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Do Platinum(II) and Palladium(II) Phosphinito Phosphinous Acids Generate the Same Type of Reactive Intermediate in Alkyne Coordination? A Gas-Phase Study with Phenylethyne and Propargyl Acetate Magdalena Karanik,† Denis Lesage,‡ Yves Gimbert,*,† Paola Nava,§ Stephane Humbel,§ Laurent Giordano,§ Gerard Buono,§ and Jean-Claude Tabet‡ †
Universite Joseph Fourier, UMR-CNRS 5250, ICMG-FR 2607, BP 53, 38041 Grenoble Cedex 9, France UPMC Universite Paris 06, UMR-CNRS 7201, IPCM, FR2769, 4, place Jussieu, 75252 Paris Cedex 05, France § Universite Aix-Marseille & ECM, ISM2, UMR-CNRS 6263, Centre Saint-Jer^ome, 13397 Marseille Cedex 20, France ‡
bS Supporting Information ABSTRACT: The structure and behavior of the catalytic species involved in the PAPd- and PAPt-catalyzed formal [2 + 1] cycloaddition between an alkyne and norbornadiene have been studied by combined mass spectrometry experiments and density functional theory calculations. The first step of this reaction, the coordination of the catalyst with the alkyne, is also investigated under collision-activated reaction conditions. Two different structures are proposed for the key intermediates: an acetylide structure for Pd and a metallacarbene for Pt.
’ INTRODUCTION Secondary phosphine oxides (SPOs) in equilibrium with phosphinous acids (PAs) are excellent preligands for transition metals, leading to several efficient catalytic processes.1,2 We recently disclosed unprecedented phosphinito phosphinous acid ligand systems coordinated to Pd and Pt which catalyze [2 + 1] cycloadditions of alkynes with norbornadiene derivatives to yield alkylidene or arylidene and vinylidene cyclopropanes (Scheme 1).3,4 In these Pd and Pt catalytic precursor complexes a chiral phosphinous acid (PA) and a phosphito ligand provided by the SPOs are associated by a strong, symmetric hydrogen bridge to afford a bidentate anionic ligand. Nevertheless, the knowledge of the coordination chemistry of SPO systems and associated mechanisms is still poor. We recently reported a mass spectrometry mechanistic study5 for the Pd catalytic process. In that study, we showed how the coordination of the alkyne on the metal could give rise to three types of activated complexes (Scheme 2). We established that the initial coordination state η2 (form A) of the phenylethyne to the metal evolves toward a coordination state η1 (form B), where the phenylethyne has transferred a proton on the phosphinato ligand and is coordinated as an acetylide. A second transformation of the initial form A implies a formal 1,2-proton shift to give rise to a metallocarbene complex (form C). For energy reasons, we concluded that B was formed. Addition of norbornadiene (NDB) led to a new adduct, D (Scheme 3). This final complex, which was obtained after two consecutive collision-activated reaction (CAR) experiments, r 2011 American Chemical Society
could not be further fragmented. Its structure was proposed only on the basis of calculations (Scheme 3) In the present work, we address two important questions related to the structure of this D complex. (i) For the Pd catalyst, do calculations agree with the observed fragmentations? (ii) Do the conclusions for the Pd catalyst remain valid for the Pt catalyst? To do this, we planned two sets of experiments in a FT-ICR mass spectrometer. In the first set, we prepared for both Pd and Pt catalysts a solution with the catalyst and associated reagents (NBD and phenylacetylene). The mass spectrometry experiments were carried out directly on this solution, allowing us to analyze the structure of the proposed D complex. These experiments reveal significant differences between the aforementioned Pd and Pt catalysts. The first step of this reaction, namely the coordination of the alkyne on the catalyst, is crucial. It shall be noted that earlier condensed-phase experiment, involving alkyne propargyl acetate compound, indicate a loss of the OAc group in the final structure.3 One might wonder if this loss can occur at the initial coordination of the alkyne on the catalyst. To address this question, we conducted an additional set of experiments with the Pd/Pt catalysts and the tertiary alkyne propargyl acetate, without NBD. These experiments on the evolution of adducts involving a tertiary acetate derivative confirmed the different behaviors of the two transition metals Pd and Pt. Density functional theory (DFT) calculations were Received: March 17, 2011 Published: August 26, 2011 4814
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Scheme 1. Formal [2 + 1] Cycloaddition between Alkyne and Norbornadiene with Phosphinito Phosphinous Acid Pd and Pt Catalyst Precursors
Scheme 2. Different Coordination Modes of Phenylethyne with the Rp*,Rp* Palladium Catalyst
Scheme 3. Formation of Complex D from B after Coordination of Norbornadiene
performed throughout this work to rationalize the experimental evidence and to support the hypotheses of fragmentation paths. Experiments and calculations were performed on complexes
including phosphinous acid (PA) and phosphinato ligands with the same configuration on the phosphorus atom (diastereomer Rp*,Rp*; Scheme 2). 4815
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Scheme 4. Species Involved in the Formation of Complexes among Pd/Pt Catalyst, Phenylethyne, and NBD
’ EXPERIMENTAL SECTION Theoretical Treatment. Full geometry optimizations were performed with the Gaussian03 program package6 at the B3LYP level,7 as implemented in this program. Scalar relativistic effective core potentials (RECP) were employed for Pd and Pt, together with the associated lanl2dz basis sets.8 For consistency, we used lanl2dz basis sets for all the other atoms and we added d polarization functions with the exponents 0.75, 0.85, and 0.37 for carbon, oxygen, and phosphorus, respectively.9 Reported energies include unscaled zero point energy (ZPE) and basis set superposition error (BSSE) corrections computed by the counterpoise method.10 Mass Spectrometry Experiments. ESI mass spectra were recorded using a modified FT-ICR mass spectrometer (7 T hybrid FTICR Apex-Q spectrometer, Bruker Daltonik GmbH, Bremen, Germany) combined with an ion funnel geometry to transfer the formed ions. The proposed ion structures were proven by high-resolution measurements. The transfer hexapole (H2) was converted into a reactive collision cell by adjunction of a new gas inlet, allowing us to perform additional consecutive CAR experiments on the important species (see the Supporting Information). To introduce the vaporized reagent into the collision cell (H2), the organic reagent was placed in a glass flask. The flask was connected to the collision cell by means of a micrometric valve and a rotary pump to remove residual air. Precursor ions were isolated using the first quadrupole Q1 to react with the reagent gas in H2, and the CAR product ions were then isolated into the ICR cell to decompose under sustained off-resonance irradiation (SORI) experiments. Norbornadiene and phenylethyne were purchased (Aldrich) and used without any treatment. Pd and Pt complexes and 2-methylbut3-yn-2-yl acetate were prepared according to the literature.3,4 The stock solutions (1 mg/mL of Pd and Pt catalyst precursors, norbornadiene, or phenylethyne) were made in acetonitrile (ACN), and the working standards were prepared by further diluting the stock solution with ACN to achieve a final concentration of 100 ng/mL of each analyte. The sample solution (complex alone or in mixture with the organic partners) was introduced into the ESI source of the mass spectrometer using an infusion pump (Cole Parmer, 74900 series) at a flow rate of 120 μL/h. Tertiary propargyl acetate was used as the target in the H2 collision cell to perform CAR experiments.
Figure 1. SORI spectra of 106Pd (a) and 195Pt (b) complexes observed under positive ESI conditions and generated from a solution containing a mixture of catalyst, phenylethyne, and norbornadiene (the excitation power was set to 0.8% and 1.5% for Pd and Pt complexes, respectively). Selected precursor ions are shown in black boxes. The ESI capillary voltage was maintained at 4.5 kV, and the skimmers 1 and 2 were kept at 60 and 10 V, respectively, for complex ion desolvation. Nitrogen was used as the desolvation and nebulization gas. The source and desolvation temperatures were kept at 250 °C. For SORI experiments the pulse gas was set to 0.25 s, P(Ar) = 9 mbar, and the excitation power was adjustable in the range 0.82.5%. 4816
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Scheme 5. Computed Energy Data (in kcal/mol, at T = 0 K, Including ZPE) for the Fragmentation Ions m/z 663 (Pd) and 752 (Pt) of Form Da
a
The BSSE corrections (5.1 kcal/mol for Pd and 5.2 kcal/mol for Pt) are calculated for D and applied to the dissociated fragments of path i.
’ RESULTS AND DISCUSSION Phenylethyne Adducts. In a first set of experiments, the sample solution (a mixture of catalyst precursor, phenylethyne, and NBD) was directly introduced into the ESI source (Scheme 4). Each catalyst precursor was dissociated to afford a cationic catalyst with m/z 469 and 558 for Pd and Pt, respectively. The adduct ions (catalyst + HCCPh + NBD) were produced in 37 and 100% abundances for m/z 663 (106Pd) and 752 (195Pt), respectively. The SORI-CID decompositions of these ions are reported in Figure 1 and show dramatic differences. In the case of the Pd catalyst (Figure 1a), only one fragment ion was observed, at m/z 481. This corresponds to the loss of one phosphinous acid (PA) ligand, HOP(Ph)(t-Bu) (Mw 182). It is worth noting that the regeneration of the catalyst is clearly not observed (no peak at m/z 469). Therefore, it appears that in complex D (Scheme 3), it is so difficult to break the CPd bond that the complex would rather lose a ligand.11,12 The situation is totally different in the case of the Pt catalyst (Figure 1b). Here, no peaks corresponding to the loss of a single ligand (PA, NBD, or phenylethyne) were observed, but only the ion m/z 558 corresponding to the catalyst (195Pt) was detected. This result argues for a loss of benzylidenecyclopropane adduct (1; Scheme 1). Mass spectrometry experiments show that the structures of the Pd and Pt complexes with phenylethyne and NBD substantially differ. We addressed this question with DFT calculations. Supposing that both Pd (m/z 663) and Pt (m/z 752) ions have a structure of form D (Scheme 3), we compared the estimated energies required for the loss of phosphine and for the acidolysis process (Scheme 5). For Pd, the energy required to lose a phosphine ligand (17.2 kcal/mol)13 is smaller than the energy required to transfer an H from the ligand to the organic moiety (TSPd at 31.2 kcal/mol). It is worth noting that the phosphine ligand is the only source of acidic proton available to perform the acidolysis of the MC bond.11 The modeling is consistent with the fact that the Pd m/z 663 ion, form D, preferentially loses a phosphine (path i) rather than cleaves the CPd bond and restores the catalyst at m/z 471 (path ii). For Pt, the computations show that form D could not evolve to the product of the acidolysis (and the consequent release of the catalyst, TSPt =33.6 kcal/mol) but rather to the loss of a phosphine (22.8 kcal/mol). Nevertheless, we know that the
Pt m/z 752 ion does release the catalyst fragment in the SORI decomposition. The confrontation of these results suggests that form D is not valid for Pt. Thus, we studied alternative coordination modes for the Pt complex, as we did for Pd:5 forms A (corresponding to η2 coordination), B (acetylide), and C (carbene) (Scheme 6). The processes leading to intermediates B and C are both exothermic with respect to the separated reactants. However, the process to the Macetylide intermediate (form B) goes through a transition state TSAB situated +2.4 kcal/mol above the reactants, while the transition state going from A to C is at 0.4 kcal/mol, which is below the energy for separated reactants. The reaction to give C should thus proceed smoothly in the gas phase. The Ptcarbene intermediate is stable,14 while the Pdcarbene species has a higher energy (+4.3 kcal/mol) than the separated reactants.5 This feature that metalcarbenes are more stable as we move down the periodic table can be attributed to relativistic effects.15,16 Four different approaches of the NBD to the Ptcarbene complex are possible, depending on the position of the NBD relative to the phenyl substituent and depending on the NBD orientation (E). Those would lead only to two diastereomers for the endo and for the exo products due to the like-P-stereogenic phosphinito phosphinous acid system. The four approaches are computationally found to be competitive, and we present here the most exothermic path (Scheme 7). Interested readers should refer to the Supporting Information for energetics and coordinates of the four approaches. From E, the platinacycle F is obtained via a transition state (TSEF) located 5.4 kcal/mol above E. This step is highly exothermic (by 29.9 kcal/mol). The product P in its exo configuration with the cationic platinum situated on the Re diastereoface, as proposed for the condensed phase,3 is generated through TSFP. This step requires an activation energy of 17.5 kcal/mol and is exothermic by 8.1 kcal/mol. From P,17 the Pt catalyst can be released with a dissociation cost of 18.7 kcal/mol. The computed energy profiles (Schemes 57) provide a reasonable explanation for the results observed in the SORI spectrum. 2-Methylbut-3-yn-2-yl Acetate Adducts. In a second set of experiments, the catalyst precursor was directly introduced into the source, while 2-methylbut-3-yn-2-yl acetate was added in a collision cell (H2) under CAR conditions. Again, the results differed dramatically depending on the metal, as shown in Figure 2. 4817
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Scheme 6. Computed Energy Data (in kcal/mol, at T = 0 K, including ZPE) for Species Involved in the Formation of Pt Complexes of Forms ACa
a
The BSSE correction (3.1 kcal/mol) is calculated for A and is applied to the dissociated fragments [Pt catalyst]+ and PhCCH.
Scheme 7. Computed Energy Profile (in kcal/mol, at T = 0 K, Including ZPE) for the Reaction between Norbornadiene and the Form C Pt Carbene (Scheme 6) To Give the Cyclopropanation Producta
a
As shown in the representation of P, the computed cyclopropanation product is in its exo configuration. The energy scale is the same as for Scheme 6. BSSE corrections are computed for E (3.5 kcal/mol) with respect to the dissociation to C and norbornadiene and for P (4.7 kcal/ mol) with respect to the dissociated fragments (P0 + catalyst dissociated). The BSSE corrections are applied to correct the relative energies of the corresponding associations/dissociations.
In the case of Pd (Figure 2a), the main fragment ion m/z 471 (108Pd) corresponds to the release of the catalyst, following the loss of acetate substrate in the original complex (m/z 597). A minor peak was observed at m/z 537. This fragment ion is produced after the loss of an acetic acid molecule from the original complex. For the Pd complex the acetate substrate/ acetic acid release ratio is close to 40.18 We repeated this experiment with a catalyst where the bridging hydrogen associating the two oxygens of the ligands was replaced by deuterium. We
Figure 2. Consecutive CAR/SORI spectra of 108Pd (a) and 194Pt (b) complexes formed by incorporation of 2-methylbut-3-yn-2-yl acetate ligand. Precursor ions (m/z 471 108Pd or m/z 557 194Pt) prepared under positive ESI conditions were selected using Q1 (m/z 2 width) to react with 2-methylbut-3-yn-2-yl acetate in the H2 collision cell. Corresponding adduct ions (m/z 597.14 or 683.19) were selected for SORI processes (the excitation power was set to 2.5% and 1.5% for Pd and Pt complexes, respectively). It should be noted that 108Pd and 194 Pt isotope selection was preferred (instead of the major isotopes 106 Pd and 195Pt) to avoid 13C contribution during deuterium labeling experiments.
used MeOD as the ESI solvent. The peak at m/z 471 was partially shifted to m/z 472 in a 1/1 ratio. This result is similar to those of the CAR/CID experiments between the identically deuterated Pd complex and phenylethyne involving the form B (Scheme 2).5 4818
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Organometallics The same experiment was conducted with the Pt catalyst, and again, the fragmentation spectra were very different from those observed with Pd. The relative heights of the peaks corresponding to the loss of acetic acid (loss of 60) and loss of acetate (loss of 126) are reversed. Here, the (acetate substrate/acetic acid release) ratio is close to 0.2. In the deuterated Pt complex, formed using CH3CN/D2O solvent, the deuterium atom is not involved in the loss of acetic acid (the deuterium atom remains on the catalyst). Scheme 8. Structures Proposed for Ions m/z 597, 683, and 623 after SORI Experiments
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By analogy with phenylethyne results,5 we propose a structure of form B for the Pd ion m/z 597 (Scheme 8a). This structure is consistent with the fragmentations observed and the partial displacement of the ion m/z 471 to 472 with deuterated catalyst, while the formation of a carbene by loss of AcO from an acetylide is not favored under our conditions. The very different spectrum obtained for the Pt catalyst suggests a structure of form C for ion m/z 683 (Scheme 8b). This kind of structure could easily lose an acetic acid in a process that does not involve the proton between the two O atoms of the PA ligands. Indeed, as noted above, the use of deuterated catalyst does not generate a deuterated acetic acid. In order to compare the ability of different catalysts (Pd or Pt) to form a carbene intermediate and then to lose an acetic acid, we finally modeled the reaction pathway through an intermediate of type C (Scheme 9). Overall, the Pt process (in black) is significantly less energetically demanding than the Pd process (in red). The coordination of the acetate with the metal is strongly exothermic (by 18.5 and 22.9 kcal/mol for Pd and Pt, respectively), leading to the complex G where the sp2 O atom of the acetate is coordinated to the metal. The change of coordination to a complex of type A requires 7.6 and 6.3 kcal/mol in the case of Pd and Pt catalysts, respectively. Form A f C transformations are endothermic in both cases, but for Pd the activation energy (TSAC) is above the energy of the separated reactant (6.7 kcal/mol) and the process leads to a carbene (form C) that is 3.7 kcal/mol less stable than the separated reactants.19 Moreover, even if C could be produced, the process to achieve the metal allenylidene structure, with the loss of acetic acid, is energetically above the reactants.
Scheme 9. Computed Energy Profiles (in kcal/mol, at T = 0 K, Including ZPE) for the Formation of Metal Allenylidenes from 2-Methylbut-3-yn-2-yl Acetate and either Pd (in Red) or Pt (in Black) Catalysta
a
BSSE corrections are computed for both G (4.9 kcal/mol for Pd and 5.5 kcal/mol for Pt) and H (4.4 kcal/mol for Pd and 4.6 kcal/mol for Pt) fragments applied to correct the relative energies with respect to the corresponding associations/dissociations. 4819
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Organometallics We can therefore expect that, for Pd, form A would fragment to restore the reagents, rather than evolve toward the formation of C. We therefore understand that it is difficult to observe this species with the Pd catalyst. These calculations also illustrate that the loss of acetic acid is not favored compared to the acetate release. This simple cleavage is largely reinforced due to a lower kinetic shift compared to the rearrangement process (TSAC). The situation is very different in the case of Pt, since the whole process is below the energy of the separated reactants. The endothermicity of the A f C transformation is less pronounced (6.1 kcal/mol for Pt vs 14.6 kcal/mol for Pd), and the evolution of the carbene toward the elimination of acetic acid and the formation of allenylidene Pt complex is energetically less demanding than reversion to form A. We can reasonably confirm a carbene of form C as the structure of the Pt ion m/z 683 and form B as the structure of the Pd ion m/z 597.
’ CONCLUSION In this paper, we performed a study on the nature of catalytic intermediates with Pd and Pt catalysts involving phosphinato phosphinous acid ligands (obtained from chiral tert-butyl(phenyl)phosphine oxide). We showed that while the Pd catalyst intermediate can be assigned to a Pd acetylide intermediate, its Pt analogue can be assigned to a Pt carbene, which would be kinetically favored over the acetylide. SORI decomposition showed a protonated phosphine loss only for the Pd catalyst, which is compatible with a proton shift from the acetylene to the phosphinito ligand. Conversely, for the Pt catalyst this experiment showed an organic moiety loss rather than a phosphine loss, which is compatible with a low-energy 1,2 proton shift within the η1-acetylene coordinated to the catalyst. Additional computations rationalized this behavior that occurs before the NBD addition. An acetate-substituted acetylene was used in complementary mass experiments and confirmed this tendency. By using tertiary propargyl acetate with Pd and Pt catalysts, we demonstrated also for these cases that the nature of the intermediates depends on the nature of the metal: the coordination of the alkyne with Pd catalyst prefers an acetylide form, while the Pt complex evolves to a carbene. These conclusions are strongly supported by CAR/SORI mass spectrometry experiments, reinforced by computations. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures and text giving experimental mass spectrometry details and all computational data. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: 33(0)4 76 51 43 44. Fax: 33 (0)4 76 63 57 54.
’ ACKNOWLEDGMENT This work was supported by the computing facilities of the CRCMM, ”Centre Regional de Competences en Modelisation Moleculaire” in Marseille, France. Funds were provided by the ANR project BLAN07-1_190839 SPOs Preligands.
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’ REFERENCES (1) (a) Ackermann, L. Synthesis 2006, 1557–1571. (b) Ackermann, L.; Born, R.; Spatz, J. H.; Althammer, A.; Gschrei, C. J. Pure Appl. Chem. 2006, 78, 209–214. (c) Dubrovina, N. V.; B€orner, A. Angew. Chem., Int. Ed. 2004, 43, 5883–5886. (d) Ackermann, L. Chiral Secondary Phosphine Oxides and Heteroatom-Substituted Secondary Phophine Oxides as Preligands. In Phosphorus Ligands in Asymmetric Catalysis; B€orner, A, Ed.; Wiley-VCH: Weinheim, Germany, 2008; Vol. 2, pp 831847. (e) Nemoto, T.; Hamada, Y. Tetrahedron 2011, 67, 667–687. (2) For reviews on phosphinous acids see: (a) Roudhill, D. M.; Sperline, R. P.; Beaulieu, W. B. Coord. Chem. Rev. 1978, 26, 263–279. (b) Walther, B. Coord. Chem. Rev. 1984, 60, 67–105. (c) Appleby, T.; Woolins, J. D. Coord. Chem. Rev. 2002, 235, 121–140. (3) Bigeault, J.; Giordano, L.; Buono, G. Angew. Chem., Int. Ed. 2005, 44, 4753–4757. (4) Bigeault, J.; Giordano, L.; De Riggi, I.; Gimbert, Y.; Buono, G. Org. Lett. 2007, 9, 3567–3570. (5) Thota, R.; Lesage, D.; Gimbert, Y.; Giordano, L.; Humbel, S.; Milet, A.; Buono, G.; Tabet, J.-C. Organometallics 2009, 28, 2735–2743. (6) Frisch, M. J. et al. Gaussian 03, Revision D.02; Gaussian Inc., Wallingford, CT. (7) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (8) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284–298. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (d) Dunning, T. H. Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 128. (9) The exponents for polarization were obtained from the D95 basis set, following the Gaussian 03 implementation of D95(d): Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 128. (10) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553–556. (11) Introduction of AcOH in H2, to promote the acidolysis of CPd bond, failed to give a hydroalkynylation compound. (12) In the condensed phase, the disconnection (by acidolysis) of the PdC(sp3) bond in this type of complex (form D) is known to occur easily, for example at the end of the hydroalkynylation process using the HermannBeller catalyst: Tenaglia, A.; Giordano, L.; Buono, G. Org. Lett. 2006, 8, 4315–4318. (13) The loss of the phosphine ligand is a barrierless process (see the Supporting Information). (14) CAR/CID experiments for the Pt catalyst and PhCCD were performed as for Pd.5 For Pt, only a low deuterium incorporation into the catalyst was observed (see the Supporting Information). This suggests that B exists as well but is not the major form in the gas phase. The fact that both B and C exist (especially under high-energy CAR/ CID conditions) is in agreement with calculations presented in Scheme 6, which reveal a small energetic difference between TSAB and TSAC. (15) Heinemann, C.; Hertwig, R. H.; Wesendrup, R.; Koch, W.; Schwarz, H. J. Am. Chem. Soc. 1995, 117, 495–500. (16) Experimental mass spectrometry data on the subject have been compiled a few years ago by Armentrout, and high-level computations have been recently performed by Zhang et al. They also showed that B3LYP functional and scalar relativistic effective core potentials (RECP) can be efficient tools to account for the relativistic effects at work in the metal dC bonding: (a) Armentrout, P. B. Int. J. Mass Spectrom. 2003, 227, 289–302. (b) Zhang, X.; Schwarz, H. Chem. Eur. J. 2010, 16, 5882–5888. (17) Bigeault et al. proposed that the phenylmethylidenecyclopropane adduct 1 is coordinated by the Pt catalyst in the endo position of the double bond to avoid steric hindrance: Bigeault, J.; de Riggi, I.; Gimbert, Y.; Giordano, L.; Buono, G. Synlett 2008, 1071–1075. (18) A peak at m/z 413 arises from the loss of a tBu group of the catalyst at m/z 471 (not shown). Note that the formation of the m/z 485.083 27 fragment ion (108Pd complex + CH2) is unexplained and the 4820
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m/z 529.073 12 fragment ion (unknown structure) is certainly an artifact due to an isobaric ion. (19) We checked the deprotonation process of G by a phosphinous acid to yield form B in Scheme 9. For Pd, the transition state is 2 kcal/mol lower than TSAC. For Pt, the transition state for the deprotonation of G by a phosphinous acid is 7.4 kcal/mol higher than TSAC. These values agree with the fact that, in the case of the Pd, starting from G, the process evolves towards a form B, rather than C. In the case of the Pt catalyst, the situation is exactly the reverse: form C is strongly preferred.
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