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Dioxirane Oxidations: Taming the Reactivity-Selectivity Principle

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Dioxirane Oxidations: Taming the Reactivity-Selectivity Principle Pure & Appl. Chem., Vol. 67, No. 5, pp. 81 1-822, 1995. Printed in Great Britain. (6 1995 IUPAC Dioxirane oxidations: Taming the reactivity-selectivity principle Ruggero Curci,* Anna Dinoi,? and Maria F. Rubino Centro C.N.R. "M.I.S.0.'I, Dipartimento Chim'ca, Universitb di Ban, via Amendola 173, Bari, ltaty w:Dioxiranes (1). a new class of powerful oxidants, have been employed to carry out a variety of synthetically useful transformations. Dioxirane reactivity appears to be earmarked by the propensity for easy electrophilic 0-atom transfer to nucleophilic substrates, as well as for 0-insertion into "unactivated" hydrocarbon C-Hbonds. For a number of substrates of varying electron-donor power, ranging from a$-unsaturated carbonyls, to alkenes to sulfoxides to sulfides, the reactivity of dimethyldioxirane (la) exceeds that of peroxybenzoic acid by a factor of the order of 102; upon increasing substrate nucleophilicity, kinetic data show that the selectivity is not diminished, and actually appears to be enhanced. Despite its exceptional reactivity, high regio- and stereoselectivities can be attained in oxyfunctionalizations at hydrocarbon C-H bonds using the powerful methyl-(trifluoromethy1)dioxirane (lb); this has been applied to accomplish remarkably high regio- and stereoselective epoxidations and oxyfunctionalizations of target molecules. In these reactions, stringent steric and stereoelectronic requirements for 0-insertion seem to dictate selectivity; adoption of an FMO model provides a likely rationale. INTRODUCTION The stage for the extensive developments of dioxirane chemistry nowadays witnessed was set by the discovery, first reported in 1979 (la), that the reaction of simple ketones with potassium caroate KHSOs at a pH close to neutrality provides an easy access to dioxiranes (1).Kinetic, stereochemical and 180-labeling data stringently indicated the involvement of dioxiranes in these reactions (I). Later, the feat (24 of isolation of a few dioxiranes from the caroateketone system allowed authentic representatives of this family of peroxides to become fully characterized spectroscopically (2). Then, the availability of dioxiranes in the isolated as well as in situ form spurred an intensive utilization of these powerful oxidants to carry out a variety of RIGA0 I synthetically useful oxidations under mild conditions (3). Po Some of the established transformations (3) performed by dioxiranes are 1 summarized by the rosette in Figure 1. These include the oxidation of chloride ( 1 a:R'= w?= CH, ion to the hypochlorite ion (transformation l), of pyridine to its N-oxide 1b: R1= CK:-. F?= CFql-. (transformation 2), of sulfides to sulfoxides or of sulfoxides to sulfones (transformation 3), epoxidation of alkenes (transformation 4), and oxidation of alkynes (transformation 5); the latter reaction is envisaged to proceed via an oxirene intermediate. Also, oxidations amounting to 0- insertion into C-H bond of aldehydes (transformation 6), of secondary or primary alcohols (transformation 7), of acyclic and cyclic ethers (transformation 8), and of Si-H bond of shes(transformation 9) have been described (3). The efficient oxyfunctionalization of simple, "unactivated" C-H bonds of alkanes (transformation 10) under extremely mild conditions (3,4) undoubtedly counts to date among the highlights of dioxirane chemistry. t In partial fulfillment of the requirements for the Ph.D. Degree. 81 1 812 R. CURCl eta/. Ri Fig. 1. Some primary transformations using dioxiranes Dioxiranes have also been used for a rapid and quantitative oxidation of primary amines to nitro compounds (transformation 111; a stepwise process involving hydroxylamines and nitroso compounds was postulated, since both are oxidized by dimethyldioxirane to their nitro derivatives (3c). Although the mechanism of the "autodecomposition" of dioxiranes (i.e. their fate in the absence of any substrate) is still object of debate, rearrangement into esters (transformation 12) represents a main pathway (3). In fact, although the thermal reactions of dimethyldioxirane (hereafter DMDO) (la) are rather complex, methyl acetate is normally the main decomposition product (5). Also, methyl trifluoroacetate is produced in the thermal or photochemical decomposition (6) of methyl(trifluoromethy1)dioxirane (hereafter TFDO) (lb). Because of their versatile reactivity, dioxiranes are today widely adopted as a new class of efficient oxygen-transfer reagents, selective (chemo-, regio- and stereo-) in their action, bland toward the oxidation product, and capable of performing under mild, strictly neutral conditions. Despite the rapid proliferation of dioxirane applications in synthesis (31, comparatively few investigations appear to have been directed toward establishing their reaction mechanisms. For instance, in some mechanistic studies (7) the gathering of unambiguous evidence concerning dioxirane electrophilic character appears to have been 0' R'a..,/S) R211...,' hindered by the dichotomy existing between them and bis(oxy1) w?' '0 R' @ \o. diradicals 2. The latter represent a class of elusive entities that have 1 2 so-far escaped direct observation. Whenever substrates were tested (e.g., (Et0)3P, (Et0)zS ) that - in a parallel with 1,2-dioxetane chemistry (8) - might have yielded at low temperatures (-50 "C) insertion products (i.e., cyclic phosphoranes or sulfuranes) these were not observed (9).Actually, theoretical studies (10) confirmed that dioxirane 1 is the lowest energy form of RlRzCOz isomers; calculations indicate that the diradical 2 is at least 15 kcal mol-1 higher. In view of the rather low activation barrier estimated (1-2 kcal mol-l ) for the reclosure, the strained dioxirane 1 has kinetic stability, and persists in the absence of oxidizable substrates. Just the unimolecular rearrangement of dioxirane into ester (transformation 12) seems to demand the intermediacy of the diradical form 2; such 0 1995 IUPAC, Pure and Applied Chemistry, 67.81 1-822 Dioxirane oxidations 813 preliminary conversion, in fact, would be essential for a symmetry-allowed process (3). However, the thermal decomposition of dioxiranes - as experimentally determined in solution - is not significant at 25 "C (Ea = 23-25 kcal/mol) (9,ll).Hence, the unimolecular rearrangement into esters still requires a sizable activation energy, in spite of the high exothermicity of this process (AH 2 -80 kcal/mol). REACTION MECHANISMS Electroohilic oxyggn-atom t ransfen It was noted that the most characteristic and distinctive feature of dioxiranes consists in their propensity for easy 0-atom transfer to a variety of donor substrates (S:), yielding oxidation products SO and ketone (eq 1). 0 s:+ I>(;;*" so +o< 0 - The conversion of pyridine into pyridine N-oxide by the dioxirane generated in situ (from cyclohexanone and caroate) is among the first examples to be reported by Edwards et d.;a careful kinetic analysis of the various steps that obtain in the in situ oxidizing system was canied out (12). A positive Hammett p value of about +2 has been estimated (34 for the oxidation of an electron-donor dye by a series of substituted aryl methyl dioxiranes generated in situ; this is that expected on grounds of the dioxirane behaving as the electron donor. TABLE 1. Hammett "Rho' Values of Some Dioxirane Reactions Entry Reaction DE- Solvent T, "C p ref. DS-Me sulfoxide la acetone 20.5 -0.77 13 5 x- - 0 ,@-Me sulfone la acetone 20.5 -0.76e 13 6x- .. - 7 la CHC13 0 - 1.0 14 8 lb CHC13 0 -0.35 14 a Value of p+; cf., epoxidation by peracetic acid in AcOH (25 OC) gives p+= -1.2, and p = -1.6. Carefully dried (Na2S04) dioxirane solution. For substituted styrenes (entry l), the p+ value is -1.0 in slightly moist (0.15 mole fraction t+O) acetone (ref. 15). E stereoisomer. Cf., for oxidation of sulfides X-C&i4-SPh by H202 (MeOH, 25.0 "C),p = -1.2 (ref. 21). Oxidation by peroxybenzoic acid PhCO3H in dioxane at 25.0 "C yields p = - 0.55 (ref. 21). 0 1995 IUPAC, Pure and Applied Chemistry, 67,811-822 814 R. CURCl eta/. TABLE 2. Secondorder rate constants for the Oxidation of some electron- donor substrates by dimethyldloxirane (DMDO) 1a and by peroxybenzoic acid (PBA) in CH2C1za at 25.0 OC. & (M'S.') EntV Substrate DMW PEA ( zo) 1 6 0.11 x10" - 3 0 1.4 1.9x lo'* 74. 4 0.. 3.9 3.5 x 1(I2 111. 8. x 325. 6 Hj: -4,:8 1.1 X102C 0.33 333. C Ha 7 8.9 x Idd 25.d 356. a Unless noted otherwise. In most of the cases, rate constant values were obtained from second-order rate-law integrated plots that were linear to over 80% reaction; unless noted otherwise, kinetics were performed by following oxidant consumption by a titrimetic (iodometv) technique (cf., ref. 4). Initial concentrations of reagents generally varied from 1.5 x 10-2 to 0.5 x 10-3 M. Estimated error ranged from 2% to 20% (for the faster kinetics). In acetone/CHpClp ca. 1:1, from stopped-flow kinetics experiments carried out by shot-injection in a mixing chamber of aliquots (1 mL) of DMDO and sulfide solutions that were from 0.08 to 0.10 M; the decay of DMDO concentration with time was monitored spectrophotometrically at 334 nm during ca. 850 ms, at 0.1 ms time intervals. From competition experiments (experimental procedure. d. ref. 4) , wherein p-tolyl methyl sulfide and excess (from 30 to 100 times) dimethylsulfoxide were allowed to react with DMDO GC analysis (Freon A1 12 internal standard) allowed to determine the concentrations of oxidized products, i.e. p-Tol-SO-CH3 and MerSOp, hence the relative rate k, I (kTolSMe / @do). From this, and the rate constant value for the oxidation of dimethylsulfoxide (entry 6), the absolute rate constant for p-tolyl methyl sulfide was estimated. As for reactions involving dioxiranes isolated in solution of the parent ketone, the kinetics were found to obey a clean overall second-order rate-law (order one each in dioxirane and substrate) in almost every case that they were studied (3).
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