Ionic Transformations in Extremely Nonpolar Fluorous Media: Easily Recoverable Phase‐Transfer Catalysts for Halide‐Substitution Reactions

Abstract

Solutions of the fluorous alkyl halides R(f8)(CH(2))(m)X (R(fn)=(CF(2))(n-1)CF(3); m=2, 3; X=Cl, Br, I) in perfluoromethylcyclohexane or perfluoromethyldecalin are inert towards solid or aqueous NaCl, NaBr, KI, KCN, and NaOAc. However, halide substitution occurs in the presence of fluorous phosphonium salts (R(f8)(CH(2))(2))(R(f6)(CH(2))(2))(3)P(+)X(-) (X=I (1), Br (3)) and (R(f8)(CH(2))(2))(4)P(+)I(-) (10 mol %), which are soluble in the fluorous solvents under the reaction conditions (76-100 degrees C). Stoichiometric reactions of a) 1 with R(f8)(CH(2))(2)Br and b) 3 with R(f8)(CH(2))(2)I were conducted under homogenous conditions in perfluoromethyldecalin at 100 degrees C and yielded the same R(f8)(CH(2))(2)I/R(f8)(CH(2))(2)Br equilibrium ratio ( approximately 60:40). This shows that ionic displacements can take place in extremely nonpolar fluorous phases and suggests a classical phase-transfer mechanism for the catalyzed reactions. Interestingly, the nonfluorous salt (CH(3)(CH(2))(11))(CH(3)(CH(2))(7))(3)P(+)I(-) (4) also catalyzes halide substitutions, but under triphasic conditions with 4 suspended between the lower fluorous and upper aqueous layers. NMR experiments established very low solubilities in both phases, which suggests interfacial catalysis. Catalyst 1 is easily recycled, optimally by simple precipitation onto teflon tape.

Palladium‐Catalyzed Decarbonylative Iodination of Aryl Carboxylic Acids Enabled by Ligand‐Assisted Halide Exchange

“…The outer sphere chloride anion could then readily substitute the iodide of alkyl iodides 2 or 10 through a nucleophilic substitution reaction. [35] Indeed, when we tried a stoichiometric reaction to detect the mono‐phosphonium chloride form of Xantphos ( 9 ), traces of the salt were unambiguously observed by 31 P{ 1 H} NMR together with a significant amount of the deactivation complex, (Xantphos)PdCl 2 (Scheme 3 b , see Supporting Information). We next used a different route to access phosphonium chloride 9 in preparative quantities, starting from chlorobenzene under nickel catalysis.…”

Catalysis of Intramolecular Morita–Baylis–Hillman and Rauhut–Currier Reactions by Fluorous Phosphines; Facile Recovery by Liquid/Solid Organic/Fluorous Biphase Protocols Involving Precipitation, Teflon® Tape, and Gore‐Rastex® Fiber

“…They also significantly extend the number of catalyst systems demonstrated to be recyclable by precipitation onto Teflon tape, [5,24] and provide the first use of Gore-Rastex fiber for recovery. The latter fluoropolymer gives slightly better results, consistent with a greater surface area.…”

3,5‐Bis(n‐perfluorooctyl)benzyltriethylammonium Bromide (F‐TEBA): An Efficient, Easily Recoverable Fluorous Catalyst for Solid‐Liquid PTC Reactions

“…The ease of recovery and the recycling efficiency of PT fluorous catalysts were thus demonstrated. In addition, reactivities and selectivities comparable to those observed with classical PT agents could be achieved through careful design of the fluorous catalysts, even in the case of reactions run in extremely non‐polar perfluorocarbon phases 2c,3b…”

“…The NEt 3 + polar head of F‐TEBA , which was instrumental for its use as a PT agent, was likely to be responsible for this particular behaviour. Indeed, related ammonium salts, 3 ,5b 4 ,2c and 5 (Supporting Information) with a NH 3 + polar head attached to a 3,5‐bis (perfluoroalkylated)aryl ring (Figure 1) were found to be soluble in perfluorocarbons. Unfortunately, the reactive nature of the NH 3 + group imposes serious limitations on the applicability of such salts in many relevant PTC processes, especially those involving basic reagents.…”