W. von E. Doering

Kinetics of the cope rearrangement of 1,1-dideuteriohexa-1,5-diene☆☆☆

Abstract The degenerate Cope rearrangement of 1,1-dideuteriohexa-1,5-diene to 3,3-dideuteriohexa-1,5-diene was studied kinetically at four temp. 207·1, 217·8, 237·1 and 258·2°. The rate constant is given by the expression log k1 = 10 ·36 − 34·3/θ where θ = 4·575 T × 10−3: ΔH‡ = 33·5 ± 0·5 kcal/mole and ΔS‡ = − 13·8 ± 1 e.u. The kinetics of the formation of propylene and phenylbut-1-ene from hexa-1,5-diene and toluene at five temperatures over the range 387·1–437·1° are given by log k1 = 12·03 − 51·0/θ.

Delocalization resonance energy of the allylic radical from the geometrical isomerization of hexa-1,3,5-trienes☆

Abstract Thermal cis, trans geometrical isomerization theoretically involves a 90° twisted, singlet diradical-like transition state which may serve as a base for the examination of structural perturbations. Although thermal rearrangement of hexa-1, trans-3,5-triene (and all-trans octa-2,4,6-triene) to the cis isomer cannot be followed directly owing to subsequent cyclization and 1,5 hydrogen shifts, activation parameters for disappearance have been determined. Experimental complications and mechanistic uncertainties which make interpretation difficult are removed in the bicyclic hexatriene, cyclopentenylidenecyclopentene. These geometrical isomers undergo uncomplicated thermal cis, trans isomerization in vessels of lead-potash glass: log k1 = 12·03±0·32−41·7±0·8/(0·004575Tabs). Extraction of a value for allylic delocalization energy from the behavior of hexa-1, trans-3,5-triene [log k1 = 12·91 ± 0·47−44·3±1·2/(0·004575 Tabs)] requires corrections of the Dewar-Schmeising type for changes in hybridization of the σ bonds. Depending on whether ethylene or trans-butene is taken as standard, values of 12·2 and 13·1 kcal/mol are obtained (estimated uncertainty ± 2 kcal/mol).

Stereochemistry of the methylenecyclopropane rearrangement—II: Exclusion of the concerted 45° pivot mechanisms

Abstract Mechanistic options for the thermal rearrangement of methylenecyclopropane are narrowed further by experiments with the four racemic diastereoisomeric 2-cyano-3-methyl-ethylidenecyclopropanes. The explicit assumption being made that these structural perturbations have neither weakened nor invalidated the mechanistic restrictions imposed by previously published work, it has been concluded that concerted birotational mechanisms involving transition states such as the 45° parallel twisted allylic (Woodward-Hoffmann, Mobius controlled) or the 45° perpendicularly twisted allylic (Berson-Salem subjacent orbital controlled) can be excluded from the presently acceptable set of structurally compatible, chirality-retaining, configuration-inverting mechanisms. A competitive rearrangement of the corresponding carboxylic esters to 1-methyl-2-ethyl-4-methoxyfuran makes these molecules unsuitable for study of the methylenecyclopropane rearrangement. Assignment of stereochemistry to the cyano compounds has been accomplished by application of base-catalyzed equilibration of the cyano center and photochemical iodine-catalyzed equilibration between geometrical isomers.

Conformational control of stereochemistry in the non-concerted, degenerate rearrangement of a vinylcyclopropane: (−)-Δ3-Thujene

Abstract The stereochemistry of the degenerate, thermal vinylcyclopropane rearrangement of (−)-2,2,3-trideuterio-Δ 3 -thujene has been elucidated. Determination of partitioning among the three products of enantiomerization and rearrangement to (+) and (−) 5,5,6-trideuterio-Δ 3 -thujene reveals that the major path is forbidden by orbital symmetry and is therefore non-concerted, while the minor paths are equally divided between a forbidden and an allowed path. The high degree of stereoselectivity of this non-concerted rearrangement is explained by a small barrier to conformational equilibration. The rate of the racemization is given by the expression log k (sec −1 ) = 14·33–43·4/2·303 RT.

Delocalization energy of the cyanomethyl radical: Kinetics of thermal diastereoisomerization of cis- and trans-1,2-dicyano- and 1 ,2-dicyano-1-methylcyclopropanes

Abstract Arrhenius parameters for the thermal first-order geometrical isomerization of 1,2-dicyanocylopropanes(I) have been determined in naphthalene solution over the range 208.0–259.5° in both directions: where θ = 4.575T × 10−3 and k is in sec−1. Since this enthalpy of activation is lower than that of the geometrical isomerization of 1,2 - dideuterocyclopropane by 17.8±0.4 kcal, it may be concluded that replacement of hydrogen by the cyano group leads to an energy lowering of 8.9 kcal mol . Kinetic parameters have been determined in the gas-phase at two temperatures, 217.8° and 259.5°: log kt,c = 13.73– 45.64/θ; log kc,t =13.86–44.43/θ. The rates of cis-trans interconversion of 1,2 - dicyano - 1 - methyl - cyclopropane(II) relative to those of I have been obtained by examination of mixtures of both substances in t-butylbenzene solution at 259.5°: 1.2-dicyano, kt,c= 1.25 and kc,t = 3.53; 1,2 - dicyano - 1 - methyl, kt,c = 8.09 and kc,t = 22.35 × 10−5 sec−1. The rate acceleration by methyl amounts to a factor of 6.4, corresponding to ΔΔG≠ = 1.96 kcal mol . A preliminary examination of optically active material leads to a minimum RA = 1.37 favoring rotation of (CN)(H) over (CN)(CH3).

CONSTITUTIONAL AND CONFIGURATIONAL ASPECTS OF THE THERMAL REORGANIZATION OF 1-CYANO-2-VINYLCYCLOBUTANE (I)

Kinetics and Arrhenius parameters have been determined for three processes: automerization, ring expansion and cycloreversion, occurring in the thermal reorganization of racemic cis and trans-I. Through the reorganization of optically active materials, automerization is trisected into enantiomerization and two epimeric diastereomerizations, and ring expansion is dissected into processes leading to enantiomeric 4-cyanocyclohexenes (II). Evidence pointing to concertedness being undiscernible, the system of reorganizations is classified as not obviously concerted and discussed in terms of relative propensities of groups to rotate internally. Comparison with similar cyclopropane systems reveals no qualitative differences (beyond cycloreversion) and only one substantial quantitative difference; that is, an enhanced tendency in the cyclobutane toward ring expansion.

MECHANISM IN THE SYSTEM OF DIMERS OF BUTADIENE

The question of concert or lack of concert in the Diels -Alder reaction can be approached through the construction of an energy diagram, on which are placed the transition state of the reaction and as many approximations to the hypothetical intermediate of the non-concerted process as possible. Depending on the extent to which the two processes differ significantly in energy, more or less definitive answers to the question may be obtained. New work on racemization and rearrangement of deuterium in optically active 4-vinylcyclohexene labelled with deuterium in the vinyl group affords a good fix on the energy required to make the octa-1, 7-dien-3, 6-diyl diradicals. When these data are combined with those obtainable from trans -1, 2-divinylcyclobutane, cycloocta-1, 5-diene, and the dimerization of butadiene itself, a small but not entirely secure concert is discernible in this Diels-Alder reaction. The butadienebutadiene, ethylene-butadiene and cyclopentadiene systems will be discussed in terms of a mechanism in which the concert of the Diels-Alder is viewed as a perturbation on the fundamental dimerization of olefins to non-interacting singlet diradicals in their most stable conformation.