Catalytic fluoride triggers dehydrative oxazolidinone synthesis from CO₂

Abstract

Herein, catalytic fluoride (F⁻) is demonstrated to be a trigger for dehydrative immobilization of atmospheric pressure CO₂, such that reaction of CO₂ with β-amino alcohols derived from natural amino acids gives optically pure oxazolidinones in high yields. A synergistic combination of fluoride and organosilicon agents (e.g., Bu₄NF + Ph₃SiF or siloxanes) enhances the catalytic activity and functional group compatibility. This system lies at the interface between homogenous and heterogeneous catalysis, and may prove useful for the development of recoverable/reusable siloxane-based CO₂ immobilization materials.

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The gradual depletion of fossil fuel resources has led to a search for other sources of carbon.¹ Carbon dioxide (CO₂) is one potential candidate, as it is the most abundant carbon source in the earth's atmosphere (around 300 billion tons), and is a cheap and non-toxic source of C₁.^1a Thus, data concerning the exploitation of CO₂ as a starting material in both the industrial and research laboratory is greatly needed. In order to understand the incorporation of CO₂ into organic frameworks² in the presence of catalysts, a quest for knowledge concerning the physical (reversible) and chemical (irreversible) interactions between CO₂ and catalyst is the foremost endeavor. Such electron donor (X:)/acceptor (CO₂) interactions accommodate X:–CO₂ adducts,³ in which the X: part may be an alkoxide,^1g,k amine,⁴ (e.g., 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a guanidine), N-heterocyclic carbene (NHC),⁵ or low-valent transition metal (M_T) complex⁶ (giving an η¹-M_T–CO₂ interaction). A wide range of X:–CO₂ adducts has been investigated theoretically and experimentally in conjunction with the catalytic transformation of CO₂. In contrast, fluorinated components (BF₄⁻, PF₆⁻, and N(SO₂CF₃)₂⁻) of ionic liquids⁷ as well as neutral perfluorinated organic surfaces⁸ can interact weakly with or physically adsorb CO₂. Fluoropolymers have been synthesized effectively in supercritical CO₂.⁹ Thus “fluorine” in a wider sense is a potential catalyst candidate; however, the catalytic activity of these compounds has thus far been poorly investigated with respect to the chemical transformation of atmospheric pressure (1 atm) CO₂.¹⁰ We report here that catalytic fluoride (F⁻) is indeed a potent trigger for CO₂ immobilization: CO₂ (1 atm) was incorporated dehydratively into β-amino alcohols derived from natural amino acids (Scheme 1), with functional group tolerance. As a result, a variety of functionalized N-unsubstituted oxazolidinones was obtained with retention of the stereogenic centers. Oxazolidinones are among the most widely used chiral auxiliaries in natural product and asymmetric synthesis.¹¹ Nevertheless, a simple and salt-free approach to oxazolidinone synthesis starting from optically pure β-amino alcohol and CO₂ has yet to be established.^12,13a This might be due to the thermodynamic disadvantage of the reaction^13a and catalyst deactivation caused by the coproduction of water.^1k In contrast, our work represents a successful synthesis of a variety of chiral oxazolidinones, which is otherwise difficult to achieve via previous dehydrative transformation of CO₂ (1 atm).^12f

Scheme 1 Oxazolidinone synthesis from atmospheric pressure CO₂ in the presence of catalytic fluoride. Fluorine is the most abundant halogen in the earth's crust: 0.065 wt%.³²

We recently reported that dialkyl carbonates^13b,c were produced using a stoichiometric amount of CsF upon reaction of CO₂ (1 atm) with alcohols and CH₂Cl₂.^13c In addition, during research on dehydrative oxazolidinone synthesis using alkali metal salts as precatalysts,^13a CsF slightly but positively affected CO₂ immobilization (Table 1, entry 1).^10a,b In contrast, use of Cs(OAc) or Cs₂SO₄ in place of CsF was completely unsatisfactory (2a: <2%). Hence, we changed our attention to the counteranion F⁻, and eventually found that when L-valinol ((S)-1a:[1a]₀ = 0.90 M) and a fluoride source, tetrabutylammonium difluorotriphenyl silicate (TBAT)¹⁴ (10 mol%: [TBAT]₀ = 90 mM), in DMSO-d₆ was exposed to 1 atm of CO₂ (purity = 99.9%) at 150 °C for 12 h, (S)-2a was obtained in 58% yield. Use of 20 mol% of TBAT produced 2a almost quantitatively (isolated yield: 90%,¹⁵ entry 9).^13d The yield of 2a was only 6% or ∼1% when CsF was replaced with KF or NaF, respectively (entries 2 and 5). Based on this trend, effective solvation of the Cs cation (Cs⁺) by DMSO¹⁶ leading to a more “naked”¹⁷ F⁻ appears to be playing an important role in the reactivity. Hence, for KF, [2.2.2]cryptand was used as an additive (10 mol%) to promote the dissociation of F⁻ from K⁺, and in fact, the product yield improved to 39% (entry 3). The role of 18-crown-6 (entry 4) is for solubilizing KF rather than for cation/anion separation,¹⁸ and indeed the result was comparable to that obtained using KF alone. Tetrabutylammonium fluoride (TBAF)¹⁹ bearing a more naked F⁻ was a better promoter (24% yield), but the reaction effectively stopped within 3 h (entry 7). Thermal decomposition of TBAF, whose trihydrate is known to undergo Hoffmann degradation at 40–77 °C,^19c was only slightly detected in DMSO at 150 °C, with or without 1 atm of CO₂, as judged by the marginal generation of 1-butene and Bu₃N via ¹³C NMR measurements.²⁰ This suggests that a strong base (“F⁻”), under the present conditions involving DMSO, is converted into a less basic species. To avoid fragmentation of the tetraalkylammonium cation, commercial Me₄NF²¹ was tested as received, but showed poorer activity (entry 8). A more anhydrous Me₄NF obtained following the literature procedure^19d did not improve the result (2a: 13%). Indeed, the moisture present in commercial DMSO did not severely interfere with the action of the catalyst. Use of tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF),²² which is more basic than TBAF,^17b was totally unsatisfactory (entry 6), probably due to its rapid degradation.^22c In cases where the formation of 2a was undetected or only moderately detected via ¹H NMR analysis, the carbamic acid of 1a (HOCH₂CH(ⁱPr)NHCO₂H: ESI, Fig. S1 and S2†) consistently remained unreacted in the reaction mixture.²³

Table 1 Synthesis of oxazolidinone (S)-2a from (S)-1a^a

Entry	Cat.	Yield^b (%)
a The reaction was performed using (S)-1a (1 mmol) in anhydrous DMSO-d6 (1 mL) with an initial CO2 pressure of 1 atm at 25 °C.b Determined by ^1H NMR (anisole as internal standard); number in parentheses is the isolated yield.c 10 mol% each.d See ref. 19 on how TBAF was used.e 20 mol% TBAT.f 100 mol% DBU.
1	CsF	13
2	KF	6
3	KF + [2.2.2]cryptand^c	39
4	KF + 18-crown-6^c	8
5	NaF	∼1
6	TASF	2
7	Bu4NF^d (TBAF)	24
8	Me4NF	16
9	Bu4N[Ph3SiF2] (TBAT)	58; 94^e (90)
10	Bu4N[Ph3SnF2]	14
11	Ph3SiF + TBAF^c^,^d	48
12	Ph3SiF + Me4NF^c	45
13	Ph3SiF	<1
14	Ph3SiF + KF^c	38
15	Ph3SiF + DBU^c	31
16	DBU	<5, 15^f
17	Ph3SiF + Bu4NOH^c	55
18	None	<1

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We thus turned our attention to “less naked” and hence less basic fluorides,¹⁶ which should be more nucleophilic, and therefore more thermally stable. Among fluoride sources of the form M_a⁺[M_bF_n]⁻ (M_a = alkali metal or tetraalkylammonium; M_b = element other than alkali metal; e.g., Na₂SiF₆, K₂SiF₆, K₂TaF₇, Bu₄NPF₆, Bu₄N[Ph₃SnF₂]) that were screened, only TBAT effectively promoted this reaction (entry 9). When TBAT was generated in situ by adding TBAF to Ph₃SiF, comparable reactivity was observed (entry 11). A similar result was also obtained by adding Me₄NF, incapable of degradation below 235 °C,^21b to Ph₃SiF (entry 12). Ph₃SiF alone gave no activity, but the combined use of it with KF or DBU resulted in a reaction rate of about half that of TBAT (entries 13–15). Yields of 2a were marginal in control experiments using DBU alone (10 mol% or 100 mol%: entry 16). DMSO was the best solvent screened for the TBAT reaction: 2a was not provided in better yields using DMF (25%), NMP (18%), DMI (34%), DMA (5%) or MeCN (<1%) under otherwise identical conditions. A 0.5–2 mL of DMSO per mmol of 1a ([TBAT]₀ = 45–180 mM) is recommended to obtain acceptable results. When no catalyst was present (entry 18), only a trace amount of the desired product was obtained (<1%).

Under the optimized conditions using commercial TBAT, various amino alcohols were tested (Table 2). According to theoretical calculations,^13a the reaction with ethanolamine 1b (R¹ = H) faces a thermodynamic disadvantage, and experimental observations are in agreement. Indeed, when 1b was used, only 20% of desired product 2b was produced (entry 1). Oxazolidinones, especially those widely used as Evans auxiliaries, were obtained in satisfactory yields (entries 5 and 7). Use of CO₂ (5 atm) with the original TBAT load (20 mol%) slightly improved the yield of 2i and 2j to 72% and 66%, respectively. A higher load of TBAT (40 mol%) also led to better yields (2: 76–92%) with 5 atm of CO₂, when β-amino alcohols derived from cysteine (1i: R¹ = BnSCH₂), tyrosine (1k: R¹ = p-HO(C₆H₄)CH₂), and tryptophan (1l: R¹ = 3-indolylmethyl) were used (entries 11, 14 and 17). Since a rather acidic phenolic OH was tolerated, the overall CO₂-incorporation pathway likely involves weak acid–weak base cooperative catalysis. Retention of the absolute configuration at the β-positions of β-amino alcohols 1a, 1c–l was consistently observed, giving products 2a and 2c–l as single enantiomers (ESI, S7–S14†). Sterically more demanding 1m and 1o, and α-phenyl-β-amino alcohols 1n, showed less or scant reactivity (entries 19–21).

Table 2 Substrate scope with catalytic TBAT^a

Entry	Amino alcohol 1	Oxazolidinone 2	Yield^b [%]
a Unless otherwise specified, the reaction conditions were: 1 (0.5 mmol), TBAT (20 mol%: [TBAT]0 = ca. 180 mM), CO2 (PCO2 = 1 atm; CO2 balloon) in anhydrous DMSO-d6 (0.5 mL) at 150 °C for 9 h.b Determined by ^1H NMR (anisole as internal standard); values in parentheses are isolated yield.c TBAT (40 mol%) was used.d DMSO (1 mL) and PCO2 = 5 atm were used.
1			20
2	(S)-1c (R^1 = Me)	(S)-2c (R^1 = Me)	72 (69)
3	(S)-1d (R^1 = Et)	(S)-2d (R^1 = Et)	95 (93)
4	(S)-1e (R^1 = ^iBu)	(S)-2e (R^1 = ^iBu)	86 (77)
5	(S)-1f (R^1 = Bn)	(S)-2f (R^1 = Bn)	73 (64)
6		(S)-2g (R = ^sBu)	94 (92)
7	(S)-1h (R^1 = ^tBu)	(S)-2h (R^1 = ^tBu)	93 (81)
8	(S)-1i (R^1 = Ph)	(S)-2i (R^1 = Ph)	69 (60)
9			85 (76)^c^,^d
10	(R)-1j (R^1 = BnSCH2)	(R)-2j (R^1 = BnSCH2)	48 (42)
11			92^c^,^d
12		(S)-2k (R^1 = p-HO(C6H4)CH2)	42 (38)
13			52^c
14			76^c^,^d
15		(R)-2k (R^1 = p-HO(C6H4)CH2)	36 (32)
16		(S)-2l (R^1 = 3-indolylmethyl)	80^c (69)
17			90^c^,^d
18		(R)-2l (R^1 = 3-indolylmethyl)	61 (54)
19			27 (23)
20			41 (36)
21			<3

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Carbamic acid, of the general formula HOCH(R²)CH(R¹)NHCO₂H, can be formed from amino alcohols when exposed to CO₂ and was detected in DMSO-d₆ as a more favorable structure than the ammonium carbamate.²³ For example, carbamic acid (R¹ = Me; R² = H, Scheme 2) has two hydroxy groups (the alcohol OH and the acid OH); one acts as a nucleophile and the other is liberated. Generally, the alcohol OH acts as the nucleophile in C–O bond formation.²⁴ Nonetheless, selectivity of the leaving OH can be altered with steric control.^25,26 Thus, control experiments were carried out to clarify which OH is liberated as water (Scheme 2). In contrast to the complete retention at the β-carbon of the β-amino alcohols, the chirality at the α-position can provide useful information concerning which OH group (the alcohol or the acid OH) is liberated. In this case, (R)-1-aminopropan-2-ol ((R)-1p) was used under similar reaction conditions. Chiral GC analysis of the product showed complete retention of the parent chiral center bearing the methyl substituent, demonstrating exclusive formation of (R)-2p (ESI, S13†). In addition, when C¹⁸O₂ was reacted with 1a, only one ¹⁸O was incorporated predominantly into 2a.²⁷ These results suggest that the alcohol OH acts as a nucleophile and the acid OH is a leaving group.

Scheme 2 Reaction with (R)-1-aminopropan-2-ol ((R)-1p). Two possible pathways.

The conversion of (S)-1a upon reaction with CO₂ in the presence of 10 mol% of TBAT effectively ceased after 9 h. ¹⁹F NMR (DMSO-d₆, ppm) spectra of the mixture over the course of the reaction (5–9 h) showed a singlet at δ −117, δ −124, and δ −145 ± 3 (Fig. S5, ESI†), which converged into two singlets at δ −124 and δ −145 ± 3 (>9 h, Fig. S6, ESI†), presumably corresponding to the catalytically incompetent F⁻(H₂O)_n and FHF⁻, respectively.²⁰ The species at δ −117, which was retained during ongoing catalysis, was suspected to be PhSiF₄⁻.²⁸ Hence, Bu₄NF/PhSiF₃ (10 mol% each) were tested separately as precatalysts, but effectively no reaction took place. A competent catalyst clearly maintained its activity for approximately 1–4 h, so a DMSO-d₆ solution of TBAT was heated at 150 °C for 3 h in a closed vessel under N₂, and the resulting mixture was evaluated via ¹H NMR (ppm) at 25 °C. The Si–Ph bonds were cleaved, and benzene-d₁ was detected as an intense singlet. The benzene-d₁ formation was reconfirmed by low-resolution mass spectroscopy (ESI, Fig. S3 and S4†). Although the intricate structures of other Si-decomposition species were not identified, it is suspected that at high temperatures, an Si–O bond may be formed in the presence of the H₂O that is gradually generated as the reaction proceeds. Hence, silicon sources bearing an Si–O bond (silanol and siloxane derivatives) instead of Ph₃SiF were mixed with TBAF in a 1 : 1 molar ratio, and the two-component systems were tested as catalysts (Table 3). The system involving (Me₂SiO)₃ (3a) was among the best silanol (siloxane) derivatives screened thus far, showed more sustainable activity, and even only 3.3 mol% of 3a and 10 mol% of TBAF yielded not less than 90% of (S)-2a (entry 4). No reaction was observed using 3a alone. Although TBAF/acyclic siloxane (entries 1 and 2) showed catalytic activity over that of TBAF alone, they are less active than TBAF/(Me₂SiO)_n (n = 3 (3a), 4 (3b), 5 (3c)) (entries 4–6). A silsesquioxane^29a–k,30 series, (MeSiO_1.5)₈ (3d)/TBAF and 3e/TBAF, showed more sustainable activity than TBAF or TBAT alone, with the yield of 2a even increasing by extending the reaction time from 12 h to 24 h (entries 4, 7 and 8). Silyl ethers of the general formula (RO)_nSiMe_4−n (n = 1–4) (entries 11–14) and (EtO)₃SiF (entry 15) were less satisfactory; however, (RO)₂SiMe₂ and (RO)₃SiMe showed better reactivity than other (RO)_nSiMe_4−n (n = 1, 4) compounds. Since the two-component system Ph₃SiF/Bu₄NOH showed a synergistic effect (Table 1, entry 17) similar to that of Ph₃SiOH/Bu₄NF (2a: 48%, entry 10), the role of F⁻ is unlikely to generate the conjugate base of the silanol, Ph₃SiO⁻Bu₄N⁺. Potent catalysis by a “Me₃SiO⁻” species was also ruled out by the sluggish reaction observed with Me₃SiOH/Bu₄NOH (10 mol% each), yielding 2a in 13% yield after a reaction time of 12 h. Me-substituted silicon centers were better cocatalysts than Ph-substituted ones (Me₂SiO vs. Ph₂SiO, and Me₃SiOH vs. Ph₃SiOH) (entries 3, 4, 9 and 10, respectively). To summarize, the platform structure [–SiMe_n–O–] (n = 1 or 2) serves as a more critical catalyst. The precatalyst loads of 3a and TBAF were successfully decreased to 1.7 mol% and 5 mol%, respectively, under 5 atm of CO₂, giving (S)-2a in an NMR yield of 98% after a reaction time of 24 h.

Table 3 Screening of Si–O agents in the reaction of (S)-1a with CO₂ (1 atm) and catalytic TBAF^a

Entry	Si–O agent	Yield^b (%)
a Unless otherwise specified, the reaction was performed at 150 °C for 12 h using (S)-1a (1 mmol), TBAF (10 mol%), and the Si–O agent (10 mol%) in anhydrous DMSO-d6 (1 mL) with an initial PCO2 = 1 atm (25 °C).b Determined by ^1H NMR (anisole as internal standard).c 5 mol% Si–O agent.d Me4NF instead of TBAF.e 3.3 mol% 3a.f 24 h.g 1.25 mol% 3d.
1	Me2HSiOSiHMe2	80 (79)^c
2	Me3SiOSiMe3	67
3	(Ph2SiO)3	67
4	(Me2SiO)3 (3a)	85 (89)^d (91)^e^,^f
5	(Me2SiO)4 (3b)	84
6	(Me2SiO)5 (3c)	83
7		66 (79)^f (62)^f^,^g
8		58 (74)^f
9	Me3SiOH	63
10	Ph3SiOH	48
11	(EtO)4Si	44
12	(EtO)3SiMe	65
13	(EtO)2SiMe2	68
14	(EtO)SiMe3	38
15	(EtO)3SiF	30

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With potent partners F⁻ and siloxane 3a ([Si]_app : [F⁻]₀ = 1 : 1) in hand, the net [F⁻]₀ sufficed to catalyze the reaction can be decreased to half or even one quarter the amount of F⁻ incorporated in TBAT (Table 4). β-amino alcohols (S)-1c, 1f, 1j and 1l, (R)-1p and 1m, 1o, which showed poor reactivity when reacted with TBAT, were converted to 2 more easily or with greatly improved yields (entries 5–7) under one atmosphere of CO₂.

Table 4 Substrate scope using TBAF and (Me₂SiO)₃ (3a) under 1 atm CO₂^a

Entry	1	3a (mol%)	TBAF (mol%)	2, Yield^b [%] (er)^c
a Unless otherwise specified, reaction conditions were: [1] : [F^−] : [3a] = 15 : 3 : 1, [F^−]0 = 90–100 mM; PCO2 = 1 atm (CO2 balloon) in anhydrous DMSO-d6 (1 mL) at 150 °C for 24 h.b Determined by ^1H NMR (anisole as internal standard).c Enantiomeric ratio.
1	(S)-1c	3.3	10	(S)-2c, 72 (99)
2	(S)-1f	6.6	20	(S)-2f, 78 (99)
3	(S)-1j	6.6	20	(S)-2j, 60 (99)
4	(S)-1l	6.6	20	(S)-1l, 79 (99)
5	1m	6.6	20	2m, 63 (—)
6	1o	6.6	20	2o, 72 (—)
7	(R)-1p	6.6	20	(R)-2p, 71 (99)

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Conclusions

In summary, catalytic fluoride, with the aid of a silicon agent, triggers competent catalysts that can promote the chemoselective production of optically pure oxazolidinones from CO₂ (1 atm) and β-amino alcohols derived from natural amino acids. The reactants shown here, as well as silicon and fluorine are all ubiquitous resources. The catalyst system is viable in the presence of a small amount of water, and without any dehydrating agent. Thus, a strict in-advance dehydration procedure for tetraalkylammonium fluoride hydrates, which is usually conducted prior to incorporation of nucleophilic fluoride into an organic framework, is no longer required. The synthetic protocol reported here is simple, straightforward, and can be easily replicated. Ongoing work involves the molecular design of both the tetraalkylammonium^21b,31 and siloxane components, in an effort to expand the substrate and reaction scope of the dehydrative CO₂ transformations. These results provide a basis for constructing a fluoride/siloxane-containing catalyst lying at the interface between homogeneous and heterogeneous, for use in the chemical immobilization of CO₂. The potential materials that could be recovered and reused encompass silicones,^30b mesoporous organosilica,^29l,m inorganic silica gel,^29n,o and ionic liquids.⁷ These attempts will open a new avenue for dehydrative chemical immobilization of atmospheric pressure CO₂.

Acknowledgements

This work was supported by funds from Asahi Glass Foundation, Asahi Glass, Asahi Kasei Chemicals, Kuraray, Sumitomo Chemical, Nissan Chemical Industries, Mitsubishi Chemical, Mitsui Chemicals, as well as the JST research area “Creation of Advanced Catalytic Transformation for the Sustainable Manufacturing at Low Energy, Low Environmental Load (ACT-C)”. The authors wish to thank Prof. R. Noyori (NU & RIKEN) for fruitful discussions. We are also grateful to K. Oyama and Y. Maeda (Chemical Instrument Room, NU), in addition to Profs S. Yamaguchi and A. Fukazawa (NU), for technical support and T. Noda, H. Natsume, and H. Okamoto (NU) for their support in the production of reaction vessels.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization of new compounds. See DOI: 10.1039/c4ra09609f

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