Patemo-Biichi photocycloaddition oxetanes

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. 

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... 

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]. 

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]. 

Closure of the biradical leads to a four-membered ring containing oxygen, which is known as an oxetane. The overall 1,2-photocycloaddition reaction is often called the Patemo-Biichi reaction (equation 12.60). The Patemo-Biichi reaction provides synthetic entry into highly strained ring systems containing oxygen atoms, as exemplified by equations 12.61 and 12.62 (where Ar is 2-naphthyl). ... 

Another photoreaction between ketones and olefins or dienes, which has often been connected to the involvement of exciplexes, is the Patemo-Biichi reaction [4,5], i.e., the photocycloaddition of C=C double bonds to carbonyl C=0 bonds under formation of oxetanes [17,78,207-217], Especially for electron-rich olefins such as ethyl vinyl ether or 1,2-diethoxyethylene, intermediary exciplexes have often been postulated [212], with the consequence of a diminished legioselectivity and stereospecificity for oxetane formation. On the other hand, electron-deficient olefins such as a,P-unsaturated nitriles react with a high legioselectivity and stereospedlicity due to a well defined transition state, which is based on the electronic leqniranents of n,7t -excited ketones [17]. 

The Patemo-Biichi reaction of 5-substituted adamantan-2-ones 55 with fumaro-nitrile to form oxetanes 56 and 57 (Scheme 20) has been studied by Turro and coworkers [93,94]. In the CD-free photocycloaddition performed in acetonitrile or water, the antf-oxetane 56 formed through the 5jn-face attack of fumaronitrile is slightly favored, with the 57/56 ratio varying from 47/53 to 36/64. When the photocycloaddition reaction is carried out in the presence of -CD in aqueous solution, the stereochemical preference is switched to 5jn-oxetane 57 and the... 

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