Platinum(II) Complexes with Coordinated Electron-Withdrawing

Review pubs.acs.org/Organometallics

Platinum(II) Complexes with Coordinated Electron-Withdrawing Fluoroalkyl and Fluoroaryl Ligands: Synthesis, Reactivity, and Catalytic Activity Paolo Sgarbossa,† Alessandro Scarso,‡ Giorgio Strukul,‡ and Rino A. Michelin*,‡ †

Dipartimento di Processi Chimici dell’Ingegneria, Università di Padova, Via F. Marzolo 9, 35131 Padova, Italy Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari di Venezia, Dorsoduro 2137, 30123 Venezia, Italy

‡

ABSTRACT: The synthesis, reactivity, and catalytic activity of Pt(II) phosphine complexes containing σ-metal-bound fluoroalkyl and fluoroaryl ligands are surveyed. The increased Pt−C(RF) chemical stability has allowed the isolation and investigation of a series of stable transition-metal hydride, hydroxo, and peroxo complexes. The relative inertness of these complexes coupled with an increased Lewis acidity of the metal center led to the catalytic oxidation of organic substrates (e.g., epoxidation and hydroxylation) in the presence of hydrogen peroxide.

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with their σ-alkyl counterparts5 and that the perfluoroalkyl groups in transition-metal complexes are quite resistant to chemical attack, paralleling the known inertness of fluorocarbon compounds.6 In the following we will survey the most significant features involving the synthesis, reactivity, and catalytic activity of the class of fluorinated compounds reported in Chart 1.

INTRODUCTION The role of the transition-metal−carbon bond is pivotal in organometallic chemistry.1 The stability (thermodynamic and kinetic) of the M−C σ bond greatly determines the chemical reactivity and the catalytic activity of the transition-metal complexes. Many years ago some of us entered this research area by investigating a series of hydrido cyanoalkyl complexes of the type [PtH(RCN)P2] (with RCN = CH2CN, C2H4CN and P2 = two monodentate tertiary phosphines or chelating diphosphine),2 since they can be considered as model compounds involved in several organometallic processes such as activation of the C−H bond, catalytic hydrogenation of olefins, β-hydrogen abstraction, etc. It was found that the Pt(II) hydrido cyanoalkyl complexes, in either trans or cis geometry, are much more thermally stable compared to the corresponding hydrido alkyl derivatives.3 No reductive elimination processes of the cyanoalkane were observed at ambient temperature unless in the presence of added nucleophiles such as CO, alkynes, and phosphines2c or under photolytic conditions.4 Following the indications of those initial investigations, we have been interested in considering other electronegative groups, in particular fluorine, synthesizing a wide series of fluoroalkyl and fluoroaryl Pt(II) complexes of the types depicted in Chart 1.

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SYNTHESIS OF THE COMPLEXES Fluoroalkyl Complexes. Fluoroalkyl platinum(II) complexes of the general formula [PtX(RF)P2] (X = halide; RF = fluoroalkyl; P2 = two monodentate tertiary phosphines or one chelating diphosphine) have been generally prepared using oxidative addition procedures, the most relevant of which are described hereby. Perfluoroalkyl iodo compounds of the type trans-[PtI(RF)(PPh3)2] (RF = CF3, C2F5, C3F7) have been prepared by oxidative addition of perfluoroalkyl iodides RFI on the zerovalent platinum complex [Pt(PPh3)4] (Scheme 1, route a) in benzene at room temperature for short reaction times (up to 90 min).7 Compounds containing a chelating diphosphine of the type [PtI(R F )(dppe)] (dppe = Ph2PCH2CH2PPh2) have been obtained similarly starting from [Pt(dppe)2]. However, these reactions have been observed to be accompanied by the formation of variable amounts of the diiodide derivatives [PtI2(PPh3)2] and [PtI2(dppe)], respectively, as byproducts.8 The complex cis-[PtI(CH2CF3)(PPh3)2] (Scheme 1, route b) was obtained in high yield by oxidative addition of CF3CH2I

Chart 1

Special Issue: Fluorine in Organometallic Chemistry

It is known that σ-perfluoroalkyl complexes of the transition metals show a remarkably high thermal stability as compared © 2012 American Chemical Society

Received: October 10, 2011 Published: January 19, 2012 1257

dx.doi.org/10.1021/om200953z | Organometallics 2012, 31, 1257−1270

Organometallics

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Scheme 1. Synthesis of Pt(II) Fluoroalkyl Complexes by Oxidative Addition of RFX (X = I, Br) to a Zerovalent Metal Complex

Scheme 3. Synthesis of Pt(II) Trifluoromethyl Complexes by Ligand Metathesis

addition of CF 3 I to the Pt(II) complexes [Pt(CH 3 ) 2 (PMe2Ph)2] and [PtI(CH3)(PMe2Ph)2]. Fluoroaryl Complexes. The most convenient synthesis of Pt(II) pentafluorophenyl complexes starts with the preparation of the dimeric species [Pt(μ-Cl)(C 6 F 5 )(tht)]2 (tht = tetrahydrothiophene) by means of a lithium aryl reaction on [PtCl2(tht)2],16 which then undergoes a ready displacement of tht by appropriate ligands, in particular a chelating diphosphine (Scheme 5). Following this route, Hughes et al. reported the synthesis, molecular structures and chemistry of some Pd(II) and Pt(II) of the type [MCl(C6F5)(L−L)] (M = Pd, Pt; L−L = alkyl diphosphine or diamine).17 We have extended the procedure to the preparation of a homologous series of complexes of the type [PtCl(C6F5)(P−P)] with a wide range of diaryl diphosphine ligands.18 Treatment of the binuclear complex in acetone with 2 equiv of diaryl diphosphine (Scheme 5) led in all cases to the formation of the corresponding neutral complexes [PtCl(C6F5)(P−P)] in good to high yields (70−99%).

to [Pt(PPh3)4].9 Only the complex with a cis geometry was formed in this reaction. The trifluoromethyl bromo Pt(II) complex trans-[PtBr(CF3)(PPh3)2] has been reported to be formed in good yield by treatment of [Pt(PPh3)4] with gaseous CF3Br in benzene at room temperature for longer reaction times (8 days) (Scheme 1, route c). Treatment of trans[PtBr(CF3)(PPh3)2] with AgBF4 in CH2Cl2/acetone at room temperature and subsequent addition of LiCl afforded the corresponding chloro derivative trans-[PtCl(CF3)(PPh3)2]. Complexes of the type trans-[PtCl(RF)(PMePh2)2] were prepared in good yields by initial oxidative addition of perfluoroacyl chlorides RFCOCl (RF = CF3, C2F5) to [Pt(PMePh2)4] to afford the corresponding platinum(II) perfluoroacyl derivatives followed by thermal decarbonylation under vacuum (Scheme 2).10

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REACTIVITY Transition-Metal−σ-Fluoroalkyl Bonding and Trans Influence of the Trifluoromethyl Ligand. As we have already stressed above, σ-perfluoroalkyl complexes of the transition metals show a remarkable thermal stability as compared to their σ-alkyl counterparts.5 The nature of the transition-metal−σ-fluoroalkyl bonding is controversial, particularly with regard to the extent of metal−fluoroalkyl M(dπ)− RF(σ*) back-bonding.10,19 Specifically, PtII−CF3 complexes show Pt−C(RF) bond lengths shorter than the analogous Pt−CH3 bonds. Although this shortening can be interpreted in terms of Pt(dπ)−CF3(σ*) back-bonding, it seems more likely that the Pt−CF3 bond contraction is primarily a consequence of the positive charge induced on the σ-carbon atom by the electronegative fluorine atoms. Thus, the electronegative fluorocarbon group strengthens the metal−carbon bond by covalent−ionic resonance, e.g. M−CF3 ↔ M+CF3−, and the higher positive charge induced on the metal atom may contract the metal σ orbitals, thus improving overlap with the small carbon σ orbitals. In addition, the presence of highly electronegative fluorine atoms may increase the s character of the carbon σ orbital, resulting in its contraction with a consequent shortening of the Pt−CF3 bond and an improvement of the overlap with the metal σ orbital.20 The trans influence, i.e. the ability of a ligand to weaken the bond of a group trans to it,21 of CF3− compared to that of other σ-carbon donor ligands in Pt(II) square-planar complexes has

Scheme 2. Synthesis of Pt(II) Perfluoroalkyl Complexes from Decarbonylation of Perfluoroacyl Compounds

A wide series of Pt(II) trifluoromethyl chloro complexes of the type trans-[PtCl(CF3)(PR3)2] (PR3 = PMePh2, PMe2Ph, PMe3), [PtCl(CF3)(P−P)] and [PtCl(CF3)(P−P)*] (P−P = chelating diphosphine: P−P* = chiral chelating diphosphine) have been prepared by ligand metathesis (Scheme 3) of the two coordinated PPh3 ligands in trans-[PtCl(CF3)(PPh3)2] with more basic monodentate phosphines or chelating diphosphines, also chiral, in n-heptane at room temperature.9,11−14 The complex trans-[PtI(CF3)(PMe2Ph)2] was prepared by pyrolysis of the Pt(IV) trifluoromethyl complexes [PtI(CH3)2 (CF 3)(PMe 2 Ph) 2] and [PtI 2 (CH 3)(CF 3 )(PMe 2Ph) 2 ] at 180 and 225 °C (Scheme 4), respectively, under vacuum.15 The Pt(IV) complexes were in turn prepared by oxidative 1258


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Scheme 4. Synthesis of trans-[PtI(CF3)(PMe2Ph)2] Complexes via Pyrolysis of Pt(IV) Trifluoromethyl Compounds

Scheme 5. Synthesis of Pt(II) Pentafluorophenyl Complexes

Table 2. Selected IRa and NMRb Data for trans[PtH(X)(PPh3)2] Complexes

been largely discussed on the basis of spectroscopic and structural data. IR data based on ν(PtCl) in complexes of the type trans-[PtCl(X)(PMePh2)2] (X = CH3, CF3) indicate that the trans influence of CH3− is higher than that of CF3−, since the ν(PtCl) values are 272 and 302 cm−1, respectively.10 The structural data based on Pt−Cl bond lengths in complexes of the type trans-[PtCl(X)(PPh3)2] (Table 1)22 allow a Table 1. Pt−Cl Bond Lengths for Some [PtCl(X)(PPh3)2] Complexes X

Pt−Cl (Å)

ref

Me Ph CH2CN C6F5 Cl

2.440(4) 2.414(2) 2.390(3) 2.370(1) 2.300(1)

22a 22b 22c 22d 22e

X

ν(PtH) (cm−1)

Me Ph C6 F 5 CH2CN CF3 Cl

1968 1966 2047 2041 2069 2234

J(PtH) (Hz)

δ(H)

656 600 716c 746 544 1192

−3.77 −5.71 −6.17c −7.32 −8.34 −16.13

1

a In CH2Cl2 solution. bIn CD2Cl2 solution. cIn acetone-d6 solution; unpublished results.

ligand, thus paralleling the behavior shown by the previously mentioned chloro complexes. Conversely, the NMR transinfluence order based on 1J(PtH) values is CF3− > C6H5− > CH3− > CH2CN− > Cl−. The higher trans influence of CF3− relative to CH3− results also from 1J(PtH) data for the series of complexes trans-[PtH(RX)(PCy3)2]: e.g., RX = CF3− (569 Hz)11 > CH3− (648 Hz).3b The opposite position of CF3− in the two series of trans influences has been tentatively explained by the different mechanisms operating on 1J(PtH) and ν(PtH). The former depends predominantly on the s character of the platinum hybrid orbital used in the Pt−H bond, while the latter is sensitive also to electrostatic effects induced by the electronegative fluorine atoms. The aforementioned properties of the Pt(II)−trifluoromethyl bonds are also substantiated by their reactivity. In fact, it is observed that the presence in the metal coordination sphere of the Pt(II) center of a trifluoromethyl ligand or an alkyl ligand bearing in the alkyl chain electronegative substituents appears to stabilize metal species containing reactive groups such as hydride, carbene, hydroxo, hydroperoxo and tert-butylperoxo. In the following we report some of their features. Platinum(II) Hydrido Fluoroalkyl Complexes. The Pt(II) hydrido trifluoromethyl complex trans-[PtH(CF3)(PPh3)2] was

comparison between different alkyl/aryl groups and fluorinated ones. Again, it has been observed that the methyl group has the highest structurally based trans influence. Unfortunately, no data are available for the corresponding CF3 derivative. We have investigated such effects considering ν(PtH), 1J(PtH), and δ(H) in the hydrido complexes trans-[PtH(X)(PPh3)2] (X = Me, Ph, CH2CN, CF3, Cl).11 These are summarized in Table 2. The Pt−H stretching frequency in trans-[PtH(X)P2] complexes is very dependent on the trans ligand X usually decreasing with increasing trans influence of X.21 The Pt−H stretching in trans-[PtH(RX)(PPh3)2] (RX = σ-carbon donor ligand) (Table 2) displays values that are significantly lower than in trans-[PtH(Cl)(PPh3)2], as expected for the higher trans influence of σ-C-donor ligands relative to that of halides.9 For the RX ligands, the trans-influence order is as follows: CH3− ≈ C6H5− > CH2CN− > CF3−, with the methyl and phenyl ligands having a higher trans influence than the trifluoromethyl 1259


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instance, the complex cis-[PtH(CH2CF3)(PPh3)2], obtained readily from the reaction of the cationic solvento species cis[Pt(CH2CF3)(PPh3)2(solv)]+ with NaBH4 at 0 °C, undergoes slow thermal reductive elimination of CH3CF3 at room or slightly above room temperature, in contrast with the highly unstable hydrido methyl analogue cis-[PtH(CH3)(PPh3)2] (Scheme 8).25

prepared according to Scheme 6 starting from trans-[PtCl(CF3)(PPh3)2], treated with AgBF4 to afford the labile solvento Scheme 6. Synthesis of the Hydrido Trifluoromethyl Complex trans-[PtH(CF3)(PPh3)2]

Scheme 8. Mechanism of Reductive Elimination of 1,1,1Trifluoroethane from cis-[PtH(CH2CF3)(PPh3)2]

The rates of reductive elimination (e.g., at 40 °C in benzene the k1 value is 4.67 × 10−4 s−1) are unaffected by the presence of other added free ligands such as PhCCPh or PPh3 as observed in other alkane elimination processes. The kinetic investigation is in agreement with a mechanism involving primary concerted unimolecular reductive elimination with rupture of both Pt−H and Pt−C bonds during the rate-determining step with concomitant C−H bond formation. Fluorinated and other electron-withdrawing substituted alkyls stabilize hydrido carbene species of the type [PtH(RX)(carbene)(PPh3)] (RX = CF3, CH2CF3, CH2CN; carbene = diaminocarbene ligand) having adjacent Pt−H and Pt−RX bonds (Scheme 9).26 The reaction sequence involves initial

cationic species trans-[Pt(CF3)(PPh3)2(solv)][BF4] (solv = acetone, CH2Cl2) and subsequently with NaBH4 in EtOH at 0 °C.23,9 trans-[PtH(CF3)(PPh3)2] is a white crystalline solid, very stable both in the solid state and in solution. The hydridotrifluoromethyl complexes trans-[PtH(CF3)P2] (P = PMePh2, PMe2Ph, PMe3, P(CH2Ph)Ph2, P(CH2Ph)2Ph, P(CH2Ph)3, PEtPh2, P(C6H11)3, P(C6H4Me-4)3) are obtained from trans-[PtH(CF3)(PPh3)2] in high yield by ligand exchange with 2 equiv of more nucleophilic phosphines, in n-heptane at room temperature. Similar exchange reactions between trans[PtH(CF3)(PPh3)2] and equivalent amounts of diphosphine, P−P = cis-Ph2PCHCHPPh2 (diphoe), Ph2PCH2CH2PPh2 (dppe), Me2PCH2CH2PMe2 (dmpe), Ph2PCH2CH2CH2PPh2 (dppp), lead to the formation of the corresponding [PtH(CF3)(P−P)] compounds (Scheme 7).

Scheme 9. Synthesis and Reactivity of Some Pt(II) Hydrido Alkyl Diaminocarbene Complexes

Scheme 7. Ligand Exchange of Monodentate Tertiary Phosphine and Chelating Diphosphine Ligands To Afford Pt(II) Hydrido Trifluoromethyl Complexes

The hydridotrifluoromethyl complexes with P−P = cisPh2PCHCHPPh2, Ph2PCH2CH2PPh2 can be prepared also by reaction of the parent chloro derivatives [PtCl(CF3)(P−P)] with NaBH4 in EtOH at room temperature. The hydrido trifluoromethyl complexes, in particular those with adjacent Pt−C and Pt−H bonds, are quite thermally stable and no reductive elimination processes of H−CF3 are observed.11 This is markedly in contrast with the behavior of the cis-hydridomethylplatinum(II) phosphine complexes cis[PtH(CH3)(PPh3)2]3 and [PtH(CH3)(P−P)] (P−P = dppe, dppp).24 In particular, the former is stable only at very low temperature (−80 °C), but already at −25 °C it decomposes with CH4 elimination and formation of Pt(0) species. The stability increases with the presence in the alkyl chain of electron-withdrawing stabilizing substituents. 2,12,23 For

reaction of the hydrido alkyl complexes [PtH(RX)(PPh3)2] (Scheme 9, A) with the isocyanide RNC (R = p-MeOC4H4) to afford the parent isocyanide precursors [PtH(RX)(CNR)(PPh3)] (B), as previously reported for cyanomethyl derivatives.2b Further reaction of compounds B with an amine such as azetidine gives the diaminocarbene complexes C in high yield. 1260


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These latter appear to be stable in the solid state and in solution. No reductive elimination of HRX is observed in refluxing THF for several hours, even in the presence of a 5-fold excess of diphenylacetylene. On the other hand, complexes of type C (RX = CH2CF3, CH2CN) react with 1 equiv of PPh3 at room temperature to give complexes D and H−RX as the only reaction products. It is worthwhile noting that complexes C with RX = CF3 were found to be completely unreactive under the same experimental conditions. The formation of the alkanes HRX is not due to a reductive elimination process involving Pt−H and Pt−RX bond ruptures. Isotopic experiments with derivatives C having Pt−D and N−D deuterated bonds indicate instead that what actually occurs is the protonolysis (possibly intramolecular as depicted in Scheme 9) of the Pt−RX bond by the N−H aminocarbene proton. In contrast with this behavior, the trifluoromethyl hydrido diaminocarbene complex C reacts with diphosphines, undergoing selective cleavage of the metal−carbene bond with formation of formamidines and the stable cis hydrido trifluoromethyl complexes E. Also in this case, isotopic experiments with Pt−D and N−D derivatives indicate that the Pt−H(D) bond is not involved in the metal−carbene cleavage, which is conversely due to an intramolecular 1,2-hydrogen transfer of the aminocarbene proton leading to the formamidine. Electrophilic Cleavage of C−F Bonds. It is known that the perfluoroalkyl groups in transition-metal complexes are quite resistant to chemical attack, paralleling the known inertness of fluorocarbon compounds.6 However, C−F bonds α to the transition metal are susceptible to electrophilic attack under mild conditions by proton and Lewis acids to give carbene and carbonyl complexes.27 Such reactivity has been explained by a weakening of the C−F bonds α to the transition metal, which accounts for the reduced C−F stretching frequencies and increased bond lengths in comparison with those in aliphatic compounds. Trifluoromethyl complexes of platinum(II) are no exception, and the CF3 group in trans[PtX(CF3)P2] complexes (X = H, P = PPh3; X = Cl, P = PMe2Ph) can be converted to a carbene ligand by reaction at room temperature with an ethereal solution of HBF4 in dry Et2O in the presence of a protic nucleophile, according to Scheme 10.28 Similarly, the reactions of trans-[PtH(CF3)P2] (P = PPh3, PCy3) with HBF4 in the presence of H2O led to the conversion of CF3 to a CO group.28b The reactions in Scheme 10 have been suggested to proceed through a difluorocarbene intermediate, which was not isolated because of a rapid reaction with the protic nucleophile to give the carbene or carbonyl products but was detected by lowtemperature 1H, 19F, and 31P NMR spectroscopy. The X-ray crystal structure of trans-[PtH(CF3)(PPh3)2] reveals that the Pt−CF3 bond length of 2.009(8) Å is considerably shorter than that found in the hydrido cyanomethyl complex trans[PtH(CH2CN)(PPh3)2] (Pt−C = 2.16(1) Å) and also in other transition-metal fluoroalkyl complexes. The C−F distances are 1.391(9), 1.410(8), and 1.411(9) Å (average 1.404(9) Å). They are comparable with the Cα−F distances in trans-[PtCl(CαF2‑CβF3)(PMePh2)2] of 1.387 Å (average),10 but they are significantly longer than those found for Cβ−F (range 1.302(15)−1.363(18) Å; 1.332 Å average). The average Cβ−F distance is close to that observed in gaseous CF3CF2I (average 1.338(4) Å) by electron diffraction.29 The C−F bond cleavage in trans-[PtH(CF3)(PPh3)2] occurs also with other electrophiles EX such as HPF6, NOBF4, and

Scheme 10. Electrophilic Cleavage of C−F Bonds in Pt(II) Trifluoromethyl Complexes by Proton Acids and Formation of Carbene Complexes

p-toluenesulfonic acid. When the reactions are carried out in wet Et2O, the corresponding carbonyl derivatives trans-[PtH(CO) (PPh3)2][X] are isolated. The reactions in Scheme 10 have also been shown to be influenced by steric effects due to the entering nucleophile (i.e., t-BuOH) or to the coordinated phosphines (e.g., PCy3, PBz3, PBzPh2), affording in the former case only the carbonyl species or in the latter case, upon reaction with MeOH, different amounts of the dimethoxycarbene products. Platinum(II) Hydroxo and Alkoxo Complexes. Hydroxo alkyl complexes of the general formula [Pt(OH)(RX)P2] (RX = CH2CN, CF3, CH2CF3; P2 = 2 PPh3, diphosphine) were prepared by treating the cationic solvento species [Pt(solv)(RX)P2][BF4] with aqueous KOH in MeOH (Scheme 11).9,30 Scheme 11. Preparation of Pt(II) Hydroxo and Alkoxo Complexes

The corresponding methoxo derivatives [Pt(OMe)(RX)P2] were obtained by metathesis of [PtCl(Rx)P2] species with a methanolic solution of NaOMe in benzene at room temperature or at 50 °C for a few hours (Scheme 11). Both hydroxo and methoxo complexes are white crystalline solids. Methoxo complexes slowly undergo hydrolysis in wet solvents to afford the corresponding hydroxo species. It is observed that prolonged heating in benzene solution of [Pt(OMe)(RX)P2] complexes does not yield the corresponding well-known hydrido complexes2a,23 by β-hydrogen abstraction from the methoxo ligand. Hydroxo and methoxo complexes undergo facile formal insertion of small unsaturated molecules such as CO, COS, 1261


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CS2, and SO2 into Pt−OH and Pt−OMe bonds. The formal insertion reactions of CO are reported in Scheme 12, and they

Scheme 14. Condensation Reactions of Pt(II) Hydroxo Complexes

Scheme 12. Formal CO Insertion Reactions in Pt(II) Hydroxo Complexes

P = monophosphine). These authors have suggested that the dissociation ability of these compounds results largely from the electron-donating power of the trans anionic ligand R (trans influence). We have performed pH determinations of some representative hydroxo trifluoromethyl, hydroxo methyl, and hydroxo phenyl complexes carried out in MeOH/H2O = 9/1 (v/v) using different concentrations.34 In order to compare the basicities of the various complexes, the pH was extrapolated at 10−3 M from pH vs −log [Pt] plots (Table 3).

afford hydroxycarbonyl (organometallic carboxylic acids) or alkoxycarbonyl derivatives, respectively. These latter species can be formed also by treating the hydroxo complexes with CO the corresponding alcohol ROH in the presence of NEt3.31 Similar reactions of hydroxo complexes with COS, CS2, and SO2 in MeOH/NEt3 yield the corresponding O-methylthiocarbonato, O-methyldithiocarbonato, and methoxysulfinato complexes, respectively (Scheme 13).9

Table 3. pH Measurements of Pt−OH Complexes in a MeOH/H2O Mixture (9/1 v/v)

Scheme 13. Formal Insertion Reactions of COS, CS2, and SO2 in Pt(II) Hydroxo Complexes

complex

pHa

[Pt(OH)(CF3)(diphoe)] [Pt(OH)(CF3)(dppe)] [Pt(OH)(CH3)(diphoe)] trans-[Pt(OH)(CF3)(PPh3)2] trans-[Pt(OH)(Ph)(PPh3)2]

7.6 9.4 11.3 8.4 10.7

a pH extrapolated graphically at [PtOH] = 10−3 M from pH vs −log [Pt] plots.

The data reported in Table 3 generally indicate that hydroxo trifluoromethyl complexes are modest bases compared to the corresponding methyl and phenyl derivatives. They also do not suggest a correlation between complex geometry and basicity, the latter reflecting simply the electron-donating (-withdrawing) character of the RX ligand. Although spectroscopic parameters indicate a considerable covalent character for the Pt−O bond under anhydrous conditions,35 the data in Table 3 indicate the important solvolytic role of water in promoting ionic dissociation of OH−: i.e., [Pt(OH)(RX)P2] ⇆ [Pt(RX)P2] + OH−. This behavior was confirmed by molar conductivity data taken in anhydrous solvents or in solvents mixed with water. In all cases, it is observed that addition of 10% H2O to the solvent (THF, MeOH) produced an increase in the molar conductivity.34 Similarly, it was observed that the 1JP−Pt value (3317 Hz) of the phosphorus trans to −OH in [Pt(Ph)(OH)(diphoe)] increases by about 300 Hz (3612 Hz) in THF on going from dry solvent to solvent containing 25% water. The aforementioned equilibrium is completely shifted to the right in the presence of good donor ligands (such as phosphines), even under anhydrous conditions. For instance, addition of a 4 mol excess of P(nBu)3 to a 10−3 M solution of [Pt(OH)(CF3)(diphoe)] in 1,2dichloroethane shows molar conductivity values corresponding to a complete dissociation with the formation of the cationic species [Pt(P(n-Bu)3)(CF3)(diphoe)][OH]. Platinum(II) Hydroperoxo and Alkylperoxo Complexes. Hydroperoxo complexes can be readily obtained by

The Pt(II) hydroxo complexes behave as bases (organometallic bases, see further), and they undergo acid−base reactions, reacting with a variety of acids, including fairly weak sulfur-, carbon-, and nitrogen-containing acids such as hydrogen sulfide, thiols, phenylacetylene, acetamide, and methylaniline, leading to the corresponding condensation complexes (Scheme 14a).9 A similar reaction occurs also with weakly acidic Pt−COOH complexes to give a series of CO2-bridged dimeric complexes as indicated in Scheme 14b.31 The reactions cited above with weak acids represent a clear, albeit indirect, evidence of the basic behavior of [Pt(OH)(Rx)L2] in solution. On the other hand, the only direct measurements of OH− dissociation were first reported by Otsuka and co-workers32 and by Arnold and Bennett33 on compounds of the type trans-[Pt(OH)(R)P2] (R = H, Me, Ph; 1262


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reaction with H2O2, taking advantage of the basic nature of the Pt−OH complexes and the slight acidity of aqueous hydrogen peroxide, as illustrated in Scheme 15.36 However, it must be

Table 4. Coupling Constants Involving [Pt(CF3)(X)(diphoe)] Complexes34

195

Pt for Some

JP−Pta,b (Hz)

1

X OH OOH OO-t-Bu

Scheme 15. Pt(II) Hydroperoxo Complexes a

3263 3139 3075

In CD2Cl2 solution. bPhosphorus trans to oxygen.

Table 5. Coupling Constants Involving 195Pt for Some trans[Pt(CF3)(X)(PPh3)2] Complexes34 JF−Pta (Hz)

2

X OH OOH OO-t-Bu

noted that these reactions seem to be fairly sensitive to the nature of the alkyl ligand bound to the metal (Scheme 15). In the case of CF3 a quantitative amount of the final product can be obtained, whereas for CH2CN or CH2CF3, the system is unreactive toward H2O2 and the starting hydroxo complexes can be recovered unaltered. In the case of CH3, we found an intermediate situation, since the IR data of the final solid reveal the presence of both hydroxo and hydroperoxo species.36b The hydroperoxo complexes show typical O−H stretchings in the 3517−3585 cm−1 region corresponding to a 70−100 cm−1 decrease as compared to the parent metal hydroxo complexes. The hydroperoxidic proton in the 1H NMR spectra could not be located in the range +7 to −5 ppm, probably due to hydrogen bond association. Pt(II) tert-butylperoxo complexes are prepared similarly by condensation reactions of hydroxo complexes with t-BuOOH; in Scheme 16 are reported the syntheses of some tert-butylperoxo complexes of trans geometry.35,36b

a

578 585 555

In CD2Cl2 solution.

The main goal for which the Pt(II) hydroperoxo and tertbutylperoxo complexes were prepared was to test their oxygen transfer properties toward olefins. All of these complexes, except for tert-butylperoxo complexes of trans geometry (see further), were inactive toward both linear and cyclic olefins in a temperature range varying between 25 and 100 °C. On the other hand, the hydroperoxo trifluoromethyl complex [Pt(OOH)(CF3)(diphoe)] was shown to undergo an oxygen transfer reaction to other inorganic and organic substrates such as PPh3, CO, and C6H5CHO to afford the corresponding oxidized products OPPh3, CO2, and C6H5COOH, respectively, restoring the Pt(II) hydroxo species.35b CO2 was obtained also by the reaction of [Pt]−OOH with [Pt]−COOH species ([Pt] = Pt(CF3)(diphoe)). The reaction yields also [Pt(CF3)(diphoe)]2(μ-CO2) similarly to the process reported in Scheme 16 and is likely to proceed via an organometallic peroxyacid species, Pt−C(O)OOH, as was suggested by an 18O labeling experiment.31 The MOOH species are more versatile oxygen transfer agents than the corresponding MOO-t-Bu species, the latter being reactive only with CO to give CO2 at room temperature. However, as previously mentioned, tert-butylperoxo complexes of trans geometry are found to be active in the stoichiometric oxygen transfer to 1-octene to give 2-octanone as the only product (Scheme 17).36

Scheme 16. Synthesis of Pt(II) trans-tert-Butylperoxo Complexes

Scheme 17. Oxygenation Reactions of 1-Octene by Pt(II) tert-Butylperoxo Complexes Since some of these condensation reactions result in the formation of an equilibrium mixture of the tert-butylperoxo product and the starting hydroxo complex, the use of anhydrous Na2SO4 appears to overcome this problem. The yields range between 50 and 80%. The peroxo species were characterized by spectroscopic techniques. Significantly, they show in their IR spectra the peroxidic stretching band around 890 cm−1. The trans influence exerted by the OOH and OO-t-Bu ligands is slightly higher than that of the OH ligand, as can be seen by inspecting both the 1JP−Pt data for a series of [Pt(CF3)(X)(diphoe)] complexes (Table 4) and the 2JF−Pt data for a series of trans-[Pt(CF3)(X)(PPh3)2] complexes (Table 5).35 An X-ray crystal structure determination was carried out for trans-[Pt(OO-t-Bu)(Ph)(PPh3)2];36 unfortunately, due to crystal decomposition and vibrational disorder of the peroxo group, the crystal data were of poor quality.

In Table 6 are reported the yields obtained in some selected reactions of tert-butylperoxo complexes with 1-octene. Complexes of cis geometry of the type cis-[Pt(OO-t-Bu)(RX)(PPh3)2] (RX = Me, Bz) or hydroperoxo complexes of the type 1263


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Table 6. Oxygenation of 1-Octene with trans-[Pt(OO-tBu)(RX)L2] Complexesa complex

yield (%)

trans-[Pt(OO-t-Bu)(CF3)(PPh3)2] trans-[Pt(OO-t-Bu)(CF3)(PPh2Me)2] trans-[Pt(OO-t-Bu)(CF3)(PPhMe2)2] trans-[Pt(OO-t-Bu)(Ph)(PPh3)2] trans-[Pt(OO-t-Bu)(Ph)(PPh2Me)2] trans-[Pt(OO-t-Bu)(Ph)(PBz3)2] trans-[Pt(OO-t-Bu)(Ph-o-CN)(PPh3)2]

80 78 75 46 38 55

Scheme 18. Examples of Oxidation Reactions with Hydrogen Peroxide Catalyzed by PtII Complexes

Reaction conditions: [Pt] = 4.6 × 10−2 M; [olefin] = 1.6 M; solvent, DCE; T = 80 °C; time, 16 h. a

trans-[Pt(OOH)(RX)(PPh3)2] (RX = CF3, Ph) proved inactive in the oxygenation reactions of Scheme 17 under the same reaction conditions. The oxygen transfer by tert-butylperoxo complexes appears to be sensitive to both electronic and steric effects. The former are due primarily to the RX ligand; in fact, as the electronwithdrawing character of RX increases (Ph < Ph-o-CN < CF3), the efficiency of the oxygen transfer increases. This is generally considered to be strictly related to the nature of the O−O bond and the electrophilic character of the peroxidic oxygen in this class of complexes.37,38 Steric requirements are also important, as shown by the inactivity of the PBz3 derivative and by the already noticed olefin-size effect on the yield of the oxygentransfer reaction.36b A plausible reaction pathway for the oxygen-transfer reactions by t-butylperoxo species involves the formation of a five-coordinate intermediate which evolves to the final products likely via a quasi-peroxymetallacycle as proposed by Mimoun et al. for related systems.39

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free H2O2, eventually leading to their decomposition to unidentified Pt species.34 Thus, complexes of the type [Pt(CF3)(P−P)(solv)]+ or [Pt(CF3)(OH)(P−P)] (P−P = various diphosphines) have proved to be good catalysts for the selective (>99%) epoxidation of simple terminal olefins, using as oxidant commercial solutions of H2O2 in concentrations ranging between 72% and 5%. The reaction can be carried out at 25 °C either in one-phase media (e.g., THF/H2O, EtOH/H2O) or in twophase media (CH2Cl2/H2O), and some examples of the reactivity observed are reported in Table 7.40 As shown, consistent

CATALYTIC OXIDATIONS

Platinum(II) trifluoromethyl hydroxo complexes of the type [Pt(OH)(CF3)(diphoe)] and especially the trifluoromethyl solvento cationic species [Pt(CF3 )(diphoe)(solv)][BF4 ] (solv = CH2Cl2) were found to be efficient catalysts in oxygen transfer reactions, such as the selective epoxidation of terminal alkenes, the oxidation of α,β-unsaturated ketones, the hydroxylation of aromatics, and the Baeyer−Villiger oxidation of cyclic ketones with dilute H2O2 as the primary oxidant in protic media (Scheme 18). Since the application of the aforementioned complexes in the Baeyer−Villiger oxidation has been recently reviewed by us,37 in the present work we will focus on the other reactions. Use of Trifluoromethyl Derivatives. The epoxidation of alkenes catalyzed by PtII−CF3 complexes represented the first reported example of epoxide formation with a group VIII metal/hydroperoxide system.40 The phenyl and methyl hydroxo complexes [Pt(OH)(RX)(diphoe)] (RX = Ph, Me) were found to be poorly active (RX = Ph) or completely inactive (RX = Me) in comparison with the trifluoromethyl derivatives.34 This behavior may reflect the ease with which the Pt−C bond is hydrolyzed because of the slight acidity of H2O2. In this respect, the Pt−C bond in fluoroalkyl complexes is known to be very stable toward hydrolysis. Hydroxo complexes of trans and cis geometry with monophosphine ligands such as trans-[Pt(OH)(RX)P2] (RX = CF3, Ph, Me, Bz; P2 = PPh3, PPh2Me, PBz3) and cis-[Pt(OH)(Bz)(PPh3)2] did not show any catalytic activity. They undergo phosphine ligand dissociation, which is then rapidly oxidized in the presence of

Table 7. Epoxidation of Terminal Alkenes with H2O2 Catalyzed by [Pt(CF3)(diphoe)(solv)]+a olefin

time

TON

propene 1-hexene 1-octene styrene cyclohexene

35 min 1h 1h 48 h 48 h

325 115 84 9 =

Experimental conditions: [Pt] = 1.7 × 10−2 M; [1-octene] = 7.4 × 10−1 M; solvent, CH2Cl2; 35% H2O2 1/1 with respect to substrate.

a

with the above observation concerning the nucleophilic nature of the peroxygen, the normally less reactive terminal olefins give better results and this behavior complements what is generally observed with d0 transition-metal systems, such as e.g. Mo(VI) or W(VI) centers, where the opposite reactivity trend occurs.41 Several factors such as the nature of the catalyst and that of the solvent appear to influence significantly the efficiency of the catalytic reaction reported in Scheme 18. Cationic solvento species afford higher initial reaction rates compared to the corresponding neutral hydroxo derivatives, and this was explained by the fact that the former species may give higher concentrations of the metal−olefin complex [Pt(CF3)(diphoe)(ol)]+, which appears to be a key reactive intermediate in the formation of the epoxide by external nucleophilic attack on the 1264


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modifying the catalytic system by introducing either a different olefin activator or a different H2O2 activator. The former point was addressed by using a Pd+/PtOH complex mixture,44 with the [Pd(CF3)(dppe)(CH2Cl2)]+ complex as the olefin activator. The individual roles of the two metals in the catalytic system were safely attributed, since it was found by 19F NMR that the equilibrium below was completely shifted to the left.

coordinated olefin by a suitable oxidant. The use of watermiscible solvents such as THF and EtOH slows down the reaction, as the solvent may compete favorably to bind at the vacant coordination site of the [Pt(CF3)(diphoe)]+ species. From this point of view, the use of a biphasic reaction medium such as CH2Cl2/H2O appeared to be very useful, since waterimmiscible solvents are generally very weak donor ligands and allow a considerable efficiency to be observed even at 5% H2O2 concentration. The same epoxidation reaction was carried out also in the enantioselective version, by modifying the [Pt(CF3)(P−P)(solv)]+ catalyst with chiral diphosphines such as diop, prophos, chiraphos, and pyrphos. Chiral discrimination up to 41% was obtained in the case of propylene.42 The mechanism of the epoxidation reaction is shown in Scheme 19. Evidence for many of the organometallic

Pd+ + Pt−OH ⇌ Pd−OH + Pt+ Interestingly, as can be seen from Table 8, under these conditions the selectivity of the system is completely changed, Table 8. Oxidation of Olefins in the Presence of a 1/1 [Pd(CF3)(dppe)(solv)]+/[Pt(CF3)(OH)(diphoe)] Catalyst Mixturea

Scheme 19. Proposed Mechanism for the Epoxidation of Terminal Olefins by Pt(II) Trifluoromethyl Complexes

olefin

time (h)

turnover

ketone (%)

epoxide (%)

1-octene butyl vinyl ether cyclooctene styrene

24 6 24 6

118 158 17 128

66 84 48 72

18 31 14

Experimental conditions: [Pd+] = [Pt−OH], 2.0 × 10−3 M; [olefin], 1.4 M; [H2O2], 0.7 M; solvent THF; T = 65°C.

a

ketones becoming the preferred products. This can be explained by the known ability of Pd(II) to promote β-hydride elimination−addition processes, which occurs in the intermediate B in Scheme 19, now containing Pd. A similar shift in selectivity can also be obtained by using t-BuOOH as oxidant. This catalytic reaction reproduces what was already observed stoichiometrically and reported in Table 7. Under the experimental conditions reported in Table 2, the reaction requires higher temperatures (83 °C) and the use of DCE as the solvent, yielding in many cases (1-hexene, 1-octene, styrene) complete selectivity to methyl ketones, albeit with lower catalytic activity. 45 In this case the β-hydride elimination−addition process, disfavored in Pt complexes, is most likely induced by the steric hindrance of the t-Bu group in the key peroxy intermediate (Scheme 19). After the transfer of the peroxidic moiety onto the coordinated olefin, the evolution of the intermediate species to products is controlled basically by the position of the open-chain vs cyclic configurational equilibrium shown in Scheme 20. Product formation is

intermediates and the individual steps involved in Scheme 19 was gained from IR studies, 19F NMR investigations, and especially designed experiments including studies of the acidity effect.43 In particular, in the case of the 1-octene epoxidation catalyzed by [Pt(CF3)(diphoe)(CH2Cl2)](BF4) in CH2Cl2, the 19 F NMR spectrum in CD2Cl2 performed on the working catalytic system shows the coexistence of [Pt(CF3)(diphoe)(CH2Cl2)]+ (δ −29.36 ppm, 2JF−Pt = 518 Hz), [Pt(CF3)(diphoe)(1-oct)]+ (δ −21.07 ppm, 2JF−Pt = 517 Hz) and [Pt(CF3)(OOH)(diphoe)] (δ −27.32 ppm, 2JF−Pt = 554 Hz). Scheme 19 illustrates also the basic mechanistic principles with which these Pt(II) complexes operate as oxidation catalysts. Two different catalytic cycles are involved, one leading to the activation of the substrate and the other to the regeneration of the actual oxidizing species (A). The two cycles intersect in the rate-determining step (the oxygen transfer), in agreement with the second-order dependence on Pt concentration observed in the kinetics. The intermediate (B) evolves into products according to the rearrangement indicated by the arrows. The relative weight of the two cycles depends on the prior hydrolysis equilibrium, which is determined essentially by the acidity of the medium. In fact, it is possible to maximize the reaction rate by finely tuning the acidity of the system.43 A noteworthy feature of this catalytic system is the bifunctional amphoteric role of platinum. The cationic species behaves as a Lewis acid, increasing the electrophilicity of coordinated olefin, while the conjugated base Pt−OH increases the nucleophilicity of hydrogen peroxide by forming Pt− OOH.34 This bifunctional role suggests the possibility of

Scheme 20. Equilibrium Governing the Selectivity in the Oxidation of Olefins Catalyzed by Pd and Pt Complexes

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Scheme 21. Catalysts Used in the Study of the Ring-Shape, Ring-Size Effect on the Epoxidation of 1-Octene

accounted for via either oxirane ring closure or β-hydride elimination−addition. The choice between the two pathways applies only to Pt complexes, depending on the experimental conditions (oxidant, temperature, etc.), while for all Pd complexes reported the β-hydride elimination−addition appears to be overriding in any case, and the ketone is always the preferred product independent of the oxidant used.39a,b,44 The fine tuning of the steric requirements related to the coordination of the olefin and the rearrangement of intermediate B was examined in the case of 1-octene with a series of different [Pt(CF3)(X)(P−P)] (X = OH, CH2Cl2) complexes (Scheme 21). By varying systematically the shape and size of the diphosphine−metal ring, a strong influence on the catalytic activity was evidenced.46 Thus for example, a 2 order of magnitude difference in catalytic activity was observed when going from dppe to dppb. Flat five-membered rings appear to be best suited to maximize the activity of the catalyst, probably optimizing the steric requirements for olefin coordination to the metal and subsequent binding of the peroxy oxygen from the axial position, both factors being critical in determining the catalytic activity. The epoxidation of α,β-unsaturated ketones (Weitz−Scheffer oxidation) in alkaline media is one of the oldest applications of hydrogen peroxide as oxidant in synthetic organic chemistry.47 The reaction involves as the rate-determining step the nucleophilic attack of the hydroperoxide anion to the carbon−carbon double bond made electrophilic by the electron-withdrawing carbonyl substituent.48 For this reason and on the basis of Scheme 19, Pt−CF3 complexes seemed to be particularly suited to promote this catalytic reaction, even in its enantioselective version. The enantioselective version of this reaction was first reported by Wynberg utilizing a chiral organic base (a quininium salt) and hydrogen peroxide under phasetransfer conditions.49 Thus, the catalytic epoxidation of a series of α,β-unsaturated ketones (2-cyclohexen-1-one, 2-pentylcyclopenten-1-one, isophorone, mesityl oxide, 1,4-naphthoquinone, etc.) with hydrogen peroxide was accomplished with a variety of [Pt(CF3) (P−P)(solv)]+ complexes (P−P = a variety of diphosphines, including chiral diphosphines).50 The reaction can be carried out at room temperature under mild conditions and is only moderately selective. After the initial, selective formation of the epoxy ketone, loss of chemical selectivity is observed due to postcatalysis consecutive reactions induced by the acidity of the

medium, as shown for example in Scheme 22 in the case of mesityl oxide. Scheme 22. Oxidation of Mesityl Oxide: Reaction Composition after 82 h

The enantioselective transformation can be easily accomplished with chiral diphosphine modified catalysts, but a significant loss of enantioselectivity is observed during the course of the reaction, again due to the acidity of the medium. If the reaction is carried out stoichiometrically the ee observed is higher and stable with time. For example by adding [Pt(CF3)(OOH)(pyrphos)] to [Pt(CF3)(pyrphos)(2-pentylcyclopenten-1-one)]+ formed in situ from the cationic solvento complex and excess unsaturated ketone, the epoxide is formed with 63% ee and is stable for several hours. Electrophilic metalation is thought to be the key step in the acetoxylation of aromatics using PdII salts via Wacker chemistry.51 The bifunctional nature of Pt−CF3 derivatives in epoxidation reactions and the observation that [Pt(CF3)(Ph)(dppe)] can be easily converted into the corresponding phenoxy complex by oxidation with [Pt(CF3)(OOH)(dppe)] opened the possibility of obtaining phenols directly from aromatic compounds via catalytic hydroxylation using H2O2 by exploiting the electrophilic nature of the (P−P)Pt(CF3)(solv)+ complexes.52 Some representative data are reported in Table 9. As shown, the reactivity is moderate and the substrate trend parallels that expected for an electrophilic substitution with the exclusive formation of ortho and para substitution products. The mechanism of the hydroxylation reaction is shown in Scheme 23, and in its essential features it closely resembles that of the epoxidation (again two parallel cycles and a prior hydrolysis equilibrium).52 The major difference is in the way in which the substrate is activated which, in agreement with the above consideration and consistent with the results of Table 9, 1266


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Table 9. Hydroxylation of Aromatic Compounds with H2O2 Catalyzed by [Pt(CF3)(dppe)(solv)]+ a

Scheme 24. Epoxidation of Terminal Alkenes with Pt(C6F5) Derivatives

products (mmol) substrate

time (h)

ortho

para

chlorobenzene toluene phenol anisol m-cresol 1,3-dimethoxybenzene

4 7 6 7 3 2

0.01 0.04 0.34 1.40 0.55 0.36 (o,o)

0.07 0.25 0.19 2.39 (o,p)

others 0.09 0.04

0.02

a

Experimental conditions: Pt, 0.06 mmol; substrate, 60 mmol; 70% H2O2, 6 mmol; T = 85°C; solvent, DCE/i-PrOH mixtures if necessary to solubilize the catalyst.

Scheme 23. Mechanism of the Hydroxylation of Arenes Catalyzed by [Pt(CF3)(P−P)(CH2Cl2)]+ Complexes with Details of the Ortho-Metalation Step of the Aromatic Ring

correlate the catalytic activity (initial rate) with the Lewis acidity of the complexes modulated by the different ligands and measured by IR using 2,6-dimethylphenyl isocyanide as the probe molecule53 gave only the rough indication that an intermediate Lewis acidity was the best option. A better correlation was found with other spectroscopic parameters: namely, the 1JP−Pt value for the phosphorus trans to coordinated water as a measurement of the lability of the latter and the Δδ (δcoord − δfree) value of the same donor caused by the shielding effect on P due to the metal and C6F5, reflecting both conformational changes and steric interactions. The correlation of the initial rate with these parameters clearly speaks for an epoxidation reaction catalyzed by Pt(II) complexes where both the steric hindrance of the ligand and the ability to exchange water play a key role, with five-membered-ring complexes as the most active catalysts. Among others, the dppe derivative emerges as the most active species and the chiral version with chiraphos is only slightly less productive.18 The enantioselective transformation of simple terminal alkenes was studied in more detail with a series of chiral catalysts (Scheme 24), with the chiraphos derivative being the most effective catalyst and resulting in exceptional enantioselectivies for this class of reluctant substrates. Some examples are reported in Table 10.54

consists of an electrophilic metalation of the arene from the Pt+ complex. Table 9 indicates that the amount of ortho products largely exceeds what should normally be expected from the thermodynamic composition of the electrophilic substitution and suggests a specific role of Pt in the selectivity of the system. The use of PtOH or PtOPh complexes in comparison with Pt+ in the hydroxylation of phenol increases the cathechol/ hydroquinone selectivity from 5/1 to 15/1, although the reaction rate decreases significantly. Since most successful substrates already contain an oxygen atom donor, this suggests an ortho metalation of the aromatic ring as in Scheme 23 is responsible for the peculiar ortho selectivity of this catalytic system. Use of Pentafluorophenyl Derivatives. More recently the epoxidation of alkenes with hydrogen peroxide was further investigated with a variety of [Pt(C 6 F 5 )(P−P)(H 2 O)] + complexes (Scheme 24).18 With respect to CF3 derivatives here the synthetic procedure is much simpler. Again, linear terminal alkenes were the preferred substrate for epoxidation with hydrogen peroxide, and high yields and rates could be observed with e.g. 1-hexene or 1-octene. An attempt to

Table 10. Epoxidation of Terminal Alkenes with 35% Hydrogen Peroxide using [Pt(C6F5)(chiraphos)(H2O)]+ as Catalystb

a

Only terminal epoxide is produced. bExperimental conditions: substrate, 0.83 mmol; H2O2, 0.83 mmol; cat., 0.016 mmol (2%); solvent, 1 mL of dichloromethane (DCM); T = −10 °C.

The system displayed an exceptional regioselectivity in the case of dienes bearing both terminal and internal carbon− carbon double bonds.55 An example is reported in Scheme 25 and shows that with respect to traditional stoichiometric 1267


Organometallics

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Scheme 25. Regioselectivity in the Epoxidation of Terminal Alkenes Mediated by Pt(C6F5) Complexes

oxidants such as, for example, m-chloroperbenzoic acid (MCPBA) the regioselectivity is completely inverted. This complete regioselectivity for terminal epoxides is in some cases superior to that of the best catalysts reported so far: i.e., V(III)containing polyoxometallates.56 The mechanism55 of the epoxidation reaction with these second-generation catalysts (Scheme 26), determined by a

catalysis, although micelles can play many roles at the same time:57 first, they improve the solubilization of organic reagents in water; second, they favor compartmentalization of reagent, thus improving local concentration and reactivity; third, in some cases they impart unique chemo-, regio-, and stereoselectivities. 58 While hydrogenation, hydroformylation, and C−C coupling reactions have been already investigated in water and in micellar media,57a,59 oxidation reactions need to be thoroughly explored: in particular, stereoselective oxidation reactions in water or in micellar media still include only a few contributions.60 [Pt(C6F5)(chiraphos)(H2O)]+ was tested at room temperature in the enantioselective epoxidation of a variety of terminal alkenes in water in the presence of different surfactants,61 with Triton-X100 giving the best results. The catalyst can be separated from the reaction mixture by simple hexane two-phase extraction and recycled a few times. An example is shown in Scheme 27. Enantioselectivity as well as yield and recyclability are all features that result from a critical balance between catalyst, substrate, and surfactant properties. This methodology represents a viable way for carrying out asymmetric epoxidation in water with “ordinary” transitionmetal catalysts, avoiding the need to modify the catalyst with hydrophilic functional groups. The extremely mild and environmentally friendly experimental conditions (room temperature, low catalyst loading, use of water as solvent, use of 35% H2O2 as oxidant) together with the easy isolation of the enantioenriched product and the possibility of recycling the catalyst are all key features of the present system that demonstrate the viability of this “greener” synthetic method. Yields and enantioselectivities were moderate to good, and in some cases the aqueous medium allowed a significant improvement in the asymmetric induction compared to that found for organic solvents. This is particularly interesting for a class of substrates that has been only sparingly investigated.62

Scheme 26. Proposed Mechanism for the Epoxidation of Terminal Alkenes by Pt(II) Pentafluorophenyl Complexes

thorough kinetic analysis, is different with respect to the CF3 derivatives, especially as far as H2O2 activation is concerned. Here the nucleophilic properties of the oxidant are enhanced via hydrogen bonding with the fluorine atoms of the C6F5 ligand, for which experimental evidence was found by in situ 19 F NMR spectroscopy. The study of the epoxidation of terminal alkenes catalyzed by [Pt(C6F5)(P−P)(H2O)]+ complexes was extended also to aqueous micellar media. These are unusual solvents in

Scheme 27. Enantioselective Epoxidation in Micellar Media and Recycling Experiments

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Review

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SUMMARY The historical survey of platinum(II) fluoroalkyl and fluoroaryl complexes described in the previous sections shows how the introduction of σ-metal-bound fluorinated ligands can bring about many changes in physical properties, such as an increased chemical stability and an enhanced Lewis acidity. Concerning the former aspect, the PtII−C(RF) stability has allowed the isolation and investigation of a series of stable transition-metal hydride, hydroxo, and peroxo complexes, which are generally difficult to prepare and isolate. Furthermore, the relative inertness of the PtII fluoroalkyl and fluoroaryl phosphine complexes reveals as an advantage making β-hydride elimination more difficult, thereby allowing the oxidation of olefins to epoxides (no Wacker chemistry) and ketones into esters. The cationic fluorinated complexes [Pt(RF)(P−P)(solv)]+ (RF = CF3, C6F5), owing to the presence of a positive charge on the metal, possess a significant Lewis acidity, which is enhanced by the presence of the electron-withdrawing fluoroalkyl or fluoroaryl ligand. These features allowed the catalytic oxidation of organic substrates (epoxidation, Baeyer−Villiger oxidation of ketones, hydroxylation) in the presence of hydrogen peroxide, which is a process attracting a great deal of interest from both the scientific and industrial points of view, because in principle it is environmentally safe, since the reduction product of H2O2 is water. A typical feature displayed by the metal in the latter reactions is the ability to increase the reactivity of the substrate by coordination, thereby making it more susceptible to nucleophilic attack by the peroxidic oxidant. On the other hand, a detailed kinetic study of the epoxidation of 1-octene with hydrogen peroxide catalyzed by PtII complexes of the type [Pt(OH)(CF3)(diphoe)] and [Pt(CF3)(diphoe)(solv)]+ revealed that two individual metal species are involved in the oxygen transfer: i.e. a Pt−OOH and a Pt−olefin complex. This illustrates well the bifunctional role of platinum, which, on the one hand, enhances olefin reactivity by making it susceptible to nucleophilic attack and, on the other hand, increases H2O2 nucleophilicity through coordination of a formal HOO− ligand.

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ACKNOWLEDGMENTS We wish to thank all the people (colleagues, students, postdocs) who have contributed to the development of the reaction chemistry described in this review. We also acknowledge financial support by the Università di Padova, Università Ca’ Foscari di Venezia, and Miur (PRIN 2008).

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Platinum(II) Complexes with Coordinated Electron-Withdrawing

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