Hydroamination of Unactivated Alkenes Catalyzed ... - ACS Publications

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Hydroamination of Unactivated Alkenes Catalyzed by Novel Platinum(II) N-Heterocyclic Carbene Complexes Peng Cao,† José Cabrera,† Robin Padilla,† Daniel Serra,† Frank Rominger,‡ and Michael Limbach*,†,§ †

CaRLaCatalysis Research Laboratory, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany Organisch-Chemisches Institut, Im Neuenheimer Feld 270, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany § BASF SE, Basic Chemicals Research, GCS/C − M313, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany ‡

S Supporting Information *

ABSTRACT: Cationic platinum(II) complexes with bi- or tridentate (pincer) functionalized NHC ligands were found to be catalytically active in the hydroamination of unactivated alkenes. In some cases, the presence of water had an activating effect on the complexes. Reactions with the Nnucleophilic substrate morpholine led to a noncatalytic reaction in which the deprotonation product of the key cationic β-aminoalkyl platinum complex could be isolated and characterized. Surprisingly, attempted protonation of this complex did not give the expected N-alkylated product, indicating either the thermodynamic unfavorability of C−Pt bond cleavage or its kinetic inertness.

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INTRODUCTION In petrochemistry, the catalytic hydroamination of alkenes is the most direct and atom economical access to alkylamines starting from substrates available from the steam cracking process. Besides a plethora of heterogeneous systems,1 there are various homogeneous catalysts available, for example, those based on coinage metals (Ag,2 Au3), platinoids (Rh, Ir,4 Ru,5 Pd6), group 12 metals (Zn7), Ni,8 and lanthanides.9 Platinumbased complexes in particular have served as excellent models for this reaction1e,10−12 and fall into three classes: (a) anionic systems, such as K[PtCl3(C2H4)]·H2O (Zeise’s salt), K[PtBr3(C2H4)] (Chojnacki’s salt), or Brunet’s and RodriguezZubiri’s salts prepared in situ from K 2[PtBr4] and a phosphonium halide; 13 (b) neutral systems, such as [PtCl2(C2H4)2]2, that have been introduced by Widenhoefer14 and others;15 and, last, (c) cationic systems with chelating olefin,16 diamine,17 or pincer ligands.18 Although significant progress has been made in the hydroamination of activated alkenes and intramolecular hydroamination reactions of functionalized alkenes, the intermolecular hydroamination of nonactivated olefins, such as ethylene or propylene, remains a challenge19 with the scope of nucleophiles often limited to weakly basic amines, such as electron-deficient anilines, arylsulfonamides, and carboxamides.3h,4,13c,e,20 We recently reported that platinum(II) complexes with CNC pincer ligands could act as selective catalysts for the hydrovinylation of ethylene.21 Changing our focus from carbon to amine nucleophiles, we reasoned that these CNC pincerbased complexes as well as some new Pt(II) complexes with Nfunctionalized N-heterocyclic carbene ligands (Scheme 1) would also serve as promising catalysts in the intermolecular hydroamination of unactivated alkenes, such as ethylene. © 2012 American Chemical Society

RESULTS AND DISCUSSION Complex Synthesis and Structure. The neutral platinum(II) complexes 3 and 4 bearing an N-functionalized NHC and the cationic CNC pincer complexes 5 and 621 were synthesized (Scheme 1) from PtBr2 by metal exchange with the in situ generated silver carbenes of the corresponding imidazoliumhydrobromides 1 (n = 0)22 and 2 (n = 1).23 The yields obtained for complexes 3 and 4 were remarkable (40% and 45%) if one considers that bidentate picoline-functionalized benzimidazolium-2-ylidene ligands (those are structurally analogous to 1 and 2) coordinate 2-fold to form the coordinatively saturated Pt(II) center.24 The NHC−PtCl2 complex 7 was synthesized according to a literature procedure.25 The protons of the methylene bridge in 4 are diastereotopic and resonate at δ 5.49 as a dd system with a 2J coupling constant of 16 Hz, which is close to those of related systems.24 This property has been observed for analogous platinum21 and palladium26 pincer complexes, due to the diastereotopicity that results from the complexes’ twisted conformation. Crystals of 3 (five-membered chelate, n = 0) and 4 (sixmembered chelate, n = 1) were grown from CHCl3 and CH2Cl2 (Figure 1). Both complexes give satisfying elemental analyses, the latter one if one considers that the elementary cell contains 2 equiv of complex 4 and 1 equiv of CH2Cl2. Both X-ray structures prove the cis configuration of the bromido ligands and the slightly distorted square-planar geometry of the platinum center. The chelating pyridinylNHC ligand of 3 has a typical bite angle of 79.6(1)° deviated from 90° due to the geometrical strain. The pyridine and Received: October 11, 2011 Published: January 18, 2012 921

dx.doi.org/10.1021/om200964u | Organometallics 2012, 31, 921−929

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Scheme 1. Synthesis of Platinum Pincer Complexes 3 and 4 and Structures of Complexes 5−7 Employed in This Study

Figure 1. X-ray structure of 3 and 4 with thermal ellipsoids drawn at 50% probability and superimposition of both structures. Hydrogen atoms have been omitted for clarity.

93.10(13)°) bond angles of 3 are significantly smaller if compared to an analogous Pd complex.22e Solvent Dependence of the Hydroamination Reaction. In the presence of ethylene and benzamide, complex 5 was reacted with 2 equiv of AgBF4 to form in situ the corresponding dicationic platinum(II) complex and the hydroamination product N-ethylbenzamide (Scheme 2, Table 1).

imidazole rings are almost coplanar, as reflected by the interplanar angle of 5.3°, whereas the mesityl ring is almost perpendicular to the imidazole ring (interplanar angle of 86.8°). Thus, the five-membered chelate ring in 3 formed by coordination of Pt, NPy, and Ccarbene is nearly planar. In contrast, the analogous six-membered ring chelate in 4 is nonplanar with a boatlike conformation. The superposition of both complexes (cf. Figure 1) supports that the ligand conformations are completely different, but the coordination geometries of both platinum centers are rather similar. Both, for 3 and 4, the Pt−Ccarbene (1.954(3) vs 1.959(4) Å) and the cisPt−Br bonds (2.4869(4) vs 2.4874(4) Å) have similar lengths, indicating similar donor properties of both ligands, and do not differ significantly from the distances in similar palladium NHC complexes described by Crabtree et al.22e (Pd−Ccarbene, 1.978(6) Å; cis-Pd−Br, 2.4816(8) Å). The same is true for the Pt−NPy (2.037(3) vs 2.042(3) Å) and the trans-Pt−Br bonds (2.4127(4) vs 2.4208(4) Å). The angles (Br−Pt−Br, 89.019(4)° vs 89.606(15)°; N−Pt−Br, 79.58(13)° vs 87.84(13)°) are larger for 4 if compared to 3, which reflects the larger chelate ring size on the ligand side and the reduced steric demands due to the tilted ligand rings on the bromine side. The Pt−NPy (2.037(3) vs 2.062(5) Å) and the trans-Pt− Br (2.4127(4) vs 2.4159(8) Å) bonds as well as the Br−Pt−Br (89.019(4)° vs 89.12(3)°) and N−Pt−Br (79.58(13)° vs

Scheme 2. Influence of Solvent on the Hydroamination of Benzamide with Ethylenea

a

For details, see Table 1.

The reaction outcome depends strongly on the solvent employed. In polar, noncoordinating solvents, such as nitromethane (εr = 39.4 [20 °C]) and nitrobenzene (εr = 35.7 [20 °C]), the catalyst shows the highest relative activity (92% and 100% conversion, respectively) and N-ethylbenzamide is obtained in high selectivity (90% and 92%, respectively, Table 1, entries 8 and 9). As a consequence of the higher conversion in nitrobenzene, overalkylation to N,N-diethylbenzamide was also observed. 922


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Table 1. Influence of Solvent on the Hydroamination of Benzamide with Ethylenea entry

solvent

t (h)

conv (%)b

1 2 3 4 5 6 7 8 9

1,4-dioxane-d8 THF-d8 DMSO-d6 CD2Cl2 CDCl3 tetrachloroethane-d2 C6D4Cl2 CD3NO2 C6D5NO2

96 96 48 96 96 96 96 96 96

0 9 0 60 7 41 35 92 100

Table 2. Effect of the Silver Salt on Conversion and Selectivitya

sel (%)b 89 63 61 66 71 90 92

entry

solvent

AgX

conv (%)b

sel (%)b

1 2 3 4 5 6 7

CD3NO2 C6D5NO2 C6D5NO2c C6D5NO2 C6D5NO2 C6D5NO2 C6D5NO2

AgBF4 AgBF4 AgBF4 AgOTf AgSbF6 AgPF6 AgN(Tf)2

53 75 60 37 66 55 71

85 88 92 78 98 95 87

a

Conditions: A high-pressure tube was charged with 5 (9.6 mg, 12.2 μmol), benzamide (15 mg, 122 μmol), the corresponding silver salt (22.4 μmol), and deuterated solvent (0.5 mL) in the glovebox under argon. The tube was degassed and then pressured with ethylene (5.5 bar). After standing at r.t. for 2 h, the tube was heated to 100 °C for 24 h. bDetermined by GC, acetophenone as an internal standard. cWith 0.5 equiv of H2O.

a

Conditions: A high-pressure tube was charged with 5 (9.6 mg, 12.2 μmol), benzamide (15 mg, 122 μmol), AgBF4 (4.8 mg, 22.4 μmol), and deuterated solvent (0.5 mL) in the glovebox under argon. The tube was degassed and then pressured with ethylene (5.5 bar). After standing at r.t. for 2 h, the tube was heated to 100 °C for the given time. bDetermined by GC, acetophenone as an internal standard.

Scheme 3. Influence of Ag Loading on the Hydroamination of Benzamide with Ethylenea

In chlorinated solvents, the catalyst activity (i.e., conversion) was low. It does not follow the polarity (i.e., relative dielectric constants εr) observed for nitroalkanes or nitroarenes and decreases in the order CD2Cl2 (εr = 8.93 [25 °C]) > C2D2Cl4 (εr = 10.36 [25 °C]) > C6D4Cl2 (εr = 2.8 [53 °C]) > CDCl3 (εr = 4.8 [20 °C]) (60, 41, 35, and 7% conversion after 96 h at 100 °C). The selectivity in chlorinated solvents was also lower (27− 71%) when compared with that of nitroalkanes or nitroarenes. The major byproduct was benzonitrile (Table 1, entries 1, 3, 6, and 9), which suggests a (Lewis) acid catalyzed consecutive reaction of N-ethylbenzamide either initiated by decomposition of the chlorinated solvents (i.e., dichlorocarbenes are known active catalysts in the dehydration of amides to nitriles)27 or as a consequence of the intrinsic Lewis acidity of the complexes.28 We attribute the low or, in some cases, nonexistent reactivity of 5 in polar, aprotic solvents, such as dioxane (εr = 2.25 [20 °C]), THF (εr = 7.58 [25 °C]), and DMSO (εr = 46.7 [20 °C]) (entries 1−3), to the ability of those solvents to coordinate as strong O donors21 to the cationic platinum species, thus blocking the free coordination site for further reactions. Influence of Ag Source on the Hydroamination Reaction. The choice of silver salt had a minor effect on the selectivity of the reaction (85−98%) but influenced the catalyst’s activity significantly. With salts of non- or weakly coordinating anions (AgBF4, AgPF6, AgSbF6, AgN(Tf)2), a higher conversion was obtained than with a silver salt bearing a coordinating anion (e.g., 37% conv with AgOTf vs 75% with AgBF4; cf. Scheme 2 and Table 2, entries 2 and 4). It is most likely that, with triflate in situ, the corresponding Pt−OTf complex21 forms readily, and the alkene, a rather poor donor, competes for a free coordination site on platinum. Surprisingly, the system was found to be rather tolerant to moisture (vide infra, Table 2, entry 3). Influence of the Molar Ratio [Ag]/[Pt] and the Type of Pt Complex on the Reactivity. For complex 3, conversion and selectivity in N-ethylbenzamide was maximized with 1 equiv of AgBF4 with respect to the complex (83% and 95%, respectively; cf. Scheme 3 and Table 3, entry 2). A blind test confirmed that no hydroamination reaction occurred without any silver salt or platinum complex. A 2-fold excess of AgBF4 (with respect to the complex) led by NMR to quick decomposition of 3, producing N-ethylbenzamide in low

a


Table 3. Influence of Ag Loading on the Hydroamination of Benzamide with Ethylenea entry

Pt complex

AgBF4 (mol %)

T [°C]

conv (%)b

1 2 3 4d 5 6 7 8 9 10 11 12 13 14 15d

3 3 3 3 4 4 5 5 6 7 7 7 3 5 5

0 10 20 20 10 20 10 20 20 0 10 20 10 20 20

100 100 100 100 100 100 100 100 100 100 100 100 150 150 100

0 83 26 74 (86, 38) 75 15 0 75 21 0 0 0 99 100 60 (59, 45)

sel (%)c,d 95 31 85 85 13

[0] [0] (92, 53) [0] [0]

88 [0] 57 [0]

88 [5] 86 [12] 92 (93, 67)

a

Conditions: A high-pressure tube was charged with the complex (12.2 μmol), benzamide (15 mg, 122 μmol), the corresponding silver salt, and deuterated solvent (0.5 mL) in the glovebox under argon. The tube was degassed and then pressured with ethylene (5.5 bar). After standing at r.t. for 2 h, the tube was heated to 100 °C for 24 h. b Determined by GC. cSelectivity toward N,N-diethylbenzamide is indicated in square brackets. d0.5 (1, 2) equiv of H2O was added.

conversion and selectivity (26% and 31%, respectively, Table 2, entry 3). Complex 4 turned out to be only slightly less reactive (75% vs 83%) and selective (85% vs 95%) than 3 under the same conditions (Scheme 3; Table 3, cf. entries 2 and 5). The cationic CNC pincer complexes 5 and 6 required 2 equiv of the Ag source per platinum center for optimal reactivity (75% and 21%, respectively) and selectivity (88% and 57%, respectively). 923


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Once again, the platinum(II) complexes 3 and 5, where the geometry of the ligand is coplanar with the metal coordination plane, performed superior to complexes 4 and 6 (75% vs 21% conversion and 88% vs 57% selectivity, respectively; cf. Table 3, entries 8 and 9), where the ligands pucker out of the metal’s coordination plane. Interestingly, neither the neutral complex 7 bearing a monodentate NHC ligand nor its cationic form showed any reactivity in the hydroamination reaction (Table 3, entries 10−12). Increasing the reaction temperature from 100 to 150 °C under otherwise identical conditions enhanced the activity of complex 3 in benzamide (99% conversion vs 83% at 100 °C) at the expense of N-ethylbenzamide selectivity. Despite the still high selectivity for N-ethylbenzamide (88%), at 150 °C, N,Ndiethylbenzamide was formed with 5% selectivity (cf. Table 3, entries 13 and 2), whereas at 100 °C, only monoalkylated product was formed with 95% selectivity. The conversion obtained with complex 5 was even more sensitive to the temperature increase (100% vs 75% conversion). Although the selectivity to the target product N-ethylbenzamide was similar for complexes 3 and 5 (86% vs 88% selectivity, cf. Table 3, entries 14 and 8), the latter complex formed more than twice the amount of N,N-diethylbenzamide by overalkylation (5% vs 12%, Table 3, entries 13 and 14). Reduction of the catalyst loading from 10 to 5 mol % increased both the activity and the selectivity of complex 5 (75% conv, 88% sel vs 98% conv, 93% sel; cf. Table 3, entry 8, vs Table 4, entry 7). At high catalyst loadings, in the 1H NMR of the crude reaction product, reasonable amounts of signals, which we assigned to a species of type C (Scheme 6), were found, which could explain the differences in activity and selectivity. Influence of Water on the Hydroamination Reaction. Interestingly, we observed an activating effect of water on the hydroamination reaction, but only for reactions catalyzed by complex 3: Under rigorous exclusion of H2O, N-ethylbenzamide was formed with moderate activity and selectivity (26% and 31%, 2 equiv of AgBF4, C6H5NO2, 100 °C, 24 h). Upon addition of H2O (0.5 and/or 1 equiv with respect to 3), conversion (selectivity) increased to 74 (85%) and even 86 (92%) but again dropped significantly to 38 (53%) in the presence of 2 equiv of H2O (cf. Scheme 3 and Table 3, entries 3 and 4). No or even the opposite effect on conversion (selectivity) was found for complex 5 (0/0.5/1/2 equiv of H2O = 75% conversion [88% selectivity], 60 [92%], 59 [93%], 45 [67%]). In the crude reaction mixture, a monomeric (or μ-OH bridged dinuclear) Pt complex derived from 3 was identified by ESI-HRMS (HRMS for C17H18N3OPt calculated: 475.10935. Found: 475.10918), albeit the integration and multiplicity of its 1 H NMR spectrum did not match. Cationic μ-OH-bridged dimers of the type {[Pt(μ-OH)(L-L)]2}2+(L = ligand) have previously been described for bidentate phosphine ligands and NCN pincer ligands,29 and some of them, apart from their increased reactivity toward ethylene in H2O,30 play a key role in the platinum-catalyzed Baeyer−Villiger oxidation.31,32 Alternatively, in the case of a bidentate ligand system, such as in 3, 1 equiv of H2O could stabilize a tricoordinated (alkenebound) intermediate, whereas a second equivalent of H2O could block the catalytically active site to form a more robust PtL4 complex. In the case of a tridentate ligand, such as in 5, already the first equivalent of H2O could occupy the fourth coordination site and hence has an inhibiting effect.

Table 4. Substrate Scope of the Hydroamination of Ethylene by Platinum Pincer Complexesa,b,c,d,e,f

a Condition A: 5 (9.8 mg, 10 mol %), AgBF4 (4.9 mg, 20 mol %), amide (122 μmol), and nitrobenzene-d5 (0.5 mL) were charged in the high-pressure NMR tube. Condition B: 3 (7.5 mg, 10 mol %), AgBF4 (2.4 mg, 10 mol %), amide (122 μmol), and nitrobenzene-d5 (0.5 mL) were charged in the NMR tube. bGC conversion. cNMR or GC yield. d Numbers in parentheses refer to selectivity for diethylamide. eYield of N-ethylpyrrolidin-2-one, as starting material and product were unseparable. f5 mol % catalyst, 10 mol % Ag salt.

Substrate Scope and Limitations. The scope of amide substrates was investigated under conditions optimal for each complex class (i.e., condition A for complex 5 and condition B for complex 3, Scheme 4 and Table 4). Under condition A, Scheme 4. Substrate Scope of the Hydroamination of Ethylene by Platinum Pincer Complexesa

a


primary alkylamides, for example, cyclohexanecarboxamide, benzamide, and heptamide, were converted with high activity and selectivity. The highly reactive substrates cyclohexanecarboxamide and heptamide were hydroaminated twice to form considerable amounts of N,N-diethylamide (Table 4, entries 1 and 2, with 20% and 38% selectivity, respectively). In turn, under condition B, primary sulfonamides, for example, ptoluenesulfonamide, were significantly more reactive and selective (45% vs 20% conversion, 96% vs 85% selectivity, Table 4, entry 6) than under condition A. In the case of secondary amides, that is, lactams, the reactivity depends very much on their ring size. The rate 924


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increased from γ-butyrolactam over δ-valerolactam to εcaprolactam under condition A, but the relative reactivity reversed under condition B. All three N-alkylated lactams were formed in fair to good conversion (38−98%) and in high selectivity (90−97%, Table 4, entries 3, 4, and 5). Difficult substrates,33,34 that is, N-nucleophiles with pKa > 4.5 (e.g., aniline, morpholine, or ammonia), did not react with ethylene to give the corresponding hydroamination products. With aniline at 150 °C, for example, Friedel−Crafts alkylation was the major reaction pathway under both conditions A and B (29% and 48% conversion, respectively).35 A similar reactivity has previously been observed by Brunet et al. with the PtBr2/nBu4PBr system.13c Regarding alkenes other than ethylene, the reactivity of propylene and styrene toward benzamide was lower than for ethylene (Scheme 5). In both cases, 5 (condition A) was more

Scheme 6. Proposed Mechanism of the Platinum-Catalyzed Hydroamination of Alkenes

Scheme 5. Catalytic Hydroamination of Alkenes Other Than Ethylene with Ratios of Branched (b) and Linear (l) Products

solution of 8 under ethylene pressure (5.5 bar) in a pressurized NMR tube, 9 (cf. C in Scheme 6) was formed by nucleophilic attack of morpholine on π-complex 8, but no hydroamination product (i.e., N-ethylmorpholine) was formed (vide supra). Cationic species of type C have been described and isolated for other secondary amines, such as diethylamine.34,38 When a solution of 9 was heated to 80 °C for 2 h, the reaction reversed, and upon cooling the reaction back down to room temperature (r.t.), the cationic species 9 appeared again after 24 h, pointing to a temperature-dependent equilibrium between 8 and 9. On the other hand, 8 was irreversibly converted to complex 10 (cf. D in Scheme 6) by treatment with K2CO3. Upon protonation of the β-aminoalkyl complex 10 with HCl, the nucleophilic addition reversed, giving C−N bond dissociation instead of Pt− C cleavage.18a Suitable crystals for X-ray analysis were obtained by slow diffusion of Et2O into a CH2Cl2 solution of 10 (Figure 2). As expected from the 1H NMR experiment, all four methyl substituents in ortho- and both para-substituted methyl substituents of the mesityl rings in 10 are each equivalent due to the presence of the alkylamine moiety attached to platinum. As with other Pt complexes with the same CNCpincer ligand,21 complex 10 shows a distorted square-planar geometry, and the geometry at the platinum(II) center is defined by two trans-carbenes (Pt−Ccarbene, 2.025(20) and 2.021(9) Å), with the amine moiety in a trans position to the pyridine (Pt−NPy, 1.991(4) Å). The newly formed Pt−C bond is elongated (2.068(8) Å).

active (54% vs 40% conv.) and selective (82% vs 80%) than 3 (condition B), and in both cases, the Markovnikov-addition product was formed with high selectivity (7−10:1 for propylene). Styrene, as a relatively activated substrate, astonishingly exhibited the lowest selectivity for the desired hydromination products (A, 54%; B, 22%) due to competing oligomerization reactions.21 Thus, low yields of alkylated benzamide were obtained (A, 27%; B, 5%). Nevertheless, in the hydroamination reaction, exclusively the Markovnikovaddition product was formed (Scheme 5). Mechanistic Discussion. Although an inner-sphere mechanism could be operational, involving, first, the coordination of the nucleophile to the Pt center (or the unlikely oxidative addition of the nucleophile to Pt(0)36), and subsequent 1,2migratory insertion of an alkene into the Pt−nucleophile bond and subsequent protonolysis (or reductive elimination), the generally accepted mechanism for the platinum-catalyzed hydroamination (Scheme 6)10d,37 involves coordination of an alkene to an electrophilic Pt center A to give platinum complex B with η2-bound ethylene (step 1). This step is followed by an outer-sphere attack by the protic nucleophile to form cation C (step 2) and protonolysis of the newly formed Pt−C bond in D (steps 3 and 4). The stoichiometric reaction of 5 with 2 equiv of AgBF4 in nitromethane under an atmosphere of ethylene (5 bar) gave ethylene complex 8 (cf. B in Scheme 6), which has been isolated and characterized previously (Scheme 7).21 When 1.5 equiv of morpholine was added to an ethylene-saturated

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CONCLUSIONS In conclusion, a novel class of platinum(II) complexes has been obtained based on bidentate N-heterocyclic carbene ligands. The corresponding mono- and dicationic platinum(II) species catalyzed the hydroamination of unactivated alkenes, such as ethylene, propylene, and styrene, with weakly basic carbox- and sulfonamides with exclusive or very high Markovnikov selectivity. Water increased the reactivity of certain complexes in the hydroamination reaction. Unproductive substrates, such as morpholine, enabled the isolation and X-ray characterization of a cationic species, which originates from deprotonation of the key β-zwitterionic species and further supports the commonly accepted mechanistic pathway. The search for complexes that enable the transformation of difficult amines in 925


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Scheme 7. Isolation of Intermediates in the Hydroamination of Alkenes with Amides, That Is, 821 and β-Aminoalkylammonium Complex 10

oil on nylon loops and cooled immediately on the goniometer head. Data collections were performed with Mo Kα radiation (graphite monochromator) on a Bruker Smart CCD or a Bruker APEX at 200 K. Structures were solved by direct methods and refined by full-matrix least-squares against F2. All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were included in calculated positions. Calculations were performed using the SHELXTL software package.39 GC analyses were carried out on an Agilent 6890N modular GC base equipped with a split-mode capillary injection system and a flame ionization detector using a standard HP-5 capillary column (30 m × 0.32 mm × 0.25 mm; He flow, 2.0 mL/min; program, 50 °C (1 min), 20 °C (20 °C/min to 250 °C (10 min)). GCMS analyses were carried out on an Agilent 19091S-433 modular GC base equipped with a split-mode capillary injection system and a flame ionization detector using a standard HP-5 capillary column (30 m × 0.25 mm × 0.25 mm; He flow, 0.6 mL/min; program, 50 °C (1 min), 20 °C (20 °C/min to 250 °C (10 min)). Melting points were measured on a Stuart SMP30 apparatus. Elemental analyses and mass spectra were obtained from the Organic Chemistry Institute of the University of Heidelberg. Ethylene and propylene were purchased with 99% purity from Aldrich. Morpholine, aniline, and styrene were distilled under an argon atmosphere before being dried with CaH2. All other starting materials were purchased in reagent-grade purity from Acros, Aldrich, Fluka, or Strem and used without further purification. Complexes 5,21 6,21 and 725 and ligand precusors 122 and 223 were synthesized according to literature procedures. Dibromo[1,3-dihydro-1-(2-pyridinyl)-3-(2,4,6-trimethylphenyl)-2H-imidazol-2-ylidene]platinum(II) (3). To a solution of 1-mesityl-3-(pyridin-2-yl)-1H-imidazol-3-ium bromide (1, 500 mg, 1.45 mmol) in CH2Cl2 (50 mL) was added Ag2O (168.2 mg, 0.73 mmol). After stirring at r.t. for 16 h, the crude reaction was filtered through Celite into a Schlenk tube under an inert atmosphere. PtBr2 (515.5 mg, 1.45 mmol) was added to the clear filtrate, and the reaction mixture was stirred at r.t. for 24 h. After addition of CH2Cl2 (30 mL), the solution was filtered through Celite and the solvent was removed. The residue was precipitated with CHCl3 and dried under vacuum to give 3 as pale yellow solid (356 mg, 40%). mp 330 °C. 1H NMR (200 MHz, DMSO-d): δ 9.76 (br s, 1H), 8.61 (d, J = 2 Hz, 1H), 8.45 (t, J = 8 Hz, 1H), 8.22 (d, J = 8 Hz, 1H), 7.66 (t, J = 6 Hz, 1H), 7.56 (d, J = 2 Hz, 1H), 6.97 (s, 2H), 2.29 (s, 3H), 2.03 (s, 6H). 13C NMR (50 MHz, DMSO-d): δ 152.1, 149.2, 142.2, 138.5, 135.2, 134.3, 128.5, 125.4, 123.4, 117.1, 112.4, 20.7, 17.5. Elemental Anal. Calcd for C17H17N3Br2Pt: C, 33.03; H, 2.77; N, 6.80. Found: C, 33.39; H, 2.96; N, 6.77.

Figure 2. X-ray structure of complex 10 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms have been omitted for clarity.

the hydroamination of unactivated alkenes is a field of further investigation in our lab.

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EXPERIMENTAL SECTION

General. All reactions and manipulations were performed under an argon atmosphere using standard Schlenk techniques. Hexane, diethyl ether (Et2O), tetrahydrofuran (THF), and dichloromethane (CH2Cl2) were dried with an MBraun solvent purification system. Nitrobenzene was distilled under an argon atmosphere before being dried with CaH2. Dimethylsulfoxide (DMSO) and nitromethane (MeNO2) were purchased in anhydrous solvent purity. All solvents were degassed with argon prior to use. All deuterated solvents were degassed via freeze− pump−thaw cycles and stored over molecular sieves (4 Å). 1H and 13 C{1H} NMR spectra were recorded at room temperature (r.t.) on a Bruker 250 spectrometer operating at 200 and 50 MHz, respectively, with chemical shifts (δ, parts per million) reported relative to the solvent peaks (1H NMR, 13C NMR). For X-ray crystal structure analyses, suitable crystals were mounted with perfluorinated polyether 926


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Yellow crystal (polyhedron); dimensions, 0.21 × 0.16 × 0.06 mm3; crystal system, triclinic; space group, P1̅; Z = 2; a = 7.8478(2) Å, b = 9.1844(3) Å, c = 15.5550(5) Å, α = 75.848(1)°, β = 79.524(1)°, γ = 89.988(1)°; V = 1067.86(6) Å3; ρ = 2.187 g/cm3; T = 200(2) K; θmax = 27.47°; radiation, Mo Kα; λ = 0.71073 Å; 0.3° ω-scans with a CCD area detector, covering a whole sphere in reciprocal space; 10815 reflections measured, 4822 unique (R(int) = 0.0271), 4334 observed (I > 2σ (I)). Intensities were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using SADABS40 based on the Laue symmetry of the reciprocal space, μ = 10.57 mm−1, Tmin = 0.22, Tmax = 0.57. The structure solved by direct methods and refined against F2 with a full-matrix least-squares algorithm using the SHELXTL (version 2008/4) software package; 238 parameters refined. Hydrogen atoms were treated using appropriate riding models. Goodness of fit, 1.06 for observed reflections; final residual values, R1(F) = 0.023, wR(F2) = 0.052 for observed reflections; residual electron density, −0.95 to 0.90 e Å−3. Dibromo[1,3-dihydro-1-(2-pyridinylmethyl)-3-(2,4,6-trimethylphenyl)-2H-imidazol-2-ylidene]platinum(II) (4). To a solution of 1-(2-pyridinylmethyl)-3-mesitylimidazolium bromide (2, 107 mg, 299 μmol) in CH2Cl2 (20 mL) was added Ag2O (35 mg, 152 μmol). After stirring at r.t. for 16 h, the crude reaction was filtered through Celite to a Schlenk tube under an inert atmosphere. PtBr2 (106 mg, 299 μmol) was added to the clear filtrate, and the reaction mixture was stirred at r.t. for 72 h. After addition of CH2Cl2 (30 mL), the solution was filtered through Celite, and the solvent was concentrated (5 mL). The residue was precipitated with Et2O and dried under vacuum to give 4 as a white powder (85 mg, 45%). mp 328 °C. 1H NMR (200 MHz, DMSO-d): δ 9.29 (d, J = 6 Hz, 1H), 8.15 (t, J = 8 Hz, 1H), 7.78 (d, J = 8 Hz, 1H), 7.70 (d, J = 2 Hz, 1H), 7.53 (t, J = 6 Hz, 1H), 7.27 (d, J = 2 Hz, 1H), 6.95 (d, J = 6 Hz, 2H), 5.49 (ABd, J = 16 Hz, 2H), 2.27 (s, 3H), 2.12 (s, 3H), 1.82 (s, 3H). 13 C NMR (100 MHz, DMSO-d): δ 155.1, 152.8, 141.1, 139.9, 137.8, 135.3, 135.2, 133.9, 128.6, 128.5, 125.9, 125.2, 123.6, 121.4, 54.6, 20.6, 18.5, 17.6. HRMS (FAB) for C18H19N3Br2Pt (m/z): calcd 631.9573. Found 631.9557. Elemental Anal. Calcd for C37H42N6Br4Pt2 × CH2Cl2: C, 34.19; H, 3.03; N, 6.65. Found: C, 32.77; H, 3.06; N, 6.05. Colorless crystal (polyhedron); dimensions, 0.26 × 0.20 × 0.08 mm3; crystal system, monoclinic; space group, P21/c; Z = 4; a = 8.591(2) Å, b = 19.517(5) Å, c = 13.841(3) Å, α = 90°, β = 92.285(6) °, γ = 90°; V = 2318.9(10) Å3; ρ = 2.035 g/cm3; T = 200(2) K; θmax = 28.34°; radiation, Mo Kα; λ = 0.71073 Å; 0.3° ω-scans with a CCD area detector, covering the asymmetric unit in reciprocal space with a mean redundancy of 2.9 and more than 99% completeness; 17094 reflections measured, 5762 unique (R(int) = 0.0372), 4898 observed (I > 2σ(I)). Intensities were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using SADABS40 based on the Laue symmetry of the reciprocal space, μ = 9.60 mm−1, Tmin = 0.19, Tmax = 0.51. The structure was solved by direct methods and refined against F2 with a full-matrix least-squares algorithm using the SHELXTL (version 2008/4) software package; 258 parameters refined. Hydrogen atoms were treated using appropriate riding models. Goodness of fit, 1.06 for observed reflections; final residual values, R1(F) = 0.032, wR(F2) = 0.070 for observed reflections; residual electron density, −1.19 to 1.79 e Å−3. General Procedure for the Hydroamination of Unactivated Alkenes: NMR Experiments. A thick-walled NMR tube was charged with platinum complex (12.2 μmol), AgBF4 (24.4 μmol), amide (122 μmol), and the deuterated solvent (0.5 mL) in the glovebox. After degassing via freeze−pump−thaw cycles, the tube was pressured with ethylene or propylene (5.5 bar) and then shaken. After 2 h at r.t., the mixture was heated at 150 °C for 24 h. After cooling, the gas was released and the mixture was filtered through Celite. The solution was diluted to 2 mL with deuterated MeOH, followed by addition of Nphenylacetamide (20 mg, inner standard). Conversion was determined by GC and yield by 1H NMR. General Procedure for the Hydroamination of Unactivated Alkenes: Autoclave Experiments. An autoclave (100 mL) was charged with complex 5 (122 μmol), AgBF4 (244 μmol), Nnucleophile (1.22 mmol), and dry nitrobenzene (10 mL) in the

glovebox. The system was pressured with ethylene (5.5 bar) and stirred (600 rpm) at r.t. for 2 h and was then heated to 150 °C for 20 h. Consumption of starting material was monitored by GC-MS analysis. The crude reaction mixture was purified by column chromatography to give the N-alkylated nucleophile. N-Ethylcaprolactam: Complex 5 (96 mg, 122 μmol), AgBF4 (48 mg, 244 μmol), caprolactam (138 mg, 1.22 mmol). Column chromatography (hexane/AcOEt = 2:1) gave N-ethylcaprolactam as a pale oil (123.5 mg, 72%). 1H NMR (200 MHz, CDCl3): δ 3.39 (q, J = 6 Hz, 2H), 3.32 (br s, 2H), 2.46 (b, 2H), 1.58−1.67 (m, 6H), 1.07 (t, J = 6 Hz, 3H). 13C NMR (75 MHz, CD3Cl): δ 175.1, 48.9, 42.7, 37.1, 28.6, 23.3,13.0. HRMS(EI) for C8H5ON (m/z): calcd 141.1154. Found 141.1165. N-Ethylbenzamide: 1H NMR (200 MHz, CDCl3): δ 7.74−7.79 (m, 2H), 7.37−7.52 (m, 3H), 6.21 (br s, 1H), 3.50 (qd, J = 8, 2 Hz, 2H), 1.25 (t, J = 8 Hz, 2H). Other analytical data match those previously published.20a N-Ethylhexylamide: 1H NMR (200 MHz, CDCl3): δ 5.52 (br s, 1H), 3.24 (q, J = 8 Hz, 2H), 2.11 (t, J = 8 Hz, 2H), 1.59 (m, 2H), 1.24 (m, 4H), 1.06 (t, J = 8 Hz, 3H), 0.85 (t, J = 8 Hz, 3H). 13C NMR (75 MHz, CD3Cl): δ = 173.1, 36.8, 34.3, 31.4, 22.4, 14.9, 13.9. Other analytical data match those previously published.41 N-Ethylpyrrolidin-2-one: Complex 3 (75 mg, 122 μmol), AgBF4 (24 mg, 122 μmol), pyrrolidin-2-one (104 mg, 1.22 mmol). Column chromatography (pentane/acetone = 2:1) gave N-ethylpyrrolidin-2one as a pale oil (80 mg, 59%). 1H NMR (200 MHz, CDCl3): δ 3.24− 3.38 (m, 4H), 2.35 (t, J = 8 Hz, 2H), 1.90−2.05 (m, 2H), 1.08 (t, J = 8 Hz, 3H). Other analytical data match those previously published.42 N-Ethyl-4-methylbenzenesulfonamide: 1H NMR (200 MHz, CDCl3): δ 7.75 (d, J = 10 Hz, 2H), 7.31 (d, J = 10 Hz, 2H), 4.39 (br s, 1H), 3.01 (m, 2H), 2.43 (s, 3H), 1.10 (t, J = 8 Hz, 3H). Other analytical data match those previously published.43 N-(1-Phenylethyl)benzamide. Complex 3 (19.8 mg, 24.4 μmol), AgBF4 (9.6 mg, 48.8 μmol), styrene (125 mg, 1.2 mmol), benzamide (30 mg, 0.244 mmol), and nitrobenzene (0.5 mL) were added to a Schlenk tube under an argon atmosphere. After stirring at r.t. for 2 h, the mixture was heated at 150 °C for 48 h. Column chromatography of the crude product (hexane/AcOEt = 5:1) gave the title compound as a colorless oil (14.9 mg, 27%). 1H NMR (200 MHz, CDCl3): δ 7.75− 7.79 (m, 2H), 7.44−7.32 (m, 8H), 6.35 (br s, 1H), 5.32 (quint, J = 8 Hz, 1H), 1.58 (d, J = 8 Hz, 3H). Other analytical data match those previously published.14b [(2,6-Pyridinediyl-κN)bis[3-(2,4,6-trimethylphenyl)-1H-imidazol-1-yl-2(3H)-ylidene-κC2]]-[2-(4-morpholinyl)ethyl]platinum(II) Tetrafluoroborate (10). A mixture of 3 (41 mg, 0.05 mmol) and AgBF4 (20 mg, 0.1 mmol) was dissolved in dry MeNO2 in a thick-walled NMR tube. After degassing via freeze−pump−thaw cycles, the tube was pressured with ethylene (5.5 bar) and then shaken. After standing for 12 h at r.t., the reaction mixture was filtered through Celite into a Schlenk flask under an inert atmosphere. The solvent was removed, the residue was dissolved in dry CH2Cl2 (2 mL), and morpholine (6.5 mg, 0.075 mmol) was added. The solution was stirred at r.t. for 12 h, during which time the color changed from yellow to brown. K2CO3 (69 mg, 0.5 mmol) was then added. After 3 h, the mixture was filtered through Celite and the solvent removed. The residue was recrystallized from CH2Cl2/Et2O to give 10 as a yellow solid (15 mg, 36%). mp 235 °C. 1H NMR (200 MHz, CDCl3): δ 8.45 (t, J = 4 Hz, 1H), 8.15 (s, 2H), 7.88 (d, J = 4 Hz, 2H), 7.10 (s, 4H), 7.01 (s, 2H), 3.64−3.75 (m, 4H), 2.42−2.47 (m, 2H), 2.32 (s, 6H), 2.14 (s, 12H), 2.05 (br s, 2H), 1.04−1.26 (m, 4H). HRMS(FAB) for C35H41ON6F4BPt (m/z): calcd 843.3019. Found 843.3029. Yellow crystal (polyhedron); dimensions, 0.12 × 0.10 × 0.05 mm3; crystal system, triclinic; space group, P1̅; Z = 2; a = 11.3575(5) Å, b = 13.0886(6) Å, c = 14.5169(7) Å, α = 89.591(1)°, β = 73.009(1)°, γ = 73.607(1)°; V = 1973.22(16) Å3; ρ = 1.545 g/cm3; T = 200(2) K; θmax= 24.71°; radiation, Mo Kα; λ = 0.71073 Å; 0.3° ω-scans with a CCD area detector, covering a whole sphere in reciprocal space; 16578 reflections measured, 6710 unique (R(int) = 0.0922), 4811 observed (I > 2σ(I)). Intensities were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using 927


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SADABS40 based on the Laue symmetry of the reciprocal space, μ = 3.62 mm−1, Tmin = 0.67, Tmax = 0.84. The structure was solved by direct methods and refined against F2 with a full-matrix least-squares algorithm using the SHELXTL (version 2008/4) software package; 450 parameters refined. Hydrogen atoms were treated using appropriate riding models. Goodness of fit, 0.99 for observed reflections; final residual values, R1(F) = 0.058, wR(F2) = 0.116 for observed reflections; residual electron density, −2.01 to 2.19 e Å−3.

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(c) Dochnahl, M.; Löhnwitz, K.; Lühl, A.; Pissarek, J.-W.; Biyikal, M.; Roesky, P. W.; Blechert, S. Organometallics 2010, 29, 2637−2645. (d) Jenter, J.; Lühl, A.; Roesky, P. W.; Blechert, S. J. Organomet. Chem. 2011, 696, 406−418. (8) Fadini, L.; Togni, A. Chem. Commun. 2003, 30−31. (9) (a) Ryu, J.-S.; Marks, T. J.; McDonald, F. E. Org. Lett. 2001, 3, 3091−3094. (b) Kim, Y. K.; Livinghouse, T.; Horino, Y. J. Am. Chem. Soc. 2003, 125, 9560−9561. (c) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673−686. (d) Zi, G. Dalton Trans. 2009, 42, 9101−9109. (10) Reviews: (a) Brunet, J.-J.; Chu, N.-C.; Rodriguez-Zubiri, M. Eur. J. Inorg. Chem. 2007, 4711−4722. (b) Liu, C.; Bender, C. F.; Han, X.; Widenhoefer, R. A. Chem. Commun. 2007, 3607−3618. (c) Fürster, A.; Davies, P. W. Angew. Chem. 2007, 119, 3478−3519. (d) Chianese, A. R.; Lee, S. J.; Gagné, M. R. Angew. Chem., Int. Ed. 2007, 46, 4042− 4059. (11) Müller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675−704. (12) Brunet, J.-J.; Neibecker, D. Catalytic Hydroamination of Unsaturated Carbon-Carbon Bonds. In Catalytic Heterofunctionalization; Togni, A., Grützmacher, H., Eds.; Wiley-VCH: Weinheim, Germany, 2001; pp 91−141. (13) (a) Brunet, J.-J.; Cadena, M.; Chu, N. C.; Diallo, O.; Jacob, K.; Mothes, E. Organometallics 2004, 23, 1264−1268. (b) Brunet, J. J.; Chu, N. C.; Diallo, O. Organometallics 2005, 24, 3104−3110. (c) Rodriguez-Zubiri, M.; Anguille, S.; Brunet, J.-J. J. Mol. Catal. A 2007, 271, 145−150. (d) Brunet, J.-J.; Chu, N.-C.; Rodriguez-Zubiri, M. Eur. J. Inorg. Chem. 2007, 4711−4722. (e) Dub, P. A.; RodriguezZubiri, M.; Daran, J.-C.; Brunet, J.-J.; Poli, R. Organometallics 2009, 28, 4764−4777. (14) (a) Wang, X.; Widenhoefer, R. A. Organometallics 2004, 23, 1649−1651. (b) Qian, H.; Widenhoefer, R. A. Org. Lett. 2005, 7, 2635−2638. (c) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127, 1070−1071. (d) Bender, C. F.; Hudson, W. B.; Widenhoefer, R. A. Organometallics 2008, 27, 2356−2358. (e) Toups, K. L.; Widenhoefer, R. A. Chem. Commun. 2010, 46, 1712−1714. (15) Lavery, C. B.; Ferguson, M. J.; Stradiotto, M. Organometallics 2010, 29, 6125−6128. (16) Karshtedt, D.; Bell, A. T.; Tilley, T. D. J. Am. Chem. Soc. 2005, 127, 12640−12646. (17) (a) Barone, C. R.; de Pinto, S.; Lorusso, G.; Di Masi, N. G.; Maresca, L.; Natile, G. Organometallics 2010, 29, 4036−4040. (b) Hoover, J. M.; DiPasquale, A.; Mayer, J. M.; Michael, F. E. J. Am. Chem. Soc. 2010, 132, 5043−5053. (18) (a) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organometallics 2002, 21, 1807−1818. (b) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc. 2002, 124, 9038−9039. (c) Campbell, A. N.; Gagné, M. R. Organometallics 2007, 26, 2788− 2790. (19) (a) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2006; pp 657−658. (b) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; University Science Books: Sausalito, CA, 2009; pp 713−714. (20) (a) Wang, X.; Widenhoefer, R. A. Organometallics 2004, 23, 1649−1651. (b) Mothes, E. Organometallics 2004, 23, 1264−1268. (c) Yi, C. S.; Yun, S. Y. Org. Lett. 2005, 7, 2181−2183. (d) Cadena, M.; Chu, N. C.; Diallo, O.; Baudequin, C.; Brunet, J.-J.; RodriguezZubiri, M. Organometallics 2007, 26, 5264−5266. (e) Brunet, J.-J.; Jacob, K.; Dub, P. A.; Rodriguez-Zubiri, M.; Baudequina, C.; Poli., R. Green Chem. 2010, 12, 1392−1396. (21) Serra, D.; Cao, P.; Cabrera, J.; Padilla, R.; Rominger, F.; Limbach, M. Organometallics 2011, 30, 1885−1895. (22) (a) Gründemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2002, 10, 2163−2167. (b) Gründemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473−10481. (c) Kaufhold, O.; Hahn, F. E.; Pape, T.; Hepp, A. J. Organomet. Chem. 2008, 693, 3435−3440. (d) Pažický, M.; Loos, A.; Ferreira, M. J.; Serra, D.; Vinokurov, N.; Rominger, F.; Jäkel, C.; Hashmi, A. S. K.; Limbach, M. Organometallics 2010, 29, 4448−4458. (e) Loch, J. A.;

ASSOCIATED CONTENT

* Supporting Information S

Files CCDC 848068 (3), 848069 (4), and 797319 (10) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. Experimental details and spectral characterization for Pt complexes 1−4 and 10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS P.C., J.C., R.P., D.S., and M.L. work at CaRLa of Heidelberg University, being cofinanced by Heidelberg University, the state of Baden-Württemberg, and BASF SE. Support of these institutions is greatly acknowledged.

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