Photo Induced DABCO Mediated HAT en route to Exocyclic Alkenyl Ethers Enabled by EDA Complex

Maji, Rohan (2023) Photo Induced DABCO Mediated HAT en route to Exocyclic Alkenyl Ethers Enabled by EDA Complex. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Rohan Maji (18MS084)) Thesis_18MS084.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

Derivatization of cyclic ethers and functionalized maleimides is a highly sought-after goal in contemporary research because they form the backbones of a great deal of natural products and other physiologically & pharmacologically active compounds. As such, considering its frequent need in late-stage modification and function-oriented synthesis, direct and specific C−H bond elaboration in such organic molecules still remains the undiscovered Holy Grail in synthetic chemistry due to the higher bond dissociation energy of the C−H bond to be functionalized. Existing protocols suffer from harsh peroxide-based conditions or use of external oxidant or UV light or highly expensive transition metal catalysts, which makes such methods non-green and non-viable for medicinal and industrial purposes. In this work, a novel robust but mild synthetic protocol is disclosed for the generation α-alkoxy radical from ethers by dissecting the C(sp3)−H bond. C(sp3)−H olefination was performed by mixing the equimolar mixture of DABCO and maleimides or other electron-deficient olefins and dissolving the components in mixture of solvents, containing THF (or other ethers)/ MeCN (7:1). The combination of DABCO and maleimide forms an EDA complex in situ which is executed as the HAT reagent for the diastereoseletive exocyclic alkenylation of cyclic ethers. Starting materials were prepared using simple reported procedures, followed by optimization studies that ascertained the reaction vial to expose to 467 nm 40W blue LED for 36 h under oxygen atmosphere (1 atm) using balloon as the optimum condition, generating 55 such desired alkenylated ether as the product, upto maximum 94% isolated yield. A wide variety of substrates are enlisted to demonstrate its diverse potential applicability and practical viability. Thorough mechanistic studies and photophysical experiments assist to establish the mechanism. Radical trapping studies provided strong evidence for a radical-mediated route and UV-Vis absorbance spectra and Cyclic Voltammetry data lent strong credence for the creation of an EDA complex with a desirable binding stoichiometry of 1:1. DABCO plays a magical role in this uncoventional C−H functionalization by exempting this method from the requirement of external photocatalyst, transition metals or stoichiometric peroxides and oxidants.

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