Supercritical Carbon Dioxide: A Promoter of Carbon-Halogen Bond Heterolysis

Generation of Carbocations under Photoredox Catalysis: Electrophilic Aromatic Substitution with 1-Fluoroalkylbenzyl Bromides

A novel Friedel-Crafts-type alkylation of arenes to access valuable 1-fluoroalkyl-1,1-biaryl compounds is established under mild conditions. The key to success is the efficient generation of a destabilized benzylic carbocation intermediate via two consecutive single-electron transfer processes by virtue of visible-light photoredox catalysis. This unique activation pattern avoids using strong Lewis acids and high temperatures that are required for generation of destabilized carbocations in traditional Friedel-Crafts reactions. This protocol demonstrates the first example of photoredox-catalyzed heterolysis of electronically deactivated benzylic C-Br bonds for the formation of destabilized carbocation intermediates.

Concerning the Role of Supercritical Carbon Dioxide in SN 1 Reactions

A series of S_N 1-type reactions has been studied under various conditions to clarify the role of supercritical carbon dioxide (scCO₂ ) as reaction medium for this kind of transformations. The application of scCO₂ did not result in higher yields in any of the experiments in comparison to those under neat conditions or in the presence of other inert compressed gases. High-pressure UV/Vis spectroscopic measurements were carried out to quantify the degree of carbocation formation of a highly S_N 1-active alkyl halide as a function of the applied solvent. No measureable concentration of carbocations could be detected in scCO₂ , just like in other low polarity solvents. Taken together, these results do not support the previously claimed activating effect via enhanced S_N 1 ionization due to the quadrupolar moment of the supercritical fluid.

SN1 reactions in supercritical carbon dioxide in the presence of alcohols: the role of preferential solvation

Ethanol () inhibits SN1 reactions of alkyl halides in supercritical carbon dioxide (scCO2) and gives no ethers as products. The unexpected behaviour of alcohols in the reaction of alkyl halides with 1,3-dimethoxybenzene () in scCO2 under different conditions is rationalised in terms of Brønsted and Lewis acid-base equilibria of reagents, intermediates, additives and products in a singular solvent characterised by: (i) the strong quadrupole and Lewis acid character of carbon dioxide, which hinders SN2 paths by strongly solvating basic solutes; (ii) the weak Lewis base character of carbon dioxide, which prevents it from behaving as a proton sink; (iii) the compressible nature of scCO2, which enhances the impact of preferential solvation on carbon dioxide availability for the solvent-demanding rate determining step.

On the ionizing properties of supercritical carbon dioxide: uncatalyzed electrophilic bromination of aromatics

scCO₂, a non-polar solvent with a dielectric constant lower than n-pentane, promotes the electrophilic bromination of aromatics as efficiently as strongly ionizing solvents such as aqueous acetic and trifluoroacetic acids.