The Heat of Formation of Cyclobutadiene

Abstract

Cyclobutadiene (1) and its derivatives have beckoned to chemists ever since Kekules deduction of the structure of benzene and his attempt to synthesize 1 in 1872.[1] Pioneering efforts by Willstätter and Finkelstein were followed by a century of studies which produced many remarkable findings. Room temperature stable derivatives such as tri-tert-butylcyclobutadiene [2] and tetra-tert-butylcyclobutadiene [3] were successfully prepared and spectroscopically characterized, whereas the parent compound was found to be more elusive. It dimerizes in solid matrices at ! 35 K, is a transient reactive intermediate in solution, and has a lifetime of only 2 ms at 0.1 Torr in the gas phase.[1a] Nevertheless, trapping [4] and spectroscopic results have revealed that cyclobutadiene has a ground-state singlet configuration and adopts a rectangular D 2h structure which rapidly undergoes automerization. An isolable and room temperature stable complex consisting of 1 in the cavity of a spherical crown ether (that is, a hemicarceplex) has even been prepared, [5] but the thermodynamic stability of cyclobutadiene remains experimentally unknown.Conventional calorimetric methods are precluded when it comes to cyclobutadiene and its simple derivatives because of their high reactivity. Electrochemical, [6] pK a , [7] and kinetic [8] measurements of model compounds have been carried out and interpreted as indicating that 1 has a negative resonance energy of at least 50-67 kJ mol À1 . This conclusion is in accord with many of the early computational findings and a

The Thermal Retro[2+2+2]cycloaddition of Cyclohexane Activated by Triscyclobutenannelation: Concerted All‐Disrotatory versus Stepwise Conrotatory Pathways to Fused [12]Annulenes

“…In summary, with hydrocarbon 2, we have synthesized and structurally characterized the first example of the long-sought all-cis triscyclobutenocyclohexane framework, along with its hexadehydro (6 a) and benzofused analogues (3)(4)(5) cycloreversion, 2 appears to prefer stepwise conrotatory cyclobutane ring openings as the mechanism of disassembly. The barriers for both processes are close-lying, in particular for the mixed systems 3 and 4.…”

“…[2c,d] In contrast, the double bonds in 1 should not only activate by introducing further strain, [4] but also enable additional orbital symmetry control in the form of stepwise conrotatory cyclobutene ring openings, conflicting stereochemically with a concerted all-disrotatory unraveling of the cyclohexane ring. While conrotatory motion is seemingly constrained in bicyclo[n.2.0]alkenes, leading to much speculation regarding the feasibility of forbidden disroratory pathways in such systems, [5] cis-bicyclo[4.2.0]oct-7-ene, the pertinent subunit of 1 (highlighted), chooses this option.…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…[3,4] Thus they have provided a solid foundation for the discussion on the thermochemical consequences of antiaromaticity, and I take this occasion to reflect on recent developments in this field and to point out the pitfalls with which the resulting discussion is fraught.…”

“…If one compares the recently determined heat of formation of CBD (429 AE 16 kJ mol À1 ) [3] with that obtained by adding four polyenic C d -(H)(C d ) group increments (113 kJ mol À1 ), [18] one is left with a whopping destabilization energy of 316 AE 16 kJ mol À1 . One then faces the problem of how to figure out which part of this energy results from some form of strain, and which results from antiaromatic destabilization.…”

“…In this case the antiaromatic destabilization energy is given by the enthalpy change for the isodesmic Reaction (1). [3] However, the above assumption merits some consideration. The fact that the strain energy increases by a mere 4 kJ mol À1 on going from cyclobutane to cyclopropane indicates that the energetic cost of angular deformation (Baeyer strain) cannot account for this property alone.…”