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The Role of Alkane Coordination in C–H Bond Cleavage at a Pt(II)

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The Role of Alkane Coordination in C–H Bond Cleavage at a Pt(II) The role of alkane coordination in C–H bond cleavage SPECIAL FEATURE at a Pt(II) center George S. Chen, Jay A. Labinger†, and John E. Bercaw† The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, 1200 East California Avenue, Mail Code 127-72, Pasadena, CA 91125 Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved February 15, 2007 (received for review December 11, 2006) The rates of COH bond activation for various alkanes by [(N– electrophilic - ؍ O A O A O ؍ ؉ O N)Pt(Me)(TFE-d3)] (N N Ar N C(Me) C(Me) N Ar; Ar Cl OH2 i Cl CH3 II PtII + CF3CD2OD) were studied. Both Pt + CH4 + H ؍ 3,5-di-tert-butylphenyl; TFE-d3 H O Cl H O Cl linear and cyclic alkanes give the corresponding alkene-hydride 2 2 -cation [(N–N)Pt(H)(alkene)]؉ via (i) rate determining alkane coor iii O ␴ + ii IV 2- dination to form a C H complex, (ii) oxidative cleavage of the CH3OH + H [Pt Cl6] coordinated COH bond to give a platinum(IV) alkyl-methyl-hydride CH3Cl electron transfer SN2 displacement intermediate, (iii) reductive coupling to generate a methane ␴ - II 2- H2O Cl [Pt Cl4] complex, (iv) dissociation of methane, and (v) ␤-H elimination to - Cl CH3 Cl PtIV form the observed product. Second-order rate constants for cy- H2O Cl Cl cloalkane activation (CnH2n), are proportional to the size of the ring ϳ (k n). For cyclohexane, the deuterium kinetic isotope effect [PtII(OH ) Cl ] net: CH + [PtIVCl ]2- + H O 2 2 2 CH -OH + [PtIICl ]2- + 2HCl (kH/kD) of 1.28 (5) is consistent with the proposed rate determining 4 6 2 3 4 alkane coordination to form a COH ␴ complex. Statistical scram- Scheme 1. bling of the five hydrogens of the Pt-methyl and the coordinated methylene unit, via rapid, reversible steps ii and iii, and inter- CHEMISTRY change of geminal COH bonds of the methane and cyclohexane strate the potential promise of homogeneous catalytic alkane COH ␴ adducts, is observed before loss of methane. conversion. Work in our group has been centered on the Shilov system for alkane functionalization ͉ COH activation ͉ catalysis ͉ organometallic selective hydroxylation of alkanes (1, 2, 4). Detailed studies estab- chemistry lished the three-step mechanism and overall stoichiometry shown in Scheme 1, in which platinum(II) catalyzes the oxidation of he selective activation and functionalization of saturated alkane alkanes to alcohols by platinum(IV) at 120°C (9–12). Although it TCOH bonds represent important areas of research (1–5). is currently impractical because of low reaction rates, expensive Alkanes are the main constituents of oil and natural gas; hence the oxidant, and catalyst instability, the system does exhibit useful ability to efficiently transform alkanes to more valuable products is regioselectivity (1° Ͼ 2° Ͼ 3°) and chemoselectivity. highly desirable (1, 2). Unfortunately, alkanes are relatively inert at The COH activation step (step i in Scheme 1) is responsible ambient temperatures and pressures, due to their high homolytic for determining both activity and selectivity; however, direct bond strengths and very low acidity and basicity (1, 5). Partial detailed study of its mechanism is not possible in the ‘‘real’’ oxidations of alkanes (hydroxylation, oxidative coupling, and oxi- Shilov system because of its complexity and interfer- dative dehydrogenation) are among the few processes that could, in ing side reactions. Accordingly, we turned to model systems, principle, provide valuable products (alcohols, higher alkanes, and generalized in Scheme 2. The platinum methyl cations alkenes, respectively) in thermodynamically favorable transforma- tions, but such reactions are difficult to carry out with high selectivity at high conversion (1, 2, 4). More traditional high- + temperature routes often proceed by free radical mechanisms, for Ar H Ar Ar + H2O N CH3 -CH N CH3 N + CH3 which the products derived from alkanes are virtually guaranteed to Pt 4 Pt Pt N CF CH OH N N be more reactive than the alkane itself, placing an inherent con- Ar CH3 3 2 Ar TFE Ar OH2 (= TFE) straint on selectivity (2–4). R-H - CH4 In contrast, low-temperature homogeneous activations of COH Ar Ar bonds need not, and often do not, involve radicals, and may lead to N + TFE H2O N + OH Pt Pt 2 more selective reactions than those promoted by heterogenous N N R R catalysts operating at high temperatures. Although many transition Ar Ar metal complexes have been shown to activate COH bonds, the Scheme 2. development of a practical catalyst to transform alkanes to value- added products remains elusive (1, 2). The key problem lies in finding a catalyst system that has both adequate reactivity and Author contributions: G.S.C., J.A.L., and J.E.B. designed research; G.S.C. performed re- selectivity while tolerating oxidizing and protic conditions (4, 5). search; G.S.C. contributed new reagents/analytic tools; G.S.C., J.A.L., and J.E.B. analyzed In recent years, several oxidation catalysts based on platinum(II), data; and G.S.C., J.A.L., and J.E.B. wrote the paper. palladium(II), and mercury(II) salts have been shown to function- The authors declare no conflict of interest. alize COH bonds, leading to good yields of partially oxidized This article is a PNAS Direct Submission. products (4, 6–8). For example, [(2,2Ј-bipyrimidine)PtCl2] catalyzes †To whom correspondence may be addressed. E-mail: [email protected] or [email protected]. the selective oxidation of methane in fuming sulfuric acid to give This article contains supporting information online at www.pnas.org/cgi/content/full/ methyl bisulfate in 72% one-pass yield at 81% selectivity based on 0610981104/DC1. methane (8). This system, although not yet practical, does demon- © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0610981104 PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6915–6920 Downloaded by guest on September 27, 2021 Ar 8000.0 N + CH 4 3 - CH D + Pt [CF3CD2OB(C6F5)3] Ar 7 3 N 7000.0 C6H 21 TFE-d3 N CH3 B(C6F5)3 Ar Pt N Ar 0. 6000 CF3CD2OD N + Ar CH3 3 CH2D - 1 (= TFE-d3) CH + Pt [CF3CD2OB(C6F5)3] 7 4 N Ar TFE-d3 ) 5000.0 CMe -1 3 C6D 21 "2-d0.43" Ar = 0 000. 4 Kobs (s CMe3 3000.0 Scheme 3. .0 2000 1000.0 [(NON)Pt(Me)(solv)]ϩ (NONϭArONAC(Me)AC(Me)A NOAr; solv ϭ TFE ϭ 2,2,2-trifluoroethanol) react with a 0 variety of R-H groups (Ar-H, benzyl-H, indenyl-H, 0 0 1. 2.0 3.0 0.4 5.0 [S bu ts r ]eta )M( Me3SiCH2-H, etc.) to afford the corresponding organoplati- num products, and have proved to be particularly well suited Fig. 1. Plot of kobs vs. [hydrocarbon] for C6H12 (diamonds) and C6D12 (squares) for mechanistic investigations (13–18). The relative reactivities at 40°C. of chemically differing COH bonds are of particular impor- tance for determining selectivity. We report herein on an investigation of the rate and selectivity of COH bond activa- peak at ␦ ϭ 4.9 and a distinctive platinum hydride peak at ␦ ϭ 195 tion for various linear and cyclic alkanes. Ϫ22.2 with Pt satellites (JPt-H ϭ 1,320 Hz). Cyclopentane, cycloheptane, and cyclooctane all react similarly with 2 to Results generate the corresponding species 4, 5, and 6 (Scheme 4). ؉ Preparation of [(N–N)Pt(Me)(TFE-d3)] (2) and Reactions with Cyclic Alkanes. Protonolysis of 1 with B(C6F5)3 in anhydrous TFE-d3 Kinetics of Cycloalkane C-H Activation by 2. The kinetics for reac- gives platinum(II) monomethyl cation 2, trifluoroethoxy- tions of 2-d0.43 with C6H12 or C6D12 at 40°C were examined by tris(pentafluorophenyl)borate, and methane (Scheme 3). Al- following the disappearance of 2-d0.43 (methyl backbone signal at ␦ ϭ 1 though only 1 equivalent of B(C6F5)3 is required by the stoichi- 2.0) and appearance of 3 by H NMR. Reactions displayed ometry of the protonation, 2 equivalents are needed to cleanly clean first-order kinetics for the disappearance of [2] and first- Ϫ3 generate 2. Both 2 (2-CH3 and 2-CH2D) and the released order dependence on [C6H12] (Fig. 1), with k2 ϭ 1.59(4) ϫ 10 Ϫ1 Ϫ1 methane (CH3D and CH4) are obtained as a statistical mixture M s . The methane isotopologs CH3D and CH4 were gener- of isotopologs, the result of fast H/D scrambling among the seven ated in the ratio of Ϸ1:2. Formation of 3 was accompanied by a ϩ positions of the [(NON)Pt(D)(CH3)2] intermediate before much slower background decomposition reaction (18), indicated methane dissociation (18). by both the appearance of additional new 1H NMR signals and Ϫ5 Ϫ1 Cyclohexane, cyclopentane, cycloheptane, and cyclooctane the nonzero intercept of Fig. 1 [kdecomp ϭ 2.52(5) ϫ 10 s ]. react cleanly with the platinum methyl cation 2 at 40°C, as shown The rate constant for the reaction of 2-d0.43 with C6D12 at 40°C Ϫ3 Ϫ1 Ϫ1 in Scheme 4. Addition of cyclohexane to a solution of 2-d0.43 in (Fig. 1) was found to be k2 ϭ 1.24(4) ϫ 10 M s [with a Ϫ5 Ϫ1 TFE-d3 produces a single species 3 over the course of several similar nonzero intercept, kdecomp ϭ 2.14(8) ϫ 10 s ], corre- sponding to a kinetic deuterium isotope effect of kH/kD ϭ 1.28(5).
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