Kinetic and equilibrium study of gas-phase interconversions of 1,3,6-cyclooctatriene, 1,3,5-cyclooctatriene and bicyclo[4.2.0]octa-2,4-diene

The gas-phase equilibrium and rate constants for the isomerizations of 1,3,6-cyclooctatriene (136COT) to 1,3,5-cyclooctatriene (135COT) [reaction (1)] and bicyclo[4.2.0]octa-2,4-diene (BCO) to 135COT [reaction (-2)] have been measured between 390 and 490 K and between 330 and 475 K, respectively. The rate constant of reaction (1) obeys the Arrhenius equation The corresponding equilibrium constant is given by the van′t Hoff equation The strain energy of the 136COT ring is calculated to be 31.7 kJ/mol, based on the known value of 37.2 kJ/mol for 135COT, and ΔH(298 K) for gaseous 136COT is 196.3 kJ/mol. The rate constant of reaction (-2) obeys the Arrhenius equation The equilibrium constant for 135COT ? BCO fits the van′t Hoff equation The strain energy of the BCO skeleton is calculated to be 108.3 kJ/mol, and ΔH(298 K) for gaseous BCO is 183.3 kJ/mol.     

Isomerization of bicyclo[5.1.0]-2,4-octadieneiron tricarbonyl to bicyclo[4.2.0]-2,4-octadieneiron tricarbonyl

It has been shown that bicyclo[5.1.0]-2,4-octadieneiron tricarbonyl undergoes thermal isomerization to bicyclo[4.2.0]-2,4-octadieneiron tricarbonyl. The first-order rate constant for this process at 120.6° was determined to be 1.8 X 10^?5 s^?1 which corresponds to ΔF³ = 31.8 kcal/mol. Gas chromatographic results coupled with a kinetic analysis very strongly suggest that 1,3,5-cyclooctatrieneiron tricarbonyl is an intermediate in this isomerization. Two plausible mechanisms are offered to explain this rearrangement.     

Direct and sensitized photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase

The direct photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase at 280–300 nm produces mainly 1,3,5-cyclooctatriene and benzene plus ethylene. The yield of the former product is enhanced by added gases, and it is proposed that it is formed in a vibrationally excited state which can revert to bicyclooctadiene unless the excess energy is removed in collisions. Computer modelling of the direct photolysis yielded quantitative agreement with the experimental results, but only when large, arbitrary adjustments were made to the calculated rate constants for the interconversion of cyclooctratriene and bicyclooctadiene. The Hg(6³P₁) sensitized reaction of bicyclooctadiene produces mainly benzene plus ethylene, a process which is also enhanced by added gases.     

Probing the formation of bicyclo[4.2.0]octan-1-ols

Reaction of lithium enolates of simple ketones with (+/-)-phenyl vinyl sulfoxide has potential for the convergent construction of complex fused ring systems containing a bicyclo[n.2.0]alkan-1-ol. The formation of sulfinylbicyclo[4.2.0]octan-1-ols 1-3 from the lithium enolate of cyclohexanone with (+/-)-phenyl vinyl sulfoxide or (R)-(+)-p-tolyl vinyl sulfoxide 18 was used to probe the mode of this novel cyclization reaction. Using phenyl vinyl sulfoxide, variations in the reaction lighting and solvent were investigated, in conjunction with radical trapping (TEMPO) and isotope labeling (deuterium) experiments. Cyclization to form sulfinylbicyclooctanols 1-3 is likely to proceed via an intermediate that ring closes to the bicycloalkanol anion 11 and was presently favored by the use of solvents such as THF or DME.     

Polar substituent effects in the bicyclo[1.1.1]pentane ring system: acidities of 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids

Experimental gas-phase acidities are reported for a series of 3-substituted (X) bicyclo [1.1.1]pent-1-yl carboxylic acids (1, Y = COOH). A comparison with available calculated data (MP2/6-311++G**// B3LYP/6-311+G**) reveals good agreement. The relative substituent effects are shown to be adequately described by a much lower level of theory (B3LYP/6-31+G*). Various correlations are presented which clearly point to polar field effects as being the origin of the relative acidities.     

A Facile Synthesis of Bicyclo[4.2.0]octa-1,3,5-trien-3-ol

Bicyclo[4.2.0]octa-1,3,5-trien-3-ol (2) has been synthesized in good yield via a Baeyer-Villiger reaction of the corresponding aldehyde 5 with permonophosphoric acid.     

The synthesis and photochemistry of bicyclo[4.2.0]octa-2,4,7-trienes

Two new crystalline bicyclo[4.2.0]octa-2,4,7-trienes have been prepared by photodecarbonylation of the appropriate tricyclo[4.2.1.0^2,5]nona-3,7-dien-9-ones and their structures confirmed by ¹³C-n.m.r. The title compounds all undergo thermal ring opening to cyclooctatetraenes, but photolysis is substituent dependent.     

"Endo" and "exo" bicyclo[4.2.0]-octadiene isomers from the electrocyclization of fully substituted tetraene models for SNF 4435C and D. control of stereochemistry by choice of a functionalized substituent

[reaction: see text] A tandem electrocyclic closure, perceived as the key step in a biomimetic approach to SNF 4435C and D, was tested with 1,1,8-trisubstituted tetraene substrates. The ratio of endo:exo products could be controlled by the choice of the RZ substituent at C-1. On the basis of these results, a short stereoselective route to an advanced SNF 4435 intermediate was devised.     

Effects of annulation on the mass spectral behaviour of monosubstituted bicyclo[4.3.0]-3-nonenes and bicyclo[4.3.0]-3,7-nonadienes

The mass spectra of the title compounds differ considerably depending on the annulation of the bicyclic skeleton. For a series of alkyl derivatives the cis-annulated compounds show a higher abundance of the products due to the retro Diels-Alder cleavage than the corresponding trans isomers. The cis-annulated esters and alcohols lose a molecule of ethanol and water, respectively, more easily than the corresponding trans isomers. Effects of conformation and configuration are discussed.     

Synthesis of unsymmetrical 2,4-dialkylpyrazolo[1,5-a]-1,3,5-triazines

The reaction of 3-aminopyrazole with imidate esters such as ethyl acetimidate, gave N-(pyrazol-3-yl)acetamidine (1) rather than the isomeric 2-acetamidoyl-3-aminopyrazole. Ring closure of 1 with orthoesters such as ethyl propionimidate, afforded unsymmetrically substituted 2.4-dialkylpyrazolo[1,5-a]-1,3,5-triazines such as 4-ethyl-2-methylpyrazolo[1,5-a]-1,3,5-triazine (3). The structure of 1 was confirmed by several alternate syntheses. The unique feature of this two-step synthetic approach to the synthesis of pyrazolo[1,5-a]-1,3,5-triazines is that it is a convenient method of preparing fused triazines based on available pyrazoles rather than the less accessible dialkyltriazines.     

Microwave-assisted intramolecular [2 + 2] Allenic cycloaddition reaction for the rapid assembly of bicyclo[4.2.0]octa-1,6-dienes and bicyclo[5.2.0]nona-1,7-dienes

Microwave irradiation of alkynyl allenes affords an intramolecular [2 + 2] cycloaddition reaction. This cycloaddition provides an efficient route to bicyclomethylenecyclobutenes. The reaction occurs with complete regioselectivity for the distal double bond of the allene for the selective formation of a variety of hetero- and carbocyclic substrates. Bicyclo[4.2.0]octadienes and bicyclo[5.2.0]nonadienes have been prepared in high yield. [reaction: see text]     

Thermal [2+2] cycloaddition of allenynes: easy construction of bicyclo[6.2.0]deca-1,8-dienes, bicyclo[5.2.0]nona-1,7-dienes, and bicyclo[4.2.0]octa-1,6-dienes

The simple refluxing of allenynes, having a phenylsulfonyl functionality on the allenyl group, in xylene (or mesitylene) without microwave irradiation resulted in the efficient formation of bicyclo[5.2.0]nona-1,7-dienes and bicyclo[4.2.0]octa-1,6-dienes in high yields. This method was shown to be successfully applicable to the first construction of bicyclo[6.2.0]deca-1,8-dienes. Construction of the corresponding oxa- and azabicyclo[m.2.0] frameworks could also be attained. This thermal ring-closing reaction involves the formal [2+2] cycloaddition in which the distal double bond of an allenyl moiety exclusively served as one of the pi-components regardless of the position of the phenysulfonyl functionality on the allenyl moiety.     

Ring closure of cyclooctatetraenetricarbonyliron complexes: Crystal structure of a bicyclo[4.2.0]-2,4,7-octatrienetricarbonyliron compound

Mono- or di-substituted cyclooctatetraenetricarbonyliron complexes on heating yield isomeric bicyclo[4.2.0]-2,4,7-octátriene compounds; in contrast, the carbonylruthenium analogues afford polynuclear species.     

Synthesis of Si- and N-containing bicyclo[4.2.1]nona-2,4-dienes and bicyclo[4.2.1]nona-2,4,7-trienes

A [6π+2π] cycloaddition of Si- and N-containing alkynes and 1,2-dienes to cyclohepta-1,3,5-trienes in the presence of the two-component catalytic system (acac)₂TiCl₂-Et₂AlCl gives rise to the corresponding bicyclo[4.2.1]nona-2,4-diene and bicyclo[4.2.1]nona-2,4,7-triene derivatives.     

Deuterium kinetic isotope effects and mechanism of the thermal isomerization of bicyclo[4.2.0]oct-7-ene to 1,3-cyclooctadiene

The thermal conversion of cis-bicyclo[4.2.0]oct-7-ene to cis,cis-1,3-cyclooctadiene might involve a direct disrotatory ring opening, or it might possibly take place by way of cis,trans-1,3-cyclooctadiene. This cis,trans-diene might possibly form the more stable cis,cis isomer through a [1,5] hydrogen shift or a trans-to-cis isomerization about the trans double bond. Deuterium kinetic isotope effect determinations for the isomerizations of 2,2,5,5-d(4)-bicyclo[4.2.0]oct-7-ene and 7,8-d(2)-bicyclo[4.2.0]oct-7-ene rule out these two alternatives because the observed effects are much smaller than would be anticipated for these mechanisms: k(H)/k(D)(d(4)) at 250 degrees C is 1.17 (1.04 per D), and k(H)/k(D)(d(2)) at 238 degrees C is 1.20 (1.10 per D). The direct disrotatory ring opening route remains the preferred mechanism.     

Substituent effects in the interconversion of phenylcarbene, bicyclo[4.1.0]hepta-2,4,6-triene, and 1,2,4,6-cycloheptatetraene

The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functional theory. It is found that substituents have a large effect on both the thermochemistry and activation energy of these rearrangements. For instance, para-substitution yields a range of overall activation energies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH(2) and NO(2) substituents, respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTE species are more exothermic than that of the parent PC. The proximity of the substituent to the carbene center can also affect the overall chemistry as in the case of ortho-substituted species. Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures, can compete with the ring-expansion process. Also, as calculated herein, the ortho substituents can, by a combination of mesomeric and steric interactions with the carbene center, affect the overall barrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens (F and Cl) raise the activation barrier to ring expansion by approximately 5 kcal/mol. This is reminiscent of the effect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.     

Long-range deuterium isotope effects on fluorine-19 chemical shifts of 4-substituted bicyclo[2.2.2]octyl- and bicyclo[2.2.1]heptyl-1-fluorides : Deuterium as a substituent

²H/¹H isotope effects on the ¹⁹F chemical shifts of 4-substituted bicyclo[2.2.2]oct-1-yl and bicyclo[2.2.1]hept-1-yl fluoride are significantly shielding and deshielding, respectively. This result is consistent with deuterium being viewed as an $\text{electronegative}$ substituent relative to hydrogen when attached to an sp³ hybridised carbon.