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 Gröger
Hardcover, 440 Pages
First Edition, 2005
ISBN: 3-527-30517-3 - Wiley-VCH
An N-heterocyclic carbene-catalyzed oxidative LUMO activation of the β-cabons of saturated carboxylic esters enables an efficient asymmetric access to lactams and lactones. The method introduces functional groups at the typically inert β-sp3 carbons of saturated esters. The use of HOBt as an additive improves both yields and enantioselectivities of the reactions.
B. Liu, W. Wang, R. Huang, J. Yan, J. Wu, W. Xue, S. Yang, Z. Jin, Y. R. Chi, Org. Lett., 2018, 20, 260-263.
An isothiourea-catalyzed enantioselective formal [4+3] cycloaddition of various α,β-unsaturated carboxylic acid derivatives with 2-aminothiophenols proceeds via a reversible sulfa-Michael addition to α,β-unsaturated acylammonium intermediates, followed by an enantioselective formation of a seven-membered ring. This method enables a facile and divergent synthesis of optically active 2- and 3-substituted 1,5-benzothiazepines.
Y. Fukata, K. Yao, R. Miyaiji, K. Asano, S. Matsubara, J. Org. Chem., 2017, 82, 12655-12668.
Tetraarylphosphonium salts (TAPS) catalyze the [3 + 2] coupling reaction of isocyanates and epoxides to provide a range of oxazolidinones, including enantioenriched N-aryl-substituted oxazolidinones. These Brønsted acid/halide ion bifunctional catalysts accelerate epoxide ring opening with high regioselectivity.
Y. Toda, S. Gomyou, S. Tanaka, Y. Komiyama, A. Kikuchi, H. Suga, Org. Lett., 2017, 19, 5786-5789.
Bioinspired ortho-quinone catalysts have been applied to oxidative synthesis of benzimidazoles, quinoxalines and benzoxazoles from primary amines in high yields under mild conditions with oxygen as the terminal oxidant.
R. Zhang, Y. Qin, L. Zhang, S. Luo, Org. Lett., 2017, 19, 5629-5632.
1,2,4,3-triazaphospholenes halides catalyze the 1,2 hydroboration of imines and α,β unsaturated aldehydes with pinacolborane, including examples that did not undergo hydroboration by previously reported diazaphospholene systems. DFT calculations support a mechanism where a triazaphospholene cation interacts with the substrate.
C.-H. Tien, M. R. Adams, M. J. Ferguson, E. R. Johnson, A. W. H. Speed, Org. Lett., 2017, 19, 5565-5568.
A chiral phosphine-catalyzed enantioselective [1 + 4] annulation of Morita-Baylis-Hillman carbonates with β,γ-unsaturated α-keto esters and α,β-unsaturated ketones provides a wide range of optically active 2,3-dihydrofurans in high yields with excellent asymmetric induction via a Michael alkylation process.
Y. Cheng, Y. Han, P. Li, Org. Lett., 2017, 19, 4774-4777.
A synergistic merger of photoredox, nickel, and hydrogen atom transfer catalysis enables direct aldehyde C-H functionalization. This mild, operationally simple protocol provides a broad range of ketones in excellent yield from commercially available aldehydes, along with aryl or alkyl bromides.
X. Zhang, D. W. C. MacMillan, J. Am. Chem. Soc., 2017, 139, 11353-11356.
A readily accessible thiourea organocatalyst catalyzes reductive condensations of alcohols with aldehydes/ketones to provide ethers without homocoupling of the carbonyl component in the presence of HCl and 1,1,3,3-tetramethyldisiloxane as a convenient reducing reagent. This strategy is applicable to challenging substrate combinations and exhibits functional group tolerance.
C. Zhao, C. A. Sojdak, W. Myint, D. Seidel, J. Am. Chem. Soc., 2017, 139, 10224-10227.
The use of tert-butyl nitrite as the co-catalyst in a 2-azaadamantane-N-oxyl (AZADO)- and 9-azanoradamantane-N-oxyl (nor-AZADO)-catalyzed efficient aerobic oxidation of primary alcohols in MeCN instead of the previously reported AcOH provides the corresponding aldehydes selectively. The addition of saturated aqueous NaHCO3 after the completion of the reaction suppresses the overoxidation of the product during the workup.
M. Shibuya, K. Furukawa, Y. Yamamoto, Synlett, 2017, 28, 1554-1557.
A N-heterocyclic carbene (NHC) catalyzed transformylation of 1°, 2°, and 3° alcohols in the presence of methyl formate as the formyl transfer reagent provides a broad range of formates in good yields. The reaction employs low catalyst loadings, is insensitive to common nitrogen and oxygen protecting groups and can be achieved in the presence of a number of heterocycles.
J. E. M. Fernando, A. Levens, D. Moock, D. W. Lupton, Synthesis, 2017, 49, 3505-3510.
A N-heterocyclic carbene catalyzed formal [3 + 2] annulation of enals with 2-aminoacrylates provides various γ-lactam derivatives in good yields with excellent diastereo- and enantioselectivities. In this process, two consecutive stereocenters and a quaternary carbon center are constructed.
X.-S. Li, L.-L. Zhao, X.-K. Wang, L.-L. Cao, X.-Q. Shi, R. Zhang, J. Qi, Org. Lett., 2017, 19, 3943-3946.
The direct oxidative N-acylation reaction of primary amides with aryl/α,β-unsaturated aldehydes was achieved in the presence of an azolium salt and an inorganic base using 3,3′,5,5′-tetra-tert-butyldiphenoquinone as the oxidant. The reaction provides an efficient approach for the synthesis of N-sulfonylcarboxamides, N-sulfinylcarboxamides, and dicarboxyimides in good yield.
C. Zheng, Y. Liu, C. Ma, J. Org. Chem., 2017, 82, 6940-6945.
Peroxy trichloroacetimidic acid, in situ generated from aqueous hydrogen peroxide and trichloroacetonitrile, acts as a competent electrophilic oxygenating agent for a direct enantioselective α-hydroxylation of oxindoles in the presence of a chiral 1,2,3-triazolium salt as a phase-transfer catalyst.
K. Ohmatsu, Y. Ando, T. Ooi, Synlett, 2017, 28, 1291-1294.
A pyrene-substituted imidazolium-based ionic liquid (PIL) serves as organic catalyst for the SN2 fluorination using CsF. In this system, the PIL significantly enhanced the reactivity of the metal fluoride due to the phase-transfer catalytic effect of the imidazolium moiety as well as the metal cation-π (pyrene) interactions. Furthermore, PIL can easily be separated from the reaction mixture using reduced graphene oxide by π-π stacking.
A. Taher, K. C. Lee, H. J. Han, D. W. Kim, Org. Lett., 2017, 19, 3342-3345.
4-Mesityl-2,6-diphenylpyrylium tetrafluoroborate (MDPT) and 4-mesityl-2,6-di-p-tolylpyrylium tetrafluoroborate (MD(p-tolyl)PT) are highly robust photoredox catalysts, and exhibit some of the highest oxidation potentials reported. Their utility was demonstrated in the mild and efficient generation of carbonyl ylides from benzylic epoxides.
E. Alfonzo, F. S. Alfonso, A. B. Beeler, Org. Lett., 2017, 19, 2989-2992.
A water-soluble photocatalyst promotes an aerobic oxidative hydroxylation of arylboronic acids to furnish phenols in excellent yields. This transformation uses visible-light irradiation under environmentally friendly conditions.
H.-Y. Xie, L.-S. Han, S. Huang, X. Lei, Y. Cheng, W. Zhao, H. Sun, X. Wen, Q.-L. Xu, J. Org. Chem., 2017, 82, 5236-5241.
1,2,2,3,4,4-hexamethylphosphetane catalyzes a deoxygenative N-N bond-forming Cadogan heterocyclization of o-nitrobenzaldimines and o-nitroazobenzenes with good functional group compatibility in the presence of phenylsilane as terminal reductant.
T. V. Nykaza, T. S. Harrison, A. Ghosh, R. A. Putnik, A. T. Radosevich, J. Am. Chem. Soc., 2017, 139, 6839-6842.
A highly enantioselective cascade sulfa-Michael/Julia-Kocienski olefination reaction between 2-mercaptobenzaldehydes and β-substituted vinyl PT-sulfones provies a wide range of 3,4-unsubstituted 2H-thiochromenes with excellent enantioselectivities. This reaction is catalyzed by diphenylprolinol TMS ether and proceeds through an aromatic iminium intermediate.
A. K. Simlandy, S. Mukherjee, J. Org. Chem., 2017, 82, 4851-4858.
A metal-free catalytic strategy for the facile synthesis of biologically relevant indolizines and imidazopyridines scaffolds is promoted by amine and N-heterocyclic carbene (NHC) relay catalysis via Michael addition-[3 + 2] fusion of simple azaarenes and α,β-unsaturated aldehydes.
H. Li, X. Li, Y. Yu, J. Li, Y. Liu, H. Li, W. Wang, Org. Lett., 2017, 19, 2010-2013.
N-Heterocyclic olefins (NHOs) are promising organocatalysts with strong nucleophilicity and Brønsted basicity. NHOs are efficient promoters for a direct dehydrogenative silylation of alcohols or hydrosilylation of carbonyl compounds. Preliminary results of an asymmetric dehydrogenative silylation are also discussed.
U. Kaya, U. P. N. Tran, D. Enders, J. Ho, T. V. Nguyen, Org. Lett., 2017, 19, 1398-1401.
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