7694-30-6

The outcome of photosensitized [2+2]-cycloaddition reactions of various cinnamates has been compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λmax=455 nm) irradiation and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. Moreover, while for reactions run in homogeneous solution oxygen needs to be excluded, no erosion in yield is observed when the photoadditions were run in aerated gel media.

The organoborane-catalyzed reductive carbocyclization of homoallylic alcohols has been developed by using hydrosilanes as reducing reagents to provide a range of 1,2-disubstituted arylcyclobutanes. The reaction proceeds in a cis-selective manner with high efficiency under mild conditions. Mechanistic studies, including deuterium scrambling and Hammett studies, and DFT calculations, suggest a dual ring-closing pathway.

An efficient method for the synthesis of substituted cyclobutanes from cinnamates, chalcones, and styrenes has been developed utilizing a visible-light triplet sensitisation mode. This reaction provides a diverse range of substituted cyclobutanes in high yields under mild conditions without the need of external additives. Good regioselectivity is obtained due to strong π-π-stacking of arene moieties, whereas diastereoselectivity relies on the electronic effects or ortho-substitution of the arene substrate. The utility of this transformation is demonstrated by the formal synthesis of the lignane natural product (±)-Tanegool.

2,4,6-Triphenylpyrylium cation encapsulated within zeolite Y promotes highly selective transformation of olefins to ketones with molecular oxygen, under visible light (λ > 400 nm) irradiation at room temperature. The Royal Society of Chemistry 2006.

Photochemical deazetation of 3,6-diphenyl-3,4,5,6-tetrahydropyrazine (1) was re-examined; evidence was obtained for trapping of a diradical intermediate (2) by oxygen when (1) was photolysed in the absence of solvent.

Regiochemistry and rectivities in the thermal ring-splitting reactions of diarylcyclobutanes (1-5) have been studied and shown to depend on the stable conformations and rotational mobilities of the aryl substituents.The reactions of 1 and 2 result in a regiospecific symmetric cleavage to give indene or styrene along with significant isomerization of 2 to 3.In the cases of 3-5 both the symmetric and unsymmetric cleavages competitively occur with decreasing symmetric-to-unsymmetric ratios with an increase in methyl substitution.The olefin products from 4 are mixtures of cis- and trans-2-butene, cis- and trans-β-methylstyrene, and trans-stilbene.Thermochemical analyses combined with product analyses indicate that the symetric cleavage of 1 and the unsymmetric cleavage of 3 proceed with a concerted mechanism, whereas 1,4-biradicals are involved in the other reactions.Structure-reactivity relationships of the present reactions are discussed in terms of mixing of the ?* character in a bonding MO by specific ?-?* interactions, depending on the conformational situations of the aryl groups and in terms of the steric effects which destabilize 1,4-biradicals as well as transition states of the biradical fragmentation to the olefins.

The photolyses of cis-transoid-cis-cyclobutadiindene (1), cis-2-phenylcyclobutindene (2), cis-1,2-diphenylcyclobutane (3), the trans isomer (4), and its dimethyl (5) and tetramethyl (6) derivatives have been investigated to explore stereoelectronic factors controlling the excited-state reaction courses for the ring cleavages.The symmetric ring cleavage occurs regiospecifically in the cases of 1-3 and with 98percent specificity in the case of 4.The photolysis of 5 results in both the symmetric and the asymmetric cleavages in a ca. 1:3 ratio accompanied by substantial losses of stereointegrity in the olefin formation, and the regiospecific asymmetric cleavage occurs in the case of sterically more congested 6.The ring-cleavage efficiencies systematically decrease in the order 1>2>3>4>5>6 and reveal negative temperature dependences, particularly in the cases of 1, 5, and 6.The features of the reactions are in parallel with stable conformations and conformational mobilities of the aryl groups associated with different ?-? orbital interactions, while the spectroscopic properties of the cyclobutanes are essentially identical with those of benzene chromophores with little electronic perturbation.The photolitic ring-cleavage reactions are discussed in terms of crossing from the localized 1?,?* state to a ?* hypersurface, in wich conformation-controlled orbital interactions play important roles.The final products are formed via a pericyclic minimum which might be biradicaloid in nature.

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Photosensitized reactions of cis- and trans-1,2-diphenylcyclobutane and cis-transoid-cis-cyclobutadiindene by 1-cyanonaphthalene, 9,10-dicyanoanthracene, 1,4-dicyanonaphthalene, and chloranil, which mainly give the ring-splitting product (styrene or indene), have been investigated to explore steric and electronic requirements of the reactions associated with charge-transfer and excitation-resonance contributions of exciplex intermediates.The reaction efficies increase with an increase in the electron-accepting power of the sensitizers irrespective of the spin states.The formation of 1-phenyltetralin in the photosensitized reactions of the diphenylcyclobutanes occurs from the polar singlet exciplex with the dicyanoarenes but not at all from the less polar exciplex with 1-cyanonaphthalene nor from the triplet chloranil exciplex of high charge-transfer nature.Exciplex reactivities are discussed in terms of roles of charge-transfer contributions in exciplex decay channels as well as in terms of configurational and conformational effects on orbital interactions in the cyclobutanes.

Photosensitized reactions of cis-1,2-diphenylcyclobutane by aromatic nitriles, which mainly give styrene along with the trans isomer and 1-phenyltetralin revealed divergent solvent effects depending on the sensitizers.

The photosensitized reactions of cis-1,2-diphenylcyclobutane by 1,4-dicyanonaphthalene and 9,10-dicyanoanthracene gave styrene, the trans isomer, and 1-phenyltetralin whereas the photosensitization by 1-cyanonaphthalene effected the ring-splitting and cis

Addition of 2,2-diphenyl-1-picrylhydrazyl DPPH to styrene at 75 deg C results in little change in the amounts of cis- and trans-1,2-diphenylcyclobutane and 1,2,3,4-tetrahydro-1-phenylnaphthalene, in a slightly smaller amount of 1-phenylnaphthalene, and in a considerable increase in the amount of 1,2-dihydro-1-phenylnaphthalene that are formed.Formation of the styrene trimer 1,2,3,4-tetrahydro-1-phenyl-4-(1-phenylethyl)naphthalene is eliminated completely.Products derived from DPPH are 1-(4-nitrophenyl)-1-phenyl-2-picrylhydrazine 9 and 1--2,2-diphenylhydrazine, 10.DPPH intercepts the thermally formed Diels-Alder dimer of styrene as soon as it is formed to give the 1,2,3,4-tetrahydro-4-phenyl-1-naphthyl radical.Thermolysis of bis(1,2,3,4-tetrahydro-4-phenyl-1-naphthyl)diazene in the presence of DPPH yields 20percent of 10, 10percent of 1--2-(4-nitrophenyl)-2-phenylhydrazine and 9 while thermolysis of 1,2-bis(1-phenylethyl)diazene in the presence of DPPH yields 9 and 44percent of 1--2,2-diphenylhydrazine showing that substituted benzyl radicals efficiently displace the 4-nitro group of DPPH.The nitro group is transferred to an unsubstituted phenyl group of DPPH.