Photoexcited nitroarenes for the oxidative cleavage of alkenes

Abstract

The oxidative cleavage of olefins is an integral process that converts feedstocks into highvalue synthetic intermediates 1,2,3 . The most viable method to oxidatively cleave C-Cπ bonds in one chemical step is with ozone 4,5,6,7 , which however poses technical and safety challenges owing to the explosive nature of ozonolysis products 8,9 . Herein, we present a distinct approach to achieve oxidative cleavage of olefins using nitroarenes and purple light irradiation. We demonstrate that photoexcited nitroarenes are effective ozone surrogates that undergo facile radical [3+2] cycloaddition with olefins. The resulting "Ndoped" ozonide cycloadducts are safe to handle and lead to the corresponding carbonyl products under mild hydrolytic conditions. These features have enabled the controlled cleavage of all types of olefins in the presence of a broad array of commonly used organic functionalities. Furthermore, by harnessing electronic, steric, and mediated polar effects, the structural and functional diversity of nitroarenes has provided a modular platform to obtain site-selectivity in substrates containing more than one olefin.Olefins are feedstock materials harvested on ton-scale from petroleum and vegetable biomass routinely exploited by the bulk chemical industry to access oxygen-enriched synthetic intermediates 1,2,3 . Ozonolysis is a widely adopted method to achieve this and requires a specialised apparatus for the conversion of molecular oxygen (O2) into highly reactive ozone (O3) 6,7 . This species undergoes a [1,3] dipolar cycloaddition with the olefin converting a stable chemical into a high-energy 1,2,3-ozonide A from which cycloreversion is immediate. The consequent C-C σ-bond cleavage event generates carbonyl oxide B and carbonyl compound C which recombine to give 1,2,4-ozonide D. Depending on reaction solvent and work-up procedure, B or D can lead to aldehydes/ketones as well as carboxylic acids or alcohols 4,5 (Fig. 1a).Despite its attractive synthetic versatility, ozone toxicity (lethal at 5 ppm), explosivity, and extreme oxidising power (E0 = 2.07 V) raise critical safety, technical, and chemical concerns 8,9 .As a result, ozonolytic strategies are often challenging to translate into the fine chemical industry 10,11,12 , particularly in the discovery sector which heavily relies on parallel and highthroughput screening platforms 13 . Consequently, alternative strategies for olefin oxidation based on high-valent heavy-metal oxides have been devised. However, these approaches can yield mixtures of products of various oxidation degrees, and cause trace metal contaminations

Palladium Hydride-Enabled Hydroalkenylation of Strained Molecules

“…Synthetic usefulness of the obtained alkenylated gem -difluorocyclopropanes was highlighted by the following transformations (Scheme C). Epoxidation and photoinduced oxidative cleavage of the alkene proceeded smoothly, furnishing epoxide 3ag and aldehyde 3ah , respectively. Dibromination of the olefin provided product 3ai in good yield with two new functionalizable reaction sites.…”

Oxidative Cleavage of Alkenes by Photosensitized Nitroarenes

“…In contrast, Hammett plot analysis of nitroarenes containing multiple substitutions at the arene ring allowed Leonori and co‐workers to identify a series of reactive nitroarenes suitable for engaging various unactivated alkenes. While electron deficient nitroarenes generally showed high reactivities, performing the reaction at low temperature (−30 °C) was crucial for selective formation of “N‐doped” ozonides in high yields [10] . In an overall one‐pot two‐step approach, those “N‐doped” ozonides were converted to the corresponding carbonyl compounds by hydrolytic work‐up at high temperature.…”

“…The recent independent reports by the research groups of Parasram and Leonori have now addressed this longstanding problem by exploring the excited state reactivities of nitroarenes with alkenes under mild purple light photosensitization conditions. [9,10] Unlike powerful mercury lamps, mild photosensitization conditions allowed formation of the "N-doped" ozonide and a controlled cleavage of the alkenyl CÀ C bond to generate the corresponding carbonyl compounds in high yields (Scheme 2C). Parasram and co-workers identified 4-cyanonitrobenzene as an effective one step oxygen transfer agent for styrenes at room temperature.…”

“…While electron deficient nitroarenes generally showed high reactivities, performing the reaction at low temperature (À 30 °C) was crucial for selective formation of "N-doped" ozonides in high yields. [10] In an overall one-pot two-step approach, those "N-doped" ozonides were converted to the corresponding carbonyl compounds by hydrolytic work-up at high temperature.…”

Hydrogen-Borrowing Reduction/Dehydrogenative Aromatization of Nitroarenes through Visible-Light-Induced Energy Transfer: An Entry to Pyrimidoindazoles and Carbazoles

“…On the basis of the above-described experimental results and previous reports, we propose the mechanism illustrated in Figure for this sequential visible-light photocatalyzed dehydrogenation/cyclization. Under visible-light irradiation, [Ir III ]-catalyst photosensitizes 3a into its exited state * 3a via energy transfer (ET) . Photoactivated * 3a proceeds through hydrogen atom transfer (HAT) and generates intermediate I-1 , , which undergoes single electron transfer (SET) to the intermediate I-2 .…”