Oxetanes, photochemical formation

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

Vargas, F., Rivas, C. Photochemical formation of oxetanes derived from aromatic ketones and substituted thiophenes and selenophenes. InternationalJournal of Photoenergy 2000, 2, 97-101. 

Table 8. The Influence of Carbonyl Substituent R on the Diastereoselectiv-ity of Photochemical Formation of Oxetanes...

Further examples using rapidly equilibrating screwed/helical/spiral molecules that happen to crystallize in chiral space groups have been provided by Sakamoto. [11c] These are photochemical formations of chiral oxetanes, / -lactames, oxazolidine-2,4-diones, aziridines, oxazolines, phthalides, and -thio-lactames. [11c] In most cases the ee values had to be improved by going to -78 °C or to low conversion . Whereas the latter technique is frequently preferred it is often dubious for the normal selective irradiations (only the educt absorbing the light), if the crystal reacts from its surface down into the bulk, multilayer by multilayer as these become transparent at very advanced local conversions. [14, 15] Thus, no basis exists for assumptions that the optical yield may increase to quantitative values at very low conversions, while the error limits will not allow to secure such claims. Nevertheless, there is the risk that... 

Cyanoalkenes are also substrates used for oxetane formation. Thus benzil (in its lowest triplet state) adds to the alkene 349, to yield the oxetane 350 (in 54% yield) as the main product. A detailed study of the photochemical formation of oxetanes by the addition of E -l,2-dicyanoethene and Z-l,2-diethoxyethene to the adamantanones 351 has been reported . In addition Turro, Le Noble and coworkers have studied the photochemi-... 

Gotthardt, H. and Lenz, W., Unusually high asymmetric induction in the photochemical formation of oxetanes, Angew. Chem. Int. Ed. Engl., 18, 868, 1979. 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

The other photochemical reactions of simple carbonyls mentioned earlier in this chapter—type I cleavage (a-cleavage) and oxetane formation—will be discussed in Chapter 4. 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

The formation of 3-pyrrolylcarbinols (280) from the photochemically induced reaction of pyrrole, or its 1-alkyl derivatives, with aliphatic aldehydes and ketones is thought to proceed via an oxetane intermediate (279) (79JOC2949). In contrast, the analogous reaction of 1 -phenylpyrrole with benzophenone leads to the formation of the diphenyl(2-pyrrolyl)car-binol, whilst the oxetane (281) has been isolated from the photoaddition of 1-benzoylpyrrole and benzophenone (76JHC1037, B-77MI30500). 2-Benzoyl-1-methylpyrrole undergoes a normal Paterno-Buchi photocyclization with 2,3-dimethylbut-2-ene, via the n -> v triplet... 

Photochemical isomerizations by intramolecular 4n + An cycloaddition of carbon oxygen linked bichromophoric anthracenes to give oxetane derivatives have not been reported yet. Upon irradiation (X > 400 nm) in either toluene or ethyl acetate, the methoxycarbonyl substituted carbon oxygen linked bichromophoric anthracene 14 indeed isomerizes smoothly and efficiently (cp — 0.45). However, the two products, obtained in an approximate ratio of 5 1, are anthrone derivatives 15 and 16 whose formation can be rationalized by migration of the anthryloxy moiety [60], An analogous photolytic rearrangement has been found for 9-anthryloxy substituted dianthrylethylenes (see Section III.A). 

Another important class of cycloaddition reactions is the formation of oxetane rings between a photoexcited carbonyl compound and an unsaturated molecule. These reactions also occur probably through an exciplex although these exciplexes are non-fluorescent as they are formed from the triplet state of the ketone or aldehyde. The formation of the four-membered oxetane ring is an interesting example of a typical photochemical reaction... 

PATERNO-BUCHI REACTION. Formation of oxetanes by photochemical cycloaddition of carbonyl compounds to olefins. 

The synthesis of oxetanes by the photochemical 1,2-cycloaddition of the carbonyl function in aldehydes and ketones to alkenes [Eq. (69)] was first reported by Paterno274 in 1909, and later reinvestigated by Biichi 275 in 1954. This reaction has recently been extensively reviewed.276, 277 The formation of the oxetane is apparently the result of addition of excited n, n triplet carbonyl to an alkene, although for certain aromatic aldehydes and ketones the mechanism is less clear.278... 

Intramolecular cycloaddition, particularly in y,unsaturated ketones,348 has also been reported. Recent examples include the photochemically induced conversion of the cyclohexanone (397) to the two possible adducts (398 and 399)349 ancj the formation of the oxetans (400) from 5-acylnorbomenes (401 ).350 The introduction of an ether oxygen into the unsaturated ketone... 

FMO calculations using PM3-C1 were used to investigate the regioselectivities obtained by the photochemical reactions between 2-pyridone and pcnta-2,4-dienoate.46 The hard and soft acid-base principle has been successfully used to predict product formation in Patemo-Buchi reactions.47 The 2 + 2-photo-cycloaddition of homobenz-valene with methyl phenylglyoxylate, benzyl, benzophenone, and 1,4-benzoquinone produced the corresponding Patemo-Buchi products.48 The photo-cycloaddition of acrylonitrile to 5-substituted adamantan-2-ones produces anti- and svn-oxetanes in similar ratios irrespective of the nature of the 5-substituent49... 

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