Zirconacyclopentanes preparation

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

A further example of a zirconacyclopentane with an indenyl ligand system is the compound rac-(ebi)Zr(C4H8) 21 (ebi = ethylenebisindenyl) prepared by the treatment of a tetrahydrofuran (THF) solution of (ebi)ZrCl2 at -78 °C with magnesium... 

Interestingly, zirconacyclopentane 246 formed by the reaction of 1,6-heptadiene with the Zr complex has the firms ring junction mainly [108]. It should be noted that the preparation of the trans ring junction in the bicyclo[3.3.0]octane system by other means is difficult. Carbonylation of 246 affords trans-fuzed bicyclo[3.3.0]octanone 247 [109,111]. The diacetoxy compound 248 is obtained by oxidative cleavage of 246. Protonation affords the frans-dimethylcyclopentane skeleton. Similar reactions occur with 1,6-enynes, and Pauson Khand-type cyclopentenone synthesis is possible by carbonylation. 

Mashima, K., Yamakawa, M., Takaya, H. Preparation, characterization and thermal reactions of 1-oxa-2-zirconacyclopentanes and 1-oxa-2-zirconacyclohexane crystal structures of [ Zr(C5H5)2(OCH2CH2CHMe) 2] and [Zr(C5Me5)2 OCH2(CH2)2CH2 ]. Journal of the Chemical Society, Dalton Transactions Inorganic Chemistry (1972-1999) 1991,2851-2858. 

Analogous dienyl zirconium compounds can be prepared by addition of unconjugated dienes that contain enol ethers to 105 (Scheme 19).56-58 These reactions are believed to proceed by initial coordination of the terminal olefin followed by isomerization via zirconocene allyl hydride intermediates to eventually yield zirconacyclopentanes that... 

In order to prepare very clean unsymmetrical zirconacyclopentadienes, the use of ethene is a prerequisite [14] (Eq. 2.4). An excess of ethene stabilizes the intermediates such as zirconacyclopentane 5a and zirconacyclopentene 4. Such a transformation from a metallacyclopentane to a metallacyclopentene was first demonstrated by Erker in the case of the hafnium analogues [15],... 

Cleavage of the p,P-carbon—carbon bonds of zirconacyclopentanes and zirconacyclopen-tenes is very often observed. This cleavage reaction is useful for the preparation of various zirconacycles. As examples, various transformations of zirconacyclopentenes involving P,P-carbon—carbon bond cleavage are shown in Eq. 2.69 [13,48],... 

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

For the preparation of 5-membered zirconacycles, although it is possible to use di-Grignard or dilithium reagents for the preparation of zirconacyclopentanes [ 10], the reductive coupling of unsaturated organic substrates, either intramolecular or intermolecular, on a zirconocene(II) species is the most common way. 

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). 

Insertion of isocyanides into zirconacyclopentanes and zirconacyclopentenes affords the corresponding zirconocene q -imine complexes. Cyclopentylamine derivatives could be prepared by trapping of the insertion intermediates using alkenes, alkynes, and carbonyl compounds [50]. For example, insertion of phenylisocyanide into bicyclic zirconacyclopentenes affords iminoacyl complexes that rearrange to give a, )0-unsaturated zirconocene q -imine complexes. These complexes react with alkenes or alkynes to give cyclopententylamines (Eq.48) [51]. 

All substituted and all different substituted pyridine derivatives could be prepared by the C-C bond cleavage of the zirconacyclopentane reaction in one-pot from two different alkynes and one nitrile with excellent selectivity and high yields [15]. This is the only method for the penta-substituted and all different substituted pyridine from two unsymmetrical internal alkynes and one nitrile so far. 

Alternatively, functionalized zirconacyclopropene and zirconacyclopropane can be easily prepared by using the Negishi reagent (Cp2ZrCl2 + 2 -BuLi) [47]. In the presence of an additional unsaturated system, a carbocyclization reaction occurs to lead to the corresponding zirconacyclopentene or zirconacyclopentane derivatives [47]. When the substrate possesses a stereocontrol unit such as an amide moiety, a diastereoselective carbocyclization occurs [48] (Scheme 12.35). 

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