Role of hyperconjugation in determining carbon-oxygen bond dissociation enthalpies in alkylperoxyl radicals

Organic Letters

Volumn 5, Issue 4, 2003, Pages 387-390

  Pratt, Derek A ^a   Porter, Ned A ^a  

^a VANDERBILT UNIVERSITY SCHOOL OF MEDICINE   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALKYL GROUP; CARBON; METHYL GROUP; OXYGEN; RADICAL; FREE RADICAL; PERHYDROXYL RADICAL; PEROXIDE;

ARTICLE; CALCULATION; CHEMICAL BINDING; CHEMICAL REACTION; ELECTRON; ENTHALPY; HYPERCONJUGATION; POLARIZATION; THEORY; CHEMICAL STRUCTURE; CHEMISTRY; THERMODYNAMICS;

FREE RADICALS; MODELS, MOLECULAR; PEROXIDES; THERMODYNAMICS;

EID: 0037772316     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol027094x     Document Type: Article

References (23)

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5. Kranenburg, M.; Ciriano, M. V.; Cherkasov, A.; Mulder, P. J. Phys. Chem. A 2000, 104, 915 and references therein.
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7. Hoffmann, R.; Radom, L.; Pople, J. A.; Schleyer, P. v. R.; Hehre, W. J.; Salem, L. J. Am. Chem. Soc. 1972, 94, 6221. When σ* is a π-acceptor, this is generally termed negative hyperconjugation.
8. Knyazev, V. D.; Slagle, I. R. J. Phys. Chem. A 1998, 102, 8390.
9. CRC Handbook of Chemistry and Physics, 83rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2002 and references therein.
10. Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1999, 110, 4703. Performed with the Gaussian-98 suite of programs. Gaussian 98, rev. A.7; Gaussian, Inc.: Pittsburgh PA, 1998.
11. Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1999, 110, 4703. Performed with the Gaussian-98 suite of programs. Gaussian 98, rev. A.7; Gaussian, Inc.: Pittsburgh PA, 1998.
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14. See ref 8 and citations therein.
15. 3 are also unlikely to be additive (10.9 + 6.4 = 17.3 kcal/mol).
16. If hyperconjugation is operative, the C-O bond should be somewhat longer in the cases where Y is strongly ED. Indeed, r(C-O) in aminomethylperoxyl is 0.04 Å longer than that in methylperoxyl, 1.491 vs 1.450 Å. Moreover, the dioxygen moiety becomes more negative (-0.429) compared with methylperoxyl (-0.197), consistent with charge-transfer resonance structures; see the graphical abstract.
17. • bond in trimethylaminoperoxyl weaker than in methylperoxyl by the difference of 3.7 (28.7 vs 32.4) kcal/mol.
18. Font-Sanchis, E.; Allaga, C.; Focsaneanu, K.-S.; Scaiano, J. C. Chem. Commun. 2002, 1576.
19. (b) Bejan, E. V.; Font-Sanchis, E.; Scaiano, J. C. Org. Lett. 2001, 3, 4059.
20. (c) Scaiano, J. C.; Martin, A.; Yap, G. P. A.; Ingold, K. U. Org. Lett. 2000, 2, 899.
21. It is well-documented that the benzylic and allylic radical stabilization enthalpies are very similar. See, for example: Hrovat, D. A.; Borden, W. T. J. Phys. Chem. 1994, 98, 10460-10464.
22. Although in 2 the radical spin is delocalized into the aromatic ring, addition of oxygen to the ortho and/or para positions does not occur in benzylic radicals because of the energetic penalty incurred for breaking aromaticity. Pratt, D. A.; Mills, J. H.; Porter, N. A. In preparation.
23. Hyperconjugation by definition requires that this interaction will exist between any electron-donating substituent and polarized C-X bond where X is more electronegative than C. However, it is usually not important to the homolytic reactivity of these compounds because their C-X bonds are relatively strong. This is not true of alkylperoxyl radicals that have very weak C-O bonds.