Synthesis and photochemistry of 3,5-dialkyl-3,5-dihydro-3,5-diphenyl-4H-pyrazol-4-ones

Oxyallyls have been proposed as reactive intermediates in a number of classical organic reactions. Nevertheless, the nature of the electronic ground state of oxyallyls (singlet or triplet state) remains unclear. In this connection, the photochemistry of trans-3,5-dihydro-3,5-dimethyl 3,5-diphenyl-4 H-pyrazol-4-one (trans-1a) as a source of 2,4-diphenyl-3-pentanone-2,4-diyl (1,3-dimethyl-1,3-diphenyloxyallyl, DMDPOXA) was studied in both steady state and transient experiments.; According to the photophysical data, trans-1a has the lowest excited triplet state. One pathway of its decomposition is the expulsion of nitrogen forming the triplet DMDPOXA ( T-DMDPOXA). T-DMDPOXA was observed in the femtosecond TR UV-VIS spectra of trans-1a and its lifetime estimated. Subsequent ISC in the T-DMDPOXA leads to the singlet DMDPOXA with the zwitterionic nature (S-DMDPOXA). The zwitterionic S-DMDPOXA was observed in the femtosecond TR UV-VIS spectra of trans-1a in carbon tetrachloride-methanol. The lifetime of the zwitterionic oxyallyl (S-DMDPOXA) was estimated for the first time. Disrotatory ring closure of the S-DMDPOXA leads to cis and trans-2,3-dimethyl-2,3-diphenylcyclopropanone (cis and trans-14). The formation of cis and trans-14, phenyldiazoethane (13) and phenylmethylketene (7) was confirmed by the nanosecond TRIR spectroscopy during the photodecomposition of trans-1a. Disrotatory ring opening of cis and trans-14 with subsequent controtary electrocyclic ring closure forms 8. The hydrogen in 8 leads to 9, which then forms cis and trans-1,3-dimethyl-1-phenyl-2-indanone (cis and trans-10) by the keto-enol tautomerisation.; The products of the retro-1,3-dipolar cycloaddition of the excited triplet state of trans-1a (another pathway of decomposition): 7 and 13 were observed in the nanosecond TRIR spectra. Similar fragmentation was observed for trans-4,5-dihydro-3,5-dimethyl-3,5-diphenyl-3 H-pyrazol-4-ol (trans-2a) forming 13 in the TRIR spectra.