Photochemical Diels-Alder reaction

As final examples, the intramolecular cyclopropane formation from cycloolefins with diazo groups (S.D. Burke, 1979), intramolecular cyclobutane formation by photochemical cycloaddition (p. 78, 297f., section 4.9), and intramolecular Diels-Alder reactions (p. 153f, 335ff.) are mentioned. The application of these three cycloaddition reactions has led to an enormous variety of exotic polycycles (E.J. Corey, 1967A). 

A second synthesis of cobyric acid (14) involves photochemical ring closure of an A—D secocorrinoid. Thus, the Diels-Alder reaction between butadiene and /n j -3-methyl-4-oxopentenoic acid was used as starting point for all four ring A—D synthons (15—18). These were combined in the order B + C — BC + D — BCD + A — ABCD. The resultant cadmium complex (19) was photocyclized in buffered acetic acid to give the metal-free corrinoid (20). A number of steps were involved in converting this material to cobyric acid (14). 

Perfluorotetramethylthiadiphosphanorbornadiene and bis(trifluoromethyl) thiadiphosphole can be prepared by thermolysis of an adduct of methanol and hexakis(trifluoromethyl)-l,4-diphosphabarrelene with sulfur [113] (equation 23) Pyrolysis of the adduct of hexafluorinated Dewar benzene and phenyl azide results in ring expansion giving azepine, which photochemically yields an intramolecular 2-1-2 adduct, a good dienophile for the Diels-Alder reaction [114, //5] (equation 24) Thermolysis of fluonnated derivatives of 1,5-diazabicyclo-... 

In contrast with the [4 + 2]- --electron Diels-Alder reaction, the [2 + 2 thermal cycloaddition between two alkenes does not occur. Only the photochemical [2 + 2] cycloaddition takes place to yield cyclobutane products. 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

The intramolecular Diels-Alder reactions of photochemically generated trans-cycloalk-enones [162]... 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

Hoomaert has studied Diels-Alder reactions of pyridine oquinodimethane analogs generated from functionalized o-bis(chloromethyl)pyridines <96T(52)11889>. The photochemical cycloaddition of 2-alkoxy-3-cyano-4,6-dimethylpyridine with methacrylonitrile gives a bicyclic azetine, 6-alkoxy-3,5-dicyano-2,5,8-trimethyl-7-azabicyclo[4.2.0]octa-2,7-diene, in moderate yield <96CC1349>. Regiospecific hydroxylation of 3-(methylaminomethyl)pyridine to 5-(methylaminomethyl)-2-(17/)-pyridone by Arthrobacter ureafaciens has been reported <96MI173>. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

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... 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

Formation of bicyclo[3.1,0]hex-2-enes The photochemical Diels-Alder reaction ... 

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

The non-nitrogenous carbene precursor (102) was used for the photochemical generation of the carbene (103) without complications due to reactions of diazirine or diazo species. In the presence of alkenes, carbene (103) gave rise to cyclopropanes and in the absence of alkenes was proposed to undergo [1,2]-C shift to form (104), which suffered retro-Diels-Alder reaction to give a triene. 

Vinylindote radical-cations, for example that derived from 64, take part in a Diels-Alder reaction with alkenes. Subsequent oxidation of the initial product with loss of two protons and dimethylamine gives the pyrido[l,2a]indoie. Reaction is achieved either by direct electrochemical oxidation or by photochemical electron... 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Zhang W, Guo YP, Liu ZG, Jin XL, Yang L, Liu ZL (2005) Photochemically catalyzed Diels-Alder reaction of arylimines with N-vinylpyrrohdinone and N-vinylcarbazole by 2,4,6-triphenylpyrylium salt synthesis of 4-heterocycle-substituted tetrahydroquinoline derivatives. Tetrahedron 61 1325-1333... 

Diels-Alder, imino dienophiles, 65, 2 Diels-Alder, intramolecular, 32, 1 Diels-Alder, maleic anhydride, 4, 1 [4 -h 3], 51, 3 of enones, 44, 2 of ketenes, 45, 2 of nitrones and alkenes, 36, 1 Pauson-Khand, 40, 1 photochemical, 44, 2 retro-Diels-Alder reaction, 52, 1 53, 2 [6-h4], 49, 2 [3-h2], 61, 1 Cyclobutanes, synthesis ... 

In common with other azodicarboxylic acid derivatives, the uses of 4-phenyl-l,2,4-triazoline-3,5-dione are many. It undergoes a Diels-Alder reaction with most dienes11-14 and is, in fact, the most reactive dienophile so far reported.15 16 As with the formation of all Diels-Alder adducts the reaction is reversible, and in the case of the adduct with 3-j3-acetoxy-17-cyano-5,14,16-androstatriene, the reverse reaction can be made to proceed under especially mild conditions.14 An instance has also been reported of the dione photochemically catalyzing other retro Diels-Alder reactions.17 Along with the proven use of azodicarboxylic ester,18-18 the dione should be potentially important in the preparation of strained ring compounds. 

In analogy to olefins, Cjq undergoes a broad variety of cycloadditions (see Chapter 4 and Scheme 14.3). In many cases cycloadducts of Cjq exhibit the same stability as the corresponding non-fullerene based adducts. These reactions are very useful for the introduction of fimctionat groups. Among the most important cycloadditions are [4-1-2] cycloadditions such as Diels-Alder and hetero-Diels-Alder reactions, where Cjq reacts always as dienophile, [3-1-2] cycloadditions with 1,3 dipoles, thermal or photochemical [2-1-2] cycloadditions, [2-t-l] cycloadditions and others, for example, [8-1-2] cycloadditions. Among these general reactions several examples deserve special attention, since they reflect characteristic chemical properties of Cjq [36] ... 

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

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