Diphenylmethyl Cations Research Articles

Ultrafast non-adiabatic dynamics of excited diphenylmethyl bromide elucidated by quantum dynamics and semi-classical on-the-fly dynamics.

Carbocations and carboradicals are key intermediates in organic chemistry. Typically UV laser excitation is used to induce homolytical or heterolytical bond cleavage in suitable precursor molecules. Of special interest hereby are diphenylmethyl compounds (Ph2CH-X) with X = Cl, Br as a leaving group as they form diphenylmethyl radicals (Ph2CH˙) and cations (Ph2CH+) within a femtosecond time scale in polar solvents. In this work, we build on our methodology developed for the chlorine case and investigate the photodissociation reaction of Ph2CH-Br by state-of-the-art theoretical methods. On the one hand, we employ specially adapted reactive coordinates for a grid-based wave packet dynamics in reduced dimensionality using the Wilson G-matrix ansatz for the kinetic part of the Hamiltonian. On the other hand, we use full-dimensional semiclassical on-the-fly dynamics with Tully's fewest switches surface hopping routine for comparison. We apply both methods to explain remarkable differences in experimental transient absorption measurements for Cl or Br as the leaving group. The wave packet motion, visible only for the bromine leaving group, can be related to the crucial role of the central carbon atom, which undergoes rehybridization from sp3 to sp2 during the photoinduced bond cleavage. Comparable features are the two consecutive conical intersections near the Franck-Condon region controlling the product splitting to Ph2CH˙/Br˙ and Ph2CH+/Br- as well as the difference in delay time for the respective product formation.

Molecular features in complex environment: Cooperative team players during excited state bond cleavage.

Photoinduced bond cleavage is often employed for the generation of highly reactive carbocations in solution and to study their reactivity. Diphenylmethyl derivatives are prominent precursors in polar and moderately polar solvents like acetonitrile or dichloromethane. Depending on the leaving group, the photoinduced bond cleavage occurs on a femtosecond to picosecond time scale and typically leads to two distinguishable products, the desired diphenylmethyl cations (Ph2CH+) and as competing by-product the diphenylmethyl radicals (). Conical intersections are the chief suspects for such ultrafast branching processes. We show for two typical examples, the neutral diphenylmethylchloride (Ph2CH–Cl) and the charged diphenylmethyltriphenylphosphonium ions () that the role of the conical intersections depends not only on the molecular features but also on the interplay with the environment. It turns out to differ significantly for both precursors. Our analysis is based on quantum chemical and quantum dynamical calculations. For comparison, we use ultrafast transient absorption measurements. In case of Ph2CH–Cl, we can directly connect the observed signals to two early three-state and two-state conical intersections, both close to the Franck-Condon region. In case of the , dynamic solvent effects are needed to activate a two-state conical intersection at larger distances along the reaction coordinate.

ChemInform Abstract: Photo-Heterolysis and -Homolysis of Substituted Diphenylmethyl Halides, Acetates, and Phenyl Ethers in Acetonitrile: Characterization of Diphenylmethyl Cations and Radicals Generated by 248-nm Laser Flash Photolysis

Para-substituted diphenylmethyl halides, acetates, and ethers RPh(R'Ph)CH-X (R, R' = CF3 to OCH3), upon photolysis with -250-nm light in acetonitrile solutions, undergo homolysis and heterolysis of the C-X bond to give the radicals, RPh(R'Ph)CH' (abbreviated as C), and the cations, RPh(R'Ph)CH+ ((2'). Whereas the quantum yields for homolysis (0.2-0.4) are rather independent of the nature of the substituent on the benzene ring, those for heterolysis increase with increasing electron-donator strength from 50.07 for CF3 to 0.3 for OMe. The cationxadical ratios are also dependent on the nucleofugal properties of X. For the halides, the observed hetero1ysis:homolysis ratios correlate with the pK, values of the conjugate acids HX and not with the electron affinities of X'. In acetonitrile, heterolysis is much less endothermic than homolysis. Homolysis and heterolysis can also be effected indirectly by reaction with triplet acetophenone (produced by 308-nm photolysis). Unless stabilized by one or more MeO, the cations decay predominantly by reaction with acetonitrile to give nitrilium ions. However, since this reaction is reversible (shown for the benzhydryl cation), the nitrilium ion contributes only to an insignificant degree to the formation of the final (cation-derived) products, which result from reaction with trace water (main product, benzhydryl alcohol; minor, benzhydrylacetamide). The rate constants for addition of C+ to CH$N are in the range 3.5 X lo5 to 3.8 X IO7 s-I for the cations with R = R' = Me to R = H, R' = CF3. The rate constants for reaction of C+ with halides (ion recombination) are -2 X 1O1O M-l s-' (diffusion control). The radicals C' disappear by dimerization and disproportionation, for which a complete mass balance has been achieved by product analysis for the case of the benzhydryl system. At laser-pulse powers > IO mJ electronically excited radicals, C, are additionally formed in many cases, via absorption of a light quantum by ground-state C'.

ChemInform Abstract: Azulene Analogues of Triphenylmethyl Cation. Extremely Stable Hydrocarbon Carbocations.

Abstract A series of azulene analogues of triphenylmethyl cation; tri(1-azulenyl)methyl cation, di(1-azulenyl)phenylmethyl cation, (1-azulenyl)diphenylmethyl cation, and their methyl and methoxycarbonyl derivatives were synthesized. Their pKR+ values showed that the stability of these cations dramatically increases with the number of azulene rings, and the pKR+ values of tri(1-azulenyl)methyl cation (11.3) and its trimethyl derivative (11.4) are the highest ever reported for methyl cations substituted with hydrocarbon groups.

Benzyl ions from 1,1‐(2,2′‐dimethoxyphenyl)‐substituted 2‐methylpropanes under electron ionization

The electron ionization (EI)-induced fragmentations of a series of 1,1-(2,2'-dimethoxyphenyl)-substituted 2-methylpropanes (1-20) in both 70 eV and mass-analyzed ion kinetic energy (MIKE) spectra have been investigated. The EI-MS spectra of these compounds are characterized by the presence of abundant benzyl ions. These ions result from competitive hydrogen migration from the 2- and 2'-methoxy groups on the carbenium center of the diphenylmethyl cations formed by benzylic cleavage of the molecular ions. The relative abundances of the benzyl ions arising from such competitive processes are discussed and rationalized. The steric effect of the 3- or 3'-substituents is the main discriminating factor between the two competitive processes. The structural information, arising either from the 70 eV or the MIKE spectra, is discussed.

Dications of Fluorenylidenes. The Relationship between Redox Potentials and Antiaromaticity for Meta- and Para-Substituted Diphenylmethylidenefluorenes

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.

Regioselective Superacid-Catalyzed Electrocyclization of Diphenylmethyl Cations to Fluorenes, Phenanthrols and Benzofurans

Cationic electrocyclization of α-benzoyldiphenylmethanols in the presence of superacid provides fluorenes, phenanthrols and benzofurans in good to moderate yields. A single substitution leads to regioselective cationic electrocyclizations.

C‐N vs. C‐C Bond Photodissociation in N‐(tert‐butyldiphenylmethyl)aniline

The direct photolysis of the title aniline compound Ph2(Me3C) C‐NHPh in acetonitrile with 248 nm laser light results in the dissociation of the C‐N bond giving the corresponding (tert‐Bu)diphenylmethyl radical 1a• and the anilino radical PhNH• detectable by ns‐laser flash photolysis (LFP) and ESR spectroscopy. The same radical 1a• is also produced from the corresponding (tert‐Bu)diphenylmethyl chloride Ph2(Me3C)C‐Cl either photolytically in MeCN through C‐Cl bond homolysis or pulse radiolytically in THF; the (tert‐Bu)diphenylmethyl cation is also detected, however, at sorter time scales because of its high reactivity in the solvent MeCN. At higher laser pulse intensities the radical 1a• is reexcited by a second photon leading through electrocyclization to the 4a,4b‐dihydro‐9‐tert‐butylfluorenyl radical (DHBF•) and finally to fluorenyl products. Product analyses of steady state photolyses using GC and GC‐MS gave as principal products (tert‐Bu)diphenylmethane, aniline and various 9‐substituted fluorenyl derivatives, corroborating further the primary C‐N bond cleavage and the formation of the radicals 1a• and PhNH•. Other conceivable dissociation pathways leading to the C‐CMe3 or N‐H bond rupture were not observed.

Solvent reorganization controls the rate of proton transfer from neat alcohol solvents to singlet diphenylcarbene.

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 lambda(max) approximately 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 proton-transfer 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.

Studies of 9-fluorenyl carbocations. intramolecular hydride migration in a substituted 9-fluorenyl carbocation.

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.

Studies in organic mass spectrometry. Part 25. Benzyl ion formation in chemical ionisation (methane or isobutane) of some ortho‐alkylhetero‐substituted diphenylcarbinols

The behaviour of some ortho-alkylhetero-substituted diphenylcarbinols, including deuterium labelled derivatives, under chemical ionisation (methane or isobutane) conditions has been investigated. It has been determined that ortho-alkylhetero diphenylmethyl cations formed by water elimination from the protonated molecule undergo consecutive rearrangements which afford benzyl (or tropylium) ions previously observed for ortho-substituted diphenylcarbenium ions generated by electron ionisation. This reaction also occurs under low-energy collision conditions. Copyright © 2000 John Wiley & Sons, Ltd.

Studies in organic mass spectrometry. Part 25. Benzyl ion formation in chemical ionisation (methane or isobutane) of some ortho-alkylhetero-substituted diphenylcarbinols

The behaviour of some ortho-alkylhetero-substituted diphenylcarbinols, including deuterium labelled derivatives, under chemical ionisation (methane or isobutane) conditions has been investigated. It has been determined that ortho-alkylhetero diphenylmethyl cations formed by water elimination from the protonated molecule undergo consecutive rearrangements which afford benzyl (or tropylium) ions previously observed for ortho-substituted diphenylcarbenium ions generated by electron ionisation. This reaction also occurs under low-energy collision conditions. Copyright 2000 John Wiley & Sons, Ltd.

ChemInform Abstract: Synthesis and Properties of Extremely Stable Tris(6‐methoxy‐1‐azulenyl)methyl Cation and a Series of Di(1‐azulenyl)phenylmethyl and (1‐Azulenyl)diphenylmethyl Cations Stabilized by Methoxy Substituents.

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Effect of concentrations of radical-ion intermediates in the photochemical redox reaction of the diphenylmethyl radical

Effect of concentrations of radical ion intermediates in photoinduced electron transfer (PET) reactions was examined for the redox reaction of diphenylmethyl radical. The irradiation of sym-tetraphenylacetone in the presence of aromatic donor and acceptor in MeOH–MeCN (1:9) leads to (i) the simultaneous generation of diphenylmethyl radical and a pair of radical ions, (ii) the electron transfers between them affording diphenylmethyl cation and anion, and (iii) the formation of Ph 2CHOMe and Ph 2CH 2 by the trapping reactions with methanol. The product ratios of Ph 2CH 2/Ph 2CHOMe are shown to be dominated by the steady-state concentrations of radical ions. Thus, pulsed irradiations resulted in quite different product ratios depending on the pulse intervals. The relative rates extrapolated to infinite, which are directly related to the intrinsic electron transfer rates with radical ions, are shown to follow the Rehm–Weller-type relationship. The importance of the persistent radical ion effect for PET reactions, just as the Ingold–Fischer persistent radical one, is discussed.

Tris[6-(dimethylamino)-1-azulenyl]methyl Hexafluorophosphate. Extremely Stable Methyl Cation with the Highest pKR+ Value

Extremely stable carbocations, tris[6-(dimethylamino)- and 6-morpholino-1-azulenyl]methyl (2e and 2f), bis[6-(dimethylamino)-1-azulenyl][4-(dimethylamino)phenyl]methyl (3e), and [6-(dimethylamino)-1-azulenyl]bis[4-(dimethylamino)phenyl]methyl (4e) cations, were prepared, and their properties were fully characterized. These cations showed extremely high stabilities with high pKR+ values. The values of 2e and 3e were determined spectrophotometrically in DMSO/water as 24.3 ± 0.3 and 21.5 ± 0.2, respectively, which are higher than those of tri(1-azulenyl)methyl and di(1-azulenyl)phenylmethyl cations (2a and 3a) by 13.0 and 11.0 pK units. The value of 4e was determined to be 14.0 ± 0.1 in 50% aqueous acetonitrile and 14.3 ± 0.2 in DMSO/water, which is higher than that of (1-azulenyl)diphenylmethyl cation (4a) by 11.0−11.3 pK units. The extreme stability of these methyl cations is attributable to the dipolar structure of the azulene rings, in addition to the contribution of the mesomeric effect of three dimethy...

Characterization of Persistent Intermediates Generated upon Inclusion of 1,1-Diarylethylenes within CaY Zeolite: Spectroscopy and Product Studies

Inclusion of 1,1-diarylethylenes within activated CaY results in the formation of colored samples that are stable for prolonged periods. For example, diphenylethylene generates a green color within CaY. Two reactive intermediates have been shown to be responsible for this color. The species with the short wavelength (λmax 410 nm) absorption has been established to be the diphenylmethyl cation. The identity of the species with the long wavelength ((λmax 620 nm) absorption has been debated for three decades. We establish that the diarylethylene radical cation is not responsible for the long wavelength absorption. Laser flash photolysis studies show that a number of diarylethylene radical cations have a characteristic absorption in the 400 nm region in both solution and in NaY zeolite. The absence of this absorption for diarylethylene-CaY samples unequivocally eliminates the possibility that these radical cations are responsible for color formation. This is substantiated by the fact that a number of 2-methyl-substituted 1,1-diarylethylenes generate only the short wavelength cation absorption upon inclusion in CaY; radical cation formation would be expected to be equally facile for the methyl-substituted derivatives. Our results suggest that an allylic dimer cation is the most likely source of the long wavelength ((λmax 620 nm) absorption within CaY.

Synthesis and Properties of Extremely Stable Tris(6-methoxy-1-azulenyl)methyl Cation and a Series of Di(1-azulenyl)phenylmethyl and (1-Azulenyl)diphenylmethyl Cations Stabilized by Methoxy Substituents

Abstract Extremely stable carbocations, tris(6-methoxy-1-azulenyl)methyl (8), bis(6-methoxy-1-azulenyl)(4-methoxyphenyl)methyl (9a), and (6-methoxy-1-azulenyl)bis(4-methoxyphenyl)methyl (10a) cations and a series of di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having methoxy substituents on each phenyl group, i.e., di(1-azulenyl)(4-methoxyphenyl)methyl (9b) and (1-azulenyl)bis(4-methoxyphenyl)methyl (10b) cations and 3-methyl-1-azulenyl (9cand 10c) and 3,6-di-t-butyl-1-azulenyl (9d and 10d) analogues, were synthesized by hydride abstraction from the corresponding methane derivatives and their properties were fully characterized. The pKR+ values of 8 and 9a were well beyond 14.0. The value of 10a was determined as 13.2, which is higher by 10.2 pK units than that of (1-azulenyl)diphenylmethyl cation. The value also considerably increased by the methoxy substitution on each phenyl group. The values of 9b—d (pKR+ 11.7—13.4) and 10b—d (pKR+ 5.2—7.0) are higher by 1.0—1.4 and 2.2—2.4 pK units than those of the corresponding analogous benzyl and diphenylmethyl cations. The electrochemical reduction of 8, 9a—d, and 10a—d showed a wave at −0.88, −0.71 — −0.83, and −0.56 — −0.71 V (V vs. Ag/Ag+), respectively, upon cyclic voltammetry (CV). The relatively high reduction potentials also exhibited the stabilization of the methyl cations by the methoxy substituents. The oxidation of 8 in acetonitrile exhibited barely separated two-step, one-electron oxidation waves at a potential range of +0.90 — +0.98 V upon CV, although 9a—d and 10a—d did not show two similar waves at a narrow potential range. The wave is ascribed to the oxidation of two azulene rings to generate a trication species.

Studies in organic mass spectrometry; Part 26. Unimolecular decomposition of ortho-methoxy-substituted diphenylmethyl cations

A double rearrangement (hydrogen migration followed by carbon–carbon displacement) constitutes the main fragmentation process of the 2-methoxydiphenylmethyl cations formed by benzylic cleavage of the molecular ion of ortho-methoxy-substituted 1,1diphenylalkanes with a wide range of internal energy. This has been demonstrated by recording mass analysed ion kinetic energy (MIKE) spectra, calculation of approximate activation energies (obtained by appearance energy difference) and calculation of a degrees-of-freedom effect. A geometry of the transition state, which accounts for the observed substituent effects, is proposed .

Superacid-Catalyzed Electrocyclization of Diphenylmethyl Cations to Fluorenes. Kinetic and Theoretical Revisit Supporting the Involvement of Ethylene Dications

In superacid media, diphenylmethyl cations bearing an α-carbonyl group generate fluorene compounds. We have obtained chemical evidence showing the acidity dependence of this fluorene cyclization process. A linear relationship was found between the rate of the fluorene cyclization and the acidity of the reaction media in kinetic studies, strongly supporting the intervention of an additional proton transfer to the diphenylmethyl monocation in the fluorene cyclization, that is, the involvement of a dication in the form of the diphenylmethyl cation bearing an O-protonated carbonyl group. Ab initio calculations employing Becke3-LYP density functional theory provided theoretical support for the idea that the dicationic fluorene cyclization is energetically more favorable than monocationic electrocyclization to give the benzofuran or the fluorene. The magnitudes of the predicted activation barriers of the dicationic fluorene cyclization are compatible with the experimental values for the fluorene cyclization and...

ChemInform Abstract: Syntheses and Properties of Extremely Stable Di(1‐azulenyl) phenylmethyl and (1‐Azulenyl)diphenylmethyl Cations Having Dimethylamino Substituents on Their Phenyl Groups.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.