Mechanism of Ketone and Alcohol Formations from Alkenes and Alkynes on the Head-to-head 2-Pyridonato-Bridged cis-Diammineplatinum(III) Dinuclear Complex

Abstract

Reactions of the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having nonequivalent two platinum atoms, Pt(N(2)O(2)) and Pt(N(4)), with p-styrenesulfonate, 2-methyl-2-propene-1-sulfonate, 4-penten-1-ol, and 4-pentyn-1-ol were studied kinetically. Under the pseudo first-order reaction conditions that the concentration of the Pt(III) dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed: the olefin pi-coordinates preferentially to the Pt(N(2)O(2)) in the first step (step 1), followed by the second pi-coordination of another olefin molecule to the Pt(N(4)) (step 2). In the next step (step 3), the nucleophilic attack of water to the coordinated olefin triggers the pi-sigma bond conversion on the Pt(N(2)O(2)), and the second pi-bonding olefin molecule on the Pt(N(4)) is released. Finally, reductive elimination occurs to the alkyl group on the Pt(N(2)O(2)) to produce the alkyl compound (step 4). The first water substitution with olefin (step 1) occurs to the diaqua and aquahydroxo forms of the complex, whereas the second substitution (step 2) proceeds either on the coordinated OH(-) on the Pt(N(4)) (path a) or on the coordinatively unsaturated five-coordinate intermediate of the Pt(N(4)) (path b), in addition to the common substitution of H(2)O (path c). The reactions of p-styrenesulfonate and 2-methyl-2-propene-1-sulfonate proceed through paths b and c, whereas the reactions of 4-penten-1-ol and 4-pentyn-1-ol proceed through paths a and c. This difference reflects the difference of the trans effect and/or trans influence of the pi-coordinated olefins on the Pt(N(2)O(2)). The pentacoordinate state in path b is employed only by the sulfo-olefins, because these exert stronger trans effect. The steps 3 and 4 reflect the effect of the axial alkyl ligand (R) on the charge localization (R-Pt(IV)(N(2)O(2))-Pt(II)(N(4))) and delocalization (R-Pt(III)(N(2)O(2))-Pt(III)(N(4))-OH(2)); when R is p-styrenesulfonate having an electron withdrawing group, the charge localization in the dimer is less pronounced and the water molecule on the Pt(N(4)) atom is retained (R-Pt(III)(N(2)O(2))-Pt(III)(N(4))-OH(2)) in the intermediate state. In both routes, the alkyl group undergoes nucleophilic attack of water, and the oxidized products are released via reductive elimination.

Kinetics and mechanisms of the axial ligand substitution reactions of platinum(III) binuclear complexes with halide ions

“…Most of the complexes of group (1) and (2) are considerably stable in solid and/or in solution, and have been widely studied. However, no detailed analysis in the reactivity and the reaction mechanism has been reported for platinum(III) dimers except one before our recent studies [27][28][29][30][31][32].…”

“…Furthermore, the reactions of some of the diaqua complexes with olefins proceed with similar rate constants (k 1 ¼ 10 2 210 3 M À1 s À1 ) [30,31]. These results suggest that the first ligand substitution of the diaqua complex (k 1 path) proceeds via a dissociative interchange mechanim (I d ) [46].…”

“…Furthermore, the kinetic analyses in both reports are insufficient to clarify the detailed reaction mechanism. Thus, re-investigation was carried out on the axial water substitution reaction of the HPO 4 -bridged lantern-type complex with halide and pseudo-halide ions (X À ¼ Cl À , Br À , SCN À ) in acidic aqueous solution (Scheme 7) [32], and the detailed mechanisms were determined based on the mechanistic information obtained from the amidato-bridged platinum(III) binuclear complexes [27][28][29][30][31].…”

“…According to the study on the axial ligand substitution reactions involving amidato-bridged cis-diammineplatinum(III) binuclear complexes, all of the diaqua Pt(III) dimers were found to be in an acid dissociation equilibrium in acidic aqueous solution [27][28][29][30][31]. However, the acid dissociation constant of [Pt 2 (m-HPO 4 ) 4 (OH 2 ) 2 ]…”

Axial ligand effect on the reaction mechanism of head-to-head pivalamidato-bridged Pt(III) binuclear complex containing an equatorial bromide ligand with acetone

“…These complexes act as catalysts in the oxidation reactions of olefins to aldehydes, ketones, epoxides, and a,b-diols [2][3][4][5]. In a series of studies on the reaction mechanisms of amidato-bridged Pt III binuclear complexes with organic compounds, we have reported the mechanisms of their reactions with olefins [6,7], phenol [8], isoprene [9], and acetone [10] in acidic aqueous solutions. In our previous report [10], we proposed a mechanism for the reaction of HH tetraammine amidato-bridged platinum(III) binuclear complexes with acetone in strongly acidic aqueous solutions, where acetonylplatinum(III) binuclear complex is formed successively in two steps, as shown in Scheme 1.…”