Zirconacyclopentenes

Scheme 1.56. Displacement of ethylene from zirconacyclopentenes by n-compounds.

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

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 zirconacydopentane 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]. 

The first protonation occurs much more rapidly at the sp3 carbon attached to zirconium than at the sp2 carbon. Reaction at the sp2 carbon attached to the zirconium in the zirconacyclopentene is very slow with methanol. As regards the second protonation of the organozirconocene, even the sp3 carbon does not react with MeOH (Eq. 2.13) [23]. 

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. 

Cleavage of the (3,(3-carbon—carbon bonds of zirconacydopentanes 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 (3,(3-carbon—carbon bond cleavage are shown in Eq. 2.69 [13,48],... 

Elimination of an alkoxy group or of a halogen in the case of zirconacyclopentenes has been investigated in combination with the (3, (3-carbon—carbon bond-cleavage reaction. As shown in Eq. 2.73, an OR group or a halogen at a (3-position is eliminated and trapped by the zirconium metal center [53],... 

Reductive elimination of a zirconacycle to give a four-membered ring is very rare. Only one example has been reported in the case of a-alkynylated zirconacyclopentenes, as shown in Eq. 2.75 [54]. 

When an alkynylsilyl group is present at the a-position of zirconacycles such as zircona-cyclopropene, zirconacyclopentene, or zirconacyclopentadiene, an unusual rearrangement proceeds to give zirconacyclosilacyclobutene derivatives 132, as shown in Eq. 2.78 [57]. The zirconacyclosilacyclobutene fused complex 132 and the zirconacydohexadiene silacy-clobutene fused complex 137 have been characterized by X-ray analysis. 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

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. 

The most versatile synthesis involves the transmetallation of a zirconiacyclopentadiene231,232 with a tin halide under catalysis with CuCl233 (and an equivalent reaction occurs with other halides of other elements of Groups 13, 14, 15, and 16) (Equation (77)). Stannacyclopentenes can be prepared by analogous reactions of zirconacyclopentenes.233... 

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... 

Zirconacyclopentenes (equation 151)974. decompose by iodine in a highly chemoselective fashion... 

Similarly, Takahashi and coworkers reported that treatment of alkynes with zirconocene-ethylene complex (9) and homoallylic bromides gave allylcyclo-propane derivatives [21]. Therefore, the possibility of y-elimination of the halogen atom in zirconacyclopentene intermediates appeared to be more general as expected. The plausible mechanism is shown in Scheme 13. Carbozirconation... 

Homoallylic ethers appeared much less reactive towards the f-elimination reaction with the zirconocene-alkyne complexes or zirconacyclopentenes [21]. No -elimination products were observed in these cases. In contrast, Szymo-... 

The synthetic potential of organozirconocenes is greatly expanded by their easy transformation into other organometallics. Therefore, transmetalation-based approaches to cyclopropane synthesis have been reported. The reaction of zirconacyclopentene with phthaloyl chloride in the presence of CuCl was used for the preparation of cyclopropylenolate derivatives in moderate yields (Scheme 29) [39]. 

Takahashi et al. have also developed an alternative approach to dienyl zir-conocene by ethenylzirconation of vinyl ethers. As shown in Scheme 9, the reaction of alkynes with Cp2ZrEt2 24 gives zirconacyclopentenes. Unsaturated compounds such as vinyl ethers can easily replace the ethylene moiety of the zir-conacycle to afford potentially two regioisomers 37 and 38 [47-49] (Scheme 18). As 37 and 38 are in equilibrium, 38 undergoes a / -elimination reaction to give 39 as a unique isomer. This sequence maybe considered as a vinylzirconation reaction of alkynes. Although substituted alkenyl ethers, such as 40 and 41, did... 

Oxazirconacycloheptenes, generated in situ by the reaction of a zirconacyclopentene with an aldehyde, can be reacted with a second aldehyde in the presence of CuCl. After hydrolysis, a tetrahydrofuran derived from four different components, an alkyne, ethylene, and two different aldehydes, is obtained in good isolated yield (Scheme 76) <2004T1417>. 

Zirconacyclopentenes with BICeFsIs give zwitterionic complexes of the type 94 <1997JA11165, 2001ACR309, 2004JOM(689)4305>. Zirconacyclopentanes also form zwitterionic complexes <1999OM3094>. 

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