A novel solid-phase equivalent to the triflate group and its application to traceless linking and cross-coupling release strategies Adapted from: Andrew N. Cammidge et al, www.rsc.org/chemcomm 1. Read the text
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We wished to develop a solid-phase triflate equivalent using robust and simple chemistry and focused on aryl sulfonates. Polymer supported (simple) phenyl sulphonates have been employed in attempts to perform related chemistry but limited success was achieved.The catalytic process required for reduction or cross-coupling relies on Pd (0) insertion into the Ar – O bond. Successful reactions require this bond to be weakened by either electron withdrawing groups on the Ar or on the sulphonate ester. Efficient reductive cleavage was therefore only observed when electron withdrawing groups were present on the aryl residue.
A more detailed solution phase study has been carried out by Cabri who examined the effect of electron withdrawing groups on the sulphonate, ranking efficiency CF p-FArAr , indicating the potential for polyfluorophenyl sulphonates. To establish the number of fluorines required for efficient Pd(0) insertion we performed further solution phase model reactions using 2,4-difluorophenylsulphonyl chloride.The corresponding esters were prepared from phenylphenol and reduced using standard (for triflates)conditions. Both reactions proceeded smoothly to give biphenyl as the only product (Scheme 1). The rates of the reactions were significantly different with the 2,4-difluorophenyl sulphonate requiring 6 h. The reaction employing the pentafluorophenyl sulphonate, in contrast, was complete in less than 1 h. This result indicates that 2 fluorine substituents are sufficient to enable reduction (Pd insertion) on electron-neutral residues, but the more powerfully electron deficient sulphonate permits reaction under milder conditions.
