The Effects of Substituents on the Degenerate Cope Rearrangement of Semibullvalenes and Barbaralanes

Abstract

The activation parameters [∆G (200 K), ∆H , and ∆S ] for compounds, together with those for related compounds previously reported in the literature, show that conjugating the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a subsituents (e. g. aryl, CN, CO 2 R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas number of their 3,7-and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF 3 , SO 2 Ar) retard the reaction. spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, Substituents in the 3,7 positons have little effect or are rateretarding. The presence of 1,5-tri-and -tetramethylene and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5hϪ8h. Careful attention has been given to the bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former. measurement of temperatures. The data for these

A C2v-Symmetric Barbaralane

“…This is confirmed by consideration of the HOMOs and the electron localization function (ELF; see below). Note that for some substituted semibullvalenes it was previously observed that shortening of the 1,5‐bridge is accompanied by opening of the cyclopropane ring,4 as can be seen in tetraphosphabarbaralane in Figure 2. Thus, the symmetrical phosphabarbaralane is achieved by two geometrical changes: 1) the C1‐C9‐C5 bridge is shortened, and 2) the angle at C1 is enlarged, that is, the C2C8 bond is elongated.…”

“…Much effort was made to find a symmetrical ground‐state structure by studying various derivatives experimentally and theoretically (e.g., semiempirical MO calculations2, 3). The most valuable overview was given by Quast et al4 Additional reviews can be found in refs. 1, 5, 6 and references cited therein.…”

“…From the possible derivatives of 1 we chose to replace the 2,4,6,8‐positions of barbaralane by heteroatoms (Figure 1). The use of nitrogen or oxygen has been suggested 4. A derivative of 2,4,6,8‐tetraazabarbaralane ( 2 a ) was studied before; it does not give a symmetrical ground state ( 2 b or 2 c ), but the activation barrier of the Cope rearrangement is lowered relative to that of the carbon analogue 7.…”

“…7, made for Δ G ≠ , and added it to the experimental value of Δ H ≠ from ref. 4. However, for estimating the increment Moskau et al7 already made the following assumptions: 1) a methoxy is comparable to an ethoxy group, 2) methyl groups in positions 1 and 5 are negligible, 3) the situation in the semibullvalene is compared to the case of barbaralane, and 4) substitution at positions 3 and 7 is equivalent to substitution at positions 2 and 6.…”

The Barrier to Rotation around N−O Bonds in 4-Substituted N-Isopropoxythiazole-2(3H)-thiones and in N-Isopropoxy-2(1H)-pyridinethione

“…[3Ϫ9] These values were calculated from coalescence temperatures in variable temperature NMR experiments and therefore, strictly speaking, refer only to a single temperature. Recent advances in NMR technology, especially in currently available devices for controlling temperature stability in variable temperature NMR experiments, [10] have encouraged us to readdress the problem of investigating rotational NϪO barriers over a broader temperature range in order to derive more significant activation parameters (∆H ‡ , ∆S ‡ ) for this dynamic process. In view of the above mentioned issues we selected a small set of newly available heterocyclic N-alkoxides as probes [N-Isopropoxy-2(1H)-pyridinethione (1) and the 4-substituted thiazolethiones 2 and 3, Figure 1 …”