Lanthanide propanolates

Diene polymerization via lanthanide propanolate-based catalytic systems was first reported by U. Pedretti et al. [162], Several types of binary and ternary initiator systems have been developed since the late 1970s (see Table 7), including zz- and zsopropanolalc ligands [162], homo- and heteroleptic lanthanide species [163-165], and different types of organoaluminum cocatalysts [162,166-168]. Despite such extensive studies, structural evidence of the formation of alkylated rare-earth metal centers remained scarce. 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

Complexes of alcohols like methanol, ethanol, 2-propanol and n-butanol (116-122), and ethers like Diox (47,120,123-125) and THF (126-128) have been prepared. The bonding between these ligands and the metal ions is considered to be very weak. In recent years, complexes of the lanthanides with a few macrocyclic polyethers have been reported. Cassol et al. (129) have prepared the complexes of benzo-15-crown-5 and dibenzo-18-crown-6 with lanthanide nitrates and isothiocyanates. King and Heckley (130) have also reported the complexes of these ligands with lanthanide nitrates. The heavier lanthanide nitrate complexes of dibenzo-18-crown-6... 

Meerwein-Ponndorf-Verley-Type Reduction Reduction of ketones by 2-propanol or related alcohols, known as Meerwein-Ponndorf-Verley (MPV) reduction, is promoted by various metal alkoxides, typically aluminum 2-propoxide [2a,d,281]. The C2 hydrogen of 2-propanol is transferred directly to the carbonyl carbon through a six-membered pericyclic transition state [284], Earlier, a stoichiometric quantity of a metal alkoxide was required for this purpose, but recently, lanthanide [285] and aluminum [286] complexes acting as excellent catalysts have been reported. 

To avoid the complication of hydrolysed product or partial decomposition during dehydration, anhydrous diketonate complexes can be prepared by the reaction of diketone with lanthanide 2-propanolate [47] in benzene, with a lanthanide hydride or with metallic europium [48]. 

Lanthanides form complexes with Schiff bases or imines quite readily. The important ligands are obtained by the condensation of an amine with salicylaldehyde derivative or a /6-diketone. Schiff base complexes have been prepared by the reaction of ligand with a lanthanide 2-propanolate. The solvent medium is benzene and the liberated propanol during the complexation reaction is removed by distillation as alcohol-benzene azeotrope [57,58]. 

Overall stability constants for the formation of lanthanide chloro- and bromo-complexes in anhydrous methanol, ethanol, and propanol, as determined by spectrophotometry. 

In the case of photolysis of acidic aqueous solutions of Eu2+ ion at 366 nm, the excited state results from 4f —> 5d transitions localized on the metal centre. These excited states have also considerable MLCT character because of strong mixing of metal 5d orbitals with ligand orbitals. In the case of the reaction of Eu3+ with H2 which occurs on photolysis at 254 nm, the photo reaction is due to the formation of an LMCT excited state. This process has been successfully used in the photochemical separation of Eu from other members of the lanthanides because Eu2+ is the only member of the lanthanide series which is at suitably low energy that an LMCT state is accessible [98]. Yb3+ and Sm3+ ions behave in a similar fashion to Eu3+ as far as their photochemical behaviour is concerned. Aqueous solutions of Sm3+ or Yb3+ containing 2-propanol on photolysis at 185 nm give hydrogen and acetone as products probably by a mechanism similar to Eu3+ ion. 

High catalytic activities, with turnovers of up to 9(X) cycles min , is displayed in the transfer hydrogenation of a,p-unsaturated ketones, such as benzylideneacetone and chalcone, using 2-propanol and catalytic amounts of [Ir(3,4,7,8-Me4-phen)COD]Cl (phen = 1,10-phenanthroline COD = 1,5-cyclo-octadiene) in a weakly alkaline medium. Other Ir-chelated complexes are also active catalysts in this reaction, with over 95% selectivity for the 1,4-reduction mode. Divalent lanthanide derivatives, such as Sml2 or Ybh in stoichiometric quantities, in THF and t-butyl alcohol or methanol reduce ethyl cinnamate and cinnamic acid to give the saturated derivatives. " Similarly, 3-methylcyclohexenone is reduced to 3-methylcyclohexen-l-ol in 67% yield, but a,p-unsaturated aldehydes are nonselectively reduced with these systems. 

Chirally modified lanthanide alkoxides [prepared in situ from Ln(0-/-Bu)3 and chiral ligands] give higher asymmetric induction in the reduction of acetophenone (for La. 32%ee) than the normally used aluminum or alkali metal alkoxides208. However, the decrease of ee values with increasing reaction time shows that kinetic control is not maintained with 2-propanol as the reducing agent. 

The selectivities of rare earth oxide catalysts for dehydration of 2-propanol and butanols have been examined recently by Bernal and Trillo (1980) as a function of reaction temperature. No definite variations of selectivity have been observed along the lanthanide series, but the highest percentage of 1-butene was found on H02O3 and LU2O3 and the lowest on La20j. This is opposite to what was expected on the basis of the respective basicities of these oxides. Bernal and Trillo (1980) have noted that the reaction temperature influences product distributions, and that care has to be exerted in establishing trends of a series of related catalysts because of these temperature effects. 

The FTICR-MS examination of the reactivity of rare earth cations with methanol, ethanol, and /-propanol was later extended to aU the lanthanide series cations by Carretas et al. (2004). All the lanthanide ions reacted to form LnO" " and LnOH" " ions as main primary products, showing strong similarities with the results obtained by Geribaldi and coworkers with the same alcohols. The primary product ions participated in subsequent reactions, which through intermediate species such as Ln(OH)2 and Ln(OH)(OR)" led mainly to Ln(OR)2(HOR) ions, where n = 0-3 as is often the case, Eu constituted an exception as it only formed Eu(OR)(HOR) species, due to the stability of its formal oxidation state 2+. The overall efficiency of the reactions along... 

The most active metals, such as lanthanides, receive really just a support for the direct interaction with alcohols (2-propanol in this case) from the applied anodic potential supposedly via the elimination of the oxide barrier. The electric current yields (the ratio of the alkoxide obtained to the total charge that passed through the system) often exceed 100%. High concentrations of soluble conductive additives (LiX or R4NX, where X = Cl, Br), which contaminate the product have to be removed by repeated recrystallization from hydrocarbon solvents. 

Complexes of the bisethoxydithiophosphinate ion [(C2HsO)2PS2] for all the lanthanides are readily obtained by adding the tetraphenylarsonium ion to an ethanolic solution of the ligand and the hydrated lanthanide chlorides (Pinkerton, 1974). Recrystallization from ethanol/iso-propanol mixtures yields the tetrakis complexes [(C6Hs)4As]RL4. The sodium salts can also be prepared although they... 

Lanthanide compounds such as yttrium and lanthanum alkoxides have been reported to yield high-molecular-mass polyesters under mild conditions. The yttrium alkoxide-initiated polymerization of CL proceeded rapidly at room temperature [27-29], while the use of bulky groups reduced the transesterification reaction such that polymers with a narrow molecular mass were obtained (Scheme 11.3). For example, the bulky phenoxide ligands of the yttrium or lanthanide catalyst were exchanged for the smaller alcohol (2-propanol), followed by coordination and insertion of the monomer (CL) [27]. 

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