Synthesis of δ-oxo-1,1-bis(triflyl)alkanes and their acidities.

The reaction of 1,1-bis(triflyl)ethylene generated in situ with enolizable carbonyls yielded δ-oxo-1,1-bis(triflyl)alkane derivatives. Their acidities in both the gas and solution phases were determined.

1. Introduction

The bis(triflyl)methyl (Tf2CH; Tf = CF3SO2) group is known to be a strong C–H acidic functionality due to the gem-disubstitution of a carbon atom by two triflyl groups. This type of carbon acid (C–H acid) shows notably strong acidity not only in the gas-phase [1,2] but also in solution-phase [3]. For example, the gas-phase acidity ΔGacid of Tf2CH2 (1) has been determined to be 300.6 kcal mol−1. Compared to the value of sulfuric acid (302.2 kcal mol−1), this somewhat lower value means that Tf2CH2 1 performs as a superacid in the gas-phase. The pKa of 1 in DMSO is also measured as 2.1 and it works as a better proton donor relative to trifluoroacetic acid (pKa in DMSO = 3.45). On the basis of this feature, some powerful Brønsted acid catalysts containing Tf2CH functionalities such as Tf2CHC6F5 [4,5,6], Tf2CHCH2CHTf2 [7,8,9,10], and multiple carbon acids [11,12,13] were developed. Compared to the corresponding nitrogen acid Tf2NH and oxygen acid TfOH, these carbon acids show excellent catalyst performance in several synthetic reactions, including the Mukaiyama aldol reaction, the Friedel–Crafts acylation, and esterification. However, the synthesis and purification of such strongly acidic carbon acids are not so easy [14]. For example, Koshar and co-workers reported that in situ-formation of 1,1-bis(triflyl)ethylene (2) by the reaction of Tf2CH2 (1) with paraformaldehyde in the presence of CaSO4 and the subsequent one-pot reaction with diethyl malonate (3a) gave the bis(triflyl)ethylated malonate 4a in poor yield (Scheme 1) [15].

Scheme 1

Koshar’s synthesis of bis(triflyl)ethylated malonate 4a.

Since this reaction required harsh conditions for the effective generation of the alkene intermediate 2, the yield of 4a was not very high. To overcome this problem, we reported that 1,1,3,3-tetrakis(triflyl)propane (5, Figure 1) [16,17] can be used as not only an acid catalyst, but also a very effective reagent for in situ-generation of Tf2C=CH2 (2) via a retro-Michael type reaction. Recently, zwitterion 6 (Figure 1) was also developed for the same use [18]. These reagents can be easily prepared on multi-gram scale from commercially available Tf2CH2 (1) in one step.

Figure 1

Structures of 1,1-bis(triflyl)ethylating reagents.

Herein we report an improved synthesis of δ-oxo-1,1-bis(triflyl)alkanes via bis(triflyl)ethylation reaction of enolizable carbonyls with tetrasulfone 5. Furthermore, both gas-phase acidity and pKa values in a DMSO solution of some of the prepared carbon acids were determined.

2. Results and Discussion

2.1. Improved Synthesis of δ-Oxo-1,1-bis(triflyl)alkanes

Keeping Koshar’s original work in mind, we first examined the 2,2-bis(triflyl)ethylation reaction of diethyl malonate (3a, Scheme 2). Notably, the reaction of 3a with Tf2CHCH2CHTf2 (5) was smoothly completed within 3 h at 80 °C. In this case, we observed complete consumption of tetrasulfone 5 and quantitative formation of Tf2CH2 (1) and the desired carbon acid 4a by 19F-NMR analysis of the crude mixture. This mixture was successfully purified by bulb-to-bulb distillation (150 °C at 5 mmHg) using a Kugelrohr oven to give acceptably pure carbon acid 4a in 84% yield. Compared to Koshar’s procedure, the use of tetrasulfone 5 instead of Tf2CH2/paraformaldehyde resulted in a better yield of 4a (50% vs. 84%) within a shorter reaction time.

Scheme 2

Improved synthesis of bis(triflyl)ethylated malonate 4a.

Smooth formation of the carbon acid 4a in the present case could be attributed to rapid formation of alkene intermediate 2 in solution phase from tetrasulfone 5. For instance, 1H-NMR analysis of a solution of 5 in CDCl3 at 40 °C revealed very rapid formation of 2 in a reversible manner (Figure 2). When this mixture was left for 20 min at 40 °C, tetrasulfone 5 partly decomposed to Tf2CH2 (1) and alkene 2 to give an equilibrium mixture between 5 and 1/2 without formation of any side products and its equilibrium constant Keq was calculated as 3.21 × 10−3.

Figure 2

Reaction profile in a 0.01 M solution of Tf2CHCH2CHTf2 (5) in CDCl3 at 40 °C (squares, Tf2CHCH2CHTf2 (5); diamonds, Tf2CH2 (1); triangles, Tf2C=CH2 (2)).

Since the product yield in the bis(triflyl)ethylation with Tf2CHCH2CHTf2 (5) was better than that in Koshar’s original procedure using Tf2CH2 (1) and paraformaldehyde, we carried out the reaction of several active methylenes with tetrasulfone 5. Selected results are summarized in Table 1. When some dialkyl malonates such as 3b and 3c were treated with tetrasulfone 5 at 80 °C in 1,2-dichloroethane, the desired bis(triflyl)ethylated products 4b and 4c were obtained in 84% and 98% yield, respectively (entries 1 and 2). In the case of dimethyl malonate (3b), the product 4b was isolated after standard bulb-to-bulb distillation (method A). Although distillation of dibenzyl derivative 4c was problematic due to its high boiling point, acceptably pure 4c was obtained by removal of Tf2CH2 (1) and remaining 3c by a Kugelrohr oven (120 °C at 2 mmHg; method B). Under similar conditions, carbon acid 4d derived from phosphonyl acetate 3d was isolated in 57% yield using method A (entry 3). The reactions of β-ketoesters and of 1,3-diketones proceeded under more mild conditions in CH2Cl2. For example, the products 4e–g derived from β-ketoesters were obtained in excellent yields by the reaction at 40 °C (entries 4–6). Likewise, the reactions of 1,3-diketones 3h and 3i with tetrasulfone 5 completed at room temperature to give the corresponding products 4h and 4i in 74% and 80% yields, respectively (entries 7 and 8).

Table 1

Reaction of Tf2CHCH2CHTf2 5 with 1,3-dicarbonyl compound 3.

Entry	3		Temp. (°C)	Time (h)	Method a	4	Yield b (%)
1 c	3b	CH2(CO2CH3)2	80	8	A	4b	84
2 c	3c	CH2(CO2Bn)2	80	8	B	4c	98
3 c	3d	CH2(CO2CH3)P(O)(OCH3)2	80	5.5	A	4d	57
4	3e	CH2(COt-Bu)CO2CH3	40	2	A	4e	93
5	3f	CH2(COt-Bu)CO2Et	40	5	A	4f	82
6	3g	CH2(COPh)CO2Et	40	4.5	B	4g	86
7	3h	CH2(COt-Bu)2	Rt	4	A	4h	73
8	3i	CH2(COi-Pr)2	Rt	2.5	A	4i	80

Method A; Product was isolated by bulb-to-bulb distillation. Method B; Product was purified by distillative removal of Tf2CH2 1 and remaining 3 using a Kugelrohr oven; Isolated yield; Reaction was carried out in 1,2-dichloroethane.

As shown in Scheme 3, it should be noted that less reactive triester 3j smoothly converted to the corresponding bis(triflyl)ethylated product 4j in 78% yield under the present conditions. In this case, we found that the use of acetonitrile instead of 1,2-dichloroethane gives a better yield of 4j.

Scheme 3

Bis(triflyl)ethylation of triester 3j.

2.2. Gas-Phase and Solution-Phase Acidities of Carbon Acids

The gas-phase acidity ΔGacid established with the use of the FT-ICR technique [1,2] is known as an extensive scale for strong acids. We measured ΔGacid values of some select carbon acids (Table 2). The values of triflylated methanes were reduced by increasing in the number of triflyl group (TfCH3, 339.8 kcal mol−1 [19]; Tf2CH2 (1), 300.6 kcal mol−1 [1]; Tf3CH, 289.0 kcal mol−1 [2]) (entries 1, 2, and 7). The value of Tf2CHCH2CHTf2 (5) was recently revised to 290.2 kcal mol−1 and its acidity is notably stronger than that of Tf2CH2 1 [20]. On the other hand, δ-oxo-1,1-bis(triflyl)alkanes 4b, 4e, and 4h showed very similar acidities in gas-phase compared to Tf2CH2 (1). That is, established ΔGacid values of these compounds were 299.6 kcal mol−1, 300.3 kcal mol−1, and 300.4 kcal mol−1, respectively (entries 3–5). This finding suggests that the difference of carbonyl functionalities in the structures of δ-oxo-1,1-bis(triflyl)alkanes is not critical factor for their gas-phase acidities. Therefore, the symmetrical structure of 5 plays an important role for its significantly enhanced acidity (the statistical effect). In addition, the pKa value of carbon acid 4b in DMSO solution was determined as 2.16 by the voltammetric method [21,22]. This also means that the 4 is comparable in the acidity to 1 (pKa = 2.1) in DMSO solution.

Table 2

The gas-phase acidities of carbon acids.

Entry	Carbon Acid	ΔGacid (kcal mol−1)
1 a	TfCH3	339.8
2 b	Tf2CH2 (1)	300.6
3	Tf2CHCH2CH(COt-Bu)2 (4h)	300.4
4	Tf2CHCH2CH(COt-Bu)CO2CH3 (4e)	300.3
5	Tf2CHCH2CH(CO2CH3)2 (4b)	299.6
6 c	Tf2CHCH2CHTf2 (5)	290.2
7 d	Tf3CH	289.0

Ref. [19]. Ref. [1]. Ref. [20]. Ref. [2].

3. Experimental

3.1. General

All reactions were carried out under Ar atmosphere. Melting points were uncorrected. 1H- (400 MHz) and 13C-NMR (100 MHz) spectra were taken on a Bruker DPX 400 spectrometer, and chemical shifts were reported in parts per million (ppm) using CHCl3 (7.26 ppm) in CDCl3 for 1H-NMR, and CDCl3 (77.01 ppm) for 13C-NMR as an internal standard, respectively. 19F-NMR spectra were taken on a Varian Mercury 300 spectrometer (282 MHz for 19F), and chemical shifts were reported in parts per million using trifluoromethylbenzene (0 ppm) as a standard. Mass spectra were recorded by an electrospray ionization-time of flight (ESI-TOF) mass spectrometer (Micromass LCT). IR spectra were recorded by a JASCO FT/IR 4100 spectrometer. Column chromatography was performed on neutral silica gel (75–150 μm). Tf2CHCH2CHTf2 (5) was prepared from Tf2CH2 (1) by the reported procedure [15].

3.2. General Procedure for Bis(triflyl)ethylation Reaction of Enolizable Carbonyls

To a solution of carbonyl compound (1.0–2.0 equiv) in CH2Cl2 or 1,2-dichloroethane, Tf2CHCH2CHTf2 (5, 0.50 mmol) was added at room temperature. After stirring at room temperature to 80 °C, the reaction mixture was concentrated under reduced pressure. The resultant residue was purified by bulb-to-bulb distillation using a Kugelrohr oven to give bis(triflyl)ethylated product 4.

Diethyl 2-(2,2-bis(trifluoromethylsulfonyl)ethyl)malonate (4a). According to the general procedure, this compound was obtained in 84% yield (190 mg, 0.420 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 286 mg, 0.500 mmol) with diethyl malonate (83.5 μL, 0.55 mmol) in CH2Cl2 (0.10 mL) at 80 °C for 3 h and the following bulb-to-bulb distillation (140–160 °C at 5 mmHg). Colorless oil; IR (neat) ν 2989, 2944, 1746, 1397, 1214, 1115, 1024 cm−1; 1H-NMR (CDCl3) δ 1.29 (6H, t, J = 7.1 Hz), 2.91–2.98 (2H, m), 3.93 (1H, t, J = 7.8 Hz), 4.19–4.33 (4H, m), 5.73 (1H, t, J = 6.6 Hz); 13C-NMR (CDCl3) δ 13.9, 24.6, 47.5, 62.7, 74.4, 119.2 (q, JC-F = 329.6 Hz), 167.5; 19F-NMR (CDCl3) δ−10.1 (6F, s); MS (ESI-TOF) m/z 453 [M+H]+; HRMS calcd. for C11H15F6O8S2 [M+H]+, 453.0113; found, 453.0054. Anal. calcd. for C11H14F6O8S2: C, 29.21; H, 3.12. Found: C, 29.24; H, 3.19.

Dimethyl 2-(2,2-bis(trifluoromethylsulfonyl)ethyl)malonate (4b). According to the general procedure, this compound was obtained in 84% yield (178 mg, 0.420 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 286 mg, 0.500 mmol) with dimethyl malonate (62.9 μL, 0.55 mmol) in CH2Cl2 (0.20 mL) at 80 °C for 8 h and the following bulb-to-bulb distillation (160–170 °C at 5 mmHg). Colorless crystals (Et2O-hexane); Mp. 44.2–46.8 °C; IR (neat) ν 3007, 2962, 2854, 1747, 1440, 1395, 1215, 1114, 1036, 1004, 705 cm−1; 1H-NMR (CDCl3) δ 2.92–2.98 (2H, m), 3.81 (6H, s), 3.99 (1H, t, J = 7.8 Hz), 5.69 (1H, t, J = 6.6 Hz); 13C-NMR (CDCl3) δ 24.7, 47.3, 53.5, 74.3, 119.2 (q, JC-F = 329.8 Hz), 167.8; 19F-NMR (CDCl3) δ −10.0 (6F, s); MS (ESI-TOF) m/z 425 [M+H]+; HRMS calcd for C9H11F6O8S2 [M+H]+, 424.9800; found, 424.9825. Anal. calcd. for C9H10F6O8S2: C, 25.48; H, 2.38. Found: C, 25.82; H, 2.57.

Dibenzyl 2-(2,2-bis(trifluoromethylsulfonyl)ethyl)malonate (4c). According to the general procedure, this compound was obtained in 98% yield (56.7 mg, 984 μmol) by the reaction of dibenzyl malonate (28.4 mg, 0.10 mmol) with Tf2CHCH2CHTf2 (5, 64.0 mg, 0.112 mmol) in CH2Cl2 (0.15 mL) at 80 °C for 8 h and the following removal of Tf2CH2 using a Kugelrohr oven (140–150 °C at 5 mmHg). Colorless oil; IR (neat) ν 3068, 3036, 2954, 1747, 1498, 1456, 1396, 1302, 1215, 1114, 750, 697 cm−1; 1H-NMR (CDCl3) δ 2.98 (2H, t, J = 7.2 Hz), 4.05 (1H, t, J = 7.2 Hz), 5.15 (2H, d, J = 12.3 Hz), 5.18 (2H, d, J = 12.3 Hz), 5.66 (1H, t, J = 7.2 Hz), 7.24–7.27 (4H, m), 7.30–7.34 (6H, m); 13C-NMR (CDCl3) δ 24.5, 47.6, 68.3, 74.3, 119.2 (q, JC-F = 329.7 Hz), 128.4, 128.7, 128.8, 134.3, 167.2; 19F-NMR (CDCl3) δ −10.0 (6F, s); MS (ESI-TOF) m/z 599 [M+Na]+; HRMS calcd. for C21H18F6NaO8S2 [M+Na]+, 599.0245; found, 599.0247. Anal. calcd. for C21H18F6O8S2: C, 43.75; H, 3.15. Found: C, 44.09; H, 3.35.

Methyl 2-(dimethoxyphosphoryl)-4,4-bis(trifluoromethylsulfonyl)butanoate (4d). According to the general procedure, this compound was obtained in 57% yield (68.2 mg, 0.144 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 145 mg, 0.253 mmol) with methyl 2-(dimethoxyphosphoryl)acetate (47.2 mg, 0.259 mmol) in 1,2-dichloroethane (0.40 mL) at 80 °C for 5.5 h and the following bulb-to-bulb distillation (190–210 °C at 5 mmHg). Colorless crystal (Et2O); Mp. 80.5–83.2 °C; IR (neat) ν 2964, 1741, 1394, 1342, 1216, 1115, 1053 cm−1; 1H-NMR (CDCl3) δ 2.86–3.04 (2H, m), 3.63 (1H, dt, JH-P = 23.8 Hz, JH-H = 7.8 Hz), 3.818 (3H, s), 3.821 (3H, d, JH-P = 10.9 Hz), 3.85 (3H, d, JH-P = 10.5 Hz), 6.21 (1H, t, J = 6.4 Hz); 13C-NMR (CDCl3) δ 23.2, 40.1 (d, JC-P = 130.5 Hz), 53.5, 53.9 (d, JC-P = 7.1 Hz), 54.0 (d, JC-P = 6.4 Hz), 74.5, 119.2 (d, JC-F = 329.8 Hz), 167.3 (d, JC-P = 6.9 Hz); 19F-NMR (CDCl3) δ −10.3 (3F, s), −10.0 (3F, s); MS (ESI-TOF) m/z 475 [M+H]+; HRMS calcd. for C9H14F6O9PS2 [M+H]+, 474.9721; found, 474.9714. Anal. calcd. for C9H13F6O9PS2: C, 22.79; H, 2.76. Found: C, 22.63; H, 3.04.

Methyl 2-(2,2-bis(trifluoromethylsulfonyl)ethyl)-4,4-dimethyl-3-oxopentanoate (4e). According to the general procedure, this compound was obtained in 93% yield (73.4 mg, 0.163 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 100 mg, 0.175 mmol) with methyl 4,4-dimethyl-3-oxopentanoate (31 μL, 0.19 mmol) in CH2Cl2 (0.15 mL) at 40 °C for 2 h and the following bulb-to-bulb distillation (150–160 °C at 5 mmHg). Colorless crystals (Et2O-hexane); Mp. 55.1–55.9 °C; IR (KBr) ν 2975, 2911, 2880, 1742, 1712, 1481, 1439, 1396, 1350, 1214, 1115, 970, 699, 676 cm−1; 1H-NMR (CDCl3) δ 1.21 (9H, s), 2.60 (1H, ddd, J = 15.0, 9.6, 4.5 Hz), 2.95 (1H, ddd, J = 15.0, 10.5, 3.6 Hz), 3.74 (3H, s), 4.53 (1H, dd, J = 10.5, 4.5 Hz), 5.68 (1H, dd, J = 9.6, 3.6 Hz); 13C-NMR (CDCl3) δ 25.8, 25.9, 45.8, 47.3, 53.2, 74.6, 119.20 (q, JC-F = 329.8 Hz), 119.23 (q, JC-F = 329.7 Hz), 169.2, 208.6; 19F-NMR (CDCl3) δ −10.2 (3F, s), −10.1 (3F, s); MS (ESI-TOF) m/z 451 [M+H]+; HRMS calcd. for C12H17F6O7S2 [M+H]+, 451.0320; found, 451.0320. Anal. calcd. for C12H16F6O7S2: C, 32.00; H, 3.58. Found: C, 31.63; H, 3.63.

Ethyl 2-(2,2-bis(trifluoromethylsulfonyl)ethyl)-4,4-dimethyl-3-oxopentanoate (4f). According to the general procedure, this compound was obtained in 82% yield (223 mg, 0.480 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 336 mg, 0.587 mmol) with ethyl 4,4-dimethyl-3-oxopentanoate (104 μL, 0.59 mmol) in 1,2-dichloroethane (0.50 mL) at 40 °C for 5 h and the following bulb-to-bulb distillation (160–170 °C at 5 mmHg). Colorless crystals (Et2O-hexane); Mp. 34.7–36.5 °C; IR (KBr) ν 2979, 2942, 2911, 2876, 1738, 1712, 1480, 1397, 1350, 1213, 1114, 847, 781, 678 cm−1; 1H-NMR (CDCl3) δ 1.20 (9H, s), 1.24 (3H, t, J = 7.1 Hz), 2.58 (1H, ddd, J = 15.1, 10.3, 4.4 Hz), 2.95 (1H, ddd, J = 15.1, 11.2, 3.5 Hz), 4.18 (2H, q, J = 7.1 Hz), 4.50 (1H, dd, J = 11.2, 4.4 Hz), 5.71 (1H, dd, J = 10.3, 3.5 Hz); 13C-NMR (CDCl3) δ 13.7, 25.7, 25.9, 45.7, 47.4, 62.5, 74.6, 119.20 (q, JC-F = 329.6 Hz), 119.24 (q, JC-F = 329.8 Hz), 168.7, 208.7; 19F-NMR (CDCl3) δ –10.3 (3F, s), –10.0 (3F, s); MS (ESI-TOF) m/z 465 [M+H]+; HRMS calcd. for C13H19F6O7S2 [M+H]+, 465.0476; found, 465.0496. Anal. calcd. for C13H18F6O7S2: C, 33.63; H, 3.91. Found: C, 33.47; H, 4.17.

Ethyl 2-benzoyl-4,4-bis(trifluoromethylsulfonyl)butanoate (4g). According to the general procedure, this compound was obtained in 86% yield (86.1 mg, 0.178 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 119 mg, 0.208 mmol) with ethyl 3-oxo-3-phenylpropanoate (44.0 mg, 0.229 mmol) in CH2Cl2 (0.15 mL) at 40 °C for 4.5 h and the following removal of Tf2CH2 (1) using a Kugelrohr oven (120 °C at 5 mmHg). Colorless oil; IR (neat) ν 3068, 2990, 2932, 1738, 1688, 1598, 1449, 1396, 1293, 1215, 1114, 1028, 774, 689 cm−1; 1H-NMR (CDCl3) δ 1.14 (3H, t, J = 7.1 Hz), 2.95–3.08 (1H, m), 3.08–3.16 (1H, ddd, J = 15.8, 8.8, 5.7 Hz), 4.16 (2H, q, J = 7.1 Hz), 5.02 (1H, dd, J = 8.8, 6.4 Hz), 5.62 (1H, dd, J = 8.0, 5.7 Hz), 7.53 (2H, t, J = 7.0 Hz), 7.64–7.68 (1H, m), 8.00 (2H, d, J = 7.0 Hz); 13C-NMR (CDCl3) δ 13.7, 24.7, 49.3, 62.7, 74.8, 119.22 (q, JC-F = 329.8 Hz), 119.25 (q, JC-F = 329.8 Hz), 128.9, 129.0, 134.5, 135.1, 168.1, 193.0; 19F-NMR (CDCl3) δ −10.1 (3F, s), −9.9 (3F, s); MS (ESI-TOF) m/z 485 [M+H]+; HRMS calcd. for C15H15F6O7S2 [M+H]+, 485.0163; found, 485.0156.

4-(2,2-Bis(trifluoromethylsulfonyl)ethyl)-2,2,6,6-tetramethylheptane-3,5-dione (4h). According to the general procedure, this compound was obtained in 73% yield (72.8 mg, 0.153 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 119 mg, 0.208 mmol) with 2,2,6,6-tetramethylheptane-3,5-dione (48 μL, 0.23 mmol) in CH2Cl2 (0.30 mL) for 4 h at room temperature and the following bulb-to-bulb distillation (150–170 °C at 5 mmHg). Colorless crystals (CHCl3); Mp. 51.0–52.2 °C; IR (KBr) ν 2973, 2911, 2877, 1713, 1481, 1398, 1213, 1114, 1152, 677 cm−1; 1H-NMR (CDCl3) δ 1.22 (18H, s), 2.76 (2H, t, J = 7.1 Hz), 5.04 (1H, t, J = 7.1 Hz), 5.17 (1H, t, J = 7.1 Hz); 13C-NMR (CDCl3) δ 25.6, 27.3, 44.9, 52.3, 74.7, 119.2 (q, JC-F = 329.8 Hz), 210.4; 19F-NMR (CDCl3) δ −10.0 (6F, s); MS (ESI-TOF) m/z 477 [M+H]+; HRMS calcd. for C15H23F6O6S2 [M+H]+, 477.0840; found, 477.0842.

4-(2,2-Bis(trifluoromethylsulfonyl)ethyl)-2,6-dimethylheptane-3,5-dione (4i). According to the general procedure, this compound was obtained in 80% yield (62.4 mg, 0.139 mmol) by the reaction of Tf2CHCH2CHTf2 (5, 100 mg, 0.175 mmol) with 2,6-dimethylheptane-3,5-dione (33 μL, 0.19 mmol) in CH2Cl2 (0.15 mL) for 2.5 h at room temperature and the following bulb-to-bulb distillation (150–165 °C at 5 mmHg). Colorless oil; IR (neat) ν 2979, 2941, 2880, 1725, 1469, 1396, 1213, 1114, 1024, 689 cm−1; 1H-NMR (CDCl3) δ 1.14 (6H, d, J = 6.7 Hz), 1.16 (6H, d, J = 7.0 Hz), 2.69–2.82 (4H, m), 4.73 (1H, t, J = 7.3 Hz), 5.32 (1H, t, J = 6.9 Hz); 13C-NMR (CDCl3) δ 17.8, 18.4, 24.5, 41.5, 57.3, 74.7, 119.2 (q, JC-F = 329.7 Hz), 208.1; 19F-NMR (CDCl3) δ −10.3 (6F, s); MS (ESI-TOF) m/z 449 [M+H]+; HRMS calcd. for C13H19F6O6S2 [M+H]+, 449.0527; found, 449.0508.

Triethyl 3,3-bis(trifluoromethylsulfonyl)propane-1,1,1-tricarboxylate (4j). According to the general procedure, this compound was obtained in 88% yield (183 mg, 0.349 mmol) by the reaction of triethyl methanetricarboxylate (92.2 mg, 0.397 mmol) with Tf2CHCH2CHTf2 (5, 295 mg, 0.515 mmol) in acetonitrile (0.40 mL) at 80 °C for 13 h and the following removal of Tf2CH2 using Kugelrohr oven (100 °C at 5 mmHg). Colorless oil; IR (neat) ν 2988, 2943, 2911, 1745, 1393, 1220, 1110, 1018, 860, 701 cm−1; 1H-NMR (CDCl3) δ 1.28 (9H, t, J = 7.2 Hz), 3.45 (2H, d, J = 5.0 Hz), 4.27 (6H, q, J = 7.2 Hz), 6.53 (1H, t, J = 5.0 Hz); 13C-NMR (CDCl3) δ 13.6, 27.3, 62.1, 63.5, 73.9, 119.3 (q, JC-F = 330.2 Hz), 165.4; 19F-NMR (CDCl3) δ −8.5 (6F, s); MS (ESI-TOF) m/z 525 [M+H]+; HRMS calcd. for C14H19F6O10S2 [M+H]+, 525.0324; found, 525.0299.

4. Conclusions

In summary, we successfully found that δ-oxo-1,1-bis(triflyl)alkanes are obtained in good to excellent yields by the reaction of enolizable carbonyls with Tf2CHCH2CHTf2 (5). NMR study of a solution of tetrasulfone 5 in CDCl3 revealed smooth formation of reactive 1,1-bis(triflyl)ethylene (3) in a reversible manner. On the basis of this reaction, incorporation of Tf2CH functionality into a wide range of 1,3-dicarbonyl compounds was realized. Furthermore, gas-phase acidities of some δ-oxo-1,1-bis(triflyl)alkanes thus obtained were determined by the FT-ICR technique. The present work is a notable extension of our synthetic methodology for Tf2CH type carbon acids. Further studies on this reaction and catalysis of the δ-oxo-1,1-bis(triflyl)alkanes are under progress in our laboratory.