Oxetanes carbonyl compounds

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

Two different alkenes can be brought to reaction to give a [2 -I- 2] cycloaddition product. If one of the reactants is an o, /3-unsaturated ketone 11, this will be easier to bring to an excited state than an ordinary alkene or an enol ether e.g. 12. Consequently the excited carbonyl compound reacts with the ground state enol ether. By a competing reaction pathway, the Patemo-Buchi reaction of the 0, /3-unsaturated ketone may lead to formation of an oxetane, which however shall not be taken into account here ... 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

By reaction with the appropropriate aryl halides can be prepared a variety of aryltin compounds that are not accessible from the reactions involving arylmagnesium halides and organotin halides (88,89) there is evidence that an aryne intermediate may be involved (90). However, for some purposes, such as the addition to carbonyl compounds, ox-iranes, and oxetanes, to give hydroxyalkyltin compounds, the Sn-Mg reagents may have advantages (see Section II,E) (91-93). 

Grignard reagents having bulky alkyl groups react with trialkyltin hydrides to give compounds having a Sn-Mg bond, and are synthetically useful as a source of nucleophilic RsSn in particular, they react with carbonyl compounds, oxiranes, and oxetanes to give the -, jS-, or... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Photochemical reactions of carbonyl compounds with alkenes give the oxetanes (Scheme 30). The stereochanical course depends on the substituents of the alkenes [16]. The reactions proceed with the retention of the configuration of the alkenes for the electron accepting substituent, e.g., CN. The stereochemical integrity is lost for the donating group, e.g., OCH. ... 

Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

The reaction of carbonyl compounds to olefins often yields products difficult to obtain synthetically by other routes. The excellent yields obtainable under proper conditions make this reaction of definite preparative interest. Examples of some synthetic applications of oxetane formation follow ... 

The Photochemistry of Simple Carbonyl Compounds Type I Cleavage and Oxetane Formation... 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

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. 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

On irradiation with alkenes, carbonyl compounds undergo photocycloaddition to give oxetanes in the Paterno-Biichi reaction ... 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

Biichi, G. Inman, C. G. Lipinsky, E. S. J. Am. Chem. Soc. 1954, 76,4327. George H. Biichi (1921—1998) was bom in Baden, Switzerland. He was a professor at MET when he elucidated the structure of oxetanes, the products from the light-catalyzed addition of carbonyl compounds to olefins, which had been observed by E. Paterno in 1909. Btichi died of heart failure while hiking with his wife in his native Switzerland. 

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]. 

Carbonyl Compounds, The Photocycloaddition of, to Unsaturated Systems The Syntheses of Oxetanes (Arnold). 

VII. Tabular Survey of Photocycloaddition Reactions of Carbonyl Compounds to Unsaturated Systems which Yield Oxetanes. 349... 

The photocycloaddition of a carbonyl compound to an olefin with the formation of an oxetane 3 was first observed by Paterno and Chieffi in 1909.1 They reported obtaining a compound in good yield from the irradiation of benzophenone in a petroleum mixture rich in 2-methyl-2-butene, for which they proposed the oxetane structure. Although Paterno and Chieffi had suggested the correct structure, it was not until 1954 that Buchi and his collaborators reinvestigated this reaction and... 

If the triplet energy of the carbonyl compound is below that of the diene, triplet-triplet transfer will become inefficient and photocycloaddition may occur. For example, the triplet energy of 1,4-benzo-quinone is about 50 kcal mole - V41 and photocycloaddition to dienes can indeed occur.42 The products are spiro-pyrans 13 (not vinyl oxetanes), which may arise via the allylic radical intermediate 12. 

In general, carbonyl compounds that are reactive in the photocycloaddition reaction are also reduced upon irradiation in isopropyl alcohol.17 Subject to the limitation of triplet-triplet transfer to the olefin mentioned previously, the converse is also true. That is, carbonyl compounds that are photoreduced in isopropyl alcohol can form oxetanes unless their triplet energies are high enough for the olefins to act as quenchers. Thus, the two reactions are characteristic of the same type of excited state. (This is not an exclusive generalization.) The quenching experiments mentioned on pp. 308-311 provide evidence that the reactive state can be the triplet and, in some cases, only the triplet. Evidence for this state being n,ir comes from the fact that carbonyl compounds which are reactive usually emit from the n,n triplet, while those which are unreactive emit from some other excited state. 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

The photocycloaddition of carbonyl compounds to unsymmetrical olefins (electron rich) can give two products however, usually one predominates. For example, the photocycloaddition of benzophenone to isobutylene gives a mixture of the two oxetanes 30 and 31 in the ratio of 9 1.17 This ratio is consistent with the preferential formation and/or closure of the intermediate 30a relative to 31a. The diradical 30a is more stable than 3la since a tertiary radical is more stable than a primary radical by about 8 kcal.62 Many of the examples listed in Section VII are consistent with this apparent generalization there are, however, exceptions. 

One important reaction which competes with photocycloaddition is hydrogen abstraction. The n,ir state can both abstract hydrogen from, and add to olefins. If the starting olefin or carbonyl compound has easily abstractable hydrogens, this competing reaction can become so important that oxetane formation is excluded. 

The importance of the carbonyl n,tr state as a reagent for the photocycloaddition reaction has been mentioned (p. 305). This represents a major limitation, for many carbonyl compounds attain states other than the n,it which are either unreactive, react inefficiently, or undergo other photochemical reactions. It is impossible, at this time, to provide rigorous rules for predicting when a carbonyl compound will attain, and maintain long enough to react, an n,n state. It is perhaps best to mention additional examples of carbonyl compounds that do not yield oxetanes upon irradiation in the presence of simple olefins, presumably because the n,n state is not attained and/or maintained, and to point out how, in some cases, it has been possible to exert some control over this limitation. 

The irradiation of , 3-unsaturated carbonyl compounds in the presence of olefins does not usually lead to oxetanes. In some cases, however, a photocycloaddition reaction takes place, yielding a cyclobutane ring. This has proved to be a useful reaction which has warranted recent review.71 These carbonyl compounds typically are not reduced upon irradiation in isopropanol, nor do they show any phosphorescence emission. The mechanism of this reaction has been discussed 72,73 however, the nature of the excited state involved (n,n or 7r,7r singlet or triplet) is still in question. 

There are some cases where both types of photocycloaddition take place. For example, cinnamaldehyde and crotonaldehyde yield, upon irradiation with 2-methyl-2-butene, both the oxetane and the cyclobutane products.26 In marked contrast, mesityl oxide, as similar as it would appear to be to crotonaldehyde (Table I), is stable to irradiation in the presence of both isobutylene and isopropanol.37,74 These differences in reactivity of a,/9-unsaturated carbonyl compounds have been attributed to conformational (that is, s-cis or s-trans) differences.74... 

For carbonyl compounds that ordinarily would react in the n,n triplet state, an important reaction which can compete with photocycloaddition is energy transfer to the unsaturated system. When the triplet energy of the unsaturated system is below that of the carbonyl compound, triplet-triplet transfer may take place to the exclusion of oxetane formation. Such is the case when irradiation of a carbonyl compound in the presence of a diene results in isomerization and dimerization of the diene, while no direct photocycloaddition takes... 

If the starting carbonyl compound or olefin has other chromo-phores which can undergo photochemistry, this may lead to complications. In the case of the photocycloaddition of 9-anthraldehyde to 2-methyl-2-butene (pp. 305, 324), prolonged irradiation of the oxetane 53 leads to anthracene-type dimers 87.25 When fluorenone is irradiated... 

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