Notable effect of an electron-withdrawing group at C3 on the selective formation of alkylidenecyclobutanes in the thermal denitrogenation of 4-spirocyclopropane-1-pyrazolines. Nonstatistical dynamics effects in the denitrogenation reactions.

A detailed study of the thermal denitrogenation of 3-carbomethoxy-substituted 4-spirocyclopropane-1-pyrazolines 6 was conducted. Alkylidenecyclobutane derivatives 7 were selectively formed in a stereospecific manner. Unrestricted density functional calculations for a 1-pyrazoline 10a indicated that the concerted cleavage of two C-N bonds is the energetically favored process for the denitrogenation reaction to give the 2-spirocyclopropyl 1,3-diyl, followed by a conrotatory ring-closure process, which was calculated to be the energy minimum pathway, to afford a spiropentane derivative.

A matrix isolation study of 2-isopropylidenecyclopentane-1,3-diyl (Berson-type diradical).

The photodenitrogenation of diazene 5 in an argon matrix at 10 K permitted the first observation of an IR spectrum of 2-isopropylidenecyclopentane-1,3-diyl 2 (a Berson-type diradical). A comparison of the IR spectrum with a vibrational simulation at the B3LYP/6-31G(d) level of theory revealed that the diradical 2 has a planar structure. The oxygen-trapping reaction of 2 produced regioselectively fused peroxide 6 in an oxygen-doped argon matrix at 10 K.

Experimental probe for hyperconjugative resonance contribution in stabilizing the singlet state of 2,2-dialkoxy-1,3-diyls: Regioselective 1,2-oxygen migration.

A detailed study of the regioselectivity of 1,2-oxygen migration was conducted using the unsymmetrically substituted singlet 2,2-dialkoxy-1,3-diarylcyclopentane-1,3-diyls 5. The alkoxy group selectively migrates to the electron-donating p-methoxyphenyl-substituted carbon. The regioselective migration of oxygen clearly indicates a hyperconjugative resonance structure, that is, zwitterionic characteristics, in singlet 2,2-dialkoxy-1,3-diyls.

Novel substituent effects on the mechanism of the thermal denitrogenation of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives, stepwise versus concerted.

The substituent effect on the thermal denitrogenation mechanism of 7,7-disubstituted 2,3-diazabicyclo[2.2.1]hept-2-enes, concerted versus stepwise, has been investigated in detail.

Theoretical calculations of the effects of 2-heavier group 14 element and substituents on the singlet-triplet energy gap in cyclopentane-1,3-diyls and computational prediction of the reactivity of singlet 2-silacyclopentane-1,3-diyls.

UDFT and CASSCF calculations with the 6-31G(d) basis set were performed to investigate the heavier group 14 element (M) effect on the ground-state spin multiplicity of cyclopentane-1,3-diyls and their reactivity. The calculations find that 2-metallacyclopentane-1,3-diyls (M = Si, Ge) that possess a variety of substituents (X = H, Me, F, OR, SiH(3)) at M(2) are singlet ground-state molecules. The energies of the 1,3-diphenyl-substituted singlet 2-silacyclopentane-1,3-diyls are calculated to be ca.

2-Silyl group effect on the reactivity of cyclopentane-1,3-diyls. Intramolecular ring-closure versus silyl migration.

Generation of singlet and triplet 2-silylcyclopentane-1,3-diyls and their reactivity have been investigated in the thermal and photochemical denitrogenation of 2,3-diaza-7-silylbicyclo[2.2.1]hept-2-ene.

Mechanism of stereo- and regioselectivity in the Paternò-Büchi reaction of furan derivatives with aromatic carbonyl compounds: importance of the conformational distribution in the intermediary triplet 1,4-diradicals.

Temperature and substituent effects on the stereo- and regioselectivity have been investigated in the photochemical [2 + 2] cycloaddition reaction, the so-called Paternò-Büchi (PB) reaction, of unsymmetrically substituted furans 2a,b (2-methyl- and 3-methylfuran) with aromatic carbonyl compounds 1a,b (benzaldehyde and benzophenone). The regio-random but stereoselective (exo/endo > 97/3) formation of lower substituted oxetane 3a and higher substituted oxetane 4a is found in the reaction with benzaldehyde (1a). The exclusive stereoselectivity is not dependent on the position of methyl substituent on the furan ring and the reaction temperature.

Effects of spiroconjugation on the calculated singlet-triplet energy gap in 2,2-dialkoxycyclopentane-1,3-diyls and on the experimental electronic absorption spectra of singlet 1,3-diphenyl derivatives. Assignment of the lowest-energy electronic transition of singlet cyclopentane-1,3-diyls.

The effect of a 2,2-ethylene-ketal functionality on the singlet-triplet energy gap (Delta E(ST)) and on the first electronic transition in singlet cyclopentane-1,3-diyls (1) has been investigated. UDFT calculations predict a significant increase in the preference for a singlet ground state in the diradical with the cyclic ketal at C2 (1g; Delta E(ST) = -6.6 kcal/mol in C(2) symmetry and -7.

Mechanism of an alcohol-trapping reaction in the direct and benzophenone-sensitized photodenitrogenation of a spiroepoxy-substituted azoalkane: solvent effects and stereochemical deuterium labeling.

The spin-state-dependent reactivity, singlet versus triplet, of the 2-spiroepoxy-1,3-cyclopentane-1,3-diyl DR2 has been assessed through alcohol-trapping reactions for which the effect of solvent acidity on the product distribution of the alcohol trapping products 2 versus 3 + 4 and stereochemical deuterium-labeling studies have been performed. The proposed mechanism for the solvent effect on the product ratio (2/3 + 4) reveals the importance of the hydrogen-bonded intermediates I1 and I2 in the trapping reactions; the stereochemical deuterium-labeling results clarify the dipole structure trapped by the alcohol. The dipoles DP1 and DP2, in which the configuration between the epoxide oxygen and the deuterium atoms is retained, are inferred for the direct photodenitrogenation reactions (singlet state), whereas for the benzophenone-sensitized photoreactions (triplet state), after ISC, the ring-opened dipole DP3 is implied as the intermediate that is trapped by the alcohol.

DFT prediction of ground-state spin multiplicity of cyclobutane-1,3-diyls: notable effects of two sets of through-bond interactions.

DFT calculations (UB3LYP/6-31+G**) have been performed to predict the substituent effect on the ground-state spin-multiplicity and the singlet-triplet energy gap in cyclobutane-1,3-diyls, CB-DR. The ground state is calculated to be largely dependent on the substituents (X, Y) at the C2 and C4 positions. The substituent effects can be reasonably explained by the two sets of through-bond (TB) interactions which result from the coupling between the symmetric nonbonding molecular orbital (Psi(S)) and the C-X (Y) sigma and sigma* orbitals.