Carbonyl compounds Paterno-Biichi reaction

Cycloadditions of olefins and carbonyl compounds (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)... 

A clear division of Paterno-Biichi reactions into several distinct categories is possible on the basis of the type of reacting carbonyl compound (alkyl or aromatic), the excited state responsible for reaction (n—71 or Ti—n, singlet or triplet), and the type of olefin (electron deficient or electron-rich). Some examples of these reactions are given in Eqs. 7—11, where only the oxetane products are shown. 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The photocycloaddition of a carbonyl compound to an alkene was discovered as early as 1909 by Paterno and Chiefifi [78] who employed sunlight as the irradiation source. In the 1950s the reaction was more intensively investigated by Biichi et al. [79] using artificial light sources. The Paterno-Biichi reaction has been studied mechanistically [80] and some important aspects are summarized in Scheme 37. Upon n r -excitation (1=280-350 nm), aldehydes... 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

The reactivity and selectivity trends described for the intermolecular version of the Paterno-Biichi reaction are also valid for the intramolecular version. Special aspects of intramolecular photocycloadditions have been reviewed [157]. A crucial role for the reactivity mode of unsaturated carbonyl compounds is the tether length, i.e., the distance between reactive carbonyl and C-C double bond. Conjugated systems, i.e., oc,(3-unsaturated carbonyls, prefer photodeconjugation or photocycloaddition invovling the C-C double... 

Cycloadditions may proceed via concerted or nonconcerted mechanisms. Photochemical [2 + 2] cycloadditions take place via triplet intermediates [20]. Photochemical cycloaddition reactions of olefins to carbonyl compounds known as Paterno-Biichi reactions [21]... 

Carbonyl Compounds Oxetane Formation (Paterno-Biichi Reaction)... 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

In the previous sections the multiplicity of the excited state has been discussed. In general, electron-deficient alkenes react preferentially with alkanones in the nit singlet state preserving the stereochemistry of the alkene (see Section I.6.I.4.3.2.)6,, 3 27,47-49. Note that ZjE isomerization of the alkenes can precede and as a consequence change the stereoselectivity of oxetane formation61 J 27. Nevertheless, the majority of Paterno- Biichi reactions are concerned with electron-rich olefins and here triplet states of the carbonyl compounds are most likely involved. As briefly discussed in Section 1.6.1.4.3.1., 1,4-diradicals play a decisive mechanistic role and consequently also control the stereochemical outcome some examples have already been given11 28 29 32-J6. The controlling factors can be seen in the mechanistic scheme shown in... 

Asymmetric induction in the Paterno Biichi reaction has been increasingly studied since the first successful example, reported in 1979, which gave up to 53% de51. Since then high dia-stereoselectivities with either optically active carbonyl compounds or alkenes have been reported. The most extensive studies have been performed by Scharfs group1 ,28 29, who found that the degree of stereoselectivity depends not only on the structure of the chiral auxiliary (mainly chiral alcohols bound to a-oxo acids) but also on the olefinic partner, the reaction conditions and, in particular, on the temperature. The following items will be discussed here. 

A general mechanism of the Paterno-Biichi reaction through either a singlet or triplet excitation state of the carbonyl compound is illustrated below, regardless of stereochemistry. 

Another photocycloaddition reaction that has been known for a long time is the Paterno-Biichi reaction, which involves the formation of oxetanes through the addition of an excited carbonyl compound to olefins ... 

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. ... 

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