Mechanism of the Gibbs reaction. Part 4. Indophenol formation via N- chlorobenzoquinone imine radical anions. The aza-S(RN)2 chain reaction mechanism. Chain initiation with 1,4-benzoquinones and cyanide ion

Journal of Organic Chemistry

Volumn 64, Issue 18, 1999, Pages 6530-6540

  Pallagi, István ^a   Toró, András ^b   Horváth, Gyula ^a  

^a Institute for Drug Research Budapest   (Hungary)

^b UNIVERSITÉ DE SHERBROOKE   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

1,4 BENZOQUINONE DERIVATIVE; ANION; CYANIDE; INDOPHENOL;

ARTICLE; ELECTRON TRANSPORT; PROTON NUCLEAR MAGNETIC RESONANCE; REACTION ANALYSIS; REACTION TIME; SYNTHESIS;

EID: 0032886581     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo982113v     Document Type: Article

References (90)

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3. Kinetics shows that only the anion of phenol reacts with N-chloroimines 1 and phenol itself does not (see ref 1).
4. For a compilation of this assay on various para-substituted phenols and for proposed mechanisms see ref 1.
5. When the direct combination and the chain reaction run simultaneously, by terminating the latter with 2,2,6,6-tetramethylpyperidine-N-oxyl (TEMPO), the much slower former reaction can be monitored, proving that the initiating SET is the slowest step.
6. In some casea (e.g., R= methoxy), the final oxidation involving the second mole of phenol is the rate-determining step.
7. The hypothetical second-order rate vs time function was found to be an excellent indicator. Its horizontal straight line or a deviation from that reflects reliably the nature of the reaction, the acceleration, and the influence of the additives. The acceleration is termed linear when the virtual rate of the accelerated reaction remains second-order.
8. The second-order kinetics alone does not confirm the direct combination. To comply with this requirement, the reaction rate should not be affected by radical scavenger TEMPO, i.e., identical rate constants in the presence and absence of TEMPO indicate an exclusive direct combination for the dye formation reaction.
9. (a) Denney, D. B.; Denney, D. Z. Tetrahedron 1991, 47, 6577-6600.
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11. (c) Denney, D, B.; Denney, D, Z. Tetrahedron 1997, 53, 9835-9846.
12. RN1 mechanism was proposed in 1966 by Kornblum and Russell for aliphatic systems (Kornblum, N.; Michel, R. E.; Kerber, R. C. J. Am. Chem. Soc. 1966, 88, 5662-5663. Russell, G. A.; Danen, W. C. J. Am. Chem. Soc. 1966, 88, 7463-7464) and in 1970 by Bunnett for aromatic systems (see ref 39a).
13. RN1 mechanism was proposed in 1966 by Kornblum and Russell for aliphatic systems (Kornblum, N.; Michel, R. E.; Kerber, R. C. J. Am. Chem. Soc. 1966, 88, 5662-5663. Russell, G. A.; Danen, W. C. J. Am. Chem. Soc. 1966, 88, 7463-7464) and in 1970 by Bunnett for aromatic systems (see ref 39a).
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20. For conversion plots, see Supporting Information.
21. Later we show that there are two reasons why the rate constant is increased. First, a chain reaction resulting in a nonlinear acceleration and second, in certain cases, the chain carrier, in a terminating step, produces imine 6, which is more reactive than N-chloroimine 1 but reacts along the same second-order kinetics, resulting in an increase of the rate constant, i.e., linear acceleration is observed (see ref 7).
22. The observed slight decrease of the rate constant in time is caused by a side reaction of quinone 11 with imine 6 formed dominantly in the case of the more reactive N-chloroimines 1.
23. 1H NMR signals upon addition to the NMR solutions of spin trapping adducts prepared in a different way proved their structure. (For the preparation, isolation and identification of these products see Experimental Section).
24. The disproportionation can also occur in an acidic medium (see: Patai, S. The Chemistry of the Quinonoid Group. Part 2. John Wiley & Sons: New York, 1974; pp 746-769). Formation of more 1,4-hydroquinone (4b) within a prolonged time in an alkaline medium before acidifying the solution ruled out this potential artifact.
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35. RN2 when it is the N-chloroimine radical anion 7. For the sake of clarity, structures of imine radicals 10 are also included.
36. z on N-atom of imine radical 10a are 0.48, 0.08, and 0.28, respectively). However, an intramolecular π → σ transformation on N-haloimine radical anion 7 may also be presumed, as a similar intramolecular π → σ transformation has already been reported for haloaryl radical anions (see ref 24).
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38. The linear accelerating effect of quinones 11 were perfectly simulated by in situ generated imines 6 (see Supporting Information).
39. -3) and found that the acceleration effect caused by the cyanide contamination was equal to that caused by the same quantity of cyanide added directly to cyanide-free acetonitrile. Thus, the observed acceleration caused by acetonitrile was clearly due to its cyanide contamination.
40. -3. (Determined according to Standard Methods for the Examination of Water and Wastewater APHA, AWWA, and WPCF: 1989; XVII edition, pp 4-31.
41. Ninety percent of conversion was achieved in a blank experiment with N-chloroimine 1b in 40 min. During this time, the second-order rate constant was found to be steady.
42. (a) Behar, D.; Neta, P. J. Am. Chem. Soc. 1981, 103, 103-106.
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48. The products from the reaction of these phenols and N-chloroimine 1b are formed partially by a chain reaction mechanism even without initiators at this pH, but as a result of the low reactivity of the nucleophile, the decomposition of radical anion 7 is significant, resulting in a lower conversion. When initiators were applied the rate increased and the products were formed quantitatively.
49. -5, respectively, (b) The proportions of chain reaction mechanism for phenols 4d-g were found to be 34%, 48%, 52%, and 59%, respectively. Thus, even the higher proportion of chain reaction mechanism cannot compensate the lower reactivity of the phenols 4f,g.
50. RN2 mechanism thermodynamically favoring N-chloroirmne radical anion 7a as an intermediate over imine radical 10a with ca. 117 kcal/ mol. Heat of formations calculated with the PM3 method in kcal/mol are 7a: -60.8, 7b: -42.4, 10a: +56.2, 10b: +51.8. For a complete energy diagram see ref 1.
51. Reaction of phenols with imine 6b proceeds with a second-order kinetics similarly to N-chloroimines 1 (direct combination, comparable concentration of reactants), with the difference that its total rate is the sum of the rates of four reactions: (a) reaction of the neutral imine with the anion of 4, (b) reaction of the neutral imine with phenol 4, (c) reaction of the protonated imine with the anion of 4, and (d) reaction of the protonated imine with phenol 4 (see ref 20). The primacy among these reactions depends on pH; thus, in the pH range 6-8, the majority of the dye is formed by reaction c. As the protonated imine is by several orders of magnitude more reactive (2.4 x 106 times) than the neutral form, the total rate increases with decreasing pH.
52. 4 times more reactive than neutral phenol in their reaction with p-benzoquinone diimine (see ref 20). (b) The rate of this reaction was the same at pH 12.1 with or without TEMPO. The conversion was also the same, but in both cases, it was about 5% lower than in the blank experiment conducted in borate buffer. This suggests that, similarly to the reaction of phenol 4a, at high pH imine 6b is not formed because of the fast decomposition of radical anion 7b.
53. Upon monitoring the reaction until 90% conversion for about 8 h, the second-order rate constant was found to be steady.
54. Indophenol 3a can also be prepared by the reaction of N-chloroimine 1a and phenol 4a, and this reaction was used for calculating the amount of the dye in these experiments. Thus, in the presence of TEMPO, at pH 9.57, 41% of indophenol 3a is observed, indicating that the chain reaction was also a significant route (59%) in the case of this phenol. It was also established that in the pH range 9.0-9.7 the yield of indophenol 3a was only 74% as a result of the decomposition of N-chloroimine radical anion 7b. A formation of 100% dye was detected when applying cyanide additive.
55. (a) Monitoring the reaction until 90% conversion for about 20 h, the rate constant was found to be steady, (b) The rate constant calculated was corrected with the decay of the N-chloroimine radical anion 7b.
56. Though the stability of the other two possible reactants N-chloroimine 1b and imine 6b is somewhat lowered by increasing the pH, the extreme decrease of the dye formation cannot be explained only by this reduced stability. However, it indicates the role of the chain reaction mechanism.
57. The chain reaction was anticipated as N-chloroimine 1b reacts even with phenol 4a in this way (> 95%) to form indophenol 3. Because phenol 4g is less reactive than phenol 4a, after the SET, the direct combination in the cage is suppressed and the escape becomes favored.
58. 36b (b) The linear acceleration is indicative of the influence of imine 6a on the rate. The pH independence can be rationalized by the decreased basicity caused by the negative inductive effect of the chlorine substituants in imine 6a (in contrast with imine 6b), which inhibits protonation at this pH.
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90. 2).