Selective CarbonâCarbon Bond Activation of Epoxides by a

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Selective Carbon−Carbon Bond Activation of Epoxides by a Bisphosphine Pt(0) Complex Susanne Neumann,† Thomas W. Gerl,†,# Frank Rominger,† Gunter Scherhag,† Claudia Meier,† Markus Metz,† Alberto Albinati,‡ and Peter Hofmann*,†,§ †

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Department of Chemistry, University of Milan, Via C. Golgi 19, I-20133 Milan, Italy

‡

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S Supporting Information *

ABSTRACT: Contrasting established bond activation chemistry of oxiranes (epoxides), the unprecedented insertion of a Pt(0) complex into the carbon−carbon bonds of ethylene oxide and other epoxides, generating 3-oxaplatinacyclobutanes under remarkably mild conditions, has been found. Pt(II) neopentyl hydride complex (dtbpm-κ2P)Pt(Np)H (1) undergoes first-order reductive elimination of neopentane at ambient temperature in solution, forming a highly reactive species of an overall composition [(dtbpm)Pt(0)] (C). This intermediate inserts into epoxide C−C bonds or, in the absence of substrates, dimerizes to d10−d10 Pt(0) species D. The epoxide activation products have been fully characterized including X-ray structure determinations. Various experiments have been conducted in order to decipher mechanistic details of this unusual chemistry. Theoretical studies on various levels do not allow a reliable conclusion as to the actual nature of the reactive intermediate C. Both conceivable structures C1 (ring-opened [(dtbpm-κ1P)Pt(0), d10-ML, 12 VE]) and C2 (chelate, [(dtbpm-κ2P)Pt(0)], d10-ML2, 14 VE) are minimum-energy structures with a very small (method-dependent) energy difference.

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INTRODUCTION Chiral and achiral epoxides (oxiranes), due to their broad accessibility and high reactivity, play an important role as valuable building blocks in organic chemistry, both for the synthesis of fine chemicals and for large-scale bulk chemical production.1 Generally, the chemistry of these strained threemembered oxygen heterocycles is characterized by ring opening reactions exclusively of one of their polar C−O bonds (C−O bond dissociation energy in oxirane itself: 285 kJ/mol),2 induced by nucleophilic, electrophilic (Scheme 1),3

important role during the past few decades. Much attention has been paid to epoxide bond activation and ring-enlargement reactions4 at transition metal centers. A fascinating lowtemperature C−H activation of ethylene oxide at Rh(I) in solution was reported by Bergman5 et al., leading to an intermediate oxiranyl hydrido Rh(III)-species. It rearranges to a Rh(III) enolate at higher temperature. The insertion chemistry of transition metals into intra-ring bonds of epoxides has been investigated, because the potentially resulting metallaoxetanes were considered as promising precursor compounds for further functionalization steps in stoichiometric or catalytic reactions. A recent comprehensive review by Dauth and Love6 has summarized the history and role of 2metallaoxetanes A in reaction development. Not surprisingly, in accord with expectations, in all cases reported so far, reactive transition metal species MLx, being nucleophilic or electrophilic in nature, have exclusively led to carbon−oxygen-activated 2-metallaoxetanes A (Scheme 2).7 Direct insertion reactions of transition metal reagents into the carbon−carbon bonds of epoxides, yielding 3-metallaoxetanes B, to the best of our knowledge, have never been observed. On the other hand, the synthesis and detailed investigation of easily accessible and abundantly available 2-metallaoxetanes A for a broad variety of metals has been a continuous focus of active research, because inter alia they are supposed to be key

Scheme 1. C−O Bond Opening of Oxirane by Nucleophilic or Electrophilic Attack

or radical attack. The regioselectivity of C−O bond opening in substituted epoxides can be directed by the specific reagent employed, as well as by the electronic and steric features of the respective epoxide structure. Among the large variety of nucleophiles and electrophiles (and radicals) which may be employed to harvest the synthetic potential of epoxides, transition metals have played an © XXXX American Chemical Society

Received: July 3, 2019

A

DOI: 10.1021/acs.organomet.9b00450 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 3. In Situ Generation of Pt(0) Intermediate C from 1 and Its Dimerization to Eight-Membered Dinuclear d10− d10 Pt(0) Complex D in Absence of a Reactive Substrate

Scheme 2. Formation of 2-Metallaoxetanes (A) and 3Metallaoxetanes (B) by C−O and C−C Bond Activation of Oxiranes with Transition Metal Complexesa

a

Subscripts x and n indicate potentially different ligand environments.

intermediates in important oxygen-transfer reactions.6 In addition, less oxophilic metals are able to undergo insertion reactions of small molecules into their M-O bonds, resulting in ring expansion or epoxide functionalization reactions.4,6 Not by direct C−C bond activation of epoxides, but by independent synthesis, Hoover and Stryker have synthesized the first and so far unique examples of 3-platinaoxetanes B, L 2 Pt[(CH 2 ) 2 O] (L = PPh 3 , PMe 3 and L 2 = bis(diphenylphosphino)ethane, respectively). They are accessible via intramolecular dehydration of cis-bis(hydroxymethyl) platinum precursor complexes.8a,b These 3-oxaplatinacyclobutanes B are quite stable, and no insertions into their Pt−Cbonds are observed. Interestingly, the reaction of (PMe3)2Pt[(CH2)2O] with 1 equiv of the strong π-acceptor olefin tetracyanoethylene (TCNE) at room temperature led to quantitative formation of the known olefin complex (Me3P)2Pt(η2-TCNE)8c and free ethylene oxide as the exclusive organic product. Transition metal induced C−C bond activation reactions of strained sp2−sp2 bonds in biphenylene using Ir fragments9a and quite recently of sp−sp2 carbon−carbon bonds by a Pt(0) bisphosphine complex9b have been reported by Jones et al., referencing other cases, but not C−C bonds of epoxides. C−C Bond Activation: Results and Discussion I. We report here unprecedented and surprisingly selective direct C− C bond activation reactions of ethylene oxide and several of its alkyl substituted derivatives. An easily accessible, highly reactive, coordinatively and electronically unsaturated Pt(0) species of the overall composition [(dtbpm)Pt(0)] (C) is the active intermediate (vide infra). We had shown earlier, that the sterically congested, electronrich, chelating bisphosphine dtbpm (dtbpm = bis(di-t butylphosphino)methane = tBu2P−CH2−PtBu2)10 is capable to tailor electronic structures and reactivity patterns of various transition metal fragments for facile bond activation reactions of C−H, C−C, Si−H, CC, C−O, CO, C−Si, and C−F bonds9c−i and for homogeneous catalysis.9j−n The reactive Pt(0) intermediate [(dtbpm)Pt(0)] (C) can be generated in situ from the cis-alkylhydrido Pt(II) precursor (dtbpm-κ2P)Pt(Np)H (1)9c with a chelating dtbpm ligand, by clean first-order reductive elimination of neopentane (for experimental details, see the Supporting Information) at ambient temperature in solution (Scheme 3). In the absence of a reactive substrate, it dimerizes to dinuclear Pt(0) complex D. We found that intermediate C, when generated from neopentyl hydride 1 in solution (e.g., in pentane) containing oxirane and various of its alkyl derivatives, selectively generates 3-oxaplatinacyclobutanes B (Scheme 2: LnM = (dtbpmκ2P)Pt) by smooth insertion of the metal fragment into the carbon−carbon bonds under remarkably mild conditions.

Liquid epoxides like rac-propene oxide (E2) can be used directly as solvent (Scheme 4). Scheme 4. Selective Carbon−Carbon Bond Activation of Epoxides E1−E6 to 3-Platinaoxetanes B1−B6 (Isolated Yields in Parentheses) with the Pt(0) Fragment C

As shown in Scheme 4, oxirane (ethylene oxide, E1) and epoxides E2−E6 (rac-methyloxirane = rac-propene oxide, cisand rac-trans-2,3-dimethyloxirane = 2-butene oxides, cyclopentene oxide, and cyclohexene oxide) react with [(dtbpm)Pt(0)] (C) when this intermediate is generated from (dtbpmκ2P)Pt(Np)H (1). In all cases, carbon−carbon bond activation is observed. Exclusively 3-oxaplatinacyclobutanes as C−C insertion products (B1−B6) are formed, besides variable amounts of extremely soluble dinuclear species D as the only other product. The epoxide activation products could be easily separated from D. Metallacycles B1−B6 were isolated and fully characterized as colorless, air-stable solids with relatively poor solubility in most solvents. Even with bicyclic oxiranes like cyclopentene oxide E5 or cyclohexene oxide E6, unsaturated Pt(0) intermediate C inserts exclusively into the oxirane C−C bonds, resulting in unusual bicyclic 3-oxaplatinacyclobutanes B5 and B6 and in variable amounts of dimer D. No C−O bond activation products of E5 or E6 yielding the presumably more stable (vide infra) bicyclic 2-oxaplatinacyclobutane derivatives were detected. Dissolved in toluene, B1 and B2 are remarkably stable at temperatures as high as 100 °C for several days without any sign of rearrangement to their 2-platinaoxetane isomers or of decomposition. Only strained 3-platinaoxetane B6 slowly decomposes in toluene solution after prolonged heating to 50 °C. After 5 and 17 days, respectively, almost 50% and 90% of dimer D are formed. The reactions of 3-platinaoxetanes B1 with fumarodinitrile (FDN) and of B2 with bis(carbomethoxy)acetylene, respecB


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Organometallics

diffraction, but its molecular structure was determined and reported independently by Jones et al.12d If the reductive elimination of neopentane from (dtbpmκ2P)Pt(Np)H (1) is performed in the presence of alkenes or alkynes, then C can be trapped as (dtbpm-κ2P)Pt(η2-alkene) or (dtbpm-κ2P)Pt(η2-alkyne) complexes,9c,13 with dtbpm in a chelating κ2 bonding mode. A small selection of typical examples14 is shown in Scheme 5.

tively, at room temperature led to reductive elimination reactions of the oxametallacycles with formation of the complexes (dtbpm-κ2P)Pt(η2-FDN) and (dtbpm-κ2P)Pt(η2MeO2CCCCO2Me) and of free oxiranes E1 and E2 as the exclusive organic products. X-ray structure determinations (Figure 1) of monocyclic insertion products B2−B4 confirm

Scheme 5. Representative (dtbpm-κ2P)Pt(η2-alkene) and (η2-alkyne) Complexes Formed from 1 by Reductive Elimination of Neopentane in the Presence of Different Substratesa

a

TME = tetramethylethylene, 2,3-dimethylbutene-2.

These trapping reactions might be interpreted as indicating that the Pt(0) intermediate C has a structure in which the fourmembered chelate ring geometry of its precursor 1 is retained, i.e., a species [(dtbpm-κ2P)Pt(0)] (C2) is responsible for the attack at the C−C epoxide bonds (Scheme 6). We soon had to

Figure 1. Molecular structures of 3-platinaoxetanes B2/B3 (top left/ right) and B4/B6 (bottom left/right) in the crystal. ORTEP plots, 50% probability. Hydrogens are omitted for clarity.

Scheme 6. Alternative Structures C1 and C2 for [(dtbpm)Pt(0)] C

planar four-membered 3-oxaplatinacyclobutane ring geometries as reported by Stryker et al. for (PPh3)2Pt[(CH2)2O].8a Only bicyclic 3-oxaplatinacyclobutane B6 shows a slightly folded four-membered ring, most likely caused by the influence of the pseudo-chair conformation of the seven-membered [O− C6] unit (Figure 1). Obviously, the true nature of complex C is the crucial question, if a mechanistic understanding of epoxide C−C bond activation is sought. It certainly is a short-lived, highly reactive transient. For all bond activation and other reactions studied so far by us,9c−i the rate limiting reductive elimination of neopentane from 1 always determines the overall rates. Essentially the rates do not depend upon the specific substrate C is reacting with. Once C is formed, any consecutive reaction step is extremely fast. In the absence of reactive substrates, [(dtbpm)Pt(0)] C, generated from 1 in solution or in the solid state, dimerizes to the dark red, highly soluble, thermally stable dinuclear complex D.9c This eight-membered ring system is a typical d10−d10 dimer with two interacting11 d10-Pt(0) centers. Each metal is coordinated in a linear, electronically ideal fashion by two P atoms. D itself is completely unreactive toward epoxides. For the cyclohexyl analog of D, [(dcpm)Pt(0)]2 (dcpm = bis(dicyclohexylphosphino)methane) X-ray quality crystals could be grown and its molecular structure in the crystal could be determined.12a,b By comparison, the structure of D could also be unambigously assigned by 1H NMR, 195Pt NMR, 31 P NMR, and UV−vis spectroscopy.12c We were unable to grow appropriate crystals of D for single-crystal X-ray

realize, however, that despite the well-defined structures of trapping products their formation did not allow firm conclusions with respect to the actual structure of the reactive intermediate [(dtbpm)Pt(0)] (C). In fact, C in our hands when utilized in various reactions9c−n for a long time has not been amenable to an unambiguous experimental or theoretical structure assignment. Calculations at various levels of theory suggested that C actually may exist as two individual isomeric molecular species. Given the ring strain in the four-membered PCPPt(0) chelate ring of C2 (bite angles of P−Pt−P are around 75° in all determined Pt(II) structures), this species would have an extreme deviation from the preferred linear ground state geometry of d10-ML2 systems with 14 VE. Therefore it has to be considered that C may not be a nucleophilic four-membered ring species [(dtbpm-κ2P)Pt(0)] (C2, 14 valence electrons, d10-ML2, isolobal15 to 1CH2) but rather a more electrophilic ring-opened isomer [(dtbpmκ1P)Pt(0)] (C1, 12 valence electrons, d10-ML, isolobal to H+ or LAu+). C


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experimentally to be the case in the three-coordinate γ-agostic Rh(I) complex [(dtbpm-κ2-P)Rh(Np)].20 Unfortunately the computed relative stabilities of C1 and C2 (and thus the sign of ΔΔG in Figure 2) turned out to be method dependent. At the levels of theory we have used19a in order to decide between C1 and C2, no convincing conclusion was possible. We will not discuss these computations (see the Supporting Information) in detail here, because it soon became clear that a more detailed computational study on a higher level was mandatory to reliably differentiate between C1 and C2. Therefore, an extended higher level quantum chemistry study of the reductive elimination of neopentane from 1 forming D, of the electronic structures and relative energies of C1 versus C2 and of the mechanistic features of C−C, C−H, and C−O bond activation reactions of epoxides, using the real systems which were investigated experimentally, has been performed and published separately in a theoretical paper,19b as its inclusion here would expand too much the size of this experimental report. Clearly, however, standard DFT calculations (B3LYP, TZVDP) for the two isomeric structures which could result from C−C or C−O bond activation, respectively (2- or 3platinaoxetanes, A or B, Scheme 2), clearly predicted C−O insertion products A as more stable, pointing toward a kinetic reaction control for the formation of isomers B. C−C Bond Activation: Results and Discussion II. Understanding the surprisingly selective insertion reactions of Pt(0) fragment C into the C−C bonds of ethylene oxide and its alkyl substituted derivatives of course is a mechanistic and theoretical challenge. We report here some further experimental findings which we believe to provide pieces of related information. We first note that besides using the neopentyl hydride precursor (dtbpm-κ2P)Pt(Np)H (1), the olefin complex (dtbpm-κ2P)Pt(η2-TME) (2, TME = tetramethylethylene, 2,3-dimethylbutene-2)9c (Scheme 5) also allows C−C activation of epoxides. Compound 2, which is one of the rare examples of a stable TME late transition metal complex, leads to C−C activations of oxiranes but at significantly higher temperatures (T around or above 50 °C) and longer reaction times with lower yields. NMR experiments reveal that (dtbpm-κ2P)Pt(η2-TME) (2) in solution (benzene) does not begin to decompose below temperatures of ca. 40 °C. Above this temperature, it slowly starts to lose TME and to form red, dinuclear Pt(0) complex D. In contrast, a solution of TME complex 2 in pentane quantitatively and instantaneously is converted to the corresponding η2-ethylene complex (dtbpm-κ2P)Pt(η2-C2H4) (5), releasing TME (Scheme 5) if a solution of 2 is flooded with 1 bar of ethylene gas at ambient temperature. The ethylene complex (with dominant platinacyclopropane character) itself is thermally so stable that it can be vacuumsublimed without decomposition.13 The facile olefin exchange reaction in our opinion clearly indicates an associative mechanism: The stronger (and sterically less demanding) πacceptor ethylene irreversibly liberates the much more weakly bound TME in a bimolecular substitution process. The epoxide C−C activation reactions with (dtbpm-κ2P)Pt(η2TME) (2) as a metal precursor require higher temperatures, which may be interpreted either as an also associative process of the epoxide at olefin complex 2 with a higher barrier or, more probably in our opinion, as a reaction initiated by

In our own earlier work and in other16 studies of d10transition metal complexes with metal fragments [(dtbpm)M(0)] (M = Ni, Pd, Pt) as reactive intermediates,17 the chelating structures [(dtbpm-κ2P)M(0)] of type C2 (stabilized by the Thorpe−Ingold effect through the tBu substituents)18a,b were considered more likely than C1. This seemed of course reasonable for dtbpm as ligand for other dn-electron counts than d10 with nonlinear chelate groundstate geometries. Not directly transferable to Pt(0) chemistry, but important and remarkable in the context of conceivable structural motifs C1 versus C2 for dtbpm as ligand, is an observation by Pörschke et al.18c The reaction of (dtbpm-κ1P)Ni(η2-C2H4)2, itself a three-coordinate Ni(0) complex with monodentate dtbpm-coordination as in C1 with 2 acetylene ligands, resulted in acetylene cyclotrimerization and gave the product (dtbpmκ1P)Ni(η6-C6H6). This compound is a half-sandwich with dtbpm in an open chelate ring as in C1 and a dangling phosphine arm. However, when we reacted the labile dimethyl compound (dtbpm-κ2P)Ni(CH3)29g or the trans-stilbene complex (dtbpm-κ2P)Ni(η2-trans-PhCHCHPh),16d respectively, with the acceptor-substituted alkyne bis(carbomethoxy)acetylene, a fluxional, haptotropic η2-arene complex (dtbpmκ2P)Ni(η2-C6E6) (E = CO2Me) with dtbpm in a chelating fashion is obtained. This clearly shows that (for Ni) the different π-acceptor capabilities of arenes benzene C6H6 versus C6E6 induces κ1-P (C1) or κ2-P (C2) metal coordination of dtbpm to the d10 metal. For Pt, on the basis of our own quantum chemical studies on various levels of theory for C1 versus C2 and for simplified model systems thereof19a (see the Supporting Information for details), both structures displayed in Scheme 6 turned out to be minimum energy geometries and were found to be quite close in energy within a few kJ/mol or kcal/mol. From DFT (Figure 2), ring-opened isomer C1 came out as stabilized by a δ-agostic interaction of the Pt(0) center with a C−H bond of one of the tBu methyl groups at the dangling, noncoordinated phosphorus (Figure 2, right), similar to what we have shown

Figure 2. Potential alternative minimum energy structures for intermediate C, as resulting from DFT (B3LYP, TZVDP; ΔΔG = −3.38 kcal/mol and RI-MP2, TZVDP; ΔΔG = +2.49 kcal/mol. For further computational results and for model complexes, see the Supporting Information. D


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Table 1. Experimental Activation Parameters for Reductive Elimination of Neopentane from (κ2-dtbpm)Pt(Np)H (1) compound

solvent

ln(A)

ΔG⧧ (kJ mol−1)

ΔH⧧(kJ mol−1)

ΔS⧧(J mol−1 K−1)

1 1

C6D6 TMUa

33 ± 2 31 ± 1

100 ± 7 103 ± 5

107 ± 5 103 ± 4

23 ± 16 0 ± 12

a

TMU = tetramethylurea.

dissociative pathway (Table 1). In rac-propene oxide, the rate is in the same range, indicating that again the reductive elimination of neopentane from 1 is rate-determining. The kinetic data determined for two solvents with very different polarity, benzene-d6 and TMU (polarity: 0 and 11.6 × 10−30 Cm),23 clearly point to very similar mechanistic characteristics. These experimental results collected from kinetic studies in our eyes also exclude the above-mentioned, direct oxidative C−C addition picture with an octahedral Pt(IV) species formed from 1. Some more information concerning the mechanism of oxirane C−C bond activation reactions by (dtbpm-κ2P)Pt(Np)H (1) is gained by exploring the stereochemistry of bond activation products for cis- and rac-trans-2,3-dimethyloxirane (B4, B3). We find complete retention of stereochemistry in the resulting 3-oxaplatinacyclobutanes, B4 and rac-B3, when reacting (dtbpm-κ2P)Pt(Np)H (1) in pentane at ambient temperature with E4 and rac-E3. With racemic trans-2,3dimethyloxirane, both enantiomers of platinum complex B3 are formed, and cis-2,3-dimethyloxirane gives achiral platinum complex B4 (Scheme 4). The two diastereomers, B3 and B4, can be easily distinguished by 1H and 31P NMR spectroscopy. No isomerization of the epoxides takes place during or after the reaction. GC measurements confirmed this by showing identical pre- and postreaction ratios of the two stereoisomeric 2,3-dimethyloxiranes. The configuration of complex B3 is stable, and no isomerization to B4 can be observed. Upon heating B3 to 90 °C, reductive elimination of trans-2,3dimethyloxirane and formation of red dimer D is observed. The stereospecific formation of B3 and B4 makes a concerted C−C bond activation mechanism without ionic or radical intermediates probable and supports the dissociative pathway via an intermediate [(dtbpm)Pt(0)], C1 or C2, respectively. Independent of the problem which of these two species is more stable and represents the actual C−C bond activating, highly reactive Pt(0) intermediate, the question remains which of the specific electronic features of the metal fragment and/or of oxiranes might be responsible for the unusual bond activation selectivity pattern. The electronic structure of oxirane correlated to the electronic structure of cyclopropane is displayed qualitatively in Figure 3 with KS orbital energies from DFT (B3LYP/LACVP*). Oxirane is characterized by a low-lying LUMO, which corresponds to the familiar a2′ LUMO of cyclopropane in its Walsh description. From simple perturbation arguments, it follows that (i) the ethylene oxide LUMO is lower in energy than the LUMO of cyclopropane and (ii) it is located more on the two carbon atoms than on the 3 (equivalent) cyclopropane carbons. This relation is of course just as in the familiar comparison of the π* LUMOs of olefins (e.g., ethylene) with those of carbonyl groups (e.g., formaldehyde). This qualitative picture of the C−C antibonding, empty oxirane MO, potentially relevant for C−C bond activation, is of course not method-dependent, although the LUMO shown in Figure

thermally enforced TME dissociation from Pt forming [(dtbpm)Pt(0)] (C1 or C2) in the initiating step, followed by epoxide carbon−carbon bond activation.21 Because all C−C oxirane activations using the neopentyl hydride 1 are more efficient than with the TME complex 2, we have not performed further experiments with the latter, but we have kept our focus on the reactivity of epoxides with neopentyl hydride 1. Basically, one could discuss different mechanistic scenarios for C−C bond activation. The neopentyl hydride (dtbpmκ2P)Pt(Np)H (1), which undoubtedly is a very electron-rich Pt(II) species, might react itself by direct oxidative addition of oxiranes in an associative manner, generating an octahedral 3oxaplatina(IV)cyclobutane, which eliminates neopentane in a consecutive step (perhaps induced by P-dissociation). This pathway seems unlikely simply for steric reasons: Neopentyl hydride 1 is a very crowded entity, for which a bimolecular direct attack of epoxides E1−E6 at Pt should be severely hampered. In contrast, a single-electron transfer (SET) process from the alkyl hydride (dtbpm-κ2P)Pt(Np)H (1) to an oxirane molecule, might be operative, yielding a (caged) pair of radical ions which then could undergo rapid transfer of a second electron along with elimination of neopentane and the insertion of the [(dtbpm)Pt(0)] fragment into the oxirane C−C bond. Concerning SET scenarios in the chemistry of oxiranes, the elegant work of Bartmann and Houk has shown a regioselective C−O cleavage of oxiranyl radical anions with formation of the less substituted linear carbon radicals.2,22a Under the same conditions, radical-stabilizing substituents like the phenyl ring in styrene oxide (rac-phenyloxirane) led to C− O cleavage of the substituted C−O bond. Kasai et al. observed regioselectivity of C−O bond cleavage by ESR spectroscopy at low temperature.22b,c Upon the basis of the observation that exclusively C−O bond activation and not C−C bond activation takes place when SET is involved, we consider this pathway as not operative in our systems. For that reason, we have not looked at our epoxide activation reactions by ESR. In our opinion, the most plausible mechanistic picture is a dissociative pathway, where (dtbpm-κ2P)Pt(Np)H (1) eliminates neopentane in the rate-determining step, creating intermediate C (C1 or C2), which rapidly reacts with oxiranes via oxidative addition of (insertion into) their C−C-bonds. Since, as mentioned before, dimerization of intermediate C to the eight-membered ring compound D occurs in the absence of a reactive substrate, we decided to compare the kinetics of both processes, namely, (i) the reductive elimination reaction of neopentane from 1 giving exclusively D and (ii) the oxirane activation using 1 and neat rac-propene oxide. IR-kinetic measurements (using the intense Pt−H band of pure (dtbpm-κ2P)Pt(Np)H (1) revealed a clean first-order formation of neopentane and of dimer D. Reductive neopentane elimination reactions conducted in different unreactive solvents over a broad range of polarities show that the first-order rate constants for neopentane formation from 1, regardless of the solvent, are similar (for experimental details see the Supporting Information). The measured activation parameters are in good agreement with the requirements for a E


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As clearly visible in its UV-photoelectron spectrum, the HOMO of oxirane of course is the p-type oxygen lone pair perpendicular to the ring plane, followed at lower energies by the in-plane oxygen lone pair with spn character and by the two nondegenerate MOs, which are derived from the two e′ Walsh HOMOs of cyclopropane. In qualitative terms and on the basis of these typical epoxide electronic structure features, a reactive transition metal fragment, which is optimally suited to cause C−C activation of epoxides should have a high-lying occupied MO of appropriate energy and π-symmetry for overlapping with and back-bonding to the oxirane LUMO (Scheme 7) as well as a low-lying LUMO with σ-symmetry taking out sufficient electron density from the C−C bond of an epoxide. These electronic structure features are precisely the ones of a 14 VE d10-ML2 Pt(0) fragment C2 with a small P−Pt−P bite angle (Scheme 7), isolobal to 1CH2 but with inverted HOMO and LUMO symmetry. These features have been described early on 10a and can be found in multiple literature accounts.9c,26 Thus, it is tempting to assume a direct insertion of the Pt(0) fragment C2 into C−C epoxide bonds in a symmetry-allowed reaction pathway for the chemistry we observe. In contrast, structure C1, isolobal and isoelectronic to electrophilic 12 VE L−Au+ systems, cannot be excluded as the reactive species on the basis of such an orbital-based qualitative reasoning. The electronic structure of C1 is also characterized by a low-lying, spatially rather extended, spd-hybrid acceptor orbital of σ-symmetry, which makes it a strong electrophile similar to many monocoordinate L−Au+ cations (vide inf ra). It could weaken the C−C epoxide bonds and insert into them, while back-bonding from the filled Pt d-shell at rather high energy into the oxirane LUMO assists this oxidative addition process. After the insertion step of C1, the dangling phosphine arm of the dtbpm ligand of course would immediately and without barrier re-coordinate to the then reconstituted Pt(II) center, making the reaction irreversible. Both Pt(0) units C1 and C2 certainly are high-energy fragments which could in principle react instantaneously with epoxide substrates. For both alternatives, C1 and C2, epoxide C−C activation is of course expected to be exergonic due to the formation of two strong Pt−C bonds and the relief of epoxide ring strain.

Figure 3. Relevant frontier orbitals of oxirane (left) and cyclopropane (right). Orbital energies (B3LYP/LACVP*) are in eV.

3 sometimes appears as the LUMO+1, with the LUMO then being a b2-type orbital with σ*-CH2 character. A comparative plot of the LUMO of cyclopropane and the LUMO of oxirane is inserted in Scheme 7. The gas-phase Scheme 7. Qualitative Representations of Frontier Orbitals of a Reactive Transition Metal Fragment [(dtbpmκ2P)Pt(0)] (C2) Suitable for Interacting with the σCC and σ*CC Orbitals of Oxirane

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CONCLUSION We must refrain here from assigning a “closed” (C2) or an “open” (C1) geometry to the reactive species [(dtbpm)Pt(0)] (C). Neither “qualitative” arguments on the basis of standard and reasonable electronic structure concepts for intermediates C2 and C1 and for oxirane substrates, nor standard quantum chemistry employed by us while the experimental work was done (ranging from EHT to rather high-level methods, viz. Supporting Information), nor any convincing experimental strategy could provide a clue to the true nature and reactive behavior of C. As mentioned above, the results of extensive quantum chemistry studies on an appropriate higher level, finally provided a reasonably clear and convincing overall picture, as well as with respect to unobserved C−O- and C−H-activation reactivity. These results are the subject of a separate paper in this journal.19b

(vertical) electron affinity of cyclopropane, which cannot be measured reliably by electron transmission spectroscopy (ETS), has been determined through resonant vibrational excitation by electron impact24 and amounts to Eea = −2.6 ± 0.3 eV. To the best of our knowledge, there is no experimental measurement of the related electron affinity of oxirane, but the computed value determined by Sevin et al.25 was given as Eea = −4.53 eV. These Eea values correlate to the LUMO energies of the ground states of cyclopropane and oxirane, reflecting the qualitative MO pictures of Figure 3 and Scheme 7. F


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Organometallics

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of the molecule. 31P{1H} NMR (109.4 MHz, C6D6, rt) δ = 7.3 (d+sat, 2 J(P,P) = 45.2 Hz, 1J(Pt,P) = 1294 Hz), 6.5 (d+sat, 2J(P,P) = 45.2 Hz, 1J(Pt,P) = 1178 Hz). Anal. calcd for C20H44OP2Pt (Found): C 43.08 (42.81), H 7.95 (7.56), P 11.11 (11.08). M.p. 125 °C (decompositions, turns red). MS for C20H44OP2195Pt: m/z (M+ calcd 557), 527 [M+ − CH2O], 512 [M+ − CH3CHO], 499 [M+ − propene oxide], 433 [M+ − 2 tBu], 442 [M+ − tBu − propene oxide], 328 [M+ − 3 tBu − propene oxide], 57 [tBu]. Syntheses of rac-B3, B4, B5, and B6. Syntheses were carried out in the same manner as described for B2, but liquid oxiranes racE3, E4, E5, and E6 were condensed into a stirred pentane suspension (10 mL for B4, 7.0 mL for B3, B5, and B6) of 1 (190 mg, 0.332 mmol for B4 and B5; 210 mg, 0.367 mmol for B6; 150 mg, 0.263 mmol for B3). In the case of B3 and B4, 18−27 equiv of E3 and E4 were used, and in case of B5 and B6 2−4 equiv of E5 and E6 was used. The resulting suspensions have been stirred for about 8 days at ambient temperature. In the case of B1, gaseous E1 (142 equiv) was condensed into the stirred pentane suspension (20 mL) of 1 (546 mg, 0.955 mmol), and the mixture was stirred for 2 days. (dtbpm-κ2P)Pt(−CH2−O−CH2−), B1. Isolated yield: 75% of B1. 1 H NMR (270.1 MHz, C6D6, rt) δ = 5.05 (d + sat, 3J(P,Htrans) = 2.0 Hz, 2J(Pt,H) = 75.2 Hz, PtCH2, 4H), 3.14 (t, 2J(P,H) = 7.3 Hz, PCH2P, 2H), 1.11 (d, 3J(P,H) = 12.7 Hz, C(CH3)3, 36H). 13C{1H} NMR (67.9 MHz, C6D6, rt) δ = 35.6 (t, 1J(P,C) = 9.0 Hz, PCH2P), 34.1 (t + sat, 1J(P,C) = 5.1 Hz, 2J(Pt,C) = 22.4 Hz, PC), 30.6 (t + sat, 2 J(P,C) = 2.6 Hz, 3J(Pt,C) = 12.8 Hz, CH3), 30.29 (“quint”, 2J(Pa,C) = 12.4 Hz, 2J(Pb,C) = 88.9 Hz, 2J(Pa,Pb) = −47.8 Hz, PtCH2). 31 1 P{ H} NMR (109.4 MHz, C6D6, rt) δ = 6.01 (s + sat, 1J(Pt,P) = 1309 Hz). Anal. Calcd. for C19H42OP2Pt (Found): C 41.98 (42.23), H 7.79 (7.85), P 11.40 (11.34). M.p. 125 °C (decomposition, turns red). MS for C19H42OP2195Pt: m/z 543 [M+] (calcd 543), 499 [M+ − C2H4O], 442 [M+ − C2H4O − tBu], 385 [M+ − C2H4O − 2tBu], 328 [M+ − C2H4O − 3tBu], 271 [M+ − C2H4O − 4tBu], 57 [tBu]. trans-(dtbpm-κ2P)Pt(−CH(CH3)−O−CH(CH3)−), rac-B3. Isolated yield: 36% of B3. 1H NMR (300.13 MHz, C6D6, rt) δ = 1.12 (d, 3J(P,H) = 12.7 Hz 18H, PC(CH3)3), 1.17 (d, 3J(P,H) = 12.3 Hz, 18H, PC(CH3)3), 2.10 (dd + sat, N = 17.3 Hz, 4J(P,H) = 6 Hz, 3 J(H,H) = 10.6 Hz, 3J(Pt,H) = 75.5 Hz, 6H, CH3), 3.07 (t, 2J(P,H) = 7.0 Hz, 2H, PCH2P), 5.03 (m + sat, N = 22.4 Hz, 2J(Pt,H) = 83.0 Hz, 2H, PtCH). 13C{1H} NMR (75.47 MHz, C6D6, rt) δ = 30.1 (t, 2 J(P,C) = 2.8 Hz, CH3), 30.4 (s, CH3), 31.1 (t, 2J(P,C) = 2.8 Hz, CH3), 33.5 (t, 1J(P,C) = 4.2 Hz, PC), 34.5 (t, 1J(P,C) = 4.8 Hz, PC), 35.4 (t, 1J(P,C) = 8.8 Hz, PCH2P), 36.6 (“quint”, N = 141.6 Hz, PtCH). 31P{1H} NMR (121.49 MHz, C6D6, rt) δ = 8.1 (s + sat, 1 J(Pt,P) = 1153 Hz)). Anal. calcd for C21H46OP2Pt (Found): C 44.13 (44.06), H 8.11 (8.08), P 10.84 (10.53). M.p. 184 °C (beginning of decomposition at 155 °C). FD+-MS for C21H46OP2195Pt: m/z 571 [M+] (calcd 571). cis-(dtbpm-κ2P)Pt(−CH(CH3)−O−CH(CH3)−), B4. Isolated yield: 26% of B4. 1H NMR (300.13 MHz, CD2Cl2, rt) δ = 1.37 (d, 3 J(P,H) = 9.2 or 12.4 Hz, 18H, PC(CH3)3), 1.41 (d, 3J(P,H) = 9.0 or 12.3 Hz, 18H, PC(CH3)3), 1.49 (dd + sat, N = 16.0 Hz, J = 9 Hz, J = 6 Hz, 3J(Pt,H) = 74.4 Hz, 6H, CH3), 3.56 (m, 2H, PCH2P), 4.00 (m + sat, N = 20.7 Hz, 2J(Pt,H) = 65.0 Hz, 2H, PtCH). 13C{1H} NMR (125.77 MHz, CD2Cl2, rt) δ = 30.3 (s, CH3), 30.5 (t, 2J(P,C) = 2.8 Hz, CH3), 31.5 (t, 2J(P,C) = 2.8 Hz, CH3), 34.1 (t, 1J(P,C) = 3.7 Hz, PC), 35.1 (t, 1J(P,C) = 6.0 Hz, PC), 35.4 (t, 1J(P,C) = 10.2 Hz, PCH2P), 37.1 (“quint”, N = 137.7 Hz, PtCH). 31P{1H} NMR (121.49 MHz, CD2Cl2, rt) δ = 7.3 (s + sat, 1J(Pt,C) = 1206 Hz). Anal. calcd for C21H46OP2Pt (Found): C 44.13 (43.65), H 8.11 (8.01). Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. M.p. 190 °C (beginning of decomposition at 150 °C). FD+-MS: m/z 571 [M+] (calcd 571). HR-FAB+: m/z 571. Found: 571.2641. Calcd: 571.2672 for C21H46OP2195Pt, difference: −3.1 mmu = −5.4 ppm. (dtbpm-κ2P)Pt(−CH(R1)−O−CH(R3)−) [R1 + R3 = −(CH2)3−], B5. Isolated yield: 24−34% of B5. 1H NMR (300.13 MHz, CD2Cl2, rt) δ = 1.35 (d, 3J(P,H) = 12.8 Hz, 18H, C(CH3)3), 1.44 (d, 3J(P,H) = 12.8 Hz, 18H, C(CH3)3), 1.74 (m, 1H, CH2), 1.83 (m, 2H, CH2),

COROLLARIES There is an interesting connection to recent developments in gold chemistry in the context of oxidative addition (bond activation) behavior, which we would like to finally point out here. It has been shown by Bourissou et al.27 that cationic gold complexes [(PP-κ2P)Au(I)]+ (PP = a chelating carborane bisphosphine, enforcing a P−Au−P bite angle in fivemembered chelate structures of around 90°), which are isoelectronic to the Pt fragment [(dtbpm-κ2P)Pt(0)] (C2), are the first Au(I) species capable of facile Au(I) → Au(III) oxidative addition reactions of C−I bonds with a variety of aryl iodides. This unprecedented pattern of Au(I) reactivity has been interpreted as a consequence of the bent as opposed to “normal” linear d10-ML2 geometry imposed by the chelate ligand. In contrast, monoligated cationic L−Au(I)+ intermediates, isoelectronic to the Pt fragment [(dtbpm-κ1P)Pt(0)] (C1) have been established as reactive species in a broad spectrum of Au(I)-catalyzed or Au(I)-mediated reactions.28 Related to the Pt(0)-based C−C activation chemistry of epoxides involving C1 is a recent study of Toste et al.29 It was shown that an (NHC-κ1C)Au(I)+ species, an electrophilic, monoligated cationic L−Au(I)+ complex (with NHC = IPr and SbF6− as the counterion), is prone to C−C bond activation of biphenylenes under remarkably mild reaction conditions. The Au(III) reaction products have been disclosed as stable gold(III) catalysts29 with probably useful reaction patterns.

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EXPERIMENTAL DETAILS

General Methods. All manipulations were performed under an inert atmosphere of purified argon (standard vacuum line, MBraun glovebox) using standard Schlenk techniques. All solvents were purified and dried according to literature procedures, degassed by freeze−pump−thaw cycles, and stored under an atmosphere of argon. NMR spectra were recorded on either a Bruker DRX 300 or a 500 MHz spectrometer. 1H and 13C{1H} NMR data are reported in units of δ relative to TMS referenced to the residual solvent resonance as internal reference. J(H,P) coupling constants were assigned by 1 H{31P} NMR measurements. 31P{1H} NMR spectra were externally referenced to 85% H3PO4, while low-temperature measurements were calibrated at room temperature. Mass spectra were obtained on a Jeol LMS-700 instrument and for IR spectroscopy a Bruker Equinox 55 FT-IR spectrometer was used. Preparation of rac-(dtbpm-κ2P)Pt(−CH(CH3)−O−CH2−), racB2. First, (dtbpm-κ2P)Pt(Np)H (1, 500 mg, 0.875 mmol) was dissolved in 15 mL of racemic propene oxide (rac-E2, 0.21 mol, 245 equiv) at ambient temperature. The solution was stirred for 11 days. It slowly turned red. The solvent was removed under reduced pressure, and the resulting solid was washed with 10 mL of pentane, thus removing dimer D, dissolved in toluene and after filtration through a small plug of Celite recrystallized from toluene at −30 °C to provide 76% (372 mg) of rac-B2. 1H NMR (270.1 MHz, C6D6, rt) δ = 5.10 (m + sat, 2J(Pt,H) = 39.1 Hz, PtCH, 1H), 4.94 (m + sat, 2J(Pt,H) = 33.7 Hz, PtCH2, 2H), 3.10 (td, 2J(P,H) = 3.9 Hz, PCH2P, 2H), 2.11 (“tt” + sat, 4J(P,H) = 2.0 Hz, 3J(H,H) = 10.5 Hz, 3J(Pt,H) = 73.7 Hz, CH3, 3H), 1.18 (d, 3J(P,H) = 12.2 Hz, C(CH3)3, 12H), 1.09 (d, 3 J(P,H) = 12.2 Hz, C(CH3)3, 12H), 1.08 (d, 3J(P,H) = 12.2 Hz, C(CH3)3, 12H). 13C{1H} NMR (67.9 MHz, C6D6, rt) δ = 38.3 (dd, 2 J(P,Ccis) = 13.0 Hz, 2J(P,Ctrans) = 92.8 Hz, PtCH), 35.6 (dd, 1J(Pa,C) = 7.7 Hz, 1J(Pb,C) = 7.7 Hz, PCH2P), 34.6 (dd, 3J(P,Ccis) = 2.1 Hz, 1 J(P,Ctrans) = 8.4 Hz, PC), 34.1 (dd, 3J(P,Ccis) = 2.1 Hz, 1J(P,Ctrans) = 8.4 Hz, PC), 33.7 (d, 3J(P,C) = 7.7 Hz, CH3), 31.2 (d, 2J(P,C) = 5.6 Hz, CH3), 30.6 (d, 2J(P,C) = 6.3 Hz, CH3), 30.2 (d, 2J(P,C) = 5.6 Hz, CH3), 30.1 (d, 2J(P,C) = 2.1 Hz, CH3), 28.8 (dd, 2J(P,Ccis) = 10.6 Hz, 2J(P,Ctrans) = 85.8 Hz, PtCH2). Each of the four signals belonging to the tbutyl groups could not be assigned clearly to one tbutyl group G


Article

Organometallics 2.31−2.51 (m, 3H, CH2), 3.71 (“q” + sat, N = 23.4 Hz, 2J(P,H) = 7 Hz, 3J(Pt,H) = 33.9 Hz, 2H, PCH2P), 4.45 (m + sat, N = 13.9 Hz, 2 J(Pt,H) = 75.7 Hz, 2H, PtCH). 13C{1H} NMR (125.77 MHz, CD2Cl2, 243 K) δ = 30.1 (“t”, N = 5.1 Hz, CH3), 30.3 (s, CH2), 30.5 (“t”, N = 5.1 Hz, CH3), 34.1 (“t”, N = 12.5 Hz, PC), 34.5 (“t”, N = 10.6 Hz, PC), 35.5 (t, 1J(P,C) = 10.6 Hz, 1J(P,C) = 5 Hz, PCH2P), 37.7 (s, CH2), 55.3 (“quint”, N = 122.5 Hz, PtCH). 31P{1H} NMR (121.49 MHz, CD2Cl2, rt) δ = 4.6 (s + sat, 1J(Pt,P) = 1304 Hz). Anal. calcd for C22H46OP2Pt (Found): C 45.28 (45.04), H 7.94 (8.06), P 10.61 (10.36). M.p.: decomposition begins at 166 °C. FD+-MS for C22H46OP2195Pt: m/z 583 [M+] (calcd 583). (dtbpm-κ2P)Pt(−CH(R1)−O−CH(R3)−) [R1 + R3 = −(CH2)4−], B6. Isolated yield: 32% of B6. 1H NMR (500.13 MHz, [D8]THF, 263 K) δ = 1.37 (d, 3J(P,H) = 12.2 Hz, 18H, PC(CH3)3), 1.43 (d, 3J(P,H) = 12.5 Hz, 18H, PC(CH3)3), 1.78 (m, 2H, CH2), 1.97 (m, 2H, CH2), 2.11 (m, 2H, CH2), 2.24 (m + sat, 3J(Pt,H) = 118 Hz, 2H, CH2), 3.58 (m, 1H, PCH2P), 3.83 (dt, 2J(P,H) = 7.2 Hz, 2J(H,H) = 16.8 Hz, 1H, PCH2P), 4.49 (m + sat, N = 14.5 Hz, 2J(Pt,H) = 85.5 Hz, 2H, PtCH). 13 C{1H} NMR (125.77 MHz, [D8]THF, 263 K) δ = 29.2 (s, CH2), 31.1 (t, 2J(P,C) = 2.8 Hz, CH3), 31.4 (t, 2J(P,C) = 2.8 Hz, CH3), 34.7 (t, 1J(P,C) = 4.2 Hz, PC), 35.9 (t, 1J(P,C) = 6.0 Hz, PC), 36.4 (t, 1 J(P,C) = 10.2 Hz, PCH2P), 42.5 (s, CH2), 44.4 (“quint”, N = 140.1 Hz, PtCH). 31P{1H} NMR (202.46 MHz, [D8]THF, 263 K) δ = 6.2 (s + sat, 1J(Pt,P) = 1186 Hz). Anal. calcd for C23H48OP2Pt (Found): C 46.22 (46.03), H 8.10 (8.26), P 10.36 (10.36). M.p. 169 °C. FD+MS for C23H48OP2195Pt: m/z 597 [M+] (calcd 597). X-ray Structure Determinations. All data sets were collected on a Bruker Smart CCD diffractometer at 200 K with Mo Kα radiation, λ = 0.71073 Å, and 0.3° ω-scans covered the complete reciprocal space with at least 2-fold redundancy. An empirical absorption correction was applied using SADABS.30 Structures were refined against F2 using SHELXL.30 Hydrogen atoms were treated using appropriate riding models. CCDC 1031937 (B2), 1031938 (B3), 1031939 (B4), and 1031940 (B6) contain the supplementary crystallographic data for the structure analyses presented in this paper. CCDC 1031941−1031947 and 1035697 are for related structure analyses of (dtbpm-κ2P)Pt alkene and alkyne complexes mentioned in the text. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. B2: C20H44OP2Pt, 557.58 g/mol, colorless crystal (lamina), 0.25 × 0.17 × 0.07 mm3, monoclinic, space group Cc, Z = 4, a = 20.0600(3) Å, b = 8.4866(1) Å, c = 15.4694(2) Å, beta = 117.1830(10)°, V =2342.66(6) Å3, Θmax= 25.6°, 8343 reflections measured, 3831 unique (R(int) = 0.0259), 3631 observed (I > 2σ(I)), μ = 6.13 mm−1, Tmin = 0.50, Tmax = 0.73, 235 parameters refined, Flack 0.038(5), goodness of fit 1.01, R1(F) = 0.020, wR(F2) = 0.041, all for observed reflections, residual electron density −0.62 to 0.86 e Å−3. B3: C21H46OP2Pt, 571.61 g/mol, colorless crystal (irregular), 0.20 × 0.18 × 0.16 mm3, tetragonal, space group I41cd, Z = 8, a = 16.0984(6) Å, c = 18.7262(10) Å, V = 4853.1(4) Å3, Θmax= 25.6°, 16 515 reflections measured, 2151 unique (R(int) = 0.0251), 1572 observed (I > 2σ(I)), μ = 5.92 mm−1, Tmin = 0.44, Tmax = 0.53, 125 parameters refined, Flack −0.015(8), goodness of fit 1.36, R1(F) = 0.047, wR(F2) = 0.099 all for observed reflections, residual electron density −1.09 to 1.24 e Å−3. B4: C21H46OP2Pt, 571.61 g/mol, colorless crystal (prism), 0.28 × 0.08 × 0.04 mm3, orthorhombic, space group P212121, Z =4, a =8.6348(1) Å, b =14.9163(2) Å, c =18.8908(1) Å, V =2433.12(4) Å3, Θmax= 25.6°, 15855 reflections measured, 4168 unique (R(int) = 0.0543), 3781 observed (I > 2σ(I)), μ=5.91 mm−1, Tmin = 0.48, Tmax = 0.80, 240 parameters refined, Flack 0.015(7), goodness of fit 1.04, R1(F) = 0.030, wR(F2) = 0.052, all for observed reflections, residual electron density −0.59 to 0.86 e Å−3. B6: C23H48OP2Pt, 597.64 g/mol, colorless crystal (polyhedron), 0.20 × 0.18 × 0.14 mm3, monoclinic, space group P21/n, Z = 4, a = 9.9153(2) Å, b = 22.8005(4) Å, c = 11.2772(1) Å, beta = 90.638(1)°, V = 2549.32(7) Å3, Θmax= 25.6°, 19 082 reflections measured, 4406 unique (R(int) = 0.0302), 4067 observed (I > 2σ(I)), μ = 5.64 mm−1, Tmin = 0.46, Tmax = 0.56, 281 parameters refined, goodness of fit 1.08,

R1(F) = 0.021, wR(F2) = 0.044, all for observed reflections, residual electron density −0.61 to 0.85 e Å−3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00450. Experimental details, computational details, geometries and energies of computed systems, X-ray structure analysis of 3-platinaoxetanes, mechanistic alternatives of epoxide C−C activation (PDF) Computed Cartesian coordinates (XYZ) Accession Codes

CCDC 1031937−1031947 and 1035697 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. § Peter Hofmann is deceased (died August 15, 2015). # Thomas W. Gerl is deceased (died October 29, 2017). This manuscript was written by Prof. Dr. Peter Hofmann and submitted on May 22, 2015 to Organometallics, a revised version was provided on July 25, 2015. Due to the sudden passing of Peter Hofmann, the second revised version of this manuscript had only been manually corrected by him but was not submitted. These corrections, minor editorial changes, and the final publishing procedure of the manuscript were carried out by Dr. Claudia Meier ([email protected]. de), Prof. Dr. Bernd Straub ([email protected]), and Prof. Dr. Doris Kunz ([email protected]) in honor of Peter Hofmann.

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ACKNOWLEDGMENTS Support of this work by the Deutsche Forschungsgemeinschaft, the Fonds of the German Chemical Industry and the EUVigoni Program is gratefully acknowledged by P.H. and A.A.; M.M. and P.H. thank Prof. Margareta R. A. Blomberg and Prof. Per E. M. Siegbahn, University of Stockholm, for a fruitful collaboration during early stages of the theoretical work mentioned in the paper. M.M. thanks for the hospitality of the Swedish group during his research stay in Stockholm, where he could use the computing facilities there. We also thank Dr. S. Brode, Dr. A. Schäfer, and Dr. H. Domgörgen, Computational Chemistry Group, BASF SE, for providing computer facilities and for valuable discussions. P.H. and S.N. thank BASF SE for financial support provided in the course of a collaboration project on epoxide activation chemistry.

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REFERENCES

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Organometallics

Coordination and C-H Activation of Electron-Poor Arenes. Organometallics 2002, 21, 5320−5333. (17) Similar to the facile generation of reactive Pt intermediate C from neopentyl hydride 1, the Ni analog [(dtbpm)Ni(0)] can be easily made from the rather labile dimethyl complex (dtbpm-κ2P) NiMe2 (see ref 9g above), and the related Pd species [(dtbpm)Pd(0)] is accessible from (dtbpm-κ2P)Pd(CH2-SiMe3)2, in both cases by first-order reductive alkane elimination at ambient temperature as in the case of 1. (18) (a) Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. The formation and stability of spiro-compounds. Part I. spiro-Compounds from cyclohexane. J. Chem. Soc., Trans. 1915, 107, 1080−1106. (b) Jung, M. E.; Gervay, J. gem-Dialkyl effect in the intramolecular Diels-Alder reaction of 2-furfuryl methyl fumarates: the reactive rotamer effect, the enthalpic basis for acceleration, and evidence for a polar transition state. J. Am. Chem. Soc. 1991, 113, 224−232. (c) Nickel, T.; Goddard, R.; Krüger, C.; Pörschke, K.-R. Cyclotrimerisierung von Ethin am [(η1-tBu2PCH2PtBu2)Ni0]-Komplexfragment unter Bildung eines η6Benzolnickel(0)-Komplexes. Angew. Chem. 1994, 106, 908−910; Cyclotrimerization of Ethyne on the Complex Fragment [(η1tBu2PCH2PtBu2)Ni0] with Formation of an η 6-Benzene-Nickel(0) Complex. Angew. Chem., Int. Ed. Engl. 1994, 33, 879−882. (19) (a) Metz, M. C-H-, C-C- und C-Si-Bindungsaktivierungen mit [(dtbpm)Pt(0)] als reaktive Zwischenstufe ; quantenchemische Modellstudien. Ph.D. thesis. Heidelberg University, Germany, 1999. Apart from the real ligand dtbpm, H2P−CH2−PH2 (dhpm) and Me2P− CH2−PMe2 (dmpm) were used as simplified models of the ligand in order to allow calculations on higher levels. Computational details are given in the Supporting Information. (b) Plessow, P. N.; Carbo, J. J.; Schäfer, A.; Hofmann, P. Selective Carbon−Carbon Bond Activation of Oxirane by a Bisphosphine Pt(0) ComplexA Theoretical Study. Organometallics 2015, 34, 3764−3773. (20) Urtel, H.; Meier, C.; Eisenträger, F.; Rominger, F.; Joschek, J. P.; Hofmann, P. A Neutral Three-Coordinate Alkylrhodium(I) Complex: Stabilization of a 14-Electron Species by γ-C-H Agostic Interactions with a Saturated Hydrocarbon Group. Angew. Chem. 2001, 113, 803−806; A Neutral Three-Coordinate Alkylrhodium(I) Complex: Stabilization of a 14-Electron Species by γ-C-H Agostic Interactions with a Saturated Hydrocarbon Group. Angew. Chem., Int. Ed. 2001, 40, 781−784. (21) This is consistent with our observation that the thermally very stable and less congested (in comparison to the TME complex) ethylene complex (dtbpm-κ2P)Pt(η2-C2H4) does not activate the CC or CO bonds of epoxides. (22) (a) Bartmann, E. Metallorganische Verbindungen aus Epoxiden. Angew. Chem. 1986, 98, 629−631; Organometallic Derivatives of Epoxides. Angew. Chem., Int. Ed. Engl. 1986, 25, 653−654. (b) Kasai, P. H. Na0 (epoxy)1−3 complexes and dissociative electron capture of the epoxy system: matrix isolation ESR study. J. Am. Chem. Soc. 1990, 112, 4313−4320. (c) Kasai, P. H. Dissociative electron capture of propylene oxide: matrix isolation ESR study. J. Am. Chem. Soc. 1991, 113, 1539−1544. (23) Reichardt, C.; Welton, T. Solvents and Solvent Effects in Organic Chemistry, 4th ed; Wiley-VCH, Weinheim, Germany, 2010. (24) Allan, M. Vibrational excitation by electron impact in cyclopropane. Electron affinity and σ* orbitals. J. Am. Chem. Soc. 1993, 115, 6418−6419. (25) Sevin, A.; Chevreau, H.; Dezarnaud-Dandine, C. A theoretical study of the formation of stable anion intermediates via electron capture by heterosubstituted three membered ring XCH2CY2 compounds (X = O, S, NH, PH; Y = H, F). J. Mol. Struct. (Theochem) 1996, 371, 69−78. (26) (a) Wolters, L. P.; Bickelhaupt, M. F. Nonlinear d10-ML2 Transition-Metal Complexes. ChemistryOpen 2013, 2, 106−114. (b) Hofmann, P.In Organometallics in Organic Synthesis; de Meijere, A., tom Dieck, H., Eds.; Springer Verlag, Berlin Heidelberg, Germany, 1987; pp 1−35. (c) Ziegler, T.; Tschinke, V.; Becke, A. Theoretical study on the relative strengths of the metal-hydrogen and metalmethyl bonds in complexes of middle to late transition metals. J. Am.

A.; Krüger, C.; Notheis, J. U.; Rominger, F.; Scherhag, G.; Schultz, M.; et al. Sterically crowded diphosphinomethane ligands: molecular structures, UV-photoelectron spectroscopy and a convenient general synthesis of tBu2PCH2PtBu2 and related species. New J. Chem. 2003, 27, 540−550. and references therein. (11) (a) Pyykkö, P. Strong Closed-Shell Interactions in Inorganic Chemistry. Chem. Rev. 1997, 97, 597−636. (b) Dedieu, A.; Hoffmann, R. Platinum(0)-platinum(0) dimers. Bonding relationships in a d10-d10 system. J. Am. Chem. Soc. 1978, 100, 2074−2079. (12) (a) Notheis, U. Aktivierung von C-H und C-Si Bindungen mit einem Bisphosphanplatin(0)-Intermediat. Ph.D. thesis. Technical University of Munich, Germany, 1992. (b) Ficker, R.; Hiller, W.; Hofmann, P. (2000) Private communication to the Cambridge Structural Database, deposition number CCDC 107226. (c) X-ray crystal structures of the related Palladium-dimers Pd2(dotpm)2 (dotpm = bis(diortho-tolylphosphino)methane) and Pd2(dcpm)2 (dcpm = bis(dicyclohexylphosphino)methane) are also known: Reid, S. M.; Mague, J. T.; Fink, M. J. Facile Reductive Elimination of Ethane from Strained Dimethylpalladium(II) Complexes. J. Am. Chem. Soc. 2001, 123, 4081−4082. and Lumbreras, E., Jr.; Sisler, E. M.; Shelby, Q. D. Synthesis, X-ray crystal structure, and reactivity of Pd2(μ-dotpm)2 (dotpm = bis(di-ortho-tolylphosphino)methane). J. Organomet. Chem. 2010, 695, 201−205. (d) Simhai, N.; Iverson, C. N.; Edelbach, B. L.; Jones, W. D. Formation of Phenylene Oligomers Using Platinum-Phosphine Complexes. Organometallics 2001, 20, 2759−2766. (13) Unfried, G. Bindungsaktivierungen an Platin(0): Zur Chemie eines ungewöhnlichen Bisphosphanplatin(0)-Fragments. Ph.D. thesis. Technical University of Munich, Germany, 1993. (14) Scheme 5 only displays a small number of prototype, representative examples of (dtbpm-κ2P)Pt(η2-alkene) and (dtbpmκ2P)Pt(η2-alkyne) complexes. Overall, many more have been synthesized with around a dozen of them being characterized by Xray. For the first X-ray structure analysis of (dtbpm-κ2P)Pt(η2-C2H4) and (dtbpm-κ2P)Pt(η2-TME), see ref 13. Both X-ray structures were redetermined by the following: (a) Gerl, T. W. Aktivierung organischer Moleküle an Platin(0): Präparative und mechanistische Untersuchungen. Ph.D. thesis. Technical University of Munich, Germany, 1994. (b) Scherhag, G. Koordinationschemie und Metallorganik des Platins: Hydrosilylierung, Silyl-, Silylen- und Hydridokomplexe. Ph.D. thesis. Heidelberg University, Germany, 2001. (15) (a) Hoffmann, R. Brücken zwischen Anorganischer und Organischer Chemie (Nobel-Vortrag). Angew. Chem. 1982, 94, 725−739; Building Bridges Between Inorganic and Organic Chemistry (Nobel Lecture). Angew. Chem., Int. Ed. Engl. 1982, 21, 711−724. (b) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry, 2nd ed.; Wiley& Sons, Inc.: Hoboken, NJ, 2013. (16) (a) Hofmann, P.; Perez-Moya, L. A.; Steigelmann, O.; Riede, J. η2-(C,O) Ketene coordination at nickel(0). Synthesis, bonding, and molecular structure of (dtbpm)Ni[η2-(C,O)-Ph2C2O] [dtbpm = bis(di-tert-butylphosphino)methane]. Organometallics 1992, 11, 1167−1176. (b) Ficker, R.; Hiller, W.; Regius, Ch. T.; Hofmann, P. Crystal structure of dicarbonyl[η 2-bis(di-tert-butylphosphino)methane]nickel(0), ((C4H9)2P)2CH2Ni(CO)2. Z. Kristallogr. - Cryst. Mater. 1996, 211, 62−63. (c) Ficker, R.; Hiller, W.; Regius, Ch. T.; Hofmann, P. Crystal structure of [η2-bis(trimethylsilyl)acetylene][η2bis(di-tert-butylphosphino)methane]nickel(0), ((CH3)3Si)2C2((C4H9)2P)2CH2Ni. Z. Kristallogr. - Cryst. Mater. 1996, 211, 58−59. (d) Hofmann, P.; Perez-Moya, L. A.; Krause, M. E.; Kumberger, O.; Müller, G. [η2-Bis(di-t-butylphosphino)methan](trans-stilben)nickel(0), Ni(dtbpm)(trans-PhCH = CHPh). Synthese und Molekülstruktur einer Vorstufe des 14-Elektronenfragments [Ni(dtbpm)]/[η2-Bis(di-t-butylphosphino)methane](trans-stilbene)nickel(0), Ni(dtbpm)(trans-PhCH = CHPh). Synthesis and Molecular Structure of a Precursor for the 14-Electron Fragment [Ni(dtbpm)]. Z. Naturforsch., B: J. Chem. Sci. 1990, 45b, 897−908. (e) Iverson, C. N.; Lachicotte, R. J.; Müller, C.; Jones, W. D. η2J


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Organometallics Chem. Soc. 1987, 109, 1351−1358. (d) Hay, P. J. The model reaction Pt(PH3)2+H2 revisited. Influence of outer core orbitals on electron correlation effects using relativistic effective core potentials. New J. Chem. 1991, 15, 735−740. (e) Morokuma, K.; Borden, W. T. Ab initio calculations on (H3P)2Pt(C2H4). The effect of alkene pyramidalization on internal rotation and alkene binding energies. J. Am. Chem. Soc. 1991, 113, 1912−1914. (f) Sakaki, S.; Ieki, M. Oxidative addition reactions of saturated Si-X bonds (X = H, F, C, or Si) to Pt(PH3)2. An ab initio MO/MP4 study. J. Am. Chem. Soc. 1993, 115, 2373−2381. (g) Sakaki, S.; Satoh, H.; Shono, H.; Ujino, Y. Ab Initio MO Study of the Geometry, η3⇄η1 Conversion, and Reductive Elimination of a Palladium(II) η3-Allyl Hydride Complex and Its Platinum(II) Analogue. Organometallics 1996, 15, 1713−1720. (h) Sakaki, S.; Biswas, B.; Sugimoto, M. Oxidative addition of a C− H σ bond toM(PH3)2 (M = Pd or Pt). An ab initio molecular orbital study on the chelatephosphine effect. J. Chem. Soc., Dalton Trans. 1997, 803−809. (i) Cui, Q.; Musaev, D. G.; Morokuma, K. Density Functional Study on the Mechanism of Palladium(0)-Catalyzed Thioboration Reaction of Alkynes. Differences between Pd(0) and Pt(0) Catalysts and between Thioboration and Diboration. Organometallics 1998, 17, 1383−1392. (j) Sakaki, S.; Biswas, B.; Sugimoto, M. A Theoretical Study of the C-H Activation of Methane Derivatives. Significant Effects of Electron-Withdrawing Substituents. Organometallics 1998, 17, 1278−1289. (k) Cui, Q.; Musaev, D. G.; Morokuma, K. Why Do Pt(PR3)2 Complexes Catalyze the Alkyne Diboration Reaction, but Their Palladium Analogues Do Not? A Density Functional Study. Organometallics 1998, 17, 742−751. (l) Dedieu, A.; Ingold, F. Monohapto- oder Dihapto-Koordination von CO2 in Bis(amin)-Ni0 -Komplexen: Eine ab-initio-Untersuchung. Angew. Chem. 1989, 101, 1711−1712; Monohapto versus Dihapto CO2 Coordination in Bis(amine)Ni0 Complexes: A CAS-SCF Study. Angew. Chem., Int. Ed. Engl. 1989, 28, 1694−1695. (m) Deeth, R. J.; Smith, A.; Hii, K. K.; Brown, J. M. The Heck olefination reaction; A DFT study of the elimination pathway. Tetrahedron Lett. 1998, 39, 3229−3232. (n) Su, M. D.; Chu, S.-Y. Theoretical Study of Oxidative Addition and Reductive Elimination of 14-Electron d10 ML2 Complexes:A ML 2 +CH 4 (M = Pd, Pt; L = CO, PH 3 , L2=PH’2CH2CH2PH2) Case Study. Inorg. Chem. 1998, 37, 3400− 3406. (o) Uddin, J.; Dapprich, S.; Frenking, G.; Yates, B. F. Nature of the Metal-Alkene Bond in Platinum Complexes of Strained Olefins. Organometallics 1999, 18, 457−465. (27) Joost, M.; Zeineddine, A.; Estévez, L.; Mallet-Ladeira, S.; Miqueu, K.; Amgoune, A.; Bourissou, D. Facile Oxidative Addition of Aryl Iodides to Gold(I) by Ligand Design: Bending Turns on Reactivity. J. Am. Chem. Soc. 2014, 136, 14654−14657. (28) Wang, Y.-M.; Lackner, A. D.; Toste, F. D. Development of Catalysts and Ligands for Enantioselective Gold Catalysis. Acc. Chem. Res. 2014, 47, 889−901. (29) Wu, C.-Y.; Horibe, T.; Jacobsen, C.-B.; Toste, F. D. Stable gold(III) catalysts by oxidative addition of a carbon−carbon bond. Nature 2015, 517, 449−454. (30) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

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