Stereoselective synthesis of α-disubstituted cyclopentanones by palladium-catalyzed rearrangement of allenylcyclobutanols with aryl halides

Wacker Oxidation of Methylenecyclobutanes: Scope and Selectivity in an Unusual Setting

Methylenecyclobutanes are found to undergo Wacker oxidation via a semi-pinacol-type rearrangement. Key to a successful process is the use of tert-butyl nitrite as oxidant, which not only enables efficient catalyst turn-over but also ensures high Markovnikov-selectivity under mild conditions. Thus, cyclopentanones (26 examples) can be accessed in an overall good yield and excellent selectivity (up to 97 % yield, generally >99 : 1 ketone:aldehyde ratio). Stereochemical analysis of the reaction sequence reveals migration aptitudes in line with related 1,2-shifts. By introducing a pyox ligand to palladium, prochiral methylenecyclobutanes can be desymmetrized, thus realizing the first enantioselective Wacker oxidation.

One-pot synthesis of quaternary carbon centered cyclobutanes via Pd(ii)-catalyzed cascade C(sp3)–H activations

A novel approach toward quaternary carbon centered cyclobutanes through Pd(ii)-catalyzed sequential intramolecular methylene C–H alkylation and intermolecular methine C–H bond arylation, alkenylation, alkylation, alkynylation, allylation, benzylation or alkoxylation is described.

Rh-catalyzed Reagent-Free Ring Expansion of Cyclobutenones and Benzocyclobutenones

Here we report a reagent-free rhodium-catalyzed ring-expansion reaction via C-C cleavage of cyclobutenones. A variety of poly-substituted cyclopentenones and 1-indanones can be synthesized from simple cyclobutenones and benzocyclobutenones. The reaction condition is near pH neutral without additional oxidants or reductants. The potential for developing a dynamic kinetic asymmetric transformation of this reaction has also been demonstrated. Further study supports the proposed pathway involving Rh-insertion into the cyclobutenone C-C bond, followed by β-hydrogen elimination, olefin insertion and reductive elimination.

[Development of domino reactions based on skeletal rearrangement]

Domino reactions, which enable formations of several chemical bonds and multi-step transformation in one-pot process, have received much attention as an efficient synthetic methodology to preserve chemists from time-consuming purification protocols and protection-deprotection procedures. Furthermore, the domino processes containing skeletal rearrangement have been utilized for constructions of complex molecules because of their potential that they could afford entirely different valuable structures from readily available simple substrates. We recently developed novel domino reactions including ring enlargement process, which could afford biologically important cyclopentanones and nitrogen heterocycles. In this review, we will describe 1) a novel type of palladium-catalyzed domino insertion-ring expansion reaction of dienylcyclobutanols, which could enable a stereospecific synthesis of (Z)-2-(3-aryl-1-propenyl)cyclopentanones, 2) ruthenium-catalyzed domino ring expansion-insertion reaction of 1-acetylenylcyclobutanols for a construction of 2-alkylidenecyclopentanones, and 3) DIBALH-mediated reductive ring expansion reaction of oximes and studies on the reaction mechanisms. The rationales for the observed stereoselectivities in each reaction are also discussed.

Cyclobutanes in catalysis

The exploitation of ring strain as a driving force to facilitate chemical reactions is a well-appreciated principle in organic chemistry. The most prominent and most frequently used compound classes in this respect are oxiranes and cyclopropanes. For rather a long time, cyclobutanes lagged behind these three-membered-ring compounds in their development as reactive substrates, but during the past decade an increasing number of useful reactions of four-membered-ring substrates have emerged. This Minireview examines corresponding catalytic reactions ranging from Lewis or Brønsted acid catalyzed processes to enzymatic reactions. The main focus is placed on transition-metal-catalyzed C-C bond-insertion and β-carbon-elimination processes, which enable exciting downstream reactions that deliver versatile building blocks.

Palladium-catalyzed diastereo- and enantioselective Wagner-Meerwein shift: control of absolute stereochemistry in the C-C bond migration event

Inducing absolute stereochemistry in Wagner-Meerwein shifts was examined in a ring expansion protocol. Initiated by generation of a pi-allylpalladium intermediate by hydropalladation of allenes, the ring expansion of allenylcyclobutanol substrates proceeded with excellent diastereo- and enantioselectivities. The results demonstrate that, during the C-C bond migration process, our chiral catalysts can control the stereochemistry of both the pi-allylpalladium intermediate and the corresponding migration bond. Moreover, the stereochemical outcome of the reaction can be rationalized very well with the working model of the chiral catalyst. The method provides an efficient way to synthesize highly substituted cyclopentanones with an alpha-chiral O-tertiary center which has various synthetic applications.

Palladium-catalyzed asymmetric ring expansion of allenylcyclobutanols: an asymmetric Wagner-Meerwein shift

In this study, we developed a palladium-catalyzed atom economic asymmetric Wagner-Meerwein shift of allenylcyclobutanol substrates. It is an excellent method for creating functionalized cyclopentanones with an alpha-chiral O-tertiary center by ring expansion of allenylcyclobutanols. This reaction was initiated by hydropalladation and afforded excellent enantioselectivity as well as atom economy. This method provides an efficient route toward the synthesis of natural products such as trans-kumausyne's family, spiro ring systems. In addition, we obtained excellent diastereoselectivity and enantioselectivity at the same time by using 3-monosubstituted allenylcyclobutanol.

Development of Novel Type of Transition Metal-Catalyzed Cascade Reactions Utilizing Small Ring Systems and the Applications

This review summarizes the novel type of palladium-catalyzed cascade ring expansion reactions of cyclobutanols with various unsaturated bonds. The intramolecular cascade ring expansion-cyclizations of isopropenylcyclobutanol proceeds smoothly in the presence of the palladium (II) complex. The diastereoselectivity of the obtained naphthohydrindans can be controlled by the choice of the reaction solvents. As an application utilizing this reaction, total synthesis of (+)-equilenin has been achieved. The reaction of allenylcyclobutanols with a iodoalkenyl side chain with a palladium (0) catalyst produces the cyclized products that have seven- and eight-membered rings. The reaction can be successfully applied to the stereospecific synthesis of alpha-substituted cyclopentanones with a quaternary carbon stereocenter. The cyclobutanols containing a propargylic component react with phenols in the presence of the palladium (0) catalyst to afford the phenoxy-substituted cyclopentanones via a nucleophilic addition-ring expansion process. The reaction also proceeds stereospecifically to afford the corresponding products with high efficiency.

Palladium-Catalyzed Ring Expansion Reaction of (Z)-1-(1,3-Butadienyl)cyclobutanols with Aryl Iodides. Stereospecific Synthesis of (Z)-2-(3-Aryl-1-propenyl)cyclopentanones

[reaction: see text] A novel type of cascade ring expansion process has been developed by the palladium-catalyzed reaction of (Z)-1-(1,3-butadienyl)cyclobutanols with aryl iodides. The reaction proceeds in a stereospecific manner to produce (Z)-2-(3-aryl-1-propenyl)cyclopentanones. It has also been found that regioselective alpha-arylation of alkenyl cyclopentanones proceeds to afford the alpha-arylated cyclopentanones.