Catalysis lanthanide

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

Cyclic amino-carbenes, in molybdenum carbonyls, 5, 457 Cyclic bis(phosphine) dichlorides, with iron carbonyls, 6, 48 Cyclic carbenes, as gold atom ligands, 2, 289 Cyclic carbometallation, zirconium complexes, 10, 276 Cyclic carbozirconation characteristics, 10, 276 intermolecular reactions, 10, 278 intramolecular reactions, 10, 278 Cyclic dinuclear ylides, and gold , 2, 276 Cyclic 1,2-diols, intramolecular coupling to, 11, 51 Cyclic enones, diastereoselective cuprate additions, 9, 515 Cyclic esters, ring-opening polymerization, via lanthanide catalysis, 4, 145 Cyclic ethers... 

Lanthanide catalysis was again effective in ring-opening reactions of cyclic epoxides (eq 8). Finally, MAO-activated l-RuCL provides block copolymers of ethylene and hex-l-ene. ... 

Danishefsky and co-workers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is used as an NMR shift reagent, promoted hetero Diels-Alder reactions [30] of aldehydes with siloxydienes and induced enantiomeric enrichment (Sch. 1) [31]. Suitable substituents on the dienes were introduced to improve the extent of asymmetric induction. The best result was obtained in the reaction of benzaldehyde with l-methoxy-2-methyl-3-(trimethyl-siloxy)- , 3-butadiene using 1 mol % Eu(hfc)3 the enantiomerie excess was, however, moderate (58%). The authors maintained that the major advantage of lanthanide catalysis lay in the survival of otherwise labile systems used as adducts. 

Lanthanide catalysis has been applied to inverse election demand [4+2]-cycloadditions. As above, high selectivities are observed only when double dia-stereodifferentiation operates. Posner and coworkers performed the cydoaddition of a pyrone 9.54 bearing an ester group derived from (5)-methyl lactate with an enol ether under (-)-Pr(hfc)3 catalysis [1578, 1579] (Figure 9.22). Marko and Evans [1580] used other esters, among which pantolactone 1.16 proved to be the most efficient auxiliary, and performed similar cydoadditions under (+) or (-)-Eu(hfc)3 catalysis at room temperature with excellent results (de > 95%). 

Danishefsky has also reported investigations on the cycloadditions of highly oxygenated dienes such as (32) and (38) under lanthanide catalysis. This methodology has proven to be a rapid and generai route to various dihydropyrone derivatives, inciuding C-glycoside units (39a, b). These results are summarized in Scheme 14. 

A variety of substituted dienes give high cis (endo) selectivity under lanthanide catalysis. Other examples are presented later in this chapter M. D. Bednarski, Ph.D. Thesis, Yale University, 1986. 

The synthetic potential of lanthanide catalysis has been recognized in high pressure chemistry mainly during the last decade although earlier work was concerned with the beneficial effect of the biactivation mode to overcome steric hindrance in hetero-Diels-Alder reactions [30]. In this way, alkyl pyruvates and aldehydes considered as dienophiles could react with 1-methoxy-l,3-butadiene yielding adducts which could serve as synthons in sugar synthesis [31]. 

The advantage of using a catalyst is obvious. When one reactive center of the keto compound is substituted by a methyl or phenyl group (Rz) the catalyst effect is more enhanced. Mesityl oxide (Rj = Rz = R3 = Me) requires very high pressure and efhcient lanthanide catalysis. This reaction involving a sterically congested unsaturated ketone is a prominent example emphasizing the requirement of biactivation to overcome steric hindrance. It should be added that, apart from the mesityl oxide reaction, a modest pressure (300 MPa) is sufficient to induce reactivity in these catalyzed reactions which were shown to proceed only at pressures in excess of 1200 MPa in an uncatalyzed version [33]. 

The pressure kinetic effect on the Michael reaction between methyl vinyl ketone and nitromethane was studied under various conditions including lanthanide catalysis [36] (Scheme 10.8). Table 10.8 shows some kinetic results. 

The determination of the volume profile indicates a significant electrostriction related to the formation of zwitterions generating activation volumes oscillating between —40 to —55 cm mol [37]. At ambient pressure the reaction is usually facile when using unsubstituted primary amines and unhindered acrylates. It does not require catalysis at all on account of the basicity of amines. However, its sensitivity to steric hindrance is so high that only a combination of high pressure and lanthanide catalysis is an efficient way to synthesize congested yS-aminoesters (Table 10.9) [38]. 

An important reaction is the condensation of amines with 1,3-diketones or 3-ketoesters yielding -enaminoesters which are important intermediates for the synthesis of natural products (Scheme 10.10). The step preceding the formation of the enamine involves zwitterionic intermediates such as those in the addition of amines to a,yS-unsaturated compounds. The electrostrictive effect contributes to the magnification of the pressure effect. Lanthanide catalysis and use of 300 MPa pressure can be an excellent way to produce enamino compounds in high yields [39] (Table 10.10). 

There are several reports concerning the dual action of pressure and lanthanide catalysis on Michael reactions. Kotsuki reported the case of reactions of this kind which were inaccessible under ordinary conditions, for example, the homoconjugate... 

In a previous study Kotsuki s group reported reactions of / -ketoesters with enones or acrylates under 800 MPa (Scheme 10.12) [41]. Table 10.11 compares the yields obtained under biactivation conditions (pressure -h lanthanide catalysis) on one side and in the presence of a catalytic system consisting of Yb(OTf)3 -I- silicagel at ambient pressure on the other. Both methods reveal similar efficiency, although operation under pressure reduces reaction time. However, association of pressure and lanthanide catalysis proves its usefulness when acrylates are involved as Michael acceptors. In this case, no reaction occurs with the ambient pressure method even after prolonged reaction times. 

During the last two decades, lanthanide catalysis has been extensively explored [3], considering the unique properties and the absence of toxicity of these "heavy" metals which make them environmentally friendly. Olefin transformations catalysed by organolanthanides such as oligomerisation, hydrogenation, hydrosilylation, hydroamination, polymerisation, have attracted much attention. The two latter reactions can be initiated by hydrides (which act as precatalysts, such as for MMA polymerisation [4]), but do not involve hydrides as intermediates in the catalytic cycle and therefore will not be considered in the present review. 

Extension of this chemistry eventually led to the development of a new method for the high-yield nitration of phenols using one equivalent of sodium nitrate as nitrating agent in excess HCl (Ouertani et al., 1982) lanthanide catalysis was effective under mild conditions, which leave many other aromatic systems unaltered. Three particular aspects of this study should be emphasized ... 

In general, lanthanide catalysis does not change the endo/exo product ratio as compared to the uncatalyzed reaction. Neutralization of the reaction mixture followed by extractive workup allows for recycling of the lanthanide-containing aqueous phase in subsequent reactions with no diminished catalytic effect. 

Scheme 12 Transition metal-catalyzed additions of trialkyl alanes to imines. (a) Lanthanide catalysis, (b) Addition to an A-formylimine...

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