Four-membered Cyclic Transition State Research Articles

Stereoselectivity control in Rh-catalyzed β-OH elimination for chiral allene formation

Stereoselectivity control and understanding in the metal-catalyzed reactions are fundamental issues in catalysis. Here we report sterically controlled rhodium-catalyzed SN2’-type substitution reactions of optically active tertiary propargylic alcohols with arylmetallic species affording the non-readily available enantioenriched tetrasubstituted allenes via either exclusive syn- or anti-β-OH elimination, respectively, under two sets of different reaction parameters. Detailed mechanistic experiments and density functional theory (DFT) studies reveal that the exclusive anti-Rh(I)-OH elimination is dictated by the simultaneous aid of in situ generated boric acid and ambient water, which act as the shuttle in the hydroxy relay to facilitate the Rh(I)-OH elimination process via a unique ten-membered cyclic transition state (anti-TS2_u). By contrast, the syn-Rh(III)-OH elimination in C–H bond activation-based allenylation reaction is controlled by a four-membered cyclic transition state (syn-TS3) due to the steric surroundings around the Rh(III) center preventing the approach of the other assisting molecules. Under the guidance of these mechanistic understandings, a stereodivergent protocol to construct the enantiomer of optically active tetrasubstituted allenes from the same starting materials is successfully developed.

Theoretical study on thermal degradation mechanism of polybutylene terephthalate dimer

Thermal degradation mechanism of polybutylene terephthalate (PBT) dimer was studied by density functional theory (DFT) method M06-2X/6-311G(d). Eight possible reaction paths were designed for the thermal decomposition of PBT dimer, and the thermodynamic and kinetic parameters of elementary reaction steps in each reaction path were calculated. Calculation results show that, in the initial pyrolysis process of PBT, the energy barrier of concerted reaction occurring on the main chain is significantly lower than that of the radical reaction, so terephthalic acid, monobutenyl terephthalate, dibutenyl terephthalate and diterephthalate-1,4-butadiester formed by concerted reaction are main products in PBT initial pyrolysis. The energy barrier of the main chain fracture through the six-membered cyclic transition state is lower than that through the four-membered cyclic transition state, and the main chain fracture of PBT is mainly through the concerted reaction with six-membered cyclic transition state. In addition, the secondary degradation reaction of the main products of PBT pyrolysis was also discussed. It is found that the main products such as 1,3-butadiene, tetrahydrofuran, benzene, CO2 and benzoic acid are mainly generated through concerted reaction in the processes of secondary degradation.

Catalytic effect of hydroxyl-containing compounds at the propagation step of the chain reaction of thiophenol with quinone imines

Hydroxyl-containing compounds (water, methanol, and phenol) act as efficient catalysts of the chain reactions of thiophenol with quinone imines at their propagation steps as it was found previously for thiols. The results of quantum chemical analysis of isomerization of a radical adduct into phenoxyl radical or corresponding aromatic aminyl radical are considered. The adduct was formed at the rate limiting step of the chain propagation by thiyl radical addition to quinone imine. Enthalpies and activation energies for mono- and bimolecular isomerization of the adduct with participation of hydroxyl-containing compounds were computed and matched. For comparison, analogous reactions with participation of thiols (hydrogen sulfide, metahnthiol and thiophenol) were also studied. In both the cases, the catalytic effect is caused by transforming the four-membered cyclic transition state of the monomolecular isomerization reaction into the energetically favorable six-membered transition state by the O—H or S—H groups of the indicated compounds. This considerably reduces the activation energy.

Experimental and DFT studies for the kinetics and mechanism of the pyrolysis of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes in the gas-phase

The gas-phase pyrolysis kinetics of tetrahydropyranyl phenoxy ethers: 2-(4-cyanophenoxy)tetrahydro-2H-pyran, and 2-(4-bromophenoxy)tetrahydro-2H-pyran were studied in a static system, with allyl bromide as surface-effect deactivator, and in the presence of toluene to inhibit chain reactions. The temperature and pressure were 350–440 °C and 25–89 Torr, respectively. The pyrolysis is homogeneous, unimolecular, and consistent with the first-order rate law yielding 3,4-dihydro-2H-pyran (DHP) and the corresponding 4-substituted phenol. The Arrhenius equations were found as follows:2-(4-cyanophenoxy)tetrahydro-2H-pyranlog k1 (s−1) = (14.76 ± 0.12) − (232.1 ± 1.3) kJ mol−1 (2.303 RT)−12-(4-bromophenoxy)tetrahydro-2H-pyranlog k1 (s−1) = (14.08 ± 0.15) − (209.1 ± 1.8) kJ mol−1 (2.303 RT)−1DFT theoretical calculations were performed to 2-(4-substituted-phenoxy) tetrahydro-2H-pyranes with groups H, OCH3, C(CH3)3, CN and Br. The calculated values for thermodynamic parameters using the WB97XD/6–311 + +G(d,p) level have a good agreement with the experimental values except for the cyano compound. Experimental and theoretical data suggest a mechanism with four-membered cyclic transition state (TS). The results indicate a small increment of rate constants values with raising electron donating ability groups in the phenoxy ring, and a slight decrease effect with electron withdrawing substituents. The effect of resonance structures giving by the phenyl ring seems to be the main factor in the TS stabilization. The assistance of the oxygen atom belonging to the phenoxy group in the pyranyl hydrogen abstraction exerts influence on the rate constants, but the polarization of Cδ+⋯δ−OPh bond is proposed as the rate determining process.

The keto–enol equilibrium and thermal conversion kinetics of 2- and 4-hydroxyacetophenone in the gas phase: a DFT study

ABSTRACTKeto–enol tautomeric equilibrium and the mechanism of thermal conversion of 2- and 4-hydroxyacetophenone in gas phase have been studied by means of electronic structure calculations using density functional theory (DFT). A topological analysis of electron density evidence that the structure of keto and enol forms of 2-hydroxyacetophenone are stabilised by a relatively strong intramolecular hydrogen bond. 2- and 4-hydroxyacetophenone undergo deacetylation reactions yielding phenol and ketene. Two possible mechanisms are considered for these eliminations: the process takes place from the keto form (mechanism A), or occurs from the enolic form of the substrate (mechanism B). Quantum chemical calculations support the mechanism B, being found a good agreement with the experimental activation parameters. These results suggest that the rate-limiting step is the reaction of the enol through a concerted, non-synchronous, semi-polar, four-membered cyclic transition state (TS). The most advanced reaction coordinate in the TS is the rupture of O1···H1 bond, with an evolution in the order of 79.7%–80.9%. Theoretical results also suggest a three-step mechanism for the phenyl acetate formation from 2-hydroxyacetophenone.

Kinetic Determination of the Gas-Phase Decarbonylation of Butyraldehyde in the Presence of HCl Catalyst

The gas-phase kinetics and mechanism of the homogeneous elimination of CO from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is homogeneous and follows the second-order kinetics with the following rate expression: log k1 (s−1 L mol−1) = (13.27 ± 0.36) – (173.2 ± 4.4) kJ mol−1(2.303RT)−1. Experimental data suggested a concerted four-membered cyclic transition state type of mechanism. The first and rate-determining step occurs through a four-membered cyclic transition state to produce propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and HCl gases.

Homogeneous, unimolecular gas-phase pyrolysis kinetics of 4- and 2-hydroxyacetophenone

The pyrolyses kinetics of 4- and 2-hydroxyacetophenones in the gas phase were determined in a static system, where the reaction vessel was deactivated with allyl bromide, and in the presence of the free radical inhibitor propene when necessary. The working temperature range was 464–485°C, and the pressure range was 44–133Torr. The reactions were found to be homogeneous and unimolecular and to obey a first-order rate law. The products for 4-hydroxyacetophenone are phenol and ketene, while for 2-hydroxyacetophenone are phenol and ketene, with smaller amounts of phenyl acetate and benzofuran. The Arrhenius expression of the pyrolyses was found for 4-hydroxyacetophenone: logk1 (s−1)=(13.83±0.08)−(258.1±1.1)kJmol−1(2.303RT)−1(r=0.9999); for 2-hydroxyacetophenone: logk1 (s−1)=(13.81±0.43)−(251.9±6.2)kJmol−1(2.303RT)−1(r=0.9991). Deacetylation may be considered as the primary reasonable mechanism, and the process appears to proceed through a semi-polar concerted four-membered cyclic transition state.

Computational study of the thermal decomposition of 2-methylbutyraldehyde and 2-pentanone through retro-ene reactions

In this work, a theoretical study at the MP2/6-31G(d) level of the thermal decomposition retro-ene reaction of 2-methylbutyraldehyde was carried out at a pressure of 1.5 atm. and temperatures ranging from 1110 to 1190 K. The progress of the reaction has been followed by means of the Wiberg bond indices which in turn allowed the calculation of the reaction synchronicity. Transition state theory was used to calculate the theoretical rate constant at 1150 K which was compared with the previously reported experimental value at the same conditions. We found that both values show a close agreement. The obtained computational evidence allowed us to support a reaction mechanism which proceeds in two steps: the first one with the formation of ethylene and 1-propenol via a six-membered cyclic transition state and the second one involving keto-enol equilibrium of 1-propenol to propionaldehyde via a four-membered cyclic transition state. It was found that the reaction is a highly synchronous and concerted process. The results obtained for the thermolysis of 2-methylbutyraldehyde were compared with those obtained for the thermolysis of 2-pentanone. A comparison of our results with those reported for their corresponding β-hydroxy counterparts, 3-hydroxy-2-methylpropionaldehyde and 4-hydroxy-2-butanone has also been made. A study of the thermochemistry of the compounds involved in the reactions studied has been carried out at the G3 level.

Kinetics of the Gas-Phase Elimination Reaction of Benzyl Chloroformate and Neopentyl Chloroformate

The gas-phase eliminations of benzyl chloroformate (475–523 K, 31–103 Torr) and neopentyl chloroformate (563–622 K, 37–70 Torr), in a deactivated static reaction vessel, and in the presence of a free radical suppressor, are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following Arrhenius equations: Benzyl chloroformate Neopentyl chloroformate Formation of neopentyl chloride: Formation of 2-methylbutenes: The derived kinetic and thermodynamic parameters for benzyl chloroformate decomposition indicate the reaction proceeds through a concerted four-membered cyclic transition state to give benzyl chloride and CO2 gas. Neopentyl chloroformate undergoes a parallel reaction, where neopentyl chloride formation may arise from a polar-concerted four-membered cyclic transition state, whereas the mixture of olefins, 2-methyl-2-butene, and 2-methyl-1-butene appears to be produced from a carbene intermediate. This intermediate seems to be originated from a concerted five-membered cyclic transition state of the neopentyl substrate.

Alkali-Metal Ion Catalysis and Inhibition in SNAr Reaction of 1-Halo-2,4-dinitrobenzenes with Alkali-Metal Ethoxides in Anhydrous Ethanol

A kinetic study is reported for SNAr reaction of 1-fluoro-2,4-dinitrobenzene (5a) and 1-chloro-2,4-dinitrobenzene (5b) with alkali-metal ethoxides (EtOM, M = Li, Na, K and 18-crown-6-ether complexed K) in anhydrous ethanol. The second-order rate constant increases in the order kEtOLi < kEtO− < kEtONa < kEtOK < kEtOK/18C6 for the reaction of 5a and kEtOLi < kEtONa < kEtO− < kEtOK < kEtOK/18C6 for that of 5b. This indicates that M ion behaves as a catalyst or an inhibitor depending on the size of M ion and the nature of the leaving group (F vs. Cl). Substrate 5a is more reactive than 5b, although the F in 5a is ca. 10 pKa units more basic than the Cl in 5b, indicating that the reaction proceeds through a Meisenheimer complex in which expulsion of the leaving group occurs after the rate-determining step (RDS). M ion would catalyze the reaction by increasing either the nucleofugality of the leaving group through a four-membered cyclic transition state or the electrophilicity of the reaction center through a π-complex. However, the enhanced nucleofugality would be ineffective for the current reaction, since expulsion of the leaving group occurs after the RDS. Thus, it has been concluded that M ion catalyzes the reaction by increasing the electrophilicity of the reaction center through a π-complex between M ion and the π-electrons in the benzene ring.

ChemInform Abstract: Atom‐Efficient Gold(I)‐Chloride‐Catalyzed Synthesis of α‐Sulfenylated Carbonyl Compounds from Propargylic Alcohols and Aryl Thiols: Substrate Scope and Experimental and Theoretical Mechanistic Investigation.

Gold(I)-chloride-catalyzed synthesis of α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols showed a wide substrate scope with respect to both propargylic alcohols and aryl thiols. Primary and secondary aromatic propargylic alcohols generated α-sulfenylated aldehydes and ketones in 60–97 % yield. Secondary aliphatic propargylic alcohols generated α-sulfenylated ketones in yields of 47–71 %. Different gold sources and ligand effects were studied, and it was shown that gold(I) chloride gave the highest product yields. Experimental and theoretical studies demonstrated that the reaction proceeds in two separate steps. A sulfenylated allylic alcohol, generated by initial regioselective attack of the aryl thiol on the triple bond of the propargylic alcohol, was isolated, evaluated, and found to be an intermediate in the reaction. Deuterium labeling experiments showed that the protons from the propargylic alcohol and aryl thiol were transferred to the 3-position, and that the hydride from the alcohol was transferred to the 2-position of the product. Density functional theory (DFT) calculations showed that the observed regioselectivity of the aryl thiol attack towards the 2-position of propargylic alcohol was determined by a low-energy, five-membered cyclic protodeauration transition state instead of the strained, four-membered cyclic transition state found for attack at the 3-position. Experimental data and DFT calculations supported that the second step of the reaction is initiated by protonation of the double bond of the sulfenylated allylic alcohol with a proton donor coordinated to gold(I) chloride. This in turn allows for a 1,2-hydride shift, generating the final product of the reaction.

Atom-efficient gold(I)-chloride-catalyzed synthesis of α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols: substrate scope and experimental and theoretical mechanistic investigation.

Gold(I)-chloride-catalyzed synthesis of α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols showed a wide substrate scope with respect to both propargylic alcohols and aryl thiols. Primary and secondary aromatic propargylic alcohols generated α-sulfenylated aldehydes and ketones in 60–97 % yield. Secondary aliphatic propargylic alcohols generated α-sulfenylated ketones in yields of 47–71 %. Different gold sources and ligand effects were studied, and it was shown that gold(I) chloride gave the highest product yields. Experimental and theoretical studies demonstrated that the reaction proceeds in two separate steps. A sulfenylated allylic alcohol, generated by initial regioselective attack of the aryl thiol on the triple bond of the propargylic alcohol, was isolated, evaluated, and found to be an intermediate in the reaction. Deuterium labeling experiments showed that the protons from the propargylic alcohol and aryl thiol were transferred to the 3-position, and that the hydride from the alcohol was transferred to the 2-position of the product. Density functional theory (DFT) calculations showed that the observed regioselectivity of the aryl thiol attack towards the 2-position of propargylic alcohol was determined by a low-energy, five-membered cyclic protodeauration transition state instead of the strained, four-membered cyclic transition state found for attack at the 3-position. Experimental data and DFT calculations supported that the second step of the reaction is initiated by protonation of the double bond of the sulfenylated allylic alcohol with a proton donor coordinated to gold(I) chloride. This in turn allows for a 1,2-hydride shift, generating the final product of the reaction.

Theoretical calculations on the gas-phase thermal decomposition kinetics of selected thiomethyl chloroalkanes: A new insight of the mechanism

Theoretical calculations of the gas-phase thermal decomposition kinetics of 2-thiomethyl-1-chloroethane and 4-thiomethyl-1-chlorobutane have been carried out by using Density Functional Theory (DFT), composite CBS-Q3, and Møller-Plesset second-order (MP2) methods in order to elucidate a reasonable reaction mechanism of these compounds. The enhanced reactivities of these two substrates, when compared with their parent compounds, attributed to neighboring group participation (NGP) or anquimeric assistance in the literature, were investigated. For 2-thiomethyl-1-chloroethane two dehydrochlorinaton pathways, with and without NGP were studied. The results of quantum chemical estimations of 2-thiomethyl-1-chloroethane in the gas phase show good agreement with experimental values at B3LYP/6-31++G(2d,p) level, and suggest the 1,2-elimination through non-synchronous four-membered cyclic transition state is the preferred mechanism. For 4-thiomethyl-1-chlorobutane, the significant increase in rate compared to 2-thiomethyl-1-chloroethane, together with the formation of a cyclic product tetrahydrothiophene suggest the anchimeric assistance by the CH3S group in the transition state. Best calculated parameters were obtained with CAM-B3LYP/6-31G++(2d,p). Results support the NGP of the thiomethyl group, through cyclic ion-pair type of intermediate. The bond polarization of the C–Cl, in the direction of Cδ+⋯Clδ−, appears to be the rate determining step of these decompositions.

Quantum Chemical Study of B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Group

The hydrosilylation reaction of a carbonyl group catalyzed by tris(pentafluorophenyl)borane, B(C6F5)3, is investigated by using the DFT method. M06-2X level calculations suggest the presence of a stable complex between trimethylsilane and B(C6F5)3. The attack of the carbonyl group in acetone from the back side of the Si-H bond prompts the abstraction of the hydride ion by B(C6F5)3. This reaction path is lower in free energy than the conventional carbonyl-activation path via a four-membered cyclic transition state. The silane-activation mechanism is supported in this case, in agreement with experimental results reported by Piers and by Oestreich. The calculations show, on the other hand, that the silane-activation mechanism does not apply to the reaction catalyzed by BF3. This difference in mechanisms arises from a stronger electrophilicity of the boron center in B(C6F5)3 than in BF3 toward a hydride ion, as demonstrated by an analysis of reactive orbitals. Attractions between the silane part and the fluorine atoms at the ortho positions of C6F5 groups in the Lewis acid assist the path by making up for the destabilization of the reacting system that is caused by the distortion of the B(C6F5)3 framework in forming a bond with the hydrogen of silane.

DFT and ab-initio study on the mechanism of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their isomerization

The mechanisms of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their interconversion have been examined at MP2 and DFT levels of theory. These halide substrates yield isoprene and hydrogen chloride. The results MPW1PW91 calculations agree with the experimental kinetic parameters showing the elimination reaction occurs at greater rate for 1-chloro-3-methylbut-2-ene than that for the 3-chloro-3-methylbut-1-ene isomer. The mechanism for the molecular elimination of 1-chloro-3-methylbut-2-ene suggests proceeding through an uncommon six-membered cyclic transition state for alkyl halides in the gas phase, while 3-chloro-3-methylbut-1-ene eliminates through the usual four-membered cyclic transition state. The elongation and subsequent polarization of the C-Cl bond, in the direction of C^{δ+}…Cl^{δ-}, is rate determining step of these reactions. The isomerization of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene was additionally studied. The 1-chloro-3-methylbut-2-ene converts to 3-chloro-3-methylbut-1-ene easier than the reverse reaction. This means that 1-chloro-3-methylbut-2-ene was found thermodynamically more stable than 3-chloro-3-methylbut-1-ene.

Theoretical study of the decomposition of ethyl and ethyl 3-phenyl glycidate

The mechanism of the decomposition of ethyl and ethyl 3-phenyl glycidate in gas phase was studied by density functional theory (DFT) and MP2 methods. A proposed mechanism for the reaction indicates that the ethyl side of the ester is eliminated as ethylene through a concerted six-membered cyclic transition state, and the unstable intermediate glycidic acid decarboxylates rapidly to give the corresponding aldehyde. Two possible pathways for glycidic acid decarboxylation were studied: one via a five-membered cyclic transition state, and the other via a four-membered cyclic transition state. The results of the calculations indicate that the decarboxylation reaction occurs via a mechanism with five-membered cyclic transition state.

Ab Initio Study and Nbo Analysis of the Unimolecular Decomposition Kinetics of 2,2-Dimethyloxetane

Theoretical study of the thermal decomposition kinetics of 2,2-dimethyloxetane has been carried out at the B3LYP/6-311 + G**, B3PW91/6-311 + G**, and MPW1PW91/6-311 + G** levels. The MPW1PW91/6-311 + G** method was found to give a reasonably good agreement with the experimental kinetics and thermodynamic parameters. Two parallel unimolecular reactions occur to give either isobutene and formaldehyde [reaction (1)] or ethene and acetone [reaction (2)]. The calculated data demonstrate that the polarization of the O1-C2 bond in reaction (1) and C4-O1 bond in reaction (2), are rate determining. The analysis of bond order and NBO charges, bond indices, synchronicity parameters, and IRC calculations suggest reactions (1) and (2) occur through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

DFT Calculations of the Elimination Kinetics of Silacyclobutanes and its Methyl Derivatives in the Gas-Phase

The gas-phase thermal decomposition kinetics of silacyclobutane (1), 1-methyl- silacyclobutane (2), and 1,1-dimethyl-1-silacyclobutane (3) has been theoretically studied at the B3LYP/6-311G**, B3PW91/6-311G**, and MPW1PW91/6-311G** levels. The B3LYP/6-311G** method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1–3 yields ethylene and the corresponding silene. Based on the optimized ground state geometries using B3LYP/6-311G** method, the natural bond orbital (NBO) analysis of donor-acceptor (bonding–antibonding) interactions revealed that the perturbation energies (E2) associated with the electronic delocalization from σSi1–C2 to σ*C4–Si1 orbitals increase from compounds 1 to 3. The σSi1–C2→σ*C4–Si1 resonance energies for compounds 1–3 are 1.17, 1.26, and 1.43 kcal/mol, respectively. Also, the decomposition process in these compounds is controlled by σ→σ* resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Sisp2 atom. NBO analysis shows that the occupancies of σSi1–C2 bonds decrease for compounds 1–3 as 3 < 2 < 1, and the occupancies of σ*Si1–C2 bonds increase in the opposite order (3 > 2 > 1). Moreover, these results can fairly explain the decrease of the energy barriers (ΔEo) of the decomposition reaction of compounds 1 to 3. The calculated data demonstrate that in the decomposition process of the studied compounds, the polarization of the C3–C4 bond is the rate determining factor. Analysis of bond orders, NBO charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

Kinetics and Mechanisms of the Unimolecular Elimination of 2,2-Diethoxypropane and 1,1-Diethoxycyclohexane in the Gas Phase: Experimental and Theoretical Study

The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.

Tandem mass spectrometry of poly(ethylene imine)s by electrospray ionization (ESI) and matrix‐assisted laser desorption/ionization (MALDI)

In this contribution, linear poly(ethylene imine) (PEI) polymers, which are of importance in gene delivery, are investigated in detail by using electrospray ionization-quadrupole-time of flight (ESI-Q-TOF) and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS). The analyzed PEIs with different end groups were synthesized using the polymerization of substituted 2-oxazoline via a living cationic ring-opening polymerization (CROP) and a subsequent hydrolysis under acidic conditions. The main goal of this study was to identify linear PEI polymers in a detailed way to gain information about their fragmentation pathways. For this purpose, a detailed characterization of three different linear PEIs was performed by using ESI-Q-TOF and MALDI-TOF MS in combination with collision-induced dissociation (CID) experiments. In ESI-MS as well as MALDI-MS analysis, the obtained spectra of PEIs resulted in fitting mass distributions for the investigated PEIs. In the tandem MS analysis, a 1,2-hydride shift with a charge-remote rearrangement via a four-membered cyclic transition state, as well as charge-induced fragmentation reactions, was proposed as the main fragmentation mechanisms according to the obtained fragmentation products from the protonated parent peaks. In addition, heterolytic and homolytic cleavages were proposed as alternative fragmentation pathways. Moreover, a 1,4-hydrogen elimination was proposed to explain different fragmentation products obtained from the sodiated parent peaks.