C(sp3)-Cl Bond Activation Promoted by a POP-Pincer Rhodium(I) Complex Sheila G. Curto, Laura A. de las Heras, Miguel A. Esteruelas,* Montserrat Oliván, and Enrique Oñate Departamento de Química Inorgánica – Instituto de Síntesis Química y Catálisis Homogénea (ISQCH) – Centro de Innova- ción en Química Avanzada (ORFEO-CINQA), Universidad de Zaragoza – CSIC, 50009 Zaragoza, Spain 3 i i ABSTRACT: Complex [RhCl(κ -P,O,P-{xant(P Pr2)2})] (1; xant(P Pr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) activates C(sp3)-Cl bonds of mono- and dichloroalkanes and catalyzes the dehalogenation of chloroalkanes and the homocoupling 3 i of benzyl chloride. Complex 1 reacts with chlorocyclohexane to give [RhHCl2(κ -P,O,P-{xant(P Pr2)2})] (2) and cyclohexene and promotes the dehalogenation of the chlorocycloalkane to cyclohexane using 2-propanol solutions of sodium formate as reducing 3 i agent. The oxidative addition of benzyl chloride to 1 leads to [Rh(CH2Ph)Cl2(κ -P,O,P-{xant(P Pr2)2})] (4). The dehalogenation of this chloroalkane with 2-propanol solutions of sodium formate, in the presence of 1, gives toluene and 1,2-diphenylethane. The latter is selectively formed with KOH instead of sodium formate. Complex 1 also reacts with trans-1,2-dichlorocyclohexane and 3 i dichloromethane. The reaction with the former gives [RhCl3(κ -P,O,P-{xant(P Pr2)2})] (5) and cyclohexene, whereas complex 1 3 i undergoes oxidative addition of dichloromethane to afford cis-dichloride-[Rh(CH2Cl)Cl2(κ -P,O,P-{xant(P Pr2)2})] (6a), which evolves into its isomer trans-dichloride 6b. The kinetic study of the overall process suggests that the oxidative addition is cis- concerted and the isomerization an intramolecular reaction which takes place through a σ-C-Cl intermediate with two confor- mations. implication in a wide range of interesting organic reactions,13 INTRODUCTION as well as in the dehydrocoupling and dehydropolymerization 14 Oxidative addition to transition metal complexes is one of of amine-boranes. In agreement with this, we have recently 3 the most relevant procedures for the activation of σ-bonds.1 shown that the square-planar monohydride [RhH(κ -P,O,P- i i The oxidative addition of C-X bonds of organic halides to {xant(P Pr2)2})] (xant(P Pr2)2 = 9,9-dimethyl-4,5- basic unsaturated metal centers has particular interest by its bis(diisopropylphosphino)xanthene) undergoes the sterically connection with the catalytic formation of C-C bonds2 and governed C-Cl bond cis-oxidative addition of chlorobenzene, with the metal-catalyzed degradation of these substrates.3 The chlorotoluenes, chlorofluorobenzenes and di- and trichloro- latter is a priority target from an environmental point of view, benzenes to afford rhodium(III) derivatives, which experience 3 since their accumulation is a serious health hazard.4 The C-X dehydrochlorination to give a wide range of [Rh(aryl)(κ - i 15 bond enthalpy increases as we go down the group in the peri- P,O,P-{xant(P Pr2)2})] complexes (Scheme 1). odic table. Thus, the C-Cl rupture is more challenging than the C-Br and C-I bonds activations. However, chlorides are most Scheme 1. C-Cl Bond Activation of Halogenated Arenes interesting to working due to lower cost and wider diversity. Most metal-promoted C-C coupling reactions involve aryl or alkenyl halides. Alkyl halides have been comparatively much less employed, particularly those bearing β-hydrogen atoms5 because of the resulting alkyl intermediates decompose by means of a β-hydride elimination reaction. Palladium(0) com- pounds are the most used catalysts for these reactions.6 Ac- Our interest in dehalogenation and C-C coupling processes cording to this, there is a significant amount of work centered has prompted us to study now the activation of C(sp3)-Cl about the oxidative addition of C(sp3)-X bonds to d10 metal bonds of mono- and dichloroalkanes, with and without β- centers.7 In recent years, examples proving the rhodium effi- hydrogens, promoted by [RhCl(κ3-P,O,P-{xant(PiPr ) })] (1). ciency for coupling of alkyl halides have been also reported,8 2 2 In this paper, we report the oxidative addition of these classes whereas other ones have demonstrated their capacity for of substrates to 1 and a first exploration of the behavior of this dehalogenation reactions.9 In the same vein, the study of the square-planar rhodium(I) complex in the dehalogenation of rhodium-mediated C(sp3)-X bond activation reactions is these organic compounds and towards the use of some of them awakening notable interest.10 for C-C coupling reactions. Neutral POP diphosphines are hemilabile pincer ligands, which have the ability of adapting their coordination mode to the requirements of the each particular species.11 As a conse- quence of this flexibility POP-rhodium derivatives are proving RESULTS AND DISCUSSION a noticeable efficiency in reactions of σ-bond activation12 with Monochloroalkanes. Complex 1 activates the C-Cl bond of noticeable attention as reducing agents in metal-mediated chlorocyclohexane. However, the presence of four β-hydrogen hydrodehalogenation reactions.17 atoms in the substrate destabilizes the resulting alkyl interme- diate, which undergoes a β-hydride elimination reaction (Scheme 2). Thus, complex 1 affords cyclohexene and the 3 i rhodium(III) monohydride [RhHCl2(κ -P,O,P-{xant(P Pr2)2})] (2) in chlorocyclohexane as solvent. According to the 31P{1H}NMR spectrum of the solution, the reaction is quanti- tative after 24 h, at 100 ºC. Attempts for detecting and charac- terizing the alkyl intermediate A were unsuccessful, even at room temperature. Under these conditions, complex 2 was also the only detected species, although its formation is excessively slow. Scheme 2. C-Cl Bond Activation of Chlorocyclohexane Figure 1. Molecular diagram of complex 2 (ellipsoids shown at 50% probability). All hydrogen atoms (except the hydride) are omitted for clarity. Selected bond distances (Å) and angles (deg): Rh-P(1) = 2.3054(7), Rh-P(2) = 2.3063(7), Rh-Cl(1) = 2.3438(6), Complex 2 was isolated as pale yellow crystals and charac- Rh-Cl(2) = 2.3412(6), Rh-O(1) = 2.2591(16); P(1)-Rh-P(2) = terized by X-ray diffraction analysis. Figure 1 shows a view of 163.49(2), Cl(1)-Rh-Cl(2) = 177.44(2), P(1)-Rh-O(1) = 82.26(5), the structure. As expected for a pincer coordination of the P(2)-Rh-O(1) = 83.04(5), O(1)-Rh-H(01) = 176.0(9). diphosphine, the (POP)Rh skeleton is T-shaped with the metal center situated in the common vertex and P(1)-Rh-P(2), P(1)- Rh-O(1), and P(2)-Rh-O(1) angles of 163.49(2)º, 82.26(5)º, and 83.04(5)º, respectively. Thus, the coordination polyhedron around the rhodium atom can be described as an octahedron with the hydride disposed trans to the oxygen atom of the diphosphine (O(1)-Rh-H(01) = 176.0(9)º) and the chloride Scheme 3. Dehalogenation of Chlorocyclohexane ligands laid mutually trans (Cl(1)-Rh-Cl(2) = 177.44(2)º. This is also evident in the 1H and 13C{1H} NMR spectra of the crystals, in benzene-d6, at room temperature, which display two signals for the methyl groups of the phosphine isopropyl substituents (δ1H, 1.49 and 1.45; δ13C, 21.9 and 19.6) and a signal for the methyl substituents of the central heterocycle (δ1H,1.21; δ13C, 30.9). In agreement with the presence of the hydride, the 1H NMR spectrum contains at -19.79 ppm a dou- 1 2 blet of triplets with JH-Rh and JH-P coupling constants of 13.6 i and 11.5 Hz, respectively. As expected for equivalent P Pr2 groups, the 31P{1H} NMR spectrum shows at 42.2 ppm a 1 doublet, which display a typical JP-Rh(III) coupling constant of 98.9 Hz. Chlorocyclohexane reacts with sodium formate to give cy- clohexane, NaCl, and CO2, in the presence of 1 (eq 1). The dehalogenation is catalytic. Using 1.0 mol% of complex 1 and 1.2 equiv of sodium formate in 2-propanol, the cycloalkane is formed in 85% yield, after 24 h, under reflux. Acetone is not observed during the reaction, indicating that the solvent does not participate in the dehalogenation. The formation of 2 and cyclohexene, according to Scheme 2, is consistent with this Complex 1 also activates the C-Cl bond of benzyl chloride. fact and should constitute the first part of the catalysis. Thus, Treatment of toluene solutions of this compound with 2.0 the replacement of one of the chloride ligands by the formate equiv of the chloroalkane, at room temperature, for 7 h quanti- anion could afford the intermediate B, which should release tatively leads to the benzyl derivative [Rh(CH Ph)Cl (κ3- CO to give the previously described dihydride [RhH Cl(κ3- 2 2 2 2 P,O,P-{xant(PiPr ) })] (4), as a result of the oxidative addition P,O,P-{xant(PiPr ) })] (3).12a The hydrogenation of cyclohex- 2 2 2 2 of the C(sp3)-Cl bond of the organic substrate to the metal ene by the latter would lead to the cycloalkane and to regener- center of 1 (Scheme 4). In contrast to A, complex 4 is stable ate the catalyst (Scheme 3). Formate salts are promising chem- and was isolated as a beige solid in 70% yield. The presence ical hydrogen carriers.16 As a consequence, they are receiving of a coordinated benzyl group in the new species is strongly 1 13 1 supported by the H and C{ H} NMR spectra of the obtained beige solid, in benzene-d6, at room temperature, which display 2 3 a doublet of triplets at 5.64 ( JH-Rh = 3.4 Hz and JH-P = 4.1 Hz) Scheme 5.