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
The organic photocatalyst 4CzIPN, visible light, and N-(acyloxy)phthalimides as radical precursors enable an intramolecular arene alkylation reaction. This reaction provides a diverse set of fused, partially saturated cores which are of high interest in synthetic and medicinal chemistry.
T. C. Sherwood, H.-Y. Xiao, R. G. Bhaskar, E. M. Simmons, S. Zaretsky, M. P. Rauch, R. R. Knowles, T. G. M. Dhar, J. Org. Chem., 2019, 84, 8360-8379.
Phthaloyl chloride as reagent and N-formylpyrrolidine as Lewis base catalyst enable a transformation of aldehydes into geminal dichlorides. This simple reaction offers mild reaction conditions, high levels of functional group compatibility, and scalability.
P. H. Huy, Synthesis, 2019, 51, 2474-2483.
In a catalytic system for the chlorination of alcohols under Appel conditions, benzotrichloride is used as a cheap and readily available chlorinating agent in combination with trioctylphosphane as the catalyst and phenylsilane as the terminal reductant under solvent-free conditions. In total, 27 different primary, secondary, and tertiary alkyl chlorides were synthesized in good yields.
L. Longwitz, S. Jopp, T. Werner, J. Org. Chem., 2019, 84, 7863-7870.
The combination of Brřnsted acid catalysis with visible-light induction enables a highly enantioselective synthesis of 2-substituted tetrahydroquinolines from 2-aminoenones through a relay visible-light-induced cyclization/chiral phosphoric acid-catalyzed transfer hydrogenation reaction.
W. Xiong, S. Li, B. Fu, J. Wang, Q.-A. Wang, W. Yang, Org. Lett., 2019, 21, 4173-4176.
In a triphenylphosphine oxide-catalyzed reduction of conjugated polyunsaturated ketones using trichlorosilane as reducing reagent, the α,β-C=C double bond was selectively reduced while all other reducible functional groups remained unchanged. Additionally, a tandem one-pot Wittig/conjugate-reduction reaction sequence provides γ,δ-unsaturated ketones from simple building blocks.
X. Xia, Z. Lao, P. H. Toy, Synlett, 2019, 30, 1100-1104.
In a disulfide-catalyzed electrophilic iodination of electron-rich aromatic compounds using 1,3-diiodo-5,5-dimethylhydantoin (DIH), the disulfide activates DIH as a Lewis base to promote the iodination reaction in acetonitrile under mild conditions. This system is applicable to a wide range of substrates, including anisoles, acetanilides, imidazoles, and pyrazoles.
K. Iida, S. Ishida, T. Watanabe, T. Arai, J. Org. Chem., 2019, 84, 7411-7417.
A diboron/methoxide/pyridine reaction system enables a photoactivation of an in situ generated super electron donor for an efficient, transition-metal-free borylation of unactivated aryl chlorides.
L. Zhang, L. Jiao, J. Am. Chem. Soc., 2019, 141, 9124-9128.
An efficient catalytic Staudinger reduction at room temperature provides structurally diverse amines from azides in excellent yields in the presence of catalytic amounts of triphenylphosphine and diphenyldisiloxane as terminal reducing agent. The reaction exhibits high chemoselectivity and tolerates nitriles, alkenes, alkynes, esters, and ketones.
D. C. Lenstra, J. J. Wolf, J. Mecinović, J. Org. Chem., 2019, 84, 6536-6545.
The use of N-oxides in butanol as solvent enables a site-selective oxidation of vicinal bis(boronates) with good efficiency and selectivity across a range of substrates to provide 2-hydro-1-boronic esters, which are shown to be versatile intermediates in the synthesis of chiral building blocks.
L. Yan, J. P. Morken, Org. Lett., 2019, 21, 3760-3763.
The Doebner-Knoevenagel condensation enables a selective synthesis of acrylamides under mild conditions at ambient temperatures. The scalable reaction tolerates a wide range of functional groups, and affords the E-isomer with high selectivity. An organocatalytic mechanism has been proposed.
M. J. Zacuto, J. Org. Chem., 2019, 84, 6465-6474.
Phosphazene P4-tBu effectively catalyzes a nucleophilic substitution reaction of gem-difluoroalkenes with ketene silyl acetals under mild conditions to provide monofluoroalkenes possessing an alkoxycarbonylmethyl group in high yields with high Z selectivities.
A. Kondoh, K. Koda, M. Terada, Org. Lett., 2019, 21, 2277-2280.
The phosphazene base BEMP catalyzes an intramolecular hydroamidation of propargylic ureas to provide imidazolidin-2-ones and imidazol-2-ones with excellent chemo- and regioselectivities under ambient conditions and short reaction times.
A. Casnati, A. Perrone, P. P. Mazzeo, A. Bacchi, R. Mancuso, B. Gabriele, R. Maggi, G. M. E. Motti, A. Stirling, N. Della Ca', J. Org. Chem., 2019, 84, 3477-3490.
A unique combination of organic catalysts promotes the transfer of electrons and hydrogen atoms in a mild, modular, and practical direct hydroarylation of vinyl amine derivatives with a wide range of aryl halides (including aryl chlorides). This general and highly chemoselective protocol delivers a broad range of arylethylamine products with complete regiocontrol.
A. J. Boyington, C. P. Seath, A. M. Zearfoss, Z. Xu, N. T. Jui, J. Am. Chem. Soc., 2019, 141, 4147-4153.
A H8-binaphthyl-linked hemisquaramide catalyst accelerated the synthesis of cyclic carbonates from epoxides and CO2 under mild and solvent-free conditions. Kinetic resolution was also achieved at −20°C (s = 3.0).
K. Takaishi, T. Okuyama, S. Kadosaki, M. Uchiyama, T. Ema, Org. Lett., 2019, 21, 1397-1401.
A mild and transition-metal-free decarboxylative coupling of aryl aldehydes and tertiary or secondary alkyl carboxylic acid-derived redox-active esters is catalyed by a N-heterocyclic carbene to provide aryl alkyl ketones.
T. Ishii, Y. Kakeno, K. Nagao, H. Ohmiya, J. Am. Chem. Soc., 2019, 141, 3854-3858.
An aerobic oxidation of a wide range of aldehydes to carboxylic acids in both organic solvent and water under mild conditions is catalyzed by 5 mol % N-hydroxyphthalimide (NHPI) as the organocatalyst in the presence of oxygen as the sole oxidant. No transition metals or hazardous oxidants or cocatalysts were involved.
P.-F. Dai, J.-P. Qu, Y.-B. Kang, Org. Lett., 2019, 21, 1393-1396.
CHAMPO, a new aminoxyl radical catalyst, enables an electrochemical diazidation of a broad range of alkenes in good yields. Mediated by an anodically generated charge-transfer complex in the form of CHAMPO-N3, radical diazidation was achieved without the need for a transition metal catalyst or a chemical oxidant.
J. C. Siu, J. B. Parry, S. Lin, J. Am. Chem. Soc., 2019, 141, 2825-2831.
An N-heterocyclic carbene (NHC)-catalyzed intramolecular Stetter reaction enables a transition-metal-free and mild cross-dehydrogenative coupling of 2-cinnamoyl benzaldehydes to provide 2-aryl naphthoquinones. The intramolecular Stetter reaction is followed by an aerobic oxidation to reinstall the C-C double bond.
A. Ghosh, A. Patra, S. Mukherjee, A. T. Biju, J. Org. Chem., 2019, 84, 1103-1110.
A mild, late-stage conversion of primary sulfonamides to several other other functional groups occurs via initial reductive deamination of sulfonamides to sulfinates via an NHC-catalyzed reaction of transiently formed N-sulfonylimines. This method tolerates common functional groups, as exemplified by the late-stage derivatization of several complex pharmaceutical compounds.
P. S. Fier, K. M. Maloney, J. Am. Chem. Soc., 2019, 141, 1441-1445.
C2-symmetric chiral-substituted 1,16-dihedroxytetraphenylenes (DHTP) were used as organocatalysts in asymmetric allylboration of ketones under very mild conditions. Several tertiary alcohols were isolated in good yields with high ees.
G.-L. Chai, B. Zhu, J. Chang, J. Org. Chem., 2019, 84, 120-127.
A N-heterocycle carbene (NHC)-catalyzed dual Stetter cascade reaction enables a coupling of β-nitrostyrene with phthalaldehyde under mild conditions to furnish valuable aryl-naphthoquinones, whereas the reaction of salicylaldehydes with β-nitrostyrene provides functionalized dihydroisoflavanones.
R. N. Mitra, K. Show, D. Barman, S. Sarkar, D. K. Maiti, J. Org. Chem., 2019, 84, 42-52.
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