Zirconacycles, three-membered

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 36 Preparation of three-membered zirconacycles via (1 -butenejzirconocene.

Scheme 1.15. Synthesis of other three-membered zirconacycles.

Another major protocol for the generation of three-membered zirconacycles was initially devised by Erker [47—49] and was extensively developed by Buchwald [36—44] (Erker—Buchwald protocol) (Scheme 1.14). No alkenes or alkynes are used as temporary ligands in this protocol. Unless hydrozirconation is used to generate the initial organyl-zirconocene derivatives, even final alkene or alkyne ligands are not usually derived from the corresponding ji-compounds. Thus, the synthetic values of the two representative protocols are quite different and often complementary to each other. 

In addition to the methods discussed above, transmetallation and migratory insertion also provide useful routes to three-membered zirconacycles (Scheme 1.15). 

Although 16-electron three-membered zirconacycles are generally unsuitable for X-ray analysis, their complexes with phosphines, such as PMe3, or some ethers, such as THF, have often yielded crystalline compounds suitable for X-ray analysis. Thus, their existence and identity have been firmly established. 

Stoichiometric intermolecular cyclic carbozirconation of three-membered zirconacycles... 

Until recently, the structures of the five-membered zirconacycles had been proposed on the basis of NMR data and identification of the final organic compounds, especially the products of deuterolysis, iodinolysis, and carbonylation. Determination of their structures by X-ray analysis proved to be more difficult than that of three-membered zirconacycles, largely because attempts to obtain their stable 18-electron derivatives led to ring-contraction to give three-membered zirconacycles, as in the last example in Scheme 1.56. This difficulty was overcome by the use of bulky Cp derivatives that permitted the formation of stable, crystalline, 16-electron, five-membered zirconacycles such as 5 [198] and (tBujQHj Z Ch (6) [199] (Scheme 1.59). 

In these early studies, however, the concept of c-bond metathesis most probably did not exist, and the results were presented just as observed facts. Mainly in the 1990s, a wide variety of c-bond metathesis reactions of both three- and five-membered zirconacycles were reported. In Scheme 1.4, the reaction of the five-membered zirconacycle with EtMgBr via c-bond metathesis followed by another c-bond metathesis (p-H abstraction) produces the ethylmagnesation product along with ethylene-zirconocene [51], Some representative examples of c-bond metathesis reactions of three-membered zirconacycles are shown in Scheme 1.69. These are examples of stoichiometric c-bond metathesis reactions from which the products have been identified. 

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

Another pathway to generate reactive three-membered zirconacycles and zirconaheterocycles is the thermolysis of diphenylzirconocene or related complexes. Many different stractures with or without heteroatoms are readily obtained. These strongly constrained compounds are very reactive towards a large variety of polar or nonpolar species. 

SSJ2 Formation of Zirconacyclopentadienes, Zirconacyclopentenes and Zirconacyclopentanes via Three-membered Zirconacycles... 

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. ... 

The precise mechanisms of the conversion of zirconacycloptt nes 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-zirconocene 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. 

When three-membered zirconacycle phosphonates are treated with terminal alkynes, and the reaction mixture is hydrolyzed, two isomeric dienylphosphonates are obtained. In all cases the alkyne coupling occurs mainly on C-1, probably because of the steric interactions. On hydrolysis, the die-nylphosphonate having the (1Z,3 ) configuration is isolated in 57-73% yields as the major product, whereas the minor product (IE,3E configuration) is obtained in 11-20% yields (Scheme 1.30)2 2... 

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