Visible Light Promoted, Catalyst‐Free Radical Carbohydroxylation and Carboetherification under Mild Biomimetic Conditions

Abstract

Metal and catalyst‐free carbohydroxylations and carboetherifications at room temperature have been achieved by a combination of beneficial factors including high aryl diazonium concentration and visible light irradiation. The acceleration of the reaction by visible light irradiation is particularly remarkable against the background that neither the aryldiazonium salt nor the alkene show absorptions in the respective range of wavelength. These observations point to weak charge transfer interactions between diazonium salt and alkene, which are nevertheless able to considerably influence the reaction course. As highly promising perspective, many more aryldiazonium‐based radical arylations might benefit from simple light irradiation without requiring a photocatalyst or particular additive.

Current Advances in Meerwein-type Radical Alkene Functionalizations

“…Photocatalytic aryl radical generation using the established Ru(bpy) 3 catalyst and visible light irradiation could be improved by the addition of potassium carbonate. This effect will be discussed in a later part of this review (Section 3.2), as also related studies 22 showed the benefits of mild basic conditions for visible light irradiation. After aryl radical addition to the double bond of acrylamide 12 , an ipso -attack of the resulting alkyl radical leads to the formation of the spirocyclic intermediate X .…”

“…As shown in Scheme 16 , heating is no longer necessary if visible light (450–475 nm) is applied instead, thus turning the overall transformation into a mild biomimetic process. 22 The fact that no photocatalyst is required although none of the reactants absorbs light in the visible wavelength range can be rationalized by the formation weak and transient, but still light absorbing charge-transfer complexes between diazonium ion and styrene. Due to an efficient chain transfer (c.f.…”

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

“…Recently, Heinrich published a catalyst-free carboetherification of α-methylstyrene using aryl diazonium salts and alcohols under visible light irradiation (Scheme 4). 53 A broad scope of primary alcohols was used to form α-tertiary ethers in good yields with useful pendant functionality, including olefins. The reaction was found to proceed without irradiation, albeit in lower yields, which is consistent with prior work on this transformation.…”

Visible-Light-Mediated Radical Arylations Using a Fluorescein-Derived Diazonium Salt: Reactions Proceeding via an Intramolecular Forth and Back Electron Transfer

“…11 Depending on the strength of the charge transfer complexes, which are typically exploited in such catalyst-free transformations, comparably long reaction times may be necessary. 12,13 Against this background, and in the context of our own contributions to catalyst-free arene and alkene functionalization, 12,13 how strongly light-absorbing diazonium salts would behave in such arylation reactions arose as an interesting question (Scheme 1B).…”