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DOI:10.15227/orgsyn.068.0206

Organic Syntheses, Coll. Vol. 8, p.31 (1993); Vol. 68, p.206 (1990).

IMINIUM ION-BASED DIELS–ALDER REACTIONS: N-BENZYL-2AZANORBORNENE[2-Azabicyclo[2.2.1]hept-5-ene, 2-(phenylmethyl)-]

Submitted by Paul A. Grieco and Scott D. Larsen1.
Checked by V. Ramamurthy and Bruce E. Smart.

1. Procedure
A 100-mL, round-bottomed flask equipped with a Teflon-coated magnetic stirring bar is charged
with 24 mL of deionized water and 8.6 g (60.0 mmol) of benzylamine hydrochloride (Note 1). To thishomogeneous solution is added 6.3 mL (84 mmol) of 37% aqueous formaldehyde solution (Note 2)
followed by 9.9 mL (120 mmol) of freshly prepared cyclopentadiene (Note 3). The flask is stoppered
tightly (Note 4) and stirred vigorously at ambient temperature. After 4 hr, the reaction mixture is poured
into 50 mL of water and washed with ether–hexane, 1 : 1 (2 × 40 mL). The aqueous phase is made basic
by the addition of 4.0 g of solid potassium hydroxide and extracted with ether (3 × 60 mL). The
combined ether extracts are dried over anhydrous magnesium sulfate and filtered. The solvent isremoved under reduced pressure (15–20 mm) to give 11.2 g (100%) of N-benzyl-2-azanorbornene as avery-pale-yellow oil (Note 5). The crude product is distilled at 80–85°C (0.05 mm) (Note 6) through ashort-path apparatus to provide 10.1–10.2 g (91–92%) of pure product (Note 7) as a colorless oil (Note8).

2. Notes
1. Benzylamine hydrochloride is commercially available from Aldrich Chemical Company, Inc.
2. Aqueous formaldehyde solution (37%) is commercially available from Mallinckrodt, Inc.
3. Cyclopentadiene is prepared by heating commercial dicyclopentadiene (available from Aldrich
Chemical Company, Inc.) at 160°C in a distillation apparatus,. Cyclopentadiene distills smoothly at 39–45°C.2
4. The heterogeneous reaction mixture is stoppered tightly to avoid loss of cyclopentadiene.
5. This crude material is essentially pure product contaminated by trace amounts of ether. N-Benzyl-2azanorbornene has the following spectrum: 1H NMR (300 MHz, CDCl3) δ: 1.42 (dm, 1 H, J = 8), 1.52
(dd, 1 H, J = 2, 8.5), 1.64 (dm, 1 H, J = 8), 2.94 (bs, 1 H), 3.18 (dd, 2 H, J = 3, 8.5), 3.34, 3.58 (AB, 2H, J = 13), 3.83 (m, 1 H), 6.09 (dd, 1 H, J = 2, 6), 6.38 (ddd, 1 H, J = 2, 3, 6), 7.2–7.4 (m, 5 H).
6. Attempted distillation at 15–20 mm resulted in extensive decomposition.
7. The submitters obtained 10.8 g (97%) of analytically pure product, bp 80–85°C (0.05 mm). Anal.
calcd. for C13H15N: C, 84.28; H, 8.16; N, 7.56. Found: C, 84.68; H, 8.36; N, 7.59.
8. On prolonged standing in air at room temperature discoloration of the product accompanied by slow
evolution of cyclopentadiene takes place.

3. Discussion
Simple unactivated iminium salts generated in situ from formaldehyde and primary alkyl amines
undergo a facile aza Diels–Alder reaction with cyclopentadiene at ambient temperature3 to afford novel
N-alkylated 2-azanorbornenes. The procedure described above is general and can be applied to anumber of primary alkyl amines. Yields of N-alkyl substituted 2-azanorbornenes are good to excellent. Use ofammonium chloride and formaldehyde in the above reaction produces 2azanorbornene in modest (40–50%) yield.
2-Azanorbornene (3) has been previously prepared by reaction of cyclopentadiene with chlorosulfonyl isocyanate, which
provides a single N-chlorosulfonyl βlactam (1). Exposure of 1 to an aqueous solution of sodium sulfite gives rise (25–30%)
to 2azanorbornen-3-one (2) which upon reduction with lithium aluminum hydride affords (ca. 80%) 2azanorbornene (3).
4References and Notes
1.
2.
3.
4.

Department of Chemistry, Indiana University, Bloomington, IN 47405.
Moffett, R. B. Org. Synth., Coll. Vol. IV, 1963, 238.
Larsen, S. D.; Grieco, P. A. J. Am. Chem. Soc. 1985, 107, 1768.
Malpass, J. R.; Tweddle, N. J. J. Chem. Soc., Perkin Trans. I, 1977, 874.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);(Registry Number)
ether (60-29-7)
ammonium chloride (12125-02-9)
sodium sulfite (7757-83-7)
formaldehyde (50-00-0)
potassium hydroxide (1310-58-3)
magnesium sulfate (7487-88-9)lithium aluminum hydride (16853-85-3)
hexane (110-54-3)
CYCLOPENTADIENE (542-92-7)
dicyclopentadiene (77-73-6)
CHLOROSULFONYL ISOCYANATE (1189-71-5)
2-Azabicyclo[2.2.1]hept-5-ene, 2-(phenylmethyl)-,N-Benzyl-2-azanorbornene (112375-05-0)

benzylamine hydrochloride (3287-99-8)
2-azanorbornene
2-azanorbornen-3-one
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