Preparation of Zirconacyclopentadienes

The first reported zirconacydopentadiene was 2,3,4,5-tetraphenyl-l-zirconacyclopenta-diene (la), prepared from two molecules of diphenylacetylene and a low-valent zircono-cene [3]. The low-valent zirconocene spedes was produced by the reaction of Cp2ZrCl2 with sodium/naphthalene (Eq. 2.1). 

There are several methods for generating low-valent zirconocene species in situ. Although the conventional method has been the reduction of Cp2ZrCl2 with Na/Hg or Mg/Hg, more convenient methods have since been developed for the preparation of symmetrical zirconacyclopentadienes such as Cp2ZrBu2 2 [the so-called Negishi reagent (Eq. 2.2)] [12]. 

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

In order to prepare very clean unsymmetrical zirconacyclopentadienes, the use of ethene is a prerequisite [14] (Eq. 2.4). An excess of ethene stabilizes the intermediates such as zirconacydopentane 5a and zirconacyclopentene 4. Such a transformation from a metallacyclopentane to a metallacyclopentene was first demonstrated by Erker in the case of the hafnium analogues [15]. 

Hydrozirconation of alkynes with the Schwartz reagent, Cp2Zr(H)Cl (6), and subsequent methylation is also a general method (Eq. 2.5) [16]. 

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Highly functionalized benzenes and naphthalenes have been prepared by cycloaddition of zirconacyclopentadiene 32 and its benzoderivative 33 [38] with... 

Hydrolysis of zirconacyclopentadienes provides, in a stereocontrolled manner, 1,2,3,4-tetrasubstituted dienes 13. In particular, unsymmetrical diene derivatives 14 can be prepared by this method (Eq. 2.10) [14]. 

Preparation and Reaction of Zirconacyclopentadienes Table 2.1. Cross-coupling reactions of alkynes on zirconocene... 

As mentioned above, for more than 20 years after the first preparation of zirconacyclopen-tadiene, no systematic carbon—carbon bond-forming reactions were investigated. The major reason was the low nucleophilicity of the zirconacyclopentadienes. Indeed, such was the reputation of zirconacyclopentadienes in the 1980s that they were referred to as dead-end compounds . This statement clearly emphasizes the assumption that zirconacyclopentadienes were completely inert with regard to the creation of carbon—carbon bonds. In this context, transmetalation of zirconacyclopentadienes to copper and subsequent carbon—carbon bond formation represents a milestone in zirconacyclopentadiene chemistry. 

Eq. 2.41. Preparation of a tricyclic cyclooctatetraene from a bicyclic zirconacyclopentadiene and a diiododiene. 

Eq. 2.51. Preparation of a benzene derivative from a zirconacyclopentadiene in the presence of NiX2(PPh3)2. 

Dibromoterphenyl compounds (72i) can be prepared from zirconacyclopentadienes. They polymerize in the presence of a Ni catalyst to afford polyphenylene derivatives 87, as shown in Eq. 2.58 [42]. 

For a long time, zirconacyclopentadiene was considered to be inert towards carbon-carbon bond formation. However, through transmetalation to Cu, Ni, Zn, Li, and Al, various kinds of carbon—carbon bond-formation methodologies have been widely developed. One major advantage of this chemistry is that zirconacyclopentadienes can be conveniently prepared in situ, in high yields and with excellent selectivities, from two different alkynes. Combination of the selective formation of zirconacyclopentadienes and the chemistry of Cu, Ni, Zn, Li, or Al provides useful tools in organic synthesis. In the near future, it can be expected that more metals will be used for the transmetalation reactions of zirconacycle derivatives and some catalytic reactions will hopefully be developed. 

General Procedure for the Formation of Benzene Derivatives (see Eq. 2.48) At 0°C, dimethyl acetylenedicarboxylate (284 mg, 2 mmol) and CuCl (198 mg, 2 mmol) were added to a solution of zirconacyclopentadiene (1 mmol) in THF, prepared in situ according to the known procedure [12]. The reaction mixture was then allowed to warm to room temperature and was stirred for 1 h. After hydrolysis with 3 n HC1, the mixture was extracted with diethyl ether. The combined extracts were washed sequentially with water, aq. NaHC03 solution, brine, and water, and then dried over MgS04. Concentration in vacuo followed by flash-chromatography eluting with a mixture of hexane and diethyl ether (10 %) afforded benzene derivatives. 

Typical Procedure for the Preparation of Benzene Derivatives from Three Different Alkynes using NiCI2(PPh3)2 (see Eq. 2.53) To a solution of a zirconacyclopentadiene (1.0 mmol) [4] prepared from two different alkynes in THF (20 mL), the third alkyne (1.5 mmol)... 

Once single zirconacyclopentadienes 20 are formed, only the regiospecific reaction of one carbon-zirconium bond with an electrophile could lead to the stereoselective preparation of metalated dienes 26 (Scheme 13). 

Cyclopentadienes can be prepared by double Michael addition of zirconacyclopentadienes with propynoates <1997CC2069> or nucleophilic attack of zirconacyclopentadienes on acyl halides mediated by copper(l) chloride and accompanied by elimination <1995CC1503, 1996TL7521>. Tetraethylzirconacyclopentadiene with benzal chloride in THF in the presence of copper(l) chloride and DMPU yield l,2,3,4-tetraethyl-5-phenylcyclopenta-l,3-diene <2000TL7471>. A series of other similar compounds were prepared from tetra- -propylzirconacyclopenta-diene and 0 ,Q -dichlorotoluene, tetra- -butylzirconacyclopentadiene and 0 ,Q -dichlorotoluene or 0 ,Q -dibromotoluene, as well as some other combinations. Tetraethylzirconacyclopentadiene reacts with l,l-dibromo-l-alkene-3-ynes under the same conditions (copper(l) chloride and DMPU) to yield alkynylfulvenes. 

Earlier studies of the preparation of five-membered zirconacycles either do not discuss the intermediacy of three-membered zirconacycles or merely suggest their intermediacy. Such studies include those on die formation of zirconacyclopentadienes,zirconacyclopentenes,and ziicona-cyclopentanes. ... 

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