Cationic ions photochemical reactions

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

Thermal and photochemical decomposition of triarylmethyl azides proceeds by loss of molecular nitrogen. In contrast, the electron impact-induced fragmentation occurs by loss of N3, possibly because of the great stability of triarylmethyl cations. However, decomposition of the molecular ion of azidophenylacetic acid involved loss of N2 and CO2 as in the photochemical reaction (138). In the mass-spectrometric... 

The reaction pathway of the present photochemical reaction is not clear but presumably proceeds as shown in Scheme 31. The radical ion pair of 73 and DCN is formed on photoirradiation. Electron transfer then occurs between the radical anion of EXTN and cyclohexanone, forming radical ion pair of CR73 and AR14b. The radical cation CR73 cleaves into methoxystannane and aryloxymethyl radical R73, which couples with AR14b to give the enolate (or enol) of 74. 

Photochemical addition reactions may also occur as electron-transfer reactions involving a radical ion pair. An illustrative example is the photochemical reaction of 9-cyanophenanthrene (154) with 2,3-dimethyl-2-butene, which, in nonpolar solvents, gives good yields of a [2 + 2] cycloadduct via a singlet exciplex, while in polar solvents radical ions are formed in the primary photochemical process. The olefin radical cation then undergoes deprotonation to yield an allyl radical or suffers nucleophilic attack by the solvent to produce a methoxy alkyl radical. Coupling of these radicals with... 

A number of reviews and texts whose topics include photochemical reactions of aromatic compounds have been published during the year and will be mentioned here. A new volume of the Organic Photochemistry series has appeared and contains a chapter detailing the photochemical reactions of aromatic and heteroaromatic cations (e.g. cyclopropenlum ions, tropylium ions, pyrilium ions and... 

In polar solvents the excited state of sufficiently electron deficient arenes will accept an electron from donors. The fates of the radical ion pairs produced include formation of products of addition to the arene ring. A new example of this mode of reactivity is the photochemical reaction of 1,4-dicyanonaphthalene with benzyl methyl ether in acetonitrile. This yields stereoisomers of the addition product (120). The reaction most likely involves electron transfer from the ether to the naphthalene excited state and subsequent ionisation of a proton from the benzyl ether radical cation. This produces a benzyl ether radical which adds to the naphthalene derivative. An analogous sequence is proposed to explain the photochemical formation of (121)-(124) from ultra-violet light irradiated solutions of naphthalene-1,2-dicarboxylic acid anhydride in methanolic benzene or acetonitrile containing isobutene, 2-butene or 2-methyl-2-butene. Here it is suggested that the alkene radical cation, formed by electron transfer to the excited state of the naphthalene, is attacked by methanol deprotonation... 

The photochemical reaction of the acridinium ion 8.203 in methanol is an example of a general reaction type in which radical ions and radicals are created by electron transfer to or from the excited state. As we saw earlier in the discussion about Fig. 8.10, the HOMO of an excited state is a low-energy, half-filled orbital, into which another electron can be fed from a filled orbital of suitably close energy. In this case, the lone pair in methanol transfers an electron to the HOMO of the excited state 8.204, giving the radical 8.205 and the radical cation 8.206. The latter easily loses a proton to give the radical 8.207, and the C—C coupling of this... 

Photochemically induced walk rearrangements were observed for several 2-hydroxyhomotropylium cations. On irradiation, these cations isomerize to the corresponding 1-hydroxy ions (124b, 127). The stereochemical course, though not known for the thermal walk rearrangement, was clarified for the photochemical reaction by using the diastereomeric cations exo- and endo-85 (124b, 127). 

A review of the photochemical properties of copper complexes includes a survey of the photocatalysed reactions of copper-olefin complexes. The addition of acetonitrile to norbornene may be induced by irradiation in the presence of silver ions. The reaction appears to involve excitation of a LMCT excited state of the norbomene-silver complexes and the formation of norbornene radical cations. 

The ions (216) can be prepared by protonation of the corresponding 2,3-homotropones in fluorosulphonicacid. Irradiation (A > 360 nm, — 70°C)ofthe ions leads to their isomerization. The selectivity shown in the photoisomerizations is attributed to the circumambulatory migration of the C-8 group via intermediates such as (217—219). Photoisomerization about the C-1—O and about the C-2—C-3 bond of the allyl cations (220—222) has been reported cf. the earlier preliminary report. Childs has reviewed the photochemical reactions of protonated unsaturated compounds. 

For the insertion of the iron ion into the porphyrin a variety of general procedures have been described and reviewed. In most cases, these methods lead to the formation of Fe " complexes, which are then used to prepare Fe Fe , Fe, and Fe porphyrins. The most commonly employed methods for synthesizing Fe porphyrins are described below. The preparation of the Fe and Fe complexes from the iron (III) porphyrins by chemical or electrochemical means and the oxidized iron porphyrins (Fe TT-cation radicals, Fe, Fe TT-cation radicals, and Fe ) by chemically or electrochemically oxidizing the iron(III) porphyrins is described in more detail in the sections on the corresponding iron porphyrins below. Whereas Fe porphyrins can be photochemically reduced to Fe porphyrins, only a few examples of photooxidations of the iron center are known, which include laser photolysis of the co-condensation products of PFe at 15K to produce PFe = O. Typical photochemical reactions of iron and other metalloporphyrins have been summarized by Suslick and Watson. ... 

Diamine complexes. The pK values determined for a range of cobalt(iii)-amine chelates indicated enhanced acidity for the protic chelate complexes compared with those with related unidentate ligands." Complete resolution of rac-[Co(en)3] cations has been achieved by paper electrophoresis in a background of d-tartrate and AlC. The crystal structure of (-H)D-[Co(en)3](N03)3 has been determined. The very low quantum yields for the photoreactions of [Co(en)3] have been explained as due to the lack of ligand field excitation for this ion. However, convincing evidence is presented for the reaction of [Co(en)3] " and [Fe(CN)g] under photochemical conditions indicating that this is an efficient photochemical reaction of the cobalt-amine complex in a ligand field excited state (quantum yield ca. 0.2). ... 

We came to this area quite by chance. Our interest in nucleophilic functionalization of aromatics led us to consider photochemical reactions for this purpose. In several cases, such reactions involve ionization of the substrate. Furthermore, we were impressed by the work of Arnold and his co-workers showing that SET often occurs upon photoexcitation yielding an ion radical pair. In view of this fact, one of the experiments we carried out involved irradiation of the photochemical oxidant 1,4-naphthalenedicarbonitrile (DCN) in the presence of toluene and cyanide in deareated acetonitrile. Arnold s work had shown that cation radicals of alkenes add nucleophiles under this condition, and we wanted to test whether a similar reaction with... 

Arsenic and Antimony. Three studies of reaction mechanisms for tetrahedral antimony(v) compounds are reported. These are of the reaction of trimethylantimony sulphide (MegSbS) with alkyl halides, where a four-centre transition state seems possible, the reaction of R4Sb+ cations with alkoxide ions, and the ageing of antimonic acid in aqueous solution. Both thermal and photochemical decomposition of pentaphenylanti-mony have been investigated. Whereas the products of the photochemical reaction are numerous, though all derived from phenyl radicals, the... 

Of the obvious routes to nitrogen pentafluoride, the NF3 + F2 -> NF5 reaction will not work, as it is very unfavorable thermodynamically. The other superficially promising approach, the reaction of the known NF4 with fluoride ion (as HF2 ) gave as the only detectable result attack by F on the F, rather than the N, of the cation. Photochemical reaction of NF3 with F2, although it has been tried, seems worth repeating. 

This type of reaction has been used for the determination of anionic rather than cationic species in inorganic analysis. Thus, halides have been determined using their inhibitory effect on the oxidation of organic compounds by metal ions, as well as in photochemical reactions on the other hand, the oxidizing power of some metal ions such as cerium(IV) and vanadium(V) has been exploited for their own determination. Photochemical reduction reactions have also been used for the determination of anions such as nitrate in effluents. Most of the few reported simultaneous kinetic determinations relying on redox reactions are based on the reduction of heteropoly acids formed between molybdate and silicate, phosphate, or germanate ions. 

---