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Since cyclopropanes are much more reactive than cyclobutanes despite their nearly identical strain energy, it has become clear that rate enhancement in cyclopropanes is partly due to strain release but also partly due to an additional factor about whose nature there is no consensus. Activation and equilibrium energies for a series of nucleophilic reactions of MeO-, MeS-, MeNH-, Me2N-, and MePH- with strained rings ranging from epoxide to aza-cubane were computed at the HF/631 + G* level. Using the Marcus equation, in combination with the computed intrinsic barrier for the identity reactions of the same nucleophiles with the strain free reference compounds (e.g., MeO- with Me-O-Me and Me2N- with Me3N), enabled a quantitative determination of the individual contributions of the partial strain release and the additional factor to the overall lowering of the transition state energy. Analysis of the data reveals the following: (a) There is no contribution of the additional factor to rate enhancement in four membered rings for first row elements (O and N) and only a small contribution (approximately 2 kcal) for second row elements (S and P). This is to be compared with a contribution of 7-17 kcal for three membered rings. (b) A significant synergistic effect is observed. Thus, in housane, for example, the additional factor amounts to 12.7 kcal which is more (by nearly 5 kcal) than the sum of the individual contributions of the isolated three and four membered rings. (c) The magnitude of the additional factor was found to be Periodic Table row dependent.

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6,6’-Dibromoindigo is a major component of the historic pigment Tyrian purple, arguably the most famous dye of antiquity. Over the last century, chemists have been interested in developing practical syntheses of the compound We describe herein a new, reasonably simple and efficient synthesis of Tyrian purple which opens the way to the production of large quantities of the dye with minimal hazards and at low cost.

This paper reports computational data for the energetics of internal attacks, both in ring-opening reactions (eq 3) where strain energy is released and in model, strain-free systems (eq 4). A comparison is drawn with the corresponding bimolecular processes. The exothermicity of three-membered ring-opening reactions is significantly larger than that of the four-membered ring systems. However, using the Marcus equation, it is shown that the higher reactivity of the three-membered rings is intrinsic to the system and does not stem only from a higher thermodynamic driving force. The intrinsic barriers for the strain-free reactions are shown to be dominated by the position of the nucleophilic and nucleofugic atoms in the periodic table, as in the bimolecular SN2 reactions, although a pi rather than a sigma bond is formed in these reactions.

Radical (neutral) and electrophilic (cationic) ring opening reactions were studied computationally in order to probe the difference in reactivity between three and four membered rings. Using the Marcus equation we have shown that the activation energy for the four membered ring opening is close to the Marcus predicted barrier whereas three membered rings display much higher reactivity than that predicted by the Marcus equation. Thus, the reactivity of the three membered rings is enhanced, in addition to the strain release, by another factor which is not operative in the four membered rings. It is clear also that this factor is not charge dependent. The possible origin of this effect is discussed.

Over the past century, various synthetic approaches have been suggested to the most famous dye of antiquity, Tyrian purple (6,6′-dibromoindigo). These synthetic routes have been exhaustively surveyed and critically evaluated from the perspective of convenience, cost, safety and yield.