Samarium systems

Block copolymers of olefins and acrylates or vinyl esters can be obtained with lanthanoid compounds as catalysts. Living polyethylene-biscyclopenta-dienyl samarium systems can continue the polymerization of cyclolactone monomers by ROMP (273, 274). 

Block copolymers of olefins and acrylates or vinylesters can be obtained by lanthanoid compounds. Living polyethylene-biseyclopentadienyl samarium systems can continue the polymerization with acrylate monomers by group transfer polymerization or cyclolactone monomers by ring opening polymerization [216, 217]. 

To avoid secondary reductive steps, low concentrations of the active low-valent metal are advantageous, i.e. the use of catalytic one-electron reduction cycles [31]. Low-valent vanadium species are efficient catalysts for pinacolization. After the primary electron transfer, the oxidized vanadium catalyst can be reduced by Zn(0) or by aluminum [32], TrialkyIsilanes have to be added to decomplex the oxidized vanadium catalyst from the pinacol [33]. The same catalytic system can also be used for the synthesis of 1,2-diamines via coupling of aldimines [34]. Alternatively, a titanium (Il)-samarium system gives the 1,2-diamines with moderate D,L-selectivity [35]. 

Fig. 23. (1 lattice spacings in the cerium-samarium system. The straight line represents the Vegard s law relationship for the a lattice spacing in this system based on the data given in table 2. 

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. 

An example, where two C-C-bonds are formed and one C-C-bond is broken is the synthesis of the tricycle 3-285, which has some similarity with the eudesmane framework 3-286, developed by Kilburn and coworkers (Scheme 3.72) [113]. Thus, exposure of the easily accessible methylenecyclopropyl-cyclohexanone 3-281 to samarium(II) iodide led to the generation of ketyl radical 3-282, which builds up a six-membered ring system with simultaneous opening of the cyclopropane moiety. Subsequent capture of the formed radical 3-283 by the adjacent alkyne group afforded the tricycle 3-285 via 3-284 as a single diastereoisomer in up to 60% yield. It should be noted that in this case the usual necessary addition of HMPA could be omitted. 

Opening of a cyclobutane ring fused to a quinolizine system under reductive conditions has been described. Thus, the previously mentioned compound 128 was obtained by treatment of 132 with samarium diiodide (Equation 8)... 

The (C Me ) Sm(THF) metal vapor product provided the first opportunity ta see if Smdl) complexes (y =3.5—3.8 Ufi) could be characterized by H NMR spectroscopy (24). Fortunately, the paramagnetism doesn t cause large shifting and broadening of the resonances and hence samarium provides the only Ln(III)/Ln(II) couple in which both partners are NMR accessible. Once the existence and identity of (C Mej- SmdHF) was known, a solution synthesis was developed from KC Me and Sml THF) (44). This system is the preferred preparative route and also provides another soluble organosamarium(II) complex, [(C Me )Sm(THF)2(u-I)]2, under appropriate conditions. This is another xample of how solution studies subsequently catch up to the research targets often identified first in metal vapor reactions. 

Yahiro, H. Eguchi, Y. Eguchi, K. Arai, H. 1988. Oxygen ion conductivity of the ceria samarium oxide system with fluorite structure. I. Appl. Electrochem. 18 527-531. 

Samarium-mediated Barbier-type reactions of carbonyl compounds were reported in a similar reaction system (Equation (6)).22 THF is the key solvent to obtain the product 33. In MeOH, pinacol coupling-type reaction proceeded predominantly, while the reaction failed to produce any desired compound in CH3CN.23... 

Lugmair GW, Marti K, Scheinin NB (1978) Incomplete mixing of products from r-, p-, and s-process nucleosynthesis Sm-Nd systematics in Allende inclusion EK141. Lunar Planet Sci IX 672-673 Lugmair GW, Shimamura T, Lewis RS, Anders E (1983) Samarium-146 in the early solar system evidence from neodymium in the Allende Meteorite. Science 222 1015-1018 Lugmair GW, Shukolyukov A (1998) Early solar system timescales according to Mn- Cr systematics. Geochim Cosmochim Acta 62 2863-2886... 

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