In:
Helvetica Chimica Acta, Wiley, Vol. 57, No. 6 ( 1974-01), p. 1815-1845
Abstract:
When α,β‐unsaturated γ‐dimethoxymethyl cyclohexenones are excited to the S 2 (π,π*) state, certain unimolecular reactions can be observed to compete with S 2 → S 1 internal conversion. These reactions do not occur from the S 1 ( n ,π*) or the lowest T(π,π* and n ,π*) states. They comprise the radical elimination of the formylacetal substituent ( cf . 8 , 9 → 32 + 33 ), γ → α formylacetal migration ( cf . 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position ( cf . 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05 M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*‐induced transformations is placed energetically higher than the S 1 ( n ,π*) state, and it is either identical with the thermally equilibrated S 2 ( n ,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*‐induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross‐conjugated cyclohexadienones.
Type of Medium:
Online Resource
ISSN:
0018-019X
,
1522-2675
DOI:
10.1002/hlca.19740570631
Language:
English
Publisher:
Wiley
Publication Date:
1974
detail.hit.zdb_id:
74-7
detail.hit.zdb_id:
1475013-2
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