Dimerization and Intermolecular Additions

5 Dimerization and Intermolecular Additions Dimerization of cyclohexa-1,3-diene under aryl nitrile sensitization affords the four dimers (174) - (177). The detailed results are interpreted as providing evidence for the involvement 

Kaupp and Ringer have reported the photochemical addition 

Ferris and Guillemin have demonstrated that the irradiation of 

5-tricyano benzene. This product is also formed on 

An efficient cation-radical chain process is involved in the 

The dimerization of acenaphthylene (187) in faugasites has been studied. The irradiation gives a mixture of cis- and trans-dimers. The work has shown that the dimers are formed from the singlet state when the faugasite has low atomic weight cations 

Photochemical dimerization of the diene (160) has been carried out in the solid state. The styry1isoxazoles (161) are also photoreactive and yield (2+2)-dimers. Oxidation of these affords the a-truxillic acids derivatives (162). The photochemical dimerization of (163) has been reported to afford a cyclobutane derivative when irradiated in the solid phase. A detailed review has surveyed the conformation effects influencing photochemical solid state reactions.  

A mixture of four dimers (8X) is obtained from the benzophenone-sensitized irradiation of the triene (164). Vacuum sublimation of this mixture affords the dimer (165) which was subsequently converted by conventional chemical paths to the hydrocarbon (166).Photosensitized (Rose Bengal) dimerization of aceanthrylene (167) has been studied in methanol solution. This affords both the syn- and anti-head-to-head and head-to-tail stereoisomers. The results indicate that there is a slight preference for the formation of the syn-adduc t s.  

Kaupp and Ringer have described the photoaddition of trans-stilbene to the heterocyclic compound (168) to yield the adducts (169) and (170). 

Interest in reactions within constrained environments continues apace. A report has described the results of irradiating complexes of the stilbene (7) with y-cyclodextrin. Rather than c ,/ra/w-isomerization, dimerization occurs to afford (165) and (166) in 79% and 19% yield, respectively. These results are different 

The synthesis of so-called paddlanes has been reported following the irradiation of the divinylbenzene derivative (167). The product isolated from this reaction was identified as the cycloadduct (168), and interestingly this intermole-cular cycloaddition only occurs with the o-divinyl moiety the corresponding p-derivative fails to yield the excepted product. The intermolecular photocycloaddition, brought about by irradiation through Pyrex of a ben ne solution, of the diene (169) results in the formation of the adducts (170) and (171) in 15 and 13% yield, respectively.  

Photochemical cycloadditions of the alkyne (174) to the dienedione (175) occurs sequentially and affords the two adducts (176) and (177). The latter was chemically transformed into the [l.ljparacyclophane (178) which was then studied photochemically. The photochemical cycloaddition of ethyne to the dehydrodoanthracene (179) results in the formation of the (2+2)-cycloadduct 

Calculations have studied the (2 + 2)-photocycloaddition of ethene, either under direct conditions or on a Si (100) surface. 

Inokuma et al have reported a further example of additions with crown ether derivatives. In this instance the additions are intermolecular and involve the dimerization of the vinylbenzene derivative (123) to afford the two adducts (124) and (125). The ion-complexing capabilities of the adducts were assessed. A layered ternary solid is formed between 1,2-dihydroxybenzene and trans- -(2-pyridyl)-2-(4-pyridyl)ethylene. Within this, the stilbene is held in a head-to-tail arrangement. Irradiation brings about the formation of a cyclobutane identified as (i )-c/ y,/ra y,/ra -l,3-bis(2-pyridyl)-2,4-bis(4-pyridyl)cyclobutane. An extension of this work to the use of 5-methoxyresorcinol as the template has demonstrated that quantitative yields of ladderanes (126) can be obtained by irradiation of the solid-state units represented as (127). The diazastilbene derivative (128) readily forms complexes with the tetra-acid (129). This acts as a supramolecular template and holds the ethene systems close enough for photochemical dimerization.  

The styrylpyrilium salt derivatives (130) undergo (2 + 2)-cycloaddition to afford the corresponding dimers. The magnetic properties of the radical cations formed from these dimers were compared with those formed from (130). Styryl dyes of the type shown as (131) undergo E-Z isomerization on irradiation at 436 nm. The dyes align themselves in the pattern shown in (132, where the filled blob represents the crown ether complex). These undergo dimerization on irradiation to afford compounds (133), from which the magnesium ions can be removed. 

Cross-photodimerization of ( )-2-(2-phenylethenyl)benzoxazole and ( )-4-[2-(4-chlorophenyl)ethenyl]-pyridine in sulfuric acid solution results in the formation of a product identified as (7 )-cw,traw, raw -l-(2-benzoxazolyl)-2-(4-chlorophenyl)-4-phenyl-3-(4-pyridinyl)cyclobutane.  

Nishimura and his eo-workers have reported the synthesis of the calixa-rane derivatives (134). These are formed by photoaddition of ethene to the eorresponding alkene derivative. 

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The structure of the reagent, the mechanism of epoxide opening, deoxygenations, dimerizations and intermolecular additions will be discussed first before covering the preparatively much more important cyclization reactions [36]. 

The benzyl anion generated from benzyl chloride gave a low yield of phenyl-acetic acid 305). The radical anion dimer of benzalacetone 75 reacts simultaneously by intramolecular addition to the carbonyl group and intermolecular addition to C02 to the cyclopentane derivative 76 306 (Eq. (135) ). 

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

Dimerization allows the kinase activity of both intracellular chains to encounter target sequences on the other, linked receptor molecule. This enables the intermolecular cross-phosphorylation of several tyrosine residues (Figure 8.2). The phosphorylated dimer then constitutes the active receptor. It possesses an array of phosphotyrosines that enable it to bind proteins to form receptor signaling complexes. Additionally, the dimerized and phosphorylated receptor has the potential of phospho-rylating its targets. 

Chiral methylzinc aminoalkoxides 146a-c were obtained from the reaction of ZnMe2 with aminoalcohols, having chiral centers in their carbon backbones (Scheme 92).211 The methylzinc aminoalkoxides crystallize dimeric and trimeric with the formation of intermolecular zinc-oxygen bonds and creation of additional chiral centers. 

In this article, the features and mechanism of the crystal-to-crystal reactions of 1,3-diene compounds are described on the basis of the molecular packing structure and intermolecular interactions in the crystals for starting materials and products. The dimerization and isomerization of unsaturated compounds as well as addition polymerization via a chain reaction mechanism are ideal sohd-state reactions, because they produce no leaving group during the reac-... 

Photochemically induced dimerization224 readily occurs both in the solid state and in concentrated solution, and is, in general, the result of the intermolecular addition of an excited molecule to a second molecule in the ground state. Intramolecular reactions compete with dimerization in more dilute solution. 

An interesting addition reaction to silenes was recently described by Oehme and coworkers. They found that at high concentrations of LiBr the dimerization of the transient silene 371 yields 33% of the head-to-head dimer 373 and the head-to-tail dimer 407 in a 1 5.6 relative ratio108. The formation of the unexpected dimer 407 was rationalized by the addition of LiBr to the Si=C bond and intermolecular cyclization of the w-lithiosilyl bromide 408 or its reaction with the transient silene 371 with subsequent cyclization to the 1,3-disilacyclobutane 407 (equation 126)108. 

The comparison between calculated spectra of the dimer and of the isolated species showed a behaviour which compares favourably with the experimental data. This behaviour can be correlated with the predicted changes of the intramolecular geometry, and in particular with a displacement of the equilibrium BLA parameter, modulated by dipole-dipole intermolecular interaction. Moreover, a simple model based on the explicit introduction of an additional intermolecular dipole-dipole interaction term in the potential allowed to understand the frequency shifts caused by the formation of the dimer. 

These adducts consist of cyclic dimers comprising folded M2Si2 backbones. The spectroscopic data reveal mainly ionic interactions between alkali metal cations and hypersilanide anions. Additionally, oligo-hapto coordination of the metal ions by the toluene molecules is observed. Intra- and intermolecular agostic interactions to C-H bonds finally govern the peculiarities of both, the molecular and the crystal structures of these compoimds. 

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