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Proceeding Paper

Alternative Synthesis of Phosphonate Derivatives of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide †

by
Marco Bortoluzzi
1,*,
Stefan Ghincolov
1 and
Lodovico Agostinis
2
1
Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155, 30170 Mestre, Veneto, Italy
2
Aimplas, Plastic Technology Center, Valencia Parc Tecnologic, C/Gustave Eiffel, 4, 46011 Valencia, Spain
*
Author to whom correspondence should be addressed.
†
Presented at the 27th International Conference on Synthetic Organic Chemistry, 15–30 November 2023; Available online: https://ecsoc-27.sciforum.net/.
Chem. Proc. 2023, 14(1), 19; https://doi.org/10.3390/ecsoc-27-16176
Published: 15 November 2023
(This article belongs to the Proceedings of 27th International Electronic Conference on Synthetic Organic Chemistry)
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Versions Notes

Abstract

:
The phosphonates 6-methoxy-6H-dibenzo[1,2]oxaphosphinine-6-oxide and 6-(allyloxy)-6H-dibenzo[1,2]oxaphosphinine 6-oxide were synthesised in a single step under mild conditions from the H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), following a method based on the oxidation of the reactant with I2 in the presence of the reactant alcohol as solvent and triethylamine as base. The compounds, of potential interest in the field of non-halogenated flame retardants, were isolated with high purity and the formulations were confirmed via multinuclear NMR spectroscopy.

1. Introduction

The organophosphorus H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is a molecule of great interest in the field of non-halogenated flame retardants for plastics, and the peculiar reactivity of the P-H bond opens the possibility of different types of functionalization while maintaining the flame-retardant activity both in gas and condensed phase [1,2,3,4,5,6,7,8,9]. Related phosphinates where the P-H bond is formally replaced with a P-C bond can be obtained following approaches such as the nucleophilic attack of the conjugate base of DOPO on electron-poor carbon atoms [10,11,12,13,14,15], the phospha-Michael addition [16,17,18,19,20] and the Michaelis–Arbuzov rearrangement of related phosphonites [21,22,23]. On the other hand, the formation of a P-N or a P-O bond in place of the P-H one, with consequent isolation of phosphonamidates and phosphonates from DOPO, is commonly carried out through the intermediate synthesis of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-chloride (DOPO-Cl). Such a species can be obtained on the basis of the Atherton–Todd reaction using CCl4 as reactant in combination with a suitable base [24,25,26,27,28,29], even if alternative chlorinating agents such as sulfuryl chloride, trichlorocyanuric acid, chlorine gas, and N-chlorosuccinimide are revealed to be suitable [30,31,32,33,34]. In terms of the preparation of DOPO-based aliphatic phosphonates, examples of other synthetic approaches are the esterification of the phosphonic acid 6-hydroxydibenzo[1,2]oxaphosphinine-6-oxide [35] and the electrochemical dehydrogenative coupling of the H-phosphinate with alcohols [36].
In recent years, our research group investigated the coordinating behaviour as oxygen donors towards metal centres, such as manganese (II) and zinc (II), of the simple phosphonate O=P(OPh)2Ph (diphenyl phenylphosphonate) and of the H-phosphinate DOPO, with the aim of obtaining multifunctional compounds, thanks to the luminescence exhibited by the resulting complexes [37,38]. These studies prompted the investigation of alternative approaches for the conversion of DOPO in related phosphonates, and we recently patented a one-pot approach for the formal replacement of the P-H bond of DOPO with P-OR bonds, working under mild conditions [39]. Herein, we report as examples of this research, the syntheses and characterization data of the compounds 6-methoxy-6H-dibenzo[1,2]oxaphosphinine-6-oxide (DOPO-OMe) and 6-(allyloxy)-6H-dibenzo[c,e][1,2]oxaphosphinine-6-oxide (DOPO-OAllyl)

2. Materials and Methods

The reactants and solvents were Merck products with the exception of DOPO, purchased from Fluorochem, and they were used as received. Elemental analyses were carried out using an Elementar Unicube microanalyzer. Infrared spectra (IR) were registered using a Perkin-Elmer SpectrumOne spectrophotometer between 4000 and 450 cm−1. The products were dispersed between KBr windows. Absorption spectra in dichloromethane were recorded with a Yoke 6000Plus double-beam spectrophotometer. Mono- and bidimensional nuclear magnetic resonance (NMR) spectra were collected by employing a Bruker Avance 400 instrument operating at 400.13 MHz of 1H resonance. 1H NMR spectra are referred to the partially non-deuterated fraction of the solvent, itself quoted with respect to tetramethylsilane. 31P{1H} chemical shifts are reported with respect to 85% H3PO4, with downfield shifts considered positive. 13C{1H} NMR spectra refer to the solvent signal, quoted with respect to tetramethylsilane.

Synthesis of 6-Methoxy-6H-dibenzo[1,2]oxaphosphinine-6-oxide (DOPO-OMe) and 6-(Allyloxy)-6H-dibenzo[c,e][1,2]oxaphosphinine-6-oxide (DOPO-OAllyl)

In a typical preparation, DOPO (1.08 g, 5.0 mmol) was dissolved in 15 mL of methanol or allyl alcohol. Triethylamine (1.4 mL, 10.0 mmol) was added to the solution, then solid iodine (1.26 g, 5.0 mmol) was slowly added to the stirred solution. The reaction mixture was kept under stirring conditions at room temperature for 5 h. The solvent was then evaporated and dichloromethane (20 mL) was added. Triethylammonium iodide and other by-products were removed via extraction with water (2 × 20 mL). The organic phase was dried with anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure and the product was selectively dissolved with hot aliquots (3 × 20 mL) of cyclohexane. After the rapid filtration of the hot solutions, the solvent was removed under reduced pressure and the products were isolated in oily form. Yields: 30% (0.369 g) for DOPO-OMe; 33% (0.449 g) for DOPO-OAllyl.
Characterization of DOPO-OMe: Anal. calcd for C13H11O3P (246.2 g mol−1,%): C, 63.42; H, 4.50. Found (%): C, 63.17; H, 4.52. IR (KBr, cm−1): 1273 νP=O, 1035 νP=O. UV-Vis (CH2Cl2, r.t., nm): <330, 300 sh, 290, 267, 260. 1H NMR (CDCl3, 300 K): δ 7.98 (ddd, 1H, JHH = 7.4 Hz, JHH = 1.6 Hz, JPH = 14.4 Hz, arom), 7.97 (dd, 1H, JHH = 8.1 Hz, JPH = 6.4 Hz, arom), 7.94 (dd, 1H, JHH = 8.0 Hz, JHH = 1.7 Hz, arom), 7.73 (tdd, 1H, JHH = 7.6 Hz, JHH = 1.5 Hz, JPH = 1.1 Hz, arom), 7.52 (tdd, 1H, JHH = 7.6 Hz, JHH = 1.1 Hz, JPH = 3.7 Hz, arom), 7.40 (tdd, 1H, JHH = 7.7 Hz, JHH = 1.5 Hz, JPH = 1.3 Hz, arom), 7.30–7.24 (m, 2H, arom), 3.80 (d, 3H, JPH = 11.5 Hz, CH3). 31P{1H} NMR (CDCl3, 300 K): δ 11.36 (s). 13C{1H} NMR (CDCl3, 300 K): δ 149.92 (d, JPC = 7.9 Hz, arom-Cipso), 137.07 (d, JPC = 7.0 Hz, arom-Cipso), 133.57 (d, JPC = 2.5 Hz, arom-CH), 130.54 (s, arom-CH), 130.26 (d, JPC = 9.2 Hz, arom-CH), 128.29 (d, JPC = 15.4 Hz, arom-CH), 125.25 (d, JPC = 1.1 Hz, arom-CH), 124.77 (s, arom-CH), 124.08 (d, JPC = 12.1 Hz, arom-CH), 122.57 (d, JPC = 12.0 Hz, arom-Cipso), 121.85 (d, JPC = 181.1 Hz, arom-Cipso), 120.17 (d, JPC = 6.7 Hz, arom-CH), 52.93 (d, JPC = 6.6 Hz, CH3).
Characterization of DOPO-OAllyl: Anal. calcd for C15H13O3P (272.24 g mol−1,%): C, 66.18; H, 4.81. Found (%): C, 65.95; H, 4.79. IR (KBr, cm−1): 1271 νP=O, 1009 νP=O. UV-Vis (CH2Cl2, r.t., nm): <330, 300 sh, 290, 267, 260. 1H NMR (CDCl3, 300 K): δ 7.94 (ddd, 1H, JHH = 7.6 Hz, JHH = 1.6 Hz, JPH = 14.4 Hz, arom), 7.91 (dd, 1H, JHH = 8.2 Hz, JPH = 6.6 Hz, arom), 7.88 (dd, 1H, JHH = 8.0 Hz, JHH = 1.6 Hz, arom), 7.66 (tdd, 1H, JHH = 7.7 Hz, JHH = 1.4 Hz, JPH = 1.4 Hz, arom), 7.46 (tdd, 1H, JHH = 7.6 Hz, JHH = 1.0 Hz, JPH = 3.7 Hz, arom), 7.34 (tdd, 1H, JHH = 7.8 Hz, JHH = 1.5 Hz, JPH = 1.4 Hz, arom), 7.24–7.17 (m, 2H, arom), 5.82 (ddt, 1H, JHH = 17.2 Hz, JHH = 10.4 Hz, JHH = 5.5 Hz, CH2-CH), 5.20 (dq, 1H, JHH = 17.2 Hz, JHH = 1.6 Hz, =CH2), 5.13 (dq, 1H, JHH = 10.4 Hz, JHH = 1.2 Hz, =CH2), 4.61 (ddt, 2H, JHH = 5.5 Hz, JPH = 9.5 Hz, JHH = 1.4 Hz, CH2-CH). 31P{1H} NMR (CDCl3, 300 K): δ 10.50 (s). 13C{1H} NMR (CDCl3, 300 K): δ 149.84 (d, JPC = 8.0 Hz, arom-Cipso), 136.98 (d, JPC = 7.0 Hz, arom-Cipso), 133.54 (d, JPC = 2.6 Hz, arom-CH), 132.27 (d, JPC = 6.0 Hz, CH2-CH), 130.46 (s, arom-CH), 130.16 (d, JPC = 9.3 Hz, arom-CH), 128.26 (d, JPC = 15.5 Hz, arom-CH), 125.23 (d, JPC = 1.0 Hz, arom-CH), 124.74 (s, arom-CH), 124.04 (d, JPC = 12.2 Hz, arom-CH), 122.49 (d, JPC = 11.8 Hz, arom-Cipso), 122.02 (d, JPC = 181.5 Hz, arom-Cipso), 120.16 (d, JPC = 6.7 Hz, arom-CH), 118.52 (s, =CH2), 67.02 (d, JPC = 6.3 Hz, CH2-CH).

3. Results and Discussion

According to the recently published patent [39], the iodine-based functionalization of DOPO allowed the isolation of related phosphonates such as DOPO-OMe and DOPO-OAllyl under mild conditions, as depicted in Scheme 1. A supporting base is required for the reaction, and triethylamine was used because of its inability to behave as a nucleophile. The alcohols considered in this communication are methanol and allyl alcohol, which were used as solvents.
The proposed formulations, corroborated by the elemental analysis data, were confirmed by the NMR spectra (see Figure 1, Figure 2, Figure 3 and Figure 4) that showed, besides the aromatic signals of the phenyl rings in the aromatic regions of 1H and 13C{1H} NMR spectra, resonances attributable to the O-containing substituents. In particular, a doublet at 3.80 ppm with 3JPH coupling constant of 11.5 ppm was observed for DOPO-OMe, correlated to a doublet (2JPC = 6.6 Hz) at 52.93 ppm in the 13C{1H} NMR spectrum. DOPO-OAllyl showed signals at 5.82, 5.20, and 5.13 ppm for the vinylic protons and a multiplet at 4.61 ppm corresponding to the O-bonded CH2, with a 3JPH coupling constant of 9.5 Hz. The assignment was confirmed by the 13C{1H} and HSQC spectra, with three 13C{1H} NMR resonances for the allyl fragment, those involving the C=C bond at 132.27 and 118.57 ppm, and that corresponding to O-CH2 at 67.02 ppm. The signals of two carbon atoms closer to the phosphorus centre are doublets because of the coupling with 31P. Both the compounds showed only one 31P{1H} NMR sharp singlet in the 11.5–10.5 ppm range.
The IR spectra of DOPO-OMe and DOPO-OAllyl showed νP=O stretching around 1272 cm−1, shifted by more than 30 cm−1 towards higher wavenumbers with respect to DOPO, as highlighted in Figure 5. The electron-withdrawing behaviour of the oxygen-containing substituents probably increases the electrophilicity of the phosphorous atom, causing an enforcement of the P=O double bonds. On the other hand, the UV-Vis spectra did not show any meaningful variation with respect to DOPO, with absorptions below 330 nm composed by two groups of bands, respectively, centred around 290 and 265 nm. Such an outcome indicates that the π-delocalised electronic structure of the aromatic moiety is scarcely affected by the substitution at the phosphorus atom.
To conclude, in this communication, we reported the straightforward synthesis of two phosphonates starting from DOPO, operating under mild conditions and avoiding the use of aggressive reactants. The changes in the {P=O} moiety with respect to DOPO revealed by the IR spectra make DOPO-OMe a phosphonate of interest as a ligand for hard transition metal centres, with the aim of developing luminescent coordination compounds suitable as multifunctional materials. On the other hand, DOPO-OAllyl is a potentially reactive flame retardant thanks to the presence of the terminal double bond [40], which opens the possibility of co-polymerization with suitable monomers and the development of plastics with intrinsic flame-retardant features.

4. Patents

The data provided in this work were obtained on the basis of the 2023 patent WO2023094526A1, entitled “Preparation process of P(=O)-heteroatom derivatives of dibenzooxaphosphacycles”, presented by our research group.

Author Contributions

Conceptualization, M.B.; methodology, M.B.; validation, M.B., S.G. and L.A.; formal analysis, M.B. and S.G.; investigation, S.G. and M.B.; resources, L.A.; data curation, M.B and S.G.; writing—original draft preparation, M.B.; writing—review and editing, S.G. and L.A.; visualization, M.B. and S.G.; project administration, L.A.; funding acquisition, L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available on request due to restrictions related to the patent above reported.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemproc 14 00019 sch001
Scheme 1. Synthesis of DOPO-OMe and DOPO-OAllyl.
Scheme 1. Synthesis of DOPO-OMe and DOPO-OAllyl.
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Figure 1. 1H, 31P{1H}, and 1H{31P} NMR spectra of DOPO-OMe (CDCl3, 300 K).
Figure 1. 1H, 31P{1H}, and 1H{31P} NMR spectra of DOPO-OMe (CDCl3, 300 K).
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Figure 2. 13C{1H} and 1H-13C HSQC NMR spectra of DOPO-OMe (CDCl3, 300 K). Cross-peaks in odd CHx fragments are blue coloured.
Figure 2. 13C{1H} and 1H-13C HSQC NMR spectra of DOPO-OMe (CDCl3, 300 K). Cross-peaks in odd CHx fragments are blue coloured.
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Figure 3. 1H, 31P{1H}, and 1H{31P} NMR spectra of DOPO-OAllyl (CDCl3, 300 K).
Figure 3. 1H, 31P{1H}, and 1H{31P} NMR spectra of DOPO-OAllyl (CDCl3, 300 K).
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Figure 4. 13C{1H} and 1H-13C HSQC NMR spectra of DOPO-OAllyl (CDCl3, 300 K). Cross-peaks in even and odd CHx fragments are red and blue coloured, respectively.
Figure 4. 13C{1H} and 1H-13C HSQC NMR spectra of DOPO-OAllyl (CDCl3, 300 K). Cross-peaks in even and odd CHx fragments are red and blue coloured, respectively.
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Figure 5. IR spectra of DOPO-OMe and DOPO-OAllyl and comparison with DOPO.
Figure 5. IR spectra of DOPO-OMe and DOPO-OAllyl and comparison with DOPO.
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MDPI and ACS Style

Bortoluzzi, M.; Ghincolov, S.; Agostinis, L. Alternative Synthesis of Phosphonate Derivatives of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. Chem. Proc. 2023, 14, 19. https://doi.org/10.3390/ecsoc-27-16176

AMA Style

Bortoluzzi M, Ghincolov S, Agostinis L. Alternative Synthesis of Phosphonate Derivatives of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. Chemistry Proceedings. 2023; 14(1):19. https://doi.org/10.3390/ecsoc-27-16176

Chicago/Turabian Style

Bortoluzzi, Marco, Stefan Ghincolov, and Lodovico Agostinis. 2023. "Alternative Synthesis of Phosphonate Derivatives of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide" Chemistry Proceedings 14, no. 1: 19. https://doi.org/10.3390/ecsoc-27-16176

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