Comments on “The Transition Metal Catalyzed [.pi.2s + .pi.2s + .sigma.2s + .sigma.2s] Pericyclic Reaction: Woodward−Hoffmann Rules, Aromaticity, and Electron Flow" (J. Am. Chem. Soc. 2020, 142, 19033-19039.): An Orbital Phase Theory Approach

Recently, Goddard and Stoltz reported a theoretical investigation of the intramolecular coupling of two 1-allyl ligands on Pd atom (J. Am. Chem. Soc. 2020, 142, 19033-19039.). They proposed that the symmetry-allowed [.pi.2s + .pi.2s + .sigma.2s + .sigma.2s] orbital interaction is essential to achieve the reaction. However, there are four issues of misunderstanding: (i) Two .sigma.Pd-C orbitals, which are in phase combination of the hybrid orbitals, are always and unexeptionaly associated with two out-of-phase-combined .sigma.*Pd-C’s. Thus, total of four orbitals, two .sigma.Pd-C’s and .sigma.*Pd-C’s, are to be generated from only three orbitals, one dPd orbital and two nC’s; (ii) Two .pi.2s orbitals and .sigma.2s ones are all bonding and occupied orbitals so that interaction among them should lead to destabilization, not stabilization; (iii) Two .pi.2s orbitals are combined out of phase so that it is not appropriate to make a bond between them; (iv) Normally, two .eta.1-allyl ligands are regarded as two electron donors, while the Pd atom is an acceptor. From the orbital phase theory perspective, cyclic orbital interactions among d*Pd-.pi.2/allyl-.pi.2/allyl-, d*Pd-.pi.*3/allyl-.pi.2/allyl- and d*Pd-.pi.2/allyl-.pi.*3/allyl- are phase-continuous so that they can produce stabilization at the TS. The HOMO at the TS should consist of superimposition of these interactions.