Experimental and DFT Study of the Tautomeric Behavior of Cobalt‐Containing Secondary Phosphine Oxides

Abstract

New cobalt-containing secondary phosphine oxides [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(==O)(H)(R)}] (8 a: R=tBu; 8 b: R=Ph) were prepared by reaction of secondary phosphine oxides PhC[triple chemical bond]CP- (==O)(H)(R) (6 a: R=tBu; 6 b: R=Ph) with dppm-bridged dicobalt complex [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(6)] (2). The molecular structures of 8 a and 8 b were determined by single-crystal X-ray diffraction. Although palladium-catalyzed Heck reactions employing 8 b as ligand gave satisfying results, 8 a performed poorly in the same reaction. Judging from these results, a tautomeric equilibrium between 8 b and its isomeric form [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(OH)(Ph)}] 8 b' indeed takes place, but it is unlikely between 8 a and [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(OH)(tBu)}] (8 a'). The DFT studies demonstrated that reasonable activation energies for the tautomeric conversions can be achieved only via a bimolecular pathway. Since a tBu group is much larger than a Ph group, the conversion is presumably only feasible in the case of 8 bright harpoon over left harpoon8 b', but not in the case of 8 aright harpoon over left harpoon8 a'. Another cobalt-containing phosphine, namely, [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(NEt(2))(tBu)}] (7 a), and its oxidation product [(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4){mu,eta-PhC[triple chemical bond]CP(==O)(NEt(2))(tBu)}] 7 a' were prepared from the reaction of PhC[triple chemical bond]CP(NEt(2))(tBu) (5 a) with 2. The molecular structures of 7 a and 7 a' were determined by single-crystal X-ray diffraction. The phosphorus atom is surrounded by substituents in a tetrahedral environment. A P--N single bond (1.676(3) A) is observed in the molecular structure of 7 a. Heck reactions employing 7 a/Pd(OAc)(2) as catalyst system exhibited efficiency comparable to that of 8 a/Pd(OAc)(2).

Coordination chemistry and catalysis with secondary phosphine oxides

“…15 In 2004, Pietrusiewicz, Duddeck and co-workers suggested 16 from NMR studies that the fast kinetics of the tautomeric equilibrium is due to the migration of an acidic proton. In this line, Hong and co-workers 17 proposed two reaction pathways for the transformation of SPOs to the corresponding PA (Scheme 2), based on DFT calculations. Scheme 2.…”

Computational and Experimental Studies on Dual Bridged Bimetallic Catalyst Enabled by the Synergistic Effect of Lewis Acid and eg Orbital Interaction

“…Tautomerization from P V to P III is a well-accepted mechanism for SPO transformation 43 . However, our initial calculations indicate the reactivity of intermolecular H-transfer may vary from different substituents as shown in Figure 1a.…”

Palladium(II)-catalyzed Heck reaction of aryl halides and arylboronic acids with olefins under mild conditions

“…Previously, we also reported the synthesis of cobalt-containing SPO ligands and their palladium complex. This was successfully applied as a catalytic precursor in oxidative Heck reactions [32]. However, these reactions were carried out at high temperatures with limited substrate scope.…”