Palladium-Catalyzed Aerobic Synthesis of Terminal Acetals from Vinylarenes Assisted by π-Acceptor Ligands

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.

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.