Cobalt-Catalyzed Z to E Isomerization of Alkenes: An Approach to (E)-β-Substituted Styrenes

N2 Dissociation vs Reversible 1,2-Methyl Migration in PCNHCP Cobalt(I) Complexes in the Stereoselective Isomerization (E/Z) of Allyl Ethers

With growing efforts pushing toward sustainable catalysis, using earth-abundant metals has become increasingly important. Here, we present the first examples of cobalt PCNHCP pincer complexes that demonstrate dual stereoselectivity for allyl ether isomerization. While the cationic cobalt complex [((PCNHCP)Co)2-μ-N2][BAr4 F]2 (3) mainly favors the Z-isomer of the enol ether, the corresponding methyl complex [(PCNHCP)CoMe] (4) mostly gives the E-isomer. The dichotomy in selectivity was investigated computationally, revealing important contributions from the substituents on the metal (N2 vs Me), including a 1,2-alkyl migration from cobalt to the N-heterocyclic carbene (NHC) of the methyl substituent, which is further explored in this report.

Photoswitched Stereodivergent Synthesis of Allylic Sulfones

The present Letter demonstrates a photoswitched stereodivergent synthesis of allylic sulfones from sodium sulfinates, triphenylvinylphosphonium chloride, and (hetero)aromatic aldehydes in a single step. Mechanistically, cis-allylic sulfones, generated from the unstabilized ylide intermediates and aldehydes in situ, could be finally converted to trans-allylic sulfones via photochemical isomerization in the presence of a catalytic amount of bis(2-thienyl) ketone.

Ligand-Controlled Cobalt-Catalyzed Regiodivergent and Stereoselective Ring-Opening Isomerization of Vinyl Cyclopropanes

A ligand-controlled regiodivergent and stereoselective ring-opening isomerization of vinylcyclopropane was developed with cobalt catalysis. Employing the commercially available Xantphos ligand, the reactions afforded exclusively linear-type 1,3-dienes as the products. Interestingly, when switching the ligand to an amido-diphosphine ligand (PNP), branched-type 1,3-dienes were obtained with high regioselectivity and stereoselectivity. Preliminary mechanistic investigations suggested that a π-allyl metal and a metal-hydride species are involved as key intermediates in the two transformations, respectively.

A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Earth-abundant-metal catalyzed double bond transposition offers a sustainable and atom-economical route toward the synthesis of internal alkenes. With an emphasis specifically on internal olefins and ethers, the isomerization of allylic amines has been particularly under represented in the literature. Herein, we report an efficient methodology for the selective isomerization of N-allylic organic compounds, including amines, amides, and imines. The reaction is catalyzed by a neutral PCNHCP cobalt(I) pincer complex and proceeds via a π-allyl mechanism. The isomerization occurs readily at 80-90 °C, and it is compatible with a wide variety of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction to furnish a series of diversely substituted heterobiaryls, which is further discussed in this report.

Transition-metal-free and additive-free intermolecular hydroarylation of alkenes with indoles in hexafluoroisopropanol

Hydroarylation of alkenes is one of the most straightforward and atom-economical strategy for the construction of multi-aryl-substituted alkanes, but systematic studies have been limited to transition metal catalysis. Here we report a hexafluoroisopropanol (HFIP)-promoted hydroarylation of alkenes with indoles without the presence of transition metal catalysts or any additive. HFIP was the only reagent used in this work, and could be easily removed via evaporation, and recovered via distillation in industry settings. This reaction was shown to provide an efficient, clean and operationally simple procedure with a remarkable substrate scope and versatile transformations, delivering a variety of multi-aryl alkanes incorporating the indole motif. In preliminary studies, several of these products showed biologically activity against cells from an array of human cancer cell lines. A mechanistic study was also carried out and suggested that the quinone methide might be the key intermediate. And in contrast to the conclusions of a previous report, the current work suggested that protonation by HFIP might not be the rate-determining step.

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3 )Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3 )Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

Cobalt-Catalyzed Z to E Geometrical Isomerization of 1,3-Dienes

An efficient cobalt-catalyzed geometrical isomerization of 1,3-dienes is described. In the combination of a CoCl₂ precatalyst with an amido-diphosphine-oxazoline ligand, the geometrical isomerization of E/Z mixtures of 1,3-dienes proceed in a stereoconvergent manner, affording (E) isomers in high stereoselectivity. This facile transformation features a broad substrate scope with good functional group tolerance and could be scaled up to the gram scale smoothly with a catalyst loading of 1 mol %.

Cobalt-Catalyzed Isomerization of Alkenes

Catalytic isomerization of alkenes is a highly atom-economical approach to upgrade from lower- to higher-value alkenes. Consequently, tremendous attention has been devoted to the development of this transformation, approaches which exploit cobalt catalysis are particularly attractive. This short review focuses on the cobalt-catalyzed alkene isomerization, including positional isomerization, geometric isomerization, and cycloisomerization. Three main types of reaction mechanism have been discussed to help the reader to better understand and make meaningful comparison between the different transformations.

1 Introduction

2 Positional Isomerization

3 Geometric Isomerization

4 Cycloisomerization

5 Conclusion and Outlook

Catalytic Cycloisomerization onto a Carbonyl Oxygen

We have found that terminal N-vinylindoles bearing cycloalkanone substituents are excellent hydrogen atom acceptors, generating α-aminyl radicals with a variety of catalysts (Co(II)/H2 or Co(III)Cl precatalysts with silane reductants). These radicals can be converted to internal vinylindoles but eventually add to the oxygen of the cycloalkanone substituents. These cyclizations eventually furnish a densely functionalized dihydrofuran (a net cycloisomerization). The internal vinylindoles are slowly converted to the dihydrofurans, but the final cycloisomerization/isomerization ratio is affected by the size of the cycloalkanone ring (seven- and eight-membered rings give the highest ratio). These results demonstrate how HAT can isomerize substrates in nonintuitive ways, here leading to the first HAT-promoted formation of a C-O bond.

E-Z isomerization of 3-benzylidene-indolin-2-ones using a microfluidic photo-reactor

Here, we report controlled E-Z isomeric motion of the functionalized 3-benzylidene-indolin-2-ones under various solvents, temperature, light sources, and most importantly effective enhancement of light irradiance in microfluidic photoreactor conditions. Stabilization of the E-Z isomeric motion is failed in batch process, which might be due to the exponential decay of light intensity, variable irradiation, low mixing, low heat exchange, low photon flux etc. This photo-μ-flow light driven motion is further extended to the establishment of a photostationary state under solar light irradiation.

Cobalt-Catalyzed E-Selective Isomerization of Alkenes with a Phosphine-Amido-Oxazoline Ligand

An efficient method to access (E)-trisubstituted alkenes is reported via cobalt-catalyzed isomerization of 1,1-disubstituted alkenes using a phosphine-amido-oxazoline ligand. The reaction could also convert mono- and 1,2-disubstituted alkenes to (E)-internal alkenes with benzylic selectivity. This protocol is atom-economy and operationally simple and uses readily available starting materials with good functional tolerance. This catalytic system could be scaled up to gram scale smoothly with a catalyst loading of 0.1 mol %.