Coarctate and Pseudocoarctate Reactions: Stereochemical Rules

Abstract

Coarctate reactions are a distinct class of concerted reactions. They differ from other concerted reactions, such as S(N)2, E2, or pericyclic reactions, in such a way that atoms are involved in the transition state at which two bonds are made and two bonds are broken simultaneously. Similar to pericyclic reactions, an electron count can be used to predict reactivity and stereochemistry. The "coarctate rules" are derived from a topological variant of the aromaticity of the transition state principle. Pseudocoarctate transition states exhibit disconnections in the topology of orbital overlap and do not necessarily follow the coarctate rules.

Helical orbitals and circular currents in linear carbon wires

“…In the broad sense, they are fully delocalized molecular orbitals spanning the full length of the molecule. The rectilinear porbitals of the unsubstituted [4]cumulene ( Fig. 1a) do not extend from the p-orbital on rst carbon atom onto the porbital of the last carbon atom, the helical p-orbitals of the substituted molecule do (Fig.…”

“…In the C 2 point group there are no degenerate irreducible representations and consequently the HOMO and HOMOÀ1 are explicitly non-degenerate in the disubstituted molecule. However, every pair of helical p-orbitals remain quasi-degenerate; in the case of S-1,5-dichloro- [4]cumulene the energy difference between the HOMO and HOMOÀ1 is only 6 meV. The energetic splitting between the near-degenerate pairs of helical orbitals is important for several reasons.…”

“…In Fig. 3, the three optimized conformations of S-1,5-diamino- [4]cumulene are shown. The two hydrogen atoms on each amine bend, which is generally perceived as the consequence of the nitrogen having a lone-pair orbital.…”

“…While the substituted molecule is one of two enantiomers, here the S-enantiomer, both chiralities are present in the electronic structure. The highest occupied molecular orbital (HOMO) of S-1,5-dichloro- [4]cumulene Fig. 1 Optimized molecular structure and the two highest occupied molecular orbitals of [4]cumulene and S-1,5-dichloro- [4]cumulene.…”

“…2 As we presented in recent work, the electronic structure of even [n]cumulenes is best understood by considering them as coarctate Möbius systems. 3,4 The helical molecular orbitals are an intrinsic property of such systems when helicogenic symmetry is present, and they can be derived using a simple Hückel model and group theory. 2 Beautiful and conceptually interesting as the helical molecular orbitals are, they are not experimental observables.…”