Scandium complex 85, activity

In recent years, many chiral catalysts for the enantioselective synthesis of optical active 1,5-dicarbonyl compounds have been developed, such as chiral crown ethers with potassium salt bases and chiral palladium complexes, including bimetallic systems. Nakajima and coworkers reported on enantioselective Michael reactions of S-keto esters to a,/3-unsaturated carbonyl compounds in the presence of a chiral biquinoline N,N dioxide-scandium complex, which catalyzed the additions in high yields and with enan-tioselectivities up to 84% ee . Kobayashi and coworkers found that the combination of Sc(OTf)3 with the chiral bipyridine ligand 149 (equation 41) was also effective as a chiral catalyst for asymmetric Michael additions of 1,3-dicarbonyl compounds 147 to a,/3-unsaturated ketones 148. The corresponding Michael adducts 150 were obtained in good to high yields with excellent enantiomeric excesses in most cases (Table 10). 

The base-free dimethyl Sc complex 121 was a highly active catalyst precursor for ethylene polymerization under B(C6F5)3, trityl borate, or methylaluminoxane (MAO)-type activation. The catalytic activity of 121 was similar to those observed of Group 4 metallocene complexes [81]. Generally, cationic scandium complexes are believed to be the active species. Activation of the catalyst was studied by reacting 120 and 121 with various equivalences of B(C6F5)3. The monomeric bulky rBu-substituted dimethyl complex 121 reacted with 1 equiv of B(C6F5)3... 

Additional catalytic investigation of p-diketiminate scandium complexes by Piers and coworkers showed that well-characterized complexes 121 and 122 with the bulky ligand L27 were highly active catalysts for intramolecular hydroamination to form nitrogen heterocycles. The catalytic reaction was monitored by determining starting material and product with NMR. Both the neutral complex 121 and the CIP complex 122 are effective catalysts (10 mol%) for the intramolecular hydroamination of 5-phenyl-4-pentyl-l-amine (R = H, R = Ph, n = 1 in Scheme 42). However, they are not active catalysts for the potential application to the intermolecular hydroamination of 1-hexyne with alkylamines [82],... 

Ishihara, K., Kubota, M., and Yamamoto, H. 1996. Anew scandium complex as an extremely active acylation catalyst. Synlett 265-266. 

In this section we reviewed the latest theoretical studies on the metathesis reaction. In general, it has been shown that the metathesis reaction at first yields adduct complex then activation of the H-R bond takes place with a four-center transition state, which leads to the second adduct. It occurs more easily for scandium complexes than for lutetium analogues. The a-metathesis reactivity between R-H and M-R bonds decreases in the order of R C2H... 

SCHEME 13.1 Activation of a scandium complex forming a dicationic, C3-symmetric scandinm catalyst that is capable of polymerizing 1-hexene to isotactic poly(l-hexene) (TMS = trimethylsilyl). 

Very recently, Hou and coworkers have reported that rare earth half-metallocene complexes (Figure 14.16, 85-88) activated with [Ph3C] [B(C6F5)4] afford highly active systems for syndio-specific styrene polymerization, producing sPS with high syndiotacticities rrrr > 99%) and rather narrow polydispersities (Mw/Mn = 1.29-1.55). The activity of scandium complex 85 is comparable with that for the most active titanium catalysts (1.36 x 10 g sPS/(mol Sc-h)). The neutral allyl lanthanide complexes 89-92 (Figure 14.16) in the absence of a cocatalyst are also active for the syndiospecific polymerization of styrene (rrrr > 99%), but with lower activities that are in the order... 

Amino complexes of rare-earth elements can undergo intramolecular alkene insertion rather than alkene activation followed by nucleophilic attack (Scheme 6.59). Treatment of the amino diene 6.170 with a scandium complex 6.174 resulted in insertion of one double bond to give a d5-pyrrolidine 6.171. Raising the reaction temperature then caused insertion of the second alkene to give the pyrrolizidine 6.172. The thiophene group had been chosen to activate the first alkene it also serves as a surrogate alkyl group, and the synthesis was completed by desulfurization-reduction. The product is the alkaloid, xenovenine 6.173 (see Schemes 6.65 and 6.74 for related syntheses, and Scheme 9.46 for a different approach.)... 

The exploration of the chemistry of terphenyl derivatives of the group 3 metals is due mainly to Rabe and coworkers [13-18]. The ligands used were of the formula C6H3-2,6-Ar2 (Ar = CeHs, C6H2-2,4,6-Mc3, 1-naphthyl, or 3-MeO-C6H4). The halide complexes 1-3 could readily be obtained by simple salt metathesis from the reaction of the terphenyl lithium with anhydrous metal trichlorides MCI3 (M = Sc or Y) in THF at room temperature [13,14], The yttilum complexes 2 and 3 were isolated in moderate yield (ca. 50%) however, only a low yield of the scandium complex 1 could be obtained, most likely because of C-H bond activation as indicated by NMR spectroscopy. These metatheses reactions did not proceed in aromatic solvents or hexanes probably as a result of the low solubility of the reactants in these media. Complex 1 decomposes slowly in THF solution, while 2 and 3 are considerably more stable. 

A cationic scandium complex, [(C5Me4SiMe3)Sc(CH2SiMe3)] [B(QF5)4] , showed remarkable catalytic activity for syndiospecific styrene polymerization, and copolymerization with ethylene afforded multiblock copolymers... 

Scandium A computational study on the C-H addition of a-picoline (2-MeC5H3N) and other pyridine derivatives to terminal olefins RCH=CH2, catalysed by cationic scandium complexes, has demonstrated that the reaction rate is controlled by generation of the active metal-pyridyl species and an insertion step. In agreement with the experimental observation, formation of the branched product 6-[RCH(Me)-(2-MeC5H3N)] is both kinetically and energetically favourable over that of the linear product. ... 

The guanidinate-supported titanium imido complex [Me2NC(NPr02l2Ti = NAr (Ar = 2,6-Me2C6H3) (cf. Section IILB.2) was reported to be an effective catalyst for the hydroamination of alkynes. The catalytic activity of bulky amidinato bis(alkyl) complexes of scandium and yttrium (cf. Section III.B.l) in the intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentenylamine has been investigated and compared to the activity of the corresponding cationic mono(alkyl) derivatives. 

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