Cycloaddition Reactions of Carbonyl Compounds with Alkenes

5 Cycloaddition Reactions of Carbonyl Compounds with Alkenes 

The photochemical [2 + 2]-cycloaddition reactions of carbonyl compounds with alkenes are known as the Paterno-Buchi reactions This reaction was first reported by the group of E. Patemo and G. Buchi on the reaction of benzaldehyde with 2- 

The Study of the mechanism of the reactions indicated that for aromatic carbonyl compounds, the reaction occurs through a triplet excited state of the carbonyl compound, whereas for aliphatic carbonyl compounds through both singlet and triplet excited states of the carbonyl compound. The reaction is stereospecific for aliphatic carbonyl compounds and gives syn adduct. For cyclic alkenes, kinetically controlled endo-isomer is the major product. The regioselectivity of this cycloaddition reaction depends on the stability and steric interactions of the intermediate diradical. In the reaction of benzophenone with isobutene, the major product is derived from the stable diradical. 

The bfetime of Sj is too short and so oxetane formation is much faster than C-C bond rotation. The formation of oxetane can be explained from FMO approach. The frontier orbital interactions between half occupied n orbital of carbonyl oxygen atom (LUMO) with the k orbital of electron rich alkene (HOMO) take place to form a C,C-diradical. 

The reaction of benzophenone with dihydrofuran gives regioselective and almost single product 111 [64]. 

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Scheme 6.9. Photochemical Cycloaddition Reactions of Carbonyl Compounds with Alkenes... 

Photochemical cycloaddition reactions of carbonyl compounds with alkenes... 

The photochemically induced [2+2] cycloaddition (Patemo-Btlchi reaction) of carbonyl compounds with alkenes gives oxetanes. ... 

Anhydro-4-hydroxyoxazoIium hydroxides, such as compound (231), behave as carbonyl ylides (232) in cycloaddition reactions, yielding bicyclic adducts with alkenes and carbonyl compounds (Scheme 24). The adducts produced by combination with alkynes fragment spontaneously in a retro-Diels-Alder reaction, giving furans (equation 57). The formation of a furan by the action of DMAD on the 4(5//)-oxazolone (233) shows that the latter exists in equilibrium with the mesoionic tautomer (234 equation 58) (79JOC626). 

U nder thermal or Lewis acid catalyzed conditions, reactions of fluoroalkyl carbonyl compounds with alkenes usually provide the ene adduct. " One example of a [2 + 2] cycloaddition between hexafluoroacetone and an alkyne has been reported. ... 

Mechanism of Ozonolysis (Criegee mechanism) The initial step of the reaction involves a 1,3-dipolar cycloaddition of ozone to the alkene leading to the formation of the primary ozonide (molozonide or 1,2,3-trioxolane), which decomposes to give a carbonyl oxide and a carbonyl compound. The carbonyl oxides are similar to ozone in being 1,3-dipolar compounds and undergo 1,3-dipolar cycloaddition to the carbonyl compound with the reverse regio-chemistry, leading to a relatively stable secondary ozonide (1,2,4-trioxolane) (Scheme 5.47). 

Compared to the hetero-Diels-Alder reaction of carbonyl compounds and derivatives with dienes, where only a limited number of catalytic and enan-tioselective reactions have been reported, the number of asymmetric hetero-Diels-Alder reactions in which the ketone or imine functionality is part of a heterodiene is much higher. In contrast, there are only a few examples of using a,p-unsaturated aldehydes in inverse hetero-Diels-Alder reactions. In the case of the inverse electron demand hetero-Diels-Alder reaction, the ketone or imine functionality is part of an a,(I-unsaturated system, which reacts in a cycloaddition reaction with an electron-rich alkene. The inverse electron demand hetero-Diels-Alder reaction is primarily controlled by a LUMOdiene-HOMOdienophiie interaction, which can be found, for example, in the reactions... 

An extensive review of the synthesis of a wide variety of five-membered heterocyclic compounds, via the formal 3-1-2-cycloaddition reactions of aziridines with alkenes, alkynes, nitriles, carbonyl groups, and heterocumulenes, has been presented." Supercritical CO2 has been used as the solvent in the formal 3-1-2-cycloaddition reactions of A-benzyl- and A-cyclohexyl-2-benzoyl-3-phenylaziridines with allenoates to yield pyrrole derivatives." The Lewis acid-catalysed intramolecular formal 3-1-2-cycloaddition reactions of 2-methyleneaziridines with tethered alkenes or alkynes (23) yielded cw-octahydrocyclopenta[c]pyrroles (24) after reductive workup. The reaction mechanism proceeds in a stepwise manner via a 2-aminoallyl cation (Scheme The Cu(I)/DTBM-Segphos-catalysed 1,3-dipolar cycloaddition reactions of a-silylimines and activated alkenes yielded highly enriched 5-unsubstituted a-quaternary proline cycloadducts with excellent diastereo- and enantio-selectivities (73-99% The... 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

Scheme 30a,b [2 + 2] Cycloaddition reactions of excited carbonyl compounds with the alkenes substituted by electron-accepting (a) and -donating (b) groups... 

Isosydnones (146) react with alkynes to give pyrazoles (150). For example, 4,5-diphenylisosydnone (146, R = R = Ph) and ethyl phenyl propiolate gives 4-ethoxycarbonyl-l,3,5-triphenylpyrazole (150, R = R = R = Ph, R = CO Et) identical with the product from 4,5-diphenylsydnone (1, R = R = Ph). The rate of 1,3-cycloaddition for isosydnones (146) is relatively slow in comparison with sydnones (1).2o, 04 number of other cycloaddition reactions of isosydnones with alkenes, alkynes, and carbonyl compounds have been reported. ... 

By analogy with the formation of dihydropyrans from unsaturated carbonyl compounds and alkenes (see Section 2.24.2.7.l(i)), the synthesis of 4//-pyrans from the [4 + 23-cycloaddition of unsaturated carbonyl compounds and alkynes would seem to offer some potential. Such a reaction has indeed proved of value, but examples are largely restricted to the use of ynamines as the dienophile (76BSF987). 

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