(E,E)-1,5-Diethoxy-1,5-diphenylpenta-1,4-dien-3-one

Abstract

The title compound has been fully characterised for the first time. Fully assigned ¹H and ¹³C NMR spectra and the X-ray structure are presented.

1. Introduction

The simple compound 1,5-diethoxy-1,5-diphenylpenta-1,4-dien-3-one 2 was first mentioned in a literature paper [1] over 50 years ago, where it was characterised by IR, elemental analysis, and its melting point. It was prepared by the two-fold addition of sodium ethoxide in ethanol to the triple bonds of 1,5-diphenylpenta-1,4-diyn-3-one 1 (Scheme 1). More recently, it was considered as a possible intermediate in the synthesis of thio-, seleno-, and telluropyranones 4 by treatment of acetylenic ketone 1 with sodium ethoxide and in situ generated sodium chalcogenide in ethanol [2], but this was ruled out in favour of the intermediate being the monoethoxy acetylenic ketone 3 based on mechanistic evidence. It was noted that formation of 3 as the major product was accompanied by <10% of 2 by NMR, but no data were given [2].

In the course of recent studies on the synthesis of thio-, seleno-, and telluropyranones we have frequently obtained compound 2 as a side-product separated by column chromatography, and present here its full characterisation by ¹H and ¹³C NMR as well as the X-ray crystal structure, which shows that it is obtained exclusively as the (E,E)-isomer.

2. Results

The use of diynone 1 to prepare the chalcogenapyranones 4 has been extensively investigated by Detty and coworkers [3,4], and we prepared the diynone 1 as described [3], by reaction of phenylacetylene with ethylmagnesium bromide followed by 0.5 equiv. of ethyl formate and subsequent dichromate oxidation. This was then reacted with sodium ethoxide and sodium chalcogenides prepared in situ from elemental sulfur, selenium, or tellurium to give the products 4 after column chromatography. However, in every case, a side-product was obtained from the chromatography, eluting from the column before the main product, which turned out to be the compound 2. This could also be obtained in a low yield by heating a solution of 1 with 2 equiv. of sodium ethoxide in ethanol followed by evaporation and chromatography. The compound was obtained as yellow needles, which showed good agreement with the literature melting point. Its ¹H and ¹³C NMR data (see Supplementary Materials), recorded for the first time, indicated a single symmetrical stereoisomer with the expected alkene (δ_H 5.23) and ethyl (δ_H 3.54, 1.23) signals in the proton spectrum and a highly polarised C=C double bond (δ_C 166.8, 105.8) resulting from the combined effects of the electron-donating OEt and electron-withdrawing C=O substituents. The data are summarised in Figure 1.

Since it was not possible to distinguish (E,E)- and (Z,Z)-isomers by NMR and the compound formed crystals suitable for X-ray diffraction upon careful evaporation of the chromatography fractions, we have determined its X-ray structure. As shown in Figure 2, this clearly showed the (E,E) structure resulting from attack of EtO⁻ at the side of the triple bonds remote from C=O in 1.

Half of the molecule appears in the asymmetric unit with the central carbonyl unit located along a two-fold rotation axis (

Table 1. Molecular dimensions for 2 (Å, °).

Bond Lengths		Angles
O(1)–C(1)	1.368(3)	C(4)–C(1)–O(1)	108.1(2)
C(1)–C(4)	1.484(4)	O(1)–C(1)–C(2)	122.6(2)
C(1)–C(2)	1.346(4)	C(2)–C(1)–C(4)	129.2(2)
C(2)–C(3)	1.469(3)	C(1)–C(2)–C(3)	129.1(3)
C(3)–O(3)	1.230(4)	C(2)–C(3)–O(3)	123.79(16)
		C(2)–C(3)–C(2′)	112.4(3)

). The central C=C–C(=O)–C=C unit is essentially planar (root-mean-squared deviation of those atoms from the plane of 0.011 Å), with the ethoxy and phenyl groups bending away slightly from the plane (torsions O1–C1–C2–C3 172.8(2)°, C4–C1–C2–C3 8.9(5)°). The phenyl group is twisted away from the central plane, with an angle between planes of 45.82(8)°. There is a weak non-classical intramolecular hydrogen bonding interaction between the ketone oxygen (O3) and the ortho-hydrogen (H9) of the adjacent benzene rings (

Table 2. Hydrogen bonding parameters for 2 (Å, °).

D—H···A	D—H	H···A	D···A	D—H···A	Symmetry
C(8)–H(8)···O(3) *	0.95	2.52	3.398(4)	154	1−x, 2−y, 1−z
C(9)–H(9)···O(3)	0.95	2.48	2.881(3)	105

). Reciprocal weak non-classical intermolecular hydrogen bonding between the meta-hydrogen (H8) of one benzene ring and the ketone oxygen (O3) of an adjacent molecule form $R_2^2$(16)-type rings, with each molecule forming two of these rings with two adjacent molecules. This forms chains of molecules down the crystallographic c axis (Figure 3) which further pack together into sheets across the ac plane.

In a search of the Cambridge Crystallographic Database (CSD), we have been unable to locate any previous structures of 1,5-dialkoxy-1,5-diarylpenta-2,4-dien-3-ones. There are numerous structures of 1,5-diarylpenta-2,4-dien-3-ones and these invariably involve the aryl groups being (E) to the ketone: as a typical example compound 5, isomeric with 2, has the two (E) double bonds oriented as shown [5], while the unsubstituted diphenyl analogue 6 adopts the more linear structure [6] (Figure 4). It is interesting to note that a range of 1,5-diarylpentane-1,3,5-triones such as 7 [7], 8 [8], and 9 [9] are found to exist in the solid state exclusively as the (Z,Z)-bis(enol) forms shown, but these structures are dictated by the possibility of twofold hydrogen bonding between OH and C=O.

In summary, we have been able to record and assign the ¹H and ¹³C NMR spectra for compound 2 for the first time. The X-ray structure confirms the (E,E) configuration and shows a symmetrical largely planar structure with weak intra- and intermolecular non-classical hydrogen bonding.

3. Experimental

3.1. General Experimental Details

Melting points were recorded on a Reichert hot-stage microscope (Reichert, Vienna, Austria) and are uncorrected. NMR spectra were obtained using a Bruker AV III 500 instrument (Bruker, Billerica, MA, USA). Spectra were run with internal Me₄Si as the reference and chemical shifts are reported in ppm to high frequency of the reference. NMR spectra were processed using iNMR reader, version 6.3.3 (Mestrelab Research, Santiago de Compostela, Spain). 1,5-Diphenylpenta-1,4-diyn-3-one 1 was prepared as reported [3].

3.2. Synthesis of (E,E)-1,5-Diethoxy-1,5-diphenylpenta-1,4-dien-3-one 2

A solution of 1,5-diphenylpenta-1,4-diyn-3-one 1 (1.00 g, 8.69 mmol) in ethanol (50 mL) was added dropwise to a cooled solution of sodium ethoxide in ethanol (0.44 M, 40 mL, 17.6 mmol) and the reaction mixture was heated under reflux for 4 h. The mixture was then added to water (100 mL) and extracted with diethyl ether (3 × 75 mL). The combined extracts were dried and evaporated to give a solid which was purified via silica gel chromatography, (SiO₂, Et₂O) to afford the product (290 mg, 21%) as yellow crystals, m.p. 120–122 °C (lit. [1] 121–123 °C). ¹H NMR (500 MHz, CDCl₃): δ 7.42–7.30 (10H, m, Ph), 5.23 (2H, s, =CH), 3.54 (4H, q, J = 7.0 Hz, CH₂), 1.25 (6H, t, J = 7.0 Hz, CH₃); ¹³C NMR (125 MHz, CDCl₃): δ 191.4 (C=O), 166.8 (O–C=), 136.1 (C), 129.6 (CH), 129.2 (2CH), 127.8 (2CH), 105.8 (=CH), 64.3 (CH₂), 14.2 (CH₃). HRMS (ESI): found 323.1638, C₂₁H₂₃O₃ (M+H) requires 323.1647; found 345.1460, C₂₁H₂₂NaO₃ (M+Na) requires 345.1467.

3.3. X-Ray Structure Determination of 2

X-ray diffraction data for compound 2 were collected at 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å), Tokyo, Japan]. Data for the compound were collected (using a calculated strategy) and processed (including correction for Lorentz, polarisation, and absorption) using CrysAlisPro [10]. The structure was solved by dual-space methods (SHELXT) [11] and refined by full-matrix least-squares against F² (SHELXL-2019/3) [12]. All calculations were performed using the Olex2 [13] interface.

Crystal data for C₂₁H₂₂O₃, M = 322.38 g mol⁻¹, yellow platy-needle, crystal dimensions 0.06 × 0.02 × 0.007 mm, monoclinic, space group C2/c (No. 15), a = 22.1569(10), b = 10.1522(3), c = 7.8310(4) Å, β = 99.997(4)°, V = 1734.75(13) ų, Z = 4, D_calc = 1.234 g cm⁻³, T = 100 K, goodness of fit on F² 1.091, 16,803 reflections measured, 2098 unique (R_int = 0.1256), which were used in all calculations. The final R₁ [I > 2σ(I)] was 0.0781 and wR₂ (all data) was 0.1535. Data have been deposited at the Cambridge Crystallographic Data Centre as CCDC 2429078. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/structures.