Carbonyl group photochemistry

The Tunnel Effect Theory was designed to model the carbonyl group photochemistry and to fulfill the criteria mentioned for chemical models. We find beauty and order in the simplicity of this theory and in its quantitative explanation of the major features of ketone photoreactivity. 

In the case of compound 4 photolysis with a medium pressure mercury lamp results in exclusive cleavage of the C—Sn bond fi to the carbonyl group, while the terminal C—Sn bond remains intact. The cyclic ketone 5 and the a, /Lunsaturated ketone 6 are produced in equal if low yields (18%)19. The reduced reactivity of remote C—Sn bonds in such compounds appears to be a general feature of their chemistry, and is also observed in acyclic systems. Thus the photochemistry of organotin compounds with carbonyl groups in the /3-position can be classified as follows ... 

A study of the photochemistry of 4-acetyl- and 4-benzoyl-5-methyl-1,2,3-triazoles shows that the nature and lifetime of the lowest triplet state depends on the nature of the 1- and 4-substituents. 4-Benzoyl-5-methyl-1,2,3-triazole has a high rate constant for triplet deactivation, which is attributed to interaction of the nitrogen lone pairs with the excited carbonyl function. The compound forms a pinacol derivative when irradiated in propan-2-ol and undergoes cycloaddition, involving the carbonyl group, with 2-methylpropene, giving an oxetane derivative. 

The photochemistry of biacetyl has been extensively studied, both in the vapor phase and in solution. In the vapor phase the products include carbon monoxide, ethane, methane, acetone, ketene, and 2,3-pentanedione. It has been shown that the primary process is cleavage of the carbon-carbon bond between the two carbonyl groups to yield acyl radicals, which on further reaction give the observed products.14,43... 

Carbonyl compounds, 121-128 acids and electrophiles, 97 electrophilic attack, 121-123 photochemistry Norrish type I, 215-217 Norrish type II, 213-215 stability of, 127-128 UV spectra, 214 Carbonyl group... 

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. 

It should be noted that all three possess a carbonyl group situated in a five-membered ring. It is not surprising that the photochemistry of these ketones shows conspicuous similarities to the photochemistry of cy clopentanone. 

Not surprisingly, the introduction of a carbonyl group into the heterocycle has a profound effect on the photochemistry of the system. The formation of photoproducts arising by a Type I reaction involving 2,3-bond homolysis... 

Acyl silanes display a range of behaviour upon irradiation, depending upon their structure and the reaction conditions. The interesting photochemistry displayed by acyl silanes has been attributed to the low-energy n - it carbonyl group transition. 

Coumarin photochemistry has been recently employed to demonstrate that a frozen axial chirality can be used to induce the absolute configuration of stereogenic centers. Coumarin 103 was obtained as a single atropisomer by spontaneous crystallization (Scheme 6.37). Upon warming powdered crystals of 103 in MeOH to —20 °C, sensitized [2 + 2]-photocydoaddition to ethyl vinyl ether gave the almost enantiomerically pure products 104. The approach to the coumarin double bond occurred preferentially from the less-shielded face to which the amide carbonyl group... 

This process is an important tool in synthetic organic photochemistry for the preparation of small rings. The difficulties in the preparation of medium-size and larger cyclic compounds, caused by entropic factors, have been circumvented by the presence of a rigid molecular scaffold (cyclopropanes, aromatic rings, cyclohexenes and cis-alkenes), which prevents hydrogen abstraction from positions close to the carbonyl group (Scheme 9.39) [64—66]. 

The special case of hydrogen abstraction has been discussed in connection with the simple model for the n-n excited state. However, it was noted in 1959-62 by Zimmerman 1 -3,7) that a major fraction of the organic photochemistry of carbonyl compounds was explicable on the basis of simple basic principles and this simple model for the carbonyl group. 

When one takes up a polyfunctional molecule which contains at least one carbonyl group, one knows which reactions of that carbonyl group are possible. One does not know how well these reactions will compete with various physical and chemical interactions of that excited carbonyl group with other functional groups in the molecule. It is this aspect of carbonyl photochemistry which requires and deserves extensive future research. The uniquely well understood chemistry of the carbonyl group can serve as a monitor for studying interactions in electronically excited polyfunctional molecules. 

The past decade has witnessed a remarkable growth in the field of organic photochemistry as evidenced by the appearance of numerous textbooks, review serials, and primary publications (1). Examination of the chemical literature reveals that this growth is due, in large measure, to research into the nature of the excited-state chemistry and physics of the carbonyl group (e.g., ketones, aldehydes, and carboxylic acids and esters). 

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