Triplet-sensitized Irradiation Research Articles

Substituent Effect on Reactivity of Triplet Excited State of 2,3-Diazabicyclo[2.2.1]hept-2-enes, DBH Derivatives: α C–N Bond Cleavage versus β C–C Bond Cleavage

The photoreaction of a series of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, in which various substituents (X) were introduced at the methano bridge carbon of C(7), was investigated under direct (>290 nm) and triplet-sensitized (Ph2CO, >370 nm) irradiation conditions of the azo chromophore (−Cβ–Cα–N═N–Cα–Cβ−). The azo compounds offered a unique opportunity to see the substituent X effect at the remote position of the azo chromophore on the reactivity of the triplet excited state of bicyclic azoalkanes. The present study led to the first observation of the unusual Cα–Cβ bond-cleavage reaction without the ring stiffness, that is, the structural rigidity, in the triplet state of the cyclic azoalkanes. The stereoelectronic effects were found to play an important role in lowering the activation energy of the Cα–Cβ bond-cleavage reaction. NBO analyses at the M06-2X/cc-PVDZ level of theory confirmed the stereoelectronic effect.

ChemInform Abstract: A Novel Photochemical Vinylcyclopropane Rearrangement Yielding 6,7-Dihydro-5H-benzocycloheptene Derivatives.

1-Substituted 1-(2,2-diphenylvinyl)cyclopropanes with electron-accepting groups at C1 undergo a novel rearrangement to benzocycloheptenes on triplet-sensitized irradiation. In some instances competition between rearrangement to cyclopentenes and formation of cycloheptenes takes place. When electron-acceptor groups are not present at C1, conventional ring opening to cyclopentenes occurs exclusively. A mechanism involving intramolecular SET from the diphenylvinyl unit to the electron-acceptor group is proposed for this novel photoreaction.

Remarkable Observations on Triplet-Sensitized Reactions. The Di-π-methane Rearrangement of Acyclic 1,4-Dienes in the Triplet Excited State

Previous studies have led to the conclusion that a large majority of acyclic 1,4-dienes do not undergo photochemical di-pi-methane (DPM) rearrangement under triplet-sensitized irradiation. The results of a detailed analysis of these processes demonstrate that a series of these compounds do indeed undergo highly efficient DPM rearrangement from their triplet excited states when suitable triplet sensitizers are used.

Studies on Cycloaddition of Cyclohexa-2,4-dienones and Transformation of Adducts: A General, Stereoselective Route to Multifunctional Bicyclo[2.2.2]octanes and Diquinanes

The cycloaddition of in situ generated cyclohexa-2,4-dienones with vinyl ethers, vinyl acetate, and phenyl vinyl sulfone leading to variously functionalized bicyclo[2.2.2]octanes has been examined. Functional group manipulation in the resulting adducts gave bicyclo[2.2.2]octanes endowed with a β,γ-enone chromophore. Triplet-sensitized irradiation of bicyclooctenones followed by reductive cleavage provided a stereoselective route to diquinane frameworks having diverse functionalities.

CASPT2//CASSCF Study on the Photolysis Mechanism of 2,3-Diazabicyclo[2.1.1]hex-2-ene: α C−N versus β C−C Cleavage

A CASPT2//CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.1.1]hex-2-ene under direct irradiation and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces and surface crossing points for several low-lying excited states, we have been able to provide a tentative description on the photophysical and photochemical processes of this compound. According to our calculations, on the S(1) surface one C-N bond is broken first to generate the diazenyl biradicals, which then result in the photoproduct bicyclobutane through a concerted C-N cleavage and C-C coupling process. The intersystem crossing (ISC) from the S(1) state to the triplet state was found to be quite efficient, which could account for two other photoproducts (butadiene and 1,2-diazabicyclo[3.1.0]hex-2-ene) on direct irradiation. On the T(1) surface, the C-C bond cleavage could readily take place to generate the triplet hydrazonyl biradical. One deactivation path of this intermediate is to convert into the corresponding singlet biradical by an ISC process. Then this biradical undergoes a barrierless C-N bond formation to form the photoproduct 1,2-diazabicyclo[3.1.0]hex-2-ene. Another path of the triplet hydrazonyl biradical is to form the intermediate 2-allyl-diazomethane through a C-N cleavage. This intermediate will break another C-N bond to produce a terminal carbene species, which is easily converted into butadiene and bicyclobutane by 1,2-hydrogen shift reaction and addition to C=C double bond, respectively.

Ring Contraction of Bridgehead Sultams by Photoinduced Di-π-methane Rearrangement

Triplet-sensitized irradiation of 8-thia-9-azatricyclo[7.2.1.0(2,7)]dodeca-2,4,6,10-tetraene (16) in acetone solution gives rise exclusively to tetracyclic sultam 20. This strong preference for benzo-vinyl bridging distal to the sulfonamide functional group has also been observed in eight derivatives carrying chemically diverse functional groups at C-10. In none of these examples is regiospecificity eroded. This overall result suggests that the added substituents act in harmony with the electronic rebonding pathway found operative in the parent system. From the structural perspective, this transformation constitutes a facile means for accomplishing the ring contraction of a bridgehead sultam.

Theoretical Study on the Photolysis Mechanism of 2,3-Diazabicyclo[2.2.2]oct-2-ene

A CASPT2/CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) under direct and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces including intermediates, transition states, conical intersections, and singlet/triplet crossing points, for the first excited singlet (S(1)) and the low-lying triplet states (T(1), T(2), and T(3)), we provide satisfactory explanations of many experimental findings associated with the photophysical and photochemical processes of DBO. A key finding of this work is the existence of a significantly twisted S(1) minimum, which can satisfactorily explain the envelope of the broad emission band of DBO. It is demonstrated that the S(1) (n-pi*) intermediate can decay to the T(1) (n-pi*) state by undergoing intersystem crossing (rather inefficient) to the T(2) (pi-pi*) state followed by internal conversion to the T(1) state. The high fluorescence yield and the extraordinarily long lifetime of the singlet excited DBO are due to the presence of relatively high barriers, both for intersystem crossing and for C-N cleavage. The short lifetime of the triplet DBO is caused by fast radiationless decay to the ground state.

Novel photoreactions of 2-aza-1,4-dienes in the triplet excited state and via radical-cation intermediates. 2-aza-di-pi-methane rearrangements yielding cyclopropylimines and N-vinylaziridines.

Triplet-sensitized irradiation of 2-aza-1,4-dienes affords N-cyclopropylimines via 2-aza-di-pi-methane (2-ADPM) rearrangement pathways. In the case of the pentaphenyl-substituted azadiene 1, irradiation leads to formation of cyclopropylimine 2 as well as N-vinylaziridine 3. The transformations represent the first examples of di-pi-methane rearrangement reactions that yield three-membered heterocyclic products. SET-sensitized irradiation of 2-aza-1,4-dienes, by using 9,10-dicyanoanthracene (DCA) as an electron-acceptor sensitizer and biphenyl as cosensitizer, brings about regioselective formation of N-vinylaziridines. Under these conditions, azadiene 1 also affords cyclopropylimine 37, resulting from an aryl-di-pi-methane rearrangement. This result demonstrates that di-pi-methane reactions can also take place via radical-cation intermediates. In some instances, imine and olefin centered cation-radical intermediates, generated by SET-sensitized irradiation, undergo alternative reactions to produce isoquinoline and benzoazepine products.

A novel photochemical vinylcyclopropane rearrangement yielding 6,7-dihydro-5H-benzocycloheptene derivatives

[reaction: see text] 1-Substituted 1-(2,2-diphenylvinyl)cyclopropanes with electron-accepting groups at C1 undergo a novel rearrangement to benzocycloheptenes on triplet-sensitized irradiation. In some instances competition between rearrangement to cyclopentenes and formation of cycloheptenes takes place. When electron-acceptor groups are not present at C1, conventional ring opening to cyclopentenes occurs exclusively. A mechanism involving intramolecular SET from the diphenylvinyl unit to the electron-acceptor group is proposed for this novel photoreaction.

Triplet-sensitized irradiation of a main-chain liquid crystalline poly(aryl cinnamate) in three different phases

Visible light irradiation of thin films of a main-chain liquid crystalline poly(aryl cinnamate) using ketocoumarins as triplet sensitizers leads to photochemical crosslinking and UV-vis and FTIR spectroscopic changes associated with saturation of the cinnamate double bond, most likely by 2 + 2 photocycloaddition. The triplet sensitizers are themselves photolabile and are lost by photochemical reactions during the sensitization process. A new ketocoumarin sensitizer with decyloxy substituents and a reduced tendency to phase separate from the polymer is reported. A simple calculation of the sensitization stoichiometry shows that a single molecule of this ketocoumarin sensitizes the destruction of approximately 90 cinnamate chromophores in the “as cast” films below Tg and about 300 chromophores in the more-ordered glassy nematic films and in “as cast” films of poly(vinyl cinnamate). Triplet sensitization of fluid nematic films leads, upon initial irradiation, to UV-vis hyperchromism that is attributed to disruption of chromophore aggregation and, possibly, to disruption of the nematic mesophase as photoproducts begin to form. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 134–144, 2001

Preparation and characterization of DNA containing a site-specific nonadjacent cyclobutane thymine dimer of the type implicated in UV-induced -1 frameshift mutagenesis.

One mechanism for the origin of UV-induced -1 deletion mutations involves the bypass of a nonadjacent cis-syn cyclobutane pyrimidine dimer containing a single intervening nucleotide. To begin to investigate this mechanism, we required a method for obtaining a single, site-specific, nonadjacent dimer. One approach to the preparation of a nonadjacent dimer is to irradiate a DNA duplex containing a centrally located TNT sequence in which the two T's are paired to an AA sequence in an otherwise fully complementary strand. Triplet-sensitized irradiation of the duplex formed between the 13-mer d(GAGTATCTATGAG) and the 12-mer d(CTCATAATACTC) on ice gave a major product that could be reverted to the parent 13-mer by 254 nm irradiation. Proton NMR experiments established the major product to be the nonadjacent cis-syn cyclobutane dimer formed between the two T's of the TCT sequence. Melting temperature studies show that the nonadjacent dimer is more destabilizing to DNA duplex structure than a normal cis-syn dimer and is as stable as the parental bulged DNA duplex. The nonadjacent dimer-containing 13-mer was ligated into a 51-mer and used as a template for primer-extension studies by DNA polymerases. The nonadjacent dimer could not be bypassed by Sequenase Version 2.0 and terminated synthesis primarily prior to and opposite the 3'-T of the dimer. In contrast, approximately 30% of the dimer was bypassed by an exonuclease-deficient (exo-) Klenow fragment, and termination occurred primarily opposite the 3'- and 5'-T's of the dimer. Bypass of the nonadjacent dimer by exo(-) Klenow fragment led primarily to a single-nucleotide deletion mutation as well as small amounts of a full-length product and a four-nucleotide deletion that could be explained by a primer misalignment mechanism.

Unexpected Oxadi-π-methane Rearrangement of β,γ-Unsaturated Aldehydes

The oxadi-π-methane rearrangement (ODPM) is considered to represent the normal photochemical behavior of β,γ-unsaturated ketones in the triplet excited π,π* state. However, the usual photoreactivity reported for the majority of β,γ-unsaturated aldehydes is decarbonylation. There are only two published reports of β,γ-unsaturated aldehydes that undergo the ODPM rearrangement. We now report efficient ODPM rearrangement in the triplet-sensitized irradiation of twelve cyclic and acyclic β,γ-unsaturated aldehydes, namely, 2,2-dimethyl-4,4-diphenyl-3-butenal (6), 1-methyl-3-phenyl-2-cyclohexene-1-carbaldehyde (7), 1-methyl-3-phenyl-2-cyclopentene-1-carbaldehyde (14a), 1-methyl-3-phenyl-2-cycloheptene-1-carbaldehyde (14b), 2,2-dimethyl-4-phenyl-3-butenal (18), 2-(3,4-dihydro-2-naphthyl)-2-methylpropanal (23), 3-(9-fluorenylidene)-2,2-dimethylpropanal (24), 5-cyclopentylidene-2,2-dimethyl-3-pentenal (27), 2,2,6-trimethyl-3,5-heptadienal (28), 2,2,4,4-tetraphenyl-3-butenal (35), 2-methyl-4,4-diphenyl-2-vinyl-3-bute...

In vivo evidence that UV-induced C .fwdarw. T mutations at dipyrimidine sites could result from the replicative bypass of cis-syn cyclobutane dimers or their deamination products

The major mutations induced by UV light are C-->T transitions at dipyrimidines and arise from the incorporation of A opposite the C of dipyrimidine photoproducts. The incorporation of A has most often been explained by the known preference of a polymerase to do so opposite noninstructional DNA damage such as an abasic site (A rule). There are also mechanisms that suppose, however, that cis-syn dipyrimidine photodimers are instructional. In one such mechanism (tautomer bypass), the incorporation of A is directed by the tautomer of a C of a dimer that is equivalent in base-pairing properties to U [Person et al. (1974) Genetics 78, 1035-1049]. In another mechanism (deamination bypass), the incorporation of A is directed by a U of a dimer that results from the deamination of the C of a dimer [Taylor & O'Day (1990) Biochemistry 29, 1624-1632]. The viability of these mechanisms was tested by obtaining the mutation spectrum of a TU dimer in Escherichia coli by application of a standard method for site-directed mutagenesis. To this end, a 41-mer containing a site-specific TU dimer was constructed via ligation of a dimer-containing decamer that was produced by triplet-sensitized irradiation and used to prime DNA synthesis on a uracil-containing (+) strand of an M13 clone containing a double mismatch opposite the dimer. The reaction mixture was used to transfect a uracil glycosylase proficient, photoproduct repair deficient E. coli host, and all progeny phage weakly hybridizing to the parental (+) or (-) strands were sequenced. Under non-SOS conditions the TU dimer almost completely blocked replication, while under SOS conditions it directed the incorporation of two As with much higher specificity (96%) than would an abasic site. The implications of these results to the mechanism of the UV-induced TC-->TT mutation, and by extension to the CT-->TT, CC-->TC, CC-->CT, and the tandem CC-->TT mutations, are discussed.

ChemInform Abstract: PHOTOCHEMISTRY OF DIENONES. PART 12. PHOTOCHEMISTRY OF (E)‐α‐ AND (E)‐β‐DAMASCONE

AbstractDie Bestrahlung von (Era‐Damascon (I) mit 254, 300 oder 350 nm in verschiedenen Lösungsmitteln führt zu den Produkten (Il)‐( IV), wobei bei verlängerter Bestrahlung (IIIa) und (IIIb) Endprodukte sind.

Arylcyclopropane photochemistry. Part 4. The photochemistry of some 2-arylcyclopropanecarboxylates

The photochemistry of some 2-arylcyclopropanecarboxylates has been studied. Both the direct (λ 254 nm) and triplet-sensitized (λ 313 nm) irradiation of the trans-isomers yields only the corresponding cis-isomers. The direct irradiation of methyl cis-2-phenylcyclopropanecarboxylate and of cis-2-phenylcyclopropanecarboxylic acid yields products which are formed via 1,3-dipolar intermediates. The difference in photoreactivity between these cis-isomers and their trans-isomers is explained in terms of intramolecular exciplex formation with the cis-isomers which cannot occur with the trans-isomers. The difference in photoreactivity between the cis-compounds without a methoxy-group (2a,b) and those with a methoxy-group attached to the phenyl nucleus (2c,d) is explained in terms of the occurrence of a chemical reaction of the intramolecular exciplex of (2a,b) and the non-occurrence of any chemical reaction of the intramolecular exciplex of (2c,d). The intramolecular exciplex formation with the methoxy-substituted cis-compounds (2c,d) only leads to an increased ϕisc of these cis-compounds relative to that of the corresponding trans-isomers (1c,d).

Photochemical cleavage of the cyclopropane ring of 6,20-epoxylathyrol {1,11-diacetoxy-3,6,6,14-tetramethyl-13-phenylacetoxy(tricyclo[10.3.0.0]pentadec-3-ene-10-spiro-2′-oxiran)-2-one}

The products of direct and triplet-sensitized irradiation of 6,20-epoxylathyrol (1a) and the corresponding parent alcohol (1b) have been investigated. The major product of direct irradiation is a furan derivative, arising from cis-trans-isomerisation of the 11,12-double bond followed by cleavage at the cyclopropane ring and insertion of the resulting carbene into the carbonyl group. The nature of the electronic species involved in this reaction scheme is discussed.