Azulene-to-naphthalene rearrangement: the Car-Parrinello metadynamics method explores various reaction mechanisms.

We studied the thermal intramolecular and radical rearrangement of azulene to naphthalene by employing a novel metadynamics method based on Car-Parrinello molecular dynamics. We demonstrate that relatively short simulations can provide us with several possible reaction mechanisms for the rearrangement. We show that different choices of the collective coordinates can steer the reaction along different pathways, thus offering the possibility of choosing the most probable mechanism. We consider herein three intramolecular mechanisms and two radical pathways. We found the norcaradiene pathway to be the preferable intramolecular mechanism, whereas the spiran mechanism is the favored radical route. We obtained high activation energies for all the intramolecular pathways (81.5-98.6 kcal mol(-1)), whereas the radical routes have activation energies of 24-39 kcal mol(-1). The calculations have also resulted in elementary steps and intermediates not yet considered. A few attractive features of the metadynamics method in studying chemical reactions are pointed out.     

The formation of naphthalene, azulene, and fulvalene from cyclic C5 species in combustion: an ab initio/RRKM study of 9-H-fulvalenyl (C5H5-C5H4) radical rearrangements

Chemically accurate ab initio Gaussian-3-type calculations of the C(10)H(9) potential energy surface (PES) for rearrangements of the 9-H-fulvalenyl radical C(5)H(5)-C(5)H(4) have been performed to investigate the formation mechanisms of polycyclic aromatic hydrocarbons (PAHs) originated from the recombination of two cyclopentadienyl radicals (c-C(5)H(5)) as well as from the intermolecular addition of cyclopentadienyl to cyclopentadiene (c-C(5)H(6)) under combustion and pyrolytic conditions. Statistical theory calculations have been applied to obtain high-pressure-limit thermal rate constants, followed by solving kinetic equations to evaluate relative product yields. At the high-pressure limit, naphthalene, fulvalene, and azulene have been shown as the reaction products in rearrangements of the 9-H-fulvalenyl radical, with relative yields depending on temperature. At low temperatures (T < 1000 K), naphthalene is predicted to be the major product (>50%), whereas at higher temperatures the naphthalene yield rapidly decreases and the formation of fulvalene becomes dominant. At T > 1500 K, naphthalene and azulene are only minor products accounting for less than 10% of the total yield. The reactions involving cyclopentadienyl radicals and cyclopentadiene have thus been shown to give only a small contribution to the naphthalene production on the C(10)H(9) PES at medium and high combustion temperatures. The high yields of fulvalene at these conditions indicate that cyclopentadienyl radical and cyclopentadiene more likely represent significant sources of cyclopentafused PAHs, which are possible fullerene precursors. Our results agree well with a low-temperature cyclopentadiene pyrolysis data, where naphthalene has been identified as the major reaction product together with indene. Azulene has been found to be only a minor product in 9-H-fulvalenyl radical rearrangements, with branching ratios of less than 5% at all studied temperatures. The production of naphthalene at low combustion temperatures (T < 1000 K) is governed by the spiran mechanism originally suggested by Melius et al. At higher temperatures, the alternative C-C bond scission route, which proceeds via the formation of the cis-4-phenylbutadienyl radical, is competitive with the spiran pathway. The contributions of the previously suggested methylene walk pathway to the production of naphthalene have been calculated to be negligible at all studied temperatures.     

The carbon-skeleton rearrangement in tropane alkaloid biosynthesis

High-level quantum chemistry calculations have been performed to examine the carbon-skeleton rearrangement of the tropane alkaloid littorine to hyoscyamine. Two pathways involving radical and carbocation intermediates have been investigated in this regard, namely, stepwise (or fragmentation-recombination) and concerted. The fragmentation products are calculated to be of high energy for both the radical- and carbocation-based mechanisms (136.3 and 170.9 kJ mol(-1), respectively). Similarly, the rearrangement barrier for the radical-based concerted pathway is calculated to be quite high (135.6 kJ mol(-1)). In contrast, the carbocation-based concerted pathway is found to be associated with a relatively low barrier (47.4 kJ mol(-1)). The ionization energy of the substrate-derived radical 3a is calculated to be 7.01 eV, suggesting that its oxidation to generate the substrate-derived carbocation 3b ought to be facile. In an attempt to investigate how an enzyme might modulate the rearrangement barriers, the separate and combined influences of partially protonating the migrating group and partially deprotonating the spectator OH group of the substrate were investigated. Such interactions can lead to significant reductions in the rearrangement barrier for both the radical- and carbocation-based concerted pathways, although the carbocation pathway continues to have significantly lower energy requirements. Also, the relatively high (gas-phase) acidity of the OH group of the product-related carbocation 4b indicates that the direct formation of hyoscyamine aldehyde (6) is a highly exothermic process. Although we would not wish to rule out alternative possibilities, our calculations suggest that a concerted rearrangement mechanism involving carbocations constitutes a viable low-energy pathway for the carbon-skeleton rearrangement in tropane alkaloid biosynthesis.     

An ab initio G3-type/statistical theory study of the formation of indene in combustion flames. II. The pathways originating from reactions of cyclic C5 species-cyclopentadiene and cyclopentadienyl radicals

Chemically accurate ab initio Gaussian-3-type calculations of various rearrangements on the C10H11 potential energy surface have been performed to investigate the indene formation mechanism originating from the reactions of two abundant cyclic C5 species, cyclopentadiene and cyclopentadienyl radicals. Using the accurate ab initio data, statistical theory calculations have been applied to obtain high-pressure-limit thermal rate constants within the 300-3000 K temperature range, followed by calculations of relative product yields. Totally, 12 reaction pathways leading to indene and several azulene precursors, 1,5-, 1,7-, 1,8a-, and 1,3a-dihydroazulene, have been mapped out, and the relative contributions of each pathway to the formation of reaction products have been estimated. At temperatures relevant to combustion, the indene has been found as the major reaction product (>50%) followed by 1,5-dihydroazulene (25-35%), whereas all other products demonstrate either minor or negligible yields. The results of the present study have been combined with our previous data for rearrangements of the 9-H-fulvalenyl radical on the C10H9 potential energy surface to draw the detailed picture of radical-promoted reaction mechanisms leading from c-C5 species to the production of indene, naphthalene, azulene, and fulvalene in combustion. The suggested mechanism and computed product yields are consistent with the experimental data obtained in the low-temperature pyrolysis of cyclopentadiene, where indene and naphthalene have been found as the major reaction products.     

On the formation of phenyldiacetylene (C6H5CCCCH) and D5-phenyldiacetylene (C6D5CCCCH) studied under single collision conditions

The crossed molecular beam reactions of the phenyl and D5-phenyl radical with diacetylene (C(4)H(2)) was studied under single collision conditions at a collision energy of 46 kJ mol(-1). The chemical dynamics were found to be indirect and initiated by an addition of the phenyl/D5-phenyl radical with its radical center to the C1-carbon atom of the diacetylene reactant. This process involved an entrance barrier of 4 kJ mol(-1) and lead to a long lived, bound doublet radical intermediate. The latter emitted a hydrogen atom directly or after a few isomerization steps via tight exit transition states placed 20-21 kJ mol(-1) above the separated phenyldiacetylene (C(6)H(5)CCCCH) plus atomic hydrogen products. The overall reaction was determined to be exoergic by about 49 ± 26 kJ mol(-1) and 44 ± 10 kJ mol(-1) as determined experimentally and computationally, thus representing a feasible pathway to the formation of the phenyldiacetylene molecule in combustion flames of hydrocarbon fuel.     

Enthalpies of formation, bond dissociation energies and reaction paths for the decomposition of model biofuels: ethyl propanoate and methyl butanoate

The complete basis set method CBS-QB3 has been used to study the thermochemistry and kinetics of the esters ethyl propanoate (EP) and methyl butanoate (MB) to evaluate initiation reactions and intermediate products from unimolecular decomposition reactions. Using isodesmic and isogeitonic equations and atomization energies, we have estimated chemically accurate enthalpies of formation and bond dissociation energies for the esters and species derived from them. In addition it is shown that controversial literature values may be resolved by adopting, for the acetate radical, CH3C(O)O(.-), DeltaH(o)(f)298.15K) = -197.8 kJ mol(-1) and for the trans-hydrocarboxyl radical, C(.-)(O)OH, -181.6 +/- 2.9 kJ mol(-1). For EP, the lowest energy decomposition path encounters an energy barrier of approximately 210 kJ mol(-1) (approximately 50 kcal mol(-1)), which proceeds through a six-membered ring transition state (retro-ene reaction) via transfer of the primary methyl H atom from the ethyl group to the carbonyl oxygen, while cleaving the carbon-ether oxygen to form ethene and propanoic acid. On the other hand, the lowest energy path for MB has a barrier of approximately 285 kJ mol(-1), producing ethene. Other routes leading to the formation of aldehydes, alcohols, ketene, and propene are also discussed. Most of these intramolecular hydrogen transfers have energy barriers lower than that needed for homolytic bond fission (the lowest of which is 353 kJ mol(-1) for the C(alpha)-C(beta) bond in MB). Propene formation is a much higher energy demanding process, 402 kJ mol(-1), and it should be competitive with some C-C, C-O, and C-H bond cleavage processes.     

Rearrangement and hydrogen scrambling pathways of the toluene radical cation: a computational study

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.     

Halo-substituted 1-buten-3-ynes

Conclusions Some halo-substituted 1-buten-3-ynes were synthesized by the reaction of alkoxy-, alkylthto-, and alkyl-1-buten-3-ynes with alkaline metal hypohalites.Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 2353–2354, October, 1972.     

Identification of the 1,3,2,4-Dithiadiazolyl RCNSNS(*) Radical Dimers in Solution, Their Dimeric Concerted Photochemically Symmetry-Allowed Rearrangement to 1,2,3,5-Dithiadiazolyl RCNSSN(*) by the Net Exchange of Adjacent Cyclic Sulfur and Nitrogen Atoms, and the Photolysis of RCNSNS(*)

The 1,3,2,4-dithiadiazolyl RCNSNS(*) radicals undergo an unprecedented concerted rearrangement to the thermodynamically more stable 1,2,3,5-dithiadiazolyl RCNSSN(*) radicals by the net exchange of adjacent cyclic sulfur and nitrogen atoms. The UV-visible spectra of RCNSNS(*) (R = Ph, p-O(2)NC(6)H(4), 3,5-(O(2)N)(2)C(6)H(3), CF(3)) in solution show bands at 250 nm (strong) and 680 nm (very weak) attributable to monomer and two dimer bands at 376 and 480 nm, the positions of which are independent of the substituents, providing direct identification of the radical dimers in solution. The dimerization equilibrium constant (K(298) approximately 0.7 for R = Ph) at room temperature was derived from the enthalpy and entropy changes for the dimerization of PhCNSNS(*) (DeltaH(d) degrees = -19.0 kJ/mol, DeltaS degrees = -66.5 J/mol) estimated by a variable-temperature ESR spectroscopic study. In addition, RCNSNS(*) (R = Bu(t), Ph) undergo an apparent unimolecular photolysis to RCN and possibly SNS(*) (analogue of ONO(*)). The photochemical rearrangement and dissociation (for R = Ph and 3,5-(O(2)N)(2)C(6)H(3)) were shown to proceed by irradiation of the radical dimer (376 and 480 nm) and monomer (250 nm), respectively. Thus, the radical rearrangement reasonably occurs via a concerted dimeric pathway shown by molecular orbital calculations (CNDO) to be photochemically symmetry-allowed. In addition, we propose that the radical dissociation proceeds via a concerted unimolecular photochemically symmetry-allowed process.     

A crossed beam and ab initio investigation on the formation of vinyl boron monoxide (C2H3BO; X1A') via reaction of boron monoxide (11BO; X2Σ+) with ethylene (C2H4; X1A(g))

The reaction dynamics of the boron monoxide radical ((11)BO; X(2)Σ(+)) with ethylene (C(2)H(4); X(1)A(g)) were investigated at a nominal collision energy of 12.2 kJ mol(-1) employing the crossed molecular beam technique and supported by ab initio and statistical (RRKM) calculations. The reaction is governed by indirect scattering dynamics with the boron monoxide radical attacking the carbon-carbon double bond of the ethylene molecule without entrance barrier with the boron atom. This addition leads to a doublet radical intermediate (O(11)BH(2)CCH(2)), which either undergoes unimolecular decomposition through hydrogen atom emission from the C1 atom via a tight transition state located about 13 kJ mol(-1) above the separated products or isomerizes via a hydrogen shift to the O(11)BHCCH(3) radical, which also can lose a hydrogen atom from the C1 atom. Both processes lead eventually to the formation of the vinyl boron monoxide molecule (C(2)H(3)BO; X(1)A'). The overall reaction was determined to be exoergic by about 40 kJ mol(-1). The reaction dynamics are also compared to the isoelectronic ethylene (C(2)H(4); X(1)A(g)) - cyano radical (CN; X(2)Σ(+)) system studied earlier.     

Radical-promoted Stone-Wales rearrangements

The mechanism of the known Stone-Wales rearrangement of bifluorenylidene to dibenzo[g,p]chrysene is assessed with the aid of B3LYP/6-31G(d) density functional calculations, and it is shown that a radical-promoted mechanism involving a sequence of homoallyl-cyclopropylcarbinyl rearrangement steps gives a realistic activation energy and can explain experimental observations, whereas a unimolecular mechanism has an improbably high activation energy. Radical-promoted mechanisms are then applied to the hypothetical Stone-Wales rearrangements of diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene and C(60) itself. Severe steric constraints in these cases raise the activation energy for the radical-promoted pathways substantially, but they are still strongly preferred to uncatalyzed, unimolecular pathways     

1,2-C4H6→2-C4H6异构化反应机理的理论研究

采用自洽场分子轨道UHF/6-31G**从头算法,研究了1,2-C4H6→2-C4H6异构化反应机理,优化了基态势能面上反应物、过渡态、中间体和产物的几何构型,并对各驻点能量进行了零点能校准.结果表明该反应经历一个1-甲基环丙烯生成产物比经两步氢迁移反应历程更易发生.     

S-to-αC Radical Migration in the Radical Cations of Gly-Cys and Cys-Gly

The radical cations of Cys-Gly and Gly-Cys were studied using ion-molecule reactions (IMR), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. Homolytic cleavage of the S–NO bond of nitrosylated precursors generated radical cations with the radical site initially located on the sulfur atom. Time-resolved ion-molecule reactions showed that radical site migration via hydrogen atom transfer (HAT) occurred much more quickly in Gly-Cys^•+ than in Cys-Gly^•+. IRMPD and DFT calculations indicated that for Gly-Cys, the radical migrated from the sulfur atom to the α-carbon of glycine, which is lower in energy than the sulfur radical (–53.5 kJ/mol). This migration does not occur for Cys-Gly because the glycine α-carbon is higher in energy than the sulfur radical (10.3 kJ/mol). DFT calculations showed that the highest energy barriers for rearrangement are 68.2 kJ/mol for Gly-Cys and 133.8 kJ/mol for Cys-Gly, which is in agreement with both the IMR and IRMPD data and explains the HAT in Gly-Cys.     

Rhodium-catalyzed carbothiolation reaction of 1-alkylthio-1-alkynes

A rhodium complex derived from RhH(PPh₃)₄ and Me₂PhP catalyzed the carbothiolation reaction of 1-alkylthio-1-alkynes and 1,4-diaryl-1,3-butadiynes giving (Z)-4-alkylthio-4-aryl-3-arylethynyl-3-buten-1-ynes. Terminal alkynes such as 1-decyne and (t-butylthio)acetylene underwent the carbothiolation reaction using a RhH(PPh₃)₄-dppb catalyst. The reactions proceeded via cis-addition with C-C bond formation at the less hindered acetylene carbon.     

Toward an improved understanding of the glutamate mutase system

High-level quantum chemistry calculations have been used to examine the catalytic reactions of adenosylcobalamin-dependent glutamate mutase (GM) with the natural substrate (S)-glutamic acid. We have also examined the rearrangement of (S)-2-hydroxyglutaric acid, (S)-2-thiolglutaric acid, and 2-ketoglutaric acid, all of which have previously been shown to react as substrates or inhibitors of the enzyme. Our calculations support the notion that the 100-fold difference in kcat between glutamate and 2-hydroxyglutarate is associated with the relatively high energy of the glycolyl radical intermediate compared with the glycyl radical. More generally, calculations of radical stabilization energies for a variety of substituted glycyl radical analogues indicate that modifications at the radical center can profoundly affect the relative stability of the resulting radical, leading to important mechanistic consequences. We find that the formation of a thioglycolyl radical, derived from (S)-2-thiolglutaric acid, is highly dependent on the protonation state of sulfur. The neutral radical is found to be of stability similar to that of the glycolyl radical, whereas the S- form of the thioglycolyl radical is much more stable, thus providing a rationalization for the inhibition of the enzyme by the substrate analogue 2-thiolglutarate. Two possible rearrangement pathways have been examined for the reaction of GM with 2-ketoglutaric acid, for which previous experiments had suggested no rearrangement took place. The fragmentation-recombination pathway is associated with a fragmentation step that is very endothermic (by 102.2 kJ mol-1). In contrast, the addition-elimination pathway has significantly lower energy requirements. An alternative possibility, namely, that 2-ketoglutaric acid is bound in its hydrated form, 2,2-dihydroxyglutaric acid, also leads to a pathway with relatively low energy requirements, suggesting that some rearrangement might be expected under such circumstances.     

Enthalpy of formation of the cyclopentadienyl radical: photoacoustic calorimetry and ab initio studies

The gas-phase C-H bond dissociation enthalpy (BDE) in 1,3-cyclopentadiene has been determined by time-resolved photoacoustic calorimetry (TR-PAC) as 358 +/- 7 kJ mol(-1). Theoretical results from ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)) are reported. The CCSD(T) prediction for the C-H BDE of 1,3-cyclopentadiene (353.3 kJ mol(-1)) is in good agreement with the TR-PAC result. On the basis of the experimental and the theoretical values obtained, we recommend 355 +/- 8 kJ mol(-1) for the C-H BDE of 1,3-cyclopentadiene and 271 +/- 8 kJ mol(-1) for the enthalpy of formation of cyclopentadienyl radical.     

A ring walk of methylene groups in toluene radical cations. An extension of the toluene-cycloheptatriene rearrangement of aromatic radical cations. Theory and experiment

The minimum energy reaction pathway (MERP) of the toluene-cycloheptatriene radical cation rearrangement (TOL/CHT-rearrangement) has been calculated by the UHF and DFT model at the level UHF/6-311+G(3df,2p)//UHF/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYp/6-31G(d), respectively, including the ring walk of the substituent by a 1,2-shift around the aromatic ring. This ring walk corresponds to interconversion of distonic ions and norcaradiene radical cations (the two intermediates of the TOL/CHT-rearrangement) by making and breaking of the external C-C bonds of the cyclopropane moiety of the intermediate norcaradiene structure. For toluene radical cation 1, UHF calculations adequately reproduce earlier results(4) and show, that the ring walk of the CH(3)-substituents requires slightly more energy than formation of the cycloheptatriene radical cation. By the DFT model, the distonic ion, which is formed initially by a 1,2-H shift from CH(3) to the benzene ring, is not stable but the transition state of an interconversion of norcaradiene radical cations along a ring walk of the CH(3) substituent. The activation energy for this ring walk exceeds that for formation of the cycloheptatriene radical cation by c. 30 kJ mol(-1). Thus, isomerization of 1 by a ring walk of the CH(3)-substituent competes with the TOL/CHT-rearrangement likely only for excited 1. The calculation was repeated for the MERPs of a TOL/CHT-rearrangement of para-xylene radical cation 5 and ethylbenzene radical cation 2, yielding basically the same results as for 1. According to the calculation, polar substituents alter significantly the relative energies of the competing routes of isomerization. For benzylcyanide 3 (X = CN), the activation energy for a ring walk of the NC-CH(2)-substituent is distinctly below that of a ring enlargement. For benzyl methyl ether 4 (X = OCH(3)), the distonic intermediate along the UHF-MERP is unusually stable. Further, the 7-methoxy-norcaradiene radical ion is unstable and corresponds to a transition state between isomeric distonic intermediates differing by a 1,2-shift of the side chain. In contrast, the 7-methoxy-norcaradiene radical ion is the only intermediate of the DFT-MERP, and the distonic ion is the transition state for a 1,2-shift of the cyclopropane ring. A ring walk of the CH(3)OCH(2)-substituent is much more favorable than formation of a 7-methoxy-cycloheptatriene radical cation in both MERPs. The findings of the theoretical calculation are substantiated by the mass spectrometric fragmentations of meta- and para-methoxymethylated 1-phenylethanols 8 and 9 and of para-methoxymethyl substituted benzyl ethyl ether 10 and benzyl n-propyl ether 11. Important fragmentation routes of metastable molecular ions of these compounds correspond to elimination of alcohols. Use of deuterated derivatives shows that the elimination occurs by a "false" ortho-effect which requires migration of a ROCH(2)-substituent around the benzene ring. Results of particular interest are obtained for the asymmetric bis-ethers 10 and 11. Here, the MIKE spectra of the molecular ions of deuterated analogs reveal a selective ring walk of the C(2)H(5)OCH(2)- and n-C(3)H(7)OCH(2)-side chain, respectively.     

钐(II)类卡宾CH_3SmCH_2I与乙烯环丙烷化反应及溶剂化效应(THF)对反应途径影响的理论研究

用密度泛函B3LYP方法研究了过渡金属钐类卡宾与乙烯的环丙烷化反应的机理.对钐类卡宾试剂CH3SmCH2I和CH2CH2反应的反应物、中间体、过渡态和产物构型的全部结构几何参数进行了优化,并计算了THF溶液的溶剂化效应,用内禀反应坐标(IRC)计算和频率分析方法,对过渡态进行了验证.结果表明:CH3SmCH2I与CH2CH2环丙烷化反应按亚甲基转移机理(通道A)和卡宾金属化机理(通道B)都可以进行,与锂类卡宾的反应机理相同,通道A比通道B反应的势垒降低了14.65kJ/mol.溶剂化效应使通道B比通道A的反应势垒大幅度提高,更有利于反应沿通道A进行,而不利于通道B.     

Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.     

Two pathways for the protolysis of 1-(alkythio)-1-buten-3-ynes and the effect of cis-trans isomerism

Conclusions We have used a spectrophotometric method to examine the kinetics of the hydrolysis of 1-(alkylthio)-1-buten-3-ynes to (alkylthio)butenones and acetoacetaldehyde; the rate of reaction depends markedly on the reactant configuration.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 153–159, January, 1978.