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5 Cascade Reactions Initiated by Addition of Higher Main Group (V)-Centered Radicals to Alkynes

5 Cascade Reactions Initiated by Addition of Higher Main Group (V)-Centered Radicals to Alkynes

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38



INTERMOLECULAR RADICAL ADDITIONS TO ALKYNES



However, their intermolecular addition reactions with alkynes are mostly aimed at

synthesizing substituted alkenes,69,70 and only very few cascade reactions that are

initiated by P radical addition to C:C triple bonds have been reported. Renaud and

coworkers developed a simple one-pot procedure for the cyclization of terminal

alkynes mediated by dialkyl phosphites (Scheme 2.35).71 In this radical chain

procedure, dialkyl phosphite radicals, (RO)2P ¼O, undergo addition to the C:C

triple bond in 190, which triggers a radical translocation (1,5-HAT)/5-exo cyclization

cascade. The sequence is terminated by hydrogen transfer from dialkyl phosphite to

the intermediate 194 and regeneration of P-centered radicals.

.



O



RO



( )n



P



O



H



RO

(RO2)P(O)H, DLP



MeO2C CO2Me



( )n

H

CO2Me



MeO2C



190

(n = 1, 2)



191

n = 1, R = Me: 71%, d.r. 85:15

n = 2, R = Et: 84%, d.r. 61:39



DLP = dilauryl peroxide

via:



P(O)(RO)2 O

O



O



190



( )n



(RO)2P



P(O)(RO)2 O

1,5-HAT



( )n



MeO2C CO2Me



MeO2C CO2Me



192



193



(RO)2(O)P



H



5-exo



O

( )n



MeO2C



H

CO2Me



(RO2)P(O)H

O



191



(RO)2P



194



SCHEME 2.35



.



Diphenylphosphanyl radicals, Ph2P , generated from diphenylphosphane in the

presence of a radical initiator were used to cyclize the alkynyl-substituted carbohydrate derivative 195 in a radical addition/5-exo cyclization sequence to give the

bicyclic deoxysugar derivative 196 (Scheme 2.36).66 Ph2P have also been used as

promoters for the cyclization of alkynyl b-lactam 181 for a highly efficient, diastereoselective synthesis of bicyclic b-lactams (see Scheme 2.33).67

.



AcO



O



O



37%



AcO



O



Ph2PH, AIBN AcO



O



AcO

196

Z/E = 6:1



195



PPh2



via:

AcO



O



O



5-exo AcO

PPh2



AcO



O



O



Ph2PH



AcO



197



198



SCHEME 2.36



PPh2



Ph2P



196



39



REFERENCES

O



O



1) Ph2PH, Ph2PCl,

V-40, Et3N

15%



199



S

P

Ph Ph



S

P

Ph Ph



2) S8



V-40 = 1,1'-azobis-(cyclohexanecarbonitrile)



200



via:



Ph2PH + Ph2PCl



Et3N

− HCl



Ph2P−PPh2



O

199



Ph2P

O



O



Ph2P

PPh2



S8



Ph2P−PPh2



5-exo

201



V-40



202



Ph2P



200



PPh2



Ph2P



PPh2



203



SCHEME 2.37



A radical diphosphanylation of alkynes using tetraorganodiphosphanes as precursors for phosphanyl radicals has been applied to the synthesis of a doubly

phosphinated diene 200 (Scheme 2.37).72 Tetraphenyldiphosphane was generated

in situ from diphenylphosphane with an excess of chlorodiphenylphosphane in the

presence of triethylamine. Addition of the phosphanyl radical to one C:C triple bond

in the dialkyne 199 leads to vinyl radical 201, which undergoes a 5-exo cyclization

(out of a Z-configured vinyl radical to minimize sterical hindrance in the cyclization) to

give vinyl radical intermediate 202. The latter is trapped by a second phosphine moiety

in a radical substitution step. The radical cyclization product is ultimately isolated as

bis-phosphane sulfide 200 after treatment of the intermediate phosphane 203 with

sulfur.



REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.



McCarroll, A. J.; Walton, J. C. Angew. Chem., Int. Ed. 2001, 40, 2224–2248.

Parsons, P. J.; Penkett, C. C.; Shell, A. J. Chem. Rev. 1996, 96, 195–206.

Albert, M.; Fensterbank, L.; Lacoˆte, E.; Malacria, M. Top. Curr. Chem. 2006, 264, 1–62.

Radicals in Organic Synthesis; Sibi, M.; Renaud, P., Eds.; Wiley: Weinheim, 2001; Vols 1

and 2.

Yet, L. Tetrahedron 1999, 55, 9349–9403.

Amiel, Y. In The Chemistry of Functional Groups, Supplement C; Patai, S.; Rappoport, Z.,

Eds.; John Wiley & Sons, Ltd., 1983; pp 917–944.

Fischer, H. In Free Radicals in Biology and Environment; Minisci, F., Ed.; Kluwer

Academic Publishers, 1997; pp 63–78.

Fischer, H.; Radom, L. Angew. Chem., Int. Ed. 2001, 40, 1340–1371.

Beckwith, A. L. J.; O’Shea, D. M. Tetrahedron Lett. 1986, 27, 4525–4528.

Heiba, E. I.; Dessau, R. M. J. Am. Chem. Soc. 1967, 89, 3772–3777.

Suzuki, A.; Miyaura, N.; Itoh, M. Synthesis 1973, 305–306.



40



INTERMOLECULAR RADICAL ADDITIONS TO ALKYNES



12. Ichinose, Y.; Matsunaga, S.; Fugami, K.; Oshima, K.; Utimoto, K. Tetrahedron Lett. 1989,

30, 3155–3158.

13. Curran, D. P.; Kim, D. Tetrahedron 1991, 47, 6171–6188.

14. Montevecchi, P. C.; Navacchia, M. L. Tetrahedron 2000, 56, 9339–9342.

15. Leardini, R.; Nanni, D.; Tundo, A.; Zanardi, G. Tetrahedron Lett. 1998, 39, 2441–2442.

16. Montevecchi, P. C.; Navacchia, M. L.; Spagnolo, P. Tetrahedron 1997, 53, 7929–7936.

17. Oka, R.; Nakayama, M.; Sakaguchi, S.; Ishii, Y. Chem. Lett. 2006, 35, 1104–1105.

18. Fernandez, M.; Alonso, R. Org. Lett. 2005, 7, 11–14.

19. Fernandez-Gonzales, M.; Alonso, R. J. Org. Chem. 2006, 71, 6767–6775.

20. Lenoir, I.; Smith, M. L. J. Chem. Soc., Perkin Trans. 2000, 1, 641–643.

21. Wille, U.; Plath, C. Liebigs Ann./Recueil 1997, 111–119.

22. Wille, U. Unpublished.

23. Wille, U.; Dreessen, T. J. Phys. Chem. A 2006, 110, 2195–2203.

24. Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical Reactions: Concepts,

Guidelines and Synthetic Applications; Wiley-VCH Verlag: Weinheim, 1996.

25. Example: Felix, D.; Schreiber, J.; Ohloff, G.; Eschenmoser, A. Helv. Chim. Acta 1971, 54,

2896–2912.

26. Wille, U.; Lietzau, L. Tetrahedron 1999, 55, 10119–10134.

27. Lietzau, L.; Wille, U. Heterocycles 2001, 55, 377–380.

28. Wille, U.; Lietzau, L. Tetrahedron 1999, 55, 11465–11474.

29. Stademann, A.; Wille, U. Aust. J. Chem. 2005, 57, 1055–1066.

30. Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc. 1915, 107, 1080–1106.

31. Li, C. H. Honours Thesis; The University of Melbourne, 2006.

32. Wille, U. Org. Lett. 2002, 2, 3485–3488.

33. Wille, U. Tetrahedron Lett. 2002, 43, 1239–1242.

34. Wille, U. J. Am. Chem. Soc. 2002, 124, 14–15.

35. Jargstorff, C.; Wille, U. Eur. J. Org. Chem. 2003, 3173–3178.

36. Sigmund, D.; Schiesser, C. H.; Wille, U. Synthesis 2005, 1437–1444.

37. Kim, S.; Lim, C. J.; Song, S. E.; Kang, H. Y. Synlett 2001, 688–690.

38. Wille, U.; Andropof, J. Aust. J. Chem. 2007, 60, 420–428.

39. See, for example: (a) Koenig, T. W.; Barklow, T. Tetrahedron 1969, 25, 4875–4886;

(b) Dayan, S.; Ben-David, I.; Rozen, S. J. Org. Chem. 2000, 65, 8816–8818.

40. Tan, K. J.; Wille, U. Chem. Commun. 2008, 6239–6241.

41. Yang, N. C.; Libman, J. J. Org. Chem. 1974, 39, 1782–1784.

42. Griesbaum, K.; Dong, Y.; McCullough, K. J. J. Org. Chem. 1997, 62, 6129–4136.

43. Examples: (a) Wolf, M. E. Chem. Rev. 1963, 63, 55–64; (b) Neale, R. S. Synthesis 1971,

1–15; (c) Minisci, F. Synthesis 1973, 1–24; (d) Chow, Y. L. Acc. Chem. Res. 1973, 6,

354–360; (e) Minisci, F. Acc. Chem. Res. 1975, 8, 165–171; (f) Chow, Y. L.; Danen, W. C.;

Nelsen, S. F.; Rosenblatt, D. H. Chem. Rev. 1978, 78, 243–274; (g) Stella, L. Angew.

Chem., Int. Ed. Engl. 1983, 22, 337–350; (h) Chow, Y. L. Can. J. Chem. 1965, 43,

2711–2714; (i) Chow, Y. L.; Colo´n, C. Can. J. Chem. 1967, 45, 2559–2568.

44. Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; John Wiley & Sons/

Masson: Chichester, 1995.



REFERENCES



45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

71.

72.



41



Wille, U.; Kr€uger, O.; Kirsch, A.; L€uning, U. Eur. J. Org. Chem. 1999, 3185–3189.

Wille, U.; Heuger, G.; Jargstorff, C. J. Org. Chem. 2008, 73, 1413–1421.

Early example: Stork, G., Mook, R. Jr. Tetrahedron Lett. 1986, 27, 4529–4532.

Review: Oshima, K. Advances on Metal Organic Chemistry; JAI Press, Ltd., 1991; Vol. 2,

pp 101–141.

Bernardoni, S.; Lucarini, M.; Pedulli, G. F.; Vaglimigli, L.; Gevorgyan, V.; Chatgilialoglu,

C. J. Org. Chem. 1997, 62, 8009–8014.

Nozaki, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1987, 60, 3465–3467.

(a) Beckwith, A. L. J.; Schiesser, C. H. Tetrahedron Lett. 1985, 26, 373–376;

(b) Spellmeyer, D. C.; Houk, K. N. J. Org. Chem. 1987, 52, 959–974.

Baldwin, J. E.; Adlington, R. M.; Robertson, J. Tetrahedron 1991, 47, 6795–6812.

Bosch, E.; Bachi, M. D. J. Org. Chem. 1993, 58, 5581–5582.

Nishida, A.; Takahashi, H.; Takeda, H.; Takada, N.; Yonemitsu, O. J. Am. Chem. Soc. 1990,

112, 902.

Journet, M.; Rouillard, A.; Cai, D.; Larsen, R. D. J. Org. Chem. 1997, 62, 8630–8631.

Broka, C. A.; Reichert, D. E. C. Tetrahedron Lett. 1987, 28, 1503–1506.

S-Centered Radicals; Alfassi, Z. B., Ed.; John Wiley & Sons: Chichester, 1999.

Montevecchi, P. C.; Navacchia, M. L. Recent Res. Dev. Org. Bioorg. Chem. 1997, 1–13.

Montevecchi, P. C.; Navacchia, M. L.; Spagnolo, P. Tetrahedron Lett. 1997, 38,

7913–7916.

Tan, K. J. PhD Thesis, University of Melbourne, 2009.

Keck, G. E.; Wagner, T. T. J. Org. Chem. 1996, 61, 8366–8367.

Triestad, G. K.; Jiang, T.; Fioroni, G. M. Tetrahedron: Asymmetry 2003, 14, 2853–2856.

Lachia, M.; Denes, F.; Beaufils, F.; Renaud, P. Org. Lett. 2005, 7, 4103–4106.

Denes, F.; Beaufils, F.; Renaud, P. Org. Lett. 2007, 9, 4375–4378.

Leardini, R.; Nanni, D.; Zanardi, G. J. Org. Chem. 2000, 65, 2763–2772.

Brumwell, J. E.; Simpkins, N. S.; Terrett, N. K. Tetrahedron 1994, 50, 13533–13552.

Alcaide, B.; Rodrıguez-Campos, I. M.; Rodrıguez-Loˆpez, J.; Rodrıguez-Vicente, A. J. Org.

Chem. 1999, 64, 5377–5387.

Tsuchii, K.; Doi, M.; Hirao, T.; Ogawa, A. Angew. Chem., Int. Ed. 2003, 42, 3490–3493.

Jessop, C. M.; Parsons, A. F.; Routledge, A.; Irvine, D. J. Tetrahedron: Asymmetry 2003,

14, 2849–2851.

Antczak, M. I.; Montchamp, J.-L. Synthesis 2006, 3080–3084.

Beaufils, F.; Denes, F.; Renaud, P. Angew. Chem., Int. Ed. 2005, 44, 5273–5275.

Sato, A.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2005, 44, 1694–2696.



3

RADICAL CATION

FRAGMENTATION REACTIONS

IN ORGANIC SYNTHESIS

ALEXANDER J. PONIATOWSKI AND PAUL E. FLOREANCIG

Department of Chemistry, University of Pittsburgh,

Pittsburgh, PA, USA



3.1 INTRODUCTION

Radical cations, the products of single electron oxidation reactions, are high-energy

intermediates that can react through numerous pathways, including cycloaddition,

nucleophilic addition, metathesis, and fragmentation processes.1 The potential for

reaction through multiple pathways leads to speculation that radical cations are not

viable intermediates for complex molecule synthesis. Careful mechanistic studies,

however, have provided an understanding of the structural features that promote

selective reactions and applications of this information have provided spectacular

transformations. For example, nucleophilic addition reactions to radical cation

intermediates have been applied to natural product synthesis and asymmetric bond

formation (Scheme 3.1).2 In this chapter, we will discuss the manner in which

fundamental mechanistic studies on radical cation fragmentation reactions have

provided the framework for the development of an electron transfer-initiated cyclization reaction3 that has been applied to the synthesis of a wide array of complex

structures.



Carbon-Centered Free Radicals and Radical Cations, Edited by Malcolm D. E. Forbes

Copyright Ó 2010 John Wiley & Sons, Inc.



43



44



RADICAL CATION FRAGMENTATION REACTIONS IN ORGANIC SYNTHESIS

O

NMe



Bn



N



1. Fe(III)



+



N

O



N



O



2. NaBH4



68% yield

82% ee



Ph

OH



Ph

O

NMe

Bn



Ph



+



N



O



Ce(IV)



Ph



BF3K



72% yield

94% ee



H



TBSO



O



H



anodic

TBSO



oxidation

O



85–90%

yield

TBSO



O



anodic

TBDPSO

O



OTBS

TBSO



SCHEME 3.1



oxidation

MeOH



H

TBDPSO

O H



70% yield

OTBS



MeO



Addition reactions through radical cation intermediates.



3.1.1 Oxidative Carbon–Carbon Bond Cleavage

Arnold’s demonstration4 that oxocarbenium ion intermediates can be formed through

homobenzylic ether radical cation fragmentation reactions shows that mild oxidizing

conditions can be used to prepare important reactive intermediates.5 Scheme 3.2

illustrates a critical observation in the development of an explanatory model that

allows for the application of radical cation fragmentation reactions in complex

molecule synthesis. In Arnold’s seminal work, cleavage of the benzylic carbon–

carbon bond in substrate 1 is promoted by 1,4-dicyanobenzene (DCB) with photoirradiation by a medium-pressure mercury vapor lamp. With methanol as the solvent,

the resulting products were diphenylmethane (2) and formaldehyde dimethyl acetal (3).

Arnold’s proposed mechanism6 for these oxidations is shown in Scheme 3.3.

Photoexcitation of 1,4-dicyanobenzene produces a potent single electron oxidant



OMe

1



SCHEME 3.2



hν, DCB, 10ºC

CH3CN, MeOH



+

2



OMe



MeO

3



Arnold’s oxidative carbon–carbon bond fragmentation.



45



INTRODUCTION



(Step 1) that removes an electron from the substrate to form the radical cation (Step 2).

The benzylic bonds in the radical cation are substantially weakened relative to those in

the neutral substrate, allowing for mesolytic bond cleavage to form the diphenylmethyl radical and the oxocarbenium ion (Step 3). The oxocarbenium ion is subsequently captured by MeOH to form the acetal product (Step 4). The radical anion of

dicyanobenzene is a potent electron donor and can reduce the diphenylmethyl radical

to form the diphenylmethyl anion (Step 5), which can be protonated by MeOH to form

diphenylmethane (Step 6). Notably, no diphenylmethyl ether product was detected,

indicating that the fragmentation reaction proceeds exclusively to form the diphenylmethyl radical and the oxocarbenium ion. This suggests that cleavage provides the

more stable cationic fragment.

(Step 1)

CN



CN







*



NC



NC



(Step 2)

CN



Ph

OMe



Ph



+



*

transfer



NC



CN



Ph



electron



+



OMe



Ph







NC



(Step 3)

Ph



Ph



fragmentation



+



OMe



Ph



OMe



Ph



(Step 4)

OMe



+



(Step 5)

Ph



CN

+



Ph



cation



MeOH



OMe



+







H+



CN



Ph



electron

transfer



NC



MeO



capture



+

Ph



NC



(Step 6)

Ph



+



Ph



SCHEME 3.3



MeOH



Ph



proton

transfer



+



MeO–



Ph



Mechanism for photosensitized carbon–carbon bond cleavage.



Additional examples of oxidatively initiated cleavage reactions demonstrated that

heteroatom stabilization of the cationic fragment is not necessary and that reactivity can

be tuned by manipulating the strength of the benzylic carbon–carbon bond (Scheme 3.4).

Entry 1 shows that substrate 4 with a benzyl rather than a diphenylmethyl group is

completely stable toward oxidative cleavage. Substrate 5, in which the methoxymethyl

group is replaced by a diphenylmethyl group, also does not undergo oxidative cleavage at

room temperature (Entry 2) but does cleave at elevated temperature (Entry 3). These

experiments show that oxidative cleavage reactivity can be enhanced by weakening the

benzylic carbon–carbon bond through the addition of phenyl groups or alkoxy groups and

that alkoxy groups are particularly effective at promoting fragmentation.7



46



RADICAL CATION FRAGMENTATION REACTIONS IN ORGANIC SYNTHESIS



(Entry 1)

OMe



Ph

(Entry 2)



N. R.

CH3CN, MeOH



4

Ph

Ph



Ph



hν, DCB, 10ºC



N. R.



CH3CN, MeOH



Ph

(Entry 3)



hν, DCB, 80ºC



5

Ph

Ph



Ph

Ph



Ph



hν, DCB, 80ºC



MeO



Ph



+



CH3CN, MeOH



Ph



Ph

2



5



6



SCHEME 3.4 Tuning reactivity by manipulating benzylic bond strength.



Substitution of the aromatic ring was also shown to be an important factor in

determining the facility of bond cleavage (Scheme 3.5).8 Cyano-substituted ether 7

cleaved under oxidizing conditions, while methoxy-substituted ether 9 did not. These

results are contrary to the ease of radical cation formation, but are consistent with

fragmentation proceeding more readily from nonstabilized radical cations.



hν, DCB, 10ºC

OMe



+



OMe



MeO



CH3CN, MeOH



NC



NC

7



8



hν, DCB, 10ºC

OMe



3



NR



CH3CN, MeOH



MeO

9



SCHEME 3.5



Reactivity as a function of oxidation potential.



3.1.2 Thermodynamic and Kinetic Considerations

These studies taken together show that, from a thermodynamic perspective, three

factors can be utilized to predict the bond dissociation energy of a homobenzylic ether

radical cation (BDE(RC)) and thereby its propensity to fragment: (1) the bond

dissociation energy of the benzylic carbon–carbon bond in the neutral substrate

(BDE(S)), (2) the oxidation potential of the substrate (Epa(S)), and (3) the oxidation



47



INTRODUCTION



potential of the radical that corresponds to the eventual electrophilic fragment

(Epa(E)). These factors combine to yield equation 1 as illustrated in Scheme 3.6

by using a representative Arnold mesolytic cleavage. The same equation explains the

enhanced reactivity of the diphenylmethyl-containing substrates relative to their

monophenyl counterparts because of their lower bond dissociation energies. The low

reactivity of the radical cation of methoxyphenyl-containing substrate is also

explained by this equation because of its low oxidation potential. Finally, the

selectivity of the cleavage to form the oxocarbenium ion rather than the diphenylmethyl radical can be explained by the lower oxidation potential of alkoxyalkyl

radicals compared to diarylmethyl radicals.9



BDE(S)



+



OMe



OMe



+ e–



OMe



– Epa(S)



OMe



+ e–

OMe



Epa(E)



OMe



+



OMe



BDE(RC)



OMe



BDE(RC) = BDE(S) – Epa(S) + Epa(E)



SCHEME 3.6



(Equation 1)



Thermodynamics of radical cation fragmentation.



A subsequent study10 from the Arnold group showed an intriguing stereoelectronic

effect in oxidative benzylic carbon–hydrogen bond cleavage reactions of substrates 8

and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the

presence of a nonnucleophilic base. Radical cation formation also weakens benzylic

carbon–hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic

hydrogens yields benzylic radicals that can be reduced by the radical anion

of dicyanobenzene to form benzylic anions that will be protonated by solvent.

This sequence of oxidation, deprotonation, reduction, and protonation provides a

sequence by which epimerization can be effected at the benzylic center. In this study,

trans isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in

Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions

were repeated in deuterated solvents to assure that these observations resulted from

kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of

deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete

deuterium incorporation. The authors attributed this difference in reactivity to



48



RADICAL CATION FRAGMENTATION REACTIONS IN ORGANIC SYNTHESIS



conformational effects. The dihedral angle between the phenyl ring and the benzylic

carbon–hydrogen bond in 10 was calculated to be 1 while the corresponding dihedral

angle in 11 was calculated to be 34 . These studies indicate that benzylic fragmentation is possible only when the cleaving bond can overlap with the p-orbitals of the

arene radical cation. The role of orbital overlap in determining fragmentation

propensity has also been discussed extensively by other research groups.11

(Equation 1)

hν, 1,4-dicyanobenzene

N.R.



H



2,4,6-collidine, CH3CN



H3C

10

(Equation 2)



H



hν, 1,4-dicyanobenzene



H



2,4,6-collidine, CH3CN



H3C



H3C



11



10



(Equation 3)

hν, 1,4-dicyanobenzene

N.R.



H

2,4,6-collidine

CH3CN, CH3OD



H3C

10

(Equation 4)



H

hν, 1,4-dicyanobenzene



D



2,4,6-collidine

CH3CN, CH3OD



H3C



H3C

12



11



SCHEME 3.7



Stereoelectronic effects.



This work and related studies provided the basis for the stereoelectronic model for

homobenzylic ether cleavage in Fig. 3.1. This model includes overlap of the benzylic

carbon–carbon bond with the SOMO of the aromatic ring (structure 13),

thereby stabilizing the benzylic radical upon cleavage. Additionally, overlap of a

heteroatom lone pair and the benzylic sà orbital was shown to be necessary for

cleavage (structure 14).



R'



CH3



H

Ph



O

H



H



CH3



H

O



σ∗



R' (H)

13



FIGURE 3.1



14



Preferred conformation for radical cation carbon–carbon bond cleavage.



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