HOMOGENEOUS HYDROCARBON CH BOND ACTIVATION AND FUNCTIONALIZATION WITH PLATINUM

Facial-Meridional Isomerization and Reductive Elimination in [(R2P-o-C6H4)2N]PtMe3 (R = Ph, iPr)

Treatment of PtMe₃I in tetrahydrofuran with either in situ prepared [R-PNP]Li ([R-PNP]^- = [(R₂P-o-C₆H₄)₂N]^-; R = Ph, iPr) or H[R-PNP] in the presence of triethylamine at room temperature affords quantitatively fac-[R-PNP]PtMe₃. Thermolysis of fac-[R-PNP]PtMe₃ in benzene solutions generates mer-[R-PNP]PtMe₃ and ultimately [R-PNP]PtMe and ethane. Complexes mer-[R-PNP]PtMe₃ represent the first meridional trialkylplatinum(IV) derivatives to date.

Platinum(IV) complexes of a bulky and exceptionally donating alkyl: 2-adamantyl

The 2-adamantyl (2-Ad) group is a particularly challenging alkyl to install on a transition metal, with the only reported 2-Ad complexes of platinum being those of Pt(II). The question of whether 2-Ad complexes with platinum in the oxidation state +IV can be made and whether they are stable is answered here. The Pt(IV) compound trans-[(bpy)PtMe2(2-Ad)I] (bpy = 2,2′-bipyridine) was synthesized via oxidative addition of iodomethane to [(COD)PtMe(2-Ad)] (COD = 1,5-cyclooctadiene) in the presence of bpy. Iodide abstraction with silver triflate (triflate anion, OTf− = CF3SO3−) produced trans-[(bpy)PtMe2(2-Ad)(OTf)]. Both complexes were unambiguously characterized by 1D NMR (1H, 13C) spectroscopy and X-ray crystallography. Both show evidence for the large steric bulk of the 2-Ad group (through the conformation of the complex adopted) and also for its exceptionally pronounced trans-influence (very long bonds to iodide/triflate trans to adamantyl). Both of the new platinum(IV) adamantyl complexes were extremely stable, even to prolonged heating.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

Platinum complexes have been often considered as the least reactive of the group 10 triad metals. Slow kinetics are behind this lack of reactivity but, still, some industrially relevant catalytic process are dominated by platinum compounds and sometimes different selectivities can be found in comparison to Ni or Pd. Nevertheless, during the last years, it has been reported that the catalytic behaviour of well-defined platinum derivatives can be improved through a judicious choice of their electronic and steric properties, leading to highly electrophilic or low-electron count platinum systems. In this feature article, we highlight some catalytic processes in which well-defined electrophilic platinum complexes or coordinatively unsaturated systems play an important role in their catalytic activity.

An H-Substituted Rhodium Silylene

Divergent reactivity of organometallic rhodium(I) complexes, which led to the isolation of neutral rhodium silylenes, is described. Addition of PhRSiH₂ (R=H, Ph) to the rhodium cyclooctene complex (^iPr NNN)Rh(COE) (1-COE; ^iPr NNN=2,5-[iPr₂ P=N(4-iPrC₆ H₄ )]₂ N(C₆ H₂ )^- , COE=cyclooctene) resulted in the oxidative addition of an Si-H bond, providing rhodium(III) silyl hydride complexes (^iPr NNN)Rh(H)SiHRPh (R=H, 2-SiH₂ Ph; Ph, 2-SiHPh₂ ). When the carbonyl complex (^iPr NNN)Rh(CO) (1-CO) was treated with hydrosilanes, base-stabilized rhodium(I) silylenes κ² -N,N-(^iPr NNN)(CO)Rh=SiRPh (R=H, 3-SiHPh; Ph, 3-SiPh₂ ) were isolated and characterized using multinuclear NMR spectroscopy and X-ray crystallography. Both silylene species feature short Rh-Si bonds [2.262(1) Å, 3-SiHPh; 2.2702(7) Å, 3-SiPh₂ ] that agree well with the DFT-computed structures. The overall reaction led to a change in the ^iPr NNN ligand bonding mode (κ³ →κ² ) and loss of H₂ from PhSiRH₂ , as corroborated by deuterium labelling experiments.

Homogeneous Functionalization of Methane

One of the remaining "grand challenges" in chemistry is the development of a next generation, less expensive, cleaner process that can allow the vast reserves of methane from natural gas to augment or replace oil as the source of fuels and chemicals. Homogeneous (gas/liquid) systems that convert methane to functionalized products with emphasis on reports after 1995 are reviewed. Gas/solid, bioinorganic, biological, and reaction systems that do not specifically involve methane functionalization are excluded. The various reports are grouped under the main element involved in the direct reactions with methane. Central to the review is classification of the various reports into 12 categories based on both practical considerations and the mechanisms of the elementary reactions with methane. Practical considerations are based on whether or not the system reported can directly or indirectly utilize O₂ as the only net coreactant based only on thermodynamic potentials. Mechanistic classifications are based on whether the elementary reactions with methane proceed by chain or nonchain reactions and with stoichiometric reagents or catalytic species. The nonchain reactions are further classified as CH activation (CHA) or CH oxidation (CHO). The bases for these various classifications are defined. In particular, CHA reactions are defined as elementary reactions with methane that result in a discrete methyl intermediate where the formal oxidation state (FOS) on the carbon remains unchanged at -IV relative to that in methane. In contrast, CHO reactions are defined as elementary reactions with methane where the carbon atom of the product is oxidized and has a FOS less negative than -IV. This review reveals that the bulk of the work in the field is relatively evenly distributed across most of the various areas classified. However, a few areas are only marginally examined, or not examined at all. This review also shows that, while significant scientific progress has been made, greater advances, particularly in developing systems that can utilize O₂, will be required to develop a practical process that can replace the current energy and capital intensive natural gas conversion process. We believe that this classification scheme will provide the reader with a rapid way to identify systems of interest while providing a deeper appreciation and understanding, both practical and fundamental, of the extensive literature on methane functionalization. The hope is that this could accelerate progress toward meeting this "grand challenge."

Silver-Catalyzed C(sp3)-H Chlorination

A silver-catalyzed chlorination of benzylic, tertiary, and secondary C(sp³)-H bonds was developed. The reaction proceeded with as low as 0.2 mol % catalyst loading at room temperature under air atmosphere with synthetically useful functional group compatibility. The regioselectivity and reactivity tendencies suggest that the chlorination proceeded through a radical pathway, but an intermediate alkylsilver species cannot be ruled out.

Divergent reactivity of platinum(ii) and palladium(ii) methylperoxo complexes and the formation of an unusual hemi-aminal complex

The 6,6''-diaminoterpyridine palladium(ii) methylperoxo complex eliminates methyl hydroperoxide and reacts with acetone to form a novel hemi-aminal palladium complex, whereas the analogous platinum(ii) complex generates formaldehyde and a platinum(ii) hydroxo complex.

Carbon–sulfur bond reductive coupling from a platinum(ii) thiolate complex

Complex 1 was reacted with different alkyl halides at room temperature. This reaction proceeded via formation of binuclear intermediate complex 3 and through C–S bond reductive coupling to produce alkyl sulfides and corresponding halide complexes 2.

[RuCl2(η6-p-cymene)] complexes bearing phosphinous acid ligands: preparation, application in C–H bond functionalization and mechanistic investigations

A series of [RuCl₂(η⁶-p-cymene)] complexes bearing phosphinous acid (PA) ligands have been straightforwardly prepared from the dimer [RuCl₂(p-cymene)]₂ and secondary phosphine oxides (SPOs) and fully characterized.

Oxidation of dimethylplatinum(II) complexes with malonyl peroxide derivatives

The reaction of the cyclic peroxides cyclobutane malonoyl peroxide and cyclopentane malonoyl peroxide to dimethylplatinum(II) complexes with the bidentate nitrogen donor ligand 4,4′-di-t-butyl-2,2′-bipyridine (bubpy) gave a mixture of the products of cis and trans oxidative addition, [PtMe2{(O2C)2C(C3H6)}(bubpy)] and [PtMe2{(O2C)2C(C4H8)}(bubpy)], respectively. The cis isomers exist with a chelating dicarboxylate ligand, forming a six-membered ring, while the trans isomers are thought to exist as oligomers, which may react with water or solvent to give complexes containing a monodentate dicarboxylate ligand. Equilibration of the product of cis oxidative addition, cis-[PtMe2{(O2C)2C(C3H6)}(bubpy)], with oxalic or phthalic acid to give equilibria with the oxalate cis-[PtMe2(O4C2)(bubpy)] or phthalate cis-[PtMe2{(O2C)2C6H4}(bubpy)] derivative, respectively, indicated the expected sequence of chelate stability oxalate > cyclobutane malonate > phthalate.

Electrophilic Pt(II) complexes: precision instruments for the initiation of transformations mediated by the cation-olefin reaction

A discontinuity exists between the importance of the cation–olefin reaction as the principal C–C bond forming reaction in terpene biosynthesis and the synthetic tools for mimicking this reaction under catalyst control; that is, having the product identity, stereochemistry, and functionality under the control of a catalyst. The main reason for this deficiency is that the cation–olefin reaction starts with a reactive intermediate (a carbocation) that reacts exothermically with an alkene to reform the reactive intermediate; not to mention that reactive intermediates can also react in nonproductive fashions. In this Account, we detail our efforts to realize catalyst control over this most fundamental of reactions and thereby access steroid like compounds. Our story is organized around our progress in each component of the cascade reaction: the metal controlled electrophilic initiation, the propagation and termination of the cyclization (the cyclase phase), and the turnover deplatinating events. Electrophilic Pt(II) complexes efficiently initiate the cation–olefin reaction by first coordinating to the alkene with selection rules that favor less substituted alkenes over more substituted alkenes. In complex substrates with multiple alkenes, this preference ensures that the least substituted alkene is always the better ligand for the Pt(II) initiator, and consequently the site at which all electrophilic chemistry is initiated. This control element is invariant. With a suitably electron deficient ligand set, the catalyst then activates the coordinated alkene to intramolecular addition by a second alkene, which initiates the cation–olefin reaction cascade and generates an organometallic Pt(II)-alkyl. Deplatination by a range of mechanisms (β-H elimination, single electron oxidation, two-electron oxidation, etc.) provides an additional level of control that ultimately enables A-ring functionalizations that are orthogonal to the cyclase cascade. We particularly focus on reactions that combine an initiated cyclization reaction with a turnover defining β-hydride elimination, fluorination, and oxygenation. These latter demetalation schemes lead to new compounds functionalized at the C3 carbon of the A-ring (steroid numbering convention) and thus provide access to interesting potentially bioactive targets. Progress toward efficient and diverse polycyclization reactions has been achieved by investing in both synthetic challenges and fundamental organometallic reactivity. In addition to an interest in the entrance and exit of the metal catalyst from this reaction scheme, we have been intrigued by the role of neighboring group participation in the cyclase phase. Computational studies have served to provide nuance and clarity on several key aspects, including the role (and consequences) of neighboring group participation in cation generation and stabilization. For example, these calculations have demonstrated that traversing carbonium ion transition states significantly impacts the kinetics of competitive 6-endo and 5-exo A-ring forming reactions. The resulting nonclassical transition states then become subject to a portion of the strain energy inherent to bicyclic structures, with the net result being that the 6-endo pathway becomes kinetically favored for alkene nucleophiles, in contrast to heteroatom nucleophiles which progress through classical transition states and preferentially follow 5-exo pathways. These vignettes articulate our approach to achieving the desired catalyst control.

Stability and dynamic processes in 16VE iridium(III) ethyl hydride and rhodium(I) σ-ethane complexes: experimental and computational studies

Iridium(I) and rhodium(I) ethyl complexes, (PONOP)M(C2H5) (M = Ir (1-Et), Rh (2-Et)) and the iridium(I) propyl complex (PONOP)Ir(C3H7) (1-Pr), where PONOP is 2,6-(tBu2PO)2C5H3N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)Ir(H)(R) (1-(H)(Et)(+) and 1-(H)(Pr)(+)), respectively. Dynamic (1)H NMR characterization of 1-(H)(Et)(+) establishes site exchange between the Ir-H and Ir-CH2 protons (ΔG(exH)(‡)(-110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic (13)C NMR spectroscopy, the exchange barrier between the α and β carbons ("chain-walking") was measured (ΔG(exC)(‡)(-110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (-40 °C), to be compared with the reported barrier to methane loss in 1-(H)(Me)(+) of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)Rh(EtH) (2-(EtH)(+)), was prepared by protonation of 2-Et at -150 °C. The barrier for ethane loss (ΔG(dec)(‡)(-132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, 2-(MeH)(+), (ΔG(dec)(‡)(-87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of 2-(EtH)(+) is reported, a key feature of which is the upfield signal at -31.2 ppm for the coordinated CH3 group in the (13)C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (-132 °C) is observable by (13)C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η(2)-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex.

True and masked three-coordinate T-shaped platinum(II) intermediates

Although four-coordinate square-planar geometries, with a formally 16-electron counting, are absolutely dominant in isolated Pt(II) complexes, three-coordinate, 14-electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Although very few authenticated three-coordinate Pt(II) complexes have been characterized, a much larger number of complexes can be described as operationally three-coordinate in a kinetic sense. In these compounds, which we have called masked T-shaped complexes, the fourth position is occupied by a very weak ligand (agostic bond, solvent molecule or counteranion), which can be easily displaced. This review summarizes the structural features of the true and masked T-shaped Pt(II) complexes reported so far and describes synthetic strategies employed for their formation. Moreover, recent experimental and theoretical reports are analyzed, which suggest the involvement of such intermediates in reaction mechanisms, particularly C-H bond-activation processes.