[Show abstract][Hide abstract] ABSTRACT: Equilibria for the reactions of benzhydryl cations (Ar2CH+) with phosphines, tert-amines, pyridines, and related Lewis bases were determined photometrically in CH2Cl2 and CH3CN solution at 20 °C. The measured equilibrium constants can be expressed by the sum of two parameters, defined as the Lewis Acidity (LA) of the benzhydrylium ions and the Lewis basicities (LB) of the phosphines, pyridines, etc. Least-squares minimization of log K = LA + LB with the definition LA = 0 for (4-MeOC6H4)2CH+ gave a Lewis acidity scale for 18 benzhydrylium ions covering 18 orders of magnitude in CH2Cl2 as well as Lewis basicities (with respect to C-centered Lewis acids) for 56 bases. The Lewis acidities (LA) correlated linearly with the quantum chemically calculated (B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G(d,p) level) methyl anion affinities of the corresponding benzhydrylium ions, which can be used as reference compounds for characterizing a wide variety of Lewis bases. The equilibrium measurements were complemented by isothermal titration calorimetry (ITC) studies. Rates of SN1 solvolyses of benzhydryl chlorides, bromides, and tosylates derived from E(13-33)+, i. e., from highly reactive carbocations, correlate excellently with the corresponding Lewis acidities and the quantum chemically calculated methyl anion affinities. This correlation does not hold for solvolyses of derivatives of the better stabilized amino-substituted benzhydrylium ions E(1-12)+. In contrast, the correlation between electrophilic reactivities and Lewis acidities (or methyl anion affinities) is linear for all donor-substituted benzhydrylium ions E(1-21)+, while the acceptor-substituted benzhydrylium ions E(26-33)+ react more slowly than expected from their thermodynamic stabilities. The boundaries of linear rate-equilibrium relationships were thus defined.
Preview · Article · Jan 2015 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: A series of α,β-unsaturated iminium ions derived from substituted cinnamaldehydes and C2- and C5-substituted chiral imidazolidin-4-ones were isolated and characterized in solution and in the solid state. The kinetics of the reactions of the iminium ions with silyl ketene acetals were determined in dichloromethane at 20 °C. The resulting second-order rate constants were used to estimate the electrophilicity E of the iminium ions according to the linear free energy relationship log k2(20 °C)=sN(N+E). The kinetics for the reactions of two of the iminium ions with tributylphosphine were studied by laser flash spectroscopy and their second-order rate constants were found to agree within a factor of 2.2 with those calculated by using the linear free energy relationship above.
No preview · Article · Apr 2014 · Asian Journal of Organic Chemistry
[Show abstract][Hide abstract] ABSTRACT: The (1) H NMR chemical shifts of the C(α)H protons of arylmethyl triphenylphosphonium ions in CD2 Cl2 solution strongly depend on the counteranions X(-) . The values for the benzhydryl derivatives Ph2 CHPPh3 (+) X(-) , for example, range from δH =8.25 (X(-) =Cl(-) ) over 6.23 (X(-) =BF4 (-) ) to 5.72 ppm (X(-) =BPh4 (-) ). Similar, albeit weaker, counterion-induced shifts are observed for the ortho-protons of all aryl groups. Concentration-dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl(-) > Br(-) ≫ BF4 (-) > SbF6 (-) . For the less bulky derivatives PhCH2 PPh3 (+) X(-) , we also find CH⋅⋅⋅Ph interactions between C(α)H and a phenyl group of the BPh4 (-) anion, which result in upfield NMR chemical shifts of the C(α)H protons. These interactions could also be observed in crystals of (p-CF3 -C6 H4 )CH2 PPh3 (+) BPh4 (-) . However, the dominant effects causing the counterion-induced shifts in the NMR spectra are the CH⋅⋅⋅X(-) hydrogen bonds between the phosphonium ion and anions, in particular Cl(-) or Br(-) . This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh4 (-) anions. The concentration dependence of the (1) H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD2 Cl2 solution. The cation-anion interactions increase with the acidity of the C(α)H protons and the basicity of the anion. The existence of CH⋅⋅⋅X(-) hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X-ray analyses of the crystals. The IR spectra of the Cl(-) and Br(-) salts in CD2 Cl2 solution show strong red-shifts of the CH stretch bands. The CH stretch bands of the tetrafluoroborate salt PhCH2 PPh3 (+) BF4 (-) in CD2 Cl2 , however, show a blue-shift compared to the corresponding BPh4 (-) salt.
No preview · Article · Sep 2013 · Chemistry - A European Journal
[Show abstract][Hide abstract] ABSTRACT: The nucleophilicity parameters N and sN, as defined by the linear free-energy equation log k(20 °C) = sN(N + E), of the 2-imidazolines 1a–d and the related N-heterocyclic compounds 2–5 have been determined by studying the rates of their reactions with differently substituted benzhydrylium ions in dichloromethane at 20 °C by stopped-flow or laser flash photolysis techniques. It is demonstrated that the N and sN parameters thus obtained can be used to reliably predict the rate constants for their reactions with Michael acceptors of known electrophilicity E. A comparison of the nucleophilicity parameters of the imidazoline derivatives 1 with other commonly used nucleophilic organocatalysts shows that they are 10 to 103 times less nucleophilic than PPh3, 1,8-diazabicyclo[5.4.0]undec-7-ene, or 4-(dimethylamino)pyridine. The structure–reactivity relationships of these heterocycles are discussed.
No preview · Article · Jun 2013 · European Journal of Organic Chemistry
[Show abstract][Hide abstract] ABSTRACT: Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.
[Show abstract][Hide abstract] ABSTRACT: Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV-vis spectroscopic measurements. The rate constants for the reaction of the 4,4'-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4-(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the second-order rate constants, we derived the nucleophilicity parameters N and s(N) for the substituted pyridines, as defined by the linear free energy relationship, log k(2) = s(N)(N + E).
No preview · Article · Jul 2012 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: Second-order rate constants k(2) for the reactions of various donor- and acceptor-substituted benzhydrylium ions Ar(2)CH(+) with π-nucleophiles in CH(2)Cl(2) were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log k(2)(20 °C) = s(N)(N + E). The new correlation analysis also leads to the N and s(N) parameters of 18 π-nucleophiles, which have only vaguely been characterized previously. The correlations of log k(2) versus E are linear well beyond the range where the activation enthalpies ΔH(++) of the reactions are extrapolated to reach the value of ΔH(++) = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect. A flattening of the correlation lines only occurs for k(2) > 10(8) M(-1) s(-1) when the diffusion limit is approached.
No preview · Article · Jul 2012 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) yields benzhydryl cations Ar(2)CH(+) and/or benzhydryl radicals Ar(2)CH(•). The efficiency and mechanism of the photo-cleavage were studied by nanosecond laser flash photolysis and by ultrafast spectroscopy with a state-of-the-art femtosecond transient spectrometer. The influences of the photo-electrofuge (Ar(2)CH(+)), the photo-nucleofuge (PPh(3) or P(p-Cl-C(6)H(4))(3)), the counterion (X(-) = BF(4)(-), SbF(6)(-), Cl(-), or Br(-)), and the solvent (CH(2)Cl(2) or CH(3)CN) were investigated. Photogeneration of carbocations from Ar(2)CH-PAr(3)(+)BF(4)(-) or -SbF(6)(-) is considerably more efficient than from typical neutral precursors (e.g., benzhydryl chlorides or bromides). The photochemistry of phosphonium salts is controlled by the degree of ion pairing, which depends on the solvent and the concentration of the phosphonium salts. High yields of carbocations are obtained by photolyses of phosphonium salts with complex counterions (X(-) = BF(4)(-) or SbF(6)(-)), while photolyses of phosphonium halides Ar(2)CH-PPh(3)(+)X(-) (X(-) = Cl(-) or Br(-)) in CH(2)Cl(2) yield benzhydryl radicals Ar(2)CH(•) due to photo-electron transfer in the excited phosphonium halide ion pair. At low concentrations in CH(3)CN, the precursor salts are mostly unpaired, and the photo-cleavage mechanism is independent of the nature of the counter-anions. Dichloromethane is better suited for generating the more reactive benzhydryl cations than the more polar and more nucleophilic solvents CH(3)CN or CF(3)CH(2)OH. Efficient photo-generation of the most reactive benzhydryl cations (3,5-F(2)-C(6)H(3))(2)CH(+) and (4-(CF(3))-C(6)H(4))(2)CH(+) was only achieved using the photo-leaving group P(p-Cl-C(6)H(4))(3) and the counter-anion SbF(6)(-) in CH(2)Cl(2). The lifetimes of the photogenerated benzhydryl cations depend greatly on the decay mechanisms, which can be reactions with the solvent, with the photo-leaving group PAr(3), or with the counter-anion X(-) of the precursor salt. However, the nature of the photo-leaving group and the counterion of the precursor phosphonium salt do not affect the rates of the reactions of the obtained benzhydryl cations toward added nucleophiles. The method presented in this work allows us to generate a wide range of donor- and acceptor-substituted benzhydryl cations Ar(2)CH(+) for the purpose of studying their electrophilic reactivities.
No preview · Article · May 2012 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The identification of the transition state or a short-lived intermediate of a chemical reaction is essential for the understanding of the mechanism. For a direct identification typically transient optical spectroscopy is used, preferentially with high temporal resolution. We combine broad-band femtosecond transient absorption measurements and on-the-fly molecular dynamics calculations to decipher the microscopic evolution of the geometry and solvation of photogenerated benzhydryl cations (Ar(2)CH(+), Ar = phenyl, p-tolyl, m-fluorophenyl, or m,m'-difluorophenyl) in bulk solution. From the high level quantum chemical calculations on the microsolvated cation we can deduce a narrowing and blue shift of the cation absorption that is nearly quantitatively equal to the experimental finding. The roughly 300 fs initial increase in the absorption signal found for all investigated combinations of benzhydryl chlorides or phosphonium salts as benzhydryl cation precursors and solvents is therefore assigned to the planarization and solvation of the nascent fragment of the bond cleavage. The actual cleavage time cannot directly be deduced from the rise of the spectroscopic signal. For alcohols as solvent, the cation combines on the picosecond time scale either with one of the solvent molecules to the ether or to a lesser degree geminately with the leaving group. The study shows that the absorption signal attributable to a species like the benzhydryl cation does not mirror the concentration during the first instances of the process. Rather, the signal is determined by the geometrical relaxation of the photoproduct and the response of the solvent.
No preview · Article · Apr 2012 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: A series of p-substituted benzhydryl fluorides (diarylfluoromethanes) were prepared and subjected to solvolysis reactions, which were followed conductometrically. The observed first-order rate constants k(1)(25 °C) were found to follow the correlation equation log k(1)(25 °C) = s(f)(N(f) + E(f)), which allowed us to determine the nucleofuge-specific parameters N(f) and s(f) for fluoride in different aqueous and alcoholic solvents. The rates of the reverse reactions were measured by generating benzhydrylium ions (diarylcarbenium ions) laser flash photolytically in various alcoholic and aqueous solvents in the presence of fluoride ions and monitoring the rate of consumption of the benzhydrylium ions by UV-vis spectroscopy. The resulting second-order rate constants k(-1)(20 °C) were substituted into the correlation equation log k(-1) = s(N)(N + E) to derive the nucleophilicity parameters N and s(N) for fluoride in various protic solvents. Complete Gibbs energy profiles for the solvolysis reactions of benzhydryl fluorides are constructed.
No preview · Article · Feb 2012 · The Journal of Organic Chemistry
[Show abstract][Hide abstract] ABSTRACT: Kinetics versus thermodynamics: Methyl groups increase the nucleophilic reactivity of the substituted position of hydrazines and reduce the nucleophilicity of the adjacent nitrogen center. As a result, the tertiary nitrogen atom of 1,1-dimethylhydrazine is 3000 times more reactive than the NH2 group, but under thermodynamic control substitution of an NH2 proton occurs (see picture).
No preview · Article · Feb 2012 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: Kinetik contra Thermodynamik: Methyl-Gruppen erhöhen die nucleophile Reaktivität der substituierten Position von Hydrazinen und verringern die Nucleophilie des benachbarten Stickstoff-Atoms. Infolgedessen ist das tertiäre Stickstoff-Atom von 1,1-Dimethylhydrazin 3000-mal reaktiver als die NH2-Gruppe, jedoch wird unter thermodynamischer Kontrolle ein NH2-Proton ersetzt.
No preview · Article · Feb 2012 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Zwei auf einen Streich: α,β-Ungesättigte Iminium-Ionen lassen sich durch Laserblitzphotolyse von Enaminophosphonium-Ionen erzeugen (siehe Schema). Die Geschwindigkeitskonstanten ihrer Reaktionen mit Nukleophilen ermöglichen den ersten direkten Vergleich der Elektrophilie von Iminium-Ionen, die sich von MacMillans Katalysatoren der I. und II. Generation ableiten.
No preview · Article · Oct 2011 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Two at a time: α,β-Unsaturated iminium ions can be generated by laser flash photolysis of enaminophosphonium ions. The rate constants of their reactions with nucleophiles provide the first direct comparison of the electrophilicities of iminium ions derived from MacMillan's first- and second-generation catalysts.
No preview · Article · Oct 2011 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of encapsulation on the dynamics after photoinduced bond cleavage of a diphenylmethyl phosphonium salt in acetonitrile reverse micellar nanopools by femtosecond UV/Vis transient absorption. The small volume of the nanopool is just large enough to accommodate one precursor molecule and therefore eliminates the effects of diffusion present in bulk solution. The tight environment keeps the fragments together and prolongs the time for geminate recombination to occur. We therefore observe an enhanced yield of this bimolecular reaction of the ground state photofragments.
Full-text · Article · Aug 2011 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: For benzhydryl chloride compounds we observe that photodissociation only leads to radical pairs. The typically observed cations are formed by subsequent electron transfer. Reactions of cations in neat alcohols can then occur within 2.6 ps.
[Show abstract][Hide abstract] ABSTRACT: The cumyl cation was generated by laser flash photolysis of cumyl tris(4-chlorophenyl)phosphonium tetrafluoroborate in CH2Cl2 and identified by its UV spectrum. From the decay of its absorbance at λ = 335 nm in the presence of variable concentrations of several nucleophiles with CC double bonds, rate constants for the reactions of the cumyl cation with these π-nucleophiles were determined. The linear free energy relationship log k20°C = s(N + E) (eq 1) was used to calculate the electrophilicity parameter E = 5.74 of the cumyl cation from the rate constants determined in this work and the previously reported N and s parameters of the nucleophilic reaction partners. Substitution of E of the cumyl cation and of the previously reported N and s parameters of α-methylstyrene into eq 1 predicts the temperature-independent rate constant of the addition of the cumyl cation to α-methylstyrene (1.2 × 108 M−1 s−1), which is relevant for the cationic polymerization of α-methylstyrene.