Scandium half-sandwich complexes

Scheme 7.4 Activation of scandium half-sandwich complexes.

Scandium and Ytrium A new a-alkylation of pyridines via C—H addition to terminal alkenes RCH=CH2, styrenes, and conjugated dienes has been attained, with cationic Sc and Y half-sandwich complexes as catalysts in conjunction with (QH5)3B. The method provides a straightforward access to alkylated pyridine derivatives, namely the branched a-PyCH(R)Me with Cp ScX2 as catalyst, and linear PyCH2CH2Ar using styrenes and Cp YX2. ... 

In contrast, the 1.8 A diameter of the cavity in the tetradentate 12-crown-4 ring is too small for a [ScCy ion to fit, so an extra-cavity (half-sandwich) structure (12) has been suggested for [ScCl2(12-crown-4)(MeCN)] SbCl6, though no diffraction data are available. Unlike the intracavity crown ether complexes already mentioned, a MeCN is believed to coordinate, again reflecting scandium s preference for seven-coordination. ... 

Tardif reported recently that the cationic half-sandwich lanthanide amido complexes [(Ind)Ln N(SiMe3)2 ][B(C6Fs)4] (29, Fig. 3) were also highly efficient and c/x-l,4-selective for butadiene polymerization [115]. Meanwhile, Visseaux demonstrated that the half-sandwich scandium borohydride complex Cp Sc(BH4)2(THF) (30, Fig. 3) combined with [Ph3C][B(C6Fs)4] and TIBA led to the very active and highly stereoselective isoprene polymerization (>90% c/x-1,4. Table 11) as well as styrene (>99.9% syndio, Table 12). Improvement of the control of the polymerization was performed at lower temperature at — 10°C that the cm-1,4-ratio increased up to 97.2% followed by the decrease of PDI down to 1.7 [116]. This... 

Fig. 17 Structures of half-sandwich scandium alkyl complexes 97 to 102...

The first molecular structure of half-sandwich lanthanide porphyrins was reported by Schaverien and Orpen (1991). The lutetium alkyl complex Lu(OEP)[CH(SiMc3)2] has a structure resembling that of the scandium analog (Arnold et al. 1993), exhibiting a square pyramidal geometry. The five-coordinated lutetiiun atom lies 0.918 A above the N4 plane, and the porphyrin ring also displays a saucer shape. 

Scheme 48 A direct, enantioselective alkylation of pyridines by a cationic half-sandwich scandium complex.

The half-sandwich scandium dialkyl complex combined with [Ph3C][B(C6Fs)4] can also catalyze the syndiospe-cific copolymerization of styrene with isoprene. Because of the excellent living character of this catal5dic system, the... 

Hitzbleck, I, Beckerle, K., Okuda, J. Half-sandwich dibenzyl complexes of scandium Synthesis, structure, and styrene polymerization activity. J. Organomet. Chem., 692,4702-4707 (2007). 

Recently Hou reported the first example of efficient COG synthesis by rare earth metal catalysts. Cationic rare earth (group 3 and lanthanide) metal alkyls is an emerging new class of catalysts for the polymerization and copolymerization of various olefins including cyclic olefins. The combination of half-sandwich scandium bis (alkyl) complexes such as... 

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