Designer Lewis-acid catalysts

Designer Lewis Acid Catalysts Modern Organic Synthesis... 

Scheme 18.9 Selective reduction assisted by designer Lewis-acid catalysts . ...

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

The ammonolysis of phenol (61—65) is a commercial process in Japan. Aristech Chemical Corporation (formerly USS Chemical Division of USX Corporation) currently operates a plant at Ha verb ill, Ohio to convert phenol to aniline. The plant s design is based on Halcon s process (66). In this process, phenol is vapori2ed, mixed with fresh and recycled ammonia, and fed to a reactor that contains a proprietary Lewis acid catalyst. The gas leaving the reactor is fed to a distillation column to recover ammonia overhead for recycle. Aniline, water, phenol, and a small quantity of by-product dipbenylamines are recovered from the bottom of the column and sent to the drying column, where water is removed. 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

In the Eastman process for 2,5-dihydrofuran production, the situation is different and the problem of heavy products has been tackled in a highly original manner. [31] The oligomers formed in the process are highly polar and insoluble in alkanes. The ionic liquid, [P(oct)3C18H37]I and the Lewis acid catalyst, [Sn(oct)3]I, which are non toxic (LD50 > 2 g kg"1 for each), non-flammable (flammability 1) and non-corrosive (340 stainless steel is used for the reactor), have been designed to be soluble in... 

The most important development within the field of Diels-Alder chemistry during the past two decades must be considered to be the design and application of chiral Lewis acid catalysts. From the mid 80s on, the number of literature reports about the design and application of chiral Lewis acids in the synthesis of chiral Diels-Alder adducts from achiral precursors grew exponentially, but it started to level off and decrease again in the mid 90s. Several excellent reviews about the application of chiral Lewis acids in Diels-Alder reactions have been published41,43 44. In this section, the recent literature about the chiral Lewis acid catalyzed all-carbon Diels-Alder reactions of dienes with dienophiles is reviewed, which, as such, has not been reviewed before. 

Since Curran and Kuo and Schreiner and coworkers reported that urea and thiourea derivatives act like Lewis acid catalysts, several chiral urea and thiourea catalysts have been designed by Jacobsen et al. and Takemoto et al. ... 

Several asymmetric versions of cycloaddition reactions with nitrones in the presence of optically active metal complexes as Lewis-acid catalysts have been reported [15]. Because of a lack of suitable chiral catalysts, however, the asymmetric design of this reaction was found to be difficult when using a,/(-unsaturated aldehydes as substrates, because these compounds are poor substrates for metal catalysts, probably because of preferential coordination of the Lewis acid catalyst to the nitrone in the presence of monodentate carbonyl compounds. Consequently, inhibition of the catalyst occurs. 

The first catalytic asymmetric allylation of imines has been reported using allyltri- -butyltin in the presence of a chiral 7r-allylpalladium complex.179 Zirconium is also demonstrated as a metal center for the design of chiral Lewis acid catalysts that are suitable for the activation of bidentate imino compounds.180 Jprgensen reports high enantio-selective allylation of ct-imino esters (Equation (52)).181... 

Another approach is to design homogeneous Lewis acids which are water-compatible. For example, triflates of Sc, Y and lanthanides can be prepared in water and are resistant to hydrolysis. Their use as Lewis acid catalysts in aqueous media was pioneered by Kobayashi and coworkers [144-146]. The catalytic activity is dependent on the hydrolysis constant (Kh) and water exchange rate constant (WERC) for substitution of inner sphere water ligands of the metal cation [145]. Active catalysts were found to have pKh values in the range 4-10. Cations having a pKh of less than 4 are easily hydrolyzed while those with a pKh greater than 10 display only weak Lewis acidity. 

Interactions of the carbonyl group with Lewis acids are colorful and varied, but by no means unpredictable. The structural information gathered in this chapter points, fairly consistently, to the same set of principles. Rules of complexation such as coordination syn to the small substituent, s-trans preference due to gearing effects and distortions towards the ir-cloud in response to steric strain are among these principles. Section 1.10.7 illustrated a few examples of how the predictive power of such structural rules have been applied to the design of novel and highly selective Lewis acidic catalysts. 

Sc(() l f) ( is an effective catalyst of the Mukaiyama aldol reaction in both aqueous and non-aqueous media (vide supra). Kobayashi et al. have reported that aqueous aldehydes as well as conventional aliphatic and aromatic aldehydes are directly and efficiently converted into aldols by the scandium catalyst [69]. In the presence of a surfactant, for example sodium dodecylsulfate (SDS) or Triton X-100, the Sc(OTf)3-catalyzed aldol reactions of SEE, KSA, and ketene silyl thioacetals can be performed successfully in water wifhout using any organic solvent (Sclieme 10.23) [72]. They also designed and prepared a new type of Lewis acid catalyst, scandium trisdodecylsulfate (STDS), for use instead of bofh Sc(OTf) and SDS [73]. The Lewis acid-surfactant combined catalyst (LASC) forms stable dispersion systems wifh organic substrates in water and accelerates fhe aldol reactions much more effectively in water fhan in organic solvents. Addition of a Bronsted acid such as HCl to fhe STDS-catalyzed system dramatically increases the reaction rate [74]. 

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