Photochemically Induced Transformations

(a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. ( /) Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

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To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. This hv/Pd°-promoted process allows the synthesis of five-membered cyclic y-keto esters 5-119 from 5-iodoalkenes 5-117 in the presence of CO and an alcohol 5-118 as a nucleophile (Scheme 5.24) [41]. The yields are high, and differently substituted iodoalkenes can be employed. 

To date, no examples have been found in the literature for the combination of photochemically induced transformations with reductive/oxidative or enzymatic processes. 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. 

There are examples of photochemically induced transformations of pyrimidines into imidazoles. In benzene or methanol solution, substituted pyrimidine 1-oxides give low yields of imidazoles (Scheme 108). Such reactions have been explained by invoking oxaziridines, 1,2,4-oxadiazepines and zwitterionic species as intermediates. 

Burrell RA, Cox JM, Savins EG (1973) Quinoxaline precursors of fungitoxic benzimidazolyl-carbamates syntheses and photochemically-induced transformations. J Chem Soc, Perkin I 2707-2713. doi 10.1039/P19730002707... 

Burrel, A., Cox, J.M., and Savins, e.g., Quinoxaline precursors of fungitoxic benzimidazolylcarbamates syntheses and photochemical induced transformations,/. Chem. Soc., Perkin Trans. I, 2707, 1973. 

Collectively, these examples illustrate the diversity of transformations of xenobiotics that are photochemically induced in aquatic and terrestrial systems. Photochemical reactions in the troposphere are extremely important in determining the fate and persistence of not only xenobiotics but also of naturally occurring compounds. A few illustrations are given as introduction ... 

The same group has developed another synthetically useful photochemically induced domino transformation. Irradiation of the enaminecarbaldehydes 5-40a or 5-40b in the presence of acrylic acid ester 5-41a or acrylonitrile 5-41b afforded the quinolizidines 5-45a and 5-45b as well as the pyrido[l,2-a]azepines 5-45c and 5-45d, respectively, with high stereoselectivity [14]. Only very small amounts of the corresponding diastereomers 5-46a-d were detected. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Various types of photochemically induced 1,3-shifts have been observed in nitrogen containing heterocycles. Concerted [1,3] suprafacial sigmatropic reactions are photochemically allowed processes, but many of the reported transformations especially those which arise by n->n excitation un-... 

A photo-induced transformation of a molecular entity in which there is a spectral change (usually, but not always, in the visible region of the spectrum). Such processes are photochemically or thermally reversible. 

The fundamental types of thermally and photochemically induced intramolecular transformations are summarized in Scheme 1. All reactions of this class involve intermediates in which aromaticity is lost hence they are most common in the classes of less aromaticity, i.e. polyhetero rings, cationic rings, rings containing carbonyl groups. However, polysubstitution, especially by bulky groups, can also induce reactions by strain relief in transition states. Most of the reactions known are photochemical. 

As mentioned above, the photochemically induced Pr - Pfr transformation manifests itself in the bilatriene chromophore, when still attached to the undecapeptide fragment, solely by the Z,E isomerism of the C(15) double bond in the Pr and Pfr froms (cf. Figure 5) [42,43], It is not yet known whether the chromophore in the intact Pfr chromoprotein differs additionally from the Pr chromophore with regard to the configuration of any of the other two exocyclic double bonds and/or the conformation. 

Photochemically induced nitrogen-oxygen bond cleavage is also thought to be the initial step in the conversion of 6-chloro-3-phenyl-anthranil (84) into 3-benzoyl-5-chloro-2-methoxy-3 -azepine (85) in methanol.67 Analogous transformations are reported in 3-methyl-and 6-chloroanthranil.67... 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Intramolecular photochemical reactions need the presence of light sensitive groups such as carbonyls to induce transformations by oc-cleavage (Norrish I reaction) or by y-H-abstraction (Norrish II reaction). 

The photochemically induced aryl-aryl coupling reaction of the diphenolic amide 28 was used as the key step in the synthesis of crassifolazonine (2). In this case, in addition to small amounts of the reduced derivative 29, indoline 30 was obtained as the result of N-attack (Scheme 6). Cyclization took place exclusively at the position para to the hydroxyl group to give a low yield of 31, which was transformed by the usual two-step procedure to crassifolazonine (2) (15). 

The fundamental types of thermally and photochemically induced inter- and intramolecular transformations proceed, in general, with loss of heteroaromaticity. This leads mostly to metastable intermediates prone to further interconversions. 

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