Isolation and X-ray structures of labile benzoic- and acetic-acidium carbocations

Journal of Organic Chemistry

Volumn 70, Issue 8, 2005, Pages 3263-3266

  Lindeman, S V ^a   Neretin, I S ^a   Davlieva, M G ^a   Kochi, J K ^a  

^a University of Houston   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

AROMATIC COMPOUNDS; CRYSTAL STRUCTURE; ELECTRONIC STRUCTURE; POSITIVE IONS; SALTS; X RAY ANALYSIS;

BENZOYLIUM SALTS; CARBOCATIONIC SALTS; CRYSTALLINE FORM; X-RAY STRUCTURES;

ACETIC ACID;

ACETIC ACID; ACETIC ACIDIUM; BENZOIC ACID; BENZOIC ACIDIUM; CARBENOID; CARBOXYLIC ACID; CARBOXYLIC ACIDIUM; DIHYDROXYCARBENIUM; UNCLASSIFIED DRUG;

ARTICLE; CHEMICAL REACTION; CHEMICAL STRUCTURE; DRUG ISOLATION; PROTON TRANSPORT; SYNTHESIS; X RAY CRYSTALLOGRAPHY;

EID: 17444399691     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo047847o     Document Type: Article

References (31)

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10. However, crystalline salts of both acidium and diacidium cations of oxalic acid have been isolated by similar procedure: Minkwitz, R.; Hartfeld, N.; Hirsch, C. Z. Anorg. Allg. Chem. 1999, 625, 1479.
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15. We note that the two fluorines in the 2,6-difluoro derivative are inequivalent owing to differences in the hydrogen bonding to the counteranion, and the overall carbocation structure is asymmetric. Intramolecular hydrogen bonding is also responsible for the small dihedral angle of α = 3-6° despite the presence of two o-fluorines.
16. a = 1.31 and 1.23 Å for the carbonyl and hydroxy groups. Search through the CSD database yields an average of 1.30 and 1.23 Å, respectively.
17. 1 = 1.45 Å (K = 0.50 Å), which includes the correction for the change in covalent radius with hybridization (see, e.g., Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. Coord. Chem. Rev. 2000, 200, 831). The calculated bond orders are presented in the Table 2 with the esds in parentheses.
18. 1 = 1.45 Å (K = 0.50 Å), which includes the correction for the change in covalent radius with hybridization (see, e.g., Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. Coord. Chem. Rev. 2000, 200, 831). The calculated bond orders are presented in the Table 2 with the esds in parentheses.
19. It is noteworthy that the length of the C-O bond (a) in the acidium cation is essentially the same as those in dialkoxyphenylcarbenium cations (see: Childs, E. F.; Frampton, C. S.; Kang, G. J.; Wark, T. A. J. Am. Chem. Soc. 1994, 116, 8499).
20. + is desirable to establish this point. Unfortunately, we have been unable as yet to isolate these crystalline acidium salts (suitable for X-ray crystallography) due to the exceedingly delicate balance between hydration of the acylium cation and the further hydrolysis of the acidium salt.
21. (a) Such a minor quinonoidal distortion is shown by the shortening of the endocyclic d bond (1.378 Å) as compared with the c bond (1.405 Å) and the e bond (1.408 Å) in Table 1.
22. (b) For discussion of this type of quinonoidal distortion in other aromatic cations, see: Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 843. Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012. Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Organometallics 2001, 20, 115. For the major quinonoidal distortions in benzylic cations, see: Laube, T.; Olah, G. A.; Bau, R. J. Am. Chem. Soc. 1997, 119, 3087.
23. (b) For discussion of this type of quinonoidal distortion in other aromatic cations, see: Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 843. Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012. Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Organometallics 2001, 20, 115. For the major quinonoidal distortions in benzylic cations, see: Laube, T.; Olah, G. A.; Bau, R. J. Am. Chem. Soc. 1997, 119, 3087.
24. (b) For discussion of this type of quinonoidal distortion in other aromatic cations, see: Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 843. Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012. Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Organometallics 2001, 20, 115. For the major quinonoidal distortions in benzylic cations, see: Laube, T.; Olah, G. A.; Bau, R. J. Am. Chem. Soc. 1997, 119, 3087.
25. (b) For discussion of this type of quinonoidal distortion in other aromatic cations, see: Lindeman, S. V.; Rosokha, S. V.; Sun, D.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 843. Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012. Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Organometallics 2001, 20, 115. For the major quinonoidal distortions in benzylic cations, see: Laube, T.; Olah, G. A.; Bau, R. J. Am. Chem. Soc. 1997, 119, 3087.
26. Electronic isolation between the acidium and aryl group is also indicated by the structural parameters in the skewed structure of mesitoic-acidium carbocation (which has dihedral angle of α = 41°) relative to those in planar acidium structures with α ∼ 5° in Table 2.
27. + substituent parameter (see: Lowry, T. H.; Richardson, K. S. Mechanisms and Theory in Organic Chemistry, 2nd ed.; Harper and Row: New York, 1981; p 134.) However, our X-ray studies rule out this structure in favor of the dioxonium form (structure II) as the resonance hybrid (Chemical Equation Presented) in which the cationic acidium functionality is largely isolated from the aromatic ring and thus insensitive to changes in substituents. By contrast, structure I is related to the cumyl cation and will be strongly affected by p-substituents, e.g. Compare also: Kochi, J. K.; Hammond, G. S. J. Am. Chem. Soc. 1953, 75, 3445, 3452.
28. + substituent parameter (see: Lowry, T. H.; Richardson, K. S. Mechanisms and Theory in Organic Chemistry, 2nd ed.; Harper and Row: New York, 1981; p 134.) However, our X-ray studies rule out this structure in favor of the dioxonium form (structure II) as the resonance hybrid (Chemical Equation Presented) in which the cationic acidium functionality is largely isolated from the aromatic ring and thus insensitive to changes in substituents. By contrast, structure I is related to the cumyl cation and will be strongly affected by p-substituents, e.g. Compare also: Kochi, J. K.; Hammond, G. S. J. Am. Chem. Soc. 1953, 75, 3445, 3452.
29. Davlieva, M. G.; Lindeman, S. V.; Neretin, I. S.; Kochi, J. K. New J. Chem. 2004, 28, 1568.
30. Sheldrick, G. M. SADABS v. 2.03, Bruker/Siemens Area Detector Absorption and Other Corrections; Bruker Analytical X-ray Systems, Madison, WI, 2000.
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