+
+
Decay Kinetics of Three (Z)-1-Arylethanediazoates
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7135
according to the procedure of Moss.7 The diazoates were contaminated
1
in every case with <10% potassium ethoxide according to H-NMR.
The solid diazoates were unstable even under high vacuum in a
desiccator so that all experiments were performed on the compounds
within a few hours of their synthesis. Potassium (Z)-2,2,2-trifluoro-1-
phenylethanediazoate: 1H-NMR (DMSO-d6) δ 7.23 (m, 5H), 5.91 (q,
1H). Potassium (Z)-2,2,2-trifluoro-1-(4-methylphenyl)ethanediaz-
oate: 1H-NMR (DMSO-d6) δ 7.13 (d, 2H), 7.00 (d, 2H), 5.90 (q, 1H),
2.22 (s, 3H). Potassium (Z)-2,2,2-trifluoro-1-(4-methoxyphenyl)ethane-
diazoates: 1H-NMR (DMSO-d6) δ 7.16 (d, 2H), 6.78 (d, 2H), 5.93 (q,
1H), 3.69 (s, 3H).
Ethyl-N-nitroso-N-((1-aryl)-2,2,2-trifluoroethyl)urethanes. These
were synthesized from the parent urethanes using 2 equiv of NOBF4
and 1 equiv of 2,6-di-tert-butyl-4-methylpyridine, as described previ-
ously.3 After 30 min to 2 h of reaction, most of the solvent was
evaporated off and the reaction was quenched with pentane. The
resulting slurry was filtered, and the eluant was collected and combined
with pentane washings. Pentane was evaporated by means of an argon
stream, and the residue was dissolved in a minimum of 1/1 pentane/
dichloromethane and chromatographed on silica using an eluant of
identical composition. The first yellow band was collected, and the
product was obtained as a yellow oil after the solvent was removed by
means of an argon stream. The yields were typically ∼60-40%.
Urethanes. These were made from the reaction of 2,2,2-trifluoro-
1-arylethanamines with ethyl chloroformate analogous to the method
described previously.3
Figure 1. Plot of log ko, the buffer-independent rate constant for decay
of (Z)-2,2,2-trifluoro-1-arylethanediazoates at 25 °C in aqueous media,
4% 2-propanol by volume, ionic strength 1 M (NaClO4), against pH.
Points are defined as follows: b, unsubstituted; 2, 4-CH3; 9, 4-CH3O.
of the crude reaction indicated that the tautomerization was complete
under these conditions in all cases. The product imines were
subsequently hydrolyzed in aqueous 2 M HCl, and acetophenone was
removed by extracting this reaction mix with diethyl ether. The amines
were subsequently isolated by basifying the aqueous acid solution using
concentrated aqueous NaOH and extracting with diethyl ether. The
final products were purified by distillation under reduced pressure.
1-Phenyl-2,2,2-trifluoroethylamine: bp 84 °C (19 Torr) (lit.8 bp 35 °C
1
(0.3 Torr)); H-NMR (CDCl3) δ 7.40 (m, 5H), 4.38 (q, 1H), 1.76 (b,
2,2,2-Trifluoro-1-arylethanamines. These were generated via the
reactions in the scheme below. This method gave adequate yields of
the final desired product, whereas the more direct hydrogenolysis of
precursor oximes8 failed to give yields greater than 5% in our hands.
2H). 1-(4-Methylphenyl)-2,2,2-trifluoroethylamine: bp 92-93 °C (18
1
Torr); H-NMR (CDCl3) δ 7.32 (d, 2H), 7.19 (d, 2H), 4.36 (q, 1H),
2.36 (s, 3H), 1.73 (b, 2H). 1-(4-Methoxyphenyl)-2,2,2-trifluoro-
1
ethylamine: bp 78 80 °C (3 Torr); H-NMR (CDCl3) δ 7.36 (d, 2H),
6.91 (d, 2H), 4.36 (q, 1H), 3.82 (s, 3H), 1.73 (b, 2H).
Deuterium Incorporation into Product Alcohols. These experi-
ments were carried out by decomposition of the diazoates (∼0.005 M),
either with conventional or stopped-flow conditions as appropriate (5
mL reaction volumes). Products were isolated by extraction with 2 ×
10 mL ether, and the ether phase was treated with 1 × 10 mL of 0.2
M DClO4 in D2O. The ether phase was then washed with 3 × 10 mL
of NaCl-saturated H2O and concentrated, and the isotopic analysis of
the product alcohols was carried out by GC-MS.
2,2,2-Trifluoro-1-arylethanones. These were synthesized by lit-
erature methods.9 1-Phenyl-2,2,2-trifluoroacetaldehyde: bp 69-73 °C
(43 Torr) (lit.9 54-56 °C (12 Torr)); 1H-NMR (CDCl3) δ 8.08 (d, 2H),
7.73 (m, 1H), 7.56 (t, 2H). 1-(4-Methylphenyl)-2,2,2-trifluoro-
Product analysis by 1H-NMR was carried out subsequent to the
decay, upon addition of 1 equiv of DClO4, of (Z)-2,2,2-trifluoro-1-(4-
methylphenyl)ethanediazoate in 0.05 M NaOD in D2O/20% CD3CN
by volume and containing tert-butyl methyl ether as an internal standard.
Kinetics. The instrumentation and general procedures followed were
the same as those previously described.4
1
acetaldehyde: bp 65-66 °C (13 Torr) (lit.10 69 °C (13 Torr)); H-
NMR (CDCl3) δ 7.98 (d, 2H), 7.35 (d, 2H), 2.47 (s, 3H). 1-(4-Meth-
oxyphenyl)-2,2,2-trifluoroacetaldehyde: bp 87 °C (7 Torr) (lit.11 bp
98-99 °C (17 Torr)); 1H-NMR (CDCl3) δ 8.06 (d, 2H), 7.02 (d, 2H),
3.92 (s, 3H).
Imines. These were prepared by methods analogous to literature
procedures. In a typical procedure, to 20 g (0.11 mol) of 1-(4-
methylphenyl)-2,2,2-trifluoroacetaldehyde and 40 g (0.33 mol) of
1-phenylethylamine in 400 mL of dry toluene was added 55 mL of a
1 M solution of TiCl4 in toluene. Precipitation occurred immediately,
and subsequent to 30 min stirring at room temperature, the solution
was filtered, the solid discarded, and the filtrate evaporated to a residue
that was purified by distillation under reduced pressure. N-(1-
Phenylethyl)-1-phenyl-2,2,2-trifluoroethanimine: bp 119-120 °C (4
Results
The decay of (Z)-2,2,2-trifluoro-1-(4-methylphenyl)ethanedia-
zoate in 0.05 M NaOD in predominantly aqueous media (20%
CD3CN by volume) gave a 95% yield of 2,2,2-trifluoro-1-(4-
methylphenyl)ethanol.
The decay of the (Z)-2,2,2-trifluoro-1-arylethanediazoates in
aqueous media, 4% 2-propanol by volume, ionic strength 1 M
(NaClO4) at 25 °C, exhibited good first-order behavior for more
than four half-lives of reaction. Values of the rate constants,
ko, for the buffer-independent rate constant for decomposition
were determined as the intercepts of plots of kobsd against buffer
concentration. The standard error in the values was typically
less than (5%. Plots of log ko as a function of pH are presented
in Figure 1.
Activation parameters for the decay of (Z)-2,2,2-trifluoro-1-
(4-methylphenyl)ethanediazoate were determined, at pH ) 5.51
(pH measured at 25 °C), to be ∆H# ) 14.9 ((0.1) kcal mol-1
and ∆S# ) -6.9 ((0.3) cal deg-1 mol-1. These experiments
were carried out between 5 and 50 °C (13 temperatures) in 0.05
M morpholinoethanesulfonate buffer (20% acid) in which it was
separately determined that the contribution to the reaction rate
constant due to buffer catalysis was less than 5% of kobsd at 25
and 50 °C.
1
Torr) (lit.12 bp 99-101 °C (0.5 Torr)); H-NMR (CDCl3) δ 7.28 (m,
10H), 4.54 (q, 1H), 1.45 (d, 3H). N-(1-Phenylethyl)-1-(4-methyl-
phenyl)-2,2,2-trifluoroethanimine: 1H-NMR (CDCl3) δ 7.30 (m, 9H),
4.57 (q, 1H), 2.41 (s, 3H), 1.44 (d, 3H). N-(1-Phenylethyl)-1-(4-
methoxyphenyl)-2,2,2-trifluoroethanimine: bp 142 °C (2 Torr), 1H-
NMR (CDCl3) δ 7.31 (m, 5H), 7.14 (d, 2H), 6.97 (d, 2H) 4.61 (q, 1H),
3.86 (s, 3H), 1.45 (d, 3H).
Tautomerization and Hydrolysis. The imines were tautomerized
by heating, by means of an oil bath, at 120 °C for 24 h using 5 mol %
1,8-diazabicyclo[5.4.0]undec-7-ene as a catalyst.13 1H-NMR analysis
(7) Moss, R. A. J. Org. Chem. 1966, 31, 1082.
(8) Wang, Y.; Mosher, H. S. Tetrahedron Lett. 1991, 32, 987.
(9) Diderich, G. HelV. Chim. Acta 1972, 55, 2103.
(10) Stewart, R.; Van der Linden, R. Can. J. Chem. 1960, 38, 399.
(11) Hatanaka, Y.; Hashimoto, M.; Kurihara, H.; Nakayama, H.;
Kanaoka, Y. J. Org. Chem. 1994, 59, 383.
(12) Pirkle, W. H.; Hauske, J. R. J. Org. Chem. 1977, 42, 2436.
(13) Kukar, V. P.; Soloshonok, V. A.; Galushko, S. V.; Rozhenko, A.
B. Dokl. Akad. Nauk. SSSR 1990, 310, 886.