Bronsted acids, as catalysts

Although the discussion to this point has been concerned with the explanation of the behavior of Bronsted acids as catalysts, there is an enormous range of reactions in which catalysis by acids and bases occurs. Many of the important types of organic reactions involve catalysis by acids or bases. In this section, several reactions will be mentioned, but the mechanistic details will not be presented in this book on inorganic chemistry. The discussion is intended to show the scope of catalysis by acids and bases. 

There has been a continuing effort to make the Baylis-Hillman reaction a catalytic asymmetric process. Scott Schnauss of Boston University recently reported (J. Am. Chem. Soc. 125 12094, 2003) an elegant solution to this problem, based on the use of Binol-derived Bronsted acids as catalysts. The product hydroxy enones such as 6 are interesting in themselves, and also as substrates for further transformation, for instance by Claisen rearrangement. 

An alternative method for the organocatalytic reduction of imines employs Bronsted acids as catalysts and Hantzsch dihydropyridine as a reducing reagent. This topic is described in Chapter 11 (on Bronsted acids) [86]. 

The use of Lewis and Bronsted acids as catalysts in oleic acid esterification reactions it was assessed. Although being homogeneous catalysts, both compoimds can be recovered and reused in the reactions via a... 

Recently, the first example of an environmentally benign enantioselective BV oxidation, utilizing a strong chiral Bronsted acid as catalyst was reported [42]. With a chiral phosphoric acid and 30% aqueous H2O2 as the oxidant, various of 3-substituted cyclobutanones were converted into their corresponding y-Iactones in excellent yields and enantioselectivities up to 93% ee. 

Several Bronsted acids as catalysts were tested in a model Mannich-type reaction in water, and it was found that dodecylbenzenesulfonic acid (DBSA) afforded the desired product in... 

Repas, M., Supported Bronsted acids as catalysts of propylene oligomerization, Chem. Pnim., 15, 543, 1965. 

During the last two decades the synthesis of levofloxacin and its S-(-)-pre-cursors has been improved considerably, and new approaches have been advanced [243-255], In particular, kinetic resolution of 7,8-difluoro-2,3-dihydro-3-methyl-4H-[l,4]-benzoxazine racemate using naproxen, N-[sulphonylsubstituted]-(/ )-proline and (2S)-(6-methoxynapht-2-yl)propionyl chloride, has been advanced [256-261], The optically active (5)-isomer obtained by this method has been used for the synthesis of levofloxacin (5)-(-)-80 [256]. Also a new synthetic approach to (5)-isomer through catalytic reduction of 7,8-difluoro-3-methyl-2 (-l,4-benzoxazine with use of chiral Bronsted acids as catalyst and substituted dihydropyridine as a source of hydrogen has been described [262],... 

Table 2.3 CO formylation of aromatic compounds using various Lewis- and Bronsted acids as catalysts...

The use of chiral Bronsted acids as catalysts for enantioselective hetero-Diels-Alder reactions has also been examined by Akiyama [132, 133]. A BINOL-derived phosphoric acid [19,128, 134] was found to promote enantioselective cycloadditions of Brassard s diene 265 [135] to imines (Equation 27). Use of the corresponding pyridinium phosphate 266 resulted in higher yields along with high enantioselectivity for a range of imines, as exemplified by the formation of 267 (97% ee, 90% yield) [132]. 

Figure 4.10 is plot of the Bronsted relationship for hydrolysis of an enol ether. The plot shows that the effectiveness of the various carboxylic acids as catalysts is related to their dissociation constants. In this particular case, the constant a is 0.79 ... 

Scheme 11 Plausible role of Bronsted acid co-catalyst as supported by computational studies...

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

A number of reactions that are traditionally carried out using either Bronsted or Lewis acids as catalysts have been shown to take place in the presence of EG A formed either in situ or ex situ. The reactions do not involve charge-consuming conversions of the substrates. 

Finally in Chapters 11-13, some of the more recent discoveries that have led to a renaissance in the field of organocatalysis are described. Included in this section are the development of chiral Brdnsted acids and Lewis acidic metals bearing the conjugate base of the Bronsted acids as the ligands and the chiral bifunctional acid-base catalysts. 

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. 

Surprisingly, in some cases, simple Lewis or Bronsted acids as the catalysts can replace the PtCl2 catalyst... 

In contrast to chiral amines, phosphorus-based chiral catalysts were less developed for asymmetric MBH transformations. As with amine-based reactions, the selectivity of the addition in phosphine-mediated reactions depends clearly on the nature of the complementary Bronsted acid co-catalysts used. 

As a result of these suggested differences in isomorphic substitution mechanism, SAPO and MCM materials should have different ion-exchange and catalytic properties. SAPO s are cation exchangers and potential BrCnsted acid catalysts. MCM s are expected to be cation and/or anion exchangers and are potential Bronsted acid, Brensted base or Bronsted acid/base catalysts. 

Reactions in SCCO2 have also been used for the production of minor lipid components, such as tocopherols and sterol esters. The synthesis of D, L, ot-tocopherol in SCCO2 and nitrous oxide by condensation of trimethyUiydroquinone with iso-phytol in the presence of various Bronsted or Lewis acids as catalysts resulted in... 

As is described in more detail in subsequent sections, it is well established that iron (III) porphyrins can be reduced by photochemical, electrochemical, and radiochemical methods in organic or aqueous solutions to give Fe and Fe complexes.Recently, it has also been shown that such reactions lead to the formation of complexes with the iron in the zero oxidation state. Such Fe° porphyrins are, for example, formed electrochemically when Fe porphyrins ([PFe ]+) are reduced in three successive steps in DMF in the presence of a weak Bronsted acid as co-catalyst to yield the iron (II) porphyrin ([PFe ]), the iron(I) porphyrin ([PFe ] ) and the iron(O) porphyrin 503-506 jj-on(0) porphyrins are... 

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