Enantioselective cyanosilylation of aldehydes catalyzed by a multistereogenic salen–Mn(III) complex with a rotatable benzylic group as a helping hand

Abstract

A multistereogenic salen–Mn(III) complex bearing an aromatic pocket and two benzylic groups as helping hands was found to be efficient in the catalysis of asymmetric cyanosilylation. The salen–Mn catalyst partially mimics the functions of biocatalysts by reasonably utilizing the steric and electronic properties of the catalytic center to interact with the substrate.

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In the past few decades, naturally occurring enzymatic systems have been revealed, in which effective stereochemical communication between the catalyst and the substrate is essential for attaining enantioselective control in catalytic asymmetric reactions.¹ The enantioselective transformations catalyzed by enzymes, in fact, take place rapidly under mild conditions because of the existence of an activated complex that is responsible for stereo- or enantioselectivity.² As a family of biocatalysts containing metal ions, enzymes are characterized by a metallic site that hosts a flexible structure and inside channels that originate from the assembly of the protein macromolecules.³ This is one of the reasons why chemists active in the area of biomimetic catalysis have attempted to design non-enzymatic or chemical catalyst systems according to the principle of enzymatic catalysis, including the reasonable utilization of the right steric properties in the catalytic center and the right electronic properties to interact with the substrate inside the channel/space of the catalyst.⁴ Inspired by the lessons derived from simplified biocatalysis to efficiently mimic enzymes, it is now acceptable that an ideal enantioselective catalyst needs to possess a metal complex/catalytic center, and a sophisticated and tunable structure with a suitable size.⁵ In particular, according to the concepts reported by Zecchina and co-workers,⁶ nanoscale selective catalysts (1–1.5 nm range) can be considered as nanoreactors designed to promote organic transformations with high activity and selectivity. However, the design and synthesis of chiral ligands for metal-based catalysts that are highly efficient, inexpensive, and selective with a wide range of substrates remains a challenging goal in asymmetric catalysis. Over the past few decades, salen-type C₂-symmetric N,O-ligands have drawn most attention as important chiral ligands in catalytic asymmetric reactions. Such ligands are very flexible molecules to modify and are easy-to-prepare, fulfilling important criteria such as cost-effective synthesis and ease of derivatization.⁷ Many groups have devoted their efforts to developing highly reactive salen–metal catalysts by modifying salen ligands (Fig. 1).⁷ In this context, Katsuki⁸ and Jacobsen⁹ have developed various salen–metal complexes from multistereogenic salen ligands containing steric bulky groups for a versatile array of organic reactions. For example, the Katsuki salen ligands and corresponding metal complexes can adopt a square planar geometry, and the stereomeric configuration as well as the steric repulsion were beneficial to the achievement of high enantioselectivity and the following dissociation for the next catalytic cycle of the metal complex.^8a Previous works on salen–metal based asymmetric catalysis showed that the rigid geometry of salen–metal complexes could be manipulated by substituting the phenyl group on the axial chiral BINOL backbone.^7–9 As a result, the crowded salen ligand around the metal center has the ability to change the Lewis acid character of the metal and therefore this could be effectively used to increase the reactivity of the resulting salen–metal complex.

Fig. 1 The structures of a chiral diamine derived symmetrical salen ligand.

We recently reported new nanoscale salen–Cu(II) and salen–Co(III) complexes with the distinguishing features of an enzyme for the asymmetric Henry reaction of aromatic aldehydes and asymmetric fluorination.¹⁰ For the salen–copper complex, it was found that the complex had good catalytic performance for benzaldehyde and halogen-substituted aromatic aldehydes in terms of conversion and enantioselectivity (90–99% yields and up to 91% ee).^10a We speculated that such a salen ligand with two benzylic groups as helping hands would be a possible supramolecular catalyst system bearing a large aromatic π-wall. Besides the inherent deficiency of the catalytic copper center, the remarkable discrimination of different aldehydes in the catalytic Henry reaction might be due to the considerable catalyst–substrate interaction, including noncovalent interactions between the copper ligand and the aldehydes. Very recently, on the basis of salen–Cu catalysis, a new type of chiral salen–Co(III) catalyst that features a multistereogenic center and a larger molecular space has been developed for highly enantioselective Henry/nitroaldol reactions (up to 93% yield and up to 98% ee).^10c Given the potential importance of cyanosilylation reactions¹¹ and the important role of salen–Mn complexes in biomimetic catalysis,¹² the continued investigation of Ar-BINMOL-derived salen–Mn complexes in cyanosilylation reactions would be highly desirable. Such a synthetic method could be of great value for both synthesis of optically pure cyanohydrins, which are versatile synthetic intermediates, and structural–activity relationship (SAR) studies of metal catalysts. Notably, a previous study suggested that salen–Mn(III) plays a critical role in asymmetric Mn-catalyzed cyanosilylation reactions,¹³ however it gave the desired products in only moderate to good yields with low to moderate enantiomeric excess (ee). Thus in this report, we present a preliminary study of multistereogenic salen–Mn(III) complex-catalyzed cyanosilylation of aldehydes, in which the well-established active center impacted by the salen ligands was combined with a nanoscale cavity. Good to excellent enantioselective cyanosilylation reactions were achieved (up to 90% ee) in the addition of cyanide to aldehydes, the success of which relied exclusively on the special structure of the Ar-BINMOL-derived backbone, which had a chirality that matched the multistereogenic centers.

Various Ar-BINMOL-derived salen ligands and related salen–Mn(III) complexes were easily prepared according to previous methods, in which the key step is neighboring lithium-assisted [1,2]-Wittig rearrangement (NLAWR).^10,14 Exploratory studies were then conducted with 2-fluorobenzaldehyde (1a) and were guided initially by previous reports that cyanosilylation of aldehydes in the presence of both a catalytic amount of salen–Mn(III) complex and triphenylphosphine oxide gave the desired products.¹³ The effect of solvent on the salen–Mn(III) complex (C1a) catalyzed cyanosilylation of 2-fluorobenzaldehyde (1a) with trimethylsilyl cyanide (TMSCN) was revealed in Table S1 (see ESI†). It was found that DCE was better than other solvents, such as DCM, toluene, Et₂O, THF, and the other solvents evaluated in Table S1.† In this case, good yield and promising enantiomeric excess could be achieved at room temperature (79% ee and 91% yield). We then found that this cyanosilylation was impacted largely by temperature, with the best enantioselectivity and yield of the desired product being obtained at −10 °C (Table S2,† 88% ee and 94% yield). Higher and lower temperatures led to inferior enantioselectivity and yield in the presence of the same salen–Mn(III) catalyst. Encouraged by these findings, we continued to investigate possible enhancement of the enantioselectivity with additives. As shown in Table S3 (see ESI†), the use of various phosphine oxides and chiral Lewis bases could not improve the enantioselectivity in this reaction. Most of the additives evaluated in this work resulted in low enantioselectivity and yield. Examination of additive effects on the cyanosilylation of 2-fluorobenzaldehyde (1a) in the presence of the salen–Mn(III) complex (C1a) showed a strong dependence of the product yield and transformation on the additive type, with triphenylphosphine oxide giving the best results. In addition, the most suitable amount of triphenylphosphine oxide has been further confirmed as 20 mol% for the activation of TMSCN. After considering options to improve the catalytic performance, especially the enantioselectivity, in the cyanosilylation of 2-fluorobenzaldehyde (1a), we continued to modify the multistereogenic Ar-BINMOL-derived salen ligand and corresponding salen–Mn(III) complex by changing the anionic group on the salen–Mn(III) complex and increasing the steric bulk around the benzylic moiety of the binaphthyl backbone. As shown in Scheme 1, in addition to C1a, seven Ar-BINMOL-derived salen–Mn(III) complexes were synthesized for investigation of the steric repulsion of the ligand (nanocavity), the chirality matching or mismatching, and the electronic effect of salen–Mn(III) induced by anionic groups.

Scheme 1 Various Ar-BINMOL-derived salen–Mn(III) complexes for catalytic cyanosilylation of aldehyde 1a.

Upon screening a number of the various Ar-BINMOL-derived salen–Mn complexes (entries 1–8, Table 1), we found that the salen–Mn (C1a) catalyst, derived from (R)-BINOL and (1R,2R)-cyclohexane-1,2-diamine, favored the efficient formation of the desired product, (S)-3-(2-fluorophenyl)-3-(trimethylsilyloxy)propanenitrile, in excellent yield (>94% yield) and high enantioselectivity (88% ee). Unfortunately, the introduction of substituted groups, such as tert-butyl, methyl, and naphthyl groups, on the benzylic moiety of the salen ligand led to inferior conversion and enantioselectivity in this reaction (entries 2–4). These results showed that the bulky benzylic groups, with possible aromatic interaction and steric repulsion, had a large impact on the capacity of the metallic center (Mn). Similarly, a change of anion on the salen–Mn complex from Cl to OAc or OTf also disfavored the cyanosilylation reaction of the aldehyde (entries 5 and 6). As such, the electronic effect of the anion was suggested to be important for the catalytic performance of Mn. In addition, the configuration of the salen ligand proved to be crucial to the enantioselectivity. For example, both of the salen–Mn complexes C2 and C3, which were prepared from (S)-BINOL and two isomers of cyclohexane-1,2-diamine, respectively, resulted in low enantioselectivities (entries 7 and 8, 31–59% ee). The experimental results also supported that the cavity of the Ar-BINMOL-derived salen ligand that was derived from (S)-BINOL and (1R,2R)-cyclohexane-1,2-diamine was smaller than those obtained from (R)-BINOL with (1R,2R)-cyclohexane-1,2-diamine or (1S,2S)-cyclohexane-1,2-diamine.¹⁵ Interestingly, even though the chiral cyclohexane-1,2-diamine impacted the enantioselectivity largely in this cyanosilylation reaction, it was discovered that the chirality of BINOL governed the absolute configuration of reaction product 2a/3a, because the chiral salen–Mn complexes C2 and C3 gave (R)-3-(2-fluorophenyl)-3-(trimethylsilyloxy)propanenitrile and salen–Mn complex C1a gave (S)-3-(2-fluorophenyl)-3-(trimethylsilyloxy)propanenitrile. Thus the salen–Mn complex C1a was selected as a promising catalyst for further optimizing the catalyst load. As shown in Table 1 (entries 9–12), the use of 3 mol%, 1 mol%, 0.5 mol%, and 0.1 mol% of salen–Mn was evaluated in this reaction and the corresponding yields were 93%, 86%, 83% and <40%, respectively. Notably, 0.5 mol% of salen–Mn complex C1a gave promising yield and good enantioselectivity (80% ee), so the salen–Mn catalyst proved to be quite effective in this reaction. However, the salen–Mn(III) catalyst is still difficult to recycle and reuse for the cyanosilylation reaction. In comparison to previous work that was reported by Kim and co-workers,¹³ our salen–Mn catalyst system could be better than previous salen–Mn complexes, including Katsuki-like salen–Mn complexes^13a and Jacobsen salen–Mn complexes,^13c in term of enantioselectivity for the cyanosilylation reaction of aldehydes.

Table 1 Ar-BINMOL-derived salen–Mn catalyzed cyanosilylation reaction of aldehyde 1a

Entry^a	Catalyst	Yield^b (%)	ee^c (%)
a Reaction conditions: aldehyde (0.5 mmol), salen–Mn(III) catalyst (5 mol%), TMSCN (1.5 eq.), triphenylphosphine oxide (20 mol%), at −10 °C, in DCE.b Yield of isolated product 3a was calculated based on the starting material 1a (two step).c Determined by chiral HPLC, and the absolute configuration was confirmed in comparison to that of previous reports.^11d 2 mol% of salen–Mn (C1a) was used in this case.e 1 mol% of salen–Mn (C1a) was used in this case.f 0.5 mol% of salen–Mn (C1a) was used in this case.g 0.1 mol% of salen–Mn (C1a) was used in this case.h 3 mol% of salen–Mn (C1a) was used in this case.i 0.01 mol% of salen–Mn (C1a) was used in this case, and almost no desired product was obtained.
1	C1a	94	88
2	C1b	80	59
3	C1c	85	73
4	C1d	<30	—
5	C1e	30	37
6	C1f	NR	—
7	C2	80	−31
8	C3	92	−59
9^d	C1a	93	82
10^e	C1a	86	78
11^f	C1a	84	80
12^g	C1a	<40	—
13^h	C1a	93	88
14^i	C1a	Trace	—

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With the optimized reaction conditions in hand, we investigated the scope of this cyanosilylation protocol. As shown in Scheme 2, salen–Mn C1a exhibited excellent activity and high functional-group tolerance. Aromatic aldehydes containing methyl, halide, trifluoromethyl, phenyl, and nitro groups, were reacted with TMSCN efficiently to give the corresponding products (2) and subsequent desilylated products (3) in high yields (the yields shown in Scheme 2 were calculated based on the starting material (1) for the two-step transformations) and good enantioselectivities (determined by chiral HPLC for product 3). The fact that up to 90% ee of cyanohydrin could be obtained by salen–Mn complex C1a is of interest, because there are no previous reports of salen–Mn(III) complex-catalyzed cyanosilylation of aldehydes with such high enantioselectivity. Although the true reason could not be concluded completely at present, we suggested that the substituted benzyl group on the multistereogenic salen ligand played a considerable role in the enantioselective induction of the salen–Mn(III) catalyst system. Under the optimized reaction conditions, the Ar-BINMOL-derived salen–Mn(III) complex C1a was also effective for the cyanosilylation of alkyl aldehydes (1q–t), in which high yields of the desired product were achieved (81–92% yields). Unfortunately, the ee values of these products could not be determined by chiral HPLC.

Scheme 2 Ar-BINMOL-derived salen–Mn catalyzed cyanosilylation of aldehydes.

On the basis of the experimental results shown in the text as well as in the ESI,† the proposed transition state was provided in Fig. 2, which featured dual-activation catalysis. Similarly to previous Ar-BINMOL-derived salen–metal complexes,¹⁰ the salen–Mn complex acted as a chiral Lewis acid to activate the aldehyde by binding the oxygen atom of the carbonyl group, while the triphenylphosphine oxide worked as a Lewis base for activation of TMSCN through the formation of a pentacoordinated intermediate and subsequent transfer to a hexacoordinated intermediate with the substrate. The attack of cyanide (CN) would prefer the Re face of the carbonyl group of the aldehyde to afford R-cyanohydrin, and the Si face is blocked by the steric repulsion of the salen–Mn complex.

Fig. 2 Proposed transition state involved in the enantioselective cyanosilylation of aldehyde by dual-activation catalysis in the cage of salen–Mn complex C1a.

In summary, inspired by the lessons of nanoscale enzyme mimics, we have developed an efficient enantioselective salen–Mn(III) complex catalyzed cyanosilylation of aldehydes with TMSCN. In comparison to previous efforts in this area, it was found firstly that the construction of a macromolecular Ar-BINMOL-derived salen–Mn(III) catalyst bearing an aromatic pocket and two benzylic groups as helping hands exhibited excellent yields and good enantioselectivities in this reaction (up to 94% yield and 90% ee). Our Ar-BINMOL-derived salen–Mn complex exhibited superior activity over other salen–Mn catalysts in terms of yield and enantioselectivity. In addition, the multistereogenic salen–Mn complex with multifunctional groups partially mimics the functions of biocatalysts in term of reasonable utilization of the steric properties of the catalytic center and the electronic properties to interact with the substrate inside the channel/space of the catalyst.

Acknowledgements

This project was supported by the National Natural Science Founder of China (no. 21173064, 51203037 and 21472031), and the Zhejiang Provincial Natural Science Foundation of China (LR14B030001) is appreciated.

Notes and references

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15. It was found that the salen–Mn(III) complexes C2 and C3 were difficult to obtain under the same reaction conditions in comparison to salen–Mn(III) complex C1a. On the basis of this finding, we can conclude that the cavity of the Ar-BINMOL derived salen ligand was largely impacted by the multistereogenic configuration of the two chiral sources. Considering the critical phenomenon of coordination between the Mn(II) salt and the salen ligand, we suggested that the estimated cavity might be about 3.6–4.0 Å (ionic radius of Mn²⁺ is 0.9 or 0.96 Å) for these Ar-BINMOL derived salen ligands.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12884b

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