The Valence Isomerization of Cyclooctatetraene to Semibullvalene

The Journal of Physical Chemistry A, 1998

We report herein the results of a high-level ab initio study of the D 2d and C 2h structures of 1,3,5,7cyclooctatetraene, COT, and bicyclo[4.2.0]octa-2,4,7-triene, BOT. This information has been used to obtain, seemingly for the first time, the standard heat of formation of COT in the gas phase. Values obtained by means of standard atomization reaction are higher than the experimental value (4.7 kcal mol -1 at the G2 level). When isodesmic or homodesmotic reactions are used, the computational and experimental values agree well within the experimental uncertainties.

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 Organic Chemistry, 1990

In an effort to understand the substantially lower activation energy for the 3,3-sigmatropic shift of cis-1,2divinylcyclopropane relative to that of cis-1,2-divinylcyclobutane, we determined the secondary deuterium kinetic isotope effects (KIEs) at the vinyl termini of both materials. For the cyclopropane derivative, kH/kd4 = 1/1.29 (*0.09) at 10 "C in deuteriochloroform. For the cyclobutane derivative, kH/kd4 = 1/1.04 (*0.02) at 77.7 "C in hexachlorobutadiene solvent. These KIEs indicate that there is much more bonding at the terminal carbons in the transition state for the rearrangement of the cyclopropane analogue and relatively little bonding at the termini in the rearrangement of the cyclobutane material. (8) See ref 3c and see: Giinther, H.; Pawliczek, J. B.; Ulman, J.; Grimme, W. Angew. Chem., Int. Ed. Engl. 1972,11,517; Chem. Ber. 1975, 108,3141. See also: Bicker, R.; Kessler, H.; Ott, W. Ibid. 1975,108,3151. (9) (a) Gunther, H.; Ulmen, J. Tetrahedron 1974,30,3781 and reference contained therein. (b) For the first synthesis of bullvalene, see: Schroder, G. Angew. Chem., Int. Ed. Engl. 1963,2, 481. (10) (a) Cheng, A. K.; Anet, F. A. L.; Mioduski, J.; Meiwald, J. J. Am. Chem. SOC. 1974,96,2887. (b) For the first synthesis of semibullvalene, see: Zimmerman, H. E.; Grunewald, G. L. Elsevier: New York, 1987; Vol. 7, p 61. There are unfortunate typos in Appendix B: the ratio of the hydrogen-deuterium equilibrium constants must be reversed, and the equations refer to two deuteriums, not four. (12) Engel, P. S. Chem. Reu. 1980,80, 99. 1990,55,679-684 679

The Journal of Organic Chemistry, 1988

The thermal equilibration between cis,cis,cis and cis,cis,trans isomers of 2,4,6-octatriene at 111 "C occurs both by an indirect route, with cis,cis-1,3,5-octatriene as reactive intermediate, and by a direct path. In some of the earliest and most influential quantitative kinetic and stereochemical work related to thermal electrocyclic isomerizations of 1,3,5-hexatrienes to 1,3-cyclohexadienes, two groups reported in 1965 that the trans,cis,trans isomer of 2,4,6-octatriene gave cis-5,6-dimethylcyclohexa-1,3-diene with high stereoselectivity, reflecting the disrotatory course of this allowed pericyclic conversion.'P2 The cis,cis,cis and cis,cis,trans isomers, however, interconverted with one another so rapidly that they could not be used to gain sure stereochemical evidence on the slower electrocyclic rea~tions.~ Marvell and co-workers4 postulated that this facile reversible isomerization between cis,cis,cis-and cis,cis,trans-trienes 1 and 3 occurred through [1,7] sigmatropic hydrogen shifts and an unstable, undetectable intermediate, cis,cis-1,3,5-octatriene (2).

Chemical Physics Letters, 1987

Ab initio calculations up to the MP4SDQ16-31G* level are reported for the chair Cope rearrangement of 1,5-hexadiene. These are consistent with a recent AMI study, which showed the reaction to take place via a biradicaloid intermediate. Recent AM 1 [ I] calculations [ 21 seem to have established that the chair [ 31 Cope rearrangements [4] of 1,5-hexadiene (1) and its derivatives take place by the non-synchronous mechanism suggested by Doering et al. [ 51, involving a biradicaloid #' derived from the 1 ,Ccyclohexylene biradical , as a symmetrical intermediate (SI) . These reactions had earlier been assumed to be typical synchronous "allowed" pericyclic processes [ 6 ] taking place via aromatic [ 71 transition states (TS; 3), a mechanism supported by a recent ab inito MC SCF study [ 81 of the chair rearrangement of 1. The two mechanisms can be distinguished by the (equal) lengths of the forming (CC,) and breaking (C,C,) bonds in the SI, these being expected [ 21 to be > 2 8, in an aromatic SI (3) but < 1.7 A in the biradicaloid (2). The lengths found in the AM1 [ 21 and ab initio [ 81 studies were 1.65 and 2.06 A, respectively. / 2 3

Journal of Molecular Structure: THEOCHEM, 1997

This contribution describes the elucidation of two consecutive phototransformations of the radical cation ofcyclooctatetraene (COT.+) which were first noted by Shida and Iwata in 1972 from changes in the optical spectra. Here we highlight some particular features of the electronic and molecular structure of the involved species. In addition to aspects which were already discussed in our original publications on the subject (cf. 14-71) we focus on the question of localized versus delocalized structures for the primary photoproduct of COT.'. i.e. the radical cation of bicyclo[3.3.0]octa-2,6-diene-4&diyl (BOD,').

2008

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.

Angewandte Chemie International Edition, 2010

In the quest to harness solar energy for power generation, most efforts are centered around photoinduced generic charge separation, such as in photovoltaics, water splitting, other small-molecule activation, and biologically inspired photosynthetic systems. In contrast, the direct collection of heat from sunlight has received much less diversified attention, the bulk devoted to the development of concentrating solar thermal power plants. An attractive alternative would be to trap solar energy in the form of chemical bonds, ideally through the photoconversion of a suitable molecule to a higher energy isomer, which, in turn, would release the stored energy by thermal reversal. Such a system would encompass the essential elements of a rechargeable heat battery, with its inherent advantages of storage, transportability, and use on demand. The underlying concept has been explored extensively with organic molecules (such as the norbornadienequadricyclane cycle), often in the context of developing photoswitches, although a functioning device has yet to be reported. Organometallic complexes have remained rela-tively obscure in this capacity. A highly promising organometallic system is the previously disclosed, robust photothermal fulvalene (Fv) diruthenium couple 1Q2 (Scheme 1). [7a] Kinetic, stereochemical, and attempted trapping experiments led, in the absence of compelling evidence to the contrary, to the postulation of concerted pathways for these isomerizations. Here we present a theoretical investigation, in conjunction with corroborating experiments, of the mechanism for the heat-releasing step of 2!1 and its Fe and Os relatives, painting a surprising picture.

Computational and Theoretical Chemistry, 2012

The mechanism for the thermal isomerization reactions of 2-methyl-4,5-dihydrofuran was investigated using quantum chemical calculations at B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), and [PBE/6-31G(d,p), PBE/6-31++G(d,p)] levels of theory. It was found that 2-methyl-4,5-dihydrofuran isomerizes to acetylcyclopropane, and by a parallel reaction a slower isomerization to give 3-pentene-2-one. The acetylcyclopropane formation occurs through unimolecular electrocyclic mechanism. The 3-penten-2-one formation also takes place through electrocyclic mechanism, involving [1,2] hydrogen migration. The isomerization reaction of acetylcyclopropane to 3-penten-2-one occurs by step-wise mechanism, with the formation of an intermediate product 2-hydroxy-2,4-pentadiene, which subsequently isomerizes to the keto form, 3-penten-2-one. The stepwise acetylcyclopropane isomerization to 3-penten-2-one has lower energy of activation than the direct conversion of 2-methyl-4,5-dihydrofuran to 3-penten-2-one. Reasonable agreement was found between experimental and calculated energies of activation using B3PW91/6-311G(d,p) and MPW1PW91/6-311G(d,p) methods. Results suggest that both isomerization reactions pathways are possible under the experimental conditions reported. However, the lower energy of activation of the rate determining step of the step-wise mechanism favors this process over the single step mechanism.