WITHDRAWN: Chiral Borohydride Complex Crystals: Synthesis, Structure and Reduction Application of Acetophenone

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WITHDRAWN: Chiral Borohydride Complex Crystals: Synthesis, Structure and Reduction Application of Acetophenone

https://doi.org/10.21203/rs.3.rs-3917884/v1

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In the current search; two types of crystal synthesis were carried out. Sodium boron hydride (NaBH₄) was used in the synthesis of both crystals. One of the crystals was synthesized with diglyme (diethylene glycol dimethyl ether) (1) and the other with 3,5-lutidine (3,5-dimethylpyridine) (2). NaBH₄:Diglyme (1) crystals were employed to reduce acetophenone. The existence of an enantiomeric selectivity as a result of reduction was investigated. However, the reduction occurred with 36% efficiency and a racemic mixture was formed. The other crystal, NaBH₄:3,5-lutidine (2), was obtained newly, and its structure was analyzed by X-ray single crystal diffraction. The structure has triclinic space group P-1 with Z = 4 features. Additionally, asymmetric unit, which contains two sodium atoms coordinating two 3,5-lutidine ligands each; the sodium atoms are connected via bridging BH₄ to form chiral 2D polymer.

The nature of crystals

Crystals can be encountered at many moments in our lives as nutrients or chemical formulations. It occurs over a crystal unit cell which the structure repeats itself with an identical orientation and three-dimensional arrays [1]. If the exact information of the unit cell is known then it will be determined lots of properties about crystal. First it is collected the reflections and electron density map to locate elements into the correct position. Thereby the exact image can be obtained for the unit cell as a crystallographic data.

Of course, it is important that the crystallization process is carried out correctly and effectively in order to refine the crystal structure in the best possible way. The most commonly used methods are liquid-liquid diffusion and vapor diffusion. (Fig. 1) The main point of getting well growth crystals is solvent selection. It should be chosen polar solvent for the nonpolar solution or vice versa to assist the crystals forming when they interact opposite solvent [2–4].

Enantiomeric Properties of Crystals: Crystals are separated three classes according to their enantiomeric properties. These can be classified as ‘racemic crystals’, ‘conglomerates’, and ‘Solid Solutions’. (Fig. 2) Racemic crystals have equal amount of R and S enantiomers, conglomerates have individually R and S; solid solutions have unequal distribution of R and S enantiomers. The common aspects of getting optical resolution of racemates are preferential crystallization and the diastereomeric salt formation methods during the crystallization. It has been still unknown which of the mechanisms is followed for chiral separation of enantiomers among the racemates [5].

The Preferential Crystallization

As it can be understood from the title the keynote for this method is controlling or performing the crystallization whatever we want by seeding one enantiomer of conglomerate. For example if you add minus crystals to supersaturated solution, the minus crystals will tend to precipitate together. On the other hand, if the plus seeding is attempted, then the similar case will happen. However, the yield for conglomerate by this method has been achieved less than 10% even it requires very easy operation.

There are three processes to decide if a racemate is a conglomerate or not. It’s compared with their melting point, solubility and IR/XRD spectrum between racemic and optically active phase of racemate. It’s usually determined higher melting point and lower solubility in the case of a conglomerate. And also the XRD and IR spectral data show the same results [5].

Absolute Asymmetric Synthesis: The point for Absolute Asymmetric Synthesis (AAS) is developing optical activity from achiral precursors. This can be differed from asymmetric synthesis (AS) by using none of the chiral catalysts, solvents or reagents [5–9]. The term was first introduced by Bredig in 1923. Polarized light in the present of magnetic field and irradiation with electrons originating from radioactive decay are the only examples described for AAS. These three don’t constitute a preparative process [7, 10–16]. Absolute asymmetric synthesis refers to the creation of optically active products exclusively from achiral or racemic precursors. Historically deemed implausible, it holds significance in the exploration of biomolecular homochirality origins. One potential approach to achieving absolute asymmetric synthesis is through total spontaneous resolution. This method is applicable to stereochemically labile substances that crystallize as conglomerates, wherein enantiomers form distinct crystals [17].

Total Spontaneous Resolution: The phenomenon known as total spontaneous resolution or crystallization-induced asymmetric transformation arises from the combination of spontaneous resolution, preferential crystallization, and stereo-chemical lability in solution or melt.[12] In the case of a stereo-chemically labile compound crystallizing in a Sohncke space group, slow crystallization may be induced by a single nucleus. When enantiomers undergo rapid interconversion in solution, the 1:1 ratio of enantiomers remains unaffected during crystallization. If the rate of enantiomer interconversion surpasses the rate of crystal growth, and both the rates of secondary nucleation and crystal growth significantly outpace the rate of primary nucleation, the entire solute quantity may crystallize as a single enantiomer [18]. (Fig. 3)

In the context of all the above, many organic compounds are synthesized by new types of molecules by reduction and oxidation methods. In this manner, obtained structures act as precursors of many targeted biological or synthetic reactions. However, as a result of some reduction processes, the non-chiral structure turns into a chiral feature. Chiral structures may contain enantiomeric mixtures. If it contains both (R) and (S) enantiomers together, then a racemic mixture occurs. Furthermore, sometimes the structure used as a reductant reduces the organic structure to a single geometrical isomer, that is, only (R) or (S). In this case, the enantioenriched structure is obtained.

Homochirality of organic compounds has been investigating for many years by the scientist to learn the ratio and function of D-, L- sugars in the different systems. The changes of the racemic mixture are shown by the duration of achiral molecules reaction [19]. The racemic compounds are differed from the racemates with having only equimolar enantiomers in the crystal lattice. And also the conglomerates are described as mechanical mixtures of same enantiomer crystals. It can be obtained only 5–10% of whole racemates even it’s significantly important for optical resolution experiments [20, 21].

Ligands having donor atoms such as N, O, S etc. are interacted with metal center as a monohapto (η¹), dihapto (η²), trihapto (η³), tetrahapto (η⁴) fashion [22]. So [BH₄]⁻ can also coordinate these types of fashion to the alkali metals which provide acidic cations to form proper symmetry and energy [23–25].

In the present work, single crystals were obtained by sodium borohydride with diglyme (diethylenglycol dimethyl ether) (1) and 3,5-lutidine (3,5-dimethylpyridine) (2) compounds, separately. One of them (1) was employed for reduction of acetophenone. Another crystal compound (2) was structurally enlightened.

All experiments were performed under inert atmosphere using Schlenk techniques. Hexane and THF were distilled from sodium/benzophenone shortly prior to use. All of the other reagents were used as supplied commercially. Crystal data was collected by using a Rigaku R-AXIS II area detector with graphite monochromated Mo Kα radiation from a Rigaku RU-H3R rotating anode, operating at 50 kV and 70 mA. The structure was solved by direct methods using SHELXS-97 and refined against F2 by full matrix least-squares using SHELXL-97 [26, 27]. Hydrogen atoms attached to carbon were placed in calculated positions. HPLC analyses carried out on a Varian system with a 9010 Solvent Delivery System and a 9050 UV detector coupled with a PDR-Chiral Inc. Advanced Laser Polarimeter.

Synthesis of NaBH₄:Diglyme Crystals

The synthesis of crystals of sodium borohydride with diglyme (diethylenglycol dimethyl ether) was synthesized according to literature [24]. NaBH₄ (0.4 g, 13.2 mmol) was added to diglyme (20 mL). After stirring for 1 h the turbid solution was filtered and the filtrate was reduced to half of its volume at 50°C under vacuum. The remaining solution was layered with methyl cyclohexane (6 mL) and stored at − 20°C. Crystals separated within a few days. The crystals are hygroscopic and quite air sensitive. The structural illustration is given in Fig. 4.

Synthesis of NaBH₄:3,5-Lutidine Crystals

These crystals was synthesized by using the similar process according to previous research [24]. NaBH₄ (0.03 g 0.8 mmol) was added to 3,5-lutidine to get a saturated solution at 50 ^oC. Then the suspension was filtered and layered with methyl cyclohexane. After few days, yellowish crystals which are not air stable were precipitated.

Asymmetric synthesis is performed by using chiral auxiliaries, catalysts or reagents. The goal is to create optical activity with the designing asymmetric synthesis from achiral or racemic precursors. Chiral crystals are relied on according to literature either by creating a chiral environment for a reaction in a crystal, or through reaction with enantiomerically pure crystalline reagents [28]. There are 230 space groups (possible combinations of symmetry elements) in crystal structures; 65 of these are Sohncke space groups and they contain only rotation or screw axes.We can summarize their group numbers: 1, 3-5, 16-24, 75-80, 89-98, 143-146, 149-155, 168-173, 177-182, 195-199 and 207-214. An enantiomerically pure sample must crystallize in Sohncke space group, while racemates, as well as achiral molecules, can crystallize either in all space groups [28].

Reduction Experiment of Acetophenone with (1) [C₆H₁₄NaO₃⁺, BH₄^-]

The structure of sodium borohydride with diethylenglycol dimethyleter (1) was also found as sodium hydroborato diglyme polymeric complex by A.S.Antsyshkina et. al.[23]. The crystals of 1 (Figure 4) are is highly hygroscopic and dissolves in air. As molecule of 1 crystallizes in a Sohncke space group, they form a conglomerate. Our aim was to use crystals of 1 to reduce acetophenone and to get enantiopure 2-phenylethanole. To carry out this purpose; one crystal (approximately 20 mm length, 5.85 mm³volume) was separated and kept under nitrogen atmosphere. The crystal was powdered in a tube which was immersed in liquid nitrogen. Acetophenone was added as a solution in dry hexane and the mixture was stirred constantly for 3 days at 318 K. After 3 days the mixture was neutralized with 1 M HCl, extracted with hexane/ isopropyl alcohol (IPA) (90:10) and analyzed with HPLC. The suggested reaction mechanism was depicted in Figure 5. The powder of sodium borohydride:diglyme complexes was used for reduction experiment at 45 ^oC and compared with the single crystals’ results. The conversions to the chiral alcohol were significantly obtained.

Acetophenone was reduced to its corresponding alcohol in the solid phase with a yield of 36 %. Reduction in the solid phase using single crystals, unfortunately gave racemic product. The results determined by HPLC chiral stationary phase with an advanced laser polarimeter, revealed that two enantiomers were formed (Figure 6) along with small amount of acetophenone. But there is a little bit more conversion to (R)-2-phenylethanole enantiomer than (S)-2-phenylethanole. (Figure 6) This is not sufficient to evaluate that our crystals (1) significantly provide an enantiomerically selectivity during the reduction process. Both of the products were optically active as seen in Figure 7. The existence of chiral alcohol was also confirmed with the ¹H NMR spectrum. The quartet peak at 4.8 ppm corresponds to C-H for the chiral carbon (see Figure 8). One explanation of the racemic product can be that the single crystals were not enantiopure.

Single crystals of (2) were obtained from a highly concentrated 3,5-lutidine solution of NaBH₄ by layering methyl cyclohexane on it. (Figure 9) The experiment to synthesize complex between sodium borohydride and 3,5-lutidine was conducted in mild conditions such as dissolving salt in 3,5-lutidine then layering with methyl cyclohexane. The crystals were mounted carefully under N₂ for characterization by single crystal X-ray diffraction.

The compound crystallizes in triclinic space group P-1 with Z = 4. Figure 10 shows the asymmetric unit, which contains two sodium atoms coordinating two 3,5-lutidine ligands each; the sodium atoms are connected via bridging BH₄ to form chiral 2D polymer. (See Figure 11).

The distances between Na(1)-N(1), Na(1)-N(2), Na(1)-B(2) were observed 2.426, 2.405, 2.629 Å respectively when they compared with Na(2)-N(3), Na(2)-N(4), Na(2)-B(2) values of 2.427, 2.412, 2.681 Å. The angles were obtained 96.66^o, 107.8^o, 110.0^o between N(2)-Na(1)-N(1), N(2)-Na(1)-B(2), N(1)-Na(1)-B(2) atoms around the first Na atom and 105.7^o, 106.5^o, 126,7^o, 121,4^o between N(4)-Na(2)-B(2), N(3)-Na(2)-B(2), B(1)-Na(2)-B(2), Na(1)-B(2)-Na(2) atoms around the second Na atom.

The packing and unit cell structure can be seen in Figure 12 and also the data is summarized in Table 1 and Table 2.

Table 1

Crystallographic data and the structure-refinement statistics for 2
Parameters
Empirical formula	C₂₈H₄₄B₂N₄Na₂
M	504,28
Crystal system	Triclinic
Space group	P-1
a(Å)	8.501 (2)
b(Å)	9.260 (2)
c(Å)	20.708 (5)
α(º)	90.093(10)
β(º)	89.974(13)
γ(º)	89.187(15)
Volume (Å³⁾	2277.9
Z	4
D_c/g cm^− 3	1.03
Absorption Coefficient (Mo Kα)mm^− 1	0.083
Crystal size(mm)	0.3×0.3×0.2
Theta range for data collection(θ Range/°)	2.20 to 25.50º
Index Ranges	-9 < = h<=9, -10 < = k<=11, -25 < = l<=24
Reflections collected	10754
Unique reflections, R_int	5563 [R(int) = 0.1759]
Completeness of theta = 25.50º	91.5%
Refinement Method	Full matrix least-square on F²
Data / restraints parameters	5563 / 0 / 369
Goodness of fit on F²	0.425
Final R indices [I > 2sigma(I)]	R1 = 0.0853, wR2 = 0.1723
R indices	R1 = 0.1756, wR2 = 0.2154
Largest Diff. peak and hole(e.Å⁻³)	0.230 and − 0.311

Table 2

Bond lengths [Å] and angles [^o] for C₂₈H₄₄B₂N₄Na₂ (2)
C(15)-N(3) 1.366(6) C(15)-C(16) 1.386(7) C(16)-C(17) 1.383(7) C(16)-C(20) 1.515(6) C(17)-C(18) 1.389(7) C(18)-C(19) 1.385(7) C(18)-C(21) 1.528(7) C(19)-N(3) 1.349(6) C(22)-N(4) 1.339(7) C(22)-C(23) 1.383(8) C(26)-C(25) 1.375(8) C(26)-N(4) 1.377(6) C(5)-N(1) 1.367(7) C(5)-C(4) 1.366(7) C(25)-C(24) 1.381(7) C(25)-C(28) 1.505(7) C(24)-C(23) 1.409(7) C(27)-C(23) 1.523(8) C(4)-C(3) 1.408(7) C(4)-C(7) 1.520(7) C(12)-N(2) 1.378(6) C(12)-C(11) 1.382(7) C(14)-C(11) 1.514(6) C(2)-C(1) 1.360(7) C(2)-C(3) 1.390(7) C(2)-C(6) 1.527(7) C(11)-C(10) 1.409(7) C(10)-C(9) 1.396(7) C(9)-C(8) 1.370(7) C(9)-C(13) 1.515(7) C(8)-N(2) 1.363(6) C(1)-N(1) 1.369(6) N(4)-Na(2) 2.412(5) N(2)-Na(1) 2.405(5) N(1)-Na(1) 2.426(5) N(3)-Na(2) 2.427(4) Na(1)-B(2) 2.629(9) Na(1)-B(1)#1 2.680(8) Na(2)-B(1) 2.637(8) Na(2)-B(2) 2.681(9) B(1)-Na(1)#2 2.680(8) N(3)-C(15)-C(16) 123.3(5) C(17)-C(16)-C(15) 118.2(5) C(17)-C(16)-C(20) 122.1(5) C(15)-C(16)-C(20) 119.7(5) C(16)-C(17)-C(18) 120.2(5) C(19)-C(18)-C(17) 117.9(5) C(19)-C(18)-C(21) 120.7(5) C(17)-C(18)-C(21) 121.4(5) N(3)-C(19)-C(18) 123.8(5) N(4)-C(22)-C(23) 125.8(5) C(25)-C(26)-N(4) 124.5(5)	N(1)-C(5)-C(4) 123.6(5) C(26)-C(25)-C(24) 117.6(5) C(26)-C(25)-C(28) 122.2(5) C(24)-C(25)-C(28) 120.2(5) C(25)-C(24)-C(23) 120.6(6) C(22)-C(23)-C(24) 116.4(5) C(22)-C(23)-C(27) 123.2(5) C(24)-C(23)-C(27) 120.4(6) C(5)-C(4)-C(3) 119.1(5) C(5)-C(4)-C(7) 119.6(5) C(3)-C(4)-C(7) 121.2(5) N(2)-C(12)-C(11) 124.2(5) C(1)-C(2)-C(3) 119.4(5) C(1)-C(2)-C(6) 119.8(5) C(3)-C(2)-C(6) 120.8(5) C(12)-C(11)-C(10) 117.7(5) C(12)-C(11)-C(14) 121.7(5) C(10)-C(11)-C(14) 120.7(5) C(9)-C(10)-C(11) 119.5(5) C(8)-C(9)-C(10) 118.5(5) C(8)-C(9)-C(13) 121.3(5) C(10)-C(9)-C(13) 120.1(5) N(2)-C(8)-C(9) 124.6(5) C(2)-C(3)-C(4) 118.0(5) C(2)-C(1)-N(1) 124.1(5) C(22)-N(4)-C(26) 115.2(5) C(22)-N(4)-Na(2) 122.8(3) C(26)-N(4)-Na(2) 121.4(4) C(8)-N(2)-C(12) 115.5(5) C(8)-N(2)-Na(1) 123.0(4) C(12)-N(2)-Na(1) 121.1(3) C(5)-N(1)-C(1) 115.7(5) C(5)-N(1)-Na(1) 119.6(4) C(1)-N(1)-Na(1) 124.6(4) C(19)-N(3)-C(15) 116.7(4) C(19)-N(3)-Na(2) 124.5(3) C(15)-N(3)-Na(2) 118.8(3) N(2)-Na(1)-N(1) 96.66(16) N(2)-Na(1)-B(2) 107.8(3) N(1)-Na(1)-B(2) 110.0(3) N(2)-Na(1)-B(1)#1 104.9(3) N(1)-Na(1)-B(1)#1 106.0(2) B(2)-Na(1)-B(1)#1 127.1(3) N(4)-Na(2)-N(3) 96.73(17) N(4)-Na(2)-B(1) 107.0(2) N(3)-Na(2)-B(1) 110.1(2) N(4)-Na(2)-B(2) 105.7(3) N(3)-Na(2)-B(2) 106.5(2) B(1)-Na(2)-B(2) 126.7(3) Na(2)-B(1)-Na(1)#2 121.4(3) Na(1)-B(2)-Na(2) 121.4(3)

Symmetry transformations used to generate equivalent atoms: #1 x + 1,y,z #2 x-1,y,z

Experiments achieved in this study, focused on the asymmetric synthesis, employing chiral auxiliaries, catalysts, and reagents with the aim of generating optical activity from achiral or racemic precursors. The investigation focused on the role of chiral crystals, particularly those crystallizing in Sohncke space groups, in creating a chiral environment or reacting with enantiomerically pure crystalline reagents.

The reduction experiment of acetophenone using the [C₆H₁₄NaO₃⁺, BH₄⁻] complex yielded promising results in the solid phase, showcasing a 36% yield in the corresponding chiral alcohol. However, when utilizing single crystals, the unfortunate emergence of racemic products raised questions about the enantiopurity of the crystals. HPLC analyses revealed the formation of two enantiomers, with a slight preference for the (R)-2-phenylethanole. The synthesis and structural analysis of C₂₈H₄₄B₂N₄Na₂ involved obtaining single crystals from a concentrated 3,5-lutidine solution of NaBH₄. The resulting compound crystallized in a triclinic space group (P-1), forming a chiral 2D polymer with sodium atoms coordinating 3,5-lutidine ligands and interconnected via bridging BH₄ groups.

In summary, while the reduction experiment exhibited notable success in generating chiral alcohols, the complexity of enantiomeric outcomes in single crystal experiments highlights the need for further exploration and understanding of chiral crystal behaviors in asymmetric synthesis processes. The synthesis and structural analysis of (2) provided valuable insights into the formation of chiral polymers, contributing to the broader understanding of chiral structures in crystallography.

Acknowledgements

I would like to thank Prof. Mikael Håkansson, Susanne Olsson and Per Martin Björemark, for providing facilities to research in Gothenburg University and their generous support.

Author Contributions

Salih PAŞA carried out the whole synthesis and characterization of crystal structures. Then he conducted the reduction experiments and data analysis. Then he wrote the main manuscript according to journal template.

Data Availability

Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Center as CCDC 2330973 for sodium borohydride complex 2. The crystallographic data for this paper can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.

Conflict Of Interest

The authors declare that they have no conflicts of interest.

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WITHDRAWN: Chiral Borohydride Complex Crystals: Synthesis, Structure and Reduction Application of Acetophenone

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Editorial Note

Abstract

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Introduction

Experimental

Synthesis of NaBH₄:Diglyme Crystals

Synthesis of NaBH₄:3,5-Lutidine Crystals

Results and Discussion

Reduction Experiment of Acetophenone with (1) [C₆H₁₄NaO₃⁺, BH₄^-]

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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Version 1

WITHDRAWN: Chiral Borohydride Complex Crystals: Synthesis, Structure and Reduction Application of Acetophenone

Status:

Version 1

Editorial Note

Abstract

Figures

Introduction

Experimental

Synthesis of NaBH4:Diglyme Crystals

Synthesis of NaBH4:3,5-Lutidine Crystals

Results and Discussion

Reduction Experiment of Acetophenone with (1) [C6H14NaO3+, BH4 -]

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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Synthesis of NaBH₄:Diglyme Crystals

Synthesis of NaBH₄:3,5-Lutidine Crystals

Reduction Experiment of Acetophenone with (1) [C₆H₁₄NaO₃⁺, BH₄^-]