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The palladium-catalyzed cyclopropanation of strained alkenes with 3-pinacolatoboryl-1-arylallyl carboxylates was explored. The reactions proceeded smoothly under mild conditions, and the cyclopropanation products were obtained in good to high yields with high diastereoselectivities if CsF, 18-crown-6, and molecular sieves were used as additives. The reaction was hypothesized to proceed through the formation of a putative palladacyclobutene intermediate that needed to be considered to explain the observed stereochemistry.

A general and effective palladium-catalyzed cross-coupling of organostibines with styrenes to give (E)-olefins was disclosed. By the use of an organostibine reagent, this method can produce unsymmetric (E)-1,2-diarylethylenes and (1E,3E)-1,4-diarylbuta-1,3-dienes in good yields with high E/Z selectivity and good functional group tolerance. Resveratrol and DMU-212 were synthesized in high yield. The protocol can be extended to the synthesis of (1E,3E,5E)-1,6-diphenylhexa-1,3,5-triene in 40% yield. Products 5e, 5f, and 7a showed good photoluminescence quantum yields ranging from 72 to 99%.

A nickel-catalyzed hydrocyanation triggered by hydronickelation of the carbon-carbon double bonds of allenes followed by cyclopropane cleavage is described. The observed regio- and stereochemistries in the products are strongly influenced by the initial hydronickelation step, and allenyl- and methylenecyclopropanes reacted smoothly to promote the cleavage of cyclopropane. In contrast, this cleavage was not observed with vinylidenecyclopropanes, because the initial hydronickelation does not give a suitable intermediate for cleavage of the cyclopropanes. (Figure presented.).

Pd(0)-complexed 3-aryl or 2,3-diaryl propenylcarbenes generated from α-silyl-, α-germyl-, or α-boryl-σ-allylpalladium intermediates undergo self-dimerization to provide 1,6-di- or 1,2,5,6-tetraarylhexa-1,3,5-trienes in good to high yields. This method allows the use of a π-allylpalladium intermediate for a carbenoid precursor. Furthermore, the obtained 1,2,5,6-tetraarylhexa-1,3,5-trienes exhibit aggregation-induced emission enhancement property.

The rhodium(I)-catalyzed reaction between arylboronic esters and excess 1,2-dichloroethene selectively afforded (2-chlorovinyl)arenes. Double arylation yielding 1,2-diarylethenes was observed when 1,2-dibromoethene was reacted with 2.5 equivalents of aryl

The mechanism of the Pd-catalyzed vinylation of aryl halides with vinylalkoxysilanes in water has been studied using different catalytic precursors. The NaOH promoter converts the initial vinylalkoxysilane into a highly reactive water-soluble vinylsilanolate species. Similarly, deuterium-labeling experiments have shown that, irrespective of the catalytic precursor used, vinylation occurs exclusively at the CH vinylic functionality via a Heck reaction and not at the C-Si bond via a Hiyama cross-coupling. The involvement of a Heck mechanism is interpreted in terms of the reduced nucleophilicity of the base in water, which disfavors the transmetalation step. The Heck product (β-silylvinylarene) undergoes partial desilylation, with formation of a vinylarene, by three different routes: (a) hydrolytic desilylation by the aqueous solvent (only at high temperature); (b) transmetalation of the silyl olefin on the PdH Heck intermediate followed by reductive elimination of vinylarene; (c) reinsertion of the silyl olefin into the PdH bond of the Heck intermediate followed by β-Si syn-elimination. Both the Hiyama and Heck catalytic cycles and desilylation mechanisms b and c have been computationally evaluated for the [Pd(en)Cl2] precursor in water as solvent. The calculated Gibbs energy barriers support the reinsertion route proposed on the basis of the experimental results.

Allylphosphonates of allylic alcohols were converted to conjugated dienyl phosphonates in a one-flask reaction, comprising a ring-closing metathesis (RCM), a base-induced ring-opening, and an alkylation. The ring-opening proceeds with very high diastereoselectivity, giving exclusively the (1Z,3E)-configured dienes. Single diastereomers and mixtures of diastereomers can be used as starting materials without noticeable effect on the diastereoselectivity of the sequence.

Molybdenum or tungsten monoaryloxide pyrrolide (MAP) complexes that contain OHIPT as the aryloxide (hexaisopropylterphenoxide) are effective catalysts for homocoupling of simple (E)-1,3-dienes to give (E,Z,E)-trienes in high yield and with high Z selectivities. A vinylalkylidene MAP species was shown to have the expected syn structure in an X-ray study. MAP catalysts that contain OHMT (hexamethylterphenoxide) are relatively inefficient.

The palladium-catalyzed cyclopropanation of strained alkenes with 3-trimethylsilyl- or 3-pinacolatoboryl-1-arylallyl acetate derivatives is described. This reaction gives cyclopropanation products in good to high yields with a single diastereomer, and the key step is likely to involve the formation of a palladacyclobutene complex from the α-trimethylsilyl- or α-pinacolatoboryl-σ-allylpalladium complex. Copyright

[Chemical equation presented] Readily available cinnamic acid derivatives such as ferullc acid couple with β-bromostyrenes and 1-bromo-4- phenylbutadiene under palladium catalysis accompanied by decarboxylation to produce the corresponding α,β-diarylbutad

The Wittig reaction involving direct olefination of triphenylphosphonium ylides (Ph3PCHR) with aldehydes is arguably the most often used method for alkene synthesis, but in general it yields mixtures of Z- and E-alkenes for semistabilized triphenylphosphonium ylides (R = aryl or vinyl). We have developed a simple and efficient protocol to improve the stereoselectivity significantly by replacing the aldehydes used in the Wittig reaction with N-sulfonyl imines, which possess distinct electronic and steric properties relative to aldehydes. A broad range of aromatic, α,β-unsaturated, and aliphatic imines bearing appropriate N-sulfonyl groups smoothly undergo olefination reaction with various benzylidenetriphenylphosphoranes or allylidenetriphenylphosphoranes under mild reaction conditions to afford an array of both Z- and E-isomers of conjugated alkenes in good to excellent yields and with greater than 99:1 stereoselectivity. Moreover, this tunable protocol has been successfully applied to the highly stereoselective synthesis of two anticancer agents, DMU-212 and its Z-isomer.

Given the particular importance of dye photostability for single-molecule investigations, fluorescence fluctuation spectroscopy, and laser-scanning microscopy, refined strategies were explored for enhancing the fluorescence signal by selectively quenching

Disrotatory bicycle pedals: Irradiation of the title compound in the solid state gives the all-trans isomer directly in a crystal-to-crystal reaction. This threefold cis-trans photoisomerization is proposed to proceed by a two-stage mechanism that is consistent with two simultaneous bicyclepedal processes occurring in disrotatory fashion about the central bond.

Reaction of azolylacroleins with phosphoranes bearing a conjugated double bond was found to yield either azolyltrienes in?a Wittig reaction, or to undergo cyclization to a dihydrobenzene containing the azole substituent. Transformation with an aza-Wittig reagent gave tetrazolylpyridines. The ambident reactivity was found to be dependent on the substituent of the phosphorane, which was rationalized by ab initio (DFT) calculation of the atomic charges of the reaction centres.

Chloroalkenes, important building blocks for cross-coupling reactions, are prepared in one step by the Suzuki reaction of 1,1-dichloroethylene and 1,2-dichloroethylene with alkenyl- and arylboronic acids. Under the proper reaction conditions, it is possible to obtain the monocoupling reaction to provide the corresponding chloroalkenes with good yields. This method represents an excellent route for the preparation of chloroalkenes and chlorodienes from commercially available starting materials.

A number of 1,6-diphenyl-1,3,5-hexatrienes of varying alkene geometries were stereoselectively prepared from just two starting materials: iodobenzene and 4,4,6-trimethyl-2-vinyl-1,3,2-dioxaborinane via a series of Heck, Suzuki-Miyaura and stereocontrolled iododeboronation reactions. These results demonstrate how 4,4,6-trimethyl-2-vinyl-1,3,2-dioxaborinane can be used as a genuine two-carbon vinyl-dianion building block in stereocontrolled polyene synthesis. The Royal Society of Chemistry 2005.

(trans) 1-Chloro-2-iodoethylene (3), (trans) 1-bromo-2-iodoethylene (4), (trans) 1,2-diiodoethylene (5) and (cis and trans) 1,2-dibromoethylene (11) were reacted under Suzuki, Sonogashira and Negishi cross-coupling conditions using Pd catalysis to obtain mono coupled products. Only olefin template 3 provided the desired coupling products reliably under all reaction conditions. Compound 5 did not provide cross coupled products under any of the reaction conditions used. The Negishi reaction was the only one that worked for templates 4 and 11. Studies indicate that oxidative addition of the most reactive carbon-halogen bond to Pd(0) is followed by elimination of the second halide, when the second halide is a bromide or an iodide. This happens to a much lesser degree when the second halogen is a chloride. Graphical Abstract.

Irradiation of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) in degassed acetonitrile (AN) gives ctt- and tct-DPH, relatively inefficiently, mainly via isomerization in the singlet excited state. The triplet contribution to the photoisomerization is small due to a very low intersystem crossing yield (φis = 0.01). Central bond isomerization is quenched in the presence of air by a factor of 1.4, consistent with the expected quenching of the lowest singlet and triplet excited states by oxygen. However, the presence of air enhances terminal bond photoisomerization by nearly twofold. Triplet-sensitized ttt-DPH photoisomerization favors tct-DPH formation and is quenched by oxygen. It follows that the interaction of singlet-excited ttt-DPH with O2 suppresses isomerization to tct-DPH but opens a new isomerization pathway to ctt-DPH. The presence of dimethylfuran, a singlet O2 trap, has no effect on the photoisomerization, eliminating the possible involvement of singlet O2 in this new reaction, ttt-DPH radical cations are ruled out as intermediates because the presence of fumaronitrile, which leads to their formation, suppresses both central and terminal bond photoisomerizations. In contrast to acetonitrile, ctt-DPH formation is quenched by oxygen in methylcyclohexane, suggesting the requirement of a polar environment. Strikingly different deuterium isotope effects distinguish the direct and O2-induced photoisomerization pathways. A comparative study of ttt-DPH-d0 with ttt-DPH-d 2 and ttt-DPH-d4, involving deuteration of one and both terminal double bonds, reveals an inverse kinetic isotope effect (k. Hox/kDox = 0.92) for the O 2-induced reaction. An attractive mechanism for the new oxygen-induced photoisomerization involves charge transfer from the S 1 state of ttt-DPH to oxygen followed by collapse of the exciplex to either a zwitterionic or a biradicaloid species through bonding at one of the benzylic positions. Rotation about the new single bond in this intermediate followed by reversion to DPH and O2 gives the observed result.

The relationship of structure to optical spectral properties was determined for five types of fluors in a search for an optimum-wavelength shifter to be used as part of the detection systems for high-energy particles from accelerators. In a search for photostable fluors to serve as waveshifters in plastic fibers it was found that the wavelengths of interest, absorption max 410 ± 10 nm and fluorescence emission max 480 ± 20 nm, along with other properties, such as high solubility and short fluorescence decay time, could be obtained from fluorophors composed of aromatic rings and vinyl groups only by using amino groups as auxochromes to give bathochromic shifts of wavelengths. Since primary, monoalkyl, and dialkylamino groups were not sufficiently photostable, a number of fluorophores bearing diarylamino groups were investigated. Syntheses of the fluors made use of the Buchwald amination, an improved version of the Emmons-Horner reaction, and other common reactions. The fluor types were the following: a 2-benzoxazolyl-7-(4-diarylamino)fluorene 7, 2-(4-cyanophenyl)-5-(4-aminophenyl)oxazoles 14 and 20, 1,3,5-hexatrienes 24a-d and 26a-c, 1,4-distyrylbenzenes 31d-g and 32a-e, and 2,7-distyrylfluorenes 40a,d-e. The unsymmetrical fluors 7, 14, and 20 were not as bright as the best hexatrienes, distyrylbenzenes, and distyrylfluorenes, which were all symmetrical. Where the 1,6-diaryl-1,3,5-hexatrienes 24a-d had high fluorescence quantum yield (Φf), the 1,1,6,6-tetraryl-1,3,5-hexatrienes 26a-c had both lower ε and Φf. Where the 1,4-distyrylbenzenes 31d-g had high Φf, the 1,4-bis(2-phenylstyryl)benzenes 32a-e had (Φf = 0. Diarylamino groups as auxofluors conferred higher photochemical stability than dialkylamino groups on similar fluorophores. The 1,4-distyrylbenzenes 31d,e and the 2,7-distyrylfluorenes 40d,e had the most desirable properties overall, which included fast decay times of 2 ns. Computer simulations predicted absorption and emission wavelengths fairly well, but were of little help for the prediction of brightness, stability, Φf, or decay time.

Diazo coupling reactions are capable of forming E,E,E-trienes from cinnamaldehydes in good yield. An efficient methodology is reported for the production of styryldiazomethanes that are subsequently used with catalysis for coupling and for cyclopropanatio

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1,6-Dibromohexa-1,3,5-triene, previously described by us and easily obtained from 5-bromopenta-2,4-dienal by condensation with bromomethylene triphenylphosphorane, is a versatile precursor for the synthesis of conjugated 1,3,5-trienic derivatives of contr

The relative energies of the ground state isomers of 1,6-diphenyl- 1,3,5-hexatriene (DPH) in benzene are determined from the temperature dependence of the equilibrium isomer composition obtained with the use of diphenyl diselenide as isomerization catalyst. In the triplet state, DPH exists as an equilibrium mixture of all-trans (ttt), trans,cis,trans (tct), cis,trans,trans (ctt), and cis,cis,trans (cct) isomers. Under degassed conditions, photoisomerization of the triplets is primarily bimolecular, involving a quantum chain process. Oxygen eliminates the quantum chain process by efficient deactivation of DPH triplets thereby revealing the triplet state isomeric composition. The temperature dependencies of the fluorenone-sensitized photoisomerization quantum yields and photostationary states for DPH in air-saturated benzene provide two independent measures of the temperature dependence of the equilibrium contribution of the isomeric triplets. They reveal the relative energies of the DPH triplet isomers. Together with the known 34 kcal/mol triplet energy of ttt-DPH, these results define points on the potential energy surfaces of the ground and triplet states corresponding to the equilibrium geometries of the four observed DpH isomers. At these geometries the two surfaces roughly parallel each other. Complete equilibration of isomeric triplets within 100 ns requires that the energies of triplet biradical transition states be no higher than 40.3 kcal/mol. Estimated radical stabilization energies give 40.2 and 41.6 kcal/mol for the energies of biradical transition states for central and terminal bond isomerization, respectively, in the ground state of ttt-DPH.

Direct irradiation of (E,E,E)-1,6-bis(4-formylphenyl)hexa-1,3,5-triene in chloroform under argon atmosphere led to a fast equilibrium between E,E,E(62%) and Z,E,E(38%) isomers. Neither isomers nor photoproducts were detected by HPLC in the photostationary mixture. This is entirely different from the reaction of (E,E,E)-1,6-diphenylhexa-1,3,5-triene on irradiation under similar conditions, in which dimers and solvent adducts were formed via various kinds of Z isomers. The high stereoselectivity observed in the Z,E-photoisomerization of the formyl-substituted diphenylhexatriene can be explained by the assumption that a terminal twisted transient state has a zwitterionic character which is stabilized by the polarization of carbonyl groups.

Substituted 1,3,5-hexatrienes 7 can be prepared in excellent yields and with good stereoselectivity from diallylic sulfones 6 employing a modified Ramberg-Baecklund reaction.

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NbCl5 reacts with one equivalent of methyllithium or metallic potassium to give a reagent for the transformation of aldehydes, ketones, benzalchloride, and benzotrichloride to symmetrically substituted ethene derivatives (Tab.1).The reagent NbCl5/K (2) is distinctly more reactive than NbCl5/MeLi (1).For the reductive carbonyl dimerization with niobium reagents a mechanism is proposed.Key Words: Reductive carbonyl dimerization / Niobium pentachloride

Reaction of tricarbonyl(η4-vinyl ketone)iron(0) complexes (CO)3(PhCH=CHC(R)=O)Fe(O) (R = Me, 1; Bun, 3; But, 5; Ph, 7) with organolithium reagents under an atmosphere of carbon monoxide gave products that were identified as tricarbonyl (η4-vinylketene) iron(0) complexes (CO)3(PhCH=CHC(R)=C=O)Fe(0) (R = Me, 2; Bun, 4; But, 6; Ph, 8) based on a crystal structure analysis of 2. Complex 2 crystallizes in space group P21/c with a = 13.998 (6) A?, b = 7.955 (5) A?, c = 12.935 (5) A?, β = 113.04 (3)°, and Z = 4. A reaction pathway that postulates the intermediacy of a tricarbonyl(η3-vinylcarbene)iron(0) complex, 20, is discussed. Heating the tricarbonyl(η4-vinylketene)iron(0) complexes 6 and 2 with either tert-butyl, cyclohexyl, or 2,6-xylyl isocyanide at 80°C gave tricarbonyl (η4-vinylketenimine) iron(0) complexes (CO)3(PhCH=CHC(R1)= C=NR2)Fe(0) (R1 = R2 = But, 23; R1 = Me, R2 = But, 24; R1 = Me, R2 = C6H11, 25; R1 = Me, R2 = (2,6-Me2)C6H3, 26) via intermediate dicarbonyl(isocyanide)(η4-vinylketene)iron(0) complexes. The intermediacy of tricarbonyl(isocyanide)(η1-vinylcarbene)iron(0) complexes in this reaction is proposed.

Pulse radiolysis of tetrahydrofuran (THF) solutions of all-trans α,ω-diphenyl-substituted polyenes, such as 1,4-diphenylbuta-1,3-diene, 1,6-diphenylhexa-1,3,5-triene and 1,8-diphenylocta-1,3,5,7-tetraene, has been undertaken in the absence and presence of Bu4NPF6 and NaBPh4.In the presence of the salts, the absorption maxima of the diphenylpolyene radical anions are shifted to shorter wavelengths by ion pairing with Bu4N+ and Na+ as well as that of the trans-stilbene radical anion previously investigated.When Ph(CH=CH)nPH.- (n=1-4) is paired with Bu4N+, the magnitude of the spectroscopic shift is larger for n=2-4 than for n=1.On the other hand, the magnitude of the spectroscopic shift due to the ion pairing with Na+ decreases with increasing n and becomes very small in the case of n=3 or 4.The decay of the radical anions, which is due to neutralization reactions with THF(H+), is retarded by the addition of the salts.The retarding effect of the salts is attributed to the ion pairing of the reactant ions with the counterions derived from the salts.In acetonitrile solution the absorption spectra and the decay rates of the radical anions are not affected by the addition of the salts, demonstrating that the ion pairing is not important in such a polar solvent.Results for the radical anions of pyrene, perylene and triphenylethylene are presented for the sake of comparison.The appreciable spectroscopic shift due to the ion pairing with the large Bu4N+ ion was found to be characteristic of the diphenylpolyene radical anions.

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