Photochemical nucleophilic displacement

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

The 3-(4-chloro-2-nitrophenyl)triazole derivative (110) undergoes nucleophilic displacement of chloride by azide ion <89JCS(P2)1425>. The thermal and photochemical decomposition of 3-nitro-triazolone have been shown to occur via radical cations <9UPC5509>. 

Unactivated aryl halides also undergo nucleophilic displacement via electron transfer in the initial step the so-called SRN1 mechanism. It is now clear that in the case of heteroaromatic compounds, nucleophilic substitution by the Srn process often competes with the addition-elimination pathway. The SRN reactions are radical chain processes, and are usually photochemically promoted. For example, ketone (895) is formed by the SRN1 pathway from 2-chloroquinoxaline (894) (82JOC1036). 

Both neutral and anionic species of DNOC show absorption shoulders at wavelengths >300 nm (Schwarzenbach et al. 1988). However, photolysis of DNOC in water involving nucleophilic displacement of the nitro group by the hydroxyl group does not seem likely (EPA 1979). The photochemical reduction of the nitro group in DNOC is possible in water in the presence of a reducing agent (e.g., ascorbic acid or ferrous ions) and a sensitizer, such as chlorophyll (EPA 1979). However, there is no experimental evidence of the photochemical reduction of DNOC in water. 

Recently, the thermally and photochemically induced one-electron transfers from the metal-carbonyl anions A[Mn(CO)5], A[Co(CO)4], and A[Re(CO)s] (A = Na and PPN) to fullerene have been studied. In the case of the thermal reactions of the Mn and Re anions, initial electron transfer to form Ceo and the corresponding 17e M(CO)s radical is followed by the competitive collapse to the metal dimer and formation of the tj -Ceo complexes A [Mn(CO)4(jj -Ceo)]. Under photochemical conditions, redissociation of the dimer results in exclusive formation of the jj -complexes, which are thought to be formed by nucleophilic displacement... 

Deoxy-6-thio-D-a yfo-hexose forms a 1,6-anhydro compound analogous to 270. S-Acetyl-5-deoxy-l,2-0-isopropylidene-6-thio-a-D-xyZo-hexofuranose is obtained either by nucleophilic displacement on the corresponding 6-p-tolylsulfonyloxy compound with potassium thioacetate or by the photochemical addition of thioacetic acid to... 

Synthesis of Precursors for Photochemical and Cationic Cyclizations. Allyl(2-chloroethyl)dimethylsilane (4) was converted to thiol 15 by nucleophilic displacement with thiolacetic acid and treatment of the resultant thioacetate with ammonia. UV irradiation of 15 gave exclusively the 7-endo cyclization product 16 (eq 8). ... 

Bipyridyl complexes of Ru(II) can be used for the production and storage of multiple photochemical redox equivalents by attachment to a soluble polymer. The photoactive complexes have been bound to a styrene/chloromethylstyrene copolymer by nucleophilic displacement of the chloro groups from the chloromethyl-styrene. Because the excited state properties are relatively unaffected in the mixed-valence polymers, it is possible to build up a chain of approximately 30 oxidative equivalents with a (bpy)2Ru or a (bpy)20s moiety bonded to each strand of the polyma . Excited state donors and acceptors can also be immobilized using a sol-gel technique. Such a technique has been s plied to the 1,4-dimethoxyl-benzene mediated photoinduced electron transfer between bipyridyl complexes of ruthenium and iridium. Metallopolymeric films have also been used for energy... 

The aryl-thallium bond is thus apparently capable of displacement either by electrophilic or by suitable nucleophilic reagents. Coupled with its propensity for homolytic cleavage (spontaneous in the case of ArTlIj compounds, and otherwise photochemically induced), ArTlXj compounds should be capable of reacting with a wide variety of reagents under a wide variety of conditions. Since the position of initial aromatic thallation can be controlled to a remarkable degree, the above reactions may be only representative of a remarkably versatile route to aromatic substitution reactions in which organothallium compounds play a unique and indispensable role. 

Radical cations can be formed by irradiation of unsubstttuted aromatic hydrocarbons such as naphthalene, and this makes possible the photochemical displacement of hydride ion by a nucleophile such ascyanide f3.10). Oxygen is not necessary for the success of this type of reaction if a good electron-acceptor is present, such as p-dicyanobenzene (3.11), which enhances the initial photoionization and also provides for reaction with the displaced hydrogen. 

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