Promoters by lanthanides

Aqueous aza-Diels-Alder reactions of chiral aldehydes, prepared from carbohydrates and with benzylamine hydrochloride and cyclopentadiene, were promoted by lanthanide triflates (Eq. 12.65).137 The nitrogen-containing heterocyclic products were further transformed into aza sugars, which are potential inhibitors against glycoprocessing enzymes. 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

Immediately following Danishefsky s pioneering work, several reports of cyclocondensations promoted by lanthanide shift reagents appeared in the literature. The Eu(fod)3/hetero-Diels-Alder methodology offers a novel approach to highly functionalized 8-lactones (Castellino and Sims, 1984a). Such a scheme provides access to natural products via synthetically useful intermediates. 

Early Examples with Esters Having Complex Mechanisms for Methanolysis Promoted by Lanthanides and a Triazacyclododecane Zn" Complex... 

Meerwein-Ponndorf-Verley-Type Reduction Reduction of ketones by 2-propanol or related alcohols, known as Meerwein-Ponndorf-Verley (MPV) reduction, is promoted by various metal alkoxides, typically aluminum 2-propoxide [2a,d,281]. The C2 hydrogen of 2-propanol is transferred directly to the carbonyl carbon through a six-membered pericyclic transition state [284], Earlier, a stoichiometric quantity of a metal alkoxide was required for this purpose, but recently, lanthanide [285] and aluminum [286] complexes acting as excellent catalysts have been reported. 

The reductive disproportionation of C02 to give CO and C03 , promoted by electron-rich transition metals [9, 101] or to produce C20 using lanthanides such as Sm [102, 103], has been observed in a number of systems. 

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... 

Catalytic Asymmetric Diels-AIder Reactions and Hetero Diels-Alder Reactions Promoted by Chiral Lanthanide Catalysts... 

Enantioselective Mukaiyama Aldol Reaction Promoted by Chiral Lanthanide Complexes... 

Although the development of a variety of Lewis acids has enabled the reahzation of a wide range of catalytic asymmetric reactions, most of the catalysts have limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activates only one side of the substrate in an intermolecular reaction, whereas the latter not only can activate both sides of the substrate but also can control the orientation of the substrate. If this kind of synergistic cooperation could be realized in synthetic asymmetric catalysis, it would open up a new field in asymmetric synthesis, and a wide range of applications might well ensure. In this section we discuss asymmetric two-center catalysis promoted by chiral lanthanide complexes with Lewis acidity and Brpnsted basicity [44,45]. 

In catalytic processes with enzymes such as o-oxynitrilase and (K)-oxynitrilase (mandelonitrilase) or synthetic peptides such as cyclo[(5)-phenylalanyl-(5)-histidyl], or in reaction widi IMS-CN promoted by chiral titanium(IV) reagents or with lanthanide trichlorides, hydrogen cyanide adds to numerous aldehydes to form optically active cyanohydrins. The optically active Lewis acids (8) can also be used as a catalyst. Cyanation of chiral cyclic acetals with TMS-CN in the presence of titanium(IV) chloride gives cyanohydrin ethers, which on hydrolysis lead to optically active cyanohydrins. An optically active cyanohyrMn can also be prepared from racemic RR C(OH)CN by complexation with... 

Molander, G. A., Etter, J. B. Lanthanides in organic synthesis. 8. 1.3-Asymmetric induction in intramolecular Reformatskii-type reactions promoted by samarium diiodide. J. Am. Chem. Soc. 1987, 109, 6556-6558. 

Scheme 60 illustrates the proposed catalytic cycles for a samarium(II) silylamide precursor. The first step in both reactions is supposed to be the formation of a bimetallic samarium bis(amidinate) species originating from the reductive coupling of carbodiimide promoted by the Ln(II) complex. The active species is proposed to be a samarium guanidinate and a lanthanide amidinate [70]. 

The new diphenylalanine-derived oxazolidinone, 15, is particularly effective when used as an auxiliary on radical 29. The auxiliary can be used in a propagation sequence that involves radical addition followed by trapping of the addition radical with allylstannane or allylsilanes, Eq. (21). Excellent yield and diastereoselectivity are observed if the reaction is carried out in the presence of Lewis acids such as magnesium bromide or lanthanide triflates at —78°C. The reaction promoted by magnesium bromide, for example, provides a diastereomer mixture in excess of 100 1 with a yield of 85%. Sc, Yb, Y, La, or Sm triflates provide similar results in reactions usually carried out in ether. 

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