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Samarium alkoxides

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. [Pg.70]

In 1993, Molander found that in the presence of catalytic Fe(III) salts, Sml2 mediates intramolecular Barbier additions to esters to give cyclic ketones (or cyclic hemiketals, if they prove to be stable).135 Double addition to the ester is not observed, nor is reduction of the cyclic ketone product. This suggests that the tetrahedral intermediate, a samarium alkoxide of a cyclic hemiketal, is partially stable to the reaction conditions and the ketone group is not revealed until work-up. Molander found that both alkyl and allyl halides could be used in the additions (Scheme 5.83).135... [Pg.124]

More recently, Concellon has reported a stereoselective method for the formation of ( )-a,p-unsaturated esters that exploits a Sml2 Reformatsky reaction followed by an elimination.141 For example, ethyl dibromoacetate reacts with benzaldehyde in the presence of Sml2 to form samarium alkoxide 126, which is reduced further to give a second Sm(III) enolate 127. Elimination then affords ( )-a,p-unsaturated ester 128 in good yield (Scheme 5.90).141... [Pg.128]

In 2002, Skrydstrup reported the diastereoselective construction of functionalised prolines using a Sml2-mediated aldol cyclisation.162 Treatment of p-lactam-derived a-benzoyloxy esters, such as 155, with Sml2 led to the generation of a Sm(III) enolate 156, aldol cyclisation and addition of the resultant samarium alkoxide to the (3-lactam carbonyl. The efficient sequential reaction gave proline derivatives, such as 157, with high diastereoselectivity and in good yield (Scheme 5.103).162 This example illustrates how the presence of a protic cosolvent does not necessarily interfere with the intramolecular aldol reaction and can in fact be crucial to the success of the cyclisation. [Pg.135]

Reactions of organosamarium(II) halides with aldehydes are sranewhat more complicated and synthetically less useful than those with ketones. The ability of Sm species to serve as strong reducing agents introduces a number of alternative reaction pathways. For example, reaction of EtSmI with benzalde-hyde provides a mixture of benzyl alcohol, benzoin, hydrobenzoin, and benzyl benzoate in low yields. Die first three products presumably arise from benzaldehyde ketyl, generated by single-electron transfer from the Sm reagent to benzaldehyde. The benzyl benzoate apparently is derived frmn a Tischenko-type condensation reaction between a samarium alkoxide species and benzaldehyde. [Pg.254]

Aldehydes rapidly, react with alkyl halides (e.e. ICH ) giving a mixture of products (15). This reaction has no synthetic utility, and was explained by a very fast Meerwein-Ponndorf reaction reaction at the level of intermediate secondary samarium alkoxides. This hypothesis was recently confirmed and gave rise to new developments for the catalyzed Op-penauer-Meerwein-Ponndorf-Verley reaction (see in IV). By taking very reactive halides (benzylic or allylic) it is possible to overcome the side-reaction and to get good yields of alcohols (23) (eq.p4j). [Pg.57]

The lanthanide (especially samarium) alkoxides serve as highly effective cata-lysts ° ° for Oppenauer-type oxidation of alcohols to aldehydes and ketones (Eq. 2.349) ... [Pg.148]

New methods of synthesis of samarium alkoxides catalytically active in Meerwein-Ponndorf-Verley reductions and Oppenauer oxidations resulted from careful reexamination of diiodosamarium-promoted alkylations of alddiydes (Namy et al., 1984). [Pg.365]

As expected, suitable selection of a metal combination for each targeted reaction was important to achieve high stereoselectivity. For a sy -selective nitro-Maimich-type reaction, a heterobimetallic complex prepared from Cu(OAc)2, Sm(0-/Pr)3, and dinucleating Schiff base la was the best [12]. Other metal combinations resulted in much lower selectivity. Either Cu or Sm alone also resulted in poor reactivity and selectivity. Thus, both Cu and Sm are essential for high catalytic activity and selectivity. After optimization studies, the addition of achiral phenol source and the use of oxo-samarium alkoxide, Sm50(0-jPr)i3, with a well-ordered structure, gave the superior reactivity and stereoselectivity. Under the optimized reaction cmiditions, 1-10 mol% of the Cu/Sm catalyst promoted asymmetric... [Pg.3]

It was not possible to get clean reactions between aldehydes and alkyl halides in the presence of Smij. The reason is the very fast Meerwein-Ponndorf reaction quoted in section 4.3.5. It was, however, found that halides such as allyl iodide or benzyl bromide are so reactive towards aldehyde groups that the secondary samarium alkoxide which appears in the solution cannot compete for the remaining aldehyde (Souppe et al., 1982). In this way several homoallylic or homobenzylic alcohols could be obtained. Some examples are listed in table 10. The reaction failed on aromatic aldehydes because the pinacol formation is faster. [Pg.557]

Chiral induction was also observed in lanthanide(III)-alkoxide-mediated MPV reductions. The optically active ligand (Fig. 35C) was used in enantioselec-tive samarium-catalyzed MPV reductions of arylmethyl ketones (Scheme 30) [253], The resulting mixed alkoxide-iodide complex shows higher reactivity than (rBuO)SmI2. It was pointed out that the tridentate, secondary alkoxide ligand is not oxidized under the reaction conditions and that tridendate ligands... [Pg.216]

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... [Pg.349]

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. [Pg.121]

Sml2X that is produced as a result of this process then serves to open the epoxide, generating the iodohy-drin. Although this appears to be a likely scenario, a more direct route involving a Finkelstein reaction between the brsamarium iodide salts - cannot be ruled out (Scheme 2). [Pg.261]

Other a-alkoxylithium carbanions have been generated by tin-lithium exchange at low temperature, by metallation of (alkoxymethyl)trimethylsilane, or by using samarium iodide. These reagents do not directly form epoxides due to the poor nucleofiigicity of alkoxide anion. However, the resultant protected diols can be considered as particularly robust precursors to epoxides when deprotected and manipulated in any number of standard ways. A notable example of this strategy is shown in Scheme 13. ... [Pg.829]

The proposed mechanism for this samarium-catalyzed transformation is illustrated in Fig. 3-4. The pathway parallels that depicted in Fig. 3-3 for the chromium-catalyzed pinacol coupling reaction, with the exception of the timing of the silylation step. Based on the fact that pinacol reactions with catalytic and stoichiometric Sml2 furnish the same diastereoselection, Endo believes that carbon-carbon bond formation precedes silylation of the alkoxide. [Pg.82]


See other pages where Samarium alkoxides is mentioned: [Pg.272]    [Pg.258]    [Pg.272]    [Pg.258]    [Pg.254]    [Pg.258]    [Pg.7220]    [Pg.379]    [Pg.555]    [Pg.272]    [Pg.258]    [Pg.272]    [Pg.258]    [Pg.254]    [Pg.258]    [Pg.7220]    [Pg.379]    [Pg.555]    [Pg.46]    [Pg.57]    [Pg.1803]    [Pg.1003]    [Pg.191]    [Pg.725]    [Pg.49]    [Pg.58]    [Pg.66]    [Pg.144]    [Pg.188]    [Pg.795]    [Pg.1872]    [Pg.2089]    [Pg.829]   
See also in sourсe #XX -- [ Pg.60 ]

See also in sourсe #XX -- [ Pg.60 ]

See also in sourсe #XX -- [ Pg.32 , Pg.148 , Pg.212 ]




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