Zirconacyclopentanes

Interestingly, Hoveyda and coworkers observed a second-order dependence of the reaction rate on the concentration of zirconium in these reactions, suggesting that the zirconacyclopentane is formed from a bimetallic alkene-zirconate complex such as A in Fig. 1 [21]. This finding suggests that olefin alkylations and substitutions occur via reaction of a nucleophilic alkene unit [23]. 

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. 

Zirconacyclopentanes can readily undergo skeletal rearrangements, exemplified by those shown in Scheme 41. Although the process involving Zr reveals only the final zirconacycle, examination by NMR spectroscopy of the corresponding Hf reaction shows the formation and decay of the 2,5- and 2,4-dimethylhafnacyclopentanes. All of the three hafnacycles as well as the zirconacyclic product are >98% dl.229... 

Although a-bond metathesis of five-membered zirconacydes 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],... 

Scheme 1.71. a-Bond metathesis reaction of zirconacyclopentanes with alkyllithiums. 

Actually, while zirconacyclopentadienes alone do not react with CO at —78 °C, the addition of BuLi allows the reaction to proceed, thereby leading to cyclopentenones 42 (Eq. 2.29) [10]. A similar transmetalation to Li has been proposed for zirconacyclopentanes. 

Zirconacyclopentadiene shows a different reactivity towards CO as compared with zirco-nacyclopentane and zirconacyclopentene. Zirconacyclopentane and zirconacydopentene readily react with CO at low temperature to give cyclopentanone and cyclopentenone, respectively. The different reactivity of zirconacyclopentadienes can be explained by comparing the reactivity of the Zr—Csp2 bond with that of the Zr—Csp3 bond. Insertion of CO into the Zr—C bond proceeds readily at low temperature and therefore zirconacydopentane and zirconacyclopentene, which contain Zr—Csp3 bonds, react directly with CO as shown in Eq. 2.65 [45], Zirconacyclopentadienes, on the other hand, do not. 

Insertion of phenyl, trimethylsilyl, and nitrile-stabilized metalated epoxides into zircona-cyclcs gives the product 160, generally in good yield (Scheme 3.37). With trimethylsilyl-substituted epoxides, the insertion/elimination has been shown to be stereospecific, whereas with nitrile-substituted epoxides it is not, presumably due to isomerization of the lithiated epoxide prior to insertion [86]. With lithiated trimethylsilyl-substituted epoxides, up to 25 % of a double insertion product, e. g. 161, is formed in the reaction with zirconacyclopentanes. Surprisingly, the ratio of mono- to bis-inserted products is little affected by the quantity of the carbenoid used. In the case of insertion of trimethylsilyl-substituted epoxides into zirconacydopentenes, no double insertion product is formed, but product 162, derived from elimination of Me3SiO , is formed to an extent of up to 26%. 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

General structure 24 is used throughout to indicate a wide variety of zirconacyclopentanes and zirconacyclopentenes. Generally, these are unsubstituted on alkyl carbons a to zirconium, whereas alkenyl carbons generally have an alkyl, aryl, or trimefhylsilyl substituent a to the zirconium. 

These complexes show an interesting chemistry, e. g. they undergo coupling with ethene to give zirconacyclopentane 71 or with water to give zirconoxane 72, or they can undergo insertion of carbon dioxide with formation of the complexes 73 and 74. In all of these reactions, the pyridine moiety is restored. With acids, complex 73 liberates the corresponding carbonic acids 75 or esters 76. 

The remarkable effect that CuCl has on the reaction of the zirconacyclopentadienes with R2SnCl2 or SnCU (Section 3.14.9) extends to reactions involving zirconacyclopentenes and zirconacyclopentanes, providing a route to stannacyclopentenes and stannacyclopentanes (Equations (123) and (124)).233... 

Carbocupration of alkynes by zirconacyclopentane derivatives can be performed according to the same procedure. Thus, the zirconocyclopentane 135, obtained by treatment of Bu2ZrCp2 with 1,6-heptadiene, reacts at room temperature with phe-nylacetylene to afford the adduct 136 through a carbocupration-reductive elimination mechanism. Cross-coupling followed by intramolecular carbocupration takes place in the case of the reaction with 1-bromohexyne, producing 137 (Scheme 2.66) [143]. 

Scheme 2.66. Copper-catalyzed reactions of zirconacyclopentane derivatives.

Zirconacyclopentanes are readily formed by the reductive coupling of ot-olefins. [Cp2ZrN2]2N2 reacts with ethene at 25 °C in toluene solution to afford the zircona-cyclopentane 15, in nearly quantitative yield.21 In addition, 15 can be obtained by reacting CpljZrE with ethene at 25 °C. Reaction of CpijZrF with excess... 

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... 

The parent zirconacyclopentane 27a can be synthesised by either the reaction of Cp2ZrCl2 with BrMg(CH2)4MgBr as reported by Takahashi and co-workers or by warming dibutylzirconocene (Cp2ZrBu2) to room temperature under an ethene atmosphere 32... 

Negishi and co-workers33 report that zirconacyclopentanes of type 27 react with benzaldehyde at or below 25 °C to give seven-membered oxazirconacycles 28, which are then converted into alcohols on hydrolysis with HC1 ... 

Similar results have been reported by Takahashi and co-workers.34 These authors report that zirconacyclopentanes react with PhCHO under a slightly positive pressure of ethylene to form the oxazirconaheptacycle 28a as the major product at room temperature, and that the reaction carried out in the absence of additional ethylene produces the five-membered oxazirconacyclopentane 29 instead [Eq. (10)]. Reaction of 27a with benzaldehyde at higher temperatures of 50 °C was also found to yield the five-membered oxazirconacyclopentane as the major product. 

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. 

A further extension involves the readily synthesised zirconacyclopentanes, which react with the same reagents to give to give novel 18-electron azazirconacycles. These are very stable towards hydrolysis [75], but hydrogenolysis leads to the formation of a primary amine. 

Cyanosilanes can be isocyanide sources since a tautomeric equilibrium exists between cyanosilanes and the corresponding isocyanides. The equilibrium largely favors the cyano tautomer. The use of such dilute isocyanide donors realizes efficient Ti(n)- and Ni(0)-catalyzed cyclizations of enynes to iminocyclopentenes via metallacyclopentene intermediates (Scheme 19).266,266 266b Treatment of zirconacyclopentanes and -pentenes with Me3SiCN provides zirconocene-imine complexes, which serve for carbon-carbon bond formation with various unsaturated bonds.267... 

Whitby, Blagg, and coworkers have used a similar method for zirconaaziridine synthesis (Scheme 1). They inserted phenyl isocyanide into a Zr-C bond of the zirconacyclopentane 11 thermal rearrangement gave the zirconaaziridine 12. 

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