Rate constants and Arrhenius functions for rearrangements of the 2,2-dimethyl-3-butenyl and (2,2-dimethylcyclopropyl)methyl radicals

Newcomb, Martin; Glenn, Anne G.; Williams, William G.

doi:10.1021/jo00272a042

Cited by 59 publications

(22 citation statements)

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“…Before considering the experimental 12 C/ 13 C KIEs as evidence for tunneling in the ring opening of cyclopropylcarbinyl radical 18 to 19 , it is interesting to ask how the geminal methyl groups at one ring carbon of 22a affect which ring bond cleaves, when the ring opening reaction occurs by tunneling. Experiments conducted at slightly above room temperature find that ring opening of 22a to 23a is favored over opening to 24a by a ratio of 6.5:1 . This result is hardly surprising since, as shown in Figure , in 23a the radical appears at a tertiary carbon; whereas, in 24a the radical center is primary.…”

Section: Recent Studies Of Reactions Involving Tunneling By Carbon Anmentioning

confidence: 94%

Reactions that involve tunneling by carbon and the role that calculations have played in their study

Borden

2015

WIREs Comput Mol Sci

137

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Section: Recent Studies Of Reactions Involving Tunneling By Carbon Anmentioning

confidence: 94%

Reactions that involve tunneling by carbon and the role that calculations have played in their study

Borden

2015

WIREs Comput Mol Sci

137

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“…2 Professor Athel Beckwith made many important contributions to the study of this ring opening reaction, 3 including measuring 25 the effects of substituents on it. For example, in 1989 Newcomb 4 and Beckwith 5 published back-to-back papers on the ring opening of 2,2-dimethylcyclopropylcarbinyl radical (1b). Although the tertiary radical center in 2b makes it by far the thermodynamically preferred product, ring opening of 1b to 2b is 30 only favored kinetically over ring opening to 2c by a factor of about 6.5 at both 25°4 and 60°.…”

mentioning

confidence: 99%

“…Therefore, since the calculated difference in E a values for formation of 2b and 2c is a little too high, the 20 calculated product ratio of 2b to 2c of 25.4 at 300 K is about a factor of four larger than the observed value. 4,5 The results of our CVT + SCT calculations, comparing the rates of ring opening of 1a and 1b at 20 K, are given in Table 2. Tunnelling rates are expected to decrease exponentially with the square root of the effective tunnelling mass.…”

mentioning

confidence: 99%

Effects of geminal methyl groups on the tunnelling rates in the ring opening of cyclopropylcarbinyl radical at cryogenic temperature

et al. 2011

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CVT + SCT calculations on the rate of tunnelling at 20 K in the ring opening of cyclopropylcarbinyl radical, substituted with geminal methyl groups at a ring carbon (1b), have been performed. The calculations predict that, contrary to 10 expectations based on the effect of mass on the rate of tunnelling, the geminal methyl substituents in 1b should make the rate of ring opening to 1,1-dimethyl-3-butenyl radical (2b) 10 4 times faster than the rate of ring opening of unsubstituted cyclopropylcarbinyl radical (1a) to 3-butenyl radical (2a) and 15 almost 10 6 times faster than the rate of ring opening of 1b to 2,2-dimethyl-3-butenyl radical (2c). The reasons for these unexpected findings are discussed.The ring opening of cyclopropylcabinyl radical (1a) to 3-butenyl radical (2a), shown in Scheme 1, has been extensively 20 investigated. 1 The extreme rapidity of this reaction has resulted in its widespread use as a "radical clock" for timing the rates of other free radical reactions. 2 Professor Athel Beckwith made many important contributions to the study of this ring opening reaction, 3 including measuring 25 the effects of substituents on it. For example, in 1989 Newcomb 4 and Beckwith 5 published back-to-back papers on the ring opening of 2,2-dimethylcyclopropylcarbinyl radical (1b). Although the tertiary radical center in 2b makes it by far the thermodynamically preferred product, ring opening of 1b to 2b is 30 only favored kinetically over ring opening to 2c by a factor of about 6.5 at both 25°4 and 60°. 5 The temperature dependence of the ratio of 2b/2c is apparently small, and 1 kcal/mol probably is an upper limit on the difference between the E a values for these two reactions. The results of several different types of ab initio 35 calculations, performed by Schlegel and Newcomb, 6 also give values of slightly less than 1 kcal/mol for the difference between the barrier heights for ring opening of 1b to 2b and 2c.The rate constants for the ring opening of the unsubstituted radical (1a) have been measured at temperatures as low as 128 K 7 45 and as high as 395 K. 8 An Arrhenius plot over this very wide temperature range is reasonably linear and gives E a = 7.05 kcal/mol and log A = 13.15 s -1 . 9 The linear Arrhenius plot and the high A factor both suggest that, at least at the high end of this temperature range, the ring opening of 1a to 2a proceeds largely 50 by passage over the reaction barrier, rather than by tunnelling through it.We have been interested in the possibility that, at cryogenic temperatures, 1a might undergo rapid ring opening by tunnelling, despite the fact that a CH 2 group, rather than a hydrogen atom, 55 would have to tunnel in this reaction. Although not common, there are now several reactions in which experiments have shown that tunnelling by carbon can occur and occur rapidly. 10 The two requirements for tunnelling by carbon to be rapid are a reaction barrier that is both low and narrow. 10 These requirements 60 are met in the ring opening of 1a. Indeed our previous calculatio...

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“…This value of k f is a factor of 3.6 slower than that of the carbon analog 41 64 but both rates are enhanced by the Thorpe-Ingold effect. (2) and (3) above, their exothermicities should resemble those of the acyclic cases, if ring strain is taken into account.…”

Section: Reactions Of Radicals With Azoxyalkanesmentioning

confidence: 82%

The free radical chemistry of the azoxy group

Engel¹,

Duan²,

Shu-Lin³

et al. 2003

Arkivoc

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The azoxy functional group, though relatively uncommon, is found in several natural products and in liquid crystalline compounds. Azoxyalkanes are generally very stable to heat and light and are not subject to attack by radicals. Intramolecular radical reactions, however, can lead to cyclic aminyl nitroxides and hydrazyls, which rearrange further or undergo fragmentation. The azoxy group greatly stabilizes an attached carbon-centered radical but the chemistry of the resulting α-azoxy radicals is not completely understood.

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Rate constants and Arrhenius functions for rearrangements of the 2,2-dimethyl-3-butenyl and (2,2-dimethylcyclopropyl)methyl radicals

Cited by 59 publications

References 1 publication

Reactions that involve tunneling by carbon and the role that calculations have played in their study

Reactions that involve tunneling by carbon and the role that calculations have played in their study

Effects of geminal methyl groups on the tunnelling rates in the ring opening of cyclopropylcarbinyl radical at cryogenic temperature

The free radical chemistry of the azoxy group

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