Theoretical Studies on the Photoinduced Rearrangement Mechanism of α‐Santonin

Abstract

α-Santonin is the first organic compound observed to feature a photoinduced rearrangement and is now known to undergo a series of photochemical processes under UV irradiation. On the basis of the considerable interest of this system as a prototype, and of the yet limited insights reached for the basic photo mechanisms, we calculate the high-level electronic structures and explore the potential energy surfaces (PES) of α-santonin in the ground and lowest-lying excited states, their couplings, and the possible photoinduced isomerization pathways. The calculations identify the low-lying singlet excited state (1)(nπ*) accessible under light irradiation, which decays to the low-energy (3)(ππ*) state through an intersystem crossing in the Franck-Condon region to initiate the photoinduced rearrangement. The initial reaction from the C3-C5 bond coupling, which takes place on the (3)(ππ*) state potential energy surface, leads to a three-membered alkyl-ring compound intermediate state INT. The following photochemical reactions have the possibility to arise from two distinct C-C bond cleavages, C4-C5 and C3-C4, denoted as path A and path B. Path A is favored both dynamically on the excited-state PES and thermodynamically on the ground-state PES in vacuo. Experiments show that it also becomes the dominant photoinduced rearrangement process in the crystal, which can be explained by considering the requirement for less space and the stacking effect under the confined environment. Path B is dynamical advantaged both on the ground- and excited-state PESs in a weak polar solvent, such as dioxane. Once the biradical intermediate B-INT is accessible on the ground-state PES, the formation of the product B-P is almost barrier free.

Divergent Photocyclization/1,4-Sigmatropic Rearrangements for the Synthesis of Sesquiterpenoid Derivatives

“…The 6methyl-and 6-cyano-substituents formed the respective derivatives of 16, containing adjacent chiral quaternary centers, in good yield using visible light (entries 16, 17). Upon switching the solvent to acetonitrile, 15 could be observed in the course of the irradiation with visible light however, the ratio of 15:16 decreased until only 16 remained after full conversion of starting material (entries [18][19][20]. Similar results are obtained using UV-A and UV-B requiring significantly shorter reaction times (entries 21-23) while UV-C was optimal for obtaining 15 (entries 24-26).…”

“…Recent MCSCF computations on the santonin-lumisantonin rearrangement show that intersystem crossing (ISC) to the singlet state can occur either at this stage, or following subsequent C-C cleavage. 19 The formation of Woodward-Hoffmann disallowed products 14 and 16 is inconsistent with singlet ground state reactivity (i.e. a Zimmerman type A zwitterion), and suggests ISC occurs after C-C cleavage.…”

“…In accordance with Zimmerman’s seminal experimental findings, C3–C5 bond formation in triplet states of cyclohexadienones 11 and 12 is facile (barriers of 3.6 and 6.1 kcal/mol, respectively). Recent MCSCF computations on the santonin–lumisantonin rearrangement show that intersystem crossing (ISC) to the singlet state can occur either at this stage or following subsequent C–C cleavage . The formation of Woodward–Hoffmann disallowed products 14 and 16 is inconsistent with singlet ground-state reactivity (i.e., a Zimmerman type A zwitterion) and suggests ISC occurs after C–C cleavage.…”

“…Recent MCSCF computations on the santonin−lumisantonin rearrangement show that intersystem crossing (ISC) to the singlet state can occur either at this stage or following subsequent C−C cleavage. 19 The formation of Woodward−Hoffmann disallowed products 14 and 16 is inconsistent with singlet ground-state reactivity (i.e., a Zimmerman type A zwitterion) and suggests ISC occurs after C−C cleavage. Therefore, assuming that cyclopropyl ring-opening takes place in the triplet state, we obtained competing transition structures (TSs) in which the cleavage of either C−C bond takes place to give the two product skeletons.…”

Synthesis of 4- Desmethyl -Rippertenol and 7- Epi -Rippertenol via Photoinduced Cyclization of Dienones

“…Moreover, these studies mainly focus on 2,5-dienones and the case for linear dienone is rare. [44][45][46][47] As shown in Table 1, αcyclopropyl dienone 9 was initially employed as the model substrate. Exposing this compound to UV light (365 nm) in either nonpolar (CCl 4 , benzene or toluene) or polar (MeCN or tert-butanol) solvents resulted in slow decomposition of the substrate and produced unidentified mixtures (Entries 1-5).…”