Zirconocene complexes isomerization

Then, contrary to what was reported previously, the olefin dissociates from the zirconium metal complex. This conclusion was further supported by other experimental observations. However, it cannot be completely excluded that competition between dissociative and direct rearrangement pathways could occur with the different isomerization processes studied up to now. Note that with cationic zirconocene complexes [Cp2Zr-alkyl], DFT studies suggest that Zr-alkyl isomerizations occur by the classical reaction route, i.e. 3-H transfer, olefin rotation, and reinsertion into the Zr-H bond the olefin ligand appears to remain coordinated to the Zr metal center [89]. 

Internal RCH=CHR 2-Hexene MeCH=CHC3H7 is not isomerized by complex 1 to 1- or 3-hexene, nor is its cis trans ratio changed. No olefin complexes or coupling products are obtained. The corresponding zirconocene complexes 2 likewise did not show any isomerization activity [15]. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Zirconocene complexes of two strained cyclopropenes, bicyclo[3.1.0]hex-5-ene and bicyclo[4.1.0]hept-6-ene, are prepared similarly by warming benzene solutions of the corresponding mixtures of the stereoisomeric exo-exo, exo-endo and endo-endo bis-(bicycloalkyl)zirconocenes (equation 218)41. Since only the exo-exo and exo-endo isomers have at least one cis jS-hydrogen required for the elimination, the product mixture contains two isomeric cyclopropene complexes and the unreactive endo-endo bis(bicycloalkyl)-zirconocenes. 

So, the combination of the isomerization process described in Scheme 33 with the elimination reaction described in Scheme 29 would lead to an efficient preparation of stereodefined metalated zirconocene complexes 95 from simple unsaturated enol ether 94 (Scheme 34). 

In most cases, complexes of primary alkyl ligands are more stable than the isomeric complexes of secondary or tertiary alkyl ligands. For example, Reger has shown that the secondary butyl iron complex in Equation 3.18 isomerizes to the corresponding primary n-butyl complex, and that the isopropyl palladium complex in Equation 3.19 isomerizes to the more stable -propyl isomer. Likewise, secondary zirconocene alkyl complexes isomerize to the linear isomers (Equation 320), as shown many years ago by Schwartz, and Labinger and Bercaw have recently shown that the sec-butyl complex of zirconocene, generated by the hydrozirconation of cis-2-butene, isomerizes in several hours to the corresponding n-butyl complex. ... 

Brandenburg JG, Bender G, Ren J, Hansen A, Grimme S, Eckert H, Daniliuc CG, Kehr G, Erker G (2014) Crystal packing induced carbon-carbon double-triple bond isomerization in a zirconocene complex. Organometallics 33 5358-5364... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Some of the bicyclo[3.1.0]hexene zirconocene derivatives, especially those containing a phenyl group, rearrange quantitatively to the corresponding 4-vinyl-l-metalla-2-cyclobutene derivatives when heated in toluene to 60-80 °C for several hours (equation 131). The 6,6-dimethyl-2,3-diphenyl-2-zirconabicyclo[3.1.0]hex-3-ene derivative prefers ring-opening to the isomeric j/. -allylic complex upon thermolysis. 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

Organozirconates. Zwitterionic zirconocene-ate see Ate Complexes) intermediates have been evoked to explain some reaction mechanisms (see Section 2.3.3). In fact, electron transfer from an organic ligand to 16-electron metal center permits the generation of a carbocationic center which can, then, undergoes isomerization and rearrangement characteristic of carbocations. It is now possible to isolate and characterize stable zwitterionic phosphonium zirconate complexes (eqnation 18). ... 

Chiral C2-symmetric early-late complexes can be obtained via similar complexation starting from bis phosphinoenolato zirconocene. Assembly of phenyl-substituted phospha-metallocene and [(binap)Rh(cod)] + leads also to a chiral hetero dinuclear complex. The unprecedented thermal room temperature isomerization ofZr derivative was exploited to achieve its dynamic resolution producing enantiopure bimetallic ansa metallocene. ... 

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. 

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