Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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N‑Heterocyclic Germylene and Stannylene Catalyzed Cyanosilylation and Hydroboration of Aldehydes Rajarshi Dasgupta,† Shubhajit Das,‡ Shweta Hiwase,† Swapan K. Pati,*,‡ and Shabana Khan*,† †
Department of Chemistry, Indian Institute of Science Education and Research Pune, Dr. Homi Bhabha Road, Pashan, Pune 411 008, India ‡ New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur-Bangalore 560064, India
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S Supporting Information *
ABSTRACT: Recent years have witnessed a significant growth in the area of low-valent main-group compounds due to their potential to activate small molecules. However, there is a paucity of examples of low-valent main-group compounds being used as single-site catalysts for organic transformations. This study represents the hydroboration and cyanosilylation reactions of a range of aldehydes by a benzannulated heavier N-heterocyclic germylene (1) and stannylene (2) under mild conditions. A wide variety of substrate scope was studied. The mechanistic pathway of the cyanosilylation reaction is initiated through the coordination of TMSCN with the catalyst followed by the attack of aldehydes. Conversely, hydroboration proceeds via formation of a donor−acceptor adduct between HBpin and the catalyst. Experimental and theoretical studies were performed to establish the mechanism.
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INTRODUCTION The recent developments of low-valent main-group catalysts aim to replace the more expensive and toxic transition-metal catalysts, which are being used for the facile activation of catalytically relevant small molecules, in a cheaper, less toxic, and eco-friendly manner.1 Hydroboration reactions of unsaturated bonds serve as efficient tools for the preparation of various organic intermediates ever since their discovery by Brown et al. in 1956.2 Since then, there have been numerous reports on the hydroboration of aldehydes and ketones using transition-metal and s-block catalysts.3 A number of research groups have started using compounds with p-block elements (Chart 1, A−I) as single-site catalysts due to their costeffective and nontoxic nature for homogeneous catalytic hydroboration reactions of aldehydes and ketones.4−16 Recently in 2018 a greener catalyst and solvent-free hydroboration of aldehydes have also been reported.17 However, most of these catalysts reduce both aldehydes and ketones under the same conditions. Selective hydroboration of an aldehyde over a ketone has been known only for a few cases, such as Fe(acac)3,17 [Fe-N2S2]2,18 [(p-cymene)RuCl2]2,19 B,5 F,8 and G.9 In these cases, the ketones either cannot be reduced (G and H) or can be hydroborated by increasing the catalyst loading, reaction temperature, or reaction time. Hence, chemoselective hydroboration of aldehydes using a main-group compound is limited. Parallel to hydroboration, there has been a flurry of recent research activity on the cyanosilylation of aldehydes and ketones by s- and p-block compounds (Chart 2, A, J−N)20−26 but examples of compounds with low-valent p-block elements © XXXX American Chemical Society
Chart 1. p-Block Elements That Catalyze Hydroboration of Aldehydes and Ketones (A−I)
are rare. There is only one report so far which employs a basestabilized germylene (N) for cyanosilylation; however, the substrate scope is very limited. Moreover, to the best of our knowledge, no cyanosilylation has been reported with lowvalent tin derivatives. Therefore, we turn our interest toward the previously reported N-heterocyclic germylene (1) and stannylene (2),27,28 which have not been reported so far for any catalytic application. Herein, we report the catalytic potential of 1 and 2 for the cyanosilylation reaction of aldehydes. We also have Received: September 13, 2018
A
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Chart 2. p-Block Elements That Catalyze Cyanosilylation of Carbonyl Compounds (J−N)a
Scheme 1. Group 14 Heavier N-Heterocyclic Tetrylene (NHGe (1), NHSn (2)) Catalyzed Cyanosilylation of Aldehydes
Scheme 2. Substrate Scope for Cyanosilylation of Aldehydes Using Catalysts 1 and 2a
2 is the first example of a tin derivative used for cyanosilylaton reactions.
a
shown that both of these compounds are capable in the selective hydroboration of aldehydes. For both cyanosilylation and hydroboration reactions the purity of the compounds was analyzed through 1H NMR spectroscopy. The reaction conditions were optimized and monitored by NMR spectroscopy, and the yields were calculated on the basis of the integration area of the product and starting material in 1 H NMR spectra using mesitylene as an internal standard (Table 1). Only aromatic aldehydes were tested for Table 1. Optimization of Reaction Conditionsa for Cyanosilylation Reaction Using Catalysts 1 and 2
a
Reaction conditions for cyanosilylation reaction: benzaldehyde (0.25 mmol), HBpin (0.25 mmol), and toluene (2 mL) as solvent at room temperature. The catalyst loading (1 mol %) is relative to benzaldehyde. 1H NMR spectroscopy was used to determine the yield using mesitylene as an internal standard. The superscripts 1 and 2 denote the yield with the corresponding catalysts 1 and 2, respectively.
b
entry
catalyst
amt (mol %)
time (h)
yield (%)
1 2 3 4 5 6 7 8c 9 10 11 12 13 14 15 16c
1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2
0.5 1 2 5 1 1 1 1 0.5 1 2 5 1 1 1 1
2 2 2 2 0.25 1 8 2 2 2 2 2 0.25 1 8 2
41 87 81 73 34 83 68 62 28 86 79 75 31 81 74 57
Selective cyanosilylation of aldehyde functionalities was observed intramolecularly in the presence of other unsaturated groups, e.g. the olefinic bond for cinnamaldehyde (3i) and the CN group for 4-cyanobenzaldehyde (3e), and also intermolecularly in the presence of acetophenone (Scheme 3). The mechanistic pathway for the cyanosilylation reaction was investigated through NMR (1H, 29Si) studies. The success in using 1 and 2 in the cyanosilylation of aldehydes led us to investigate them for the hydroboration of aldehydes and ketones. Although the hydroboration of aldehydes and ketones using main-group catalysts has received
a
Reaction conditions for cyanosilylation reaction: benzaldehyde (0.25 mmol), HBpin (0.25 mmol), and toluene (2 mL) as solvent at room temperature. The catalyst loading (1 mol %) is relative to benzaldehyde. b1H NMR spectroscopy was used to determine the yield using mesitylene as an internal standard. cTHF was used as the solvent.
Scheme 3. Selective Cyanosilylation of Aldehydes: Intermolecularly and Intramolecularly Using Catalysts 1 and 2 (with the Optimized Protocol)
cyanosilylation reactions (Scheme 1), and it was seen both electron-donating and electron-withdrawing groups were well tolerated to give the respective cyanohydrin trimethylsilyl ethers (Scheme 2). Very mild reaction conditions were required for sterically demanding substrates such as 2,6dimethylbenzaldehyde, α-naphthaldehyde, etc. No dearomatization took place for furfuraldehyde during the cyanosilylation reaction (Scheme 2). B
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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Organometallics intense attention, the selective hydroboration of aldehydes is not common. Compounds 1 and 2 successfully catalyze the hydroboration reaction of aldehydes with HBpin at room temperature to yield alkoxy pinacol boronate esters (Scheme 4) in good to excellent yields. A brief screening to optimize the
Scheme 5. Substrate Scope for Hydroboration of Aldehydes Using Catalysts 1 and 2a
Scheme 4. Group 14 Heavier N-Heterocyclic Tetrylene (NHGe (1), NHSn (2)) Catalyzed Hydroboration of Aldehydes
reaction conditions shows that a good yield of product conversion takes place using 2 mol % catalyst loading at room temperature within 4−6 h using toluene as a solvent (Table 2). Table 2. Optimization of Reaction Conditionsa for Hydroboration Reactions Using Catalysts 1 and 2 entry
catalyst
amt (mol %)
time (h)
yield (%)b
1 2 3 4 5 6 7 8c 9 10 11 12 13 14 15 16c
1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2
0.5 1 2 5 2 2 2 2 0.5 1 2 5 2 2 2 2
4 4 4 4 0.5 6 overnight 4 4 4 4 4 0.5 6 overnight 4
24 93 81 79 19 90 71 53 26 91 80 82 22 87 78 48
a
Reaction conditions for hydroboration reaction: benzaldehyde (0.25 mmol), HBpin (0.25 mmol), and toluene (2 mL) as solvent at room temperature. The catalyst loading (2 mol %) was relative to benzaldehyde. 1H NMR spectroscopy was used to determine the yield using mesitylene as an internal standard. The superscripts 1 and 2 denote the yield with the corresponding catalysts 1 and 2, respectively. Asterisks denote that the reactions of the substrates take 6 h for completion.
Scheme 6. Selective Hydroboration of Aldehydes: Intermolecularly and Intramolecularly Using Catalysts 1 and 2 (with the Optimized Protocol)
a
Reaction conditions for hydroboration reaction: benzaldehyde (0.25 mmol), HBpin (0.25 mmol), and toluene (2 mL) as solvent at room temperature. The catalyst loading (2 mol %) is relative to benzaldehyde. b1H NMR spectroscopy was used to determine the yield using mesitylene as an internal standard. cTHF was used as the solvent.
No increment in the yield was observed upon heating the reaction mixture at 60 °C. The hydroboration of aromatic aldehydes with electron-donating and -withdrawing substituents at different positions was achieved in good to high yields (Scheme 5). The hydroboration reactions using catalysts 1 and 2 are highly chemoselective in nature. Chemoselectivity was displayed both intermolecularly and intramolecularly (Scheme 6). The specific formation of alkoxy pinacol boronate ester took place even in the presence of acetophenone, aniline, and phenol, and even addition of another 1 equiv of HBpin did not lead to any changes in the 1H NMR spectrum. Intramolecularly aromatic aldehydes having a second unsaturated substituent containing a cyano (4e), amide (4m), or ester (4n) group or instead an olefinic bond (4i) also react selectively with HBpin, leading to the formation of alkoxy pinacol boronate ester C
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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Organometallics specifically while the second site of unsaturation remains unreacted. This selective hydroboration of aldehydes can be used for various organic transformations which are of biological relevance: for instance, N-benzyl-2-oxoacetamide (4o). N-Benzyl-2-oxoacetamide is the primary building block for peptide chains and contains an aldehyde functional group which can be selectively converted to an alkoxy pinacol boronate ester with the peptide linkage kept intact (Scheme 6). Catalysts 1 and 2 are very successful in the hydroborations of differently bromo substituted aromatic aldehydes which are quite sluggish with other catalysts.29 No dehydrocoupling takes place for 2-hydroxybenzaldehyde, whereas in the absence of catalyst it shows significant dehydrocoupling in accordance with a recent report from Bertrand and co-workers.30 Sterically demanding substrates such as α-naphthaldehyde also require similar reaction conditions using catalysts 1 and 2. To gain more insights into the reaction mechanism a detailed NMR (1H, 11B, 29Si, 13C) study and DFT calculations were performed. The mechanism of cyanosilylation and hydroboration using density functional theory (DFT) calculations was determined with the model tetrylene catalysts 1m/2m, and benzaldehyde (PhCHO) was used as the substrate. 1m initially coordinates to the nitrile-N atom of TMSCN, forming INT1Ge (Figure 1), which lies 2.5 kcal/mol below 1m
Figure 2. 13C NMR of the 1:1 reaction of TMSCN and 1.
Figure 3. 29Si NMR (C6D6) of the 1:1 reaction of TMSCN and 1.
reaction of INT1Ge with 1 equiv of benzaldehyde by 1H, 13Cm and 29Si NMR, we observed the characteristic peak for −CH at 5.46 ppm in the 1H NMR and a new peak at 24.3 ppm for −OSiMe3 in the 29Si NMR (see Table S1 for details). Our computational results suggest that the CN group is transferred to the carbonyl carbon of benzaldehyde through a fourmembered TS, TS1Ge, which lies at 29.5 kcal mol−1. 1m anchors more strongly to TMSCN in TS1Ge, as is apparent from the shorter Ge−N distance (2.424 vs 2.929 Å in INT1Ge) and greater extent of N−Ge donor−acceptor interactions. Upon dissociation of the product from the complex INT2Ge, cyanohydrin is formed, and the catalyst is regenerated. The free energy activation barrier for the cyanosilylation, between PhCHO and TMSCN, without the involvement of any catalyst is found to be 34.1 kcal/mol. Thus, anchoring the 1m leads to a decrease in the activation barrier. This effect is much more pronounced in the 2m-catalyzed reaction, in which the TS for cyanosilylation, TS1Sn, is found to be at 24.4 kcal mol−1. Nevertheless, the computed activation barriers seem to be slightly higher (particularly for the 1mcatalyzed reaction) considering the mild reaction conditions. Despite our sincere efforts we were not able to find any other reasonable mechanistic pathway consistent with the experimental results that proceeded with a low activation barrier.
Figure 1. Catalytic cycle and proposed mechanism for cyanosilylation of benzaldehyde using the model catalysts 1m and 2m. Relative solvent-corrected free energy values of various reaction intermediates/ TS are also shown. The separated reactants, i.e. 1m/2m, TMSCN, and PhCHO molecules, are used as the zero-energy reference.
+ TMSCN in the free energy profile (see the Supporting Information). INT1Ge features a donor−acceptor interaction between the N lone pair and empty p orbitals on the Ge center (Figure 1). This is confirmed from the 1H, 29Si, and 13C NMR spectra of the solution containing 1m and TMSCN (see the Supporting Information for details). The 13C NMR shows a clear shift of the −CN carbon from 127.5 to 147.1 ppm (Figure 2), indicating the coordination of an N atom to 1m. The 29Si NMR reveals a slight shift from −14.3 to −11.7 ppm (Figure 3). The new resonances are indicative of the formation of the weakly bound adduct INT1Ge. By monitoring the D
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Instead, we think that the anchoring could be more efficient (see also the mechanism of hydroboration) if two catalyst molecules coordinate to the nitrile-N atom of TMSCN. This might lead to more facile transfer of the CN to the carbonyl atom through a lower activation barrier. We have indeed located such intermediates featuring the nitrile-N atom weakly bound to two 1m/2m molecules in the reaction pathway (Figure S149; see the Supporting Information for structures), although all our attempts to locate a corresponding TS structure were unsuccessful. Therefore, we refrain from making any quantitative kinetic prediction regarding this mechanism for cyanosilylation at this point. Rather, we focus on the mechanism of hydroboration. The hydroboration begins with the coordination of HBpin and 1m, leading to the formation of INT3Ge, which lies 4.0 kcal/mol below 1m + HBPin. Since there are two oxygens, two catalyst molecules are found to anchor into HBpin in INT3Ge. NBO analysis reveals donor−acceptor interactions between the LPs of oxygens in HBpin and empty p orbitals on the Ge center in INT3Ge (Figure 4). This was supported by observing
Figure 5. Stacked 11B decoupled NMR of free HBpin, HBpin, and 1 (1:2) and the final product.
Figure 6. 1H NMR (CDCl3) (shifts of −tBu and −CH2− of 1 and −Me of HBpin in ppm) of the 1:2 reaction of HBpin and 1.
of 1m units to HBPin is even stronger in the TS structure, as indicated by the Ge−O distances, which drop down to 2.497 and 2.828 Å in comparison to 3.018 and 3.197 Å in INT3Ge. TS2Ge initially relaxes to INT4Ge, in which 1m anchorings become much weaker (Ge−O distances 2.957 and 3.155 Å). Finally, INT4Ge dissociates to release the catalyst molecules and yields the desired alkoxy pinacol boronate ester (PhCH2OBPin). 2m-catalyzed hydroboration of PhCHO proceeds by a similar pathway following a similar intermediate sequence; 2m + HBPin + PhCHO → INT3Sn → TS2Sn → INT4 Sn → PhCH2OBPin + 2m (see the Supporting Information for structures). The activation barrier for the hydroboration between HBpin and PhCHO without the involvement of a catalyst is calculated to be 31.1 kcal/mol. The computed barriers with the coordinated 1m/2m show the utility of the anchoring mechanism to reduce the activation barrier. Considering anchoring of only one catalyst molecule (Figure 7), the TSs for 1m- and 2m-catalyzed hydroboration lie at 23.6 and 28.6 kcal/mol, respectively. Another pathway which includes oxidative addition of B−H bond of pinacol borane to the catalysts was also taken into account. However, this pathway was found to be energetically highly disfavorable (ΔGTSOX(1m) = 45.7 kcal/mol; ΔGTSOX(2m) = 60.1 kcal/ mol) (see the Supporting Information for details, Figure
Figure 4. Catalytic cycle and proposed mechanism I for hydroboration of benzaldehyde using model catalyst 1m or 2m. Relative solvent-corrected free energy values of various reaction intermediates/ TS are also shown. The separated reactants, i.e. 1m/2m, HBPin, and PhCHO molecules, are used as the zero-energy reference.
the 2:1 reaction of 1 and HBpin. The 1H NMR and 11B NMR spectra of the reaction mixture reveal significant shifts in comparison to the parent compounds (Figures 5 and 6). A decoupled 11B NMR spectrum of a 1:2 mixture of HBpin and 1 reveals a new peak at 20.20 ppm with the disappearance of the 28.83 ppm peak of HBpin. Similarly, the 1H NMR spectrum of the mixture also disclosed considerable shifts of the tBu and −CH2 protons of 1 and Me protons of HBPin (Figure 6; see the Supporting Information for complete spectra and details, Table S2). However, 1H NMR and 11B NMR spectra for the 1:1 reaction between 1 and benzaldehyde did not show any significantly noticeable shift (see the Supporting Information for NMR data, Figure S140). Upon addition of benzaldehyde, the carbonyl oxygen makes a nucleophilic attack to the boron center in INT3Ge while the hydride is transferred to the carbonyl carbon via a fourmembered TS, TS2Ge, lying at 22.1 kcal mol−1. The anchoring E
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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General procedure for cyanosilylation and hydroboration reactions, standardization of reaction conditions for cyanosilylation and hydroboration reactions, substrate scope and experimental data to propose mechanistic pathways for cyanosilylation and hydroboration reactions, kinetic studies, and computational details (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.K.P.:
[email protected]. *E-mail for S.K.:
[email protected]. ORCID
Swapan K. Pati: 0000-0002-5124-7455 Shabana Khan: 0000-0002-6844-3954 Notes
The authors declare no competing financial interest.
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Figure 7. Catalytic cycle and proposed mechanism II for hydroboration of benzaldehyde using model catalyst 1m or 2m. Relative solvent-corrected free energy values of various reaction intermediates/ TS are also shown. The separated reactants, i.e. 1m/2m, HBPin, and PhCHO molecules, are used as the zero-energy reference.
ACKNOWLEDGMENTS S.K. thanks the SERB (India), BRNS (37(2)/14/23/2017), and IISER Pune for the financial support. S.K. also thanks the DST-FIST for a single-crystal X-ray diffractometer. R.D. and S.D. thank the CSIR (India) for fellowships. We acknowledge Dr. Jeetender Chugh, IISER Pune, for kinetic experiments.
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S142). These results indicate that the anchoring is more efficient with two catalyst molecules instead of one in reducing the barrier for the hydroboration reaction. 1m and 2m both perform well as catalysts for cyanosilylation and hydroboration reactions of aromatic aldehydes. The kinetic experiments further indicated a linear correlation of catalyst concentration with kobs which upon further increase in the concentration of catalyst shows a zero-order kinetics (Figures S141 and S142). Of the two mechanistic possibilities considered above, pathway II (Figure 4) is slightly preferred. Both the hydroboration and cyanosilylation reactions are found to be highly exergonic in toluene. The NMR studies of the solution of a 1:1 reaction mixture of 1 and HBPin further supported the proposed reaction mechanism passing through the anchoring pathway (see the Supporting Information for details).
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CONCLUSION In summary, we demonstrated two very important basic organic catalytic reactions, namely hydroboration and cyanosilylation, for the reduction of aromatic aldehydes catalyzed by N-heterocyclic germylene (1) and stannylene (2), under mild conditions that exhibit excellent chemoselectivity. Our experimental and theoretical results reveal a weak adduct formation between TMSCN and catalysts, leading to a polar Si−C bond which is prone to be attacked by a carbonyl group. A similar catalytic pathway was proposed for the hydroboration reaction as well. The catalytic activity of both the germanium (1) and tin (2) derivatives can be attributed to the higher Lewis acidity of group 14 heavier congeners and are reflected in the respective low free energy barriers in both the reactions. These catalytic transformations have opened up a new domain for using main-group compounds as catalysts for various basic organic reactions.
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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/acs.organomet.8b00673. F
DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.8b00673 Organometallics XXXX, XXX, XXX−XXX
