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Org. Synth. 1983, 61, 14
DOI: 10.15227/orgsyn.061.0014
OXIDATION OF 5-AMINOTETRAZOLES: BENZYL ISOCYANIDE
[Benzene, isocyanomethyl]
Submitted by Gerhard Höfle and Bernd Lange1.
Checked by Orville L. Chapman and Thomas C. Hess.
1. Procedure
Caution! This preparation should be conducted in an efficient hood because of the obnoxious odor of the isocyanide.
A. 5-Benzylaminotetrazole. Freshly distilled benzaldehyde (21.2 g, 0.2 mol) is added in one portion to a warm (50°C) solution of 5-aminotetrazole (17.2 g, 0.2 mol) (Note 1) and triethylamine (20.2 g, 0.2 mol) in 100 mL of absolute methanol. After 15 min the reaction mixture is cooled to room temperature, transferred to an autoclave, and hydrogenated with agitation at room temperature over Pd (10%) on carbon (1 g) for 18 hr at 500 psi (pounds per square inch) of hydrogen. The catalyst is removed by filtration and all volatile material is removed at 60°C under aspirator pressure. The gummy tan solid is triturated with 250 mL of hot water. Aqueous 20% HCl is added until pH 3 is reached. The mixture is cooled to room temperature and the solid collected, washed with water, and dried over-night at room temperature under reduced pressure (100 μ); yield: 27.5 g (80%), mp 183.5–185°C (lit.2 mp 183°C).
B. Benzyl isocyanide. In a 500-mL, round-bottomed flask equipped with a magnetic stirring bar and a pressure-equalizing funnel are placed 5-benzylaminotetrazole (10.5 g, 60 mmol), 100 mL of 10% sodium hydroxide solution, and 70 mL of dichloromethane. The mixture is cooled to 0°C and a solution of NaOBr in water (165 mL, 65 mmol) (Note 2) is added with vigorous stirring over a 15-min period (Note 3). The dichloromethane layer is separated and the aqueous phase extracted with five 50-mL portions of dichloromethane. The combined dichloromethane extracts are dried over anhydrous MgSO4, the drying agent is removed by filtration, and the dichloromethane is removed by simple distillation. The pressure is then reduced to ~20 mm with an aspirator and benzyl isocyanide is distilled at 98–100°C; yield: 5.91 g (84%) (Note 4) and (Note 5).
2. Notes
1. 5-Aminotetrazole monohydrate is available from Aldrich Chemical Company, Inc.; it was dehydrated by heating over P2O5 at 100°C under reduced pressure (100 μ) for 4 hr.
2. The NaOBr solution was prepared according to a procedure described in Organic Syntheses.3 Bromine [12.6 g (4 mL, 79 mmol)] was added dropwise with vigorous stirring to 150 mL of a 10% NaOH solution at −10°C. Enough 10% NaOH solution was added to the yellow solution to give 200 mL of reagent.
3. During addition of the NaOBr solution the mixture warms to 20°C. The reaction is virtually instantaneous and can be monitored by the liberated nitrogen.
4. The product was pure by IR and NMR spectroscopy. The IR spectrum showed a very strong band at 2150 cm−1, the NMR spectrum a distorted triplet at δ 4.5 (2 H) and a broad singlet at δ 7.3 (5 H).
5. Glassware can be freed from the odor of isocyanide by rinsing with a 1 : 10 mixture of concentrated hydrochloric acid and methanol.
3. Discussion
By this method high yields of isocyanides are obtained by an oxidation process. Since this oxidation can also be performed anodically or with bromine or lead tetraacetate and triethylamine in the absence of water (see Table I),4 it represents a valuable alternative to other procedures: dehydration reactions,5,6,7 the alkylation of silver cyanide8,9 or the carbylamine (isocyanide) reaction.10 The starting materials, 5-aminotetrazoles, can be readily obtained by reductive alkylation of 5-aminotetrazole2 or from monosubstituted thioureas and sodium azide.11 A limitation of the reaction is that the substituent R must be stable toward oxidation. From a mechanistic point of view the oxidation of 5-aminotetrazoles is a two step process with a pentaazafulvene as an unstable, undetectable intermediate.
TABLE I
PREPARATION OF ISOCYANIDES (R-N=C) BY OXIDATION OF 5-AMINOTETRAZOLES

Yield (%)

R

NaOBra

Pb(OAc)4/NEtb3

Br2/Netb3

Anodic Oxidationa


C6H5

92

70

43

39

C4H9

75

C6H5CH2

84

48


a In 2N sodium hydroxide solution.

b In dichloromethane.

Benzyl isocyanide is a useful precursor of compounds containing the α-benzylamino moiety. Substituted styrenes, vinyl isocyanides, 2-oxazolines, 1-pyrrolines, imidazoles, and α-amino acids and ketones can be obtained by metalation of isocyanides with butyllithium12 or copper salts,13 and subsequent reaction with various electrophiles.12

References and Notes
  1. Institut für Organische Chemie, Technische Universität Berlin, D-1000 Berlin 12, Strasse des 17, Juni 135. This work was supported by the Deutsche Forschungsgemeinschaft.
  2. Henry, R. H.; Finnegan, W. G. J. Am. Chem. Soc. 1954, 76, 926–928. The present procedure represents a modification of the procedure described herein.
  3. Allen, C. F. H.; Wolf, C. N. Org. Synth., Coll. Vol. IV 1963, 45.
  4. Höfle, G.; Lange, B. Angew. Chem. 1976, 88, 89; Angew. Chem., Int. Ed. Engl. 1976, 15, 113–114.
  5. Ugi, I.; Meyr, R.; Lipinski, M.; Bodesheim, F.; Rosendahl, F. Org. Synth., Coll. Vol. V 1973, 300; Ugi, I.; Meyr, R. Org. Synth., Coll. Vol. V 1973, 1068.
  6. Niznik, G. E.; Morrison, W. H., III, Walborsky, H. M. Org. Synth., Coll. Vol. VI 1988, 751.
  7. Schuster, R. E.; Scott, J. E.; Casanova, J., Jr. Org. Synth., Coll. Vol. V 1973, 772.
  8. Jackson, H. L.; McKusick, B. C. Org. Synth., Coll. Vol. IV 1963, 438.
  9. Engemyr, L. B.; Martinsen, A.; Songstad, J. Acta. Chem. Scand., Ser. A 1974, 28, 255–266.
  10. Gokel, G. W.; Widera, R. P.; Weber, W. P. Org. Synth., Coll. Vol. VI 1988, 232.
  11. Stolle, R. J. Prakt. Chem. 1982, 134, 282–309; Garbrecht, W. L.; Herbst, R. M. J. Org. Chem. 1953, 18, 1269–1282.
  12. Hoppe, D. Angew. Chem., Int. Ed. Engl. 1974, 13, 789–804.
  13. Saegusa, T.; Ito, Y.; Kinoshita, H.; Tomita, S. J. Org. Chem. 1971, 36, 3316–3323.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

Pd

hydrochloric acid,
HCl (7647-01-0)

methanol (67-56-1)

hydrogen (1333-74-0)

sodium hydroxide,
NaOH (1310-73-2)

bromine (7726-95-6)

nitrogen (7727-37-9)

benzaldehyde (100-52-7)

carbon (7782-42-5)

Benzyl isocyanide,
Benzene, isocyanomethyl (10340-91-7)

sodium azide (26628-22-8)

dichloromethane (75-09-2)

NaOBr

magnesium sulfate (7487-88-9)

butyllithium (109-72-8)

silver cyanide (506-64-9)

triethylamine (121-44-8)

5-aminotetrazole (4418-61-5)

5-Benzylaminotetrazole (14832-58-7)

5-Aminotetrazole monohydrate

pentaazafulvene

phosphorus pentoxide (1314-56-3)

lead tetraacetate (546-67-8)