Dry Synthesis Under Microwave Irradiation: A rapid and efficient coupling of naphthols

Iron (III) chloride hexahydraae without solvent induce coupling of naphthols were quickly and efficiently under focused microwave irradiation. Coupling of naphthols bearing an electronwithdrawing group which are very slow under classical conditions became possible under these conditions. We are interesaed by the synthesis of substituted binaphthols because the C2 symmetric 11'-bi-2-naphthol (BINOL) I and the 2'-bis (diarylphosphino)-1,1'-binaphthyl (BINAP) 2 ligands have emerged as important chiral auxiliaries and ligands in organic synthesis. For these synthesis of binaphthols, oxidative coupling seems to be a method of choice. Oxidative coupling of phenols has long been recognized as being importânt in biosynthetic pathway and in biomimetic synthesis 3. Various oxidants can bring about the coupling of phenols in solution 4 but the oxidative coupling of phenols ofaen leads to low yields of the desired products or suffer from disadvantages due to oxidant or phenol insolubility, or to difficulties in separating inorganic and organic products. Recently Toda et al5 have described the possibility of oxidative phenol coupling without solvent (dry conditions). But the reaction in solution is slow with naphthol and does not take place when an electron withdrawing group is present on the naphthol. Ultrasound irradiation was not very efficient to accelerate'these r actions. 5 and also the thermal activation is inefficienr The yields obtained under conventional heating and under iradiation with a commercial microwave oven or in resonance cavity were compared in table l. With a commercial microwave oven results are difficult to reproduce when the reactions are conducted with small samples, but the results are reproducible with larger quantitiesg [Eg: (1a) 0.138 mol., 280W, 420s1. Table 1. Coupling of naphthols (1) into binaphthols (2) (conditions, yield) naphthola heatingb microwavesc microwavesd l a l b l c 1 d l e l f 70oC.lh,42% 280W, 30s,55Vo 40W, 30s, 96% 280'W, 20s,76% 40W,20s,95% 280W, 100q34% 40W, lfi)s, 857o 70"C, Lh,>37o 140W, 140g3% 40W, 140s, 40% 70oC, lh,SVo 140W, 140s, lUVo 40W, 140s, 62% 140rW, 140s,347o 40W, 140s, 84% a) Coupling of (1) (7 mmol.); b) heating with a oil bath; c) irradiation with a commercial microwave oven Toshiba ER 8930FC; d) irradiation in a resonance cavity 7. Several mechanistic pathways which involve oxidation of the naphthol to naphthoxy radical and dimerization are generally proposed. Because of rapid reaction, the variety of functional groups toleraaed and the good yield observed, in this procedure provides a very useful synthetic method to binaphthol. Under the similar conditions p-cresol gave a mixture of 6 products containing 2,2'-dihydroxy-5,5'-dimethylbiphenyl (52Vo) and traces of Pummerer's ketone (4Vo). The 2,6-disubstitued phenols (3a-c) gave diphenoquinones (4a-c) according the Scheme 2. 2a+ b)R1=OH; RaeH d) R1=H; R5PO(OC2H5)2 f)R1=H;R5COOCH3 R / A FeCl3,6 H2O ' \ / F o " : U( rnrcrowaves, \ 140w,120s 3a-c R We have already reported that a great number of dry reactions are activaaed by microwanes 6. We now report that microwave inadiation 7 accelerated greatly the oxidation of naphthol (la-f) with iron (IIf chloride hexahydraae without solvent. Under these conditions coupling of naphthol with an electron withdrawing group (ld-f) ( COOH, COOCH3, PO(OR)) takes place. In control experiments, no reaction is observed afaer 24 h with (ld-f) at room temperature without microwave a, R=GH3 (95%);b, R=l-Bu (92%l; c, R=Cl(85%)