Formation of Zirconacycles

The reactions of complex (25) with carbonyl compounds show a strong dependence on the substituents leading to the substitution or elimination of the silyl-substituted acetylene or the insertion into the Zr-C bond. Moreover, the reaction of methacrolein with (25) depends strongly of the solvent used in hexane elimination of bis(trimethylsilyl)acetylene leads to the formation of dimer (33) while in THF insertion with the formation of zirconacycle (34) is observed. 

A reaction pathway for the formation of zirconacycles, involving the Zr-carboryne Cp2Zr( j -C2BioHio), has been proposed (Scheme 7.12). In general, electron-withdrawing aryl substituents go to the a position (via intermediate M),... 

Copper-catalyzed or mediated carbon-carbon bond formation reactions of zirconacycles and alkenylzirconocenes 97YGK958. 

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). 

For a long time, zirconacyclopentadiene was considered to be inert towards carbon-carbon bond formation. However, through transmetalation to Cu, Ni, Zn, Li, and Al, various kinds of carbon—carbon bond-formation methodologies have been widely developed. One major advantage of this chemistry is that zirconacyclopentadienes can be conveniently prepared in situ, in high yields and with excellent selectivities, from two different alkynes. Combination of the selective formation of zirconacyclopentadienes and the chemistry of Cu, Ni, Zn, Li, or Al provides useful tools in organic synthesis. In the near future, it can be expected that more metals will be used for the transmetalation reactions of zirconacycle derivatives and some catalytic reactions will hopefully be developed. 

The protonolysis of the equally substituted zirconacycle 27 with weak acids such as ethanol leads to the monoprotonated dienyl zirconocene compound 28, which is subsequently converted into substituted diene 29 by a palladium crosscoupling reaction with aryl iodide (Scheme 14) [44]. Selective halogenation of such zirconacycle leads, as well, to the formation of zirconated diene 30 and then to the 1,4-dihalogenodiene 31 (Scheme 15) [45]. 

Insertion of isonitriles into zirconacycles affords )] -iminoacyl complexes that smoothly rearrange to stable f-irame zirconocenes. In the case of 2-phosphanozirconaindenes, rearrangement proceeds leading to -imine zirconocenes (Scheme 35). Subsequent formation of a /3-phosphano indenoimine is obtained by elimination of Cp2Zr fragment for the 2,6-dimethylphenyl isocyanide." ... 

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. 

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). 

Quntar, A.A.A., and Srebnik, M., Carbon-carbon bond formation of alkenylphosphonates by aldehyde insertion into zirconacycle phosphonates, J. Org. Chem.. 66. 6650, 2001. 

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