Metallocenes addition reactions

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Polymerization Reactions. Polymerization addition reactions are commercially the most important class of reactions for the propylene molecule and are covered in detail elsewhere (see Olefin polymers, polypropylene). Many types of gas- or liquid-phase catalysts are used for this purpose. Most recently, metallocene catalysts have been commercially employed. These latter catalysts requite higher levels of propylene purity. 

On the basis of these considerations, it is possible to classify and to understand the potential of metallocene-assisted reactions for organic syntheses. Additionally, it is possible to investigate which functional groups are tolerated in a considered reaction. 

Teuben et al. reported a related reaction, in which the thermolysis of alkyl-bis(pentamethylcyclopentadienyl)titanium(III) caused a dissociation of alkane with formation of 64. further heating at 150°C resulted in hydrogen dissociation and formation of 70 as a diamagnetic material. Subsequent reaction with acetophenone gave chelate complex 71 in 60% yield, which was fully characterized spectroscopically as well as by an X-ray structure analysis (Scheme 10.24). Interestingly, the authors do not mention the presence of two diasteromers, although the alkyl chain bears an asymmetric carbon atom in addition to the planar chirality of the metallocene. Similar reactions were observed with pyridine derivatives [72]. 

Following this theme, a series of ruthenium metallocene complexes were phosphinated using a microwave-assisted approach (Scheme 4.181) [271]. The yield of the arylphos-phine was moderate and the reactions were quite fast. While only a single set of substrates were screened in the microwave-assisted protocol, it does provide the proof of concept for the design of additional reactions. It should be noted that the temperature reached in this reaction is far above the boiling point for dichloromethane. Appropriate safety measures should be taken, and caution should be used when opening the reactor vials. 

Cole and co-workers study" of the electrochemical ionization of metallocenes in ES-MS cites several examples of the occurence of chemical follow-up reactions. For example, the electrochemical oxidation products of ruthenocene (Cp2Ru ) and osmocene (Cp20s ) are known to lack solution-phase stability and to be very susceptible to nucleophilic addition reactions. The ES mass spectra of these metallocenes sprayed at a relatively slow flow rate (1.6pL/min) from a methylene chloride/0.5-1.0% trifluoroacetic acid (TEA) solution include peaks corresponding to the chloride ion addition products ([Cp2RuCl]and [CP2OSCI] ) and, in the case of osmocene, the trifluoroacetate addition product ([CP2OS - - trifluoroacetate] ). 

Xu, X. Nolan, S. P Cole, R. B. Electrochemical oxidation and nucleophiUc addition reactions of metallocenes in electrosjaay mass spectra. AnaL Chem. 1994, 66, 119 125. 

The studies summarized above clearly bear testimony to the significance of Zr-based chiral catalysts in the important field of catalytic asymmetric synthesis. Chiral zircono-cenes promote unique reactions such as enantioselective alkene alkylations, processes that are not effectively catalyzed by any other chiral catalyst class. More recently, since about 1996, an impressive body of work has appeared that involves non-metallocene Zr catalysts. These chiral complexes are readily prepared (often in situ), easily modified, and effect a wide range of enantioselective C—C bond-forming reactions in an efficient manner (e. g. imine alkylations, Mannich reactions, aldol additions). 

By using different Cp ligands (Cp, Cp, ebthi), additional ligands (THF, pyridine, acetone), and metals (Ti, Zr), a fine-tuning of the reactions of these complexes has been feasible. Additional influences are exerted by, e. g., the substituents on the substrate, the stoichiometry used, the solvents, and other reaction conditions. Complexes of this type have also been prepared and used in connection with a multitude of substrates, and in many cases the products differ markedly from those obtained with conventional metallocene sources. 

In addition to the previously described coupling of two diynes at a single metal center, there are examples of coupling reactions of two diynes between two metallocene fragments (complexes 22 and 23). Acidolysis of complex 22 gives the trans-3,4-dibenzyli-dene-l,6-diphenyl-l-hexen-5-yne 29 [26], The corresponding reaction of complex 23 (Fig. 10.4) does not lead to the expected radialene 30, which seems to be unstable, but forms, after an H-shift, the ds-3,4-dibenzylidene-l,6-diphenyl-l-hexen-5-yne 31 (Fig. 10.5) [26],... 

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

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