1676
P. Uriz et al. / Tetrahedron Letters 43 (2002) 1673–1676
Aprotic
Solvent
O
Acknowledgements
MeOH
R - N - C - OMe
H
OH
MM - K10
H
This work was supported by Repsol-YPF. The authors
are indebted to Juan Antonio Delgado for his contin-
ued interest and his invaluable contributions.
+
O
R - N - C - OMe
H
H
OH
O-
OMe
MM - K10
MM - K10
(a)
H
References
O
Filtration
R - N = C - OMe
RN=C=O
1. (a) Wirpsza, Z. Polyurethanes: Chemistry, Technology and
Applications; Ellis Horwood: London, 1993; (b) Verstee-
gen, R. M.; Sijbesma, R. P.; Maijer, E. W. Angew. Chem.,
Int. Ed. 1999, 38, 2917.
OH
H+
MM - K10
(b)
2. (a) Okuda, S., Japanese Patent, JP 57158746 1983; Chem.
Abstr. 1983, 98, 144386b; (b) Okuda, S., Japanese Patent
JP 57158747 1983; Chem. Abstr. 1983, 98, 160402j; (c)
Yagii, T., US Patent, 5789614 1998; (d) Gerhard, L., D.
E. Patent, 19541384 1997.
3. (a) Valli, V. L. K.; Alper, H. J. Org. Chem. 1995, 60, 257;
(b) Butler, D. C. D.; Alper, H. Chem. Commun. 1998,
2575.
Scheme 3.
In agreement with the above results, the increase of
Bro¨nsted acid centers of the clay can favor the dealco-
holysis. Taking into account how the number of Bro¨n-
sted acid centers affects the catalytic process, we
prepared a more acidic clay than montmorillonite K-10,
by acidification of montmorillonite K-10 with HNO3 to
provide H+-M with 7.7×10−4 meq H+/m2. However,
although the conversion of dicarbamate 1 on iso-
cyanates was still almost quantitative, the selectivity on
diisocyanate 3 and the isolated yield decreased proba-
bly because more of the product was retained on the
clay surface, (entry 4, Table 4). It therefore seems that
montmorillonite K-10 has the optimum number of
Bro¨nsted acid centers to perform the dealcoholysis of
dicarbamates efficiently.
4. Bal’on, Y. G. J. Org. Chem. USSR (Engl. Transl.) 1980,
16, 2233.
5. (a) Greber, G.; Kricheldorf, H. R. Angew. Chem., Int. Ed.
Engl. 1968, 7, 941; (b) Pirkle, W. H.; Hoekstra, M. S. J.
Org. Chem. 1974, 39, 3904; (c) Pirkle, W. H.; Hoekstra,
M. S. J. Org. Chem. 1977, 42, 2781; (d) Mironov, V. F.;
Kozyukov, V. P.; Orlov, G. I. J. Gen. Chem. USSR
(Engl. Transl.) 1981, 51, 1555; (e) Chong, P. Y.; Janicki,
S. Z.; Petillo, P. A. J. Org. Chem. 1998, 63, 8515.
6. Gastaldi, S.; Weinreb, S. M.; Stien, D. J. Org. Chem.
2000, 65, 3239.
7. (a) McKillop, A.; Young, D. W. Synthesis 1979, 401; (b)
McKillop, A.; Young, D. W. Synthesis 1979, 481.
8. Balogh, M.; Laszlo, P. Organic Chemistry Using Clays;
Springer-Verlag: Berlin, 1993.
9. Vaughau, D.; Lusier, R.; Magere, J., US Patent, 4175090,
1979.
10. Gonzalez, F.; Pesquera, C.; Benito, I.; Herrero, E.; Pon-
cio, C.; Casuscelli, S. Appl. Catal. A: General 1999, 181,
71.
11. (a) Cornelis, A.; Laszlo, P. Synlett 1994, 3, 155; (b)
Dewan, S. K.; Varma, U.; Malik, S. D. J. Chem. Res. (S)
1995, 21.
The catalytic cycle shown in Scheme 3 is a plausible
pathway for coupling this chemistry. Montmorillonite
K-10 may act as an acid catalyst if its Bro¨nsted acid
centers interact with the carbamate by protonation of
the carbonyl functional group of the substrate to give
the ionic intermediate (a), followed by the elimination
of the amidic proton (b) and subsequent formation of
the isocyanate. The reaction temperature allows the
elimination of MeOH from the clay, in order to regen-
erate the active surface.
12. Campanati, M.; Fazzini, F.; Fornasari, G.; Piccolo, O.;
Tagliani, A.; Vaccari, A. Catalysis of Organic Reactions;
Dekker: New York, 1998; p. 307.
13. Montmorillonite K-10 (Fluka) was used as received.
14. Othmer, K. Encyclopaedia of Chemical Technology, 4th
ed.; Wiley: New York, 1997; Vol. 24, p. 695.
15. Bentonite (Majorbenton B, AEB Iberica S. A.) was used
as received.
In summary, montmorillonte K-10 is an efficient cata-
lyst for synthesizing mono- and diisocyanates from
mono- and dicarbamates, removing the alcohol
efficiently since this is the driving force behind the
reaction. Also its low cost, easy accessibility and
reusability are additional advantages to take into con-
sideration for large-scale production.