Paul Dowd

Novel free radical ring-expansion reactions

Abstract A novel free radical initiated ring expansion of haloalkyl β-keto esters is described. Following alkylation of the β-keto ester with the appropriate dihalide, the resulting halide is treated at reflux with tri-n-butyltin hydride. Rearrangement to the homologated γ-keto ester occurs smoothly. An oxy radical intermediate is proposed for the reaction.

On the mechanism of the intramolecular samarium Barbier reaction. Probes for formation of radical and organosamarium intermediates

Abstract A new type of mechanistic probe for the intramolecular samarium Barbier reaction has been designed, and two different probe substrates have been investigated in detail. Remarkably, no unambiguous evidence could be obtained in favor of any of the obvious intermediates (free alkyl or alkoxy radicals, ketyls, organosamarium species) that are postulated for this reaction. Several possibilities for modified mechanisms are suggested.

Free radical ring-expansion leading to novel six- and seven-membered heterocycles

Abstract Free radical promoted ring-expansion of nitrogen-, oxygen- and sulfur-containing heterocyclic β-keto esters is described. Treatment of the derived phenylselenomethyl derivatives with tri-n-butyltin hydride leasds to smooth one-carbon ring expansion.

Trimethylenemethane: Activation energy for ring-closure of the diradical

Abstract The energy of activation for ring-closure of ground state triplet trimethylenemethane (I) to methylenecyclopropane has been measured by following the rate of dissappearance of the electron spin resonance spectrum over the temperature range –155° to –140°, in a series of frozen solid matrices. The experiments described make use of 3-methylenecyclobutanone and methylenecyclopropane as precursors to trimethylene-methane. Kinetic results obtained starting from methylenecyclopropane were most satisfactory and lead to an energy of activation for ring-closure of 7 kcal/mole. This value is significantly smaller than the aprpox. 20 kcal/mole barrier estimated on the basis of theoretical models. Truncation of the barrier by a tunnelling mechanism is made unlikely by the finding that trimethylenemethane-d6(I-d6) undergoes ring-closure with the same 7 kcal/mole energy of activation as the parent I.

Convenient Preparation of Sodium and Potassium Phenyl Selenides. Reduction of the Selenium-Selenium Bond by Sodium Hydride and Potassium Hydride

Abstract Sodium and potassium phenyl selenides are conveniently prepared by brief reflux of diphenyl diselenide with sodium hydride and potassium hydride, respectively.

Unusual cyclopropane formation following free radical ring expansion

Abstract Free radical ring expansion of cyclobutanones by a propenyl radical leads to β, γ-cyclopropyl ketanes.

Transition structures of the ene reactions of cyclopropene.

The transition structures for the ene reactions of cyclopropene with ethylene, propene, and cyclopropene have been located with ab initio molecular orbital calculations and the 6-31G* basis set and by DFT calculations with the Becke3LYP functional and the 6-31G* basis set. Several of the transition structures have also been located with CASSCF calculations. Energies of all stationary points were also evaluated with second-order Møller-Plesset theory using the RHF/6-31G* optimized geometry. The geometries of each transition structure and the energetics of each reaction are discussed and compared to the ene reaction of propene with ethylene. Calculations show that the cyclopropene ene reactions have much lower activation barriers than the propene-ethylene ene reaction, in agreement with experimental results. The transition structures have varying degrees of asynchronicity. The stabilities of the possible radical intermediates for each reaction are reflected in the geometries of the transition structures. The relief of strain in a cyclopropene, when acting as the enophile, accounts for the energetic differences in these reactions. The endo transition structure for the dimerization is lower in energy than the exo transition structure by 2.7 kcal/mol at the Becke3LYP/6-31G* + ZPE level of theory. Secondary orbital overlap of a CH bond of the enophile with the π-system at the central carbon of the ene is proposed to account for the preference for the endo transition structure. Barely stable diradical intermediates have been found for both endo and exo cyclopropene dimerization reactions, but it is likely that they are artifacts of the current level of theory.

INTRAMOLECULAR 2+2 CYCLOADDITION AND SEQUENTIAL RING EXPANSION

Abstract Free radical ring expansion of fused-cyclobutanones gives bicyclic ketones expanded by three and four carbons, with the carbonyl group β to the ring junction. Four-carbon ring expansion shows stereoselectivity favoring the trans-fused product 6-trans . The radical precursor cyclobutanones are readily prepared by intramolecular [2 + 2] cycloaddition of ketenes or keteniminium salts to olefins.

Homologation of large rings

Abstract Free radical-promoted, one-carbon, ring expansion of twelve, fourteen-, and fifteen-membered cyclic β-keto esters is described. The method is then extended to include a three-carbon ring expansion of cyclododecanone, the targets being (±)-muscone and naturally occurring (R)-(−)muscone. Free radical promoted one-carbon ring expansion of twelve-, fourteen- and fifteen-membered cyclic β-keto esters is described. The method is then extended to include a three-carbon ring expansion of cyclododecanone, the targets being (±)-muscone and naturally occuring (R)-(−)muscone.

Synchronized distortions of bicyclobutanes

Abstract The linear relationship between flap angle, θ, and bridging bond length, r 13 , in bicyclobutanes, found empirically by Irngartinger through x-ray analysis, is reproduced by ab initio STO-3G calculations. Additional distortions of the bicyclobutane geometry which are synchronized with flap angle variations are predicted.