Chemistry of exciplexes. IV. Orientation of photocycloaddition of 1,3-dienes to anthracenes

Abstract

Sir:The conservation of orbital symmetry plays an important role in the orientation of cycloadditions between unsaturated systems. The Woodward-Hoffmann rule of conservation of orbital symmetry predicts that suprafacial photocycloadditions involving 4n x-electrons are allowed processes while those involving 4n + 2 x-electrons are forbidden.' Anthracene reacts photochemically with either 2,5-dimethyl-2,4-hexadiene (DMHD), an acyclic s-transoid 1,3diene, or 1,3-cyclohexadiene (CHD), an alicyclic s-cisoid 1,3-diene, to give 4x, + 4 xS adducts l a and 2a, respectively, while 9-cyanoanthracene under similar conditions reacts with CHD to give the 4x, + 4irs adduct 2e but with acyclic s-transoid 1,3-dienes to give 4xs + 2?r, adducts such as 3e.2.3 The photocycloadditions of DMHD and other s-transoid 1,3-dienes to 9-cyanoanthracene constitute an "apparent forbidden" process according to the Woodward-Hoffmann rule, yet they proceed stereospecifically and regiospecifically in both high chemical and quantum yield^.^ Several explanations have been proposed for the orientation of these photocycl~additions,~-~ but no systematic correlations have been presented so far. In our investigation on the nature and behavior of [arene*: 1,3-diene] exciplexes, we studied simultaneously the exciplex fluorescence6 and photochemistry of substituted anthracenes with 1,3-dienes. In this communication, we wish to report the relationship between the nature of exciplex and the orientation of photocycloaddition in arene: 1,3-diene systems.The photocycloadditions were usually carried out in 0.5% solutions of the arene in reagent grade benzene containing 1 M of the diene unless otherwise specified. The light source used was a Hanovia 450-W medium pressure Hg lamp with a cylindrical uranyl glass filter to cut off the light emission below 330 nm from the Hg lamp. The progress of the reaction was monitored by the uv spectrum of the irradiation mixture, and the products were isolated by column chromatography on activated alumina. The photochemical additions of DMHD and C H D to 9,lO-difluoroanthracene (DFA), 9,lO-dichloroanthracene (DCA), 9,10-dibromoanthracene (DBA), 9,10-dicyanoanthracene (DCNA), and acridine were investigated. The quantum yields of the dis-appearance of various arenes in the presence of 1,3-dienes were determined with degassed samples and a conventional merry-go-round apparatus at 365 nm and ambient temperature (21 fThe results obtained are summarized in Table I together with the available spectroscopic data of the corresponding exciplexes and compared with those from published results of anthracene, 9-cyanoanthracene (CNA), and naphthalene.2.3-s Since all the reactions in this study except that of DFA were carried out under conditions such that 87.5-99+% of the anthracene fluorescence was quenched by the diene, i.e., most of the singlet excited anthracene was intercepted by the diene as the exciplex, most of the photocycloaddition must proceed via exciplex as the intermediate.Judging from the results in Table I, we have found that ...

The Role of Biradicaloid Geometries in Organic Photochemistry

“…Stereochemical information could easily be lost in such a non-concerted process. The 4 + 2 photocycloaddition to anthracenes (Yang et at., 1975) is presumably of this type, but no detailed theoretical investigations appear to have been performed. rationalized with confidence.…”

Supramolecular Photocatalysis by Utilizing the Host–Guest Charge‐Transfer Interaction: Visible‐Light‐Induced Generation of Triplet Anthracenes for [4+2] Cycloaddition Reactions

“…This result suggests that while the direct excitation using UV light generated a species which favored [4+4] cycloaddition pathway, a different excited species of anthracene which favored [4+2] cycloaddition pathway was selectively generated in the presence of [2+2] BTH‐F . It is assumed that the former is the singlet state and the latter is the triplet state of anthracene.…”

Multireference exciplex binding energies: Basis set convergence and error

“…The other two complexes represent exciplexes that remain theoretically and experimentally uncharacterized: the cis ‐butadiene‐benzene exciplex (BdBz) * and the benzene‐naphthalene exciplex (BzNa) * . The cis ‐butadiene‐naphthalene exciplex has been examined using Hückel theory, and it has been suggested that the cis ‐butadiene‐anthracene exciplex represents an intermediate in the photocycloaddition involving the two molecules . Nonetheless, (BdBz) * , the smallest diene‐aromatic exciplex, has not been investigated in any work to date.…”

“…The cis-butadiene-naphthalene exciplex has been examined using Hückel theory, [38] and it has been suggested that the cis-butadiene-anthracene exciplex represents an intermediate in the photocycloaddition involving the two molecules. [39] Nonetheless, (BdBz) * , the smallest diene-aromatic exciplex, has not been investigated in any work to date. Similarly, while the benzene and naphthalene excimers represent important benchmark systems for excited state electronic structure methods, the spectroscopic parameters for (BzNa) * and the degree of excited state stabilization remain unknown.…”