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Synthesis of enones

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α,β-unsaturated compounds (C-C Coupling)

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An eco-friendly and mild aerobic oxidation of allylic alcohols using Fe(NO₃)₃·9H₂O/TEMPO/NaCl as catalysts under atmospheric pressure of oxygen at room temperature provides a convenient pathway to the synthesis of stereodefined α,β-unsaturated enals or enones with the retention of the double-bond configuration.
J. Liu, S. Ma, Org. Lett., 2013, 15, 5150-5153.

Allylic alcohols were oxidized into aldehydes or ketones in the presence of oxygen and Et₃N using Pd(OAc)₂ as catalyst. Diols with one allylic function were selectively oxidized, with one of the hydroxyl groups remaining untouched.
F. Batt, E. Bourcet, Y. Kassab, F. Fache, Synlett, 2007, 1869-1872.

Ruthenium-catalyzed oxidation of multisubstituted allyl alcohols in the presence of benzaldehyde gives enals or enones in good yields via an intramolecular hydrogen transfer. This reaction offers an efficient, mild, and high-yielding access to substituted α,β-unsaturated compounds.
K. Ren, B. Hu, M. Zhao, Y. Tu, X. Xie, Z. Zhang, J. Org. Chem., 2014, 79, 2170-2177.

Orangoselenium catalysis enables an efficient route to 3-amino allylic alcohols in excellent regio- and stereoselectivity in the presence of a base. In the absence of bases α,β-unsaturated aldehydes were formed in excellent yield. The hydroxy group is crucial for the direct amination.
Z. Deng, J. Wei, L. Liao, H. Huang, X. Zhao, Org. Lett., 2015, 17, 1834-1837.

2-Iodoxybenzenesulfonic acid, which can be generated in situ from 2-iodobenzenesulfonic acid sodium salt, is a much more active catalyst than modified IBXs for the oxidation of alcohols with Oxone. Highly efficient and selective methods for the oxidation of alcohols to carbonyl compounds such as aldehydes, carboxylic acids, and ketones were established.
M. Uyanik, M. Akakura, K. Ishihara, J. Am. Chem. Soc., 2009, 131, 251-262.

The combination of TEMPO and CAN can be used for the aerobic oxidation of benzylic and allylic alcohols into their corresponding carbonyl compounds. However, steric hindrance has been observed to impede the reaction with some substituted allylic systems. The present method is superior to others currently available due to its relatively short reaction times and excellent yields.
S. S. Kim, H. C. Jung, Synthesis, 2003, 2135-2137.

Pd/C in aqueous alcohol with molecular oxygen, sodium borohydride, and potassium carbonate efficiently oxidized benzylic and allylic alcohols. Sodium borohydride allows a remarkable reactivation of active sites of the Pd surface.
G. An, M. Lim, K.-S. Chun, H. Rhee, Synlett, 2007, 95-98.

A ZnCl₂/LiCl/H₂O-mediated transformation of styrenes provides α-benzyl-substituted conjugated enals. On the basis of experimental and computational studies, an underlying mechanism including electrophilic addition and hydride transfer with iminium cations has been proposed.
E. S. Kudriashova, M. A. Yarushina, A. E. Gavryushin, I. D. Grishin, Y. B. Malysheva, V. F. Otvagin and A. Yu. Fedorov, Org. Lett., 2023, 25, 4996-5000.

A simple, efficient, and environmentally beneficient disulfide-catalyzed photocatalytic regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes offers mild reaction conditions, excellent regioselectivity, and compatibility with a wide range of functional groups.
R. A. Fernandes, P. Kumar, A. Bhowmik, D. A. Gorve, Org. Lett., 2022, 24, 3436-3439.

Iron(III) sulfate mediates a simple and efficient regioselective oxidative cleavage of 1-arylbutadienes in the presence of oxygen. The reaction offers good yields, excellent regioselectivity, and good functional group tolerance.
A. Bhowmik, R. A. Fernandes, Org. Lett., 2019, 21, 9203-9207.

An efficient and economical ligand-free palladium-based oxidation system using molecular oxygen as the sole oxidant enables the Tsuji-Wacker oxidation of terminal olefins and especially styrenes to methyl ketones. In addition, this system achieves a tandem Wacker oxidation-dehydrogenation sequence of terminal olefins to yield α,β-unsaturated ketones.
Y.-F. Wang, Y.-R. Gao, S. Mao, Y.-L. Zhang, D.-D. Guo, Z.-L. Yan, S.-H. Guo, Y.-Q. Wang, Org. Lett., 2014, 16, 1610-1613.

A sequential PdCl₂/CrO₃-promoted Wacker process followed by an acid-mediated dehydration enables the synthesis of β-substituted and β,β-disubstituted α,β-unsaturated methyl ketones from homoallyl alcohols with a terminal double bond, whereas internal homoallyl alcohols delivered regioselectively nonconjugated unsaturated carbonyl compounds under the same protocol.
V. Bethi, R. A. Fernandes, J. Org. Chem., 2016, 81, 8577-8584.

A method for generating (E)-α,β-unsaturated aldehydes from Z- or E-allylic alcohols involves a Cu-catalyzed oxidation followed by an organocatalytic Z/E-isomerization with N,N-dimethylaminopyridine (DMAP).
D. Könning, W. Hiller, M. Christmann, Org. Lett., 2012, 14, 5258-5261.

A mild oxidation of alkyl enol ethers to enals employs low loadings of a palladium catalyst and tolerates a diverse array of functional groups, while allowing the formation of di-, tri-, and tetrasubtituted olefins. The application of this methodology to intramolecular reactions of alkyl enol ethers containing pendant alcohols provides furan and 2,5-dihydrofuran products.
M. G. Lauer, W. H. Henderson, A. Awad, J. P. Sambuli, Org. Lett., 2012, 14, 6000-6003.

DABCO-mediated Meinwald rearrangements of 2-aryl-, alkenyl-, and alkynylepoxides provide the corresponding methyl ketones chemospecifically in good yields. The reaction offers readily accessible starting materials, a wide substrate scope, a transition-metal- and acid-free environment, and chemospecificity in the isomerization of epoxides.
S. Li, Y. Shi, P. Li, J. Xu, J. Org. Chem., 2019, 84, 4443-4450.

A one-step reaction of trisubstituted olefins provides enones in good yields with high regioselectivity under visible-light irradiation in the presence of molecular oxygen and a photocatalyst. The reaction tolerates oxygen- and nitrogen-containing functional groups, heteroaromatic rings, and cyclopropanes.
S. Harada, D. Matsuda, T. Morikawa, A. Nishida, Synlett, 2020, 31, 1372-1377.

A palladium-catalyzed oxidative dehydrogenation enables an efficient synthesis of 1,4-enediones from saturated ketones in the presence of molecular oxygen as the sole oxidant. This atom- and step-economic process offers broad substrate scope, good functional group tolerance, and complete E-stereoselectivity.
B.-Y. Zhao, X.-L. Zhang, R.-L. Guo, M.-Y. Wang, Y.-R. Gao, Y.-Q. Wang, Org. Lett., 2021, 23, 1216-1221.

Olefin substrates can be converted to the corresponding enones or 1,4-enediones in very good yields in short reaction times using a Cu(II) 2-quinoxalinol salen complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant via allylic activation. The reaction tolerates many additional functional groups.
Y. Li, T. B. Lee, T. Tang, A. V. Gamble, A. E. V. Gorden, J. Org. Chem., 2012, 77, 4628-4633.

Dirhodium(II) caprolactamate effectively catalyzes the allylic oxidation of a variety of olefins and enones with tert-butyl hydroperoxide as terminal oxidant. The reaction is completely selective, tolerant of air and moisture, and can be performed with as little as 0.1 mol % catalyst in minutes.
A. E. Lurain, A. Maestri, A. R. Kelli, P. J. Carroll, P. J. Walsh, J. Am. Chem. Soc., 2004, 126, 13622-13623.

Oxoammonium salts enable a practical and highly efficient oxidative rearrangement of tertiary allylic alcohols to β-substituted α,β-unsaturated carbonyl compounds. Acyclic substrates as well as medium membered ring substrates and macrocyclic substrates can be oxidized.
M. Shibuya, M. Tomizawa, Y. Iwabuchi, J. Org. Chem., 2008, 73, 4750-4752.

The use of cationic silver (AgSbF₄) as a catalyst for intra- and intermolecular alkyne-carbonyl coupling is described. Intermolecular coupling proceeds stereoselectively to afford trisubstituted enones.
J. U. Rhee, M. J. Krische, Org. Lett., 2005, 7, 2493-2495.

Readily available indenylbis(triphenylphosphine)ruthenium chloride in conjunction with an indium cocatalyst and Brønsted acid isomerizes primary and secondary propargylic alcohols in good yields to provide trans enals and enones exclusively. The presence of indium triflate and camphorsulfonic acid gives the best turnover numbers and reactivity with the broadest range of substrates.
B. M. Trost, R. C. Livinston, J. Am. Chem. Soc., 2008, 130, 11852-11853.

DBU catalyzes a rearrangement of diarylated secondary propargylic alcohols to give α,β-unsaturated carbonyl compounds in excellent yields. The typical 1,3-transposition of oxy functionality, characteristic of Meyer-Schuster rearrangements, is not observed. This method offers a broad substrate scope, functional-group tolerance, operational simplicity, and complete atom economy.
R. De, A. Savarimuthu, T. Ballav, P. Singh, J. Nanda, A. Hasija, D. Chopra, M. K. Bera, Synlett, 2020, 31, 1587-1592.

The Au-catalyzed hydrative rearrangement of 1,1-diethynylcarbinol acetates in wet CH₂Cl₂ produces either 5-acetoxy-2-alkyl-2-cyclopentenones or acetoxymethyl α-alkylallenones as a major product depending on the temperature, reaction time, and catalyst loading.
C. H. Oh, S. Karmakar, J. Org. Chem., 2009, 74, 370-374.

Catalytic (1,4-diazabicyclo[2.2.2]octane (DABCO) was found to be effective for the isomerization of electron-deficient propargylic alcohols to E-enones under mild conditions. When the substrate is conjugated with an amide, addition of sodium acetate catalyzed the isomerization.
J. P. Sonye, K. Koide, J. Org. Chem., 2006, 71, 6254-6257.

A new and simple method is described for the one-step oxidation of α,β-enones to 1,4-enediones in good yields using t-butylhydroperoxide as stoichiometric oxidant and 20% Pd(OH)₂ on carbon as catalyst. The same reagents have been used to convert ethylene ketals of α,β-enones to the corresponding monoethylene ketals of 1,4-enediones. The mechanism is discussed.
J.-Q. Yu, E. J. Corey, J. Am. Chem. Soc., 2003, 125, 3232-3233.

trans-2-Aryl-3-nitro-cyclopropane-1,1-dicarboxylates undergo ring-opening rearrangement and the Nef reaction in the presence of BF₃·OEt₂ to give aroylmethylidene malonates. The products are potential precursors for heterocycles, such as imidazoles, quinoxalines, and benzo[1,4]thiazines.
T. Selvi, K. Srinivasan, J. Org. Chem., 2014, 79, 3653-3658.

The gold(I) complex MeAuPPh₃ is a highly effective catalyst for the hydrative cyclization of 1,6-diynes to yield 3-methyl hex-2-enone derivatives with very good yield. A mechanism is proposed.
C. Zhang, D.-M. Cui, L.-Y. Yao, B.-S. Wang, Y.-Z. Hu, T. Hayashi, J. Org. Chem., 2008, 73, 7811-7813.

The water-soluble μ-oxo-bridged hypervalent iodine trifluoroacetate reagent [(PhI(OCOCF₃)]₂O enables aqueous oxidations of phenolic substrates to dearomatized quinones in excellent yields in most cases compared to conventional phenyliodine(III) diacetate and bis(trifluoroacetate).
T. Dohi, T. Nakae, N. Takenaga, T. Uchiyama, K.-i. Fukushima, H. Fujioka, Y. Kita, Synthesis, 2012, 44, 1183-1189.

An efficient synthesis of α-iodo/bromo-α,β-unsaturated aldehydes/ketones directly from propargylic alcohols is catalyzed collaboratively by Ph₃PAuNTf₂ and MoO₂(acac)₂, and Ph₃PO as an additive helps suppress undesired enone/enal formation. Notable features of this method include low catalyst loadings, mild reaction conditions, and mostly good diastereoselectivity.
L. Ye, L. Zhang, Org. Lett., 2009, 11, 3646-3649.

A catalytic amount of Au(PPh₃)NTf₂ converts readily accessible propargylic acetates into versatile linear α-iodoenones in good to excellent yields. Very good Z-selectivities are observed for aliphatic propargylic acetates.
M. Yu, G. Zhang, L. Zhang, Org. Lett., 2007, 9, 2087-2090.

Bis(pyridine) iodonium tetrafluoroborate (Barluenga’s reagent) promotes the rearrangement of propargylic alcohol derivatives under mild conditions to provide β-unsubstituted, β-monosubstituted, and β,β-disubstituted α-iodoenones in high yields. β-Substituted α-iodoenones are obtained with excellent (Z)-selectivity.
T. Suárez-Rodríguez, Á. L. Suárez-Sobrino, A. Ballesteros, J. Org. Chem., 2018, 83, 12575-12583.

A vanadium-catalyzed coupling of allenylic alcohols with electrophilic halide sources provides α-halo-α',β'-unsaturated ketones. Fluorine, chlorine, and bromine electrophiles can be utilized, and the resulting products can give rise to the introduction of nitrogen, oxygen, sulfur, and iodine nucleophiles α to the ketone through substitution chemistry.
B. M. Trost, J. S. Tracy, T. Yusoontorn, Org. Lett., 2019, 21, 1207-1211.

The generation of a tribromomethyl radical from tetrabromomethane (CBr₄) with the assistance of a persulfate salt (K₂S₂O₈) enables a single step conversion of terminal alkynes to gem-dibromoenones in the presence of water. The reaction features readily available chemicals, a broad substrate scope, a green solvent, and mild reaction conditions.
X. Zeng, Y. Xu, J. Liu, Y. Deng, Org. Lett., 2021, 23, 9058-9062.

A novel domino copper-catalyzed trifluoromethylated Meyer-Schuster rearrangement reaction with Togni’s reagent provides α-trifluormethyl enones with moderate to good yields. Furthermore, these α-CF₃ enones can be transformed toward interesting trifluoromethyl-substituted heterocycles in a one-pot reaction.
Y.-P. Xiong, M.-Y. Wu, X.-Y. Zhang, C.-L. Ma, L. Huang, L.-J. Zhao, B. Tan, X.-Y. Liu, Org. Lett., 2014, 16, 1000-1003.

A domino palladium-catalyzed nitration of Meyer-Schuster intermediates, which were generated in situ from propargylic alcohols, with t-BuONO provides α-nitro enones in very good yields at room temperature with a broad functional group tolerance.
Y. Lin, W. Kong, Q. Song, Org. Lett., 2016, 18, 3702-3705.

A sequence of two gold(I)-catalyzed isomerization steps allows the synthesis of functionalized acetoxy bicyclo[3.1.0]hexenes from 5-en-2-yn-1-yl acetates. Acetoxy bicyclo[3.1.0]hexene products can be further transformed to 2-cycloalkenones by simple methanolysis.
A. Buzas, F. Gagosz, J. Am. Chem. Soc., 2006, 128, 12614-12615.

A highly efficient carbon-carbon triple bond cleavage reaction of (Z)-enynols offered a new route to highly substituted butenolides through a gold(I)-catalyzed tandem cyclization/oxidative cleavage.
Y. Liu, F. Song, S. Guo, J. Am. Chem. Soc., 2006, 128, 11332-11333.