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allenic Alder-ene reaction, ene-allene 75a was subjected to 10 mol% [Rh(CO)2Cl]2 in DCE;
reaction did not occur at rt. However, upon heating to 90 °C, the starting material was consumed
and tetrahydroazepine 189 was isolated as the sole product in 43% yield. It was determined that
the low isolated yield was due to decomposition of the product during purification. Minimal
exposure of the crude reaction mixture to silica gel resulted in 95% yield of 189 (Scheme 3.42).
The presence of an enamide olefin in 189 is characterized with two resonances in the 1 H NMR
spectrum (in CDCl3) at 5.03 ppm (dd, J = 8.1, 2.5 Hz) and 4.53 ppm (dd, J = 8.1, 4.1 Hz)
(Scheme 3.43). In addition, the exocyclic olefin displays a signal at 5.52 ppm (q, J = 6.8 Hz).
Scheme 3.42 Formation of tetrahydroazepine 189.
Bz
N
•
MeO2C Bn
10 mol %
[Rh(CO) 2Cl] 2
DCE
Bz
MeO 2C
N
Bn
75a 189
conditions: A. rt, no reaction.
B. 90 o C, 2.2 h, 95%.
MeO 2C
Scheme 3.43 Selected data from the 1 H NMR spectrum of 189 in CDCl3.
Bz
MeO 2C
H a
N
Bn
189
H b
H c
2
5
H a 5.03 (dd, J = 8.1, 2.5 Hz)
H b 4.53 (dd, J = 8.1, 4.1 Hz)
H c 5.52 (q, J = 6.8 Hz)
Bz
N
Bn
190 (not observed)
Furthermore, 189 was isolated from the reaction as mainly one stereoisomer l with respect to the
C2 and C5 relative stereochemistry, and E-geometry of the exocyclic olefin, which is predicted
based on a mechanistic hypothesis (see Scheme 3.45). m The relative stereochemistry of the C2
and C5 stereocenters was not determined.
l The 1 H NMR spectrum of the reaction mixture after passing through a short silica gel column displayed additional
peaks, which may be attributed to small amounts of a second isomer (estimated ~10%).
m Additional examples of azepines prepared by Dr. Hongfeng Chen were also obtained as the E isomer of the
exocyclic olefin: Brummond, K. M.; Chen, H.; Mitasev, B. Org. Lett. 2004, 6, 2161.
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