Synthesis of novel dihydroxydiphosphines and dihydroxydicarboxylic acids having a tetra(thio-1,3-phenylene-2-yl) backbone

Abstract

Novel tetradentate PPh₂–OH hybrid ligands 5 and CO₂H–OH hybrid ligands 6 have been successfully synthesised from tetra(thio-5-tert-butyl-2-hydroxy-1,3-phenylene) (2 ₄ ) by replacing the hydroxy groups at both the 2-position and either the 2′-, 2″- or 2‴-position with diphenylphosphino or carboxy groups, after converting into bistriflates 8; the substitution positions of newly introduced substituents are denoted hereafter by superscript 1,n as 5 ^{1, n} . Bistriflates 8 ^1,3 and 8 ^1,4 can be readily prepared by the regioselective detriflation of tetrakistriflate 7 with tetrabutylammonium fluoride (TBAF), conducted under different conditions. On the other hand, the preparation of bistriflate 8 ^1,2 requires a four-step process through protection/deprotection. Thus, the silylation of tetraol 2 ₄ with an excess of 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane gives O,O′- and O″,O‴-disiloxane-1,3-diyl-capped derivative 9. One of the two disiloxane bridges is removed by the treatment with 0.5 mol equiv. of TBAF to give diol 10. Triflation of diol 10, followed by the removal of the remaining disiloxane bridge, affords bistriflate 8 ^1,2 . Bistriflates 8 are subjected to palladium-catalysed phosphorylation, followed by the reduction of the resulting phosphine oxide 12 ^{1, n} to give PPh₂–OH hybrid ligands 5 ^{1, n} (n = 2–4), while CO₂H–OH hybrid ligands 6 ^{1, n} (n = 3,4) are obtained from 8 via acetylation of the remaining hydroxy groups, followed by palladium-catalysed methoxycarbonylation of the TfO moieties, and subsequent hydrolysis of the resulting tetraesters 14. X-ray structural analyses of dicarboxylic acids 6 ^1,3 and 6 ^1,4 reveal that they form quite different 3D network structures to each other. Interestingly, 6 ^1,4 constructs a porous channel with a potential for serving as a supramolecular host, by the stacking of its cyclic dimer that is formed by intermolecular hydrogen bondings between the carboxy groups.

Keywords:

• tetra(thio-m-phenylene)
• regioselective synthesis
• hydroxyphosphine
• hydroxyacid