Transesterification Kinetics Investigation of R‐Substituted Phenyl Benzoates with 4‐Methoxyphenol in the Presence of K2CO3 in DMF

Transesterification of R‐substituted phenyl benzoates 1–5 with 4‐methoxyphenol 6 was kinetically investigated in the presence of K₂CO₃ in dimethylformamide (DMF) at various temperatures. The Hammett plots for the reactions of the 1–5 demonstrate good linear correlations with σ⁰ constants. Low magnitude of ρ_LG values indicate that the leaving group departure occurs after the rate‐determining step. The Brønsted coefficient values for the reactions (?0.2, ?0.16, ?0.13 at 15, 24, 36°C, respectively) demonstrate the weak effect of leaving group substituent on the reactivity of R‐substituted phenyl benzoates 1–5 for the reactions with 4‐methoxyphenol 6 in the presence of K₂CO₃ in DMF. The leaving group substituent effect on free energy (ΔG^≠), enthalpy (ΔH^≠), and entropy (ΔS^≠) of activation was examined. It was shown that the activation parameters obtained depend weakly on the leaving group substituent effect. The reaction is entropy controlled in case the leaving group substituent becomes electron withdrawing.     

Die Konformativen Umwandlungen des Ungesättigten Siebenringes in 4.5.6-Trithia-1.2-Benzocycloheptenen-(1) Konformative Beweglichkeit Flexibler Ringsysteme—XVI. Untersuchungen mit Hilfe Der Protonenresonanzspektroskopie

The conformational equilibria and conversions of 4.5.6-trithia-1.2-benzocycloheptene-(1) ( 1 ) and the 3′.6′-dimethoxy-, 3′.6′-dimethyl- and 3′.6′-diphenyl- derivatives ( 2, 3 and 4 ) were investigated by NMR spectroscopy. Solutions of these substances are equilibrium mixtures of two conformers, one presumably having a chair form and the other a boat form. The free enthalpy of the boat conformer ΔG_B is dependent on the size of the substituents (R) in the 3′ and 6′ positions. The ΔG_B values for R = H, OCH₃, C₆H₅ and CH₃ are 1,03, 0,82, 0,50 and ?0,19 kcal/moles, respectively. By slow crystallization one conformer of the substituted trithiabenzocycloheptenes may be obtained in a pure crystalline form. The dimethoxy derivative crystallizes in the chair form, whereas the dimethyl and the diphenyl derivatives crystallize in the boat form. After dissolving the crystals, the conformational equilibrium is restored; at 0°C the half-lifes range from 2 to 15 minutes. By means of the temperature dependence of the NMR spectra two different types of conformational changes may be distinguished experimentally: the slower one is assigned to the inversion of the seven membered ring and the faster one to its pseudorotation. The free enthalpy of activation ΔG_v^≠ of the inversion was determined for 4.5.6-trithia-1.2-benzocycloheptene-(1) by the ‘line-shape’ method and for the diphenyl derivative by the ‘equilibration’ method. Both methods were applied to the other derivatives. The ΔG_v^≠ values obtained by the two different methods agree well with one another. The free enthalpy of activation of the inversion ΔG_v^≠ and of the pseudorotation ΔG_p^≠ both depend on the nature of the substituents. The ΔG_v^≠ values range from 17,9 to 20,5 kcal/mole and the ΔG_p^≠ values are equal to or lower than 11,4 kcal/mole.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Substituent effects on the reactions of aromatic triflones with para‐X‐anilines in methanol: Low intrinsic reactivity and transition state imbalances

A kinetic study is reported for S_NA_r reactions of 2,4,6‐tris(trifluoromethanesulfonyl) anisole 1a with a series of para‐X‐substituted anilines 2a–e in a methanol solution at various temperatures. The substituent effects on free energy (Δ^≠G), enthalpy (Δ^≠H), and entropy (Δ^≠S) of activation are examined. Aniline addition to triflone 1a is characterized by a β_X=0.57, α_Z=0.31, and an imbalance of I = α_Z–β_X=?0.26. The imbalance shows that resonance development lags behind C? N bond formation at the transition state. Interestingly, analysis of the results in terms of Marcus theory reveals that these S_NAr are associated with some extremely low intrinsic reactivity (log k_o=?1.25& © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 255–262, 2011     

Structures and Dynamic Solution Behavior of Cationic,Two‐Coordinate Gold(I)–π‐Allene Complexes

A family of seven cationic gold complexes that contain both an alkyl substituted π‐allene ligand and an electron‐rich, sterically hindered supporting ligand was isolated in >90 % yield and characterized by spectroscopy and, in three cases, by X‐ray crystallography. Solution‐phase and solid‐state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two‐term rate laws of the form rate=k₁[complex]+k₂[complex][allene] consistent with allene‐independent and allene‐dependent exchange pathways with energy barriers of ΔG^≠₁=17.4–18.8 and ΔG^≠₂=15.2–17.6 kcal mol^?1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG^≠=8.9–11.4 kcal mol^?1) intramolecular exchange of the allene π faces through η¹‐allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)₂o‐binaphthyl}Au(η²‐4,5‐nonadiene) ]⁺SbF₆^? ( 5 ), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG^≠_298K=17.4 kcal mol^?1), which ruled out the participation of a η¹‐allylic cation species in the low‐energy π‐face exchange process for this complex.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

NMR analysis of restricted internal rotation in 2-substituted-2,3-dihydro-3-o-tolyl(chlorophenyl)-4(1H)-quinazolinones

Steric interactions between aryl and heterocyclic moieties in 2-substituted-2,3-dihydro-3-o-tolyl(chlorophenyl)-4(1H)-quinazolinones 1a-j produce sufficient restriction to rotation about the aryl C? N bond that the presence of torsional isomers may be detected at room temperature. Diastereomeric population and free energy of activation for rotation have been calculated by ¹H nmr spectra. Probably due to a preferred axial position of R² substituent no dramatic variation both in A /B ratio and in ΔG≠ value has been observed for 1a-f . The comparison between 1a and 1j ΔG^≠ values allows to formulate a hypothesis on the structure of the transition state.     

Compensation effect in reactions between <Emphasis Type="Italic">trans</Emphasis>-4,4'-dinitrostilbene oxide and arylsulfonic acids

The effect structure and temperature have on the rate and free activation energy of reactions between trans-4,4'-dinitrostilbene oxide and Y-substituted arylsulfonic acids YC₆H₄SO₃H in a mixture of dioxane with 1,2-dichloroethane (7: 3 vol/vol) at 265, 281, and 298 K is studied. It is found that as a result of the nonadditivity of the joint effect of substituents Y and temperature on the rate of the process of oxirane ring opening, the cross reaction series exhibits isoparametric properties in the aspect of enthalpy–entropy compensation. This allows the experimental determination of an isoparametric point with respect to the constant of substituent Y (σ_Y^IP= 0.52), in which activation entropy ΔS^≠ = 0 and free activation energy ΔG^≠ do not depend on temperature (ΔG^≠ = ΔH^≠), and to conduct the transition through this point with inversion of the order of the effect temperature has on the value of ΔG^≠ as a result of reversing the sign of ΔS^≠: in the series Y (σ_Y) = 4-OCH₃ (–0.27), 4-CH₃ (–0.17), H (0), 4-Cl (0.23), and 3-NO₂ (0.71), the values of ΔS^≠ (J/(mol K)) are–140,–119,–85,–42, and 44, respectively. The possibility of using isoparametric points as quantitative mechanistic criteria is demonstrated.     

铀(Ⅵ)与不对称希夫碱配合物的合成和热分解反应动力学

邻苯二胺与5－氯－2－羟基二苯酮、邻香草醛作用合成了一种不对称希夫碱配体C₂₇H₂₁N₂O₃Cl（H₂L）。在正丁醇和甲醇体系中硝酸铀酰与该配体反应合成了一种固体希夫碱配合物[UO₂(HL)(NO₃)(H₂O)]·H₂O。通过元素分析、IR、UV、¹H NMR、TG-DTG及摩尔电导率分析等手段对合成的配合物进行了表征,用非等温热重法研究了铀(Ⅵ)配合物的热分解反应动力学,推断出第三步热分解的动力学方程为：d α /d t = A · e^{- E/RT} ·3/2[(1- α )^-1/3-1]^-1,得到了动力学参数E和A。并计算出了活化熵△S^¹和活化吉布斯自由能△G^¹。     

Dynamic NMR studies of ferrocenylsulphide-palladium(II) and -platinum(II) complexes

The stereodynamics of ferrocenylsulphide-palladium(II) and -platinum(II) complexes, Fe(C₅H₄SR)₂MX₂, (M = Pd^II, Pt^II; X = Cl, Br; R = Ph, i-Pr and i-Bu), have been examined by variable temperature NMR. At temperatures down to ca. ?100° C, the pyramidal inversion of the S atoms could be slowed down sufficiently to yield accurate energy data, while the reversal of the ferrocenophane ring remained fast on the NMR time scale. ΔG^≠ data for the S inversion process were in the range 47 to 65 kJ mol^?1 and were influenced to varying extents by the nature of the transition metal, the halogen, and the R substituent on the sulphur.     

Kinetic Studies of the Thermal cis-to-trans Isomerization of Dioxaphospholanes

By following a previously reported method,¹ the synthesis of r-2-alkoxy-cis-4-cis-5-dimethyl-1,3,2-λ³-dioxaphospholanes ligands (1 and 3) was carried out. The purpose of this work is the kinetic study of the inversion barrier at phosphorus for 1 and 3 and the comparison with the already informed dioxaphospholane 2. The kinetic measurements of the thermal isomerization cis-to-trans were performed by ³¹P NMR spectroscopy, observing a first order kinetics for both compounds. The energy of activation (E_a) for the epimerization of compounds cis-1 and cis-3 was calculated to be 16.0 ± 0.6 and 11.8 ± 0.8 kcal/mol, respectively. The values of the thermodynamic parameters of the transition state (Δ H^≠, Δ S^≠, Δ G^≠) suggest that the inversion at phosphorus not only depends on the spatial requirements of the alkoxy substituent but also on entropic effects. The thermodynamic parameters Δ H°, Δ S°, and Δ G° were also evaluated and they show that the cis isomers are preferred from enthalpic point of view, but entropic effects dominate the equilibrium trans ? cis leading to the entropically favored trans isomers. Furthermore, the results are supported by density functional theory calculations of 1–4 at the B3LYP/6-31G** level.     

Probing the Rotational Dynamics of meso‐(2‐Substituted)aryl Substituents in A2B‐Type Subporphyrins

A₂B‐type B‐methoxy subporphyrins 3 a – g and B‐phenyl subporphyrins 7 a – c , e , g bearing meso‐(2‐substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable‐temperature (VT) ¹H NMR spectroscopy. In these subporphyrins, the rotation of meso‐aryl substituents is hindered by a rationally installed 2‐substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH^≠ and ΔS^≠ in ΔG^≠. 2‐Methyl and 2‐ethyl groups of the meso‐aryl substituents in subporphyrins 3 e , 3 f , and 7 e induce larger rotational barriers than 2‐alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4‐dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B‐phenyl subporphyrins than for B‐methoxy subporphyrins indicate a negligible contribution of S_N1‐type heterolysis in the rotation of meso‐aryl substituents.     

Substituentenabhängigkeit der Ringinversion des Ungesättigten Siebenringes bei Benzocycloheptenderivaten. Konformative Beweglichkeit flexibler Ringsysteme—XIV: Untersuchungen mit Hilfe der Protonenresonanzspektroskopie

NMR spectroscopy has been used to investigate the ring inversions of the unsaturated seven membered ring system in a total of 20 benzocycloheptene derivatives with 1, 2 and 3 pairs of geminal substituents. For all compounds the inversion of the ring at ? 80°C is ‘frozen’ and at this temperature only one conformation is present in detectable quantity, presumably that of the chair form. The free activation enthalpies ΔG^≠ for the chair inversions lie between 9·9 and 13·7 kcal/mole. For disubstituted and tetrasubstituted benzocycloheptenes the ΔG^≠ values vary according to the positions of the ligands: for disubstituted derivatives ΔG^≠ is largest for the 5-position and smallest for the 3-position. For the tetrasubstituted derivatives the inversion of the ring—compared to that in the comparable dimethyl derivatives—is made more difficult when the ligands are in the 3,6- or 3,7- positions, but is facilitated when in the 3,5- or 4,6- positions. The effect observed in the 3,5- and 4,6- substituted rings is due to transanular repulsion of synaxial substituents, which leads to a flattening of the ring. Such a repulsion does not occur when the ligands are in the 3,6- positions. On the other hand, when the ligands are in 3,7- positions the transanular repulsion leads to a stronger puckering of the chair; the inversion could be hindered by this. For benzocycloheptene the activation energies for the inversions between chair, boat and twist (S, W, T) conformations were determined from model calculations. The best route for the inversion of the chair is the version way S → W via the transitional conformation V₄₅ and V₅₆. The calculated activation energy for this (14·6 kcal/mole) agrees well with the experimentally determined value (13 ± 1·5 kcal/mole). For the pseudorotation W → T a slightly lower calculated value of 11·1 kcal/mole was found.     

Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism

We report that 2,6‐lutidine?trichloroborane (Lut?BCl₃) reacts with H₂ in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl₂. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl₃ was calculated to react with H₂ and form the ion pair, [LutH⁺][HBCl₃^?], with a barrier of ΔH^≠=24.7 kcal mol^?1 (ΔG^≠=29.8 kcal mol^?1). Metathesis with a second molecule of Lut?BCl₃ produced Lut?BHCl₂ and [LutH⁺][BCl₄^?]. The overall reaction is exothermic by 6.0 kcal mol^?1 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})BCl₂]. Barriers for addition of H₂ across the Lut/LutBCl₂⁺ pair and the boracycle B?C bond are substantially higher (ΔG^≠=42.1 and 49.4 kcal mol^?1, respectively), such that these pathways are excluded. The barrier for addition of H₂ to the boracycle B?N bond is comparable (ΔH^≠=28.5 and ΔG^≠=32 kcal mol^?1). Conversion of the intermediate 2‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) to Lut?BHCl₂ may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl₃. Intramolecular protodeboronation, which could form Lut?BHCl₂ directly, is prohibited by a high barrier (ΔH^≠=52, ΔG^≠=51 kcal mol^?1).     

Low temperature 13C NMR study of 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkyl- aminocyclohexanes

Cyclohexane and piperidine ring reversal in 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkylaminocyclohexanes was investigated by ¹³C NMR. An unusually low conformational energy ΔG = 0.59 kJ mol^?1 and activation parameters ΔG^≠₂₁₈ = 43.8 ± 0.4 kJ mol^?1, ΔH^≠ = 48.9 ± 2.5 kJ mol^?1 and ΔS^≠ = 23 ± 9 J mol^?1 K^?1 were found for the diequatorial to diaxial transition of the cyclohexane ring in the trans-pyrrolidinyl derivative. In the trans-piperidinyl derivative, ΔG₂₂₂ = 44.7 ± 0.5 KJ mol^?1, ΔH^≠ = 55.7 ± 6.3 kJ mol^?1 and ΔS^≠ = 51 ± 21 J mol^?1 K^?1 was found for the piperidine ring reversal from the non-equivalence of the α-carbons.     

Activation of the aromatic system by the SO2CF3 group: Kinetics study and structure–reactivity relationships

The rate constants for the reaction of 2,6‐bis(trifluoromethanesulfonyl)‐4‐nitroanisole with some substituted anilines have been measured by spectrophotometric methods in methanol at various temperatures. The data are consistent with the S_NAr mechanism. The effect of substituents on the rate of reaction has been examined. Good linear relationships were obtained from the plots of log k₁ against Hammett σ_para constants values at all temperature with negative ρ values (?1.68 to ?1.11). Activation parameters Δ^≠H varied from 41.6 to 54.3 kJ mol^?1 and Δ^≠S from ?142.7 to ?114.6 J mol^?1 K^?1. The δΔ^≠H and δΔ^≠S reaction constants were determined from the dependence of Δ^≠H and Δ^≠S activation parameters on the σ substituent constants, by analogy with the Hammett equation. A plot of Δ^≠H versus Δ^≠S for the reaction gave good straight line with 177°C isokinetic temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 203–210, 2010     

Determination of ΔH≠ and ΔS≠ by simultaneous 1H and 13C dynamic n.m.r. studies: Importance of the accuracy of temperature measurement

From ΔG_Tc^≠ values obtained by ¹H and ¹³C dynamic nuclear magnetic resonance studies of the same dynamic process, it is possible to estimate ΔH^≠ and ΔS^≠. Nevertheless, the accuracy of the temperature measurement is a factor which limits the applicability of this method. A very simple procedure for calibrating the usual temperature sensors is described, which can be applied to all types of n.m.r. probes. By the use of this procedure it is possible to measure coalescence temperatures in ¹H and ¹³C n.m.r. with such an accuracy that ΔS^≠ can be effectively determined from the difference between ΔG_Tc^≠ values.     

Investigation of pyramidal inversion of nitrogen atom in carbamate and thiocarbamate ions formed at the reaction of СО<Subscript>2</Subscript>, СOS,CS<Subscript>2</Subscript> with 2-aminoethanol

Dynamic NMR was applied to measuring the value ΔG^≠ characterizing the height of the barrier to the pyramidal inversion of the nitrogen atom in carbamate and thiocarbamate anions formed at the reaction of 2-aminoethanol with CO₂ and COS. The refinement was introduced in formerly suggested cyclic structures of anions containing an intramolecular hydrogen bond NH···O(S), which contradicted the found large values of the barrier of inversion (ΔG ^≠ ～ 70 kJ mol^–1). The hydrogen bond in the cyclic anions of carbamates and thiocarbamates is two-electron and three-center. Analogous cyclic structure with a multicenter hydrogen bond does not form in the case of dithiocarbamate anion that is the product of 2-aminoethanol reaction with CS₂.     

Solid-State Confinement Effects in Selective exo-H/D Exchange in the Rhodium σ-Norbornane Complex [(Cy2PCH2CH2PCy2)Rh(η2:η2-C7H12)][BArF4]

Density functional theory calculations modelling selective exo-H/D exchange observed in the Rh σ-alkane complex [(Cy₂PCH₂CH₂PCy₂)Rh(η²:η²-endo-NBA)][BAr^F₄], [1-NBA][BAr^F ₄ ] , are reported, where Ar^F=3,5-C₆H₃(CF₃)₂ and NBA=norbornane, C₇H₁₂. Two models were considered 1) an isolated molecular cation, [1-NBA]⁺ and 2) a full model in which [1-NBA][BAr^F ₄ ] is treated in the solid state through periodic DFT. After an initial endo-exo rearrangement, both models predict H/D exchange to proceed through D₂ addition and oxidative cleavage followed by a rate-limiting C−H activation of the norbornane through a σ-CAM step to form a [1-Rh(D)(η²-HD)(norbornyl)]⁺ intermediate. HD rotation followed by a σ-CAM C−D bond formation, HD reductive coupling and HD loss then complete the H/D exchange process. exo-H/D exchange is facilitated by a supporting agostic interaction and is consistently more accessible kinetically than the potentially competing endo-H/D exchange (isolated cation: ΔG^≠_exo=+15.9 kcal/mol, ΔG^≠_endo=+18.4 kcal/mol; solid state: ΔG^≠_exo=+22.1 kcal/mol, ΔG^≠_endo=+25.1 kcal/mol). The solid-state environment has a significant impact on the computed energetics, with barriers increasing by ca. 7 kcal/mol, while only the solid-state model correctly predicts the endo-bound NBA complex to be the resting state of the system. These outcomes reflect solid-state confinement effects within the pocket occupied by the [1-NBA]⁺ cation and defined by the pseudo-octahedral array of neighbouring [BAr^F₄]⁻ anions. The asymmetry of the solid-state environment also requires a second H/D exchange pathway to be defined to account for reaction at all four exo-C−H bonds. These entail slightly higher barriers (ΔG^≠_exo= +24.8 kcal/mol, ΔG^≠_endo=+27.5 kcal/mol) but retain a distinct preference for exo- over endo-H/D exchange.     

Activation of Strong Boron–Fluorine and Silicon–Fluorine σ‐Bonds: Theoretical Understanding and Prediction

The oxidative addition of BF₃ to a platinum(0) bis(phosphine) complex [Pt(PMe₃)₂] ( 1 ) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF₃ to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol^?1, respectively) and require large Gibbs activation energies (ΔG°^≠=56.3 and 38.9 kcal mol^?1, respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF₃ to 1 in the presence of a second BF₃ molecule occurs with ΔG°^≠ and ΔG° values of 10.1 and ?4.7 kcal mol^?1, respectively. ΔG°^≠ becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF₃, occurs from the dissociating fluoride to the second non‐coordinated BF₃. This CT interaction stabilizes both the transition state and the product. The B?F σ‐bond cleavage of BF₂Ar^F (Ar^F=3,5‐bis(trifluoromethyl)phenyl) and the B?Cl σ‐bond cleavage of BCl₃ by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF₄ to 1 easily occurs in the presence of a second SiF₄ molecule via the formation of a hypervalent Si species.