Integrating Regioselective <i>E</i>→<i>Z</i> Isomerization of Trienones with Cascade Sequences under Photosensitizer‐Free Direct Irradiation at 390 nm

Maity, Rajib; Ghosh, Subhadeep; Das, Indrajit

doi:10.1002/chem.202300421

Cited by 5 publications

(1 citation statement)

References 96 publications

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“…Close examination of the exo transition states revealed a fascinating stabilizing effect; the ketone distal to the Lewis acid coordination not only enabled the 5-exo addition but also participated in an n→π* donation to the bound carbonyl ( 8a ) 69 . This ouroboros-like activation 70 , 71 is evidenced by the short C = O→C = O bond (1.53 Å in 8a ) and the tetrahedral geometry of the carbon of the bound C = O (C16). Such interactions have been described for static protein structure 72 and utilized for the synthesis of Lewis acid/base heteroaromatics 69 , but to our knowledge has not been proposed as a stabilizing factor in reaction catalysis 73 .…”

Section: Resultsmentioning

confidence: 98%

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones

Peng,

Isho,

May

2024

Nat Commun

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The chemical synthesis of molecules with closely packed atoms having their bond coordination saturated is a challenge to synthetic chemists, especially when three-dimensional control is required. The organocatalyzed asymmetric synthesis of acyclic alkenylated, alkynylated and heteroarylated quaternary carbon stereocenters via 1,4-conjugate addition is here catalyzed by 3,3´-bisperfluorotoluyl-BINOL. The highly useful products (31 examples) are produced in up to 99% yield and 97:3 er using enediketone substrates and potassium trifluoroorganoborate nucleophiles. In addition, mechanistic experiments show that the (Z)–isomer is the reactive form, ketone rotation at the site of bond formation is needed for enantioselectivity, and quaternary carbon formation is favored over tertiary. Density functional theory-based calculations show that reactivity and selectivity depend on a key n→π* donation by the unbound ketone’s oxygen lone pair to the boronate-coordinated ketone in a 5-exo-trig cyclic ouroboros transition state. Transformations of the conjugate addition products to key quaternary carbon-bearing synthetic building blocks proceed in good yield.

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Section: Resultsmentioning

confidence: 98%

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones

Peng,

Isho,

May

2024

Nat Commun

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Thermally Activated Geometrical Regioselective E→Z Isomerization‐Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

Saha

Banerjee

Malo

et al. 2023

Chemistry A European J

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The geometrical regioselective E → Z isomerization of a conjugated alkene under thermal activation poses a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E → Z isomerization with subsequent cyclization cascade. Thus, linearly conjugated dienals in a mixture of toluene‒alcohol (2:1) solvents or only with alcohol at 60‐70 °C can be converted to γ‐alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E‒isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E → 2Z,4E isomerization of dienal is a thermodynamically facile (ΔG < 0) process. The thermodynamic plausibility of the subsequent cascade reaction from the Z‐isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the conjugate addition pathway.

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A Torquoselective Thermal 6π‐Electrocyclization Approach to 1,4‐Cyclohexadienes via Solvent‐Aided Proton Transfer: Experimental and Theoretical Studies

Saha,

Banerjee,

Malo

et al. 2024

Chemistry A European J

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The thermal 6π‐electrocyclization of hexatriene typically delivers 1,3‐cyclohexadiene (1,3‐CHD). However, there is only limited success in directly synthesizing 1,4‐cyclohexadiene (1,4‐CHD) using such an approach, probably due to the difficulty in realizing thermally‐forbidden 1,3‐hydride shift after electrocyclic ring closure. The present study shows that by heating (2E,4E,6E)‐hexatrienes bearing ester or ketone substituents at the C1‐position in a mixture of toluene/MeOH or EtOH (2:1) solvents at 90‐100 °C, 1,4‐CHDs can be selectively synthesized. This is achieved through a torquoselective disrotatory 6π‐electrocyclic ring closure followed by a proton‐transfer process. The success of this method depends on the polar protic solvent‐assisted intramolecular proton transfer from 1,3‐CHD to 1,4‐CHD, which has been confirmed by deuterium‐labeling experiments. There are no reports to date for such a solvent‐assisted isomerization. Density functional theory (DFT) studies have suggested that forming 1,3‐CHD and subsequent isomerization is a thermodynamically feasible process, regardless of the functional groups involved. Two possible successive polar solvent‐assisted proton‐transfer pathways have been identified for isomerization.

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Integrating Regioselective E→Z Isomerization of Trienones with Cascade Sequences under Photosensitizer‐Free Direct Irradiation at 390 nm

Cited by 5 publications

References 96 publications

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones

Thermally Activated Geometrical Regioselective E→Z Isomerization‐Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

A Torquoselective Thermal 6π‐Electrocyclization Approach to 1,4‐Cyclohexadienes via Solvent‐Aided Proton Transfer: Experimental and Theoretical Studies

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