· (MW 101.49)
(functions as a potent electrophile in reactions with a wide variety of ketenophiles including alkenes,3 alkynes,4 arylaldehydes,5 and imines6-8)
Alternate Name: CCK.
Preparative Methods: chlorocyanoketene readily undergoes self-condensation and must be generated in situ prior to usage. By generating the ketene in the presence of a ketenophile, concentration-related difficulties are circumvented and yields are greatly improved.2 A most convenient route to the title compound is afforded by the thermal decomposition of pseudo esters of azidofuranones. Starting with commercially available mucochloric acid the desired furanone is synthesized in a two-step process: etherification and azidation (eq 1). Alcohols with at least three carbons, such as isopropanol, are desirable for the etherification in order to minimize the detonation capability of the subsequent azide.2
Cycloadditions to Alkenes.
Cycloadditions of chlorocyanoketene (CCK) to alkenes give good yields of the corresponding cyclobutanone and include additions to di-, tri-, and tetrasubstituted alkenes. The addition process is in complete accord with a concerted p2
s + p2 a mechanism. Cyclobutanones (1), (2), and (3) are obtained by a highly stereoselective route as evidenced by the formation of their respective single diastereomers (eqs 2-7).
Alkenic ketenophiles with higher nucleophilic character react with CCK in a dipolar fashion. For example, treatment of cyclopentadiene with CCK gives a 55:45 mixture of diastereomers (eq 8). Further exemplifying this dipolar mode is the reaction of the ketene with dihydropyran (eq 9), whose product likely arises from a proton transfer process involving a zwitterionic intermediate.3
Cycloadditions to Alkynes.
CCK cycloadds to alkynes to give cyclobutenones. Reactions using terminal alkynes give complex mixtures, while employment of internal alkynes affords the corresponding cyclobutenones in good yields (eqs 10-13).4
4-Chloro-4-cyanobutenones, by virtue of the initial cycloadditions, are in equilibrium with their respective vinylketenes at room temperature, and these reactive intermediates can be intercepted by other ketenophiles, as exemplified in eq 14.4
Cyclohexadienones are the result of trapping the vinylketenes with an alkyne. Reductive elimination of these products gives highly functionalized phenols.
Cycloadditions to Arylaldehydes.
Chlorocyanoketene cycloadds to arylaldehydes to form b-lactones which consequently decarboxylate under the reaction conditions to give exclusively the (E) isomer of 1-halo-1-cyano-2-phenylethylenes. The initial cycloaddition is a nonconcerted dipolar process in which the cyanoketene uniquely functions as an electrophile and the aldehyde as the nucleophile (eq 15).5
Cycloadditions to Imines.
CCK cycloadds to a variety of imines to form both b- and d-lactams depending on the particular imine used.6-8 Among the most studied imines are cinnamylideneamines and benzylideneamines. In the former series of cycloadditions, it is generally found that 2-azetinone formation is enhanced when the N-substituent of the imine is small. Stereochemical control is realized through the bulk of this substituent as well as the substitution pattern of the imine (eq 16). Within the benzylidene series, cycloadditions are stereospecific where the N-substituent is an aryl group resulting in a trans relationship between the 3-cyano group and the proton at the 4 position. The yield of corresponding cis diastereomer is
progressively increased by increasing steric bulk (eq 17). As in the case with arylaldehydes, stereochemical outcomes in the cycloadditions of imines to CCK are best rationalized by invoking a dipolar intermediate.
- 1. Moore, H. W.; Gheorghiu, M. D. CSR 1981, 10, 289.
- 2. Fishbein, P. L.; Moore, H. W. OS 1990, 69, 205.
- 3. Fishbein, P. L.; Moore, H. W. JOC 1984, 49, 2190.
- 4. Fishbein, P. L.; Moore, H. W. JOC 1985, 50, 3226.
- 5. Moore, H. W.; Mercer, F.; Kunert, D.; Albaugh, P. JACS 1979, 101, 5435.
- 6. Moore, H. W.; Hughes, G.; Srinvasachar, K.; Fernandez, M.; Nghi, V.; Schoon, D.; Tranne, A. JOC 1985, 50, 4231.
- 7. Moore, H. W.; Hughes, G. TL 1982, 23, 4003.
- 8. Moore, H. W.; Hernandez, L., Jr.; Chambers, R. JACS 1978, 100, 2245.
Philip S. Turnbull & Harold W. Moore
University of California, Irvine, CA, USA
Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.