J . Org. Chern., Vo1. 38, No. 4,1975 675
REACTIONS OF AZIDESWITH ISOCYANATES
Reactions of Azides with Isocyanates.
Cycloadditions and Cyeloreversions
J-M. VANDENSAVEL, G. SMETS,*AND G. L'ABBI$* Laboratory of Macromolecular and Organic Chemistry, University of Louvain, B-9050 Heverlee, Belgium Received September 18, 1Q7g
Reactions of alkyl azides with aryl isocyanates, acyl isocyanates, carboalkoxy isocyanates, and sulfonyl isocyanates, as well as reactions of aryl azides with sulfonyl isocyanates, provide a convenient method for the synthesis of 1,Cdisubstituted A2-tetraxolin-5-ones (5, 6 , and 9). l-Alkyl-4-sulfonyl-A2-tetrasolin-5-ones (sa-e) undergo cycloreversion upon thermolysis. The forward and reverse reactions are accelerated by the introduction of electron-withdrawing groups on the isocyanate moiety. The kinetics of the cycloreversion reaction of selected examples were studied; energies of activation and heats of reaction were determined.
Only a few reactions of azides with isocyanates have been reported. The lack of interest in this topic probably originates from the recognition that alkyl and aryl isocyanates can be prepared from acyl azides by the well-known Curtius rearrangement, and, hence, do not react with their precursors. Photochemically, however, aroyl azides yield aroylnitrenes,2 which were shown by Lwowski3 to react with ethyl isocyanate to give 2-aryl-4-ethyl-l,3,4-oxadiazolin-5-ones (1). TherEtN=C=O
+
-
p.XC,H,CON,
-Nz
EtN-C=O I
I
N,\
/o
C
I
P-C~HJ 1
molysis and photolysis of azidoformates in the presence of alkyl isocyanates also led to oxadiazolinones, but the major products in this case were 1-carboalkoxy-2,4dialkylurazoles (2). The intermediacy of l-carboalkoxy-4-alkyl-A2-tetrazolin-&ones (3) in this reaction might be postulated but is questionable. ZEtN=C-0
+
EtN-C=O
Et02CN3
I
-Nz
I
EtO&N\C,NEt
tion of aluminum azide with aryl isocyanates in tetrah y d r ~ f u r a n . ~Sodium azide failed to react with isocyanates, while hydrazoic acid reacted in a different manner to give carbamoyl a ~ i d e s . ~ 1,4-Disubstituted A2-tetrazolin-5-ones, though synthesized on several occasions,G were until now not accessible by a general and direct method. We now wish to report that several 1,4-disubstituted A2-tetrazolina0 a 82 120-124.5 55 25 OCe& 56-58 100 11 73 C~HG CeHs 100 7 82 9&99 8 75 113-125 ClCHz 55 C1,C 25 6 >80 a OCsHs. 55 25 87 111-113 solids which hydrolyze in contact with atmo-
the results and also illustrates the rate enhancement by the introduction of electron-withdrawing substituents on the isocyanate. The structures 6a-n were ascertained by spectral analyses. Worth mentioning are the typical ir absorptions a t 1770-1810 and at 1720-1730 cm-l corresponding respectively to the side chain C=Os and ring C=O stretching vibrations. To test thermal stability, one selected example (6n) was heated in nitrobenzene at 150" for 3 days and recovered in almost quantitative yield. The ir spectrum of the crude product, however, showed the presence of (7) R . B. Woodward and R . Hoffman, Anpew. Chem., 81, 797 (1969); Anpew Chem., Int. Ed. Enpl., 8 , 781 (1969). Photochemically induced elimination of nitrogen from 1,4-dimethyl-A~-tetraoolin-5-one bas been reported; Bee ref 6d. (8) H. A. Staab, Anpew. Chem., 74, 407 (1962); Anpew. Chem., Int. Ed. E'npl., 1, 351 (1962).
reaction of 1,5-diazidopentane with trichloroacetyl isocyanate and subsequent methanolysis of the crude mixture. Compound 8 was isolated in 65% overall yield. The existence of 7 and 8 in the keto form rather than in the tautomeric hydroxytetrazole form has been established previou~ly.~ Additions to Sulfonyl Isocyanates.-Sulfonyl isocyanates were found to react readily with alkyl and aryl azides to give 1-alkyl (or aryl) 4-sulfonyl-A2-tetrazolin5-ones (9) in good yields (see Table 111). BistetrazosBa
RN,
+
R'SO,N=C=O
t9
--+
N=N, R i \C/ NS02R'
II
0 9
linones were also prepared by this method from diisocyanates or diazides. Thus, 9 j was obtained from m-phenylenedisulfonyl isocyanate and n-butyl azide ( 5 5 O , 3 days), and 9k resulted from the interaction of p-tolylsulfonyl isocyanate and l,5-diazidopentane (55" 6 days).
E"=y n-C4H9N C ''
N=N\ NS02C,H,SOzN
