Nucleophilic attack, electron transfer reactions

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Figure 3 Proposed CODH mechanism. This mechanism proposes that the active state is Credl, consistent with recent electrochemical studies,which contains a hridghig hydroxide at the hinuclear NiFe center that serves as the nucleophile to attack a Ni-bound carbonyl forming a Ni-carboxylate. This OH, which is formed by acid-hase catalysis by indicated acid-base residues, is in the position of the bridging sulfide in the C. hydrogenoformans structure. The enzyme is proposed to remain in the Credl redox state until formation of CO2 when it becomes two-electrons reduced to the Cred2 state. Conversion of Credl to Cred2 occurs faster than electron transfer from Cred2 to the FeS clusters, which in turn reduce external electron acceptors. The electron transfer reactions are proposed to occur through a diamagnetic Cint state. ...

Nucleophiles also add to radical cations of arylcyclopropanes to give anti-Markownikoff-type adducts [40-43]. There are two possible structures for the radical cations of arylcyclopropanes i.e., ring opened 1,3-radical cation and ring closed cyclopropane radical cation. Dinnocenzo reported that the photoinduced electron transfer reaction of chiral trans-l-methyl-2-phenylcyclopropane in methanol gives chiral 3-methoxy-l-phenylbutane via the radical cation [44], This result indicates that the radical cation in the ring closed form is attacked by methanol. The fact that this cyclopropane does not isomerize under the photochemical reaction conditions supports this conclusion (Scheme 11). 

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 strong dependence on the leaving group is also noted in preparation of Cp(CO)3-MRe(CO)s (M = Mo, W) . These differing product mixtures result from three different mechanisms direct nucleophilic attack [reaction (f)], electron transfer [reaction (g)], and a two-electron transfer [reaction (h)] . Reaction of Fe(CO)4" with Re(CO)sBr gives the heterobimetallic complex ... 

Sulfur nucleophiles give reduced products consistent with either nucleophilic attack at a sulfur atom or electron transfer reactions <90PS(53)425>. The dithiolate anions formed by these reactions can be characterized as nickel complexes <89JAP(K)63222188>. Thiocyanate ion attacks at S(2) in 1,2-dithiole-3-thiones <88jhci223>. 

Finally it should be noted that the radical products obtained by nucleophilic attack on thianthrene radical cation are characteristically oxidized by this radical cation in a one-electron transfer reaction. This process is presented in detail in the subsequent section on nucleophilic attack. 

The reactions of TH with other nucleophiles have been reviewed previously [10, 86-88]. As mentioned above potential nucleophiles may preferentially undergo electron-transfer with TH. Thus nucleophilic attack and electron transfer reactions compete with each other. Prediction of which of these reactions occurs is an interesting but, as yet, unresolved issued [89]. Clearly an important factor is the oxidation potential of the nucleophile. However, electron transfer may occur by an outer sphere or inner sphere mechanism [90,91] thereby complicating the analysis of electron transfer. Future work may provide the insights necessary to fully understand this issue. 

The highly nucleophilic carbonylate [Re(CO)s]" displaces [Mn(CO)5] rapidly from [Mn2(CO)io] in THF (and [ 0(00)4]" and [CpMo(CO)3]" from [Co(CO)4]2 and [CpMo(CO)3]2, respectively) in second-order reactions that probably involve nucleophilic attack at a C atom of a CO ligand. The ultimate dinuclear product is [Re2(CO)io] and no intermediate mixed metal carbonyls are detected. Nucleophilic attack on [M(CO)6], [M(CO)5L], and [Mn(CO)4(PPh3)2] (M = Mn, Re L = various P-donors) by a range of Co-, Mn-, Fe-, and Re-containing carbonylates has been studied. The ultimate thermodynamic products are a mixture of dinuclear carbonyls formed by one-electron transfer reactions, but in over half the reactions the first step appears to involve... 

C. Electron Transfer Reactions Mimicking Nucleophilic Attack. There are several reports in which 02 nominally appears to react as a nucleophile but, in fact, involve an initial electron-transfer from 02 followed by reaction of O2 with the radical anion generated from the organic substrate. 

Upon oxidation, the subsequent radical cation can decompose in a number of different ways to generate reactive intermediates (Scheme 10.1). The first possibility involves direct H-atom abstraction of the a-C-H bond of the oxidized amine (I) to generate an iminium ion (II), which is susceptible to nucleophilic attack via polar reaction mechanisms (pathway a). Deprotonation of I may also form a carbon-centered radical species (III) that can react with typical radical traps, such as olefins or arenes (pathway b). Generation of the iminium ion may also occur indirectly through oxidation of III via SET to the photocatalyst or another oxidant (pathway c). Finally, radical cation I can undergo non-productive pathways such as back-electron transfer with the reduced photocatalyst (PC" ) to re-generate the neutral amine and PC" (pathway d). 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

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