Alkene Isomerization–Hydroboration Catalyzed by First-Row Transition-Metal (Mn, Fe, Co, and Ni) N-Phosphinoamidinate Complexes: Origin of Reactivity and Selectivity

Abstract

We describe the results of our combined experimental and computational investigation of structurally analogous (N-phosphinoamidinate)metal(N(SiMe 3 ) 2 ) precatalysts ((PN)M; M = Mn 2+ , Fe 2+ , Co 2+ , and Ni 2+ ; d 5 −d 8 ) in the isomerization−hydroboration of 1-octene, cis-4-octene, or trans-4-octene (1a−c) with HBPin. As part of this investigation, the synthesis and crystallographic characterization of diamagnetic (PN)Ni, ((PN)NiH) 2 , (PN)NiH(L) (L = pyridine or DMAP), and (PN)Ni(NHdipp) (dipp = 2,6-iPr 2 C 6 H 3 ) are reported. Divergent catalytic reactivity and selectivity was noted for members of the (PN)M series; (PN)Mn and (PN)Ni afforded poor hydroboration yields, whereas the use of (PN)Fe or (PN)Co afforded high conversion and selectivity for the terminal borylation product, (n-octyl)BPin (2a). DFT calculations involving (PN)M as well as stoichiometric reactivity studies featuring (PN)Ni confirmed that (PN)MH intermediates generated upon reaction of (PN)M with HBPin represent viable catalytic species whereby formation of putative (PN)Ni(H 2 BPin) is reversible. Conversely, poor catalytic performance was noted for ((PN)NiH) 2 and (PN)NiH(L) (L = pyridine or DMAP).Using DFT calculations, the relative reactivity of (PN)M precatalysts was found to be a function of their spin-state energy gaps. For reaction of (PN)MnH with trans-4-octene (1c) there is no viable spin crossover mechanism and migratory insertion is slow, resulting in poor reaction yields. In contrast, (PN)FeH can access a lower barrier through spin crossover, whereas (PN)CoH has a very low migratory insertion barrier from its low spin state. While (PN)NiH has a reasonable migratory insertion barrier, it is plausible that off-catalytic cycle intermediates are responsible for the diminished reaction rate and product yields that are observed experimentally. On the basis of the computed isomerization and borylation energy landscapes, a Curtin−Hammett-type scenario with fast isomerization through β-hydride elimination and migratory insertion steps is proposed, giving rise to a catalytic equilibrium of isomeric (PN)M(octyl) resting states, followed by slow product-forming borylation. The significantly lower barriers calculated for borylation of terminal (PN)M(n-octyl) species versus isomeric internal (PN)M(CHR 2 ) intermediates provides a rationale for the experimentally observed terminal isomerization−hydroboration selectivity.

La[N(SiMe3)2]3-Catalyzed Deoxygenative Reduction of Amides with Pinacolborane. Scope and Mechanism

“…Next, a HBpin molecule approaches the catalyst complex. Hydride transfer from the La-coordinated hemiaminal-hydroborate species to the coordinated HBpin forms La-coordinated [H 2 Bpin] − , a species often proposed to facilitate hydride transfer in HBpin-mediated reductions. ,,− With reorganization, and the approach and subsequent coordination of an additional amide substrate after TS1 , the complex then forms species D with a stabilization energy of 18.4 kcal/mol. A significant portion of this stabilization energy coming from amide coordination alone (Figure ).…”

Electronic Spin Moment As a Catalytic Descriptor for Fe Single-Atom Catalysts Supported on C2N

“…Theoretical calculations were performed at the density functional theory (DFT) level of the M06-L functional using the Gaussian 16 software package, a computational configuration which has been widely used for Fe-containing organometallic systems. , The 6-311G­(d, p) basis set was used for H, C, O, and N atoms, and the Fe atom was described by the Stuttgart/Dresden effective core potentials (SDD) basis set . Geometries for minima and transition states were optimized in the liquid phase.…”

Hydrosilylative Reduction of Tertiary Amides to Amines Catalyzed by N‐(Phosphinoaryl)anilido Complexes of Iron and Cobalt

“…Synthesis of an Fe(II) Amido Complex . Our previous work has identified Fe hexamethyldisilazide complexes as useful pre‐catalysts for carbonyl hydrosilylation applications, possibly via reactive Fe‐hydride intermediates generated under catalytic conditions . In an effort to access such a hexamethyldisilazide complex featuring anilido phosphine ancillary ligation, FeCl 2 was treated with two equiv.…”