Siloxane based syntheses of fluorous ethenes and their tandem Heck reactions with aryl iodides

Abstract

Perfluoroalkyl-ethenes (RfnCHCH2, 6a–c; a, n=4; b, n=6; c, n=8) were prepared in good isolated yields (67–89%) and high purity (GC assay>98%) from various fluorinated organosilanes in fluoride-anion assisted protodesilylation reactions. The environmentally more benign ‘KF/NEt3/H2O’ reagent combination introduced here was found as an effective substitute for the commonly used tetrabutylammonium-fluoride trihydrate (TBAF·3H2O) as a fluoride source. Fluorous styrenes ((E)-RfnCHCHAr, 8) were then prepared in good isolated yields (58–93%/iodoarene) and purities (GC assay>95%) with the Pd(0) catalyzed Heck coupling of iodoarenes (Ar-I, 7) and perfluoroalkyl-ethenes generated in situ by the fluoride assisted cleavage of (β-perfluoroalkyl-α-iodo-ethyl)-siloxane ([RfnCH2CH(I)SiMe2]2O, 3) precursors in DMF solution at elevated temperatures. They are accessible by the one-pot reaction of dimethylvinylchlorosilane (CH2CHSiMe2Cl, 2) and perfluoroalkyl iodides (Rfn-I, 1) as we reported earlier. Similarly, the radical chain addition of C8F17I to CH2CHSi(OMe)3 (9) gave (β-perfluorooctyl-α-iodo-ethyl)-trimethoxysilane ([C8F17CH2CH(I)]Si(OMe)3, 10) in good yield, which then was reacted with silica gel in dry toluene to obtain an SiO2-bonded (perfluorooctyl)ethene surrogate [silica(O)3SiCH(I)CH2C8F17, 11]. The fluoride assisted cleavage of 11 and tandem Heck reaction with iodobenzene afforded the appropriate cross-coupled product (E)-C8F17CHCHPh.