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Wei CH, Wu CE, Huang YL, Kultyshev RG, Hong FE: Experimental and DFT study of the tautomeric behavior of cobalt-containing secondary phosphine oxides. Chemistry. 2007;13(5):1583-93.
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). |
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