BLA catalysts

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Yamamoto et al. have developed a catalytic enantioselective carbo-Diels-Alder reaction of acetylenic aldehydes 7 with dienes catalyzed by chiral boron complexes (Fig. 8.10) [23]. This carbo-Diels-Alder reaction proceeds with up to 95% ee and high yield of 8 using the BLA catalyst. The reaction was also investigated from a theoretical point of view using ab-initio calculations at a RHF/6-31G basis set. 

The four different transition states in Fig. 8.10 were considered with BF3 as a model for the BLA catalyst in the theoretical calculations. It was found that the lowest transition-state energy for the BF3-catalyzed reactions was calculated to be 21.3 kcal mol for anti-exo transition state, while only 1.5 kcal mol higher in energy the syn-exo transition state, was found. The uncatalyzed reaction was calculation to proceed via an exo transition state having an energy of 37.0 kcal mol . The calculations indicated that the reaction proceeds predominantly by an exo transition-state structure and that it is enhanced by the coordination of the Lewis acid. 

Yamamoto and co-workers have introduced a conceptually interesting series of catalysts that incorporate an acidic proton into the active catalyst. Termed Bronsted acid-assisted chiral Lewis acid (BLA), catalyst 14 selectively catalyzes a number of diene-aldehyde cycloadditions reactions (Scheme 16) [67]. While extremely selective for the substrates shown, no aldehydes lacking an a-substitu-ent were reported to be effective in this reaction. This feature was addressed in... 

The use of arylboronic acids with electron-withdrawing substituents such as 3,5-(CFj)2C6H3B(OH)2 in the preparation of BLA greatly enhances its catalytic activity and asymmetry-inducing ability. Ishihara and Yamamoto et al. have developed a more practical BLA (10), which has a greater catalytic activity in the enantioselective cycloaddition of both a-substituted and a-unsubstituted a,P-enals with various dienes (Equation 24) [24b,dj. Notably, the presence of a Bronsted acid in BLA catalysts clearly... 

Ishihara K, Yamamoto H (1994) Brqnsted acid assisted chiral Lewis acid (BLA) catalyst for asymmetric Diels-Alder reaction. J Am Chem Soc 116 1561-1562... 

A more recent contribution from the Corey group deserves special appreciation. These above-mentioned BLA catalysts can be viewed as borane Lewis acids with protonated secondary amine as part of their ligands, while combination with Brpnsted acid is responsible for the enhanced Lewis acidity. A related oxazabor-olidine catalyst with cationic tertiary amine as part of its ligand can also be used in enantioselective Diels-Alder reaction (Fig. 4) [42]. This single functional acid catalyst was shown to give similar or superior performance compared with related combined acid catalysts, though its preparation seems to be more difficult. 

In 2007, the Corey group reported the use of AlBr3 as Lewis acid activator for a number of oxazaborolidine catalyzed reactions (Fig. 6) [44, 45]. In this study, complete complexation between AlBr3 and oxazaborolidine was observed by NMR, which might be the reason for the higher reactivity of this LLA catalyst compared with previously reported BLA catalysts. 

In 1994 Yamamoto et al. developed a novel catalyst which they termed a "Brmsted acid-assisted chiral Lewis acid" (BLA) [10] (Scheme 1.14, Table 1.3). The catalyst 7 was prepared from (R)-3,3 -dihydroxyphenyl)-2,2 -dihydroxy-l,l -binaphthyl by reaction with B(OMe)3 and removal of methanol [10a, dj. The Brmsted acid is essential for both the high reactivity of the Lewis acid and the high enantioselectivity - the... 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Bronsted-acid-assistcd chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phcnylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5A(trifluoromethy I) - be nzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,(3-enals and various dienes.62 The interesting aspect is the role of water, THF, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

Yamamoto and co-workers found that BLA 76a is one of the best catalysts for the enantioselective and selective cycloaddition of a-substituted ajl-enals to highly reactive dienes such as cyclopentadiene. The results in the presence of (J )-76a are summarized in Table 5-2. The major enantiomer has, in several cases, been demonstrated to have the predicted absolute configuration. 

The results suggest that BLA 77 is the best catalyst. For example, the reaction of ( )-methyl acrolein with cyclopentadiene catalyzed by 77 (5 mol%) gives the adduct at 96% yield with 99% ee [(S configuration]. 

Yamamoto had earlier reported that Lewis acid activation of valine-derived oxazaborolidine 60 yielded a highly reactive and moisture-tolerant LLA catalyst 61 for the Diels-Alder reaction (Scheme 5.76) [145]. In later studies, activation of 60 with the super Bronsted acid, C,sF5CHTf2, was found to produce the even more reactive catalytic species BLA 62. During studies toward an enantioselective route to Platensimycin [146], BLA 62 was found to catalyze the Diels-Alder reaction between various monosubstituted dienes and ethyl acrylate to afford adducts... 

The catalyst (10 mg) was dissolved in toluene (25 ml) and added under argon to the solution of a mixture (1.1 g) of avermectin Bla (96%) and avermectin Bib (4%) and of m9 °f triphenylphosphine in toluene (25 ml) in a stainless steel autoclave. This starting material was then hydrogenated at 88°C under a hydrogen pressure of 20 bar with stirring of the solution. After 10 hours, HPLC analysis revealed a content of 86% dihydro-avermectin Bla and of 4 % dihydroavermectin Blb, and also of 3% tetrahydroavermectin Bla. 

The catalyst is added to a solution of 4.3 g of avermectin (Bla and Blb mixture) in 25 ml of a mixture of acetone and cyclohexane in a ratio of 2 1. After addition of 51.4 mg of tris-(mexylphenyl)phosphine, the hydrogenation is carried out in a steel autoclave at a hydrogen pressure 5 bar and at 88°C. After a hydrogenation time of 4 hours, 8.9% of starting material, 89.9% of ivermectin (Bla and Blb mixture), tetrahydroavermectin content <0.1% was obtained (according to HPLC analysis). 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

We have found that chiral boronate complexes with BLA (Brpnsted acid-assisted chiral Lewis acids) to give new catalysts for enantioselective synthesis which achieve selectivity by a double effect of intramolecular hydrogen-bonding interaction and attractive ji-ti donor-acceptor interaction in the transition state by a hydroxy aromatic group [27a],... 

BLA 4 (Fig. 5) is one of the best catalysts for the enantio- and earo-selective cycloaddition of a-substituted a,/3-enals with highly reactive dienes such as cyclopen-tadiene. The corresponding reactions of a-unsubstituted a,)8-enals such as acrolein and crotonaldehyde are, however, characterized by low enantioselectivity and/or reactivity. The range of dienophiles that can be used with less reactive dienes is quite limited. The use of arylboronic acid with electron-withdrawing substituents such as [3,5-bis(trifluoromethyl)phenyl]boronic acid in the preparation of BLA greatly enhances... 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

Some quantity of solution HAuCl (twice the required for this reaction) was added to the pattern catalyst. After reaction, the catalyst thoroughly water-washing was dried in an oven at 120 C all night. The bimetallic catalyst was activated whether by direct reduction under hydrogen at 500 C for 2h (catalysts Alb, 61b, Clb) or calcined under air at 300 C for 2h, and reduced at 500 C (catalysts Ala, Bla, Cla). 

Fig. 8. Effect of NO2/NO1 feed ratio on the steady-state NO, conversion over a commercial V206-W03/Ti02 catalyst for vehicle SCR applications at 160°C (full squares), 175°C (blank squares), 200°C (full circles), 225°C (bla circles), 275°C (full triangles), 350°C (blank triangles), and 425°C (spheres). Feed NHa/NO = 1 1. IVcat = 80 mg, Q = 280...

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