One-Pot Synthesis of Dioxime Oxalates

Abstract

Dioxime oxalates, a type of carbonyl oximes, are well-known as clean sources of iminyl radicals that undergo key organic chemistry transformations. A series of dioxime oxalates is reported in this manuscript, obtained by the reaction of the corresponding oximes with oxalyl chloride and Et₃N at room temperature. This one-pot method afforded three novel dioxime oxalates and the crystal structure of cyclopentanone dioxime oxalate analysis is also presented.

1. Introduction

Since the synthesis and isolation of the first organic free radical by Moses Gomberg at the beginning of the nineteenth century, chemists have been interested in studying and forming radical species emphasizing their applications in life and material sciences [1]. Eventually, in the 1970s, preliminary studies on the chemistry of nitrogen-centered free radicals indicated that iminyl radicals were slightly unreactive and no suitable methods to generate them were available [2,3,4]. That changed during the last few decades. Today, iminyl radicals are valuable intermediates involved in many organic reactions, including additions to π-systems to produce reactive radicals (Scheme 1a) [5,6], transformation into carbon-centered radicals by H transfers (Scheme 1b) [7], and the formation of new C-N bonds in the synthesis of nitrogen heterocycles [8,9,10,11,12,13] (Scheme 1c).

Iminyl radicals can also be formed by thermal fragmentation of N-O bonds and under milder photoredox methodologies in which visible light is used for the excitation process [14], or by triplet energy transfer (EnT) [15].

Oximes and their derivates are arguably the most used compounds for homolytic N-O bond fragmentation to obtain N• and O• centered radicals [16]. For instance, oxime esters form iminyl radicals via single electron transfer (SET) or triplet energy transfer (EnT) [17] (Scheme 2a). Consequently, dioxime oxalates have been recognized as atom-efficient and clean sources of iminyl radicals, considering that the main by-product in the processes is CO₂ (Scheme 2b). Examples have been described by Forrester et al. to produce radicals during UV photolysis [18], and by Walton and co-workers during the study of photochemical reactions of a range of symmetrical and unsymmetrical oxime oxalates for synthesizing dihydropyrroles and phenanthridines [19]. Generally, the reported syntheses of dioxime oxalates consist of two steps. The first step requires a nucleophilic substitution between an oxime and oxalyl chloride at low temperatures (−60 °C) to afford an O-(chlorooxalyl)oxime, which can be isolated and undergo a Beckmann rearrangement for the synthesis of nitrilium salts. In contrast, if a second molecule of oxime is added, it reacts with the O-(chlorooxalyl)oxime, yielding the dioxime oxalate (Scheme 2c) [20].

We report herein the one-pot synthesis of dioxime oxalates under simple reaction conditions; a method that is helpful with cyclic and acyclic oximes. We also report three new oxalates and a brief structural analysis of the cyclopentanone dioxime oxalate crystal structure.

2. Results

Non-commercially available ketoximes (1) may be obtained from the classic condensation reaction between the corresponding ketones and hydroxylamine hydrochloride under basic conditions. Acetophenone oxime 1a was employed as a model substrate in developing the one-pot method for preparing dioxime oxalate. The first experiment was carried out by adding dropwise 1.8 mmol of oxalyl chloride to a solution of (1a, 1.5 mmol) and Et₃N (1.65 mmol) in anhydrous MeCN at room temperature. Whereas the reaction was complete after 1 h, the crude was a brown viscous oil that showed difficulties in the purification processes. A second reaction was made under the same conditions, replacing MeCN with dry DCM. The result of this experiment was an easily handled crude and the desired dioxime oxalate 2a was obtained in 92% yield; the NMR data agreed with previous descriptions [20] (Scheme 3).

With those results in hand, the same conditions were explored on cyclic oximes, affording another four dioxime oxalates with moderate to good yields (

Table 1. Reaction with other oximes.

Entry	Oxime (1)	Dioxime Oxalate (2), Yield (%) 1
1		(2b), 60
2		(2c), 60 2
3		(2d), 89
4		(2e), 94

¹ Isolated yield after chromatographic purification. ² NMR data agreed with previous descriptions.

, Entries 1–4). Remarkably, to our knowledge, compounds (2b), (2d), and (2e) were not reported until now.

It is well known that dioxime oxalates can be stored for long periods under ambient conditions, and are stable to moderate heat and hydrolysis [9]. Moreover, they are solids. Considering these physicochemical properties, crystallization was attempted. A successful crystallization process was achieved with cyclopentanone dioxime oxalate via slow diffusion of Et₂O through a saturated dissolution of the product in AcOEt. The obtained crystal confirmed the molecular structure by X-ray crystal analysis.

3. Discussion

The crystal structure of cyclopentanone dioxime oxalate was determined from X-ray single-crystal measurements. A search in the CSD database version 5.41 (date of the search: September 2022) using the ConQuest software version 2020.1 for molecules with the same molecular core did not generate any results about its crystallographic analysis. Crystal data, data collection, and structure refinement details are summarized in

Table 2. Crystallographic data of cyclopentanone dioxime oxalate.

Crystal Data
Chemical Formula	C12H16N2O4
Mr	252.27
Crystalline system, space group	Monoclinic, I2/c
a, b, c (Å)	8.9695 (11), 11.2015 (11), 13.3635 (15)
α, β, γ (°)	90, 109.087 (13), 90
Volume, (Å3)	1268.8 (3)
ρ, kg m−3	1.321
Z	4
Temperature, (K)	298 (2)
Radiation type	Cu Kα
μ (mm−1)	0.84
Theta range for data collection	5.278° < 2θ < 76.337°
Index range	−11 ≤ h ≤ 11,
	−13 ≤ k ≤ 14,
	−15 ≤ l ≤ 16
Data collection
Diffractometer	SuperNova, Dual, Cu at zero, Atlas
Absorption correction	Multi-scan method CrysAlis PRO 1.171.41.119a (Rigaku Oxford Diffraction, 2021)
Tmin, Tmax	0.908, 1.000
No. of measured, independent, and observed reflections [I > 2σ(I)]	5200, 1314, 1156
Rint	0.034
(sin θ/λ)max (Å−1)	0.630
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.048, 0.142, 1.09
No. of reflections	1314
Refined parameters	83
H-atoms treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.23, −0.18

.

The molecular structure drawn as ellipsoids described with anisotropic thermal vibrations (ORTEP style) is shown in Figure 1a. Cyclopentane moieties are out of the plane defined by the dioxime oxalate fragment, with dihedral angles between their weighted least-squares mean planes of 23.53° (Figure 1b). In the crystal, the supramolecular structure lacks classic hydrogen bonds. For this reason, no short contacts are detected. However, C5-H5···O1 hydrogen interactions (H···O distance: 2.70 Å; symmetry code: 3/2 − x,1/2 + y,1 − z) connect molecules to form sheets stacked along the [101] direction (Figure 1c), which are further connected by C6-H6···O2 hydrogen interactions along the [101] direction (H···O distance: 2.73 Å; symmetry code: 1 − x,1 − y,1 − z). The molecular orientations in the crystal leave the dioxime oxalate fragments oriented along the [100] direction (Figure 1d), with distances between neighboring fragments of 4.49 Å.

The supramolecular structure is mainly driven by long C-H···O interactions observed in the Hirshfeld surface (HS) mapped over d_norm (Figure 2). No red spots are detected, suggesting no interactions smaller than the sum of two consecutive atoms’ van der Waals radii. These sorts of structures are considered networks built by weak interactions. Nevertheless, from the 2D-fingerprint plots, H···O/O···H interactions represent 26.0% of the total HS, supporting the importance of these contacts in the formation of the crystal. Other interactions, such as H···N/N···H, constitute 11.3% of the total HS and can be observed due to the closeness caused by the shorter C5-H5···O1 interactions in the formation of the molecular sheets (Figure 1c). The high proportion of H···H non-covalent interactions (51.2% of the HS) results from the critical role that dispersion forces play in crystal growth.

4. Materials and Methods

4.1. Materials

All the reactions were performed under a nitrogen atmosphere. Dichloromethane was dried over anhydrous CaH₂ and distilled before use. All the reagents were used as received from commercial suppliers (Alfa Aesar, Massachusetts, United States). Reaction progress was monitored by thin-layer chromatography (TLC) performed on aluminum plates coated with silica gel F254, with 0.2 mm thick TLC plates visualized using ultraviolet (UV) light at 254 nm or stained with p-anisaldehyde, vanillin, or KMnO₄ solutions. Flash column chromatography was performed using silica gel 60 (230–400 mesh). Acetophenone, cyclopentanone, and cyclohexanone were obtained from commercial suppliers (Merck/MilliporeSigma, Burlington, MA, United States s). 3-phenylcyclobutan-1-one [21] and 2,3-dihydro-1H-inden-1-one [22] were synthesized according to the previously reported procedures. Some compounds were purified using flash chromatography (FC). The solvents used for purification are described as follows: FC: (Solvent).

4.2. Preparation of Ketoximes

Ketone (1.25 mmol, 1 equiv.), hydroxylamine hydrochloride (3 equiv.), and sodium acetate trihydrate (3 equiv.) were mixed in absolute EtOH (12.5 mL) and then, refluxed until complete conversion was observed by TLC (pentane:AcOEt 4:1). The solvent was then evaporated under reduced pressure. Saturated NaCl solution (50 mL) was added and the aqueous phase was extracted with CH₂Cl₂ (3 × 50 mL). The combined organic phases were dried over anhydrous MgSO₄, filtered, and concentrated. The product was purified by flash column chromatography on silica gel (pentane:AcOEt 4:1) to give the desired oxime. Oximes NMR data agree with previous literature reports descriptions.

4.2.1. Acetophenone Oxime (1a)

According to the general procedure, acetophenone (1.20 g, 10 mmol) was used to obtain the acetophenone oxime as a white solid (1.15 g, 85%) [23].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.01 (brs, 1H), 7.63 (m, 2H), 7.39 (m, 3H), 2.31 (s, 3H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 156.2, 136.5, 129.4, 128.7, 126.1, 12.5.

4.2.2. Cyclopentanone Oxime (1b)

According to the general procedure, cyclopentanone (1.00 g, 11.9 mmol) was used to obtain the cyclopentanone oxime as a white solid (1.02 g, 87%) [23].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.44 (brs, 1H), 2.46 (t, J = 7.0 Hz, 2H), 2.36 (t, J = 6.7 Hz, 2H), 1.70–1.81 (m, 4H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 167.7, 31.0, 27.2, 25.3, 24.7.

4.2.3. Cyclohexanone Oxime (1c)

According to the general procedure, cyclohexanone (986 mg, 10.05 mmol) was used to obtain the cyclohexanone oxime as a white solid (1.04 g, 91%)[23].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 2.50 (t, J = 6.0 Hz, 2H), 2.21 (t, J = 6.2 Hz, 2H), 1.54–1.72 (m, 6H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 161.1, 32.3, 27.0, 25.9, 25.8, 24.5.

4.2.4. 3-Phenylcyclobutan-1-one Oxime (1d)

According to the general procedure, 3-phenylcyclobutan-1-one (300 mg, 2.05 mmol) was used to obtain the 3-phenylcyclobutan-1-one oxime as pale-yellow solid (272 mg, 82%) [24].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.26 (brs, 1H), 7.21–7.39 (m, 5H), 3.66–3.56 (m, 1H), 3.27–3.54 (m, 2H), 2.99– 3.11 (m, 2H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 156.7, 144.1, 128.7, 126.7, 39.5, 38.3, 32.9.

4.2.5. 2,3-Dihydro-1H-Inden-1-one Oxime (1e)

According to the general procedure, 2,3-dihydro-1H-inden-1-one (500 mg, 3.78 mmol) was used to obtain the 2,3-dihydro-1H-inden-1-one oxime as pale-yellow solid (446 mg, 80.1%) [22].

¹H NMR (400 MHz, CDCl₃): δ (ppm)7.67–7.66 (d, 1H), 7.50 (brs, 1H), 7.30–7.38 (m, 2H), 7.20–7.30 (m, 1H), 3.02–3.12 (m, 2H), 2.92–3.01 (m, 2H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 164.50, 148.64, 136.19, 130.55, 127.15, 125.77, 121.62, 28.66, 25.91.

4.3. Preparation of Dioxime Oxalates

To a solution of 1.5 mmol ketoxime in 5 mL of anhydrous dichloromethane, the Et₃N (1.65 mmol) was added. Then, oxalyl chloride (1.8 mmol) was added dropwise at room temperature under stirring and a nitrogen atmosphere. The reaction was stirred until completion was indicated by TLC (pentane:AcOEt 9:1) (the total reaction time is around 1 h). Water (15 mL) was added and the mixture was extracted with DCM (3 × 10 mL). The combined extracts were washed with brine (2 × 25 mL), dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. Purification of the crude by flash chromatography (pentane:AcOEt 9:1) afforded the corresponding oxalate.

4.3.1. O,O′-Oxalylbis(1-phenylacetophenone oxime) (2a)

According to the general procedure, 1-phenylethan-1-one oxime (203 mg, 1.5 mmol) was used to obtain acetophenone dioxime oxalate as a white solid (180 mg, 74%) [20].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.71 (d, J = 7.5 Hz, 2H), 7.39–7.47 (m, 3H), 2.48 (s, 3H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 164.4, 133.7, 131.2, 128.7, 127.1, 14.7.

FT-IR (neat) ν (cm⁻¹ ): 2927, 2858, 1789, 1307, 763, 694.

mp = 50–52 °C

4.3.2. O,O′-Oxalyldicyclopentanone Oxime (2b)

According to the general procedure, cyclopentanone oxime (297 mg, 3.0 mmol) was used to obtain the cyclopentanone dioxime oxalate oxime as a colorless solid (230 mg, 60%).

The product was crystallized through a liquid–liquid slow diffusion technique. In total, 20 mg of 2b was dissolved in 5 mL of ethyl acetate, and 10 mL of diethyl ether was layered and cooled to −5 °C. After 15 days, the single crystals grow colorless. Finally, the crystals were washed with cold diethyl ether.

¹H NMR (400 MHz, CDCl₃): δ (ppm) 2.60–2.68 (m, 2H), 2.54–2.60 (m, 2H), 1.79–1.88 (m, 4H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 178.4, 31.7, 30.0, 25.3, 24.6.

mp = 98–100 °C

FT-IR (neat) ν (cm⁻¹): 2962, 2885, 1789, 1654, 1261, 1126, 840, 790.

HRMS (ESI): calculated for C₁₂H₁₇N₂O₄ (M+H) 253.1183; found 253.1183.

4.3.3. O,O′-Oxalyldicyclohexanone Oxime (2c)

According to the general procedure, cyclohexanone oxime (300 mg, 2.65 mmol) was used to obtain the cyclohexanone dioxime oxalate as a pale-yellow solid (222 mg, 60%) [20].

¹H NMR (400 MHz, CDCl₃): δ (ppm) 2.61 (t, J = 6.4 Hz, 2H), 2.39 (t, J = 6.3 Hz, 2H), 1.60–1.81 (m, 6H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 171.6, 32.1, 27.4, 26.9, 26.0, 25.4.

mp = 68–69 °C

FT-IR (neat) ν (cm⁻¹): 2642, 1666, 1616, 1431, 1234, 713, 559.

HRMS (ESI): calculated for C₁₅H₂₅N₂O₅ (M + CH₃OH + H) 313.1758; found, 313.1758.

4.3.4. O,O′-Oxalylbis(3-phenyl-3-phenylcyclobutan-1-one oxime) (2d)

According to the general procedure, 3-phenylcyclobutan-1-one oxime (161 mg, 1 mmol) was used to obtain the 3-phenylcyclobutanone dioxime oxalate as a colorless solid (168 mg, 89%).

¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.31–7.41 (m, 2H), 7.23–7.31 (m, 3H), 3.65–3.74 (m, 1H), 3.48–3.65 (m, 2H), 3.09–3.28 (m, 2H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 168.6, 142.6, 128.9, 127.2, 126.4, 39.8, 39.5, 32.4.

FT-IR (neat) ν (cm⁻¹): 3028, 2927, 1755, 1496, 1107, 698.

mp = 118–120 °C

HRMS (ESI): calculated for C₂₂H₂₄N₃O₄ (M + NH₄), 394, 1761; found, 394, 1766.

4.3.5. O,O′-Oxalylbis(2,3-dihydro-1H-2,3-dihydro-1H-inden-1-one oxime) (2e)

According to the general procedure, 2,3-dihydro-1H-inden-1-one (60 mg, 0.408 mmol) was used to obtain the 2,3-dihydro-1H-inden-1-one dioxime oxalate as a colorless solid (67 mg, 94%).

¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.83–7.93 (m, 1H), 7.41–7.51 (m, 1H), 7.27–7.41 (m, 2H), 3.04–3.22 (m, 4H).

¹³C NMR {¹H} (101 MHz, CDCl₃): δ (ppm) 172.65, 150.5, 133.6, 132.9, 127.6, 125.9, 123.6, 28.7, 28.3.

FT-IR (neat) ν (cm⁻¹): 1762, 1631, 1338, 1123, 1045, 883, 760.

mp = 140–145 °C

HRMS (ESI): calculated for C₂₀H₁₇N₂O₄ (M + H), 349.1183; found, 349.1187.

4.4. Characterization Techniques

All the ¹H NMR and ¹³C NMR spectra were recorded using a BRUKER Avance III HD Ascend 400 spectrometer. Chemical shifts are given in parts per million (ppm, δ), referenced to the TMS (¹H and ¹³C). When necessary, the solvent peak of residual CDCl₃ was defined at δ = 7.26 ppm (¹H NMR) and δ = 77.16 (¹³C NMR). Coupling constants are quoted in Hz (J). Splitting patterns of ¹H NMR were designated as singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m). Splitting patterns that could not be interpreted or easily visualized were designated multiplet (m) or broad (br). Neat infrared spectra were recorded using a THERMO NICOLET-NEXUS (FT-IR) with PIKE MIRacle ATR cell. Wave numbers (ν_max) are reported in cm⁻¹. High-resolution mass spectrometry was recorded using an Agilent 5973 (70 eV) spectrometer, using electrospray ionization (ESI). GC-MS was recorded in a Thermo Scientific Trace 1300. Copies of NMR spectra are provided as Supplementary Materials. The X-ray intensity data were measured at room temperature, 298 (2) K, using CuKα radiation (λ = 1.54184 Å), and ω scans in an Agilent SuperNova, Dual, Cu at Zero, Atlas four-circle diffractometer equipped with a CCD plate detector (Rigaku Americas Corporation, The Woodlands, Texas, USA). The collected frames were integrated with the CrysAlis PRO software package (CrysAlisPro 1.171.39.46e, Rigaku Oxford Diffraction, 2018). Data were corrected for the absorption effect using the CrysAlis PRO software package by the empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm (CrysAlisPro 1.171.39.46e, Rigaku Oxford Diffraction, 2018). The structure of the cyclopentanone dioxime oxalate 2b was solved using an iterative algorithm [25] and then, completed by a difference Fourier map.

5. Conclusions

We developed a one-pot method for synthesizing dioxime oxalates, starting from the corresponding oximes. Compared with the previously described methods, our methodology does not isolate the O-(chlorooxalyl)oxime intermediate and does not need cryogenic baths. The procedure is applicable for acyclic and cyclic ketoximes, allowing three novel dioxime oxalate compounds. Finally, we report the structural analysis of the cyclopentanone dioxime oxalate from X-ray single-crystal data. The crystal growth is controlled by a combination of dispersion forces and weak C-H···O hydrogen interactions, equal to or longer than the sum of the van der Waals radii. Hirshfeld’s surface maps support this observation.