Oxidative cleavage of a,b-unsaturated compounds by pentachlorohydroxoplatinate(IV) in alkaline medium Biswajit Pal* Department of Chemistry, St. Paul’s C. M. College, 33/1 Raja Rammohan Roy Sarani, Kolkata-700009, West Bengal, India Kalyan K. Sen Gupta and Pratik K. Sen Department of Chemistry, Jadavpur University, Kolkata-700032, West Bengal, India Received 17 Feburary 2005; accepted 01 March 2005 Abstract The kinetics of oxidation of a,b-unsaturated compounds by platinum(IV) in the presence of alkali {[OH ) ] ¼ (1–9) · 10 )3 mol dm )3 } have been investigated over the 303–318 K temperature range . The rate of the reaction is dependent on the first power of the concentrations of substrates, oxidant, and alkali. The rate constant increases with an increase in ionic strength and also with increasing dielectric constant of the medium. The oxidation rates follow the order: ACN > ACONH 2 > ACOO ) . The values of the third order rate constant (k 3 ) for the oxidation of acrylonitrile, acrylamide and acrylate are 1.24, 0.826 and 0.628 mol )2 dm 6 s )1 respectively, at 303 K. The oxidations of the substrates by PtCl 5 (OH) 2) take place by an inner-sphere mechanism. Platinum(IV) is reduced to platinum(II) by the substrates in a one-step two-electron transfer process to give reaction products. The major reaction product, HCHO, is identified from the reaction mixture using i.r. spectrometry, n.m.r. and C, H, N analysis. A tentative reaction mechanism, leading to the formation of products, has been suggested. The activation parameters of the reaction have been evaluated. Introduction The p-electrons of alkenes are more loosely held [1] by their carbon nuclei than are the electron pairs of single bonds, and they are susceptible to direct attack by electrophilic reagents. A few olefinic oxidations by cobalt(III) have been studied kinetically by Bawn and his co-workers [2] and seem to involve direct attack on the p-bond. Uncomplexed manganese(III) may behave similarly but some other oxidants are so inert towards olefins that radical polymerization of the latter can be used to demonstrate the presence of transient radicals from oxidation of other substrates [3]. The reactions of some unsaturated compounds with thallium(III) acetate in methanol and in acetic acid have been reported [4, 5]. Several other oxidants, like thallium(III) nitrate in methanol [6], lead(IV) [7], cerium(IV) [8], quinolinium dichromate(QDC) [9] and peracetic acid [10], have been used to oxidize unsaturated compounds. Chloro(tetraphenylporphyrinato)chromium(III), abb- reviated as (TPP)CrCl, has been used as a catalyst in conjugation with hydrogen peroxide as the oxygen source, to convert styrene into benzaldehyde in reason- ably good yields [11, 12]. When reacted with (TPP) CrCl/-iodosylbenzene, styrene underwent cleavage to give benzaldehyde [13, 14]. Using dodecylmethyl ammo- nium bromide (DDAB) as the phase transfer catalyst, styrene in 1,2-dichloroethane containing ruthenium chloride trihydrate was oxidized by hydrogen peroxide to give a good yield of benzaldehyde [15]. Ruthenium polyoxytungstates have been used as catalysts in the liquid phase oxidation of styrene by sodium periodate [16], resulting in the formation of benzaldehyde. Styrene has also been oxidized to benzaldehyde in good yields by hydrogen peroxide catalyzed by metallo-silicate xerogels [17] of the type MO X –SiO 2 , where M is Ti IV ,V V , Mo VI , W VI or Se IV . Molecular oxygen was used for the liquid phase oxidation of styrene, taken in dimethylphthalate. The products obtained were benzaldehyde and styrene oxide in a 1:1 ratio [18]. Permanganate has also been used to cleave the carbon–carbon double bond in cinnamic acid and 4-nitrocinnamic acid to yield benz- aldehyde and 4-nitrobenzaldehyde [19] respectively, in the presence of moist alumina as the solid and dichlo- romethane as solvent. Kinetic investigations of oxidations of organic [20–23] and inorganic substrates [24–28] by platinum(IV) have been reported. Recently, Arendse et al. [29] studied the reduction of the platinum(IV) complexes [PtCl 2 (OH) 2 (N, N-dmen)]by sodium ascorbate with the isolation of the oxalatoplatinum(IV) complex, Pt(C 2 O 4 )Cl(OH)(N,N- dmen)]  H 2 O, identified by an X-ray diffraction study. The reduction of the platinum(IV) complex can be * Author for correspondence: E-mail: palbiswajit@yahoo.com; Tel: +91-33-2350-3682 Transition Metal Chemistry (2005) 30:593–600 Ó Springer 2005 DOI 10.1007/s11243-005-4037-z