Lanthanide reagent

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

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

Addition of the chiral lanthanide reagent tris-(3-trifluoromethyl-hydroxymethylene-rf-camphorato)europium(m) to a solution of (+)-... 

Lanthanide reagents are meanwhile well established in organic synthesis [5, 227]. While the reactivity of low valent species generally resembles that of the... 

Other more conventional detectors that might ostensibly outperform CD in selectivity are nmr and mass spectrometry, and in fact they do for the analysis of diastereomers, although quantitation is a much more difficult task. They cannot compete with chiroptical methods for the distinction between enantiomers. In nmr detection, derivatization to diastereomers is a prerequisite to enantiomer analysis, and chiral forms of lanthanide reagents can been used with good effect [16,17]. For the analysis of mixtures by either nmr or mass spectrometry, total chromatographic separation is a necessity, so the completeness of the baseline separation is the limiting step not the detector. In contrast CD can be applied to the analysis of enantiomers in mixtures in methods that require no prior separation. 

A similar effect was observed when the electrophile was a lanthanide reagent reacting with isomeric pyridines 144 and 145 (85TL4669, 85TL4673) (Scheme 109). 

The addition of sp3 or sp2 carbon-centred radicals to unsaturated carbon carbon bonds is yet another class of synthetically useful reactions promoted by the low-valent lanthanide reagent Sml2. Halides and sulfones are the most common functional groups used as precursors to radicals, although other groups have also been successfully employed. Although the intermolecular variant of this reaction has found only limited application, intramolecular variants can be highly successful. In most of these cyclisation events, the... 

Some salient points to note are (i) the model is in accord with the experimental results, (ii) nuclei at a and yS positions to the coordinating atom show deviations from the model due to contributions of other shift mechanisms. Thus the best procedure for the elucidation of molecular structure by using lanthanide reagents consists of (i) to obtain the relative magnitude of geometrical function, G for different ligand nuclei from the slopes of A bT versus 1 / T plots, (ii) since temperature dependence of ytterbium complexes conforms to the model, use of ytterbium complexes is prudent, (iii) in cases where the temperature dependence is interfered with effects due to chemical equilibrium or exchange, data for a number of lanthanides at room temperature may be obtained and plots of equation... 

G. A. Molander, Chem. Rev., 1992, 92, 29 (application of lanthanide reagents in organic synthesis). 

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

Lanthanide reagents produce shifts through complexation, dipolar and contact effects. Complexation shifts are small and are measured using diamagnetic La(III) or Lu(III) chelates. ... 

The high sensitivity of lanthanide reagents to steric factors is also observed in the cyanosilylation reaction of ketones catalyzed by ytterbium cyanide, Yb(CN)3 (Eq. 7) [10], Other reactions, for example epoxide and the aziridine opening by tri-methylsilyl cyanide, TMSCN, are also efficiently catalyzed by Yb(CN)3 [11]. This Yb reagent is not regarded as a Lewis acid but as the active species in these reactions. 

An interesting result of control of acyclic stereochemistry is reported by Nagano et al. [57], who showed that efficient 1,2-asymmetric induction can be achieved in radical-mediated allylation of diethyl (25,35)-3-bromo-2-oxo-succinates stereoselectively. In the Eu(fod)3 (1.1 equivalent) photocatalyzed reaction of bromohydroxy compound (4) diastereoselectivity is reversed with respect to the simple photoreaction. On the other hand, substitution with silyl groups tends to enhance diastereoselectivity up to 8.6 1. The effect is still operative to a lesser extent with catalytic amounts of the lanthanide reagent (0.1 equivalents, threo/erythro [5/6] = 3 1) (eq. (2)) [57]. 

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