research papers
the sp-C—Hꢀ ꢀ ꢀN hydrogen bond between dipyridines and
diethynylbenzenes controlled the formation of cocrystals. In
contrast, the sp-C—Hꢀ ꢀ ꢀO(carbonyl) hydrogen-bonding
interaction was less efficient, in our hands, as a driving force
for cocrystallization of different molecules which was signifi-
cantly less predictable (Bosch, 2016). In a separate study, we
investigated the structure of two ortho-nitrophenylacetylenes
and observed unusual C—Hꢀ ꢀ ꢀO interactions (Bosch &
Jeffries, 2016), so we decided to further investigate C—
Hꢀ ꢀ ꢀO2N interactions in three 4,5-dialkoxy-1-ethynyl-2-nitro-
benzenes, namely 1-ethynyl-2-nitro-4,5-dipropoxybenzene,
(1), 1,2-dibutoxy-4-ethynyl-5-nitrobenzene, (2), and 1-ethynyl-
2-nitro-4,5-dipentoxybenzene, (3) (see Scheme).
bis(triphenylphosphane)palladium(II) chloride (73 mg,
0.103 mmol), and copper(I) iodide (6.8 mg, 0.03 mmol) in tri-
ethylamine (12 ml) was stirred under an argon atmosphere for
3 min. Trimethylsilylacetylene (0.414 g, 0.6 ml, 4.2 mmol) was
added, the flask sealed, and the reaction mixture stirred under
an inert atmosphere for 24 h at 298 K. The reaction mixture was
added to dichloromethane (150 ml), washed four times with
water (50 ml), dried over magnesium sulfate, and the solvent
evaporated to yield a dark-brown crude product. This crude
product was purified using flash chromatography to give
trimethyl-(2-nitro-4,5-dipropoxyphenylethynyl)silane as an off-
white solid. 1H NMR (400 MHz, CDCl3): ꢀ 7.60 (s, 1H), 6.98 (s,
1H), 4.03 (q, J = 8.0 Hz, 4H), 1.87 (m, 4H), 1.04 (m, 6H), 0.26 (s,
9H); 13C NMR (100 MHz, CDCl3): ꢀ 152.94, 149.06, 143.21,
124.32, 110.49, 105.00, 69.41, 69.24, 19.06, 19.08, 13.72, 0.00.
Trimethyl(2-nitro-4,5-dipropoxyphenylethynyl)silane was de-
protected using sodium hydroxide in methanol to give (1) as a
light-brown solid. The product was purified using flash chro-
matography. 1H NMR (400 MHz, CDCl3): ꢀ 7.62 (s, 1H), 7.02 (s,
1H), 4.04 (t, J = 6 Hz, 4H), 3.46 (s, 1H), 1.93–1.84 (m, 4H), 1.07
(t, J = 7 Hz, 3H); 13C NMR (100 MHz, CDCl3): ꢀ 152.72, 149.03,
142.936, 117.17, 110.97, 108.77, 83.28, 79.54, 70.87, 22.19, 22.18,
10.26, 10.25.
2.1.3. 1,2-Dibutoxy-4-iodo-5-nitrobenzene. 1H NMR
(400 MHz, CDCl3): ꢀ 7.59 (s, 1H), 7.37 (s, 1H), 4.06 (t, J =
7 Hz, 2H), 4.04 (t, J = 7 Hz, 2H), 1.84 (m, 4H), 1.51 (m, 4H),
1.00 (t, J = 8 Hz, 3H), 0.99 (t, J = 8 Hz, 3H); 13C NMR
(100 MHz, CDCl3): ꢀ 153.19, 149.05, 144.90, 124.46, 110.67,
69.61, 69.43, 31.06, 31.04, 19.30, 19.28, 13.97, 13.95.
2. Experimental
2.1. Synthesis and characterization
The Williamson ether synthesis was used to prepare each of
1,2-dipropoxybenzene, 1,2-dibutoxybenzene, and 1,2-dipen-
toxybenzene from catechol, 2.2 equivalents of KOH and
2.5 equivalents of the corresponding 1-bromoalkane in
acetone. Each of these compounds was then iodinated using
mercuric iodide and iodine (Bacher et al., 1999) to form the
corresponding 1,2-dialkoxy-4-iodobenzene. The resultant di-
alkoxyiodobenzenes were nitrated with nitric acid in acetic
acid and then subjected to Sonogashira cross-coupling with
trimethylsilylacetylene. Finally, base-promoted deprotection
provided the dialkoxyethynylnitrobenzenes. The full proce-
dure is given for the dipropoxy derivative.
2.1.1. 1-Iodo-2-nitro-4,5-dipropoxybenzene. 1-Iodo-3,4-
dipropoxybenzene (1.96 g, 6.12 mmol) was dissolved in glacial
acetic acid (4 ml) and the mixture cooled to 273 K. A mixture
of nitric acid (1 ml, 24 mmol) and acetic acid (1.3 ml) was then
added dropwise to the reaction mixture, which was stirred at
273 K for 40 min. The reaction mixture was dissolved in di-
chloromethane (100 ml) and washed twice with water
(100 ml), and then 5% aqueous sodium bicarbonate (75 ml).
The dichloromethane solution was dried over magnesium
sulfate and the solvent evaporated. The crude solid was
recrystallized from a propan-2-ol/water mixture (100 ml,
1
2.1.4. 1,2-Dibutoxy-4-ethynyl-5-nitrobenzene (2). H NMR
(400 MHz, CDCl3): ꢀ 7.62 (s, 1H), 7.02 (s, 1H), 4.08 (t, J = 6 Hz,
4H), 3.45 (s, 1H), 1.87–1.80 (m, 4H), 1.50 (sextet, J = 6 Hz, 4H),
0.98 (t, J = 7 Hz, 3H); 13C NMR (100 MHz, CDCl3): ꢀ 153.00,
149.33, 143.23, 127.39, 110.23, 109.01, 83.50, 79.83, 69.47, 69.45,
32.03, 19.30, 19.28, 13.96, 13.94.
2.1.5. 1-Iodo-2-nitro-4,5-dipentoxybenzene. 1H NMR
(400 MHz, CDCl3): ꢀ 7.59 (s, 1H), 7.37 (s, 1H), 4.05 (t, J =
7 Hz, 2H), 4.03 (t, J = 7 Hz, 2H), 1.84 (m, 4H), 1.44 (m, 8H),
0.94 (t, J = 7 Hz, 3H), 0.93 (t, J = 7 Hz, 3H); 13C NMR
(100 MHz, CDCl3): ꢀ 153.17, 149.03, 144.90, 124.43, 110.64,
69.89, 69.70, 28.72, 28.70, 28.24, 28.21, 22.55, 22.53, 14.17.
2.1.6. 1-Ethynyl-2-nitro-4,5-dipentoxybenzene, (3). 1H NMR
(400 MHz, CDCl3): ꢀ 7.62 (s, 1H), 7.02 (s, 1H), 4.07 (t, J = 7 Hz,
4H), 3.46 (s, 1H), 1.86 (quintet, J = 7 Hz, 4H), 1.51–1.36 (m, 8H),
0.94 (t, J = 7 Hz, 6H); 13C NMR (100 MHz, CDCl3): ꢀ 152.96,
149.28, 143.18, 117.33, 111.20, 108.95, 83.52, 79.81, 69.72, 69.69,
28.68, 28.67, 28.22, 28.21, 22.52, 22.51, 14.13.
2.2. Refinement
Crystal data, data collection, and structure refinement
details are summarized in Table 1. The phenyl and alkynyl H
atoms involved in C—Hꢀ ꢀ ꢀO hydrogen bonds were refined
with distance restraints of C—H = 0.93 (2) (phenyl) and
1
9:1 v/v) as thin yellow needles (yield 0.89 g, 39%). H NMR
(400 MHz, CDCl3): ꢀ 7.59 (s, 1H), 7.37 (s, 1H), 4.02 (m, 4H),
1.87 (m, 4H), 1.06 (m, 6H); 13C NMR (100 MHz, CDCl3): ꢀ
152.97, 148.83, 124.30, 110.51, 71.09, 70.93, 22.27, 13.74.
2.1.2. 1-Ethynyl-2-nitro-4,5-dipropoxybenzene, (1). A mix-
ture of 1-iodo-2-nitro-4,5-dipropoxybenzene (0.88 g, 2.4 mmol),
˚
0.95 (2) A (alkynyl), and with Uiso(H) = 1.2Ueq(C). The
methylene, methyl, and remaining phenyl H atoms were
treated as riding in geometrically idealized positions, with
ꢁ
Acta Cryst. (2017). C73, 814–819
Oburn and Bosch
Three homologous 4,5-dialkoxy-1-ethynyl-2-nitrobenzenes 815