Samarium catalytic

In comparison to samarium and ytterbium salts, there were few examples for cerium, praseodymium, and other the rare earth metals catalyzed aldol reaction (214,215). In 2000s, Samarium salts, especially Sml2, have been used in versatile aldol reactions, for example, direct aldol of aldehydes and substituted oxi-ranyl ketones (216), nitro aldol reaction (217,218), intramolecular aldol reaction (219), and other aldol reaction of special carbonyl compounds (220-222). However, catalytic asymmetric samarium-catalytic aldol reaction was not reported so far. In the asymmetric version of the aldol reaction, ytterbium exhibited promising enantioinduction. In the first example of the asymmetric ytterbium-catalyzed aldol reaction, moderate levels of enantioselectivities were achieved (Scheme 56) (223). Subsequently, Mlynarski and co-workers improved enatioinduction ability of the ytterbium-catalyzed aldol reaction by using catalytic amount of Yb(OTf)3... 

Other catalytic uses of rare-earth compounds have not reached the same development. Neodymium salts are, however, used for mbber manufacturing (22). Divalent samarium haHdes are employed in organic synthesis (23). 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

Ni-cermet type anodes have been improved by substituting the YSZ by ceria, gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC). Ceria seems to increase the catalytic activity of the cermet for hydrogen oxidation, while SDC and GDC improve the ionic conductivity of the anode. Ni-ceria cermets are considered the main candidate for low-temperature SOFC [74],... 

Recently, rare-earth metal complexes have attracted considerable attention as initiators for the preparation of PLA via ROP of lactides, and promising results were reported in most cases [94—100]. Group 3 members (e.g. scandium, yttrium) and lanthanides such as lutetium, ytterbium, and samarium have been frequently used to develop catalysts for the ROP of lactide. The principal objectives of applying rare-earth complexes as initiators for the preparation of PLAs were to investigate (1) how the spectator ligands would affect the polymerization dynamics (i.e., reaction kinetics, polymer composition, etc.), and (2) the relative catalytic efficiency of lanthanide(II) and (III) towards ROPs. 

The diastereoselective cycloaddition of 2-phenyl-4-dimethylamino-l-thia-3-azabuta-l,3-diene with a choice of dienophiles and in the presence of a Lewis acid provides a convenient route to 5,6-dihydro-4//-l,3-thiazines <2002TL6067, 2004T1827>. The more stable /ra r-adducts are produced exclusively. The approach using (4A)-3-acryloyl-4-benzyloxazolidin-2-one 198 provides access to the chiral 5,6-dihydro-4//-l,3-thiazine 199 <2004T1827>. The exceptional level of selectivity is only achieved when magnesium bromide is used. The chiral auxiliary was removed by reaction with lithium benzoxide to give the benzyl ester 200, and reaction with catalytic amount of samarium triflate and methanol provides the methyl ester 201 (Scheme 21). 2-Substituted-5,6-dihydro-l,3-thiazines are conveniently synthesized from nitriles or thiocyanates and 4-mercapto-2-methylbutan-2-ol to produce... 

Aliphatic and aromatic carboxylic esters are also directly converted, in one step, to oxazolines using amino alcohols. As expected, harsh conditions are required for this transformation. Typically, the reaction is performed in refluxing xylene in the presence of catalytic quantities of a Lewis acid such as dibromo- " or dichloro-dimethylstannane. More recently, lanthanide chloride and samarium chloride have also been reported as useful catalysts for this one-pot transformation in refluxing toluene. Representative examples are shown in Table 8.2 (Scheme 8.2). ... 

Samarium(ll) triflate, a halogen-free samarium(ll), can also be prepared by disproprotionation of samarium(iii) triflate and samarium(O) in DMF in the presence of a catalytic amount of mercury. 

The alloy can also be used as a reductant to recycle samarium(iii) to samarium(ii). Because pinacolate ligand exchange from samarium to light lanthanides proceeds smoothly, addition of MesSiCl is not necessary to complete the catalytic cycle (Schemes 4 and 5). 

Diastereomeric ratios of pinacol dimers obtained by coupling of benzaldehyde and acetophenone with samarium reagents are summarized in Tables 3 and 4, respectively. In general, the stereochemical control of pinacol coupling reactions with a stoichiometric or catalytic samarium(ll) reagent is poor, giving nearly a 1 1 dljmeso mixture. 

Evans reported an enantioselective Meerwein-Ponndorf-Verley reduction using a catalytic amount of chiral samarium complex 26 prepared from samarium (III) iodide and a chiral amino diol (Scheme 9.16) [34], Even when a partially resolved ligand (80% ee) was used, the enantiopurity of the resulting alcohol 27 reached 95% ee, which is the same value as that obtained when the enantiopure amino diol was used. 

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