Bronsted cooperative catalysis

These experimental results suggested a hydrogen-bonding mediated cooperative Bronsted acid catalysis mechanism (Scheme 6.28). Thiourea cocatalyst 9 is viewed to coordinate to mandelic acid 20 through double hydrogen-bonding, stabilizes the acid in the chelate-hke cis-hydroxy conformation, and acidifies the a-OH proton via an... 

By applying a new mode of cooperative catalysis involving the combination of a chiral Bronsted acid and a -symmetric biaryl saturated imidazolium precatalyst, Lee and Scheldt disclosed a highly enantioselective NHC-cata-lyzed [3 + 2] annulation reaction between a,p-alkynals and a-keto esters to generate the desired y-crotonolactones in high yields and excellent levels of enantioselectivity (up to 92% yield, 92% ee). The authors proposed that NHC-bound allenolate underwent addition to the a-keto ester activated by the chiral Bronsted acid derived co-catalyst (Scheme 7.43). 

Using menthol as a chiral Bronsted acid source, upon the Lewis acid activation with the Au(I) center, cooperative catalysis was achieved to prepare substituted pyrrohdine heterocycles with excellent enantiomeric excesses (Scheme 15.83) [300],... 

In a tandem isomerization/Prins strategy utilizing cooperative catalysis between an iridium(III) catalyst and a Bronsted acid, indole 182 underwent an isomerization/protonation sequence via a Prins-type oxocarbenium intermediate, with subsequent C—C bond formation to give oxepane-fused indole 183 (13AGE12910). Various anthranilic acids were coupled with chiral a-haloacids to afford N-acylated anthranilic acid intermediates which underwent cyclization to (3R)-3-alkyl-4,l-benzoxazepin-2,5-diones... 

Tang, W. Johnston, S. Li, C. Iggo, J. A. Bacsa, J. Xiao, J. Cooperative catalysis Combining an achiral metal catalyst with a chiral bronsted acid enables highly enantioselective hydrogenation of imines. Chem. 2013,19,14187-14193. 

Yang T, Ferrali A, Sladojevich F, Campbell L, Dixon DJ (2009) Bronsted base/lewis acid cooperative catalysis in the enantioselective conia-ene reaction. J Am Chem Soc 131 (26) 9140-9141... 

A cooperative catalysis with the non-chiral Knolker iron complex (227) and a chiral Bronsted acid (223) has also been utilised for asymmetric reductive amination of ketones. Various ketones including aromatic, heteroaromatic and aliphatic ketones (228) have been transformed to the corresponding chiral amines (229) with high enantioselectivities up to 99% ee (Scheme 61). ... 

A highly enantioselective catalysis has been developed for [3 + 2] an-nulation reaction with a,p-allqmals (278) and a-ketoesters (279). A new mode of cooperative catalysis involving a combination of the chiral Bronsted acid (281) and a Ci-symmetric biaryl saturated-imidazolium precatalyst (282) was required to generate the desired y-crotonolactones (280) in high yields and levels of enantioselectivity (Scheme 76). ° ... 

Scheme 43.6 Enantioselective bromocycloetherification by Lewis base/chiral Bronsted acid cooperative catalysis.

Scheme 43.14 N-heterocyclic carbene and Bronsted acid cooperative catalysis for asymmetric synthesis of trons-y-lactams.

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

The small-molecule catalysts are covered in Chapters 5 and 6. In Chapter 5, Joshua Payette and Hisashi Yamamoto discuss the importance of polar Bronsted-acid-type catalysts as well as cooperative effects in hydrogen bonding catalysis. Chapter 6 by Mike Kotke and Peter Schreiner is then devoted to the single most popular small-molecule catalyst types, the thiourea catalysts. Chapter 6, the longest of all chapters, also provides an excellent overview of the history and development of the field of small-molecule hydrogen bond catalysis. 

The nature of the cooperativity was further characterized based on results from kinetic measurements. The two HisH+-His pairs in helix II catalyzed the hydrolysis of mono-p-nitrophenyl fumarate at pH 5.1 and 290 K with second-order rate constants of 0.01 M-i s-i (JNI, His-26, His-30) and 0.055 M s (JNII, His-30, His-34), respectively, and a rate constant ratio JNII/JNI of 5.5. The pKa values of both His residues in JNII are the same, so for the analysis it does not matter which residue is the nucleophile and which one is the acid. In JNI, however, the pKa values are 6.9 for His-26 and 5.6 for His-30. The rate constant ratio of 5.5 should therefore arise due to the difference in nucleophihcity or due to the difference in acidity, or if both residues in the pair can be both nucleophile and acid, from a mixture of the two. If His-30 functions as a general acid in JNI, then the rate constant ratio should arise from the difference in nucleophUicity between two nucleophiles with the pKa values 5.6 and 6.9. We can, however, calculate the reactivity difference as in Section 5.2.3 to find that 10 - = 1.8, one third of the observed ratio of 5.5. If, on the other hand, the rate constant ratio is due to a difference in general-acid catalysis by two residues with pKa values of 5.6 and 6.9, then the the Bronsted equa-hon for general-acid catalysis can be applied... 

Discussions of OH groups in the context of catalysis normally focus on their role as active centers in a number of reactions. The work by Haag et al. (94) constitutes a classic example the authors estahhshed a linear relationship between the concentration of aluminum in HZSM-5 (which imphes an equal concentration of bridging hydroxyls) and the activity for cracking of -hexane. It was concluded that aU protonic acid sites in the zeohte are characterized by the same turnover frequency. Many other correlations between catalytic properties of materials and the strength and/or density of their Bronsted acid sites are well estabHshed. We will not discuss this aspect in detail and recommend instead a number of recently pubhshed reviews (59,60,87). Two more points are worth mentioning. One point is that the cooperative action of Bronsted and Lewis acid sites has been demonstrated. The second is that, of course, OH groups must not necessarily be involved in a catalytic conversion in fact, they can even block the catalyt-icaUy active sites. 

---