Radical lanthanides

Keywords Nitroxide radicals Lanthanide complexes Low-temperature absorption spectroscopy Low-temperature luminescence spectroscopy Molecular magnetism... 

KAGAN-MOLANDER Samanum reagent Lanthanide reagents, speoflcally samanum, lor generation of free radicals useful in cyclizations, reductions... 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

Metal-catalyzed hydrophosphination has been explored with only a few metals and with a limited array of substrates. Although these reactions usually proceed more quickly and with improved selectivity than their uncatalyzed counterparts, their potential for organic synthesis has not yet been exploited fully because of some drawbacks to the known reactions. The selectivity of Pt-catalyzed reactions is not sufficiently high in many cases, and only activated substrates can be used. Lanthanide-catalyzed reactions have been reported only for intramolecular cases and also sulfer from the formation of by-products. Recent studies of the mechanisms of these reactions may lead to improved selectivity and rate profiles. Further work on asymmetric hydrophosphination can be expected, since it is unlikely that good stereocontrol can be obtained in radical or acid/base-catalyzed processes. 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Entries 10 to 12 are examples of oxidative generation of radicals, followed by tandem cyclization. The reaction in Entry 10 includes a lanthanide catalyst. Entry 11... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Molecular hydrogen is rather unreactive at ambient conditions, but many transition and lanthanide metal ions are able to bind and therefore activate H2, which results in transformation into H (hydride) 11 (hydrogen radical) or H+ (proton), and subsequent transfer of these forms of hydrogen to the substrate.7,8 In this context, not only metal hydride but also dihydrogen complexes of transition metal ions, play a key role,9 10 especially since the first structural characterization of one of these species in 1984 by Kubas.11... 

It is interesting to stress that the spin chirality observed in the gadolinium radical chains differs from the more usual one that characterizes anisotropic materials and is solely due to the significant strength of NNN interactions between lanthanide ions that are very far apart. The mechanism responsible for this interaction remains unclear and the complexity of the system has, up to now, hampered an ab initio investigation of the phenomenon. 

In this sub-chapter, selected examples of lanthanide-based chains are described. We have chosen to comment on only systems with structural characterization and significant dynamic properties. Chains with a sole lanthanide ion as spin carrier are described first. 3d-4f and 3d-3d -4f heterometallic chains follow. Finally, chains comprising lanthanide and radical ligands conclude this chapter. 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

A more recent report by Sibi and co-workers displayed the utility of chiral lanthanide Lewis acids for addition-trapping reactions [150]. An exhaustive screening of lanthanide Lewis acids and several chiral ligands revealed that Y(OTf)3 and proline derived ligand 138 was optimal (data not shown). Upon further optimization it was discovered that achiral additives 139 and 212 increased ee s (Scheme 56, entries 2 and 3). Bulkier radicals were found to decrease the enantioselectivity (entries 4 and 5). Also, larger aryl substituents on the ligand gave similar ee s as observed for 138 (compare entries 1, 6, and 7). 

Scheme 56 Radical allylations using chiral lanthanide Lewis acids... 

Complexes of the lanthanides with amine oxides, nitroso compounds, and nitroxide radicals fall under this category. However, only the complexes with the amine oxides have received wide attention. Four comprehensive reviews have appeared on the complexes of amine oxides with various metal ions 142-145). Only a few complexes of the lanthanides have been quoted in these reviews. 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

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