Photochemistry of Carbonyl Groups

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 two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collisions, whereas in the liquid phase, the energy is rapidly transferred to the solvent or to other components of the solutions. Solution photochemistry will be emphasized here, since most organic chemists interested in either mechanistic studies or preparative photochemistry have studied this aspect of the problem. 

An alternative excited state available to carbonyl compounds involves promotion of a bonding n--electron to the antibonding ir -orbital. This is called a tt-tt transition, and is most likely to occur when the ketone group is conjugated with an extensive 7r-bonding system. 

It is not possible to draw unambiguous Lewis structures of excited states of the sort that are so useful for depicting ground-state chemistry. Instead, it is common to asterisk the normal carbonyl structure and provide information about the multiplicity of the excited state if it is available  

Many aromatic ketones react by hydrogen abstraction from solvent or some other a hydrogen atom by the oxygen of the carbonyl group or with cleavage of the carbon-carbon bond adjacent to the carbonyl group  

The hydrogen atom abstraction can be either intramolecular or intermolecular. Many aromatic ketones undergo hydrogen abstraction from solvent or some other hydrogen donor, followed by coupling of the resulting a-hydroxy radicals. Such reactions go best in solvents that have easily abstractable hydrogens  

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Photochemical reactions have been used for the preparation of various olefin, and acetylene complexes (7). Application to the coordination of dienes as ligands has not been used extensively, so far. In this article the preparative aspects of the photochemistry of carbonyls of the group 6 and group 7 elements and some key derivatives, with the exception of technetium, with conjugated and cumulated dienes will be described. Not only carbonyl substitution reactions by the dienes, but also C—C bond formation, C—H activation, C—H cleavage, and isomerizations due to H shifts, have been observed, thereby leading to various types of complexes. 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

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. 

The group VI compounds M(CO)4(DAB) were first prepared by Tom Dieck and his coworkers in the 1960s and their strong solvatochromism arising from the antiparallel dipole of the excited state was reported shortly thereafter. Resonance Raman studies established the relationship between the MLCT absorption and several molecular vibrations, most notably the carbonyl stretching frequencies. The resonance Raman spectra of this class of compounds has been reviewed. Vlcek has published a detailed review of the photophysics and photochemistry of the group VI diimine compounds. ... 

Nevertheless, we were able to develop a transient absorption apparatus involving IR probe radiation that is suitable for gas phase studies, as have a number of other groups either coincident with or subsequent to our work [1]. In the remainder of this article we will discuss the apparatus and the results of our studies on three prototypical metal carbonyl species Fe(C0>5, Cr(C0>5 and Mn2(CO)] o The discussion in this article will center on the nature of the photolytically generated coordinatively unsaturated species, their kinetic behavior and photophysical information regarding these species. This latter information has enabled us to comment on the mechanism for photodissociation in these systems. Since most of the results that will be discussed have been presented elsewhere [3-10], we will concentrate on a presentation of data that illustrates the most important features that have come out of our research and directly related research regarding the kinetics, photophysics and photochemistry of coordinatively unsaturated metal carbonyls. 

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 ... 

The majority of such systems involves the coordination of a Group 14 organometallic to a transition metal, in most cases a carbonyl complex. This section falls into two categories—the formation of such complexes by photochemical means, and their photochemistry once formed. 

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. 

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