Carbon dioxide as a primary oxidant and a C1 building block

Abstract

2,3-Dihydro-2,2,2-triphenylphenanthro[9,10-d]-1,3,2-λ⁵-oxazaphospholes react with carbon dioxide in an overall second order reaction at room temperature to give 3H-phenanthro[9,10-d]oxazol-2-ones and triphenylphosphine oxide in good yields. Nucleophilic attack of the phenolate on CO₂ and formation of Ph₃P=O was found.

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Carbon dioxide chemistry (in particular, capture and conversion) has attracted much attention from the scientific community due to global warming associated with positive carbon accumulation.¹ The most widely used chemical absorption technique for carbon capture and storage/sequestation would be essentially adopting amino-containing absorbents through formation of C–N bonds in terms of mechanistic considerations.² However, extensive energy input in desorption and compression process would be a crucial barrier to realize it practically. On the other hand, CO₂ is very attractive as an environmentally friendly feedstock to replace the hazardous phosgene route for making commodity chemicals, fuels, and materials from a standpoint of green chemistry, whereas the reactions involving CO₂ are commonly carried out at high pressure, which may not be economically suitable and also pose safety concerns. The challenge is to develop catalysts that are capable of activating CO₂ under low pressure (preferably at 1 bar), and thus incorporating CO₂ into organic molecules catalytically or find new energy-rich reagents for these reactions.³ If we confine ourselves to reactions of CO₂ with N- or O-nucleophiles these include mainly compounds such as ammonia,⁴ amines,⁵ azides,⁶ aziridines,⁷ inorganic oxides,⁸ a terminal P=O,⁹ alcohols¹⁰ and other various terminal oxide anions.¹¹ With aziridines carbon dioxide react to give oxazolidinones¹² or polyurethanes in the presence of a catalyst and the latter in its absence. However, in both cases elevated CO₂ pressures and higher temperatures are required. Oxazolidinones can also be prepared from propargylalcohol and amines in ionic liquid at 25 bar CO₂ pressure and CuCl as catalyst.¹³ Quinazolinediones are formed from aminobenzonitriles in excellent yields in water.¹⁴ CO₂ would be a good and abundant C1 source for chemistry and its elimination from the atmosphere very desirable. Its reactions to valuable compounds are very limited, especially in industrial context, due to its low reactivity on the one side, and otherwise its high concentration in the atmosphere, which poses big environmental problems.

1,2-Quinones react with tertiary phosphines in a [4 + 1] electrocyclic reaction giving dioxaphosphole heterocyclic rings.¹⁵ Monoimines of 1,2-quinones (1a–d) give 1,3,2-oxazaphospholes (2) by the use of tertiary phosphines in good yields.¹⁶ Where the monoimines are unstable they can also be prepared from the corresponding quinones and triphenylphosphine in a sealed tube under ammonia. Electron-releasing groups on the quinones make the nucleophilic replacement of the C=O bond with NH₃ to a C=NH functionality difficult or impossible (e.g. R = NMe₂, OR).

1,3,2-Oxazaphospholes exhibit tautomerism as shown in Scheme 1 between the oxazaphosphole form (2) and the imino phosphorane form (3) dependent on the consistency or the solvent of solution (Scheme 1).¹⁷ Oxazaphospholes (2) seem to be stable in the solid form. In solution, however the tautomeric compounds of the iminophosphorane form (3) are also present. ³¹P NMR spectra in the range of 39–51 ppm are characteristic for 2 and those of the form 3 have a shift in the range from −23 to −8 ppm. In more basic solvents the oxazaphosphole form 2 is dominant. In solution they decompose readily in air due to dioxygen or moisture.

Scheme 1 The tautomeric forms of 1,3,2-oxazaphospholes and their reaction with CO₂.

2,3-Dihydro-2,2,2-triphenylphenanthro[9,10-d]-1,3,2λ⁵-oxaza-phospholes (2a,b) react with carbon dioxide in acetonitrile at room temperature slowly (24 h) and at reflux temperature in a fast reaction (2 h) to give 3H-phenanthro[9,10-d]oxazol-2-one (4a,b) in 78–85% yields (Scheme 1). The same reaction products can be obtained when the corresponding 9,10-phenanthrenequinone monoimines are used with a stoichiometric amount of triphenylphosphine and CO₂. The endproducts crystallise from the solution and Ph₃PO can be obtained from the filtrate purified by column chromatography using methanol as eluent. X-ray structure determination of 4b secured the structure with bond distances of C=O 1.196(7), C–N 1.378(8), and C–O 1.374(7) Å comparable to those found in similar molecules (Fig. 1).¹⁸ The other product formed (38%), compared with an authentic sample, was found to be triphenylphosphine oxide with ν(P=O) at 1191 cm⁻¹ in the FTIR spectrum¹⁹ and a signal at 25.5 ppm in the ³¹P NMR spectrum.‡²⁰

Fig. 1 X-ray crystal structure of 3H-phenanthro[9,10-d]oxazol-2-one (4b). Red O, blue N, black C, white H. Selected bond distances (Å) and angles (°): C1–O2 1.196(7), C1–O1 1.374(7), C1–N1 1.378(8), C2–C15 1.350(8), C2–N1 1.391(7), C15–O1 1.402(7), O2–C1–O1 125.0(6), O2–C1–N1 129.0(6), O1–C1–N1 106.1(6), C15–C2–N1 106.0(6), N1–C2–C3 131.3(6), C2–C15–O1 109.4(5).

Kinetic measurements under pseudo first order condition (constant CO₂ concentration, 1 bar) (Fig. 2) and with different 1,3,2-oxazaphosphole concentrations resulted in a second order overall rate eqn (1).

Reaction rate = k₂ [1,3,2-oxazaphosphole][CO₂] (1)

Fig. 2 Time courses of the reactions of the initial 2,3-dihydro-2,2,2-triphenyl phenanthro[9,10-d]-1,3,2λ⁵-oxazaphosphole (2b) concentration with CO₂. [2b]₀ = 40 mM (^), 20 mM (◇), 10 mM (▲) by 1 bar of CO₂ pressure.

The 2,7-dinitro and 2,7-dibromo derivatives (2c,d) did not react with CO₂. However, the 2,7-di-tert-butyl derivative 2a resulted in a faster reaction to give 4a. Probably the nucleophilic power of their deprotonated forms of the iminophosphoranes (5c,d) do not reach the energy level necessary for the reaction (E^°_1/2 = 213 and 351 mV, respectively vs. Fc/Fc⁺). The elevated rate of the latter reaction is based on the enhanced nucleophilic character of the O-nucleophile formed by deprotonation of 3a due to the two tert-butyl electron-releasing groups. The addition of stoichiometric amount of triethylamine to the reaction mixture resulted also in faster reaction rate as a result of higher deprotonation of 3b (k₂ = 6.81 ± 0.12 and k₂^Et₃N = 28.69 ± 2.42 M⁻¹ s⁻¹). According to literature data, iminophosphoranes do not react with carbon dioxide.²¹ However, phenolates²² and also carbanions²³ are potent nucleophiles. They react even with triplet dioxygen when their electron affinity lies below 20 kcal mol⁻¹.²⁴ That is in agreement mostly with those of the nucleophilic reactions on CO₂.

Reaction of 3b with C¹⁸O₂ (∼50 : 50 C¹⁶O₂ : C¹⁸O₂) under similar conditions revealed that the carbonyl O-atom of 4b and the O-atom of triphenylphosphine oxide originates from the labeled C¹⁸O₂. The ¹⁸O : ¹⁶O ratio in both products based on GC-MS measurements are in a ratio of 45 : 55 and 43 : 57 respectively. The reaction product triphenylphosphine oxide have been identified by FTIR spectroscopy with the help of its ν(P=O) stretching frequencies of 1184 and 1152 cm⁻¹, Δν = 32 cm⁻¹ (calculated for P=¹⁸O 1184 cm⁻¹, Δν = 32 cm⁻¹), (P=¹⁶O and P=¹⁸O), 25.5 ppm in the ³¹P NMR spectrum. Mass spectrometric analysis of the reaction product indicated a molecular formula C₁₈H₁₅¹⁶OP (m/z [M]⁺: 278.28 and 280.28) for Ph₃P=¹⁶O and Ph₃P=¹⁸O. 3H-Phenanthro[9,10-d]oxazol-2-one (4b) showed in FTIR ν(C=¹⁶O and C=¹⁸O) 1752 and 1726 cm⁻¹, Δν = 26 cm⁻¹ (calculated for C=¹⁸O 1726 cm⁻¹, Δν = 26 cm⁻¹) and mass spectroscopic data of the molecular formula C₁₅H₉N¹⁶O₂ (m/z [M]⁺: 235.24 and 237.24) for the ¹⁶O- and ¹⁸O-derivative (Fig. 3).

Fig. 3 The GC-MS spectrum of the reaction product of 2,3-dihydro-2,2,2-triphenylphenanthro[9,10-d]-1,3,2λ⁵-oxazaphosphole (4b) prepared with a ∼50 : 50 mixture of C¹⁶O₂ : C¹⁸O₂ at room temperature in acetonitrile.

The mechanism of the reaction (Scheme 2) can be interpreted as a nucleophilic attack of the deprotonated iminophosphorane (5) on the carbon atom of CO₂ yielding a deprotonated semiester of carbonic acid 6, from which then the N=P bond of the iminophosphorane part of the molecule abstracts an oxygen atom to form triphenylphosphine oxide and the carbon atom of the O=C=O part closes with the imino N-atom of 6 to give the oxazol-2-one ring system 4a,b as endproduct (Route a). Route b, which is also in agreement with the kinetic data, is practically an aza-Wittig reaction.²¹ The iminophosphorane tautomer form 3 reacts with CO₂ forming the four-membered ring compound 7,²⁵ which decomposes to triphenylphosphine oxide and the isocyanate 8. This is followed with the intramolecular reaction of the isocyanate and the phenolic OH to end up in the oxazol-2-one (4). Both pathways may be operative, however the faster reaction rate when NEt₃ is added seems to support Route a and no reaction of iminophosphoranes (3) with CO₂ to form a four-membered ring is known from the literature. Recent results demonstrating a nucleophilic attack of an oxo anion are in agreement of this assumption.¹¹

Scheme 2 Possible mechanism of CO₂ fixation.

Kinetic investigations resulted in an overall second-order rate equation with k₂ value of 6.81 ± 0.12 M⁻¹ s⁻¹. Due to the large activation energy (92 kJ mol⁻¹) and the negative activation of entropy (−21 J mol⁻¹ K⁻¹) we can conclude that the rate-determining step involves a nucleophilic addition of the phenolate 5b on the carbon atom of CO₂ or the formation of the four-membered ring 7. Labeling experiments straightforward support that the O-atom of triphenylphosphine oxide and that of the carbonyl group originates from carbon dioxide.

Conclusions

In summary, we have disclosed that carbon dioxide, having the most oxidized state of carbon, the biggest obstacle for its implementation in industrial processes as a raw material in its low energy level, acts as a primary oxidant and as a C1-building block. We have found that 1,3,2-oxazaphospholes react easily with carbon dioxide even at room temperature to oxazol-2-ones and triphenylphosphine oxide. This methodology serves as a useful alternative for the photochemical preparation of oxazol-2-ones starting from benzoin.²⁵ In a coupled reaction triphenylphosphine is oxygenated to its oxide. The O-atoms of oxazol-2-ones and Ph₃PO originates from CO₂ proved by labeling experiments, and assumedly in a similar fashion like the Wittig reaction²⁶ this is the driving force for the reaction. 1,3,2-Oxazaphospholes should be rich on energy and we believe that the deprotonated form of the iminophosphorane tautomer 3 attacks the carbon dioxide in a nucleophile manner. This argumentation opens a new type of energy rich phenolate nucleophiles which are able to react with CO₂ in negative Gibbs free energy reactions.

Acknowledgements

This work was supported by a grant from The Hungarian Research Fund (OTKA) #K108489 and COST Actions CM1205, CM1201 and CM1003.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details of synthesis and structural characterisation, kinetics, electrochemical and spectroscopic data. CCDC 956401. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06462c

‡ Crystallographic data for 4b: C₆₀H₃₆N₄O₈; M_r = 940.93; crystal size 0.14 × 0.06 × 0.02 mm³; monoclinic; C2/c; a = 41.056(1), b = 10.5170(5), c = 22.3769(9) Å; β = 118.092(2)°; V = 8523.8(6) ų; Z = 8; ρ_cald = 1.466 g cm⁻¹; μ = 0.099 mm⁻¹; F(000) = 3904; T = 203(2) K; Kappa CCD area detector equipped with monochromated Mo_Kα radiation (λ = 0.71073 Å); 7254 reflections measured (Θ_max = 0.998°); 2499 independent reflexions; R₁ = 0.0865; wR₂ = 0.2074; GOF = 0.961.

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