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CHEMISTRY
Yu. V. ZEIFMAN, N. P. GAMBARYAN, Academician I. L. KNUNYANTS
IMINES OF HEXAFLUOROACETONE
Until recently, the imines of hexafluoroacetone, as well as those of other perfluorinated ketones, were unknown. Meanwhile, they are of considerable interest. First of all, the presence of two trifluoromethyl groups should increase the electrophilic character of the azomethine bond. In addition, these compounds open the way to a variety of derivatives of 2-aminohexafluoropropane. Compounds containing such a grouping had not hitherto been obtained.
Attempts to obtain imines of hexafluoroacetone by the usual methods proved unsuccessful (¹). In the interaction of hexafluoroacetone with amino compounds, relatively stable terminal oxyamines are formed, which do not undergo dehydration. Nor can they be obtained from ketals of hexafluoroacetone
\[ (\mathrm{CF_3})_2\mathrm{CO}+\mathrm{RNH_2}\to(\mathrm{CF_3})_2\mathrm{C(OH)NHR} \ \xrightarrow[-X]{-\mathrm{H_2O}}\ (\mathrm{CF_3})_2\mathrm{C{=}NR} \ \xleftarrow[-X]{\mathrm{RNH_1}}\ (\mathrm{CF_3})_2\mathrm{C(OCH_3)_2}. \]
Only recently we (¹), from triphenylphosphinephenylimine (I) by a reaction analogous to the Wittig reaction, obtained the anil of hexafluoroacetone (II). However, it remained an unstudied and not readily accessible compound. A new and simple method for obtaining anil II, which we have found, consists in the interaction of phenyl isocyanate with hexafluoroacetone at 200° in the presence of catalytic amounts of triphenylphosphine oxide:
\[ \begin{aligned} &\mathrm{C_6H_5N{=}C{=}O} \\ &\quad +\ \mathrm{(C_6H_5)_3P{=}O} \ \xrightarrow{-\mathrm{CO_2}}\ [\mathrm{(C_6H_5)_3P{=}NC_6H_5}] \ \xrightarrow{(\mathrm{CF_3})_2\mathrm{CO}}\ (\mathrm{CF_3})_2\mathrm{C{=}NC_6H_5} +\mathrm{(C_6H_5)_3P{=}O}. \end{aligned} \]
\[ \mathrm{(I)} \qquad\qquad\qquad\qquad\qquad\qquad \mathrm{(II)} \]
Without a catalyst the reaction cannot be carried out even under more severe conditions, which is not surprising, since isocyanates are electrophilic reagents, whereas the nucleophilicity of hexafluoroacetone is very low. The role of the catalyst evidently amounts to the fact that in the first stage of the reaction phenylimine I is formed, which then reacts with hexafluoroacetone. A similar catalytic action of phosphine oxides was recently observed by Monell (²), who found a new method for the synthesis of carbodiimides from isocyanates.
To obtain other imines of hexafluoroacetone, one may use N-phenyl-N′-alkyl-2,2-diaminohexafluoropropanes (III), readily obtained from anil II and amines (see below). These compounds undergo an unusual decomposition upon treatment with hydrogen chloride in ether. In this process the weaker base—aniline—is split off quantitatively, being isolated in the form of the hydrochloride, and the corresponding imine of hexafluoroacetone is formed:
\[ \begin{aligned} &\begin{array}{c} \mathrm{CF_3}\quad \mathrm{NHR}\\[-2mm] \ \ \backslash \ \ /\\[-1mm] \mathrm{C}\\[-1mm] / \ \backslash\\[-2mm] \mathrm{CF_3}\quad \mathrm{NHC_6H_5} \end{array} +\mathrm{HCl} \ \xrightarrow{0^\circ}\ \begin{array}{c} \mathrm{CF_3}\\[-2mm] \backslash\\[-1mm] \mathrm{C{=}NR}\\[-1mm] /\\[-2mm] \mathrm{CF_3} \end{array} +\mathrm{C_6H_5NH_2\cdot HCl} \end{aligned} \]
\[ \mathrm{(III)} \qquad \mathrm{R=H,\ CH_2CH(CH_3)_2,\ CH_2C_6H_5.} \]
Thus, from anil II one can proceed to any more basic imine of hexafluoroacetone. We also obtained the unsubstituted imine of hexafluoroacetone IV from triphenylphosphineimine. However, the most convenient method for obtaining imine IV consists in the thermal decomposition of N-phenyl-2,2-diaminohexafluoropropane, and here too an unusual decom-
breakdown: instead of the most volatile base (ammonia), the weaker base (aniline) is expelled, and imine IV is formed:
\[ \begin{gathered} \begin{array}{c} \mathrm{CF_3}\quad \mathrm{NH_2}\\[-2mm] \diagdown\ \ \diagup\\[-1mm] \mathrm{C}\\[-1mm] \diagup\ \ \diagdown\\[-2mm] \mathrm{CF_3}\quad \mathrm{NHC_6H_5} \end{array} \ \xrightarrow[\ 180^\circ\ ]{-\mathrm{C_6H_5NH_2}}\ \begin{array}{c} \mathrm{CF_3}\\[-2mm] \diagdown\\[-1mm] \mathrm{C}=\\[-1mm] \diagup\\[-2mm] \mathrm{CF_3} \end{array} \mathrm{NH}\leftarrow(\mathrm{C_6H_5})_3\mathrm{P}=\mathrm{NH} +(\mathrm{CF_3})_2\mathrm{CO}. \end{gathered} \tag{IV} \]
An analogous decomposition with expulsion of the weaker base is known only for \(N\)-acyl-\(N'\)-alkyl-diaminodiphenylmethanes \((^3)\):
\[ (\mathrm{C_6H_5})_2\mathrm{C} \begin{matrix} \diagup \mathrm{NHR}\\ \diagdown \mathrm{NHCOR'} \end{matrix} \ \longrightarrow\ (\mathrm{C_6H_5})_2\mathrm{C}=\mathrm{NR} + \mathrm{R'CONH_2}. \]
Imine IV, in contrast to known aliphatic imines \((^4)\), is very stable and is not changed on heating at least up to \(150^\circ\).
A distinctive feature of the imines of hexafluoroacetone is the ease with which they react with such typical nucleophilic reagents as ammonia and amines. Under mild conditions, derivatives of 2,2-diaminohexafluoropropane are thereby formed in high yields (see Table 1):
\[ \begin{gathered} \begin{array}{c} \mathrm{CF_3}\\[-2mm] \diagdown\\[-1mm] \mathrm{C}=\mathrm{NH_2^+}\ (R)\\[-1mm] \diagup\\[-2mm] \mathrm{CF_3} \end{array} + \mathrm{NH_3}\ (\mathrm{R'NH_2}) \ \longrightarrow \\[2mm] \begin{array}{c} \mathrm{CF_3}\quad \mathrm{NH_2}\ (R)\\[-2mm] \diagdown\ \ \diagup\\[-1mm] \mathrm{C}\\[-1mm] \diagup\ \ \diagdown\\[-2mm] \mathrm{CF_3}\quad \mathrm{NH_2}\ (R') \end{array} . \end{gathered} \]
The simplest of them, 2,2-diaminohexafluoropropane, is as yet the only representative of geminal diamines unsubstituted at nitrogen. Other compounds of this type are unknown in the free state. Products of addition of ammonia and amines to azomethines had previously been obtained only in the case of \(N\)-acylimines of benzophenone \((^3)\), although they probably are formed as intermediates in the reamination of ald- and
Table 1
Derivatives of 2,2-diaminohexafluoropropane
\[ \mathrm{CF_3{-}C(NHR_1)(NHR_2){-}CF_3} \]
| Starting imine | \(R_1\) | \(R_2\) | b.p. or m.p., °C, mm | m.p., °C | Yield, % | \(n_D^{20}\) | \(d_4^{20}\) | Found, % C | Found, % H | Found, % F | Found, % N | Calculated, % C | Calculated, % H | Calculated, % F | Calculated, % N |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| IV | H | H | 91–92 | 86 | 1.3320 | 1.5152 | 19.62 | 2.41 | 62.31 | 14.58 | 19.76 | 2.23 | 62.60 | 15.38 | |
| IV | H | \(\mathrm{CH_2CH(CH_3)_2}\) | 71–73 (80) | 80 | 1.3560 | 1.3560 | 42.21 42.48 |
3.11 2.90 |
48.01 47.56 |
11.79 11.82 |
41.86 | 3.10 | 47.90 | 11.78 | |
| II | H | \(\mathrm{C_6H_5}\) | 76–77 (8) | 12–14 | 92 | 1.4431 | 1.4302 | 49.49 49.69 |
4.97 4.99 |
44.16 44.89 |
11.31 11.28 |
49.65 | 5.09 | 44.12 | 10.86 |
| II | \(\mathrm{CH_2CH(CH_3)_2}\) | \(\mathrm{C_6H_5}\) | 94–95 (5) | 67 | 1.4452 | 1.2620 | 55.70 55.53 |
5.05 4.05 |
36.44 36.02 |
7.97 | 55.20 | 4.03 | 36.30 | 8.05 | |
| II | \(\mathrm{CH_2C_6H_5}\) | \(\mathrm{C_6H_5}\) | 32–34* | 78 | 32.71 | 7.94 | 32.8 | 8.05 |
* From petroleum ether on cooling to \(-78^\circ\).
ketimines \((^5)\):
\[ \begin{aligned} &\begin{array}{c} R'\\[-2mm] \diagdown\\[-1mm] \quad C=NR'''\\[-1mm] \diagup\\[-2mm] R'' \end{array} + RNH_2 \;\longrightarrow\; \left[ \begin{array}{c} R'\\[-2mm] \diagdown\\[-1mm] \quad C\begin{array}{c}\diagup NHR\\[-1mm]\diagdown NHR'''\end{array}\\[-1mm] \diagup\\[-2mm] R'' \end{array} \right] \;\longrightarrow\; \begin{array}{c} R'\\[-2mm] \diagdown\\[-1mm] \quad C=NR\\[-1mm] \diagup\\[-2mm] R'' \end{array} + R'''NH_2 . \end{aligned} \]
It is important to note that in this process, in all cases, exchange of the more volatile base for the less volatile one occurs, irrespective of their basicity.
As was to be expected, the imine of hexafluoroacetone proved to be considerably more reactive than anil II. Thus, whereas the latter does not react with aniline at \(200^\circ\), imine IV smoothly adds aniline when heated in a sealed ampoule at \(100^\circ\); consequently, its formation from N-phenyl-2,2-diaminohexafluoropropane is, in principle, reversible.
Using II and IV as examples, we also studied a number of other reactions of hexafluoroacetone imines. Thus, lithium aluminum hydride smoothly reduces them to the corresponding amines V; addition of hydrocyanic acid leads to the nitriles of \(\alpha\)-amino- and \(\alpha\)-phenylaminohexafluoroisobutyric acid VI.
\[ \begin{aligned} &\begin{array}{c} CF_3\\[-1mm] \diagdown\\[-1mm] \quad CHNHR\\[-1mm] \diagup\\[-1mm] CF_3 \end{array} \ (V) \ \xleftarrow{\ \mathrm{LiAlH_4}\ }\ \begin{array}{c} CF_3\\[-1mm] \diagdown\\[-1mm] \quad C=NR\\[-1mm] \diagup\\[-1mm] CF_3 \end{array} \ \xrightarrow[\mathrm{C_5H_{11}N}]{\mathrm{HCN}}\ \begin{array}{c} CF_3\quad NHR\\[-1mm] \diagdown\ \diagup\\[-1mm] \quad C\\[-1mm] \diagup\ \diagdown\\[-1mm] CF_3\quad CN \end{array} \ (VI)\\[2mm] &\hspace{55mm} \begin{array}{l} a:\ R=H\\[2mm] b:\ R=C_6H_5 \end{array}\\[-1mm] &\hspace{37mm} \xdownarrow[\mathrm{C_2H_5OH}]{}\ \begin{array}{c} CF_3\quad NHR\\[-1mm] \diagdown\ \diagup\\[-1mm] \quad C\\[-1mm] \diagup\ \diagdown\\[-1mm] CF_3\quad OC_2H_5 \end{array} \ (VII) \end{aligned} \]
Whereas azomethines usually do not react with alcohol, VIIa is readily obtained by simply heating imine IV with alcohol; VIIb can be obtained only by boiling with alcohol in the presence of catalytic amounts of trifluoroacetic acid, or by the action of sodium alcoholate on anil II. Thus, in the reaction with alcohol, as in the reaction with amines, the enhanced ability of hexafluoroacetone imines to add nucleophilic reagents is manifested.
Experimental Part
Anil of hexafluoroacetone (II). Into a steel ampoule were charged 23.6 g of phenyl isocyanate, 4.2 g of triphenylphosphine oxide, and 36 g of hexafluoroacetone. The mixture was heated for 17 h at \(200^\circ\), the excess pressure was released, and the residue was distilled. This gave 43.7 g (91.5% of theory) of II, b.p. \(72—73^\circ/73\) mm, \(n_D^{22}\) 1.4152.
Interaction of hexafluoroacetone imines with ammonia and amines. With cooling by ice water, equimolecular amounts of the imine and ammonia or amine were mixed. After 10–12 h, III was isolated by distillation. The properties of the compounds obtained and the results of elemental analysis are given in Table 1.
Imine of hexafluoroacetone (IV). A) Into a stirred and cooled suspension of 29.4 g of triphenylphosphine imine \((^6)\) in 120 ml of xylene, 18 g of hexafluoroacetone was passed. On the following day IV was distilled off from the mixture. After redistillation, 11.2 g (64% of theory) of hexafluoroacetone imine with b.p. \(16—19^\circ\) was obtained.
Found, %: C 22.44; 22.67; H 0.84; 1.00; F 69.43; 69.80
\(C_3HF_6N\). Calculated, %: C 21.80; H 0.61; F 69.1
B) 10 g of N-phenyl-2,2-diaminohexafluoropropane was boiled with a reflux condenser connected to a trap cooled to \(-78^\circ\). By distillation of the condensate collected in the trap, 5 g (80% of theory) of imine IV with b.p. \(16.5—18^\circ\) was obtained. After distillation of the flask residue, 3.1 g (86% of theory) of aniline, b.p. \(182^\circ\), was obtained.
C) To a solution, cooled to \(0^\circ\), of 6.6 g of N-phenyl-2,2-diaminohexafluoropropane in 20 ml of anhydrous dibutyl ether, 0.93 g of hydrogen chloride in 7 ml of ether was added. After an hour, 3.2 g (96% of theory) of the hydrochloride-
aniline, m.p. 188–190°. From the mother liquor, by distillation, 2.5 g (60% of theory) of imine IV was isolated, b.p. 18–23°. The following were obtained analogously (in diethyl ether):
N-Benzylimine of hexafluoroacetone. Yield 76% of theory, b.p. 84–85°/40 mm, \(n_D^{20}\) 1.4230; \(d_4^{20}\) 1.3302. \(MR\) found 48.62, calculated 48.81.
\[ \begin{aligned} &\text{Found, \%: } &&\text{C }46.68;\ 46.46;\quad \text{H }2.98;\ 2.99;\quad \text{F }43.11;\ 43.07\\ &\mathrm{C}_{10}\mathrm{H}_{7}\mathrm{F}_{6}\mathrm{N}.\ \text{Calculated, \%: } &&\text{C }47.05;\quad \text{H }2.74;\quad \text{F }44.73 \end{aligned} \]
N-Isobutylimine of hexafluoroacetone. Yield 45% of theory, b.p. 85–86°, \(n_D^{20}\) 1.3270, \(d_4^{20}\) 1.1700. \(MR\) found 38.24, calculated 38.04.
\[ \begin{aligned} &\text{Found, \%: } &&\text{F }49.94;\ 49.94;\quad \text{N }6.96;\ 7.13\\ &\mathrm{C}_{7}\mathrm{H}_{9}\mathrm{F}_{6}\mathrm{N}.\ \text{Calculated, \%: } &&\text{F }51.5;\quad \text{N }6.34 \end{aligned} \]
N-Hexafluoroisopropylaniline (Vb). To a suspension of 0.44 g of lithium aluminum hydride in 100 ml of ether, with ice cooling and stirring, 5.6 g of anil II was added. The mixture was left overnight at 20°. After the usual workup, distillation gave 5.1 g (90% of theory) of amine Vb, b.p. 86–88°/50 mm, \(n_D^{20}\) 1.4168, \(d_4^{20}\) 1.3863. \(MR\) found 44.99, calculated 44.60.
\[ \begin{aligned} &\text{Found, \%: } &&\text{C }44.5;\ 44.64;\quad \text{H }2.95;\ 3.05;\quad \text{F }46.83;\ 46.5\\ &\mathrm{C}_{9}\mathrm{H}_{7}\mathrm{F}_{6}\mathrm{N}.\ \text{Calculated, \%: } &&\text{C }44.48;\quad \text{H }2.83;\quad \text{F }46.93 \end{aligned} \]
Hexafluoroisopropylamine (Va). Obtained analogously to Vb and isolated from the ethereal solution as the hydrochloride. Yield 60% of theory, m.p. 198–200° (in a sealed capillary).
\[ \begin{aligned} &\text{Found, \%: } &&\text{F }56.82;\ 56.14;\quad \text{Cl }17.94;\ 18.06\\ &\mathrm{C}_{3}\mathrm{H}_{4}\mathrm{F}_{6}\mathrm{NCl}.\ \text{Calculated, \%: } &&\text{F }56.02;\quad \text{Cl }17.44 \end{aligned} \]
Nitrile of α-aminohexafluoroisobutyric acid (VIa). To a cooled mixture of 8.2 g of hexafluoroacetone imine and 1.4 g of hydrocyanic acid, several drops of piperidine were added. The mixture was left at room temperature until the next day. Distillation gave 7.2 g (75% of theory) of aminonitrile, b.p. 45–46°/100 mm. \(n_D^{20}\) 1.3130, \(d_4^{20}\) 1.5112. \(MR\) found 24.76, calculated 24.75.
\[ \begin{aligned} &\text{Found, \%: } &&\text{C }24.41;\ 24.39;\quad \text{H }1.36;\ 1.36;\quad \text{N }14.6;\ 14.91\\ &\mathrm{C}_{4}\mathrm{H}_{2}\mathrm{F}_{6}\mathrm{N}_{2}.\ \text{Calculated, \%: } &&\text{C }24.96;\quad \text{H }1.04;\quad \text{N }14.56 \end{aligned} \]
2-Amino-2-ethoxyhexafluoropropane (VIIa). 4 g of hexafluoroacetone imine and 1 g of alcohol were heated in a sealed ampoule for 6 h at 100°. Distillation gave 4.1 g (81% of theory) of VIIa, b.p. 100–102°, \(n_D^{20}\) 1.3245, \(d_4^{20}\) 1.396. \(MR\) found 30.37, calculated 31.19.
\[ \begin{aligned} &\text{Found, \%: } &&\text{C }28.17;\ 28.03;\quad \text{H }3.53;\ 3.45;\quad \text{F }54.13;\ 53.70\\ &\mathrm{C}_{5}\mathrm{H}_{7}\mathrm{F}_{6}\mathrm{NO}.\ \text{Calculated, \%: } &&\text{C }28.42;\quad \text{H }3.32;\quad \text{F }54.03 \end{aligned} \]
2-Phenylamino-2-ethoxyhexafluoropropane (VIIb). 3.5 g of hexafluoroacetone anil was boiled with 5 ml of alcohol and several drops of trifluoroacetic acid for 6 h. Distillation gave 1.7 g (41% of theory) of VIIb, b.p. 75–77°/8 mm, m.p. 26–28° (from petroleum ether).
\[ \begin{aligned} &\text{Found, \%: } &&\text{C }45.41;\quad \text{H }4.02;\quad \text{F }40.47\\ &\mathrm{C}_{11}\mathrm{H}_{11}\mathrm{F}_{6}\mathrm{NO}.\ \text{Calculated, \%: } &&\text{C }46.01;\quad \text{H }3.84;\quad \text{F }39.75 \end{aligned} \]
Institute of Organoelement Compounds
Academy of Sciences of the USSR
Received
28 IX 1963
REFERENCES CITED
- I. L. Knunyants, N. P. Gambaryan et al., Izv. AN SSSR, OKhN, 1962, 684.
- J. J. Monagle, J. Am. Chem. Soc., 84, 4289 (1962).
- J. E. Banfield, G. M. Brown, F. H. Davey, Austr. J. Sci. Res., 1, 330 (1948).
- P. L. Pickard, J. Org. Chem., 26, 4886 (1961).
- Houben-Weyl, 11/2, 1958, p. 74.
- R. Appel, W. Buchner, E. Guth, Ann., 618, 53 (1958).