title: Theoretical and experimental studies on the mechanism of norbornadiene Pauson-Khand cycloadducts photorearrangement. Is there a pathway on the excited singlet potential energy surface?
abstract: The intermolecular Pauson-Khand reaction (PKR), a carbonylative cycloaddition between an alkyne and an alkene, is a convenient method to prepare cyclopentenones. Using norbornadiene as alkene, a myriad of tricyclo[5.2.1.0(2,6)]deca-4,8-dien-3-ones 1 can be easily prepared. The mechanism of the photochemical rearrangement of these adducts 1 into tricyclo[5.2.1.0(2,6)]deca-3,8-dien-10-ones 2 has been studied. The ground state (S(0)) and the three lowest excited states ((3)(pi pi*), (1)(n pi*), and (3)(n pi*)) potential energy surfaces (PESs) concerning the prototypical rearrangement of 1a (the cycloadduct of the PK carbonylative cycloaddition of norbornadiene and ethyne) to 2a have been thoroughly explored by means of CASSCF and CASPT2 calculations. From this study, two possible nonadiabatic pathways for the photochemical rearrangement arise: one starting on the (3)(pi pi*) PES and the other on the (1)(n pi*) PES. Both involve initial C-C gamma-bond cleavage of the enone, which leads to the formation of a bis-allyl or an allyl-butadienyloxyl diradical, respectively, that then decays to the S(0) PES through a (3)(pi pi*)/S(0) surface crossing or a (1)(n pi*)/S(0) conical intersection, each one lying in the vicinity of the corresponding diradical minimum. Once on the S(0) PES, the ring-closure to 2a occurs with virtually no energy barrier. The viability of both pathways was experimentally studied by means of triplet sensitization and quenching studies on the photorearrangement of the substituted Pauson-Khand cycloadduct 1b (R = TMS, R' = H) to 2b. Using high concentrations of either piperylene as a triplet quencher, or benzophenone as a triplet sensitizer, the reaction rate significantly slowed down. A Stern-Volmer type plot of product 2b concentration vs triplet quencher concentration showed an excellent linear correlation, thus indicating that only one excited state is involved in the photorearrangement. We conclude that, though there is a nonadiabatic pathway starting on the (1)(n pi*) PES, the reaction product is formed through the (3)(pi pi*) state because the energy barrier involved in the initial C-C gamma-bond cleavage of the enone is much lower in the (3)(pi pi*) PES than in the (1)(n pi*) PES.
authors.affiliation: Institut d'Investigacions Quimiques i Ambientals de Barcelona, CSIC, Jordi Girona, 18-26, 08034-Barcelona, Catalonia, Spain. sonqtc@cid.csic.es
authors.forename: Santiago
authors.initials: S
authors.lastname: Olivella
authors.forename: Albert
authors.initials: A
authors.lastname: Solé
authors.forename: Agustí
authors.initials: A
authors.lastname: Lledó
authors.forename: Yining
authors.initials: Y
authors.lastname: Ji
authors.forename: Xavier
authors.initials: X
authors.lastname: Verdaguer
authors.forename: Rafael
authors.initials: R
authors.lastname: Suau
authors.forename: Antoni
authors.initials: A
authors.lastname: Riera
mesh_terms: D018360:Crystallography, X-Ray; D003500:Cyclization; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009636:Norbornanes / Q000737:chemistry*; D010777:Photochemistry
chemical_list: D005609:Free Radicals; D009636:Norbornanes