Norbomadienes irradiation

When irradiated in stronger acid (FHSO3) at —60°, the tropylium ion is seen to isomerize cleanly to the norbomadien-7-yl cation (7) (Childs and Taguchi, 1970 Hogeveen and Gaasbeek, 1970). The ion 7 has been shown to be photochemically stable (Cabell and Hogeveen, 1972). However, it is known to revert to the tropylium ion thermally at temperatures above 45° (Lustgarten et al., 1967). 

Ejection of dinitrogen from the triazoline adducts to form the related aziridines was promoted by ultraviolet irradiation (300 nm, benzene) and usually proceeded in excellent yield. An exception was found in the irradiation of the triazoline substrate 59, where cleavage of the cyclobutane ring occurred as the dominant reaction pathway to form the pyridazino norbomadiene 61 (and secondary photoproducts derived therefrom), together with the triazole-4,5-diester 62. A role for the pyridazine ring and the 2-pyridyl substituents in stabilising the diradical intermediate 60 has been proposed for this abnormal outcome (Scheme 8). 

One of the interesting molecules that has been studied in considerable detail is nor-bomadiene (80). Much of this interest has been associated with the interactions between the double bonds of the system. Thus irradiation affords quadricyclane (81). This area of study will be discussed later in this chapter. The radical cation 82 can also be formed from both norbomadiene and quadricyclane by irradiation in acetonitrile/methanol solution with the DCB/phenanthrene sensitizer system. Several products (Scheme 2) are formed in low yield and it should be noted that there is little difference in the yields of products obtained from either starting material. However, it is evident that attack by methanol occurs from the exo face32. 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

Intramolecular cycloadditions can also be induced by photoelectrochemical activation. Norbomadiene, for example, can be cyclized upon irradiation of ZnO, ZnS, CdS, or Ge semiconductors [179]. 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

In their recent landmark femtosecond-resolved mass spectrometry studies, Zewail and coworkers have used mass spectrometry for monitoring the time-resolved unimolecular fragmentation of neutral norbomene and norbomadiene. In both cases, the RDA reactions occurred, but only in the norbomadiene case was the well-known H loss giving rise to C7H7+ ions found to compete. Still, non-concertedness and biradicaloid character of the intermediates is being addressed by femtosecond dynamic studies. In this context, Kompa and coworkers" have compared the expulsion of H+ from femtosecond-laser-irradiated... 

Investigations have also examined the photochemical outcome of the inclusion of other aromatic substituents onto the norbornadienes. Examples of this are the direct and sensitized irradiation of the naphthyl-substituted derivatives 249 that brings about cyclization to 250. Sensitization of the cyclization with ketones such as benzophenone leads to a much cleaner reaction. Biacetyl has also been used as the sensitizer . Cyclization also occurs with the norbomadiene 251 2. 

The incorporation of sulphonyl groups does not inhibit the cyclization process as demonstrated by Gleiter and Ohlbach, who have reported the efficient synthesis of the quadricyclane 252 by irradiation of the norbomadiene derivatives 253. A study has also been made of the sulphonyl-substituted norbomadiene derivatives 254 ... 

Another example of self-assembly of porphyrin-containing polymer was illustrated by Li et al.73 Polyacetylene functionalized with fullerene and zinc porphyrin pendant groups were synthesized by polymerizing the corresponding fullerene/porphyrin substituted alkyne monomers with rhodium(I) norbomadiene catalyst (Scheme 5.5).74 Polymers with different ratio of C60 and porphyrin were synthesized. The polymers showed photocurrent response when the thin films were irradiated with white light, which was due to the electron transfer from the photo-excited porphyrin to the C60 units. In addition, the copolymers aggregated into ellipse-shaped nanorod structures with a diameter of approximately 100 nm and a length of... 

In the case of the chromium carbonyl compounds the intermediacy of norbomadiene-Cr(CO) 4 has been demonstrated. 1,4-diphenylbuta-diene-Fe(CO) 3 and [(C0H5)3P]2Ni(CO)s have been recovered to >90% showing their catalytic function. UV irradiation of norbomadiene in the presence of Ni(CO)4 gives the following product resulting from a formal (2 + 2 + 2) cycloaddition reaction 571> ... 

One typical example is given by G. Kaupp with the [2+2] cycloaddition of diphenyl-acetylene to 1,4-dioxene leading to a bicyclic cyclobutene in good yield (cf. iv of this chapter). Another example is presented by T. Miyashi starting from naphthoquinonor-bomadiene. In an intramolecular reaction a quadricyclane derivative is quantitatively formed upon irradiation at X > 400 nm. It is assumed that this process involves the diketo-norbomadiene in its triplet state. Although the spin multiplicity of intermediates have not... 

Addition of acetone to the catalyst CrfCOjg or Cr(CO)4(norbomadiene) with UV irradiation gives both better selectivity and higher activity in the hydrogenation of trans-, rraws-hexa-2,4-diene in mixtures of hexadienes . 

However, unlike simple alkenes, this reaction produces predominately trans-isomers since the trans-isomer is much more stable that its cis-isomer. A third example is the valence isomerization of quadricyclene through a similar mechanism, which yields norbomadiene on irradiated SC particles, including CdS, TiOj, and ZnO [Eq. (4)] [105] ... 

Among possible alternatives to DPMR we discarded the cx.cx-elimination of benzidine 12 from biradical-zwitterions 2 and 3 and formation of carbene 23 based on the absence of trapped carbene products in experiments with cyclohexene and ethanethiol (Xgxc = 300 nm). However, norcaradiene 6 produces carbene 23 when subjected to 350-nm irradiation. Another plausible alternative to DPMR involves initial C-CN bond homolysis, which is energetically favorable in both excited states, followed by a photochemical reaction of Crystal Violet radical. We discarded this mechanism because none of the expected products of the photochemical reactions of Crystal Violet radical has been detected during the course of our work. Nevertheless, isolation of MGCN in the trapping experiment with ethanethiol points to the possible involvement of the C-CN bond homolysis in S. Finally, although norcaradiene, cycloheptatriene, and norbomadiene are valence tautomers and are easily interconverted into each other [188], products of the latter two kinds have been neither detected nor isolated in our study [66, 30]. 

Irradiation of the norbomadiene (41) produces the expected quadricyclane which undergoes a secondary photoreaction to give (42) by a (2 + 2 + 2) cyclisation followed by a [1.5]-hydrogen shift (Ivakhnenko et al). The efficient photodimerisation of the sterically hindered 1,4-dihydropyridine (43) in the solid... 

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