Dynamic Effects on [3,3] and [1,3] Shifts of 6-Methylenebicyclo[3.2.0]hept-2-ene (original) (raw)
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The thermal rearrangement of 7,7-dimethylbicyclo[3.2.0]hept-2-ene
Tetrahedron Letters, 1994
Abtitracti. lls title cun~ when aubjectcd to p-phase pyrolysis at 275OC. undergoes predominantly fnemengtaotOV=disoktylme. We wish to report on the thermal behavior of 7,7dimethylbicyclo[3.2.0lhept-2-ene (l),l which was prepared from 7.7-dimethylbicyclo[3.2.O]hept-2-en-6-one2 viu our standard cyclobutanone reduction3 in an overall yield of 12% based on the ketene pncursor isobutyryl chloride. We have conducted a rigorous kinetic investigation4 of the gas-phase (275oC.) pymsylate5 of the title compound using methylcyclohexane as an internal standard. The rate constant for overall loss of 1 (kd) as well as the relative distribution of products among three rearrangement modes (see Scheme 1). direct [lJ]-hydrogen shift (kt~). indirect [lJ]-hydrogen shift (kts), and retro-[2+2] cycloteversion or fragmentation (1q),6 m w in Table 1. 4 Scheme 1. Thermal rearrangement of 1 The most favorable rearrangement mode for compound 1 is fragmentation to isobutylene and cyclopentadiene. However, two [ 1 j&hydrogen shif'ts also occur. The direct [lJ]-hydrogen shift, formally a retro-enc reaction, involves migration of a hydrogen from the endo-mthyl on C-7 to C-3 with concurtent migration of the pi bond and cleavage of the sigma bond between C-1 and C-7. The indhect [ 1,5]-hydrogen shift we attribute to a diradical-mediated process involving homolysis of the sigma bond between C-l and C-7 followed by a hydtogen shift from a methyl on C-7 to C-l.
Journal of the American Chemical Society, 1976
The mechanisms of the dimerization of butadiene and piperylene and the thermal rearrangements of the corresponding dimers are investigated by kinetic and stereochemical techniques. Particular attention is given to the question whether, in the Diels-Alder dimerization of the dienes la or Ib, intermediates are involved that are common to the 1,3-sigmatropic rearrangements of the corresponding [2 + 21 dimers 3a and 3b. Substituents on the terminal vinyl position of cis-1,2-divinylcyclobutane (Sa) retard the normal stereospecific boat-like Cope rearrangement to 3,4-dimethyl-cis,cis-cycloocta-1,5-diene and permit the detection of a new "nonboat" process, whicjl leads to a stereoisomeric prbduct. The boat-like rate constant declines with increasing terminal cis-methyl sustitution in the series Sa > cTT-8 > cCT-8 > cCC-8. The total range of the effect amounts to a factor of 1.81 X lo5. The trans-1,2-dipropenylcyclobutanes also give Cope rearrangement products, but this reaction occurs exclusively by an indirect mechanism: prior epimerization to the cis isomer followed by Cope rearrangement of the latter. The rearrangement of trans-3,4-dimethyl-cis,trans-cycloocta-1,5-diene (16) to cis-3,4-dimethyl cis,cis-cycloocta-1,5-diene (lo), involving overall epimerization at one asymmetric center and geometric isomerization at one olefinic site, proceeds by a two-step mechanism in which cis.-1,2-trans,trans-dipropenylcyclobutane (cTT-8) is an intermediate. The 1,3-sigmatropic rearrangement of (lR,2R)-(+)-trans-1,2-divinylcyclobutane (3a) gives (R)-(+)-4-vinylcyclohexene (2a) with 7.7% preservation of enantiomeric purity (corrected for competing racemization of 3a). This corresponds to 54% inversion and 46% retention of configuration of the migrant carbon. By attaching stereochemical labels to the terminal vinyl positions as in optically active tTT-9 and tCT-9, the stereochemistry of the 1,3-sigmatropic rearrangement can be subdivided into the four possible pathways (Schemes IX and X), suprafacial inversion, antarafacial retention, suprafacial retention, and antarafacial inversion. In this way, it can be shown that relative rates through these four pathways are, respectively, 50.2,6.0,41.1, and 2.7 from tTT-9, and 49.5, 2.8.46.8, and 0.9 from tCT-9. These results can be fitted by a biradical mechanism, but are more fruitfully interpreted as mainly the outcome of two competing concerted reactions, one allowed (supra facial inversion) and one forbidden (suprafacial retention). The absence of any substantial antara contribution in the dipropenyl systems rules out a stereorandom biradical intermediate in the tTT-9 and tCT-9 rearrangements and makes it unlikely in the divinyl system 3a. The Diels-Alder dimerization of trans-penta-1,3-diene-t~a~s-l-d (45, Scheme XIV) in both the exo and endo orientations gives exclusively the product of reaction cis-on-the-diene,&-on-the-dienophile. This is consistent with a concerted [4s + 2s]cycloaddition and rules out common intermediates in the formation of product tT-13 and cT-12 from the two alternative pathways of Diels-Alder dimerization of piperylene and 1,3-sigmatropic rearrangement of tTT-9. 3b-2b, 3b-. 4b, and 5b-. 4b, in the piperylene dimer series. A related study concerns the possibility that cycloocta-1,5dienes with a trans double bond may play a role in these rearrangements. Cope Rearrangements of cisand trans-1,2-Dipropenylcyclobutanes. The chair-like geometry normally favored in the acyclic Cope rearrangement'8,t9 should be difficult to achieve from cis-1,2-divinylcyclobutane because the small ring wopld resist the internal rotation needed to generate the true chair, and because, even if a quasi-chair conformation could be attained, the product, &,trans-cycloocta-1 $diene, would be severely strained.21 Although the transient intermediacy of the latter substance cannot be excluded on purely energetic ground^,^ the rearrangement of cis-1,2-divinylcyclobutane (5a) to cis,cis-cycloocta-1,5-diene (4a) usually is formulat-edI4,l5JS with a boat-like transition state, the free energy of which in the acyclic system normally lies about 6 kcal/mol above that of the chair.1s.20 Our studies support this formulation and, moreover, they show how cis-1,2-dialkenylcyclobutanes can be subjected to incremental steric effects that gradually deny access even to the "second-best" boat-like reaction. cis-Cyclobutane-1,2-dicarboxylic acid anhydride (6) serves as the starting material for the syntheses of the three cis-1,2-dipropenylcyclobutanes. Dimethyl cis-cyclobutane-1,2-6
Journal of Physical Organic Chemistry, 2018
Rate constants for overall decomposition (k d) for a series of exo-7alkylbicyclo[3.2.0]hept-2-enes are relatively invariant. For the alkyl substituents ethyl, propyl, butyl, isopropyl, and t-butyl, the ratio of the rate constant for [1,3] sigmatropic rearrangement to the rate constant for fragmentation, k 13 /k f , is significantly lower than k 13 /k f = 150 observed for exo-7methylbicyclo[3.2.0]hept-2-ene. Regardless of the size and mass of the alkyl group, the stereoselectivity of the [1,3] carbon migration appears to be quite stable at 80% to 89% suprafacial inversion (si), an observation consistent with conservation of angular momentum but not conservation of orbital symmetry. This global result comports with the phenomenon of "dynamic matching" espoused by Carpenter and collaborators for [1,3] sigmatropic rearrangements in general.
Ab Initio Study of the Pathways and Barriers of Tricyclo[4.1.0.02, 7]heptene Isomerization
The Journal of Physical Chemistry A, 2010
The thermal isomerization of tricyclo[4.1.0.0 2, 7 ]heptene has been studied using computational chemistry with structures determined at the MCSCF level and energies at the MRMP2 level. Both the allowed conrotatory and forbidden disrotatory pathways have been elucidated resulting in cycloheptatriene isomers. Four reaction channels are available for the conrotatory pathway depending on which bond breaks first in the bicyclobutane moiety leading to enantiomeric pairs of (E,Z,Z)-1,3,5-cycloheptatriene and (Z,E,Z)-1,3,5-cycloheptatriene intermediates. The activation barrier is calculated to be 31.3 kcal • mol-1 for two channels and 37.5 kcal • mol-1 for the other two. The lower activation barrier leading to the (E,Z,Z)-1,3,5-cycloheptatriene enantiomeric pair is proposed to be due to resonance within the transition state. The same behavior was observed for the disrotatory pathway with activation barriers of 42.0 kcal • mol-1 and 55.1 kcal • mol-1 for the two channels, again with one transition state resonance stabilized. The barriers for trans double bond rotation of the intermediate cycloheptatrienes are determined to be 17.1 and 17.4 kcal • mol-1 , about 5 kcal • mol-1 more than that for the seven carbon diene (E,Z)-1,3-cycloheptadiene. The electrocyclic ring closure of the trans cycloheptatrienes have been modeled and barriers determined to be 11.1 and 11.9 kcal • mol-1 for the formation of bicyclo[3.2.0]hepta-2,6-diene. This structure was previously reported as the end product for thermolysis of the parent tricyclo[4.1.0.0 2, 7 ]heptene. The thermodynamically more stable cycloheptatriene can be formed from bicyclo[3.2.0]hepta-2,6-diene through a two step process with a calculated pseudo first-order barrier of 36.4 kcal • mol-1. The trans-cycloheptatrienes reported herein are the first characterization of a small sevenmembered ring triene with a trans double bond.
Organometallics, 1994
Two results argue against a diradical intermediate in the exchange of diosmacyclobutanes with free olefins. The diosmacyclobutane Osz(CO)&-propene) reacts with vinylcyclopropane to give as the sole product a diosmacyclobutane bearing an intact cyclopropane ring. Repeated exchange of truns-ethylene-l,Z-d2 with the same ligand in a diosmacyclobutane shows > 99.1% stereochemical excess per exchange half-life. These exchange reactions do not involve mononuclear olefin complexes. (4) (a) Bowry, V. W.; Lusztyk, J.; Ingold, K. U. (11) The assessment of stereochemistry in these compounds, and the determination of their structure, by 'H NMR in nematic phase solvents will be reported separately: Bender, B. R.; Hembre, R. T.; Norton, J. R. Manuscript in preparation. (12) (a) Golike, R. C.; Mills, I. M.; Person, W. B.; Crawford, B., Jr.
Re-examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7-Octatriene
Chemistry - A European Journal, 2012
As part of our ongoing efforts to understand and design metal-promoted sigmatropic shifts, [1] we set out to examine the [3, 5]-sigmatropic rearrangements of 1,3,7-octatriene (Scheme 1) with an eye toward designing a metal-promoted variant. Sigmatropic shifts are widely utilized in synthetic organic chemistry, often as key steps in complex synthetic routes, [2] and have received considerable attention from practitioners of physical organic chemistry. [3, 4] Of the many sigmatropic shifts that have been studied, the [3, 3] sigmatropic shift of 1,5-hexadienes (Cope rearrangement) has perhaps received the most attention, both from synthetic organic and physical organic [7][9] chemists. The latter, in particular, participated in vigorous debates revolving around the nature of the mechanism for the Cope reaction, that is, whether it is concerted or stepwise, and the nature of the intermediate(s) involved, if it is the latter. It is now well estab-lished that the Cope reaction of 1,5-hexadiene itself is a concerted process, [8c, 10] but we wondered if the same would be true for the orbital symmetry-allowed [4] sigmatropic shift of 1,3,7-octatriene .
The Journal of Organic Chemistry, 2005
The three potential energy surfaces of the trans-trans, cis-trans, and cis-cis divinyltetramethylene diradicals have been located with DFT calculations at the BPW91/6-311+G** levels. The three surfaces account well for the experimental results reported for the thermolysis of optically active trans-1,2-divinylcyclobutane and optically active and deuterated 4-vinylcyclohexene. The surfaces account also for the outcome of the dimerization of butadiene and the thermolysis of cis,cis-1,5cyclooctadiene. The three diradical intermediates are connected to the cyclization and dissociation products through conformations that are explored fully here. The mechanism of the Diels-Alder (DA) reaction has attracted a huge number of investigations and the concerted mechanism has emerged 1 as the lowest energy mode of union of 4π and 2π addends in keeping with the orbital symmetry rules. 2 Only in special cases do twostep mechanisms compete or even predominate. 3 In this context the dimerization of butadiene BD 1 has attracted considerable attention since the major product (>85%) at 150-200°C is the DA cycloadduct 4-vinylcyclohexene (VCH) 2 along with smaller but significant (5-10%) amounts 4 of the isomeric trans-1,2-divinylcyclobutane (DVCB) t-3 and cis,cis-1,5-cyclooctadiene (COD) 4, derived from the forbidden [2+2] and [4+4] modes of union, respectively (Scheme 1). The reaction provides an interesting prototypal case where concerted and diradical paths may compete. The diradical path should involve the trans,trans-1,4-divinyltetramethylene (DVT) diradicals tt-5, which should undergo the typical closure 5 to cyclobutanes, affording the trans DVCB t-3 and the cis DVCB c-3. The latter is known 6 to undergo a rapid Cope rearrangement under the reaction conditions to COD 4. According to thermochemical estimates, 7 the cis,trans and cis,cis DVTs ct-5 and cc-5 lie at energies progressively higher than tt-5 owing to the destabilization (ca.
The thermal rearrangement of 7-ethyl-7-methylbicyclo[3.2.0]hept-2-ene (1)
Tetrahedron Letters, 1995
The title compound 1, when subjected to gas-phase pyrolysis at 275°C, undergoes predominantly fragmentation to cyclopentadiene and 2-methyl-l-butene. We wish to report on the thermal behavior of 7-ethyl-7-methylbicyclo[3.2.0]hept-2-ene (1), 1 which was prepared from 7-ethyl-7-methylbicyclo[3.2.0]hept-2-en-6-one 2 via our standard cyclobutanone reduction 3 in typical yield. Because all attempts at epimeric separation of compound 1 have proven futile when 1 was prepared by low-temperature Wolff-Kishner reduction of the ketene cycloadduct of cyclopentadiene and ethyl methyl ketene, we have synthesized each separate epimer of 1 (la and lb, see Scheme 1) from the corresponding ketone diastereomer. 2 We have conducted a rigorous kinetic investigation 4 of the gas-phase (275°C.) pyrosylate 5 of the title compound using nonane as an internal standard. The rate constant for overall loss of I (kd) as well as the relative distribution of products among three rearrangement modes (see Scheme 1), direct [1,5J-hydrogen shift (kl,5), indirect [1,5]-hydrogen shift (kl,5,), and retro-[2+2] cycloreversion or fragmentation (kf),6, 7 are reported in Table 1. Et Me 4 3 ~S fragmentation-R2 ] R'~';X'R [ 1,5I~H, indirect ~Me(Et) 2a (2h) Scheme 1. Thermal rearrangement of I
The Journal of Organic Chemistry, 1995
We have carried out calculations at the MP2/6-31G*/lRHF/6-31G* level on bicyclo[5.1.0locta-2,4diene (BCOD), 8-oxabicyclo[5.l.0locta-2,4-diene (&oxaBCOD), 6-oxabicyclo[5.1.Olocta-2,4-diene (6-oxaBCOD), and 6,8-dioxabicylo[5.l.0locta-2,4-diene (6,8-dioxaBCOD), otherwise 2,3-epoxyoxepin, t o determine whether the remarkable instability of 6,8-dioxaBCOD with respect to the fission of both the three-and the seven-membered ring giving eZzZz-muconaldehyde-a key step in the metabolic oxidation of benzene-is already apparent in either or both monooxygen derivatives. The effect of oxygen substitution is traced from reactions in which the overall structure is conserved, i.e. the cisoid Itransoid interconversion, the degenerate Cope rearrangement, and the 1,5-hydrogen shift in the bicyclic molecules, to the fission of both rings giving acyclic isomers. Oxygen substitution has little effect on the interconversion and the 1,5-hydrogen shifi, but the Cope rearrangement of 6,8-dioxaBCOD is much slower than that of BCOD. On the other hand, oxygen substitution has an incremental destabilizing influence on the ring fission reaction with respect to both thennodynamic and kinetic parameters. Kinetically, the double substitution in 6,8-dioxaBCOD exerts a destabilizing influence over and above the combined effects of the single substitutions in 8-oxaBCOD and 6-oxaBCOD, decreasing the activation energy further by some 10 kcal mol-l. The activation energies for the fission reactions of the three-membered ring in BCOD, in which cyclooctatriene and methylcycloheptatriene are formed, are far in excess of the activation energy for the fission of both rings. These results suggest that the fission of both rings of BCOD is a cooperative process.