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Polyfluorinated carbocations stabilized by oxygen and sulfur
Journal of Fluorine Chemistry 95 (1999) 5±13
The 1998 EmeleÂus Prize Lecture
Poly¯uorinated carbocations stabilized by oxygen and sulfur Viacheslav A. Petrov*, Fred Davidson
DuPont Central Research and Development, Experimental Station, PO Box 80328, Wilmington, DE 19880-0328, USA1 Accepted 23 November 1998
Abstract Poly¯uorinated oxonium cations RfCH2OCFX and acyclic per¯uorinated sulfonium cations Rf SCF 2 are prepared by the reaction of corresponding ethers or sul®des with antimony penta¯uoride in SO2ClF solvent and characterized by 19 F and 1 H spectroscopy. Based on experimental data the relative stability of poly¯uorinated oxonium cations are determined. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Poly¯uorinated carbocations; Rf SCF 2 ; NMR spectroscopy
1. Introduction A number of stable per¯uorinated and poly¯uorinated carbocations has been reported and reviewed in the last few years [1±3]. Among them are several poly¯uorinated allyl cations containing H, Cl, Br, F and CF3 as substituents [1] along with numerous examples of allylic cations containing alkoxy groups [4±10] generated by reaction of the corresponding unsaturated compound with Lewis acids and characterized by NMR spectroscopy [1]. On the other hand, so far methoxy¯uoromethyl CH3OCFH [11] and F-4-methyl-2-benzopyrilium [12] cations remain the only two representatives of ¯uorinated carbocationic species stabilized by oxygen, being observed in a condensed phase. Per¯uorinated ethers are inert to the action of strong Lewis acids. For example, (RfO)3CF cannot be ionized by SbF5 [13], in sharp contrast to (CF3S)3CF (forming a stable salt in a reaction with AsF5 which has recently been isolated and fully characterized [14]). However, most of the partially ¯uorinated ethers RfOR have low stability to strong Lewis acids and usually readily decompose under the action of Lewis acids [15] to produce a carbonyl compound and alkyl ¯uoride. Recently, we have demonstrated that the compounds of general formula Rf CH2 OCFXRf 0 (XF, H) have unexpect-
*Corresponding author. Fax: +1-302-695-8281. 1 Publication No. 7792
edly high stability to Lewis acids and in the presence of SbF5 catalyst rapidly react with ¯uorinated ethylenes to form branched ethers [16], as exempli®ed by their reaction with tetra¯uoroethylene. SbF5
Rf CH2 OCFXR0f CF2 CF2 ! Rf CH2 OCX C2 F5 R0f yields 40ÿ85%;
Rf CF3ÿ; H CF2 CF2 n ÿ; R0f HCF2ÿ;
CF3 CFHÿ; CF3 OCFHÿ; X F; H; Cl:
(1)
Reactions catalyzed by strong Lewis acids, such as SbF5 or aluminum chloro¯uoride, usually involve generation of the corresponding carbonium cations as a result of abstracting ¯uoride anion by strong Lewis acid from the substrate [1,17,18]. Hence we had a strong reason to believe that the above process (Eq. (1)) proceeds through a step involving formation of a free carbocation. In this study, we report results concerning the reaction of SbF5 with poly¯uorinated ethers and per¯uorinated sul®des leading to the formation of stable alkoxy- and alkylthiocarbocations. 2. Results Addition of poly¯uorinated ether 1 in a homogeneous mixture of SbF5 and SO2ClF solvent at ÿ408C results in the formation of a stable clear solution (Table 1). The 19 F NMR spectrum of the mixture, along with two signals of the starting material, contains three additional signals belonging to carbocation 2. A signi®cantly deshielded AB pattern
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 6 1 - 3
6
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Table 1 Preparation of polyfluorinated ethers Compound
Method
Temperature (8C)
Time (h)
Yield (%)
Bp (8C)
1 3
A A
150 150
14 14
78 62
4
A
150
14
58
16
B
120
24
62
18
B
50
16
80
19 20
B B
100 120
20 16
92 58
10±12 44 (45.5 [32]) 92±94 (99 [32]) 55±56 (56.7 [30]) 72 (72 [33]) ±a 80±82 (82 [34])
a
Compound of >98% purity is isolated and used without distillation.
Fig. 1. AB pattern of
19
F NMR spectrum of cation 2.
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13 Table 2 NMR data of polyfluorinated ethers
(15.73, 14.77 (1F, d, 274 Hz), 8.56, 7.61 (1F, d, 274 Hz)) corresponds to two ¯uorines directly connected to the carbocationic center (see Fig. 1), and the slightly deshielded triplet at ÿ73.53 (3F, t; 6 Hz) of the CF3CH2± group (see Tables 2 and 3). The ratio of signals (1:1:3) and a signi®cant down®eld shift of ¯uorine resonances (76 ppm) are indicative of the formation of cation 2 existing in equilibrium with the starting material (see Fig. 2).
19
Compound a
1 3 4
13b
(2) Similar changes observed by NMR for solutions of poly¯uoroole®ns in a SbF5/SO2ClF mixture have been proven to be the result of formation of the corresponding poly¯uorinated allylic cations [1]. Magnetic non-equivalence of two ¯uorines at cationic carbon in 2 is a clear indication of an unusually high barrier of rotation around the C±O bond in this species. Upon warming the sample to 08C, only the line broadening of the AB pattern is observed. This process is completely reversible, and lowering of the sample temperature regenerates the original spectrum. In the 1 H NMR spectrum of this mixture, a shifted down®eld quartet is present (5.13 ppm), along with the signal of the starting material (3.56 ppm). Compounds 3 and 4 in a mixture with excess SbF5 behave similar to 1, giving cations 5 and 6 observed in equilibrium with the starting material
16 18
19
20
a b
1
F NMR ( ppm, J Hz)
ÿ63.1 (3F q; 1.7), ÿ75.20 (3F qt; 8; 1.7) ÿ62.63 (3F, s) ÿ124.43 (2F, t; 12) ÿ139.45 (2F, d; 53) ÿ62.38 (3F, s) ÿ120.85 (2F, t; 11) ÿ125.55 (2F, s) ÿ130.17 (2F, m) ÿ137.58 (2F, dm; 52) ÿ75.17 (3F, t; 8) ÿ86.73 (2F, d; 67) ÿ80.01 (3F, d) ÿ80.44 (1F, dd) ÿ81.75 (1F, dd) ÿ141.60 (1F, qdd) ÿ74.83 (3F br s) ÿ92.43 (2F, br s) ÿ137.18 (2F d, 53) ÿ73.73 (3F, t; 7.5) ÿ74.49 (3F m) ÿ80.63 (1F dm; 145) ÿ82.58 (1F dm; 145) ÿ211.05 (1F dm; 47) ÿ60.52 (3F d; 5) ÿ75.15 (3F tt; 8; 2) ÿ90.80 (2F m) ÿ145.71 (1F m; 58) ÿ74.96 (3F t; 8) ÿ88.60 (2F m) ÿ154.42 (1F dt; 49; 12)
11
H NMR ( ppm, J Hz)
3.56 (q; 8) 4.46 (2H, tt) 6.05 (1H, tt; 53) 4.38 (2H, t; 12) 6.05 (1H, tt; 52.5)
4.02 6.16 5.77 6.25
(2H, (1H, (1H, (1H,
q; 8) t; 67) t; 67) dd)
4.33 (2H, q; 8) 5.82 (1H, tt; 55; 4) 4.35 (2H, q; 7.5) 4.85 (1H, dm; 47; 6)
4.41 (2H, q; 8) 5.95 (1H, dt; 58; 3) 4.29 (2H, q; 8) 6.12 (1H, dt; 49; 7)
From Ref. [35]. From Ref. [31].
H CF2 n CH2 O CF2 Sbn ÿ F5n1
H CF2 n CH2 OCF3 nSbF5 3 4
7
n2 n4
where the ratios of 3:576:24 and 4:684:16.19 F NMR spectra of cations 5 and 6 are similar to those of cation 1; each spectrum contains a shifted down®eld AB pattern (see Table 3). It is noteworthy that some per¯uorinated sulfur-containing compounds behave similar to ethers in reaction with SbF5, producing the corresponding sulfonium cations. For example, interaction of bis(tri¯uoromethyl) sul®de (7) and SbF5 in SO2ClF solvent results in generation of cation 8 in equilibrium with the starting material (the 7±8 ratio is 78:22). Changes in NMR spectrum are very similar to those observed in the case of cations 2, 5 and 6, and a set of three new signals with intensities 1:1:3 is observed in the ¯uorine NMR spectrum. Like cation in 2, ¯uorine substituents connected to the carbon carrying a positive charge in sulfonium cation 8 are magnetically non-equivalent (J258 Hz) and signi®cantly shifted down®eld, appearing
(3)
5 6 as an AB pattern between 60 and 80 ppm (see Fig. 2). CF3 SCF3 nSbF5
ÿ
CF3S CF2 Sbn F5n1
(4)
8
7
In contrast to 2, an equal coupling constant between each ¯uorine and the CF3 group is present in the 19 F spectrum of 8. Disul®de 9 in reaction with SbF5 also gives the corresponding sulfonium cation 10, which seems to be more stable than 8, based on the cation-to-starting material ratio 58:42 (see Table 4). CF3 SSCF3 nSbF5 9
ÿ
CF3SS CF2 Sbn F5n1 10
(5) Interestingly, the 19 F NMR spectrum of 10 (similar to that of 8) contains another AB pattern of low intensity (see
8
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Table 3 NMR data of polyfluorinated carbocations RfCHYCOCFXa Rf
X
Y
19
CF3
F
H
5d
HCF2CF2
F
H
6e
H(CF2CF2)2
F
H
12f
CF3
H
H
14g,h
CF3
H
F
ÿ73.53 (3F, t; 6) 15.73; 14.77 (1F, d; 274) 8.56; 7.61 (1F, d, 274) ÿ122.14 (2F, t; 11) ÿ135.42 (2F, d; 52) 14.63; 13.67 (1F, d; 272) 7.83, 6.87 (1F, d; 272) ÿ137.83 (2F, d; 51) ÿ130.12 (2F, m) ÿ124.93 (2F, m) ÿ119.71 (2F, m) 15.83; 14.86 (1F, d; 272) 8.85; 7.89 (1F, d; 272) Major: ÿ70.56 (3F, t, 6) 60.71 (12F, d, 92) Minor: ÿ72.15 (3F, t; 6) 56.61 (1F, d; 92) Major: ÿ82.81 (3F, dt; 5, 2) ÿ139.21 (1F, dt; 45, 5) 56.59 (1F, dd; 84, 5) Minor: ÿ82.95 (3F, d; 5) ÿ137.52 (1F, dd; 56, 5) 65.81 (1F, d; 87) ÿ73.72 (3F, t; 6) ÿ128.29 (2F, dd; 50, 12) 54.28 (1F, t; 12) ÿ72.73 (3F, dt; 13, 4) ÿ73.82 (3F, t; 6) ÿ207.62 (1F, dm; 42) 61.28 (1F, d; 25) ÿ61.33 (3F d; 3) ÿ73.65 (3F t; 6) ÿ135.83 (1F dd; 51; 11) 55.43 (1F d; 11) ÿ73.43 (3F, t; 6) ÿ159.80 (1F, dd; 56, 17) 45.66 (1F, d; 17)
Cation 2
b,c
17
CF3
CF2H
H
21
CF3
CFHCF3
H
22
CF3
CFHOCF3
H
23
CF3
CHFCl
H
F NMR ( ppm, J Hz)
1
H NMR ( ppm, J Hz)
5.13 (q; 6)
Major: 5.29 (2H q; 6) 9.05 (1H, d, 92) Minor: 5.38 (2H q; 6) 9.35 (1H, d; 89) Major: 6.78 (1H d, 44) 9.31 (1H d, 84) Minor: 6.85 (1H d; 45) 9.35 (1H d; 87) 5.05 (2H q; 6) 5.99 (1H t; 49) 5.13 (2H, q; 6) 5.66 (1H, dq; 42; 4)
4.95 (2H q; 7) 6.38 (1H d; 54)
a
At ÿ408C. Cf19 Fg: 120.61 (s), 121.54 (t, 125 Hz), 153.26 (s). c Equilibrium; ratio of starting ether to cation 63:37. d Equilibrium; ratio of starting ether to cation 76:24. e Equilibrium; ratio of starting ether to cation 84:16. f Mixture of two isomers; ratio 73:27. g Equilibrium; ratio of starting ether to cation 16:84. h Mixture of two isomers; ratio 65:35. b 13
Table 4), the origin of which is not clear at this point. It should be emphasized that chemical shifts of the ¯uorine substituent connected to the carbon bearing a positive charge and the difference in chemical shifts between the starting material and cation ( values) are in good agreement with those reported for a cyclic sulfonium cation generated by the reaction of 2,2,4,4-tetra¯uoro-1,3ditiethane with SbF5 (67.25; 119.75) [19].
The 19 F NMR spectrum for a solution of ether 11 in SbF5/ SO2ClF at low temperature does not contain signals of the starting material, but only the signals of the corresponding cation. This indicates that the equilibrium on NMR timescale is completely shifted towards 12, which indicates higher stability of 11 compared to 2, 5, 6 (see Section 3 for data on the relative stability of poly¯uorinated alkoxycarbocations).
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Fig. 2. AB pattern of
19
(6)
Both 19 F and 1 H spectra of cation 12 contain two sets of signals (ratio 73:27), suggesting that 12 exists as a mixture of two isomers, probably due to a high rotation barrier around the C±O bond. Unfortunately, the absence of additional coupling constants makes it impossible to assign structure to a speci®c isomer. However, based on the data reported for CH3OCXH [11] methoxy¯uorocyclopropenyl [5] and methoxy¯uorocyclobutenyl cations [6], the isomer containing ¯uorine and an alkyl group in trans-position should be favored. Chemical shifts of the ±CH2±O± and ± OCFH groups in the 1 H NMR spectrum of 12 are in good agreement with the values of the corresponding groups reported for CH3OCHX cations [11].
9
F NMR spectrum of cation 8.
Replacement of hydrogen by ¯uorine in CF3CH2± moiety of ether 11 only slightly affects the stability of the corresponding carbocation. Although the 19 F NMR spectrum of a mixture of ether 13 and SbF5 at ÿ408C contains signals of both cation 14 and the starting material, the equilibrium is Table 4 19 F NMR data of sulfur-containing cations 8 and 10 RfSCF2a Cation
Rf
19
8b
CF3, 109 ppm
10c
CF3S, 118 ppm
ÿ40.05 (3F, t, 13) 62.24, 63.15 (1F, dq, 258, 13) 77.62, 77.60 (1F, dq, 258, 13) ÿ39.29 (3F, d, 5) 81.21, 80.24 (1F, dq, 279, 5) 65.49, 64.50 (1F, dq, 279, 5) 57.28, 56.40 (1F, d; 252) 73.06, 73.98 (1F, d; 252)d
a
F NMR ( ppm, J Hz)
At ÿ508C. Equilibrium; ratio of starting material to cation 78:22. c Equilibrium; ratio of starting material to cation 42:58. d Second set of signals of low intensity. b
10
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
shifted towards the cation (the 13±14 ratio is 16:84).
(7)
Similar to 12, cation 14 exists as a mixture of two isomers (ratio 65:35); however, without NOE experimental data the structural assignment of these isomers is not possible. No formation of the corresponding carbocation is observed when ether 15 is treated with SbF5. CF3 CFHOCF2 C2 F5 nSbF5
poly¯uorinated allylic cations [1] is employed. A mixture of two different ethers is reached with SbF5 under the conditions where starting compounds are competing for a Lewis acid. The stability of cations is determined from the ratio of the two resulting species (experimental details and ratio are given in Table 5). Results of these experiments are arranged into the following sequence going from stable, long-lived cations 12, 14 and 17 to less stable, existing in equilibrium with precursors, cations 2, 5, 6. Cation 24, containing C2F5 group attached to the carbon carrying positive charge, is the least stable in this series, since attempt to observe it by NMR spectroscopy at low temperature failed.
12
15
ÿ
CF3 CFHO C FC2 F5 Sbn F5n1
(8)
14
CF3 CH2 OCF2 CF2 H nSbF5
CF3 CH2 O C F2 H CF2 2 CH2 O C F2 5
6
17
According to 19 F NMR addition of ether 16 to excess of SbF5 dissolved in SO2ClF solvent results in total disappearance of signals of starting material and appearance of a new set of three signals with chemical shifts at 54.28 to ÿ73.72 and ÿ128.29 ppm, integrated as 1:3:2 (see Table 3). The multiplicity of the up®eld signal (HCF2, doublet of doublets, J50 and 12 Hz) along with chemical shift and value of coupling constant of a down®eld signal (54.28 ppm, J12 Hz) indicates the formation of carbocation 17. In general, cations of the formula CF3CH2OCFCFHX (XF±, Cl, CF3±, CF3O±) containing a poly¯uoroalkyl group in a-position to the carbocationic center are more stable than cations 2, 5, 6 (bearing two ¯uorine substituents at carbocationic center). Formation of the corresponding cations is also observed in the reaction of ethers 18, 19 and 20 with excess SbF5. These cations also exist as stable, longlived species, since in each case, signals of starting materials are not present in 19 F NMR spectra of the corresponding mixture.
> H CF2 4 CH2 O C F2 H CF2 2 CH2 O C FC2 F5 (11)
CF3 CH2 O C FCF2 H Sbn ÿ F5n1
16
17
2
Surprisingly, the reaction of partially ¯uorinated ether 16 with antimony penta¯uoride under similar conditions leads to the formation of stable long-lived carbocation 17.
SO2 ClF; ÿ40 C
24
(9)
3. Discussion Experimental data obtained in this study are suf®cient to conclude that the onium structure with a high degree of the double-bond character between carbon and oxygen (or sulfur) predominates in the resonance structure of poly¯uorinated cations 2, 5, 6, 8, 10, 12, 14, 17, 21±23. These species can be viewed as methyl cations stabilized (along with two other substituents) by oxygen or sulfur. Obviously, their stability is affected by all the three substituents. The fragment connected to oxygen has a signi®cant in¯uence on the stability of poly¯uorinated oxonium cations. Per¯uorinated alkyl groups Rf, having a strong electron-withdrawing effect, signi®cantly destabilize the cation, and make it impossible to observe these species by means of NMR spectroscopy. On the other hand, groups such as CH3± are able to stabilize carbocations due to a positive inductive effect; however, the ability of partially ¯uorinated ethers RfOR to
CF3 CH2 O C FCFHX Sbn F5n1
CF3 CH2 OCF2 CFHX 18 19 20
CF3 CH2 O C FH CF3 CFHO C FH CF3 CH2 O C FCF2 H
X CF3 X OCF3 X Cl
21 22 23
The chemical shifts and coupling constants of cations 21± 23 are quite similar to those of 17, containing a signal at 45± 62 ppm (see Table 3), coupled with the ¯uorine of the ±CFH group, as demonstrated in 19 FfHg experiments. To ®nd the relative stability of poly¯uorinated oxonium cations, a method used earlier for determining stability of
(10)
eliminate alkyl ¯uoride makes them sensitive towards Lewis acids and complicates the observation by NMR spectroscopy of the corresponding cation (for example, CH3OCFH readily decomposes at ÿ208C [11]). The reactivity of ether 25 towards SbF5 is another example. Rapid decomposition of this material is observed when it is added
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13 Table 5 Competitive reactions of ethers 1, 3, 4, 11, 13, 16 with SbF5a Entry
Ethers (ratio)
Cations (ratio)
1 2 3 4 5
11, 13 (43:57) 1, 16 (45:55) 1, 4 (50:50) 3, 4 (50:50) 11, 16 (44:66)
12, 14 (61:39) Only 17 2, 6 (37:63) 5, 6 (88:12) 12, 17 (40:60)
a
Samples were made using: 1 g(0.01, 4.6 mmol) of SbF5, 10 mmol (0.1) of SO2ClF, 1.5±2(0.3) mmol of each ether; 19 F NMR at ÿ408C.
to a solution of SbF5 in SO2ClF (even at ÿ408C) leading to CF3CH2Cl and CF3H as the only ¯uorine-containing products. ClCF2 CH2 OCF2 CF2 H 25
CF3 CH2 Cl CF3 H 26
(12) A striking difference in the stability of compounds 16 and 25 is probably the result of a signi®cantly lower barrier for decomposition of cation 27, which may be a result of the additional stabilization effect of b-chlorine, making ClCF2 CH 2 a better leaving group than CF3 CH2 and leading to rapid decomposition of ether 25 (see Scheme 1). Formation of CF3H in this reaction is the result of the well-documented low temperature degradation of CF2HC(O)F [20], while the presence of compound 26 in the reaction mixture probably is an indication of the formation of cation 28, which then captures a ¯uoride anion to CF3CH2Cl. From this standpoint, a substituent such as CF3CH2±, is almost ``ideal''. Its electron-withdrawing effect is weaker than that of a per¯uoroalkyl group (I values for CF3CH2± and CF3CF2± groups are 0.14 and 0.41, respectively [21]) but at the same time it is a poor leaving group (CF3 CH 2 is calculated to be 37 kcal/mol less stable than CH3 CH 2 [21]). An alkoxy substituent actively participates in stabilization of the carbocation, donating a p-electron pair of oxygen on the vacant orbital of a positively charged carbon and magnetic non-equivalence of ¯uorines in the ÿOCF 2 group of cations 2, 5, 6 is a re¯ection of this effect. A relatively small down®eld shift of the carbon carrying a positive charge in cation 2 (153.26 ppm vs. 216±224 ppm for positively
11
charged carbon atoms in ROCH 2 cations [22]) is an additional indication of a signi®cant participation of substituents, including oxygen, in stabilization of the carbocation. The in¯uence of two other substituents on the stability of methyl cations 2, 5, 6, 12, 14, 17, 21±24 is not as straightforward. In general, the presence of maximum p-donor substituents (able to stabilize a carbocation by the ``backdonation'' mechanism) at the carbocationic center is bene®cial. Long-lived trihalomethyl X3C (XCl, Br, I) [23], trialkoxymethyl [24] cations, along with stable salts of (CF3S)3C [14] are examples. However, poly¯uorinated methyl cations deviate from this rule. Among trihalomethyl cations X3C (XCl, Br, I) observed in condensed phase [13,25,26], the tri¯uoromethyl cation is the least stable and so far has been observed only in the gas phase. The relative stability of cations 2, 5, 6, 12, 14, 17, 21±23 (see Eq. (11)) observed in this study indicates that introduction into cation CF3CH2OCFX of ¯uorine substituents signi®cantly reduces the stability of the cation, whereas the in¯uence of poly¯uoroalkyl substituents (such as ±H, ± CF2H, ±CFHCF3 or ±CFHOCF3 is less pronounced. A reasonable explanation of this phenomena may be based on the relative electronegativities of different substituents. Since in cation CF3 CH2 OCF 2 both ¯uorine substituents cannot participate equally in stabilization through ``backdonation'' of p-electrons on the vacant orbital of the carbocation, one of the ¯uorine substituent starts acting as a powerful electron-withdrawing group causing destabilization of the cation through the inductive mechanism. A per¯uoroalkyl group has very similar, although, more pronounced effect on the stability of oxonium cations. The similarity in in¯uence of ¯uorine and C2F5± substituents on the stability of cations H(CF2CF2)nCH2OCFX (XF, C2F5) may be a result of high electronegativity of both substituents (I0.52 and 0.42 for F and C2F5, respectively). On the other hand, poly¯uoroalkyl groups such as ±CF2H have less pronounced electron-withdrawing ability (I0.32 for CF2H vs. 0.42 for C2F5 [21]) and the stabilizing effect of RfCH2O± substituents is suf®cient to overcome the destabilizing effect of the poly¯uoroalkyl group. Higher electronegativity of ¯uorine substituent vs. electronegativity of a carbocationic center is also probably responsible for signi®cantly lower stability of elusive CF 3 [25,26] and
Scheme 1.
12
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
FCO [27] carbocations compared to the relatively stable and observed in condensed phase CCl 3 [23] and ClCO [27,28]. Due to the same effect di¯uoromethyl cations XCF 2 (XH, Cl, Br, I) are expected to have signi®cantly lower stability compared to corresponding trihalomethyl cations CX 3 (XCl, Br, I).
nel, and 16 g (0.079 mol) of CF3CHClOCF2H is slowly added. After the end of the exothermic reaction the mixture is stirred for 1 h, the product (10 g, 62.5%, 98% purity) is transferred into a cold trap (ÿ788C) and used without further puri®cation. 19 F NMR parameters of isolated 13 are in good agreement with the values reported [31].
4. Experimental
Acknowledgements
All the NMR spectra were recorded on G.E. Omega 300 MHz spectrometer (276 MHz for 19 F and 300 MHz for 1 H). Chemical shifts are given relative to CFCl3 and TMS, respectively. All ¯uorinated alcohols used in this study for the preparation of ethers and compounds 7, 10, 11, 15 are commercially available (PCR) and used without further puri®cation. Ethers 1, 3, 4 are obtained using a slight modi®cation of the procedure described in [29] (see below), whereas ethers 16, 18±20 are prepared by base catalyzed addition of tri¯uoroethanol to the corresponding ole®ns [30]. NMR data not available for certain materials are listed in Table 1. Antimony penta¯uoride (Ozark-Mahoning) is distilled under nitrogen and stored and handled inside a dry box.
The authors thank Dr. B.E. Smart, Dr. C.G. Krespan and Dr. A.C. Sievert for stimulating discussions and valuable comments, Dr. W. Qiu for sample of ether 25 and Boris Vekker for the help in the preparation of the paper.
4.1. Preparation of ethers 1, 3, 4 (method A) A 1 gal Hastelloy autoclave is charged with 1 mol of the corresponding alcohol, 1 mol of CCl4, 15 g of BF3 and 400 ml of anhydrous HF. The mixture is agitated at 1508C for 14±16 h. Water (1 l) is injected into the reactor, it is cooled down to 08C, and the reaction mixture transferred into a polyethylene container. An organic layer (lower) was separated, dried over MgSO4 and distilled. Reaction conditions and yields are shown in Table 1. 4.2. Preparation of ethers 16, 18±20 (method B) A Hastelloy reactor (400 ml) is charged with 1 mol of corresponding alcohol and a solution of 6±25 g of KOH in 20±75 ml of water. The reactor is closed, cooled down to ÿ788C, evacuated, and 1 mol of the corresponding ole®n introduced into the reactor (in the case of tetra¯uoroethylene, addition is conducted in two steps: 0.5 mol of ole®n is added after the initial 0.5 mol has been consumed). The reaction mixture is agitated at 50±1208C for 16±24 h. The crude reaction mixture is poured into water, the organic layer is separated, dried over P2O5 and distilled. Reaction conditions and product yields are shown in Table 1. 4.3. Synthesis of compound 13 Fifty g (0.23 mol) of SbF5 is placed in a ¯ask equipped with a thermometer, dry-ice condenser and additional fun-
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
V.I. Bakhmutov, M.V. Galakhov, Uspekhi Khimii 57 (1988) 839. G.G. Belen'kii, J. Fluorine Chem. 77 (1996) 107. V.A. Petrov, V.V. Bardin, Topics Curr. Chem. 192 (1997) 39. I.L. Knunyants, Yo.G. Abduganiev, E.M. Rokhlin, P.O. Okulevich, N.I. Karpushina, Tetrahedron 29 (1973) 595. B.E. Smart, J. Org. Chem. 41 (1976) 2377. B.E. Smart, G.S. Reddy, J. Am. Chem. Soc. 78 (1976) 5593. R.D. Chambers, A. Parkin, R.S. Matthews, J. Chem. Soc. Rerkin 1 (1976) 2107. V.F. Snegirev, M.V. Galakhov, K.N. Makarov, V.I. Bakhmutov, Izv. AN SSSR. Ser. Khim. (1985) 2302. V.F. Snegirev, M.V. Galakhov, V.A. Petrov, K.N. Makarov, V.I. Bakhmutov, Izv. AN SSSR. Ser. Khim. (1989) 1318. M.V. Galakhov, V.A. Petrov, G.G. Belen'kii, L.S. German, E.I. Fedin, V.F. Snegirev, V.I. Bakhmutov, Izv. AN SSSR. Ser. Khim. (1986) 1063. G.A. Olah, J.M. Bollinger, J. Am. Chem. Soc. 89 (1967) 2993. V.M. Karpov, T.V. Mezhenkova, V.E. Platonov, G.G. Yakobson, Izv. An SSSR. Ser. Khim. (1991) 745. G.A. Olah, A. Burrichter, G. Rasul, A. Yudin, G.K.S. Prakash, J. Org. Chem. 61 (1996) 1934. R. Boese, A. Haas, C. Kruger, G. Moller, A. Waterfeld, Chem. Ber. 127 (1994) 597. D.C. England, J. Org. Chem. 49 (1984) 4007. V.A. Petrov (to DuPont) US Patent pending. G.G. Belen'kii, L.S. German, in: M.E. Vol'pin (Ed.), Soviet Scientific Reviews, Chem. Rev. (1984) 183. C.G. Krespan, V.A. Petrov, Chem. Rev. 96 (1996) 3269. A. Waterfeld, R. Mews, Chem. Ber. 118 (1995) 4997. G.A. Olah, A. Germain, H.C. Lin, J. Am. Chem. Soc. 97 (1975) 5481. B.E. Smart, in: S. Patai, Z. Rappoport (Eds.), The Chemistry of Halides, Pseudo-Halides and Azides. Part 1, Wiley, New York, 1983, p. 603. G.A. Olah, S. Yu, G. Liang, G.D. Matseescu, M.R. Bruce, D.J. Donovan, M. Arvanaghi, J. Org. Chem. 46 (1981) 571. G.A. Olah, L. Heiliger, G.K.S. Prakash, J. Am. Chem. Soc. 111 (1989) 8020. B.G. Ramsey, R.W. Taft, J. Am. Chem. Soc. 88 (1966) 3058. G.K.S. Prakash, G. Rasul, A. Burrichter, K.K. Laali, G.A. Olah, J. Org. Chem. 61 (1996) 9253. G. Frenking, S. Fau, C.M. Marchand, H. Grutzmacher, J. Am. Chem. Soc. 119 (1997) 6648. K.O. Christe, B. Hoge, J. Sheehy, W. Wilson, X. Zhang, Abstracts of the 216th ACS National Meeting, 1998, p. O36.
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13 [28] B. Hoge, J. Sheehy, G.K.S. Prakash, G.A. Olah, K.O. Christe, Abstracts of the 12th European Symposium on Fluorine Chemistry, 1998, p. B39. [29] C.G. Krespan, M. Rao, US Patent 5 382 704 (1995), to DuPont, CA 122 (1995) 239184. [30] A.L. Henne, M.A. Smook, J. Am. Chem. Soc. 72 (1950) 4378.
13
[31] K. Ramig, A. Krishnaswami, L.A. Rozov, Tetrahedron 52 (1996) 319. [32] P.E. Aldrich, W.A. Sheppard, J. Org. Chem. 29 (1964) 11. [33] V.A. Gubanov, A.V. Tumanova, I.M. Dolgopol'skii, Zh. Obshch. Khim. 35 (1965) 398. [34] R.E.A. Dear, E.E. Gilbert, J. Chem. Eng. Data 14 (1969) 493. [35] K.K. Johri, D.D. DesMarteau, J. Org. Chem. 48 (1983) 242.
The 1998 EmeleÂus Prize Lecture
Poly¯uorinated carbocations stabilized by oxygen and sulfur Viacheslav A. Petrov*, Fred Davidson
DuPont Central Research and Development, Experimental Station, PO Box 80328, Wilmington, DE 19880-0328, USA1 Accepted 23 November 1998
Abstract Poly¯uorinated oxonium cations RfCH2OCFX and acyclic per¯uorinated sulfonium cations Rf SCF 2 are prepared by the reaction of corresponding ethers or sul®des with antimony penta¯uoride in SO2ClF solvent and characterized by 19 F and 1 H spectroscopy. Based on experimental data the relative stability of poly¯uorinated oxonium cations are determined. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Poly¯uorinated carbocations; Rf SCF 2 ; NMR spectroscopy
1. Introduction A number of stable per¯uorinated and poly¯uorinated carbocations has been reported and reviewed in the last few years [1±3]. Among them are several poly¯uorinated allyl cations containing H, Cl, Br, F and CF3 as substituents [1] along with numerous examples of allylic cations containing alkoxy groups [4±10] generated by reaction of the corresponding unsaturated compound with Lewis acids and characterized by NMR spectroscopy [1]. On the other hand, so far methoxy¯uoromethyl CH3OCFH [11] and F-4-methyl-2-benzopyrilium [12] cations remain the only two representatives of ¯uorinated carbocationic species stabilized by oxygen, being observed in a condensed phase. Per¯uorinated ethers are inert to the action of strong Lewis acids. For example, (RfO)3CF cannot be ionized by SbF5 [13], in sharp contrast to (CF3S)3CF (forming a stable salt in a reaction with AsF5 which has recently been isolated and fully characterized [14]). However, most of the partially ¯uorinated ethers RfOR have low stability to strong Lewis acids and usually readily decompose under the action of Lewis acids [15] to produce a carbonyl compound and alkyl ¯uoride. Recently, we have demonstrated that the compounds of general formula Rf CH2 OCFXRf 0 (XF, H) have unexpect-
*Corresponding author. Fax: +1-302-695-8281. 1 Publication No. 7792
edly high stability to Lewis acids and in the presence of SbF5 catalyst rapidly react with ¯uorinated ethylenes to form branched ethers [16], as exempli®ed by their reaction with tetra¯uoroethylene. SbF5
Rf CH2 OCFXR0f CF2 CF2 ! Rf CH2 OCX C2 F5 R0f yields 40ÿ85%;
Rf CF3ÿ; H CF2 CF2 n ÿ; R0f HCF2ÿ;
CF3 CFHÿ; CF3 OCFHÿ; X F; H; Cl:
(1)
Reactions catalyzed by strong Lewis acids, such as SbF5 or aluminum chloro¯uoride, usually involve generation of the corresponding carbonium cations as a result of abstracting ¯uoride anion by strong Lewis acid from the substrate [1,17,18]. Hence we had a strong reason to believe that the above process (Eq. (1)) proceeds through a step involving formation of a free carbocation. In this study, we report results concerning the reaction of SbF5 with poly¯uorinated ethers and per¯uorinated sul®des leading to the formation of stable alkoxy- and alkylthiocarbocations. 2. Results Addition of poly¯uorinated ether 1 in a homogeneous mixture of SbF5 and SO2ClF solvent at ÿ408C results in the formation of a stable clear solution (Table 1). The 19 F NMR spectrum of the mixture, along with two signals of the starting material, contains three additional signals belonging to carbocation 2. A signi®cantly deshielded AB pattern
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 6 1 - 3
6
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Table 1 Preparation of polyfluorinated ethers Compound
Method
Temperature (8C)
Time (h)
Yield (%)
Bp (8C)
1 3
A A
150 150
14 14
78 62
4
A
150
14
58
16
B
120
24
62
18
B
50
16
80
19 20
B B
100 120
20 16
92 58
10±12 44 (45.5 [32]) 92±94 (99 [32]) 55±56 (56.7 [30]) 72 (72 [33]) ±a 80±82 (82 [34])
a
Compound of >98% purity is isolated and used without distillation.
Fig. 1. AB pattern of
19
F NMR spectrum of cation 2.
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13 Table 2 NMR data of polyfluorinated ethers
(15.73, 14.77 (1F, d, 274 Hz), 8.56, 7.61 (1F, d, 274 Hz)) corresponds to two ¯uorines directly connected to the carbocationic center (see Fig. 1), and the slightly deshielded triplet at ÿ73.53 (3F, t; 6 Hz) of the CF3CH2± group (see Tables 2 and 3). The ratio of signals (1:1:3) and a signi®cant down®eld shift of ¯uorine resonances (76 ppm) are indicative of the formation of cation 2 existing in equilibrium with the starting material (see Fig. 2).
19
Compound a
1 3 4
13b
(2) Similar changes observed by NMR for solutions of poly¯uoroole®ns in a SbF5/SO2ClF mixture have been proven to be the result of formation of the corresponding poly¯uorinated allylic cations [1]. Magnetic non-equivalence of two ¯uorines at cationic carbon in 2 is a clear indication of an unusually high barrier of rotation around the C±O bond in this species. Upon warming the sample to 08C, only the line broadening of the AB pattern is observed. This process is completely reversible, and lowering of the sample temperature regenerates the original spectrum. In the 1 H NMR spectrum of this mixture, a shifted down®eld quartet is present (5.13 ppm), along with the signal of the starting material (3.56 ppm). Compounds 3 and 4 in a mixture with excess SbF5 behave similar to 1, giving cations 5 and 6 observed in equilibrium with the starting material
16 18
19
20
a b
1
F NMR ( ppm, J Hz)
ÿ63.1 (3F q; 1.7), ÿ75.20 (3F qt; 8; 1.7) ÿ62.63 (3F, s) ÿ124.43 (2F, t; 12) ÿ139.45 (2F, d; 53) ÿ62.38 (3F, s) ÿ120.85 (2F, t; 11) ÿ125.55 (2F, s) ÿ130.17 (2F, m) ÿ137.58 (2F, dm; 52) ÿ75.17 (3F, t; 8) ÿ86.73 (2F, d; 67) ÿ80.01 (3F, d) ÿ80.44 (1F, dd) ÿ81.75 (1F, dd) ÿ141.60 (1F, qdd) ÿ74.83 (3F br s) ÿ92.43 (2F, br s) ÿ137.18 (2F d, 53) ÿ73.73 (3F, t; 7.5) ÿ74.49 (3F m) ÿ80.63 (1F dm; 145) ÿ82.58 (1F dm; 145) ÿ211.05 (1F dm; 47) ÿ60.52 (3F d; 5) ÿ75.15 (3F tt; 8; 2) ÿ90.80 (2F m) ÿ145.71 (1F m; 58) ÿ74.96 (3F t; 8) ÿ88.60 (2F m) ÿ154.42 (1F dt; 49; 12)
11
H NMR ( ppm, J Hz)
3.56 (q; 8) 4.46 (2H, tt) 6.05 (1H, tt; 53) 4.38 (2H, t; 12) 6.05 (1H, tt; 52.5)
4.02 6.16 5.77 6.25
(2H, (1H, (1H, (1H,
q; 8) t; 67) t; 67) dd)
4.33 (2H, q; 8) 5.82 (1H, tt; 55; 4) 4.35 (2H, q; 7.5) 4.85 (1H, dm; 47; 6)
4.41 (2H, q; 8) 5.95 (1H, dt; 58; 3) 4.29 (2H, q; 8) 6.12 (1H, dt; 49; 7)
From Ref. [35]. From Ref. [31].
H CF2 n CH2 O CF2 Sbn ÿ F5n1
H CF2 n CH2 OCF3 nSbF5 3 4
7
n2 n4
where the ratios of 3:576:24 and 4:684:16.19 F NMR spectra of cations 5 and 6 are similar to those of cation 1; each spectrum contains a shifted down®eld AB pattern (see Table 3). It is noteworthy that some per¯uorinated sulfur-containing compounds behave similar to ethers in reaction with SbF5, producing the corresponding sulfonium cations. For example, interaction of bis(tri¯uoromethyl) sul®de (7) and SbF5 in SO2ClF solvent results in generation of cation 8 in equilibrium with the starting material (the 7±8 ratio is 78:22). Changes in NMR spectrum are very similar to those observed in the case of cations 2, 5 and 6, and a set of three new signals with intensities 1:1:3 is observed in the ¯uorine NMR spectrum. Like cation in 2, ¯uorine substituents connected to the carbon carrying a positive charge in sulfonium cation 8 are magnetically non-equivalent (J258 Hz) and signi®cantly shifted down®eld, appearing
(3)
5 6 as an AB pattern between 60 and 80 ppm (see Fig. 2). CF3 SCF3 nSbF5
ÿ
CF3S CF2 Sbn F5n1
(4)
8
7
In contrast to 2, an equal coupling constant between each ¯uorine and the CF3 group is present in the 19 F spectrum of 8. Disul®de 9 in reaction with SbF5 also gives the corresponding sulfonium cation 10, which seems to be more stable than 8, based on the cation-to-starting material ratio 58:42 (see Table 4). CF3 SSCF3 nSbF5 9
ÿ
CF3SS CF2 Sbn F5n1 10
(5) Interestingly, the 19 F NMR spectrum of 10 (similar to that of 8) contains another AB pattern of low intensity (see
8
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Table 3 NMR data of polyfluorinated carbocations RfCHYCOCFXa Rf
X
Y
19
CF3
F
H
5d
HCF2CF2
F
H
6e
H(CF2CF2)2
F
H
12f
CF3
H
H
14g,h
CF3
H
F
ÿ73.53 (3F, t; 6) 15.73; 14.77 (1F, d; 274) 8.56; 7.61 (1F, d, 274) ÿ122.14 (2F, t; 11) ÿ135.42 (2F, d; 52) 14.63; 13.67 (1F, d; 272) 7.83, 6.87 (1F, d; 272) ÿ137.83 (2F, d; 51) ÿ130.12 (2F, m) ÿ124.93 (2F, m) ÿ119.71 (2F, m) 15.83; 14.86 (1F, d; 272) 8.85; 7.89 (1F, d; 272) Major: ÿ70.56 (3F, t, 6) 60.71 (12F, d, 92) Minor: ÿ72.15 (3F, t; 6) 56.61 (1F, d; 92) Major: ÿ82.81 (3F, dt; 5, 2) ÿ139.21 (1F, dt; 45, 5) 56.59 (1F, dd; 84, 5) Minor: ÿ82.95 (3F, d; 5) ÿ137.52 (1F, dd; 56, 5) 65.81 (1F, d; 87) ÿ73.72 (3F, t; 6) ÿ128.29 (2F, dd; 50, 12) 54.28 (1F, t; 12) ÿ72.73 (3F, dt; 13, 4) ÿ73.82 (3F, t; 6) ÿ207.62 (1F, dm; 42) 61.28 (1F, d; 25) ÿ61.33 (3F d; 3) ÿ73.65 (3F t; 6) ÿ135.83 (1F dd; 51; 11) 55.43 (1F d; 11) ÿ73.43 (3F, t; 6) ÿ159.80 (1F, dd; 56, 17) 45.66 (1F, d; 17)
Cation 2
b,c
17
CF3
CF2H
H
21
CF3
CFHCF3
H
22
CF3
CFHOCF3
H
23
CF3
CHFCl
H
F NMR ( ppm, J Hz)
1
H NMR ( ppm, J Hz)
5.13 (q; 6)
Major: 5.29 (2H q; 6) 9.05 (1H, d, 92) Minor: 5.38 (2H q; 6) 9.35 (1H, d; 89) Major: 6.78 (1H d, 44) 9.31 (1H d, 84) Minor: 6.85 (1H d; 45) 9.35 (1H d; 87) 5.05 (2H q; 6) 5.99 (1H t; 49) 5.13 (2H, q; 6) 5.66 (1H, dq; 42; 4)
4.95 (2H q; 7) 6.38 (1H d; 54)
a
At ÿ408C. Cf19 Fg: 120.61 (s), 121.54 (t, 125 Hz), 153.26 (s). c Equilibrium; ratio of starting ether to cation 63:37. d Equilibrium; ratio of starting ether to cation 76:24. e Equilibrium; ratio of starting ether to cation 84:16. f Mixture of two isomers; ratio 73:27. g Equilibrium; ratio of starting ether to cation 16:84. h Mixture of two isomers; ratio 65:35. b 13
Table 4), the origin of which is not clear at this point. It should be emphasized that chemical shifts of the ¯uorine substituent connected to the carbon bearing a positive charge and the difference in chemical shifts between the starting material and cation ( values) are in good agreement with those reported for a cyclic sulfonium cation generated by the reaction of 2,2,4,4-tetra¯uoro-1,3ditiethane with SbF5 (67.25; 119.75) [19].
The 19 F NMR spectrum for a solution of ether 11 in SbF5/ SO2ClF at low temperature does not contain signals of the starting material, but only the signals of the corresponding cation. This indicates that the equilibrium on NMR timescale is completely shifted towards 12, which indicates higher stability of 11 compared to 2, 5, 6 (see Section 3 for data on the relative stability of poly¯uorinated alkoxycarbocations).
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
Fig. 2. AB pattern of
19
(6)
Both 19 F and 1 H spectra of cation 12 contain two sets of signals (ratio 73:27), suggesting that 12 exists as a mixture of two isomers, probably due to a high rotation barrier around the C±O bond. Unfortunately, the absence of additional coupling constants makes it impossible to assign structure to a speci®c isomer. However, based on the data reported for CH3OCXH [11] methoxy¯uorocyclopropenyl [5] and methoxy¯uorocyclobutenyl cations [6], the isomer containing ¯uorine and an alkyl group in trans-position should be favored. Chemical shifts of the ±CH2±O± and ± OCFH groups in the 1 H NMR spectrum of 12 are in good agreement with the values of the corresponding groups reported for CH3OCHX cations [11].
9
F NMR spectrum of cation 8.
Replacement of hydrogen by ¯uorine in CF3CH2± moiety of ether 11 only slightly affects the stability of the corresponding carbocation. Although the 19 F NMR spectrum of a mixture of ether 13 and SbF5 at ÿ408C contains signals of both cation 14 and the starting material, the equilibrium is Table 4 19 F NMR data of sulfur-containing cations 8 and 10 RfSCF2a Cation
Rf
19
8b
CF3, 109 ppm
10c
CF3S, 118 ppm
ÿ40.05 (3F, t, 13) 62.24, 63.15 (1F, dq, 258, 13) 77.62, 77.60 (1F, dq, 258, 13) ÿ39.29 (3F, d, 5) 81.21, 80.24 (1F, dq, 279, 5) 65.49, 64.50 (1F, dq, 279, 5) 57.28, 56.40 (1F, d; 252) 73.06, 73.98 (1F, d; 252)d
a
F NMR ( ppm, J Hz)
At ÿ508C. Equilibrium; ratio of starting material to cation 78:22. c Equilibrium; ratio of starting material to cation 42:58. d Second set of signals of low intensity. b
10
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
shifted towards the cation (the 13±14 ratio is 16:84).
(7)
Similar to 12, cation 14 exists as a mixture of two isomers (ratio 65:35); however, without NOE experimental data the structural assignment of these isomers is not possible. No formation of the corresponding carbocation is observed when ether 15 is treated with SbF5. CF3 CFHOCF2 C2 F5 nSbF5
poly¯uorinated allylic cations [1] is employed. A mixture of two different ethers is reached with SbF5 under the conditions where starting compounds are competing for a Lewis acid. The stability of cations is determined from the ratio of the two resulting species (experimental details and ratio are given in Table 5). Results of these experiments are arranged into the following sequence going from stable, long-lived cations 12, 14 and 17 to less stable, existing in equilibrium with precursors, cations 2, 5, 6. Cation 24, containing C2F5 group attached to the carbon carrying positive charge, is the least stable in this series, since attempt to observe it by NMR spectroscopy at low temperature failed.
12
15
ÿ
CF3 CFHO C FC2 F5 Sbn F5n1
(8)
14
CF3 CH2 OCF2 CF2 H nSbF5
CF3 CH2 O C F2 H CF2 2 CH2 O C F2 5
6
17
According to 19 F NMR addition of ether 16 to excess of SbF5 dissolved in SO2ClF solvent results in total disappearance of signals of starting material and appearance of a new set of three signals with chemical shifts at 54.28 to ÿ73.72 and ÿ128.29 ppm, integrated as 1:3:2 (see Table 3). The multiplicity of the up®eld signal (HCF2, doublet of doublets, J50 and 12 Hz) along with chemical shift and value of coupling constant of a down®eld signal (54.28 ppm, J12 Hz) indicates the formation of carbocation 17. In general, cations of the formula CF3CH2OCFCFHX (XF±, Cl, CF3±, CF3O±) containing a poly¯uoroalkyl group in a-position to the carbocationic center are more stable than cations 2, 5, 6 (bearing two ¯uorine substituents at carbocationic center). Formation of the corresponding cations is also observed in the reaction of ethers 18, 19 and 20 with excess SbF5. These cations also exist as stable, longlived species, since in each case, signals of starting materials are not present in 19 F NMR spectra of the corresponding mixture.
> H CF2 4 CH2 O C F2 H CF2 2 CH2 O C FC2 F5 (11)
CF3 CH2 O C FCF2 H Sbn ÿ F5n1
16
17
2
Surprisingly, the reaction of partially ¯uorinated ether 16 with antimony penta¯uoride under similar conditions leads to the formation of stable long-lived carbocation 17.
SO2 ClF; ÿ40 C
24
(9)
3. Discussion Experimental data obtained in this study are suf®cient to conclude that the onium structure with a high degree of the double-bond character between carbon and oxygen (or sulfur) predominates in the resonance structure of poly¯uorinated cations 2, 5, 6, 8, 10, 12, 14, 17, 21±23. These species can be viewed as methyl cations stabilized (along with two other substituents) by oxygen or sulfur. Obviously, their stability is affected by all the three substituents. The fragment connected to oxygen has a signi®cant in¯uence on the stability of poly¯uorinated oxonium cations. Per¯uorinated alkyl groups Rf, having a strong electron-withdrawing effect, signi®cantly destabilize the cation, and make it impossible to observe these species by means of NMR spectroscopy. On the other hand, groups such as CH3± are able to stabilize carbocations due to a positive inductive effect; however, the ability of partially ¯uorinated ethers RfOR to
CF3 CH2 O C FCFHX Sbn F5n1
CF3 CH2 OCF2 CFHX 18 19 20
CF3 CH2 O C FH CF3 CFHO C FH CF3 CH2 O C FCF2 H
X CF3 X OCF3 X Cl
21 22 23
The chemical shifts and coupling constants of cations 21± 23 are quite similar to those of 17, containing a signal at 45± 62 ppm (see Table 3), coupled with the ¯uorine of the ±CFH group, as demonstrated in 19 FfHg experiments. To ®nd the relative stability of poly¯uorinated oxonium cations, a method used earlier for determining stability of
(10)
eliminate alkyl ¯uoride makes them sensitive towards Lewis acids and complicates the observation by NMR spectroscopy of the corresponding cation (for example, CH3OCFH readily decomposes at ÿ208C [11]). The reactivity of ether 25 towards SbF5 is another example. Rapid decomposition of this material is observed when it is added
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13 Table 5 Competitive reactions of ethers 1, 3, 4, 11, 13, 16 with SbF5a Entry
Ethers (ratio)
Cations (ratio)
1 2 3 4 5
11, 13 (43:57) 1, 16 (45:55) 1, 4 (50:50) 3, 4 (50:50) 11, 16 (44:66)
12, 14 (61:39) Only 17 2, 6 (37:63) 5, 6 (88:12) 12, 17 (40:60)
a
Samples were made using: 1 g(0.01, 4.6 mmol) of SbF5, 10 mmol (0.1) of SO2ClF, 1.5±2(0.3) mmol of each ether; 19 F NMR at ÿ408C.
to a solution of SbF5 in SO2ClF (even at ÿ408C) leading to CF3CH2Cl and CF3H as the only ¯uorine-containing products. ClCF2 CH2 OCF2 CF2 H 25
CF3 CH2 Cl CF3 H 26
(12) A striking difference in the stability of compounds 16 and 25 is probably the result of a signi®cantly lower barrier for decomposition of cation 27, which may be a result of the additional stabilization effect of b-chlorine, making ClCF2 CH 2 a better leaving group than CF3 CH2 and leading to rapid decomposition of ether 25 (see Scheme 1). Formation of CF3H in this reaction is the result of the well-documented low temperature degradation of CF2HC(O)F [20], while the presence of compound 26 in the reaction mixture probably is an indication of the formation of cation 28, which then captures a ¯uoride anion to CF3CH2Cl. From this standpoint, a substituent such as CF3CH2±, is almost ``ideal''. Its electron-withdrawing effect is weaker than that of a per¯uoroalkyl group (I values for CF3CH2± and CF3CF2± groups are 0.14 and 0.41, respectively [21]) but at the same time it is a poor leaving group (CF3 CH 2 is calculated to be 37 kcal/mol less stable than CH3 CH 2 [21]). An alkoxy substituent actively participates in stabilization of the carbocation, donating a p-electron pair of oxygen on the vacant orbital of a positively charged carbon and magnetic non-equivalence of ¯uorines in the ÿOCF 2 group of cations 2, 5, 6 is a re¯ection of this effect. A relatively small down®eld shift of the carbon carrying a positive charge in cation 2 (153.26 ppm vs. 216±224 ppm for positively
11
charged carbon atoms in ROCH 2 cations [22]) is an additional indication of a signi®cant participation of substituents, including oxygen, in stabilization of the carbocation. The in¯uence of two other substituents on the stability of methyl cations 2, 5, 6, 12, 14, 17, 21±24 is not as straightforward. In general, the presence of maximum p-donor substituents (able to stabilize a carbocation by the ``backdonation'' mechanism) at the carbocationic center is bene®cial. Long-lived trihalomethyl X3C (XCl, Br, I) [23], trialkoxymethyl [24] cations, along with stable salts of (CF3S)3C [14] are examples. However, poly¯uorinated methyl cations deviate from this rule. Among trihalomethyl cations X3C (XCl, Br, I) observed in condensed phase [13,25,26], the tri¯uoromethyl cation is the least stable and so far has been observed only in the gas phase. The relative stability of cations 2, 5, 6, 12, 14, 17, 21±23 (see Eq. (11)) observed in this study indicates that introduction into cation CF3CH2OCFX of ¯uorine substituents signi®cantly reduces the stability of the cation, whereas the in¯uence of poly¯uoroalkyl substituents (such as ±H, ± CF2H, ±CFHCF3 or ±CFHOCF3 is less pronounced. A reasonable explanation of this phenomena may be based on the relative electronegativities of different substituents. Since in cation CF3 CH2 OCF 2 both ¯uorine substituents cannot participate equally in stabilization through ``backdonation'' of p-electrons on the vacant orbital of the carbocation, one of the ¯uorine substituent starts acting as a powerful electron-withdrawing group causing destabilization of the cation through the inductive mechanism. A per¯uoroalkyl group has very similar, although, more pronounced effect on the stability of oxonium cations. The similarity in in¯uence of ¯uorine and C2F5± substituents on the stability of cations H(CF2CF2)nCH2OCFX (XF, C2F5) may be a result of high electronegativity of both substituents (I0.52 and 0.42 for F and C2F5, respectively). On the other hand, poly¯uoroalkyl groups such as ±CF2H have less pronounced electron-withdrawing ability (I0.32 for CF2H vs. 0.42 for C2F5 [21]) and the stabilizing effect of RfCH2O± substituents is suf®cient to overcome the destabilizing effect of the poly¯uoroalkyl group. Higher electronegativity of ¯uorine substituent vs. electronegativity of a carbocationic center is also probably responsible for signi®cantly lower stability of elusive CF 3 [25,26] and
Scheme 1.
12
V.A. Petrov, F. Davidson / Journal of Fluorine Chemistry 95 (1999) 5±13
FCO [27] carbocations compared to the relatively stable and observed in condensed phase CCl 3 [23] and ClCO [27,28]. Due to the same effect di¯uoromethyl cations XCF 2 (XH, Cl, Br, I) are expected to have signi®cantly lower stability compared to corresponding trihalomethyl cations CX 3 (XCl, Br, I).
nel, and 16 g (0.079 mol) of CF3CHClOCF2H is slowly added. After the end of the exothermic reaction the mixture is stirred for 1 h, the product (10 g, 62.5%, 98% purity) is transferred into a cold trap (ÿ788C) and used without further puri®cation. 19 F NMR parameters of isolated 13 are in good agreement with the values reported [31].
4. Experimental
Acknowledgements
All the NMR spectra were recorded on G.E. Omega 300 MHz spectrometer (276 MHz for 19 F and 300 MHz for 1 H). Chemical shifts are given relative to CFCl3 and TMS, respectively. All ¯uorinated alcohols used in this study for the preparation of ethers and compounds 7, 10, 11, 15 are commercially available (PCR) and used without further puri®cation. Ethers 1, 3, 4 are obtained using a slight modi®cation of the procedure described in [29] (see below), whereas ethers 16, 18±20 are prepared by base catalyzed addition of tri¯uoroethanol to the corresponding ole®ns [30]. NMR data not available for certain materials are listed in Table 1. Antimony penta¯uoride (Ozark-Mahoning) is distilled under nitrogen and stored and handled inside a dry box.
The authors thank Dr. B.E. Smart, Dr. C.G. Krespan and Dr. A.C. Sievert for stimulating discussions and valuable comments, Dr. W. Qiu for sample of ether 25 and Boris Vekker for the help in the preparation of the paper.
4.1. Preparation of ethers 1, 3, 4 (method A) A 1 gal Hastelloy autoclave is charged with 1 mol of the corresponding alcohol, 1 mol of CCl4, 15 g of BF3 and 400 ml of anhydrous HF. The mixture is agitated at 1508C for 14±16 h. Water (1 l) is injected into the reactor, it is cooled down to 08C, and the reaction mixture transferred into a polyethylene container. An organic layer (lower) was separated, dried over MgSO4 and distilled. Reaction conditions and yields are shown in Table 1. 4.2. Preparation of ethers 16, 18±20 (method B) A Hastelloy reactor (400 ml) is charged with 1 mol of corresponding alcohol and a solution of 6±25 g of KOH in 20±75 ml of water. The reactor is closed, cooled down to ÿ788C, evacuated, and 1 mol of the corresponding ole®n introduced into the reactor (in the case of tetra¯uoroethylene, addition is conducted in two steps: 0.5 mol of ole®n is added after the initial 0.5 mol has been consumed). The reaction mixture is agitated at 50±1208C for 16±24 h. The crude reaction mixture is poured into water, the organic layer is separated, dried over P2O5 and distilled. Reaction conditions and product yields are shown in Table 1. 4.3. Synthesis of compound 13 Fifty g (0.23 mol) of SbF5 is placed in a ¯ask equipped with a thermometer, dry-ice condenser and additional fun-
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