Zirconacyclopentadienes

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

Takahashi and co-workers have designed a [4 + 3]-cycloaddition based on their previously reported [4 + 4]- and [4 + 5]-reactions (Sections 10.13.2.4 and 10.13.2.7) involving zirconacyclopentadienes as four-carbon components. The zirconacyclopentadienes are prepared by the coupling of two alkynes with Cp2ZrR2 (where R = Et, Bu, or... 

Based on their interest in medium-size ring synthesis, Takahashi and co-workers used a copper(l) salt to mediate the [4+ 4]-reaction of zirconacyclopentadienes 82a, 82b and 82d and l,2-bis(bromomethyl)arenes (Scheme 31).85 The reaction works with substoichiometric amounts of copper(l) chloride (10 mol%), but in general higher yields and faster reaction times are observed with 2 equiv. 

As an extension of their work on the [4+ 4]-reaction of zirconacyclopentadienes and l,2-bis(bromomethyl)arenes (Section 10.13.2.4), Takahashi and co-workers reported a [5 + 4]-reaction based on the use of l,8-bis(bromomethyl)-naphthalene as the five-carbon component.85 As with the [4 + 4]-reaction, the [5 +4]-reaction works with a catalytic amount of CuCl, but higher yields and faster reactions result when stoichiometric CuCl and 1,3-dimethyl-3,4,5,6-tetrahydro-2(l//)-pyrimidinone(iV,iV -dimethylpropyleneurea) (DMPLJ) are used (Scheme 42). 

Scheme 1.68. A suggested mechanism for the formation of 11 through a-bond metathesis of zirconacyclopentadienes with H2ZrCp2.

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]. 

The regiochemistry can be controlled by the nature of the substituents. With a tri-methylsilyl-substituted acetylene, the trimethylsilyl groups are placed in a positions of zirconacyclopentadienes with excellent selectivity (Eq. 2.8) [20]. With a phenyl-substituted alkyne, regioselective reactions are usually observed, although in some cases a mixture of two isomers may be formed. 

The simplest reaction of zirconacyclopentadienes is their hydrolysis. It is characteristic for organo-early transition metal compounds the metal—carbon bond is easily hydrolyzed with acids to give free organic compounds. Similarly, deuterolysis of zirconacyclopentadienes, rather than protonolysis, affords deuterated compounds as expected (Eq. 2.9). The position of the deuterium is indicative of the position of the metal—carbon bond in the organozirconium compound. 

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... 

Halogenolysis of zirconacyclopentadienes affords 1,4-dihalodienes (Eq. 2.14) [20]. Solvent effects on halogenolysis are remarkable. Indeed, whereas the iodination of zirconacyclopentadienes in THF with 2 equivalents of I2 affords mainly the monoiodinated diene, the diiododiene can be the major product in CH2C12. For example, 2,3,4,5-tetraethylzirco-nacyclopentadiene reacts with 2 equivalents of I2 in THF at room temperature to give the monoiodinated 3-iodo-4,5-diethylocta-3,5-diene 23 in 70% yield along with only an 18% yield of 3,6-diiodo-4,5-diethylocta-3,5-diene 24 (Eq. 2.14). 

Transmetalation of zirconacyclopentadienes to copper will be discussed later. However, it is noteworthy here that when the iodination is carried out in THF in the presence of 2 equivalents of CuCl, the product of diiodination (24) is obtained in 95 % yield without any trace of the monoiodinated adduct 23 (Eq. 2.16) [24],... 

This transmetalation of zirconacyclopentadienes to copper is very effective. Table 2.2 shows several examples of diiodination reactions of zirconacyclopentadienes performed in the presence of CuCl. 

Selective mixed halogenation of zirconacyclopentadienes is attractive from a synthetic point of view. Indeed, the successive treatment of zirconacyclopentadienes with N-chloro-succinimide (NCS) followed by iodine, N-bromosuccinimide (NBS)/I2, or NCS/NBS selectively affords chloroiododienes, bromoiododienes, and chlorobromodienes, respectively (Eq. 2.17) [25],... 

An important area of progress in zirconacyclopentadiene chemistry has been the heteroatom transfer developed by Fagan and Nugent, leading to five-membered heterocydes (Eq. 2.19) [26],... 

Me2SiCl2 does not react with either monocyclic or bicyclic zirconacyclopentadienes (Eq. 2.22) [7g,7h]. 

Phosphole formation from zirconacyclopentadienes is useful for preparing novel organic materials. As shown in Eq. 2.25, this method can provide monomers (32 and 33) or oligomers for electropolymerization reactions [29]. 

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. 

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