Lanthanide-catalyzed reaction

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. 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Besides the already described Pd-, Rh- and Ru-catalyzed transformations, many other transition metals have also been used in domino processes, albeit to a lesser extent. Co- and also Ni-catalyzed transformations constitute the largest group in this section, though other examples include Cu, W, Mo, Fe, Ti, Cr, Au, Pt, Zr, and Lanthanide-catalyzed reactions. 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

The types of dienophiles which have been studied most are acrylic aldehydes, acrylates and 3-acryloyl-l,3-oxazolidines. The latter have been used predominantly in copper, magnesium, zinc and lanthanide catalyzed reactions in which the chiral Lewis acid binds in an rj2 fashion to the dienophile (complexation to both carbonyls). 

In contrast to the epoxides, preparative routes to the aziridines are fairly evenly split between the [C=N + C] and the [C=C + N] routes. Among contributions in the former category, aziridine carboxylate derivatives 110 can be prepared through the lanthanide-catalyzed reaction of imines with diazo compounds, such as ethyl diazoacetate (EDA). In this protocol, iV-benzyl aryl aldimines and imines derived from aromatic amines and hindered aliphatic aldehydes are appropriate substrates <99T12929>. An intramolecular variant of this reaction (e.g.. Ill —> 112) has also been reported <990L667>. 

It is believed that bidentate coordination (ex. Yb(III)-3-(2-alkenoyl)-l,3-oxazolidin-2-one) is necessary to obtain high selectivities in many chiral lanthanide-catalyzed reactions [101]. These results are very interesting and promising because it has been shown that even monodentate coordination can achieve good selectivities by using the hetero-chiral Yb(III) catalyst... 

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