Five-membered zirconacycle

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. 

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... 

The structure of 2a and 3a. First, two possible isomers of the zirconacycle intermediates 2a and 3a have been fully optimized (Fig. 7). The results indicate that the seven-membered structure 3a is —1.7 kcal/mol more stable than the five-membered structure 2a. Therefore, 3a should be dominant under room temperature. The transition state of the isomerization from 2a to 3a has also been located (TS-2a-3a). The isomerization barrier is only 4.1 kcal/mol. The low barrier of this isomerization indicates that the seven-membered zirconacyclocumulene may be formed via the five-membered zirconacyclic intermediate [43,44]. 

Considering the experimental data and DFT calculations disclosed above, a possible reaction mechanism for the synthesis of cumulenols is outlined in Scheme 5 First of all, a fast equilibrium between seven-membered zirconacycle 3 and five-membered zirconacycle 2 exists in the reaction mixture, in which 3 is more stable, whereas 2 is more reactive. The addition of carbonyl group to the propargyl zirconium moiety in 2 proceeds to generate a nine-membered oxazirconacycle 7 via a cyclic Se2 process, and hydrolysis of 7 affords the cumulenol 5. 

There have been many reports on the preparation and applications of five-membered zirconacycles, namely, zirconacyclopentanes, zirconacyclopentenes and zirconacyclopentadienes [1-5]. 

Compared with five-membered metallacycles, relatively fewer reports are known on the preparative methods and reaction chemistry of six-membered metallacycles. Whitby and coworkers have systematically investigated insertion of carbenoids into five-membered zirconacycles and developed a number of interesting six-membered zirconacycles [17, 26]. Isonitriles, 1-halo-l-lithioalkenes, allenyl carbenoids, allyl carbenoids, propargy carbenoids, benzyl carbenoids, and 1-nitrile-1-lithio epoxides can all insert into zirconacyclopentanes and zirconacyclopentenes to afford various six-membered zirconacycles (Eqs. 23,24). 

Takahashi and coworkers have reported three types of formation of cyclopropane derivatives from five-membered zirconacycles. One is via reaction of carbenes or carbenoids with carbon-carbon double bond in zirconcy-clopentadienes (Eq. 30) [30], the second via jS-elimination from zirconacy-clopentenes (Eq. 31) [31] and the third via intramolecular Michael addition (Eq.32) [32]. 

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