Ketones, Paterno-Biichi reaction

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

With many other ketones and aldehydes, reaction between the photoexcited carbonyl chromophore and alkene can result in formation of four-membered cyclic ethers (oxetanes). This reaction is often referred to as the Paterno-Biichi reaction.127 128 129... 

Sauers and coworkers have applied the Paterno-Biichi reaction to engeneral formula 427 (Scheme XXXIV) Reductive cleavage of these products with lithium aluminium hydride is also regioselective and leads, following oxidation, to ketones... 

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

The Paterno-Biichi reaction has been employed in the synthesis, often in high yield, of a large variety of substituted oxetanes. In addition to simple aliphatic and aromatic alkenes, cycloaddition of ketones to, for example, fumaronitrile,284 l,3-diacetylimidazolin-2-one288 [Eq. (73)], and allenes286 has been reported. Allenes yield both 1,5- and l,6-dioxaspiro[3.3]heptanes as well as the 2-alkylidene-oxetane this is illustrated for benzophenone and tetramethylallene in Eq. (74). Cycloaddition of ketones to ketenimines to form 2- and... 

The intramolecular equivalent of the Paterno-Biichi reaction has been observed in a number of unsaturated ketones. A series of y,S-unsaturated ketones (278) are converted in this way into the oxabicyclo[2.2.0]hexanes (279) and the oxabicyclo[2.1.1]hexanes (280), both photoproducts being the result of 1,2-cycloaddition.303... 

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. 

The Paterno-Biichi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes.1108,1109 The excited state of the ketone is n-n, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. Often it is the triplet state which is involved,1110 but occasionally the singlet state is important. The orientation usually observed for C- and X-substituted alkenes is usually counter-thermodynamic, with the more-substituted atoms bonded to each other.1111,1112... 

The following tables contain a representative survey of inter- and intramolecular Paterno-Biichi reactions with the emphasis on cycloadditions of aldehydes (Tables 9 and 14) and ketones (Tables 10 and 14). In addition, examples of cycloadditions of 1,2-diketones, a-oxocarboxylic esters, simple esters, and imides are presented in Tables 11 -13. The formation of side products and further transformations to consecutive products are denoted when possible. 

Photocyclization is not limited to two alkene partners, and one important reaction involves cycloaddition of a carbonyl and an alkene. This reaction was discovered by Paterno and Chieffi in 19093U and expanded by Biichi et al. 3l3 thus it is called the Paterno-Biichi reaction.319 The product is an oxetane (378) and the reaction is believed to proceed via a diradical intermediate (377). The regioselectivity of the reaction can be explained by molecular orbital considerations, however. Electron-rich carbonyls of both aldehydes or ketones can be used. The alkene moiety is usually electron rich but this is not a requirement. 

An extensive review of the selective formation of oxetanes using photochemical 2 + 2-cycloaddition reaction (Paterno-Biichi reaction) has been published. Methods that describe the control of stereo- and regio-selectivity of synthetically important oxetanes have been discussed. The bicyclic guanidine, l,5,7-triazabicyclo[4.4.0]dec-5-ene, catalyses the 2 -l- 2-cycloaddition of allenoates with trifluoromethylaryl ketones to yield highly substituted 3-alkyl-oxetan-2-ylidenes in high yields. ... 

In the presence of a olefin, photo excited ketone add to it and yields oxetane, the reaction is known as Paterno Biichi reaction. 

The reaction is initiated by low-lying n,K state. Most of the Paterno-Biichi reactions are reported to involve (n, t ) ketones. In some conjugated ketones it is difficults to distinguish between and Ti,n transitions. Evidence support reactions by both of the transition states in conjugated ketones. It is assumed that the radical like oxygen atom of (n, n ketone adds to the olefin to give preferentially the most stable biradical intermediate. In the addition of (n,K ) benzophenone to trimethylethylene the element of choice lies between a secondary radical and a tertiary radical, since both are same in other respects the tertiary radical is more stable and this mode of addition is preferred. 

The biradical hypothesis is helpful in predicting the major product in Paterno-Biichi reaction. It is proposed that the reaction involves an exciplex (a complex of excited ketone and olefin), that break to the biradical. 

Two side reactions can limit the utility of the Paterno- Biichi reaction. The presence of active hydrogen [allylic hydrogen atom] on the olefin makes hydrogen abstraction by excited ketone which complete with Paterno-Biichi reaction. If the triplet energy of ketone is comparable to the triplet excitation energy of olefin, energy transfer will compete with the reaction. 

Ketones with homotopic faces have been widely used to determine the stereoselectivity in Paterno-Biichi reactions with E- and Z-isomers of electron-rich and electron-poor olefins. The photocycloaddition of... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

The Patemo-Biichi reaction is a milestone in organic photochemistry (Scheme 3.1). Paterno and Chieffi [1] reported obtaining oxetanes from the photocycloaddition of ketones to olefins in 1909, but this reaction was recognized as an important synthetic reaction only after the work of Biichi et al. [2]. 

In the ketone (481), the steric hindrance provided by the gem dimethyl group is not enough to change the regioselectivity all it does is to cause the appearance of some Paterno-Biichi product (with the usual regioselectivity for that reaction). 

The low quantum yield indicates that intermediate complexes and diradicals decay unproduc-tively. In general, Norrish type II photoreactions and other hydrogen abstraction processes must be overcome in order to achieve successful cycloaddition. Only in a few reported cases is intramolecular hydrogen abstraction a serious competitive reaction path. For example, the cycloheptenyl-substituted ketone 5 yields an oxetane 6 in addition to a cyclobutanol derivative 758, whereas the unsaturated cycloheptanone 8 only gives oxetane products 9 and 10 on irradiation59. The main product 10 was converted in two steps to azulene in 25 % overall yield. The reaction sequence 11 - 12 - 13 also demonstrates the high synthetic potential of the intramolecular Paterno-Biichi reaction61. 

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