Alkenes, Patemo-Biichi reaction

The [2+2] photocycloaddition of an aldehyde or a ketone to an alkene to form an oxetane (e. g. Ill + IV —> V, Scheme 1) is a process that is known as the Patemo-Biichi reaction 8 it is a reaction that was first reported by Patemo in 19094 and was confirmed approximately forty-five years later by Biichi.Sa A particularly interesting variant of the Patemo-Biichi reaction is illustrated in... 

First reported in 1909, the photochemical [2+2] cyclization between alkenes and carbonyl compounds, known as the Patemo-Biichi reaction, is one of the most commonly used methods of synthesizing oxetanes. The scope of the reaction is however limited and only occurs readily between electron-rich alkenes and electron-poor carbonyls. The importance of the reaction is that, with careful selection of alkenes and carbonyl compounds, high regio- and stereoselectivities can be achieved (see CHEC(1984) and CHEC-II(1996) for previous examples) < 1984CHEC(7)363, 1996CHEC-II(1)721>. 

The Patemo-Biichi reaction is a photocycloaddition reaction of a n,ji carbonyl compound to an alkene in the ground state from either the Si or the rl i state. The reaction can occur through the initial C O bond formation or through a previous formation of the C—C bond. A frontier orbitals approach can be used to explain the formation of oxetanes. We can observe the HSOMO-LUMO interaction in which the half-occupied ji carbonyl orbital interacts with the unoccupied ji molecular orbital of an electron-deficient alkene, and a C,0-biradical is formed. The LSOMO-HOMO interaction in which the half-occupied n orbital of the carbonyl O atom interacts with the ji orbital of an electron-rich alkene, and a C,C-biradical is formed [13, 14]. 

The [2 + 2] photocycloaddition of two alkenes is widely used to form cyclobutanes. The reaction proceeds in the forward direction because the product cannot absorb light of the wavelengths that the starting material can absorb. In the Patemo-Biichi reaction, one of the two-atom components is a ketone or an aldehyde instead of an alkene. Why these cycloadditions require light to proceed will be discussed later. 

In addition to alkenes and acetylenes, carbonyl-containing compounds also participate in [2+2] cycloaddition reactions with indoles. The first example of oxetane formation via the photoreaction of indoles with a carbonyl group (a type of Patemo-Biichi reaction) was reported by Machida and coworkers [34, 35] (Scheme 16), who carried out the intramolecular cyclization of iV-acetylindole with phthalimide tethered with appropriate linkers 67a-d. Interestingly, only the... 

The Patemo-Biichi reaction of carbonyl compounds to alkenes fails when the energy difference between the triplet and groimd states of carbonyl compound is greater than that between the corresponding states of the alkene. In such case, the... 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls. This result suggests that the reactive excited state is a singlet for aliphatics and a triplet for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible radicals by bond formation between oxygen and the alkene. Some examples of Patemo-Biichi reactions are given in Scheme 6.9. 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

Griesbeck and co-workers" described the first examples of oxazoles reacting as the alkene component in a Patemo-Biichi photocycloaddition reaction with aldehydes and a-ketoesters. The authors envisioned that such a process would generate... 

In 1909, Patemo and Chieffi noted that mixtures of tri- or tetra-substituted olefins and aldehydes formed trimethylene oxides when exposed to sunlight. Biichi later repeated Patemo s experiments by irradiating 2-methyI-2-butene in the presence of benzaldehyde, butyraldehyde, or aeetophenone and rigorously purifying and identifying the resulting products. The reaction thus bears the name of its two primary pioneers and has come to represent any photo-catalyzed [2 + 2] electrocyclization of a carbonyl and an alkene. 

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