Four-center Transition State Research Articles

ChemInform Abstract: COMPETITIVE UNIMOLECULAR REACTIONS AT LOW PRESSURES. THE PYROLYSIS OF CYCLOBUTYL CHLORIDE

The thermal decomposition of cyclobutyl chloride has been investigated over the temperature range of 892–1150 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via two competitive unimolecular channels, one to yield ethylene and vinyl chloride and the other to yield 1,3-butadiene and hydrogen chloride, with the latter being the major reaction under the experimental conditions. With the usual assumption that gas-wall collisions are «strong,» RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k1(sec−1) = (14.8 ± 0.3) − (61.1 ± 1.0)/Θ for vinyl chloride formation and log k2(sec−1) = (13.6 ± 0.3) − (55.7 ± 1.0)/Θ for 1,3-butadiene formation, where Θ = 2.303 RT kcal/mol. The A factors were assigned from previous high-pressure low-temperature data of other workers assuming a four-center transition state for 1,2-HCl elimination and a chlorine-bridged biradical transition state for vinyl chloride formation. The activation energies are in good agreement with the high-pressure results which were obtained with a conventional static system. The difference in critical energies is 4.6 kcal/mol.

A Molecular-orbital Calculation of Chemically-interacting Systems. Interaction between Two Radicals. Self-reaction of Peroxyl Radicals

Abstract The path of the termination reaction of peroxyl radicals (self-reaction) was studied by means of a perturbational method and its reaction mechanism was elucidated theoretically. It was ascertained from this calculation that the self-reaction of tertiary alkylperoxyl radicals yielding a tetroxide does not have a four-centered transition state with four oxygen atoms of the two peroxyl radicals participating, but has a two-centered transition state with only the terminal oxygen atoms. The self-reaction of primary or secondary alkylperoxyl radicals is found to have a six-centered transition state. The self-reaction of acylperoxyl radicals is also shown to have a six-centered transition state. The mechanism of these self-reactions is clarified in terms of the mode of the particular orbital interaction.

Molecular decomposition of 2,2,3,3-tetramethylbutane

In the presence of O2, the decomposition of 2,2,3,3-tetramethylbutane (TMB) gives 98 % of isobutene in the temperature range 420–540°C through reactions (4) and (5). (CH3)3 CC(CH3)3= 2(CH3)3C (4). (CH3)3C + O2=(CH3)2CCH2+ HO2(5). Approximately 1 % of isobutane is also obtained, and the rate of isobutane formation at a given temperature is directly proportional to [TMB], and is independent of [O2], N2 addition, and vessel diameter. It is shown that these results require the molecular reaction (3). (CH3)3 CC(CH3)3=(CH3)3 CH +(CH3)2 CCH2(3). Studies over the range 420–540°C give log10(A3/s– 1)= 13.89 ± 0.10, E3= 275 ± 1.4 kJ mol– 1. The A factor is consistent with a four-centre transition state.

The chemistry of nitroso compounds. Part 12. The mechanism of nitrosation and nitration of aqueous piperidine by gaseous dinitrogen tetraoxide and dinitrogen trioxide in aqueous alkaline solutions. Evidence for the existence of molecular isomers of dinitrogen tetraoxide and dinitrogen trioxide

Detailed quantitative results are reported for the interaction of aqueous piperidine in aqueous 0.1M-NaOH at 25° with gaseous N2O4 and N2O3. Both reagents rapidly give substantial amounts of N-nitrosopiperidine, plus smaller amounts of N-nitropiperidine in the case of N2O4, in addition to hydrolysis products such as NO2–. All these reactions are considered to occur predominantly in the aqueous phase and to be complete in a few seconds. With excess amine, yields of N-nitrosopiperidine reach maximum values corresponding to 100% for N2O3 but only ca. 50% for N2O4. The yield of N-nitropiperidine from N2O4, however, shows no maximum even at the highest [Piperidine]. The dependence of product yields on initial [Piperidine] and [N2Ox] suggests that N-nitrosopiperidine formation follows Rate =kp[Piperidine][N2Ox]. The concurrent hydrolysis of N2O3 and N2O4 is not significantly catalysed by HO– and is considered to involve H2O only. On a molar basis, piperidine is more reactive than H2O towards nitrosation by N2O3 and N2O4 by factors of 3 300 and 2 000, respectively. The results are discussed in relation to the existence of two molecular isomers for both N2O3 and N2O4, and the mechanisms by which these entities react with amines. For N2O4, the more stable symmetrical O2N–NO2 is considered to form only N-nitropiperidine, probably via a four-centre transition state: N-nitrosopiperidine results from concurrent reaction by the less stable ON–ONO2 isomer formed in aqueous solution by dimerisation of NO2 from the gaseous phase. For N2O3(which is fully dissociated in the gaseous phase) recombination of NO with NO2 in aqueous solution produces the less stable, symmetrical ON–ONO rather than the more stable ON–NO2 isomer present in aqueous HNO2. The existence of two isomers explains the higher reactivity of gaseous N2O3 towards weakly basic amines. N-Nitrosopiperidine formation with gaseous N2O3 results predominantly from nucleophilic attack by the amine on the ON–ONO isomer. Analysis of the data suggests that the formation of both ON–ONO2 and ONONO from their radical components in solution may be the rate-limiting step for the reactions leading to N-nitrosopiperidine.

Chemistry of metal hydrides. Part XX. The nature of the insertion rearrangement of acetylenes with platinum(II) hydrides

The reactions of platinum(II) hydrides, trans-PtHXL2, where X = NO3, Cl or CH3OH, and L = PEt3 or tricyclohexylphosphine, with disubstituted acetylenes, R1C≡CR2 have been examined under a variety of conditions. From spectroscopic parameters, the gross geometry at platinum as well as the geometry of the resulting alkenyl groups in the products have been determined. The observation that all alkenyl groups have a cis geometry is consistent with a migratory insertion step involving a four-centred transition state, in which the disposition of the acetylene relative to the Pt—H bond appears to be dependent on the Pt—H bond polarity. Evidence is presented to demonstrate that this polarity depends on several factors including solvent, and ligands on Pt(II). For L = tricyclohexylphosphine, the occurrence of a side reaction which competes with insertion and which leads to a Pt(0) acetylene complex is also demonstrated, and other interesting reactions are described.

Limited alkoxy group exchange in tetraalkoxyborohydrides: Evidence against the four-centre transition state in the borohydride reduction of ketones

Limited alkoxy group exchange in tetraalkoxyborohydrides: Evidence against the four-centre transition state in the borohydride reduction of ketones

Thermal decomposition of alcohols. III. 1-Methylcyclohexanol

The thermal decomposition of 1-methylcyclohexanol has been studied over the temperature range 448-506�C. Decomposition is homogeneous and involves two competing first-order reactions, dehydration and ketone (propanone and butanone) formation. Errors are quoted as 90% confidence limits, and R = 8.31 J mol-1 K-1. Dehydration is a concerted, unimolecular elimination; calculations suggest that the four-centre transition state is 'tight'. Ketone formation involves several free-radical reactions initiated by either carbon-carbon bond fission at the tertiary carbon or hydrogen atom abstraction from the alcohol.

Are arylcopper compounds intermediates in the exchange reaction between aryl halides and copper(I) salts?

exchange between aryl halides and anions of copper salts haL; generally been rationalized in terms of a copper-assisted nucleophilic displacement reaction involving a four-center transition state or intermediate 1,2 , eq. 1. At-Hal + XCuL, Z [Ar:$:C”L,] ; ArX + HalCuL3

Allowed and Forbidden Reaction Paths for the H2 + D2 Exchange Reaction

Symmetry arguments and abinitio s.c.f. calculations (double-zeta basis set) are used to show that the exchange reaction H2+ D2 → 2HD could proceed in a concerted fashion through a six-center transition state. The computed barrier height of 90 kcal/mol for this process lies below the experimental dissociation energy of H2 (but above the computed dissociation energy) and also below the energy required for exchange through a four-center transition state. Either the termolecular(2 + 2 + 2 ) or bimolecular(4 + 2 ) cycloadditions are thermally allowed. The presence of a transition metal would allow the reaction to proceed through a four-center geometry, leading to the formation of a possibly stable metal-H4 complex.

The mechanism of the β-ketosilane to siloxyalkene thermal rearrangement

The mechanism of the β-ketosilane to siloxyalkene rearrangement has been investigated. The reaction followed first order kinetics for a wide variety of compounds. Activation parameters have been determined for six compounds. For a series of p-substituted triphenylsilylacetophenones log k vs. σ p gave a Hammett p value of −0.78. The data are consistent with an intramolecular concerted rearrangement involving a four-centre transition state where silicon—oxygen bond formation cannot significantly precede silicon—carbon bond breaking. Molecular orbital calculations were consistent with this view.

Synthesen mit verbindungen R 3M-Hg-MR 3 und (R 2M-Hg) n(M = C, Si, Ge, Sn) : XI. Mercurierung und demercurierung mittels di-t-butylquecksilber, (CH 3) 3C-Hg-C(CH 3) 32

Me 3CHgCMe 3 (I) adds to strongly polar CC groups as well as to some CC and NN groups resp., forming t-BuHg containing cis-products. Its reactivity is higher than that of Et 2Hg and equals that of Me 3SiHgSiMe 3 to some extent. In most examples, polar four-center transition states are assumed, but radical reactions occur, too. In the Hg containing adducts investigated, the t-BuHg group can be replaced by the Me 3Sn group easily, by using Me 3SnH. Hg is split off, and i-C 4H 10 is evolved quantitatively.

Kinetics and mechanism of the morpholine–borane reduction of substituted acetophenones and benzaldehydes

Rates of reduction of substituted acetophenones and benzaldehydes by morpholine–borane in aqueous ethanol are enhanced by the introduction of electron-withdrawing groups in the carbonyl compound are are relatively independent of the water : ethanol ratio. Reduction by morpholine–[2H3]borane, is somewhat slower (kH/kD= 1·23 and 1·47 for acetophenone and benzaldehyde respectively) and a pronounced retardation of rate is observed for reduction by morpholine–cyanoborane. A four-centre transition state is proposed and it is suggested that hydride transfer from boron to the carbonyl carbon atom may be energetically important in forming the activated complex. No significant intramolecular catalysis occurs on substitution of a hydroxy-group ortho to the carbonyl carbon atom.

Kinetics and mechanism of the reactions of triethylaluminium with phenylacetylene

The kinetics of the reactions between triethylaluminium and phenylacetylene have been studied in cyclohexane between 313 and 333 K. Two reactions are observed; metallation, which proceeds by the bimolecular reaction of dimeric triethylaluminium and phenylacetylene with a rate coefficient k2= 106.8 ± 1.5 e–(64000 ± 2900K/8.314T) dm3 mol–1 s–1, and addition which proceeds by way of an alkyne π-complex (I). The equilibrium constant for the formation of (I) has been estimated. The rate controlling step of the addition reaction is a rearrangement of (I), through a four-centre transition state, with a rate coefficient k1= 1011.6 ± 0.9 e–(94000 ± 2500K)/8.314T) s–1.

Thermolysis of S-methoxymethyl thioacetates and derivatives; a novel β-elimination reaction in the vapour phase

S-Methoxymethyl thioacetates are cleaved into ester and (thio)aldehyde in the vapour phase, apparently via a four-centre transition state.

The thermal decompositions of carbamates. II. Methyl N-methylcarbamate

Methyl N-methyloarbamate decomposes in the range 370-422� to give methyl isocyanate and methanol. The reaction is first order in carbamate, and the variation of the rate constants with temperature is given by the equation. k = 1012.39 exp(-4806O/RT) (s-l; activation energy in cal mol-l) Rate constants are unaffected by the addition of isobutene or by increase in the surface to volume ratio of the reaction vessel. The addition of alcohols or amines does not reverse the process. The decomposition is considered to be a homogeneous, unimolecular gas-phase reaction, probably proceeding through a four-centred transition state.

Reactions of trans-cinnamyltriethyl- and trans-cinnamyltriphenyltin with tribromoborane, preferential transfer of the cinnamyl group from tin to boron

The syntheses of trans-cinnamyltriethyl- and trans-cinnamyltriphenyltin and their reactions with tribromoborane are described. The trans-cinnamyl residue on the tin is found to be transferred preferentially to boron, probably through a four-centered transition state or a six-membered cyclic transition state. The double bond in trans-cinnamyl group is attacked later by tribromoborane.

Kinetics and mechanism of 1,3-cycloadditions of benzonitrile N-oxides to thiobenzophenones

The kinetics of the cycloadditions of 2,4,6-trimethyl- and 4-chloro-2,6-dimethyl-benzonitrile N-oxide to thiobenzophenones to yield 1,4,2-oxathiazoles, have been studied. The reaction rate shows a small substituent effect and is slightly and irregularly influenced by the solvent polarity. ΔS‡ is ca.–26 cal mol–1 K–1. The results suggest that the reaction is a concerted process, as predicted by orbital symmetry selection rules for 1,3-dipolar cycloadditions. A four-centre transition state with charge separation is suggested.

Reactions of group III metalalkyls in the gas phase. Part 3.—Kinetics of the intramolecular elimination of but-1-ene from di-n-alkyl-n-butyl aluminium

The gas phase kinetics and the mechanistics of the unimolecular elimination of but-1-ene from a mixture of methyldi-n-butylaluminium and n-butyldimethylaluminium have been studied in the presence of excess ethylene in the temperature range 401–478 K. The observed rate constants for the apparently homogeneous elimination reaction yield (with standard errors) log k(s–1)=(10.85 ± 0.18)–(27.80±0.36)/θ, where θ equals 4.58 × 10–3T in kcal mol–1[=(10.85 ± 0.18)–(116.3±1.5)/19.15 × 10–3T in kJ mol–1] resulting in a negative intrinsic entropy of activation of 13.0 cal K–1 mol–1[54.4 JK–1 mol–1]. These data when compared with the corresponding activation parameters reported earlier for tri-isobutylaluminium substantiate the concept of a tight polar four-centre cyclic transition state. The apparent stabilizing effect of a methyl group on the positively charged carbon centre in the transition state is only 1.2 kcal mol–1. Available thermodynamic data are used to derive intrinsic ΔH°f-increment values for this class of compounds, resulting in an estimated activation energy for the back reaction, the addition of 1-butene to di-n-alkylaluminium hydride, of ∼5.2 kcal mol–1.

Reactions of group III metalalkyls in the gas phase. Part 4.—Kinetics of the homogeneous dimerization of ethylene to 1-butene, catalyzed by gaseous triethylaluminium

The kinetics of the homogeneous dimerization of ethylene into but-1-ene, catalyzed by triethyl-aluminium (Alet3) in the gas phase was studied in Teflon coated reactors for temperatures ranging between 160 and 233°C. The only significant side reaction is the isomerization of but-1-ene to but-2-ene. The reaction is second order in ethylene pressure and the catalyst Alet3, can practically quantitatively be recovered from the product mixture. Consistent second order rate constants have been obtained despite large variations in the individual reaction parameters, yielding the Arrhenius relationship log k1(l. mol–1 s–1)=(6.15 ± 0.68)–(16.3±1.5)/θ where θ equals 4.58 × 10–3T in kcal mol–1 or log k(l. mol–1 s–1=(6.15 ± 0.68)–(6.82 ± 0.63)/θ where θ equals 1.914 × 10–3T in kJ mol–1). The rate controlling step is the addition of ethylene to Alet3 to form n-butyl-diethyl-aluminium and the detailed reaction mechanism involves the establishment of a pre-equilibrium between Alet3 and ethylene to form a Alet3· C2H4π-complex prior to the formation of the four-centre cyclic transition state.

The Retentive Solvolysis. VII. Structural Effect of the Leaving Group on the Steric Course of the Sn1 Phenolysis of 1-Phenylethyl Systems

Abstract A series of optically active 1-phenylethyl systems with various leaving groups has been phenolyzed in the phenolic solvents. The 1-phenylethyl systems with a negative leaving group (i.e., 1-phenylethyl chloride, bromide, 3,5-dinitrobenzoate, and tosylate) afforded 1-phenylethyl phenyl ether with a net retained configuration. In contrast, the 1-phenylethyl systems with a positively-charged leaving group (i.e., the 1-phenylethyldiazonium ion, the diethyl-(1-phenylethyl)-oxonium ion, and the protonated 2,6-di-t-butyl-4-methyl-4-(l-phenylethyl)-cyclohexadien-l-one) .gave the 1-phenylethyl phenyl ether with a net inverted configuration. These results provide further support for the view that the retentive Sn1 phenolysis proceeds via a four-center transition state between the phenol molecule and the ion-pair intermediate. Besides the phenolic ether, the o- and p-(1-phenylethyl)-phenols with net inverted configurations were obtained from all optically active substrates except 1-phenylethyl 3,5-dinitrobenzoate and the 1-phenylethyldiazonium ion. The 3,5-dinitrobenzoate gave the completely racemic o-alkylated phenol, along with the inverted p-alkylated phenol, whereas the diazonium ion afforded the o-alkylated phenol with a net retained configuration, together with the inverted p-alkylated phenol. The stereochemical and mechanistic implications of the C-alkylation are also discussed on the basis of the results of the acidic rearrangement of the phenolic ether under the Sn1 phenolysis conditions.