and Ferracyclic Complexes Containing Isocyanide ... - ACS Publications

Article Cite This: J. Am. Chem. Soc. 2018, 140, 4119−4134

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Disilaruthena- and Ferracyclic Complexes Containing Isocyanide Ligands as Effective Catalysts for Hydrogenation of Unfunctionalized Sterically Hindered Alkenes Yusuke Sunada,†,§ Hajime Ogushi,‡ Taiji Yamamoto,‡ Shoko Uto,‡ Mina Sawano,‡ Atsushi Tahara,† Hiromasa Tanaka,† Yoshihito Shiota,† Kazunari Yoshizawa,† and Hideo Nagashima*,†,‡ †

Institute for Materials Chemistry and Engineering, and ‡Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan S Supporting Information *

ABSTRACT: Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3′ and 4′, were synthesized and characterized by spectroscopy and crystallography. Both 3′ and 4′ showed excellent catalytic activity for the hydrogenation of alkenes. Compared with iron and ruthenium carbonyl analogues, 1′ and 2′, the isocyanide complexes 3′ and 4′ were more robust under the hydrogenation conditions, and were still active even at higher temperatures (∼80 °C) under high hydrogen pressure (∼20 atm). The iron complex 3′ exhibited the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among currently reported iron catalysts. Ruthenium complex 4′ catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H2. The remarkably high catalytic activity of 4′ for hydrogenation of unfunctionalized tetrasubstituted alkenes was especially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activation of H2 assisted by the metal−silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of MCNC (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of sterically hindered tetrasubstituted alkenes.

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INTRODUCTION Transition metal-catalyzed hydrogenation of alkenes is a fundamental reaction promoted by transition metals that provides practical methods for synthesizing organic compounds on laboratory and industrial scales.1 Although numerous homogeneous catalysts have been developed using noble metals such as Ru, Rh, Ir, Pd, and Pt, achieving efficient hydrogenation of unfunctionalized tri- and tetrasubstituted alkenes remains problematic. As summarized in a review by Burgess, several transition metal complexes were reported as catalysts for hydrogenation of trisubstituted alkenes.2 Functional groups at the allylic or homoallyic position, which are capable of coordinating to the metal in the catalyst, accelerate hydrogenation.2,3 However, hydrogenation of unfunctionalized trisubstituted alkenes requires harsh reaction conditions, such as temperatures higher than 100 °C and pressures higher than 100 atm of H2.2,3 Hydrogenation of tetrasubstituted alkenes is more challenging than that of trisubstituted, for which only a limited number of transition metal complexes were effective. Furthermore, the catalyst turnover was often low, even under harsh conditions. One of the few examples of a catalyst showing good activity is Crabtree’s catalyst, (η4-COD)Ir+(κ1-pyridine)[κ1-P(cyclohexyl)3] (COD = 1,5-cyclooctadiene), which has been shown to efficiently promote hydrogenation of tri- and © 2018 American Chemical Society

tetrasubstituted alkenes. The high activity of Crabtree’s catalyst was demonstrated by hydrogenation of 2,3-dimethyl-2-butene, which proceeded effectively with 0.1 mol % of catalyst loading at 0 °C under 600 mmHg (0.79 atm) of H2.4 The highest TOF (h−1) reported for this reaction was 4000; however, catalyst deactivation occurred concomitantly, which prevented full conversion of sterically hindered alkenes (40% conversion, TON = 400).4 Crabtree-type chiral iridium complexes were initially reported by Pfaltz and co-workers, which provided efficient iridium-catalyzed asymmetric hydrogenation of trisubstituted alkenes.5 Despite these advances the hydrogenation of tetrasubstituted alkenes still needs to be improved.6 Several recent trials to modify the structures of iridium catalysts have achieved limited success.7,8 Effective catalysts for hydrogenation of unfunctionalized triand tetrasubstituted alkenes are limited mainly to iridium complexes.9 Trials to find more cost-effective ruthenium catalysts have been performed by Shvo,10a Stephan,10b and Leitner.10c In particular, a recent report by Stephan showed that a ruthenium complex containing cyclic bent allene ligands was a highly reactive catalyst for the hydrogenation of trisubstituted Received: January 22, 2018 Published: March 5, 2018 4119

DOI: 10.1021/jacs.8b00812 J. Am. Chem. Soc. 2018, 140, 4119−4134

Article

Journal of the American Chemical Society alkenes; the TON reached >105 at room temperature under 20 atm of H2. However, the TON for the hydrogenation of 2,3dimethyl-2-butene, a typical tetrasubstituted alkene, was as low as 12 (with 0.5 mol % of the catalyst, 6% conversion of the substrate after 0.5 h). Titanium and zirconium catalysts exhibited a good catalytic activity toward tri- and tetrasubstituted alkenes, but the reactions require a high pressure of H2 (>50 atm) even with high catalyst loadings (5 mol %).2,9a,b Recent concerns about the growing scarcity of chemical elements and the need for environmentally benign chemical processes, have resulted in iron-catalyzed organic reactions receiving much attention from chemists.11 Iron catalysts active for catalytic hydrogenation of alkenes, especially those composed of well-defined iron complexes, are surprisingly rare.12 Among iron complexes reported, iron complexes bearing a bis(imino)pyridine ligand reported by Chirik and co-workers possessed a catalytic activity toward mono- and disubstituted alkenes, but not for tri- and tetrasubstituted alkenes.12b Although improved iron catalysts having bis(arylimidazol-2ylidene)pyridine ligands performed better for several trisubstituted alkenes, their activity toward hydrogenation of 2,3dimethyl-1H-indene was low (TON = 13 at 68% conversion) and no activity was observed for reaction of 2,3-dimethyl-2butene at room temperature under 4 atm of H2.13a Another iron catalyst investigated was Fe(CO)5, which displayed low catalytic activity even at 180 °C, but catalyzed the hydrogenation of alkenes at room temperature under photoirradiation.14 Another was a Ziegler-type catalyst composed of iron salts and organoaluminum or magnesium reagents, which was one of the first reported iron catalysts for hydrogenation of alkenes, but the catalytically active species was ill-defined.15,16 Using these two catalysts, hydrogenation of tri- and tetrasubstituted alkenes were examined, but the activity was not high.14,15 Although Chirik and co-workers reported that cobalt complexes bearing bis(imino)pyridine or diphosphine ligands showed some catalytic activity toward hydrogenation of tri- or tetrasubsituted alkenes,13c,17b−d other cobalt complexes were effective catalysts only for sterically unhindered alkenes.16b,c,17a,e−j Among the references dealing with ironand cobalt-catalyzed hydrogenation of tri- and tetrasubstituted alkenes, seven of them reported the hydrogenation of unfunctionalized alkenes,13,16a,17b−d which includes asymmetric hydrogenation of unfunctionalized trisubstituted alkenes.13c,17c Previous studies reported two disilaferra- and ruthena cyclic carbonyl complexes, 1 and 2, as depicted in Chart 1, which showed good performance for the catalytic hydrogenation of alkenes.18 These two complexes have several interesting structural features and potential hydrogenation mechanisms. According to the 18-electron rule, iron complex 1 has a disilaferracyclic moiety, two cis-CO ligands, and two η2-H-Si ligands derived from 1,2-bis(dimethylsilyl)benzene (BDSB) [Chart 1, Si-M(η2-H-Si) forms]. The ruthenium complex 2 has the same ligand arrangement, except the two CO groups are in the trans-configuration. In contrast, DFT calculations suggested that optimized structures of these carbonyl complexes contain a Si4MH2 core encompassing four M−Si bonds and two M−H bonds. There are secondary interactions between the H and Si atoms, which form Si···H···Si SISHA (the secondary interaction between silicon and hydrogen atoms)19,20 structures shown as 1′ and 2′ in Chart 1, SISHA forms. The structures, 1′ and 2′, are consistent with the structures suggested by crystallography and spectroscopy. These complexes showed dynamic behavior derived from fluctuation of hydrogen atoms among the silicon

Chart 1. Molecular Structures of Disilametallacyclic Carbonyl and Isocyanide Complexes; Those Depicted by σSi-M(η2-H-Si) Forms (Left, 1−4) and Those Described as a Si4MH2 Core with SISHA Forms (Right, 1′−4′)

atoms bonded to the metal center, and species 1 and 2 are isomers of 1′ and 2′. Further calculations revealed that dissociation of the nonclassically coordinated BDSB ligand of 1′ and 2′ and subsequent coordination of H2 and ethylene, generated the species in the catalytic hydrogenation of alkenes. The catalytic cycles were proposed from calculations, in which activation of H2 occurs on the metal−silicon bond and not by oxidative addition on the metal center. This mechanism involving the H−H cleavage assisted by M−Si bond is a variation of σ-bond assisted metathesis (σ-CAM), proposed by Perutz and Sabo-Etienne.21 Of particular importance to the catalytic performance of 1′ and 2′ was the successful hydrogenation of several tri- and tetrasubstituted alkenes at room temperature under 1 atm of H2. However, in the hydrogenation of a typical tetrasubstituted alkene, 2,3-dimethyl-2-butene, the product yield was only 20% and catalyst turnover numbers (TON) was as low as 4. It is desirable to develop the hydrogenation catalysts, which achieve full conversion of tetrasubstituted alkenes with high TON. Investigations into improving catalytic activity were performed, revealing that 1′ and 2′ were unstable under the hydrogenation conditions, and decomposition of the catalytically active species resulted in a low activity. This manuscript describes a solution to this problem by replacing the CO ligands in 1′ and 2′ with isocyanides. Analogous to the structure of 2′, new disilaferraand disilaruthenacyclic complexes, 3′ and 4′ having a Si4MH2 core with Si···H···Si SISHA and two mesityl isocyanide ligands in a trans-configuration have been prepared (Chart 1). Both complexes were durable within the temperature range of 25−80 °C under 1 to 20 atm of H2, and hydrogenated not only mono-, di-, and trisubstituted alkenes, but also tetrasubstituted alkenes. The reactions of tetrasubstituted alkenes were especially important; >99% conversion of the substrate was achieved, and TON reached 100−1000 in the hydrogenation of three tetrasubstituted alkenes. The reason for the high catalytic 4120


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Journal of the American Chemical Society activity of 3′ and 4′ toward sterically hindered alkenes such as tetrasubstituted alkenes was investigated by DFT calculations. Two potential catalytic cycles were proposed; one being a σCAM-type mechanism through a Si2M(η2-H2)(η2-alkene) intermediate (Si = the Me2Si group in disilametallacycle, M = Fe, Ru) used for the explanation for the hydrogenation of alkenes by 1′ and 2′, whereas the other is a modified σ-CAMtype mechanism involving a Si2M(η2-H2)2 intermediate, which is the most feasible mechanism for the hydrogenation of 2,3dimethyl-2-butene.

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RESULTS AND DISCUSSION Synthesis and Characterization of Disilaferra- and Disilaruthenacyclic Complexes, 3′ and 4′. As reported previously,18a the photochemical reaction of (η4-C6H8)Fe(CO)3 with 2 equiv of 1,2-bis(dimethylsilyl)benzene led to the formation of disilaferracyclic dicarbonyl complex 1′. A search for isocyanide analogues of (η4-C6H8)Fe(CO)3 led to the discovery of (η4-C8H8)Fe(CNR)3,22 prepared in situ from (ηC8H8)2Fe and mesityl isocyanide, which reacted with 2 equiv of 1,2-bis(dimethylsilyl)benzene in hexane under 1 atm of H2 under photoirradiation by high pressure mercury lamp. Complex 3′ was isolated in 29% yield as colorless crystals after recrystallization of the crude sample (Scheme 1). Complex

Figure 1. Molecular structures of 3′ (left) and 4′ (right) with 50% probability ellipsoids.

= 2.5132(12) and Ru(1)−Si(2) = 2.5161(12) Å for 4′], and were slightly longer than those in previously reported disilaferra- or ruthena(II)cyclic complexes.24 Second, the H− Si bond distances in 3′ and 4′ [1.79(2) − 1.94(4) Å] were longer than those found in the η1- or η2-(H−Si) moiety in several transition metal complexes, but within the range of secondary interactions (SISHA) (1.8−2.4 Å).19,20 The spectroscopic data obtained in this study further confirmed the molecular structures of 3′ and 4′. A singlet due to the Si···H···Si moiety in 3′ appeared at −11.5 ppm in C6D6 at room temperature, which was shifted slightly to higher field compared with that of 1′ (−10.8 ppm). In contrast, the Si···H···Si signal in 4′ appeared at −7.09 ppm as one slightly broadened singlet at room temperature in the 1H NMR spectrum, which sharpened at −10 °C. The Si···H···Si signals in 3′ at r.t. and 4′ at −10 °C accompanied by a satellite signal coupled with the 29Si nucleus (JH−Si = 16.5 Hz for 3′, and 17.9 Hz for 4′), comparable to those found in the dicarbonyl analogues 1′ (JH−Si = 13.2 Hz) and 2′ (JH−Si = 16.8 Hz). The JH−Si signals of uncoordinated H−Si bonds appeared in the spectrum in the area above 150 Hz, whereas typical η1- and η2(H−Si) bonds were observed at 40−140 Hz.19 The relatively small JH−Si for 3′ and 4′ suggests a weak secondary interaction between the silicon and hydrogen atoms (SISHA).24 While JH−Si values smaller than 40 Hz are recognized as good evidence for SISHA, assignment of IR absorption due to the coordinated H−Si bond is not clear in the literature.19 Corey reported that intense νSi−H stretching band of nonclassical σ-interaction of a Si−H bond to a metal appears at 1650−1800 cm−1, but that for SISHA is weak and does not fit consistently.19d For the assignment of νSi···H···Si signal for 3′ and 4′, their metal deuteride analogues 3′-d2 and 4′-d2 (96% D was incorporated) were synthesized by the reactions of 3′ or 4′ with 1,2(Me2SiD)2C6H4. In the IR spectrum of the ruthenium complex 4′ showed one weak absorption band at 1918 cm−1, which is assignable to the Si···H···Si moiety in 4′. This signal was not visible in the IR spectrum of 4′-d2. The signal due to the Si··· D···Si moiety in 4′-d2 should appear at around 1380 cm−1, and in fact, a small signal at 1374 cm−1 was visible, though this may be overlapping with absorptions due to the other groups. Assignment of νSi···H···Si in 3′ was unfortunately ambiguous. From the analogy to the IR spectrum of 4′, a weak peak at 1915 cm−1 may be the signal due to the Si···H···Si moiety, but a

Scheme 1. Synthesis of Disilaferra- and Disilaruthenacyclic Complexes 3′ and 4′

4′, a ruthenium homologue of 3′, was prepared by treatment of (COD)Ru(η3-methallyl)2 with 2 equiv of BDSB in the presence of 2 equiv of the isocyanide ligand in DME at 55 °C for 18 h. The product was isolated as white powder in 42% yield (Scheme 1).23 The molecular structures of 3′ and 4′ were determined by Xray diffraction analysis, and their ORTEP drawings are shown in Figure 1. Selected bond distances and angles are listed in the Supporting Information. The molecular structures show that both 3′ and 4′ adopt an octahedral coordination geometry with the two isocyanide ligands located in the trans position. Both the iron and ruthenium atoms of 3′ and 4′ are located on a crystallographic inversion center, and the entire molecular structure possesses C2v-symmmetric octahedral coordination geometry. Two hydrogen atoms were detected in the Fourier map, positioned between two Si groups with Fe−H and Ru−H bond distances of 1.49(2) Å for 3′ and 1.48(4) Å for 4′. Similar to the carbonyl analogues 1′ and 2′, the following data reinforced that the structures of 3′ and 4′ included an H2MSi4 core with two Si···H···Si SISHA. First, the two M−Si bond distances in 3′ or 4′ were nearly identical [Fe(1)−Si(1) = 2.4339(7) and Fe(1)−Si(2) = 2.4361(6) Å for 3′; Ru(1)−Si(1) 4121


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Journal of the American Chemical Society Table 1. Hydrogenation of Alkenes to Alkanes Catalyzed by 3′ or 4′a entry

alkene

cat.

solvent

cat. loading (mol %)

H2 (atm)

temp (°C)

time (h)

yield (%)b

1 2 3 4 5 6 7 8 9 10

styrene styrene 1-octene 1-octene 1-methyl-1-cyclohexene 1-methyl-1-cyclohexene 1-methyl-1-cyclohexene styrene 1-octene 1-methyl-1-cyclohexene

3′ 3′ 3′ 3′ 3′ 3′ 3′ 4′ 4′ 4′

toluene DME toluene DME toluene toluene DME toluene toluene toluene

0.5 0.5 0.5 0.5 1 1 1 0.5 0.5 1

10 5 10 1 10 20 10 1 1 1

80 80 80 80 80 80 80 25 25 25

0.5 2 2 2 17 22 2 1 2 3

97 >99 97 >99 8 78 >99 >99 >99 >99

a All reactions were carried out with the alkene (1 mmol) in the presence of catalytic amounts of 3′ or 4′ in toluene or DME (0.5 mL). bYields of the products were determined by GC in the presence of an internal standard.

Table 2. Hydrogenation of Alkenes Catalyzed by 3′ in DMEa

a

All reactions were carried out with alkene (1 mmol) in the presence of catalytic amount of 3′ in DME (0.5 mL). bYield of the product was determined by GC in the presence of an internal standard, which was identified to conversion of the starting material. cIsolated yield after chromatographic purification. d10 mmol of 1-octene was used.

Table 3. Hydrogenation of Alkenes Catalyzed by 4′ in Toluenea

a All reactions were carried out with the alkene (1 mmol) in the presence of a catalytic amount of 4′ in toluene (0.5 mL). bYields of the product was determined by GC in the presence of an internal standard. cIsolated yields after chromatographic purification. d10 mmol of 1-octene was used. e30 mmol of 1-octene was used.

Catalytic Hydrogenation of Mono-, Di-, and Trisubstituted Alkenes Catalyzed by Disilametallacyclic Complexes. Both the iron and ruthenium disilacyclic complexes, 3′ and 4′, showed a good catalytic activity toward mono-, di-, and trisubstituted alkenes. Representative results are shown in Table 1, and additional details are summarized in Table S2 and

similar peak was also seen in 3′-d2. In contrast to the intense absorption due to coordinated CNR, the signal from SISHA is relatively weak as mentioned by Corey. In 3′ and 3′-d2, the νSi···H···Si signal may be too weak to be detected or overlapped with shoulder peaks of νCNR. 4122


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Journal of the American Chemical Society

trisubstituted alkenes at 23 °C under 4 atm of H2 for 1 h using 5 mol % of catalyst.13a Among them, an iron catalyst, (MesCNC)Fe(N2)2, containing a bis(arylimidazol-2-ylidene) ligand, showed good activity toward 1,2-diphenyl-1-propene, 2-methyl-2-butene, and 1-methyl-1-cyclohexene; reaction was complete in 1 h (TON = TOF = 20). For hydrogenation of ethyl 3,3-dimethyl acrylate, one of the bis(imino)pyridine iron catalysts, [(MePDI)Fe(N2)]2(μ-N2)], gave better results (TON = TOF = 20)12b than the (MesCNC)Fe(N2)2 catalyst.13a As shown in entries 7 and 8, the disilaferracyclic iron catalyst 3′ showed higher TON (100) and TOF (50) values at 80 °C under 10 atm of H2. Hydrogenation of the disilaruthenacyclic complex 4′ was more efficient than that of 3′. As shown in Table 3 (entry 1), reaction of 1-octene in the presence of 0.5 mol % of the catalyst resulted in complete conversion to n-octane within 2 h at room temperature under 1 atm of H2. The lowest catalyst loadings (0.005 mol %, 45 ppm Ru to the charged 1-octene) were examined at 80 °C under 10 atm of H2; reaction was complete after 2.5 h (TON = 20 000, TOF = 4000) (entry 3). Other monosubstituted alkenes (entries 4−8) and disubstituted alkenes (entries 10−12) also underwent hydrogenation to give the corresponding alkanes quantitatively within 6 h at room temperature under 1 atm (or 10 atm for entry 12) of H2. Experiments confirming functional group compatibility were performed (entries 6−9); results showed that bromo, cyano, and pyridine did not disrupt the reaction, whereas the reaction of 4-nitrostyrene was slow. Although most transition metal complexes showed low catalytic activity for the hydrogenation of trisubstituted alkenes under mild conditions, 4′ was an efficient catalyst at room temperature under 1 atm of H2 (entries 13 and 14). The diastereoselective hydrogenation of terpinen-4-ol has been a widely discussed topic since it was realized by using Crabtree’s iridium catalyst.25 Hydrogenation using 2.5 mol % of (η4-COD)Ir+(κ1-pyridine)[κ1-P(cyclohexyl)3] afforded the product as a single stereoisomer, with the methyl and isopropyl groups in a cis-orientation (Scheme 2). Ruthenium complexes

S3 in the Supporting Information. The iron complex 3′ acted as an effective catalyst for the hydrogenation of styrene in toluene at 80 °C under 10 atm of H2 No reaction took place when the hydrogenation was conducted at 80 °C under 1 atm of H2 or at 25 °C under 10 atm of H2 (Table S2 in SI). Under optimized conditions, full conversion of styrene to ethylbenzene was achieved with 0.5 mol % of 3′ after 2 h (entry 1, Table 1). In toluene, 1-octene was hydrogenated to n-octane at 80 °C under 10 atm of H2 (entry 3, Table 1), while the reactivity of a representative trisubstituted alkene, 1-methyl-1-cyclohexene, was lower under the same conditions (entry 5), which was improved to an extent by increasing the H2 pressure from 10 to 20 atm (entry 6). Full conversion of 1-methyl-1-cyclohexene was not accomplished. A breakthrough was achieved when DME was used as the solvent instead of toluene. In DME, the reactions of all three alkenes occurred at 80 °C under 1−10 atm of H2 in the presence of 0.5−1 mol % of 3′, and were complete in 2 h as shown in entries 2, 4, and 7. Hydrogenation of 1octene proceeded under 1 atm of H2, whereas full conversion of 1-methyl-1-cyclohexene was achieved under 10 atm of H2 after 2 h. The ruthenium complex 4′ displayed excellent catalytic activity toward hydrogenation of 1-octene, styrene, and 1methyl-1-cyclohexene in toluene at room temperature under 1 atm of H2, as shown in Table 1 (entries 8−10). Conversion of the charged alkene to the corresponding alkane reached >99% within a few hours, and the reaction completion time was unchanged for mono- and trisubstituted alkenes. In sharp contrast to the hydrogenation catalyzed by 3′, using DME as the solvent did not improve the reactivity, but may have hindered the reaction by coordination to the ruthenium center (Table S3 in SI). Tables 2 and 3 show the results for hydrogenation of several mono-, di-, and trisubstituted alkenes using 3′ or 4′ as the catalyst. For hydrogenation by 3′, DME was used as the solvent, while hydrogenation by 4′ was performed in toluene. In the presence of 0.5 mol % 3′, 1-octene was hydrogenated at 80 °C under 1 atm of H2. The reaction was complete after 2 h, and n-octane was obtained as the sole product (Table 2, entry 1).The reaction also was complete within 8 h at 40 °C under 10 atm of H2 (entry 2). Two other monosubstituted alkenes, a disubstituted alkene, and two trisubstituted alkenes, which included substrates containing conjugated (entry 8) and unconjugated (entry 5) carboalkoxy groups, were subjected to hydrogenation at 80 °C under 5−10 atm of H2. In all cases, the reaction was complete in 2 h, and TON reached 100−200. Only a few examples have been reported for successful hydrogenation of alkenes by well-defined iron complexes. Chirik reported the most active iron complex catalyst, which was a dinitrogen complex of [bis(imino)pyridine]Fe that achieved hydrogenation of 1-hexene at 22 °C under 4 atm of H2. In the presence of 0.3 mol % of the catalyst, the initial activity was extremely high (TOF (/h) = 1814); however, preparative scale hydrogenation with 0.04 mol % catalyst, required 19 h [TON = 2500, TOF (/h) = 132] for complete conversion of 1-hexene to n-hexane.12b As shown in entry 3, hydrogenation at lower catalyst loadings (0.05 mol %) resulted in quantitative formation of n-octane after 6 h at 80 °C under 20 atm of H2 [TON = 2000, TOF (/h) = 333]. Catalytic activity of Chirik’s bis(imino)pyridine catalyst toward hydrogenation of trisubstituted alkenes was lower than that for the reaction of 1-hexene.12b The same group reported improved iron catalysts for hydrogenation of four

Scheme 2. Hydrogenation of Terpinen-4-ol Catalyzed by Crabtree’s Catalyst or Disilametallacyclic Catalysts 3′ and 4′

bearing bent cyclic allene ligands were reported by Stephan and co-workers as a highly reactive catalyst for trisubstituted alkenes and showed similar cis-selective hydrogenation of terpinen-4ol.10b In contrast, hydrogenation of the same substrate by a catalytic amount of 3′ or 4′ afforded a 1:9 mixture of diastereomers, with the major product different from that obtained by Crabtree’s catalyst. Stereoselectivity of hydrogenation by 3′ and 4′ was similar to that obtained by hydrogenation over Pd/C.25 The high cis-selectivity is thought to originate from coordination of the hydroxyl group in terpinen-4-ol that directed the orientation of addition of hydrogen to the carbon−carbon double bonds. Thus, active species generated from disilametallacyclic complexes 3′ and 4′ 4123


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Journal of the American Chemical Society Table 4. Hydrogenation of Tetrasubstituted Alkenes Catalyzed by 3′ or 4′a

a Reactions were carried out with the alkene (1 mmol) in the presence of catalytic amounts of 3′ or 4′ in toluene or DME (0.5 mL). bYields of the products were determined by GC in the presence of an internal standard. cIsolated yield after chromatographic purification. dReaction was performed with 5 mmol of substrate in toluene (0.1 mL).

°C under 10 atm of H2 to form the corresponding product quantitatively. Other Features of the Disilametallcyclic Catalysts. Three experiments were carried out for understanding the features of 3′ and 4′ as hydrogenation catalysts. The first being alkene isomerization, which preceded hydrogenation of 1octene. When hydrogenation catalyzed by 3′ was quenched after 3 h under the conditions those shown in Table 2, entry 3, all of the 1-octene was found to be consumed and two products, n-octane and 2-octene, were detected in 78 and 22% yields, respectively. Further hydrogenation of this mixture afforded n-octane quantitatively. This shows the reaction was accompanied by hydrogenation of 2-octene formed by the concomitantly occurring alkene isomerization. In the hydrogenation catalyzed by 4′, the reaction was too rapid to be quenched at the early stage of the reaction and to detect internal octenes. Instead, we performed the reaction of 1octene with D2 under the same conditions shown in Table 3, entry 3. The hydrogenation took place completely to form a mixture of deuterated octanes. 13C NMR of the product showed four signals at δ 14.7, 23.3, 29.9, and 32.5 due to 1CH3, 2-CH2, 3-CH2, and 4-CH2 groups, respectively. It is important that all of these four signals accompanied by signals at higher chemical shifts indicating the existence of deuterium atoms bonded with the carbon (see, the Supporting Information). Typical examples are triplets with JC‑D = 16−20 Hz at δ 29.3 and 31.9. This clearly indicates that deuterium atoms are distributed to all carbons of the n-octane product, which is in accordance with integral values of signals seen in 1H NMR spectra of the n-octane product. Mass spectroscopy of the product revealed M+ = 114, 115, 116, 117, 118, 119, 120, 121, and 122 in a ratio of 1.0:1.1:2.1:3.0:3.1:2.0:1.0:0.3. Apparently, the product was a mixture of C8H18−nDn, where n = 0−8. These spectroscopic results suggesting deuterium scrambling are explained by the hydrogenation accompanied by alkene isomerization, in which addition of M−H (or D) to a CC bond followed by β-H (or D) elimination to regenerate a CC bond at different positions sequentially occurred. The second experiment is concerning the interesting solvent effect of DME for 3′-catalyzed hydrogenation reactions as described in a previous section. Further studies to elucidate the DME effect revealed that even small amounts of DME was enough to improve the catalytic activity in the hydrogenation catalyzed by 3′. Table S4 showed that the hydrogenation of 1methyl-1-cyclohexene in toluene when a small amount of DME or other oxygen-, nitrogen-, or sulfur-containing compounds (1 to 2 equiv to 3′) was added as the additive (PH2 = 10 atm, at 80 °C). There was no difference in reactivity between the

did not interact with the hydroxyl group, and the diastereoselectivity was determined by steric factors. Catalytic Hydrogenation of Tetrasubstituted Alkenes Catalyzed by Disilametallacyclic Complexes. Highly active hydrogenation catalysts for tetrasubstituted alkenes are a desirable target.2,6−10,13−18 As mentioned in the introduction, Chirik’s bis(arylimidazol-2-ylidene) complex catalyzed the hydrogenation of 2,3-dimethyl-1H-indene; however, only ca. 60−68% conversion of the substrate occurred (TON = 12− 13.6). The same Chirik’s catalyst did not hydrogenate 2,3dimethyl-2-butene under the same reaction conditions.12b Crabtree’s iridium catalyst demonstrated excellent initial activity for 2,3-dimethyl-2-butene (TOF > 4000), but deactivation occurred quickly to prevent full conversion of the charged alkene (40% conversion, TON = 400).4 Stephan’s ruthenium complexes containing bent cyclic allene ligands showed catalytic activity toward mono-, di-, and trisubstituted alkenes, but little activity occurred toward tetrasubstituted alkenes (6% conversion, TON = 12).10b As other catalysts which were not mentioned in the introduction, hydrogenation of 2,3-dimethyl-2-butene was examined using a rhodium catalyst, but TON was as low as 70.9b A notable example is modified Crabtree’s catalyst (Pfaltz’s catalyst), which promoted hydrogenation of 2,3-dimethyl-1H-indene at ambient temperature under 50 atm of H2 pressure. After 4 h, 2,3-dihydro-cis1,2-dimethyl-1H-indene was formed in 94% yield (TON = 940).6a Table 4 shows hydrogenation of three tetrasubstituted alkenes by disilametallacyclic catalysts 3′ and 4′. With the iron catalyst 3′, quantitative conversion of 2,3-dimethyl-2butene and 2,3-dimethyl-1H-indene to the corresponding hydrogenated products was confirmed in the presence of 1 mol % 3′ at 80 °C under 10−20 atm of H2 (entries 1 and 3). A TON of 100 was better than that produced by Chirik’s bis(arylimidazol-2-ylidene). The ruthenium catalyst 4′ catalyzed hydrogenation of 2,3-dimethyl-2-butene at 25 °C under 1 atm of H2. Compared with Crabtree’s catalyst, 4′ was durable enough to accomplish complete conversion of the starting material with a TON of 200 (entry 2). Hydrogenation of 2,3dimethyl-1H-indene took place under 10 atm of H2; only a cisproduct was formed.26 With 0.3 mol % of 4′, the reaction was complete after 6 h at ambient temperature (entry 4, TON = 333). In contrast, the reaction at 80 °C was complete after 5 h with 0.1 mol % of catalyst loading (entry 5, TON = ∼1000). The TON of 4′ was comparable to that of Pfaltz’s catalyst. Hydrogenation of diethyl isopropylidenemalonate was examined using 4′ as the catalyst (entry 6). With a slightly higher catalyst loading (1 mol %), the reaction was accomplished at 25 4124


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Scheme 3. (a) Simplified Scheme of the Hydrogenation of Ethylene Catalyzed by 1′, 2′, and Their Osmium Homologues Reported in a Previous Paper; (b) Possible Pathway of the Hydrogenation of Ethylene Catalyzed by 3′ or 4′ (Cycle C; Starting from [C1M]ET)

species in the catalytic cycle. An σ-CAM-type mechanism was proposed for catalytic hydrogenation of ethylene using DFT calculations.18b,c Three possible catalytic cycles were investigated for iron, ruthenium, and osmium metallacyclic complexes. The reaction pathways were dependent on the ligand arrangement; two of them (Cycle A and Cycle B in ref 18b) were for species with the two CO ligands in the cisorientation, such as 1′ and its ruthenium and osmium homologues, whereas the other (Cycle C in ref 18c) was for trans-CO species, e.g., 2′ and its iron and osmium homologues. Since these three catalytic cycles are essentially the same, except for the orientation of the two CO ligands, a simplified cycle is outlined in Scheme 3(a). All of them involve (η1-Si)2M(CO)2(η2-H2)(η2-CH2CH2) species X1, where Si = Me2Si group in the disilametallacycle, generated from catalytic precursors 1 or 2 by replacement of a BDSB ligand with H2 and ethylene. The H−H bond of the η2-H2 moiety in X1 undergoes σ-bond metathesis of late transition metals (i.e., oxidative hydrogen migration)29 associated with an M−Si bond to form (η2-Si-H)(η1-Si)M(CO)2(η1-H)(η2-CH2CH2) (X2) (step a). This H−H splitting assisted by the M−Si bond is followed by hydride migration from the metal to a carbon of the η2-ethylene moiety to form (η2-Si-H)(η1-Si)M(CO)2(η1CH2CH3) (X3) (step b), which corresponds to insertion of ethylene to the M−H bond. Oxidative hydrogen migration involving the M−C bond and the η2-Si-H moiety in X3 subsequently occurs to generate (η1-Si)2M(CO)2(η-CH3CH3) (X4) (step c). The coordinated ethane is easily replaced by H2 and CH2CH2, resulting in formation of ethane and regeneration of X1. Since the isocyanide ligands of 3′ and 4′ were in a transconfiguration, calculations of hydrogenation mechanisms initiated by 3′ and 4′ were done using Cycle C adopted for catalysis of 2′ and its iron homologue. Scheme 3(b) illustrates a plausible catalytic cycle for ethylene hydrogenation. Generation of catalytically active species [C1M]ET from 3′ and 4′ occurs

experiments when DME was used as the solvent or the additive (1 equiv to 3′). Similarly, [Me2N(CH2)2]2O (1 equiv to 3′), furan or thiophene (2 equiv to 3′) were also effective to afford the hydrogenated product in quantitative yields (Table S4). In contrast, TMEDA, pyridine, and tetrahydrothiophene as additives did not affect the reaction rate or rather retarded it. The addition of 2,6-dithiaheptane inhibited the catalysis. It is difficult to show unequivocal experimental evidence against the argument that net catalytic species may not be molecular complexes but nanoparticles formed during the reaction. To exclude the possible involvement of nanoparticles in catalysis, the mercury drop test is often performed.27a Neither inhibition nor retardation was observed for both hydrogenation reactions of 1-octene catalyzed by 3′ and 4′ as described in the Supporting Information. The results suggest that the species responsible for the catalytic hydrogenation by the ruthenium complex 4′ is molecular. It has been claimed that the mercury drop test is not good enough evidence to exclude the involvement of iron nanoparticles in catalysis, because iron is not amalgamated with Hg.27b,c In a recent paper by de Vries on catalytic hydrogenation of alkenes by soluble iron nanoparticles, the reaction was not inhibited by mercury, but apparently slowed down.15c They have also claimed that the reaction catalyzed by iron nanoparticles was inhibited by the addition of a small amount of thiophene, which is much less than one equivalent of thiophene per iron atom. As described above, addition of two equivalents of thiophene to 3′ did not disturb the reaction but rather accelerate it. These results strongly suggest that the net catalyst species is molecular, which is supported by similarity in the catalytic performance between 3′ and 4′. For instance, both of the catalysts are effective for the hydrogenation of tetrasubstituted alkenes, and both induce hydrogenation and alkene isomerization.28 Theoretical Study to Elucidate Possible Reaction Mechanisms. We performed DFT calculations based on the assumption that disilametallacycllic intermediates are key 4125


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Figure 2. Optimized structures of [C1Fe]ET and [C1Fe]DMB with their space-filling models. Some H atoms in side view and the mesityl groups in the top view have been omitted for clarity.

through isomer 3 or 4, respectively, having η2-Si−H ligands. Intermediates 3 and 4 are slightly higher in energy than 3′ and 4′, respectively, (ΔG° = 3−4 kcal/mol) and activation energy for the reactions of 3′ to 3 and 4′ to 4 is small (ΔG‡ < 6.3 kcal/ mol, see Scheme S4). Replacement of the BDSB ligand in 3 and 4 by H2 and CH2CH2 results in formation of (η1Si)2M[trans-{CN(mesityl)}2](η2-H2)(η2-CH2CH2) species [C1Fe]ET and its ruthenium homologue [C1Ru]ET, respectively. Small ΔG° values of [C1Fe]ET from 3 and [C1Ru]ET from 4 of approximately 4 kcal/mol indicate that this ligand exchange reaction is easy. Two elementary reactions, step a and step b, occur in a concerted fashion without a discrete intermediate analogous to X2 in Scheme 3a from [C1Fe]ET or [C1Ru]ET to form [C3Fe]ET or [C3Fe]ET, which included η2-Si−H and βagostic ethyl moieties. Since the η2-H2 ligand in [C1Fe]ET or [C1Ru]ET is coplanar with the η2-CH2CH2 moiety and one of the metal−silicon bonds, the H−H splitting induces simultaneous Si−H and C−H bond formation. For step c involving M−C/η2-Si−H oxidative hydrogen migration to proceed, the geometries of [C3Fe]ET and [C3Ru]ET, where the M−C bond and the η2-Si−H moiety are located in trans-position, are not adequate. As a consequence, dissociation of the β-agostic C−H bond (step d: [C3Fe]ET to [C3′Fe]ET or [C3Ru]ET to [C3′Ru]ET) is followed by the movement of the η1-ethyl moiety close to the η2-Si−H moiety (step e: [C3′Fe]ET to [C3′′Fe]ET or [C3′Ru]ET to [C3′′Ru]ET). The M−C/η2-Si−H oxidative hydrogen

migration occurs from [C3′′Ru]ET or [C3′′Ru]ET to accomplish step c, which is followed by elimination of ethane and regeneration of [C1Fe]ET or [C1Ru]ET in contact with H2 and ethylene. These reaction pathways and energy diagrams (described in the Supporting Information) were similar to those obtained by DFT calculations for 2′ and its iron homologue. The rate-determining step of the catalytic cycle is in step e for the ruthenium-catalyzed process (ΔG‡ = 10.3 kcal/ mol), whereas it is in step d for the iron-catalyzed reaction (ΔG‡ = 16.0 kcal/mol). These ΔG‡ values were consistent with the experimental results that hydrogenation proceeds within the temperature range of 25−80 °C. When the catalytic cycle calculated for ethylene hydrogenation was adopted for reaction of the more sterically hindered 2,3-dimethyl-2-butene as a typical tetrasubstituted alkene, two factors made the reaction difficult (see Scheme S6). One was the ligand substitution of a BDSB ligand in 3 or 4 by H2 and 2,3-dimethyl-2-butene. The ΔG° values of C1Fe (denoted as [C1Fe]DMB) from 3 and that of C1Ru (denoted as [C1Ru]DMB) from 4 were 16.9 and 13.6 kcal/mol, respectively, which were significantly larger (>ca. 10 kcal/ mol) than those observed for the catalytic hydrogenation of ethylene (ΔG° of [C1Fe]ET from 3 = +3.7 kcal/mol, and that of [C1Ru]ET from 4 = +2.9 kcal/mol). Comparison of the structure of [C1Ru]ET with that of [C1Ru]DMB (Figure 2) suggests that the difference is due to the steric repulsion 4126


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Scheme 4. (a) Simplified Scheme of Hydrogenation of 2,3-Dimethyl-2-butene Proposed in This Paper; (b) Possible Pathway of the Hydrogenation of Ethylene Catalyzed by 3′ or 4′ (Cycle D; Starting from D1M)

Scheme 5. Relative Energy Diagrams for the Hydrogenation of 2,3-Dimethyl-2-butene Starting from D1Fe or D1Ru [Cycle D] (ΔG [kcal/mol])

catalyzed of 2,3-dimethyl-2-butene, the rate-determining step is step d or step e. The ΔG‡ values were +21.3 kcal/mol (step d) and +21.6 kcal/mol (step e) for 2,3-dimethyl-2-butene, whereas the value was +16.0 kcal/mol (step d) for ethylene. In contrast, the rate-determining step of ruthenium-catalyzed hydrogenation of 2,3-dimethyl-2-butene was either step d (ΔG‡ = +13.5 kcal/mol) or step e (ΔG‡ = +13.6 kcal/mol). These ΔG‡ values were higher than that for the hydrogenation of ethylene (ratedetermining step = step e; ΔG‡ = +10.3 kcal/mol). Among these two factors, the former is more influential; catalytically active species C1M is barely generated from the catalyst precursor during hydrogenation of sterically hindered alkenes,

between two of the four methyl groups in 2,3-dimethyl-2butene and the nearby SiMe2 groups. Since isocyanides have a linear structure of MCNC(Ar) bonds, a large space exists around the metal for coordination of sterically hindered alkenes. However, the structure of [C1Fe]DMB shown in Figure 2 (and [C1Ru]DMB) indicates some interaction between the methyl groups of the coordinated 2,3-dimethyl-2-butene and the methyl moieties of mesityl isocyanides. Another factor is the steric repulsion between the methyl groups in 2,3-dimethyl2-butene with those in the mesityl isocyanide ligands, which contributes to the increase in ΔG‡ in the rate-determining steps of the catalytic hydrogenation by 1.5−5 kcal/mol. In the iron4127


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Figure 3. Optimized structures of [D3Fe]ET and [D3Fe]DMB with their space-filling models. Some H atoms in side view and the mesityl groups in top view are omitted for clarity.

[D1M (M = Fe or Ru)]. This configuration results in smooth formation of (η2-Si−H)2M(trans-CNMes)2(η2-H2) species D2M. Transformation of D1M to D2M occurs via a single transition state TSD1/D2M, regardless of the metal. A difference between Fe and Ru was suggested by Mayer bond-order analyses (see Supporting Information 7−8), in which two dihydrogen ligands are activated in a concerted manner when using iron, while the cleavage of two H−H bonds occurs stepwise on a ruthenium center. Formation of D2M is followed by replacement of the η2-H2 ligand by ethylene or 2,3-dimethyl2-butene to generate D3M (step g). Two subsequent reactions corresponding to step h and step i then forms D5M through D4M. This scheme explains the facile hydrogenation of 2,3dimethyl-2-butene by 3′ and 4′. The energy diagram for the hydrogenation of 2,3-dimethyl-2-butene is shown in Scheme 5. The ethylene version of Scheme 4(b) also is reasonable for the efficient hydrogenation of ethylene as summarized by the energy diagram in the Supporting Information (Scheme S8). A feature of this scheme is the relatively small ΔG° values for the generation of catalytically active species from the catalyst precursor; D1Fe from 3 and D1Ru from 4, which were 6.1 and 6.9 kcal/mol, respectively. Metathesis of the H−H bonds with the M−Si bonds (step f) to form (η2-Si−H)2M(η2-H2) species D2M had a relatively small energy barrier; ΔG‡ from D1Fe to D2Fe and that from D1Ru to D2Ru were 4.0 and 12.2 kcal/mol, respectively. Formation of D2M (M = Fe, Ru) is followed by

according to Cycle C; this is inconsistent with the experimental results, especially those from the ruthenium-catalyzed hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes, which were hydrogenated at room temperature under 1 atm of H2. The search for reasonable mechanisms provided a new catalytic cycle. Scheme 4(a) depicts a new simplified mechanism. The catalytically active species initiating the catalytic cycle is bis(η2-H2) species Y1, formed by ligand replacement of BDSB in 3 or 4 generated from 3′ or 4′ by H2. The H−H splitting of two η2-H2 ligands occurs simultaneously with the aid of the adjacent M−Si bonds via σ-bond metathesis of late transition metals (oxidative hydrogen migration) (step f) to form intermediate Y2. The formal oxidation state is reduced by two in conversion of Y1 to Y2. The η2-H2 ligand in Y2 is subsequently substituted by ethylene to form Y3 (step g). Hydrogen migration from the η2-Si−H ligand in Y3 to the closest carbon of the η2-alkene ligand leads to formation of the M−Si bond and β-agostic alkyl moiety (step h). Oxidative hydrogen migration from η2-Si−H to the carbon bonded to the metal generates the coordinated alkane complex Y5 (step i), followed by the reaction with H2 causing elimination of an alkane and regeneration of Y1. Scheme 4(b) illustrates a catalytic cycle for hydrogenation of 2,3-dimethyl-2-butene via a disilametallacyclic intermediate having octahedral geometry with two trans-CNR ligands (Cycle D). Two M−Si bonds are coplanar with two η2-H2 ligands in the initial species, (Si2)M(trans-CNMes)2(η2-H2)2 4128


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Journal of the American Chemical Society Scheme 6. Relative Energy Diagrams for Exchange of Ligands between Two H2 and BDSB [ΔG (kcal/mol)]

provided a reasonable σ-CAM-type mechanism for hydrogenation promoted by disilametallacyclic catalysts, but also prompted a question about the difference in catalytic activity between the ruthenium and iron catalysts. Since the ratedetermining step of Cycle D for the iron-catalyzed hydrogenation of ethylene or 2,3-dimethyl-2-butene is step g, which involves dissociation of H2 from D2Fe, the energy barrier of the catalytic hydrogenation was 10.7 kcal/mol, regardless of the alkene used. In contrast, that of ruthenium-catalyzed hydrogenation reactions, the rate-determining step is step f, which corresponds to metathesis of two Ru−Si bonds and two η2-H2 ligands. For hydrogenation of ethylene and 2,3-dimethyl-2butene, the energy barrier was 12.2 kcal/mol. The calculations predicted a higher energy barrier for ruthenium than for iron. Thus, the iron-catalyzed hydrogenation reactions should proceed more quickly than the ruthenium-catalyzed reactions, after the catalytic cycle starts. This is inconsistent with the data shown in Tables 2−4, which indicate that the rutheniumcatalyzed hydrogenation occurred at room temperature under 1 atm of H2, whereas elevated temperatures and higher hydrogenation pressure were necessary for the iron-catalyzed reactions. A clue for understanding this inconsistency was found in the induction period observed for the hydrogenations catalyzed by 3′ or 4′. The η-(Si−H) ligands generally are weakly coordinated to the metal center, and are easily replaced by other compounds able to bind to the metal center, such as H2 and alkenes. The hypothesis to explain the induction period involves η2-(Si−H) ligands in BDSB more strongly coordinated to the metal center than expected, especially to the iron center in 3. This disrupts the generation of catalytically active species D1Fe. The calculations provided evidence supporting this hypothesis. Catalyst precursor 3′ or 4′ is coordinatively

replacement of the η2-H2 ligand by 2,3-dimethyl-2-butene to generate [D3M]DMB (step g). Reaction from D2M to [D3M]DMB proceeds by dissociation of H2 to form coordinatively unsaturated species D2M-H2, followed by coordination of 2,3dimethyl-2-butene. The energy barrier of D2M to [D3M]DMB through D2M-H2 was 10.7 (M = Fe) and 2.9 (M = Ru) kcal/ mol. In the following two steps, step h and step i, the ironcatalyzed reactions proceed with essentially no energy barrier, whereas ΔG‡ of the ruthenium-catalyzed reactions are as low as 4.5 kcal/mol.30 The rate-determining step of the iron-catalyzed hydrogenation is in step g, with an energy barrier of 10.7 kcal/ mol, whereas the energy barrier of the ruthenium-catalyzed hydrogenation in step f is 12.2 kcal/mol. Consequently, generation of catalytically active species from the catalyst precursor is easy, and the energy barrier for the catalytic reactions was less than 12.2 kcal/mol in Cycle D for the ruthenium-catalyzed reaction. This indicates that Cycle D is more reasonable than Cycle C to explain the hydrogenation of 2,3-dimethyl-2-butene under mild conditions. Figure 3 shows why Cycle D is preferable for hydrogenation of 2,3-dimethyl-2-butene. In the intermediate D4M, ligand arrangement is trigonal bipyramidal, and two η2-Si−H bonds and 2,3-dimethyl-2-butene are in the equatorial plane. The dimethylsilyl groups, which sterically affect the coordinated alkene in intermediate C1M, are remote from the coordinated 2,3-dimethyl-2-butene. Four methyl groups of the coordinated 2,3-dimethyl-2-butene are located where the methyl group in the isocyanide ligand avoid the steric repulsion. These ligand arrangements minimize the barrier for sterically hindered alkenes such as 2,3-dimethyl-2-butene to coordinate to the metal center. Discrepancy in the Energy Barrier of Catalytic Hydrogenation for 3′ and 4′. The calculations described 4129


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both 3′ and 4′ exhibited induction periods, suggesting that significant difficulty exists in generating the catalytically active species from 3′ and 4′ by dissociation of BDSB. Other disilametallacycles, which include Si2M(CNR)2(L)2 complexes with weakly coordinating L, are now being explored for more efficient hydrogenation catalysts, in which the activation of H2 occurs on a metal−silicon bond rather than on a metal center.

saturated, and replacement of BDSB by an alkene and H2 is necessary. A reasonable pathway involves dissociation of two Si−H groups in BDSB from the metal center giving Si2M(transCNR)2 (E1M) with 14-electron configuration, followed by coordination of two hydrogen molecules. The energy difference between 3′ and E1Fe and between 4′ and E1Ru was calculated and showed that the former was higher than 10 kcal/mol. Scheme 6 shows the energy diagrams from the catalyst precursor to D1M, suggesting that the activation energy of 12.1 kcal/mol from 4′ to D1Ru through E1Ru is in a range allows the reaction to proceed at room temperature. In contrast, the process from 3′ to D1Fe through E1Fe requires extra thermal energy to overcome the high activation energy of 23.6 kcal/mol, or application of higher hydrogen pressure to promote generation of the hydrogen-coordinated complex E1Fe. The effect of DME as the solvent for hydrogenation of alkenes catalyzed by 3′ (Table 1, 2 and 4) can be explained by assuming that the E1Fe intermediate is moderately stabilized by coordination of DME to the iron center, which is enough to lower ΔG‡ from 3′ to E1Fe. Furthermore, the coordination ability of DME is not strong enough for replacement by H2 to generate D1Fe. The following experiments supported that E1Fe could be stabilized by certain weakly coordinating ligands. Coordination of these additives to the iron center lowered the energy of E1Fe. Similar additive effects were not observed when 4′ was used as the hydrogenation catalyst, perhaps due to the nature of ruthenium, which is softer than iron. The DFT calculations suggests that the dissociation of BDSB is the initial step of the catalytic cycle. Attempted replacement of BDSB in 4′ by styrene (10 equiv) in C6D6 at room temperature under N2 atmosphere resulted in recovery of 4′. Monitoring a C6D6 solution of 4′ by 1H NMR under H2 (1 atm) at room temperature showed generation of BDSB and decomposition of 4′. The reaction of 4′ with H2 triggered dissociation of BDSB, while efforts to detect the resting state species of the hydrogenation of styrene catalyzed by 4′ (and 3′, too) were so far unsuccessful. These are ascribed to unexpectedly strongly coordinated BDSB to the metal center. The long induction period is a weak point for the hydrogenation reactions catalyzed by 3′ and 4′. It seems reasonable that higher temperature and pressure for the dissociation of BDSB are necessary to generate catalytically active species during the hydrogenation. The induction period disturbed kinetic studies. Apparently, it is the next target to seek for catalytic precursors, which easily form coordinatively unsaturated disilametallacycles.31

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EXPERIMENTAL SECTION

General Methods. Manipulation of air and moisture sensitive compounds was carried out under a dry nitrogen atmosphere using Schlenk tube techniques associated with a high-vacuum line or in the glovebox which was filled with dry nitrogen. All solvents were distilled over appropriate drying reagents prior to use (DME, toluene, diethyl ether, hexane, pentane; Ph2CO/Na). 1H, 13C, 29Si NMR spectra were recorded on a JEOL Lambda 400 or a Lambda 600 spectrometer at ambient temperature. 1H, 13C, 29Si NMR chemical shifts (δ values) were given in ppm relative to the solvent signal (1H, 13C) or standard resonances (29Si: external tetramethylsilane). Elemental analyses were performed by a PerkinElmer 2400II/CHN analyzer. IR spectra were recorded on a JASCO FT/IR-550 spectrometer. The starting compounds, (COT)2Fe,32 1,2-bis(dimethylsilyl)benzene,33 (COD)Ru(η3-methallyl)2,34 and 2,3-dimethyl-1H-indene9a were synthesized by the method reported in the literature. Alkenes other than and 2,3dimethyl-1H-indene were purchased from Tokyo Chemical Industries Co., Ltd., and were used after purification by distillation. Mesityl isocyanide and 2,6-diisopropylphenyl-isocyanide were synthesized by the method reported in the literature,35 whereas tert-butylisocyanide and 1-isocyanoadamantane were purchased from Tokyo Chemical Industries Co., Ltd. or Sigma-Aldrich, and were used without further purification. Synthesis of Fe[o-(SiMe2)2(C6H4)]2(H)2[CN-2,4,6-Me3(C6H2)]2 (3′). In a 100 mL Schlenk tube, (COT)2Fe (40 mg, 0.15 mmol) was dissolved in toluene (10 mL), then mesityl isocyanide (65 mg, 0.45 mmol) was added to this solution at room temperature. The solution was stirred at room temperature for 2 h. The dark brown solution turned to dark red, then the solvent was evaporated in vacuo. The obtained solid was dissolved in hexane (30 mL), then 1,2bis(dimethylsilyl)benzene (60 mg, 0.31 mmol) was added to this solution. The atmosphere was replaced by H2, and the mixture was stirred at room temperature for 36 h under irradiation of the high pressure mercury lamp. The solution was centrifuged to remove the small amount of insoluble materials. The supernatant was collected, and the solvent was evaporated under vacuum. The remaining solid was washed with cold pentane (5 mL × 2), then white powder of 3′ was obtained in 40% yield (44 mg). This white powder was dissolved in toluene (3 mL), and layered with pentane (10 mL) at −30 °C, from which the single crystals of 3′ were obtained as colorless crystals in 29% yield (32 mg). 1H NMR (600 MHz, r.t., C6D6) δ = −11.54 (s, 2H, H−Si, with a satellite signal due to the coupling with 29Si, JSi−H = 16.5 Hz), 0.90 (s, 24H, SiMe2), 1.81(s, 6H, para-CH3 of mesityl), 1.91(s, 12H, ortho-CH3 of mesityl), 6.30 (s, 4H, C6H2), 7.30−7.32 (m, 4H, C6H4) 7.72−7.74 (m, 4H, C6H4). 13C NMR (150 MHz, r.t., C6D6) δ = 9.9 (br s, SiMe2), 19.4 (s, C6H2Me3), 20.8 (s, C6H2Me3), 128.7 (s, Ar), 129.4 (s, Ar), 131.2 (s, Ar), 134.8 (s, Ar), 137.0 (s, Ar), 156.8 (s, C6H4-ipso), 177.6 (s, CN). 29Si NMR (119 MHz, r.t., C6D6); no peak was observed. IR (ATR) νCN = 2056 cm−1, νSi−H = 1956 cm−1. Anal. Calcd for C40H56N2Si4Fe1: C, 65.54; H, 7.70; N, 3.82. Found: C, 65.36; H, 7.56; N, 3.98. Synthesis of Ru[o-(SiMe2)2(C6H4)]2(H)2[CN-2,4,6-Me3(C6H2)]2 (4′). In a 20 mL Schlenk tube (COD)Ru(η3-methallyl)2 (200 mg, 0.63 mmol) was dissolved in DME (10 mL), then mesityl isocyanide (182 mg, 1.26 mmol) and 1,2-bis(dimethylsilyl)benzene (244 mg, 1.26 mmol) were added to this solution at room temperature. The solution was stirred at 55 °C for 18 h. The orange solution turned to dark brown, and pale yellow solid precipitated. This solid was collected by filtration, and was washed with pentane (5 mL × 3) to give the product 4′ as white powder in 42% yield (206 mg, 0.27 mmol). Crystals suitable for X-ray diffraction analysis was obtained by cooling

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CONCLUSION New disilaferra- and ruthenacyclic isocyanide complexes 3′ and 4′ possessed an excellent catalytic activity toward alkenes, especially tri- and tetrasubstituted alkenes. In particular, both of the catalysts achieved hydrogenation of three tetrasubstituted alkenes with full conversion of the substrates and TON reached 100−1000. The DFT calculations clearly highlighted the importance of the η2-(H2) ligands, which induced facile M− Si/η2-(H−H) and η2-(H−Si)/M−C(alkene) metathesis to provide catalytic cycles with a low energy barrier. The isocyanide and the η2-(H−Si) ligands did not experience steric repulsion between the ligand and substituents on the alkenes; this key feature explains the good catalytic performance of 3′ and 4′ for the hydrogenation of sterically hindered alkenes. It is a shortcoming of the present disilametallacycic catalysts that 4130


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Journal of the American Chemical Society the saturated ether solution of 4′ at −30 °C. 1H NMR (600 MHz, 20 °C, THF-d8) δ = −7.27 (brs, 2H, H−Si), 0.73 (s, 24H, SiMe2), 1.77 (s, 12H, ortho-CH3 of mesityl), 2.13 (s, 6H, para-CH3 of mesityl), 6.70(s, 4H, C6H2), 7.27−7.34 (m, 4H, C6H4) 7.64−7.70 (m, 4H, C6H4). 1H NMR (600 MHz, 20 °C, C7D8) δ = −7.09 (brs, 2H, H−Si), 0.98 (s, 24H, SiMe2), 1.71 (s, 12H, ortho-CH3 of mesityl), 1.78 (s, 6H, paraCH3 of mesityl), 6.17(s, 4H, C6H2), 7.32−7.37 (m, 4H, C6H4) 7.76− 7.80 (m, 4H, C6H4). 1H NMR (600 MHz, −50 °C, C7D8) δ = −7.11 (s, 2H, H−Si, with a satellite signal due to the coupling with 29Si, JSi−H = 17.9 Hz), 1.04 (s, 24H, SiMe2), 1.67 (s, 12H, ortho-CH3 of mesityl), 1.73 (s, 6H, para-CH3 of mesityl), 6.02(s, 4H, C6H2), 7.36−7.41 (m, 4H, C6H4) 7.76−7.82 (m, 4H, C6H4). 13C NMR (150 MHz, r.t., THFd8) δ = 9.6 (s, SiMe2), 18.8 (s, C6H2Me3), 20.7 (s, C6H2Me3), 128.5 (s, Ar), 129.2 (s, Ar), 131.3 (s, Ar), 135.6 (s, Ar), 138.9 (s, Ar), 148.8 (s, C6H4-ipso), 156.3 (s, CN). 29Si NMR (119 MHz, r.t., C7D8); no peak was observed. IR (ATR) νCN = 2082 cm−1, νSi−H = 1918 cm−1. Anal. Calcd for C40H56N2Si4Ru1: C, 61.83; H, 7.25; N, 3.60. Found: C, 61.98; H, 7.32; N, 3.68. General Procedure for Hydrogenation of Alkenes Catalyzed by 3′ or 4′. In a 50 mL Schlenk tube, catalyst 3′ or 4′ (0.5−1 mol %), and alkene (1 mmol) were dissolved in toluene (0.5 mL) or DME (0.5 mL). For the experiments under 1 atm of H2, the atmosphere in the Schlenk tube was replaced by hydrogen. Alternatively, the catalyst solution was transferred into a stainless autoclave, and the atmosphere was replaced by 5−20 atm of hydrogen. The resulting mixture was stirred at 25 or 80 °C for the time indicated in the table. Then anisole (108 μL, 1.0 mmol, internal standard) was added, and the conversion of alkene and the yield of the product were determined by GC analysis. Detail conditions are described in the footnotes of Tables 1−4. Large Scale Experiments for the Hydrogenation of 1Octene. In a 50 mL Schlenk tube, the catalyst 3′ or 4′ (0.005 mmol, 0.05 mol %) and 1-octene (1.12 g, 10 mmol) were dissolved in the solvent (0.5 mL). The mixture was transferred into a stainless autoclave, and the atmosphere was replaced by hydrogen. The resulting mixture was stirred at 80 °C. In the reaction using 3′ as the catalyst, DME was used as solvent, and the reaction was performed under 20 atm of H2 for 6 h. In the reaction using 4′ as the catalyst, toluene was used as solvent, and the reaction was performed under 10 atm of H2 for 3 h. After cooling to room temperature, anisole (108 μL, 1.0 mmol, internal standard) was added, and the conversion of 1octene and the yield of n-octane were determined by GC analysis. The quantitative formation of n-octane was confirmed, and TON reached 2000. The reaction with lower catalyst loading was carried out without solvent at 80 °C for 2.5 h under 10 atm of H2 in the presence of 4′ (1.2 mg, 0.0015 mmol) and 1-octene (3.36 g, 30 mmol). n-Octane was obtained quantitatively (TON = 20 000). Hydrogenation of Terpinen-4-ol Catalyzed by 3′ or 4′. With the same hydrogenation procedures as those described above, hydrogenation of terpinen-4-ol (154 mg, 1 mmol) was carried out with 3′ (7.3 mg, 0.01 mmol) or 4′ (7.8 mg, 0.01 mmol). The reaction with 3′ was performed under 10 atm of H2 at 80 °C for 10h in DME (0.5 mL), whereas that with 4′ was performed under 1 atm of H2 at 25 °C for 6h in toluene (0.5 mL). After yield was determined by GC analysis with anisole as the internal standard, the crude product was purified by alumina column eluting with pentane/ether. Isolated yields were 134 mg (86%) for the reaction using 3′ and 141 mg (90%) for the reaction using 4′. The ratio of cis to trans was estimated by the integral ratio in the 13C NMR spectrum (90:10 for 3′ and 4′). 4Methyl-1-(1-methylethyl)-cyclohexanol (the mixture of cis and trans isomers): 1H NMR (395 MHz, CDCl3) δ = major isomer: 0.81 (d, 6H, JH−H = 6.8 Hz, −CH(CH3)2), 0.75−0.85 (m, 2H), 0.90−1.05 (m, 1H), 1.07−1.36 (m, 5H), 1.37−1.84 (m, 5H); minor isomer: 0.79 (d, 6H, JH−H = 6.8 Hz, −CH(CH3)2), other peaks are overlapped with the major peaks. 13C NMR (99 MHz, CDCl3) δ = major isomer: 17.1, 22.6, 30.5, 32.5, 33.9, 38.8, 72.8; minor isomer: 16.3, 20.7, 25.2, 30.7, 31.4, 34.4, 73.4. Large Scale Experiments for the Hydrogenation of 2,3Dimethyl-1H-indene. In a similar manner as above, the hydrogenation of 2,3-dimethyl-1H-indene was performed by using 4′ (3.9

mg, 0.005 mmol) and 2,3-dimethyl-1H-indene (720 mg, 5 mmol) in toluene (0.1 mL) at 80 °C for 5h under 10 atm of hydrogen. The yield of the product was determined by GC analysis with anisole as the internal standard. 2,3-Dihydro-cis-1,2-dimethyl-1H-indene: 1H NMR (395 MHz, CDCl3) δ = 0.97 (d, 3H, JH−H = 7.3 Hz, −CH2CHCH3), 1.13 (d, 3H, JH−H = 7.3 Hz, −CHCH3), 2.52−2.62 (m, 2H, −CH2−), 2.93−3.01 (m, 1H, −CH2CHCH3), 3.15 (quint, 1H, JH−H = 7.3 Hz, −CHCH3), 7.11−7.21 (m, 4H, C6H4). 13C NMR (99 MHz, CDCl3) δ = 14.6, 15.2, 37.8, 39.4, 42.4, 123.6, 124.4, 126.0, 126.8, 143.0, 148.8. X-ray Data Collection and Reduction. X-ray crystallography was performed on a Rigaku Saturn CCD area detector with graphite monochromated Mo Kα radiation (λ = 0.71075 Å). The data were collected at 123(2) K using ω scan in the θ range of 3.21 ≤ θ ≤ 27.48 deg (3′), 3.21 ≤ θ ≤ 27.48 deg (4′), 3.39 ≤ θ ≤ 27.47 deg (5′), 3.70 ≤ θ ≤ 27.47 deg (6′), 1.67 ≤ θ ≤ 30.85 deg (7′). The data obtained were processed using Crystal-Clear (Rigaku) on a Pentium computer, and were corrected for Lorentz and polarization effects. The structures were solved by direct methods,36 and expanded using Fourier techniques.37 Hydrogen atoms were refined using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on 4570 observed reflections and 218 variable parameters for 3′, 4637 observed reflections and 218 variable parameters for 4′, 5262 observed reflections and 245 variable parameters for 5′, 3895 observed reflections and 173 variable parameters for 6′, 4781 observed reflections and 227 variable parameters for 7′. Neutral atom scattering factors were taken from Cromer and Waber.38 All calculations were performed using the CrystalStructure39 crystallographic software package except for refinement, which was performed using SHELXL-97.40 Details of final refinement as well as the bond lengths and angle are summarized in Tables S3−S7, and the numbering scheme employed is also shown in Figures S1−S5, which were drawn with ORTEP at 50% probability ellipsoids. CCDC numbers 1556196 (3′), 1503533 (4′), 1503534 (5′), 1503536 (6′) and 1503537 (7′) 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. DFT Calculations. All of the calculations were performed using the Gaussian 09 program to search for all intermediates and transition structures on potential energy surfaces.41 For optimization, the M06 functional42 was selected based on the previous report.18b,c We also employed the SDD (Stuttgart/Dresden pseudopotentials)43 and 631G** basis sets44 for Ru atoms and the other atoms, respectively [BS1]. All stationary-point structures were found to have the appropriate number of imaginary frequencies. An appropriate connection between a reactant and a product was confirmed by intrinsic reaction coordinate (IRC)45 and quasi-IRC (qIRC) calculations. In the quasi-IRC calculation, the geometry of a transition state was first shifted by perturbing the geometries very slightly along the reaction coordinate, and then released for equilibrium optimization. To determine the energy profile of the proposed catalytic cycle, we performed single-point energy calculations at the optimized geometries using the SDD (Stuttgart/Dresden pseudopotentials) and 6-311+G**46 basis sets for Ru atoms and the other atoms, respectively. Solvent effects of toluene (ε = 2.379) were evaluated using the polarizable continuum model (PCM) [BS2].47 Energy profiles of the calculated reaction pathways are presented as Gibbs free energy changes (ΔG) involving thermal corrections at 298.15 K. All of the optimized structures (ball-and-stick models) and optimized geometries in XYZ file format are summarized in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b00812. Experimental data and compound characterization including the details for additional experiment, the molecular structures of 3′, 4′, 5′, 6′, and 7′, and details 4131


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of crystallographic studies, details of theoretical calculations, and the copy of the actual NMR chart for all products (PDF) X-ray diffraction data (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yusuke Sunada: 0000-0002-8954-181X Kazunari Yoshizawa: 0000-0002-6279-9722 Hideo Nagashima: 0000-0001-9495-9666 Present Address §

Institute of Industrial Science, The University of Tokyo, 4−6− 1 Meguro-ku, Komaba, Tokyo 153−8505, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Core Research Evolutional Science and Technology (CREST) Program of Japan Science and Technology Agency (JST) Japan, Integrated Research Consortium on Chemical Sciences (IRCCS), and Grant in Aid for Scientific Research (B) (No. 16H04120), Young Scientist (A) (No. 24685011), and Young Scientist (B) (No. 15K21222) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, the Naito Foundation, and the Hattori Hokokai Foundation.

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particles. If the rate retardation is insignificant, it is difficult to rule out the possibility that dibenzo[a,e]cyclooctatetraene does not bind to the molecular catalytic species effectively. Crabtree’s test for the hydrogenation of 1-octene catalyzed by 3′ and 4′ in the presence of dibenzo[a,e]cyclooctatetraene was negative and small (∼15% retardation) as shown in the Supporting Information. The mercury drop test for the ruthenium catalyst 4′ excludes the nanoparticle catalysis, and is inconsistent with the Crabtree’s test. We consider that dibenzo[a,e]cyclooctatetraene is not a good substrate, which coordinates to the catalytically active species in the hydrogenations shown in this paper and affects the reaction rate. See: (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855−859. (b) Consorti, C. S.; Flores, F. R.; Dupont, J. J. Am. Chem. Soc. 2005, 127, 12054−12065. (29) (a) Oxgaard, J.; Goddard, W. A., III. J. Am. Chem. Soc. 2004, 126, 442−443. (b) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; III; Periana, R. P. J. Am. Chem. Soc. 2004, 126, 352−363. (c) Tenn, W. J.; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A.; III; Periana, R. P. J. Am. Chem. Soc. 2005, 127, 14172−14173. (30) As described in the Introduction, iridium catalysts developed by Crabtree and Pfaltz are active toward hydrogenation of hindered alkenes. The mechanisms are not fully investigated, but theoretical studies for Pfaltz’s catalysts suggest the involvement of Ir+(H)2(η2-H2) (η2-alkene)(L̂ L) [L̂ L = bidentate ligands] composed of phosphorous and nitrogen moieties or those of NHC and nitrogen moieties).28a,b Recently, Ir+(H)2(η2-alkene)(L̂ L) as a resting state of the catalytic hydrogenation was experimentally evidenced.28c It should be noted that DFT calculations by Burges, Hall, and their co-workers suggests hydrogen atom migration from the η2-H2 ligand to η2-alkene, which is similar to step h in Scheme 4, hydrogen atom migration from η2-Si−H ligand to η2-alkene. See: (a) Fan, Y.; Cui, X.; Burgess, K.; Hall, M. B. J. Am. Chem. Soc. 2004, 126, 16688−16689. (b) Brandt, P.; Hedberg, C.; Andersson, P. G. Chem. - Eur. J. 2003, 9, 339−347. (c) Gruber, S.; Pfaltz, A. Angew. Chem., Int. Ed. 2014, 53, 1896−1900. (31) The effect of DME described in the text indicates that a disilaferracycle having two CNR ligands and DME may be a good candidate for the improved catalyst. DME is more easily replaceable by H2 and an alkene than BDSB. However, DFT calculations suggest that this species (R = mesityl) is higher in energy than the starting complex 3′ (ΔG° = +14.6 kcal/mol). Experiments to attempt the replacement of BDSB in 3′ by DME under several conditions so far unsuccessful. (32) (a) Gerlach, D. H.; Schunn, R. A. Inorg. Synth. 2007, 15, 1−4. (b) Garbonaro, A.; Greco, A.; Dallasta, G. J. Organomet. Chem. 1969, 20, 177−186. (33) Nagashima, H.; Tatebe, K.; Ishibashi, I.; Nakaoka, A.; Sakakibara, J.; Itoh, K. Organometallics 1995, 14, 2868. (34) Genêt, J. P.; Pinel, C.; Ratovelomanana-Vidal, V.; Mallart, S.; Pfister, X.; Caño De Andrade; Laffitte, J. A. Tetrahedron: Asymmetry 1994, 5, 665−674. (35) Tanabiki, M.; Tsuchiya, K.; Kumanomido, Y.; Matsubara, K.; Motoyama, Y.; Nagashima, H. Organometallics 2004, 23, 3976−3981. (36) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Siliqi, D.; Spagna, R. J. Appl. Crystallogr. 2012, 45, 357−361. (37) DIRDIF99: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF99 program system; Technical Report of the Crystallography Laboratory; University of Nijmegen, Nijmegen, The Netherlands, 1999. (38) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4. (39) CrystalStructure 4.0: Crystal Structure Analysis Package; Rigaku Corporation: Tokyo, Japan, 2000−2010. (40) SHELX97: Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; 4133


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