Reactions of a Platinum(III) Dimeric Complex with Alkynes in Water: Novel Approach to α-Aminoketone, α-Iminoketone, and α,β-Diimine via Ketonyl−Pt(III) Dinuclear Complexes

Reaction of the platinum(III) dimeric complex [Pt2(NH3)4((CH3)3CCONH)2(NO3)2](NO3)2 (1),
prepared in situ by the oxidation of the platinum blue complex [Pt4(NH3)8((CH3)3CCONH)4](NO3)5 (2) with
Na2S2O8, with terminal alkynes CH⋮CR (R = (CH2)nCH3 (n = 2−5), (CH2)nCH2OH (n = 0−2), CH2OCH3,
and Ph), in water gave a series of ketonyl−Pt(III) dinuclear complexes [Pt2(NH3)4((CH3)3CCONH)2(CH2COR)](NO3)3 (3, R = (CH2)2CH3; 4, R = (CH2)3CH3; 5, R = (CH2)4CH3; 6, R = (CH2)5CH3; 7, R = CH2OH;
8, R = CH2CH2OH; 9, R = (CH2)2CH2OH; 10, R = CH2OCH3; 11, R = Ph). Internal alkyne 2-butyne reacted
with 1 to form the complex [Pt2(NH3)4((CH3)3CCONH)2(CH(CH3)COCH3)](NO3)3 (12). These reactions show
that Pt(III) reacts with alkynes to give various ketonyl complexes. Coordination of the triple bond to the
Pt(III) atom at the axial position, followed by nucleophilic attack of water and hydrogen shift from the enol
to keto form, would be the mechanism. The structures of complexes 3·H2O, 7·0.5C3H4O, 9, 10, and 12
have been confirmed by X-ray diffraction analysis. A competitive reaction between equimolar 1-pentyne
and 1-pentene toward 1 produced complex 3 and [Pt2(NH3)4((CH3)3CCONH)2(CH2CH(OH)CH2CH2CH3)](NO3)3 (14) at a molar ratio of 9:1, suggesting that alkyne is more reactive than alkene. The ketonyl−Pt(III)
dinuclear complexes are susceptible to nucleophiles, such as amines, and the reactions with secondary
and tertiary amines give the corresponding α-amino-substituted ketones and the reduced Pt(II) complex
quantitatively. In the reactions with primary amines, the once formed α-amino-substituted ketones were
further converted to the iminoketones and diimines. The nucleophilic attack at the ketonyl group of the
Pt(III) complexes provides a convenient means for the preparation of α-aminoketones, α-iminoketones,
and diimines from the corresponding alkynes and amines.