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Organic Letters

Volumn 8, Issue 26, 2006, Pages 5923-5925

Bond strengths: The importance of hyperconjugation

(2) Ingold, K U a Dilabio, Gino A a

a NONE

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EID: 33846136417 PISSN: 15237060 EISSN: None Source Type: Journal
DOI: 10.1021/ol062293s Document Type: Article

Times cited : (39)

References (19)

1
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- Gronert, S. J. Org. Chem. 2006, 71, 1209-1219.
- (2006) J. Org. Chem , vol.71 , pp. 1209-1219
- Gronert, S.¹

2
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- See also
- See also, Gronert, S. J. Org. Chem. 2006, 71, 7045-7048.
- (2006) J. Org. Chem , vol.71 , pp. 7045-7048
- Gronert, S.¹

3
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- Wodrich, M. D.; Schleyer, P. v. R. Org. Lett. 2006, 8, 2135-2138.
- (2006) Org. Lett , vol.8 , pp. 2135-2138
- Wodrich, M.D.¹ Schleyer, P.V.R.²

4
- 33846146791
- This is also the process that gives the methyl radical's three hydrogen atoms negative spin, i.e, spin of the opposite sign as the spin at its C •
- •.

5
- 33846145464
- All EPR data are from: Fischer, H. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; II, Chapter 19.
- All EPR data are from: Fischer, H. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. II, Chapter 19.

6
- 0001190197
- See, for example:, and references therein. 1998
- See, for example: Brocks, J. J.; Beckhaus, H.-D.; Beckwith, A. L. J.; Rüchardt, C. J. Org. Chem. 1998, 63, 1935-1943 and references therein.
- (1935) J. Org. Chem , vol.63
- Brocks, J.J.¹ Beckhaus, H.-D.² Beckwith, A.L.J.³ Rüchardt, C.⁴

7
- 33846179783
- The theoretical 13C hfcc for a system in which 100% of an unpaired electron occupies this carbon's 2s orbital is 1110 G.4 Thus, the 13C hfcc at the alkyl radical centers (Table 1) imply that only ca. 4% of the unpaired electron is in the 2s orbital of C•. Small changes in 13C• hfcc between alkyl radicals are difficult to interpret, and this difficulty is compounded by the fact that the potential energy surface for the in-and-out umbrella motion of, e.g, methyl, has a single minimum, whereas the umbrella potential for tert-butyl has a double minimum.7
- 7

8
- 0001637204
- Griller, D.; Ingold, K. U.; Krusic, P. J.; Fischer, H. J. Am. Chem. Soc. 1978, 100, 6750-6752.
- (1978) J. Am. Chem. Soc , vol.100 , pp. 6750-6752
- Griller, D.¹ Ingold, K.U.² Krusic, P.J.³ Fischer, H.⁴

9
- 33846131015
- If 100% spin is attributed to hyperconjugation, a straight-line fit of the BDE data would predict the propene C-H BDE to be 94.2 kcal/mol. too high by 8.4 kcal/mol, and if 39% of the spin is attributed to hyperconjugation, the predicted BDE is too low by 8.4 kcal/mol.
- If 100% spin is attributed to hyperconjugation, a straight-line fit of the BDE data would predict the propene C-H BDE to be 94.2 kcal/mol. too high by 8.4 kcal/mol, and if 39% of the spin is attributed to hyperconjugation, the predicted BDE is too low by 8.4 kcal/mol.

10
- 33846128212
- In the ethyl radical, the conventional valence bond picture of hyperconjugation ascribes significant double bond character to the H 3C-CH2• bond. However, the amount of double bond character in this and in the C-C• bond of other alkyl radicals must be very small because the barriers to rotation about these bonds are very much less (∼1 kcal/mol) than the barrier to rotation in the allyl radical (15.7 kcal/mol).10 Hyperconjugative interactions in the ethyl radical are maximzed when the nominally planar CH2 • moiety is perpendicular to the plane defined by a H, the C atom of the β-H3C group, and the C atom of the CH 2• group. However, upon rotation of the CH 2• group, overlap can be achieved between the radical center's singly occupied p-orbital and linear combinations of the β-H 3C group's σ-and σ
- 3C group's σ-and σ*-orbitals, resulting in a very small barrier to rotation. Molecular orbital calculations also provide evidence for the importance of hyperconjugation because deleting the principal natural bond orbital hyperconjugative interactions in the ethyl radical causes the HF/6-31.G(d) C-C bond length to increase by 0.05 Å.

11
- 0000446885
- Korth, H.-G.; Trill, H.; Sustmann, R. J. Am. Chem. Soc. 1981, 103, 4483-4489.
- (1981) J. Am. Chem. Soc , vol.103 , pp. 4483-4489
- Korth, H.-G.¹ Trill, H.² Sustmann, R.³

12
- 0009662262
- See also
- See also: Walton, J. C. Rev. Chem. Intermed. 1984, 5, 249-291.
- (1984) Rev. Chem. Intermed , vol.5 , pp. 249-291
- Walton, J.C.¹

13
- 33846124346
- NIST Structures and Properties Database and Estimation Program; U.S. Department of Commerce: Washington, DC
- Stein, S. E. Thermochemical Data Base 25, NIST Structures and Properties Database and Estimation Program; U.S. Department of Commerce: Washington, DC, 1992.
- (1992) Thermochemical Data Base 25
- Stein, S.E.¹

14
- 33846175470
- Calculated using the B3P86/6-311G(2d,2p)//B3P86/6-31G(d) method described in ref 13. This approach has been shown to predict X-H bond dissociation enthalpies in very good agreement with experimental values, as is also seen in this example.
- Calculated using the B3P86/6-311G(2d,2p)//B3P86/6-31G(d) method described in ref 13. This approach has been shown to predict X-H bond dissociation enthalpies in very good agreement with experimental values, as is also seen in this example.

15
- 0344272337
- Johnson, E. R.; Clarkin, O. J.; DiLabio, G. A. J. Phys. Chem. A 2003, 107, 9953-9963.
- (2003) J. Phys. Chem. A , vol.107 , pp. 9953-9963
- Johnson, E.R.¹ Clarkin, O.J.² DiLabio, G.A.³

16
- 0000985048
- Experimental values as high as 124.7 and as low as 109.0 kcal/mol have been published; see: Ingold, K. U., Wright, J. S. J. Chem. Educ. 2000, 77, 1062-1064.
- Experimental values as high as 124.7 and as low as 109.0 kcal/mol have been published; see: Ingold, K. U., Wright, J. S. J. Chem. Educ. 2000, 77, 1062-1064.

17
- 0037149752
- Ruscic, B.; Wagner, A. F.; Harding, L. B.; Asher, R. L.; Feller, D.; Dixon, D. A.; Peterson, K. A.; Song, Y.; Qian, A.; Ng, C.-Y.; Liu, J.; Chen, W.; Schwenke, D. W. J. Phys. Chem. A 2002, 106, 2727-2747.
- (2002) J. Phys. Chem. A , vol.106 , pp. 2727-2747
- Ruscic, B.¹ Wagner, A.F.² Harding, L.B.³ Asher, R.L.⁴ Feller, D.⁵ Dixon, D.A.⁶ Peterson, K.A.⁷ Song, Y.⁸ Qian, A.⁹ Ng, C.-Y.¹⁰ Liu, J.¹¹ Chen, W.¹² Schwenke, D.W.¹³

18
- 33846128543
- Calculated from heats of formation from the NIST database: Afeefy, H. Y.; Liebman, J. F.; Stein, S. E. Neutral Thermochemical Data. In NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; June 2005; National Institute of Standards and Technology: Gaithersburg MD, 20899 (http://webbook.nist.gov).
- Calculated from heats of formation from the NIST database: Afeefy, H. Y.; Liebman, J. F.; Stein, S. E. Neutral Thermochemical Data. In NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; June 2005; National Institute of Standards and Technology: Gaithersburg MD, 20899 (http://webbook.nist.gov).

19
- 33846130698
- Using the isodesmic scheme developed in reference 2 reveals that C-H BDEs in ethane, propane, and butane decrease solely as a result of hyperconjugation. A similar analysis of the C-C BDEs in of propane, isobutane, and neopentane, relative to ethane, reveals that hyperconjugative stabilizations are present in the radicals but these are effectively cancelled by attractive branching interactions present in both the parent and radical species. For additional information, see Supporting Information of ref 2
- Using the isodesmic scheme developed in reference 2 reveals that C-H BDEs in ethane, propane, and butane decrease solely as a result of hyperconjugation. A similar analysis of the C-C BDEs in of propane, isobutane, and neopentane, relative to ethane, reveals that hyperconjugative stabilizations are present in the radicals but these are effectively cancelled by attractive branching interactions present in both the parent and radical species. For additional information, see Supporting Information of ref 2.

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