Wolfgang R. Roth

Thermal Isomerisation of Substituted Semibullvalenes and Cyclooctatetraenes – A Kinetic Study

Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclooctatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irreversibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarbonitriles 14 and 15. Arrhenius and Eyring parameters of these thermal isomerisation reactions have been determined for the gas phase (1b 2b) and [D6]benzene solutions (1b 2b, 13 14 + 15). Furthermore, the activation parameters of the known rearrangement of octamethylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determined. – The data for these compounds, together with those for related compounds previously reported in the literature, show that substituents not only influence the relative stabilities of the semibullvalene and cyclooctatetraene systems but also the height of the energy barrier for their interconversion. Those substituents that lower the barrier toward the degenerate Cope rearrangement of semibullvalenes simultaneously accelerate their rearrangement to cyclooctatetraenes thus limiting the thermal stability of the former.

7-Methyl- and 7-phenylcyclohepta-1, 3, 5-trienes from benzvalene via 3, 3a, 4, 5, 6, 6a-hexahydro-4, 5, 6-methenocyclopentapyrazoles and tetracyclo[4. 1. 0. 0.0] heptanes

Abstract The addition of benzvalene (1) to diazomethane, diazoethane, 2-diazopropane, phenyldiazomethane, and diphenyldiazomethane afforded the 1-pyrazolines 2a-g in good yields. By means of competition experiments, the relative reactivities of benzvalene (1) and norbornene with regard to diazomethane and 2-diazopropane have been determined. The fact that benzvalene reacts about twice as fast as norbornene with both diazoalkanes cannot be rationalized on the basis of frontier orbital energies. On direct photolysis, the pyrazolines 2a-g were converted into the tetracyclo[4. 1. 0. 02,4. 03,5] heptanes 4a-g exclusively. These compounds gave the 1, 3, 5-cycloheptatrienes 5a, b, d, e, g in high yields on treatment with silver ions, thus providing better access to 7, 7-dimethyl-(5d) and 7, 7-diphenylcycloheptatriene (5g) than before. Surprisingly, the latter compound is in equilibrium with a substantial quantity of the norcaradiene form. - The heat of reaction for the rearrangement of 4a to 5a has been determined, which allows to derive the heat of formation of tetracyclo[4. 1. 0. 02,4. 03,5] heptane (4a).

The Barrier to Rotation about the Double Bond in Methylenecyclopropane

Activation parameters have been determined in the gas phase for the geometrical isomerization of the double bonds in two phenyl-substituted methylenecyclopropanes (2 and 8). Comparison with the analogous values for three sterically non-restricted styrene derivatives (15, 16 and 17) shows that the enthalpies of activation for the methylenecyclopropanes are lower by 3.7 kcal mol−1. This value, which is an upper limit to the amount of strain energy released by pyramidalization of the ring carbon atom in the orthogonal diradical transition state when undergoing geometrical isomerization, is considerably smaller than the value of 12-14 kcal mol−1 by which the strain energy of methylenecyclopropane is larger relative to that of cyclopropane. Our kinetics experiments thus show that the angle strain, associated with incorporating a trigonal carbon atom into the three-membered ring, is not responsible for the majority of the additional strain energy of methylenecyclopropane, relative to that of cyclopropane.

6,7-Dimethyltricyclo[3.2.2.0]non-6-ene at 120 K, a [3.2.2]propellane derivative

The title compound belongs to the group of [3.2.2]-propellanes. Bond angles inside the cyclobutane and cyclobutene units involving the carbon atoms C1 and C2 are: C2—C1—C6 89.81 (4), C1—C2—C5 89.93 (4), C2—C1—C4 86.07 (4) and C1—C2—C3 85.93 (4)°.

(E)-3,4-Diisopropyl-2,5-dimethylhex-3-ene at 125K

The title compound, C16H32, belongs to the family of sterically crowded alkenes. The molecule has a normal C3—C4 double-bond length of 1.3686 (14) A. The torsion angles around this bond are in good agreement with the results of force-field calculations.

2,2,3,3-Tetrachloro-1,4-diphenylbicyclo[2.2.0]hexane, a Strained Bicyclic System

The title compound belongs to the family of bicyclo[2.2.0]hexanes. The bond lengths C1—C4 and C3—C4 of the two four-membered rings are significantly longer than the average bond length for such ring systems [1.554 (21) A] which is obviously a result of angular strain. Experimental bond lengths in A: C1—C4 1.607 (3), C3—C4 1.592 (3), C2—C3 1.588 (3), C1—C6 1.577 (3), C4—C5 1.543 (3), C1—C2 1.535 (3), C5—C6 1.535 (4).

cis-8-Methylbicyclo[4.2.0]octa-1,3,5-triene-7-carboxylic Acid at 293K

The molecules of the title compound are associated as dimers by hydrogen bonds H(1)—O(2)′. Additional characteristic atom distances of the dimer: O(1)—O(2)′ 2.650 (3) (intermolecular), H(1)—O(2)′ 1.74 (intermolecular), O(1)—H(1) 0.91, C(10)—O(2) 1.225 (4) and C(10)—O(1) 1.293 (3) A. The C(10)—O(2) bond distance is slightly longer than the standard distance range for such dimers of 1.205 to 1.215 A whereas the C(10)—O(1) bond distance is slightly shorter than the standard distance of 1.308 A.