Scheme 1
a = 16.1420(3), b = 18.9857(4) and c = 11.1379(2) A, a =
b = g = 901, U = 3413.40(11) A3, T = 120(2) K, space group
Pna21, Z = 8, m = 0.069 mmꢀ1, 51849 reflections measured,
6697 unique (Rint = 0.079), final R indices [I 4 2s(I)]: R1 =
0.0609, wR2 = 0.1277; R indices (all data): R1 = 0.1577,
wR2 = 0.1658.z
with dichloromethane. Purification of the products by column
chromatography over silica gel (100–200 mesh) using a mixture
(1 : 99) of ethyl acetate and hexane as the eluent gave MCBE
(30%) and MCB (30%).
MCBE: Mp 107–108 1C. IR nmax (KBr): 2965, 2225, 1584,
1506, 1248, 1175, 1020, 963, 818 cmꢀ1 UV (toluene): lmax 360
nm (e = 4842 Mꢀ1cmꢀ1); 1H NMR (CDCl3, 300 MHz): d 1.04
(t, 3H, CH3), 2.64–2.71 (q, 2H, CH2), 3.77 (s, 3H, OCH3), 6.52
(d, 1H, olefinic, J = 11 Hz), 6.63 (d, 1H, olefinic, J = 15 Hz),
6.83 (d, 2H, aromatic, J = 8 Hz), 6.92–7.01 (dd, 1H, olefinic,
J = 15 Hz), 7.35 (d, 2H, aromatic, J = 8.7 Hz), 7.47 (d, 2H,
Syntheses
Preparation of the phosphonate ester of p-cyanobenzylbromide
(3). A mixture of p-cyanobenzyl bromide (100 mg, 0.5 mmol)
and triethyl phosphite (1 mmol) was heated at 100 1C for 24
h.23 After cooling, the unreacted triethyl phosphite was dis-
tilled out under reduced pressure, leaving behind the phospho-
nate ester of p-cyanobenzylbromide 3, which was obtained as a
viscous liquid. IR (KBr): nmax 2990, 2913, 2227, 1615, 1512,
aromatic, J = 8.5 Hz), 7.55 (d, 2H, aromatic, J = 8.3 Hz); 13
C
NMR (CDCl3, 75 MHz): d 14.06, 22.85, 55.34, 110.96, 114.14,
119.18, 122.77, 126.41, 127.88, 129.92, 130.07, 132.18, 134.92,
140.27, 146.71. MS (EI): m/z 289 (M+, C20H19NO).
1
1445, 1393, 1249, 1166, 1043, 971, 863, 785 cmꢀ1; H NMR
Results and discussion
(CDCl3, 300 MHz): d 1.25 (t, 6H, CH3), 3.20 (d, 2H, CH2), 4.10
[q, 4H, (CH2)2], 7.43 (d, 2H, aromatic), 7.61 (d, 2H, aromatic).
13C NMR (CDCl3, 75 MHz): d 16.1, 20.59, 32.93, 34.75, 62.25,
110.64, 118.45, 130.36, 132.01, and 137.45.
Syntheses, structure and formation mechanism
Alkoxy-cyano-substituted diphenylbutadienes have been re-
ported to possess liquid crystalline phases and our initial
interest was to investigate photoinduced isothermal phase
transitions in these materials.24 These derivatives were synthe-
sized by base-catalyzed condensation of the phosphonate of
p-cyanobenzyl bromide with p-alkoxycinnamaldehydes. These
materials also exhibited strong solid state fluorescence.9
We were interested in 1-(p-N,N-dimethylaminophenyl)-4-
(p-cyanophenyl)buta-1E,3E-diene (ACB), to study the effect
of donor strength on the solid state fluorescence of these
chromophores. Braatz et al. have studied the solution phase
fluorescence and photoisomerization properties of ACB.25 For
the preparation of ACB, we used the HWE reaction, with
triethyl phosphite as the phosphonating reagent (Scheme 1).
Interestingly, apart from the expected product, ACB, the
reaction also yielded 1-(p-N,N-dimethylaminophenyl)-4-
(p-cyanophenyl)-4-(ethyl)buta-1E,3E-diene (ACBE). The
structure of ACBE was unequivocally established on the basis
of spectroscopic techniques and X-ray crystallographic
analysis (Fig. 1).
Kucerovy et al. have observed a similar product formed
during the synthesis of 5-[2-(2,5-dimethoxyphenyl)ethyl]-2-hy-
droxybenzoate.26 To carry out an HWE reaction they used
dimethyl phosphite as the phosphonating reagent and obtained
trace amounts of an unexpected product bearing a methyl
substitution on the unsaturated bond. The intermediate methyl
phosphonate has been indicated to be the methylating agent.
The formation of ACBE may be rationalized in terms of the
pathways shown in Scheme 2. The initial step, an Arbuzov
reaction, involves the formation of the phosphonate 3, which
was isolated from the reaction mixture by column chromato-
graphy. The structure of 3 was confirmed using 1H NMR,
which clearly showed the presence of a two-proton doublet at d
3.20 that corresponds to the benzylic protons. The phospho-
nate carbanion of 3, formed in the presence of NaH, reacts
with p-(N,N-dimethylamino)cinnamaldehyde, leading to the
formation of ACB. The formation of ACBE will require an
ethyl group at the benzylic carbon of the phosphonate carba-
nion. The only source of an ethyl group in the reaction mixture
is triethyl phosphite. A probable route to the formation of
ACBE involves the formation of the phosphonate 5 via an
Preparation of 1-(p-N,N-dimethylaminophenyl)-4-(p-cyano-
phenyl)-4-(ethyl)buta-1E,3E-diene (ACBE). To the phospho-
nate ester 3, NaH (61 mg, 2.5 mmol) was added, followed by
dropwise addition of dry THF (10 mL) with efficient stirring.
After about 10 min, a THF solution (10 mL) of p-N,
N-dimethylaminocinnamaldehyde (89 mg, 0.5 mmol) was
added dropwise and the resultant mixture was refluxed for 24
h under an argon atmosphere. The solvent was removed under
reduced pressure and the residue was washed with water. The
crude products were extracted using dichloromethane. Purifi-
cation of the products by column chromatography over silica
gel (100–200 mesh) using a mixture (1 : 19) of ethyl acetate and
hexane as the eluent gave ACB (36%) and ACBE (4%).
ACBE: Mp 142–143 1C; IR (KBr): nmax 3744, 3036, 2921,
2355, 2213, 1573, 1512, 1357, 1175, 960, 811 cmꢀ1 UV (to-
luene): lmax 396 nm (e = 87 336 Mꢀ1cmꢀ1); 1H NMR (CDCl3,
300 MHz): d 1.10 (t, 3H, CH3), 2.69–2.77 (q, 2H, CH2), 2.99 [s,
6H, N(CH3)2], 6.59–6.68 (m, 2H, olefinic), 6.68–6.71 (m, 2H,
aromatic), 6.92–7.01 (dd, 1H, olefinic, J = 15.0 Hz), 7.36 (d,
2H, aromatic, J = 8.7 Hz), 7.52 (d, 2H, aromatic, J = 8.6 Hz),
7.59 (d, 2H, aromatic, J = 8.5 Hz); 13C NMR (CDCl3, 75
MHz): d 14.01, 22.73, 40.36, 109.57, 112.33, 119.26, 120.70,
125.59, 126.22, 127.80, 130.50, 132.12, 135.83, 138.60, 146.87,
150.35. MS (EI): m/z 302 (M+, C21H22N2).
Preparation
of
1-(p-methoxyphenyl)-4-(p-cyanophenyl)-
4-(ethyl)buta-1E,3E-diene (MCBE). To the phosphonate ester
3, NaH (61 mg, 2.5 mmol) was added and the mixture was
heated at 100 1C for 24 h. NaH (61 mg, 2.5 mmol) was added,
followed by dropwise addition of a THF solution (20 mL) of p-
methoxycinnamaldehyde (83 mg, 0.5 mmol) and the resultant
mixture was refluxed for 24 h under an argon atmosphere. The
solvent was removed under reduced pressure and the residue
was washed with water. The crude products were extracted
nj/b4/b408748h/ for crystallographic data in .cif or other electronic
format.
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 3 6 8 – 1 3 7 2
1369