Catalyst yttrium-based

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

More recently, a very efficient yttrium-based catalyst, yttrium 2-methoxyeth-oxide, has been applied successfully for /f-butyrolactone polymerisation which proceeded easily at room temperature [99], It is worth mentioning that rare-earth metal alkoxides (derived from yttrium and lanthanum) exhibit outstanding efficiency as catalysts for the polymerisation of cyclic esters such as e-caprolactone [132] and lactide [133]. 

This yttrium-based catalyst was then applied to silylation of alcohols (Scheme 12.77) [175]. A variety of alcohols reacted with hexamethyldisilazane... 

Actinide, lanthanide, and yttrium-based catalyst systems showing characteristics of reversible chain transfer in ethylene polymerization are summarized in Table 3. Samsel and Eisenberg claimed to observe the characteristics in ethylene polymerization with several metallocenes of actinides, such as the bis(pentamethylcyclopentadienyl) thorium complex 5 in combination with aluminum alkyl reagents. These systems catalyze the production of aluminum alkyl chain growth products at lower temperatures than those required by the uncatalyzed Ziegler process. The systems were limited to production of low-molecular-weight PE oligomers. 

Kempe reports CCTP in ethylene polymerization using the yttrium-based catalyst system 8/borate combined with a variety of aluminum alkyls.Although many aluminum alkyls show... 

Table 3 Actinide-, lanthanide-, and yttrium-based catalyst systems for CCTP of ethylene...

In 1994, a highly efficient yttrium-based catalyst-yttrium 2-methoxyethoxide-was applied successfully to the polymerization of BL, with the reaction proceeding readily at room temperature [55]. When compared to diethylzinc/water or diethyl-zinc/methanol catalytic systems, the enantioasymmetric polymerization of racemic BL was also carried out in the presence of a diethylzinc/(R)-(-)-3,3-dimethyl-l,2-butanediol catalytic system (Scheme 9.18) [56]. 

Me3Si)2NH, Me3SiCl, Pyr, 20°, 5 min, 100% yield. ROH is a carbohydrate. Hexamethyldisilazane (HMDS) is one of the most common silylat-ing agents and readily silylates alcohols, acids, amines, thiols, phenols, hydroxamic acids, amides, thioamides, sulfonamides, phosphoric amides, phosphites, hydrazines, and enolizable ketones. It works best in the presence of a catalyst such as X-NH-Y, where at least one of the groups X or Y is electron withdrawing." Yttrium-based Lewis acids also serve as catalysts. ... 

Lanthanide-based catalysts, despite finding a lot of application in homogeneous catalysis, can be rather problematic due to the lability of some ligand types and the versatility of their coordination chemistry in the -1-3 oxidation state this makes the controlled synthesis of single-site Ln complexes a quite ambitious goal [92]. McLain and coworkers first demonstrated the high potential of a homoleptic yttrium complex Y(OCH2CH2NMe2)3 as ROP catalyst for the preparation of PLA from rac-lactide and that it promotes a rapid and controlled polymerization... 

An yttrium-based strong Lewis acid, prepared by treatment of aqueous solutions of yttrium nitrate and zirconyl nitrate (molar ratio 16 84) with aqueous ammonia (28%), was an active catalyst of a wide range of Diels-Alder reactions [51], Among the reactions investigated was that of acrolein with dihydropyran (Scheme 2). In contrast with reactions with H-form zeolites [11] no regio- and stereoisomers were formed. 

Broderick and co-workers developed one of such alkoxide based catalysts for LA ROP reaction by using ferrocene derivatives, 31-32 [35]. They showed that by using yttrium t-butoxide, LA ROP reaction could be proceeded with 31 but clogged with 32. Consequently, they designed a catalytic system that stopped by addition of FcBAr and resumed by addition of CoCp. Nevertheless, if the metal alkoxide changed to indium phenoxide, this effect will be reversed. 

Some examples of sequence-controlled polymers prepared by ring opening polymerization of cyclic esters have been reported. For instance, Thomas, Coates, and coworkers reported a very original method for the synthesis of alternating aliphatic polyesters.These authors used a stereoselective yttrium-based organometallic catalyst to control comonomer sequences. It had been previously shown that the polymerization of a racemic mixture of p-butyrolactone in the presence of this catalyst led to the formation of... 

A large variety of rare earth derivatives have been used to initiate ROP of cyclic carbonates. Their usually high reactivity must be emphasized, as exemplified by the polymerization of TMC with Ln(OAr)3 (Ln = Lanthanide = f-block elements) under mild conditions, in contrast to the long reaction time and high temperature required when tin-based or aluminum-based catalysts were used. As rare earth metals La (lanthanum), Ce (cerium), Nd (neodymium), Sm (samarium), Gd (gadolinium), Dy (dysprosium), Er (erbium), Yb (ytterbium). Sc (scandium), and Y (yttrium) are most often applied. ... 

The major drawback of the nickel catalyst is poisoning of Ni surface by the interaction with CO to form nickel carbonyl at low temperature. Noble metal based catalysts are more active and stable catalyst in comparison with Ni based catalysts [100]. Among Ru nanoparticle dispersed on various supports (Al Oj, MgAljO, MgO, C, etc ), Ru/Al Oj showed highest catalytic activity (turnover frequency, TOF = 16.5 x 10 s" ) [101,102]. Yttrium addition to the Ru-based catalyst enhances the activity and stability for methanation reaction [103], Pd/Mg-SiO and platinum titanate nanotubes were also foimd... 

Intermolecular Hydrosilylation of Internal Alkynes 10.17.3.5.1 Yttrium catalysts for regiocontrol based on sterics... 

The addition of trimethylsilyl (TMS) cyanide to aldehydes produces TMS-protected cyanohydrins. In a recent investigation a titanium salen-type catalyst has been employed to catalyse trimethylsilylcyanide addition to benzaldehyde at ambient temperature1118]. Several other protocols have been published which also lead to optically active products. One of the more successful has been described by Abiko et al. employing a yttrium complex derived from the chiral 1,3-diketone (41)[119] as the catalyst, while Shibasaki has used BINOL, modified so as to incorporate Lewis base units adjacent to the phenol moieties, as the chiral complexing agent11201. 

The supported silylamide complexes display lower activities than the homogeneous counterparts Ln[N(SiMe3)2]3, with the yttrium (12c) and samarium surface complexes (12d) revealing the worst performance. Conversely, the yttrium sUyl-amide-based material 12c gave complete conversion of hept-l-yne after 15 h at 100 °C. It was speculated that the relatively high tail-to-head selectivity of 82% Pj, obtained for both fresh and recovered catalyst 12c, could be due to the bulky siloxo surface ligand [117]. 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

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