Oxidation of a Dimethylplatinum (II) Complex with Oxaziridines: A

Jun 21, 2013 - Hemiaminal Intermediate but No Oxo Complex. Kyle R. Pellarin and Richard J. Puddephatt*. Department of Chemistry, University of Western...
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Oxidation of a Dimethylplatinum(II) Complex with Oxaziridines: A Hemiaminal Intermediate but No Oxo Complex Kyle R. Pellarin and Richard J. Puddephatt* Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7 S Supporting Information *

ABSTRACT: The complex [PtMe2(bipy)] (1; bipy = 2,2′-bipyridine) fails to react with the oxaziridine derivatives RCHON-t-Bu (R = H, Ph, 2-pyridyl) in acetone at room temperature, but an easy reaction occurs in methanol solution to give the platinum(IV) complex [Pt(OH)(OMe)Me2(bipy)] and the corresponding imine RCHN-t-Bu. In the case with R = Ph, the hemiaminal intermediate [Pt(OMe)(OCHPhNH-t-Bu)Me2(bipy)] was formed and then reacted only slowly to form [Pt(OH)(OMe)Me2(bipy)] and PhCHN-t-Bu. Computational studies indicate that the reaction of 1 with oxaziridines occurs to give a charge transfer complex but that further reaction requires assistance from hydrogen bonding and coordination with specific methanol molecules. The work provides strong support for the theory that oxygen atom transfer to 1 from either dioxiranes or oxaziridines must be coupled to proton transfer and does not involve oxoplatinum(IV) intermediates.

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INTRODUCTION The oxygenation of both organic and inorganic compounds may be catalyzed by platinum or its complexes, and platinum oxides, hydroxides, peroxides, or alkoxides are often invoked as reaction intermediates.1 This commercial significance has naturally stimulated research into the synthesis and properties of complexes containing these platinum−oxygen-bonded functional groups.1−5 The reagent dimethyldioxirane can act as an oxygen atom donor, and it has yielded the unique fourcoordinate oxoplatinum(IV) complex A (Scheme 1),2b,c formulated as containing a PtO double bond, whereas most known oxoplatinum complexes contain Pt2(μ-O) groups with Pt−O single bonds. 2a However, the reaction of dimethyldioxirane with the dimethylplatinum(II) complex

[PtMe 2 (bipy)] (1; bipy = 2,2′-bipyridine) gave the dihydroxoplatinum(IV) complex B and not the potential oxoplatinum(IV) complex C.6,7 The dimethyldioxirane reagent was prepared as a dilute solution in acetone, and this evidently contained enough water impurity to give the observed product B.6 No oxoplatinum complex could be detected, and it was suggested that the oxygen atom transfer reaction was coupled to proton transfer from water to give the hydroxo complex directly.6 The oxo group in C would be highly reactive according to theory, because there is no suitable orbital on platinum to form a π bond to the oxo group.6,8 The oxoplatinum(IV) complex A, for which platinum−oxygen π bonding is possible,2b reacted only slowly with water to give the corresponding dihydroxoplatinum(IV) complex.2c This article reports reactions of oxaziridines of the formula OCHRN-t-Bu (Scheme 2) with complex 1. Oxaziridines are related to dioxiranes by replacement of one oxygen atom by an NR group. They are considerably less reactive than dioxiranes

Scheme 1. Reactions of Dimethyldioxirane with Organoplatinum(II) Complexesa

Scheme 2. Reactions of Dioxiranes (E = O) or Oxaziridines (E = NR) To Form Sulfoxides or Epoxides by Oxygen Atom Transfer

a

Received: January 29, 2013

The oxo complex C is not observed. © XXXX American Chemical Society

A

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room temperature, but no evidence for the intermediacy of complex 4a or 4c (Scheme 3) was obtained. Hence, if these hemiaminal intermediates are formed at all, they must decompose more quickly than they are formed. The final products 55d,7a,14 and 2a−c9,10 are known compounds and so could be identified easily by their NMR spectra (Figure 1 and Figure S1 (Supporting Information)).

but give many similar reactions, including oxygen atom transfer reactions, though with a smaller range of substrates (Scheme 2).9 One complication is that transfer of the NR group, rather than the O atom transfer, may occur unless R is a bulky group.9 Many of these reactions of oxaziridines and dioxiranes occur by similar mechanisms.9b,c The oxaziridines have a major advantage for the present work because they are easily prepared in pure, anhydrous form, and so the formation of hydrated product B (Scheme 1) would not be possible. At the outset, three outcomes appeared possible: formation of the oxoplatinum(IV) complex C or a product derived from it,6 a new reaction type, or no reaction at all. This article describes the chemistry, including the formation and characterization of a long-lived hemiaminal intermediate in one case.

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RESULTS AND DISCUSSION Synthesis and Characterization. The new chemistry is shown in Scheme 3. The oxidation of the known imine Scheme 3. Oxidation Reactions with Oxaziridinesa

Figure 1. 1H NMR spectra (400 MHz, CD3OD) of (a, boxed areas) [Pt(OH)(OMe)Me2(bipy)] (5) and (b) the mixture of [Pt(OD)(OCD3)Me2(bipy)] (5-d4) and PhCHN-t-Bu, (2b), formed by decomposition of complex 4b over a period of 5 days.

a

For example, complex 5 is characterized by a single methylplatinum resonance at δ 1.74 (2J(Pt−H) = 71 Hz), a PtOMe resonance at δ 2.63 (3J(Pt−H) = 39 Hz), and four bipyridine resonances (Figure 1a). The mixture of products 5* and 2b from reaction in CD3OD after several days (Figure 1b) gave resonances essentially identical with those for 5 except that the methoxyplatinum resonance was absent. A parallel reaction was carried out in CH3OH, and the product was isolated and redissolved in CD3OD, to confirm that the methoxyplatinum group was present. The imine resonances were identified similarly by comparison with an authentic sample.9 The singlet imine proton resonance, PhCHaN at δ 8.39 in CD3OD solution, is characteristic (Figure 1b). The characterization of complex 4b was more challenging because it is a new compound and it could not be isolated in pure form. The structure was deduced from the 1H, 1H COSY, and 1H−13C HSQC NMR spectra. The 1H NMR spectrum of [Pt(OCD3)(OCHPhND-t-Bu)Me2(bipy)] (4b-d4), prepared by reaction of complex 1 with oxaziridine 3b, is shown in Figure 2. The carbon atom PtOC*HaPhN is chiral, and so the complex has only C1 symmetry and the two pyridyl rings and two methylplatinum groups are nonequivalent. Two closely spaced, equal-intensity methylplatinum resonances were observed at δ 1.82 (2J(PtH) = 66 Hz) and 1.83 (2J(PtH) = 68 Hz). Similarly, two sets of resonances were observed for the pyridyl group protons, though some were overlapped. The PtOCH proton appeared as a singlet at δ 4.69 (3J(PtH) = 41

The potential intermediates 4a,c were not observed.

derivatives t-BuNCHR (2a, R = H; 2b, R = Ph; 2c, R = 2pyridyl) with m-chloroperbenzoic acid (MCPBA) gave the corresponding oxaziridine derivatives 3a−c.10,11 The reactions of these oxaziridines 3a−c with [PtMe2(2,2′-bipyridine)] (1)12 were initially monitored by 1H NMR spectroscopy in solution in either acetone-d6 or methanol-d4. In acetone-d6 solution at room temperature, no reaction was observed over a period of several days. However, when an oxaziridine was added to a yellow-orange solution of complex 1 in methanol-d4,13 the color of the solution was bleached immediately as oxidation to a platinum(IV) complex occurred. The 1H NMR spectra, obtained immediately after mixing, showed that the products from reaction of 1 with 3a or 3c were a 1:1 mixture of [Pt(OD)(OCD3)Me2(bipy)] (5-d4) and the corresponding imine 2a or 2c. However, the reaction of 1 with 3b gave a new complex identified as the hemiaminal derivative [Pt(OCD3)(OCHPhND-t-Bu)Me2(bipy)] (4b-d4), which decomposed only slowly to give 5-d4 and 2b as major products (Scheme 3). Complex 5 was also formed if excess methanol was added to a solution of 1 in acetone. Details of the characterization are given below. The reaction of complex 1 with 3a or 3c was studied by low-temperature 1H NMR spectroscopy, after mixing the reagents in CD3OD at −80 °C. Spectra were recorded at 20 °C intervals as the sample was warmed slowly to B

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Scheme 4. Potential Reactions of 1 with 3 in the Absence of Protic Solvent

Figure 2. 1H NMR spectrum of complex 4b-d4 (peaks marked # are due to impurities, and those marked * are 195Pt satellites). The inset shows the methoxyplatinum resonance of complex 4b (the peak marked + is due to the decomposition product 5).

Hz), and the coupling constant is similar to that observed for the methoxyplatinum group in 5, which gives 3J(PtH) = 39 Hz, indicating that the alkoxide group is trans to an oxygen donor; a smaller coupling constant would be expected if the alkoxy group was trans to carbon or nitrogen.5 Of course, there was no 1 H NMR resonance for the PtOCD3 group in 4b-d4; therefore, the nature of this group was not established. Fortunately, a sample of 4b was prepared by reaction of 1 with oxaziridine 3b in methanol, followed immediately by removal of the solvent under vacuum. Some decomposition to complex 5 occurred during this procedure, but when the sample was dissolved in CD3OD, the methoxyplatinum resonance of 4b was observed at δ 2.51 (3J(PtH) = 40 Hz); the magnitude of the coupling constant indicates that this methoxy group is trans to oxygen (Figure 2, inset).5 The 1H−13C NMR HSQC spectrum showed a correlation of the PtOCHa proton with a 13C resonance at δ 105.9, in the region expected for a hemiaminal derivative.15 Together, these data define the structure of the complex 4b. The phenyl protons (Figure 2) are significantly more shielded than in the precursor oxaziridine 3b, probably as a result of π stacking with the 2.2′-bipyridine ligand. Computational Studies. Some insight into the reaction mechanism was gained by carrying out DFT calculations on potential products and intermediates of the reactions. These calculations were carried out on the basis of the BLYP functional, with double-ζ basis set and first-order scalar relativistic corrections, and were carried out for the gas phase and for acetone and methanol solutions, using the conductor like screening model (COSMO) to treat solvation effects.16 Some potential reactions which do not involve specific methanol molecules are shown in Scheme 4, with selected structures, frontier orbitals, and relative energies for the case with R = Ph shown in Figures 3 and 4 and with selected atomic and bond parameters in Table 1. The DFT calculations predict that a weak charge transfer complex 6b may be formed (Figure 3g), with 1 as nucleophile (using the platinum 5dz2 orbital, Figure 3e) and 3b as the electrophile (using the σ*(NO) molecular orbital, Figure 3c).

Figure 3. Calculated structure (left), HOMO (center), and LUMO (right) for compounds 3b (a−c), 1 (d−f), and 6b (g−i).

Figure 4. Calculated relative energies (methanol solvent, kJ mol−1) and structures of compounds in Scheme 4.

This leads to lengthening of the N−O bond of 3b and charge transfer from platinum to the nitrogen and oxygen atoms of the oxaziridine (Table 1). We have attempted to detect this complex by recording low-temperature NMR spectra in acetone-d6 or a CD2Cl2 solution of a mixture of 1 and 3b, but no charge transfer complex was detected. Presumably it does not compete with solvent interactions.17 Nevertheless, 6b is a likely intermediate and, in the absence of protic solvent, it could give the metallacycle 7b or 8b, which are calculated to be C

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Table 1. Calculated Bond Distances and Hirshfeld Atomic Charges 1 Pt−O/Å O−N/Å O−C/Å N−C/Å Q(Pt)/e Q(O)/e Q(N)/e

3b (gas) 1.64 1.51 1.48

0.07 −0.16 −0.04

6b (gas)

6b (Me2CO)

6b (MeOH)

7b (MeOH)

8b (MeOH)

4b (MeOH)

2.34 2.00 1.50 1.46 0.30 −0.21 −0.11

2.29 2.06 1.50 1.46 0.30 −0.21 −0.17

2.29 2.07 1.50 1.46 0.31 −0.22 −0.18

2.20 2.34 1.49 1.52 0.43 −0.43 −0.19

2.10 2.31 1.52 1.48 0.46 −0.32 −0.22

2.13 2.43 1.48 1.48 0.46 −0.29 −0.17

perhaps more useful is to consider the changes in bond distances as the charge transfer complex 6b interacts with specific methanol molecules (Table 2). Formation of 4b requires addition of a proton to the oxaziridine nitrogen atom and a methoxide ion to the platinum center of 6b. The methanol solvent molecules in 6b·2MeOH-N,Pt (see Table 2 for the nomenclature used to define solvation) cooperate in aiding this transformation by coordination of the oxygen atom of one solvent molecule to platinum and by hydrogen bonding of the second solvent molecule to nitrogen. This push−pull effect increases the extent of the charge transfer, leading to oxidation of platinum(II) in 1 or 6b to platinum(IV) in 4b. The conversion of 4b to 5 formally requires a proton transfer from nitrogen to oxygen, with loss of the imine PhCHN-t-Bu, but the reaction is most likely mediated by further methanol solvent molecules and the reaction is slow in this case. In principle, 5 could be formed directly from a complex such as 6b·2MeOHO,Pt or 6b·3MeOH-N,O,Pt, in which a methanol molecule is hydrogen-bonded to the oxaziridine oxygen atom. This does not occur to a significant extent in the reaction of 1 with the oxaziridine 3b, but it might be the dominant route in the reactions with 3a and 3c in which no intermediate 4a or 4c could be detected during the formation of 5.

thermodynamically favorable products, whereas formation of the oxo complex C is unfavorable (Figure 4). However, no evidence was found for formation of any of these complexes or their decomposition products. The high activation energy for metallacycle formation is attributed to the weak charge transfer interaction in 6b, the absence of a suitable acceptor orbital on platinum(II), and the rigididity of the [PtMe2(bipy)] unit to the distortion needed to form 7b or 8b.18 The calculations for reactions of 1 with 3a or 3c gave similar results. There are challenges in modeling the mechanisms of reactions involving solvent methanol molecules, because of likely entropic effects and differences in hydrogen bonding. The energies of potential intermediates and products shown in Figure 5 are calculated on the basis of the energy of the

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CONCLUSIONS

The most striking observation of the present work is that the oxaziridines 3a−c do not react with complex 1 at room temperature in acetone solution, whereas they react very rapidly with 1 in methanol solution. This lends strong support to the theory that oxygen atom transfer to 1 from either dioxiranes or oxaziridines to give the oxo complex C (Scheme 1) is unfavorable.6 It follows that the easy reaction in methanol solution involves proton coupling, and it is likely that the slower reaction of 1 with dioxygen in methanol solution also involves proton coupling.5−7 Similar proton-coupled mecha-

Figure 5. Calculated relative energies (kJ mol−1, methanol solution) and structures of potential intermediates 6b·xMeOH (in 6b·2MeOHN there is a hydrogen bond between the uncoordinated methanol molecule and the oxaziridine nitrogen atom) and the products 4b and 5. The conversion of 4b to 5 is favored by entropy and by solvation by individual methanol molecules.

methanol molecules in the hydrogen-bonded cyclic hexamer (MeOH)6,19 but this is an arbitrary approximation. What is

Table 2. Trends in Calculated Bond Distances and Hirshfeld Atomic Charges with Specific Methanol Solvation in Methanol Solvent for Complexes 6b·xMeOH (x = 1−3)a 6b Pt−Ob/Å O−Nb/Å O−Cb/Å N−C/Å Pt−Oc/Å Q(Pt)/e Q(N)/e Q(O)b/e

6b·MeOH-N

6b·MeOH-O

2.29 2.07 1.50 1.46

2.24 2.12 1.51 1.45

2.24 2.16 1.52 1.45

0.31 −0.18 −0.22

0.34 −0.11 −0.21

0.35 −0.17 −0.18

6b·MeOH-Pt

6b·2MeOH-N,O

2.17 2.27 1.54 1.43 2.50 0.41 −0.28 −0.26

2.24 2.18 1.53 1.44 0.36 −0.10 −0.19

6b·2MeOH-N,Pt

6b·2MeOH-O,Pt

6b·3MeOH-N,O,Pt

2.14 2.31 1.55 1.43 2.41 0.43 −0.17 −0.26

2.14 2.35 1.60 1.41 2.40 0.43 −0.27 −0.22

2.11 2.41 1.63 1.41 2.34 0.44 −0.17 −0.23

a

N or O indicates N···HOMe or O···HOMe hydrogen bonding, and Pt indicates Pt···OHMe bonding. bO atom of oxaziridine. cO atom of solvent methanol. D

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may be catalyzed by either acid or base and that the reagents or products may act as base catalysts; thus, this is not implausible.9,10,15,20 Second, it is possible that complex 5 is formed without the intermediacy of complex 4a or 4c, as illustrated in Scheme 6. The nitrogen atom in 6a or 6c is the

nisms may apply in other important catalytic reactions, such as in the oxidation of water.1 The reaction of complex 1 with oxaziridine 3b is the best defined of those studied here because it gives the long-lived hemiaminal complex 4b. On the basis of the experimental and computational studies, the formation of 4b is proposed to occur according to Scheme 5. The easy, reversible formation of the

Scheme 6

Scheme 5. Proposed Mechanism of Formation of Complex 4b and Its Further Reaction To Give the Imine 2b and Complex 5

most basic center; therefore, hydrogen bonding of methanol to nitrogen is expected in preference to hydrogen bonding to the oxaziridine oxygen atom. Nevertheless, hydrogen bonding can also occur to the oxygen atom, and this can lead to direct formation of complex 5 as shown in Scheme 6. The dual reactivity pattern is analogous to the reaction of oxaziridines with alkenes, which can give either epoxidation, with loss of an imine (Scheme 2), or oxyamination of the double bond.22

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EXPERIMENTAL SECTION

Reactions were carried out using standard Schlenk techniques, unless otherwise stated. NMR spectra were recorded using a Varian Mercury 400 or Varian INOVA 400 or 600 MHz spectrometer. NMR labeling is shown in Chart 1. Assignments were confirmed by recording COSY

charge transfer complex 6b is enhanced by the push−pull effect of methanol molecules coordinating weakly to platinum and by hydrogen bonding of a second methanol molecule to nitrogen in 6b·2MeOH-N,Pt. This complex is then proposed to lose methanol, by loss of a proton from the Pt···OHMe group and methoxide from the N···HOMe group, to give 4b. This last step is likely to be promoted by chains or clusters of methanol molecules, since neither individual step is easy in isolation. Similarly, the slow conversion of 4b to 5, with loss of the imine 2b, is likely to be mediated by one or more methanol molecules, as indicated in Scheme 5. Organic hemiaminals are usually detected only as transient intermediates unless protected in a cavitand or metal−organic framework host, so it is remarkable that the organoplatinum derivative 4b is so long-lived.15,20 In addition, complex 4b appears to be the first example of a platinum(IV) dialkoxide complex, though several platinum(IV) complexes with one alkoxide group are known.21 Finally, we note that the reactions of 1 with 3a or 3c occur rapidly to give complex 5 and the corresponding imine 2a or 2c and that no intermediate hemiaminal complex analogous to 4b could be detected in these cases. There appear to be two possible explanations for this. First, it is possible that the reaction occurs according to Scheme 5 but that the corresponding hemiaminal complex 4a or 4c is too shortlived to detect. It is known that imine loss from hemiaminals

Chart 1. NMR Labeling Scheme

and HSQC spectra, but assignments of nonequivalent pyridyl peaks are arbitrary. Mass spectrometric analysis was carried out using an electrospray PE-Sciex mass spectrometer (ESI-MS) coupled with a TOF detector. The complexes [Pt2Me4(μ-SMe2)2], [PtMe2(bipy)] (1), and an authentic sample of [Pt(OH)(OMe)Me2(bipy)] (5) were prepared according to the literature.5h,12,14 The imine and oxaziridine derivatives were prepared by using the known methods.9−11 DFT calculations were carried out by using the Amsterdam Density Functional program based on the BLYP functional, with double-ζ basis set and first-order scalar relativistic corrections.16 General solvation E

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effects were treated by using the conductor like screening model (COSMO).16 The energy minima were confirmed by vibrational frequency analysis in each case. [Pt(OCD3)(OCHPhND-t-Bu)Me2(bipy)] (4b-d4). To a solution of complex 1 (9.0 mg, 0.0236 mmol) in methanol-d4 (0.5 mL) was added 2-tert-butyl-3-phenyloxaziridine (4.2 mg, 0.0236 mmol) dissolved in the same solvent (0.2 mL). The complex was identified by its spectroscopic properties. NMR in methanol-d4: δ(1H) 1.82 (s, 3H, 2 J(PtH) = 66 Hz, PtMe), 1.83 (s, 3H, 2J(PtH) = 68 Hz, PtMe), 4.69 (s, 1H, 3J(PtH) = 41 Hz, PtOCHa), 6.66 (d, 3J(HoHm) = 7 Hz, Ho), 6.90 (dd, 3J(HmHo) = 7 Hz, 3J(HmHp) = 7 Hz, Hm), 7.01 (t, 3J(HpHm) = 7 Hz, Hp), 7.80 (m, 2H, H5/H5′), 8.16 (dd, 1H, 3J(H4H5) = 8 Hz, 3 J(H4H3) = 8 Hz, H4), 8.37 (d, 1H, 3J(H3H4) = 8 Hz, H3), 8.22 (dd, 1H, 3J(H4′H5c′) = 8 Hz, 3J(H4′H3′) = 8 Hz, H4′), 8.41 (d, 1H, 3 J(H3′H4) = 8 Hz, H3′), 9.01 (m, 2H, H6/H6′). δ(13C) 0.1, 0.2 (PtMe), 30.9 (MeC), 44.8 (MeC), 60.1 (OMe), 105.9 (CHO), 125.85 (Co), 127.9 (Cm), 127.6 (Cp), 127.1, 127.2 (C5/C5′), 137.5(Cipso), 140.4 (C4′), 140.0 (C4), 124.0 (C3′), 124.2 (C3), 147.2, 147.3 (C6/C6′). ESIMS in methanol-d4: m/z 596 [4b + D]+. A sample of [Pt(OCH3)(OCHPhNH-t-Bu)Me2(bipy)] (4b) was prepared in the same way, but using methanol solvent. The solvent was removed immediately under high vacuum (IR: ν(NH) 3340 cm−1), and the product was redissolved in methanol-d4. The 1H NMR spectrum was as above but with an additional resonance at δ(1H) 2.51 (s, 3H, 3J(PtH) = 40 Hz, PtOMe); about 15% decomposition of 4b to 5 and 2b occurred during the evaporation and redissolution. The 1H NMR spectrum of the methanol-d4 solution of complex 4b (prepared in methanol as above) was monitored for several days. After 2 days, conversion to complex 5 and imine 2b was complete. NMR in CD2Cl2: complex 5, δ(1H) 1.62 (s, 6H, 2J(PtH) = 73 Hz, PtMe), 2.68 (s, 3H, 3J(PtH) = 41 Hz, OMe), 7.71 (m, 2H, 3J(H5H6) = 6 Hz, 3 J(H5H4) = 8 Hz, 4J(H5H3) = 1 Hz, H5), 8.11 (m, 2H, 3J(H4H5) = 8 Hz, 3J(H4H3) = 8 Hz, 4J(H4H6) = 2 Hz, H4), 8.30 (d, 2H, 3J(H3H4) = 8 Hz, H3), 8.99 (dd, 2H, 3J(H6H5) = 6 Hz, 4J(H6H4) = 2 Hz, H6); compound 2b, δ(1H) 1.32 (s, 9H, tBu), 7.45 (m, 3H, Hm/Hp), 7.76 (m, 2H, Ho), 8.38 (s, 1H, NCH). Complex 1 with Aziridine 3c. To a solution of [PtMe2(bipy)] (1; 10.2 mg, 0.0267 mmol) in methanol-d4 (0.5 mL) was added 2-tertbutyl-3-pyridinyloxaziridine (3c; 4.8 mg, 0.0267 mmol) dissolved in the same solvent (0.2 mL). The reaction mixture showed the formation of complex 5 and the parent imine 2c. NMR in methanold4: complex 5-d4, δ(1H) 1.74 (s, 6H, 2J(Pt−H) = 71 Hz, Pt-Me), 7.86 (dd, 2H, 3J(H5H6) = 5 Hz, 3J(H5H4) = 8 Hz, H5), 8.30 (dd, 2H, 3 J(H4H5) = 8 Hz, 3J(H4H3) = 8 Hz, H4), 8.68 (d, 2H, 3J(H3H4) = 8 Hz, H3), 9.05 (d, 2H, 3J(H6H5) = 5 Hz, H6); compound 2c, δ(1H) 1.34 (s, 9H, tBu), 7.47 (dd, 1H, (H5′H6′) = 5 Hz, 3J(H5′H4′) = 7 Hz, H5′), 7.91 (m, 1H, 3J(H4′H5′) = 7 Hz, 3J(H4′H3′) = 7 Hz, H4′), 8.05 (d, 1H, 3J(H3′H4′) = 7 Hz, H3′), 8.38 (s, 1H, NCH), 8.60 (d, 1H, 3 J(H6′H5′) = 5 Hz, H6′). A similar reaction, at lower concentration, was carried out at −80 °C, with NMR monitoring at 20° intervals as the solution was warmed to room temperature, but only resonances for starting materials and the above products were observed. A similar reaction was carried out in methanol solvent, and the product 5 was isolated and characterized by comparison of its 1H NMR spectrum with that of an authentic sample. In particular, the methoxyplatinum resonance was observed at δ 2.68 (s, 3H, 3J(PtH) = 41 Hz, OMe). The isolated yield of 5 was 86%. Complex 1 with Oxaziridine 3a. To a solution of complex 1 (12.3 mg, 0.0322 mmol) in methanol-d4 (0.5 mL) was added 2-tertbutyloxaziridine (3a; 3.3 mg, 0.0267 mmol) dissolved in the same solvent (0.2 mL). The 1H NMR spectrum of the reaction mixture showed the formation of complex 5* (spectrum as above) and the parent imine 2a. NMR in methanol-d4: compound 2a, δ(1H) 1.16 (s, 9H, t-Bu), 7.25 and 7.44 (AB multiplet, each 1H, 2J(HaHb) = 16 Hz, CHaHb). Over several days, the resonances for 2a decayed as a further reaction to give the trimer occurred, but resonances of complex 5 remained unchanged. A similar reaction was carried out in methanol, and the product 5 was isolated and characterized as above. Yield: 79%.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1, giving the 1H NMR spectrum for the reaction of complex 1 with oxaziridine 3c. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.J.P.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. REFERENCES

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