Carbonyl compounds photochemical reactions

The most useful intermolecular [2tt+2tt] cycloadditions from a synthetic point of view involve alkenes and cyclic a,6-unsaturated carbonyl compounds. These reactions are discussed in more detail in Section 6.3.2 of Part B. Scheme 12.1 lists some examples of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. 

Addition reactions such as addition of enolates to carbonyl compounds, photochemical cydoaddition, radical addition and elimination reactions such as dehydration can be carried out in microflow reactors. Addition-elimination reactions such as Wittig reaction, Homer-Wadsworth-Emmons reaction, Baylis-Hillman reaction. 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

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). 

Azirine, trans-2-methyl-3-phenyl-racemization, 7, 33, 34 1-Azirine, 2-phenyl-reactions, 7, 69 with carbon disulfide, S, 153 1-Azirine, 3-vinyl-rearrangements, 7, 67 Azirines, 7, 47-93 cycloaddition reactions, 7, 26 fused ring derivatives, 7, 47-93 imidazole synthesis from, 5, 487-488 photochemical addition reactions to carbonyl compounds, 7, 56 photolysis, 5, 780, 7, 28 protonated... 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

The photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

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. 

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... 

Fig. 15 Possibilities for photochemically induced reactions, using a carbonyl compound as example. U and IT = electron spins parallel and antiparallel respectively.

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. ... 

Four-membered ring formation between unsaturated carbon bonds and carbonyl compounds is a photochemical reaction [25], This is an excited-state reaction in the delocalization band (Scheme 6). 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

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. 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

An interesting application of this Norrish type II reaction consists in the photochemical oxydation of alcohols to carbonyl compounds by irradiation of their pyruvic esters (4.11) 412). 

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,... 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

The existence of the neutral rhenium carbonyl [Re(C0)4] was first claimed in 1965 206 but, although it is easily sublimed, it has not yet been characterized by mass spectrometry and the value of n is still not known. This colourless substance [v (CO) 2055 and 1995 cm-1 in CHC13] has been obtained as a by-product in the synthesis of Re2(CO)i0 starting from Re2S7, copper powder, and carbon monoxide at 85 atm, 200 °C206>. There has also been a report of the compound Re4(CO)10(PPh2Me)6, which can be considered to be a substitution product of the hypothetical species, Re4(CO)i6 it has been obtained by a photochemical reaction between Re2(CO)j0 and PPh2Me194. In both cases, and particularly in the phosphine derivative, a tetrahedral structure seems improbable because of steric constraints. 

Photochemical Cycloaddition Reactions between Unsaturated Carbohydrate Derivatives and Carbonyl Compounds... 

A photochemical reaction initiated by a-cleavage in a carbonyl compound is called a Type I process.45 Photolysis of terf-butyl-3,4-0-iso-... 

Three types of photochemical reaction of carbohydrate acetals have been investigated. Early studies centered on the photochemical fragmentation of phenyl glycosides, and the photolysis of o-nitrobenzyli-dene acetals. (The latter reactions will be discussed with the photolysis of other nitro compounds see Sect. VII,1.) Later experiments were concerned with hydrogen-abstraction reactions from acetal carbon atoms by excited carbonyl compounds. 

The photochemical reactions of xanthides are quite complex. They are solvent-, concentration-, temperature-, wavelength-, and time-de-pendent.130 The most thoroughly studied of these compounds is compound 64, whose irradiation (through a Corex filter) in cyclohexane under nitrogen produces tetrasulfide 69 (37% yield), xanthate 70 (35%), l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (71, 13%), sulfur, and carbonyl sulfide. Irradiation of a dilute solution of 64 produced only 70 (in 74% yield). The most intriguing finding from irradiation of the xanthides 64-66 is the fact that 66 produces a xanthate... 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

Explain the role of the photochemical reactions of carbonyl compounds in the photoinitiated polymerisation of vinyl monomers and cross-linking in polymers. 

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