Abstract:The surfaces for cis–trans isomerization and racemization of the cyclopropane radical cation (1+) are determined using abinitio calculations. It is found that polarization functions and electron correlation have large effects on the relative energies of 1+. Orbital symmetry arguments predict that conrotatory closure of the 0,0 conformer of 1+ should be preferred. These predictions are supported by the calculations; however, even the conrotatory closure has a significant activation barrier. Conformational chang… Show more
“…Hence, TS1 represents the weakly bonding situation as a superposition of the [R···H···E + ] and [ +• R···H···E • ] VB-states and resembles the structures of the isobutane (R−H) and trimethylene (H−E) radical cations , in an averaged [ δ+• R···H···E δ+• ] structure. In an alternative view, and this explains the ET pathway B, the cyclopropane moiety in TS1 resembles that of its radical cation where one C−C bond is substantially elongated , (an electron is removed from the respective C−C bonding orbital). This activation mode is an example for an H-coupled ET from the alkane to the electrophile along the C−H activation path.…”
The mechanisms for the reactions of isobutane and adamantane with polyhalogen electrophiles (HHal(2)(+), Hal(3)(+), Hal(5)(+), and Hal(7)(+), Hal = Cl, Br, or I) were studied computationally at the MP2 and B3LYP levels of theory with the 6-31G (C, H, Cl, Br) and 3-21G (I) basis sets, as well as experimentally for adamantane halogenations in Br(2), Br(2)/HBr, and I(+)Cl(-)/CCl(4). The transition structures for the activation step display almost linear C...H...Hal interactions and are characterized by significant charge transfer to the electrophile; the hydrocarbon moieties resemble the respective radical cation structures. The regiospecificities for polar halogenations of the 3-degree C-H bonds of adamantane, the high experimental kinetic isotope effects (k(H)/k(D) = 3-4), the rate accelerations in the presence of Lewis and proton (HBr) acids, and the high kinetic orders for halogen (7.5 for Br(2)) can only be understood in terms of an H-coupled electron-transfer mechanism. The three centered-two electron (3c-2e) electrophilic mechanistic concept based on the attack of the electrophile on a C-H bond does not apply; electrophilic 3c-2e interactions dominate the C-H activations only with nonoxidizing electrophiles such as carbocations. This was shown by a comparative computational analysis of the electrophilic and H-coupled electron-transfer activation mechanisms for the isobutane reaction with an ambident electrophile, the allyl cation, at the above levels of theory.
“…Hence, TS1 represents the weakly bonding situation as a superposition of the [R···H···E + ] and [ +• R···H···E • ] VB-states and resembles the structures of the isobutane (R−H) and trimethylene (H−E) radical cations , in an averaged [ δ+• R···H···E δ+• ] structure. In an alternative view, and this explains the ET pathway B, the cyclopropane moiety in TS1 resembles that of its radical cation where one C−C bond is substantially elongated , (an electron is removed from the respective C−C bonding orbital). This activation mode is an example for an H-coupled ET from the alkane to the electrophile along the C−H activation path.…”
The mechanisms for the reactions of isobutane and adamantane with polyhalogen electrophiles (HHal(2)(+), Hal(3)(+), Hal(5)(+), and Hal(7)(+), Hal = Cl, Br, or I) were studied computationally at the MP2 and B3LYP levels of theory with the 6-31G (C, H, Cl, Br) and 3-21G (I) basis sets, as well as experimentally for adamantane halogenations in Br(2), Br(2)/HBr, and I(+)Cl(-)/CCl(4). The transition structures for the activation step display almost linear C...H...Hal interactions and are characterized by significant charge transfer to the electrophile; the hydrocarbon moieties resemble the respective radical cation structures. The regiospecificities for polar halogenations of the 3-degree C-H bonds of adamantane, the high experimental kinetic isotope effects (k(H)/k(D) = 3-4), the rate accelerations in the presence of Lewis and proton (HBr) acids, and the high kinetic orders for halogen (7.5 for Br(2)) can only be understood in terms of an H-coupled electron-transfer mechanism. The three centered-two electron (3c-2e) electrophilic mechanistic concept based on the attack of the electrophile on a C-H bond does not apply; electrophilic 3c-2e interactions dominate the C-H activations only with nonoxidizing electrophiles such as carbocations. This was shown by a comparative computational analysis of the electrophilic and H-coupled electron-transfer activation mechanisms for the isobutane reaction with an ambident electrophile, the allyl cation, at the above levels of theory.
“…921 This is in marked contrast to the phenylcyclopropane radical cation where the structure with two long C-C bonds is a minimum due to benzylic stabilization of both the radical and the cation site. 922,923 The ring-opened form of c-C 3 H 6 •+ , the "trimethylene radical cation", 924 is, despite a number of claims about its formation, 925,926 not a minimum 913 but rather the TS (∆H ‡ ) 30 kcal mol -1 ) en route to the propene radical cation, which is 8.2 kcal mol -1 more stable than c-C 3 H 6 •+ (MP2/6-31G(d)). 918 In contrast, nucleophilic backside attack on the terminal carbon atom of c-C 3 H 6 •+ , which was predicted theoretically, 927 is almost barrierless 885 and in agreement with the experimentally observed inversion of configuration for the ring opening of substituted cyclopropane radical cations (Scheme 24).…”
Section: Cyclopropanementioning
confidence: 99%
“…The ring-opened form of c -C 3 H 6 •+ , the “trimethylene radical cation”, is, despite a number of claims about its formation, , not a minimum 913 but rather the TS ( Δ H ‡ = 30 kcal mol -1 ) en route to the propene radical cation, which is 8.2 kcal mol -1 more stable than c -C 3 H 6 •+ (MP2/6-31G( d )) . In contrast, nucleophilic backside attack on the terminal carbon atom of c -C 3 H 6 •+ , which was predicted theoretically, is almost barrierless 885 and in agreement with the experimentally observed inversion of configuration for the ring opening of substituted cyclopropane radical cations (Scheme ). , 24 Ring Opening of Substituted Cyclopropane Radical Cations in the Presence of Nucleophiles…”
“…The 2 A 1 state (orbital s singly occupied) relaxes to a trimethylene structure with one lengthened one‐electron C–C bond (type A) whereas the 2 B 2 state forms a structure type resembling a π complex (type B ; two lengthened C–C bonds). Structure type A is lowest in energy for many cyclopropane radical cations, including the parent . An ESR spectrum at 4.2 K shows two strongly coupled 1 H nuclei (isotropic hyperfine coupling constant, hfcc, a H = 20.4 G, β‐protons) and four 1 H nuclei with a smaller splitting ( a H = −11.7 G, α‐protons), clearly supporting the 2 A 1 structure.…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
