Photochemical oxetane formation

Abe, M. Photochemical oxetane formation addition to heterocycles. CRC Handbook of Organic Photochemistry and Photobiology (2nd... 

Funke, C. W., Cerfontain, H. Photochemical oxetane formation the Paterno-Biichi reaction of aliphatic aldehydes and ketones with alkenes and dienes. J. Chem. Soc., Perkin Trans. 2 1976, 1902-1908. 

Table 6. Influence of the Chiral Auxiliary on the Stereoselectivity of Photochemical Oxetane Formation"...

Mattay, J. and Buchkremer, K., Thermal and photochemical oxetane formation with a-ketoesters. 

Mattay, J. and Buchkremer, K., Thermal and photochemical oxetane formation. A contribution to the synthesis of branched-chain aldonolactones, He/v. Chim. Acta, 71, 981,1988. 

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). 

Bach and coworkers observed both regioselective and stereoselective oxetane formation during the PB reaction of acyclic vinyl ethers (Scheme 7.26) [15n], The stereoselectivity observed for such photochemical reactions cannot be explained using the Griesbeck Model, even though triplet, 14-biradicals were proposed as intermediates. Thus, the stereoselectivity was proposed to be largely dependent on product stability. 

A is exothermic. An additional advantage is that one has a wider margin to select spin-state specific photochemical transformations. Representative examples include ds-trans-olefin isomerization (Eq. 49), olefin dimerization (Eq. 50), oxetane formation (Eq. 51), dienone rearrangement (Eq. 52), di-vr-methane rearrangement (Eq. 53), azoalkane denitrogenation (Eq. 54a, b), and photocyclization (Eq. 55). 

Figure 5.34 shows the Stern-Volmer plot for triplet quenching of the photochemical addition of benzaldehyde to 2,3-dimethyl-2-butene (cf. Section 7.4.3) by piperylene. The ratio of the quantum yield 4> of oxetane formation without quencher to that with quencher is plotted against the quencher concentration [Q]. From the resulting straight line it may be concluded that the reaction proceeds via a single reactive state which is assigned as (n, r ). 

In addition to the normal photochemical reactions of saturated ketones, p,y-unsaturated carbonyl compounds undergo carbonyl migration via a [1,3] shift. Compound 139 in Scheme 45 represents a typical example. Compounds with an alkyl substituent in the P position such as 140 may also undergo a Norrish type II reaction (Kiefer and Carlson, I%7), while for ketones with electron-rich double bonds such as 141, oxetane formation is also observed (Schexnayder and Engel, 1975). 

Photo-addition of alkenes to A methylnaphthalene dicarboxamides in benzene has been studied. The structure of the arene moiety in the imide was important in determining the reaction path. Mainly cyclobutane and oxetan formation occurred. The dicarboximide (342) undergoes photochemical cyclization with incorporation of methanol to yield the two products (343) and (344) in 55 and 16% respectively. This type of cyclization appears to be quite general for such systems and is also reported for the imides (345) and (346). A variety of products resulting from aminolysis, reduction, and radical coupling is produced on irradiation of the phthalimide (347) in diethylamine. ... 

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