TETRAHEDRON LETTERS Tetrahedron Letters 43 (2002) 2543–2546 Pergamon A direct approach to selective sulfonation of triarylphosphines Henrik Gulya ´s, a A ´ ron Szo ¨ llo  sy, b Brian E. Hanson c and Jo ´ zsef Bakos d, * a Research Group for Petrochemistry, Hungarian Academy of Sciences, P.O. Box 158, H-8201 Veszpre ´m, Hungary b Department of General and Analytical Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary c Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 -0212, USA d Department of Organic Chemistry, University of Veszpre ´m, P.O. Box 158, H-8201 Veszpre ´m, Hungary Received 13 December 2001; revised 7 February 2002; accepted 15 February 2002 Abstract—A practical and convenient synthesis of mono-, di- and a trisulfonated phosphines from triphenylphosphine analogs, (2,4-dimethylphenyl) 3-n -(phenyl) n -phosphine and (4-methoxyphenyl) 3-n -(phenyl) n -phosphine (n=0–2), is described, respectively. This represents an easy way to prepare water-soluble phosphines with complete selectivity, and with essentially no phosphine oxide formation. © 2002 Elsevier Science Ltd. All rights reserved. Aqueous-organic two-phase catalysis is one of the most dynamically developing areas of molecular catalysis, since it provides a simple solution to the problem of product isolation and catalyst recovery. The catalyst- complex, modified by water-soluble ligands, can be separated from the hydrophobic products by the sepa- ration of the aqueous and the organic phases. The significance of this environmentally benign concept is demonstrated by an increasing number of industrial applications, 1 however, it has obvious drawbacks. Meeting the double requirements of good catalytic activity and quantitative catalyst recovery is not always simple. Primarily, depending on the water-solubility of the substrate, besides the basic concept, 2 several modified techniques 3 can be adopted too. Thus, a need for applicable ligands with a wide range of water-solu- bility has arisen. Sulfonated arylphosphines are used most frequently as modifying ligands in aqueous and aqueous-organic two-phase catalysis. In general, these phosphines are obtained by the direct sulfonation of the aromatic rings. 4 Selective sulfonation, controlling the number of entering SO 3 units, would be a direct way to control the hydrophilic character of the ligand. Much effort has been devoted to the selective prepara- tion of triphenylphosphine derivatives with different degrees of sulfonation. Most of these attempts have focused on the sulfonation medium, reaction conditions and work-up procedure. Two reasons must be emphasized to explain why the syntheses of sulfonated triphenylphosphine-derivatives are fairly challenging. (i) The phenyl rings are directly attached to the phosphorus. Due to the deactivating effect of the protonated phosphorus atom, the sulfona- tion requires an aggressive sulfonating medium (fuming sulfuric acid), and relatively long reaction time. As a consequence, oxidation is a common side-reaction, par- ticularly in the preparation of trisulfonated triphenylphosphine (TPPTS). 4b,4e (ii) Differences between the activation energies for the sulfonation of the non-, mono- and disulfonated species are quite small. Accordingly, controlling the sulfonation degree is difficult and recognized as another major obstacle. Due to the intensive academic and industrial interest of this area, several new methods have been developed recently. Herrmann and his co-workers have introduced the use of superacidic medium (a mixture of anhydrous sulfuric acid and orthoboric acid) for sulfonation of phosphines. 4g Although this sulfonation method needs a slightly more complex work-up procedure than the conventional one, owing to the complete protonation of the phosphorus and the lack of free SO 3 , oxidation can be mostly avoided. A new method has been elaborated by Joo ´ for the preparation of monosulfonated triphenylphosphine (TPPMS). 4i The procedure is referred to by the authors as incomplete monosulfonation, which expresses very Keywords : sulfonation; phosphines; water-soluble. * Corresponding author. Tel.: 36-88-422-022; fax: 36-88-427-492; e-mail: bakos@almos.vein.hu 0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(02)00319-2