Unexpected Selectivities in C−H Activations of Toluene and<i>p</i>-Xylene at Cationic Platinum(II) Diimine Complexes. New Mechanistic Insight into Product-Determining Factors

Johansson, Anh; Ryan, c Olav B.; Rømming, a and Christian; Tilset, a Mats

doi:10.1021/ja010277e

Cited by 123 publications

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“…The inhibition of isotopic scrambling by the addition of a Lewis base is most readily explained by an associative mechanism for exchange of ethylbenzene with THF and mirrors the observations of isotope exchange from protonation of (diimine)Pt(Me) 2 using DOTf, as well as positional o/m/p isomerization of Pt tolyl and xylyl complexes where NCMe was used as the Lewis base in both cases. 39,40 To confirm the interpretation of the results of isotopic labeling, [( , has a high calculated barrier (ΔG q = 27.5 kcal/mol), consistent with the absence of extensive H/D scrambling into the ethyl fragment. The free energy for dissociation of ethylbenzene from A 0 is calculated to be 12.5 kcal/mol (Scheme 19; these calculations are gas phase and thus the free energy of ethylbenzene dissociation is different from that in Scheme 8), while arene CÀH activation barriers are calculated to be 16.2 kcal/mol (para CÀH activation), 16.7 kcal/ mol (meta CÀH activation) and 18.7 (ortho CÀH activation).…”

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confidence: 76%

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

et al. 2011

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Cationic platinum(II) complexes [( t bpy)Pt(Ph)(L)]+ [ t bpy =4,4′-di-tert-butyl-2,2′-bipyridyl; L = THF, NC5F5, or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 °C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt–phenyl bond, and subsequent metal-mediated benzene C–H activation. Studies of stoichiometric benzene (C6H6 or C6D6) C–H/C–D activation by [( t bpy)Pt(Ph-d n )(THF)]+ (n = 0 or 5) indicate a k H/k D = 1.4(1), while comparative rates of ethylene hydrophenylation using C6H6 and C6D6 reveal k H/k D = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C–H activation is the highest energy species along the catalytic cycle. In CD2Cl2, [( t bpy)Pt(Ph)(THF)][BAr′4] [Ar′ = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [( t bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] with k obs = 1.05(4) × 10–3 s–1 (23 °C, [C2H4] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C–H activation competing with ethylbenzene dissociation.

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confidence: 76%

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

et al. 2011

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“…One important finding has been that low-temperature protonation of (diimine)Pt(II) dialkyl and Pt(II) diaryl complexes lead to observable, but thermally sensitive, Pt(IV) hydridoalkyl and hydridoaryl complexes that eliminate the respective hydrocarbons upon heating. 60,[76][77][78][79][80][81] Scheme 9 summarizes the mechanistic picture that has emerged for these reactions at (diimine)Pt(II) systems 60,63,74,[82][83][84][85] and at related Pt species with bidentate ligands. Substitution of π-benzene for an aqua (or TFE) ligand occurs as a solvent assisted process for which there is controversy over an associative mechanism.…”

Section: Methodsmentioning

confidence: 99%

Combined Low Temperature Rapid Scan and¹H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

et al. 2009

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A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)Pt II diphenyl complex (denoted as (N-N)PtPh 2 ) has been undertaken in dichloromethane solution with and without acetonitrile as a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the temperature range -80 to +27 °C allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh 2 with HBF 4 3 Et 2 O in CH 2 Cl 2 /MeCN occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces (N-N)PtPh 2 H(NCMe) + (ΔH q = 29 ( 1 kJ mol -1 , ΔS q = -47 ( 4 J K -1 mol -1 ). At higher temperatures, (N-N)PtPh 2 H(NCMe) + eliminates benzene to furnish (N-N)PtPh(NCMe) + . This reaction proceeds by rate-limiting MeCN dissociation (ΔH q = 88 ( 2 kJ mol -1 , ΔS q = +62 ( 6 J K -1 mol -1 , ΔV q = +16 ( 2 cm 3 mol -1 ). Protonation of (N-N)PtPh 2 in dichloromethane in the absence of MeCN cleanly produces the Pt(II) π-benzene complex (N-N)PtPh(η 2 -C 6 H 6 ) + at low temperatures. Addition of MeCN to a solution of the π-benzene complex causes an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by 1 H NMR (ΔH q = 39 ( 2 kJ mol -1 , ΔS q = -126 ( 11 J K -1 mol -1 ). When the stronger triflic acid is employed in dichloromethane/acetonitrile, a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe) + produces (N-N)Pt(NCMe) 2 2+ and benzene with no detectable intermediates (ΔH q = 69 ( 1 kJ mol -1 , ΔS q = -43 ( 3 J K -1 mol -1 ). The mechanisms for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(η 2 -C 6 H 6 ) + . It appears that the exchange occurs by a direct σ-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling sequence that was proposed.

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“…These differences reflect the increased steric demands of the 2,6-dimethyl-substituted systems and appear to be a common feature for N,NЈ-diaryl-substituted (N-N)PtX 2 complexes where similar trends in dihedral angles have been reported. [15,36,44,45,54,60,[62][63][64][65][66][67] This empha-sizes the steric hindrance imposed by the 2,6-dimethyl-substituted N-aryl groups: The perpendicular orientation of these N-aryl groups with respect to the coordination plane causes the methyl groups to sterically block the access to Pt from above and below the coordination plane. This has a pronounced effect on the qualitative protonation rates and on the stabilities of the Pt II π-benzene complexes, as discussed earlier.…”

Section: Synthesis and Characterization Of Metal Complexesmentioning

confidence: 99%

“…On the other hand, the η 2 -benzene complexes will be stabilized by the 2,6-dimethyl groups because the displacement of benzene (and other hydrocarbons) from these and related diimine-Pt complexes has been demonstrated to be associative reactions. [32,59,60] Scheme 4. It has been demonstrated that the kinetically preferred site of protonation is the Pt center for (Ar-DAB)PtMe 2 complexes. [17,18] We have recently presented experimental evidence that this is also the case for (Ar-DAB)PtPh 2 analogues, [32] a scenario that had already been predicted by DFT calculations.…”

Section: Synthesis and Characterization Of Metal Complexesmentioning

confidence: 99%

Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

2010

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PtII diphenyl complexes (N–N)PtPh2 [N–N = diimines Ar–N=C(An)C=N–Ar with Ar = substituted aryl groups] have been prepared and characterized by 1H, 13C, and 195Pt NMR spectroscopy. The 195Pt NMR spectroscopic data establish the electronic influence exerted by substituents at the backbone of the diimine ligand system to the metal center. When compared to diimines Ar–N=CMe–CMe=N–Ar, the electron‐withdrawing ability of the Ar‐BIAN ligand and the electron‐donating ability of the O,O‐heterocyclic Ar‐BICAT systems are demonstrated. Trends in 195Pt NMR chemical shifts suggest that electronic tuning of the metal center is better achieved through variations of the diimine backbone substituents rather than variation of the substituents at the N‐Aryl groups. Protonation of (N–N)PtPh2 in dichloromethane/acetonitrile at –78 °C furnishes the corresponding PtIV hydrides (N–N)PtPh2H(NCMe)+. The PtIV hydrides liberate benzene with the formation of (N–N)PtPh(NCMe)+ when the temperature is raised. A second protonation and rapid benzene elimination produces the dicationic PtII species (N–N)Pt(NCMe)22+ at approximately 50 °C. Protonation of (N–N)PtPh2 in the absence of acetonitrile results in the clean formation of (N–N)PtPh(η2‐C6H6)+ at temperatures that depend on the steric hindrance provided by the alkyl substituents at the diimine N‐aryl groups. These findings support the notion that the metal is the kinetically preferred site of protonation. The results qualitatively agree with a recent mechanistic study of protonation‐induced reactions of (diimine)PtPh2 complexes that bear simple methyl substituents at the diimine backbone. Several compounds have been crystallographically characterized. All complexes have the expected square planar environment at the metal. Modest variations in the metric parameters suggest that the Ar‐BICAT system has a weaker trans influence than the Ar‐BIAN and Ar‐DAB systems.

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Unexpected Selectivities in C−H Activations of Toluene andp-Xylene at Cationic Platinum(II) Diimine Complexes. New Mechanistic Insight into Product-Determining Factors

Cited by 123 publications

References 51 publications

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

Combined Low Temperature Rapid Scan and¹H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

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Unexpected Selectivities in C−H Activations of Toluene andp-Xylene at Cationic Platinum(II) Diimine Complexes. New Mechanistic Insight into Product-Determining Factors

Cited by 123 publications

References 51 publications

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

Combined Low Temperature Rapid Scan and1H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation

Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

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Combined Low Temperature Rapid Scan and¹H NMR Mechanistic Study of the Protonation and Subsequent Benzene Elimination from a (Diimine)platinum(II) Diphenyl Complex Relevant to Arene C−H Activation