Carbonyl groups photoreactions

The photoreactions of a number of a-diketones in which the carbonyl groups are held in a somewhat rigid m-configuration have been reported. These compounds undergo a variety of reactions depending upon the strain of the system and the solvent in which the reaction is carried out. 

As in five-membered heterocycles, the introduction of a carbonyl group produces a dramatic effect on the photoreactivity. Pyran-2-ones undergo other electrocyclic processes in addition to those discussed in preceding sections.213 Photorearrangement, however, does not occur in the coumarin nucleus, although a photo-Fries rearrangement of 3-benzoyloxy-6,7-... 

While the irradiation of substituted cyclopentenones in aqueous environment gives cyclobutane dimers with a ratio of head-to-head (HH) to head-to-tail (HT) of about one, the same photoreaction in potassium decylanoate (KDC) gives the cyclobutane dimer which is almost exclusively HH (Scheme XIX) 49). The dependence of the HH/HT ratio as a function of surfactant concentration shows a dramatic increase in the ratio at the reported CMC value. The regioselectivity is attributed to the carbonyl groups being oriented in the Stem layer with the olefin extending into... 

Figure 10 reveals that molecular shape and packing of photoreactive crystals are very similar. In addition, the electron-rich nitrogen or carbonyl group approaches the electron-deficient benzene ring in all the photopolymerizable crystals41,42, u-46,49,50. These two common features may govern the formation of photopolymerizable crystals and thus allow an empirical rule of similar photoreactivity between mono- and di-olefin crystals to be established (see Sect. II.a.). 

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

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. 

Hydrogen abstraction by an excited carbonyl group is the most typical photoreaction in both solution and solid states. A number of intramolecular Norrish type II hydrogen abstraction reactions in the crystalline state are already known, and the geometric requirements have been precisely discussed by Scheffer [58]. Solid-state asymmetric induction in the Norrish type II photocyclization of carbonyl compounds using supramolecular approaches has been also intensively studied. 

In general crosslinked polyesters have better light and weathering resistance than uncured polyester resins. Apparently the concentration of unsaturated double bonds is a determining factor. In our previous studies (14) we have shown that under UV irradiation of unsaturated polyesters the primary photoreactions involve excitation of conjugated structures carbonyl groups ( in ester... 

The product distribution in flie photoreactions of organosilicon compounds with electron-deficient compounds often depends on the reaction media. " In nonpolar solvents, the photocycloaddition of allylsilanes to electron-deficient aromatic compounds occurs to give cyclobutanes via exciplex intermediates. In contrast, photoallylation of the aromatic compounds or the carbonyl group takes place via free radical ions in polar solvents. Some examples are shown in Schemes 5 and 6. 

In addition to alkenes and acetylenes, carbonyl-containing compounds also participate in [2+2] cycloaddition reactions with indoles. The first example of oxetane formation via the photoreaction of indoles with a carbonyl group (a type of Patemo-Biichi reaction) was reported by Machida and coworkers [34, 35] (Scheme 16), who carried out the intramolecular cyclization of iV-acetylindole with phthalimide tethered with appropriate linkers 67a-d. Interestingly, only the... 

In contrast to the case of tryptophan the photoreactions with tyrosine and histidine probably involve hydrogen atom transfer as the primary step. There are several indications for this. First, 0-methylated tyrosine (p-methoxy phenylalanine) did not show any photo-CIDNP effect and its reactivity as a photo-reductant towards flavins is strongly reduced (19). Similarly, 1-N-methyl histidine is not polarized at high pH (> 7.5), when no abstractable hydrogen is present. Secondly, in the protein ribonuclease A, which has a well known 3-dimensional structure, the residues Tyr 92 and His 105 have exposed rings, but their OH and NH protons are hydrogen bonded to backbone carbonyl groups. 

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