Organocatalysis uses small organic molecules predominantly composed of C, H, O, N, S and P to accelerate chemical reactions. The advantages of organocatalysts include their lack of sensitivity to moisture and oxygen, their ready availability, low cost, and low toxicity, which confers a huge direct benefit in the production of pharmaceutical intermediates when compared with (transition) metal catalysts.
In the example of the Knoevenagel Condensation, it is believed that piperidine forms a reactive iminium ion intermediate with the carbonyl compound:
Another organocatalyst is DMAP, which acts as an acyl transfer agent:
Thiazolium salts are versatile umpolung reagents (acyl anion equivalents), for example finding application in the Stetter Reaction:
All of these organocatalysts are able to form temporary covalent bonds. Other catalysts can form H-bonds, or engage in pi-stacking and ion pair interactions (phase transfer catalysts). Catalysts may be specially designed for a specific task - for example, facilitating enantioselective conversions.
|An early example of an enantioselective Stetter Reaction is shown below: :||
model explaining the facial selectivity
Enantioselective Michael Addition using phase transfer catalysis:
The first enantioselective organocatalytic reactions had already been described at the beginning of the 20th century, and some astonishing, selective reactions such as the proline-catalyzed synthesis of optically active steroid partial structures by Hajos, Parrish, Eder, Sauer and Wiechert had been reported in 1971 (Z. G. Hajos, D. R. Parrish, J. Org. Chem. 1974, 39, 1615; U. Eder, G. Sauer, R. Wiechert, Angew. Chem. Int. Ed. 1971, 10, 496, DOI). However, the transition metal-based catalysts developed more recently have drawn the lion’s share of attention.
Hajos-Parrish-Eder-Sauer-Wiechert reaction (example)
The first publications from the groups of MacMillan, List, Denmark, and Jacobson paved the way in the year 1990. These reports introduced highly enantioselective transformations that rivaled the metal-catalyzed reactions in both yields and selectivity. Once this foundation was laid, mounting interest in organocatalysis was reflected in a rapid increase in publications on this topic from a growing number of research groups.
Proline-derived compounds have proven themselves to be real workhorse organocatalysts. They have been used in a variety of carbonyl compound transformations, where the catalysis is believed to involve the iminium form. These catalysts are cheap and readily accessible:
A general picture of recent developments: V. D. B. Bonifacio, Proline Derivatives in Organic Synthesis, Org. Chem. Highlights 2007, March 25.
Books on Organocatalysis
Albrecht Berkessel, Harald Gr÷ger
Hardcover, 440 Pages
First Edition, 2005
ISBN: 3-527-30517-3 - Wiley-VCH
Aryl iodides are efficient catalysts in an organocatalytic syn diacetoxylation of alkenes. A broad range of substrates, including electron-rich as well as electron-deficient alkenes, furnish the desired products in very good yields with high diastereoselectivity.
W. Zhong, S. Liu, J. Yang, X. Meng, Z. Li, Org. Lett., 2012, 14, 3336-3339.
Confined chiral Br°nsted acids catalyze asymmetric oxidations of a broad range of sulfides to sulfoxides with hydrogen peroxide. The wide generality and high enantioselectivity of the developed method is comparable even to the best metal-based systems.
S. Liao, I. Čorić, Q. Wang, B. List, J. Am. Chem. Soc., 2012, 134, 10765-10768.
A direct reductive amination of ketones using the Hantzsch ester in the presence of S-benzyl isothiouronium chloride as a recoverable organocatalyst converts a wide range of ketones as well as aryl amines to the expected products in good yields.
Q. P. B. Nguyen, T. H. Kim, Synthesis, 2012, 1977-1982.
An organocatalytic Dakin oxidation of electron-rich arylaldehydes to phenols can be performed under mild, basic conditions using flavin catalysts. Catechols are readily prepared and the oxidation of 2-hydroxyacetophenone was achieved.
S. C. M. S. Hoassain, F. W. Foss, Jr, Org. Lett., 2012, 14, 2806-2809.
The in situ generation of α-amino aldehydes followed by reaction with dimethyloxosulfonium methylide under Corey-Chaykovsky reaction conditions gives 4-hydroxypyrazolidine derivatives in high yields with excellent enantio- and diastereoselectivities. This organocatalytic sequential method enables an efficient synthesis of anti-1,2-aminoalcohols.
B. S. Kumar, V. Venkataramasubramanian, A. Sudalai, Org. Lett., 2012, 14, 2468-2471.
Ozonolysis in the presence of pyridine directly generates ketones or aldehydes through a process that neither consumes pyridine nor generates any detectable peroxides. The reaction is hypothesized to involve nucleophile-promoted fragmentation of carbonyl oxides via formation of zwitterionic peroxyacetals.
R. Willand-Charnley, T. J. Fisher, B. M. Johnson, P. H. Dussault, Org. Lett., 2012, 14, 2242-2245.
A bifunctional organocatalyst efficiently catalyzed not only enantioselective conjugate addition of aromatic ketones to nitroolefins in good yields with excellent enantioselectivities but also enantioselective conjugate addition of acetone to nitroolefins in excellent yields with high enantioselectivities.
Z.-W. Sun, F.-Z. Peng, Z.-Q. Li, L.-W. Zhou, S.-X. Zhang, X. Li, Z.-H. Shao, J. Org. Chem., 2012, 77, 4103-4110.
Organocatalytic stereospecific dibromination of various functionalized alkenes was achieved using a simple thiourea catalyst and 1,3-dibromo 5,5-dimethylhydantoin as a stable, inexpensive halogen source at room temperature. The procedure was extended to alkynes and aromatic rings and to dichlorination reactions by using the 1,3-dichloro hydantoin derivative.
G. Hernßndez-Torres, B. Tan, C. F. Barbas III, Org. Lett., 2012, 14, 1858-1861.
A nitroxyl-radical-catalyzed oxidation using diisopropyl azodicarboxylate (DIAD) allows the conversion of various primary and secondary alcohols to their corresponding aldehydes and ketones without overoxidation to carboxylic acids. 1,2-Diols are oxidized to hydroxyl ketones or diketones depending on the amount of DIAD used.
M. Hayashi, M. Shibuay, Y. Iwabuchi, J. Org. Chem., 2012, 77, 3005-3009.
An enantioselective synthesis of γ-nitroesters by a one-pot asymmetric Michael addition/oxidative esterification of α,β-unsaturated aldehydes is based on an enantioselective organocatalytic nitroalkane addition followed by an N-bromosuccinimide-based oxidation. The γ-nitroesters are obtained in good yields and enantioselectivities, and the method provides an attractive entry to optically active γ-aminoesters, 2-piperidones, and 2-pyrrolidones.
K. L. Jensen, P. H. Poulsen, B. S. Donslund, F. Morana, K. A. J°rgensen, Org. Lett., 2012, 14, 1516-1519.
A simple chiral primary amine catalyses a highly efficient reaction for the synthesis of both Wieland-Miescher ketone and Hajos-Parrish ketone as well as their analogues in high enantioselectivity and excellent yields. This procedure represents one of the most efficient methods for the synthesis of these versatile chiral building blocks even in gram scale with 1 mol% catalyst loading.
P. Zhou, L. Zhang, S. Luo, J.-P. Cheng, J. Org. Chem., 2012, 77, 2526-2530.
The of silica-coated magnetic nanoparticles allowed the construction of magnetically recoverable organic hydride compounds. Magnetic nanoparticle-supported BNAH (1-benzyl-1,4-dihydronicotinamide) showed efficient activity in the catalytic reduction of α,β-epoxy ketones. After reaction, the catalyst can be separated by simple magnetic separation and can be reused.
H.-J. Xu, X. Wan, Y.-Y. Shen, S. Xu, Y.-S. Feng, Org. Lett., 2012, 14, 1210-1213.
Activation of diphenylsilane in the presence of a catalytic amount of an N-heterocyclic carbene (NHC) enables hydrosilylation of carbonyl derivatives under mild conditions. Presumably, a hypervalent silicon intermediate featuring strong Lewis acid character allows dual activation of both the carbonyl moiety and the hydride at the silicon center. Some interesting selectivities have been encountered.
Q. Zhao, D. P. Curran, M. Malacria, L. Fensterbank, J.-P. Goddard, E. Lac˘te, Synlett, 2012, 433-437.
A bifunctional squaramide catalyzes a sulfa-Michael/aldol cascade reaction between 1,4-dithiane-2,5-diol and chalcones with a low catalyst loading to yield trisubstituted tetrahydrothiophenes with three contiguous stereogenic centers in a highly stereocontrolled manner.
J.-B. Ling, Y. Su, H.-L. Zhu, G.-Y. Wang, P.-F. Xu, Org. Lett., 2012, 14, 1090-1093.
A phosphinite derivative that can be easily prepared in two steps from commercially available aminoindanol is an effective catalyst for enantioselective acylation of diols. For the asymmetric desymmetrization of meso-1,2-diols, the corresponding monoester was obtained in high enantioselectivity.
H. Aida, K. Mori, Y. Yamaguchi, S. Mizuta, T. Moriyama, I. Yamamoto, T. Fujimoto, Org. Lett., 2012, 14, 812-815.
Commercially available and very inexpensive benzoic acids catalyze an efficient and simple isomerization of readily prepared allylic alcohols to yield cyclic products, unusual enyne, and dienols. The catalysts can be tuned for reactivity and substrate sensitivity.
J. A. McCubbin, S. Voth, O. V. Krokhin, J. Org. Chem., 2011, 76, 8537-8542.
Cinchona-alkaloid-thiourea-based bifunctional organocatalysts enable a straightforward asymmetric cycloetherification of ε-hydroxy-α,β-unsaturated ketones for the synthesis of tetrahydrofuran rings. This catalytic process represents a highly practical cycloetherification method that provides excellent enantioselectivities, even with low catalyst loadings at ambient temperature.
K. Asano, S. Matsubara, J. Am. Chem. Soc., 2011, 133, 16711-16713.
Enantioenriched N,S-acetals can be prepared using chiral BINOL phosphoric acids. The reaction combines electron-rich and electron-deficient aromatic N-acyl imines with broad range of aliphatic and aromatic thiols to generate products in excellent yield and enantioselectivity. The addition reaction could also be achieved with an exceptional substrate to catalyst (S/C) molar ratio.
G. K. Ingle, M. G. Mormino, L. Wojtas, J. C. Antilla, Org. Lett., 2011, 13, 4762-4765.
1-Alkyl-3-methylimidazolium cation based ionic liquids efficiently catalyze N-tert-butyloxycarbonylation of amines with excellent chemoselectivity. The catalytic role of the ionic liquid is envisaged as electrophilic activation of di-tert-butyl dicarbonate (Boc2O) through bifurcated hydrogen bond formation with the C-2 hydrogen of the 1-alkyl-3-methylimidazolium cation.
A. Sarkar, S. R. Roy, N. Parikh, A. K. Chakraborti, J. Org. Chem., 2011, 76, 7132-7140.
A biomimetic proton transfer catalysis with a chiral organic catalyst enabled an enantioselective olefin isomerization of a broad range of mono- and disubstituted β,γ-unsaturated butenolides into the corresponding chiral α,β-unsaturated butenolides in high enantioselectivity and yield. Mechanistic studies have revealed the protonation as the rate-determining step.
Y. Wu, R. P. Singh, L. Deng, J. Am. Chem. Soc., 2011, 133, 12458-12461.
3,3′-Br2-BINOL catalyze the enantioselective asymmetric propargylation of ketones using allenyldioxoborolane as nucleophile, in the absence of solvent, and under microwave irradiation to afford homopropargylic alcohols in good yields and high enantiomeric ratios. Diastereoselective propargylations using chiral racemic allenylboronates result in good diastereoselectivities.
D. S. Barnett, S. E. Schaus, Org. Lett., 2011, 13, 4020-4023.
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