Lanthanide-promoted reactions

The lanthanide-promoted reaction of 3 equivalents of an aldehyde with an amine affords a mixture of the 2,3,5-trialkylated- 2,3-dihydropyridinium and pyridinium products. The dihydro species can be converted to the pyridinium compound by refluxing in toluene with triethylamine <97JOC208>. 

Cameron LL, Wang SC, Kluger R (2004) Biomimetic monoacylation of diols in water. Lanthanide-promoted reactions of methyl benzoyl phosphate. J Am Chem Soc 126 10721... 

Yu LB, Chen D, Li J, Ramirez J, Wang PG (1997) Lanthanide-promoted reactions of aldehydes and amine hydrochlorides in aqueous solution. S3mthesis of... 

An approach based on samarium diiodide-promoted reactions of the iminoketone 279 with aldehydes provides access to a series of substituted or fused pyrroles bearing at least two phenyl groups, for instance, the system 280 (Equation 86) <2001T4881>. In an alternative lanthanide-catalyzed route, a series of pyrroles were constructed from imines and nitroalkenes in the presence of Sm(fPrO)3 <1999T13957>. A set of 1-dimethylaminopyrroles have also been obtained by TiCU-induced reactions between 2-acetoxypropanal hydrazones with silyl enol ethers <1995TL8007>. 

Lanthanide Triflate-Promoted Reactions in Aqueous Media [2]... 

While continuous use of LnfOTOs is possible, it is also easy to recover Ln(OTf)3 compounds themselves. Lanthanide triflates are more soluble in water than in organic solvents such as dichloromethane. Almost 100% of Ln(OTl)3 was quite easily recovered from the aqueous layer after the reaction was completed and could be reused. The reactions are usually quenched with water and the products are extracted with an organic solvent (for example, dichloromethane). The lanthanide triflate is in the aqueous layer and removal of the water is all that is required to give the catalyst which can be used in the next reaction (Scheme 14-1). It is noteworthy that lanthanide triflates are expected to solve some severe environmental problems induced by Lewis acid-promoted reactions in industry chemistry [20]. 

L. Yu, J. Li, J. Ramirez, D. Chen, and P. G. Wang, Synthesis of aza-sugars via lanthanide-promoted aza-Diels-Alder reactions in aqueous solutions, J. Org. Chem., 62 (1997) 903-907. 

For an example of a lanthanide-promoted phenol modification with imines in organic solvents, see T.S. Huang, C.J. Li, Synthesis of amino acids via a three-component reaction of phenols, glyoxylates and amines, Tetrahedron Lett. 2000, 41, 6715. 

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

The indium-mediated allylation of trifluoroacetaldehyde hydrate (R = H) or trifluoroacetaldehyde ethyl hemiacetal (R = Et) with an allyl bromide in water yielded a-trifluoromethylated alcohols (Eq. 8.56).135 Lanthanide triflate-promoted indium-mediated allylation of aminoaldehyde in aqueous media generated (i-airiinoalcohols stereoselectively.136 Indium-mediated intramolecular carbocyclization in aqueous media generated fused a-methylene-y-butyrolactones (Eq. 8.57).137 Forsythe and co-workers applied the indium-mediated allylation in the synthesis of an advanced intermediate for azaspiracids (Eq. 8.58).138 Other potentially reactive functionalities such as azide, enone, and ketone did not compete with aldehyde for the reaction with the in situ-generated organo-indium intermediate. 

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. 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

While the above indicates that Ln3 + and transition metal ions in the presence of at least 1 equi. of OR promote the alcoholysis of carboxylate and phosphate esters, sometimes by spectacular amounts, we have not presented evidence about the mechanism for the catalytic reactions. So far, the underlying theme is that the most active forms of the lanthanide ions are the Ln3 + ( OR) forms, either as a monomer (as in the case of Eu3 + ( OCH3)) or as a dimer (as in the case of La2+( OCH3)2). For the transition metal ions the most active forms are those where one face of the... 

Samarium and other lanthanide iodides have been used to promote a range of Mukaiyama aldol and Michael reactions. The syntheses show promise as enantio-selective transformations, but the precise mechanistic role of the lanthanide has yet to be elucidated. 

The first report on organolanthanide-promoted Tishchenko reactions, that is, the transformation of aldehydes or mixed aldehydes into the corresponding esters, including a mechanistic proposal appeared in 1996 [catalyst Cp2 LnCH(SiMe3)2 with Ln = La and Nd] [233]. Two years later, lanthanide silylamide complexes Ln[N(SiMe3)2]3 were found as easily accessible and even more active catalysts (Scheme 12.24) [234, 235]. 

Alumina, alkaline-earth oxides, mixed oxides (spinels), rare-earth oxides, and lanthanide ores are known additives capable of sorbing S-impurities. The properties of these materials can be manipulated to produce catalysts capable of reducing up to -80% S-emissions and meet the refiner needs. It is, however, unlikely that these systems will be capable of satisfying the more stringent environmental S-emission standards expected in the future. Details of the reaction mechanism by which additives and promoters catalyze the oxidative sorption of S-impurities and details of catalyst deactivation have not yet been proposed. This work could provide useful information to help design more efficient S-transfer catalysts. The catalytic control of S-emissions from FCC units has been described in detail in two papers appearing in this volume (46,47) and in the references given (59). 

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