The Photolysis of Aldehydes and Ketones

The photolysis of carbonyl compounds is one of the most intensively studied areas of photochemistry. Since CIDNP studies have been concerned mostly with aldehydes and ketones we shall confine these brief introductory remarks to such compounds. More extensive reviews are available (e.g., Simons, 1971). 

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (68)]. Of these, Norrish Type II cleavage and the related 

The CIDNP technique has led to similar conclusions about the precursor multiplicity for methyl benzyl ketone (Blank et al., 1971). With dibenzyl ketone and phenyl a-phenylethyl ketone (Closs, 1971a Muller 

A singlet precursor has, however, been proposed for 4,4,4-trichloro-2-methyl-1-butene (17) produced in the photolysis of isomesityl oxide (18) in carbon tetrachloride solution on the basis of its all-emission spectrum (DoMinh, 1971). There remains some ambiguity, however, about the detailed route by which 17 is formed. Moreover there is other evidence suggesting that ketone photolysis in carbon tetrachloride is different CI3C. CH2C(Me) CH2 CH3. CO. CH2. C(Me) CH2 

Norrish Type I cleavage of cyclic ketones necessarily yields biradicals, and in certain cases (e.g., cycloheptanone, camphor) strong emissions due to T i S mixing have been reported (Gloss and Doubleday, 1972). 

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Also, hydrogen transfer through ring intermediates has been suggested as a mechanism for intramolecular rearrangements in the photolysis of aldehydes and ketones 84... 

The photolysis of aldehydes and ketones has been the subject of many investigations, and is adequately dealt with in standard texts on photochemistry. In a few instances, excitation of the carbonyl function can result in cyclization and the formation of heterocyclic systems, although this is usually accompanied by other processes more characteristic of the carbonyl group. 

During the photolysis of aldehydes and ketones, the photodissociation of CH2O and CH3CHO is of major importance ... 

The photolysis of aldehydes and ketones in the (n, rr ) band has been widely investigated. (For summaries see and" ). The dominant primary steps involve cleavage of the C-C bonds next to the carbonyl ... 

Aldehydes are very reactive substances. They react with OH by the abstraction of H (almost from the chain) to produce finally bicarbonyls, e. g. dialdehydes and ketoaldehydes (the carbonyl group denoted by C=0). Nevertheless, more important is the photolysis of aldehydes and ketones that initiates radical chains. We have seen that the simplest aldehyde HCHO is photolyzed (reaction 5.46) and gains H (which turns into HO2) and the formyl radical HCO (a carbon radical), which reacts with O2 to HO2 and CO. Therefore, HCHO is an efficient radical source. In analogy, higher aldehydes are photolyzed and produce H atoms and acyl radicals RCO, for example acetyl CH3CO, which do not decompose like the formyl radical HCO and add O2, thereby giving acyl peroxo radicals R—C(0)02i... 

The term "Norrish-Type-2 cleavage" is understood as the photolysis of aldehydes or ketones at 230-330 nm, generating alkyl and acyl radicals, which stabilise themselves, for example, by intramolecular hydrogen abstraction or fragmentation. [215]... 

Gorin E. (1939) Photolysis of aldehydes and ketones in the presence of iodine vapor. J. Chem. Phys. 7 256-264. 

This procedure is an adaptation of that described by Emmons for the preparation of oxaziranes from imines using peracetic acid. Other procedures which may be more useful for oxazirane preparation in specific instances are the oxidation of imines with iw-chloroperbenzoic acid and the reaction of aldehydes or ketones with hydroxylamine 0-sulfonic acid in alkaline solution. 2-<-Butyl-3-phenyloxazirane has also been prepared by photolysis of a-phenyl-N-f-butylnitrone (a general reaction of considerable theoretical interest since it represents direct conversion of electromagnetic energy to chemical energy) and in low yields by ozonoly-sis of N-f-butylbenzaldimine. ... 

In addition to furan, other heterocycles have been examined." Thiophene undergoes efficient photocycloaddition with benzaldehyde to afford a single exo photoproduct (183) in 60% yield. As reported by Jones and coworkers,the photolysis of IV-methylpyrrole in the presence of aldehydes or ketones yields the corresponding 3-hydroxyalkyl derivative (184), even when the reaction mixture is free from any trace of acid. In order to use the pyrrole nucleus for stereoselective alkaloid synthesis (cf. caesalpinine, 185) in the fashion developed with the furan nucleus, pyrrole substituents that stabilize the presumed intermediate bicyclic oxetane must be discovered. 

The liquid phase photolysis of several alcohols was first systematically studied in the years between 1910 and 1913 by Berthelot and Gaudechon using a medium pressure mercury arc (21). They recognized that the main photolysis products of primary and secondary alcohols were H2 and an equivalent amount of aldehyde and ketone, respectively (21a,b). They also found (21d) that the part of the ultraviolet spectrum active on alcohols must lie below 250 nm, while carbonyl compounds are readily decomposed by light around 250 nm and above, yielding much CO. It was concluded (21d,22) that CO was a secondary product of the alcohol photolyses and due to the photolysis of the primarily produced carbonyl compound. In aqueous solutions of the alcohols the rate of decomposition was lower (21d) but led to the same products as those found in the neat photolyses. Similar to the liquid phase results Bates and Taylor (1927) found H2 as the major product in the methanol and ethanol gas phase photolysis (23). 

Acetone is a convenient actinometer for vapor-phase studies of aldehydes and ketones, since Ico is 1.0 at temperatures above 120° and at wavelengtl from 2500-3200 A. In solution, the photolysis of uranyl oxalate is generally useful, and Malachite green leucocyanide is useful for low intensities of light. ... 

The acyl radicals formed in ketone photolysis are excited and, therefore, rapidly splits into CO and alkyl radical (in the gas phase). Since aldehydes and ketones are products of oxidation, continuous hydrocarbon photooxidation is an autoaccelerated process. 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Figures 4.26, 4.27, and 4.28 show typical UV absorption spectra for some simple aldehydes and ketones (Rogers, 1990 Martinez et al., 1992 see also Cronin and Zhu, 1998, for n-pentanal). Formaldehyde stands out from the higher aldehydes and ketones in that it has a highly structured spectrum and furthermore, the absorption extends out to longer wavelengths. The latter difference is particularly important because the solar intensity increases rapidly with wavelength here (Chapter 3.C.1) and hence the photolysis rate constant for HCHO and the rate of production of free radicals...

Metal acetylacetonates quench triplet species generated by flash photolysis of aromatic ketones and hydrocarbons.330-333 More recently, these reactions have been studied from a synthetic standpoint. Triplet state benzophenone sensitizes photoreduction of Cu(MeCOCHCOMe)2 by alcohols to give black, presumably polymeric, [Cu(MeCOCHCOMe)] . This reacts with Lewis bases to provide complexes of the type CuL2(MeCOCHCOMe) (L = bipyridyl/2, ethylenediamine/2, carbon monoxide, Ph3P). Disubstituted alkynes yield Cu(C2 R2 XMeCOCHCOMe) but terminal alkynes form CuQR acetylides.334 The bipyridyl complex of copper(I) acetylacetonate catalyzes the reduction of oxygen to water and the oxidation of primary and secondary alcohols to aldehydes and ketones.335... 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

The photolysis of camphor (XXII) in aqueous alcoholic solution (11) has been observed to lead to a-campholenic aldehyde (XXXII) and a second isomer with a ketonic function. The structure of the latter has been found (30) to be 1,2,2-trimethyl cyclopent-3-enyl methyl ketone (XXXIII). The quantum yields at 3130 A. for the formation of the two isomers and of carbon monoxide in five different solvents have been determined (Table IV). It is interesting that the sum of the quantum... 

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