Photodimerization olefins

The cross-photodimerization reaction represents a great challenge because it requires D to selectively recognize two different olefins in a pair-wise fashion prior to irradiation, whereas the cross-coupling reaction under standard conditions shows no preference over the homo-coupling. The cross-photodimerization of acenaphthylene with substituted naphthoquinones within D resulted in exclusive formation of the cross syn-dimer only 

Snbseqnent work has established the formation of chiral derivatives of nanoreactor D throngh the nse of chiral diamine end-capping gronps instead of ethylenediamine as ligand on the Pd bnilding blocks and snbseqnent self-assembly with functionalized triazine frameworks. These chiral nanocages are able to remotely control the chiral [2h-2] photo addition of flnoranthenes and several maleimides, a reaction previously unknown to be performed in an asymmetric fashion, in np to 50% ee.  

As mentioned previously, nanoreactors can also stabilize in-situ generated reactive intermediates and labile products like in the oligomerization of trialkoxysilanes within D, which could not be isolated in the absence of this nanoreactor.  

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The examples of ex situ steady-state X-ray photodiffraction utihzed to follow the photodimerizations of olefin bonds in a single-crystal-to-single-crystal (or nearly so) manner are ubiquitous in the chemical literature. The interest of sohd-state chemists in this reaction dates back to the work of Cohen and Schmidt [30, 31], and it has become much of a guinea pig in organic solid-state photochemistry. In 1993, Enkelmann and collaborators published two seminal papers in the Journal of the American Chemical Society [32] and in Angewandte Chemie [33], where they presented a series of structures of a-tra s-cinnamic acid crystals reacted to various extents. These reports laid the way for a plethora of later studies on the olefin photodimerization reaction. The convenience of the high conversion and the simple mechanism, combined with the relatively small structural perturbation that it requires, has turned this reaction into a very useful tool to probe intermolecular... 

Takaoka, K., Kawano, M., Ozeki, T., Fujita, M., Crystallographic observation of an olefin photodimerization reaction that takes place via thermal molecular tumbling within a self-assembled host. Chem. Commun. 2006, 1625-1627. 

Salomon, R.G. and Kochi, J.K., Copper(l) triflate A superior catalyst for olefin photodimerization. Tetrahedron Lett., 2529,1973. 

The analysis of the regioselective reactivity of olefins in identical topochemical environments by three computational methods concludes that both steric factors (cavity and potential energy) and electronic factors (perturbation energy from orbital interactions) play important cooperative roles in determining which C—C double bond in a molecule reacts first in [2-1-2] photodimerization. The steric factor is considered to be effective in the movement of olefins at an early stage of the reaction, whereas the electronic factors are effective in the adduction of olefins at a later stage of the reaction. 

Similar relationships have been established with regard to the 1,3-dipolar addition and the photodimerization of olefins 62>. The HO—LU... 

Fig, 7.27 a and b. Orbital relationships in 1,3-dipolar additions and photodimerization of olefins, a) 1,3-Dipolar addition, b) Photodimerization of olefins... 

The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

The photodimerization of simple isolated olefinic bonds is rarely observed because of the absorption of these compounds in the high-energy or vacuum-ultraviolet region. One case reported is that of the photo-dimerization of 2-butene.<2) Irradiation of liquid cw-2-butene with light from a cadmium (A = 229, 227, 214 nm) or zinc (A = 214 nm) lamp was reported to lead to dimers (1) and (2) ... 

Photodimerizations of other olefins conjugated with aromatic rings are shown below<51 53) ... 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

Simple olefins can photodimerize to cyclobutanes in the presence of cuprous salts as sensitizers (4.20) 423>. Copper (I) trifluoromethanesulfonate has been utilized successfully (4.21) 424). 

As already mentioned in discussing the photodimerization of alkenes (cf. chapter 4.2.1), direct irradiation of the olefinic double bond is most often inefficient from the practical point of view as olefins do not absorb light... 

An obvious extension of the studies on photodimerization of crystalline olefins is to solid-state vinyl polymerization (with light, if absorbed, or y-irradiation), with the aim of achieving stereoregular polymers. In fact, an immense effort has been made in this direction, but with singular lack of success. The explanation is that, for various reasons, the lattice in the vicinity of the chain front becomes progressively more damaged as polymerization proceeds, so that after relatively few steps there is loss of stereochemical control. 

H-stacking interactions have also been exploited to orientate olefinic moieties in a geometry suitable for photochemical cycloaddition reactions, and have been invoked by Coates et al. to explain the photodimerization and photopolymerization of mono- and diolefins carrying phenyl and perfiuorophenyl groups [43]. Matsumoto et al. reported the photodimerization of 2-pyridone in co-crystals with naphthalene-substituted monocarboxyhc acids, where the stacking of the naphthalene rings provides carbon-carbon distances appropriate for [4+4] cycloaddition [44]. 

Photodimerization of olefins, 19 132 Photodissociation of metal-metal bonds, 19 106, 107, 124, 125 Photoelectrons... 

A great number of olefinic compounds are known to photodimerize in the crystalline state (1,2). Formation of a-truxillic and / -truxinic acids from two types of cinnamic acid crystals was interpreted by Bernstein and Quimby in 1943 to be a crystal lattice controlled reaction (5). In 1964 their hypothesis on cinnamic acid crystals was visualized by Schmidt and co-workers, who correlated the crystal structure of several olefin derivatives with photoreactivity and configuration of the products (4). In these olefinic crystals the potentially reactive double bonds are oriented in parallel to each other and are separated by approximately 4 A, favorable for [2+2] cycloaddition with minimal atomic and molecular motion. In general, the environment of olefinic double bonds in these crystals conforms to one of three principal types (a) the -type crystal, in which the double bonds of neighboring molecules make contact at a distance of -3.7 A across a center of symmetry to give a centrosymmetric dimer (1-dimer) (b) the / -type crystal, characterized by a lattice having one axial length of... 

Recent studies on the photodimerization of olefinic crystals several examples, which deviate from accepted topochemical principles, have been... 

Photodimerization involves 1 1 adduct formation between an excited nd a ground state molecule. Olefinic compounds, aromatic hydrocarbons, conjugated dienes, oc-unsaturated compounds are known to dimerize when Cxpdsed to suitable radiation. Photodimerization of olefinic compounds Can occur by either (a) 1,2-1,2 addition, (b) 1,2-1,4 addition or... 

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

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