Oxetanes preparation, photochemical

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. 

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... 

There are a wide variety of other synthetic methods for the preparation of oxetanes however, most of these are not very general. They frequently require starting materials which are difficult to prepare, and rarely give high yields (never as high as the better photochemical preparations). It is clear then that in comparison with the alternative methods of oxetane synthesis, the photocycloaddition reaction is more generally useful and convenient. This synthetic utility justifies a brief description of experimental conditions. 

Arnold, D. R., R. L. Hinman, and A. H. Glick Chemical properties of the carbonyl n,7t state. The photochemical preparation of oxetanes. Tetrahedron Letters 1964, 1425. 

Before describing the regioselective, site-selective and stereo-selective preparation of oxetanes via the PB reaction, the mechanism of the photochemical reaction will be briefly summarized. The reason for this is that an understanding of the reaction... 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

This photochemical umpolung is useful from the perspective of chemoselectivity in synthetic transformations. In addition, oxetane-containing products, prepared regioselectively, can be valuable synthetic intermediates. Both oxetane and vinyloxetane derivatives react with nucleophiles in intermolecular and intramolecular condensations and with transition metals and carbenoids. A second feature of oxetane-containing photoproducts, which has important implications, is that they can act as vehicles for subsequent manipulations and stereocontrolled synthesis. For example, the oxetane photoproduct (6)... 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Various photoreactions in which the silicon atom plays a less important role have been reported. The isolation of trans-1,2-diphenyl-4,4,5,5,6,6-hexamethyl-4,5,6-trisilacycloheptene (185), formed by irradiation of the corresponding cis-isomer (186), has been described the stability of this system is attributed to the long silicon-silicon bond lengths.Evidence for the intermediacy of a photochemically generated metastable trans-cycloalkene has also been reported in l,l,4,4-tetramethyl-l,4-disilacyclohept-2-ene. A novel [,2 + 2] photocycloaddition to give the bicycles (187) has been reported in the diallylsilanes (188) in the presence of 1,4,-dicyanonaphthalene. Intramolecular [ 2 + 2] photoaddition has also been observed in bis- and tetrakis-(4-vinyl-benzyl) silanes and their germane analogues,and the oxetanes (189) and (190) have been prepared by irradiation of 1,1-dimethyl-2,5-diphenylsilacyclopentadiene in the presence of benzophenone. 

Arnold, D. R., Hinman, R. L., Glick, A. H., Chemical Properties of the Carbonyl Normal ji State The Photochemical Preparation of Oxetanes, Tetrahedron Lett. 1964, 1425 1430. 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

Interestingly, many substituted oxetanes (oxacyclobutanes) can be prepared by an allowed generally stereospecific, photochemical addition of aldehydes... 

The photochemical preparation of oxetanes is the domain of the Patemo-BUchi reaction, but only a few examples have been published where these heterocycles were prepared in the course of a Norrish/ Yang reaction. This is presumably due to the remarkable tendency of the corresponding 1,4-biradicals to undergo Norrish type 11 cleavage. Lewis and Turro explored the photochemical behavior of a-alkoxyacetophenones as early as 1970 and found that oxetanes are generally formed as minor products only. 

When unsymmetrical carbonyls (R Rj) are used for the photochemical cycloaddition with furans, the inherent stereoselectivity should be considered, i.e., exo-oxetanes vs. endo-oxetanes (Scheme 9). Shima initially investigated the photocycloaddition reaction in 1965 and an NMR study by Whipple and Evanega later provided evidence for the preference of the exo-selective cycloaddition. The exo-selective formation of the bicyclic oxetanes and their chemical transformations are now accepted as powerful tools for preparing the synthetically and biologically important materials. " ... 

Besides their liquid crystallinity reactive mesogens exhibit an additional feature. They carry functional groups, such as oxetanes or acrylates, which can be polymerized and densely crosslinked polymer networks are formed. If the crosslinking takes place in the liquid crystalline phase of the material, then the liquid crystalline order is preserved in the resultant polymer network, which is thermally stable up to the decomposition temperature of the polymer network. Furthermore, the liquid crystalline polymer network becomes completely insoluble upon crosslinking, which allows the preparation of multilayer devices using solution processes. In many cases the crosslinking is carried out by a photochemically initiated reaction. 

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