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Advances in Heterocylic Chemestry. Vol 10 - Boulton A.J.

Boulton A.J. Advances in Heterocylic Chemestry. Vol 10 - Academic Press, 1969. - 347 p.
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27 A. J. Boulton, P. B. Ghosh, and A. R. Katritzky, J. Chem. Soc. C, 971 (1966).
28 R. K. Harris, A. R. Katritzky, and S. 0ksne, Chem,. Ind. (London) p. 990 (1961).
29 G. Englert, Z. Naturforsch. 16b, 413 (1961).
30 B. Dischler and G. Englert, Z. Naturforsch. 16a, 1180 (1961).
31 G. Englert, Z. Anal. Chem. 181, 447 (1961).
made.'3 Similar results for 4,7-dibromobenzofuroxan and 5-methyl-benzofuroxan have been reported by Englert,29'82 who provides a review of the NMR spectra of benzofuroxans up to mid-1961. Other compounds whose spectra have been analyzed at various temperatures include 5-chloro-,33 5-methoxy-,33 4- and 5-nitro-,18 4,6-13 and
5,6-13 dinitro-, 5-methyl-6-nitro-,33 5-ethoxycarbonyl,33 5-carboxy,33 5-acetamido-,33 5,6- and 4,7-dichloro-,14 and 4,7-dibromobenzo-furoxans.14, 29,82 5-Nitrobenzofuroxan, to take a typical example, at
— 31° shows spectra for the two distinct species present in solution; assignment of each spectrum to its tautomeric species was made on the basis of the chemical shifts of the protons, and it was thereby shown that the 6-nitro structure predominates to the extent of about 70% in the mixture of 5- and 6'-nitro structures at that temperature (see Fig. 2). 4-Nitrobenzofuroxan shows no trace of the 7-nitro compound at low temperatures (it would seem unlikely that the compound exists exclusively as the 7-nitro isomer—an alternative possible explanation of the spectra).13 Pyrido[2,3-cjfuroxan is nearly all in the
4-aza structure (11) with only ca. 7% 7-aza (12) at — 50°.34
Some workers in this field have used Eyring’s equation, relating first-order reaction rates to the activation energy A G*, whereas others have used the Arrhenius parameter EA. The results obtained are quite consistent with each other (cf. ref. 33); in all the substituted compounds listed above, AG* is about 14 kcal/mole (for the 4,7-dibromo compound an EA value of 6 + 2 kcal/mole has been reported,2 9 but this appears to be erroneous14). A correlation of EA values with size of substituents in the 4- and 7-positions has been suggested.14 AS* values (derived from the Arrhenius preexponential factor) are
32 G. Englert, Z. Elektrochsm. 65, 854 (1961).
33 A. J. Boulton, A. R. Katritzky, M. J. Sewell, and B. Wallis, J. Chem. Soc..
B, 914 (1967).
34 A. C. Gripper Gray, Ph.D. Thesis, University of East Anglia, 1966.
E. Physical Pboferties
Benzofuroxan is a very pale yellow crystalline solid, of melting point 72°. Its dipole moment is given by Tappi as 5.29 D, and the moments of eight other benzofuroxans, their molar refractivities, and melting points, are also recorded.47 It is appreciably steam-volatile, but far less so than its deoxygenated analog benzofurazan. Melting points of benzofuroxans are listed by Kaufman and Picard,1 and in Section X of this article.
The physical properties associated with the parachor, viz., the liquid surface tension and the density at various temperatures, for benzofuroxan and 5-methylbenzofuroxan are given by Hammick et al.5 The parachor values were reevaluated by Boyer et al.18
IV. Preparation of Benzofuroxans
Benzofuroxan may be obtained by oxidation of o-quinone dioxime.48 The first benzofuroxan derivative, 1,2-naphthofuroxan, was obtained by this method.8, 7 Suitable oxidizing agents include alkaline ferri-cyanide,6,7’48 bromine water,7 chlorine,49 and nitric acid.48-50 The method is of practical value only when the o-quinone or its monooxime (o-nitrosophenol) is readily available, and since this is not generally the case, other routes, e.g., the oxidation of o-nitroanilines51 and the thermal decomposition of o-nitrophenyl azides,52 are more commonly used.
The oxidation of o-nitroanilines to benzofuroxans was discovered by Green and Rowe,8,63 who used alkaline hypochlorite. Although this method has been used extensively,19,23, 54-57 it occasionally fails to
47 M. Milonc and G. Tappi, Atti 10th Oongr. Intern. Chim., Rome, 2, 352 (1939);
G. Tappi, Gazz. Chim. Ital. 71, 111 (1941).
48 T. Ziucke and P. Schwarz, Ann. Chem. 307, 28 (1899).
49 J. H. Boyer and G. Mamikunian, J. Org. Chem. 23, 1807 (1958).
50 S. V. Bogdanov and Â. I. Karavaev, Zh. Obshch. Khim. 23, 1757 (1953);
Chem. Abstr. 48, 13657 (1954).
51 F. B. Mallory, Org. Syn. 37, 1 (1957).
52 P. A. S. Smith and J. H. Boyer, Org. Syn. 31, 14 (1951).
53 A. G. Green and F. M. Rowe, J. Chem. Soc. 101, 2443 (1912).
54 F. M. Rowe and J. S. H. Davies, J. Chem. Soc. 117, 1344 (1920).
55 F. M. Rowe, S. II. Bannister, and R. C. Storey, ,J. Soc,. Chem. Ind. (London)
50, 79 (1931); Chem. Abstr. 25, 2424 (1931).
56 A. G. Green and F. M. Rowe, J. Chem. Soc. 103, 2023 (1913).
57 G. Tappi and P. V. Forni, Ann. Chim. Appl. 39, 338 (1949); Chem. Abstr.
46, 2540 (1952).
give the expected product. On reaction of aqueous methanolic sodium hypochlorite with 2,4-dinitroaniline, for example, the product is 5-chloro-4-methoxybenzofuroxan, rather than the 5-nitro compound38 (see Section VII,B). No benzofuroxan was obtained from
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