Addition-elimination Mechanism Research Articles

Phosphonyl transfer by the elimination-addition mechanism: accelerated formation of an alkylidineoxophosphorane (phosphene) intermediate when a P–O single bond is replaced by P–S

The phosphonochloridate R(2)CHP(O)(YEt)Cl (R(2)CH = fluoren-9-yl, Y = O or S) reacts with Pr(i)(2)NH largely or exclusively by an elimination-addition mechanism; the three-coordinate phosphene intermediate R(2)C=P(O)YEt is formed ca. 10(3) times more easily when Y = S than when Y = O.

Synthesis of Hindered Silyl Ethers via Fragmentation Reactions

Reaction of alcoholic sodium alkoxides with (Me3Si)3CSi(C6H4-OMe-p)MeX have been performed. Main products are of the type: (Me3Si)2CHSi(An)Me(OR) and Me3SiCH2Si(An)Me(OR);An = (C6H4-OMe-p) and OR = OEt, OPr, O iso-Pr, OBu, O iso-Bu,O iso-Amyl, OBenzyl, OAllyl, Ofurfuryl. It is suggested that the reaction proceeds through an elimination-addition mechanism. We thank Dr. M. Mahkam for helpful comments.

Kinetics and Mechanism of Reaction between N-Polyfluorophenylcarbonimidoyl Dichlorides and Tertiary Amines in Acetonitrile

The reaction of N-polyfluorophenylcarbonimidoyl dichlorides with tertiary amines in acetonitrile afforded chloroamidines R2NC(Cl) = NArF and alkyl chloride. The precursor of the products is the corresponding quaternary ammonium salt [R3N+C(Cl) = NArF]-Cl-. The rate of the salt formation is described by a second order equation; however with some amines a saturation effect was observed for the reaction rate with the growing amine concentration. This fact and also the influence of the amine and the substrate structure on the reaction rate suggests that reaction proceeds by addition-elimination mechanism with formation of a tetrahedral intermediate. The latter in the rate-limiting stage undergoes a stereomutation into an intermediate of a configuration favorable for conversion into a quaternary salt.

Novel insight into the mechanism of the reaction of Fe + with ethane

The reaction of Fe + ( 6D) with ethane in the gas-phase was re-examined at B3LYP/6-311++G(3df,3pd)//B3LYP/6-311+G(d,p) level of theory to explore new characteristic of the C–C and C–H bond activations in alkanes. Two different mechanisms from those described in the early works are shown: one is the concerted elimination of CH 4 against the consecutive addition–elimination mechanism, and the other is the 1,1-H 2 elimination vs. general 1,2-H 2 elimination. Although these alternative pathways are unimportant experimentally as they involve high-energy transition states, it may improve our understanding for the activation mechanism of the C–C and C–H bond in alkane.

Modeling the reaction mechanisms of the amide hydrolysis in an N-(o-carboxybenzoyl)-L-amino acid.

Reaction mechanisms of the amide hydrolysis from the protonated, neutral, and deprotonated forms of N-(o-carboxybenzoyl)-l-amino acid have been investigated by use of the B3LYP density functional method. Our calculations reveal that in the amide hydrolysis the reaction barrier is significantly lower in solution than that in the gas phase, in contrast with the mechanism for imide formation in which the solvent has little influence on the reaction barrier. In the model reactions, the water molecules function both as a catalyst and as a reactant. The reaction mechanism starting from the neutral form of N-(o-carboxybenzoyl)-l-amino acid, which corresponds to pH 0-3, is concluded to be the most favored, and a concerted mechanism is more favorable than a stepwise mechanism. This conclusion is in agreement with experimental observations that the optimal pH range for amide hydrolysis of N-(o-carboxybenzoyl)-l-leucine is pH 0-3 where N-(o-carboxybenzoyl)-l-leucine is predominantly in its neutral form. We suggest that besides the acid-catalyzed mechanism the addition-elimination mechanism is likely to be an alternative choice for cleaving an amide bond. For the reaction mechanism initiated by protonation at the amidic oxygen (hydrogen ion concentration H(0) < -1), the reaction of the model compound with two water molecules lowers the transition barrier significantly compared with that involving a single water molecule.

Theoretical study of nucleophilic substitution at the disulfide bridge of cyclo-l-cystine.

The reaction of cyclo-l-cystine with thiolate is examined at the B3LYP/6-31+G level. The two isomers of cyclo-l-cystine differ in their dihedral angle about the disulfide bond; the M isomer (with dihedral angle of -90.1 degrees) is found to be slightly lower in energy. The nucleophilic substitution reaction at sulfur follows the addition-elimination mechanism, exemplified by the hypercoordinate sulfur intermediate on the reaction surface. The reaction is exergonic (DeltaG = -6.16 kcal mol(-1)), and both the entrance and exit transition state lie below the reactant energies.

Action of HCl on 3-hydroxypyrazolo(iso)quinolines to give 1-chloropyrazoles: evidence for an addition-elimination mechanism by ab initio calculations in gas phase and water

Addition–elimination reactions involving a nucleophile and a remote leaving group [SHN(AE)tele] are well-known under basic conditions, especially amongst electron-poor six-membered heterocycles, but are less commonly encountered for five-membered heterocycles and are rare under acidic conditions. Concentrated HCl converts 1-hydroxy-1H-pyrazolo[3,4-c] isoquinoline and 1-hydroxy-1H-pyrazolo[3,4-c]quinoline into 3-chloro-1H-pyrazolo[3,4-c]isoquinoline and 3-chloro-1H-pyrazolo[3,4-c]quinoline, respectively. However, apparently neither the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)-quinolines nor the parent 1-hydroxypyrazole undergo this reaction. Additionally, all these systems are refractory under basic conditions. We present a plausible mechanism for the reaction, involving the 3-addition of Cl- to the diprotonated heterocycle, followed by the elimination of water. Calculations of the initial transition states and intermediates, using optimisation at B3LYP/6-311+G(d,p), including thermochemistry [HF/6-31+G(d)], and single-point Poisson–Boltzmann self-consistent reaction field determination of the free energy of solvation (Jaguar Poisson–Boltzmann self-consistent reaction field), support this mechanism and reproduce the observed order of reactivity, the addition step being 2–4 kcal less favourable for the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)quinolines and provide a rationalisation for the role of strong acid.

A comprehensive theoretical study on the reactions of Sc+ with CnH2n+2 (n=1-3): structure, mechanism, and potential-energy surface.

The reactions of Sc(+)((3)D) with methane, ethane, and propane in the gas phase were studied theoretically by density functional theory. The potential energy surfaces corresponding to [Sc, C(n), H(2n+2)](+) (n=1-3) were examined in detail at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(d,p) level of theory. The performance of this theoretical method was calibrated with respect to the available thermochemical data. Calculations indicated that the reactions of Sc(+) with alkanes are multichannel processes which involve two general mechanisms: an addition-elimination mechanism, which is in good agreement with the general mechanism proposed from earlier experiments, and a concerted mechanism, which is presented for the first time in this work. The addition-elimination reactions are favorable at low energy, and the concerted reactions could be alternative pathways at high energy. In most cases, the energetic bottleneck in the addition-elimination mechanism is the initial C--C or C--H activation. The loss of CH(4) and/or C(2)H(6) from Sc(+)+C(n)H(2n+2) (n=2, 3) can proceed along both the initial C--C activation branch and the Cbond;H activation branch. The loss of H(2) from Sc(+)+C(n)H(2n+2) (n=2, 3) can proceed not only by 1,2-H(2) and/or 1,3-H(2) elimination, but also by 1,1-H(2) elimination. The reactivity of Sc(+) with alkanes is compared with those reported earlier for the reactions of the late first-row transition-metal ions with alkanes.

Temperature and pressure dependent product distribution of the addition of CN radicals to C2H4

The temperature and pressure dependent product distribution of the C2H4+CN reaction is investigated using RRKM-master equation analyses over extended temperature and pressure ranges, including an extended error analysis of the predicted yields. At low pressures, formation of C2H3CN+H is the dominant product, while at higher pressures above 105 Pa collisional stabilization of C2H4CN radicals becomes the major channel. Some other products are formed in the fall-off region and in high-temperature conditions, but their summed contribution never exceeds 20%. The formation of HCN+C2H3 from the title reaction was investigated in detail, both by direct H-abstraction and an addition–elimination mechanism. No significant HCN+C2H3 formation is predicted at temperatures below 1000 K, in contrast to an earlier experimental investigation; this discrepancy is discussed.

Synthesis and electronic spectra of novel merocyanine dyes bearing a maleimide ring incorporated into the methine chains

The present article describes the synthesis of new polymethine dyes with a heterocyclic ring incorporated into the methine chain. 1-Alkyl-2 or 4-methylpyridinium salts and related compounds 1a–e reacted at the exocyclic double bond with cyano- or methoxycarbonyl heterocycles bearing a methylthio group 2a–d to give new polymethine dyes 3a–n in good yield. This reaction occurred by an addition-elimination mechanism, involving nucleophilic attack of cyclic enamines at carbon deficient atoms on the heterocycles followed by elimination of methylmercaptan. The polymethine dyes obtained are red, violet, and blue in color with the absorption peaks at 524–614 nm. Semi-empirical as well as ab-initio quantum chemical calculations were used to theoretically characterize their π−π* absorption maxima in the visible region which plays a decisive role in their color appearances.

Photoactivated isomerization of linear olefins.

The photocatalyzed (350 nm) isomerization of linear olefins catalyzed by platinum(II) bis(acetylacetonato) [Pt(acac)(2)] in the presence of silanes was studied. Catalytic activity depends on the silane. Triphenylsilane is the most reactive silane studied, with more than 98% isomerization of the alkene occurring after 20-min irradiation. There is no concomitant hydrosilylation. The mechanism has been investigated and a metal hydride addition-elimination mechanism proposed.

Phase-transfer catalysed (PTC) reactions of 1,1-dichloro-2-cyanocyclopropane with nucleophiles. Identification of intermediates

An addition–elimination mechanism for PTC reactions of phenols with 1,1-dichloro-2-cyanocyclopropane (1) is supported by trapping of intermediate cyclopropenes 3 and 5. PTC reactions of 1 with a series of phenols 2a–d, thiophenol (2e), alcohols 2f,g and dithioles 11a,b led to formation of disubstituted cyanocyclopropanes 6a–d, 6e, 6f,g or spiroproducts 12a,b, respectively. Under acidic conditions or thermally, dialkoxycyclopropanes 6f,g or spirane 12a were transformed into alkyl 3-cyanopropionates 10f,g and ketene dithioacetal 13a.

Mechanisms of photorelease of carboxylic acids from 1-acyl-7-nitroindolines in solutions of varying water content

1-Acyl-7-nitroindolines have been found to be useful photoactivated protecting groups for rapid release of carboxylates in aqueous solution. Mechanistic details of carboxylic acid photorelease from model compounds in solutions of varying H2O-CH3CN composition are now reported, using data from product studies, deuterium isotope effects, kinetic studies (UV-Vis), and nanosecond laser flash photolysis. Our data support a mechanism (via T1) that involves reaction of a critical photogenerated intermediate 21 (an acetic nitronic anhydride), obtained via photochemical transfer of the acetyl group from the amide nitrogen to one of the oxygen atoms of the nitro group. This mode of transfer has been reported in the photochemistry of a variety of N-acetyl-o-nitrodiphenyl-amines. Two competing pathways of reaction from 21 that differ in how the acyl group is cleaved are proposed to account for the products observed. In solutions of higher water content, the predominant reaction pathway of 21 is via an A(AL)1-like cleavage that results in formal intramolecular redox reaction of the aromatic ring system, to give the released carboxylic acid and 7-nitrosoindole 3 (after tautomerisation of an initially formed nitroso-3H-indole 9). In solutions of low water content, the major pathway for hydrolysis of 21 is via the standard addition-elimination mechanism (A(AC)2) with water as the nucleophile, that releases the carboxylic acid and nitroindoline 4. Laser flash photolysis studies in wholly aqueous medium gave a transient (within the 20 ns laser pulse) at lambda(max) 450 nm, assignable to 21. Single exponential decay of this species in water (k(obs) = 5 x 10(6) S(-1)) is assigned to the release of the carboxylic acid and formation of the nitroso-3H-indole 9, which is supported by time-resolved measurements of acid release using bromothymol blue. Therefore, 1-acyl-7-nitroindolines photorelease their protected functionalities at rates in the submicrosecond time scale.

Density functional theory gas- and solution-phase study of nucleophilic substitution at di- and trisulfides

Density functional calculations indicate that nucleophilic substitution in the thiolate–disulfide and thiolate–trisulfide exchange reactions proceeds by an addition–elimination pathway. Solution calculations were performed using B3LYP/6-31+G* and the polarized continuum method. These solution-phase calculations indicate that for the reactions where the sulfur under attack bears a hydrogen atom, the substitution proceeds via an addition–elimination mechanism; however, when a methyl group is attached to the sulfur under attack, the SN2 mechanism is predicted.

Degradation of pentachlorophenol by ozonation and biodegradability of intermediates

The degradation pathway of pentachlorophenol (PCP) under ozonation and the biodegradability of the resulting intermediates were investigated. The objectives were to: (1) provide mechanistic details in the ozone-mediated degradation, (2) evaluate the biodegradability of resultant intermediates at various progressive stages of ozonation, and (3) thereby, assess the feasibility for a coupled chemical–biological treatment scheme for PCP. Tests of BOD 5, COD, and E. coli toxicity along with qualitative and quantitative GC analyses were performed for aliquots withdrawn before and after various stages of ozonation and biological incubation. Ozonated PCP decomposed under the direct nucleophilic attack of ozone through an addition–elimination mechanism, resulting in tetrachloro- p-benzoquinone and tetrachloro- p-hydroquinone intermediates that were further degraded by O 3 and OH to other open-ring products including ketones and acids that eventually led to simple oxalic acid and quantitative release of chloride ion. As ozonation progressed toward but prior to complete mineralization, reaction intermediates became increasingly more biodegradable prior to complete degradation, which suggested the potential of using ozonation in conjunction with biological treatment for the effective control of chlorinated aromatics.

Reactivity of N-pyridylcarbamates in basic media

New secondary aryl N-pyridylcarbamates were prepared by reaction of the aminopyridine anion with aryl chloroformates and their hydrolysis was studied over the pH range from 12 to 13.7. The pH–rate profile points to an E1cB mechanism, involving pre-equilibrium deprotonation of the nitrogen atom to form an anion that undergoes rate-limiting decomposition into pyridyl isocyanate and a phenoxide ion. Further reaction of the highly reactive isocyanate with water affords N-pyridylcarbamic acid, which spontaneously decomposes to aminopyridine and carbon dioxide. The absence of significant base catalysis and the isolation of a new product resulting from trapping of the intermediate with the base piperidine are also consistent with an elimination–addition mechanism. Finally the observed substituent effect (σ−) gives ρ 2.45 which is in accordance with a rate-determining departure of the phenoxide group from the anion intermediate formed in a pre-equilibrium step. Blocking the E1cb mechanism of the secondary carbamates by introduction of N,N-disubstitution in the substrate led to a rate-limiting decrease of ca. 106.

Theoretical study on mechanisms of the high-temperature reactions C2H3 + H2O and C2H4 + OHElectronic supplementary information (ESI) available: Figures of optimized geometries of the fragments of C2H5O radicals, C2H5O isomers and C2H5O transition states and table of harmonic vibration frequencies of C2H3, H2O, TSR/P1, C2H4 and OH. See http://www.rsc.org/suppdata/cp/b1/b109758j/

The potential energy surface of the radical-molecule reaction C2H3 + H2O in the gas phase is explored at the 6-31G(d,p) and 6-311G(d,p) B3LYP and single-point QCISD(T)/6-311G(2df,p) levels. The most favorable channel is the direct H-abstraction from H2O to C2H3 leading to product P1 C2H4 + OH, whereas the other channels leading to the products P2 CH3 + CH2O, P3 CH3CHO + H, P4cis-CH2CHOH + H and P4′ trans-CH2CHOH + H are kinetically much less competitive. For the direct H-abstraction channel, high-level energetic calculations at the QCISD(T)/6-311G(2df,p), QCISD(T)/6-311+G(2df,2p) and G2 levels using the B3LYP/6-31G(d,p) and QCISD/6-31G(d,p) optimized geometries are further performed to estimate the thermal rate constants over a wide temperature range 200–5000 K for comparison with future laboratory measurements. The calculated barrier heights at the QCISD(T)/6-311+G(2df,2p) and G2 levels based on the QCISD/6-31G(d,p) geometries with zero-point vibrational energy (ZPVE) correction are 12.6 and 13.0 kcal mol−1, respectively. The results indicate that the C2H3 + H2O reaction might play an important role at high temperatures (T > 1800 K) in the presence of gaseous water and should be incorporated in the C2H3-modeling of hydrocarbon-fuel combustion processes. Discussions are also made in comparison with the analogous reactions C2H3 + H2 and C2H + H2O. While the addition-elimination mechanism of another important radical-molecule reaction C2H4 + OH has been the subject of extensive theoretical and experimental studies, its H-abstraction process leading to C2H3 + H2O has received little attention. For the C2H4 + OH → C2H3 + H2O channel, our calculations predict ZPVE-corrected barriers, 5.6 and 5.4 kcal mol−1, respectively, at the QCISD(T)/6-311+G(2df,2p)//QCISD/6-31G(d,p) and G2//QCISD/6-31G(d,p) levels, and reveal its importance at high temperatures (T > 560 K). In the range 720–1173 K, the calculated high-level rate constants are quantitatively in good agreement with the measured values. However, our calculated activation energy, 9.5 and 9.3 kcal mol−1 at the QCISD(T)/6-311+G(2df,2p)//QCISD/6-31G(d,p) and G2//QCISD/6-31G(d,p) levels with ZPVE correction, respectively, suggests that the experimentally determined value 4–5 kcal mol−1 may be underestimated and future rate constant measurements over a wide temperature range including T > 1200 K may be desirable.

Methylenethioxophosphorane (thiophosphene) intermediates in the reactions of diphenylmethylphosphonamidothioic chlorides with amines. Differences between the elimination–addition mechanisms of nucleophilic substitution for PS and PO substrates

In reactions with R2NH (R = Me or Et) in CHCl3 the PS substrate Ph2CHP(S)(NMe2)Cl differs markedly from its PO counterpart: the rate of substitution is increased much more by a 4-nitro substituent but is also more sensitive to the bulk of the amine, there is greater discrimination between competing nucleophiles, and with R2ND there is relatively little H–D exchange at the α carbon atom prior to substitution. Yet the PS and PO compounds both seem to react by elimination–addition mechanisms. It is suggested that whereas in the PO case the conjugate base is formed rapidly and elimination of chloride to give the methyleneoxophosphorane (phosphene) intermediate is rate limiting, the conjugate base of the PS substrate eliminates chloride rapidly giving a relatively stable methylenethioxophosphorane (thiophosphene) intermediate.

Phenylureas. Part 2.1 Mechanism of the acid hydrolysis of phenylureas

The mechanism of the hydrolytic decomposition of phenylureas in acid media is investigated. It includes, in part, knowledge already present in the literature. Over the investigated pH range the occurrence of a rate maximum in the pH curves due to the strongly reduced water activity at higher acid strengths is observed. An addition–elimination mechanism with rate-determining attack of water at the N-protonated substrate is proposed. The reversion of the substituent influence on the reaction rate with increasing acidity of the reaction medium points to a change of the hydrolytic decomposition mechanism in strongly acidic media.

Phenylureas. Part 1. Mechanism of the basic hydrolysis of phenylureas

The mechanism of the hydrolytic decomposition of phenylureas in basic media in the pH range 12 to 14 is investigated. In this pH range a levelling of the rate–pH curve is observed as well as a change of the substituent influence on the hydrolysis rate. These experimental findings suggest the formation of an unreactive side product of the phenylurea in a parasitic side equilibrium at sufficiently high pH. The urea dissociates at the aryl–NH group to give its conjugate base. For the hydrolytic decomposition of phenylureas an addition–elimination mechanism is proposed as has been established for the alkaline hydrolysis of carboxylic acid esters and amides.