1966
J. T. Lai / Tetrahedron Letters 43 (2002) 1965–1967
Scheme 3.
dropwise to keep the temperature below 20°C. After
the addition, the reaction is allowed to continue
overnight at 15°C. The reaction mixture is then
filtered, and the solid removed is rinsed with toluene.
The filtrate is washed with 50 ml water, dried over
magnesium sulfate and concentrated to yield the
crude product which is mainly imine, but can include
some unreacted aniline and ketone. Pure compound
can be obtained by distillation through a Vigraux
column.
In diimine 1i, the syn and anti isomers are present in
1
a 38:62 ratio based on H NMR.
Table 1 shows examples of selected hindered imines
made in this manner.
When R3 and R4 are different, the geometric syn and
anti isomers6 exhibit a ratio of about 15:85 from their
1H NMR spectra, with little influence from the size of
the R groups.
Acknowledgements
The author wishes to thank Miss Debby Filla for her
laboratory work, Dr. George Benedict for the 1H
Table 1.
R1
R2
R3
R4
Bp (°C/torr)
1H NMRa (l, CDCl3)
1.61 (s, 3H), 1.99 (s, 6H), 2.21 (s, 3H), 6.8–7.3 (m, 3H)
0.98 and 1.25 (2t, 18/82 ratio, 3H), 1.61 and 2.20 (2s, 82/18, 3H) 91 (64)
1.98 (s, 6H), 2.16 and 2.47 (2q, 18/82, 2H), 6.8–7.1 (m, 3H)
Yieldb (%)
1a
1b
Me
Me
Me
Me
Me
Me
Me
Et
85–8/5
84–7/1
\95 (77)
1c
1d
Me
Me
Me
Me
Me
Me
-(CH2)5-
i-Bu
118–23/7.5
–
110–2/1
128–31/0.9
1.5–2.0 (m, 8H), 2.00 (s, 6H), 2.54 (t, 2H), 6.8–7.0 (m, 3H)
0.81 and 1.05 (2d, 13/87, 6H), 1.60 and 2.20 (2s, 13/87, 3H), 2.00 62
(s, 6H), 1.9–2.4 (m, 3H), 6.8–7.0 (m, 3H)
0.83 and 0.91 (2t, 15/85, 3H), 1.1–2.3 (m, 8H), 1.62 and 2.21 (2s, 43 (20)
85/15, 3H), 1.91 and 2.45 (2t, 15/85, 2H), 1.99 (s, 6H), 6.8–7.0
(m, 3H)
\95 (79)
Me
Me
Me
1dc Me
i-Bu
n-Hex
73 (43)
1e
Me
1f
Me
Et
i-Pr
-(CH2)5-
Et
137–43/1
120–5/6
1.12 (d, 3H), 1.66 (d, 3H), 1.5–2.0 (m, 8H), 2.55 (t, 3H), 2.55
(hept, 1H), 6.9–7.1 (m, 3H)
1.00 and 1.26 (2t, 15/85, 3H), 1.13 (t, 6H), 1.64 and 2.22 (2s,
85/15, 3H), 2.00 and 2.48 (2q, 15/85, 2H), 2.2–2.3 (m, 4H),
6.9–7.1 (m, 3H)
\95 (81)
1g
Et
Me
83 ( 67)
1h
1i
Et
Et
-(CH2)5-
140–5/1
1.14 (t, 6H), 1.5–2.6 (m, 14H), 6.9–7.1 (m, 3H)
\95 (76)
(155–60)d
1.58–1.60 (2s, 38/62, 6H), 1.89 (s, 12H), 2.23 (s, 6H)
95 (40)
a Bruker WH-200 spectrometer.
b Yield as measured by GC, with the isolated yield in parenthesis. The large differences between the two yields in 1b, 1d and 1e are due to the
close proximity between the boiling points of the aniline and the imine, thus it is difficult separation. Correct mass ion from chemical ionization
mass spec by a Finnigan MAT spectrometer: 0.0009; infrared spectra of neat or KBr pellet all show peaks at 1665–1775 cm−1
c 0.4 mol of MIBK is used instead of 0.2 mol of ketone used for the rest of the syntheses.
d Melting point, after recrystallization from hexanes.
.