Elimination Of HX Research Articles

CH4 activation by PtX+ (X = F, Cl, Br, I).

Reactions of PtX+ (X = F, Cl, Br, I) with methane have been investigated at the density functional theory (DFT) level. These reactions take place more easily along the low-spin potential energy surface. For HX (X = F, Cl, Br, I) elimination, the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going as the sequence: X = F, Cl, Br, I. A reversed trend is observed in the loss of H2 for X = F, Cl, Br, while it is not favorable for PtI+ in the loss of either HI or H2. For HX eliminations, the transfer form of H is from proton to atom, last to hydride, and the mechanisms are from PCET to HAT, last to HT for the sequence of X = F, Cl, Br, I. One reason is mainly due to the electronegativity of halogens. Otherwise, the mechanisms of HX eliminations also can be explained by the analysis of Frontier Molecular Orbitals. While for the loss of H2, the transfer of H is in the form of hydride for all the X ligands. Noncovalent interactions analysis also can be explained the reaction mechanisms.

Reactivity and Mechanistic Issues in the Photocyclization of Dihalostyryl‐Naphthalenes towards Halo‐[4]helicenes: A Transposition on a Mallory Theme

AbstractOne of the most straightforward strategies nowadays for the synthesis of carbo‐ and hetero‐helicenes is the oxidative photocyclization of stilbene derivatives under Mallory conditions. In this study, the reactivity of a series of 3,4‐dihalostyryl‐naphthalenes (Hal=Br, F and Cl) has been investigated using Mallory conditions towards the corresponding halogenated [4]helicene compounds. The difluoro precursor afforded the two isomeric 2,3‐ and 1,2‐difluoro‐[4]helicenes, resulting from the ring closure on the two possible positions of the substituted benzene ring, the dichloro compound led to the formation of a mixture of the two isomeric 2,3 and 1,2‐dichloro‐[4]helicenes together with the 2‐mono‐chloro derivative, while the dibromo precursor provided, unexpectedly, only 2,3‐dibromo‐[4]helicene and the 2‐mono‐bromo derivative. DFT calculations performed on the entire series of precursors, final helicenes and intermediate dihydrohelicenes, including the non‐halogenated congeners, reveal the highly energetically favorable formation of a second dihydrohelicene intermediate following a [1,9]‐hydrogen sigmatropic transposition. Whereas the existence of this transposed dihydrohelicene intermediate, more stable than the initially formed dihydrohelicene by 15–30 kcal/mol, was until now “hidden” in the non‐substituted series, it allows herein to explain its re‐aromatization through formal elimination of HX in order to provide the 2‐mono‐halogenated‐[4]helicene derivatives.

Near-Thermal Reactions of Au+(1S,3D) and AuX+ with CH3X (X = Br, I): A Combined Experimental and Computational Analysis.

Reactions of Au+(1S,3D) and AuX+ with CH3X (X = I and Br) were performed in the gas phase by utilizing a selected-ion drift cell reactor. These experiments were done at room temperature as well as reduced temperature (∼200 K) at a total pressure of 3.5 Torr in helium. Rate coefficients, product sequencing, and branching fractions were obtained for all reactions to evaluate reaction efficiencies and higher-order processes. Reactions of both Au+ states proceed with moderate efficiencies as compared to the average dipole orientation model with these neutral substrates. Results from this work revealed that, dependent on the reacting partner, Au+(1S) exhibits, among others, halogen abstraction, HX elimination, and association. By comparison, Au+(3D) participates primarily in charge transfer and halogen abstraction. Dependent on the halogen ligand, AuX+ ions induce several processes, including association, charge transfer, halogen loss, and halogen substitution. AuI+ reacting with CH3Br resulted in association exclusively, whereas the AuI+/CH3I and AuBr+/CH3Br systems exhibited halogen loss as the dominant process. By contrast, all possible bimolecular pathways occurred in the reaction of AuBr+ with CH3I. Observed products indicate that displacement of bromine by iodine on gold is favored in ionic products, consistent with the thermochemical preference for formation of the Au+-I bond. All AuX+ reactions proceed at maximum efficiency. Potential energy surfaces calculated at the B3LYP/def2-TZVPP level of theory for the AuX+ reactions are in good agreement with the available thermochemistry for these species and with previously calculated structures and energetics. Experimental and computational results are consistent with a mechanism for the AuX+/CH3Y systems where bimolecular products occur either via direct loss of the halogen originally on Au or via a common intermediate resulting from methyl migration in which the Au center is three-coordinate.

Effect of substitution on dissociation kinetics of C2H5X, (X = F, Cl, Br and I): A theoretical study

The decomposition of alkyl halide is controversial concerning C–X bond dissociation reaction and molecular (HX) elimination reaction. In order to identify the feasibility of dissociation reaction, electronic structure calculations were performed at DFT level of theory in conjunction with BMK, B3LYP, and MPW1PW91 functional, and 6-311G** basis set. In addition to this, we have carried out the detail theoretical kinetic analysis for the unimolecular dissociation of ethyl halide (C2H5X, X = F, Cl, Br and I), in the temperature range from 500 to 1700 K and pressure range from 10−9 to 103 kPa. Our kinetic data for decomposition reaction reveals that, the order of reaction is independent on temperature, but it alters with applied pressure. Also, an analysis of the branching ratio of C–X bond dissociation and HX elimination reaction, as a function of temperature, shows that the molecular (HX) is eliminated predominantly from fluoro-, chloro- and bromo-derivative of ethane. On the other hand, the reverse trend was observed upon pyrolysis of iodoethane. Above discrepancy in dissociation behaviour has been attributed towards larger C–X bond length and higher ring strain in cyclic transition state structures involving heavier atom.

Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination.

The five unimolecular HX and DX (X = F, Cl) elimination pathways of CD2ClCHFCl* were examined using a chemical activation technique; the molecules were generated with 92 kcal mol-1 of vibrational energy in a room-temperature bath gas by a combination of CD2Cl and CHFCl radicals. The total unimolecular rate constant was 9.7 × 107 s-1, and branching fractions for each channel were 0.52 (2,1-DCl), 0.29 (1,1-HCl), 0.10 (2,1-DF), 0.07 (1,1-HF), and 0.02 (1,2-HCl). Comparison of the individual experimental rate constants to calculated statistical rate constants gave threshold energies for each process as 63, 72, 66, 73, and 70 kcal mol-1, listed in the same order as the branching fractions. The 1,1-HCl and 1,1-HF reactions gave carbenes, CD2Cl(F)C: and CD2Cl(Cl)C:, respectively, as products, which have hydrogen-bonded complexes with HCl or HF in the exit channel of the potential energy surface. These carbenes have energy in excess of the threshold energy for D atom migration to give CDCl═CDF and CDCl═CDCl, and the subsequent cis-trans isomerization rates of the dihaloethenes can provide information about energy disposal by the 1,1-HX elimination reactions. Electronic structure calculations provide information for transition states of CD2ClCHFCl and hydrogen-bonded complexes of carbenes with HF and HCl. In addition, D atom migration in both free carbenes and in complexes formed by the carbene hydrogen bonding to HCl or HF is explored.

Synthesis, Structure, and Gas‐Phase Fragmentation of Trinuclear Mo3S4 Clusters Bearing Aminophosphine Ligands: A Combined Experimental and Theoretical Study

Aminophosphine [Mo3S4X3(edpp)3]+ cluster complexes, 1+ (X = Cl) and 2+ (X = Br) {edpp = (2‐aminoethyl)diphenylphosphine} have been quantitatively prepared by treating the [Mo3S7X6]2– anion with the edpp ligand in a one‐pot synthetic procedure. Alternatively, complexes 1+ and 2+ can be synthesized using polymeric {Mo3S7X4}n phases as metal precursors. Their crystal structures show the incomplete cubane‐type Mo3S4 cluster core as well as the formation of a unique isomer in which the nitrogen atoms of the amino group and the halide atoms are located above the trimetallic plane. Gas‐phase reactivity studies on complexes 1+ and 2+ show the subsequent elimination of neutral HX (X = Cl, Br) molecules under collision induced dissociation (CID) conditions. The fragmentation pattern of 1+ and 2+ in combination with gas‐phase DFT calculations are contrary to the general ideas involving the hemilabile character of the aminophosphine ligands and give support to the formation of unsaturated molybdenum/imine Mo=NH species. On the basis of DFT calculations, two competitive mechanisms involving a proton transfer from the amino group to a halide ligand attached to the geminal or to the vicinal molybdenum center are proposed. The first mechanism, which is a proton transfer within the nitrogen and halogen atoms bound to the same metal atom, is energetically favored from a theoretical point of view.

Alternative synthetic routes to hydrofluoroolefins

Abstract A series of hydrofluoroolefins with CF CH 2 , CH CHF and CH CF 2 groups were designed and prepared via various synthetic routes, including HX or BrF elimination, Wittig-type olefination or fluorination using SF 4 .

Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF2ClSH

ABSTRACTChemically activated CF3SH, CFCl2SH, and CF2ClSH were formed through combination of SH and CF3, CFCl2, and CF2Cl radicals, respectively. The SH radical was prepared by abstraction of an H‐atom from H2S by the halocarbon radical produced during photolysis of (CF3)2C=O, (CFCl2)2C=O, or (CF2Cl)2C=O. 1,2‐HX (X = F, Cl) elimination reactions were observed from CF3SH, CFCl2SH, and CF2ClSH with products detected by GC‐MS. The combination reaction of CF2Cl radicals with SH radicals prepared CF2ClSH molecules with approximately 318 kJ/mol of internal energy. The experimental rate constants for elimination of HCl and HF from CF2ClSH were 3 ± 3 × 1010 and 2 ± 1 × 109 s−1, respectively. Comparison to Rice–Ramsperger–Kassel–Marcus (RRKM) calculated rate constants assigned the threshold energies as 171 ± 12 and 205 ± 12 kJ/mol for the unimolecular elimination of HCl and HF, respectively. Theoretical calculations using the B3PW91, MP2, and M062X methods with the 6311+G(2d,p) and 6‐31G(d',p') basis sets established that for a specific method the threshold energies differ by only 4 kJ/mol between the two different basis sets. There was wide variation among the three methods, but the M062X approach appeared to give threshold energies closest to the experimental values. Chemically activated CF3SH and CFCl2SH were also prepared with about 318 kcal mol−1 of internal energy, and the HX (X = F, Cl) elimination reactions were observed. Only HCl loss was detected from CFCl2SH, but the rate was too fast to measure with our kinetic method; however, based on our detection limit the HF elimination channel is at least 50 times slower.

Rapid differentiation of ortho-, meta-, and para-isomers of halogenated phenylmethylidene hydrazinecarbodithioates by metal complexation and electrospray ionization mass spectrometry.

Development of mass spectrometry (MS)-based methods for isomeric differentiation remains a challenging analytical task, and has attracted the interest of many research groups. It is relevant to develop a general method to differentiate the isomeric halogenated phenylmethylidene hydrazinecarbodithioates (MX, X = F, Cl, Br). Diluted CH3 CN solutions containing NiCl2 and a title isomer (MX) were analyzed by electrospray ionization tandem mass spectrometry (ESI-MS(n)) in a quadrupole ion trap instrument equipped with an ESI source. Theoretical calculations were performed using the density functional theory (DFT) method at the uB3LYP/6-31+G(2d,p) level. In MS(3) experiments, the complex [MX + SCH3 + Ni](+) ion, resulting from dissociation of the ESI-generated complex [2MX - H + Ni](+) ion, undergoes ligand-exchange reactions with residual gas molecules, such as water, acetonitrile, and nitrogen in the ion trap, and the o-isomers [Mo-X + SCH3 + Ni](+) were found to undergo the characteristic HX elimination reactions to afford several unique ions. Each set of three isomers [MX + SCH3 + Ni](+) show significantly different reactivity, which has been corroborated by MS(4) experiments and theoretical calculations. A rapid method based on metal complexation and tandem mass spectrometric (MS(n)) analysis has been developed to differentiate three sets of positional isomers of halogenated phenylmethylidene hydrazinecarbodithioates (MX, X = F, Cl, Br).

DFT Studies of Mechanism and Origin of Stereoselectivity of Palladium-Catalyzed Cyclotrimerization Reactions Affording syn-Tris(norborneno)benzenes

Pd-catalyzed cyclotrimerization reactions of enantiopure halonorbornene derivatives furnished C3 or C3v symmetric syn-tris(norborneno)benzenes with high syn selectivity. To elucidate the reaction mechanism as well as the stereoselectivity of the present Pd-catalyzed cyclotrimerization, DFT calculations were carried out. The promising reaction pathway consists of (1) sequential olefin insertion followed by an HX elimination reaction of halonorbornene with the norbornenylpalladium intermediate, (2) electrocyclization of the trienylpalladium intermediate with a lower activation barrier than a triene compound, and (3) the β-elimination of HPdX of the cyclohexadienylpalladium intermediate. In addition, the stereoselectivity would be controlled by the regioselectivity in the olefin insertion process (homo and hetero positions) and the symmetry breaking in the palladacyclic intermediate.

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.

Unexpected Pyrolytic Behaviour of Substituted Benzo[c]thiopyran and Thieno[2,3-c]thiopyran S,S-dioxides

Flash vacuum pyrolysis (FVP) of benzo[c]thiopyran S,S-dioxide (1) results in formation of indene and 2-vinylbenzaldehyde as previously described. A range of eight analogues with various substitution patterns are found to behave differently. In general, there is no extrusion of SO2 to give products analogous to indene, but unsaturated carbonyl products analogous to 2-vinylbenzaldehyde are formed in most cases by way of ring expansion to a 7-membered ring sultine, extrusion of SO, and intramolecular hydrogen atom transfer. Other processes observed include formation of anthracene via an isomeric 7-membered sultine with loss of SO, CO and methane or butane, and formation of 4-ethylidene-4,5-dihydrocyclobuta[b]thiophenes by way of SO loss, a radical rearrangement, and extrusion of acetone. The analogues with a halogen substituent at position 8 on the benzene ring require a higher temperature to react and give naphthalene resulting from net elimination of HX and SO2. The X-ray crystal structure of 1 is also reported.

Organotin(IV) compounds containing N,C,O-chelating ligand

A set of organotin(IV) compounds containing unsymmetrical N,C,O-chelating ligands {C6H3(CH2OMe)-2-(CH2NMe2)-6}− (hereafter denoted as L1) and {C6H3(CH2OtBu)-2-(CH2NMe2)-6}− (hereafter denoted as L2) was prepared. While triorganotin(IV) chlorides L1–2Ph2SnCl (1 for L1, 2 for L2) are stable, diorganotin(IV) chloride L1PhSnCl2 (3) and monoorganotin(IV) bromide L1SnBr3 (5) reacts with water under elimination of HX (X=Cl, Br). The reaction of another molecule 3 or 5 with HX (X=Cl, Br) provided “stannatrane type” compounds [{C6H3(CH2OMe)-2-(CH2N+HMe2)-6}(Ph)SnCl3] (4) and [{C6H3(CH2OMe)-2-(CH2N+HMe2)-6}SnBr4] (6), respectively. Compounds 1–6 were characterized by multinuclear NMR spectroscopy and molecular structures of 1, 3, 4 and 6 were determined by X-ray diffraction analysis.

Near-Thermal Reactions of Au+(1S,3D) with CH3X (X = F,Cl)

Reactions of Au(+)((1)S) and Au(+)((3)D) with CH(3)F and CH(3)Cl have been carried out in a drift cell in He at a pressure of 3.5 Torr at both room temperature and reduced temperatures in order to explore the influence of the electronic state of the metal on reaction outcomes. State-specific product channels and overall two-body rate constants were identified using electronic state chromatography. These results indicate that Au(+)((1)S) reacts to yield an association product in addition to AuCH(2)(+) in parallel steps with both neutrals. Product distributions for association vs HX elimination were determined to be 79% association/21% HX elimination for X = F and 50% association/50% HX elimination when X = Cl. Reaction of Au(+)((3)D) with CH(3)F also results in HF elimination, which in this case is thought to produce (3)AuCH(2)(+). With CH(3)Cl, Au(+)((3)D) reacts to form AuCH(3)(+) and CH(3)Cl(+) in parallel steps. An additional product channel initiated by Au(+)((3)D) is also observed with both methyl halides, which yields CH(2)X(+) as a higher-order product. Kinetic measurements indicate that the reaction efficiency for both Au(+) states is significantly greater with CH(3)Cl than with CH(3)F. The observed two-body rate constant for depletion of Au(+)((1)S) by CH(3)F represents less than 5% of the limiting rate constant predicted by the average dipole orientation model (ADO) at room temperature and 226 K, whereas CH(3)Cl reacts with Au(+)((1)S) at the ADO limit at both room temperature and 218 K. Rate constants for depletion of Au(+)((3)D) by CH(3)F and CH(3)Cl were measured at 226 and 218 K respectively, and indicate that Au(+)((3)D) is consumed at approximately 2% of the ADO limit by CH(3)F and 69% of the ADO limit by CH(3)Cl. Product formation and overall efficiency for all four reactions are consistent with previous experimental results and available theoretical models.

ChemInform Abstract: Hydroxycarbonyl Complexes as Key Intermediates in the Base‐Assisted Reduction of Ruthenium Carbonyls

AbstractReview: 139 refs.

Mechanistic Study on the Gas-Phase Generation of “Rollover”-Cyclometalated [M(bipy − H)]+(M = Ni, Pd, Pt)

“Rollover”-cyclometalated [Pt(bipy − H)]+ (bipy = 2,2′-bipyridine) can be easily generated in the gas phase via HX elimination brought about by collision-induced dissociation (CID) of the cationic complexes [Pt(X)(bipy)]+ with X = CH3, Cl; the latter as well as other [M(X)(bipy)]+ complexes (M = Ni, Pd; X = CH3, F, Cl, Br, I, OAc) are accessible by electrospray ionization mass spectrometry. For the nickel and palladium methyl complexes [M(CH3)(bipy)]+, upon CID, no cyclometalation occurs; rather, homolytic cleavage of the M−CH3 bond takes place. The related chloro complexes [M(Cl)(bipy)]+ (M = Ni, Pd) undergo competitive eliminations of HCl and Cl upon CID, and the branching ratios depend strongly on the collision energy. On the basis of DFT calculations, this metal- and ligand-controlled behavior is a consequence of the rather different energetic requirements for the direct loss of X versus elimination of HX (X = CH3, Cl). Deuterium-labeling experiments reveal that formation of CH4 and HCl is only for th...

An Efficient Synthesis of Substituted meta‐Halophenols and Their Methyl Ethers: Insight into the Reaction Mechanism

AbstractAn expeditious synthetic methodology leading to substituted meta‐halophenols and their corresponding methyl ether derivatives through acid‐mediated fragmentation of suitably substituted dihalonorbornyl ketones has been devised. The reaction sequence consists of TBTH‐mediated (TBTH is tri‐n‐butyltin hydride) selective bridgehead halogen reduction of easily accessible Diels–Alder adducts derived from 1,2,3,4‐tetrahalo‐5,5‐dimethoxycyclopentadiene and β‐substituted vinyl acetates, with subsequent conversion into the requisite bicyclic ketones by a two‐step hydrolysis/oxidation approach. An extensive mechanistic investigation based on isotope labeling and cross experiments has been carried out and plausible mechanistic pathways based on these results have been proposed. The absence of halogen atoms at the bridgehead positions steers the reaction through a novel pathway involving the incorporation of proton (or deuterium) followed by elimination of HX (or DX), so the described methodology also provides a reliable route to ortho‐para dideuteratedphenolic derivatives.

3-Halopropenals as β-Acylvinyl Anion Equivalents

The authors report an NHC-catalyzed reaction of 3-halopropenals with β,γ-unsaturated α-keto esters and azodicarboxylates to give butenolides and pyrazolones, respectively, in good yields. In the course of the reaction, the NHC derived from 1 and 3-halopropenals form Breslow intermediates, reacting with the corresponding electrophiles. Subsequent ring closure releases the catalyst and after elimination of HX the products are formed.

Hydroxycarbonyl complexes as key intermediates in the base-assisted reduction of ruthenium carbonyls

Since the recognition of metal-hydroxycarbonyl (or, metallacarboxylic acid) as a key intermediate in water gas shift (WGS) catalysis, numerous metal complexes incorporating hydroxycarbonyl ligands have been synthesized. In this Perspective, a brief account on the chemistry of metallacarboxylic acid and metallacarboxylate is presented. Special emphasis is placed on the hydroxycarbonyl-bridged dinuclear compounds. The relevance of hydroxycarbonyl complexes in the base-assisted reduction of ruthenium carbonyls is investigated. In particular, Lavigne's synthesis of Roper's catalyst Ru(CO)(2)(PPh(3))(3), triruthenium dodecacarbonyl Ru(3)(CO)(12), and Bera's mechanistic investigation on the preparation of the metal-metal singly bonded diruthenim(I) complex [Ru(2)(CO)(4)(CH(3)CN)(6)](2+) are described in detail. These reactions involve the decarboxylation of a "[Ru(m)(COOH)(n)]" species, followed by the reductive elimination of HX (X = H, Cl or OH), steps akin to WGS catalysis. The prospect of hydroxycarbonyl chemistry in the development of bimetallic catalysis is examined.

Stereoselective Synthesis of Cyclopropanes Based on a 1,2‐Chirality Transfer

A stereoselective route to enantiomerically enriched bicyclic cyclopropane derivatives 13 is described which is based on a conceptually novel 1,2-chirality transfer approach. The hyperconjugative interaction of an electronically excited carbonyl group with the sigma* orbital of an adjacent C--X bond in the transition state of a hydrogen abstraction causes the preference of a certain conformation and consequently the differentiation between two diastereotopic methylene groups. The 1,2-chirality transfer is completed by a subsequent HX elimination which destroys the only stereogenic center in the reactants 12. Furthermore, it was found that contrary enthalpic and entropic influences result in the existence of an inversion temperature T 0. Upon crossing T 0 the stereoselectivity is reversed. Considering this temperature dependency, chirality transfer efficiencies of up to 83 % could be achieved. The absolute configuration of most products could be unambiguously determined by VCD spectroscopy combined with DFT calculations.