Zirconacyclopropanes/zirconacyclopropenes

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... 

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. 

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. 

Alternatively, functionalized zirconacyclopropene and zirconacyclopropane can be easily prepared by using the Negishi reagent (Cp2ZrCl2 + 2 -BuLi) [47]. In the presence of an additional unsaturated system, a carbocyclization reaction occurs to lead to the corresponding zirconacyclopentene or zirconacyclopentane derivatives [47]. When the substrate possesses a stereocontrol unit such as an amide moiety, a diastereoselective carbocyclization occurs [48] (Scheme 12.35). 

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