Zirconacyclopropanes

As already indicated, the carbometallation reactions of zirconacyclopropanes and zirconacyclopropenes with alkenes and alkynes are in many ways similar to the corresponding reactions of titanacycles developed more recently. At the same time, however, there are a number of significant differences, as detailed in Section 10.06.2.2. At the present time, synthetically useful carbotitanation reactions are predominantly cyclic and stoichiometric in Ti and more so than the corresponding chemistry of Zr. It seems reasonable to state that Ti and Zr are complementary to each other more often than not. The cyclic carbozirconation may be either stoichiometric or catalytic. Frequently, the difference between the two is that the stoichiometric reactions lack one or more microsteps for completing catalytic cycles. Otherwise, they often share same stoichiometric microsteps. With this general notion in mind, many stoichiometric carbozirconation reactions have indeed been developed into Zr-catalyzed reactions, as discussed later. 

Formation and characterization of zirconacyclopropanes and zirconacyclopropenes In addition to / -H abstraction of dialkylzirconocenes discussed earlier (Schemes 5 and 6), several other methods are also available for the preparation of three-membered zirconacycles as summarized in Scheme 35. From the viewpoint of cyclic carbozirconation reactions, especially those under Zr-catalyzed conditions, /3-H abstraction, 7t-ligand substitution, and decarbometallative ring contraction are particularly important. As such, these three-membered zirconacycles are generally unstable, but they can be stabilized with phosphines, for example, PMe3, and other bases, and are fully identified. Some of the well-identified examples are shown in Scheme 36.206-209... 

Scheme 1.41. Synthesis of cyclobutenylzirconocene derivatives by the reaction of zirconacyclopropane derivatives with l-chloroalkynes.

The above intramolecular diene cyclizations are likely to proceed through a similar set of reactions as shown in Scheme 6.2 for the intermolecular variants. Thus, as depicted in Scheme 6.6, formation of the zirconacyclopropane at the less hindered terminal alkene (—> ii), generation of the tricyclic intermediate iii, Zr—Mg exchange through the intermediacy of zirconate iy and 3-H abstraction and Mg alkoxide elimination in v may lead to the formation of the observed product. Additional kinetic and mechanistic studies are required before a more detailed hypothesis can be put forward. 

A zirconium-silene complex was obtained during the course of our study on the synthesis of a zirconium-silyl complex. The zirconium-silene complex is formed from disilylzirconocene 2a, generated from Cp2ZrCl2 and 2 equivalents of Me2PhSiLi 8b. Zirconium-silene complex 3 should be in a state of equilibrium with zirconacyclopropane 3. The insertion of diaryl alkyne 14 or vinyl alkyne... 

The described reaction was applied to the synthesis of various vinylcyclo-propanes. However, obvious limitations remained in the scope of the reaction at this stage (1) only 1-substituted vinylcyclopropanes could be obtained and (2) the reaction couldn t be applied to the preparation of cyclopropane derivatives other than vinylcyclopropanes. To overcome the first limitation, the methyl-substituted zirconacyclopropane [13] was stabilized by adding PMe3... 

Substituted zirconacyclopropanes were demonstrated to be less stable than zirconacy-clopropane itself, see [8]... 

Furthermore, if we consider the carbometalative ring expansion to produce the corresponding five-membered ring zirconacycle 86, the carbon-heteroatom bond of the sp3 metalated center Cx should isomerize to produce the most stable intermediate. Such isomerization could be due to an interaction between the heteroatom moiety XR and the zirconium atom [65], which would produce a weakness of the Cj-Zr bond and would facilitate the isomerization. Thus, whatever the stereochemistry of the starting material, a conformation is always possible in which Cj-SR is antiperiplanar to C2-C3 in 86 with a trans relationship between R and the ZrCp2 fragment. The elimination reaction, or decarbo-zirconation, occurs in a concerted way to give the E-vinyl zirconium 83. Unfortunately, neither the zirconacyclopentane nor the zirconacyclopropane have been trapped as intermediates. 

The limitation of this methodology could be attributed not only to an unfavorable initial ligand exchange between (1-butene)ZrCp2 21 and the migrating olefin for steric reasons, but also to the formation of a hypothetically less stable trisubstituted zirconacyclopropane, 110 or 111, which should be obtained after the first migration of the double bond (Scheme 39). 

By analogy with the mechanistic pathway described for the enol ether 119 (Scheme 44), we believe that the transformation of 123a-c also occurs via the formation of the /J-metalated allenyl intermediate, generated from the -elimination of the corresponding zirconacyclopropane and subsequent rearrange-... 

Cp2Zr(PMc3 )2. Some complexes, as alkene- and aIkyne-ZrCp2 structures can be considered as Zr complexes, but also as zirconacyclopropanes and zirconacyclopropenes respectively, in which Zr is the +4 oxidation state. They can be represented as resonance hybrids (1) and (2). On the contrary, Zr and Zr compounds are rarer and displayed few synthetically useful transformations. 

Three-membered zirconacycles (zirconacyclopropanes and zirconacyclopropenes) undergo ring expansion through cychc carbozirconation see Carbometalation) (Scheme 26). 

Somewhat surprisingly, no report on the preparation of zirconacyclopropenes or zirconacyclopropanes as discrete, characterizable species and/or their full spectroscopic characterization was available until a few years ago. Probably, stilbene-zirconocene (42 equation 39) is the first example of such compounds spectroscopically characterized. ... 

The first isolated and fully characterized zirconacyclopropene prepared by the reaction of free alkyne and ZrCp2 is (46 equation 41 Figure 1). Independently and simultaneously, another X-ray structure of an acyclic alkyne-zirconocene (47) prepared as shown in equation (42) was also published. The first X-ray structure of a zirconacyclopropane derivative is shown in Figure 2. ... 

It has now been established that both zirconacyclopropenes and zirconacyclopropanes can be prepared as discrete and fully characterizable species. More relevant to the present discussion is that these species can be generated by the reaction of free alkynes or alkenes with ZrCp2 , as shown in equations (39) and (41). Another useful piece of information is that the formation of (46) from diphenylacetylene according to equation (41) is about 150 times as fast as that of (43) from ( )-stilbene under the same conditions, indicating that, in comparable situations, alkynes are far more reactive than alkenes towards ZrCpz . Little definitive information is currently available on the formation of related three-membered hafnacycles. 

Formation of (51) rather than the zirconacyclopropane derivative (52) is reasonable in the light of the considerably higher reactivity of alkynes relative to alkenes. Furthermore, the following intriguing results not only reveal some remarkable reactivity of five-membered zirconacycles but also support the intermediacy of (51) rather than (52 equation 48). ... 

Since 1986, the following zirconacyclopropenes as well as one zirconacyclopropane, i.e. (43), have been characterized by X-ray and other spectroscopic means. The compound (44) represents a benzyne-zirconocene complex, and (45) is an example of a cycloalkyne-zirconocene complex. Although these compounds are interesting and significant in their own right, they do not provide support for the formation of zirconacyclopropenes from alkynes and ZrCp , because the alkyne components are generated in situ and have probably never been free. 

---