Article pubs.acs.org/Organometallics
Reactivity and Mechanism in the Oxidative Addition of Allylic Halides to a Dimethylplatinum(II) Complex S. Jafar Hoseini,† Hasan Nasrabadi,† S. Masoud Nabavizadeh,‡ Mehdi Rashidi,‡ and Richard J. Puddephatt*,§ †
Department of Chemistry, Faculty of Sciences, Yasouj University, Yasouj, Iran Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran § Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7 ‡
S Supporting Information *
ABSTRACT: The complex [PtMe2(2,2′-bipyridine)], 1, reacts with allyl chloride and allyl bromide, to give [PtXMe2(CH2CHCH2)(2,2′-bipyridine)], 2, X = Cl; 3, X = Br, with 2-methylallyl chloride to give [PtClMe2(CH2CMeCH2)(2,2′-bipyridine)], 4, and with crotyl chloride (a mixture of trans- and cis-MeCHCHCH2Cl), to give a mixture of [PtClMe2(trans-CH2CHCHMe)(2,2′-bipyridine)], 5, and [PtClMe2(cisCH2CHCHMe)(2,2′-bipyridine)], 6. The complexes are formed mostly by trans oxidative addition and, for the crotyl complexes, without allylic rearrangement. Complex 1 reacts with CH2CHCHMeCl, mostly with allylic rearrangement, to give complex 5, with complexes 6 and [PtClMe2(CHMeCHCH2)(2,2′-bipyridine)], 7, as minor products. The reactions of 1 with CH2CHCH2X (X = Cl or Br) and MeCHCHCH2Cl follow second-order kinetics (first-order in both reagents), but 1 reacts with CH2CHCHMeCl by third-order kinetics (first-order in 1, second-order in CH2CHCHMeCl) and with CH2CMeCH2Cl by a mixture of second- and third-order kinetics. The second-order reactions are proposed to occur by the SN2 mechanism, and potential mechanisms of the third-order reactions are discussed.
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INTRODUCTION The oxidative addition of carbon−halogen bonds to transition metal complexes is a key step in many catalytic reactions, so the scope and mechanism of the reaction have been studied in much detail.1 The high reactivity of allylic halides toward oxidative addition2 and key mechanistic insight3 were established early. Since that time, there has been considerable synthetic work on oxidative additions of allyl halides to squareplanar d8 complexes, including those of rhodium(I),1,3,4 iridium(I),5 palladium(II),6 and platinum(II),7 but there has been relatively little study of the reaction mechanisms.1−7 The reaction of [RhCp(CO)(PPh3)], A, with allyl iodide followed second-order kinetics, and the primary step was proposed to occur by the normal SN2 mechanism, followed by carbonyl insertion to give B (Scheme 1). However, reactions
with several other allylic halides occurred by the SN1 mechanism, and, in the case of 3-chloro-1-butene, the reaction followed first-order kinetics and with allyl rearrangement to give the product C (Scheme 1). Isomerization of the 3-chloro1-butene to trans-1-chlorobut-2-ene occurred at about the same rate as the oxidative addition because, in both cases, the ratedetermining step was the chloride dissociation (Scheme 1).3 In nucleophilic substitution reactions of allylic halides, the allyl rearrangement is often observed and can occur either by the SN1 mechanism or by a modified SN2 mechanism, usually termed the SN2′ mechanism, in which the nucleophile attacks the alkene group directly (Scheme 2). The SN2 and SN2′ mechanisms are often competitive, and so mixtures of normal and rearranged allyl products may be obtained. Cyclic transition states are also possible, sometimes termed the SNi′ mechanism (Scheme 2).8 Analogous mechanisms could operate in oxidative addition reactions, with metal complexes as nucleophiles, but they have not yet been established. A related mechanism, and one that is unique to transition metal nucleophiles (Scheme 3), involves intermediate formation of a metal alkene complex, D, followed by oxidation to give a π-allyl complex, E. Nucleophilic attack on the π-allyl complex, E, then gives the alkene complex, F, which releases the substituted allyl derivative. The nucleophile normally attacks the least substituted carbon atom of the π-allyl complex, E, so, in the case given with R = alkyl, the reaction proceeds
Scheme 1. SN2 and SN1 Mechanisms of Allylic Halide Reactions
Received: December 31, 2011 Published: March 2, 2012 © 2012 American Chemical Society
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trans adduct 2a or 3a was the major component formed in 90− 95% yield, with the cis adduct 2b or 3b present in 5−10% abundance. Similarly, the reaction of 2-methylallyl chloride gave [PtClMe2(CH2CMeCH2)(bipy)], 4, but only the trans adduct 4a was detected in this case. The presence of the two isomers was established from the 1H and 13C NMR spectra. For example, in the 1H NMR spectrum, the product of trans oxidative addition of allyl chloride, 2a, gave a single methylplatinum resonance at δ = 1.28 [2JPtH = 70 Hz] and a single Pt-CH2 resonance at δ = 2.10 [2JPtH = 95 Hz, 3JHH = 8 Hz, 1JCH = 137 Hz] (Figure 1), whereas the unsymmetrical
Scheme 2. SN2′ and SNi′ Mechanisms of Allylic Substitution
Scheme 3. Oxidative Addition of Allyl Derivatives during the Tsuji−Trost Coupling Reaction
Figure 1. 1H NMR spectrum (400 MHz) of [PtClMe2(CH2CH CH2)(bipy)], 2, in the PtCH2 region. The inset shows an expansion of the PtCHaHb resonances for the minor isomer 2b.
largely with allylic rearrangement. This reaction has been most closely studied in oxidative addition to palladium(0) complexes because this is a key step in the Tsuji−Trost allyl coupling reactions, but a similar mechanism is known for iridium(I) and for other transition metal centers.5,9,10 This article reports a mechanistic study of the oxidative addition of allylic halides to the electron-rich dimethylplatinum(II) complex [PtMe2(bipy)], 1, bipy = 2,2′-bipyridine.11 Complex 1 is one of the most reactive metal-based nucleophiles, and its oxidative reactions with many alkyl halides have been reported earlier.12 Given the above discussion, the mechanism of reaction of complex 1 with allylic halides was not obvious and so worthy of investigation. There was the potential that the SN2′ mechanism might be observed for the first time in oxidative addition reactions to platinum(II).
product of cis oxidative addition, 2b, gave two methylplatinum resonances at δ = 0.44 [2JPtH = 75 Hz, trans to Br] and 1.32 [2JPtH = 70 Hz, trans to N] and two resonances for the diastereotopic PtCHaHb protons at δ = 2.85 [2JPtH = 90 Hz, 3 JHH = 2JHH = 9 Hz] and δ = 3.08 [2JPtH = 104.0 Hz, 3JHH = 2JHH = 9 Hz] (Figure 1).12,13 The structure of complex 3a was confirmed crystallographically and is shown in Figure 2. It confirms the
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RESULTS Synthesis and Characterization. The reaction of [PtMe2(bipy)], 1, bipy = 2,2′-bipyridine, with allyl chloride or bromide occurred to give the products of oxidative addition, [PtXMe2(CH2CHCH2)(bipy)], 2, X = Cl; 3, X = Br, as a mixture of isomers, as shown in Scheme 4. In each case, the
Figure 2. View of the structure of complex 3a. Selected bond distances (Å): Pt(1)−C(1) 2.045(5); Pt(1)−C(15) 2.132(4); Pt(1)−N(1) 2.150(3); Pt(1)−N(2) 2.152(3); Pt(1)−C(2A) 2.104(6); Pt(1)− Br(1) 2.6034(4); C(2A)−C(3A) 1.476(8); C(3A)−C(4A) 1.31(1). There is disorder of the allyl group, and only the major component is shown.
Scheme 4. Reaction of 1 with CH2CHCH2X to Give 2 or 3 and with CH2CMeCH2Cl to Give 4aa
a
characterization of 3a as an octahedral platinum(IV) complex formed by trans oxidative addition of allyl bromide. The reaction of complex 1 with a sample of commercial crotyl chloride, whose composition, as determined by 1H NMR, was about 85% trans-1-chlorobut-2-ene, 14% cis-1-chlorobut-2ene, and 1% 3-chlorobut-1-ene, gave a mixture of the
NN = bipy. 2358
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corresponding trans- and cis-but-2-enylplatinum(IV) complexes 5 and 6, with a barely detectable trace of 7a, according to Scheme 5. The stereochemistry about the CC bond was Scheme 5. Synthesis of the Isomeric Complexes 5−7a
a
Reagents are MeCHCHCH2Cl or CH2CHCHMeCl (see text).
most readily confirmed from the magnitude of the coupling constant 3J(HCCH), which was 15 Hz in 5a and 9 Hz in 6a, while the stereochemistry at platinum was determined as described for complexes 2 and 3. A reaction of complex 1 with one equivalent of crotyl chloride gave 5 and 6 in a ratio of about 4:1 (but with incomplete conversion), while reactions with a large excess of crotyl chloride (>40 equivalents) gave 5 and 6 in a ratio of about 2:1. A second sample of crotyl chloride analyzed as 75% trans-1-chlorobut-2-ene and 25% cis-1chlorobut-2-ene, when reacted using a 40-fold excess compared to 1, gave 5 and 6 in a ratio of about 1:1. These ratios did not change with time, even in the presence of excess crotyl chloride, indicating that there is no equilibration between the products 5 and 6. The data can be explained if the oxidative addition occurs, largely or completely, with retention of stereochemistry about the CC bond and if cis-1-chlorobut-2-ene reacts about three times faster than trans-1-chlorobut-2-ene. Complexes 5 and 6 were formed almost entirely as the trans adducts 5a and 6a, with ratios 5a:5b and 6a:6b about 98:2 in each case. Only the methylplatinum resonances of the trace isomers 5b and 6b were resolved, and the evidence for the structures is based only on the similarity of these resonances to those of complexes 2b and 3b. Recrystallization of the reaction mixture, containing 5 and 6 in about 2:1 ratio, gave single crystals shown to contain a disordered mixture of complexes 5a and 6a in about 35:65 abundance. The structures are shown in Figure 3. The bond parameters for the cis-but-2-enylplatinum isomer 6a were well defined, but those of the trans-but-2-enylplatinum isomer 5a were not accurately determined, so a comparison is not justified. The reaction of complex 1 with 3-chloro-1-butene gave a mixture of the simple product of trans oxidative addition, [PtClMe2(CHMeCHCH2)(bipy)], 7a (Scheme 2), and the complexes 5a and 5b, which are products of oxidative addition with allylic rearrangement. The rearranged product 5a is the major component, with the ratio of products 5a:6a:7a being about 11:1:1. The isomer 5b (Scheme 2) was also identified in
Figure 3. Views of the structures of complexes (a) 6a and (b) 5a, in the disordered structure. Selected bond distances (Å): Pt(1)−C(1) 2.039(5); Pt(1)−C(2) 2.054(4); Pt(1)−C(13) 2.148(15); Pt(1)− N(1) 2.149(4); Pt(1)−N(2) 2.161(3); Pt(1)−Cl(1) 2.4893(11); C(13)−C(14) 1.415(12); C(14)−C(15) 1.310(10); C(15)−C(16) 1.487(11). Refinement indicated 65% 6a and 35% 5a, and the parameters for the allyl group of the minor component 5a are not well defined.
trace quantity, but the related isomer 7b (note that 7b would be chiral at both the platinum and α-carbon centers, so the NMR spectra would be particularly complex)12,13 was not detected. Complex 7a could not be separated from its isomers 5a and 6a, so it was characterized by its NMR spectra. Because the α-carbon atom of the PtCHMeCHCH2 group is chiral, the two methylplatinum groups are nonequivalent and gave rise to two equal-intensity methylplatinum resonances in the 1H NMR spectrum at δ(1H) = 1.30 [2JPtH = 70 Hz] and 1.32 [2JPtH = 71 Hz].12,13 The α-CH resonance occurred as an apparent quintet at δ(1H) = 2.49 [3JHH = 7 Hz, 2JPtH = 88 Hz], and the βMeC resonance appeared as a doublet at δ(1H) = 0.44 [3JHH = 7 Hz, 3JPtH = 62 Hz] (Figure 4). The expected resonances for the vinyl group were observed at δ(1H) = 4.30 and 4.46 [ CHaHb] and at 5.21 [CH]. Kinetic Studies. The kinetics of the reactions of Schemes 4 and 5 were monitored in benzene or acetone by using UV− visible spectroscopy, by following the disappearance of the MLCT band of the red complex [PtMe2(bipy)], 1, at λ = 507 nm (benzene solution) or λ = 475 nm (acetone solution).12,13 The reactions were carried out using a large excess of the allyl halide so that its concentration was effectively constant during a kinetic run. The changes in the spectrum during a typical experiment and a typical set of absorbance−time curves for the reaction as a function of allyl halide concentration are shown in Figures 5 and 6, respectively, for reactions with allyl bromide. 2359
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concentration of complex 1. The pseudo-first-order rate constants (kobs) were evaluated by nonlinear least-squares fitting of the absorbance (A) vs time (t) profiles to the firstorder equation At = A∞ + (A0 − A∞) exp(−kobst). The data are given in Tables S1−S6. Two different types of kinetic behavior were observed with respect to the concentration of the allylic halide derivatives, as described below. The reactions with allyl chloride, allyl bromide, and “crotyl chloride” were first-order in the allylic halide. Thus, for allyl bromide, graphs of the observed first-order rate constants against the concentration of allyl bromide gave good straight line plots passing through the origin (Figure 7), showing that Figure 4. 1H NMR spectrum (250 MHz) of the products of reaction of complex 1 with CH2CHCHMeCl in the region of the MeC resonances. Satellites arising from coupling of MeC protons to 195Pt are indicated by *, and the singlet labeled 5b, with satellites labeled #, is from the MePt (trans to Cl) resonanance of complex 5b.
Figure 7. Plots of the observed first-order rate constants for the reaction of [PtMe2(bipy)] with C3H5Br versus the concentration of C3H5Br in benzene.
the reactions follow overall second-order kinetics, first-order in both complex 1 and allyl bromide. The second-order rate constants at different temperatures were determined, and the associated activation parameters were calculated by using the Eyring equation (Figure 8). With allyl bromide, reactions were
Figure 5. Changes in the UV−vis spectrum during the reaction of [PtMe2(bipy)] (3 × 10−4 M) and C3H5Br (2.29 × 10−2 M) in acetone at T = 20 °C: (a) initial spectrum (before adding C3H5Br), (b) spectrum at t = 0 (note partial reaction during mixing); successive spectra were recorded at intervals of 30 s.
Figure 8. Eyring plots for the reaction of [PtMe2(bipy)] with C3H5Br (a) in benzene and (b) in acetone.
studied in both acetone and benzene, but other reagents were studied only in acetone. The data for these reactions are summarized in Table 1. The crotyl chloride used was a mixture of 85% trans-1chlorobut-2-ene, 14% cis-1-chlorobut-2-ene, and 1% 3-chloro-1butene, so the observed rate constants in Table 1 are a composite of the individual rate constants. The reaction with the 1% 3-chloro-1-butene is negligible (see below), so only rate constants for trans-1-chlorobut-2-ene, k2t, and cis-1-chlorobut-2-
Figure 6. Absorbance−time curves for the reaction of [PtMe2(bipy)] with C3H5Br (0.038−0.27 M C3H5Br, increases reading downward) in benzene at 25 °C.
No long-lived intermediates were detected in any of the reactions. All of the reactions studied were first-order in the 2360
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Table 1. Second-Order Rate Constants (102k2/L mol−1 s−1)a and Activation Parameters (ΔH‡ in kJ mol−1, ΔS‡ in J K−1 mol−1)b for the Reactions of [PtMe2(bipy)] with Allylic Halidesc reagent
solvent
10 °C
15 °C
20 °C
25 °C
CH2CHCH2Br CH2CHCH2Br CH2CHCH2Cl MeCHCHCH2Cl
C6H6 Me2CO Me2CO Me2CO
8.71(8) 48.1(5)
11.1(1) 63.3(5) 0.078(1) 0.440(3)
15.2(1) 89.9(4)
20.3(2) 105.0(6) 0.136(4) 0.85(1)
30 °C
0.204(4) 1.08(1)
35 °C
ΔH‡
ΔS‡
0.274(3) 1.80(1)d
37.7(2) 35(3) 44(3) 40(2)
−132(6) −118(12) −151(10) −152(5)
a
Second-order rate constant values, and associated errors, are given based on 95% confidence limits from least-squares regression analysis. bObtained from the Eyring equation. cThe crotyl chloride contained both trans (85%) and cis (14%) isomers of MeCHCHCH2Cl. dThe value at 40 °C.
ene, k2c, need to be considered. From the data at 25 °C, the product ratio 5a:6a is approximately 2:1, and, assuming that the reactions occur with complete retention of stereochemistry about the CC bond, we can write 5a/6a = 2/1 = 0.85k2t/ 0.14k2c, and hence k2c = 3.0k2t.14 At 25 °C, the observed secondorder rate constant is 0.85 × 10−2 L mol−1 s−1 (Table 1), and hence k2t = 0.67 × 10−2 L mol−1 s−1 and k2c = 2.0 × 10−2 L mol−1 s−1.14 Surprisingly, the reaction of complex 1 with 3-chloro-1butene, MeCH(Cl)CHCH2, was found to follow overall third-order kinetics, being first-order in complex 1 and secondorder in the allylic chloride reagent. Plots of the observed firstorder rate constants versus [3-chloro-1-butene] and [3-chloro1-butene]2 are shown in Figure 9, which clearly demonstrates Figure 10. Eyring plot for the reaction of [PtMe2(bpy)] with 3-chloro1-butene in acetone.
activation is less negative than for many third-order reactions, but there are precedents for similar values.14c,d The reaction rates were not affected by the presence of free ligand 2,2′bipyridine or by the free radical scavenger p-benzophenone. The kinetics of the reaction involving the reagent 3-chloro-2methyl-1-propene, RCl, was investigated similarly at 25 and at 40 °C. The reaction followed a mixed second- and third-order rate law (Figure 11). Simulation gave the values k2 = 0.8(3) ×
Figure 9. Plots of observed first-order rate constants (acetone solution) for the reaction of [PtMe2(bipy)] with 3-chloro-1-butene (RCl) versus (above) the concentration of 3-chloro-1-butene and (below) the square of the concentration of 3-chloro-1-butene. In each case, the lines show the best fit for the overall third-order reaction.
Figure 11. Plot of observed rate constants at 40 °C for the reaction of [PtMe2(bipy)] with 3-chloro-2-methyl-1-propene (RCl) versus the concentration of RCl in acetone. The line shows the best fit to the equation given.
that the reaction is second-order with respect to 3-chloro-1butene. The resulting third-order rate constants (k3, L2 mol−2 s−1) at varying temperatures were 1.10(2) × 10−2, 25 °C; 1.80(1) × 10−2, 30 °C; 2.70(5) × 10−2, 35 °C; and 3.4(1) × 10−2, 40 °C. The Eyring plot is shown in Figure 10 and allows the activation parameters to be determined as ΔH‡ = 57.5(5.6) kJ mol−1 and ΔS‡ = −89(18) J K−1 mol−1.14 The entropy of
10−3 L mol−1 s−1 and k3 = 3.0(5) × 10−2 L2 mol−2 s−1 at 25 °C and k2 = 30(5) × 10−3 L mol−1 s−1 and k3 = 4.2(3) × 10−2 L2 mol−2 s−1 at 40 °C. For comparison, at 25 °C, the second-order rate constant for reaction with allyl chloride was 1.36(4) × 10−3 L mol−1 s−1 and the third-order rate constant for reaction with 3-chloro-1-butene was k3 = 1.10(2) × 10−2 L2 mol−2 s−1. 2361
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intermediate formation of a π-allyl intermediate (Scheme 7). No π-allyl complexes of platinum(IV) appear to be known,7
DISCUSSION The reactions of complex 1 with allyl chloride, allyl bromide, and crotyl chloride almost certainly occur by the classic SN2 mechanism (Scheme 6).1,3,12 This gives first the five-coordinate
Scheme 7. Possible Mechanisms of Isomerizationa
Scheme 6. Proposed SN2 Mechanism of Oxidative Additiona
a
a
NN = bipy, R = H or Me.
and the apparent lack of allyl rearrangement with the crotyl derivatives argues against the π-allyl mechanism. Hence the isomerization G to H appears to occur easily, but the alternative reaction of G, via the π-allyl intermediate I, to give J does not appear to occur under mild conditions. This mechanism would be expected to lead to allylic rearrangement with the crotyl derivatives (Scheme 7). Some further insight was obtained by carrying out DFT calculations (Figure 12). Complex 2a was calculated to be 15 kJ
NN = bipy.
cationic intermediate G, which can combine with the halide ion to give the product of trans oxidative addition, 2a−6a, or isomerize to H and then combine with the halide ion to give the product of cis oxidative addition, 2b−6b (Scheme 6).15 Several pieces of evidence support this proposal (Table 1): (1) The reactions follow second-order kinetics, and the activation parameters, particularly the large negative values of the entropy of activation, are typical of reactions following the SN2 mechanism.1,3,12 (2) The reaction of complex 1 with allyl bromide occurs significantly faster in acetone than in the less polar solvent benzene, indicative of a polar transition state.1,3,12,16 (3) The reactions with cis and trans isomers of MeCH CHCH2Cl appear to occur with retention of stereochemistry at the CC bond and without allylic rearrangement.8 (4) The reactivity series cis-MeCHCHCH2Cl > transMeCHCHCH2Cl > CH2CHCH2Cl is typical for known SN2 reactions of these reagents, such as in the substitution of chloride by iodide or ethoxide.8 The higher reactivity of the crotyl chloride isomers compared to allyl chloride is attributed to the stabilization of the transition state by the electron-releasing inductive effect of the methyl substituent. For example, relative rates for substitution by iodide in acetone solution at 20 °C,8 or oxidative addition to complex 1 in acetone at 25 °C, were cis-MeCHCHCH2Cl:trans-MeCH CHCH2Cl:CH2CHCH2Cl = 8.35:1.55:1 and 14:5:1, respectively. It should be noted that the isomerization between fivecoordinate complexes G and H could take place either by pseudorotation at platinum,12,15 as indicated in Scheme 6, or by
Figure 12. Calculated structures and relative energies of allyl complexes and proposed intermediates of Scheme 7.
mol−1 more stable than the corresponding cis-adduct 2b. However, for the proposed five-coordinate intermediates, complex G1 was calculated to be 22 kJ mol−1 less stable than H1. The dimensions of the allyl group in H1 indicate that the bonding is best considered as η1-allyl with weak coordination of the double bond to platinum. Evidence for the η1-allyl is that the calculated C−C distances within the allyl group are still indicative of single and double bonds (C−C = 1.49 and CC 2362
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Table 2. Calculated Relative Energies (kJ mol−1) for Methylallyl Compounds
= 1.37 Å) and that the calculated Pt−CH2 distances are very different from one another (Pt−CH2 = 2.12 and 2.90 Å). Evidence for weak coordination of the CC bond is that the angle Pt−C−C is 86°, which is much smaller than the corresponding calculated angle of 109° in G1, and that the βcarbon is within weak bonding distance to platinum (calculated Pt···C = 2.51 Å). The calculated greater stability of 2a compared to 2b is probably due mostly to lower steric hindrance in 2a, in which the allyl group sits above the flat 2,2′bipyridine ligand, and possibly to some attractive π−π-stacking forces between the vinyl and 2,2′-bipyridine ligands in 2a.7,13 The greater stability of H1 compared to G1 probably arises mostly from weak coordination of the vinyl group, which reduces the coordinative unsaturation at platinum(IV) (Scheme 7). The difference in steric factors in G1 and H1 is likely to be less than in 2a and 2b because in H1 the vinyl group is oriented toward the vacant coordination site in the square-pyramidal platinum(IV) complex cation (Figure 12). The mechanism of reaction of complex 1 with CH2 CHCHMeCl is less certain. The unusual features are that the reaction is second-order in the allyl derivative and that the oxidative addition occurs mostly with allylic rearrangement to give 5a. The normal SN2 mechanism, which would give 7a, is expected to be slow with the secondary alkyl chloride,1 and a previous study has shown that isopropyl iodide reacts with 1 mostly by a free radical chain mechanism.17 However, the kinetics of reaction of CH2CHCHMeCl with complex 1 do not show features of a free radical reaction, and the reaction is not retarded by the free radical scavenger p-benzoquinone. There is a precedent for the displacement of 2,2′-bipyridine from the complex [PtEt2(bipy)] by the alkene group of methyl acrylate at an intermediate stage of the reaction to give [Pt(CHMeCO2Me)2(bipy)] and ethylene, with the evidence being strong retardation in the presence of free 2,2′bipyridine.18 The kinetics of reaction of complex 1 with CH2CHCHMeCl is not affected by the presence of free 2,2′bipyridine, so complete displacement of this ligand does not occur during the reaction. It could be argued that the formation of 5a and 6a arises by reaction of 1 with crotyl chloride impurity in the CH2CHCHMeCl reagent. NMR analysis of the reagent showed that it contained 1% trans-crotyl chloride and a barely detectable trace of the cis isomer. The proportion of reaction occurring from the trans-crotyl chloride will be concentration dependent, and since the relevant rate constants are known, it is readily estimated that the initial proportion of product arising from the impurity is given by k2t[transMeCHCHCH2Cl]/k3[CH2CHCHMeCl]M = 0.01k2t/ k3M, where M is the total molarity of the reagent used. Under conditions used in the synthesis and kinetics, this amounts to no more than 1% of the total reaction. The ratio of products formed was different in reactions using crotyl chloride and CH2CHCHMeCl and did not change with time. The data indicate that isomerization between pairs of cis/trans isomers such as 5a/5b is relatively fast but that isomerization between 5a, 6a, and 7a does not occur under the conditions used, so that the product ratios are determined by kinetic factors. Energies calculated by DFT for products and proposed intermediates in cis/trans isomerization reactions are given in Table 2. They predict that 5a and 6a should be essentially equal in energy and that both are more stable than 7a, as expected in terms of steric effects. There are two possible kinetic schemes that are consistent with the experimental observations. The first is that it could be
R
trans-crotyl
cis-crotyl
α-methylallyl
R-Cl trans-[PtClRMe2(bipy)] cis-[PtClRMe2(bipy)] trans-[PtRMe2(bipy)]+ cis-[PtRMe2(bipy)]+
0a 0b 25 0c −17
7 −1 30 −1 −2
−5 10 42 14 −9
a,b,c
Reference states for RCl, [PtClRMe 2 (bipy)], and [PtRMe2(bipy)]+, respectively.
a genuine termolecular reaction involving complex 1 and two molecules of CH2CHCHMeCl. However, termolecular reactions are rare, and a mechanism involving a pre-equilibrium of complex 1 with one molecule of CH2CHCHMeCl to give [1·CH2CHCHMeCl] = K, followed by rate-determining bimolecular reaction of K with a second molecule of CH2 CHCHMeCl, is a priori more probable.14,19 If an intermediate 1:1 complex K is formed, its concentration is too low to be detected directly by either NMR or UV−visible spectroscopy, so possible structures were investigated by carrying out DFT calculations (Figure 13). An alkene complex K1 with the alkene
Figure 13. Structures and relative energies of potential intermediates K1−K4.
in the axial position is not favored, and the dimensions of the CH2CHCHMeCl unit are very similar to those of the free alkene. Slightly lower in energy is K2, in which the alkene displaces one nitrogen donor of the bipy ligand. However, the most favored structures K3 and K4 are formed by coordination of either face of the alkene with displacement of one methyl group to the axial position (K3 and K4 are chiral at both carbon and platinum). It is significant that in K3 and K4 the C−Cl bond is calculated to be longer (Figure 13) and more polar (Hirshfeld charges on Cl are −0.14, −0.17, and −0.21 in K1, K3, and K4, respectively) and that the CC bond is also significantly longer than in K1 or free alkene. A possible route to complex 5a through intermediate K3 is shown in Scheme 8. This invokes a cyclic intermediate or transition state L of a general kind that is commonly proposed in allylic substitution reactions (Scheme 2, for example).8 It would initially give 5b and one equivalent of either CH2 CHMeCl or MeCHCHCH2Cl and would be followed by isomerization of 5b to 5a, by the mechanism shown in Scheme 6. According to this mechanism, the chloride and trans-crotyl 2363
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is lower than for allyl chloride and that the third-order rate constant is higher than for CH2CHCHMeCl. It is therefore reasonable that the two mechanisms should be competitive. For the reaction of complex 1 with CH2CHCHMeCl, no secondorder component is detected (Figure 9), so the minor products 6a and 7a should arise from modified Scheme 8 or 9. Overall, this work has shown that the most reactive allyl chloride or crotyl chloride reacts with complex 1 by the SN2 mechanism but that steric effects of a β- or α-methyl group make the SN2 mechanism less favorable and that a new mechanism that gives overall third-order kinetics can then become competitive or dominant. Preliminary suggestions of mechanisms to account for the observations are given in Schemes 8 and 9, but further research is needed to establish the exact mechanism and so to gain insight into the catalysis involving allylic substitutions.9,10 The selectivity is controlled by both the pre-equilibrium and the rate-determining step in accord with the Curtin−Hammett principle.14
Scheme 8. Possible Mechanism of Reaction through Intermediate K3
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groups in 5a would originate from different molecules of the reagent CH2CHCHMeCl. A possible route to complex 5a through intermediate K4 is shown in Scheme 9. In this case, a cyclic mechanism is not
EXPERIMENTAL SECTION
The 1H and 13C NMR spectra were recorded by using a Varian Mercury 400 spectrometer or 250 MHz spectrometer (in CD2Cl2), with TMS as reference. Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer with temperature control using an EYELA NCB-3100 constant-temperature bath. DFT calculations were carried out by using the Amsterdam Density Functional program based on the BLYP functional, with first-order scalar relativistic corrections and double-ζ basis set.20 All ground-state structures were confirmed to be energy minima by analysis of force constants for normal vibrations.20 Because K1 contains only weak intermolecular forces, several conformations with very similar energies were found, and the most stable is shown. The complex [PtMe2(bipy)], 1, was prepared as described in the literature.11 [PtClMe2(CH2CHCH2)(bipy)], 2. To a solution of [PtMe2(bpy)] (0.04 g) in acetone (30 mL) was added excess allyl chloride (300 μL). The reaction mixture was stirred for 6 h, after which time an orangeyellow solution developed. The solvent was removed at reduced pressure, leaving a yellow solid. This was recrystallized from CH2Cl2/ pentane and dried in vacuo. Yield: 40 mg (83%), mp, 192 °C (dec). Anal. Calcd for C15H19ClN2Pt: C, 39.31; H, 4.15; N, 6.11. Found: C, 39.29; H, 4.12; N, 6.32. NMR in CD2Cl2: major isomer 2a, δ(1H) = 1.28 [s, 2JPtH = 70.0 Hz, 6H, MePt]; 2.10 [dd, 3JHH = 8 Hz, 4JHH = 1 Hz, 2JPtH = 95 Hz, 2H, PtCH2]; 4.1−4.4 [m, 2H, CH2]; 5.10 [m, 1H, −CH]; 7.58 [m, 2H, H4]; 7.97 [m, 2H, H5]; 8.23 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.80 [d, 3JH6H5 = 7 Hz, 3JPtH = 10 Hz, 2H, H6]; δ(13C{1H}) = 0.0 [s, 1JPtC = 693 Hz, MePt]; 21.2 [s, 1JPtC = 648 Hz, Pt-CH2]; 113.3 [s, 3JPtC = 51 Hz, =CH2]; 145.5 [s, 2JPtC = 64 Hz, −CH]; 127.5 [s, 3JPtC = 9 Hz, C5], 130.5 [s, 3JPtC = 14 Hz, C3], 142.5[s, C4], 150.3[s, 2JPtC = 12 Hz, C2], 159.1 [s, C6]; minor isomer 2b, δ(1H) = 0.44 [s, 2JPtH = 75 Hz, 3H, MePt trans to Cl]; 1.32[s, 2JPtH = 70 Hz, 3H, MePt trans to N]; 2.85 [m, 3JHH = 2JHH = 9 Hz, 2JPtH = 92 Hz, 1H, PtCHaHb]; 3.08 [dd, 3JHH = 2JHH = 9 Hz, 2JPtH = 104 Hz, 1H, PtCHaHb]; ca. 4.3 [m, overlapped with the related peak of major isomer, CH2]; 6.18 [m, 1H, −CH]; 7.63 [m, H4], 8.07 [m, H5], 8.26 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.85 [d, 3JPtH = 10 Hz, 1H, H6]; 8.95 [d, 3JH6H5 = 5 Hz, 3JPtH = 10 Hz, 1H, H6]; δ(13C{1H}) = 1.0 and 2.3 [s, MePt]; 18.9 [s, PtCH2]. [PtBrMe 2 (CH 2 CHCH 2 )(bipy)], 3. To a solution of [PtMe2(bpy)] (0.04 g) in benzene (30 mL) was added excess allyl bromide (180 μL). The reaction mixture was stirred for 3 h, after which time an orange-yellow solution developed. The solvent was removed at reduced pressure, leaving a yellow solid. This was recrystallized from CH2Cl2/ether and dried in vacuo. Yield: 37.5 mg (71%), mp, 220 °C (dec). Anal. Calcd for C15H19BrN2Pt: C, 35.87; H, 3.81; N, 5.57. Found: C, 35.61; H, 3.81; N, 5.82. NMR in CD2Cl2: major isomer 3a, δ(1H) = 1.37 [s, 2JPtH = 70 Hz, 6H, MePt]; 2.16 [d, 3 JHH = 9 Hz, 2JPtH = 94 Hz, 2H, PtCH2]; 4.2−4.4 [m, 2H, CH2];
Scheme 9. Possible Mechanism of Reaction through Intermediate K4
likely and the second equivalent of CH2CHCHMeCl acts by providing nucleophilic assistance for ionization of the C−Cl bond by coordination to platinum, probably through the chlorine atom. Once this is achieved, the coordinated molecule of CH2CHMeCl dissociates and is replaced by chloride ion. The loss of chloride from a position anti to the metal center, as in Scheme 9, is expected on the basis of the oxidative addition of allyl esters to palladium(0).9,10 Of course, this mechanism is also available for the intermediate K3. At present, we have no evidence that allows a distinction between the mechanisms of Schemes 8 and 9 to be made. For the reaction of complex 1 with CH2CMeCH2Cl, it appears that there is a competition between the simple SN2 mechanism observed with allyl and crotyl chlorides and the more complex mechanism observed with CH2CHCHMeCl. This leads to the mixed second- and third-order kinetics observed. We note that at 25 °C the second-order rate constant 2364
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5.12 [m, 1H, −CH]; 7.66 [m, 2H, H4]; 8.04 [m, 2H, H5]; 8.26 [d, 3 3 4 JH H = 8 Hz, 2H, H3]; 8.87 (d, 3JH6H5 = 6 Hz, 3JPtH = 12 Hz, 2H, H6]; δ(13C{1H}) = 0.0 [s, 1JPtC = 686 Hz, MePt]; 25.9 [s, 1JPtC = 639 Hz, PtCH2]; 114.2 [s, 3JPtC = 54 Hz, CH2]; 145.3 [s, 2JPtC = 65 Hz, −CH]; 127.9 [s, 3JPtC = 9 Hz, C5]; 131.0 [s, 3JPtC = 15 Hz, C3]; 142.5 [s, C4]; 151.2 [s, 2JPtC = 13 Hz, C2]; 159.2 [s, C6]; minor isomer 3b, δ(1H) = 0.56 [s, 2JPtH = 74 Hz, 3H, MePt trans to Br]; 1.41 [s, 2JPtH = 71 Hz, 3H, MePt trans to N]; 2.92 [dd, 3JHH = 9 Hz, 2JHH = 9 Hz, 2 JPtH = 93 Hz, 1H, PtCHaHb]; 3.20 [dd, 3JHH = 9 Hz, 2JHH = 9 Hz, 2JPtH = 103 Hz, 1H, PtCHaHb]; 4.3 [m, overlapped with the related peak of major isomer, CH2]; 6.21 [m, 1H, −CH]; 7.68 [m, H4]; 8.08 [m, H5]; 8.27 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.93 [d, 3JH6H5 = 7 Hz, 3JPtH = 11 Hz, 1H, H6]; 9.08 [d, 3JH6H5 = 6 Hz, 3JPtH = 11 Hz, 1H, H6]. [PtClMe2(CH2CMeCH2)(bipy)], 4a. This was prepared similarly from 2-methylallyl chloride. Yield: 68%, mp, 186 °C (dec). Anal. Calcd for C16H21ClN2Pt: C, 40.72; H, 4.49; N, 5.94. Found: C, 40.35; H, 4.12; N, 5.71. NMR in CD2Cl2: δ(1H) = 1.27 [s, 3H, MeC]; 1.36 [s, 2 JPtH = 67 Hz, 6H, MePt]; 2.21 [s, 2JPtH = 96 Hz, 2H, PtCH2]; 3.68 [br, 4JPtH = 21 Hz, 1H, CH2]; 4.05 [br, 4JPtH = 27 Hz, 1H, CH2]; 7.66 [m, 2H, H4]; 8.09 [m, 2H, H5]; 8.23 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.93 [d, 3JH6H5 = 6 Hz, 3JPtH = 8 Hz, 2H, H6]. [PtClMe2(CH2CHCHMe)(bpy)], 5 and 6. These compounds were prepared as an inseparable mixture by reaction of crotyl chloride with complex 1, in a similar way to that above. Anal. Calcd for C16H21ClN2Pt: C, 40.72; H, 4.49; N, 5.94. Found: C, 40.68; H, 4.36; N, 5.79. 5a: δ(1H) = 0.85 [d, 3JHH = 6 Hz, 5JPtH = 27 Hz, 3H, MeC]; 1.27 [s, 2 JPtH = 70 Hz, 6H, MePt]; 2.02 [d, 3JHH = 8 Hz, 2JPtH = 93 Hz, 2H, PtCH2]; 4.62 [dq, 3JHH = 15 Hz, 6 Hz, 3JPtH = 22 Hz, 1H, MeCH]; 4.69 [dt, 3JHH = 15 Hz, 8 Hz, 1H, CH2CH]; 7.58 [m, 2H, H4]; 8.00 [m, 2H, H5]; 8.21 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.87 [d, 3JH6H5 = 6 Hz, 3 JPtH = 12 Hz, 2H, H6]; δ(13C) = 0.4 [s, 1JPtC = 702 Hz, MePt]; 21.2 [s, 1 JPtC = 643 Hz, Pt-CH2]; 22.0 [s, 4JPtC = 15 Hz, MeC]; 125.2 [s, 3JPtC = 52 Hz, MeC]; 138.9 [s, 2JPtC = 66 Hz, CH2C). 5b: δ(1H) = 0.45 [s, 2 JPtH = 76 Hz, 3H, MePt]; 1.31 [s, 2JPtH = 70 Hz, 3H, MePt]. 6a: δ(1H) = 0.76 [d, 3JHH = 6 Hz, 5JPtH = 27 Hz, 3H, MeC]; 1.33 [s, 2 JPtH = 70 Hz, 6H, MePt]; 2.19 [d, 3JHH = 8 Hz, 2JPtH = 95 Hz, 2H, PtCH2]; 4.72 [dq, 3JHH = 9 Hz, 6 Hz, 1H, MeCH]; 4.78 [dt, 3JHH = 9 Hz, 8 Hz, 3JPtH = 36 Hz, 1H, CH2CH]; 7.62 [m, 2H, H4]; 8.01 [m, 2H, H5]; 8.22 [d, 3JH3H4 = 8 Hz, 2H, H3]; 8.90 [d, 3JH6H5 = 6 Hz, 3JPtH = 12 Hz, 2H, H6]; δ(13C) = 0.0 [s, 1JPtC = 696 Hz, MePt]; 14.6 [s, 1JPtC = 650 Hz, Pt-CH2]; 16.2 [s, 4JPtC = 13 Hz, MeC]; 124.1 [s, 3JPtC = 50 Hz, MeC]; 137.5 [s, 2JPtC = 65 Hz, CH2C]. 6b: δ(1H) = 0.50 [s, 2JPtH = 75 Hz, 3H, MePt]; 1.25 [s, 2JPtH = 70 Hz, 3H, MePt]. Reaction of Complex 1 with 3-Chloro-1-butene. This reaction was carried out in a similar way to that above. It gave a mixture of complexes 5, 6, and 7a. NMR data for complex 7a: δ(1H) = 0.44 [d, 3 JHH = 7 Hz, 3JPtH = 62 Hz, 3H, MeC]; 1.30 [s, 2JPtH = 70 Hz, 3H, MePt]; 1.32 [s, 2JPtH = 71 Hz, 3H, MePt]; 2.49 [quin., 3JHH = 7 Hz, 2 JPtH = 88 Hz, 1H, Pt-CH]; 4.30 [dd, 3JHH = 10 Hz, 2JHH = 2 Hz, 4JPtH = 20 Hz, 1H, CHaHb]; 4.46 [dd, 3JHH = 17 Hz, 2JHH = 2 Hz, 4JPtH = 25 Hz, 1H, CHaHb]; 5.21 [ddq, 3JHH = 17 Hz, 10 Hz, 7 Hz, 3JPtH = 24 Hz, 1H, CHCH2]; δ(13C) = 2.2 [s, PtMe]; 3.8 [s, PtMe]; 24.5 [s, MeC]; 28.2 [s, Pt-CH]. The resonances of the bipy group could not be assigned. X-ray Structure Determinations. Suitable crystals were mounted on a glass fiber, and data were collected at −123 °C using a Nonius Kappa-CCD area detector diffractometer with COLLECT software (Nonius B.V., 1997−2002) and with Mo(Kα) radiation (λ = 0.71073 Å). The unit cell parameters were calculated and refined from the full data set. Details of the crystal data and refinement are given in the CIF files.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the NSERC (Canada), Yasouj University, and Shiraz University for financial support. REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Crystallographic data in electronic CIF form only is available free of charge via the Internet at http://pubs.acs.org. Observed first-order rate constants are given in Tables S1−S6. 2365
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