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

Abstract

Amazing reaction medium: Supercritical carbon dioxide, with zero dipole moment, lower dielectric constant than pentane, and non‐hydrogen‐bonding behavior, ionizes carbon–halogen bonds, dissociates the resulting ion pairs, and escapes from capture by the carbocation intermediates at temperatures above 40 °C. These properties allow the observation of carbocation chemistry in the absence of acids.

Concerning the Role of Supercritical Carbon Dioxide in SN1 Reactions

“…We chose this particularr eaction because it was reportedt op roceed quite smoothly to give quantitative conversion under the conditions of Gonzµlez-NfflÇez and co-workers. [2] In as eries of experiments, employing the same substrate loading in twod ifferent high pressure reactors, we obtained, however,p roduct yields (9a+ 9b)r anging consistently only between 1% and8%a fter 5happlying temperatures of 60-70 8Ca tC O 2 pressures between2 13-240 bar. Significantly better results were obtained either under argon at ap ressure level of 205 bar (20 %y ield) or under CO 2 at al ower pressure of 100 bar (35 %y ield), while the temperature was held at 60 8C.…”

“…In addition, we carried out the reaction of tert-butyl chloridea nd 2 under similarr eaction conditions to those reported in ref. [2] (60 8C, 5h,4equiv of 2)i no ur batch reactor. Neitheri nt he presence (200 bar) nor in the absence of carbon dioxide did the amounto fp roducts 9a/9 b exceed more than 1% yield.…”

“…Recently,G onzµlez-NfflÇez and co-workers reported the use of supercritical carbon dioxide (scCO 2 ) [1] as solventf or S N 1r eactions of appropriate alkyl halidess uch as tert-butyl and1 -adamantyl halides (Cl and Br), as well as other potentially S N 1active secondary halides such as 1-chloro-1-phenylethane (1). [2] Based on their experimental observations, the authors concluded that scCO 2 had an activating effect also on electrophilic aromatic substitution reactions. In at ypical model reaction, aF riedel-Crafts alkylation of 1,3-dimethoxybenzene (2)w as performed at 60 8Ca nd 250 bar by pressurizing the reactionc omponents with CO 2 in as tainlesss teel reactor for 5h,w hich was reported to provide the alkylation products 3 o,p and 3 o,o in 82 %y ield (Scheme1).…”

“…Ta ken together,t hese results do not support the previously claimed activating effect via enhanced S N 1i onization due to the quadrupolarm oment of the supercritical fluid.Scheme1.Lewis acid free Friedel-Craftsa lkylation of 1,3-dimethoxybenzene as reported in ref. [2].Double-substituted isomers are not shown.[a] Dr.…”

Metal-free and VOC-free O-glycosylation in supercritical CO2

“…Furthermore, it has been recently described that scCO 2 promotes the heterolysis of carbon-halogen bonds in aromatic systems with good results in Friedel-Crafts type reactions, 16 which was justified by the cluster effect of scCO 2 in polar solute molecules. 13,17,18 Taking into account the fact that the heterolysis of an activated glycosidic bond is the initial step for the glycosylation reaction and considering that, similar to Friedel-Crafts reactions, a cation is the most common intermediate in this reaction, we hypothesized that the glycosylation reaction starting from glycosyl halides could be performed in scCO 2 and, more interestingly, in the absence of a promoter, additionally avoiding the use of VOCs (Scheme 1).…”