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... 

The zirconacyclopropene 1, which was prepared by treatment of Cp2ZrCl2 with magnesium metal in the presence of bis(trimethylsilyl)acetylene, reacted with one molecule of C02 under atmospheric pressure at room temperature to give the dimeric zirconacycle 2 in good yield (Scheme iy6>6a>6b Further insertion of C02 did not occur, although 2 has... 

Similar to the reaction of zirconacyclopropene 1, titanacyclopropene 14 reacted with C02 to give titanacycle 15 (Scheme 5) I0,i0a-i0c j-[owever5 the reaction of Cp TiC /Mg with l,4-bis(trimethylsilyl)-l,3-butadiyne did not afford a titanacyclocumulene species, but yielded titanacyclopropene instead 16, which on reaction with C02 gave the titanacycle complex 17.7 In the case of the titanium half-metallocene complex 18, the five-membered titanacyclocumulene 19 was obtained but the insertion of C02 took place only at one of the two Ti-carbon bonds, leading to the formation of 20 (Scheme 5),11 which is in contrast with what was observed in the case of the Zr analog 3. The... 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

Hydrozirconation of alkynes with the Schwartz reagent Cp2Zr(H)Cl yields the chlorovinyl zirconocene 17, which is easily converted to the methyl vinyl zirconocene 18 with either methyllithium in THF or methylmagnesium bromide in CH2C12. Compound 18 loses further methane at room temperature to form a zirconacyclopropene intermediate 19, which couples with a second alkyne and forms the metallacyclopentadiene 20 (Scheme 6) [34]. 

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. 

Phospholyl anions are also isoelectronic to Cp ligands, and these species readily complex Zr. A reduction reaction of (C4Me4P)2ZrCl2 by magnesium in THF gives transient diphosphazirconocene species which can be trapped by CO, bis trimethylsilyl acetylene, or 2-butyne to yield the corresponding dicarbonyl (67), zirconacyclopropene (68), and zirconacyclopentadiene (69). ... 

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. 

The precise mechanisms of the conversion of zirconacyclopropenes into five-membered zirconacycles are still unclear. For the reactions of alkynes and alkenes, a concerted carbometallation mechanism earlier proposed appears to be plausible, but remains only a reasonable working hypothesis. One useful piece of information is that the reaction of an in situ generated benzyne-zireonocene complex with stil-bene is stereospecific, as shown in equation (46), suggesting that, in contrast to the formation of some three-membered zirconacycles, this and other related reactions may be concerted. 

Subsequently, a one-pot protocol for the preparation of tri- and tetrasubstituted alkenylsilanes straightforwardly from alkynylsilanes was developed. Within the same carbozirconation protocol, the addition of allyloxytrimethylsilane led to its rapid reaction with the intermediate zirconacyclopropene 288 to expand the... 

Scheme 10.102 Synthesis of l-boryl-1,3-butadienes from zirconacyclopropenes stabilized by a phosphine ligand [84].

SCHEME 17.6 Synthesis of substituted aromatics by way of a zirconacyclopropene intermediate. 

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