Substituent effect on the energy barrier for ?-bond formation from ?-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls.

ABSTRACT:

Background

Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bond-cleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)-C(3) positions on the lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet diradicals.

Results

A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitrogenation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltricyclo[3.3.0.0(2,4)]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/?) was observed for the fate of the singlet diradicals DR (?max ? 580-590 nm). The activation parameters, ?H (‡) and ?S (‡), for the ring-closing reaction (?-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar'. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH2Ar' = OCH2(3,5-dimethoxyphenyl)) was the longest-lived, ?293 = 5394 ± 59 ns, among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH2Ar' = OCH3) was 299 ± 2 ns at 293 K.

Conclusion

The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar' at the C(1)-C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime.

SUBMITTER: Ye J 

PROVIDER: S-EPMC3678856 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature