Zirconacycles five-membered

A general method for the synthesis of highly substituted styrenes as 6/4-91, vinyl-cyclohexadienes and related compounds was developed by Xi, Takahashi and coworkers [301] by reacting an intermediately formed five-membered zirconacycle 6/4-89 with propargyl derivatives 6/4-90 or allyl bishalides in the presence of CuCl (Scheme 6/4.21). 

Scheme 39 Pair-selective preparation of five-membered zirconacycles via (1 -butene)ZrCp2 and (ethylene)ZrCp2.

A few other interesting and potentially important consequences of the reversible formation of five-membered zirconacycles include stereo- and regioselective skeletal rearrangement, as exemplified by Scheme 1.57 [197], and 1,3-C=C bond and Zr migration (Scheme 1.58) [191,192], supporting the associative mechanism for alkene displacement (Generalization 22 ). 

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 some of the preceding sections, the significance of interactions between three-mem-bered zirconacycles with 7T-bonds (Pattern 7 in Scheme 1.3 and Scheme 1.51) has been amply demonstrated. More recently, their c-bond analogues (Pattern 13) and variants involving five-membered zirconacycles, as shown in Scheme 1.65, have been recognized as important fundamental processes in organozirconium chemistry. 

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. 

Many alkyl carbenoids insert into saturated and unsaturated zirconacycles to afford zirco-nacydohexanes and -hexenes 126, which give the expected products 127 upon hydrolysis (Scheme 3.30) [48,50,68], There should be comparable scope for further elaboration of the six-membered zirconacycles, as has already been established for the five-membered analogues. Yields are generally high, one exception being the insertion of lithiated chloroace-tonitrile into saturated zirconacycles, where double insertion predominates [50],... 

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. 

Original metallacyclocumulenes were obtained from a 1 1 stoichiometric reaction between dialkyne and zirconocene (equation 11). The unusual molecular sttucture of the five-membered zirconacycle (32) shows an almost planar arrangement containing three carbon-carbon double bonds. 

One important aspect of the Cp2Zr-promoted cycliza-tion that is now well-established is the ready reversibility through decarbozirconation, presumably involving an interaction between the empty Zr-orbital and the C/S -Cfi bond of the five-membered zirconacycle and subsequent displacement of ethylene (equation 53). Reaction of 2-azadienes on zirconocene affords via a retro-Brook rearrangement interesting azaoxazirconacycles. ... 

Similar to the benzynezirconocene, cyclohexyne, cyclopen-tyne, alkyne, alkene, cycloaUcene zirconocenes, and related species insert various substrates such as alkynes, alkenes, aldehydes, ketones, nitriles or phosphaalkynes. They lead in general five-membered zirconacycles, which can be converted by transmetalation or exchange reactions into fused-ring aromatic or heterocyclic compounds. The extension of this chemistry to heterobenzyne complexes can be realized, for instance, in phosphinine compounds. Consequently, under mild conditions, ) -phosphabenzyne-zirconocene complexes are formed and can be isolated either as PMes adducts or as dimers when the elimination reaction is carried out without added phosphane (Scheme 28). 

A ring-expanded process (five- to six-membered zir-conacycle) can be observed by a clean insertion see Insertion) of 2,2-disnbstituted-l-hthio-l-chloro-alkenes into five-membered zirconacycles (Scheme 38). ... 

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. 

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). ... 

The /]2-phosphabenzyne zirconocene 99 (or the complex 98 a) reacts with alkynes insertion into the C2-Zr bond was always observed with formation of the expected five-membered zirconacycles 103-106 (Scheme 20). 

Although o-bond metathesis of five-membered zirconacycles with EtMgBr [51] (Scheme 1.4) and H2ZrCp2 (Scheme 1.68) has been implicated, there are as yet very few well-established examples. The reaction of zirconacyclopentanes with alkyllithiums is interesting since it involves (i) the displacement of one of the two Cp groups, and (ii) the generation of a bimetallic species, the NMR spectroscopic data of which are consistent only with a fluxional structure as shown in Scheme 1.71 [66]. 

A wide variety of five-membered zirconacycles 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cyclization takes place via the -complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/P-hydrogen elimination) (path b). Two additional routes to zirconocene r 2 -complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

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