The mechanism of base-promoted HF elimination from 4-fluoro-4-(4-nitrophenyl)butan-2-one is E1cB. evidence from double isotopic fractionation experiments

Leaving-group fluorine and secondary deuterium multiple kinetic isotope effects (KIEs) have been determined for the base-promoted HF elimination from the 4-fluoro-4-(4'-nitrophenyl)-(1,1,1,3,3-(2)H(5))butan-2-one. The fluorine KIE was determined by using the accelerator-produced short-lived radionuclide (18)F in combination with the naturally abundant (19)F. The (19)F substrate was labeled with (14)C in a remote position to enable radioactivity measurements of both substrates. The size of the determined fluorine KIE is 1.0009 +/- 0.0010 when acetate is used as base. The secondary deuterium KIEs are 1.009 +/- 0.017, 1.000 +/- 0.018, and 1.010 +/- 0.023 for formate, acetate, and imidazole, respectively. The magnitudes of these KIEs are significantly smaller compared to the corresponding KIEs that we recently reported for the protic substrate. This new data clearly demonstrates that the elimination proceeds via an E1cB mechanism.     

Carbodications. 4.(1) Hydrogen Isotope Exchange of Crotonyl Cations in Superacid

The title ion reacts in 1:1 DF-SbF(5) and exchanges up to five protium atoms with deuterium. The incorporation of label was measured by GC-MS analysis of the methyl crotonate formed by methanol quenching. The isotopomer distribution at about 60% conversion, which shows a minimum for the d(1) and a maximum for the d(4) species, indicates that the intermediate dication with the second charge at C(3) loses a proton faster from C(4) than from C(2). Formation of the pentadeuteriocrotonyl cation indicates that the 1,4-dication (acyl primary alkyl) or the 1,2-dication must intervene in the process. Computer modeling of the kinetics for the multiple exchange process to fit the experimental deuterium distribution allowed determination of the relative rate constants and isotope effects (KIEs) for the formation of the carbocations from alkenoyl cations (beta-secondary KIE) and elimination from carbodications to alkenoyl cations (primary KIE). An exceptionally large beta-secondary KIE of ca. 2.0/hydrogen was found for the formation of the dication. A small primary isotope effect of ca. 1.5 was found for elimination from the dications to the alkenoyl cations. Elimination from the 1,3-acylalkyl dication to form the nonconjugated 3-butenoyl cation is 6-7 times faster than elimination to the conjugated 2-butenoyl cation. The rate ratio for the conversion of 3-butenoyl cation to the 1,4-dication (primary alkyl cation) and 1,3-dication (secondary alkyl cation) is (0.025-0.030):1, whereas the relative rate of the formation of the 1,2-acylalkyl dication (the alternative route of achieving pentadeuteration) is zero.     

High resolution EPR spectroscopy of C(60)F and C(70)F in solid argon: reassignment of C(70)F regioisomers

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric C=C bonds of C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.     

The Osmium(VIII) Oxofluoro Cations OsO(2)F(3)(+) and F(cis-OsO(2)F(3))(2)(+): Syntheses, Characterization by (19)F NMR Spectroscopy and Raman Spectroscopy, X-ray Crystal Structure of F(cis-OsO(2)F(3))(2)(+)Sb(2)F(11)(-), and Density Functional Theory Calculations of OsO(2)F(3)(+), ReO(2)F(3), and F(cis-OsO(2)F(3))(2)(+)

Osmium dioxide tetrafluoride, cis-OsO(2)F(4), reacts with the strong fluoride ion acceptors AsF(5) and SbF(5) in anhydrous HF and SbF(5) solutions to form orange salts. Raman spectra are consistent with the formation of the fluorine-bridged diosmium cation F(cis-OsO(2)F(3))(2)(+), as the AsF(6)(-) and Sb(2)F(11)(-) salts, respectively. The (19)F NMR spectra of the salts in HF solution are exchange-averaged singlets occurring at higher frequency than those of the fluorine environments of cis-OsO(2)F(4). The F(cis-OsO(2)F(3))(2)(+)Sb(2)F(11)(-) salt crystallizes in the orthorhombic space group Imma. At -107 degrees C, a = 12.838(3) ?, b = 10.667(2) ?, c = 11.323(2) ?, V = 1550.7(8) ?(3), and Z = 4. Refinement converged with R = 0.0469 [R(w) = 0.0500]. The crystal structure consists of discrete fluorine-bridged F(cis-OsO(2)F(3))(2)(+) and Sb(2)F(11)(-) ions in which the fluorine bridge of the F(cis-OsO(2)F(3))(2)(+) cation is trans to an oxygen atom (Os-O 1.676 ?) of each OsO(2)F(3) group. The angle at the bridge is 155.2(8) degrees with a bridging Os---F(b) distance of 2.086(3) ?. Two terminal fluorine atoms (Os-F 1.821 ?) are cis to the two oxygen atoms (Os-O 1.750 ?), and two terminal fluorine atoms of the OsO(2)F(3) group are trans to one another (1.813 ?). The OsO(2)F(3)(+) cation was characterized by (19)F NMR and by Raman spectroscopy in neat SbF(5) solution but was not isolable in the solid state. The NMR and Raman spectroscopic findings are consistent with a trigonal bipyramidal cation in which the oxygen atoms and a fluorine atom occupy the equatorial plane and two fluorine atoms are in axial positions. Density functional theory calculations show that the crystallographic structure of F(cis-OsO(2)F(3))(2)(+) is the energy-minimized structure and the energy-minimized structures of the OsO(2)F(3)(+) cation and ReO(2)F(3) are trigonal bipyramidal having C(2)(v)() point symmetry. Attempts to prepare the OsOF(5)(+) cation by oxidative fluorination of cis-OsO(2)F(4) with KrF(+)AsF(6)(-) in anhydrous HF proved unsuccessful.     

Synthesis and methylation products of 4(5)-(2-furyl)imidazole

Bromination of 2-acetylfuran with copper(II) bromide in a mixture of ethyl acetate and chloroform leads selectively to furacyl bromide, the nucleophilic substitution of bromine in which by OAc and subsequent use of the Weidenhagen reaction enabled the synthesis of 4(5)-(2-furyl)imidazole. On N-methylation of this imidazole in KOH–acetone 2 isomers are formed, the 1-methyl-4- and 1-methyl-5-(2-furyl)imidazoles. It was established that, unlike alkylation of 4(5)-phenylimidazole, the main product of the reaction is 1-methyl-5-(2-furyl)imidazole.     

Synthesis and enhanced electrochemical performance of Li(2)CoPO(4)F cathodes under high current cycling

Lithium cobalt fluorophosphate, Li(2)CoPO(4)F, is successfully synthesized by a solid state reaction under Ar flow at 700 °C. X-ray diffraction and scanning electron microscopic studies are utilized to analyze the structural and morphological features of the synthesized materials, respectively. The presence of fluorine is also supported by energy-dispersive X-ray spectroscopy. The electrochemical properties are evaluated by means of Li/Li(2)CoPO(4)F half-cell configurations in both potentiostatic and galvanostatic modes. The Li/Li(2)CoPO(4)F cell delivers an initial discharge capacity of 132 mA h g(-1) at a current density of 0.1 mA cm(-2) between 2.0 and 5.1 V at room temperature. Due to the higher operating potential of the Co(2+/3+) couple in the fluorophosphate matrix, this cell shows a capacity retention of only 53% after 20 cycles, still the material delivered 108 mA h g(-1) at a high current rate of 1 C. Cyclic voltammetric studies corroborate the insertion and extraction of Li(+) ions by a single phase reaction mechanism during cycling.     

Elimination-addition mechanism for nucleophilic substitution reaction of cyclohexenyl iodonium salts and regioselectivity of nucleophilic addition to the cyclohexyne intermediate

The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (k(H)/k(D) > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as S(N)1 reactions (k(H)/k(D) approximately 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile.     

Variable-temperature 19F NMR and theoretical study of 1,9- and 1,7-C60F(CF3) and C(s-) and C1-C60F17(CF3): hindered CF3 rotation and through-space J(FF) coupling

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.     

Combined QM/MM and path integral simulations of kinetic isotope effects in the proton transfer reaction between nitroethane and acetate ion in water

An integrated Feynman path integral-free energy perturbation and umbrella sampling (PI-FEP/UM) method has been used to investigate the kinetic isotope effects (KIEs) in the proton transfer reaction between nitroethane and acetate ion in water. In the present study, both nuclear and electronic quantum effects are explicitly treated for the reacting system. The nuclear quantum effects are represented by bisection sampling centroid path integral simulations, while the potential energy surface is described by a combined quantum mechanical and molecular mechanical (QM/MM) potential. The accuracy essential for computing KIEs is achieved by a FEP technique that transforms the mass of a light isotope into a heavy one, which is equivalent to the perturbation of the coordinates for the path integral quasiparticle in the bisection sampling scheme. The PI-FEP/UM method is applied to the proton abstraction of nitroethane by acetate ion in water through molecular dynamics simulations. The rule of the geometric mean and the Swain-Schaad exponents for various isotopic substitutions at the primary and secondary sites have been examined. The computed total deuterium KIEs are in accord with experiments. It is found that the mixed isotopic Swain-Schaad exponents are very close to the semiclassical limits, suggesting that tunneling effects do not significantly affect this property for the reaction between nitroethane and acetate ion in aqueous solution.     

Rates and mechanism of fluoride and water exchange in UO(2)F(5)(3-) and [UO(2)F(4)(H(2)O)](2-) studied by NMR spectroscopy and wave function based methods

The reaction mechanism for the exchange of fluoride in UO(2)F(5)(3-) and UO(2)F(4)(H(2)O)(2-) has been investigated experimentally using (19)F NMR spectroscopy at -5 degrees C, by studying the line broadening of the free fluoride, UO(2)F(4)(2-)(aq) and UO(2)F(5)(3-), and theoretically using quantum chemical methods to calculate the activation energy for different pathways. The new experimental data allowed us to make a more detailed study of chemical equilibria and exchange mechanisms than in previous studies. From the integrals of the different individual peaks in the new NMR spectra, we obtained the stepwise stability constant K(5) = 0.60 +/- 0.05 M(-1) for UO(2)F(5)(3-). The theoretical results indicate that the fluoride exchange pathway of lowest activation energy, 71 kJ/mol, in UO(2)F(5)(3-) is water assisted. The pure dissociative pathway has an activation energy of 75 kJ/mol, while the associative mechanism can be excluded as there is no stable UO(2)F(6)(4-) intermediate. The quantum chemical calculations have been made at the SCF/MP2 levels, using a conductor-like polarizable continuum model (CPCM) to describe the solvent. The effects of different model assumptions on the activation energy have been studied. The activation energy is not strongly dependent on the cavity size or on interactions between the complex and Na(+) counterions. However, the solvation of the complex and the leaving fluoride results in substantial changes in the activation energy. The mechanism for water exchange in UO(2)F(4)(H(2)O)(2-) has also been studied. We could eliminate the associative mechanism, the dissociative mechanism had the lowest activation energy, 39 kJ/mol, while the interchange mechanism has an activation energy that is approximately 50 kJ/mol higher.     

Probing the transition states of four glucoside hydrolyses with 13C kinetic isotope effects measured at natural abundance by NMR spectroscopy

Kinetic isotope effects (KIEs) were measured for methyl glucoside (4) hydrolysis on unlabeled material by NMR. Twenty-eight (13)C KIEs were measured on the acid-catalyzed hydrolysis of alpha-4 and beta-4, as well as enzymatic hydrolyses with yeast alpha-glucosidase and almond beta-glucosidase. The 1-(13)C KIEs on the acid-catalyzed reactions of alpha-4 and beta-4, 1.007(2) and 1.010(6), respectively, were in excellent agreement with the previously reported values (1.007(1), 1.011(2): Bennet and Sinnott, J. Am. Chem. Soc. 1986, 108, 7287). Transition state analysis of the acid-catalyzed reactions using the (13)C KIEs, along with the previously reported (2)H KIEs, confirmed that both reactions proceed with a stepwise D(N)A(N) mechanism and showed that the glucosyl oxocarbenium ion intermediate exists in an E(3) sofa or (4)H(3) half-chair conformation. (13)C KIEs showed that the alpha-glucosidase reaction also proceeded through a D(N)*A(N) mechanism, with a 1-(13)C KIE of 1.010(4). The secondary (13)C KIEs showed evidence of distortions in the glucosyl ring at the transition state. For the beta-glucosidase-catalyzed reaction, the 1-(13)C KIE of 1.032(1) demonstrated a concerted A(N)D(N) mechanism. The pattern of secondary (13)C KIEs was similar to the acid-catalyzed reaction, showing no signs of distortion. KIE measurement at natural abundance makes it possible to determine KIEs much more quickly than previously, both by increasing the speed of KIE measurement and by obviating the need for synthesis of isotopically labeled compounds.     

Deuterium kinetic isotope effects on the gas-phase reactions of C2H with H2(D2) and CH4(CD4)

Kinetics of the ethynyl (C(2)H) radical reactions with H(2), D(2), CH(4) and CD(4) was studied over the temperature range of 295-396 K by a pulsed laser photolysis/chemiluminescence technique. The C(2)H radicals were generated by ArF excimer-laser photolysis of C(2)H(2) or CF(3)C(2)H and were monitored by the chemiluminescence of CH(A(2)Δ) produced by their reaction with O(2) or O((3)P). The measured absolute rate constants for H(2) and CH(4) agreed well with the available literature data. The primary kinetic isotope effects (KIEs) were determined to be k(H(2))/k(D(2)) = 2.48 ± 0.14 and k(CH(4))/k(CD(4)) = 2.45 ± 0.16 at room temperature. Both of the KIEs increased as the temperature was lowered. The KIEs were analyzed by using the variational transition state theory with semiclassical small-curvature tunneling corrections. With anharmonic corrections on the loose transitional vibrational modes of the transition states, the theoretical predictions satisfactorily reproduced the experimental KIEs for both C(2)H + H(2)(D(2)) and C(2)H + CH(4)(CD(4)) reactions.     

Synthesis of 1-(dimethylsulfamoyl)-2- and 5-imidazolecarboxaldehydes. Rearrangement of 1-(dimethylsulfamoyl)-5-imidazole-carboxaldehyde to the 4-carboxaldehyde

Lithiation of 1-(dimethylsulfamoyl)imidazole by n-butyllithium, followed by substitution with dimethylformamide provided 1-(dimethylsulfamoyl)-2-imidazolecarboxaldehyde in 19% yield. When 1-(dimethylsulfamoyl)-2-(tert-butyldimethylsilyl)imidazole was lithiated by sec-butyllithium, followed by methyl formate, there was obtained 1-(dimethylsulfamoyl)-2-(tert-butyldimethylsilyl)-5-imidazolecarbox-aldehyde (57%). Removal of the silyl group by acetic acid yielded 1-(dimethylsulfamoyl)-5-imidazolecarbxaldehyde ( 11 , 96%) as a gum. Isomerization of 11 took place slowly at room temperature (10 days), or faster in tetrahydrofuran solution containing triethylamine (2 hours) to form crystalline 1-(dimethylsul-famoyl)-4-imidazolecarboxaldehyde (12) in 68% yield. Proton and carbon-13 nmr spectra were analyzed to determine the structure of the isomers. However, only X-ray crystallography established the structure of 1-(dimethylsulfamoyl)-4-imidazolecarboxaldehyde, unequivocally. A mechanism for the isomerization of 11 to 12 is proposed.     

The kinetics and mechanisms of the gas phase elimination of 4-(methylthio)-1-butyl acetate and 1-chloro-4-(methylthio)-butane

The gas phase elimination of 4-(methylthio)-1-butyl acetate and 1-chloro-4-(methylthio)-butane has been investigated in a seasoned, static reaction vessel over the temperature range of 310–410°C and the pressure range of 46–193 Torr. The presence of the inhibitors propene, cyclohexene, and/or toluene had no effect on the rates. The reactions are homogeneous, unimolecular, and obey a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 4-(methylthio)-1-butyl acetate, log k₁(s^?1) = (12.32 ± 0.29) ? (192.1 ± 3.6) kJ/mol/2.303RT; for 1-chloro-4-(methylthio)-butane, log k₁(s^?1) = (12.23 ± 0.59) ? (175.7 ± 6.8) kJ/mol/2.303RT. The CH₃S substituent in 1-chloro-4-(methylthio)-butane has been found to participate in the elimination reaction, where tetrahydrothiophene and methyl chloride formation may result from an intimate ion-pair type of mechanism. The yield of a cyclic product in gas phase reactions provides additional evidence of an intimate ion pair mechanism through neighboring group participation in gas phase elimination of special types of organic halides.     

Theoretical Study on the Gas‐Phase SN2 Reaction of Microhydrated Fluoride with Methyl Fluoride

The rate constants for the gas‐phase S_N2 reaction of F^?(H₂O) with CH₃F have been calculated using the dual‐level variational transition state theory including multidimensional tunneling from 50 to 500 K. Tunneling was found to dominate the reaction below 200 K. The deuterium, ¹³C, and ¹⁴C kinetic isotope effects (KIEs) and solvent (D₂O) isotope effects (SKIEs) were also calculated in the same temperature range. The results indicated that the deuterium and heavy water substitutions resulted in inverse KIEs (0.6～0.8 ) while the ¹³C and ¹⁴C substitutions resulted in normal KIEs (1.0～1.2) at room temperature. The calculated carbon KIEs increased significantly below 80 K due to the differences in the magnitude of the tunneling effects for different isotopic substitutions.     

Experimental evidence for enzyme-enhanced coupled motion/quantum mechanical hydrogen tunneling by ketosteroid isomerase

Although there are considerable data demonstrating that quantum mechanical hydrogen tunneling (HT) occurs in both enzymatic and nonenzymatic systems, little data exist that address the question of whether enzymes enhance the amount of HT relative to the corresponding nonenzymatic reactions. To investigate whether 3-oxo-Delta (5)-steroid isomerase (ketosteroid isomerase, KSI) enhances HT relative to the nonenzymatic (acetate-catalyzed) isomerization of Delta (5)-androstene-3,17-dione ( 1) to Delta (4)-androstene-3,17-dione ( 3), alpha-secondary deuterium kinetic isotope effects (KIE) at C-6 of the steroid were determined for both the KSI- and acetate-catalyzed isomerizations. The normal intrinsic secondary KIE for both wild type (WT) KSI (1.073 +/- 0.023) and acetate (1.031 +/- 0.010) suggest the possibility of coupled motion (CM)/HT in both the enzymatic and nonenzymatic systems. To assess the contribution of CM/HT in these reactions, the secondary KIE were also measured under conditions in which deuterium instead of hydrogen is transferred. The decrease in secondary KIE for WT (1.035 +/- 0.011) indicates the presence of CM/HT in the enzymatic reaction, whereas the acetate reaction shows no change in secondary KIE for deuterium transfer (1.030 +/- 0.009) and therefore no evidence for CM/HT. On the basis of these experiments, we propose that KSI enhances the CM/HT contribution to the rate acceleration over the solution reaction. Active site mutants of KSI (Y14F and D99A) yield secondary KIEs similar to that of WT, indicating that mutations at the hydrogen-bonding residues do not significantly decrease the contribution of CM/HT to the KSI reaction.     

Fluoride ion donor properties of TcO2F3 and ReO2F3: X-ray crystal structures of MO2F3.SbF5 (M = Tc, Re) and TcO2F3.XeO2F2 and Raman and NMR spectroscopic characterization of MO2F3.PnF5 (Pn = As, Sb), [ReO2F2(CH3CN)2][SbF6], and [Re2O4F5][Sb2F11

The fluoride ion donor properties of TcO2F3 and ReO2F3 toward AsF5, SbF5, and XeO2F2 have been investigated, leading to the formation of TcO2F3.PnF5 and ReO2F3.PnF5 (Pn = As, Sb) and TcO2F3.XeO2F2, which were characterized in the solid state by Raman spectroscopy and X-ray crystallography. TcO2F3.SbF5 crystallizes in the monoclinic system P2(1)/n, with a = 7.366(2) A, b = 10.441(2) A, c = 9.398(2) A, beta = 93.32(3) degrees, V = 721.6(3) A3, and Z = 4 at 24 degrees C, R1 = 0.0649, and wR2 = 0.1112. ReO2F3.SbF5 crystallizes in the monoclinic system P2(1)/c, with a = 5.479(1) A, b = 10.040(2) A, c = 12.426(2) A, beta = 99.01(3) degrees, V = 675.1(2) A3, and Z = 4 at -50 degrees C, R1 = 0.0533, and wR2 = 0.1158. TcO2F3.XeO2F2 crystallizes in the orthorhombic system Cmc2(1), with a = 7.895(2) A, b = 16.204(3) A, c = 5.198(1) A, beta = 90 degrees, V = 665.0(2) A3, and Z = 4 at 24 degrees C, R1 = 0.0402, and wR2 = 0.0822. The structures of TcO2F3.SbF5 and ReO2F3.SbF5 consist of infinite chains of alternating MO2F4 and SbF6 units in which the bridging fluorine atoms on the antimony are trans to each other. The structure of TcO2F3.XeO2F2 comprises two distinct fluorine-bridged chains, one of TcO2F3 and the other of XeO2F2 bridged by long Tc-F...Xe contacts. The oxygen atoms of the group 7 metals in the three structures are cis to each other and to two terminal fluorine atoms and trans to the bridging fluorine atoms. The 19F NMR and Raman spectra of TcO2F3.PnF5 and ReO2F3.PnF5 in SbF5 and PnF5-acidified HF solvents are consistent with dissociation of the adducts into cis-MO2F2(HF)2+ cations and PnF6- anions. The energy-minimized geometries of the free MO2F2+ cations and their HF adducts, cis-MO2F2(HF)2+, have been calculated by local density functional theory (LDFT), and the calculated vibrational frequencies have been used as an aid in the assignment of the Raman spectra of the solid MO2F3.PnF5 adducts and their PnF5-acidified HF solutions. In contrast, ReO2F3.SbF5 ionizes in SO2ClF solvent to give the novel Re2O4F5+ cation and Sb2F11- anion. The 19F NMR spectrum of the cation is consistent with two ReO2F2 units joined by a fluorine bridge in which the oxygen atoms are assumed to lie in the equatorial plane. The [ReO2F2(CH3CN)2][SbF6] salt was formed upon dissolution of ReO2F3.SbF5 in CH3CN and was characterized by 1H, 13C, and 19F NMR and Raman spectroscopies. The ReO2F2(CH3CN)2+ cation is a pseudooctahedral cis-dioxo arrangement in which the CH3CN ligands are trans to the oxygens and the fluorines are trans to each other.     

F/Cl-exchange on AlCl(3)-pyridine adducts: synthesis and characterization of trans-difluoro-tetrakis-pyridine-aluminum-chloride, [AlF2(Py)4]+Cl-

Whereas liquid CCl3F reacts with solid AlCl3 exothermically under chlorine-fluorine-exchange already above -20 degrees C, no reaction takes place between CCl3F and the pyridine complexes of AlCl3 (AlCl3.Py, AlCl3.2Py, or AlCl3.3Py) up to 100 degrees C. The desired chlorine by fluorine substitution on the monomer AlCl3-pyridine adducts occurs, however, easily using Me3SiF as fluorinating agent. By reacting AlCl3.3Py with Me3SiF (even up to 10-fold stoichiometric excess) in pyridine as a solvent, only two of the three Cl atoms can be substituted by fluorine, leading in good yield to the new "mixed aluminum halide", AlF2Cl.4Py. Actually, it represents the first example of a stable solid donor-acceptor adduct of an aluminum-III halide with two different halogens of defined stoichiometry. It was characterized by multinuclear solid-state NMR (27Al and 19F), IR spectroscopy, as well as single-crystal structure analysis. The new compound has an ionic solid-state structure with helical trans-octahedral [(Py)4AlF2]+ cations and isolated Cl- anions. The comparison of its 27Al MAS solid-state NMR spectra with those of a compound bearing the analogous [(Py)4AlCl2]+ cation reveals an extreme increase in the quadrupolar coupling constants, from 0.24 MHz in case of the chlorine cation to about 16 MHz in case of the new [(Py)4AlF2]+ cation.     

Diastereoselectivity in the McLafferty rearrangement of photoionized 3-methyl valeramide

The McLafferty rearrangement of photoionized 3-methyl valeramide proceeds quasi-barrierless and with high regioselectivity. The mass spectra of the stereospecifically labeled syn- and anti-[4-D(1)]-diastereomers reveal a strong preference for activation of the gamma-hydrogen/deuterium in anti-position relative to the methyl group at C(3), which serves as a steric marker. Quantitative analysis of the fragmentation patterns of other photoionized isotopomers permits the determination of primary and secondary kinetic isotope effects (KIEs), the branching ratios of competing McLafferty reactions, and the steric effect (SE) associated with transfer of the diastereotopic H(D) atoms at C(4). While the associated KIEs of the title reaction are negligible, the steric effect (SE = 2.9) is remarkably large for the otherwise flexible, monofunctional compound. The findings can be explained by a preferentially chairlike transition structure for the initial gamma-H atom transfer.     

Gas-phase thermolysis of diallyl(4-fluorophenyl) and allyl(t-butylamino)phenyl phosphines

Dially(4-fluorophenyl)phosphine and allyl(t-butylamino)phenylphosphine were pyrolyzed in a stirred-flow reactor at 340–420°C/9–19 Torr, using toluene as carrier gas. The primary reaction products were propene, 1-(4-fluorophenyl)-1-phosphabutadiene, and 1-phenyl-2-t-butyliminophosphine. The phosphorus-containing products gave rise to [4 + 2] and [2 + 2] cycloaddition products, respectively. The consumption of these phosphines showed first-order kinetics, with the rate coefficients following the Arrhenius equations: Dially(4-fluorophenyl)phosphine: k(s⁻¹) = 10^9.00±0.32 exp (- 122 ± 4 kJ/mol RT) Allyl(t-butylamino)phenylphosphine: k(s⁻¹) = 10^9.04±0.25 exp (-113 ± 3 kJ/mol RT) The results support a six-center cyclic transition-state unimolecular elimination reaction mechanism for both reactants. © 1997 John Wiley & Sons, Inc.