2,5-Diazido-3,6-di-t-butyl-1,4-benzoquinone1

[29342-21-0]  · C14H18N6O2  · 2,5-Diazido-3,6-di-t-butyl-1,4-benzoquinone  · (MW 302.38)

(reagent for the preparation of t-butylcyanoketene,2 a versatile ketene for reactions with a variety of ketenophiles1)

Physical Data: mp 89-90 °C (dec).

Solubility: sol aromatic solvents which are used in the preparation of t-butylcyanoketene.

Analysis of Reagent Purity: IR (Nujol) 2110 (N3), 1640 (CO) cm-1; 1H NMR (CDCl3), d 1.31 (s, t-Bu).

Preparative Method: 2,5-diazido-3,6-di-t-butyl-1,4-benzoquinone (2) is readily synthesized in five steps from commercially available 2,5-di-t-butyl-1,4-benzoquinone (1) in approximately 30% overall yield (eq 1-3).2

Purification: recrystallization from chloroform/ethanol.

Handling, Storage, and Precautions: during the course of any purification method that might be employed, the diazide should not be heated above 50 °C since decomposition occurs quite noticeably at that temperature. It is best to store the pure product below -5 °C in the dark since it undergoes a facile photochemical rearrangement to a cyclopentenedione.2

Thermolysis.

Upon heating in refluxing benzene, 2,5-diazido-3,6-di-t-butyl-1,4-benzoquinone (2) loses 2 mol of nitrogen to generate 2 mol of t-butylcyanoketene (TBCK, 3) (eq 4). One advantage of this route over other classical routes to TBCK is the fact that external bases or metals are not employed. Also, when the thermolysis is accomplished in the presence of a ketenophile the concentration of TBCK is low, thus minimizing polymerization reactions.2

Cycloadditions.

TBCK does not undergo rapid self-condensation in benzene at 25 °C but rather cycloadds to a variety of ketenophiles including alkenes, imines, enol ethers and silyl enol ethers, alkynes, allenes, ketenes, isocyanides, stannylethynyl ethers, and a variety of heterocyclic compounds.

Cycloadditions with Alkenes.

Cycloadditions of TBCK to a variety of mono-, di-, and trisubstituted alkenes give good yields of the corresponding cyclobutanones. No examples of reactions of TBCK with tetrasubstituted alkenes have been reported.1 The addition of TBCK to alkenes generally is stereoselective, resulting in the bulky t-butyl group cis to the larger group on the alkene. These findings best correspond to a concerted p2a + p2s reaction mechanism.1 The reactions of TBCK with styrene and cyclohexene demonstrate this highly stereoselective route to substituted cyclobutanones (eqs 5 and 6).1,3

When stabilization of a zwitterionic intermediate is energetically favored, as in the addition of TBCK to trans,trans-2,4-hexadiene, a stepwise mechanism may be involved (eq 7).1 The major product of this reaction appears to be cyclobutanone (4) and not (5), which would be the expected product if a concerted mechanism was involved. Other interesting products result from the reaction of TBCK with cyclic dienes. Addition to cyclohexa-1,3-diene and bicyclo[2.2.1]heptadiene gives both the cyclobutanone product plus a formal oxy-Cope rearrangement product (eqs 8 and 9).1,4

Cycloadditions with Imines.

Next to alkenes, the most studied ketenophiles of TBCK are formimidates, thioformimidates, and imines.1 Reaction of a variety of imines such as S-ethyl-N-phenylthioformimidate with TBCK produces b-lactam products (7) stereospecifically; the products of these reactions are consistent with a stepwise mechanism involving a dipolar intermediate (6) (eq 10). In addition, 2:1 adducts are also frequently formed with imines (8) and (9) (eq 11).1,5

Cycloadditions with Enol Ethers and Silyl Enol Ethers.

Additions of TBCK to a variety of enol ethers typically give mixtures of stereoisomers.1 Reaction with ethyl vinyl ether generates cyclobutanone products that could only reasonably arise from a dipolar intermediate (eq 12).1

Additions to silyl enol ethers behave in an analogous fashion; a stepwise mechanism is involved.6 However, different products arise depending on the steric congestion (H vs. Me) of the zwitterionic intermediate (10) and (11) formed in the reaction (eqs 13 and 14).7

Cycloadditions with Alkynes.

Regiospecific cycloadditions of TBCK to simple alkynes give cyclobutenones, a transformation consistent with a concerted mechanism (eq 15).1

Cycloaddition with Allenes.

Cycloaddition of TBCK to allenes typically gives cyclobutanone products, as in the case of 1,1-dimethylallene (eq 16).1 The varying ratios of regio- and stereoisomeric products lend credence to a zwitterionic intermediate.

Cycloadditions with Ketenes.

Cycloaddition of TBCK to ketenes generally produces 1,3-cyclobutanedione products (eq 17), although oxetan-2-one products (14) can form if the steric demands on the zwitterionic intermediate (13) are lessened as in the case of methylketene (12) (eq 18).8

Cycloadditions with Isocyanides.

Cycloaddition of TBCK to isocyanides gives the 2:1 adducts (15), presumably by a stepwise mechanism (eq 19).9 This adduct (15) rearranges to the iminolactone (16) in refluxing benzene and further thermolysis at 130 °C gives the butenolide (17).

Cycloaddition with Stannylethynyl Ethers.

Cycloaddition of TBCK to various stannylethynyl ethers proceeds to zwitterionic intermediates (18) and (19) which are in equilibrium and can either rearrange to the stannyl enol ether (20) or can add to another molecule of TBCK to form (21) (eq 20).10

Miscellaneous Cycloadditions.

Cycloadditions of TBCK have been attempted with a host of compounds including 1-azirines,1 N-oxides,1,11 N-sulfiminoaziridines,1 spirooxaziridines,1 sulfurdiimides,12 carbodiimides,12 sulfinylamines,12 and thiooxazoles.13 All cycloadditions listed here appear to involve a stepwise dipolar mechanism.

Cycloreversions of Cyclobutanone Adducts.

A variety of cyclobutanone products derived from the cycloaddition of TBCK with alkenes have been thermolyzed to form the original ketenes and ketenophiles (eq 21).14 These retro [2 + 2] reactions appear to be concerted processes due to the lack of appreciable solvent effects.

Related Reagents.

2,6-Di-t-butyl-1,4-benzoquinone.


1. Moore, H. W.; Gheorghiu, M. D. CSR 1981, 10, 289.
2. Weyler, W. Jr.; Duncan, W. G.; Liewen, M. B.; Moore, H. W. OSC 1988, 6, 210.
3. Weyler, W.; Duncan, W. G.; Moore, H. W. JACS 1975, 97, 6187.
4. Brook, P. R.; Hunt, K. CC 1974, 1989.
5. Moore, H. W.; Hughes, G.; Srinivasachar, M. F.; Nguyen, N. V.; Schoon, D.; Tranne, A. JOC 1985, 50, 4231.
6. Al-Husaini, A. H.; Moore, H. W. JOC 1985, 50, 2595.
7. Al-Husaini, A. H.; Khan, I.; Ali, S. A. T 1991, 47, 3845.
8. Moore, H. W.; Wilbur, D. S. JOC 1980, 45, 4483.
9. Moore, H. W.; Yu, C. C. JOC 1981, 46, 4935.
10. Himbert, G.; Henn, L. LA 1987, 771.
11. Abou-Gharbia, M. A.; Joulle, M. M. JOC 1979, 44, 2961.
12. Goldish, D. M.; Axon, B. W.; Moore, H. W. H 1983, 20, 187.
13. Dondoni, A.; Medici, A.; Venturoli, C.; Forlani, C.; Bertolasi, V. JOC 1980, 45, 621.
14. Al-Husaini, A. H.; Muqtar, M.; Ali, S. A. T 1991, 47, 7719.

Michael Winters & Harold W. Moore

University of California at Irvine, CA, USA



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