Checked by B. E. Smart and R. E. Benson.
1. Procedure
2. Notes
2. A satisfactory way to introduce
chlorine with minimal loss of the gas is to seal the reaction flask with a
two-holed stopper equipped with a gas-inlet tube, reaching just above the surface of the reaction mixture, and an exit tube, connected to a U-tube filled with mineral oil which is used as a gas-flow indicator.
Chlorine is introduced from the
cylinder through a safety trap at such a rate as to maintain a small positive pressure in the reaction flask.
3. The reaction can be followed by
1H NMR spectroscopy. The original absorption for the vinyl proton disappears and two new absorption peaks appear, one in the vinyl region (
ca. δ 6.5, CDCl
3) and the other in the methine region of the spectrum. There are two products formed, presumably the
cis- and
trans-isomers, in the ratio of 95:5, respectively. The checkers also obtained the same yield when the reaction quantities were doubled.
4. An analytical sample has a m.p. of
127–129°. Additional product can be recovered from the mother liquor by addition of approximately 1 l. of water followed by filtration. The yield of this product is about
31 g. (
21%). However, it contains about 20% of the minor isomer (Note
3) that is not dehydrohalogenated under the reaction conditions employed in the next step. The second crop can be recrystallized from hot
methanol, giving predominantly the desired isomer. In some preparations the submitters did not separate the minor product and observed no significant loss in yield in the subsequent steps. The spectral properties of the product are as follows; IR (Nujol) cm.
−1; 1700 (C=O), 1600 (C=C);
1H NMR (CDCl
3), δ (multiplicity, number of protons, assignment): 1.28 [s, 9H, C(C
H3)
3], 1.37 (s, 9H, C(C
H3)
3], 4.75 (s, 1H, C
H), 6.47 (s, 1H, =C
H).
5. The reaction mixture warms slightly, resulting in the boiling of the
ether. The large amount of
diethylamine hydrochloride formed transforms the reaction mixture into a thick paste.
6. The spectral properties of the product are as follows; IR (Nujol) cm.
−1: 1680 (C=O), 1660 (C=C);
1H NMR (CDCl
3), δ (multiplicity, number of protons, assignment): 1.30 [s, 9H, C(C
H3)
3], 1.46 [s, 9H, C(C
H3)
3], 6.59 (s, 1H, =C
H).
7. The experimental setup in this reaction is exactly as that described in (Note
2).
8. The progress of the reaction is followed by
1H NMR spectroscopy. When the absorption for the vinyl proton (
ca. δ 6.6, CDCl
3) is completely absent, the reaction is stopped. Several minor products that were not identified are also formed in this step.
9. IR (Nujol) cm.
−1: 1710 (C=O);
1H NMR (CDCl
3), δ (multiplicity, number of protons, assignment): 1.1–1.4 [m, 18H, 2C(C
H3)
3], 4.68 (s, 1H, C
H), 4.87 (s, 1H, C
H).
10. Comments given in (Note
5) apply here also.
12. The mother liquor and wash solution are combined and concentrated to 200 ml. on a rotary evaporator. Upon cooling, a second crop (
15–18 g.) of product is obtained. This second crop was a semisolid material. The spectral properties of the crystalline product are as follows; IR (Nujol) cm.
−1: 1660 (C=O);
1H NMR (CDCl
3), δ (multiplicity, assignment): 1.45 [s, C(C
H3)
3].
13. Theoretically there remains about
22% of product to be isolated. Some of this material can be recovered indirectly by converting it to the diazide. The submitters diluted the mother liquor, which contains at most
23 g. (0.079 mole) of 2,5-di-tert-butyl-3,6-dichloro-1,4-benzoquinone, with
500 ml. of methanol, and then added, with swirling, a solution of
10.4 g. (0.0160 mole) of sodium azide in 30 ml. of water over a 2-minute period, turning, the yellow solution orange. It is cooled to −5° to −10°, and the resulting orange precipitate is collected, yielding
12 g. of the diazide. The minimum yield is thus
88%.
14. Water can be added to the mother liquor, and the mixture extracted with
chloroform to increase the diazide yield to
95–98%. During the course of any purification method that might be employed the diazide should not be heated above 50° since decomposition occurs quite noticeably at that temperature. It is best to store the pure product below −5° in the dark since it undergoes a facile photochemical rearrangement to the
cyclopentenedione.
15. IR (Nujol) cm.
−1: 2110 (N
3), 1640 (C=O);
1H NMR (CDCl
3), δ (multiplicity, assignment): 1.31 [s, C(C
H3)
3].
17. TLC is carried out on silica gel using 1:1 (v/v)
petroleum ether–chloroform as eluent. The
cyclopentenedione has an
Rf value about half that of the diazide, and can be detected with an UV lamp when silica gel containing fluorescent indicator is used. The ketene undoubtedly reacts with the hydroxyl groups of the silica gel and remains at the origin. The checkers found the reaction to be complete in 1.5–2 hours. The yield was established by the checkers to be ≥
95% by
1H NMR, by integration studies in the presence of an internal standard.
18. The submitters have not been successful in isolating
tert-butylcyanoketene by any method. If the solvent is removed, the ketene polymerizes. The spectral properties of the product are as follows; IR (C
6H
6) cm.
−1: 2220 (C

N), 2130 (C=C=O);
1H NMR (C
6H
6), δ (multiplicity, assignment): 0.75 [s, C(C
H3)
3].
3. Discussion
In
benzene at room temperature,
tert-butylcyanoketene (TBCK) does not undergo rapid self-condensation;
2however, it is quite reactive toward cycloaddition reactions with alkenes, allenes, ketenes, imines, and formimidates.
3 The mechanisms of a number of these cycloaddition reactions have been investigated and in some cases have been shown to involve the formation of a zwitterionic intermediate. Typical examples of TBCK cycloadditions are summarized in the formula below.
The ketene is less stable in nonaromatic hydrocarbon solvents than in aromatic solvents. For example, it has a half-life of greater than 7 days in
benzene at 25°. On the other hand, in
cyclohexane at the same temperature its half-life is only a few hours. All attempts to isolate
tert-butylcyanoketene have failed. Either removal of the solvent or cooling the solution to low temperature (−70°) causes polymerization of the ketene, a very efficient process, giving a white solid polymer which appears to have repeating ketenimine units. This assignment is consistent with the very strong absorption at 2140 cm
−1 in the IR spectrum.
4
The method described here for the synthesis of
tert-butylcyanoketene has marked advantages over other possible classical routes,
e.g., dehydrohalogenation of the corresponding acid chloride. The only other product formed is molecular
nitrogen and no external catalyst,
e.g.,
triethylamine, is necessary. In fact, when
tert-butylcyanoketene reacts with
triethylamine, or when
α-tert-butyl-α-cyanoacetyl chloride is subjected to dehydrohalogenation conditions,
1,3-di-tert-butyl-1,3-dicyanoallene is immediately formed and no ketene can be detected.
tert-Pentylcyanoketene can be prepared in an analogous fashion starting from the commercially available
2,5-di-tert-pentylbenzoquinone. This ketene seems to be very similar in stability and reactivity to the
tert-butyl homolog. Other cyanoketenes which have been prepared from azidoquinones or related compounds include dicyano-, chlorocyano-, bromocyano-, iodocyano-, methylcyano-, and
isopropylcyanoketene.
3 However, all of these must be generated
in situ since they undergo rapid self-condensation in the absence of a ketenophile.
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
3,6-Diazoido-2,5-di-tert-butyl-1,4-benzoquinone
ethanol (64-17-5)
acetic acid (64-19-7)
Benzene (71-43-2)
methanol (67-56-1)
ether,
diethyl ether (60-29-7)
chloroform (67-66-3)
sodium chloride (7647-14-5)
nitrogen (7727-37-9)
cyclohexane (110-82-7)
chlorine (7782-50-5)
diethylamine (109-89-7)
sodium azide (26628-22-8)
dichloromethane (75-09-2)
magnesium sulfate (7487-88-9)
diethylamine hydrochloride (660-68-4)
triethylamine (121-44-8)
Butanenitrile, 2-carbonyl-3,3-dimethyl-,
tert-BUTYLCYANOKETENE (29342-22-1)
cyclopentenedione
isopropylcyanoketene
2,5-di-tert-butyl-1,4-benzoquinone
2,5-Di-tert-butyl-5,6-dichloro-2-cyclohexene-1,4-dione (33611-72-2)
3-Chloro-2,5-di-tert-butyl-1,4-benzoquinone (33611-70-0)
2,5-Di-tert-butyl-3,5,6-trichloro-2-cyclohexene-1,4-dione (117257-58-6)
2,5-Di-tert-butyl-3,6-dichloro-1,4-benzoquinone,
2,5-Di-tert-butyl-3,6-dichloro-1,4-benezoquinone (33611-73-3)
3,6-diazido-2,5-di-tert-butyl-1,4-benzoquinone (29342-21-0)
α-tert-butyl-α-cyanoacetyl chloride
1,3-di-tert-butyl-1,3-dicyanoallene
tert-Pentylcyanoketene
2,5-di-tert-pentylbenzoquinone
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved