Strongly Electron Deficient Sulfonyldithioformate Based RAFT Agents for Hetero Diels-Alder Conjugation: Computational Design and Experimental Evaluation

Abstract

The synthesis of a novel class of reversible addition-fragmentation chain transfer (RAFT) agents-based on quantum-chemical ab initio calculations-having methylsulfonyl and phenylsulfonyl moieties as Z-group is described. The resulting C-sulfonyldithioformate transfer agents [benzyl methylsulfonyldithioformate (PSDTF) and benzyl phenylsulfonyldithioformate (PSDTF)] feature extremely strong electron deficient C=S double bonds and should thus be suitable to undergo rapid hetero Diels-Alder (HDA) reactions with variable dienes under mild (i.e., ambient and catalyst free) reaction conditions. It can be demonstrated via a series of model reactions, whose outcome is monitored via electrospray ionization mass spectrometry (ESI-MS), that C-sulfonyldithioformate based RAFT agents undergo HDA reactions with a series of diene-capped macromolecules (10 min\ reaction time\ 24 h) at ambient temperatures (T ≈25°C) with reaction times ranging from 24 h (for open chain dienes) to a few minutes (for cyclopentadiene) in the absence of any catalysts. Concomitantly, PSDTF is able to efficiently mediate the polymerization of isobornyl acrylate (iBA) with living characteristics (2300\ Mn (g/mol)\ 16,000, 1.08\ PDI \ 1.31). In a subsequent step, it is demonstrated that conjugates of poly(iBA) and polystyrene can be constructed under mild reaction conditions (reaction time \ 10 min, T ≈25°C, Mn≈ 6000 (g/mol), PDI≈1.3, no catalyst). In addition, we highlight that sulfonyldithioformate type RAFT agents are such effective dienophiles that they can undergo HDA reactions with certain monomers, including styrene. While such a strong HDA activity limits their use in polymerizations, it opens an avenue for catalyst free efficient surface modification reactions under mild conditions with variable dienes.