Polymeric bis(triphenylphosphine)iminium chloride as a recyclable catalyst

Abstract

Metal-free catalysts have garnered considerable interest as an environmental and economical alternative to precious metal catalysts. Bis(triphenylphosphine)iminium chloride (PPNCl) has emerged as a prominent choice due to its air and thermal stability and broad reactivity, especially in applications where a bulky cation is needed. The high phosphorus content and synthetic effort required for catalyst synthesis increase environmental impact; the recyclability of PPNCl in catalytic processes remains largely unexplored. The potential development of a polymer-supported PPNCl catalysts therefore desirable to enable this recyclability. In this work, we synthesise polymeric PPNCl (poly(PPNCl)) for the first time. Poly(PPNCl) demonstrates a comparative catalytic reactivity to its small molecule variant when employed as a catalyst in halogen-exchange reactions and CO₂/epoxide coupling. For the latter the effect of catalyst loading, CO₂ pressure, reaction time and addition of co-catalyst on conversion and selectivity was investigated. Poly(PPNCl) was easily recovered from the crude product by simple precipitation and its catalytic reactivity was well-maintained over three reaction cycles, providing environmental and economic advantages for sustainable reaction development.

Fig. 1. (a) Previous work using PPNCl as catalysts; (b) poly(PPNCl) as a recyclable catalyst for halogen exchange and CO₂/epoxide coupling reactions.

Scheme 1. (a) Synthetic procedures of P2 from phosphine-containing copolymer, m : n = 95 : 5 or 90 : 10; (i) n-butyllithium, toluene, −78 °C, 3 h; (ii) C₂Cl₆, THF, r.t., 3 h; (b) Synthesis of Ph₃PNLi from triphenylphosphine; (iii) hydroxylamine-O-sulfonic acid, methanol/DCM, r.t., 1 h; (iv) n-butyllithium, THF, initiated at −50 °C, −15 °C 2 h; (c) synthesis of poly(PPNCl) (v) THF, r.t., 3 h.

Scheme 2. (a) Halogen-exchange reaction from CNB to FNB; (b) coupling reaction of CO₂ with GC.

Fig. 2. CO₂/epoxides coupling reaction to yield cyclic carbonates. Conversions calculated by ¹H NMR spectroscopy. Conversions for GC and PO were taken from the average value of three trials; the rest substrates were conducted once. Reactions were performed in a sealed ampoule on a 12.8 mmol scale at 0.1 mol% poly(PPNCl) with or without 0.1 mol% BPh₃ (0.1 mol% as calculated from the number of expected active functional groups).

Scheme 3. The selective one-pot, two-step reaction of epoxide conversion to cyclic carbonates. General conditions: the feed ratio of GC : PO : poly(PPNCl) : BPh₃ is 1000 : 1000 : 4 : 4.

Fig. 3. Reaction conversions over three cycles (a) halogen-exchange, (b) CO₂/GC coupling, and (c) CO₂/PO coupling reactions. Conversion calculated by ¹H NMR spectroscopy. Conversions for recycled poly(PPNCl) were obtained from an average value of two trials.