Steric strain and reactivity: electrophilic bromination of trans-(1-methyl-2-adamantylidene)-1-methyladamantane

trans-(1-Methyl-2-adamantylidene)-1-methyladamantane (DMAD, 1b) reacts with Br(2) in chlorinated hydrocarbon solvents to give either a bromonium polybromide ion pair or a substitution product, depending on bromine concentration. The first intermediate is a 1:1 pi-complex having K(f) = 1.85(0.19) x 10(3) M(-)(1) at 25 degrees C, which rapidly evolves to the bromonium tribromide ion pair. At high bromine concentration, which shifts all equilibria involving the counteranion of the ion pair intermediate toward the pentabromide species, this bromonium ion is stable and unable to further evolve into products. Temperature-dependent NMR spectra indicate chemical exchange of Br(+) between the sides of the plane containing the two carbons of the bromonium ion. At very low bromine concentration, no ionic intermediate is detected and the reaction rapidly yields a rearranged substitution product, identified as 10. Under these conditions the disappearance of the pi-complex follows a first-order rate law, and the observed rate constant increases with increasing olefin concentration, showing that product formation implies Br(-) as counteranion of the ionic intermediate, whose formation is a reversible process. A comparison of the results reported here for the bromination of 1b with those previously found for the parent olefin, adamantylideneadamantane (1a), shows that steric strain markedly affects the reactivity of the double bond.     

Ab initio conformational space study of model compounds of O-glycosides of serine diamide

Relative stabilities of rotamers of the N-acetyl-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-L-seryl-N'-methyl amide (1) and eleven analogous molecules containing beta-galactose, alpha- and beta-mannose, alpha- and beta-glucose, and L-threonine were calculated to learn whether they could explain the natural preference for 1 in linkages between the carbohydrate and protein in glycoproteins. The lowest energy rotamers of four O-glycoside models of serine diamide were identified with a Monte Carlo search coupled with molecular mechanics (MM2*). These rotamers were further optimized with an ab initio level of theory (HF/6-31G(d)). Subsequently, B3LYP/6-31 + G(d) single point energies were calculated for the most stable HF structures. The most favorable interactions are present in 1 and its glucose analogue. The monosaccharide for the carbohydrate antenna is anchored to the serine residue with an AcNH...O=C-NHMe hydrogen bond in the most stable rotamers. The mannose analogue and the beta-anomers are considerably less stable according to the MM2* and especially to the ab inito energy values. The three analogues have HF/6-31 G(d) energies which are 4-6 kcal mol-1 higher; the single point B3LYP/6-31 + G(d)//HF/6-31 G(d) calculations yield preferences of 3-5 kcal mol-1 for 1. The most stable L-threonine analogues show a behaviour very similarly to the corresponding serine analogues. The ZPE and thermal correction components of the calculated delta H298 and delta G298 values are relatively small (< 0.4 kcal mol-1). However, the T delta S298 term can be as large as 2.6 kcal mol-1. The entropy terms stabilize the alpha-anomers relative to beta-anomers, and ManNAc relative to GalNAc. The largest stabilization effect is observed for one of the rotamers of the alpha-anomer of ManNAc.     

Cyclic 1,3-dipoles or acyclic phosphonium ylides? Electronic characterization of "Montréalones"

The structural features of a recently introduced class of 1,3-dipolar reagents have been computed by density functional theory and ab initio methods. The reagents are formally derived from Münchnones by replacement of the C O group with a PR3 unit. The parent species (PR3 = PH3) shows a long P...O interaction (2.55 A at the B3LYP/6-31+G(d) level), together with a nonplanar ring, and is best described as a weakly chelated acylamino-phosphonium ylide. The corresponding acyclic form, in which the P...O interaction is absent, is predicted to be 2-3 kcal mol-1 higher in enthalpy. Variation of the phosphorus substituents exerts a marked effect on the P...O distance, with electron-withdrawing groups favoring a covalent interaction [P...O 1.97 A for PR3 = PPh(catechyl)] and electron-donating groups favoring a weak interaction [P...O 3.92 A for PR3 = PPh3]. However, this variation has little effect on the relative energies of the cyclic and acyclic forms. The barriers for concerted cycloadditions with ethylene are 22.8 kcal mol-1 (PH3), 31.7 kcal mol-1 (PPh3), and 16.2 kcal mol-1 [PPh(catechyl) with axial O], which correspond with experimental observations and follow the same trend as the energies required to distort the dipole to the TS geometry.     

Cycloalkane and cycloalkene C-H bond dissociation energies

Both C-H bond dissociation energies for cyclobutene were measured in the gas phase (BDE = 91.2 +/- 2.3 (allyl) and 112.5 +/- 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out proton affinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results were compared to those for an acyclic model compound, cis-2-butene, and provide the needed information to experimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1 calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. It appears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4 kcal mol-1 too small and their pi bond strengths, as given by BDE1 - BDE2, are in error by up to 8 kcal mol-1.     

Pi-donation and stabilizing effects of pnicogens in carbenium and silicenium ions: a theoretical study of [C(XH2)3]+ and [Si(XH2)3]+ (X = N, P, As, Sb, Bi)

Quantum chemical calculations at the MP2 and CCSD(T) levels of theory are reported for cations of the general type [A(XH2)3]+ with A = C, Si and X = N, P, As, Sb, Bi. Population analysis, methyl stabilization energies (MSEs), and structural criteria were used to predict the p(pi)-donor ability of and the pi-stabilization energy exerted by this series of pnicogens. All of the substituents XH2 considered in these studies invariably stabilize the triply substituted carbenium as well as the silicenium ions. The calculated data show that the intrinsic p(pi)-donation of the group 15 atoms follows the order N < P < As < Sb < Bi. However, the trend of the stabilization energies is fully reversed. The intrinsic stabilization energies of the planar carbenium ions decrease monotonically from 161.2 kcal mol(-1) for X = NH2 to 98.0 kcal mol(-1) for X = BiH2. The effective stabilization of the pnicogens in the equilibrium structures, which also includes the energy-demanding pyramidalization of the XH2 substituents, follows the same trend, although the absolute numbers are reduced to 145.6 kcalmol(-1) for X = NH2 and 53.2 kcalmol(-1) for X = BiH2. This seemingly contrasting behavior of increasing p(pi) charge donation and decreasing stabilization has already been found for other substituents. Previous studies have shown that carbenium ions substituted by chalcogens up to the fourth row also stabilize C+ less effectively with respect to heavier substituents. Of the ions investigated in this study, only the silicenium ions that are stabilized by pnicogens from the third to the sixth row of the periodic system yield increased stabilizing energies that follow the corresponding intrinsic p(pi)-donor abilities of the respective substituent.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Conformation and stereodynamics of symmetrically ortho-disubstituted aryl carbinols and aryl ethers

By making use of low-temperature dynamic NMR spectroscopy, the rotation barriers about the sp3-sp2 bond have been determined in a number of hindered benzyl alcohols symmetrically substituted in the ortho positions, the substituents being F, Cl, Br, and Me. The free energies of activation covered the range 4.6-10.1 kcal mol-1. Ab initio computations matched satisfactorily the trend of these values and predicted the conformation adopted by these compounds. In one case, this result could be also confirmed by the X-ray diffraction structure. In the case of the corresponding methyl ethers two barriers could be measured, corresponding to the passage across two distinguishable transition states: the higher barriers covered the range 5.0-8.1 kcal mol-1 and the lower ones the range 4.7-6.2 kcal mol-1.     

Kinetic investigations of the process of encapsulation of small hydrocarbons into a cavitand-porphyrin

Exchange of guest molecules into capsule shaped host molecules is the most fundamental process in host-guest chemistry. Several examples of quantitative measurements of guest exchange rates have been reported. However, there have been no reports on the activation energies of these processes. A molecule known as cavitand-porphyrin (H2CP) has been reported to have a flexible host structure capable of facilitating moderate guest exchange rates suitable for kinetic measurements of the guest exchange process with 1H NMR. In this article, various kinetic and thermodynamic parameters related to the process of encapsulation of small hydrocarbons into H2CP in CDCl3 solution were determined by 2D exchange spectroscopy (EXSY): association and dissociation rate constants (k(ass) = 320 M-1 s-1, k(diss) = 1.4 s-1 for methane at 25 degrees C), the corresponding activation energies (E(a,ass) = 27 kJ.mol-1, E(a,diss) = 58 kJ.mol-1), and thermodynamic parameters for each process (DeltaG++(ass) = 59 kJ.mol-1, DeltaG++(diss) = 72 kJ.mol-1, DeltaH++(ass) = 25 kJ.mol-1, DeltaH++(diss) = 55 kJ.mol-1, DeltaS++(ass) = -113 J.K-1.mol-1, and DeltaH++(diss) = 58 J.K-1.mol-1 for methane). The thermodynamic parameters (DeltaG degrees = -13 kJ.mol-1, DeltaH degrees = -31 kJ.mol-1, DeltaS degrees = -60 J.K-1.mol-1 for methane) for this encapsulation equilibrium determined by EXSY were comparable to those for methane determined by 1D 1H NMR titration (DeltaG degrees = -11 kJ.mol-1, DeltaH degrees = -33 kJ.mol-1, DeltaS degrees = -75 J.K-1.mol-1 for methane). In addition, the structure of the methane encapsulation process was revealed by ab initio MO calculations. The activation energies for methane association/dissociation were estimated from MP2 calculations (E(a,ass) = 58.3 kJ.mol-1, E(a,diss) = 89.1 kJ.mol-1, and DeltaH degrees = -30.8 kJ.mol-1). These values are in accord with the experimentally determined values. The observed guest exchange rates and energies are compared with the corresponding values of various reported capsule-shaped hosts.     

Thermochemistry of methyl and ethyl nitro, RNO2, and nitrite, RONO, organic compounds

Computational quantum theory is employed to determine the thermochemical properties of n-alkyl nitro and nitrite compounds: methyl and ethyl nitrites, CH3ONO and C2H5ONO, plus nitromethane and nitroethane, CH3NO2 and C2H5NO2, at 298.15 K using multilevel G3, CBS-QB3, and CBS-APNO composite methods employing both atomization and isodesmic reaction analysis. Structures and enthalpies of the corresponding aci-tautomers are also determined. The enthalpies of formation for the most stable conformers of methyl and ethyl nitrites at 298 K are determined to be -15.64 +/- 0.10 kcal mol-1 (-65.44 +/- 0.42 kJ mol-1) and -23.58 +/- 0.12 kcal mol-1 (-98.32 +/- 0.58 kJ mol-1), respectively. DeltafHo(298 K) of nitroalkanes are correspondingly evaluated at -17.67 +/- 0.27 kcal mol-1 (-74.1 +/- 1.12 kJ mol-1) and -25.06 +/- 0.07 kcal mol-1 (-121.2 +/- 0.29 kJ mol-1) for CH3NO2 and C2H5NO2. Enthalpies of formation for the aci-tautomers are calculated as -3.45 +/- 0.44 kcal mol-1 (-14.43 +/- 0.11 kJ mol-1) for aci-nitromethane and -14.25 +/- 0.44 kcal mol-1 (-59.95 +/- 1.84 kJ mol-1) for the aci-nitroethane isomers, respectively. Data are evaluated against experimental and computational values in the literature with recommendations. A set of thermal correction parameters to atomic (H, C, N, O) enthalpies at 0 K is developed, to enable a direct calculation of species enthalpy of formation at 298.15 K, using atomization reaction and computation outputs.     

Remote substituent effects on allylic and benzylic bond dissociation energies. Effects on stabilization of parent molecules and radicals

The effect of remote substituents on bond dissociation energies (BDE) is examined by investigating allylic C-F and C-H BDE, as influenced by Y substituents in trans-YCH=CHCH2-F and trans-YCH=CHCH2-H. Theoretical calculations at the full G3 level model chemistry are reported. The interplay of stabilization energies of the parent molecules (MSE) and of the radicals formed by homolytic bond cleavage (RSE) and their effect on BDE are established. MSE values of allyl fluorides yield an excellent linear free energy relationship with the electron-donating or -withdrawing ability of Y and decrease by 4.2 kcal mol-1 from Y = (CH3)2N to O2N. RSE values do not follow a consistent pattern and are of the order of 1-2 kcal mol-1. A decrease of 4.1 kcal mol-1 is found in BDE[C-F] from Y = CH3O to NC. BDE[YCH=CHCH2-H] generally increases with decreasing electron-donating ability of Y for electron-donating groups and does not follow a consistent pattern with electron-withdrawing groups, the largest change being an increase of 3.6 kcal mol-1 from Y = (CH3)2N to CF3. The G3 results are an indicator of benzylic BDE in p-YC6H4CH2-F and p-YC6H4CH2-H, via the principle of vinylogy, demonstrated by correlating MSE of the allylic compounds with physical properties of their benzylic analogues.     

Conformational studies by dynamic NMR. 79. Dimesityl sulfine revisited: detection of the helical antipodes and determination of their enantiomerization pathways.

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2C=SO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially achiral conformations (Cs symmetry), having the Ph-CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl-CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.     

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.     

Nonradical mechanisms for the uncatalyzed thermal functionalization of silicon surfaces by alkenes and alkynes: a density functional study

We propose a new concerted mechanism for the uncatalyzed hydrosilylation of terminal alkenes and alkynes, alternative to the conventional radical-based mechanism. Density functional calculations have been carried out on these and on previously proposed alternative mechanisms for the hydrosilylation of ethylene and acetylene by suitable finite size clusters as models of the thermal functionalization of -SiH3, =SiH2, and [triple bound] SiH groups in flat Si(100) and Si(111) and porous silicon surfaces by alkenes and alkynes. For each step involved in the considered hydrosilylation pathways, we optimized the geometries of reactants and products and located the corresponding transition states. The calculated activation energies for the concerted pathways of ethylene and acetylene are, respectively, 57.6 and 60.9 kcal mol(-1) on -SiH3 and in the ranges 62-63 and 58-61 kcal mol-1 on =SiH2 and 64-66 and 56-61 kcal mol(-1) on SiH. These values are much lower than the activation energies calculated for the corresponding homolytic dissociation of the Si-H bond, which is the preliminary step in the radical path, 85.6, 82-83, and 79-81 kcal mol(-1), respectively, for -SiH3, =SiH2, and [triple bound] SiH groups. Our results thus suggest that the thermal hydrosilylation of alkenes and alkynes on silicon surfaces, for which a radical-based mechanism is currently accepted, may occur through a concerted mechanism.     

New insights into the bromination reaction for a series of alkenes--a computational study

Ab initio calculations were carried out for the reaction of Br2 with ethene, propene, isobutene, fluoroethene, chloroethene, (E)-1,2-difluoroethene, and (E)-1,2-dichloroethene. For ethene the calculations were also carried out for the reaction with 2Br2. Geometries were optimized at the HF, MP2, and B3LYP levels using the 6-31G(d) and 6-31+G(d) basis sets where for Br both the standard 6-31G and the Binning-Curtiss bromine basis sets were used. Energies were also calculated at the G3MP2 and G3MP2B3 levels. For a single Br2 one mechanism involves a perpendicular attack by Br2 to the C=C bond, and a second mechanism consists of sidewise attack by Br2. Alkenes can react with 2Br2 via several mechanisms, all leading to the dibromo product. The most likely pathway for the reaction of ethene and 2Br2 involves a trans addition of a Br atom from Br3- to one of the bromonium ion carbons. Activation energies, free energies, and enthalpies of activation along with thermodynamic properties (DeltaE, DeltaH, and DeltaG) for each reaction were calculated. We have found that the reaction of ethene with 2Br2 is favored over reaction with only Br2. There is excellent agreement between the calculated free energies of activation for the reaction of ethene and 2Br2 and experimental values in nonpolar aprotic solvents. However, the free energies of activation for the reaction with a single Br2 agrees well with the experimental results for polar protic solvents only when the reaction is mediated by a solvent molecule. A kinetic expression is proposed that accounts for the difference between bromination of alkenes in protic and nonprotic solvents. Some previously unknown heats of formation are reported.     

Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: dimesitylketone, dimesitylthioketone, and dimesitylethylene.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.     

Dimerization of the keto tautomer of acetohydroxamic acid-infrared matrix isolation and theoretical study

Dimerization of the keto tautomer of acetohydroxamic acid has been studied using FTIR matrix isolation spectroscopy and DFT(B3LYP)/6-31+G(d,p) calculations. Analysis of CH3CONHOH/Ar matrix spectra indicates formation of two dimers in which two intramolecular CO...HON bonds within two interacting acetohydroxamic acid molecules are retained. A chain dimer I is stabilized by the intermolecular CO...HN hydrogen bond, whereas the cyclic dimer II is stabilized by two intermolecular NH...O(H)N bonds. Twelve vibrations were identified for dimer I and six vibrations for dimer II; the observed frequency shifts show a good agreement with the calculated ones for the structures I and II. Both dimers have comparable binding energies (DeltaE(ZPE)(CP)I, II=-7.02, -6.34 kcal mol-1) being less stable than calculated structures III and IV (DeltaE(ZPE)(CP)III, IV=-9.50, -8.87 kcal mol-1) in which one or two intramolecular hydrogen bonds are disrupted. In the most stable 10-membered cyclic dimer III, two intermolecular CO...HON hydrogen bonds are formed at expense of intramolecular hydrogen bonds of the same type. The formation of the less stable (AHA)2 dimers in the studied matrixes indicates that the formation of (AHA)2 is kinetically and not thermodynamically controlled.     

What is the Nature of the First‐Formed Intermediates in the Electrophilic Halogenation of Alkenes,Alkynes, and Allenes?

The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series.     

Transesterification and amide cis-trans isomerization in Zn and Cd complexes of the chelating amino acid ligand Boc-Asp(Dpa)-OBzl

The amino acid derivative Boc-Asp-OBzl (Boc=N-butyloxycarbonyl; Asp=aspartic acid; Bzl=benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc-Asp(Dpa)-OBzl (-OBzl). The compound -OBzl contains three different carbonyl groups: a tertiary amide linkage between Asp and Dpa, a C-terminal benzyl ester function, and an N-terminal urethane protecting group. NMR spectra were used to compare the reactivity of these moieties. The Boc protecting group gives rise to two isomers, (E, 9%) and (Z, 91%). Coordination of Cd(NO3)2 and Zn(NO3)2 yielded the complexes and. These compounds have significantly reduced barriers to rotation about the tertiary amide C-N bond compared with the free ligand (-OBzl:18.5 kcal mol-1 in CDBr3;: 12.9 kcal mol-1 in (CD3)2CO;: 13.8 kcal mol-1 in (CD3)2CO). Both complexes readily undergo transesterification in methanol or CD3OD. Experimental pseudo-first order rate constants were determined in CD3OD and (CD3)2CO:CD3OD (3:1;). It was found that the zinc complex (k=(2.28+/-0.02)x10(-4) s-1) is significantly more reactive than the cadmium complex (k=(1.41+/-0.03)x10(-6) s-1). In order to study their tertiary amide cis-trans isomerization, the cadmium complex [(-OCH3)Cd(NO3)2] was synthesized, and the zinc complex [(-OCD3)Zn(NO3)2] was generated in situ in (CD3)2CO:CD3OD (3:1). The barriers to rotation were determined (:14.1 kcal mol-1 in CD3OD;: 13.4 kcal mol-1 in (CD3)2CO:CD3OD (3:1)). Our results show that the stronger Lewis-acid zinc(II) is significantly more active than cadmium(II) in the acceleration of the transesterification. This is in marked contrast to the tertiary amide bond rotation which is comparably fast with both metal ions.     

Aromatic bromination versus oxidation of indolylmalonates by bromine

The reactions of 5-substituted indolylmalonates (2a-e), carrying an electron-withdrawing group at the N(1) position, with bromine in CCl(4) or AcOH are reported. These substrates undergo oxidation in competition with the well-known aromatic bromination. Under the two sets of conditions, with parent indolylmalonate (2a), chemospecific oxidation is observed, whereas with 5-hydroxyindolylmalonate (2c), bromination at the 4- and 6-position is the dominating reaction. Investigation of the products composition of several 5-substituted indolylmalonates revealed the following trend: with a 5-substituted electron-withdrawing group like fluorine, the indolylmalonate undergoes oxidation rather than bromination. In contrast, with a 5-substituted electron-donating group, like a hydroxyl group, the ring bromination occurs preferentially over the oxidation. When the 5-substituent is an alkoxyl group, a significant amount of brominated-oxidized products is obtained. Monitoring the oxidation reaction by mass spectrometry allowed the characterization of the 2-bromoindolylidenemalonate intermediate. A bromonium ion is considered as possible pathway in the formation of this intermediate. The conformation of unsymmetrical methoxyl and benzyloxyl substituents was determined from (1)H NMR spectra, single-crystal X-ray diffraction and ab initio calculations.     

Reactivity of homoallylic substituted adamantylideneadamantanes with bromine. Substituent effects on the stability of the ionic and nonionic intermediates

Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species.