TBN as Organic Redox Cocatalyst for Oxidative Tiffeneau-Demjanov-Type Rearrangement Using O2 as Sole Oxidant

The Tiffeneau-Demjanov-type rearrangement is a ring-expansion reaction involving the cationic rearrangement of cyclic alcohols. The carbocation intermediates can be generated in situ via various means, including Wacker oxidation. In near recent reports on the reinvestiagtions by Wahl et al. (Sietmann, J.; Tenberge, M.; Wahl, J. M. Wacker Oxidation of Methylenecyclobutanes: Scope and Selectivity in an Unusual Setting. Angew. Chem., Int. Ed. 2023, 62, e202215381) and Zhu et al. (Feng, Q.; Wang, Q.; Zhu, J.-P. Oxidative Rearrangement of 1,1-Disubstituted Alkenes to Ketones. Science 2023, 379, 1363-1368), stoichiometric oxidants were involved. In this work, we report the Tiffeneau-Demjanov-type rearrangement can be smoothly promoted by the Pd-TBN cocatalyzed aerobic Wacker oxidation using molecular oxygen as the sole oxidant. tert-Butanol is essential for achieving high yields. Since the first report by Grigg et al. in 1977 (Boontanonda, P.; Grigg, R. J. Palladium (II)-Catalysed Ring Expansion of Methylenecyclobutanes and Related Systems. J. Chem. Soc. Chem. Commun. 1977, 17, 583-584), the five-decade journey of Pd-catalyzed Tiffeneau-Demjanov-type rearrangement returns to the aerobic again.

Alkyl nitrite-enabled palladium-catalyzed terminal selective oxidative cyclization of 4-penten-1-ols

Oxidative cyclization of 4-penten-1-ols using a Pd catalyst and n-BuONO or n-BuONO/p-benzoquinone afforded 3-hydroxy- and 3-methoxytetrahydropyrans via terminal selective nucleophilic attack. The radicals formed from n-BuONO and O2 operate as critical oxidants and ligands for Pd.

Palladium/Iron-Catalyzed Wacker-Type Oxidation of Aliphatic Terminal and Internal Alkenes Using O2

The Wacker-type oxidation of aliphatic terminal alkenes proceeds using a Pd/Fe catalyst system under mild reaction conditions using 1 atm O2 without other additives. The use of 1,2-dimethoxyethane/H2O as a mixed solvent was effective. The slow addition of alkenes is also important for improving product yields. Fe(III) citrate was the most efficient cocatalyst among the iron complexes examined, whereas other complexes such as FeSO4, Fe2(SO4)3, Fe(NO3)3, and Fe2O3 were also operative. This method is also applicable to aliphatic internal alkenes, which are generally difficult to oxidize using conventional Pd/Cu catalyst systems. The gram-scale synthesis and reuse of the Pd catalysts were also demonstrated.

Water-Soluble Pd Nanoparticles for the Anti-Markovnikov Oxidation of Allyl Benzene in Water

The catalytic activity and selectivity of two different water-soluble palladium nanoparticles capped with 5-(trimethylammonio)pentanethiolate and 6-(carboxylate)hexanethiolate ligands are investigated using the catalytic reaction of allyl benzene. The results show that the regioselective transformation of allyl benzene to 3-phenylpropanal occurs at room temperature and under atmospheric pressure in neat water via a Tsuji-Wacker type oxidation. Conventionally, the Tsuji-Wacker oxidation promotes the Markovnikov oxidation of terminal alkenes to their respective ketones in the presence of dioxygen. Water-soluble Pd nanoparticles, however, catalyze the anti-Markovnikov oxidation of allyl benzene to 3-phenylpropanal in up to 83% yields. Catalytic results of other aromatic alkenes suggest that the presence of benzylic hydrogen is a key to the formation of a p-allyl Pd intermediate and the anti-Markovnikov addition of H₂O. The subsequent b-H elimination and tautomerization contribute to the formation of aldehyde products. Water-soluble Pd nanoparticles are characterized using nuclear magnetic resonance (NMR), UV-vis spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Catalysis results are examined using ¹H NMR and/or GC-MS analyses of isolated reaction mixtures.

Synthesis of 2-hydroxytetrahydrofurans by Wacker-type oxidation of 1,1-disubstituted alkenes

1,1-Disubstituted alkenes feature high steric hindrance, which renders their Wacker-type oxidation difficult. We demonstrate the stereoselective synthesis of 2-hydroxytetrahydrofurans via the Wacker-type oxidation of 3-methyl-3-buten-1-ols by using a PdCl₂(MeCN)₂/NO/BQ catalyst system under 1 atm O₂ in H₂O or H₂O/DMF.

Palladium-catalyzed anti-Markovnikov oxidative acetalization of activated olefins with iron(III) sulphate as the reoxidant

This paper discloses the efficient palladium-catalyzed anti-Markovnikov oxidative acetalization of activated terminal olefins with iron(III) sulfate as the reoxidant. This methodology requires mild reaction conditions and shows high regioselectivity toward anti-Markovnikov products and compatibility with a wide range of functional groups. Iron(III) sulphate was the sole reoxidant used in this method. Various olefins like vinylarenes, aryl-allylethers, aryl or benzyl acrylates and homoallylic alcohols all reacted well providing anti-Markovnikov acetals, some of which represent orthogonally functionalized 1,3- and 1,4-dioxygenated compounds.

Realization of Anti-Markovnikov Selectivity in Pd-Catalyzed Oxidative Acetalization and Wacker-Type Oxidation of Terminal Alkenes

Catalytic oxidative acetalization and Wacker-type oxidation of terminal alkenes normally proceed with Markovnikov selectivity to afford internally oxyfunctionalized compounds, such as internal acetals and ketones. Thus, the realization of anti-Markovnikov (AM) selectivity in these reactions is challenging. This account focuses on our recent development of Pd-catalyzed AM oxidation of terminal alkenes (mainly styrenes and aliphatic alkenes), that is, oxidative acetalization (oxidation to terminal acetals) and Wacker-type oxidation (oxidation to aldehydes). The key factors that enhance the yield and AM selectivity of the products found in our studies are: 1) the steric bulkiness of the oxygen nucleophiles that attack on the coordinated alkenes, 2) the electron-deficient cyclic alkenes as additives that withdraw electrons from Pd, 3) the slow addition of substrates in the case of the aliphatic alkenes, which suppresses the isomerization of the terminal alkenes into internal alkenes, and 4) the halogen directing groups in the case of aliphatic alkenes.

Palladium-Catalyzed Anti-Markovnikov Oxidation of Aromatic and Aliphatic Alkenes to Terminal Acetals and Aldehydes

Catalytic anti-Markovnikov (AM) oxidation of terminal alkenes can provide terminally oxyfunctionalized organic compounds. This short review mainly summarizes our recent progress on the Pd-catalyzed AM oxidations of aromatic and aliphatic terminal alkenes to give terminal acetals (oxidative acetalization) and aldehydes (Wacker-type oxidation), along with related reports. These reactions demonstrate the efficacy of the PdCl2(MeCN)2/CuCl/electron-deficient cyclic alkenes/O2 catalytic system. Notably, electron-deficient cyclic alkenes such as p-benzoquinones (BQs) and maleimides are key additives that facilitate nucleophilic attack of oxygen nucleophiles on coordinated terminal alkenes and enhance the AM selectivity. BQs also function to oxidize Pd(0) depending on the reaction conditions. Several other factors that improve the AM selectivity, such as the steric demand of the nucleo­philes, slow substrate addition, and halogen-directing groups, are also discussed.

1 Introduction

2 Anti-Markovnikov Oxidation of Aromatic Alkenes to Terminal Acetals­

3 Anti-Markovnikov Oxidation of Aromatic Alkenes to Aldehydes

4 Anti-Markovnikov Oxidation of Aliphatic Alkenes to Terminal Acetals­

5 Anti-Markovnikov Oxidation of Aliphatic Alkenes to Aldehydes

6 Conclusion

Palladium-Catalyzed Aerobic Anti-Markovnikov Oxidation of Aliphatic Alkenes to Terminal Acetals

Terminal acetals were selectively synthesized from various unbiased aliphatic terminal alkenes and 1,2-, 1,3-, or 1,4-diols using a PdCl₂(MeCN)₂/CuCl catalyst system in the presence of p-toluquinone under 1 atm of O₂ and mild reaction conditions. The slow addition of terminal alkenes suppressed the isomerization to internal alkenes successfully. Electron-deficient cyclic alkenes, such as p-toluquinone, were key additives to enhance the catalytic activity and the anti-Markovnikov selectivity. The halogen groups in the alkenes were found to operate as directing groups, suppressing isomerization and increasing the selectivity efficiently.

Catalytic Isohypsic-Redox Sequences for the Rapid Generation of Csp3 -Containing Heterocycles

Cross‐coupling reactions catalyzed by transition metals are among the most influential in modern synthetic chemistry. The vast majority of transition‐metal‐catalyzed cross‐couplings rely on a catalytic cycle involving alternating oxidation and reduction of the metal center and are generally limited to forging just one type of new bond per reaction (e.g., the biaryl linkage formed during a Suzuki cross‐coupling). This work presents an Isohypsic‐Redox Sequence (IRS) that uses one metal to effect two catalytic cycles, thereby generating multiple new types of bonds from a single catalyst source. We show that the IRS strategy is amenable to several widely used transformations including the Suzuki–Miyaura coupling, Buchwald–Hartwig amination, and Wacker oxidation. Furthermore, each of these reactions generates value‐added heterocycles with significant sp³‐C (3‐dimensional) content. Our results provide a general framework for generating complex products by using a single metal to fulfill multiple roles. By uniting different combinations of reactions in the isohypsic and redox phases of the process, this type of catalytic multiple bond‐forming platform has the potential for wide applicability in the efficient synthesis of functional organic molecules.

Wacker Oxidation of Terminal Alkenes Over ZrO2 -Supported Pd Nanoparticles Under Acid- and Cocatalyst-Free Conditions

Highly efficient Wacker oxidation of aromatic or aliphatic terminal alkenes into methyl ketones and benzofurans is developed by using reusable Pd⁰ nanoparticles (NPs) supported on ZrO₂ under acid- and cocatalyst-free conditions. Molecular oxygen or air can be utilized as the terminal oxidant, which results in the formation of H₂ O as the only theoretical byproduct. The activation of the Pd NPs by O₂ plays an important role in promoting this reaction. Interestingly, PdO supported on ZrO₂ showed no activity. Additionally, the Pd particle size significantly affects the catalytic activity, with an apparent optimal diameter of 4-12 nm. In addition to the heterogeneous catalyst forms, the Pd NPs can be generated from a Pd⁰ complex during the reaction, and these particles are even recyclable.

Wacker-Type Oxidation Using an Iron Catalyst and Ambient Air: Application to Late-Stage Oxidation of Complex Molecules

A practical and general iron-catalyzed Wacker-type oxidation of olefins to ketones is presented, and it uses ambient air as the sole oxidant. The mild oxidation conditions enable exceptional functional-group tolerance, which has not been demonstrated for any other Wacker-type reaction to date. The inexpensive and nontoxic reagents [iron(II) chloride, polymethylhydrosiloxane, and air] can, therefore, also be employed to oxidize complex natural-product-derived and polyfunctionalized molecules.