DOI: 10.1002/chem.201501568
Communication
& Hydrosilylation
Construction of a Chiral Silicon Center by Rhodium-Catalyzed Enantioselective Intramolecular Hydrosilylation Yuki Naganawa,* Tomoya Namba, Mayu Kawagishi, and Hisao Nishiyama*[a] Abstract: Rhodium-catalyzed enantioselective desymmetrizing intramolecular hydrosilylation of symmetrically disubstituted hydrosilanes is described. The original axially chiral phenanthroline ligand (S)-BinThro (Binol-derived phenanthroline) was found to work as an effective chiral catalyst for this transformation. A chiral silicon stereogenic center is one of the chiral motifs gaining much attention in asymmetric syntheses and the present protocol provides cyclic five-membered organosilanes incorporating chiral silicon centers with high enantioselectivities (up to 91 % ee). The putative active RhI catalyst takes the form of an N,N,O-tridentate coordination complex, as determined by several complementary experiments.
Transition metal-catalyzed hydrosilylation of unsaturated C¢C bonds has been established as a fundamental protocol giving useful organosilicon compounds through C¢Si bond forming processes. The catalytic asymmetric variant using chiral transition metal catalysts is also an attractive and important subject.[1] Despite many examples of successful enantioselective hydrosilylation, efforts toward the synthesis of organosilanes incorporating chiral silicon centers have rarely focused on the synthetic utility of hydrosilylation reactions. Recently, catalytic asymmetric construction of chiral silicon centers has attracted much interest, owing to their unique chemical and physical properties.[2] Among them, only a few examples of catalytic asymmetric synthesis of chiral silicon compounds via hydrosilylation have been reported. In 1996, Tamao et al. reported a pioneering study in this field, namely the Rh-catalyzed intramolecular double hydrosilylation of bis(alkenyl)dihydrosilane 1 to give spirosilane 2 with axial chirality (Scheme 1 a).[2a] The declared purpose of this study was interest in the application of 2 as a new material, rather than the development of new reaction protocols, thus the scope of this enantioselective transformation was not explored extensively, with very limited employment of just one substrate (1). In 2012, Tomooka and co-workers performed another asymmetric synthesis of chiral silicon centers by utilizing Pt-catalyzed intermolecular hydro[a] Dr. Y. Naganawa, T. Namba, M. Kawagishi, Prof. Dr. H. Nishiyama Department of Applied Chemistry, Graduate School of Engineering Nagoya University, Chikusa, Nagoya 464-8603 (Japan) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501568. Chem. Eur. J. 2015, 21, 9319 – 9322
Scheme 1. Construction of chiral organosilanes by hydrosilylation: a) Tamao and co-workers: intramolecular double hydrosilylation to give spirosilane;[2a] b) Tomooka and co-workers: intermolecular hydrosilylation to give alkenylhydrosilane;[2b] c) this work: intramolecular desymmetrizing hydrosilylation.
silylation of alkynes 3 with dihydrosilanes 4 to give alkenylhydrosilanes 5 with chiral silicon centers with up to 86 % ee (Scheme 1 b).[2b] The enantiodiscrimination event in this reaction occurs in the oxidative addition of prochiral dihydrosilanes 4 to the chiral Pt complex. In this context, our synthetic plan is based on the utilization of enantioselective desymmetrization, which is an efficient and powerful methodology to obtain complex chiral compounds in a single operation.[3, 4] We envisioned that the enantioselective desymmetrization of prochiral silanes[5] 6 should occur by intramolecular hydrosilylation[6, 7] in the presence of a chiral transition metal catalyst, thus giving chiral silicon-containing olefins 7 (Scheme 1 c). In this reaction, enantioinduction should be controlled by selective insertion of a rhodium hydride species into one olefin under the influence of the chiral ligand. We previously developed a novel axially chiral phenanthroline ligand BinThro (Binol-derived phenanthroline; 8) and examined its activity in enantioselective organozinc addition to aldehydes (Figure 1).[8] 1,10-Phenanthroline (phen) is a classical N,N-chelating reagent and is used in various organic reactions.[9] However, studies on the development of chiral ligands
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Communication Table 1. Optimization of asymmetric desymmetrization of hydrosilane 6 a.[a]
Figure 1. BinThro ligands 8 used in this study.
utilizing the phen motif have been very limited, probably due to the difficulty of introducing an effective chiral environment with its rigid and planar structure.[10] Nevertheless, the noteworthy characteristics of our ligand 8 include its distinctive N,N,O-tridentate coordination. In addition, the natural chiralauxiliary-free molecular design[11] readily enables fine tuning of the ligand structure.[12] We report herein that the Rh complex of ligand 8 works as an effective chiral catalyst for the enantioselective desymmetrization of symmetrically substituted hydrosilanes 6 by hydrosilylation of olefins.[13] To identify effective catalytic systems, we initially examined the intramolecular desymmetrizing hydrosilylation of phenylbis(2-vinylcyclohexenyl)silane 6 a as a test substrate and screened various transition metal precursors and chiral ligands in toluene at 60 8C (Table 1). Among various transition metals applied to hydrosilylation,[1] [{Rh(cod)Cl}2] was found to be an effective transition metal complex with BinThro (S)-8 a to give the corresponding five-membered cyclic organosilane 7 a in moderate yield and with good ee (53 % yield, 70 % ee), whereas the use of Pt0, Ni0 and Pd0 led to no reaction (Table 1, entries 1–4). The screening of RhI salts determined that [{Rh(cod)OMe}2] was the most effective precursor to provide 7 a in quantitative yield and with 75 % ee (Table 1, entries 5–7). To improve the enantioselectivity, we then moved to the screening of BinThro ligands having various aromatic substituents at the C2 position of the naphthalene ring without a hydroxy group (X, Figure 1). The removal of the phenyl group at C2 in (S)-8 a (X = H; (S)-8 b) drastically diminished both yield and ee (48 % yield, 17 % ee; Table 1, entry 8). The reaction occurred smoothly in the presence of the ligand (S)-8 c, in which the aromatic group was replaced by another hydroxy group, albeit with lower enantioselectivity (92 % yield, 54 % ee; Table 1, entry 9). Concerning the substituent effect of the aromatic ring of (S)-8 a, the introduction of a methyl group at the para-position decreased the selectivity (84 % yield, 71 % ee; Table 1, entry 10). In contrast, the ligand (S)-8 e, bearing a meta-substituted 3,5-xylyl group, gave the best ee (> 99 % yield, 80 % ee; Table 1, entry 11). The replacement of the methyl substituents on the 3,5-xylyl group by electron-donating or -withdrawing substituents was ineffective in improving the enantioselectivity (Table 1, entries 12 and 13). We then investigated the combination of [{Rh(cod)OMe}2] and chiral ligands binap and diop, which were recommended by Tamao et al.[2a]as good chiral ligands (Table 1, entries 14 and 15). However, these attempts resulted in low conversion with low ee or no reaction with the recovery of the starting materials, respectively. Interestingly, simple non-chiral phen was also an ineffective ligand in this system (Table 1, entry 16). In every case, intermolecular hydrosilylation was not observed. Chem. Eur. J. 2015, 21, 9319 – 9322
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Entry
[M] (x [mol %])
Ligand
Yield [%][b]
ee [%][c]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
[Pt(dba)2] (5) [Ni(cod)2] (5) [Pd(dba)2] (5) [{Rh(cod)Cl]}2] (2.5) [{Rh(coe)2Cl}2](2.5) [Rh(cod)2][BF4] (5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5)
(S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 b (S)-8 c (S)-8 d (S)-8 e (S)-8 f (S)-8 g (S)-BINAP (R, R)-DIOP phen
n.r. n.r. n.r. 53 65 n.r. 98 48 92 84 > 99 96 95 39 n.r. trace
n.d. n.d. n.d. 70 42 n.d. 75 17 54 71 80 79 79 10 n.d. n.d.
[a] Reaction conditions: phenylbis(2-vinylcyclohexenyl)silane 6a (0.1 mmol), transition metal (5 mol % metal), chiral ligand (6 mol %), toluene (1 mL), 60 8C, 24 h; [b] yield of the isolated product after column chromatography; [c] the ee value was determined by HPLC using a chiral stationary phase. dba = dibenzylideneacetone; cod = 1,5-cyclooctadiene; coe = cyclooctene; phen = 1,10-phenanthroline; n.r. = no reaction; n.d. = not determined.
With the optimized conditions in hand, we studied the scope of this asymmetric desymmetrization of hydrosilanes 6 (Table 2). Considering the substituent effect of the phenyl ring on silicon, several hydrosilanes 6 b–d were found to tolerate the reaction conditions to furnish the corresponding five-membered chiral organosilanes 7 b–d with 80–85 % ee. Next, we investigated the reaction of hydrosilanes 6 e–g with two styrenetype olefin side chains. Compared to hydrosilanes 6 a–d, with similar six-membered side chains, intramolecular hydrosilylation of 6 e–g gave products 7 e–g with relatively lower ee values. However, we found that the enantiopurity of 7 e was increased to > 95 % ee by a simple recrystallization process in n-hexane. Fortunately, the X-ray crystallographic analysis of this enantiopure 7 e was possible and the absolute stereochemistry was determined as S (Figure 2).[14] Finally, we conducted the reactions of symmetrically substituted hydrosilanes 6 h–j with various olefin side chains. Hydrosilanes 6 h–j were converted to the corresponding tricyclic condensed structures 7 h–j with good ee. For 7 h, the highest ee value of 91 % was observed. To shed light on the structure of the RhI complex of these novel ligands, we attempted several supporting experiments. Because simple N,N-bidentate phen was an ineffective ligand
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Communication Table 2. Scope of asymmetric desymmetrization of hydrosilanes 6.
Scheme 2. The possible structures of RhI complex 9 and 9’.
tion is the generation of MeOH, which should indicate that the proton of the hydroxy group of (S)-8 a is removed by the methoxide anion in [{Rh(cod)OMe}2].[15] 1H NMR spectroscopy also identified that one olefin of the external cod ligand coordinated with the RhI center and the other one was a free ligand.[15] The monodentate cod should be readily dissociated from the RhI center and hydrosilane 6 can approach this vacant site. We also detected the presence of 9’, in which the cod ligand from 9 had been released, by HRMS.[15] To clarify the role of the hydroxy group of (S)-8, we prepared the ligand (S)-10, in which the hydroxy group was protected with a methyl group, and found that it was not effective for this transformation. This result provided further evidence that 9 was not an N,N-bidentate but an N,N,O-tridentate complex (Scheme 3). In summary, we demonstrated Rh-catalyzed enantioselective desymmetrization of symmetrically substituted hydrosilanes 6 by intramolecular hydrosilylation of unsaturated C¢C bonds. The products were five-membered organosilanes 7 with achiral silicon stereogenic centers. We expect that these organosilanes 7 can find aplication in the development of silicon-based advanced materials such as benzosiloles[2, 16] with a chiral silicon center, formed by the oxidation of 7,[17] or silicon polymers,[18] formed by polymerization via the unreacted olefin. It should be also noted that the present enantioselective transformation was successfully conducted only in the presence of our original chiral phen ligand 8, which forms an N,N,O-tridentate complex with [{Rh(cod)OMe}2], the structure having been confirmed by
[a] [{Rh(cod)OMe}2] (5 mol %), (S)-8 e (12 mol %); [b] ee value after recrystallization from n-hexane.
Figure 2. X-ray crystal structure of cyclic organosilane (S)-7 e. Thermal ellipsoids are shown at the 50 % probability level. All hydrogen atoms are omitted for clarity.
for this intramolecular hydrosilylation (Table 1, entry 16), we assumed that the in situ-generated chiral RhI complex of (S)-8 a might adopt not N,N-bidentate but N,N,O-tridentate coordination. We prepared RhI complex 9 by the reaction of [{Rh(cod)OMe}2] and (S)-8 a in [D8]toluene at 60 8C for 1 h (Scheme 2). Although we could not directly confirm the structure of 9 by X-ray crystallographic analysis, we assigned the N,N,O-tridentate coordinated structure by 1H NMR spectroscopy.[15] Strong evidence for this N,N,O-tridentate coordinaChem. Eur. J. 2015, 21, 9319 – 9322
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Scheme 3. Reaction with N,N-bidentate ligand (S)-10.
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Communication 1
H NMR spectroscopy and HRMS. To further broaden the utility of the ligands 8, we plan to study various asymmetric reactions using a wide range of metal catalysts.
Acknowledgement This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Nos. 22245014 and 25810060). Keywords: asymmetric catalysis hydrosilylation · rhodium · silicon
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desymmetrization
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Received: April 22, 2015 Published online on May 28, 2015
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Communication
& Hydrosilylation
Construction of a Chiral Silicon Center by Rhodium-Catalyzed Enantioselective Intramolecular Hydrosilylation Yuki Naganawa,* Tomoya Namba, Mayu Kawagishi, and Hisao Nishiyama*[a] Abstract: Rhodium-catalyzed enantioselective desymmetrizing intramolecular hydrosilylation of symmetrically disubstituted hydrosilanes is described. The original axially chiral phenanthroline ligand (S)-BinThro (Binol-derived phenanthroline) was found to work as an effective chiral catalyst for this transformation. A chiral silicon stereogenic center is one of the chiral motifs gaining much attention in asymmetric syntheses and the present protocol provides cyclic five-membered organosilanes incorporating chiral silicon centers with high enantioselectivities (up to 91 % ee). The putative active RhI catalyst takes the form of an N,N,O-tridentate coordination complex, as determined by several complementary experiments.
Transition metal-catalyzed hydrosilylation of unsaturated C¢C bonds has been established as a fundamental protocol giving useful organosilicon compounds through C¢Si bond forming processes. The catalytic asymmetric variant using chiral transition metal catalysts is also an attractive and important subject.[1] Despite many examples of successful enantioselective hydrosilylation, efforts toward the synthesis of organosilanes incorporating chiral silicon centers have rarely focused on the synthetic utility of hydrosilylation reactions. Recently, catalytic asymmetric construction of chiral silicon centers has attracted much interest, owing to their unique chemical and physical properties.[2] Among them, only a few examples of catalytic asymmetric synthesis of chiral silicon compounds via hydrosilylation have been reported. In 1996, Tamao et al. reported a pioneering study in this field, namely the Rh-catalyzed intramolecular double hydrosilylation of bis(alkenyl)dihydrosilane 1 to give spirosilane 2 with axial chirality (Scheme 1 a).[2a] The declared purpose of this study was interest in the application of 2 as a new material, rather than the development of new reaction protocols, thus the scope of this enantioselective transformation was not explored extensively, with very limited employment of just one substrate (1). In 2012, Tomooka and co-workers performed another asymmetric synthesis of chiral silicon centers by utilizing Pt-catalyzed intermolecular hydro[a] Dr. Y. Naganawa, T. Namba, M. Kawagishi, Prof. Dr. H. Nishiyama Department of Applied Chemistry, Graduate School of Engineering Nagoya University, Chikusa, Nagoya 464-8603 (Japan) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501568. Chem. Eur. J. 2015, 21, 9319 – 9322
Scheme 1. Construction of chiral organosilanes by hydrosilylation: a) Tamao and co-workers: intramolecular double hydrosilylation to give spirosilane;[2a] b) Tomooka and co-workers: intermolecular hydrosilylation to give alkenylhydrosilane;[2b] c) this work: intramolecular desymmetrizing hydrosilylation.
silylation of alkynes 3 with dihydrosilanes 4 to give alkenylhydrosilanes 5 with chiral silicon centers with up to 86 % ee (Scheme 1 b).[2b] The enantiodiscrimination event in this reaction occurs in the oxidative addition of prochiral dihydrosilanes 4 to the chiral Pt complex. In this context, our synthetic plan is based on the utilization of enantioselective desymmetrization, which is an efficient and powerful methodology to obtain complex chiral compounds in a single operation.[3, 4] We envisioned that the enantioselective desymmetrization of prochiral silanes[5] 6 should occur by intramolecular hydrosilylation[6, 7] in the presence of a chiral transition metal catalyst, thus giving chiral silicon-containing olefins 7 (Scheme 1 c). In this reaction, enantioinduction should be controlled by selective insertion of a rhodium hydride species into one olefin under the influence of the chiral ligand. We previously developed a novel axially chiral phenanthroline ligand BinThro (Binol-derived phenanthroline; 8) and examined its activity in enantioselective organozinc addition to aldehydes (Figure 1).[8] 1,10-Phenanthroline (phen) is a classical N,N-chelating reagent and is used in various organic reactions.[9] However, studies on the development of chiral ligands
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Communication Table 1. Optimization of asymmetric desymmetrization of hydrosilane 6 a.[a]
Figure 1. BinThro ligands 8 used in this study.
utilizing the phen motif have been very limited, probably due to the difficulty of introducing an effective chiral environment with its rigid and planar structure.[10] Nevertheless, the noteworthy characteristics of our ligand 8 include its distinctive N,N,O-tridentate coordination. In addition, the natural chiralauxiliary-free molecular design[11] readily enables fine tuning of the ligand structure.[12] We report herein that the Rh complex of ligand 8 works as an effective chiral catalyst for the enantioselective desymmetrization of symmetrically substituted hydrosilanes 6 by hydrosilylation of olefins.[13] To identify effective catalytic systems, we initially examined the intramolecular desymmetrizing hydrosilylation of phenylbis(2-vinylcyclohexenyl)silane 6 a as a test substrate and screened various transition metal precursors and chiral ligands in toluene at 60 8C (Table 1). Among various transition metals applied to hydrosilylation,[1] [{Rh(cod)Cl}2] was found to be an effective transition metal complex with BinThro (S)-8 a to give the corresponding five-membered cyclic organosilane 7 a in moderate yield and with good ee (53 % yield, 70 % ee), whereas the use of Pt0, Ni0 and Pd0 led to no reaction (Table 1, entries 1–4). The screening of RhI salts determined that [{Rh(cod)OMe}2] was the most effective precursor to provide 7 a in quantitative yield and with 75 % ee (Table 1, entries 5–7). To improve the enantioselectivity, we then moved to the screening of BinThro ligands having various aromatic substituents at the C2 position of the naphthalene ring without a hydroxy group (X, Figure 1). The removal of the phenyl group at C2 in (S)-8 a (X = H; (S)-8 b) drastically diminished both yield and ee (48 % yield, 17 % ee; Table 1, entry 8). The reaction occurred smoothly in the presence of the ligand (S)-8 c, in which the aromatic group was replaced by another hydroxy group, albeit with lower enantioselectivity (92 % yield, 54 % ee; Table 1, entry 9). Concerning the substituent effect of the aromatic ring of (S)-8 a, the introduction of a methyl group at the para-position decreased the selectivity (84 % yield, 71 % ee; Table 1, entry 10). In contrast, the ligand (S)-8 e, bearing a meta-substituted 3,5-xylyl group, gave the best ee (> 99 % yield, 80 % ee; Table 1, entry 11). The replacement of the methyl substituents on the 3,5-xylyl group by electron-donating or -withdrawing substituents was ineffective in improving the enantioselectivity (Table 1, entries 12 and 13). We then investigated the combination of [{Rh(cod)OMe}2] and chiral ligands binap and diop, which were recommended by Tamao et al.[2a]as good chiral ligands (Table 1, entries 14 and 15). However, these attempts resulted in low conversion with low ee or no reaction with the recovery of the starting materials, respectively. Interestingly, simple non-chiral phen was also an ineffective ligand in this system (Table 1, entry 16). In every case, intermolecular hydrosilylation was not observed. Chem. Eur. J. 2015, 21, 9319 – 9322
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Entry
[M] (x [mol %])
Ligand
Yield [%][b]
ee [%][c]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
[Pt(dba)2] (5) [Ni(cod)2] (5) [Pd(dba)2] (5) [{Rh(cod)Cl]}2] (2.5) [{Rh(coe)2Cl}2](2.5) [Rh(cod)2][BF4] (5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5) [{Rh(cod)OMe}2] (2.5)
(S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 a (S)-8 b (S)-8 c (S)-8 d (S)-8 e (S)-8 f (S)-8 g (S)-BINAP (R, R)-DIOP phen
n.r. n.r. n.r. 53 65 n.r. 98 48 92 84 > 99 96 95 39 n.r. trace
n.d. n.d. n.d. 70 42 n.d. 75 17 54 71 80 79 79 10 n.d. n.d.
[a] Reaction conditions: phenylbis(2-vinylcyclohexenyl)silane 6a (0.1 mmol), transition metal (5 mol % metal), chiral ligand (6 mol %), toluene (1 mL), 60 8C, 24 h; [b] yield of the isolated product after column chromatography; [c] the ee value was determined by HPLC using a chiral stationary phase. dba = dibenzylideneacetone; cod = 1,5-cyclooctadiene; coe = cyclooctene; phen = 1,10-phenanthroline; n.r. = no reaction; n.d. = not determined.
With the optimized conditions in hand, we studied the scope of this asymmetric desymmetrization of hydrosilanes 6 (Table 2). Considering the substituent effect of the phenyl ring on silicon, several hydrosilanes 6 b–d were found to tolerate the reaction conditions to furnish the corresponding five-membered chiral organosilanes 7 b–d with 80–85 % ee. Next, we investigated the reaction of hydrosilanes 6 e–g with two styrenetype olefin side chains. Compared to hydrosilanes 6 a–d, with similar six-membered side chains, intramolecular hydrosilylation of 6 e–g gave products 7 e–g with relatively lower ee values. However, we found that the enantiopurity of 7 e was increased to > 95 % ee by a simple recrystallization process in n-hexane. Fortunately, the X-ray crystallographic analysis of this enantiopure 7 e was possible and the absolute stereochemistry was determined as S (Figure 2).[14] Finally, we conducted the reactions of symmetrically substituted hydrosilanes 6 h–j with various olefin side chains. Hydrosilanes 6 h–j were converted to the corresponding tricyclic condensed structures 7 h–j with good ee. For 7 h, the highest ee value of 91 % was observed. To shed light on the structure of the RhI complex of these novel ligands, we attempted several supporting experiments. Because simple N,N-bidentate phen was an ineffective ligand
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Communication Table 2. Scope of asymmetric desymmetrization of hydrosilanes 6.
Scheme 2. The possible structures of RhI complex 9 and 9’.
tion is the generation of MeOH, which should indicate that the proton of the hydroxy group of (S)-8 a is removed by the methoxide anion in [{Rh(cod)OMe}2].[15] 1H NMR spectroscopy also identified that one olefin of the external cod ligand coordinated with the RhI center and the other one was a free ligand.[15] The monodentate cod should be readily dissociated from the RhI center and hydrosilane 6 can approach this vacant site. We also detected the presence of 9’, in which the cod ligand from 9 had been released, by HRMS.[15] To clarify the role of the hydroxy group of (S)-8, we prepared the ligand (S)-10, in which the hydroxy group was protected with a methyl group, and found that it was not effective for this transformation. This result provided further evidence that 9 was not an N,N-bidentate but an N,N,O-tridentate complex (Scheme 3). In summary, we demonstrated Rh-catalyzed enantioselective desymmetrization of symmetrically substituted hydrosilanes 6 by intramolecular hydrosilylation of unsaturated C¢C bonds. The products were five-membered organosilanes 7 with achiral silicon stereogenic centers. We expect that these organosilanes 7 can find aplication in the development of silicon-based advanced materials such as benzosiloles[2, 16] with a chiral silicon center, formed by the oxidation of 7,[17] or silicon polymers,[18] formed by polymerization via the unreacted olefin. It should be also noted that the present enantioselective transformation was successfully conducted only in the presence of our original chiral phen ligand 8, which forms an N,N,O-tridentate complex with [{Rh(cod)OMe}2], the structure having been confirmed by
[a] [{Rh(cod)OMe}2] (5 mol %), (S)-8 e (12 mol %); [b] ee value after recrystallization from n-hexane.
Figure 2. X-ray crystal structure of cyclic organosilane (S)-7 e. Thermal ellipsoids are shown at the 50 % probability level. All hydrogen atoms are omitted for clarity.
for this intramolecular hydrosilylation (Table 1, entry 16), we assumed that the in situ-generated chiral RhI complex of (S)-8 a might adopt not N,N-bidentate but N,N,O-tridentate coordination. We prepared RhI complex 9 by the reaction of [{Rh(cod)OMe}2] and (S)-8 a in [D8]toluene at 60 8C for 1 h (Scheme 2). Although we could not directly confirm the structure of 9 by X-ray crystallographic analysis, we assigned the N,N,O-tridentate coordinated structure by 1H NMR spectroscopy.[15] Strong evidence for this N,N,O-tridentate coordinaChem. Eur. J. 2015, 21, 9319 – 9322
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Scheme 3. Reaction with N,N-bidentate ligand (S)-10.
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Communication 1
H NMR spectroscopy and HRMS. To further broaden the utility of the ligands 8, we plan to study various asymmetric reactions using a wide range of metal catalysts.
Acknowledgement This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Nos. 22245014 and 25810060). Keywords: asymmetric catalysis hydrosilylation · rhodium · silicon
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desymmetrization
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Received: April 22, 2015 Published online on May 28, 2015
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