Medium Effects on the Rates of Stereomutation of a Pair of Diastereomeric Cyclopropanones. Ground State Stabilization in Nucleophilic Solvents Induces Deviation from Solvent Polarity Controlled Behavior

The quest for observation and isolation of oxyallyl derivatives

This review summarizes 50 years of attempts to observe, and finally isolate, the elusive oxyallyl derivatives.

Oxyallyl exposed: an open-shell singlet with picosecond lifetimes in solution but persistent in crystals of a cyclobutanedione precursor

Photoinduced decarbonylation of 2,4-bis(spirocyclohexyl)-1,3-cyclobutanedione 1 in the crystalline solid state resulted in formation of a deep blue transient with λ(max) = 550 nm and a half-life of 42 min at 298 K, identified as kinetically stabilized oxyallyl. Support for an open-shell singlet species was obtained by spectroscopic analysis and (4/4) CASSCF calculations with the 6-31+G(d) basis set and multireference MP2 corrections. The electronic spectrum of the singlet biradical, confirmed by femtosecond pump-probe studies in solution, was matched by coupled cluster calculations with single and double corrections.

First Direct Detection of 2,3-Dimethyl-2,3-diphenylcyclopropanone

3,5-dihydro-3,5-dialkyl-3,5-diaryl-4H-pyrazol-4-ones stimulate interest as potential precursors for 2,3-diarylcyclopropanones. Photoreactions of trans-3,5-dihydro-3,5-dimethyl-3,5-diphenyl-4H-pyrazol-4-one were studied by continuous-wave (CW) and pulsed laser UV photolysis revealing an intermediate that undergoes rearrangement to form cis- and trans-1,3-dimethyl-1-phenyl-2-indanones with the yield of ca. 60%. Steady-state photolysis (254 and 350 nm excitation) in different solvents produced an intermediate cyclohexadiene as evidenced by UV/vis, IR, and 1H NMR spectra. In contrast, the nanosecond laser pulsed photolysis at 355 nm produced 2,3-dimethyl-2,3-diphenylcyclopropanone along with two products of retro-1,3-dipolar addition phenylmethylketene and 1-phenyldiazoethane. These can be observed by time-resolved IR (TRIR) spectroscopy as characteristic absorption bands at 1814, 2101, and 2038 cm-1, respectively. Similar retro-1,3-dipolar addition showed 1-phenyldiazoethane formed following flash photolysis of 1-pyrazoline (trans-4,5-dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyrazol-4-ol). The formation of the corresponding cyclopropanone as well the products of retro-1,3-dipolar addition during photoreaction of starting pyrazol-4-one is directly confirmed by the nanosecond TRIR spectroscopy for the first time. On the basis of the CW and pulsed laser UV photolysis, a dynamic equilibrium between cyclopropanone and intermediate 2,4-diphenyl-3-pentanone-2,4-diyl (dimethyldiphenyloxyallyl) was proposed.