Butadiene zirconocene complex

Complexation of 1,3-butadiynes In the reactions of complex 1 with various butadiynes, binuclear complexes with intact C4 units between the two metal centers are found. The former diynes are transformed to zig-zag butadiene ligands or g-r (l-3),r (2-4)-trans,-trans-tetradehydrobutadiene moieties between two metallocene cores. The bond type in 19 is unknown for the corresponding zirconocene complexes. 

The [(s-trans-diene)ZrCp2] complex (s-trans-1) equilibrates with the [(s-cA-diene)ZrCp2] isomer (x-cA-l) via a reactive high lying (r 2-butadiene) metallocene intermediate (2) [A(s-trans-1 s-cis-l, 283 K) = 22.7 0.3 kcal mol-1]. Syntheses of the (butadiene)zirconocene system carried out under kinetic control invariably led to pure s-trans-1, whereas a ca. 1 1 equilibrium of s-trans-1 and. v-ci.v-l was obtained under conditions of thermodynamic control.5,6 The cr,7i-structured s-cis-l isomer undergoes a dynamic ring-flip automerization process (see Scheme 2) that is rapid on the NMR time scale [AG futom = 12.6 0.5 kcal mol ].5... 

Butadiene)zirconocene reacts with metal carbonyls in a similar way as with ketones. Carbon-carbon coupling between a butadiene terminus and the carbonyl carbon atom occurs readily to yield a metallacyclic product, only that generally a metallacyclic n-allyl metallocene system is obtained from the reactions with metal carbonyls.8 Sometimes, O-zirconated Fischer-type carbene complexes are readily formed in this way that cannot be easily obtained by other routes. A typical example is the... 

Scheme 43. The Fischer-type see Fischer-type Carbene Complexes) vanadium complexes are prepared by treatment of (butadiene)hafhocene (81) or (butadiene)zirconocene with CpV(CO)4 (Scheme 44). The equilibrium mixture reacts with cyclopentanone to give the adduct complex (82). - ...

In addition, the experimental observation of (s-/rani-ij -diene)metal-locene complexes is kineticcdly facilitated by the special appearance of the energy profile of the dienemetallocene system. For the parent system there is evidence (27-31) that the stable (butadiene)ZrCp2 isomers [about equal amounts of (s-trans- (3a) and (s-cis-T]i -butadiene)zirconocene (5a) are obtained under equilibrium conditions (22, 23)] are connected through a reactive (T -butadiene)zirconocene intermediate (4a) which rapidly equilibrates with 5-tra/u-diene complex 3a, but is separated from the (s-cis-diene)metallocene isomer 5a by a rather substantial activation barrier... 

AG ii c = 14.3 kcal/mol). ( -c -Butadiene)- and -2,3-substituted buta-diene)zirconocene complexes exhibit markedly lower barriers (Table III) 50-52). In general, the corresponding bis(Tj-cyclopentadienyl)hafnium... 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

For these complexes, the rate of carbonylation increases rapidly with increasing tr-alkyl character. Thus, (5-cis-butadiene)ZrCp2 (5a) is carbony-lated 2.5 times faster than the zirconocene complex of 1,2,5,6-tetramethyl-3,4-bis(methylene)tricyclo[3.1.0.0 ] hexane (51) (ambient temperature and 1 bar CO pressure) taken as a standard. Introduction of methyl groups at the internal carbon centers C2/C3 of the diene chain increases the carbonylation rate by a factor of 5 with phenyl groups at these positions increasing the rate by a factor of 250 (Table VIII). The organometallic reaction products have yet to be isolated or completely identified. Ultimately, zirconium enolate complexes (20) are probably formed in these... 

In view of these observations, it is not at all surprising that s-trans-jf-butadiene)zirconocene (3a), which resembles a normal olehntransition metal ir complex far more than its -cis isomer 5a, does not react with carbon-carbon bond formation when exposed to carbon monoxide. Instead, zirconocene dicarbonyl and butadiene are formed in a very slow carbonylation reaction. 

Zirconocene complexes (with s-cis geometry) of isoprene, 2,3-dimethylbutadiene, and 3-methyI-I,3-pentadiene are reported to give exclusively 1 1 addition with carbonyl substrates even if these are used in excess and the reaction temperature is fairly high (ca. 100 C). On the contrary, zirconocene complexes of 5-ci5-butadiene, 1,3-pentadiene, and 2,4-hexadiene ca, 1 1 mixture of the s-cis and s-trans isomers) easily accept (equation 53) 2 equiv. of either butanal or 3-pentanone, at low temperature ca. 30 C) in high yields (95%). This can be exploited for the stepwise insertion of two different electrophiles... 

Treatment of bis(phenylethynyl)silanes with Cp2ZrEt2 followed by addition of H20/CuC1 affords (1 ,3 )T,4-diphenyl-1,3-butadienes in moderate to high yields after hydrolysis. The zirconocene complex 697 (BTCsEG) was... 

Zirconocene complexes show much lower activity as the catalyst of 1,3-butadiene polymerization than the titanocene catalysts [15]. The catalyst composed of rac-[CH2(3-terf-butyl-l-indenyl)2]ZrCl2 and MAO promotes the 1,4-polymerization of 1,3-butadiene or (Z)-1,3-pentadiene and 1,2-polymerization of (E)-1,3-pentadiene and 4-methyl-1,3-pentadiene (Eq. 4) [16]. The bulky tert-butyl group of the ligand is essential for smooth polymeriza-... 

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