Formation mechanism of 2-methyl-2-buten-1,4-diol and 2-methyl-3-buten-1,2-diol from 2-methyl-1,3-butadiene on a head-to-head pivalamidato-bridged cis-diammineplatinum(iii) binuclear complex

Abstract

Reactions of a pivalamidato-bridged head-to-head (HH) platinum(III) binuclear complex with 2-methyl-1,3-butadiene (isoprene) and p-styrenesulfonate and of an α-pyrrolidonato-bridged HH platinum(III) binuclear complex with p-styrenesulfonate were studied kinetically using UV-vis spectrophotometry and (1)H NMR spectroscopy, and detailed reaction mechanisms are proposed. Pt(III) binuclear complexes react with p-styrenesulfonate in four successive steps with mechanisms similar to that for an HH α-pyridonato-bridged Pt(III) binuclear complex with p-styrenesulfonate. In the case of isoprene, four steps were observed on the basis of UV-vis spectrophotometry. However, the reaction kinetics for steps 1 and 2 correspond to those for the previous reaction system, and those for steps 3 and 4 do not correspond to those for the previous system or to those observed by using (1)H NMR spectroscopy for the present isoprene system. By using UV-vis spectrophotometry, it was shown that isoprene preferentially π-coordinates to the Pt(N(2)O(2)) atom via the double bond adjacent to the methyl group in step 1. In step 2, a second isoprene molecule π-coordinates to the Pt(N(4)) atom, which is the rate-determining step, followed by nucleophilic attack of a water molecule on the π-coordinated isoprene on the Pt(N(2)O(2)) atom to form two isomeric σ-complexes. In the same step, π-coordinated isoprene on the Pt(N(4)) atom of the σ-complexes is released. This is different from the reaction of the Pt(III) binuclear complexes with other olefins. In step 3, reductive elimination of the σ-complexes occurs to form two diols and the HH pivalamidato-bridged Pt(II) binuclear complex. Finally, acid decomposition of the Pt(II) binuclear complex occurs to form monomers in step 4. From (1)H NMR spectroscopic observations, fast isomerization between σ-complexes and reductive elimination of the σ-complexes occurs in step 3, and isomerization from a 1,4-diol to a 1,2-diol occurs in step 4.

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

“…As mentioned in the previous report [10], the rate constants listed in Table 1 are unusually small compared to those for the reactions of the HH amidato-bridged Pt III binuclear complexes with various ligands [6][7][8][9]. Therefore, non-reactivity of the keto form of acetone should also be considered in the present system.…”

“…In step 1, the H 2 O ligated to Pt(Br 2 O 2 ) is substituted with enol acetone to form a p-complex. Subsequently, in step 2, a p-r , Scheme 2 appears to be reasonable, considering the reactivity of the amidato-bridged Pt III binuclear complexes studied so far [6][7][8][9][10][14][15][16][17].…”

“…(8) are listed under the column ''Enol acetone'' in Table 1 and are similar to those for the other systems [6][7][8][9].…”

“…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.…”

Redox Reactions of Amidato‐Bridged cis‐Diammineplatinum(III) Dinuclear Complexes with Phenol

“…These substitutions take place on the two Pt(N 3 O) atoms, and acid dissociation occurs at one of the water molecules on the Pt(N 3 O) atoms. [9][10][11][12] The most recent study focused on the mechanism of the reaction of the HH Pt III dinuclear complexes with phenol to form 2-hydroxyphenyl Pt III dinuclear complexes in strongly acidic aqueous solution [Equation (1)]. [5][6][7][8] We have previously presented mechanistic studies on the reactions of HH and HT amidato-bridged Pt III dinuclear complexes with olefins.…”