Cyclopropylcarbinyl Radical Ring Opening Research Articles

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

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 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 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.

Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects

The intramolecular (13)C kinetic isotope effects for the ring-opening of cyclopropylcarbinyl radical were determined over a broad temperature range. The observed isotope effects are unprecedentedly large, ranging from 1.062 at 80 degrees C to 1.163 at -100 degrees C. Semiclassical calculations employing canonical variational transition-state theory drastically underpredict the observed isotope effects, but the predicted isotope effects including tunneling by a small-curvature tunneling model match well with experiment. These results and a curvature in the Arrhenius plot of the isotope effects support the recently predicted importance of heavy-atom tunneling in cyclopropylcarbinyl ring-opening.

Theoretical studies of the effects of substituents on the ring opening reactions for the cyclopropylcarbinyl radical

DFT (U)B3LYP/6-31G(d,p), (U)B3LYP/6-31+G(d,p), and (U)B3LYP/6-311+G(d,p) calculations were carried out to investigate the ring opening reactions of the cyclopropylcarbinyl radical (leading to the 3-butenyl radical) and its mono-substituted analogues containing the methyl, ethyl, propyl, isopropyl, isobutyl, fluorine, chlorine, bromine, hydroxyl, methoxy, cyanogen, nitryl, vinyl, and phenyl substituents on the ring (leading to the pseudo-secondary and primary radicals, respectively). The enthalpies of activation, rate constants, and reaction enthalpies for all ring opening reactions were calculated. The predicted enthalpy of activation, rate constant, and reaction enthalpy of the cyclopropylcarbinyl radical ring opening reaction are in excellent agreement with the corresponding experimental values. For the alkyl (methyl, ethyl, propyl, isopropyl, and isobutyl) substitutions, steric effects are found to be the dominating factor to affect the ring opening reactions, while for the rest substitutions, the stereoelectronic effects play the most important roles. Our calculations indicate that the cyanogen, nitryl, vinyl, and phenyl substituents, which contain π bonds, have much larger effects on the ring opening reactions than the other substituents.

Calculations Predict a Large Inverse H/D Kinetic Isotope Effect on the Rate of Tunneling in the Ring Opening of Cyclopropylcarbinyl Radical

B3LYP/6-31G* calculations have been performed, to compute the kinetic isotope effects (KIEs) on the rate of ring opening of cyclopropylcarbinyl radical (1) to 3-buten-1-yl radical (2) at 20 K by tunneling. The most striking result of our small curvature tunneling calculations is the prediction of a 2.7-fold increase in the rate of reaction by substitution of D(2) for H(2) at C(1). The origin of this very surprising, inverse KIE on the rate of tunneling is discussed.

Calculations Predict Rapid Tunneling by Carbon from the Vibrational Ground State in the Ring Opening of Cyclopropylcarbinyl Radical at Cryogenic Temperatures

B3LYP/6-31G(d) calculations have been performed on the ring opening of cyclopropylcarbinyl radical 1 to 3-buten-1-yl radical 2. The dynamics of the reaction have been computed with canonical variational transition state theory (CVT), both with and without inclusion of small-curvature tunneling (SCT). The CVT + SCT calculations predict that 1 should undergo rapid and temperature-independent ring opening to 2 at cryogenic temperatures, by tunneling from the lowest vibrational level of 1.

Cyclization of 5-hexenyl radicals from nitroxyl radical additions to 4-pentenylketenes and from the acyloin reaction

Photochemical Wolff rearrangements were used to form 5-substituted-4-pentenylketenes 1a–1d (RCH=CHCH2XCH2CH=C=O: 1a R = H, X = CH2; 1b R = Ph, X = CH2; 1c R = c-Pr, X = CH2; 1d R = H, X = O), which were observed by IR at 2121, 2120, 2119, and 2126 cm–1, respectively, as relatively long-lived species at room temperature in hydrocarbon solvents. These reacted with the nitroxyl radical tetramethylpiperidinyloxyl (TEMPO, TO·) forming carboxy-substituted 5-hexenyl radicals 3, which were trapped by a second nitroxyl radical forming 1,2 diaddition products 4a–4d. On thermolysis, 4a–4d underwent reversible reformation of the radicals 3, which underwent cyclization forming cyclopentanecarboxylic acid derivatives 6 or 11 as the major products. However, in the case of 1b, the cyclopentane derivative was formed reversibly and on prolonged reaction times the only product isolated was PhCH=CH-(CH2)4CO2H (8b) from hydrogen transfer to Cβ and cleavage of the TEMPO group. Cyclopropylcarbinyl radical ring opening in the cyclized radical 5c from 1c led to the 2-(4-N-tetramethylpiperidinyloxybut-1-enyl)cyclopentane derivative 11 as the major product. In a test for 5-hexenyl radical ring closure in the radical anion intermediate of the acyloin condensation, the ester CH2=CH(CH2)3CO2Et (12a) gave the acyloin 13a (76%) as the only observed product, while PhCH=CH(CH2)3CO2CH3 (12b) with Na in toluene gave 21% of the acyloin product 13b and 42% of 2-benzylcyclopentanol (15) from cyclization of the intermediate radical anion.Key words: ketenes, free radical cyclization, TEMPO, acyloin condensation.

ChemInform Abstract: Ab initio Molecular Orbital Calculations of Ring Opening of Cyclopropylcarbinyl Radicals

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Ab Initio Molecular Orbital Calculations of Ring Opening of Cyclopropylcarbinyl Radicals

Ab initio molecular orbital calculations have been performed on the ring-opening reactions of the cyclopropylcarbinyl radical and analogs containing methyl substitution on the ring. The barrier height and heat of reaction for the cyclopropylcarbinyl radical ring opening calculated at the G2 level of theory are in good agreement with experiment. Barrier heights for the ring opening of substituted cyclopropylcarbinyl radicals and relative rate constants were computed at HF, UMP2, and PMP2 levels of theory using the 6-31G* basis set. The calculated relative rates are in good agreement with the experimental data available, and the trends in the kinetics can be explained primarily by steric interactions.

Ethoxycarbonyl acceleration of cyclopropylcarbinyl radical ring opening

Rate constants for ring opening of the [ trans-(2-ethoxycarbonyl)cyclopropyl]methyl radical were determined by competition kinetics employing benzeneselenol trapping.

Picosecond radical kinetics. Rate constants for reaction of benzeneselenol with primary alkyl radicals and calibration of the 6-cyano-5-hexenyl radical cyclization

The cyclopropylcarbinyl radical ring opening was used as a radical clock to determine rate constants for benzeneselenol trapping in THF and in toluene. Hydrogen atom transfer trapping from PhSeH appeared to be partially diffusion controlled. An operational Arrhenius function for trapping in THF is log (k T .M s)=11.03-2.27/2.3RT. The recommended function for PhScH trapping in other low-viscosity organic solvents is log (k T .M s)=10.87-2.10/2.3RT. The rate constant for trapping at 25 o C is 2.1×10 9 M -1 s -1

Substituent control of cyclopropylcarbinyl radical ring opening reactions

Substituent control of cyclopropylcarbinyl radical ring opening reactions