Free-Radical Rearrangements

Beckwith, A. L. J.; Ingold, K. U.

doi:10.1016/b978-0-12-481301-4.50010-3

Cited by 114 publications

(65 citation statements)

References 826 publications

(583 reference statements)

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“…Large differences in the entropies of activation are not expected for this set of similar reactions and, where available, the measured log[A c /s -1 ] were all 10±0.5 (see Table 1 and refs. [4][5][6]. In a few cases (Table 1) activation energies were therefore estimated from reported rate constants, assuming log(A c ) of this magnitude.…”

Section: Experimental Kinetic Data For 5-exo Cyclizationsmentioning

confidence: 99%

“…This property has been exploited in many organic synthetic procedures, 1 in natural product syntheses, 2 and in numerous tandem and cascade processes. 3 The main characteristics of the reaction were delineated in a seminal article by Beckwith and Ingold in 1980. the cyclohexane ring 2 with the radical centre (X-1) approaching C-5 of the alkene at an angle close to tetrahedral. A number of computational studies of hex-5-enyl ring closures have been reported [20][21][22][23][24] and these have generally supported…”

Section: Introductionmentioning

confidence: 99%

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The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

Walton

2014

Org. Biomol. Chem.

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The reaction coordinates of an archetypical set of 5-exo cyclizations of C-, N-and O-centred radicals were investigated by computational methods. G4 theory, and DFT with the um062x functional, were able to rationalise counterintuitive factors such as the 'normal' order of rate constants being: N-centred < C-centred < O-centred radicals. The access angle between the radical centre and the double bond was identified as a key factor. Examination of its evolution during ring closure implied that rigidity at the N-ends of the chains, and the consequent extra energy needed to attain chair-like transition states, might be the reason for slow aminyl cyclizations. A novel linear correlation between cyclization activation energies and the access angles was discovered. The preference for cis-1,2-disubstituted product formation was also accounted for in terms of interaction between the hyperconjugatively delocalized SOMO and the alkene * orbital.

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Section: Experimental Kinetic Data For 5-exo Cyclizationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

Walton

2014

Org. Biomol. Chem.

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“…The class of halo radicals such as CH 2 XCH 2 • is important in a number of chemical processes [1][2][3][4] and determines the stereoselectivity of the reaction products from halogenation 1,2,5-9 of alkenes and alkanes. Their role in stereoselective control is determined by whether the radical is classical (Figure 1c) or bridged and if bridged whether the structure is symmetric ( Figure 1a) or asymmetric ( Figure 1b).…”

Section: Introductionmentioning

confidence: 99%

Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

1999

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Radicals such as CH 2 XCH 2• , where X is a halogen, play an important role in the stereochemical control observed in many chemical reactions. To elucidate the origin of the stereoselectivity, we calculated the structures and potential energy surfaces of the haloethyl radicals (CH 2 XCH 2 • , X ) F, Cl, Br, I) using ab initio quantum mechanics [HF, local MP2, DFT (both B3PW91 and B3LYP)]. We find that the CH 2 BrCH 2• and CH 2 ICH 2• radicals strongly favor the symmetrically bridged structures while the CH 2 ClCH 2 • radical leads to similar energy for symmetric bridging and classical structures. (In contrast, X ) H and F leads to dramatically different structures). This confirms the Skell hypothesis of symmetric bridging to explain the stereochemical control of the CH 2 BrCH 2• and CH 2 ICH 2 • radicals, indicating that such bridged structures play an important role in the dissociation processes involving CH 2 XCH 2• with X ) Cl, Br, and I. The trends in the rotational barriers and structural parameters are consistent with hyperconjugation between the singly occupied carbon 2p orbital and the σ*(C-X) MO. We find that the rotational barrier, bridged structure, and dissociation of the radicals are described much more accurately using DFT (with GGA) than with HF or LMP2. IntroductionThe class of halo radicals such as CH 2 XCH 2 • is important in a number of chemical processes [1][2][3][4] and determines the stereoselectivity of the reaction products from halogenation 1,2,5-9 of alkenes and alkanes. Their role in stereoselective control is determined by whether the radical is classical (Figure 1c) or bridged and if bridged whether the structure is symmetric ( Figure 1a) or asymmetric (Figure 1b). The possible minima and transition structures for rotation around the C-C bond are schematically represented in Figure 2. Anti (I) and gauche (III) rotamers are candidates for local energy minima on the rotational energy surface because Pauli repulsion between bonding pairs would be minimized at these two conformations. The other two structures (II and IV) possessing eclipsed bonds are also possible transition states on the rotational energy surface.If the structure of the radical is bridged, we expect retention of the stereochemistry. However, if the radical prefers a classical asymmetric conformation, additional conditions must be fulfilled to exert stereochemical control. Namely, a strong preference for high population of the anti conformer and nonplanarity of the radical center are required. Experimental 10-20 and theoretical 21-43 studies have led to a good understanding of the structure and energetics for the cases of X ) H, F, and Cl; however, little is known about X ) Br and I. The motivation for our studies was to elucidate the X ) Br and I systems. But we also studied X ) H, F, Cl in cases to compare to previous experiments and theory.In this paper we use four techniques of first principles quantum mechanics (QM) methods to examine the potential surface for CH 2 XCH 2 • with X ) H, F, Cl, Br, and I. These QM met...

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“…Also, no rearrangement of 2 •+ and 2-Me 8 •+ of the type corresponding to the well-known ring opening of cyclopropylcarbinyl to homoallyl cations 31,32 or to the ring-enlarging rearrangement of cyclopropylmethyl to cyclobutyl cations 33 was observed. Table 2 gives the excitation wavelengths and transition moments predicted for the dipole-allowed transitions in 2 •+ and 2a •+ by the TD-B3LYP method.…”

Section: Methodsmentioning

confidence: 93%

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

et al. 2005

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The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me 8 ) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me 8 ), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me 8 ), 2 •+ and 2-Me 8 •+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekulé hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2 •+ , which is formed upon γ-irradiation of 2 in a CFCl 3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me 8 is treated in the same way, only the rearranged radical cation 2a-Me 8 •+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a •+ and 2-Me 8 •+ , is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a •+ . Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me 8 in liquid octane, strongly suggest that 2-Me 8 •+ (and not 2a-Me 8 •+ ) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me 8 •+ f 2a-Me 8 •+ is estimated. The radical cations 2a •+ and 2a-Me 8 •+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene π-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the π-system.

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Free-Radical Rearrangements

Cited by 114 publications

References 826 publications

The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

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Free-Radical Rearrangements

Cited by 114 publications

References 826 publications

The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

The importance of chain conformational mobility during 5-exo-cyclizations of C-, N- and O-centred radicals

Conformations and Barriers of Haloethyl Radicals (CH2XCH2, X = F, Cl, Br, I): Ab Initio Studies

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

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Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies