Catalyst lanthanide

Skraup/Doebner-von Miller-type reactions with lanthanide catalysts under microwave radiation are efficient for a variety of different anilines. For example, cyclisation of aniline 44 with acetone in the presence of scandium triflate gave the desired product 45 in excellent yield. 

Because ketones are generally less reactive than aldehydes, cycloaddition reaction of ketones should be expected to be more difficult to achieve. This is well reflected in the few reported catalytic enantioselective cycloaddition reactions of ketones compared with the many successful examples on the enantioselective reaction of aldehydes. Before our investigations of catalytic enantioselective cycloaddition reactions of activated ketones [43] there was probably only one example reported of such a reaction by Jankowski et al. using the menthoxyaluminum catalyst 34 and the chiral lanthanide catalyst 16, where the highest enantiomeric excess of the cycloaddition product 33 was 15% for the reaction of ketomalonate 32 with 1-methoxy-l,3-butadiene 5e catalyzed by 34, as outlined in Scheme 4.26 [16]. 

In 1996, the first example of the catalytic enantioselective aza Diels-Alder reactions of azadienes using a chiral lanthanide catalyst was reported [4], In this article, successful examples of such catalytic reactions are surveyed. 

The most frequently encountered, and most useful, cycloaddition reactions of silyl enol ethers are Diels-Alder reactions involving silyloxybutadicncs (Chapter 18). Danishefsky (30) has reviewed his pioneering work in this area, and has extended his studies to include heterodienophiles, particularly aldehydes. Lewis acid catalysis is required in such cases, and substantial asymmetric induction can be achieved using either a chiral lanthanide catalyst or an a-chiral aldehyde. 

Ladder polymer, synthesis of, 503 Lanthanide catalysts, 288 Lanthanide compounds, 73 Latent-reactive polymers, 455 Laurolactam, 136... 

The aqueous aza-Diels-Alder reaction of an aldehyde and an amine hydrochloride with a diene is catalyzed by lanthanide(III) trifluoromethane sulfonates (Ln(OTf)3, triflates [24]). Some examples are reported in Schemes 6.12 and 6.13. With respect to uncatalyzed reactions, the lanthanide catalyst allows milder reaction conditions, increases the reaction yield and does not affect the diaster-eoselectivity of the reaction, but influences the regiochemistry as in the cycloaddition of 25 with 1,3-dimethyl-1,3-butadiene (Schemes 6.10 and 6.12). These results have been applied [24b-d] to the synthesis of azasugars (Scheme 6.14). 

These three steps all produce significant amounts of waste. First, as discussed earlier, the nitration process results in the production of spent sulfuric acid. In the past the company had been able to sell much of this material to the coke and steel industries but declining demand meant that the acid now required disposing of, at additional cost. At the time green catalytic nitration technology was becoming available with clay, zeolite and lanthanide catalysts all providing possible alternatives to the use of sulfuric acid (see below). Improved selectivity to the desired para-isomer is an added benefit of some of these catalytic systems. However on the... 

As in the P(III) chemistry above, both late metal (Pd) and lanthanide catalysts have been used for P(V)-H additions to alkynes, alkenes, aldehydes, and imines. In addition, titanium, aluminum, and zinc catalysts have been employed. Typical P(V) substrates include dialkyl phosphites P(0R)2(0)H and phosphine oxides PR2(0)H. 

The same group extended this work to a cyclic imine (Scheme 5-47) better results were obtained with heterobimetallic lanthanide catalysts than with chiral titanium alkoxides. 

Entries 6 to 9 involve reactions conducted under catalytic conditions. Entry 6 uses a lanthanide catalyst that is active in aqueous solution. Entries 7 and 8 are examples of the use of (Cp)2Ti(03SCF3)2 as a Lewis acid. Entry 9 illustrates the TMS triflate-MABR catalytic combination. 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

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... 

Entry 5 is an example of nitration in acetic anhydride. An interesting aspect of this reaction is its high selectivity for the ortho position. Entry 6 is an example of the use of trifluoroacetic anhydride. Entry 7 illustrates the use of a zeolite catalyst with improved para selectivity. With mixed sulfuric and nitric acids, this reaction gives a 1.8 1 para ortho ratio. Entry 8 involves nitration using a lanthanide catalyst, whereas Entry 9 illustrates catalysis by Sc(03SCF3)3. Entry 10 shows nitration done directly with N02+BF4, and Entry 11 is also a transfer nitration. Entry 12 is an example of the use of the N02—03 nitration method. 

Mechanism and Early Transition Metal/Lanthanide Catalysts 288... 

The utility of a new lanthanide catalyst 162 for hydroamination and hydrosilylation is highlighted below<06CC874>. Application of this new lanthanide catalyst resulted in excellent yields of piperidines such as 163 and 164 with reduced reaction times. 

Early transition metal, group III, and lanthanide catalysts 504... 

Lewis acid catalysis. Anhydrous lanthanide(III) chlorides, particularly SmCl3, can function as low-cost but efficient nonhomogeneous Lewis acid catalysts for aldol and other reactions. More rapid reactions are observed when the soluble but expensive Eu(fod)3 is used as a lanthanide catalyst.1... 

Such lanthanide catalysts were also used in hydroamination/cyclization strategies for the synthesis of the alkaloid (+)-xenovenine. This reaction of enantiomerically pure 147 leading to 148 via two C-N bond formations was used in a late step of the synthesis after a hydrogenation, the natural product was isolated (Scheme 15.46) [100]. 

The general issues from literature surveys dealing with lanthanide initiators reveal the following (1) catalyst precursors with larger lanthanide metals polymerize lactide faster than metals of smaller radii, (2) lanthanide catalysts polymerize lactide at slower rates than cyclic esters such as s-caprolactone and, in most cases. 

Hanessian and Compain have also reported a Lewis acid-promoted inverse electron demand hetero-Diels-Alder reaction between dihydrofurans and dihydropyrans with a-keto-/3,7-unsaturated phosphonates to give stmcturally related products <2002T6521>. High-pressure OTr/o-selective hetero-Diels-Alder reactions between a,/3-unsaturated aldehydes and enol ethers in the presence of lanthanide catalysts have also been reported and give 3,4-dihydro-27/-pyrans. Examples include the use of cyclic enol ethers to give 2,3,4,4a,5,8a-hexahydro-277,577-pyrano[2,3-. ]pyrans <1995T8383>. 

Cp 2YH in the same manner as the lanthanide catalysts mentioned above. 

Chiral lanthanide catalysts, (R)- and (5 )-Me2Si(Me4Cp)[(—)-menthylCp]SmCH (SiMe3)2, are effective for the asymmetric hydrosilylation of 2-phenyl-1-butene with H3SiPh, giving (R)- and (5 )-2-phenyl-2-silylbutanes with 68 and 65% ee, respectively (Scheme 23)43. 

In hetero-Diels-Alder reactions, the effect of ligand structure and acidity on the catalytic activity of lanthanide catalysts has been reviewed.191 The effect of different C(2)-symmetric bisoxazolines on the zinc(II)-catalysed hetero-Diels-Alder reaction of ethyl glyoxylate with conjugated 1,3-dienes has been investigated.192 The hetero-Diels-Alder reaction 4-dimethylamino-2-phenyl-l-thiabuta-1,3 -diene with methyl acrylate and /V-cnoyloxazolidinone produces cw-3,4-disubstituted 3,4-dihydro-2//-... 

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