709-82-0Relevant articles and documents
Solvent reorganization controls the rate of proton transfer from neat alcohol solvents to singlet diphenylcarbene
Peon, Jorge,Polshakov, Dmitrii,Kohler, Bern
, p. 6428 - 6438 (2002)
Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singlet diphenylcarbene was detected and characterized for the first time. Similar band shapes were observed in acetonitrile and in cyclohexane with λmax ≈ 370 nm. The singlet absorption decays by intersystem crossing to triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band is completely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, which is tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solvents the growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transfer from an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds each nascent carbene, the intrinsic rate of protonation in the absence of diffusion could be measured. In methanol, proton transfer occurs with a time constant of 9.0 ps, making this the fastest known intermolecular protontransfer reaction to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanol the rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanol that geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect was observed for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometry the quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reaches a maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of the photoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficiencies are observed in the other alcohols. Because the primary quantum yield for formation of singlet diphenylcarbene is still unknown, the importance of reaction channels that might exist in addition to protonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases that open new possibilities for fundamental studies of proton transfer in solution.
Electron-transfer quenching of excited diphenylmethyl radicals
Arnold, Bradley R.,Scaiano,McGimpsey
, p. 9978 - 9982 (1992)
The intermolecular reactivity of the excited diphenylmethyl radical (DPM*) has been studied with particular emphasis on electron-transfer reactions. These studies allow the determination of the rate constants for the reaction of DPM* with electron acceptors. For example, carbon tetrachloride, methyl benzoate, and benzyl bromide quench DPM* with rate constants of 3.3 × 108, 1 × 106, and 3.6 × 106 M-1 s-1 in acetonitrile. The corresponding carbocation is an observable product of the reaction, leaving no doubt that the reaction involves electron transfer. A kinetic salt effect is observed for the reaction of DPM* with carbon tetrachloride, where the carbocation yield increases from ca. 61% to ca. 100% with the addition of small concentrations of tetrabutylammonium perchlorate. The lower limits to the rates of back electron transfer (BET) and ion pair escape (ESC) for the product cation and radical anions in acetonitrile solution have been estimated using reductive dehalogenation of aryl halides as clock reactions where the rates of fragmentation have been estimated.
Solvolytic Behavior of Aryl and Alkyl Carbonates. Impact of the Intrinsic Barrier on Relative Reactivities of Leaving Groups
Mati?, Mirela,Kati?, Matija,Denegri, Bernard,Kronja, Olga
supporting information, p. 7820 - 7831 (2017/08/14)
The effect of negative hyperconjugation on the solvolytic behavior of carbonate diesters has been investigated kinetically by applying the LFER equation log k = sf(Ef + Nf). The observation that carbonate diesters solvolyze faster than the corresponding carboxylates and that the enhancement of aromatic carbonates is more pronounced indicates that the negative hyperconjugation and π-resonance within the carboxylate moiety is operative in TS. The plots of ΔG? vs approximated ΔrG° for solvolysis of benzhydryl aryl/alkyl carbonates and benzhydryl carboxylates reveal that a given carbonate solvolyzes over the higher Marcus intrinsic barrier and over the earlier transition state than carboxylate that produces an anion of similar stability. Due to the lag in development of the electronic effects along the reaction coordinate, the impact of the intrinsic barrier on solvolytic behavior of carbonates is more important than in the case of carboxylates and phenolates. Consequently, the solvolytic reaction constants (sf) are generally lower for carbonates than for carboxylates. Because of considerable lower reaction constants of carbonates, an inversion of relative reactivities between aryl/alkyl carbonate and another leaving group of similar nucleofugality (Nf) may occur if the electrofuge moiety of a substrate is switched.
Why are vinyl cations sluggish electrophiles?
Byrne, Peter A.,Kobayashi, Shinjiro,Würthwein, Ernst-Urich,Ammer, Johannes,Mayr, Herbert
, p. 1499 - 1511 (2017/02/10)
The kinetics of the reactions of the vinyl cations 2 [Ph2C=C+-(4-MeO-C6H4)] and 3 [Me2C=C+-(4-MeO-C6H4)] (generated by laser flash photolysis) with diverse nucleophile
Solvolytic Behavior of Aliphatic Carboxylates
Matic, Mirela,Denegri, Bernard,Kronja, Olga
, p. 1477 - 1486 (2015/10/05)
The leaving group abilities (nucleofugalities) of a series of aliphatic carboxylates have been obtained by determining the nucleofuge-specific parameters (Nf and sf) from solvolysis rate constants of X,Y-substituted benzhydryl carboxylates in a series of aqueous ethanol mixtures by applyication of the linear free energy relationship (LFER) equation: log k = sf (Ef + Nf). These values can be employed to compare reactivities of carboxylates with those of other leaving groups previously included in the nucleofugality scale, and also to estimate the solvolysis rates of various carboxylates. It is confirmed that the inductive effect is the most important variable governing the reactivities of halogenated carboxylates in solution. Moreover, both the Hammett correlation and the solvolytic activation parameters have revealed a strong influence of the inductive effect on the nucleofugality of alkyl-substituted carboxylates. The reaction constants (sf) indicate that carboxylate substrates with weaker leaving groups solvolyze via later, more carbocation-like, transition states, which is in accord with the Hammond postulate. In addition, due to the weaker demand for solvation of transition states that produce more strongly stabilized benzhydrylium ions, in which more efficient charge delocalization occurs, the reaction constants (sf) obtained with most of the leaving groups investigated here increase as the polarity of the solvent decreases.
Nucleophilic reactivities of tertiary alkylamines
Ammer, Johannes,Baidya, Mahiuddin,Kobayashi, Shinjiro,Mayr, Herbert
supporting information; experimental part, p. 1029 - 1035 (2011/07/09)
The kinetics of the reactions of tertiary amines, triethylamine (1a), N-methylpyrrolidine (1b), N-methylpiperidine (1c), and N-methylmorpholine (1d) with benzhydrylium ions (Ar2CH+) have been studied in acetonitrile and dichlorometha
Photohomolysis and photoionization of substituted tetraphenylethanes and C - C fragmentation of 1,1,2,2-tetra(p-R-phenyl)ethane radical cations (R = H, CH3, OCH3, Cl)
Faria, Joaquim L.,McClelland, Robert A.,Steenken, Steen
, p. 1275 - 1280 (2007/10/03)
On photolysis of a series of tetraphenylethanes in 2,2,2-trifluoroethanol (TFE) solution with 248 nm light, homolysis of the central C-C bond occurs to yield the corresponding substituted diphenylmethyl radicals, in a process requiring one quantum of light. A second process takes place under conditions of high photon fluxes, namely biphotonic photoionization to produce a radical cation, which subsequently undergoes efficient C-C scission of the aliphatic central bond to yield the radical and carbocation fragments. Photoionization and photohomolysis are the preferred processes of excited state deactivation in the solvents acetonitrile, TFE, and 1,1,1,3,3,3-hexafluoroisopropanol. The lifetime of the radical cation could be directly determined by following the formation rates of the fragments in solution. The cations were characterized by their UV absorption spectra and electrophilic reactivities.On photolysis of a series of tetraphenylethanes in 2,2,2-trifluoroethanol (TFE) solution with 248 nm light, homolysis of the central C - C bond occurs to yield the corresponding substituted diphenylmethyl radicals, in a process requiring one quantum of light. A second process takes place under conditions of high photon fluxes, namely biphotonic photoionization to produce a radical cation, which subsequently undergoes efficient C - C scission of the aliphatic central bond to yield the radical and carbocation fragments. Photoionization and photohomolysis are the preferred processes of excited state deactivation in the solvents acetonitrile, TFE, and 1,1,1,3,3,3-hexafluoroisopropanol. The lifetime of the radical cation could be directly determined by following the formation rates of the fragments in solution. The cations were characterized by their UV absorption spectra and electrophilic reactivities.
A photochemical retro-Friedel-Crafts alkylation. Rapid rearrangement of cyclohexadienyl cations
MacKnight, Earl,McClelland, Robert A.
, p. 2518 - 2527 (2007/10/03)
This paper reports the use of laser flash photolysis (LFP) techniques to show that cyclohexadienyl cations (σ complexes) of the Friedel-Crafts reaction of 1,3-dimethoxybenzene and the diphenylmethyl cation rearrange on the ns time scale without separating the aromatic compound and the electrophile. This is demonstrated through a study of the photochemical behaviour of 2-diphenylmethyl-1,3-dimethoxybenzene (4) in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP). Derivatives of 1,3-dimethoxybenzene have previously been found to selectively protonate at C2 upon excitation in HFIP, and indeed the principal products with 4 are 1,3-dimethoxybenzene (6) and Ph2CHOCH(CF3)2 (7), the species expected if the cyclohexadienyl cation formed in the C2 protonation cleaved Ph2CH+. These products are, however, accompanied by 4-diphenylmethyl-1,3-dimethoxybenzene (8), a rearranged isomer of 4. A portion of this product is explained by the combination of Ph2CH+ and 1,3-dimethoxybenzene as the latter accumulates during the irradiation. However, 11.5% of 8 is also seen upon extrapolation to zero time. LFP experiments on the ps time scale reveal that the C2 protonated cation, the 1-diphenylmethy 1-2,6-dimethoxybenzenium ion (5), is formed within 100-200 ps, and reacts with k = 9 × 108 s-1, with absorbance for Ph2CH+ growing in as 5 decays. LFP studies on the ns time scale reveal that there is a second quantity of Ph2CH+ that grows in, with k = 5.0 × 105 s-1. The precursor for this has been identified as the 1-diphenylmethyl-2,4-dimethoxybenzenium ion (10), the thermodynamically more stable isomer of 5. A mechanistic model is proposed in which excited 4 is C2 protonated in HFIP with k ≥ 1 × 1010 s-1 to form 5, which loses Ph2CH+ with k = 3 × 108 s-1 in competition with rearrangement to 10 with k = 6 × 108 s-1. The cation 10 serves as the second source of Ph2CH+, losing Ph2CH+ with k = 4 × 105 s-1; in competition 10 is deprotonated by HFIP to give 8 with k = 8 × 104 s-1. The 11.5% of the rearranged 8 that is observed at zero conversion is thus shown to come from an intramolecular pathway in which the key step is the migration of a diphenylmethyl group without separation: 4 → 4* → 5 → 10 → 8.
Picosecond kinetic study of the dynamics for photoinduced homolysis and heterolysis in diphenylmethyl chloride
Lipson, Matthew,Deniz, Ashok A.,Peters, Kevin S.
, p. 3580 - 3586 (2007/10/03)
The kinetics of both the ions and radicals formed upon photolysis of diphenylmethyl chloride, (4-methoxyphenyl)phenylmethyl chloride, and bis(4-methoxyphenyl)methyl chloride in acetonitrile and propionitrile are examined by picosecond pump - probe spectroscopy. Both radical pairs and ion pairs are formed directly from a common excited state. In addition, the geminate radical pair decays by electron transfer to form either the contact ion pair or a covalent bond, as well as undergoes diffusional separation to free radicals.
Cooperative Effect of Surface Sites on the Laser Flash Photolysis of 1,1-Diphenylacetone and 1,1,3,3-Tetraphenylacetone Adsorbed on Layered Clays. Generation of Radicals and Carbocations
Cozens, Frances L.,Garcia, Hermengildo,Gessner, F.,Scaiano, J. C.
, p. 8494 - 8497 (2007/10/02)
Laser flash photolysis of 1,1-diphenyl- and 1,1,3,3-tetraphenylacetone adsorbed onto a series of clays has been carried out under dry conditions.The photochemical behavior of the guests was investigated by using time-resolved diffuse reflectance techniques.Deposition of the organic compounds onto the solids was performed by slow removal of the solvent from dilute solutions at low temperature.Distinct transient spectra in the microsecond time domain were obtained for the guest-host composites.The formation of the diphenylmethyl radical (DPM.) was observed with montmorillonites SWy-1 (λmax = 310 nm) and STx-1 (λmax = 340 nm).The decay of DPM. on the bidimensional geometry of the layered clays was found to be much slower than within a tridimensional large-pore zeolite.By contrast, in the case of pillared Al13SWy-1 and Al13STx-1 clays, additional reflectance at 450 nm characteristic of the diphenylmethyl cation (DPM+) was present.This intermediate presumably arises from the cooperative interaction with the acid sites of the clay.These results show that these ketones are convenient probes to report on the chemical and topological properties of solid surfaces.
