Laser flash photolysis nucleophilic addition

Leigh and coworkers used laser flash photolysis to generate transient 1,1-diphenylsilene 31 from silacyclobutane 32 and measured its UV absorption34,35. The method was used to determine Arrhenius parameters for the addition of nucleophiles such as alcohols to 3125... 

Time resolved laser flash photolysis and electric spin resonance (ESR) spectroscopic investigations were used to get further insight to the reaction mechanism. Both methods demonstrate the formation of do using PET conditions [175,214,215], Upon addition of H donors the signal of do is quenched [214], The oxidation of do is followed by H abstraction from the H donor as shown in Scheme 9. Nucleophilic addition can be excluded because no alkoxyfullerenes were detected at all [173], After reduction of H-do, e.g., by electron transfer from the reduced sensitizer molecule H-do might recombine with R" to the final product. Decay experiments of do by the addition of alcohols support the proposed mechanism of H abstraction as a first step. The involved radical products reveal do as an electrophilic radical. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

The initial step in the mechanism outlined in Scheme 2 is electron transfer quenching of the singlet arene by DCNB. Nucleophilic addition of the amine to the arene cation radical followed by proton and electron transfer steps yields the adduct and regenerates the sensitizer. Adduct formation requires the use of polar solvents, and yields are higher in aqueous vs. dry acetonitrile. Adduct formation is observed in moderately polar solvents (ethers) in the presence, but not in the absence, of an added salt, n-Bu4NBF4. The solvent and salt effects were interpreted as evidence for C-N bond formation via the free arene radical cation, rather than via an ion pair (CRIP or SSRIP), However, Nieminen et ah concluded that nucleophilic attack involves a radical ion pair on the basis of their laser flash photolysis investigation. In addition to this unresolved controversy, the timing of the subsequent proton transfer and electron transfer steps remains to be established. 

Workentin et al. have recently reported the results of an extensive laser flash photolysis investigation of the reactions of the cation radicals of 9-phenyl- and 9,10-diphenylanthracene (PA and DPA, respectively) with amines. Primary amines react with both cation radicals via nucleophilic addition with rate constants which reflect both the amine basicity and a steric requirement for bond formation. Steric effects are more pronounced for addition of DPA " vs. PA ", presumably due to the presence of substituents at both the 9- and 10-position. Tertiary amines and anilines react with PA " and DPA " via electron transfer with rate constants which correlate with amine ionization potentials. Rate constants for nucleophilic addition of primary amines are faster in acetonitrile than in acetonitrile/water solution. The rate-retarding effect of water is attributed to an equilibrium between the fiee amine (reactive) and hydrated amine (unreactive). The beneficial effect of water on preparative ET-sen itized photoamination may reflect its role as a catalyst for the proton transfer processes which follow C-N bond formation (Scheme 2). Hydration of the amine also should render it less reactive in primary and secondary electron transfer processes which can compete with the formation of the arene cation radical. 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

The reaction of a series of substituted styrene radical cations with anions has recently been studied in detail by laser flash photolysis. Representative kinetic data are summarized in Tables 3 and 4 and demonstrate that most of the anions studied react with styrene radical cations with diffusion controlled rate constants. These reactions can involve either addition to the p-carbon to give a benzyl radical (Eq, 15) as discussed above or electron transfer to regenerate the precursor alkene plus the oxidized nucleophile (NU , Eq. 16). Transient absorption spectra have been used to distinguish between these two possibilities. For example, reaction of the radical cation with either bromide or chloride leads to the formation of a transient that is identified... 

In polar solvents, a-halomethyl aromatics give rise to photochemical reactions that can be explained by both radical and ionic mechanisms. Equation 12.77 shows the results for irradiation of 1-chloromethylnaphtha-lene (119) in methanol. The most direct pathway for formation of the methyl ether 120 is heterolytic dissociation of the C-Cl bond to give a chloride ion and a 1-naphthylmethyl carbocation, the latter then undergoing nucleophilic addition by the solvent. Indeed, naphthylphenylmethyl carbocations were detected spectroscopically following laser flash photolysis of (naphthylphenylmethyl)triphenylphosphonium chlorides. On the other hand, products 121, 122, and 123 appear to be formed via the 1-naphthylmethyl radical. Therefore, an alternative source of the carbocation leading to 120 could be electron transfer from the 1-naphthylmethyl radical instead of direct photochemical heterolysis of 119.215-216 jaj-g g p. 

Studied using nanosecond laser flash photolysis. It is reported that two reaction pathways are possible electron transfer or nucleophilic addition when thermodynamically feasible, the electron-transfer pathway dominates. The electronic absorption spectrum of the radical cation (25) has been obtained by y-irradiation of 6b,7,8,8a-tetrahydrocyclobut[a] acenaphthylene in a CFCI3—CF2BrCF2Br matrix at 77 At >640 nm, this is converted into an isomer (26) which is the same as the radical cation obtained by ionization and irradiation of l,4-dihydro-l,4-ethanonaphtho[l,8-de][l,2]diazepine (27) at >540 nm. The electronic absorption spectra of aU the radical... 

Lemmetyinen and coworkers have reported the mechanistic analysis for photoamination of la with ethylenediamine in the presence of p-DCB in MeCN-HjO (9 1) by laser flash photolysis [57]. During the irradiation, the short-lived transient appeared at 420 nm, followed by the formation of long-lived transient (420 nm) due to the cation radical of la. The short-lived transient absorption is due to the exciplex between la andp-DCB and/or ion pair between the cation radical of la and the anion radical of p-DCB. By the addition of ethylenediamine, the absorption of exciplex was affected but the absorption of the cation radical of la was not affected. Therefore, the nucleophilic attack of the amine occurred on the exciplex between la and p-DCB. They have concluded that nucleophilic attack of the amine to the cation radical of la seems to be impossible. 

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