenes and ortho-substituted pyridines are coupled by catalytic amounts of Cp2Zr(Me)+ in 1,2-dichloroethane under an H2 atmosphere (eq 12).7b

Lewis Acid Catalyst.

Treatment of Cp2Zr(Me)(THF)+BPh4- with t-BuOH gives Cp2Zr(O-t-Bu)(THF)+ (eq 13), which is an effective catalyst in the Diels-Alder (eq 14) and Mukaiyama aldol reactions (eq 15).4


1. (a) Jordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. (b) Jordan, R. F. J. Chem. Educ. 1988, 65, 285.
2. (a) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, B. JACS 1986, 108, 7410. (b) Collins, S.; Ward, D. G. JACS 1992, 114, 5460.
3. (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. JACS 1986, 108, 1718. (b) Bochmann, M.; Wilson, L. M. CC 1986, 1610. (c) Guram, A. S.; Guo, Z.; Jordan, R. F. JACS 1993, 115, 4902.
4. (a) Hong, Y.; Norris, D. J.; Collins, S. JOC 1993, 58, 3591. (b) Hong, Y.; Kuntz, B. A.; Collins, S. OM 1993, 12, 964. (c) Collins, S.; Koene, B. E.; Ramachandran, R.; Taylor, N. J. OM 1991, 10, 2092.
5. Borkowsky, S. L.; Jordan, R. F.; Hinch, G. D. OM 1991, 10, 1268.
6. Jordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L. OM 1987, 6, 1041.
7. (a) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. OM 1990, 9, 1546. (b) Jordan, R. F.; Taylor, D. F. JACS 1989, 111, 778.
8. (a) Cheney, A. J.; Mann, B. E.; Shaw, B. L.; Slade, R. M. CC 1970, 1176. (b) Buchwald, S. L.; Lum, R. T.; Fisher, R. A.; Davis, W. M. JACS 1989, 111, 9113.
9. Jordon, R. F.; Guram, A. S. OM 1990, 9, 2116.
10. (a) Guram, A. S.; Jordan, R. F. OM 1990, 9, 2190. (b) Guram, A. S.; Jordan, R. F.; Taylor, D. F. JACS 1991, 113, 1833. (c) Guram, A. S.; Jordan, R. F. OM 1991, 10, 3470. (d) Guram, A. S.; Jordan, R. F. JOC 1992, 57, 5994.

Takeo Taguchi & Yuji Hanzawa

Tokyo College of Pharmacy, Japan



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