Phosphine-Iminopyridines as Platforms for Catalytic Hydrofunctionalization of Alkenes

Abstract

A series of phosphine-diimine ligands were synthesized by the condensation of 2-(diphenylphosphino)aniline (PNH2) with a variety of formyl and ketopyridines. Condensation of PNH2 with acetyl- and benzoylpyridine yielded the Ph2P(C6H4)N═C(R)(C5H4N), respectively abbreviated PN(Me)py and PN(Ph)py. With ferrous halides, PN(Ph)py gave the complexes FeX2(PN(Ph)py) (X = Cl, Br). Condensation of pyridine carboxaldehyde and its 6-methyl derivatives with PNH2 was achieved using a ferrous template, affording low-spin complexes [Fe(PN(H)py(R))2](2+) (R = H, Me). Dicarbonyls Fe(PN(R)py)(CO)2 were produced by treating PN(Me)py with Fe(benzylideneacetone)(CO)3 and reduction of FeX2(PN(Ph)py) with NaBEt3H under a CO atmosphere. Cyclic voltammetric studies show that the [FeL3(CO)2](0/-) and [FeL3(CO)2](+/0) couples are similar for a range of tridentate ligands, but the PN(Ph)py system uniquely sustains two one-electron reductions. Treatment of Fe(PN(Ph)py)X2 with NaBEt3H gave active catalysts for the hydroboration of 1-octene with pinacolborane. Similarly, these catalysts proved active for the addition of diphenylsilane, but not HSiMe(OSiMe3)2, to 1-octene and vinylsilanes. Evidence is presented that catalysis occurs via iron hydride complexes of intact PN(Ph)py.

Earth-abundant transition metal catalysts for alkene hydrosilylation and hydroboration

“…New catalysts based on iron, 134–136 cobalt, 137–139 nickel, 140 manganese 141 and even magnesium 142 have been since been discovered for alkene hydroboration following the initial reports described above. Effective Earth-abundant transition metal pre-catalysts have been reported with a diverse array of ligands, including phosphine-iminopyridines, 134 β-diketiminates, 135 alkoxy-tethered NHCs 136 (iron), CCC-type NHCs pincers 137,139 and bipyridine metal organic frameworks (MOFs) 138 (cobalt), phosphine-NHCs (nickel), 140 terpyridines (manganese), 141 as well as tris -(benzimidazolyl)methyl ligands on magnesium. 142 Although no new reactivity patterns were disclosed with these examples, they nonetheless exemplify the applicability of different first row transition metals and diverse ligand architectures available for olefin hydroboration.…”

Markovnikov-Selective, Activator-Free Iron-Catalyzed Vinylarene Hydroboration

“…13,14 A number of iron catalysts have been developed for the hydroboration using HBpin, or formal hydroboration using B2Pin2 and an alkoxide, of alkene and alkynes to give the pinacol boronic esters directly (Scheme 1). [15][16][17][18][19][20][21][22][23][24] In all cases these hydroboration reactions are either highly anti-Markovnikov selective or give the Markovnikov product as a mixture with the anti-Markovnikov product. Recently reported by Webster and coworkers, the highest Markovnikov:anti-Markovnikov selectivity ranges from 60:40 to 70:30 for styrene derivatives.…”

Organophosphorus and Iron Catalysis: Good Partners for Hydrometalation of Olefins and Alkynes

“…Fe-10) 45 and NaBEt 3 H (0.1 mol%) for selective anti-Markovnikov addition of 1-octene or vinylpentamethyldisiloxane with diphenylsilane. Fe-11a was also employed for the hydrosilylation of 1-octene with Ph 2 SiH 2 (Scheme 10).…”