Scandocenes

Fontaine, F.-G and Tilley, T.D. (2005) Control of selectivity in the hydromethylation of olefins via ligand modification in scandocene catalysts. Organometallics, 24, 4340. 

There are some recent reports wherein (3-methyl elimination is directly observed with well-defined metallocene derivatives of highly Lewis-acidic early transition metals. Bridged scandocene-isobutyl complex 48 decomposes to scandocene-methyl complex 49 along with propene, which is ultimately transformed to various hydrocarbons [68]. 

A cyclobutylmethyl-metal system provides another opportunity to observe 13-carbon elimination. The ring opening process harnesses the release of the least necessitating ring strain of a four-membered ring. Scandocene hydride 63 reacts with 3-methyl-l,4-pentadiene to afford the linear Ji-allyl complex 65 [80]. The intermediacy of cyclobutylmethyl complex 64 which undergoes P-carbon elimination accommodates the observed rearrangement. 

In the presence of scandocene hydride 63,3-methyl-l,4-pentadiene is catalyt-ically converted to methylenecyclopentane and its isomer via cyclobutylmethyl-metal intermediate 64 [80]. 

While rare-earth metals are proven powerful olefin polymerization catalysts [21-24], there are only limited reports on controlled olefin oligomerizations or selective olefin dimerizations utilizing these elements [204,207,208], An ansa-scandocene [207] and the bis(indenyl)yttrium complex 41 (Fig. 25) [204] were reported to produce head-to-tail dimers from monosubstimted aliphatic alkenes (57). Complex 41 produces predominantly the tail-to tail adduct with styrene. The codimerization of an aliphatic alkene (including substrates containing various functionalities) with styrene affords tran -tail-to-tail dimers, apparently as a result of 1,2-insertion of the a-olefin followed by 2,1-insertion of styrene directed by the phenyl group (58). 

The profile for scandocene is very similar to those of its Zr counterparts, as shown in Table VI, but is at higher energies relative to the reactants due to the lack of stabilization from the ion-dipole interaction present in the... 

Finally, similar results were obtained with neutral scandocene catalysts by Piers and Bercaw. They observed similar kinetic isotope effects in the hydrocyclizations and hydrodimerizations of deuter-ated species. 

Like 3 hydrogen eliminations, 3-alkyl eliminations require an open coordination site. This site is generated in the scandocene system by dissociation of PMej and in the zirconocene and hafnocene complexes by dissociation of the borate from the zwitterionic species. - The open coordination site is generated in the platinum system by abstraction of chloride and is generated in the ruthenium complex by dissociation of the monodentate phosphme. - The mild conditions for 3-methyl elimination from the ruthenium metalla-cycle is surprising, considering that it would seem to require the propellane-type transition state shown in Equation 10.21. 

Progress on the addition of aromatic C-H bonds to olefins has been made by Periana with iridium catalysts - - and Gunnoe with ruthenium catalysts. - Both systems illustrate that the anti-Markovnikov addition products can be generated in larger quantities than the Markovnikov products, although mixtures of regioisomers are still observed. Intramolecular additions of the C-H bonds of electron-rich heterocycles to electron-deficient alkenes have also been reported (Equation 18.65). Most recently, Tilley has reported the addition of the C-H bond of methane across an olefin catalyzed by scandocene complexes. This reaction occurs, albeit slowly, with Markovnikov regiochemistry. 

In addition to zirconocenes, C -symmetric scandocenes and yttrocenes with two dimethylsilyl interannular linkers (38a-c, 39, Figure 4.18) have been synthesized and tested as polymerization catalysts for propylene and 1-pentene." " Owing to the electronics of these group 3 catalysts (14-electron, d electronic configuration) no cocatalysts are required. However, metallocene chlorides cannot be used for polymerization metallocene alkyl or hydride compounds are necessary. 

Yttrocene tetramethylaluminate complex 38a was found to be less reactive towards ethylene and a-olefins than the corresponding scandocene, 38b. This reactivity trend is consistent with the observed reactivity for 39/H2(g). Compounds 38a-c and 39/H2( ) were tested for the polymerization of propylene and 1-pentene. 1-pentene polymerizations were carried out in neat monomer, either at 0 °C or at room temperature ( 22 °C). Propylene polymerizations were carried out either in neat monomer or with a 50/50 (v/v) mixture with toluene, all at -5 °C. In general, scandocenes 38b and 38c provided higher polymer Mn values and were more active for polymerization than yttrocenes 38a and 39/H2(g). 

Representative poly( 1-pentene) tacticity data is provided in Table 4.4 representative polypropylene tacticity data is provided in Table 4.2. Poly(pentene) was produced using neat 1-pentene poly(propylene) was produced using neat propylene or a 50/50 (v/v) mixture with toluene. These scandocenes and yttrocenes provide atactic polymers. The tacticity values range from [r] = 28.62% to [r] = 52.12% for poly( 1-pentene) samples, and from [r] = 48.77% to [r] = 61.13% for poly(propylene) samples. Within experimental error, polymerization temperature and monomer dilution appear to exert a minimal effect on polymer tacticity. 

Barros N, Eisenstein O, Maron L, Tilley TD (2008) DFT investigation of the catalytic hydromethylation of olefins by scandocenes. 2. Infiuence of the ansa ligand on propene and isobutene hydromethylation. Organometallics 27 2252-2257... 

On the other hand, scandocene hydride 10 prompted isomerization of penta-1,4-diene to isoprene (2-methylbutadiene) via a cyclopropylmethylmetal intermediate (Scheme 7.4) [5]. 

Bercaw reported an interesting reversible branching reaction of 1,4-penta-diene derivatives catalyzed by a scandocene hydride complex as shown in Scheme 6 [24]. 

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