Asymmetric palladium-catalysed intramolecular Wacker-type cyclisations of unsaturated alcohols and amino alcohols

Mechanistic study on the side arm effect in a palladium/Xu-Phos-catalyzed enantioselective alkoxyalkenylation of γ-hydroxyalkenes

Recently, the asymmetric bifunctionalization of alkenes has received much attention. However, the development of enantioselective alkoxyalkenylation has posed a considerable challenge and has lagged largely behind. Herein, we report a new palladium-catalyzed enantioselective alkoxyalkenylation reaction, using a range of primary, secondary, and tertiary γ-hydroxy-alkenes with alkenyl halides. By employing newly identified Xu-Phos (Xu8 and Xu9) with a suitable side-arm adjacent to the PCy2 motif, a series of allyl-substituted tetrahydrofurans were obtained in good yields with up to 95% ee. Besides (E)-alkenyl halides, (Z)-alkenyl halide was also examined and provided the corresponding (Z)-product as a single diastereomer, supporting a stereospecific oxidative addition and reductive elimination step. Moreover, deuterium labeling and VCD experiments were employed to determine a cis-oxypalladation mechanism. DFT calculations helped us gain deeper insight into the side-arm effect on the chiral ligand. Finally, the practicability of this method is further demonstrated through a gram-scale synthesis and versatile transformations of the products.

Oxidative Cyclization of Sulfamates onto Pendant Alkenes

This communication discloses the first examples of aza-Wacker cyclizations of sulfamate esters. Within the realm of related cyclization reactions, this protocol is differential in that it forms six-membered rings in good yield and uses catalytic amounts of palladium(0) rather than palladium(II) salts. These reactions scale well, and their products are demonstrated to be valuable synthetic intermediates.

Tsuji-Wacker-Type Oxidation beyond Methyl Ketones: Reacting Unprotected Carbohydrate-Based Terminal Olefins through the "Uemura System" to Hemiketals and α,β-Unsaturated Diketones

Aerobic Pd(AcO)₂/pyridine-catalyzed oxidation of unprotected carbohydrate-based terminal alkenes was studied. In accordance with previous reports, the initial reaction step gave methyl ketones. However, our substrates partially gave subsequent α,β-water elimination and alcohol oxidation to α,β-unsaturated 2,5-diketones. Upon increasing the pressure of O₂, the reaction was shifted toward formation of α,β-epoxy-2-ketones. The reactions were stereoselective and gave up to quantitative conversions. However, isolated yields were substantially lower because of the complexity of the product mixtures.

Construction of N-Heterocyclic Systems Containing a Fully Substituted Allylic Carbon by Palladium/Phosphine Catalysis

The unique cyclization of benzamide derivatives that contain an alkyne by a Pd(0)/dialkyl(biaryl)phosphine catalytic system is reported. The reaction efficiently provides a variety of six-membered N-heterocyclic compounds that contain a fully substituted carbon center without the need for a stoichiometric additive. Mechanistic studies suggest that this unprecedented cyclization starts with the cleavage of a propargylic C-O bond, and a 1,3-diene has been identified as a relevant intermediate.

Pd(II)-Catalyzed Asymmetric Oxidative 1,2-Diamination of Conjugated Dienes with Ureas

A palladium(II)-catalyzed asymmetric 1,2-diamination of 1,3-dienes with readily available dialkylureas was established by using a chiral pyridine-oxazoline ligand. The diamination reaction exclusively occurs at the terminal C-C double bond of the 1,3-dienes to give 4-vinylimidazolidin-2-ones in high yields and with excellent levels of enantioselectivity (up to 99% yield, 97% ee). The reaction could feasibly be applied for gram-scale synthesis with a 1:1 ratio of the diene and the urea.

Stereoselective and Regioselective Synthesis of Heterocycles via Copper-Catalyzed Additions of Amine Derivatives and Alcohols to Alkenes

This Perspective describes the development of a family of copper(II)-catalyzed alkene difunctionalization reactions that enable stereoselective addition of amine derivatives and alcohols onto pendant unactivated alkenes to provide a range of valuable saturated nitrogen and oxygen heterocycles. 2-Vinylanilines and related substrates undergo alternative oxidative amination or allylic amination pathways, and these reactions will also be discussed. The involvement of both polar and radical steps in the reaction mechanisms have been implicated. Major product formation is a function of the lowest energy pathway, which in turn is a function of structural aspects of the various reaction components.

Palladium-Catalyzed Enantioselective Intermolecular Coupling of Phenols and Allylic Alcohols

An enantioselective intermolecular coupling of oxygen nucleophiles and allylic alcohols to give β-aryloxycarbonyl compounds is disclosed using a chiral pyridine oxazoline-ligated palladium catalyst under mild conditions. As opposed to the formation of traditional Wacker-type products, enantioselective migratory insertion is followed by β-hydride elimination toward the adjacent alcohol. Deuterium labeling experiments suggest a syn-migratory insertion of the alkene into the Pd-O bond. A broad scope of phenols, various allylic alcohols, and an alkyl hydroperoxide are viable coupling partners in this process.

Stereochemistry of Pd(II)-Catalyzed THF Ring Formation of ε-Hydroxy Allylic Alcohols and Synthesis of 2,3,5-Trisubstituted and 2,3,4,5-Tetrasubstituted Tetrahydrofurans

Pd(II)-catalyzed ring formation of 2,3,5-trisubstituted and 2,3,4,5-tetrasubstituted tetrahydrofurans is described. Oxypalladation of a chiral ε-hydroxy allylic alcohol provides a 5-alkenyltetrahydrofuran ring in excellent yields via a 5-exo-trigonal process. Nine substrates including six secondary allylic alcohols and three primary allylic alcohols with or without an additional secondary hydroxy substituent at the γ-position have been examined. Their structures are restricted by a 2,2,4,4-tetraisopropyl-1,3,5,2,4-trioxadisilocane ring. The stereochemistry of the resulting tetrahydrofuran products was determined by chemical transformation. The reaction mechanism is discussed on the basis of the stereochemical results. The steps in the chiral allylic alcohol directed or the nucleophilic alcohol directed facial selection for the formation of the alkene-Pd(II)-π-complex, the cis-oxypalladation, and a syn-elimination mechanism account for the observed stereochemistry of the reaction.

Palladium-catalysed cyclisation of alkenols: Synthesis of oxaheterocycles as core intermediates of natural compounds

The study of Pd-catalysed cyclisation reactions of alkenols using different catalytic systems is reported. These transformations affect the stereoselective construction of mono- and/or bicyclic oxaheterocyclic derivatives depending on a starting alkenol. The substrate scope and proposed mechanism of Pd-catalysed cyclisation reactions are also discussed. Moreover, the diastereoselective Pd-catalysed cyclisation of appropriate alkenols to tetrahydrofurans and subsequent cyclisation provided properly substituted 2,5-dioxabicyclo[2.2.1]heptane and 2,6-dioxabicyclo[3.2.1]octane, respectively. Such bicyclic ring subunits are found in many natural products including ocellenynes and aurovertines.

Enantioselective copper-catalyzed carboetherification of unactivated alkenes

Chiral saturated oxygen heterocycles are important components of bioactive compounds. Cyclization of alcohols onto pendant alkenes is a direct route to their synthesis, but few catalytic enantioselective methods enabling cyclization onto unactivated alkenes exist. Herein reported is a highly efficient copper-catalyzed cyclization of γ-unsaturated pentenols which terminates in C-C bond formation, a net alkene carboetherification. Both intra- and intermolecular C-C bond formations are demonstrated, thus yielding functionalized chiral tetrahydrofurans as well as fused-ring and bridged-ring oxabicyclic products. Transition-state calculations support a cis-oxycupration stereochemistry-determining step.