Letter Cite This: Org. Lett. 2017, 19, 5736-5739
pubs.acs.org/OrgLett
Iron-Catalyzed Carbene Insertion Reactions of α‑Diazoesters into Si−H Bonds Hoda Keipour and Thierry Ollevier* Département de Chimie, Université Laval, 1045 avenue de la Médecine, Québec, QC G1V 0A6, Canada S Supporting Information *
ABSTRACT: An efficient iron-catalyzed carbene insertion reaction of α-diazo carbonyl compounds into the Si−H bond was developed. A wide range of α-silylesters was obtained in high yields (up to 99%) from α-diazoesters using a simple iron(II) salt as catalyst.
M
catalyst appears to be an important precedent for efficient enantioselective insertion reaction into the Si−H bond.20 In this paper, we report the insertion reactions of diazo compounds into Si−H bonds and demonstrate the scope and applicability of the reaction using highly practical conditions. Given that most of the catalyzed X−H bond insertion reactions involve Rh complexes, we focused here on an efficient and green metal catalytic system to probe the reactivity of diazo compounds into Si−H bonds. Diazo compounds were prepared from commercially available esters according to known procedures.18 The reaction of methyl α-phenyl-α-diazoacetate 1a and triethylsilane (Et3SiH) in CH2Cl2 was first examined using Fe(OTf)2 as catalyst. The reaction was complete within 1 h, and the corresponding α-silylester 2a was obtained in 97% yield (Table 1, entry 1). Interestingly, this compound cannot be easily prepared using synthetic methods other than the diazo insertion reaction into the Si−H bond.9,15a Useful applications of these α-silylesters have been demonstrated.18 Using the same conditions, various FeII (Table 1, entries 2−7) and FeIII (Table 1, entries 8−12) sources were tested, but none of them afforded the same reactivity as Fe(OTf)2 (Table 1, entry 1). No conversion was obtained using Fe(OAc)2 (Table 1, entry 4), and FeCl2 was shown to be an efficient catalyst leading to 81% yield (Table 1, entry 5). Only traces of the product were observed when FeBr3 and Fe(acac)3 were used as catalysts (Table 1, entries 11 and 12). Optimization studies indicated that Fe(OTf)2 was superior to the other FeII/FeIII sources for Si−H bond insertion reaction in CH2Cl2. Fe(OTf)2 has indeed been demonstrated as a superior catalyst in various transformations.21 It was consequently selected as the FeII source for further studies. A catalyst loading of 5 mol % was effective with 2 equiv of Et3SiH, whereas the product was formed in higher yield and decreased reaction time when the quantity of Et3SiH was increased to 4 equiv (Table 2, entries 1 and 2). A catalyst loading of 2 mol % was tested, but the product was formed in lower yields over longer reaction times (Table 2, entries 3−4).
etal catalysts are fundamental tools in organic chemistry. Iron is a very abundant element; its low cost and environmentally benign character make it attractive.1 New catalytic methods taking advantage of Fe are in high demand. Compared to rare metals, Fe still needs to be developed for industrial applications.1b,2 The chemistry of Fe is thus among the most promising fields to develop.3 Transition-metalcatalyzed reactions of diazo compounds generate metal carbene or metal carbene species that can undergo diverse transformations, such as X−H (X = C, N, O, Si, S, etc.) bondinsertion reactions.4 The various oxidation states of Fe make it appealing as a catalyst in diazo chemistry.5a Various Fe catalysts have been disclosed for N−H, O−H, and S−H bond insertion reactions.5 The only Fe-catalyzed enantioselective insertion reaction reported is an O−H bond insertion reaction using water or alcohols.6 To date, RhI/RhII, CuI/CuII, IrIII, AgI, and RuII complexes have been utilized as catalysts for Si−H bondinsertion reactions. The insertion of carbene species into the Si−H bond was first reported in the reaction of diazoalkanes with silanes.7 Watanabe developed the reaction of various silanes with methyl α-diazoacetate using Cu powder.8 The RhIIcatalyzed reaction of α-diazoesters and α-diazoketones with silanes was first reported by Doyle.9 Landais disclosed the synthesis of α-alkoxysilylacetic esters by Si−H bond insertion reaction of carbenes, generated by RhII-catalyzed decomposition of α-diazoacetic esters.10 Insertion reactions of diazo compounds into the Si−H bond were subsequently disclosed using RhII catalysts,11 including in enantioselective versions.12 A RhI-catalyzed enantioselective Si−H bond insertion reaction of α-diazoesters and α-diazophosphonates has been developed.13 Chiral IrIII complexes have also proven to be efficient catalysts for enantioselective Si−H bond insertion reaction.14 CuIcatalyzed Si−H bond insertion was reported for the first time by Panek.15 Zhou also developed a CuII-catalyzed highly asymmetric Si−H bond insertion reaction using spiro diimine ligands.16 Recent insertion reactions of diazo carbonyl compounds into the Si−H bond using CuI catalysts have been reported,17 including from our group.18 Insertion of Si−H bonds using AgI as catalyst was also demonstrated.19 As FeII is above RuII in the periodic table, a cationic RuII oxazoline © 2017 American Chemical Society
Received: August 10, 2017 Published: October 13, 2017 5736
DOI: 10.1021/acs.orglett.7b02488 Org. Lett. 2017, 19, 5736−5739
Letter
Organic Letters
Hexane afforded a low yield (Table 3, entry 5). When the reaction was run in polar, coordinating solvents, such as MeCN and THF, the conversion was low, and low yields were obtained (Table 3, entries 6 and 7). However, none of these solvents gave results superior to CH 2 Cl 2 , which was consequently chosen in further studies. A variety of α-diazoesters were examined to expand the scope of the substrates used for the Si−H bond insertion reaction with Et3SiH, which was run under the optimal reaction conditions. All substrates reacted to produce the corresponding α-silylesters in good to excellent yields (Scheme 1, 2b−m, 65−
Table 1. Screening of Iron Sources for Si−H Bond Insertion of Methyl α-Phenyl-α-diazoacetate 1aa
entry
[Fe]
time (h)
yield 2ab (%)
1 2 3 4 5 6 7 8 9 10 11 12
Fe(OTf)2 Fe(ClO4)2·6H2O Fe(BF4)2·6H2O Fe(OAc)2 FeCl2 FeBr2 Fe(NTf2)2 Fe(OTf)3 Fe(ClO4)3 FeCl3 FeBr3 Fe(acac)3
1 48 18 48 18 24 48 18 24 24 48 24
97 30 15 0 81 24 15 34 10 23 10 13
Scheme 1. FeII-Catalyzed Si−H Bond Insertion Reaction of α-Aryl-α-diazoacetatesa
a Conditions: iron salts (10 mol %), Et3SiH (0.5 mmol), methyl αphenyl-α-diazoacetate 1a (0.25 mmol), slow addition (1 h) of 1a. b Conversion determined by 1H NMR.
Table 2. Screening of Catalyst Loading for Si−H Bond Insertion Reaction of 1aa
entry
Fe(OTf)2 (x mol %)
Et3SiH (y equiv)
time (h)
yield 2ab (%)
1 2 3 4
5 5 2 2
2 4 2 4
5 1 24 18
92 98 90 96
a Conditions: Fe(OTf)2 (5 mol %), Et3SiH (1 mmol), 1b−m (0.25 mmol), slow addition (1 h) of α-aryl-α-diazoacetates 1b−m.
a
Conditions: Fe(OTf)2 (x mol %), Et3SiH (y mmol), 1a (0.25 mmol), slow addition (1 h) of 1a. bYield of isolated product.
99%), regardless of the nature and position of the substituents on the aromatic ring of the α-diazoesters (Scheme 1, 2b−m). However, the reactivity of the substrate was influenced by the electronic properties of the substituents on the aromatic ring of the α-aryl-α-diazoacetate. α-Aryl-α-diazoacetates containing electron-donating groups, such as methyl and methoxy groups (Scheme 1, 2b,c) required shorter reaction times to reach complete conversions. In contrast, α-aryl-α-diazoacetates containing electron-withdrawing groups, such as F, Cl, and Br (Scheme 1, 2d−j), required longer reaction times to obtain the desired α-silylesters in moderate to excellent yields (65−99%). Next, the scope of silanes was evaluated. Different silanes, such as diphenylmethylsilane (Ph2MeSiH), triphenylsilane (Ph3SiH), and phenyldimethylsilane (PhMe2SiH), were also successfully used in this transformation, affording the desired αsilylesters in moderate to high yields (Scheme 2, 3aa−n). Inspired by the aforementioned success obtained with α-arylα-diazoacetates, we next turned our attention to the more challenging α-alkyl-α-diazoacetates and diazotrifluoroalkane substrates in an attempt to obtain the corresponding α-alkylα-silyl ester and trifluoromethyl-α-silyl derivatives (Scheme 3). To the best of our knowledge, insertion of α-alkyl-α-diazoesters and diazotrifluoroalkanes into Si−H bonds has never been explored using Fe(OTf)2 as catalyst. A series of different diazo substrates was studied with different silane sources in the presence of Fe(OTf)2. In general, the reaction proceeded efficiently to give the corresponding Si−H bond insertion reaction products in moderate to good yields (Scheme 3, 5aa−
A 5 mol % quantity of Fe(OTf)2, together with 4 equiv of Et3SiH, was selected for further studies. The use of various solvents was studied in the insertion reaction of α-diazoesters. Chloroform, 1,2-dichloroethane, diethyl ether, and toluene were all suitable solvents, and very good to high yields were obtained (Table 3, entries 1−4). Table 3. Screening of Solvents for Si−H Bond Insertion Reaction of Methyl α-Phenyl-α-diazoacetate 1aa
entry
solvent
time (h)
yield 2ab (%)
1 2 3 4 5 6 7
CHCl3 DCE Et2O toluene hexane MeCN THF
3 3 12 12 48 48 18
95 91 78 75 32 10 48
a
Conditions: Fe(OTf)2 (5 mol %), silane (1 mmol), 1a (0.25 mmol), 40 °C, slow addition (1 h) of 1a. bYield of isolated product. 5737
DOI: 10.1021/acs.orglett.7b02488 Org. Lett. 2017, 19, 5736−5739
Letter
Organic Letters Scheme 2. FeII−Catalyzed Si−H Bond Insertion Reaction of α-Aryl-α-diazoacetates 1a−n with Various Silanesa
using Rh2(OAc)4,10g no significant kinetic isotope effect was observed (kH/kD = 1.1), suggesting that the Si−H bond activation step may not be rate determining. The following mechanism can be proposed (Figure 1). The diazo decomposition catalyzed by the iron complex occurs
Figure 1. Proposed mechanism for the Fe(OTf)2-Catalyzed Insertion of α-Diazoesters with Et3SiH. a Reaction conditions: Fe(OTf)2 (5 mol %), silane (1 mmol), α-aryl-αdiazoacetate (0.25 mmol), slow addition (1 h) of 1a−n, 6−24 h.
through the complexation of the negatively polarized carbon of the diazo substrate to the FeII catalyst. Afterward, the FeII carbene is generated via an irreversible N2 elimination. The metal carbene formation appears to be dependent on the electronic effects borne by the diazo compound, as demonstrated by the lower yields obtained with the electronwithdrawing groups substituting on the aryl moiety. An electrophilic iron−carbene center through the illustrated transition state, together with a three-centered transition state, could be envisioned. The overall mechanism involves iron-mediated extrusion of N2 in the slow step to form a metal carbene complex followed by insertion of the carbene fragment between a Si−H bond to regenerate the active catalyst.22 To summarize, we have successfully developed an efficient iron-catalyzed protocol for the metal carbene insertion reaction of α-diazoesters into the Si−H bond. A wide range of αsilylesters was synthesized in good to excellent yields using Fe(OTf)2 in low catalytic loadings, i.e., 5 mol %. Moreover, this catalyst has been shown to be efficient for the metal carbene insertion reaction of α-alkyl-α-diazoesters and 4-(1-diazo-2,2,2trifluoroethyl)-1,1′-biphenyl into the Si−H bond in good yields.
Scheme 3. FeII−Catalyzed Si−H Bond Insertion Reaction of Various Diazo Compoundsa
a
Conditions: Fe(OTf)2 (5 mol %), silane (1 mmol), 4a−c (0.25 mmol), slow addition (1 h) of 4a−c. bUsing 10 mol % of Fe(OTf)2, 25 °C.
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bd). Using 10 mol % of catalyst, the insertion of 4-(1-diazo2,2,2-trifluoroethyl)-1,1′-biphenyl was performed with triethylsilane and dimethylphenylsilane to give 5ca and 5cb in 81% and 76%, respectively. To gain some insight into the mechanism of the reaction, the kinetic isotope effect (KIE) was investigated for the metal carbene insertion reaction of α-diazoester by running the competition study illustrated in Scheme 4.10g,14a,15b The reaction of 1a−c with a mixture of Et3SiH (2 equiv) and Et3SiD (2 equiv) was examined in CH2Cl2 at 40 °C using Fe(OTf)2 as catalyst. The desired α-silylesters 2a−c and 2a′−c′ were formed in a 3:2.6 ratio (Scheme 4, KIE of 1.1). On the basis of the obtained KIE and the results obtained by Landais
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02488. Experimental details, characterization details, and spectral data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Scheme 4. Kinetic Isotope Effect Experiment for Diazo Insertion into Si−H and Si−D Bonds
Thierry Ollevier: 0000-0002-6084-7954 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Centre in Green 5738
DOI: 10.1021/acs.orglett.7b02488 Org. Lett. 2017, 19, 5736−5739
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Organic Letters
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Chemistry and Catalysis (CGCC), and Université Laval for financial support of our program. H.K. thanks CGCC for a doctoral scholarship.
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DEDICATION Dedicated to the memory of Professor István E. Markó. REFERENCES
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DOI: 10.1021/acs.orglett.7b02488 Org. Lett. 2017, 19, 5736−5739
