Reversal of Orbital Symmetry Control in Electrocyclic Ring Closures through Craig‐Möbius Aromaticity

Abstract

Experimentalists are challenged to find the organometallic thermal electrocyclizations that are computationally predicted to proceed with opposite stereoselectivity compared to their metal-free parent 4n and 4n+2 π-electron systems. While ring closure of, for example, s-cis-butadiene proceeds conrotatory, an iron alkyl complex formed by replacement of a (CH) unit by an [FeH] metal fragment results in a disrotatory electrocyclization.

Duality of Orbital‐Symmetry‐Allowed Transition States for Thermal Sigmatropic Hydrogen Shifts in Transition Metal Compounds

“…Note that species 1-5 are neutral due to protonation of an effective transition-state structure fora6 p electron electrocyclic ring closure. [16,17] Indeed,i nb oth 3 and 5 (according to NBO data), there is one Fe=Cd ouble bond,i naccord with the interpretation of both 3 and 5 as fully conjugated and resonance-stabilized "aromatic" allowed transition-state structures. Further analyzing the NAO contributions to the Fe=C p-bond implies that the iron in 3 employs ad yz orbitalt om ake this bond, in line with our earlierq ualitative theoretical orbital topology hypothesis (Scheme 2), which predicted that d-type bondingr esults in aC raig-type phase dislocation compensated for by a( Zimmerman-Mçbius-like) bent structure (Figure 1), hence giving a4 n+ +2e lectron system with anti-Woodward-Hoffmann-type antarafacial approachd irection of the migrating hydrogen in the sigmatropic shift transition structure.…”

“…[13a, 14] It is cleart hat these relationshipsm usta lso be fully general and are not limited to ground-state molecules. [16] Hence, we expected that the same effect, which has not previously been considered in the application or interpretationo f Woodward-Hoffmann rules for transition-metal-mediated reactions, should also be involved in transition-state structures.…”

“…The preference for ad isrotatory course wasf urthere xplained on the basis of an orbitals ymmetry correlation diagram, in which mutuallys ymmetric or antisymmetricr eactant and product orbital combinationso fl igand and metal, respectively,a re correlated under consideration of local C s symmetry. [34] In contrast, and at least for ac yclic delocalization, we propose that an orbitalp hase "switch"f rom one side of the nearly planar p-system to the other,c aused by the involvemento f am etal do rbital, may result in reversal of the stereoselectivity (and withoutachange in the electron count), [16] similar to that observed in main group elementp ericyclic reactions [5,15] when the atomicc hain is bent or twisted to create an orbitalp hase dislocation in the Zimmerman-or Heilbronner-Mçbius-type transition-state structure. In other words, the course of, for example,a [ 1,5] Hs hift must be suprafacial via TS 1,b ecause the antarafacial pathway (via TS 2)w ould involve ah igh-energy Mçbiusa ntiaromatict ransitions tate.…”

“…[27] Hence, am etal atom would contribute, in am erely formal analogy,asingle p electron from af rontier orbital to the cyclic p system, resulting in diamagnetici nnerspherem etal complexes in allowed transition states that represent Hückel or Mçbius aromatic closed-shell species. [13,16] MO theorya llows to draw am ore realistic picture of the actualb onding situation in the transition structures 3 and 5.I n archetypical metallabenzene (PH 3 ) 3 Ir I (C 5 H 5 ), [28] the metal is considered to contributet wo electrons to the delocalized p-electron system (rather than one, as the formally replaced CH fragment would suggest) and from ad oubly occupied d xz /d z2 hybrid orbital, mixing with an empty 3p hydrocarbon ligand (frontier) orbitalt og ive at otal of six p electrons togetherw ith the four electrons from the other two occupiedl igand orbitals. [29] Ad etailed analysiso ft he quantum chemical results for 3 and 5 furtherr eveals that Fe actually employst hree sd 2 hybrid orbitals for the three s-bonds to the adjacent carbons and the hydrogen ligand.…”

“…When, for example,ametal-free parent system prefers ac onrotatory electrocyclization pathway,o rganometallic counterparts, in which aC Hg roup is replaced by an iron or ruthenium fragment, react via ac oncerted disrotatory pathway (and vice versa), involvingd istinctly aromatic transition-state structures. [16] This led us to pose the question as to whether other types of peri-cyclic reactions shows imilar behaviour when do rbitals become involved in bonding.…”