Iodine-Nitrogen Tetroxide

I2-N2O4
(I2)

[7553-56-2]  · I2  · Iodine-Nitrogen Tetroxide  · (MW 253.80) (N2O4)

[10544-72-6]  · N2O4  · Iodine-Nitrogen Tetroxide  · (MW 92.02)

(adds to carbon-carbon multiple bonds to form the corresponding b-iodo-a-nitro compounds;2 useful precursors for various nitro derivatives;3,4 experimental variation with I2/AgNO2;5,6 radical intermediates;7 provides good chemo- and regioselectivity)

Alternate Names: iodine-nitrogen dioxide; iodine-dinitrogen tetroxide.

Physical Data: N2O4: mp -11.2 °C; bp 21.2 °C; d 1.449 g cm-3; colorless substance; brown color due to NO2 (nitrogen dioxide) in equilibrium: 0.1% (bp), 20% (27 °C), 40% (50 °C).8 I2: see Iodine.

Solubility: N2O4: decomposes in H2O; sol alcohol, chloroform, CS2.

Form Supplied in: N2O4: brown liquid/gas supplied in pressure cylinders.

Purification: if necessary, N2O4 can be separated from lower nitrogen oxides and nitric acid by fractional distillation under dry oxygen. Gaseous N2O4 can be purified by passing through silica and P4O10; purified N2O4 can be resublimed at -70 °C and stored at rt in pressure vessels made of glass or V2A steel.1,8

Handling, Storage, and Precautions: N2O4 is highly corrosive and very toxic when inhaled;8b it is sensitive to moisture and decomposes with water or above 150 °C. This reagent must be handled in a well-ventilated fume hood.

General Discussion.

Nitrogen Dioxide derived from N2O4 adds to carbon-carbon multiple bonds to yield, in the presence of Iodine, b-iodo-a-nitroalkanes and -alkenes, both versatile precursors of various nitrogen-containing products. The reaction of I2/N2O4 with isolated double bonds gives the corresponding base-sensitive b-iodo-a-nitroalkanes, usually in good yield. The transformation is conveniently carried out at 0 °C or rt in an organic solvent like ether or dichloromethane.9 Reactions with unsymmetrically substituted alkenes lead to the iodonitroalkane with the nitro group at the less-substituted position (eq 1).2 Both trans- and cis-stilbene are stated to furnish the same 1-iodo-2-nitro-1,2-diphenylethane (eq 2), proposedly of erythro configuration.2

Analogous results are obtained using I2/AgNO25,6,9 or I2/NaNO2;10 these reagent combinations generate the same active species. Thermodynamic data on the equilibrium 2 O2N&bdot; + Hal2 &ibond; 2 O2N-Hal confirm that nitryl halides NO2X with X = F, Cl, Br do exist, with decreasing content in the equilibrium, but rule out the presence of detectable amounts of O2N-I (nitryl iodide).11,12

In the addition of NO2/I to C=C moieties, many other functional groups such as carboxy, acetal, ester (eq 3), or trialkylsilyl13-15 are tolerated, as is some steric hindrance, cf. the cases of camphene2 or 2-cholestene.9 In general, best results are obtained with alkyl- or aryl-substituted alkenes, but carboxy- or halo-substituted12 alkenes (including difluoroethylene16) also add I/NO2 well.

Mechanistic considerations based on product analyses lead to the conclusion that the overall addition starts with the (probably reversible) attack of NO2&bdot; on the double bond, forming the more stable radical intermediate.2,9 In the presence of iodine (or bromine or chlorine, if these do not react faster alone),12,17 the nitroalkyl radical is trapped to form the b-halo-a-nitroalkane. This mechanism accounts for both the reactivities and regioselectivities observed; in the case of electron-rich double bonds an ionic mechanism may compete, as shown by the formation of a some b-iodoalkyl nitrate.9,17 Usually, however, side reactions like dihalogenation or formation of b-iodoalkyl nitrates are not important and become significant only if the reaction is carried out inversely or without solvent (eq 4).1a,17,18

Additions of NO2/I to conjugated enynes and dienes have also been reported, but without experimental details.19 While the reaction of 1,3-enynes with I2/N2O4 proceeds selectively at the double bond (in a 3,4-mode), 1-iodo-4-nitro-2-alkenes are produced from 1,3-dienes.19 With nonconjugated dienes, selective monoaddition has been observed.4

Further transformations of iodonitro addition products (often prone to decompose on standing) are sometimes effected without isolation, and include the elimination of HI to yield the respective nitroalkene6 and reduction by Sodium Borohydride to give the corresponding nitroalkane.4 While additions with nitryl chloride- or bromide-generating reagents give similar or sometimes even better yields of adducts,1,12 the further elaboration of the adducts proceeds best with the iodo derivatives.

The reaction of I2/N2O4 with alkynes similarly leads to monoadducts in high yield,2,3 with analogous reactivity and regioselectivity. Due to steric requirements in the vinyl radical intermediates, iodonitroalkene products of varying (E/Z) ratios are obtained (eq 5).1c,2,3,15,20 Subsequent base-induced HI elimination from these b-iodonitroalkenes provides a convenient way to obtain nitroalkynes.3


1. (a) Padeken, H. G.; von Schickh, O.; Segnitz, A. MOC 1971, 10/1, 86. (b) Behnisch, R. MOC 1992, E16d/1, 167. (c) Jäger, V; Viehe, H. G. MOC 1977, 5/2a, 109, 756.
2. Stevens, T. E.; Emmons, W. D. JACS 1958, 80, 338.
3. Jäger, V.; Viehe, H. G. AG(E) 1969, 8, 273; cf. Jäger, V. Dissertation, University of Erlangen, 1970.
4. Jäger, V.; Günther, H. J. AG(E) 1977, 16, 246; cf. Günther, H. J. Dissertation, University of Giessen, 1978.
5. Birckenbach, L.; Goubeau, J.; Berninger, E. CB 1932, 65, 1339.
6. Sy, W. W.; By, A. W. TL 1985, 26, 1193.
7. McMurry, J. E.; Musser, J. H. OS 1977, 56, 65.
8. (a) Hollemann, A. F.; Wiberg, N. Lehrbuch der Anorganischen Chemie, 91st ed.; de Gruyter: Berlin, 1985; p 584. (b) The Merck Index, 11th ed.; Merck: Rahway, NJ, 1989; p 6528.
9. Hassner, A.; Kropp, J. E.; Kent, G. J. JOC 1969, 34, 2628.
10. Jew, S. S.; Kim, H. D.; Cho, Y. S.; Cook, C. H. CL 1986, 1747.
11. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; Wiley: New York, 1980; p 434.
12. Bachman, G. B.; Logan, T. J.; Hill, K. R.; Standish, N. W. JOC 1960, 25, 1312.
13. Burrows, B. F.; Turner, W. B. JCS(C) 1966, 255.
14. (a) Szarek, W. A.; Lance, D. G.; Beach, R. L. CC 1968, 356. (b) Szarek, W. A.; Lance, D. G.; Beach, R. L. Carbohydr. Res. 1970, 13, 75.
15. Rall, R. B.; Vil'davskaya, A. I.; Petrov, A. A. RCR 1975, 44, 373.
16. Hauptschein, M.; Oesterling, R. E.; Braid, M.; Tyczkowski, E. A.; Gardner, D. M. JOC 1963, 28, 1281.
17. Bachman, G. B.; Logan, T. J. JOC 1956, 21, 1467.
18. Cf. iodination of alkylbenzenes with I2/AgNO2; Sy, W. W.; Lodge, B. A. TL 1989, 30, 3769.
19. (a) Vil'davskaya, A. I.; Rall, K. B.; Petrov, A. A. ZOB 1964, 34, 3513. (b) Vil'davskaya, A. I.; Rall, K. B.; Petrov, A. A. JOU 1965, 1, 229. (c) Vil'davskaya, A. I.; Rall, K. B. JOU 1968, 4, 931.
20. Most earlier results on product distribution and configurational assignments should be taken with caution, if crude product and NMR analyses are lacking; cf. Ref. 3 and Verbruggen, R. Dissertation, University of Louvain, 1974.

Volker Jäger & Martin Kleban

Universität Stuttgart, Germany



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