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
A three-component reaction of potassium alkyltrifluoroborates, DABCO·(SO2)2 as sulfur dioxide surrogate, and alkenes under photocatalysis provides diverse sulfones in very good yields at room temperature. This reaction works efficiently under mild conditions via generation of alkyl and alkylsulfonyl radicals as key intermediates, and a reductive single-electron transfer.
T. Liu, Y. Li, L. Lai, J. Cheng, J. Sun, J. Wu, Org. Lett., 2018, 20, 3605-3608.
A practical nucleophilic substitution of alcohols furnishes alkyl chlorides, bromides, and iodides under stereochemical inversion in the presence of diethylcyclopropenone as a simple Lewis base organocatalyst and benzoyl chloride as a reagent. Moreover, acetyl chloride has been used as a stoichiometric promotor in an invertive SN-type transformation for the first time.
T. Stach, J. Dräger, P. H. Huy, Org. Lett., 2018, 20, 2980-2983.
An enantioselective decarboxylative alkylation of β-keto acids, followed by sequential cyclization and dehydration, enables an efficient asymmetric synthesis of useful 2,4-diaryl-1-benzopyrans in good yields and high enantioselectivities . This one-pot, two-step sequence offers a facile way to prepare chiral 2,4-diaryl-1-benzopyran derivatives and tolerates a wide range of functional groups.
H. J. Jeong, D. Y. Kim, Org. Lett., 2018, 20, 2944-2947.
Scope and Mechanism of a True Organocatalytic Beckmann Rearrangement with a Boronic Acid/Perfluoropinacol System under Ambient Conditions
X. Mo, T. D. R. Morgan, H. T. Ang, D. G. Hall, J. Am. Chem. Soc., 2018, 140, 5264-5271.
L-proline as a catalyst enables a simple and efficient route for the synthesis of a series of 5-substituted 1H-tetrazoles from a broad range of substrates, including aliphatic and aryl nitriles, organic thiocyanates, and cyanamides. This environmentally benign, cost effective, and high-yielding L-proline-catalyzed protocol offers simple experimental procedure, short reaction time, simple workup, and excellent yields.
S. B. Bhagat, V. N. Telvekar, Synlett, 2018, 29, 874-879.
Iminium catalysis of the challenging Morita-Baylis-Hillman reaction of cyclopenten-2-one provides allylic alcohols in excellent yields. Experimental evidence shows that secondary amines act as co-catalysts activating the enone moiety towards the nucleophilic attack at the β-position by DABCO as the Lewis base catalyst.
R. Innocenti, G. Menchi, A. Trabocchi, Synlett, 2018, 29, 820-824.
Trialkylphosphines catalyze a regioselective anti-hydroboration of internal alkynes with pinacolborane reagents to provide (E)-disubstituted alkenylboronate compounds. The alkenylboronates can be used for derivatizations, such as protodeborylations, Suzuki-Miyaura couplings, conjugate reductions, and Diels-Alder reactions.
K. Nagao, A. Yamazaki, H. Ohmiya, M. Sawamura, Org. Lett., 2018, 20, 1861-1865.
Asymmetric Michael addition-lactonization reactions of β,γ-unsaturated α-keto esters with thioesters catalyzed by a proline-derived urea provide 3,4-dihydropyranones and spiro-3,4-dihydrocoumarin-fused 3′,4′-dihydropyranones in high yield with excellent stereoselectivities via bioinspired S-to-O acyl-transfer reactions.
H. Jin, J. Lee, H. Shi, J. Y. Lee, E. J. Yoo, C. E. Song, D. H. Ryu, Org. Lett., 2018, 20, 1584-15888.
Photocatalysis enables the construction of 3-acyl-4-arylcoumarins in good yields from simple aldehydes and ynoates in the presence of inexpensive 2-tBu-anthraquinone as catalyst. The reaction proceeds through generation of an acyl radical intermediate, that reacts with ynoate, and then cyclization.
K. Kawaai, T. Yamaguchi, E. Yamaguchi, S. Endo, N. Tada, A. Ikari, A. Itoh, J. Org. Chem., 2018, 83, 1988-1996.
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.
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