[reaction: see text] Both a Co(III)-alkyl complex and a Co(III)-alkylperoxo complex were found to catalyze triethylsilylperoxidation of alkenes with O(2) and Et(3)SiH. On this basis, together with the nonstereoselectivity in the Co(II)-catalyzed peroxidation of 3-phenylindene and the formation of the corresponding 1,2-dioxolane from 2-phenyl-1-vinylcyclopropane (a radical clock), we propose a reasonable mechanism for the Co(II)-catalyzed novel autoxidation of alkenes with Et(3)SiH discovered by Isayama and Mukaiyama.
Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.
CsOH- or Ag(2)O-mediated cycloalkylation of (alkylidene)bisperoxides 3 and 1,n-dihaloalkanes (n = 3-8) provided the corresponding medium-sized 1,2,4,5-tetraoxacycloalkanes 4-8 in moderate yields. Subsequent evaluation of the antimalarial activity of the cyclic peroxides 4-8 in vitro and in vivo revealed that 1,2,6,7-tetraoxaspiro[7.11]nonadecane 4a has considerable potential as a new, inexpensive, and potent antimalarial drug.
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 -7.6 kcal/mol in C(2v) symmetry), compared to the 2,2-dihydroxy- and 2,2-dimethoxy-disubstituted diradicals (1d, Delta E(ST) = -3.6 kcal/mol in C(2) symmetry, and 1e, Delta E(ST) = -3.4 kcal/mol in C(2) symmetry). Spiroconjugation is shown to be responsible for the larger calculated value of absolute value Delta E(ST) in 1g, relative to 1d and 1e. A strong correlation between the calculated values of Delta E(ST) and the computed electronic excitation energies of the singlet diradicals is found for diradicals 1d, 1e, and 1g and for 2,2-difluorocyclopentane-1,3-diyl (1c). A similar correlation between Delta E(ST) and lambda(calcd) is predicted for the corresponding 1,3-diphenylcyclopentane-1,3-diyls 3, and the predicted blue shift in the spectrum of 3g, relative to 3e, has been confirmed by experimental comparisons of the electronic absorption spectra of the annelated derivatives 2c, 2e, and 2g in a glass at 77 K. The wavelength of the first absorption band in the singlet diradicals decreases in the order 2e (lambda(onset) = 650 nm) > 2g (lambda(onset) = 590 nm) > 2c (lambda(onset) = 580 nm). The combination of these computational and experimental results provides a sound basis for reassignment of the first electronic absorption band in singlet diradicals 2c, 2e, and 2g to the excitation of an electron from the HOMO to the LUMO of these 2,2-disubstituted derivatives of cyclopentane-1,3-diyl.
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