Picosecond to millisecond tracking of a photocatalytic decarboxylation reaction provides direct mechanistic insights

Aditi Bhattacherjee,Luke Lewis-Borrell,Andrew J Orr-Ewing,Mahima Sneha,Ian P Clark,Omri Tau

doi:10.1038/s41467-019-13154-w

Abstract

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. However, the sequential nature of this multi-step reaction renders the mechanism challenging to probe. Here, we employ a 100 kHz mid-infrared probe in a transient absorption spectroscopy experiment to track the decarboxylation of cyclohexanecarboxylic acid in acetonitrile-d3 over picosecond to millisecond timescales using a photooxidant pair (phenanthrene and 1,4-dicyanobenzene). Selective excitation of phenanthrene at 256 nm enables a diffusion-limited photoinduced electron transfer to 1,4-dicyanobenzene. A measured time offset in the rise of the CO2 byproduct reports on the lifetime (520 ± 120 ns) of a reactive carboxyl radical in solution, and spectroscopic observation of the carboxyl radical confirm its formation as a reaction intermediate. Precise clocking of the lifetimes of radicals generated in situ by an activated C-C bond fission will pave the way for improving the photocatalytic selectivity and turnover.

Highlights

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis
The arene cation is thought to be the active oxidant of the carboxylic acid, a similar role played by the photoexcited electron acceptor is not completely excluded[6]
Selective excitation of a ground-state PHEN molecule in the reaction mixture enables the study of the reaction in situ in a clean fashion, without interference from the photochemical pathways of the other reactants

Summary

Introduction

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. An excited-state, single electron transfer (SET, Step 2) from PHEN* to DCB leads to the formation of Phen+DCB−, likely born as a geminate ion pair and transforming to a solvent-separated ion pair before being fully solvated to produce free ions in solution[9] (note that ‘ions’ are often referred to as ‘radical ions’ in the literature; we adhere to the use of ‘ions’ in our discussion throughout). A second oxidative SET (Step 3) between a carboxylate anion (RCOO−) and the arene cation (PHEN+) generates a reactive carboxyl radical (RCOO) in solution. This radical precursor undergoes a unimolecular dissociation (Step 4) to form R by releasing CO2 as a byproduct. A potential role of the electron acceptor (DCB) as the active photooxidant of the carboxylate ion is invoked[6], but largely unclear

Methods

Results

Conclusion