Dimerization reaction

Photodimerization is common for anthracenes, less so for naphthalenes, and until this year only one example of this reaction with a benzenoid compound had appeared. Thus irradiation of [Pg.322]

Over the years there have been many reports that photoreactions, particularly dimerization processes, carried out in micelles, microemulsions, and cyclodextrin cavities frequently show a higher selectivity and occur at a greater rate than the corresponding process in simple solution. It is,therefore, of interest but no great surprise to note that the photodimerization of water-soluble anthracenes such as 2-anthracene sulphonate in [Pg.325]

It was reported last year that 1-(9-anthryl)-2-phenylacetylene [Pg.325]

2- di(9-anthryl)ethane (278) have been described, Biacetyl sensitized irradiation of (278) leads exclusively to the 1,2-,9, 10 -dimer (279) with a quantum yield of 0.1 and a chemical yield of 62% whereas direct irradiation produces the 9,10-,9, 10 -dimer. [Pg.326]

The dimerization processes are, not surprisingly, influenced by geometrical effects and for example, while the tetrachloro compound (280) behaves analogously to the hydrocarbon (278) in both direct and biacetyl sensitized irradiations, only the 9,10-,9, 10 -dimer is formed from the tetrachloro isomer (281) under both sets of conditions. Bouas-Laurent and co-workers recently reported that with a -Si(Me)2 unit connecting the two anthracenes at the [Pg.326]

Photodimerization reactions involving aromatic rings of two naphthalene units are not as frequently encountered as those involving anthracene. A study of the effect of high pressure on the photodimerization of methyl 3-methoxy-naphthalene-2-carboxylate shows that the rate can be doubled (at 2000 bars), but this represents a much lower activation volume than for similar thermal reactions. An intramolecular dimer is formed when the a f/-[3.3]-(l,4)naphthalenophane (182) is irradiated the cycloadduct reverts to (182) on heating or on further irradiation. The syn isomer of (182) does not undergo this reaction. [Pg.386]

Anthracene photodimers have been investigated extensively. A study of the photodissociation of the dimer of 9-methylanthracene, under both steady-state and flash conditions, suggests that the process occurs through an excited singlet [Pg.386]

Yamamoto, K. H. Grellmann, and A. Weller, Chem. Phys. Lett.. 1980, 70, 241. [Pg.386]

A group of papers dealing with the intramolecular photodimerization reaction of a,a -bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. [Pg.388]

Anthracenes linked by a much longer chain also give internal photodimers, and this is demonstrated by the formation (with reasonably high quantum yield) of photoproducts from bis(9-anthryl)polyoxa-alkanes (187) with linking chain lengths of up to 19 atoms. Kinetic parameters for this reaction have also been reported. An elaboration of this system to the cyclophane (188), which has one [Pg.388]

Interest continues in the intramolecular photodimerization reactions of polynuclear aromatic moieties joined by an alkane chain. The absorption and exciplex emission spectra of the naphthalene-anthracene sandwich pair have been previously studied this species was generated by the light-induced (254 nm) cleavage of the intramolecular photo dimer (139) of l-(9-anthryl)-3-(l-naphthyl)-propane at low temperatures in a rigid matrix.173 The generation and properties of the sandwich pair have been examined by other workers, using single crystals of the intramolecular cycloadduct (139).174 The exciplex fluorescence is broad and [Pg.385]

It has been observed that piperylene catalyses the photodimerization of 9-phenylanthracene.177 The process appears to be an example of a general phenomenon, for the reaction can be catalysed by other dienes, and improvement of dimer yields has been noted with other anthracenes, including those which undergo dimerization in the absence of dienes. From kinetic data, a scheme is proposed which involves the excimer as the reactive intermediate. [Pg.386]

The role of crystal imperfections in the dimerization of substituted anthracenes has been described in the case of l,8-dichloro-9-methylanthracene.178 Similar studies have now been conducted for the 10-methyl isomer.179 In order to explain how the topochemically forbidden / -dimer (head to tail) is produced from irradiation in the solid phase, optical and electron microscopic examinations of the (010) faces of the orthorhombic crystals of the monomer have been carried out, together with differential-enthalpic and dielectric measurements. Again it is shown that the dimer nuclei appear at emergent dislocations. [Pg.386]

It is well established that mono-meso-substituted anthracenes yield head-to-tail dimers as a result of steric and/or electrostatic repulsions between the substituents.180 The first head-to-head mixed dimers have now, however, been reported, and control of the selectivity of this addition is considered to result from charge-transfer and dipole-dipole interactions.181 Thus irradiation of a mixture of 9-methoxy- and 9-cyano-anthracenes in ether yields solely the head-to-head dimer (142), with no trace of the head-to-tail isomer. At room temperature (142) is thermally converted into the starting materials. Irradiation of the mixture of anthracenes in a polar solvent, such as acetonitrile, which is able to stabilize the [Pg.386]

Desvergne, H. Bouas-Laurent, R. Lapouyade, J. Gaultier, C. Hauw, and F. Duphy, Mol. Cryst. and Liq. Cryst., 1972,19, 23 and references therein. [Pg.386]

In contrast to the sulfenes, the [2-1-2] cycloaddition reactions of sulfines are not well known. The dimerization of ethylsulfine gives rise to a four-membered ring rra 5-3,4-diethyl-l,2-dithietane 1,1-dioxide, which is not the result of a [2-1-2] cycloaddition Similarly, reaction [Pg.14]

The dimer of trifluoromethylsulfine is obtained in the thermal generation of the sulfine from the anthracene adduct . Also, the cyclic dimers of y-acetylenic-(3,(3-disubstituted thioaldehyde S -oxides are isolated in their generation from allenic sulfinates with vinyl [Pg.14]

The mechanism of formation of the dithietane 1,1-dioxide dimers involves an opposite [3-1-2] cycloaddition to form 3, which subsequently rearranges to give 4 [Pg.15]

Diphenylsulflne reacts with dichlorocarbene to give the [2-1-1] cycloadduct 5 in 18% yield [Pg.15]

The [2-1-2] cycloadditions to sulflnes have not, as yet, been observed. However, singlet oxygen addition occurs across the C=S bond of sulflnes with formation of ketones and sulfur dioxide. [Pg.15]

Finally in this Section, it is worth noting that photoaddition and photo-dimerization reactions can yield products that reflect cyclization on to an aromatic ring, and that many cyclic systems also undergo cleavage on irradiation [an example of the last type of reaction is the photoconversion of (266) into (267) ]. Thus the cyclized product (268) is among the products obtained from irradiation of dichloromethane solutions of the aromatic thiones (269) in [Pg.360]

Anthracene dimerization is the photochemical reaction having the longest published history, and this process continues to draw attention. Saltiel and [Pg.360]

Ramesh and Ramamurthy have studied the photodimerization of 2-substituted naphthalenes (274) in anionic and cationic micellar media and compared these results with those extensively reported for the reactions in organic solutions. Again micellar solution enhances reactivity by a local [Pg.362]

Interest in the monomerization of anthracene photodimers continues the head-to-tail dimer (290) (see p. 361) produced by irradiation of 9-cyanoanthracene has been investigated by two groups. From results of differential scanning calorimetry and emission spectroscopy of cleaved surfaces of partially monomerized single crystals, it is suggested that the first exothermic peak (and the subsequent ones) arises from monomer crystallization in the photodimer host matrix. Theocharis and Jones have also examined this retroprocess and using single crystal X-ray techniques they have shown that the first step in the solid-state thermal monomerization of this dimer is a [Pg.364]

Arenes of the type ArX-Y, where X may be O, S, or NR, frequently undergo facile photoinduced homolytic cleavage of the X-Y bond with the Y radical subsequently attacking the aromatic ring. The photo-Fries reaction is the most common process of this type and has been reported within the year for aromatic esters which form part of a ptolymer chain or are pendant groups on a polymer chain. In the former case, the rearrangement of fluorene-based polyacrylates [for example, (292)] was studied. Formation of the o-hydroxybenzophenone moiety in the product (293) was monitored by u.v. and [Pg.365]

The dimerisation of acenaphthylene has been used as a probe reaction to investigate the properties of the cavities of a zeolite.In solution acenaphthylene photodimerises to the syn and anti dimers (304) and (305) the singlet excited state yields the syn isomer exclusively, while the triplet excited state produces both the [Pg.242]

Normal head-to-head 4n-i-47c photodimerisation has been found for the series of anthracenes (244)-(251) which are all linked by a three carbon chain. The dimers have structures (252)-(259), respectively, and their quantum yields of formation have been determined and are [Pg.262]

The photodlmerisation of acenaphthylene (267) yields syn and anti adducts (268) and (269). In solution the singlet excited state forms the syn dimer only, and the triplet excited state forms both [Pg.264]

The anhydride of acenaphthylenedicarboxylic acid, (271), has been reported to photodimerlse in similar manner to acenaphthylene itself, but only in the solid state and not in solution.The dimer [Pg.264]

Photolysis of the perfluoro 4,6-diisopropyltriazine (273) extrudes nitrogen and yields the azete (274) which dimerises thermally to (275) [Pg.266]

It has been reported ° that photolysis of deacetylaspidospermlne (276) yields a dimer assigned the structure (277). The reaction only proceeds when dichloromethane is used as solvent and the product is reported to be formed in 60% chemical yield and with a quantum yield of 0.92 [Pg.266]

Equilibrium constants,, for all possible dimerization reactions are calculated from the metastable, bound, and chemical contributions to the second virial coefficients, B , as given by Equations (6) and (7). The equilibrium constants, K calculated using Equation (3-15). [Pg.133]

The enthalpy changes due to dimerization are determined from the van t Hoff relation. For a dimerization reaction between species i and j... [Pg.136]

If the mixture includes organic acids, the equations of Hayden and O Connell yield equilibrium constants for all possible dimerization reactions. [Pg.303]

An active catalytic species in the dimerization reaction is Pd(0) complex, which forms the bis-7r-allylpalladium complex 3, The formation of 1,3,7-octa-triene (7) is understood by the elimination of/5-hydrogen from the intermediate complex 1 to give 4 and its reductive elimination. In telomer formation, a nucleophile reacts with butadiene to form the dimeric telomers in which the nucleophile is introduced mainly at the terminal position to form the 1-substituted 2,7-octadiene 5. As a minor product, the isomeric 3-substituted 1,7-octadiene 6 is formed[13,14]. The dimerization carried out in MeOD produces l-methoxy-6-deuterio-2,7-octadiene (10) as a main product 15]. This result suggests that the telomers are formed by the 1,6- and 3,6-additions of MeO and D to the intermediate complexes I and 2. [Pg.424]

The linear dimerization of substituted conjugated dienes is difficult, but the Pd-catalyzed intramolecular dimerization reaction of the 1,3,9,11-tetraene 13 gives the 3-propenylidene-4-allylpiperidine derivative 14, which has the 1,3,7-octatriene system. The corresponding 1,3,8,10-tetraene also affords the 3-pro-penylindene-4-allylcyclopentane derivative[18]. [Pg.425]

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. [Pg.451]

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage... [Pg.348]

Stabilization of about 6.3 kcal/mol based on thermochemical analysis of dimerization reaction. [Pg.696]

E. V. Albano. Irreversible saturation transitions in dimer-dimer reaction models of heterogeneous catalysis. J Phys A (Math Gen) 25 2557-2568, 1992 Corrigendum. J Phys A (Math Gen) 26.3661, 1993. [Pg.435]

Bis(tnfluoromethyl)-substituted nitrile ylides undergo dimerization reactions in the absence of trapping reagents [143, 168, 170] (equation 38)... [Pg.861]

Compare your results to the experimental values of -34.0 2 kcal mol for the lithium reaction and -3.6 .5 kcal mol" for the water dimer reaction. Use the same model chemistry as in Example 8.2 B3LYP/6-311+G(2df,2p) // B3LYP/6-31G(d). [Pg.185]

The dimerization reactions of 2-vinylindoles and their alcohol precursors have also been explored, giving rise to the formation of several dimeric structures, such as the 6,12-dihydroindolo[3,2-h]carbazole derivative 160, which was obtained on dimerization of 2-(a-hydroxyethyl)indole under acidic conditions (71JOC1759). [Pg.33]

The dipyrrylbutadiyne building blocks, e.g. 21, are prepared by a palladium-induced dimerization reaction of two identical pyrrolecarbaldehydes substituted at the 5-position with an acetylene residue so that the central C —C single bond of the dimer is formed. [Pg.696]

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... [Pg.35]

An illustration of this method can be found in fitting the temperature dependence of the rate constant for the following dimerization reaction ... [Pg.40]

However in 72% H2SO4 no 86 was observed, but rather dimers 87, 88 and 89 were formed. The mechanism for the dimerization reaction is... [Pg.147]

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). [Pg.148]

A more complete coverage of the literature on electronic spectra of radicals is presented in our paper submitted for publication in Fortschr. Chem. Forsch. (Topics in Current Chemistry), where theafi initio studies are also reviewed and the existing open-shell computational procedures discussed. Recently we performed semiempirical all-valence-electron calculations on ground-state properties and electronic spectra of small radicals (Zahradnik, R., and P. Carsky, Theoret, Chim. Acta, 27, 121 (1972) and Carsky, P., M. Machacek, and R. Zahradnik, Coll. Czech. Chem. Commun., in press) and on equilibrium constants of dimerization reactions of small radicals (Zahradnik, R., Z. Slanina, and P. (5arsky, to be published). [Pg.380]

In fact, the primary bisphosphines 1,10,16, and 19 (Fig. 3) are air stable solids demonstrating exceptional oxidative stabilities. Recently, a primary bisphosphine 20 produced by dimerization reaction of anthracenyl primary phosphine has been shown to possess good oxidative stability [29]. [Pg.131]

The cross-dimerization reaction is very commonly employed for the manufacture of intermediates for synthetic musks, which have become an important class of perfumery chemicals. Synthetic musks have been the target of extensive research over the years due to a conservation order placed on the musk deer. Nitro musks are being steadily replaced by non-nitro polycyclic musks becau.se of technical drawbacks and health aspects of the former, which are explosive, sensitive, and virtually nonbiodegradable. Non-nitro musks, on the other hand exhibit better stability to light and alkali, and more nearly duplicate the odour of the macrocyclic musks occurring in nature. Indian musk odorants are easily soluble in alcohol and perfume compositions. They have the added advantage of non-discoloration in soap and domestic products. In view of the low price, their future in the perfume industry appears very promising. [Pg.136]

It was shown that complexes 19 of the zwitterionic precursors of ortho-quinone methides and a bis(sulfonium ylide) derived from 2,5-di hydroxyl 1,4 benzoquinone46 were even more stable than those with amine N-oxides. The bis(sulfonium ylide) complexes were formed in a strict 2 1 ratio (o-QM/ylide) and were unaltered at —78 °C for 10 h and stable at room temperature under inert conditions for as long as 15—30 min (Fig. 6.18).47 The o-QM precursor was produced from a-tocopherol (1), its truncated model compound (la), or a respective ortho-methylphenol in general by Ag20 oxidation in a solution containing 0.50-0.55 equivalents of bis(sulfonium ylide) at —78 °C. Although the species interacting with the ylide was actually the zwitterionic oxidation intermediate 3a and not the o-QM itself, the term stabilized o-QM was introduced for the complexes, since these reacted similar to the o-QMs themselves but in a well defined way without dimerization reactions. [Pg.181]

The amine-catalyzed Diels-Alder dimerization reaction of a, 3-unsaturated ketones in water was developed by Barbas et al. to form cyclohexanone derivatives (Eq. 12.12). They believe that the reaction proceeds via the in situ formation of 2-amino-1,3-butadiene and iminium-activated enone, as the diene and dienophile, respectively. [Pg.384]

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