molecules

Review Recent Advances in Multinuclear NMR Spectroscopy for Chiral Recognition of Organic Compounds

Márcio S. Silva

Centro de Ciências Naturais e Humanas—CCNH—Universidade Federal do ABC—UFABC, Av. Dos Estados 5001, 09210-180 Santo André –SP, Brazil; [email protected]; Tel.: +55-11-4996-8358

Academic Editor: Roman Dembinski Received: 4 January 2017; Accepted: 30 January 2017; Published: 7 February 2017

Abstract: Nuclear magnetic resonance (NMR) is a powerful tool for the elucidation of chemical structure and chiral recognition. In the last decade, the number of probes, media, and experiments to analyze chiral environments has rapidly increased. The evaluation of chiral molecules and systems has become a routine task in almost all NMR laboratories, allowing for the determination of molecular connectivities and the construction of spatial relationships. Among the features that improve the chiral recognition abilities by NMR is the application of different nuclei. The simplicity of the multinuclear NMR spectra relative to 1H, the minimal influence of the experimental conditions, and the larger shift dispersion make these nuclei especially suitable for NMR analysis. Herein, the recent advances in multinuclear (19F, 31P, 13C, and 77Se) NMR spectroscopy for chiral recognition of organic compounds are presented. The review describes new chiral derivatizing agents and chiral solvating agents used for stereodiscrimination and the assignment of the absolute configuration of small organic compounds.

Keywords: chiral recognition; NMR spectroscopy; chirality; multinuclear; enantiopurity; enantiomeric excess; stereochemistry; absolute configuration

1. Introduction Stereoisomers are compounds with the same molecular formula, possessing identical bond connectivity but different orientations of their atoms in space [1]. Enantiomers are stereoisomers that are mirror images of each other, but at the same time are not superimposable. Chirality is important in chemical, physical, pharmaceutical, and biological systems, inspiring new biomimicry-based innovations [2,3]. Moreover, the need to discriminate between enantiomers and quantify enantiomeric excess (ee) is of extreme importance in the pharmaceutical industry and for asymmetric synthesis [4–6]. Nowadays, the use of chromatography separation of enantiomers on chiral stationary phases is still the approach most often applied in modern chemical research [7,8]. However, the search for new chiral discriminating procedures that allow for quick analysis, high resolution, and utility for many non-volatile or thermally unstable compounds is increasing [9–12]. Among the several stereodiscrimination methods, including X-ray, circular dichroism, fluorescence spectroscopy, and electrophoresis, nuclear magnetic resonance (NMR) spectroscopy continues to be a useful tool for determining the enantiomeric purity and assigning the absolute configuration of chiral molecules [13–21]. NMR active nuclei are isochronous in an achiral medium and do not permit their discrimination, but in a chiral environment these nuclei are anisochronous and chiral discrimination is possible. Therefore, to perform the enantiopurity analysis, a chiral derivatization or solvating agent is essential to produce a nonequivalent diastereomeric mixture and relevant differences in the NMR spectra. Chiral derivatizing agents form a covalent bond with a reactive moiety of the substrate and chiral solvating agents associate with the substrate through non-covalent interactions, such as dipole–dipole

Molecules 2017, 22, 247; doi:10.3390/molecules22020247 www.mdpi.com/journal/molecules Molecules 2017, 22, 247 2 of 21 and ion pairing. In this context, strategies based on different intermolecular reactivities, interactions, and packing orders for a pair of enantiomers are in constant development. Among the active NMR nuclei, 1H is the most important. The characteristics of 1H, such as its natural abundance (99.98%) and its high sensitivity to environmental modifications, make it immensely Molecules 2017, 22, 247 2 of 22 versatile in NMR chiral analysis [22]. Nevertheless, 1H-NMR spectroscopy poses some limitations. 1 The H-NMRdipole–dipole spectra and for ion chiral pairing. analysis In arethis severelycontext, hamperedstrategies based dueto on the different numerous intermolecular scalar couplings, and thereactivities, overlap combined interactions, with and broad packing and orders featureless for a pair spectra of enantiomers leads to enormousare in constant difficulties development. in 1H-NMR analysis, evenAmong for the small active molecules. NMR nuclei, Consequently, 1H is the most theimportant. comparison The characteristics of the enantiomers of 1H, such using as its NMR spectranatural and theabundance assignment (99.98%) of absolute and its high configuration sensitivity canto environmental be unclear. Themodifications, application make of differentit immensely versatile in NMR chiral analysis [22]. Nevertheless, 1H-NMR spectroscopy poses some NMR nuclei, mainly 19F and 31P, overcomes these limitations. The simplicity of the multinuclear 1 limitations. The H-NMR1 spectra for chiral analysis are severely hampered due to the numerous NMR spectrascalar couplings, relative and to the the overlapH and thecombined larger with shift broad dispersion and featureless make these spectra nuclei leads especially to enormous suitable for analysis.difficulties in 1H-NMR analysis, even for small molecules. Consequently, the comparison of the Inenantiomers this review, using new NMR chiral spectra derivatization and the assignment agents of (CDAs), absolute chiralconfiguration solvating can be agents unclear. (CSAs), The and modernapplication methods of for different stereodiscrimination NMR nuclei, mainly and 19F assignment and 31P, overcomes of the these absolute limitations. configuration The simplicity of organic compoundsof the multinuclear by 19F-, 31P-, NMR13C-, spectra and relative77Se-NMR to the spectroscopy 1H and the larger are shift described. dispersion The make focus these is nuclei on articles from 2007especially to the suitable present for date. analysis. Furthermore, the 2H nucleus is not described because of the necessity of In this review, new chiral derivatization agents (CDAs), chiral solvating agents (CSAs), and discussing the physical bases in order to understand the quadrupolar electric moment and residual modern methods for stereodiscrimination and assignment of the absolute configuration of organic dipolarcompounds coupling contentsby 19F-, 31P-, [23 13C-,]. and 77Se-NMR spectroscopy are described. The focus is on articles from 2007 to the present date. Furthermore, the 2H nucleus is not described because of the necessity of 19 2. F-NMRdiscussing Chiral the physical Recognition bases in order to understand the quadrupolar electric moment and residual 19dipolarF is one coupling of the contents most useful [23]. nuclei for NMR studies [24]. The fluorine element has polar and steric properties2. 19F-NMR Chiral as a substituent Recognition and the effects that fluorinated groups can have on the physical and chemical properties of molecules have increased the number of new methods for incorporating 19F is one of the most useful nuclei for NMR studies [24]. The fluorine element has polar and fluorine into target compounds. Furthermore, 19F-NMR spectroscopy has proved to be a valuable tool steric properties as a substituent and the effects that fluorinated groups can have on the physical and to studychemical the structure properties and of functionmolecules ofhave nucleic increased acids the by number incorporating of new methods fluorine for labels incorporating into DNA and RNA [25fluorine]. into target compounds. Furthermore, 19F-NMR spectroscopy has proved to be a valuable Sincetool to the study19F-NMR the structure frequency and function and sensitivity of nucleic are acids similar by incorporating to the proton fluorine (the labels fluorine into spectraDNA is at 282 MHzand and RNA the [25]. proton spectrum is at 300 MHz), it is convenient to transition from proton to fluorine NMR experiments.Since the 19F-NMR frequency and sensitivity are similar to the proton (the fluorine spectra is at A282 wide MHz varietyand the ofproton fluorine-containing spectrum is at 300 MHz), reagents it is haveconvenient been to developedtransition from for proton NMR to chiral fluorine NMR experiments. discrimination [26,27]. In 2015, Nemes et al. described the chiral recognition studies of α-(nonafluoro- A wide variety of fluorine-containing reagents have been developed for NMR chiral 19 tert-butoxy)carboxylicdiscrimination [26,27]. acid byIn F-NMR2015, Nemes spectroscopy et al. described [28]. Carboxylic the chiral acids recognition1 and 2 werestudies synthesized of to be enantiomericallyα-(nonafluoro-tert-butoxy)carboxylic enriched in order acid toby obtain19F-NMR a spectroscopy CDA (Scheme [28].1 ).Carboxylic The synthesis acids 1 ofand racemic 2 carboxylicwere acidssynthesized1 and to2 bewas enantiomeric carried outally in enriched two steps. in order Chiral to obtain recognition a CDA (Scheme studies 1). were The synthesis performed of with (S)-phenylethylamine.racemic carboxylic Inacids the 1 NMR and 2 measurements, was carried out Cin6D two6 solvent steps. Chiral was more recognition favorable studies than were CDCl 3 in the differencesperformed ( ∆withδ) of (S diastereomeric)-phenylethylamine. signals. In the TheNMR benefits measurements, of these C6D CDAs6 solvent include was more the favorable presence of an than CDCl3 in the differences (Δδ) of diastereomeric signals. The benefits of these CDAs include the acidic functional group for a strong interaction, a bulky hydrophobic group OC(CF3)3 and an aromatic presence of an acidic functional group for a strong interaction, a bulky hydrophobic group OC(CF3)3 π π 19 ring forand a an– aromaticinteraction. ring for Additionally, a π–π interaction. the Additionally,F-NMR spectra the 19F-NMR have spectra only one have strong only one singlet strong to nine chemicallysinglet equivalent to nine chemically fluorine equivalent atoms. fluorine atoms.

C CF3 F3 C C F3C CF3 F3 F3 O O CO H CO2H 2

(R)-1 (R)-2

CF3 F3C CF3 O CDCl or C D 3 6 6 Δδ = 0.008 ppm (CDCl3) NH2 Δδ CO2H + = 0.029 ppm (C6D6)

(RS)-1 (S)-MBA Scheme 1. α-(nonafluoro-tert-butoxy)carboxylic acid as an 19F {1H} NMR CDA. Scheme 1. α-(nonafluoro-tert-butoxy)carboxylic acid as an 19F{1H} NMR CDA.

Molecules 2017, 22, 247 3 of 21

Molecules 2017, 22, 247 3 of 22 Another fluorine-containing CDA that surpasses the capabilities of Mosher’s acid (MTPA) was preparedAnother by Takahashifluorine-containing et al. (Scheme CDA that2)[ surpasses29]. To the obtain capabilities the chiral of Mosher’s compounds acid (MTPA) (R)-3 wasand (S)-3, high-performanceMoleculesprepared 2017 by, 22 , Takahashi247 liquid chromatography et al. (Scheme 2) (HPLC) [29]. To wasobtain carried the chiral out. compounds The efficiency (R)-3 ofand the 3 (ofS19 )-223F-NMR , high-performance liquid chromatography (HPLC) was carried out. The efficiency of the 19F-NMR nucleus wasAnother evaluated fluorine-containing with secondary CDA that alcohols. surpasses The the absolute capabilities configuration of Mosher’s acid of chiral(MTPA) FICA was 3 was nucleus was evaluated with secondary alcohols. The absolute configuration of chiral FICA 3 was determinedprepared by by reaction Takahashi with et ( Ral.)-phenylethylamine (Scheme 2) [29]. To followingobtain the bychiral single-crystal compounds X-ray(R)-3 and crystallographic (S)-3, determined by reaction with (R)-phenylethylamine following by single-crystal X-ray crystallographic analysis.high-performance The magnitude liquid of chromatography the chiral discrimination (HPLC) was ofcarried the FICA out. The esters efficiency was larger of the than19F-NMR that of the analysis. The magnitude of the chiral discrimination of the FICA esters was larger than that of the nucleus was evaluated with secondary alcohols. The absolute configuration of chiral FICA 3 was MTPAMTPA esters esters (Scheme (Scheme2). Moreover, 2). Moreover, the the assignment assignment ofof the absolute absolute configuration configuration of the of alcohols the alcohols was was determined by reaction19 with (R)-phenylethylamine following by single-crystal X-ray crystallographic easily performedeasily performed by byF-NMR 19F-NMR spectroscopy. spectroscopy. analysis. The magnitude of the chiral discrimination of the FICA esters was larger than that of the MTPA esters (Scheme 2). Moreover,F the assignment of the absolute configuration of the alcohols was CO Me easily performed by 19F-NMR spectroscopy.2

F (R) CO Me secondary 2 alcohols R1 R2 R1 R2 FICA - 3 F or CO Me O O (R) 2 secondary FICA MTPA alcohols R1 R2 R1 R2 FICA - 3 or F (S) O O CO2Me ΔδFICA (FICA) > Δδ (MTPA)MTPA Chiral Derivatizing Agents F F

(S) 19 1 Scheme 2. 1-fluoroindan-1-carboxylic acid 3 (FICA) as a chiralΔδ derivatizingΔδ agent for F { 19H} NMR1 Scheme 2. 1-fluoroindan-1-carboxylic acid 3 (FICA) as a chiralF (FICA) derivatizing > F (MTPA) agent for F{ H} NMR spectroscopy.Chiral Derivatizing Agents spectroscopy. 19 1 SchemeA chiral 2. ionic1-fluoroindan-1-carboxylic liquid 4 was employed acid by3 (FICA) Reddy as et a al.chiral for derivatizingchiral recognition agent for of racemicF { H} NMR Mosher’s spectroscopy. Aacid chiral salt ionic using liquid19F-NMR4 was spectroscopy employed [30]. by Ionic Reddy liquids et al.(ILs) for have chiral received recognition considerable of racemic attention Mosher’s as substitutes19 for volatile organic solvents due to their remarkable properties, such as non-flammable, acid salt usingA chiralF-NMR ionic liquid spectroscopy 4 was employed [30]. by Ionic Reddy liquids et al. (ILs)for chiral have recognition received of considerable racemic Mosher’s attention as non-volatile and recyclable. Thus, a chiral ionic liquid containing D-xylose was synthesized in seven substitutesacid salt for using volatile 19F-NMR organic spectroscopy solvents [30]. due Ionic to theirliquids remarkable (ILs) have received properties, considerable such as attention non-flammable, as steps with a good overall yield (56%). The D-xylose is a cheap and readily available starting material. non-volatilesubstitutes and for recyclable. volatile organic Thus, so alvents chiral due ionic to their liquid remarkable containing properties, D-xylose such was as non-flammable, synthesized in seven The chiral recognition study was performed with the racemic Mosher’s acid silver salt in CD3CN non-volatile and recyclable. Thus, a chiral ionic liquid containing D-xylose was synthesized in seven steps with(Scheme a good 3). The overall concentration yield (56%). was varied The D-xylose to observe is the a cheapsignals andsplitting readily and line available shape of starting 19F-NMR. material. steps with a good overall yield (56%). The D-xylose is a cheap and readily available starting material. The chiralThe effectiveness recognition of study the chiral was ionic performed liquid 4 withwas also the tested racemic by using Mosher’s a non-racemic acid silver Mosher’s salt inacid CD 3CN The chiral recognition study was performed with the racemic Mosher’s acid silver salt in CD3CN19 (Schemesalt.3). The The enantiomeric concentration excess was (ee) varied of this to non-racemic observe the salt signals was determined splitting andby 19 lineF-NMR. shape Moreover, of F-NMR. (Scheme 3). The concentration was varied to observe the signals splitting and line shape of 19F-NMR. The effectivenessthe splitting pattern of the of chiral the chiral ionic ionic liquid liquid4 was 4 was also evaluated tested by by 1H- using and 13 aC-NMR non-racemic spectroscopy. Mosher’s acid The effectiveness of the chiral ionic liquid 4 was also tested by using a non-racemic Mosher’s acid 19 salt. Thesalt. enantiomeric The enantiomeric excess excess (ee) (ee) of thisof this non-racemic non-racemic saltsalt waswas determined by by 19F-NMR.F-NMR. Moreover, Moreover, the 1 13 splittingthe patternsplitting pattern of the chiralof the chiral ionic ionic liquid liquid4 wasO 4 was evaluatedOCH evaluated3 by by H-1H- andand 13C-NMRC-NMR spectroscopy. spectroscopy.

N OCH3 OH O OCH OMe N 3 X 4 OMe OMe CO N OCH3 2 CO2 + CO2 CF OH 3 CF3 CF3 OMe NCD3CN or CDCl3 X 4 OMe OMe Racemic (R)(S) CO 2 CO2 + CO2 CF 19 1 Scheme 3.3 Chiral ionic liquid 4 as a chiral solvating agent for FC F{ 3H} NMR spectroscopy.CF3 CD3CN or CDCl3 Racemic (R)(S) Huang et al. described a CSA for a variety of acids using chiral thiophosphoramide 5 derived Scheme 3. Chiral ionic liquid 4 as a chiral solvating agent for 19F {1H} NMR spectroscopy. fromScheme (1R,2S)-1,2-diaminocyclohexane 3. Chiral ionic liquid 4 as (Scheme a chiral solvating4) [31]. Th agente interaction for 19F{ via1H} ion NMR pairing spectroscopy. and hydrogen bonding provides a large split in values between the enantiomers, especially when the 19F-NMR was Huang et al. described a CSA for a variety of acids using chiral thiophosphoramide 5 derived evaluated. The authors completed the chiral recognition study with phosphonic acids by 31P-NMR Huangfrom (1R et,2S al.)-1,2-diaminocyclohexane described a CSA for (Scheme a variety 4) of[31]. acids The usinginteraction chiral via thiophosphoramideion pairing and hydrogen5 derived spectroscopy (Scheme 4). bonding provides a large split in values between the enantiomers, especially when the 19F-NMR was from (1R,2SSwager)-1,2-diaminocyclohexane and Zhao prepared amide-based (Scheme 4palladium)[ 31]. The pincer interaction complex via6 asion a scaffold pairing to andexamine hydrogen evaluated. The authors completed the chiral recognition study with phosphonic acids by 31P-NMR19 bondingthe provides chiral discrimination a large split ability in values of amines between (Scheme the enantiomers, 5) [32]. Based especially on well-known when coordination the F-NMR was spectroscopy (Scheme 4). evaluated.chemistry, The authorsthe complex completed was easily the synthesized chiral recognition in two steps. study The with chiral phosphonic sensor site that acids undergoes by 31P-NMR Swager and Zhao prepared amide-based palladium pincer complex 6 as a scaffold to examine facile ligand exchange is flanked by fluorine pendant groups that are sensitive to enantiomers. The spectroscopythe chiral (Scheme discrimination4). ability of amines (Scheme 5) [32]. Based on well-known coordination observation of split signals at precise chemical shifts that are not concentration-dependent indicated Swagerchemistry, and the Zhao complex prepared was easily amide-based synthesized palladium in two steps. pincer The complex chiral sensor6 as asite scaffold that undergoes to examine the chiralfacile discrimination ligand exchange ability is flanked of amines by fluorine (Scheme pendant5)[ 32 ].groups Based that on are well-known sensitive to coordinationenantiomers. The chemistry, the complexobservation was of easily split signals synthesized at precise in chemical two steps. shifts The that chiral are not sensor concentration-dependent site that undergoes indicated facile ligand exchange is flanked by fluorine pendant groups that are sensitive to enantiomers. The observation of split signals at precise chemical shifts that are not concentration-dependent indicated the formation Molecules 2017, 22, 247 4 of 21

Molecules 2017, 22, 247 4 of 22 Molecules 2017, 22, 247 4 of 22 of staticthe complexes formation of on static the complexes NMR time on scale. the NMR Moreover, time scale. the Moreover, accuracy the of accuracy the method of the wasmethod evaluated was by measuringtheevaluated formation theenantiomeric by measuring of static complexes the excess enantiomeric ofonthe the aminesNMR exce sstime of with thescale. differentamines Moreover, with ratios. differentthe accuracy ratios. of the method was evaluated by measuring the enantiomeric excess of the amines with different ratios. OH HO OH HO O + CO H CDCl3 HNR 2 O 2 o 1 + CO H CDCl25 C3 O HNRNR 3 F 2 F H 2 o 1 25 C O NR 3 F F Δδ = 0.23 ppmH S Δδ = 0.23 ppm S P NH N Et Ph PPhNH EtN Et Ph Ph Et 5 - CSA Cl O Cl O CDCl3 5 - CSA Cl OP OH Cl OP OH CDCl25 oC3 + HNR2 POHOH o POHOH 25 C 1 + OH OH NRHNR3 2 NR1 Δδ = 0.19 ppm 3 Δδ = 0.19 ppm Scheme 4. Chiral thiophosphoramide 6 as a CSA for a variety of carboxylic acids by 19F {1H} and 31P Scheme 4. Chiral thiophosphoramide 6 as a CSA for a variety of carboxylic acids by 19F{1H} and 31P Scheme{1H} NMR 4. spectroscopy.Chiral thiophosphoramide 6 as a CSA for a variety of carboxylic acids by 19F {1H} and 31P 1 { H} NMR{1H} NMR spectroscopy. spectroscopy.

Toluene O O + H N R O O N 2 Toluene N O O + H N R Reflux, 12 h O O Cl N Cl 2 NH N HN Reflux, 12 h R R Cl Cl NH HN R R o CH3CN 40 C, 12 h o CH3CN 40 C, 12 h O O N O O N PdN N O O N PdN N N O O CN N PdN N R R C N PdN N F C CF3 F3C CF R R 3 3 CN F C CF3 F3C CF 3 6 - Chiral Pincer Complex 3 C 6 - Chiral Pincer Complex Scheme 5. Chiral amide-based palladium complex 6 as a chiral derivatizing agent for 19F {1H} NMR 19 1 SchemeSchemespectroscopy. 5. Chiral 5. Chiral amide-based amide-based palladium palladium complex complex 6 as a a chiral chiral derivatizing derivatizing agent agent for forF { 19H}F{ NMR1H} NMR spectroscopy. spectroscopy. A practical and simple derivatization protocol for determining the enantiopurity of chiral diols by 19F-NMRA practical spectroscopy and simple was derivatiza developedtion by protocol James etfor al. determining [33]. The experimental the enantiopurity procedure of chiral consisted diols Abyof practical mixing19F-NMR an and spectroscopy equimolecular simple derivatization was amount developed of 4-fluoro-2-formylphenyl protocolby James et for al. determining [33]. The boronic experimental the acid enantiopurity 7 procedure, phenylethylamine, consisted of chiral diols 19 by F-NMRofand mixing the spectroscopy chiral an equimolecular diol in CDCl was developed3amount at 25 °C of (Scheme 4-fluoro-2-formylphenyl by James 6). The et al. focus [33]. of The theboronic experimentalmethod acid was 7, phenylethylamine, to procedure synthesize consistedthe of mixingandfluorine anthe equimolecular chiralcompound diol in7 asCDCl amounta new3 at bifunctional25 of °C 4-fluoro-2-formylphenyl (Scheme template 6). The for focus carrying of boronicthe out method the acid enantiopurity was7, phenylethylamine, to synthesize analysis the of and fluorinechiral diols. compound 7 as a ◦new bifunctional template for carrying out the enantiopurity analysis of the chiral diol in CDCl3 at 25 C (Scheme6). The focus of the method was to synthesize the fluorine chiralA diols. closely related approach was used by Chaudhari and Suryaprakash to perform the compound 7 as a new bifunctional template for carrying out the enantiopurity analysis of chiral diols. MoleculesdiscriminationA 2017closely, 22, 247ofrelated chiral approachdiacids and was chiral used alpha by methylChaudhari amines and [34]. Suryaprakash The three-component to perform chiral5 ofthe 22 discriminationderivatization protocol of chiral has diacids been developedand chiral foralpha 19F- methyl and 13C-NMR. amines These[34]. The methodologies three-component are based chiral on NH2 derivatizationa mixture of diastereomeric protocol has been iminoboronate developed for esters 19F- andwithout 13C-NMR. any racemization These methodologies or kinetic are resolution. based on aThe mixture Suryaprakash of diastereomeric method wasiminoboronateRacemic the first procedur esters withoute for determining any racemization the enantiopurity or kinetic resolution. of chiral Thediacids. Suryaprakash The general method protocol was involves the first the procedur reactione offor an determining equimolecular the amountenantiopurity of diacids of chiral with O + OH CDCl 3 F F diacids.3-fluoro-2-formylboronic The generalF protocol acid 8 involvesand an enantiopure the4A MSreaction / 25oC (R )-methylethylamineof an equimolecularN + in amountmethanol-dN of diacids4 (Scheme with 7). OH 3-fluoro-2-formylboronicOH acid 819 and an enantiopure13 (R)-methylethylamine in methanol-d4 (Scheme 7). The baseline separationB of F and C was 1.24 ppm andB O0.46 ppm, respectively,B O for the 19 13 Therac-2-methylsuccinic baseline separation OHacid (Schemeof ChiralF and 7).diol C was 1.24 ppm andO 0.46 ppm, respectively,O for the rac-2-methylsuccinic7 - CDA Template acid (Scheme 7).

19 SchemeScheme 6. Three-component 6. Three-component coupling coupling reaction reaction forfor enatio enatiopuritypurity determination determination of chiral of chiral diols diolsby F by 19F {1H} NMR spectroscopy. {1H} NMR spectroscopy.

NH2

A closely related approach was used by Chaudhari and Suryaprakash to perform the discrimination of chiral diacids and chiral alphaΔδ = 1.24methyl ppm amines [34]. The three-component chiral F O 19 F 13 F derivatization protocol has beenCO developed2H for F- and C-NMR. These methodologiesΔδ = 0.46 ppm are based + MeOD-d4 N CH3 N CH3 on a mixture of diastereomeric iminoboronateCO H esters without any+ racemization or kinetic resolution. OH 2 O O B O O 2-methyl B B OH succinic acid O O 8 - CDA Template O O Scheme 7. Three-component coupling reaction for enatiopurity determination of diacids by 19F {1H} and 13C {1H} NMR spectroscopy.

There is great potential for the use of the boronic acid derivatives mentioned above and other related procedures [35–37] in the chiral recognition of organic and water soluble compounds due to their use as new probes, mainly in biologically relevant species [38]. The boronic acids have Lewis acid characteristics and react spontaneously and reversibly with diol compounds. Furthermore, boron chemistry is closely related to the living systems. These characteristics, in addition to the usefulness of multinuclear NMR spectroscopy, represent valuable methodologies to discriminate chiral organic compounds. Another chiral fluorine-containing carboxylic acid, F-THENA 9, was recently prepared by Dolsophon et al. The rigid anisotropic structure of F-THENA 9 is useful for assigning the absolute configuration of secondary alcohols and amines by 19F-NMR spectroscopy [39]. The CDA was synthesized from a commercially available starting material in six steps (Scheme 8). To establish the absolute configuration, X-ray analysis was performed. Derivatizations of the (S) and (R) F-THENA 9 acids with oxalyl chloride in the presence of a catalytic amount of dimethylformamide allowed for the formation of (S) and (R) F-THENA-Cl. In 19F-NMR, a positive shielding effect occurs from the alcohol moiety when an aromatic group is located on the lower side of the F-THENA plane while a negative value is obtained when an aromatic group is located on the upper side of the F-THENA plane (Scheme 8). The fluorine-containing CDA provides a self-validating system, which reduces the risks of incorrect assignment. The NMR results were strongest when both configuration assignments, from 19F- and 1H-NMR, were identical. An alternative approach to assign the absolute configuration of amino acids using the Mosher’s acid was developed by Katritzky et al [40]. N-acylbenzotriazoles are stable and crystalline derivatives of carboxylic acids and have advantages mainly when compared to acid chlorides that are unstable, moisture sensitive, difficult to prepare, and need to be stored in a deep freeze. Thus, the authors synthesized the Mosher-Bt 10 reagent (Scheme 9) from chiral and racemic Mosher’s acid to employ as a new and stable CDA for amino acids and peptides. The carboxylic acid derivative 10 was easily prepared with thionyl chloride and 1H-benzotriazole (BtH) and their reactions were studied with representative water-soluble chiral amino acids and peptides in acetonitrile-water medium (Scheme 9). The assignment of the absolute configuration of (R)-phenylalanine using the Mosher-Bt 10 reagent was performed using 1H-, 19F-, and 13C-NMR spectroscopy. To demonstrate that racemization does not occur during the reaction, Mosher amides were subjected to HPLC analysis. Recently, to

Molecules 2017, 22, 247 5 of 22

NH2

Molecules 2017, 22, 247 Racemic 5 of 21

O + OH CDCl 3 F F F 4A MS / 25oC N + N OH The Suryaprakash methodOH was the first procedure for determining the enantiopurity of chiral diacids. B B O B O The general protocol involvesOH theChiral reaction diol of an equimolecularO amount of diacidsO with 3-fluoro-2- 7 - CDA Template formylboronic acid 8 and an enantiopure (R)-methylethylamine in methanol-d4 (Scheme 7). The baseline 19 13 separationScheme of F and6. Three-componentC was 1.24 coupling ppm and reaction 0.46 for ppm, enatio respectively,purity determination for the of racchiral-2-methylsuccinic diols by 19F acid (Scheme7).{1H} NMR spectroscopy.

NH2

Δδ = 1.24 ppm

F O F F CO2H Δδ = 0.46 ppm + MeOD-d4 N CH3 N CH3 CO H + OH 2 O O B O O 2-methyl B B OH succinic acid O O 8 - CDA Template O O Scheme 7. Three-component coupling reaction for enatiopurity determination of diacids by 19F {1H} Scheme 7. Three-component coupling reaction for enatiopurity determination of diacids by 19F{1H} and 13C {1H} NMR spectroscopy. and 13C{1H} NMR spectroscopy. There is great potential for the use of the boronic acid derivatives mentioned above and other Thererelated is procedures great potential [35–37] for in the the chiral use ofrecognit the boronicion of organic acidderivatives and water soluble mentioned compounds above due and to other relatedtheir procedures use as new [35 probes,–37] inmainly the chiralin biologically recognition relevant of organicspecies [38]. and The water boronic soluble acids have compounds Lewis due acid characteristics and react spontaneously and reversibly with diol compounds. Furthermore, to their use as new probes, mainly in biologically relevant species [38]. The boronic acids have boron chemistry is closely related to the living systems. These characteristics, in addition to the Lewisusefulness acid characteristics of multinuclear and reactNMR spontaneously spectroscopy, represent and reversibly valuable with methodologies diol compounds. to discriminate Furthermore, boronchiral chemistry organic is compounds. closely related to the living systems. These characteristics, in addition to the usefulnessAnother of multinuclear chiral fluorine-containing NMR spectroscopy, carboxylic represent acid, valuable F-THENA methodologies 9, was recently to discriminateprepared by chiral organicDolsophon compounds. et al. The rigid anisotropic structure of F-THENA 9 is useful for assigning the absolute Anotherconfiguration chiral of fluorine-containingsecondary alcohols and carboxylic amines by acid, 19F-NMR F-THENA spectroscopy9, was [39]. recently The CDA prepared was by Dolsophonsynthesized et al. from The a rigid commercially anisotropic available structure starting of material F-THENA in six9 stepsis useful (Scheme for 8). assigning To establish the the absolute absolute configuration, X-ray analysis was performed. Derivatizations of the (S) and (R) F-THENA 9 configuration of secondary alcohols and amines by 19F-NMR spectroscopy [39]. The CDA was acids with oxalyl chloride in the presence of a catalytic amount of dimethylformamide allowed for synthesizedthe formation from aof commercially (S) and (R) F-THENA-Cl. available In starting 19F-NMR, material a positive in shielding six steps effect (Scheme occurs8). from To establishthe the absolutealcohol configuration,moiety when an X-ray aromatic analysis group was is located performed. on the lower Derivatizations side of the F-THENA of the (S) plane and (whileR) F-THENA a

9 acidsnegativeMolecules with oxalyl 2017 value, 22,chloride 247is obtained in when the presence an aromatic of agroup catalytic is located amount on the of dimethylformamideupper side of the F-THENA allowed6 of 22 for the formationplane (Scheme of (S )8). and The (fluorine-containingR) F-THENA-Cl. CDA In 19 provF-NMR,ides a aself-validating positive shielding system, which effect reduces occurs the from the alcoholrisksunderstand moiety of incorrect when the assignment.conformational an aromatic The group NMRfeatures isresults locatedof amidewere on strongestscaffolds the lower when in sidethe both assignment of configuration the F-THENA of assignments,the absolute plane while a from 19F- and 1H-NMR, were identical. negativeconfiguration, value is obtained Ichikawa when et al. anstudied aromatic the characteri group isstic located conformation on the upperof Mosher’s side acid of the amide, F-THENA which plane was Anelucidated alternative using approach the Cambridge to assign Structural the absolute Database configuration [41]. Amides of amino are acids important using forthe biologicalMosher’s (Scheme8). The fluorine-containing CDA provides a self-validating system, which reduces the risks of acidsystems was developedand are especially by Katritzky important et al [40]. in N -acylbenzotriazolesthe pharmaceutical are industry stable and since crystalline a huge derivatives number ofof incorrectcarboxylicmolecular assignment. drugacids candidatesand The have NMR advantages have results nitrogen weremainly atoms strongest when [42,43]. compared whenFurthermore, bothto acid configurationchiral chlorides amines that are assignments,are also unstable, used as from 19 1 F- andmoisturethe H-NMR,starting sensitive, material were difficult identical.in the formation to prepare, of newand needamino to acids be stored and peptide in a deep bonds. freeze. Thus, the authors synthesized the Mosher-Bt 10 reagent (Scheme 9) from chiral and racemic Mosher’s acid to employ as a new and stable CDA for amino acids and peptides. The carboxylic acid derivative 10 was easily NH O O prepared with thionyl chloride2 andsix 1 stepsH-benzotriazole (BtH) and their reactions were studied with or representative water-solubleCO2H chiral amino acids and peptides in acetonitrile-water medium (Scheme 9). CO H HO C The assignment ofF the absolute configuration of F(R)-phenylalanine2 9 using2 theF Mosher-Bt 10 reagent 1 19 13 was performed using H-, F-, and C-NMR(R)-F-THENA spectroscopy. (98 % ee) To demonstr(S)-F-THENAate (98 that % ee)racemization does not occur during the reaction, Mosher amides were subjected to HPLC analysis. Recently, to δ Shielding effect (lower F) Less shielding effect

F F L1 O O O O O O L1

Scheme 8. Assignment of absolute configuration of secondary amines and alcohols employing the Scheme 8. Assignment of absolute configuration of secondary amines and alcohols employing the F-THENA-Cl CDA by 19F {1H} NMR spectroscopy. F-THENA-Cl CDA by 19F{1H} NMR spectroscopy.

O SOCl2 Reflux, 50 h F C MeO Ph 3 OH O + MeO Ph H N CF3 N N N O (S)-(+)-MTPA N H2N CO2H N Mosher Bt 10 (R)-Phenylalanine BtH

AcCN : H2O (2 : 1)

MeO H Ph N CF3 O HO2C Scheme 9. Assignment of the absolute configuration of amino acids by 19F {1H} and 13C {1H} NMR spectroscopy.

Cyclodextrins (CDs) were employed as a chiral recognition agent for enantiodiscrimination of emtricitabine enantiomers 11, a novel nucleoside reverse transcriptase inhibitors for treatment of HIV infection in adults and children, by 19F-NMR spectroscopy [44]. In this protocol, developed by Rao et al., the influence of the CD cavity sizes (α, β, and γ), temperature and concentration were studied. The method is based on intermolecular interaction between emtricitabine enantiomers 11 with cyclodextrins in D2O solution (Scheme 10). The hydrophobic nature of such cavities provides a chiral environment for enantiodiscrimination. The α-CD has shown better enantiodiscrimination results employing low concentrations (0.01 mM) and temperatures (298 K). Moreover, the binding constant (K) was determined using the 19F-NMR method developed and a 2D 1H ROESY NMR analysis was performed to understand the geometry of association with α-CD.

Molecules 2017, 22, 247 6 of 22 Molecules 2017, 22, 247 6 of 21 understand the conformational features of amide scaffolds in the assignment of the absolute configuration, Ichikawa et al. studied the characteristic conformation of Mosher’s acid amide, which Anwas alternative elucidated approachusing the Cambridge to assign theStructural absolute Database configuration [41]. Amides of amino are important acids using for biological the Mosher’s acid wassystems developed and are by especially Katritzky important et al [40]. inN the-acylbenzotriazoles pharmaceutical industry are stable since and a crystallinehuge number derivatives of molecular drug candidates have nitrogen atoms [42,43]. Furthermore, chiral amines are also used as of carboxylic acids and have advantages mainly when compared to acid chlorides that are unstable, the starting material in the formation of new amino acids and peptide bonds. moisture sensitive, difficult to prepare, and need to be stored in a deep freeze. Thus, the authors synthesized the Mosher-Bt 10 reagent (Scheme9) fromO chiral and racemicO Mosher’s acid to employ NH2 as a new and stable CDA for amino acidssix steps and peptides. The carboxylic acid derivative 10 was easily or prepared with thionyl chlorideCO2H and 1H-benzotriazole (BtH) and their reactions were studied with F CO2H 9 HO2C representative water-solubleF chiral amino acids and peptides in acetonitrile-waterF medium (Scheme9). The assignment of the absolute configuration(R of)-F-THENA (R)-phenylalanine (98 % ee) (S)-F-THENA using the (98 Mosher-Bt% ee) 10 reagent was 1 19 13 performed using H-, F-, and C-NMR spectroscopy.δ To demonstrate that racemization does not Shielding effect (lower F) Less shielding effect occur during the reaction, Mosher amides were subjected to HPLC analysis. Recently, to understand the conformational features of amide scaffolds in the assignmentF of the absolute configuration, Ichikawa F et al. studied the characteristic conformation of Mosher’s acid amide,L1 which was elucidated using the O O O O Cambridge Structural Database [41]. Amides are importantO for biological systems and are especially O L1 important in the pharmaceutical industry since a huge number of molecular drug candidates have nitrogen atoms [42,43]. Furthermore, chiral amines are also used as the starting material in the Scheme 8. Assignment of absolute configuration of secondary amines and alcohols employing the formation ofF-THENA-Cl new amino CDA acids by 19F and {1H} peptideNMR spectroscopy. bonds.

O SOCl2 Reflux, 50 h F C MeO Ph 3 OH O + MeO Ph H N CF3 N N N O (S)-(+)-MTPA N H2N CO2H N Mosher Bt 10 (R)-Phenylalanine BtH

AcCN : H2O (2 : 1)

MeO H Ph N CF3 O HO2C Scheme 9. Assignment of the absolute configuration of amino acids by 19F {1H} and 13C {1H} NMR Scheme 9. Assignment of the absolute configuration of amino acids by 19F{1H} and 13C{1H} NMR spectroscopy. spectroscopy. Cyclodextrins (CDs) were employed as a chiral recognition agent for enantiodiscrimination of Cyclodextrinsemtricitabine enantiomers (CDs) were 11 employed, a novel nucleoside as a chiral reverse recognition transcriptase agent inhibitors for enantiodiscrimination for treatment of of 19 emtricitabineHIV infection enantiomers in adults and11, children, a novel nucleosideby F-NMR spectroscopy reverse transcriptase [44]. In this protocol, inhibitors developed for treatment by of Rao et al., the influence of the CD cavity sizes (α, β, and γ), temperature and concentration were HIV infection in adults and children, by 19F-NMR spectroscopy [44]. In this protocol, developed by studied. The method is based on intermolecular interaction between emtricitabine enantiomers 11 Rao et al., the influence of the CD cavity sizes (α, β, and γ), temperature and concentration were with cyclodextrins in D2O solution (Scheme 10). The hydrophobic nature of such cavities provides a studied.chiral The environment method is based for enantiodiscrimination. on intermolecular interaction The α-CD has between shown emtricitabine better enantiodiscrimination enantiomers 11 with cyclodextrinsresults employing in D2O solution low concentrations (Scheme 10 (0.01). The mM) hydrophobic and temperatures nature (298 of suchK). Moreover, cavities the provides binding a chiral environmentconstant for(K) enantiodiscrimination.was determined using the The 19F-NMRα-CD method has shown developed better and enantiodiscrimination a 2D 1H ROESY NMR results employinganalysis low was concentrations performed to understand (0.01 mM) the and geometry temperatures of association (298 with K).Moreover, α-CD. the binding constant (K) was determined using the 19F-NMR method developed and a 2D 1H ROESY NMR analysis was performed to understand the geometry of association with α-CD.

Molecules 2017, 22, 247 7 of 21 Molecules 2017, 22, 247 7 of 22

H2N N O H2N N O Molecules 2017, 22, 247 Drug EMT 11 7 of 22 N O + N O Treatment of HIV Infection F F H N N O 2 S OH H2N N O S OH Drug EMT 11 (SN,R)-EMTO + (R,NS)-EMTO Treatment of HIV Infection F F S OH S OH (S,R)-EMT (R,S)-EMT

OH D2O / 298 K O HOST-GUEST O O HO OH HO OH Complex OH O D2OOH / 298 K OHO HO HOST-GUEST OO O O HO OH OH HO OH Complex α- O OH O CD OH HO O HO OH HO O OH O OH O OH α- OHOH O CD OH OHO OO O OH HO OH O OH HO OH OH OHO O Scheme 10. Chiral discrimination of emtricitabine 11 by cyclodextrin host–guest complexes using 19F Scheme 10. Chiral discriminationO of emtricitabine 11 by cyclodextrin host–guest complexes using 19F {1H} NMR spectroscopy. {1H} NMR spectroscopy. HO 3. 31SchemeP-NMR 10. Chiral Chiral Recognition discrimination of emtricitabine 11 by cyclodextrin host–guest complexes using 19F 31 3. P-NMR{1H} ChiralNMR spectroscopy. Recognition Organophosphorus is an old field of organic chemistry. Organophosphorus compounds are essential in all chemical sciences, especially the life sciences, because their structures are a key Organophosphorus3. 31P-NMR Chiral Recognition is an old field of organic chemistry. Organophosphorus compounds are essentialbuilding in all chemicalblock [45,46]. sciences, Furthermore, especially the pharmaceut the life sciences,ical and becauseagrochemical their industries structures have are prompted a key building studiesOrganophosphorus of phosphorus chemistry is an old [47]. field of organic chemistry. Organophosphorus compounds are block [45,46]. Furthermore, the pharmaceutical and agrochemical industries have prompted studies of essentialUnlike in all 19 F-NMRchemical spectroscopy, sciences, especially the main thefactors life thatsciences, govern because the 31P theirchemical structures shift values are a arekey phosphorus chemistry [47]. buildingdifferent block due to[45,46]. other Furthermore,bond contributions. the pharmaceut In this way,ical a goodand agrochemicalknowledge of industriesorganometallic have chemistry prompted 19 31 Unlikestudiescould ofhelp phosphorusF-NMR with understanding spectroscopy, chemistry those [47]. the variations. main factors that govern the P chemical shift values are different dueUnlike31P-NMR to other19F-NMR spectroscopy bond spectroscopy, contributions. is highly theuseful, main In since this factors way,compounds athat good govern from knowledge natural the 31P resources chemical of organometallic normally shift values contain chemistry are coulddifferent helpthis nucleus with due understanding toand other 31P is bond more contributions. abundant those variations. (100%), In this making way, ita gooda valuable knowledge probe for of organometallicNMR investigations chemistry [48]. 31couldRecently,P-NMR help spectroscopywithSzyszkowiak understanding and is highly Majewska those useful, variations. have since reviewed compounds the application from natural of 31P-NMR resources for determination normally contain this nucleusof the31P-NMR andabsolute31 spectroscopyP isconfiguration more abundant is highly of organic useful, (100%), compousince making compoundsnds it [49]. a valuable fromIn this natural probereview, resources for the NMR model normally investigations created contain by [48]. Hammersschmidt and Li, in which the shielding of the phosphorus atom by the phenyl group in Recently,this nucleus Szyszkowiak and 31P is and more Majewska abundant have(100%), reviewed making it the a valuable application probe offor31 NMRP-NMR investigations for determination [48]. (S)-MTPA ester caused the signal to appear at a higher field (Scheme 11),31 was explored [50]. This of theRecently, absolute Szyszkowiak configuration and Majewska of organic have compounds reviewed the [ 49application]. In this of review, P-NMR the for modeldetermination created by ofmodel, the absolute based configurationon Mosher’s esters,of organic is currently compou ndsan important[49]. In this tool review, for assigning the model the createdabsolute by Hammersschmidtconfiguration of and many Li, organic in which functionalities the shielding and structures of the phosphorus by 1H-NMR spectroscopy atom by the [51]. phenyl group in Hammersschmidt and Li, in which the shielding of the phosphorus atom by the phenyl group in (S)-MTPA ester caused the signal to appear at a higher field (Scheme 11), was explored [50]. This model, (S)-MTPA ester caused the signal to appear at a higher field (Scheme 11),MeO was explored [50]. This R1 Ph based on Mosher’s esters, is currently an importantO tool for assigningO the absolute configuration of model, based on Mosher’s esters, is currently an important3 tool for assigningCF the absolute OH 1 (R O)2(O)P 3 many organic functionalities and structuresF3C by H-NMR spectroscopy1 2 [51O ]. configuration of many organicR fu1 nctionalities andCl structures by H-NMRR spectroscopy [51]. (R3O) (O) + 2 P R2 MeO Ph + MeO (S)-(+)-MTPA-Cl (R3O) (O)P 1 MeO PhPh 2 R O O O 3 R1 CFCF3 (R O)2(O)P 3 OH 2 O F3C R 2 O R1 Cl R 3 + (R O)2(O)P 2 MeO Ph + Scheme 11. Assigning ofR the configuration of α-hydroxyphosphonates from derivatization with MeO (S)-(+)-MTPA-Cl (R3O) (O)P Ph (S)-MTPA-Cl by 31P {1H} NMR spectroscopy. 2 O R1 CF3 R2 O The commercially available amino acid derivatives were employed as a CSA to differentiate enantiomersScheme 11. ofAssigning chiral ofphosphonates, the configuration phosphinates, of α-hydroxyphosphonates phosphates, fromphosphine derivatization oxides, with and Scheme 11. Assigning of the configuration of α-hydroxyphosphonates from derivatization with phosphonamidates(S)-MTPA-Cl by 31 byP { 131H}P-NMR NMR spectroscopy. spectroscopy [52]. The method developed by Li and Raushel uses (S)-MTPA-Cl by 31P{1H} NMR spectroscopy. N-Fmoc-N’-Boc-L-tryptophan 12 in CDCl3 at 25 °C as a CSA for different phosphorus compounds The commercially available amino acid derivatives were employed as a CSA to differentiate Theenantiomers commercially of chiral available phosphonates, amino acid phosphinates, derivatives werephosphates, employed phosphine as a CSA oxides, to differentiate and enantiomersphosphonamidates of chiral phosphonates, by 31P-NMR spectroscopy phosphinates, [52]. phosphates, The method phosphine developed oxides, by Li andand phosphonamidatesRaushel uses by 31NP-NMR-Fmoc-N spectroscopy’-Boc-L-tryptophan [52]. 12 The in methodCDCl3 at developed25 °C as a CSA by Lifor and different Raushel phosphorus uses N -Fmoc-compoundsN’-Boc- L- ◦ tryptophan 12 in CDCl3 at 25 C as a CSA for different phosphorus compounds (Scheme 12). Although Molecules 2017, 22, 247 8 of 21 Molecules 2017, 22, 247 8 of 22

(Scheme 12). Although there is a lower anisochrony of the phosphorus atoms, the good baseline Molecules 2017, 22, 247 8 of 22 there isresolution a lower and anisochrony absence of ofoverlapping the phosphorus signals provid atoms,ed the an goodefficient baseline and fast resolution chiral discrimination and absence of overlapping signals provided an efficient and fast chiral discrimination procedure. (Schemeprocedure. 12). Although there is a lower anisochrony of the phosphorus atoms, the good baseline resolution and absence of overlapping signals provided an efficient and fast chiral discrimination procedure. O

OH HN N O O O O O OH HN N O O O N-Fmoc-ON'-Boc-L-tryptophan - 12

N-Fmoc-N'-Boc-L-tryptophan - 12

O O O O P P O O O O O O H3CO P P O H3CO O NH2 O O O O NO2 O P P O O O Δδ =O 0.046 ppm Δδ =O 0.054 ppm Δδ = 0.008O ppm H3CO ΔPδ P O H3CO = 0.028 ppm O NH2 NO2 O Scheme 12. N-Fmoc-N’-Boc-L-tryptophan 12 as a CSAΔδ =for 0.046 chiral ppm discriΔδmination = 0.054 ppm of phosphonates, Scheme 12. N-Fmoc-Δδ = 0.008N’-Boc- ppm L-tryptophan 12 as a CSA for chiral discrimination of phosphonates, phosphinates, phosphates, phosphineΔδ = 0.028oxides, ppm and phosphonamidates by 31P {1H} NMR spectroscopy. phosphinates, phosphates, phosphine oxides, and phosphonamidates by 31P{1H} NMR spectroscopy. Scheme 12. N-Fmoc-N’-Boc-L-tryptophan 12 as a CSA for chiral discrimination of phosphonates, Mastranzo et al. demonstrated the use of P(III) and P(V) organophosphorus deriving reagents phosphinates, phosphates, phosphine oxides, and phosphonamidates by 31P {1H} NMR spectroscopy. Mastranzofor chiral discrimination et al. demonstrated of carboxylic the acids use ofby P(III)31P-NMR and [53]. P(V) In organophosphorusthis work, the authors de derivingscribed the reagents for chiralpreparation discriminationMastranzo of C 2et symmetric al. ofdemonstrated carboxylic diamines acidsth 13e ,use which by of 31 P(III)containP-NMR and the P(V) [ 53α-phenyl-ethyl]. organo In thisphosphorus work, group the and deriving authors the cyclohexane reagents described the skeleton. The discrimination of chiral carboxylic acids was performed in an NMR tube in three preparationfor chiral of Cdiscriminationsymmetric of diamines carboxylic13 acids, which by 31 containP-NMR the[53].α In-phenyl-ethyl this work, the group authors and described the cyclohexane the reaction steps2 (Scheme 13). The chemical shift difference between diastereomers varied in the ranges preparation of C2 symmetric diamines 13, which contain the α-phenyl-ethyl group and the cyclohexane skeleton. The discrimination of chiral carboxylic acids was performed in an NMR tube in three reaction skeleton.of 0.02–2.43 The for discrimination P(III) and 0.02–2.14 of chiral for P(V).carboxylic acids was performed in an NMR tube in three steps (Scheme 13). The chemical shift difference between diastereomers varied in the ranges of 0.02–2.43 reaction steps (Scheme 13). The chemical shift difference between diastereomers varied in the ranges for P(III)of 0.02–2.43 and 0.02–2.14 for P(III) for and P(V). 0.02–2.14 for P(V).

X X X NH N N PCl3 RCO2H P P O NH N Cl N O X DEA X X NH N N R PCl3 P RCO2H P NH N Cl N O DEA O Diamine - 13 S8 R

Diamine - 13 S8

P(III) Δδ = 0.02 - 2.43 ppm P(V) Δδ = 0.02 - 2.14 ppm X N S P Δδ O P(III) = 0.02 - 2.43 ppm N O P(V) Δδ = 0.02 - 2.14 ppm X N R S P O N O Scheme 13. Preparation of organophosphorus CDA for chiral discrimination of carboxylicR acids by 31P {1H} NMR spectroscopy. Scheme 13. Preparation of organophosphorus CDA for chiral discrimination of carboxylic acids by SchemeThe 13. samePreparation approach of was organophosphorus employed by Reiner CDA et for al.chiral who used discrimination 31P-NMR to of monitor carboxylic the acidsformation by 31 P 31P {1H} NMR spectroscopy. {1H}of diastereoisomers NMR spectroscopy. by a PCl3 reagent [54]. In this procedure, the PCl3 reagent reacted with the chiral BINOLThe to same form approach phosphite was derivative employed 14 andby Reiner after the et al.alcohol who usedor amine 31P-NMR was added to monitor to obtain the formation a mixture

Theof diastereoisomers same approach by was a PCl employed3 reagent [54]. by Reiner In this procedure, et al. who the used PCl313 reagentP-NMR reacted to monitor with the the chiral formation BINOL to form phosphite derivative 14 and after the alcohol or amine was added to obtain a mixture of diastereoisomers by a PCl3 reagent [54]. In this procedure, the PCl3 reagent reacted with the chiral

BINOL to form phosphite derivative 14 and after the alcohol or amine was added to obtain a mixture of diastereomers (Scheme 14). The chiral discrimination was possible by two diastereomeric 31P-NMR peaks for both enantiomers of the compounds. The measurements were performed in less than 5 min Molecules 2017, 22, 247 9 of 22 Molecules 2017, 22, 247 9 of 21 of diastereomers (Scheme 14). The chiral discrimination was possible by two diastereomeric 31P-NMR peaks for both enantiomers of the compounds. The measurements were performed in less with only 500 µL of deuterated solvent. Moreover, the authors examined its application to on-line ee than 5 min with only 500 μL of deuterated solvent. Moreover, the authors examined its application determinationto on-line inee combinationdetermination within combination standard with catalytic standard protocols. catalytic protocols.

PCl 3 E OH O P Cl + OH O R1 R2

E = OH or NH2 (+)-BINOL Phosphite - 14

2 R2 O R O P E + P E O O R1 R1

Scheme 14. Synthesis of CDA for chiral discrimination of alcohols and amines by 31P {1H} NMR Scheme 14. Synthesis of CDA for chiral discrimination of alcohols and amines by 31P{1H} NMR spectroscopy. spectroscopy. For 31P-NMR chiral discrimination of atropoisomeric phosphine oxides, Demchuk et al. evaluated Fordifferent31P-NMR CSAs chiral [55]. discriminationThe commercially of available atropoisomeric carboxylic phosphine acids 15 and oxides, 16, which Demchuk have etprovided al. evaluated differentgood CSAs results [55 ].for The NMR commercially chiral discrimination available [56,57], carboxylic and acidsdibenzoyltartaric15 and 16, whichacids 17 have and provided18 were good applied as CSAs (Scheme 15). The 31P-NMR analyses have shown that in most cases those simple results for NMR chiral discrimination [56,57], and dibenzoyltartaric acids 17 and 18 were applied as chiral acids are very efficient CSAs for the determination of the enantiopurity of atropoisomeric 31 CSAs (Schemebis-phosphine 15). Thedioxides.P-NMR To prove analyses the accuracy have shown of the that NMR in mostmethodology cases those an electronic simple chiral circular acids are very efficientdichroism CSAs spectroscopy for the determination was applied. The of electronic the enantiopurity absorptions of and atropoisomeric chiroptical data bis-phosphine of the investigated dioxides. To provecompounds the accuracy corroborate of the the NMR NMR methodology analyses. Furthermor an electronice, computer circular calculations dichroism were carried spectroscopy out to was applied.provide The electronicdetails about absorptions the stereochemistry and chiroptical of the complexes. data of the investigated compounds corroborate the NMR analyses.Bedekar et Furthermore, al. synthesized computermodified amides calculations as NMR solvating were carried agents out for tothe provide chiral discrimination details about the stereochemistryof 1,1′-binaphthyl of the and complexes. alpha-substituted acid enantiomers [58]. In this procedure, the chiral isobornyl Moleculesamine 19 2017 was, 22 ,employed, 247 which is readily available and can be used as a starting material to prepare10 of 22 steric bulk amide 20 with three-stereogenic centers for chiral molecular recognition by 31P-NMR analysis CO2H CO2H (Scheme 16). Furthermore, the linear relationship between the observed and actual ee values of binaphthyl 21 was evaluated and confirmed the accuracy of theOH 31P-NMR analyses. The strategy was

based on the method developedH3CO by Kagan et al. [59], in which simple chiral amides were studied as CSAs for efficient determination (ofS)-Naproxen enantiomers - 15 between(S)-Mandelic several Acid types- 16 of compounds. The chiral recognition of Kagan’s amide is achieved by hydrogen-bonding using a different procedure than the O CO2H O C(O)NMe2 one outlined by Bedekar et al. The effectivenessCO H of the method was expandedCO H to alpha-carboxylic O 2 O 2 acids by 19F-NMR spectroscopy using chiral isobornyl amide 20a and a base (DMAP) to improve the O O O O intermolecular interactions (Scheme 17). In another work of Bedekar et al. a modified Kagan’s amide

was synthesized and applied(2R, 3R)-DBTA as CSA - 17 for hydrogen-b(2R, onding3R)-DBTA-MDA based - chiral18 discrimination employing 1H- and 19F-NMR spectroscopy [60]. The same research group has published the chiral discrimination of binaphthyl 21 by employing a chiral aza-macrocycle 22 [61]. The synthesis of 18 memberChiral Solvatingaza-macrocycles Agents (S,S,S)-22 and (R,R,S)-22 was carried out in three steps from the cyclohexene oxide (Scheme 18). To confirm the 3D structure, a single crystal X-ray analysis of both diastereomers was performed. The discrimination ability of aza-macrocycle 22 for binaphthyl phosphoric acid 21 ranged from 0.03 to 0.81 ppm. An additional (O)Ph (O)Tol fluorescence spectroscopy study for understandingP 2 the chiral recognitionP 2 of chiral aza-macrocycle P(O)Ph P(O)Tol showed an enantioselective quenching interaction.2 This effect was attributed2 to the deprotonation of phosphoric acid 21, which is indicated by the appearance of new peak in the UV-Vis spectra. BINAPO Tol - BINAPO

P(O)Ph2 P(O)Ph2 P(O)Ph2 P(O)Ph2

BIPHEMPO Tetra-PHEMPO

Scheme 15. Chiral carboxylic acids used as CSAs for enantiodiscrimination of bis-phosphine dioxides by Scheme 15. Chiral carboxylic acids used as CSAs for enantiodiscrimination of bis-phosphine dioxides 31P {1H} NMR spectroscopy. by 31P{1H} NMR spectroscopy. O R1 Cl Ar H R2 NH2 N Et3N 1 Chiral Isobornyl Amine - 19 O R Chiral Amide - 20

CSA

O O P O OH

rac -1,1'- binaphthyl-2,2'-diyl hydrogen phosphate - 21 Scheme 16. Synthesis of isobornyl derived amides as CSA for enantiodiscrimination of binaphthyl by 31P {1H} NMR spectroscopy.

Molecules 2017, 22, 247 10 of 22

CO2H CO2H OH

H3CO (S)-Naproxen - 15 (S)-Mandelic Acid - 16

O CO2H O C(O)NMe2 CO H CO H O 2 O 2 O O O O

Molecules 2017 22 , , 247 (2R, 3R)-DBTA - 17 (2R, 3R)-DBTA-MDA - 18 10 of 21

Bedekar et al. synthesized modified amides as NMRChiral solvating Solvating Agents agents for the chiral discrimination of 1,10-binaphthyl and alpha-substituted acid enantiomers [58]. In this procedure, the chiral isobornyl amine 19 was employed, which is readily available and can be used as a starting material to prepare 31 steric bulk amide 20 with three-stereogenicP(O)Ph centers2 for chiral molecularP(O)Tol2 recognition by P-NMR analysis (Scheme 16). Furthermore, the linearP(O)Ph relationship2 betweenP(O)Tol the2 observed and actual ee values of binaphthyl 21 was evaluated and confirmed the accuracy of the 31P-NMR analyses. The strategy was based on the method developedBINAPO by Kagan et al. [59], in which Tol - simpleBINAPO chiral amides were studied as CSAs for efficient determination of enantiomers between several types of compounds. The chiral recognition of Kagan’s amide is achieved by hydrogen-bonding using a different procedure than the P(O)Ph2 P(O)Ph2 one outlined by Bedekar et al. The effectivenessP(O)Ph2 of the methodP(O)Ph2 was expanded to alpha-carboxylic acids by 19F-NMR spectroscopy using chiral isobornyl amide 20a and a base (DMAP) to improve the BIPHEMPO Tetra-PHEMPO intermolecular interactions (Scheme 17). In another work of Bedekar et al. a modified Kagan’s amide was synthesized and applied as CSA for hydrogen-bonding based chiral discrimination employing 1H- Scheme 15. Chiral carboxylic acids used as CSAs for enantiodiscrimination of bis-phosphine dioxides by 19 and F-NMR31P {1 spectroscopyH} NMR spectroscopy. [60].

O R1 Cl Ar H R2 NH2 N Et3N 1 Chiral Isobornyl Amine - 19 O R Chiral Amide - 20

CSA

O O P O OH

rac -1,1'- binaphthyl-2,2'-diyl hydrogen phosphate - 21 Scheme 16. Synthesis of isobornyl derived amides as CSA for enantiodiscrimination of binaphthyl by 31P{1H}Scheme NMR 16. spectroscopy. Synthesis of isobornyl derived amides as CSA for enantiodiscrimination of binaphthyl by Molecules31P 2017{1H}, NMR22, 247 spectroscopy. 11 of 22

CO2H Ph F NO2 H Δδ = 0.01 ppm N DMAP

CDCl3 O NO2 O CO2H Chiral Amide - 20a F Δδ = 0.01 ppm Scheme 17. Chiral discrimination of carboxylic acids by 19F {1H} NMR spectroscopy. Scheme 17. Chiral discrimination of carboxylic acids by 19F{1H} NMR spectroscopy.

O O N The same research group has publishedO the chiralO discrimination of binaphthyl 21 by employing a chiral aza-macrocycle 22 [61]. The synthesis of 18 member aza-macrocycles (S,S,S)-22 and (R,R,S)-22 N N was carried out in three steps from the cyclohexenePh Ph oxide (Scheme 18). To confirm the 3D structure, a single crystal X-ray analysis of3 steps both diastereomersAza-macrocycle (S.S.S was) performed. The discrimination ability of CSA aza-macrocycle 22 for binaphthylO phosphoric acid22 21 ranged from 0.03Binaphthyl to 21 0.81 ppm. An additional fluorescence spectroscopy study for understanding the chiral recognition of chiral aza-macrocycle O O N showed an enantioselective quenching interaction.O ThisO effect was attributed to the deprotonation of phosphoric acid 21, which is indicated by the appearance of new peak in the UV-Vis spectra. N N

Ph Ph Aza-macrocycle (R.R.S) Scheme 18. Chiral discrimination of Binaphtyhl 21 using the aza-macrocycle by 31P {1H} NMR spectroscopy.

Based on the difficulties in performing the chiral discrimination of BINOL phosphoric acids 21 and derivatives, Nagorny and Tay have developed a simple and reliable protocol for determining the enantiopurity by 31P-NMR spectroscopy [62]. The optical rotation measurements were not reliable for these compounds. Thus, the use of chiral amines as CSAs to carry out the NMR chiral discrimination of BINOL phosphoric acids derivatives was an elegant alternative (Scheme 19). Chiral amines 23 and 24 are commercially available and by the simple mixture of reagents (10 mg of BINOL and 1.5 equiv. of CSA) in a deuterated solvent, the 31P-NMR signals of the diastereomer salts are observed. The sample concentrations ranged from 4.7 to 25 mM. In this work, the authors studied the epimerization under thermal conditions of the BINOL phosphoric acid derivatives using the 31P-NMR protocol developed.

NH2 OH R1 1 CSA - 23 R O O CSA O P O O or P XH O X NH 2 2 H N R R2 3

Δδ CSA - 24 = 0.14 - 2.69 ppm

Scheme 19. Chiral discrimination of 3,3′-substituted BINOL and H8-BINOL phosphoric acids by 31P {1H} NMR spectroscopy.

Molecules 2017, 22, 247 11 of 22

Molecules 2017, 22, 247 CO2H 11 of 22 Ph F NO2 CO2H H Δδ DMAP = 0.01 ppm N Ph

NO CDCl3 F O NO2 2 O CO2H Chiral HAmide - 20a Δδ = 0.01 ppm N DMAP F CDCl3 O NO2 Δδ = 0.01 Oppm CO2H Molecules 2017, 22, 247 11 of 21 Chiral Amide - 20a Scheme 17. Chiral discrimination of carboxylic acids Fby 19F {1H} NMR spectroscopy. Δδ = 0.01 ppm O O Scheme 17. Chiral discrimination of carboxylicN acids by 19F {1H} NMR spectroscopy. O O

N N O O Ph N Ph O O 3 steps Aza-macrocycle (S.S.S) N N CSA O 22 Binaphthyl 21 Ph Ph

3 steps Aza-macrocycleO (S.S.OS) N CSA O O 22 O Binaphthyl 21

N N O O Ph N Ph O O Aza-macrocycle (R.R.S) N N Scheme 18. Chiral discrimination of Binaphtyhl 21 using the aza-macrocycle by 31P {1H} NMR spectroscopy. Scheme 18. Chiral discrimination of BinaphtyhlPh 21Ph using the aza-macrocycle by 31P{1H} NMR Aza-macrocycle (R.R.S) spectroscopy.Based on the difficulties in performing the chiral discrimination of BINOL phosphoric acids 21 andScheme derivatives, 18. Chiral Nagorny discriminati andon Tay of Binaphtyhl have developed 21 using athe simple aza-macrocycle and reliable by 31 protocolP {1H} NMR for spectroscopy. determining Basedthe enantiopurity on the difficulties by 31P-NMR in performing spectroscopy the [62]. chiral The discrimination optical rotation of measurements BINOL phosphoric were not acids 21 reliableBased for onthese the compounds. difficulties in Thus, performing the use the of chirchiralal aminesdiscrimination as CSAs of to BINOL carry outphosphoric the NMR acids chiral 21 and derivatives,and derivatives, Nagorny Nagorny and and Tay Tay have have developed developed a simplea simple and and reliablereliable protocol for for determining determining the discrimination31 of BINOL phosphoric acids derivatives was an elegant alternative (Scheme 19). enantiopurity by P-NMR31 spectroscopy [62]. The optical rotation measurements were not reliable for Chiralthe enantiopurity amines 23 and by 24 P-NMRare commercially spectroscopy available [62]. andThe by optica the sil mplerotation mixture measurements of reagents (10were mg not of these compounds.BINOLreliable andfor these1.5 Thus, equiv. compounds. the of CSA) use of inThus, chirala deuterated the amines use of solvent, chir as CSAsal aminesthe 31 toP-NMR carry as CSAs outsignals to the carry of NMR the out diastereomer chiralthe NMR discrimination chiral salts of BINOLarediscrimination observed. phosphoric Theof acids BINOLsample derivatives phosphoricconcentrations was acids anranged deriva elegant fromtives alternative 4.7was to an 25 elegantmM. (Scheme In alternative this 19 ).work, Chiral (Scheme the amines authors 19).23 and 24 are commerciallystudiedChiral amines the epimerization 23 available and 24 are andunder commercially by thermal the simple conditions available mixture andof the ofby BINOL reagentsthe simple phosphoric (10 mixture mg ofacid of BINOL reagents derivatives and (10 1.5mgusing equiv.of of 31 CSA) intheBINOL a 31 deuteratedP-NMR and 1.5 protocol equiv. solvent, developed.of CSA) the 31inP-NMR a deuterated signals solvent, of the the diastereomer P-NMR signals salts of the are diastereomer observed. The salts sample are observed. The sample concentrations ranged from 4.7 to 25 mM. In this work, the authors concentrations ranged from 4.7 to 25 mM. In this work, the authors studied the epimerization under studied the epimerization under thermal conditions of the BINOL phosphoric acid derivatives using NH 31 thermalthe conditions 31P-NMR protocol of the BINOLdeveloped. phosphoric acid2 derivatives using the P-NMR protocol developed. OH R1 1 CSA - 23 R NH2 O CSA O O OH O P or P OR1 1 XH CSA - 23 OR X

O NH2 RO2 CSA 2 O H3N P RO O or P XH O X Δδ = 0.14 - 2.69 ppm CSANH - 24 2 2 H N R R2 3

31 Scheme 19. Chiral discrimination of 3,3′-substituted BINOL and ΔH8-BINOLδ phosphoric acids by P CSA - 24 = 0.14 - 2.69 ppm {1H} NMR spectroscopy.

31 SchemeScheme 19. Chiral 19. Chiral discrimination discrimination of of 3,3 3,30-substituted′-substituted BINOL and and H8-BINOL H8-BINOL phosphoric phosphoric acids acidsby P by 31P {1H} NMR spectroscopy. {1H} NMR spectroscopy.

The use of 31P-NMR spectroscopy in organometallics chemistry is a well-known tool for structure elucidation and evaluation of mechanisms [63]. Recently, Gorunova et al. have prepared a chiral phosphine ligand as a derivatizing agent for enantiopurity determination of CN-palladacycles using 31P-NMR [64]. The phosphine ligand was synthesized in one step from the cheap and naturally occurring chiral menthol 25 (Scheme 20). The enantiopurity determination is carried out in situ, without any complications from geometric isomerism or palladacycle dechelation. Continuing this work, the same authors applied this method to determine the absolute configuration of the CN-palladacycles [65]. The results were based on 31P-NMR chiral discrimination, density function theory (DFT) calculations and X-ray data. The DFT calculations were carried out to study the rotameric mobility of the phosphinite group and dynamiC-NMR spectroscopy and X-ray data were employed to estimate the chirality transfer efficiency in the phosphinite derivatives. Molecules 2017, 22, 247 12 of 22

The use of 31P-NMR spectroscopy in organometallics chemistry is a well-known tool for structure elucidation and evaluation of mechanisms [63]. Recently, Gorunova et al. have prepared a chiral phosphine ligand as a derivatizing agent for enantiopurity determination of CN-palladacycles using 31P-NMR [64]. The phosphine ligand was synthesized in one step from the cheap and naturally occurring chiral menthol 25 (Scheme 20). The enantiopurity determination is carried out in situ, without any complications from geometric isomerism or palladacycle dechelation. Continuing this work, the same authors applied this method to determine the absolute configuration of the CN-palladacycles [65]. The results were based on 31P-NMR chiral discrimination, density function theory (DFT) calculations and X-ray data. The DFT calculations were carried out to study the Moleculesrotameric2017, 22, 247mobility of the phosphinite group and dynamiC-NMR spectroscopy and X-ray data were 12 of 21 employed to estimate the chirality transfer efficiency in the phosphinite derivatives.

Method A Method B PPh OH O 2 OH Ph2PCl Ph2PN(CH3)2 25 25

CDA H H Pri N Pri N Pd Pd Cl 2 L 2 Scheme 20. Chiral discrimination of CN-palladacycles by 31P {1H} NMR spectroscopy. Scheme 20. Chiral discrimination of CN-palladacycles by 31P{1H} NMR spectroscopy. 4. 13C-NMR Chiral Recognition 4. 13C-NMR Chiral Recognition 13C-NMR spectroscopy is an indispensable tool for elucidating the structure of organic 13C-NMRcompounds. spectroscopy Together with is an 1H-NMR indispensable analysis, tool13C-NMR for elucidating is a routine task the structurein organic ofsynthesis organic and compounds. the Togetherstudy with of natural1H-NMR products. analysis, The application13C-NMR of is this a routinenucleus is task hampered in organic by its synthesislow abundance and relative the study of 1 19 31 naturalto products. H-, F-, and The P-NMR, application but currently of this with nucleus modern is hampered equipment (hardware, by its low magnetic abundance fields, relative probes, to 1H-, etc.) and new experiments the use of the 13C nucleus has become an attractive alternative [66–68]. 19F-, and 31P-NMR, but currently with modern equipment (hardware, magnetic fields, probes, etc.) and Moreover, the use of ab initio calculations to support the interpretation and assignment of 13C-NMR 13 new experimentsspectra is an important the use of tool the for Cunderstanding nucleus has th becomee chiral environment an attractive of organic alternative compounds [66–68 [69].]. Moreover, 13 the use of abRiguera initio et calculations al. have demonstrated to support thethe interpretationapplications and and the assignment characteristics of thatC-NMR influence spectra the is an importantassignment tool for of understanding absolute configuration the chiral by 1H-NMR environment spectroscopy of organic [70]. compoundsThe same research [69]. group has Riguerarecently etreported al. have the use demonstrated of 13C-NMR spectroscopy the applications for the andassignment the characteristics of the absolute thatconfiguration influence the assignmentof alcohols, of absolute amines, configuration carboxylic acids, by thiols,1H-NMR cyanohydrins, spectroscopy diols, and [70]. amino The samealcohols research [71,72]. groupThe has authors have examined the 13C-NMR data of a collection of chiral samples and the experimental data recently reported the use of 13C-NMR spectroscopy for the assignment of the absolute configuration of indicated a perfect correlation between the distribution of signs for 13C chemical shifts and 1H-NMR alcohols,of their amines, enantiomers. carboxylic It acids,is possible thiols, to observe cyanohydrins, that the diols,anisochrony and amino spreads alcohols for almost [71 ,72all ].carbons The authors 13 have examined(Scheme 21: the carbonsC-NMR marked data in ofblue; a collection CDAs 26 and of chiral 27). The samples 13C-NMR and data the follows experimental the same data pattern indicated a perfectas 1 correlationH-NMR, providing between a way the to distribution double check of the signs data. for Furthermore,13C chemical the 13 shiftsC-NMR and chemical1H-NMR shifts of their enantiomers.can be used It is as possible a tool for to fully observe deuterated thatthe and anisochrony non-proton-containing spreads fororganic almost compounds. all carbons (Scheme 21: carbonsMolecules markedHeo 2017 et,in 22al., blue;247have CDAscompared26 theand accuracy27). The and13 C-NMRprecision dataof HPLC, follows 1H-, theand same13C-NMR pattern methods as13 of1H-NMR, 22to providingdetermine a way the to doubleenantiomeric check purity the data. of amino Furthermore, acid derivatives the 13C-NMR [73]. As chemicalshown in shiftsScheme can 22, be three used as a isomerscarbon peaksof the ofamino (R) andacid (derivativeS) appeared with in the different CSA 28 positions. in different The amounts integration with ofa concentrationeach peak was of tool for fully deuterated and non-proton-containing organic compounds. 20automatically mM. performed by NMR software. The total average accuracy calculated from the carbons was 98% and the total average relative standard deviation was 5.23%. The accuracy and the D OR reproducibility of the 13C-NMR in comparison to HPLCO and 1H-NMR were satisfactory for determining D the enantiomeric purity of these compounds.CD3 The studyO was carried out by mixing the (R) and (S) OR D D OR D

H OR OH NHR H OR SR

O O

OCH3 OCH3 R = or H H 9-AMA MPA 26 27

13 SchemeScheme 21. Prediction21. Prediction of of∆ δΔδRSRSfor for13 C nucleus nucleus of of chiral chiral organic organic compounds. compounds.

1O N 13 Heo et al. have compared the accuracy and precision of HPLC,O H-, and C-NMR methods to determine the enantiomeric purity of amino acid derivatives [O73]. As shown in Scheme 22, three carbon O H N * peaks of (R) and (S) appeared in differentN positions. The integration of each peak was automatically N H H O O A.A. Derivative Chiral Solvating Agent 28 High accuracy and precision

Scheme 22. Chiral discrimination of amino acid derivative by 13C {1H} NMR spectroscopy.

For 13C {1H} NMR, quantification of the chiral discrimination is necessary to prevent significant errors in the NMR analysis. Firstly, the signal/noise ratio should be improved due to the low abundance of 13C nuclei (1.1%). Another important parameter is the nuclear Overhauser effect that occurs during the proton decoupling. To reduce this effect, the inverse gated decoupling pulse sequence and the optimized time acquisition must be used. These and other parameters should be evaluated before the NMR analysis to obtain precise multinuclear NMR spectra, mainly for 13C nuclei. The development of new chiral auxiliaries to determine the enantiomeric excess or to assign the absolute configuration by NMR spectroscopy is a difficult and time-consuming task. For this reason, the use of natural products as a scaffold, either directly or as a starting material to prepare new chiral agents, is an elegant alternative. Based on this strategy, Szostak et al. have employed naturally occurring Chinchona 29 alkaloid and their derivatives as CSAs for different functionalities [74]. Selective modifications of the natural products were performed by tuning their enantiodiscrimination. The structure of quinine enables modulation, mainly for carbons 9, 22, and 23 (Scheme 23). Moreover, the presence of a phosphate group supplementing the quinine frame core provides zwitterionic character because it possesses two charged functions, a quaternary ammonium cation and a phosphate anion. These characteristics were used with success for enantiodiscrimination of non-derivatized amino acids (Scheme 23). The charges increase the interaction potency between the molecules, allowing for efficient chiral discrimination in the DMSO-d6 solvent by 13C-NMR spectroscopy. The split signals of 13C-NMR ranged from 0.025 to 0.145 ppm.

Molecules 2017, 22, 247 13 of 22

isomers of the amino acid derivative with the CSA 28 in different amounts with a concentration of 20 mM.

D OR O D CD3 O OR D D OR D

H OR OH NHR H OR Molecules 2017, 22, 247 SR 13 of 21

O O

OCH3 OCH3 performed by NMR software.R = The total average accuracyor calculated from the carbons was 98% and H H the total average relative standard deviation was9-AMA 5.23%. The accuracyMPA and the reproducibility of the 13 1 C-NMR in comparison to HPLC and H-NMR26 were satisfactory27 for determining the enantiomeric purity of these compounds. The study was carried out by mixing the (R) and (S) isomers of the amino acid derivative withScheme the CSA 21. 28Predictionin different of ΔδRS amounts for 13C nucleus with of achiral concentration organic compounds. of 20 mM.

O N O O O H N * N N H H O O A.A. Derivative Chiral Solvating Agent 28 High accuracy and precision

Scheme 22. Chiral discrimination of amino acid derivative by 13C {1H} NMR spectroscopy. Scheme 22. Chiral discrimination of amino acid derivative by 13C{1H} NMR spectroscopy.

For 13C {1H} NMR, quantification of the chiral discrimination is necessary to prevent significant Forerrors13C{ in1 H}the NMR, NMR quantificationanalysis. Firstly, of the the signal/n chiraloise discrimination ratio should isbe necessary improved todue prevent to the significantlow errors inabundance the NMR of analysis. 13C nuclei Firstly, (1.1%). theAnother signal/noise important ratio parameter should is be the improved nuclear Overhauser due to the effect low abundancethat of 13Coccurs nuclei during (1.1%). the Another proton decoupling. important To parameter reduce this iseffect, the the nuclear inverse Overhauser gated decoupling effect pu thatlse sequence occursduring and the optimized time acquisition must be used. These and other parameters should be evaluated the proton decoupling. To reduce this effect, the inverse gated decoupling pulse sequence and the before the NMR analysis to obtain precise multinuclear NMR spectra, mainly for 13C nuclei. optimizedThe time development acquisition of must new bechiral used. auxiliaries These to and determine other parameters the enantiomeric should excess be evaluatedor to assign beforethe the 13 NMR analysisabsolute configuration to obtain precise by NMR multinuclear spectroscopy NMR is a di spectra,fficult and mainly time-consuming for C nuclei. task. For this reason, Thethe developmentuse of natural products of new as chiral a scaffold, auxiliaries either directly to determine or as a starti theng enantiomericmaterial to prepare excess new orchiral to assign the absoluteagents, configurationis an elegant alternative. by NMR Based spectroscopy on this strategy, is a difficult Szostak andet al. time-consuming have employed naturally task. For this reason,occurring the use Chinchona of natural 29 products alkaloid and as a their scaffold, derivatives either as directly CSAs for or asdifferent a starting functionalities material to[74]. prepare new chiralSelective agents, modifications is an elegant of the natural alternative. products Based were performed on thisstrategy, by tuning Szostaktheir enantiodiscrimination. et al. have employed The structure of quinine enables modulation, mainly for carbons 9, 22, and 23 (Scheme 23). naturally occurring Chinchona 29 alkaloid and their derivatives as CSAs for different functionalities [74]. Moreover, the presence of a phosphate group supplementing the quinine frame core provides Selectivezwitterionic modifications character of thebecause natural it possesses products two were charged performed functions, by a tuning quaternary their ammonium enantiodiscrimination. cation The structureand a phosphate of quinine anion. enables These modulation, characteristics mainly were used for carbonswith success9, 22 for, and enantiodiscrimination23 (Scheme 23). Moreover, of the presencenon-derivatized of a phosphate amino acids group (Scheme supplementing 23). The charges the increase quinine the interaction frame core potency provides between zwitterionic the charactermolecules, because allowing it possesses for twoefficient charged chiral functions, discrimination a quaternary in the DMSO- ammoniumd6 solvent cation by and 13C-NMR a phosphate anion.spectroscopy. These characteristics The split signals were of used13C-NMR with ranged success from for 0.025 enantiodiscrimination to 0.145 ppm. of non-derivatized amino acids (Scheme 23). The charges increase the interaction potency between the molecules, allowing 13 for efficient chiral discrimination in the DMSO-d6 solvent by C-NMR spectroscopy. The split signals 13 of C-NMRMolecules ranged2017, 22, 247 from 0.025 to 0.145 ppm. 14 of 22

Scheme 23. Natural alkaloid 29 employed as a CSA for chiral discrimination of non-derivatized Scheme 23. Natural alkaloid 29 employed as a CSA for chiral discrimination of non-derivatized amino amino acids by 13C {1H} NMR spectroscopy. acids by 13C{1H} NMR spectroscopy. 13C {1H} NMR spectroscopy is also a useful tool for discrimination and analysis of complex 13mixturesC{1H} NMRbecause spectroscopy of the valuable is alsoinformation a useful co toolntained for in discrimination the carbon spectrum. and analysis This strategy of complex mixturesbecomes because more of attractive the valuable mainly information when the 1H-NMR contained spectra in the demonstrate carbon spectrum. broad multiplets This strategy and small becomes chemical shift differences between the stereoisomers. As a result, Lankhorst et al. presented a simple 13C-NMR methodology for the discrimination of a complex mixture of α-tocopherol stereoisomers [75]. The traditional methods employed to perform this discrimination, such as gas and liquid chromatography, require multiple steps to prepare the sample and obtain the ideal conditions. In contrast to the 13C-NMR method, the tocopherol mixture and the chiral trifluoroethanol (TFAE) 30 are dissolved in CDCl3 to prepare the sample for NMR stereodiscrimination analysis (Scheme 24). Furthermore, the assignment of individual stereoisomers was possible by the synthesis of each tocopherol stereoisomer from stereochemically pure or enriched building blocks. In this work, the temperature-dependent behavior of the racemate in the presence of TFAE 30 was studied. The temperature ranges affected the efficiency of the 13C chemical shift differences.

R1 HO

R2 O R3 Tocopherol mixture

HO CF3 Tocopherols ratio by 13C NMR

30 TFAE

Scheme 24. Discrimination of tocopherols stereoisomers using the CSA 30 (TFAE) by 13C {1H} NMR spectroscopy.

Another route to improve the chiral recognition methodologies by NMR spectroscopy is the development of new pulse sequences. In this sense, a significant increase in the number of new NMR experiments has occurred in the last decades. Among these new experiments, “pure shift” NMR emerged as a useful tool for chiral studies. This pulse sequence and their derivatives have improved the resolution of 1D and 2D NMR experiments to simplify the typical J(H,H) multiplicity pattern of 1H signals to singlets. The Se-NMR experiments make the NMR stereodiscrimination methods simpler and more efficient, mainly for complex and overcrowded resonances. Recently, Parella et al. have exploited the application of highly resolved pure shift heteronuclear single quantum coherence (HSQC) spectra for 1H- and 13C-NMR enantiodifferentiation [76]. In this study, the authors employed a racemic mixture of lactams with (R)-PA as a CSA to show how the highly resolved 2D HSQC

Molecules 2017, 22, 247 14 of 22

Molecules 2017Scheme, 22, 247 23. Natural alkaloid 29 employed as a CSA for chiral discrimination of non-derivatized 14 of 21 amino acids by 13C {1H} NMR spectroscopy. more attractive13C {1H} mainly NMR whenspectroscopy the 1H-NMR is also a spectra useful demonstratetool for discrimination broad multiplets and analysis and of smallcomplex chemical shift differencesmixtures because between of thethe stereoisomers.valuable information As a result,contained Lankhorst in the carbon et al. presentedspectrum. aThis simple strategy13C-NMR 1 methodologybecomes formore the attractive discrimination mainly when of athe complex H-NMR mixturespectra demonstrate of α-tocopherol broad multiplets stereoisomers and small [75 ]. The chemical shift differences between the stereoisomers. As a result, Lankhorst et al. presented a simple traditional methods employed to perform this discrimination, such as gas and liquid chromatography, 13C-NMR methodology for the discrimination of a complex mixture of α-tocopherol stereoisomers require[75]. multiple The traditional steps to methods prepare employed the sample to perfor andm obtain this discrimination, the ideal conditions. such as gas In and contrast liquid to the 13 C-NMRchromatography, method, the require tocopherol multiple mixture steps to and prepare the chiral the sample trifluoroethanol and obtain the (TFAE) ideal conditions.30 are dissolved In in CDCl3contrastto prepare to the the 13C-NMR sample method, for NMR the stereodiscriminationtocopherol mixture and analysisthe chiral (Schemetrifluoroethanol 24). Furthermore, (TFAE) 30 the assignmentare dissolved of individual in CDCl stereoisomers3 to prepare the was sample possible for NMR by thestereo synthesisdiscrimination of each analysis tocopherol (Scheme stereoisomer 24). from stereochemicallyFurthermore, the assignment pure or enriched of individual building stereoisomers blocks. Inwas this possible work, by the the temperature-dependent synthesis of each tocopherol stereoisomer from stereochemically pure or enriched building blocks. In this work, the behavior of the racemate in the presence of TFAE 30 was studied. The temperature ranges affected the temperature-dependent13 behavior of the racemate in the presence of TFAE 30 was studied. The efficiencytemperature of the rangesC chemical affected shift the efficiency differences. of the 13C chemical shift differences.

R1 HO

R2 O R3 Tocopherol mixture

HO CF3 Tocopherols ratio by 13C NMR

30 TFAE

13 1 SchemeScheme 24. Discrimination 24. Discrimination of tocopherolsof tocopherols stereoisomers stereoisomers using using the the CSA CSA 30 (TFAE)30 (TFAE) by byC { 13H}C{ NMR1H} NMR spectroscopy. spectroscopy. Another route to improve the chiral recognition methodologies by NMR spectroscopy is the Anotherdevelopment route of tonew improve pulse sequences. the chiral In this recognition sense, a significant methodologies increase in by the NMR number spectroscopy of new NMR is the developmentexperiments of new has pulseoccurred sequences. in the last In decades. this sense, Among a significant these new increase experiments, in the “pure number shift” of NMR new NMR experimentsemerged has as a occurred useful tool in for the chiral last studies. decades. This pulse Among sequence these and new their experiments, derivatives have “pure improved shift” NMR the resolution of 1D and 2D NMR experiments to simplify the typical J(H,H) multiplicity pattern of 1H emerged as a useful tool for chiral studies. This pulse sequence and their derivatives have improved signals to singlets. The Se-NMR experiments make the NMR stereodiscrimination methods simpler 1 the resolutionand more of efficient, 1D and mainly 2D NMR for complex experiments and overcrowded to simplify theresonances. typical JRecently,(H,H) multiplicity Parella et al. pattern have of H signalsexploited to singlets. the application The Se-NMR of highly experiments resolved make pure theshift NMR heteronuclear stereodiscrimination single quantum methods coherence simpler and more(HSQC) efficient, spectramainly for 1H- and for 13 complexC-NMR enantiodifferentiation and overcrowded [76]. resonances. In this study, Recently, the authors Parella employed et al. have exploiteda racemic the application mixture of of lactams highly with resolved (R)-PA pure as a shift CSA heteronuclear to show how the single highly quantum resolved coherence 2D HSQC (HSQC) 1 13 spectra for H- and C-NMR enantiodifferentiation [76]. In this study, the authors employed a racemic mixtureMolecules of lactams 2017, 22, with247 (R)-PA as a CSA to show how the highly resolved 2D HSQC spectra15 of 22 can be used to detect and accurately quantify very small anisochrony values (Scheme 25). The values obtained spectra can be used to detect and accurately quantify very small anisochrony values (Scheme 25). from theThe pure values shift obtained HSQC from were the compared pure shift toHSQC traditional were compared HSQC and to traditional 1D NMR HSQC experiments, and 1D NMR confirming the advantagesexperiments, of confirming this new pulse the advant sequence.ages of this new pulse sequence.

Me O N Digital Resolution: Highly Resolved 1 NMR Spectra H = +/- 0.5 ppb 13C = +/- 2.6 ppb

HO

Pure Shift 2D HSQC NMR

SchemeScheme 25. 25.Highly Highly resolved resolved purepure shift HSQC-NMR HSQC-NMR spectra spectra of lactam. of lactam. Stereochemical NMR studies have also received attention based on the development and Stereochemicalapplication of chiral NMR oriented studies media, have such also as liquid–crystalline received attention systems based [77] and on stretched the development polymer and applicationgels [78]. of chiralThe main oriented use of these media, oriented such media as liquid–crystalline is for 2H-NMR experiments systems [due77] to and the stretched quadrupolar polymer gels [78moment]. The mainof the usedeuterium of these nucleus. oriented Chiral media oriented is for media2H-NMR is a flexible experiments approach duethat can to thebe applied quadrupolar to several organic solvents without the use of solid-state NMR [79]. For this approach, a chiral liquid–crystalline is put into an NMR tube and after addition of an organic solvent and the probe the chiral liquid crystal swells and forms a gel. The gel does not swell isotropically but anisotropically along the glass wall of the tube and the resulting align–angular information relative to the static magnetic field can be obtained. This method is already a common procedure used for biomacromolecules in aqueous solution. Based on these features, the chiral recognition by liquid–crystalline systems was extended to other nuclei. The use of 13C-NMR spectroscopy is a valuable alternative to discriminate chiral environments in oriented media due to a much larger dispersion of chemical shifts. Furthermore, the correlation between 2D NMR experiments for 13C and 2H isotopes has the benefit of different pulse sequence as the INEPT-DECANCY and DEPT-DECANCY to improve the resolution and sensitivity. These characteristics were well described by Lesot et al. in a recent review [80]. The use of 19F-NMR spectroscopy for the measurement of enantiomeric excess was successfully extended to oriented media by Phillips and Sharman [81].

5. 77Se-NMR Chiral Recognition Selenium organic compounds are related in a broader context of applications [82–84] and selenium is a fundamental component of the living cells of a variety of organisms [85]. Based on these features, the use of 77Se-NMR spectroscopy emerged as an opportunity for structure and reactivity studies [86]. The application of 77Se-NMR spectroscopy for chiral recognition has great potential for success due to the characteristics of the selenium nucleus, such as a wide spectral window, reasonably high natural abundance, especially compared with the carbon nucleus, and because the 77Se isotope is ½ and shows no significant nuclear Overhauser effect. Thus, the possibility of developing chiral probes containing selenium has several benefits. In this context, Orlov and Ananikov synthesized a chiral selenide probe as a CDA for determination of enantiomeric purity of alcohols and amines by 77Se-NMR spectroscopy [87]. The synthesis was performed in two steps achieving 80% yields of CDA 31 (Scheme 26). Selenide chiral probe 31 was successfully applied to enantiodiscrimination of a variety of alcohols and amines. Ferreira and Gonçalves have carried out an inverse planning procedure to prepare a new chiral selenide alcohol as a CDA [88]. The article describes the synthesis of chiral selenide alcohol 32 in two steps (Scheme 27). To perform the derivatization, a ”mix and shape” method was performed in an NMR tube, similar to Orlov and Ananikov’s work. Using selenide alcohol 32, chiral discrimination was achieved for different carboxylic acids.

Molecules 2017, 22, 247 15 of 21 moment of the deuterium nucleus. Chiral oriented media is a flexible approach that can be applied to several organic solvents without the use of solid-state NMR [79]. For this approach, a chiral liquid–crystalline is put into an NMR tube and after addition of an organic solvent and the probe the chiral liquid crystal swells and forms a gel. The gel does not swell isotropically but anisotropically along the glass wall of the tube and the resulting align–angular information relative to the static magnetic field can be obtained. This method is already a common procedure used for biomacromolecules in aqueous solution. Based on these features, the chiral recognition by liquid–crystalline systems was extended to other nuclei. The use of 13C-NMR spectroscopy is a valuable alternative to discriminate chiral environments in oriented media due to a much larger dispersion of chemical shifts. Furthermore, the correlation between 2D NMR experiments for 13C and 2H isotopes has the benefit of different pulse sequence as the INEPT-DECANCY and DEPT-DECANCY to improve the resolution and sensitivity. These characteristics were well described by Lesot et al. in a recent review [80]. The use of 19F-NMR spectroscopy for the measurement of enantiomeric excess was successfully extended to oriented media by Phillips and Sharman [81].

5. 77Se-NMR Chiral Recognition Selenium organic compounds are related in a broader context of applications [82–84] and selenium is a fundamental component of the living cells of a variety of organisms [85]. Based on these features, the use of 77Se-NMR spectroscopy emerged as an opportunity for structure and reactivity studies [86]. The application of 77Se-NMR spectroscopy for chiral recognition has great potential for success due to the characteristics of the selenium nucleus, such as a wide spectral window, reasonably high natural abundance, especially compared with the carbon nucleus, and because the 77Se isotope is 1 2 and shows no significant nuclear Overhauser effect. Thus, the possibility of developing chiral probes containing selenium has several benefits. In this context, Orlov and Ananikov synthesized a chiral selenide probe as a CDA for determination of enantiomeric purity of alcohols and amines by 77Se-NMR spectroscopy [87]. The synthesis was performed in two steps achieving 80% yields of CDA 31 (Scheme 26). Selenide chiral probe 31 was successfully applied to enantiodiscrimination of a variety of alcoholsMolecules and 2017 amines., 22, 247 16 of 22

OH SePh iiiSePh

CO2Et CO2Et CO2H

o CDA 31 i = PhSeCN, PBu3, Toluene, 5 C; ii = HCl/AcOH, 100 oC.

DCC, DCU NMR tube

NH2 NH2 OH

CF3 F Δδ = 6.06 ppm Δδ = 1.60 ppm Δδ = 0.46 ppm

OH NH2 OMe

O Δδ Δδ = 2.65 ppm = 2.31 ppm Scheme 26. Chiral discrimination of alcohols and amines employing the selenide CDA 32 as a probe Scheme 26. Chiral discrimination of alcohols and amines employing the selenide CDA 32 as a probe by 77Se {1H} NMR spectroscopy. by 77Se {1H} NMR spectroscopy. O O iii Ferreira and GonçalvesPh OH have carried outPh an inverseOH planning procedurePh OH to prepare a new chiral SePh selenide alcohol as a CDANH [288]. The article describesBr the synthesis of chiral selenide alcohol 32 in two steps (Scheme 27). To perform the derivatization, a ”mix and shape” methodCDA was32 performed in an NMR o i = KBr, NaNO2(aq), H2SO4(aq), 0 C; ii = PhSeSePh, 18-crown-6, NaBH4, THF. DCC, DMAP NMR tube O O O

OH OH OH Br Me Ph Δδ Δδ Δδ = 51 Hz = 22 Hz = 40 Hz Scheme 27. Chiral discrimination of carboxylic acids employing the selenide CDA 33 as a probe by 77Se {1H} NMR spectroscopy.

Recently, Silva et al. have demonstrated the application of a three-component reaction for chiral discrimination of amines 33 containing the selenium atom by 77Se-NMR spectroscopy [89]. This simple and inexpensive chiral derivatizing method was successfully employed for a variety of organic compounds by 77Se-NMR. In this protocol, the selenide amine reacts with 2-formylphenylboronic acid and the active (+)-BINOL in the presence of molecular sieves for 10 min (Scheme 28). The 1H-NMR analyses were dependent on the concentration. In highly concentrated media the signals were broader. It is possible that a self-aggregation interaction occurred during the analysis. For 77Se-NMR spectra, this concentration effect was not observed. Moreover, an HPLC analysis was performed for comparative evaluation of the 77Se-NMR results (Scheme 28). Murai has performed the synthesis of the phosphoroselenoic acid 34 as a double check chiral discrimination protocol for amines and alcohols by 77Se- and 31P-NMR spectroscopy [90]. The CDA 34 was synthesized in a one-pot procedure achieving 94% yields (Scheme 29). All CDAs obtained are stable under air and are purified by column chromatography. The absolute configuration of the CDA 34 was confirmed by X-ray analysis. The CDA 34 showed high reactivity for primary amines and alcohols. When tertiary alcohols were employed the reaction did not proceed. Moreover, most of the diastereoisomers formed by this protocol were separated by simple recrystallization. Recently, Murai and Itoh have applied these CDAs derivatives for the 77Se- and 31P-NMR discrimination of remote chirality of primary alcohols [91].

Molecules 2017, 22, 247 16 of 22

OH SePh iiiSePh

CO2Et CO2Et CO2H

o CDA 31 i = PhSeCN, PBu3, Toluene, 5 C; ii = HCl/AcOH, 100 oC.

DCC, DCU NMR tube

NH2 NH2 OH

CF3 F Δδ = 6.06 ppm Δδ = 1.60 ppm Δδ = 0.46 ppm

OH NH2 OMe Molecules 2017, 22, 247 16 of 21 O Δδ Δδ = 2.65 ppm = 2.31 ppm tube, similarScheme to Orlov26. Chiral and discrimination Ananikov’s of alcohols work. and Using amines selenide employing alcohol the selenide32, chiral CDA 32 discrimination as a probe was achievedby for 77 differentSe {1H} NMR carboxylic spectroscopy. acids.

O O iii Ph OH Ph OH Ph OH

NH2 Br SePh CDA 32 o i = KBr, NaNO2(aq), H2SO4(aq), 0 C; ii = PhSeSePh, 18-crown-6, NaBH4, THF. DCC, DMAP NMR tube O O O

OH OH OH Br Me Ph Δδ Δδ Δδ = 51 Hz = 22 Hz = 40 Hz

SchemeScheme 27. Chiral 27. Chiral discrimination discrimination of of carboxylic carboxylic acids employing employing the the selenide selenide CDA CDA 33 as 33 a asprobe a probe by by 77Se {1H} NMR spectroscopy. 77Se {1H} NMR spectroscopy. Recently, Silva et al. have demonstrated the application of a three-component reaction for chiral Recently,discrimination Silva of et amines al. have 33 demonstrated containing the the selenium application atom ofby a77 three-componentSe-NMR spectroscopy reaction [89]. forThis chiral discriminationsimple and ofinexpensive amines 33 chiralcontaining derivatizing the selenium method was atom successfully by 77Se-NMR employed spectroscopy for a variety [89 of]. organic This simple and inexpensivecompounds by chiral 77Se-NMR. derivatizing In this protocol, method the wasselenide successfully amine reacts employed with 2-formylphenylboronic for a variety of acid organic 1 compoundsand the byactive77Se-NMR. (+)-BINOL In in this the protocol, presence theof molecular selenide aminesieves for reacts 10 min with (Scheme 2-formylphenylboronic 28). The H-NMR acid analyses were dependent on the concentration. In highly concentrated media the signals were and the active (+)-BINOL in the presence of molecular sieves for 10 min (Scheme 28). The 1H-NMR broader. It is possible that a self-aggregation interaction occurred during the analysis. For 77Se-NMR analyses were dependent on the concentration. In highly concentrated media the signals were broader. spectra, this concentration effect was not observed. Moreover, an HPLC analysis was performed for 77 It is possiblecomparative that aevaluation self-aggregation of the 77Se-NMR interaction results occurred (Scheme during 28). the analysis. For Se-NMR spectra, this concentrationMurai effect has performed was not observed. the synthesis Moreover, of the phosphoroselenoic an HPLC analysis acid was 34 performedas a double forcheck comparative chiral 77 77 31 evaluationdiscriminationMolecules of 2017 the, 22 ,protocol Se-NMR247 for results amines (Scheme and alcohols 28). by Se- and P-NMR spectroscopy [90]. The17 ofCDA 22 34 was synthesized in a one-pot procedure achieving 94% yields (Scheme 29). All CDAs obtained are

stable under air and are purifiedNH2 by column Ochromatography. The absolute configuration of the CDA 34 was confirmed bySe X-ray analysis. The CDA 34 showed high reactivity for primary amines + + OH and alcohols. When tertiary alcohols were employOHed the reaction did not proceed. Moreover, most B OH of the diastereoisomers formed by this protocol were separated by simple recrystallization. Recently, Racemic - 33 OH 77 31 Murai and Itoh have applied these CDAsBoronic derivatives Acid for the Se-(+)-BINOL and P-NMR discrimination of remote chirality of primary alcohols [91].

CDCl3 4A MS / 25oC

Se Se

N + N (50.4 : 49.6) by Chiral GC O O (55.2 : 44.8) by 77Se NMR B B O O

Scheme 28. Three-component reaction for chiral discrimination of selenide amines by 77Se {1H} NMR Scheme 28. Three-component reaction for chiral discrimination of selenide amines by 77Se {1H} NMR spectroscopy. spectroscopy.

o 1) Et3N, 0 C Murai has performed the synthesis2) (+)-BINOL of the phosphoroselenoic acid 34 as a double check chiral O Se discrimination protocol for amines3) Se and, Toluene, alcohols Reflux. by 77Se- and 31P-NMRP spectroscopy [90]. The CDA 34 PCl3 O Cl was synthesized in a one-pot procedure achieving 94% yields (Scheme 29). All CDAs obtained are CDA - 34 stable under air and are purified by column chromatography. The absolute configuration of the CDA 34 was confirmed by X-ray analysis. The CDA 34 showed high reactivity for primary amines and 1) Toluene, Reflux. 2) Alcohols, 3 h. 1) Toluene, Reflux. 2) Amines, 2-18 h.

O Se P O O O Se 3 P R * 2 R O NH 1 R * R Scheme 29. Phosphoroselenoic acid derivatives for chiral discrimination of amines and alcohols by 77Se {1H} and 31P {1H} NMR spectroscopy.

6. Conclusions Numerous efficient and versatile chiral derivatizing and solvating agents have emerged in recent years. These compounds present different chiral recognition abilities since the structure and conditions can be tuned to improve the molecular interactions. With the expectation of expanding the chiral probes, using multinuclear NMR spectroscopy provides new possibilities for chiral studies and the elucidation of their chiral recognition mechanisms. 19F, 31P, 13C, and 77Se nuclei have demonstrated efficient results based on their nuclear properties and their incorporation into organic compounds through organic synthesis procedures. Although the use of multinuclear NMR spectroscopy for chiral discrimination and assignment of the absolute configuration is attractive, we need to be aware that the reliability of the multinuclear NMR analysis should be evaluated for the respective application due to the different nuclear properties and general behavior of each element.

Molecules 2017, 22, 247 17 of 22

NH2 O Se + + OH OH B OH Racemic - 33 OH Boronic Acid (+)-BINOL

CDCl3 4A MS / 25oC

Se Se

N + N Molecules 2017, 22, 247 (50.4 : 49.6) by Chiral GC 17 of 21 O O (55.2 : 44.8) by 77Se NMR B B O O alcohols. When tertiary alcohols were employed the reaction did not proceed. Moreover, most of the diastereoisomers formed by this protocol were separated by simple recrystallization. Recently, Murai and Itoh have applied these CDAs derivatives for the 77Se- and 31P-NMR discrimination of remote Scheme 28. Three-component reaction for chiral discrimination of selenide amines by 77Se {1H} NMR chirality ofspectroscopy. primary alcohols [91].

o 1) Et3N, 0 C 2) (+)-BINOL O Se 3) Se, Toluene, Reflux. P PCl3 O Cl

CDA - 34

1) Toluene, Reflux. 2) Alcohols, 3 h. 1) Toluene, Reflux. 2) Amines, 2-18 h.

O Se P O O O Se 3 P R * 2 R O NH 1 R * R Scheme 29. Phosphoroselenoic acid derivatives for chiral discrimination of amines and alcohols by Scheme 29. Phosphoroselenoic acid derivatives for chiral discrimination of amines and alcohols by 77Se {1H} and 31P {1H} NMR spectroscopy. 77Se {1H} and 31P{1H} NMR spectroscopy. 6. Conclusions 6. Conclusions Numerous efficient and versatile chiral derivatizing and solvating agents have emerged in Numerousrecent years. efficient These compounds and versatile present chiral different derivatizing chiral recognition and solvating abilities agents sincehave the structure emerged and in recent years.conditions These compounds can be tuned present to improve different the chiralmolecular recognition interactions. abilities With sincethe expectation the structure of expanding and conditions the chiral probes, using multinuclear NMR spectroscopy provides new possibilities for chiral studies can be tuned to improve the molecular interactions. With the expectation of expanding the chiral and the elucidation of their chiral recognition mechanisms. 19F, 31P, 13C, and 77Se nuclei have probes,demonstrated using multinuclear efficient results NMR based spectroscopy on their nuclea providesr properties new and possibilities their incorporation for chiral into studies organic and the 19 31 13 77 elucidationcompounds of their through chiral organic recognition synthesis mechanisms. procedures. F, P, C, and Se nuclei have demonstrated efficient resultsAlthough based the use on theirof multinuclear nuclear propertiesNMR spectrosco andpy their for chiral incorporation discrimination into and organic assignment compounds throughof the organic absolute synthesis configuration procedures. is attractive, we need to be aware that the reliability of the multinuclear AlthoughNMR analysis the use should of multinuclear be evaluated NMR for the spectroscopy respective application for chiral discriminationdue to the different and assignmentnuclear of the absoluteproperties configuration and general behavior is attractive, of each weelement. need to be aware that the reliability of the multinuclear NMR analysis should be evaluated for the respective application due to the different nuclear properties and general behavior of each element. In order to spread the use of multinuclear NMR, new pulse sequence and experimental conditions were developed to improve the time-consuming and accuracy of the NMR spectra. Furthermore, the information obtained from these new experimental procedures can be employed to complement the traditional results and/or used for specific purposes.

Acknowledgments: The author thanks the FAPESP 2014/23362-8 project for financial support and the CEM-UFABC for NMR equipment. Conflicts of Interest: The author declares no conflict of interest.

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