Intramolecular Photocycloaddition Reactions Research Articles

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

AbstractWhile 2‐alk‐ω‐enyloxy‐sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen‐9‐one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44–99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3‐position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio‐ and diastereoselectivity (2 examples, 58–79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò–Büchi reaction (2 examples, 95–98 % yield).

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions.

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio- and diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

Keeping the name clean: [2 + 2] photocycloaddition.

Crossed [2 + 2] photocycloaddition is a specific case of intramolecular photocycloaddition reaction. Recently, the term "crossed [2 + 2] photocycloaddition" is interchangeably used to represent intermolecular [2 + 2] photocycloaddition reactions of two dissimilar double bonds/alkenes. To avoid confusion and to help researchers use the correct terminologies, this perspective clarifies the terminology used for different [2 + 2] photocycloaddition processes based on prior literature with the hope of establishing a standard for addressing the diverse set of photocycloaddition reactions thatwill be helpful to the chemical community.

Photocycloadditions of Arenes Derived from Lignin.

Intramolecular photocycloaddition reactions of 3,4-alkoxybenzonitriles derived from vanillin with alkenes have been investigated. In contrast to previous reports on photochemical reactions with these compounds, mainly [2 + 3] cycloaddition has been observed. A competing [2 + 2] photocycloaddition plays a minor role. Most probably, these additions occur at the singlet state S1. In the case of a triplet reaction, a different regioselectivity of the [2 + 2] cycloaddition would be observed. Linear and angular [2 + 3] cycloadducts are formed as major products. The first isomer is transformed in the second one by a photochemical vinyl-cyclopropane rearrangement, which increases the selectivity of the reaction. The influence of the substitution pattern on the reactivity and the selectivity has also been investigated.

Dicopper(II) metallacyclophanes with photoswitchable oligoacene spacers: a joint experimental and computational study on molecular magnetic photoswitches

Dinuclear copper(II) complexes of the metallacyclophane-type, (nBu4N)4[Cu2(2,6-anba)2] (1) and (nBu4N)4[Cu2(1,5-naba)2]·4H2O (2) with photoactive 2,6-anthracene-(2,6-anba) and 1,5-naphthalenebis(oxamate) (1,5-naba) bridging ligands, are reported. They undergo a thermally reversible, solid-state photomagnetic (ON/OFF) switching between the moderately strong antiferromagnetically coupled dicopper(II) species and the corresponding magnetically uncoupled [4+4] photocycloaddition product. Density functional calculations give further insights on the intramolecular (“pseudo-bimolecular”) photocycloaddition reaction of the two facing 2,6-anthracene or 1,5-naphthalene spacers in this novel family of dicopper(II) oligoacenophanes. The unique ability of oligoacenes as photoswitchable antiferromagnetic wires between two CuII ions separated by relatively long intermetallic distances could be the basis for the development of new kinds of molecular spintronic devices, referred to as molecular magnetic switches.

Synthesis of pentacyclic compounds via intramolecular [3 + 2] photocycloaddition of cycloalkene linked naphthalenes

Intramolecular photocycloaddition reactions of cycloalkene linked naphthalenes lead to formation of pentacyclic compounds in a high yielding and stereoselective manner. Intramolecular [2 + 2] cycloadducts are formed initially in photoreactions of cycloalkene linked 1-cyanonaphthalenes. The initially formed intramolecular [2 + 2] cycloadducts absorb light at wavelengths used to promote the photochemical reactions and, as a result, they undergo efficient photocycloreversion to regenerate the starting substrates. In a less efficient competing process, the cycloalkene linked 1-cyanonaphthalenes react to form [3 + 2] intramolecular photoadducts, which do not absorb incident light under the conditions used. Consequently, these photoadducts become the major products in these processes. The relative configuration of the major diastereomers of the pentacyclic products formed from photoreactions of cyclopentene linked naphthalenes (cis-cis) differ from those (cis-trans) arising from the cyclooctene linked substrates. The results of theoretical calculations, which show that steric factors govern the preferred facial mode of intramolecular addition, are in good agreement with the stereochemical course of these photoreactions.

Intramolecular Photocycloaddition Reactions of Arylcyclopropane Tethered 1-Cyanonaphthalenes.

Intramolecular photocycloaddition reactions of 1-cyanonaphthalenes bearing an arylcyclopropane containing side chain were investigated. Photoreactions of members of this family in which the arylcyclopropane moiety is bonded at the 2-position of the 1-cyanonaphthalene ring produce head-to-head and head-to-tail 1,2-[3+2] photocycloadducts. On the other hand, substances in this family containing an arylcyclopropane side chain linked to the 4-position of the cyanoarene ring undergo photoreactions to form [4+3] photocycloadducts along with novel nine-membered ring products, which are produced by photochemically induced 10π conrotatory ring opening of the initially formed intramolecular [3+2] cycloadducts. The results of solvent effects and fluorescence investigations along with those focusing on corresponding intermolecular photoreactions demonstrate that the photocycloadducts are formed predominantly through an intramolecular exciplex mechanism and that a photoinduced intramolecular electron transfer pathway via zwitterionic biradicals might be partly responsible for the process when CH3CN is the solvent.

Intramolecular 10,10a-[2+2] photocycloaddition reactions of phenanthrenes with linked styrene

Intramolecular photocycloaddition reactions of styrene linked 9-cyanophenanthrenes led to formation of C-9 C-10 [2+2] cycloadducts which underwent cycloreversion under the prolonged irradiation conditions to regenerate the starting substrates. Unusual 8-membered ring products were irreversibly formed in these processes. Product distributions were governed by substituent controlled, donor-acceptor interactions that direct generation of two intramolecular singlet exciplexes, which serve as intermediates in the pathways to formation of 9,10- and 10,10a-[2+2] adducts.

Formation of a new benzotriquinane skeleton via intramolecular photocycloaddition reactions of a phenylethynes moiety to a 1-cyanonaphthalene ring system

Photoirradiation of 1-cyano-2-(2,2-dicyano-5-phenylpentyn-4-yl)naphthalene and its analogues promotes sequential intramolecular [2π + 2π] photocycloaddition and cyclobutene ring opening to form benzocyclooctatetraenes, which are then transformed to benzotriquinanes in good yields via a photochemical transannular cyclization process.

ChemInform Abstract: Optical Switches with Biplanemers Obtained by Intramolecular Photocycloaddition Reactions of Tethered Arenes

AbstractReview: 68 refs.

Optical switches with biplanemers obtained by intramolecular photocycloaddition reactions of tethered arenes

The dimerization of anthracene by a [4π + 4π] cycloaddition is one of the oldest and best known reactions in photochemistry. In the series of tethered bichromophoric arenes, this reaction type could be extended to anthracene-naphthalene, naphthalene-naphthalene and recently even to anthracene-benzene and naphthalene-benzene systems. Cyclophanes, which can be regarded as twofold or multiple tethered systems, are not discussed here. The cycloisomerizations are performed by irradiation at the long-wavelength absorption (λ > 270 nm), whereas shorter wavelengths (λ < 270 nm) lead to cycloreversions, which can be also achieved by a thermal route. The systems represent therefore a P- and T-type photochromism, which can be used for optical or chiroptical switches. An acceleration of the switch is possible by a singlet energy transfer (light harvesting antenna effect) in dendritic compounds. In the past 5 to 10 years many applications of these switches were studied in the context of photonic devices, sensor techniques, lithographic processes, imaging techniques, data processing and data storage.

On the Regioselectivity of the Intramolecular [2 + 2]-Photocycloaddition of Alk-3-enyl Tetronates

The simple diastereoselectivity and the regioselectivity of the [2 + 2]-photocycloaddition of alk-3-enyl tetronates were studied, depending on the nature of the substituent R in the α-position. Fourteen tetronates were synthesized and their intramolecular photocycloaddition reactions performed. If R was an alkoxycarbonyl group and if the olefin, which underwent the intramolecular addition, was sterically congested, crossed photoproducts prevailed and were formed in yields between 35 and 65%. In all other cases, the straight photoproducts were obtained as the main reaction products in yields ranging from 39 to 84%. The structures of the photochemical products were thoroughly investigated by one- and two-dimensional NMR experiments. In all cases, the diastereoselectivitiy was excellent and only a single diastereoisomer was formed. A mechanistic model is proposed, which explains the regioselectivity of the [2 + 2]-photocycloaddition reaction.

Photochemistry of ω-( o-vinylphenyl)-ω′-(phenyl/2-furyl) butadienes: New approach to 4-substituted benzobicyclo[3.2.1]octadienes

Novel 1-( o-vinylphenyl)-4-(phenyl/2-furyl) butadienes ( 4a, b), synthesized in one step, undergo intramolecular photocycloaddition reaction to benzobicyclo[3.2.1]octadiene derivatives ( 6a, b) in very good yield (70–90%). In the case of phenyl derivative ( 4a) only endo-phenyl-benzobicyclo[3.2.1]octadiene isomer ( endo- 6a) was isolated whereas the furan ( 4b) derivative resulted in a mixture of endo- and exo-bicyclic isomer ( 6b). Phenyl benzobicyclo[3.2.1]octadiene derivative ( endo- 6a) underwent further di-π-methane rearrangement leading to tricyclic structure ( endo- 7a). The isolated compound 6a and the product of the rearrangement endo- 7a were characterised by X-ray structure analysis.

Competition between Inter- and Intramolecular Photocycloaddition Reactions of 9-Substituted Anthracenes

A number of 9-[(arylmethoxy)methyl]anthracenes were prepared and their photoreactivity was studied. The intramolecular [4π+4π]cycloaddition competes with the intermolecular head-to-tail [4π+4π] cyclodimerization. The aryl substituents control the selectivity; concentration only plays a role in selectivity in certain cases. The complete reversibility of the cycloaddition makes this process suitable as a molecular switch, provided that traces of acids are avoided; when acid is present the cyclomers pursue an irreversible enol ether cleavage route.

Asymmetric Photocycloadditions with an Optically Active Allenylsilane: Trimethylsilyl as a Removable Stereocontrolling Group for the Enantioselective Synthesis of exo-Methylenecyclobutanes

Allenylsilanes undergo enantioselective intramolecular photocycloaddition reactions with enones and enoates to give adducts in 76−99% enantiomeric excess and 67−90% yield. The silyl-substituted exo-methylenecyclobutane products undergo protodesilylation to give the parent unsubstituted exo-methylenecyclobutane. Optically active allenylsilanes may thus be used in enantioselective photocycloadditions in which the silyl moiety functions as a removable stereochemical controlling group.

Synthesis of substituted phenols Via photoaddition-fragmentation-aromatic annelation sequence

Intramolecular photocycloaddition reaction of dihyro-4-pyrones to alkenes ( 1), followed by fragmentation then subsequent aromatic annelation, carried out by two alternative methods, has been performed efficiently. The sequence provides a convenient method for the construction of substituted phenols fused to aliphatic ring ( 4).

ChemInform Abstract: Enantioselective Intramolecular (2 + 2) Photocycloaddition Reaction of N‐Allyl‐3‐oxo‐1‐cyclohexenecarboxamides in Inclusion Crystals with Chiral Host Compounds.

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Pharmacokinetics of S(−)bupivacaine after IV administration in the dog

Intramolecular photocycloaddition reactions of cycloalkene linked naphthalenes lead to formation of pentacyclic compounds in a high yielding and stereoselective manner. Intramolecular [2 + 2] cycloadducts are formed initially in photoreactions of cycloalkene linked 1-cyanonaphthalenes. The initially formed intramolecular [2 + 2] cycloadducts absorb light at wavelengths used to promote the photochemical reactions and, as a result, they undergo efficient photocycloreversion to regenerate the starting substrates. In a less efficient competing process, the cycloalkene linked 1-cyanonaphthalenes react to form [3 + 2] intramolecular photoadducts, which do not absorb incident light under the conditions used. Consequently, these photoadducts become the major products in these processes. The relative configuration of the major diastereomers of the pentacyclic products formed from photoreactions of cyclopentene linked naphthalenes (cis-cis) differ from those (cis-trans) arising from the cyclooctene linked substrates. The results of theoretical calculations, which show that steric factors govern the preferred facial mode of intramolecular addition, are in good agreement with the stereochemical course of these photoreactions.

Complete assignment of the 1H and 13C NMR spectra of a pentacyclo [5.4.0.02,6.03,10.05,9]undecane derivative

AbstractThe production of compounds derived from intramolecular photocycloaddition reactions upon Diels–Alder adducts has received considerable attention, but assignment of the NMR spectra of the photoaddition products has proved problematic, rendering structural determination difficult. This paper reports the complete assignments of the proton and carbon NMR spectra of 11, 11‐dimethoxypentacyclo [5.4.0.02,6.03,10.O5,9] undecan‐8‐one and the assignment of the spectra of pentacyclo [5.4.0.02,6.03,10.05,9] undecan‐8,11‐dione by analogy.

The influence of pressure on arene–ethene photocycloaddition reactions

Pressures of as little as 500 bar have been shown to exert a substantial effect upon rates and product ratios of ortho and meta photocycloadditions of ethenes to the benzene ring. In all cases, rates are enhanced by pressure and calculated ‘pseudo-activation volumes’ for both modes of addition appear to be of similar magnitude to those of thermal cycloaddition processes. The pressure-induced discrimination between isomeric meta cycloadducts from the ethyl vinyl ether–benzene system is rationalised in terms of the mechanism involving partial charge separation in the S1 arene and is backed up by calculated molar volumes of the adducts. Similar results are observed for the intramolecular photocycloaddition reactions of arene–ethene non-conjugated bichromophores.