Catalysis Involving Fluorous Phases: Fundamentals and Directions for Greener Methodologies

Abstract

This article briefly reviews the fundamentals of fluorous chemistry, with an emphasis on aspects most relevant to catalysis. The development of “greener” fluorous solvents and ponytails (phase tags) is described. Protocols that minimize the use of fluorous solvents are emphasized. Some of these exploit the highly temperature dependent solubilites of fluorous compounds in organic solvents, which can be fine‐tuned by varying the length and number of the (CF 2 ) n segments in the ponytails. Suitably designed fluorous catalysts can thus be employed under homogeneous conditions at elevated temperatures, and recovered by solid/liquid phase separation at lower temperatures. Fluorous solid supports such as fluoropolymers and fluorous silica gel can be used to aid the recovery of small catalyst quantities, and render phase separation more efficient. The use of CO 2 pressure as a non‐thermal solubility trigger, and catalyst recovery by solid phase extraction using non‐fluorous solvents, are also detailed.

Syntheses, structures, and stabilities of aliphatic and aromatic fluorous iodine(I) and iodine(III) compounds: the role of iodine Lewis basicity

“…In contrast, only traces of product were obtained with n = 15, presumably due to the poor solubility of the alcohol in virtually any medium. However, the solubilities of fluorous compounds are often highly temperature dependent [ 28 , 34 ], and an analogous reaction at room temperature gave R f15 CH 2 OTf in 22% yield. The triflates were white solids with some solubility in acetone.…”

“…To a first approximation, these are maximized by increasing the lengths and quantities of the (CF 2 ) n segments, and decreasing the lengths and quantities of any (CH 2 ) m segments [ 1 – 3 ]. However, longer (CF 2 ) n segments are often coupled with lower absolute solubilities [ 1 , 28 ], a logical consequence as one approaches the macromolecular limit of polytetrafluoroethylene. Fluorophilicities are typically quantified by fluorous/organic liquid/liquid phase partition coefficients [ 1 – 3 ].…”

Structural, Physical, and Chemical Properties of Fluorous Compounds

“…Fluorous molecules can be adsorbed onto a variety of fluorous supports, such as fluorous silica gel and fluoropolymers, including Teflon ® and Gore-Tex ® , as illustrated by their use in various catalyst recovery protocols [47,55]. It is important to emphasize that this does not imply a significant enthalpic attraction, although a very small amount would be expected.…”

“…However, it is possible to fine tune the perfluoroalkyl segments such that the solute has essentially no solubility in a fluorous or organic solvent at room temperature (i.e., is engineered to be below a certain limit or tolerance) but appreciable solubility at 60-120 C. This phenomenon can be used to conduct homogeneous reactions at elevated temperatures, with catalyst or reagent recovery by solid/liquid phase separation at lower temperatures [44][45][46]. This topic has been reviewed [47][48][49], and some of the many catalysts thus employed include the ketone (R f8 ) 2 Fluorous molecules normally show good solubilities in supercritical CO 2 [50][51][52]. Only modest fluorine content is normally required, and hence many lightly fluorinated systems have been employed as catalysts.…”

Transition metal complexes bearing NHC ligands substituted with secondary polyfluoroalkyl groups

“…Usually, the ligands in fluorous organometallic compounds contain linear perfluoroalkyl chains with an appropriate non-fluorinated spacer to tame unwanted electron-acceptor properties. [1][2][3][4][5][6][7][8][9][10][11] Due to their strong electron-donor properties, nitrogen heterocyclic carbene (NHC) ligands belong to the most efficient group of ligands in transition metals displaying high synthetic variability. [12][13][14][15][16][17][18][19][20][21] Correspondingly, several NHC complexes of silver (1), 22 palladium (2-4) [23][24][25] and ruthenium (5) 26 have been synthesized, bearing linear polyfluoroalkylated chains (Fig.…”