Photochemical Diels-Alder

Formation of bicyclo[3.1.0]hex-2-enes the photochemical Diels-Alder... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

However, photo-activation puts so much energy into the molecule, that many pathways become available to the first electronically excited state in addition to the relatively simple pericyclic change. For this reason, none of the photochemical reactions above can be guaranteed to be pericyclic, and all that one should take from these results is the very strong and suggestive contrast with the rules for thermal cycloadditions. This contrast is accentuated by the observation that photochemical Diels-Alder reactions are very rare, in spite of the ease with which 6-membered rings are normally formed. 

Thiophene reacts with alkynes to give the corresponding benzene through a photochemical Diels-Alder reaction followed by extrusion of a sulfur atom (Scheme 9) (72TL1909 73CB674). Thienyl derivatives with electron withdrawing substituents allow adducts isolation (78CJC1970). 

The thermal Diels-Alder reaction ([4 + 2] cycloaddition) is widespread in the synthesis of fullerene derivatives. In contrast, only a few examples of the photochemical Diels-Alder reaction in solution or in the solid state are known. The first example is described by Tomioka and coworkers [249], Irradiation of ketone 73 and C6o at 10°C with a high pressure mercury lamp through a Pyrex filter led to the formation of 61-hydroxy-61-phenyl-l,9-(methano[l,2]benzenomethano) fullerene 75 (Scheme 29). This compound is unusually unstable and yields the monoalky 1-1,2-dihydrofullerene 76 either by silica gel chromatography or upon heating. 

Recently, Fukuzumi et al. reported the photochemical Diels-Alder reaction with Danishefsky s diene [252], A mixture of C6o and the stereochemically defined (lii,3Z)-l,4 disubstituted Danishefsky s diene 77 was irradiated for 9h at -30°C to avoid the thermal reaction (Scheme 30). A high pressure mercury lamp... 

The Diels-Alder reaction is a well-established synthetic method that allows the creation of two new carbon-carbon bonds and leads to the formation of six-membered rings. Eventually, the photochemical reaction can advantageously compete with the thermal process. For instance, anthracene undergoes thermal and photochemical Diels-Alder reactions with alkenes, but the photoinduced addition of maleic anhydride to the homochiral anthracene, as depicted in Scheme 9.28, is faster than the thermal reaction and occurs with excellent diastereoselectivity (only one diastereoisomer) [42]. 

Jones, S. and Atherton, J.C. (2001) Highly diastereoselective photochemical Diels-Alder reactions towards the development of a photoactivated chiral auxiliary. Tetrahedron Asymmetry, 12, 1117-1119. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

The photoinduced electron transfer pathway has been unequivocally demonstrated in a photochemical Diels-Alder reactions between C6o and Danishefsky s dienes (Scheme 11) [304, 305]. 

The transient spectra of C6o at Amax = 1080 nm formed in photoinduced electron transfer from Danishefsky s diene to Cso have been detected, accompanied by the decay of Ceo in a laser flash photolysis of the reaction system [305]. The observed rate constant agrees well with the predicted rate constant on the basis of an electron transfer mechanism shown in Scheme 11 [305], In the photochemical Diels-Alder reaction of Cgo, a stereochemicaUy defined (l , 3Z)-l,4-disubstituted Danishefsky s diene is used as a stereochemical probe. In the photochemical Diels-... 

It has been shown that the rearrangement occurs via benzohexatrienes from conrotatory ring opening, which then undergo photochemical Diels-Alder reaction. ... 

Maruyama et al.li0 effected intermolecular photocyclization to substituted, condensed thiophenes. A representative series of reactions is given in Scheme 20. This photochemical Diels-Alder reaction between dienophile 296 and diene 297, with elimination of the elements of HBr and CH3OH from the adducts, gives 298 (62%, m.p. 221.5°C) and 299 (8%). Reactions of 298 produced 300 (32%, m.p. 157°C) and 301 (60%), derivatives of the parent condensed thiophene (4). As a partial proof of structure, 298 was desulfurized to 302 (13%). Thermal Diels-Alder reaction between 297 and 1,4-naphthoquinone gave only 6% of 298 (in HOAc), probably because the diene is unstable to heat. The much larger yield of 298 than of 299 was... 

A theoretical treatment of the photochemical Diels-Alder reaction has been published.58 Pusset and Beugelmans have measured the fluorescence spectra and singlet energies of several conjugated dienes (81).59... 

The occurrence of photochemical Diels-Alder reactions has always seemed slightly at variance with Woodward-Hoffmann concepts, so it is interesting that Epiotis and Yates have now described a theoretical treatment of these processes. 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

Construct a MO correlation diagram similar to Figure 11.66 (page 737) to show that the photochemical Diels-Alder reaction is forbidden by the principles of orbital symmetry. 

What about the photochemical Diels-Alder reaction The observation that this reaction is most uncommon leads us to the immediate suspicion that there is something wrong with it. Usually, the absorption of a photon will promote an electron from the HOMO to the LUMO. In this case, the lower energy HOMO-LUMO gap is that in the diene partner. Absorption of light creates a new photochemical HOMO for the diene, 3, and now the HOMO-LUMO interaction with the dienophile partner involves one antibonding overlap. Both new bonds cannot be formed at the same time (Rg. 20.22). So this photochemical Diels-Alder reaction is said to be forbidden by orbital symmetry. ... 

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