Zirconocenes with alkenes

Scheme 1.1 Coordination and carbometallation reaction of zirconocene with alkenes...

Cyclopropane ring cleavage is also observed in the case of zirconocene 2-alkene and j 2-imine complexes with adjacent cyclopropane rings to give t/3-allyl, /3-azaallyl, and t/ -enamine complexes [29]. 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

With the advent of enantioselective zirconocene-catalyzed alkene carbomagnesiation,27 27a 27c 28 28a chirally modified zirconocenes soon were applied to asymmetric reductive diene carbocyclization.2 a c As demonstrated by the reductive cyclization of 5a,29 highly enantioselective cyclization is enabled through the use Brintzinger s chiral, mszz-zirconocene.30 30a (For the preparation and resolution of chiral tf .szz-zirconocene 6, see Refs 30,30a.) However, moderate diastereoselectivities and yields are generally observed (Scheme 5). 

The manifold chemistry of alkenes with titanocenes and zirconocenes has been reviewed in detail in some of the above mentioned contributions [1], The reactions of the metallocene sources 1—6 with alkenes have only been investigated with regard to the specific question as to whether or not complexes or coupling products could be obtained. 

Coupling of 2,3-dihydrofuran with alkene-zirconocene <2004AGE3932> or aryne-zirconocene <2005SL2513> complexes and subsequent addition of an electrophile provided rA-disubstituted homoallylic alcohols, as illustrated in Equation (130). An insertion//3-elimination pathway that involved the formation of an oxazirconacyclooctene intermediate was proposed for the reaction mechanism. 

For a study of zirconocene-catalyzed alkene carbomagnesiation with longer alkylmagnesium derivatives, see Rousset, C.J., Negishi, E., Suzxiki, N., and Takahashi, T. (1992) Tetrahedron Lett., 33, 1965-1968. 

Zirconocene ii -alkyne complexes generated by the C-H activation method may be trapped intermolecularly with alkenes, but excepting ethene and nor-bornene the yields are generally low. ... 

Norton et al. investigated cyclization of zirconocene and imine to form several types of zirconaaziridine 1-16. The rich reaction chemistiy of 1-16 includes cyclization with alkene or alkyne to afford azazirconacyclopentene 1-17 and cyclization with isocyanate or aldehyde to form azaoxazirconacycle 1-18 and 1-19. These functionalized zirconacycles could be further transformed into zirco-naoxazolidione, aUylic amine, and a-amino amide (Scheme 1.12) [43, 44]. 

Excellent reviews or chapters in books have been published dealing with chemistry of metallocene-alkene complexes, metallocene-alkyne complexes, and their corresponding three-membered metallacyclic compounds of zirconocene and titanocene [1-5]. Zirconocene(II)-alkene complexes and zirconocene (Il)-alkyne complexes are often also viewed as zirconocene (IV) comple-... 

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]. 

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

In their early studies, Schwartz and co-workers [5, 80] reported the zirconocene hydrido chloride [Cp2Zr(H)Cl] (1) as a reagent capable of reacting under mild conditions with a variey of non-functionalized alkenes to form isolable alkylzirconi-um(lV) complexes Cp2Zr(R)Cl in which the zirconium is attached to the least-hindered terminal primary carbon, irrespective of the original location of the double bond in the olefin chain. As an example, at room temperature in benzene, 1-octene, cis-4-octene and trows-4-octene all yield the n-octylzirconocene derivative (Scheme 8-6) [80]. 

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

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. 

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