Photochemical cyclo additions

From the Contents J. Michl Physical Basis of Qualitative MO Arguments in Organic Photochemistry. K.-D. Gundermann Recent Advances in Research on the Chemiluminescence of Organic Compounds. W.C. Herndon Substituent Effects in Photochemical Cyclo-addition Reactions. W.-D. Stohrer, P. Jacobs, K.H. Kaiser, G. Wiech,... 

One additional feature that distinguishes the frontier orbital method from orbital symmetry analysis is its ability to separately analyse triplet state processes (Fukui, 1971) in photochemical reactions. However, it is notably difficult in excited state reactions, even if they are stereo ecific, to be sure of concertedness. Certain stereo ecific photochemical cyclo-addition reactions, hitherto thought to be concerted, have recently been re-interpreted as radical cage recombination reactions. The triplet states of many imsaturated moleculesf are lower in energy than the corresponding singlet states because... 

Consider Fig. 5.2 it is at once apparent that the supra-supra and antara-antara interactions should lead to transition state complexes of high energy since they are of the —H type. The concerted thermochemical dimerization of two ethylenic molecules should therefore proceed by way of supra-antara or antara-supra interactions (if geometrically possible) because they provide pathways to the low energy + A-H transition state. Conversely, the first excited state photochemical cyclo-addition should occur by either supra-supra or antara-antara modes because once again we have a + A-H transition state available (i.e. the excited form of a — H transition state). 

In attempts to isolate the aforementioned irradiated products of thymine derivatives at lower temperature, the photochemical reactions were carried out in frozen aqueous solutions containing either thymine or 1,3-dimethylthymine. The resulting products were not hydrates, but had elementary analyses corresponding to the starting material. Molecular weight determination indicated that the products were dimers, and infrared and ultraviolet spectral data suggested cyclo addition across the 5,6-double bond to form a cyclobutane system... 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

This classification, valid for concerted cis-cis (or supra,supra) processes, confirmed that thermal Diels-Alder reactions (m-f-/i = 6) can be (but not, must be) one-step reactions, while predicting that photochemical 1,4-cyclo-additions should be multistep reactions 1,3-cycloadditions should behave analogously, since 1,3-dipoles are four 7r-electron systems. Common 1,2-... 

ROMP and PROMP are very useful methods to synthesize a number of novel materials with unique properties. Poly(cyclooctenes), poly(norbornenes) and poly(acetylenes) were discussed in more details and some of their properties like Tg, cristallinity, oxygen permeability, dielectric properties etc. listed. The polymerization of the cycloolefins was done either thermally or photochemically with the "old" Ru(II)-salts and the later developed Ru-phosphines as catalysts, whereas substituted acetylenes were photo-polymerized with W-, Mo- and Ta-catalysts. In addition, polymeranalogous transformations of the double bonds in ROMP polymers, (additions and cyclo-additions, epoxidation, (photo) crosslinking etc.) were discussed. We are convinced that these materials and systems will find useful applications in the near future. 

K. Jahnisch, Photochemical generation and [4 -t 2]-cyclo-addition of singlet oxygen in a falling film microreactor, Chem. Ing. Tech. 2004, 76, 630-632. 

The three possible topological interactions in [ 2 + 2] cyclo-addition reactions are shown in Fig. 5.2 again the basis molecular orbitals of the ethylene components are considered. In the supra-supra and i v antara-antara combinations there are no out of phase orbital overlaps (or two if the signs are reversed on one ethylene component). In the supra intara mode there is one out of phase overlap. Since there are four electrons involved, the Mobius type interaction (i.e. supra-antard) should be preferred the other combinations should therefore be possible under photochemical control. These results accord with the previous findings of orbital symmetry theory. 

For the various reactions in Fig. 4.3 only the 0 2jand 4 interactions require summation. The total of such terms must be odd for a thermal pericyclic reaction and even for a photochemical process. Hence, all of the reactions shown are thermally allowed processes excepr the [,2 + 2J cyclo-addition which is an allowed photochemical reaction. 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

Since the addition of methylene to an olefin should be exothermic, with the evolution of about 90 kcal/mole, isomerisations of the initially formed cyclo-propanes are very likely, since they only need about 64 kcal/mole. RRKM-studies demonstrate that this isomerisation should be faster than the rearrangement of cyclopropanes 32, 33 to the pentenes 34, 35 Numerous studies of the photochemical generation in the gas phase provided conclusive evidence in favour of these findings uo.iii.iis). 

More recently, photochemical reactions of 138 with cyclic and acyclic olefins have been described. When 138 is irradiated (Pyrex) with cyclo-heptatriene, [4 + 4]- and [4 + 6]-adducts (247-250, Scheme 16) are obtained in addition, photodimer 242 and o-dibenzoylbenzene (140) were isolated. The ratio of the [4 + 6]-adducts to the [4 + 4]-adducts [(249 + 250)/(247 + 248)] is increased in air-saturated benzene solutions compared with oxygen-free benzene solutions, and enhanced in heavy atom solvents (e.g., chloroform compared with cyclohexane) furthermore, this ratio is decreased in the presence of the triplet quencher azulene. These observations suggest that [4 + 6]- and [4 + 4]-adducts are formed by different mechanisms. 

However, through quartz at 95% conversion, a mixture of I (62%) and l-carboxymethyl-2-phenylcyclooctatetraene (IT) (38%) was obtained. Compound n could be obtained exclusively (92%) from triplet-sensitized irradiation of I with high-energy sensitizers or from the sensitized reaction of methyl phenylpropiolate with benzene. From their experiments with sensitizers, the authors concluded that the primary adduct is formed from triplet alkyne (ET < 69 kcal/mol 1) and ground-state benzene and that the formation of I also proceeds via a triplet state in a two-step radical reaction. Hanzawa and Paquette [64] have also used the photochemical addition of an alkyne to benzene to produce a derivative of tetra-cyclo[3.3.0.02,4.03 6]oct-7-ene. 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

An additional example is a two-step approach to the preparation of 1,5-cyclo-decadiene. By a photochemical cycloaddition of a substituted cyclobutene and 2-cyclohexenone, a strained tricyclo[4.4.0.02,5]decane system is generated. Thermolysis of the tricycle gave 1,5-cyclodecadiene [52]. 

Breslow and co-workers have performed some of the most outstanding work in this field and their results initiated a flurry of research. They found that the rates of reaction and selectivity in the Diels-Alder reactions are improved in an aqueous system (Figure 3.6). Additionally, the presence of salts or 3-cyclo-dextrins can enhance the hydrophobic effect, which causes organic molecules to cluster together in aqueous solution, and further accelerate the Diels-Alder reaction. It should also be noted that related photochemical [2 + 2] additions can be performed using water, and in some cases these show similar rate enhancements due to hydrophobic effects. 

Cyclopentadiene is more reactive towards diazoalkanes. It undergoes addition of diazomethane (and diazoethane) and thermal or photochemical deazetization of the adduct to give bi-cyclo[3.1.0]hex-2-ene or 6-methylbicyclo[3.1.0]hex-2-ene in good yield.Polycyclic systems incorporating strained cyclopentene units also react rapidly with diazoalkanes and cyclo-propanations via this route are relatively easy. Hence the addition of diazomethane to benz-valene 13 gave an 83% yield of the 4,5-dihydro-3//-pyrazole, which on photolysis gave tetra-cyclo[4.1.0.0 ". 0 ]heptane (27%) similar reactions are known for many other diazoalkanes. Addition of diazoethane to norborn-2-ene (14a) and thermal or photochemical... 

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