SYNTHESIS OF DMBP BY TOLUENE COUPLING
365
suggest that not all of the Pd0 formed during the reaction, coupling of toluene at room temperature. Moreover, the re-
is reoxidized to Pd2+ at high catalyst concentration. action rate is promoted by H+ when the anion is labile such
The slope of the toluene conversion curve vs. Pd(II) as OTf so that the overall reactivity of the Pd(II)/HOTf
amount (Fig. 1) may be explained by the mechanism re- system is greater than that reported by others for the Pd(II)
ported by the UBE group (14). This mechanism suggests system in weaker acids such as acetic or trifluoroacetic acid.
that three molecules of toluene will be sequestered for ev- The regioselectivity is altered by increasing acid strength to
ery Pd(II) ion initially present in the reaction mixture when favor coupling in the 3- and 4-positions.
the Pd(0) is not reoxidized. Our results agree with this
mechanism since the slope of the toluene conversion curve
is 3.08 in the stoichiometric mode (i.e., no reoxidation of
ACKNOWLEDGMENTS
the Pd).
We gratefully acknowledge the financial support from the Hoechst-
Under the reaction conditions used in the present study, Celanese Corporation (Charlotte, NC) and we also thank Dr. David A.
the main coupling product is 3,40-DMBP. At shorter reac-
Schiraldi and Dr. Jeffrey C. Kenvin (KoSa, Spartanburg, SC) for their
helpful discussions.
tion times (30 min), the selectivity to 3,40- and 4,40-DMBP
are nearly equal in agreement with the results of Davidson
and Triggs who observed the regioselectivity in the coupling
of toluene in acetic acid/perchloric acid (9). Since the reac-
tion occurs in the presence of TfOH, isomerization of the
4,40-DMBP to thermodynamically favorable 3,40- and other
DMBP isomers may account for the higher selectivity of
3,40-DMBP. In order to confirm this point, we investigated
the reaction of 4,40-DMBP dissolved in toluene at 10 mol%
in the presence of TfOH (TfOH/toluene = 1) but without
the palladium salt. It was found that ca. 60% of the 4,40-
DMBP was transformed to 3,40-DMBP in 2 h; formation of
other isomers of DMBP was less than 5% of 3,40-DMBP.
Thus, isomerization of the products by strong acid could
explain some of the regioselectivity demonstrated by this
catalyst.
We compare our results (Table 4) to those appearing in
the open literature. The reactivity of the Pd(TfO)2/TfOH
is the greatest of those listed in Table 4 when one con-
siders the values of the turnover rate and the tempera-
ture of the reaction. The Pd(AcO)2/H(acac) system shows
a smaller turnover rate (0.00021 h 1) than the present
work, when one corrects for the effect of temperature
(vide supra). All other entries show turnover rates smaller
than the present work using triflic acid regardless of
temperature.
REFERENCES
1. MacBride, J. A. H., in “Second Supplements to the 2nd Edition
of Rodd’s Chemistry of Carbon Compounds” (M. Sainsbury, Ed.),
Vol. III-D/E/F (Partilal), p. 317. Elsevier, Amsterdam, 1996; Barton,
D. H. R., Ozbalik, N., and Ramesh, M., Tetrahedron 44, 5661 (1988);
Negishi, E., Takahashi, T., and Akiyoshi, K., J. Organometal. Chem.
334, 181 (1987);Dudman, C., European Patent Application EP 206,543,
1986; Kageyama, H., Jpn. Kokai Tokkyo Koho JP 02,233,622, 1989;
Hawkins, J., Mercer, P., and Bellas, M., U.S. Patent 5,194,669, 1991.
2. Cheeseman, G. M. H., and Praill, P. F. G., in “Rodd’s Chemistry of
Carbon Compounds” (S. Coffey, Ed.), 2nd ed., Vol. III-F, p. 1. Elsevier,
Amsterdam, 1974.
3. MacBride, J. A. H., in “Supplements to the 2nd Edition of Rodd’s
Chemistry of Carbon Compounds” (M. F. Ansell, Ed.), Vol. III-E/F
(Partilal), p. 301. Elsevier, Amsterdam, 1982.
4. Glover, S. A., J. Chem. Soc., Perkin Trans. I 1338 (1980).
5. Akiba, K., Pure Appl. Chem. 68, 837 (1996).
6. March, J., “Advanced Organic Chemistry,” 3rd ed., p. 484. Wiley,
New York, 1985.
7. Balaban, A. T., and Nenitzescu, C. D., in “Friedel-Crafts and Re-
lated Reactions” (G. A. Olah, Ed.), Vol. II, Part 2, p. 979. Wiley,
1964.
8. (a) Sainsbury, M., Tetrahedron 36, 3327 (1980); (b) Shilov, A. E., and
Shul’pin, G. B., Chem. Rev. 97(8), 2679 (1997).
9. van Helden, R., and Verberg, G., Recl. Trav. Chemi. Pays-Bas 84,
1263 (1965); Davidson, J. M., and Triggs, C., Chem. Ind. (London)
457 (1966); J. Chem. Soc. A 1324 (1968).
10. Tsuji, J., “Palladium Reagents and Catalysts, Innovations in Organic
Synthesis,” Wiley, New York, 1997.
11. Iataaki, H., and Yoshimoto, H., J. Org. Chem. 38, 76 (1973).
The regioselectivity is also summarized in Table 4 for
these systems. The effect of the triflic acid is to favor the
selectivity to 3,40- and 4, 4-0DMBP (total = 85% ), whereas 12. Shiotani, A., Itatani, H., and Inagaki, T., J. Mol. Catal. 34, 57 (1986);
Shiotani, A., Yoshikiyo, M., and Itatani, H., J. Mol. Catal. 18, 23
the other systems using weaker acids as cocatalysts (HClO4
(1983).
and CF3COOH) show smaller yields of these two compo-
13. Sherman, S. C., Inetski, A. V., White, M. G., Kerwin, J. C., Schiraldi,
nents (66–75 and 52% , respectively). The systems in acetic
D. A., The oxidative couplingofmethylbenzoate, manuscript in prepa-
acid show a combined selectivity to 3,40- and 4,40-DMBP in
ration, 1998.
the range of 48–55% .
14. Yoshimoto, H., and Itatani, H., Bull. Chem. Soc. Jpn. 46, 2490 (1973);
U.S. Patents 3,895,055, 1975; 4,294,976, 1981.
15. Ronlan, A., Bechgaard, K., and Parker, V. D., Acta Chem. Scand. 27,
2375 (1973).
16. Ferrers, R. S. C., Norman, R. O. C., Thomas, C. B., and Wilson, J. S.,
JCS Perkin I 1289 (1974).
17. Gol’dshleger, N. F., and Moravskii, A. P., Russ. Chem. Rev. 62(2), 125
(1994).
18. Murata, S., and Ido, Y., Bull. Chem. Soc. Jpn. 67, 1746 (1994).
CONCLUSIONS
A direct coupling reaction of toluene for synthesis of
DMBP has been reported in the presence of a strong acid
and Pd(II) at room temperature. The presence of both
Pd(TfO)2 and TfOH is shown to be crucial for an effective 19. Xu, B.-Q., Sood, D., and White, M.G., in preparation.