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

Abstract

Herein, the Zimmerman Möbius/Hückel concept is extended to pericyclic reactions involving transition metals. While sigmatropic hydrogen shifts in parent hydrocarbons are either uniquely antarafacial or suprafacial, we have shown by theoretical orbital topology considerations and quantum chemical computations at DFT level that both modes of stereoselectivity must become allowed in the same system as a consequence of Craig-Möbius-type orbital arrays, in which a transition metal d orbital induces a phase dislocation in metallacycles. This may have fundamental implications for the understanding of reactivity and bonding in organometallic chemistry.

Expeditious Preparation of Open‐Cage Fullerenes by Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Diynes and C60: An Experimental and Theoretical Study

“…As we have not employed photoexcitation in our system, a reaction path taking place in the ground state must be operative. At this point, it is convenient to remember that the presence of metals in a formal cycloaddition can (i) transform pericyclic reactions into stepwise processes, or (ii) within a concerted pathway, modify the formal topology of orbital symmetry‐allowed mechanisms by means of the participation of the orbitals provided by the metallic center . With these ideas in mind, we investigated the transformation of A5 to A9 catalyzed by Rh( P9 ) + .…”

“…At this point, it is convenient to remember that the presence of metals in af ormal cycloaddition can (i)transform pericyclic reactions into stepwise processes, [26] or (ii)within ac oncerted pathway,m odify the formal topologyo f orbitals ymmetry-allowed mechanismsb ym eans of the participation of the orbitals provided by the metallicc enter. [27] With these ideas in mind, we investigated the transformation of A5 to A9 catalyzed by Rh(P9) + .A ss hown in Figure 3, the reaction mechanism that we found follows aRh-catalyzed di-p-methane rearrangement pathway.T hus, conversion of the cyclohexadiene-fused C 60 intermediate A5 to the [5,6] (bis)methanofullerene intermediate A7 is as tepwise process in which,f irst, ac yclopropane ring is formed to yield A6 (Gibbsb arriero f 16.5 kcal mol À1 ,G ibbs reactione nergy of 7.0 kcal mol À1 ). In A6, Rh is h 4 -coordinated to the methanofullerene (Figure 4), thus stabilizing the allyl radical formed.…”

Strict Correlation of HOMO Topology and Magnetic Aromaticity Indices in d‐Block Metalloaromatics

“…To understand role of orbital topology in dimetallacycles, we have also studied dimetalla‐cyclooctatetraenes, constrained to planarity, and with either two [Nb] or two [Rh] fragments, or with one [Nb] and one [Rh] fragment, in the ring at the 1,5‐position (Figure ). As expected, the two metallacycles with like metal in symmetry equivalent positions ( 16 and 17 ) are both Hückel systems, because the two‐phase inversions annihilate and the orbital topology of the HOMO defaults to Hückel type . Accordingly, both species are magnetically antiaromatic because they possess 12 (diniobacycle) and 16 π‐electrons (dirhodacycle), respectively (see also Figure S2 in Supporting Information).…”

The novel link between planar möbius aromatic and third order nonlinear optical properties of metal–bridged polycyclic complexes

“…The realization of small cyclic alkynes challenges synthetic chemists because the angle strain associated with the highly distorted triple bonds must be overcome 10 – 14 . Fortunately, the introduction of a metal fragment is an efficient strategy to stabilize cyclic alkynes by reducing ring strain 15 – 25 . Metalla–aromatic molecules, first proposed by Thorn and Hoffmann, are analogues of conventional aromatic molecules, in which one carbon segment is formally replaced by a transition–metal fragment 7 .…”