Reactivity and Stability of 2-Platinaoxetanes - Organometallics (ACS

Nov 6, 2009 - Reactivity by Design—Metallaoxetanes as Centerpieces in Reaction Development. Alexander Dauth and Jennifer A. Love. Chemical Reviews ...
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Organometallics 2009, 28, 6935–6943 DOI: 10.1021/om900828k

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Reactivity and Stability of 2-Platinaoxetanes Jianguo Wu and Paul R. Sharp* 125 Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211 Received September 24, 2009

The influence of ancillary ligands, Lewis acids, carbon substituents, and oxidants on the stability and reactivity of 2-platinaoxetanes was investigated through the reactions of 2-platinaoxetanes Pt(L2)(AOC7H10) (L2 = COD, 2PEt3, 2PPh3, dppe, But2bpy, Me2bpy; A = nothing or a Lewis acid; C7H10 = norbornene) with Lewis acids, alkenes, H2O2, and Br2. Addition of catalytic or stoichiometric amounts of BF3 to 2-platinaoxetanes PtL2(C7H10OBF3) (L2 = 2PEt3 (7), 2PPh3 (8), dppe (9), But2bpy (10), Me2bpy (11)) in the presence of benzonorbornene diacetate (NB2) does not give alkene exchange, as observed previously for Pt(COD)(C7H10OBF3) (2), but only norbornene extrusion. The Pt products are hydroxo complexes [PtL2(μ-OH)]2(BF3OH)2 for L = PPh3 and PEt3 or unidentified compounds. Treatment of Pt(COD)(BF3OC7H10) (2) with ethylene in the presence of catalytic or stoichiometric amounts of BF3 affords norbornene, acetaldehyde, and allyl complex [Pt(COD)(η3-CH2CHCHCH3)]X (X = BF3OH or BF4, 12). Similar reactions of 2 or 2-platinaoxetane [Pt2(COD)2(OC7H10)Cl]BF4 (1) with isobutylene give norbornene, allyl complex [Pt(COD)(η3-CH2C(CH3)CH2)]BF4 (13), Pt(COD)Cl2 (for 1 only), and presumably water. To investigate the stability of 2-platinaoxetanes with different oxygen-coordinated Lewis acids, Pt(COD)(C7H10O) (6) was treated with 1,2-C6F4(BAr2)2 (Ar = C6F5), HBF4, MeOTf, or Ph3PAuOTf, and PtL2(C7H10O) (L2 = 2PEt3 (5), 2PPh3 (18), dppe (19)) were treated with HBF4. In each case, norbornene (NB) extrusion is observed. (Complexes 18 and 19 are reported here for the first time.) In contrast, the reaction of 6 with HCl(g) in toluene/water yields Pt(COD)(Cl)(C7H10OH) (20). NB extrusion is observed in dry C6D6. Both processes occur in CD2Cl2 or CD3OD. Complex 20 also forms in the reaction of 2-platinaoxetane 1 with HCl(aq). Treatment of 6 with Br2 gives rapid elimination of norbornene oxide with formation of Pt(COD)Br2. H2O2(aq) and 6 give Pt(COD)(OH)(C7H10OH) (21), an OH analogue of 20.

Introduction 1-Metalla-2-oxacyclobutanes (2-metallaoxetanes)1 have long been thought to be important in both homogeneous and heterogeneous alkene oxidations, epoxide reactions, and other transition metal-mediated transformations.2-6 Over the years a number of transition metal 2-metallaoxetanes

have been prepared and studied,5,7-28 but complete exploration of their chemistry and validation of their involvement in

*Corresponding author. E-mail: [email protected]. (1) We limit the metallaoxetane classification to complexes with sp3 C centers in the ring. (2) Joergensen, K. A. Chem. Rev. 1989, 89, 431–58. (3) Joergensen, K. A.; Schioett, B. Chem. Rev. 1990, 90, 1483–1506. (4) Linic, S.; Barteau, M. A. J. Am. Chem. Soc. 2002, 124, 310–317. (5) de Bruin, B.; Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2004, 43, 4142. (6) Keith, J. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. J. Am. Chem. Soc. 2007, 129, 12342–12343. (7) Schlodder, R.; Ibers, J. A.; Lenarda, M.; Graziani, M. J. Am. Chem. Soc. 1974, 96, 6893–6900. (8) Lenarda, M.; Pahor, N. B.; Calligaris, M.; Graziani, M.; Randaccio, L. J. Chem. Soc., Dalton Trans. 1978, 279–282. (9) Klein, D., P.; Hayes, J. C.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 3704–3707. (10) Klein, D. P.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 3079– 3080. (11) Day, V. W.; Klemperer, W. G.; Lockledge, S. P.; Main, D. J. J. Am. Chem. Soc. 1990, 112, 2031–3. (12) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. J. Am. Chem. Soc. 1990, 112, 3234–6. (13) Zlota, A. A.; Frolow, F.; Milstein, D. J. Am. Chem. Soc. 1990, 112, 6411–13.

(14) Bazan, G. C.; Schrock, R. R.; O’Regan, M. B. Organometallics 1991, 10, 1062–7. (15) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. Organometallics 1991, 10, 3344–62. (16) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. Organometallics 1991, 10, 3326–3344. (17) Calhorda, M. J.; Galvao, A. M.; Unaleroglu, C.; Zlota, A. A.; Frolow, F.; Milstein, D. Organometallics 1993, 12, 3316–3325. (18) de Bruin, B.; Boerakker, M. J.; Donners, J. J. J. M.; Christiaans, B. E. C.; Schlebos, P. P. J.; De Gelder, R.; Smits, J. M. M.; Spek, A. L.; Gal, A. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2064–2067. (19) Huang, D.; Gerard, H.; Clot, E.; Young, V.; Streib, W. E.; Eisenstein, O.; Caulton, K. G. Organometallics 1999, 18, 5441–5443. (20) de Bruin, B.; Verhagen, J. A. W.; Schouten, C. H. J.; Gal, A. W.; Feichtinger, D.; Plattner, D. A. Chem.;Eur. J. 2001, 7, 416–422. (21) Szuromi, E.; Shan, H.; Sharp, P. R. J. Am. Chem. Soc. 2003, 124, 10522–10523. (22) Cinellu, M. A.; Minghetti, G.; Cocco, F.; Stoccoro, S.; Zucca, A.; Manassero, M. Angew. Chem., Int. Ed. 2005, 44, 6892–6895. (23) Tejel, C.; Ciriano, M. A.; Sola, E.; del Rı´ o, M. P.; Rı´ os-Moreno, G.; Lahoz, F. J.; Oro, L. A. Angew. Chem., Int. Ed. 2005, 44, 3267–3271. (24) Szuromi, E.; Wu, J.; Sharp, P. R. J. Am. Chem. Soc. 2006, 128, 12088–12089. (25) Khusnutdinova, J. R.; Newman, L. L.; Zavalij, P. Y.; Lam, Y. F.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 2174–2175. (26) del Rı´ o, M. P.; Ciriano, M. A.; Tejel, C. Angew. Chem., Int. Ed. 2008, 47, 2502–2505. (27) Wu, J.; Sharp, P. R. Organometallics 2008, 27, 1234–1241. (28) Weliange, N. M.; Szuromi, E.; Sharp, P. R. J. Am. Chem. Soc. 2009, 131, 8736–8737.

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these transition metal (TM)-meditated reactions remains to be fully achieved. Many of the reported studies of 2-metallaoxetanes are related to alkene oxidation. In some cases it has been shown that 2-metallaoxetanes can form from oxidation of alkene complexes.5,11,17,18,23 Of further relevance to TM-mediated olefin oxidation (e.g., the Wacker process) is the formation of 2-metallaoxetanes by coupling of an alkene with an oxo or hydroxo complex.21,22,25,28 In some reactions the coupling can be reversible.24,25,28 However, alkene coupling to give 2-metallaoxetanes appears to be rare for oxo and hydroxo complexes, and the factors involved are not clear. Our work in this area began with the synthesis of 2-platinaoxetane (1) by combination of an oxo complex with an alkene (eq 1).21 Most recently we have shown that this reaction is proton catalyzed and that the key species that couples with the alkene is a hydroxo complex.28

In addition, 2-platinaoxetane 1 undergoes acid-catalyzed reversible alkene extrusion from the 2-platinaoxetane ring, allowing alkene exchange in the ring (eq 2).

Complex 1 is also a useful starting material for the preparation of other 2-platinaoxetanes by displacement of the ancillary COD ligand and/or the oxygen-bonded Pt(COD)Clþ Lewis acid fragment. The resulting family of 2-platinaoxetanes27 provides an opportunity to examine changes in the chemistry as a function of the ancillary ligands and the oxygen-bonded Lewis acid. Small molecule insertion chemistry of these 2-platinaoxetanes has been reported.29 Herein, we reported our investigation of the effect of ancillary ligands, oxygen-bonded acids, alkenes, and oxidation state on the reactivity and stability of 2-platinaoxetanes. Our studies show that stability depends on the ancillary ligands, the alkenes from which they are formed, the presence or absence of an acid (or two) on the oxygen atom, and, when present, the identity of the (29) Wu, J.; Sharp, P. R. Organometallics 2008, 27, 4810–4816.

Wu and Sharp

oxygen-bonded acid. Solvent-dependent reactivity is also observed.

Results BF3 Reactions in the Presence of Alkenes. In an effort to expand the alkene exchange chemistry of eq 2 to other alkenes, the reaction of several alkenes with 2-platinaoxetanes 1 and 2 was explored. Ethylene pressurization of a colorless dichloromethane solution of 2 containing 10% BF3 results in a slow (6 h) reaction ultimately leading to a pale yellow solution (eq 3). Acetaldehyde (14% yield by NMR) is readily identified by NMR spectroscopy as one of the products. The known allyl complex [Pt(COD)(η3-(CH3)CHCHCH2)]þ (12)30 is also observed and was isolated in crude form. 19F NMR data and IR absorption bands are consistent with BF3(OH)- as the counterion for 12. (BF3OH- and related anions have been previously encountered in reactions with BF3.31-34) Only small amounts of free NB are observed. A much cleaner reaction occurs in C6D6, and only acetaldehyde (27% yield) and free norbornene are detected in solution, while allyl complex 12 precipitates as a white solid in 54% yield.

Excess isobutylene addition to a mixture of 2 and 1 equiv of BF3 in C6D6 or CD2Cl2 affords only known allyl complex [Pt(COD)(η3-CH2C(CH3)CH2)]þ (13)30 and free norbornene (eq 4). No aldehyde or ketone is observed. Allyl complex 13 precipitates as the BF4- salt (84%) in C6D6 and has been characterized by single-crystal X-ray crystallography (see Supporting Information). If only 10% BF3 is used, the reaction stops at ∼10% conversion, suggesting that the BF3 catalyst is consumed in the formation of 13. 1H NMR spectroscopic monitoring of the reaction shows no sign of alkene exchange or alkene insertion for either the 10% or 1 equiv reactions.

In contrast to 2, 2-platinaoxetane 1 reacts completely with isobutylene with only a catalytic amount of BF3 in CD2Cl2, giving again allyl complex 13 and Pt(COD)Cl2 (eq 5). In this case 1 supplies the BF4 anion and the BF3 catalyst is not consumed. Even without added (30) Boag, N. M.; Green, M.; Spencer, J. L.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1980, 1200–1207. (31) Kerber, R. C.; Reis, K. P. J. Org. Chem. 1989, 54, 3550–3553. (32) Liang, K.-W.; Li, W.-T.; Peng, S.-M.; Wang, S.-L.; Liu, R.-S. J. Am. Chem. Soc. 1997, 119, 4404–4412. (33) Kuhn, N.; G€ ohner, M.; Steimann, M. Z. Anorg. Allg. Chem. 2003, 629, 595–596. (34) Suzaki, Y.; Osakada, K. Organometallics 2006, 25, 3251–3258.

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BF3, 1 slowly reacts with isobutylene to give the same products.

NB2. The role of NB2 in the reaction is not clear, but it may bring in the needed H2O or stabilize intermediate fragments.

Following the procedure for the alkene exchange of 1 and 2, the reactions of 2-platinaoxetanes 7-11 with NB2 in the presence of BF3 3 Et2O were examined. However, the reactions did not yield alkene reaction products and were generally the same as those with only the Lewis acid BF3 and are therefore considered in the following section. Acid Reactions. 2-Platinaoxetanes with two Lewis acids on the oxygen atom have been implicated in Lewis acid-catalyzed alkene exchange reactions (eq 1).24 In an attempt to obtain a stable complex with two Lewis acid centers coordinated to the oxygen atom, the bidendate Lewis acid C6F4(BC6F5)2 (Ar = C6F5)35 was added to 2-platinaoxetane 6 (eq 6). The only identifiable products are norbornene and the anion 14, identified by comparison its 1H and 19F NMR spectrum with those previously reported.35

With 1 equiv of BF3 3 Et2O, 2-platinaoxetanes 7 and 8 again extrude norbornene. In the case of 8, the Pt-containing product is again the hydroxo complex 16, but for 7 new unidentified products that change with the solvent are obtained. In CD2Cl2, the reaction mixture remains homogeneous and shows a major 31P NMR peak at 15.3 ppm with JPt-P = 2926 Hz. In C6D6, an off-white precipitate is obtained, which dissolves in CD2Cl2 to give an unknown single 31P NMR peak at 19.0 ppm with JPt-P = 3587 Hz. 2-Platinaoxetanes 9-11 are poorly soluble in benzene, and their reactions with acids were conducted in CD2Cl2. In reactions with 10% BF3 3 Et2O, NB2 was also present but did not appear to be involved in the reactions (eq 8). Norbornene extrusion was observed in all cases (1H NMR). Removal of the volatiles in vacuo followed by washing of the residue with toluene to remove the NB2 yielded solid products, which were dissolved in CD2Cl2 or DMSO. The 1H NMR spectra of these solutions show only unidentified peaks associated with the PtL2 fragment (see Experimental Section). No NB2-derived peaks are detected.

In contrast to C6F4(BC6F5)2, excess BF3 simply converts 6 to 2-platinaoxetane 2, which is stable in the presence of BF3 but susceptible to alkene exchange via coordination of a second BF3 (eq 1).24 Stability to excess BF3 is lost when the COD ligand of 2 is replaced by phosphine ligands. Addition of 10% BF3 3 Et2O to a C6D6 mixture of 2-platinaoxetane Pt(PPh3)2(C7H10OBF3) (8) causes the 1H NMR signals for 8 to disappear, signals for free norbornene to appear, and the formation of a white precipitate (eq 7). NMR spectroscopy of the precipitate dissolved in CD2Cl2 shows it to be a salt of the hydroxo dimer [Pt(PPh3)2(μ-OH)]22þ (16).36 The same products are observed in the presence of excess NB2 or in CD2Cl2. The identity of the counterion for 16 is unknown but may be BF3OH-, formed by addition of adventitious water across the O-B bond of 8.

The PEt3 2-platinaoxetane 7 reacts similarly with BF3 3 Et2O in the presence of excess NB2 (eq 7) to give [Pt(PEt3)2(μ-OH)]22þ (15, identified by 31P NMR spectroscopy37,38) but gives a mixture of products in the absence of (35) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins, S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244–3245. (36) Belluco, U.; Bertani, R.; Meneghetti, F.; Michelin, R. A.; Mozzon, M. J. Organomet. Chem. 1999, 583, 131–135. (37) Hara, K.; Taguchi, M.; Yagasaki, A. Polyhedron 2001, 20, 1903– 1905. (38) Mintcheva, N.; Nishihara, Y.; Tanabe, M.; Hirabayashi, K.; Mori, A.; Osakada, K. Organometallics 2001, 20, 1243–1246.

Protonation reactions were also investigated. Treatment of Lewis-acid-free 6 with 1 equiv of HBF4 in C6D6 gave norbornene extrusion and a white precipitate of the BF4- salt of hydroxo complex [Pt(COD)(μ-OH)]xxþ (17), characterized by comparison of the 1H and 195Pt NMR spectroscopic data with the triflate analogue [Pt(COD)(μ-OH)]x[OTf]x (eq 9), which has a tetrameric (x = 4) solid-state structure but may be dimeric (x = 2) in solution.28

Norbornene extrusion is also observed on HBF4 addition to phosphine-coordinated 2-platinaoxetane 5 and newly prepared (see Experimental Section) PPh3 and dppe analogues 18 and 19 (eq 10). The identity of the Pt-containing products has not been established, but the known hydroxo complexes [PtL2(μ-OH)2]2þ (L2 = 2PEt3 (15), 2PPh3 (16), dppe) are apparently not formed.36-40 Similarly, addition of (39) Scarcia, V.; Furlani, A.; Longato, B.; Corain, B.; Pilloni, G. Inorg. Chim. Acta 1988, 153, 67–70. (40) Longato, B.; Corain, B.; Bonora, G. M.; Pilloni, G. Inorg. Chim. Acta 1987, 137, 75–79.

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PPh3AuOTf to a C6D6 solution of 6 liberates norbornene, but again known [(COD)Pt(μ-OAuPPh3)]22þ,41 which might be expected, could not be detected. MeOTf and 6 also gives norbornene extrusion along with an unidentified precipitate.

The course of the reaction of 2-platinaoxetane 6 with HCl(g) is solvent dependent. In benzene, HCl(g) addition to 6 gives norbornene and an unidentified precipitate (eq 11). When the reaction is performed in CD2Cl2 or CD3OD, mixtures of HCl addition product 20 and norbornene are observed, and in toluene/H2O only 20 forms. 2-Platinaoxetane 1 and HCl(aq) in CH2Cl2 also give 20.

Thermal Stability and Oxidation Reactions. In the absence of additional Lewis acids the 2-platinaoxetanes are thermally robust. 2-Platinaoxetane 6 and BF3 adducts 2 and 7-11 can be heated for days at 100 °C with excess norbornene without noticeable decomposition. 2-Platinaoxetane 1 is less stable, and exo-2,3-epoxynorbornane is detected along with Pt(COD)Cl2 and Pt(norbornene)342 on heating a mixture of 1 and excess norbornene for 7 h at 60 °C (eq 12). The yield of volatile exo-2,3-epoxynorbornane was not determined prior to workup. After workup 1H NMR analysis indicated only a small amount.

In contrast, treatment of 6 with 1 equiv of Br2 in CD2Cl2 immediately affords exo-2,3-epoxynorbornane in 70% yield and Pt(COD)Br2 in quantitative yield by 1H NMR spectroscopy (eq 13).

No reaction is observed when 6 is treated with O2 or AuCl3. However, 30% H2O2(aq) addition to a CH2Cl2 or (41) Shan, H.; James, A. J.; Sharp, P. R. Inorg. Chem. 1998, 37, 5727– 5732. (42) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346–9.

Wu and Sharp Scheme 1

toluene solution of 6 results in cleavage of the Pt-O bond, giving Pt(II) complex 21 (eq 14). Spectral data for 21 are almost identical to those for 20 except for the appearance of the Pt-bonded OH proton as a singlet at 1.37 ppm in the 1H NMR spectrum. A broad IR peak at 3455 cm-1 is assigned to the OH groups. 1H-13C COSY reveals the norbornol OH as a doublet at 3.31 ppm with JH-H = 7 Hz. Curiously, although 21 is formally an H2O addition product of 6, H2O2 is required to form 21. There is no reaction between 6 and water under similar conditions. The H2O2 may simply lower the pH, as H2O2 is more acidic than water,43 thereby initiating protonation of the 2-platinaoxetane oxygen center.

Discussion Acid Reactivity. The acid (protic and Lewis) reactivity of 2-metallaoxetanes, both those studied here and from reports in the literature, is summarized in Scheme 1. The coordination of an acid to the oxygen center of a 2-metallaoxetane can give stable adducts or adducts that decompose by one of two pathways. C-O bond cleavage (path b) results in alkene extrusion, giving either alkene complex D or free alkene and the fragment E. M-O bond cleavage (path a) gives hydroxo alkyl complex C. Including the examples reported here, the most common appears to be alkene extrusion (path b). The 2-ruthenaoxetanes, RuL4(OC(Me)(Ph)CH2) (L = a phosphine), react with a variety of protic and Lewis acids by path b, extruding R-methylstyrene and giving Ru complexes from capture of a fragment of type E.15,16 Reversible alkene extrusion is observed for the protonated 2-platinaoxetanes Pt(dpms)(HOC7H10) and Pt(dpms)(HOC8H14) (dpms = dipyridylmethanesulfonate)25 and in the alkene exchange reactions of BF3 adduct Pt(COD)(BF3OC7H10) 2 catalyzed by BF3.24 In the latter case coordination of two BF3 appears to be required for alkene extrusion. This is also true of the other BF3 adducts reported here, where the presence of additional BF3 is required for alkene extrusion. Alkene extrusion path b is also followed for most acid reactions of the other 2-platinaoxetanes reported here. It should be noted that 2-molybdaoxetanes extrude alkenes even in the absence of acids.14 These remarkably facile C-O bond cleavage reactions contrast with the acid reactions of 2-rhodaoxetanes [Rh(N4)(OCH2CH2)]þ (N4 = N,N,N-tri(2-pyridylmethyl)amine or (43) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5 ed.; John Wiley and Sons: New York, 1988.

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N-[(6-methyl-2-pyridyl)methyl]-N,N-di(2-pyridylmethyl)amine).44,45 On protonation or alkylation, these 2-rhodaoxetanes yield either stable adducts of type B or products resulting from ring-opening (path a) to complexes of type C. The C-O bond is also retained in the CO2 insertion into the Ir-O bond of 2-iridaoxetane Cp*Ir(OC(Me2)CH2).9 Insertion likely occurs through an adduct of type B that rearranges to the insertion product via C. The solvent-dependent reactivity of 2-platinaoxetane 6 with HCl is noteworthy. Under polar conditions ringopening (path a) to hydroxo alkyl complex 20 (path a) is observed, while under nonpolar conditions norbornene is extruded (path b). The dissociation state of the HCl in the hydrogen-bonding solvents could be important in the divergent reactivity, but the observation of both products in CH2Cl2 suggests that this is not a major factor. Instead, solvent stabilization of the expectedly more polar ring-opened product (C) is probably the cause. Greater solvent stabilization of the ring-opened product would favor its formation over norbornene extrusion. We recently reported spectroscopic evidence of a solvent-induced weakening of the Pt-O bond in 2-platinaoxetane 5.27 This was detected as a decrease in the 195 Pt-31P coupling for the P atom trans to the oxygen atom as the solvent was changed from CD2Cl2 to C6D6, suggesting solvent stabilization of a more polar form (Scheme 2).46 These results suggest that the chemistry of other 2-metallaoxetanes may change with solvent. The 2-ruthenaoxetane acid reactions, where alkene extrusion was observed, were conducted in nonpolar solvents and may show a shift in reactivity in polar solvents. On the other hand, the 2-rhodaoxetane acid reactions, where C-O bond retention was observed, were conducted in polar solvents. Alkene extrusion may be observed in low-polarity solvents. Solventdependent 2-metallaoxetane ring-opening has implications for Wacker alkene oxidation. If formation of a protonated 2-palladaoxetane occurs in Wacker chemistry, either by OH coordination of an intermediate hydroxyethyl complex or by internal Pd-OH attachment on a coordinated alkene,6 the stability of the C-O bond should be most favorable in polar solvents, exactly the conditions used for Wacker chemistry.47 Another potential factor in competitive ring-opening and alkene extrusion is the rigidity of the metallaoxetane ring system. The norbornene-derived 2-platinaoxetanes lack C-C bond rotation, so that even when ring-opening occurs, the oxygen atom is held in close proximity to the Pt center. This would encourage ring-reclosing and promote alkene extrusion over ring-opening. More flexible ring systems allow C-C bond rotation of the O atom away from the metal center, decreasing the opportunity for ring-reclosing. (44) de Bruin, B.; Boerakker, M. J.; De Gelder, R.; Smits, J. M. M.; Gal, A. W. Angew. Chem., Int. Ed. 1999, 38, 219–222. (45) de Bruin, B.; Boerakker, M. J.; Verhagen, J. A. W.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Chem.;Eur. J. 2000, 6, 298–312. (46) Allen, F. H.; Pidcock, A. J. Chem. Soc. A 1968, 2700–2704. (47) Takacs, J. M.; Xun-tian Jiang, J. M. Curr. Org. Chem. 2003, 7, 369.

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The ethylene-derived 2-rhodaoxetanes discussed above are in this category and have shown only ring-opening albeit in polar solvents.

The absence of β-hydride elimination products (e.g., norbornone) from our norbornene-based 2-platinaoxetanes is unexpected. This is especially so with the Lewis acid adducts, where a relatively labile Pt-O bond should result in facile ring-opening (path a, Scheme 1), making a site available for β-elimination. The rigidity of the norbornene system is certainly important in suppressing β-elimination. Putative 2-platinaoxetanes from the more flexible alkenes ethylene and propylene are not observed, while products (acetaldehyde and acetone) associated with β-elimination are. Similarly, 2-rhodaoxetanes from ethylene undergo β-elimination on protonation, giving acetaldehyde. Yet, the rigid Rh complex 22, which is a ring-opened protonated 2-rhodaoxetane of type C, undergoes β-elimination, giving norbornenone.48 The difference is likely the position of the nonbridgehead β-hydrogen atom. In our 2-platinaoxetanes, where the ring is formed by intramolecular addition of a Pt-O bond across the norbornene double bond,28 the Pt and O atoms are syn with both atoms exo on the norbornane skeleton (F). The β-hydrogen atom is therefore anti to the Pt atom and inaccessible for elimination.49 In contrast, 22 is formed by intramolecular OH- attachment on coordinated norbornadiene. As a result, the Rh and oxygen atoms are anti and the Rh and β-hydrogen atoms are syn (G), thus facilitating elimination. Interestingly, this suggests that rigid alkene oxidation through an intramolecular addition mechanism will not readily give ketones, and alternate products such as epoxides may form instead. Alkene Extrusion Products. Scheme 3 shows proposed pathways for the various products formed from BF3-induced alkene extrusion from 2-platinaoxetanes 2, 7, and 8. Previous results suggest that coordination of a second BF3 to the oxygen atom of 2 induces C-O bond scission to give intermediate 23 (L2 = COD). A similar reaction is presumed for the other 2-platinaoxetanes. The fate of 23 apparently depends on the identity of the ancillary ligands (L/L2). For COD, 2 and 23 are in equilibrium, with 2 favored, but in the presence of an alkene norbornene is displaced from 23 to give 24, which then goes on to follow one of two reaction channels depending on the presence or absence of an allylic hydrogen atom. With an allylic hydrogen atom (isobutylene) proton transfer from the alkene to the oxygen center of 24 yields observed allyl complex 13. Ethylene, lacking an allylic hydrogen atom, follows the second channel, giving acetaldehyde oxo-alkene coupling. The low yield of acetaldehyde and the formation of allyl complex 12 indicate that other (48) Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B. Organometallics 2004, 23, 4236–4246. (49) A related norbornene-derived azametallaoxetane also appears to be resistant to β-elimination: Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. J. Am. Chem. Soc. 1988, 110, 6738–44.

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Wu and Sharp Scheme 4

the PtIV 2-platinaoxetanes 29 (Scheme 4). Low stability of the PtIV oxidation state of 29 would be expected with the π-acid COD ligand and the high stability of the Pt(COD)Br2 product. These thermal reactions should be contrasted with the photolytic extrusion of acetone from the IrIII 2-metallaoxetane Cp*Ir(OC(Me2)CH2), leaving a stable methylene complex.10

Conclusions

pathways are also operative. With other L or L2, including the dppe and diimine complexes 9-11, norbornene is lost from 23 and the resulting PtL2(O(BF3)2) fragment is not captured by another alkene. Instead, the electrophilic PtL2(O(BF3)2) fragment must follow other uncharacterized reaction pathways. In the case of L = PEt3 and PPh3 the fragment scavenges adventitious water, yielding the hydroxo complexes [PtL2(μ-OH)]22þ 15 and 16. Norbornene extrusion from the 2-platinaoxetane reactions with Hþ, Meþ, and PPh3Auþ give identified products only in the case of 6 and Hþ. Known hydroxo 17 is produced presumably via the fragment [Pt(COD)(OH)]þ. With the phosphine and bypiridine complexes the known39,50,51 dimeric hydroxo complexes are not observed, suggesting that the [PtL2(OH)]þ fragment for these ligands do not survive long enough to dimerize. Similarly, known41 [Pt(COD)(μ-OAuPPh3)]22þ is not observed in the reaction of 6 with PPh3Auþ; even this would be expected to form from the [Pt(COD)(μ-OAuPPh3)]þ fragment resulting from norbornene extrusion. Epoxide Formation. Epoxides are the most desirable products from alkene oxidation. Their formation from 2-metallaoxetanes has only recently been observed and appears difficult for d8-complexes. Epoxide elimination from a d8AuIII 2-auraoxetane22 takes days at room temperature, and our d8-PtII 2-platinaoxetanes are quite stable, with only 1 showing evidence of epoxide elimination on thermolysis. Recently PtIV 2-platinaoxetanes, prepared by oxidation of PtII 2-platinaoxetanes with H2O2 or O2, were reported to eliminate epoxides.25 Even these complexes required heating for hours at 60-80 °C to observe elimination. In contrast, the Br2-induced elimination of epoxide from 6 is immediate at room temperature. Elimination presumably occurs from (50) Bushnell, G. W.; Dixon, K. R.; Hunter, R. G.; McFarland, J. J. Can. J. Chem. 1972, 50, 3694. (51) Singh, A.; Anandhi, U.; Cinellu, M. A.; Sharp, P. R. Dalton Trans. 2008, 2314–2327.

Acid reactions of 2-metallaoxetanes appear to have three possible outcomes: (1) formation of a stable oxygen adduct, (2) alkene extrusion, or (3) formation of a hydroxo alkyl complex. While the rigidity of the 2-metallaoxetane ring system may be important in the direction the reaction takes, in at least some cases reactivity can be switched from alkene extrusion to hydroxo alkyl formation by increased solvent polarity. Reversible alkene extrusion appears uncommon and has been observed only for PtII 2-platinaoxetanes with COD or dpms ancillary ligands. In the absence of acids, PtII 2-platinaoxetane are thermally robust, but oxidation to a PtIV 2-platinaoxtane facilitates epoxide reductive elimination, supporting the viability of late metal 2-metallaoxetane intermediates in alkene oxidation to epoxides.22,25

Experimental Section Experiments were performed under a dinitrogen atmosphere in a Vacuum Atmospheres Corporation drybox or on a Schlenk line. Solvents were dried by standard techniques and stored under dinitrogen over 4 A˚ molecular sieves or sodium metal. Alkenes, PPh3, dppe, BF3 3 Et2O, 30% H2O2, HBF4 3 Et2O, and MeOTf were purchased from Aldrich or Acros. Anhydrous hydrogen chloride was obtained from Scott Specialty Gases, Inc. All commercial chemicals were used as received except where noted. PPh3AuOTf was prepared by metathesis of PPh3AuCl with AgOTf in CH2Cl2. Deuterated solvents were obtained from Cambridge Isotope Laboratories. CD2Cl2 was dried over activated alumina. C6D6 and toluene-d8 were dried over sodium metal. The bidentate Lewis acid C6F4(BArF2)2 was prepared by a literature procedure.35 Complexes 1, 2, and 5-11 were synthesized as described previously.21,24,27 Complexes 18 and 19 were prepared as described below following procedures for related 5.27 GC-MS experiments were performed using an HP 6890 GC-5973 MSD instrument. NMR spectra were recorded on Bruker AMX-250, -300, or -500 spectrometers at ambient probe temperatures. Data are given in ppm relative to solvent signals referenced to TMS for 1H and 13C spectra or relative to external standards for 195Pt (K2PtCl4/D2O at -1624 ppm), 31P (85% H3PO4 at 0.0 ppm), and 19F (CFCl3 at 0.0 ppm). Atom numbering for the peak assignments is given below. IR samples were prepared by evaporation of a CH2Cl2 solution onto a NaCl plate. Desert Analytics, Inc. performed the elemental analyses (inert atmosphere). Inclusion of solvent of crystallization in the elemental analyses was confirmed by NMR spectroscopy. Pt(PPh3)2(OC7H10) (18). From Pt(COD)(OC7H10) (6). PPh3 (9.5 mg, 0.036 mmol) in 0.1 mL of C6D6 was added dropwise to

Article an agitated solution of 6 (7.5 mg, 0.018 mmol) in 0.4 mL of C6D6. After 5 min, the volatiles were removed in vacuo. The resulting oily residue was dissolved in toluene, and the mixture was evaporated to dryness and left in vacuo for 2 h. This procedure was repeated two or three times until residual COD was eliminated and a solid residue was obtained. The residue was washed with cold hexane and dried in vacuo to give white solid 18. Yield: 12.1 mg (81%). From [Pt2(COD)2(OC7H10)Cl]BF4 (1). PPh3 (12.3 mg, 0.047 mmol) in 0.1 mL of CH2Cl2 was added to a solution of 1 (8.0 mg, 0.0095 mmol) in 0.4 mL of CH2Cl2. After 10 min, excess hexane was added and a white precipitate of [Pt(PPh3)3(Cl)]BF4 formed. The clear solution was decanted off the precipitate and evaporated in vacuo to give white solid 18. Yield: 6.2 mg, 78%. Anal. Calcd (found) for C43H40OP2Pt 3 0.75CH2Cl2: C, 58.81 (58.59); H, 4.68 (5.10).

1 H NMR (300 MHz, CD2Cl2): 7.42 (m), 7.26 (m) and 7.16 (m) (30H, Ph), 6.02 (br s with poorly resolved satellites (shoulders), JPt-H ≈ 30 Hz, 1H, H1), 2.78 (d, JH-H = 10 Hz, 1H, H7), 1.91 (br s, 1H, H6), 1.17 (br s, 3H, overlapping H3, H4, and H5), 0.89 (d, JH-H = 10 Hz, 1H, H70 ), 0.66 (br m, 1H, H50 ), 0.36 (br m, 1H, H40 ), 0.23 (br s with poorly resolved satellites (shoulders), JPt-H ≈ 30 Hz, 1H, H2). Overlapping peaks detected by 1H-13C HMQC. Assignments by DEPT-135 and 1H-1H COSY (unresolved coupling between H1 and H2 observed). 13C{1H} NMR (75.5 MHz, CD2Cl2): 134.2 (d, JP-C = 11.3 Hz, Ph), 129.7 (s, Ph), 129.3(s, Ph), 127.2 (dd, JP-C = 10.1 and 30.2 Hz, Ph), 99.7 (s, C1), 45.4 (s, C6), 39.2 (s, C3), 35.5 (s, C7), 32.2 (s, C4), 22.4 (s, C5). 31P NMR (101 MHz, CD2Cl2): 20.9 (br s with satellites, JPt-P = 1502 Hz), 15.1 (br s with satellites, JPt-P = 3796 Hz). 31P NMR (101 MHz, CD2Cl2, -70 °C): 20.06 (d with satellites, JPt-P = 1527 Hz, JP-P = 4 Hz), 15.10 (d with satellites, JPt-P = 3773 Hz, JP-P = 4 Hz). 31P NMR (101 MHz, C6D6): 19.84 (br s with satellites, JPt-P = 1489 Hz), 16.54 (br s with satellites, JPt-P = 3721 Hz). 195Pt NMR (64 MHz, C6D6): -3861 (dd, JPt-P = 1522 and 3793 Hz). Pt(dppe)(OC7H10) (19). A solution of dppe (7.5 mg, 0.018 mmol) in 0.1 mL of CD2Cl2 was added dropwise to an agitated solution of Pt(COD)(OC7H10) (6) (7.3 mg, 0.018 mmol) in 0.4 mL of CD2Cl2. After 15 min, the volatiles were removed in vacuo. The resulting oily residue was dissolved in toluene, and the mixture was evaporated to dryness and left in vacuo for 2 h. This procedure was repeated two or three times until residual COD was eliminated and a solid residue was obtained. The residue was dissolved in minimum CH2Cl2, and excess hexane was added to precipitate white solid 19. The solution was carefully decanted off the product, which was washed with hexane and dried in vacuo. Yield: 10.8 mg (84%).

Anal. Calcd (found) for C33H34OP2Pt: C, 56.3 (56.0); H, 4.9 (5.3). 1H NMR (500 MHz, CD2Cl2): 7.85 (m), 7.63 (m) and 7.43 (m) (20H, Ph), 6.30 (br s with satellites, JPt-H = 45 Hz, 1H, H1), 2.68 (d, JH-H = 8.7 Hz, 1H, H7), 2.39 (m, 1H, PCH2), 2.09 (m, 1H, PCH2), 2.05 (m, 2H, PCH2), 1.98 (m, 1H, H6), 1.70 (br s with satellites, JPt-H = 39 Hz, 1H, H3), 1.41 (m, 1H, H4),

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1.28 (m, 1H, H5), 0.96 (d, JH-H = 8.7 Hz, 1H, H70 ), 0.77 (br m, 2H, H40 and H50 ), 0.66 (br m with satellites, JPt-H = 44 Hz, 1H, H2). Overlapping peaks detected by 1H-13C HMQC. Assignments by DEPT-135 and 1H-1H COSY. 13C{1H} NMR (75.5 MHz, CD2Cl2): 133.4 (dd, JP-C = 11, 4 Hz, Ph), 133.3 (s, Ph), 133.1 (s, Ph), 132.9 (s, Ph), 132.8 (s, Ph), 132.5(s, Ph), 131.0 (d, JP-C = 6 Hz, Ph), 130.8 (s, Ph), 129.2 (dd, JP-C = 10 and 2 Hz, Ph), 129.0 (s, Ph), 128.8 (s, Ph), 128.7 (s, Ph) 102.1 (s, C1), 45.6 (C6), 40.4 (C3), 36.5 (C7), 33.5 (d, JP-C = 12 Hz, C4), 30.2 (dd, 1JP-C = 38 Hz, 3JP-C = 20 Hz, CH2PPh2), 24.7 (dd, 1JP-C = 32 Hz, 3JP-C = 9 Hz, CH2PPh2), 23.4 (s, C5). 31P NMR (101 MHz, CD2Cl2): 39.0 (d with satellites, JPt-P = 1611 Hz, JP-P = 11 Hz), 29.0 (d with satellites, JPt-P = 3497 Hz, JP-P = 11 Hz). 195Pt NMR (64 MHz, CD2Cl2): -3926 (dd, JPt-P = 1643 and 3552 Hz). Reaction of Pt(COD)(BF3OC7H10) (2) with Ethylene and Catalytic BF3. A medium-walled NMR tube was charged with a solution of 2 (7.2 mg, 0.0075 mmol) and 5% BF3 3 Et2O (1.2 μL of a solution prepared by adding 15 μL of BF3 3 Et2O to 485 μL of CDCl3) in CD2Cl2 (0.5 mL). The tube was connected to a vacuum line, immersed into liquid nitrogen, and evacuated. Ethylene (5 mL, 0.2 mmol at 25 °C, 1 atm) was added by vacuum transfer, and then the tube was flame-sealed and allowed to warm to room temperature. The reaction was monitored by 1H NMR spectroscopy. Free norbornene, acetaldehyde, and a transient with a peak at 8.93 ppm (t, JH-H = 7 Hz) were observed early in the reaction. Free norbornene and the transient disappeared and acetaldehyde increased as the reaction progressed. After ∼6 h the reaction was complete, yielding ∼17% acetaldehyde by 1H NMR spectroscopy. The volatiles were removed in vacuo, and the residue was dissolved in minimum CH2Cl2. Addition of excess ether gave an off-white solid consisting of crude allyl complex [Pt(COD)(η3-CH2CHCH(CH3))]þBF3OH- (12) with other minor products. Increasing the amount of BF3 3 Et2O from 5% to 10% gave the same result. With 1 equiv of BF3 the amount of the transient increased and it became persistent, but about the same yield of acetaldehyde was obtained. Both the acetaldehyde and the intermediate disappeared when the reaction mixture was left overnight. In C6D6, the reaction of 2 and ethylene with catalytic amounts of BF3 3 Et2O gives a white precipitate of 12 (54% yield). The 1H NMR spectrum of the solution showed the presence of acetaldehyde (∼28%) and free NB. Allyl complex 12 was identified by comparison of its 1H NMR spectrum with previously reported [Pt(COD)(η3-CH2CHCH(CH3))]BF4.18 IR absorption bands at 1440, 1200-1060, 880, 802, and 3442 (br) cm-1 and 19F NMR shifts (235 MHz, CDCl3) of -151.19 (10B) and -151.24 (11B) ppm suggest that the counteranion for 12 is BF3OH-.31-34 195 Pt NMR (64 MHz, CD2Cl2): -4397. Reaction of Pt(COD)(BF3OC7H10) (2) with Isobutylene and BF3: [Pt(COD)(η3-CH2C(CH3)CH2)][BF4] (13). A mediumwalled NMR tube was charged with a solution of 2 (5.7 mg, 0.012 mmol) and BF3 3 Et2O (1.4 μL, 0.012 mmol) in C6D6 (0.5 mL). The tube was connected to the vacuum line, immersed into liquid nitrogen, and evacuated. Isobutylene (5 mL, 0.2 mmol at 25 °C) was added by vacuum transfer. The tube was then flame-sealed and allowed to warm to room temperature. The NMR tube was shaken by hand. A precipitate formed and the reaction appeared complete after ca. 20 min. Free norbornene was observed by 1H NMR spectroscopy. The volatiles were removed in vacuo. The residue was dissolved in minimum CH2Cl2, and excess diethyl ether addition gave a white precipitate of [Pt(COD)(η3-CH2C(CH3)CH2)][BF4] (13). The solution was carefully decanted off the product, which was washed with hexane and dried in vacuo. Yield: 4.5 mg (83%). Spectroscopic data match those previously reported.18 Colorless single crystals for X-ray analysis were grown by slow evaporation of a CH2Cl2 solution of 13 at -30 °C. A reaction in CD2Cl2 gave similar results. 195Pt NMR (64 MHz, CD2Cl2): -4374.

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Reaction of [Pt2(COD)2(OC7H10)Cl]BF4 (1) with Isobutylene and Catalytic BF3. A medium-walled NMR tube was charged with a solution of 1 (18 mg, 0.021 mmol) in CD2Cl2 (0.5 mL) and BF3 3 Et2O (8.5 μL of a solution of 15 μL in 485 μL of CDCl3, 0.0021 mmol). The tube was connected to the vacuum line, immersed into liquid nitrogen, and evacuated. Isobutylene (5 mL, 0.2 mmol at 25 °C) was added by vacuum transfer, and then the tube was flame-sealed and allowed to warm to room temperature. The tube was shaken, and the reaction was complete after 6 h, over which time the solution changed from colorless to pale yellow and finally yellow. The 1H NMR spectrum of the reaction mixture showed the presence of free norbornene, Pt(COD)Cl2, and allyl complex 13 (62% by 1 H NMR spectroscopy). Reaction of Pt(COD)(C7H10O) (6) with C6F4(BAr2)2 (Ar = C6F5). C6F4(BAr2)2 (14.2 mg, 0.017 mmol) was added to a solution of 6 (7.0 mg, 0.017 mmol) in 0.5 mL of CD2Cl2. After 5 min, the 1H and 19F NMR spectra showed the presence of [C6F4(BAr2)2(μ-OH)]- (14).23 Reaction of Pt(PPh3)2(C7H10OBF3) (6) with BF3. Formation of [Pt(PPh3)2(μ-OH)]22þ (16). In C6D6. Catalytic BF3 (2.5 μL of a solution prepared by adding 15 μL of BF3 3 Et2O to 485 μL of CDCl3) was added to a solution of 6 (5.5 mg, 0.0060 mmol) in 0.4 mL of C6D6. Free norbornene was observed by 1H NMR spectroscopy, and a white precipitate formed. After the solution was carefully decanted off, the precipitate was dissolved in CD2Cl2 and a 31P NMR spectrum showed the formation of 16.36 In CD2Cl2. Catalytic BF3 (2.5 μL of a solution prepared by adding 15 μL of BF3 3 Et2O to 485 μL of CDCl3) or stoichiometric BF3 (0.7 μL of BF3 3 Et2O) was added to a solution of 6 (5.5 mg, 0.0060 mmol) in 0.5 mL of CD2Cl2. Free NB was observed in the 1H NMR spectra of the mixtures, and the 31P NMR spectra showed only one peak at 7.8 ppm with JPt-P = 3745 Hz, indicating the presence of known [Pt(PPh3)2(μ-OH)]22þ (16).36 Excess NB2. Excess NB2 (19.3 mg, 7.5  10-2 mmol) was added to a solution of platinoxetane 6 (7.5  10-3 mmol) in 0.5 mL of CD2Cl2, and then 3 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3 (7.5  10-4 mmol) was added. After 10 min a 31P NMR spectrum showed the formation of [Pt(PPh3)2(μ-OH)]22þ (16).36 Reaction of Pt(PEt3)2(C7H10OBF3) (5) with BF3. 1 equiv of BF3 in CD2Cl2. One equivalent of BF3 3 Et2O (0.9 μL, 7.5  10-3 mmol) was added to a solution of 5 (4.6 mg, 7.5  10-3 mmol) in 0.5 mL of CD2Cl2. Free NB was observed in the 1H NMR spectrum of the solution, and the 31P NMR spectrum showed a major peak at 15.3 ppm with JPt-P = 2926 Hz. 1 equiv of BF3 in C6D6. As above but in C6D6 instead of CD2Cl2. An off-white precipitate was obtained, which dissolves in CD2Cl2 to give a single 31P NMR peak at 19.0 ppm with JPt-P = 3587 Hz. 10% BF3. Addition of catalytic BF3 (8 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3) to a solution of 5 (11 mg, 0.022 mmol) in 0.5 mL of CH2Cl2 gave two major 31P NMR peaks at 7.90 ppm (JPt-P = 3799 Hz) and 6.80 ppm (JPt-P = 3771 Hz). 10% BF3 and Excess NB2: Formation of [Pt(PEt3)2(μOH)]22þ (15). Excess NB2 (19.3 mg, 7.5  10-2 mmol) was added to a solution of the platinaoxetane (7.5  10-3 mmol) in 0.5 mL of CD2Cl2, and then 3 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3 (7.5  10-4 mmol) was added. After 10 min a 1H NMR spectrum of the solution showed the presence of free NB, and a 31P NMR spectrum showed the formation of [Pt(PEt3)2(μ-OH)]22þ (15) as a singlet with satellites at 7.2 ppm (JPt-P = 3481 Hz).37,38 Reaction of PtL2(C7H10OBF3) with Catalytic BF3. L2 = dppe (9). Excess NB2 (2.8 mg, 1.1  10-2 mmol) was added to a solution of 9 (2.4 mg, 3.1  10-3 mmol) in 0.5 mL of CD2Cl2, and then 0.6 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3 was added. After 20 min, a 1H NMR spectrum

Wu and Sharp (300 MHz) of the filtrate showed complete loss of the peak at 6.08 ppm for 9 and appearance of free NB. The volatiles were removed in vacuo. The resulting residue was washed with toluene (to remove NB2) and dried in vacuo. A 1H NMR spectrum of the solid in CD2Cl2 gave 1H peaks at 7.75 (m), 7.63 (m), 7.40 (m), 2.34 (s), and 2.37 (s) ppm. The 31P NMR spectrum showed peaks at 48.7, 47.9, and 34.4 ppm with no 195Pt satellites. L2 = But2bpy (10). Excess NB2 (2.4 mg, 9.2  10-3 mmol) was added to a solution of 10 (0.3 mg, 4.7  10-4 mmol) in 0.4 mL of CD2Cl2, and then 0.2 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3 was added. After 20 min, a 1H NMR spectrum of the mixture showed free NB and other new peaks. The volatiles were removed in vacuo, and the resulting residue was dissolved in minimum CH2Cl2 and excess hexane was added. A white precipitate formed, which was isolated by decantation. The solid was washed with toluene and dried in vacuo. 1H NMR (500 MHz) data for the solid in CD2Cl2: 8.86 (d, JH-H = 7 Hz, 0.67H), 8.68 (d, JH-H = 7 Hz, 0.53H), 8.35 (d, JH-H = 7 Hz, 0.78H), 8.17 (d, JH-H = 2 Hz, 0.58H), 7.89 (m, 0.58H), 7.66 (m, 0.73H), 6.78 (s, 0.37H), 6.64 (s, 0.26H), 6.30 (br s, 0.94H), 3.90 (br s, 0.55H), 3.06 (m, 1.26H), 2.72 (d, JH-H = 9 Hz, 1.25H), 2.44 (s, 0.43H), 2.34 (s, 0.48H), 2.30 (s, 0.45H), 1.54 (s, 0.84 H), 1.48 (s, 2.59H), 1.45 (s, 1.56H). L2 = Me2bpy (11). Excess NB2 (2.4 mg, 9.2  10-3 mmol) was added to a solution of 11 (2.4 mg, 3.8  10-3 mmol) in 0.4 mL of CD2Cl2, and then 0.7 μL of a solution of 15 μL of BF3 3 Et2O in 485 μL of CDCl3 was added. After 1 h, a pale yellow precipitate formed. The mixture was filtered, and a 1H NMR spectrum of the filtrate showed free NB. The precipitate was washed with toluene and dried in vacuo. A 1H NMR spectrum of the precipitate in DMSO showed only Me2bpy peaks: 9.46 (d, JH-H = 6 Hz, 1H), 9.15 (d, JH-H = 6 Hz, 1H), 8.88 (d, JH-H = 6 Hz, 1H), 9.46 (d, JH-H = 6 Hz, 1H), 8.69 (s, 1.5H), 8.64 (s, 1.5H), 8.52 (d, JH-H = 6 Hz, 1H), 8.23 (s, 1H), 7.83 (m, 3H), 7.28 (m, 1H), 2.30 (s, 6H). Reaction of Pt(COD)(C7H10O) (6) with HBF4, PPh3AuOTf, or MeOTf in C6D 6. HBF4. HBF4 3 Et2O (1.5 μL, 0.011 mmol) was added to an agitated solution of 6 (4.7 mg, 0.011 mmol) in 0.4 mL of C6D6. The clear solution immediately became cloudy. After 20 min, a yellow precipitate formed and a 1H NMR spectrum of the mixture showed the presence of free norbornene. The clear solution was carefully decanted off the precipitate, which was washed with ether and dried in vacuo. NMR data identified the precipitate (17) as the BF4- salt of [Ptx(COD)x(μ-OH)4]xþ.28 1H NMR (300 MHz, CD3NO2): 5.65 (s, JPt-H = 69 Hz, 4H, COD CH), 2.81 (br s, 4H, COD CH2), 2.37 (m, 4H, COD CH2). 195Pt NMR (64 MHz, CD3NO2): -2794. 19F NMR (235 MHz, CD3NO2): -151.4. PPh3AuOTf. PPh3AuOTf (7.4 mg, 0.012 mmol) was added to an agitated solution of 6 (5.0 mg, 0.012 mmol) in 0.5 mL of C6D6. After 10 min, a 1H NMR spectrum of the mixture showed the presence of free norbornene. A 31P NMR spectrum showed a major peak at 29.4 ppm with no satellites and a very minor peak at 44.5 ppm. The same reaction in CD2Cl2 gave a black precipitate, and a 1H NMR spectrum of the mixture showed the presence of free norbornene. A 195Pt NMR spectrum gave two peaks at -2965 and -3099 ppm. The 31P NMR spectrum showed no peaks. MeOTf. MeOTf (0.9 μL, 0.008 mmol) was added to an agitated solution of 6 (3.3 mg, 0.008 mmol) in 0.5 mL of C6D6. After 10 min, a 1H NMR spectrum indicated the reaction was only partially completed. New peaks at 5.74 (m), 4.18 (d, JH-H = 5.2 Hz), 2.18 (d, JH-H = 5.5 Hz), and 2.05 (m) together with minor free NB peaks were observed. After 3 h, a white precipitate formed and a 1H NMR spectrum of the mixture showed large amounts of free NB. Reaction of PtL2(C7H10O) with HBF4. L = PEt3 (5). Addition of 1 equiv of HBF4 3 Et2O to a solution of 5 in C6D6 results in a white precipitate and free NB. 31P NMR data for a solution of

Article the precipitate in CDCl3: 19.6 (s, JPt-P = 3585 Hz, 1P), 11.9 (s, JPt-P = 2393 Hz, 1P). L = PPh3 (18). Addition of 1 equiv of HBF4 3 Et2O to a solution of 18 in C6D6 resulted in a white precipitate and free NB. 31P NMR data for a solution of precipitate in CDCl3: 23.64 ppm (d, JPt-P = 2484 Hz, JP-P = 19 Hz, 2P), 13.02 ppm (t with satellites, JP-P = 19 Hz, JPt-P = 3826 Hz, 1P). L2 = dppe (19). HBF4 3 Et2O (1 equiv) was added to a solution of 19 in CH2Cl2, giving free NB. 31P NMR data for the mixture: 42.4 (s, JPt-P = 3585 Hz, 1P), 44.6 (d, JP-P = 30 Hz, 1P), 29.5 (d, JP-P = 30 Hz, 1P). Pt-P coupling was not observed for the last two phosphine peaks. Reaction of Pt(COD)(C7H10O) (6) with H2O/HCl: Pt(COD)(C7H10OH)Cl (20). H2O (0.8 μL, 0.044 mL) was injected into a solution of 6 (4.5 mg, 0.011 mmol) in 0.5 mL of toluene-d8. The mixture was monitored by 1H NMR spectroscopy. No reaction was observed overnight. Anhydrous HCl gas (0.24 mL, 0.011 mmol) was injected into the mixture. Complex 20 was immediately formed. The solution was evaporated to dryness to give 20 as a white solid (quantitative). Anal. Calcd (found) for C15H23ClOPt: C, 40.1 (40.9); H, 5.1 (4.4).

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Thermolysis of [Pt2(COD)2(OC7H10)Cl]BF4 (1) with Excess NB. Complex 1 (54 mg, 0.064 mmol), 4 mL of CH2Cl2, and excess NB (70 mg, 0.73 mmol) were placed in a sealable 25 mL Schlenk tube. The tube was sealed, and the mixture was heated at 60 °C for 7 h. The solvent and excess NB were removed by placing the sample under high vacuum for 4 h. The resulting solid was extracted with 2  5 mL of hexane. The combined extracts were evaporated to dryness. A proton NMR spectrum in CD2Cl2 showed the presence of Pt(NB)3 with trace amounts of exo-2,3-epoxynorbornane. A 1H and 195Pt NMR spectra of the hexane insoluble residue showed the presence of Pt(COD)Cl2. Reaction of [Pt(COD)(OC7H10) (6) with Br2. Br2 (1.4 μL, 0.027 mmol) was injected into an agitated solution of 6 (11 mg, 0.027 mmol) in 0.5 mL of CD2Cl2. A 1H NMR spectrum of the homogeneous reaction mixture showed the presence of exo-2,3epoxynorbornane in ca. 70% yield. The pale yellow mixture was reduced to ∼0.1 mL, and some precipitate appeared. Addition of excess hexane gave complete precipitation of Pt(COD)Br2. The filtrate was used for GC-MS analysis. The exo-2,3-epoxynorbornane peak showed Mþ = 110 (tR = 4.97 min) and was compared with an authentic sample. Pt(COD)(OH)(C7H10OH) (21). A 6.0 μL (0.058 mmol) amount of 30% aqueous H2O2 was added to a stirred solution of 6 (11 mg, 0.027 mmol) in 1 mL of CH2Cl2. After 5 min the reaction mixture was evaporated to dryness. The resulting residue was extracted with 2  1 mL of benzene. The extract was evaporated to dryness to give 21 as a white solid. Yield: 6.7 mg, 59%. The reaction is much slower in toluene but gives similar results.

1

H NMR (300 MHz, C6D6): 5.27 (m, 2H, COD CH), 4.15 (apparent t, JH-H = 6.9 Hz, 1H, H1), 4.08 (td with satellites, JH-H = 7.5, 3 Hz, JPt-H = 75 Hz, 1H, COD CH), 3.96 (td with satellites, JH-H = 7.5, 3 Hz, JPt-H = 75 Hz, 1H, COD CH), 3.30 (d, JH-H = 7.2 Hz, 1H, OH), 2.65 (d, JH-H = 10.0 Hz, 1H, H7), 2.52 (br s, 1H, H6), 2.20 (d with satellites, JH-H = 1.8 Hz, JPt-H = 24 Hz, 1H, H3), 2.16 (dd, JH-H = 6.3, 1.8 Hz, 1H, H2), 1.78, 1.56, 1.46, and 1.31 (m’s, 8H total, COD CH2), 1.45 (m, 2H, H4 and H5), 1.18 (d, JH-H = 10.0 Hz, 1H, H70 ), 0.99 (m, 2H, H40 and H50 ). Assignments were made by comparison to the spectra for the OH analogue 21 (see below). 195Pt NMR (64 MHz, C6D6): -3459. Reaction of Pt(COD)(C7H10O) (6) with HCl(g). Anhydrous HCl(g) (0.27 mL, 0.012 mmol) was injected into a solution of 6 (5.0 mg, 0.012 mg) in 0.5 mL of the following deuterated solvents, and the reaction was monitored by 1H NMR spectroscopy. C6D6. A precipitate formed, and only free NB was detected in solution. Complex 20, which is soluble in C6D6, was not observed. The precipitate dissolved in CDCl3 to give 1H NMR (300 MHz, CDCl3): 5.59 (s, JPt-H = 68 Hz, COD CH), 2.69 (br m, COD CH2), 2.26 (m, COD CH2). CD2Cl2. The mixture remained homogeneous. Complex 20 and free NB were detected in a 4:3 ratio by 1H NMR spectroscopy. Peaks at 5.56 (m, JPt-H ≈ 68 Hz), 2.68 (m), and 2.28 (m), overlapping with those for 20, were also observed. CD3OD. The mixture remained homogeneous, and 20 and free NB were detected in a 3:1 ratio by 1H NMR spectroscopy. Peaks at 5.54 (m, JPt-H ≈ 70 Hz), 2.58 (m), and 2.36 (m), overlapping with those for 20, were also observed. Reaction of [Pt2(COD)2(OC7H10)Cl]BF4 (1) with H2O/HCl. Dilute aqueous HCl solution (4.5 μL of a 1 mL 37% hydrochloric acid solution in 5 mL of H2O) was added to a solution of 1 (7.5 mg, 0.0089 mmol) in 0.5 mL of CH2Cl2. The mixture was stirred for 10 min. The solution was evaporated to dryness in vacuo. The resulting residue was extracted with benzene. The extraction was evaporated to dryness to give crude 20 identified by 1H NMR spectroscopy.

Anal. Calcd (found) for C15H24O2Pt: C, 41.76 (42.25); H, 5.61 (5.22). IR (cm-1): 3455 (br, νOH). 1H NMR (500 MHz, C6D6): 5.27 (m, 2H, COD CH), 4.15 (apparent br t, JH-H = 7 Hz, 1H, H1), 4.08 (m, 1H, COD CH), 3.97 (m, 1H, COD CH), 3.31(d, JH-H = 7.0 Hz, 1H, C-OH), 2.65 (d, JH-H = 9.5 Hz, 1H, H7), 2.53 (br s, 1H, H6), 2.20 (br s, 1H, H3), 2.16 (d, JH-H = 6.5 Hz, 1H, H2), 1.84, 1.72, 1.66, 1.55, and 1.33 (m’s, 8H total, COD CH2), 1.45 (m, 2H, H4 and H5), 1.37 (s, 1H, PtOH), 1.18 (d, JH-H = 9.5 Hz, 1H, H70 ), 0.99 (m, 2H, H40 and H50 ). Assignments by 1H-13C HMQC, DEPT-135, and 1H-1H COSY. 13C NMR (75 MHz, C6D6): 114.8 (COD CH), 114.7 (COD CH), 84.0 (COD CH), 83.3 (COD CH), 81.4 (C1), 50.6 (C2), 44.5 (C6), 40.2 (C3), 35.8 (C7), 32.6 (C4), 31.6 (COD CH2), 30.9 (COD CH2), 27.3 (COD CH2), 26.7 (COD CH2), 24.5 (C5). Assignments by 1H-13C HMQC and DEPT-135. 195Pt NMR (64 MHz, C6D6): -3460.

Acknowledgment. Support from the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG0288ER13880), is gratefully acknowledged. We thank a reviewer for helpful comments and Dr. C. Barnes for the X-ray structure determination. Supporting Information Available: Crystallographic data in cif format is available free of charge via the Internet at http://pubs.acs.org.