Catalytic Synthesis of an Unsymmetrical PNP-Pincer-Type Phosphaalkene Ligand

Abstract

An unsymmetrical PNP-pincer-type phosphaalkene ligand, 2-(phospholanylmethyl)-6-(2-phosphaethenyl)pyridine (PPEP), has been prepared from 2,6-bis(2-phosphaethenyl)pyridine (BPEP) by intramolecular C−H addition/ cyclization of the 2-phosphaethenyl group with a 2,4,6-tritert-butylphenyl substituent (CHPMes*). The reaction proceeds in hexane in the presence of a catalytic amount of [Pt(PCy 3 ) 2 ] (20 mol %) at 80 °C in a sealed tube, giving PPEP in 32% isolated yield, along with byproduction of 2,6bis(phospholanylmethyl)pyridine (BPMP) and a Pt(II) phosphanido complex (5). The PPEP ligand reacts with [Rh(μ-Cl)(C 2 H 4 ) 2 ] 2 and [RuCl 2 (PPh 3 ) 3 ] to afford [RhCl(PPEP)] (6) and [RuCl 2 (PPh 3 )(PPEP)] (8), respectively. Complex 6 easily undergoes C−H addition/cyclization at the other CHPMes* group to afford the 2,6-bis(phospholanylmethyl)pyridine complex [RhCl(BPMP)] (7), whereas 8 is stable against C−H addition/cyclization. Treatment of 8 with t BuOK forms [RuCl(PPh 3 )(PPEP*)] ( 9), coordinated with an unsymmetrical PNP-pincer-type phosphaalkene ligand containing a dearomatized pyridine unit (PPEP*). The X-ray structures of 5 and 9 are reported. The reaction processes from BPEP to PPEP and to 5 are discussed based on NMR observations.

A Square‐Planar Complex of Platinum(0)

“…This study is based on our previous observations on [Pt(PCy 3 )(BPEP)] ( 4 , Scheme ); its 31 P NMR signal was unexpectedly upfield ( δ =113.7 ppm) relative to other PNP‐pincer‐type phosphaalkene complexes that we have examined ( δ =180–260 ppm) . Although the formation of an unusual structure was indicated, complex 4 , which features 2,4,6‐tri‐ tert ‐butylphenyl groups (Mes*) as steric protectors of the P=C bonds, could not be isolated owing to the reactivity of the Mes*−P=CH groups towards C−H addition/cyclization . Thus we prepared complex 1 , which is stabilized by fused‐ring bulky 1,1,3,3,5,5,7,7‐octaethyl‐1,2,3,5,6,7‐hexahydro‐ s ‐indacen‐4‐yl groups (Eind) instead of Mes* groups.…”

A Square‐Planar Complex of Platinum(0)

“…This study is based on our previous observations on [Pt(PCy 3 )(BPEP)] (4,S cheme 1); [6] its 31 PNMR signal was unexpectedly upfield (d = 113.7 ppm) relative to other PNPpincer-type phosphaalkene complexes that we have examined (d = 180-260 ppm). [7] Although the formation of an unusual structure was indicated, complex 4,w hich features 2,4,6-tritert-butylphenyl groups (Mes*) as steric protectors of the P=C bonds,c ould not be isolated owing to the reactivity of the Mes* À P = CH groups towards C À Ha ddition/cyclization.…”

“…[7] Although the formation of an unusual structure was indicated, complex 4,w hich features 2,4,6-tritert-butylphenyl groups (Mes*) as steric protectors of the P=C bonds,c ould not be isolated owing to the reactivity of the Mes* À P = CH groups towards C À Ha ddition/cyclization. [6] Thus we prepared complex 1,w hich is stabilized by fusedring bulky 1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-sindacen-4-yl groups (Eind) [8] instead of Mes* groups.…”

E-Selective Synthesis and Coordination Chemistry of Pyridine-Phosphaalkenes: Five Ligands Produce Four Distinct Types of Ru(II) Complexes

“…These cyclizations generally afford phospholanes, and in the case of phosphaalkene-derived PCP pincer C , a catalytic amount of [Pt­(PCy 3 ) 2 ] promotes the formation of bis­(phospholane) D . Interestingly, the PNP analogue E was far more resistant to double cyclization, requiring forcing conditions (80 °C, 11 days) to produce F (Scheme , bottom) . However, a single cyclization of bis­(phosphaalkene) E occurs readily at metal centers, affording square-planar G and octahedral H with a modified PNP pincer featuring phospholane, pyridine, and phosphaalkene donors.…”

“…Akin to the metal complexes reported by Milstein that are supported by neutral, tridentate pincers bearing a central N-heterocyclic donor, treatment of G and H with base results in deprotonation of the benyzlic arm and subsequent dearomatization of the pyridine functionality and isolation of I and J (Scheme ). , …”