Understanding β-Hydride Eliminations from Heteroatom Functional Groups

β-hydride elimination (BHE), the [2 + 2] pericyclic hydride rearrangement, is a well-established process frequently encountered in organometallic reaction mechanisms. BHE is an extraordinarily prevalent reaction mechanism, and it is commonly presented in reports, 8 reviews, 11 and textbooks. 16 Despite its literature precedence, more can be learned about BHE, especially when heteroatoms are involved in the process. This became apparent to us while we examined the details of the Wacker process. Early reports on the Wacker process erroneously labeled the final oxidation step as a BHE from a [(1hydroxyethane-1-yl)PdCl] complex to form an η-coordinated aldehyde, [(acetaldehyde)PdCl]: Pd CH(CH3) O H* f H* Pd(CH(CH3)dO). 22 Our report onWacker oxidation and that of others investigated if BHE is a generally facile process for coordinatively saturated ligands. We found this false. For BHE from coordinated alkanes, we found expectedly low barriers. For BHE from ligands with an alcohol functional group at the β-position, we discovered prohibitive barriers insurmountable in most chemical environments. A theoretical study on Wacker-like side reactions in a palladium polymerization catalyst also observed high barriers for this type of BHE; instead, they attributed their oxidative side reaction to water-mediated deprotonation of a (2-hydroxyethane-1-yl)Pd complex, which also yielded an aldehyde. This result surprised us, and we became interested in the factors that lead to the increased barrier height for β-hydride elimination from heteroatoms. We have found another catalytic cycle in which β-hydride elimination from a β-heteroatom is proposed to be involved. In the “Hieber base reaction”, a process to create metal carbonyl hydrides, aβ-hydride elimination from an iron carboxyl complex is proposed: [(CO)4Fe C(O) O H*] f [H* Fe(CO)4] + CO2. 26,27 We state the existence of this mechanism to illustrate that authors will propose BHE from a β-heteroatom without regard to its feasibility. Our hope is that our study will prompt chemists to re-examine the likelihood of β-hydride elimination from β-heteroatoms. To begin, we should state that BHE from alkyl ligands on d metals such as Ni, Pd, and Pt is a well-understood process. The necessary conditions for BHE are self-evident: a vacant coordination site on the H-accepting metal, a hydrogen atom bound to a β-atom, and a near-planar geometry of the metal, R-atom, β-atom, and the hydrogen whic h maximizes orbital overlap. Thorn and Hoffmann, who studied the microscopic reverse of BHE, olefin insertion into a metal hydride bond, and subsequently Morokuma and co-workers were the first groups to present orbital symmetry descriptions of the BHEmechanism for β-alkyl fragments based on ab initio calculations. These studies do an excellent job of describing, in a qualitative sense, the molecular orbitals (MOs) that contribute to and facilitate β-hydride transfer. Figure 1 is a reproduction of the orbitals identified by Morokuma and co-workers. Morokuma identifies the orbitals that define the agostic metal hydride interaction, and he accurately describes the flow of orbital density throughout the reaction coordinate. The σCH and σMR orbitals, their antibonding counterparts, and their linear combination,Ψ3 , are especially important because they define the active bonds during the BHE.