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Enantiomeric Excess 1 Enantiomeric excess • Optical purity - an outdated measurement of the enantiomeric excess (amount of two enantiomers) in a solution / mixture • If a solution contains only one enantiomer, the maximum rotation is observed... H NH2 CO2H measure rotation 100% (+) enantiomer derive [α]D = +14 100% of maximum observed rotation H2N H CO2H measure rotation 100% (–) enantiomer derive [α]D = -14 100% of maximum observed rotation • The observed rotation is proportional to the amount of each enantiomer present... Advanced organic 2 Enantiomeric excess II = + 90% (+) enantiomer 10% anti- 90% clockwise 10% of major 80% of maximum 10% (–) enantiomer clockwise enantiomer is rotation observed ‘cancelled out’ = + 60% (+) enantiomer 40% anti- 60% clockwise 40% of major 20% of maximum 40% (–) enantiomer clockwise enantiomer is rotation observed ‘cancelled out’ = + 50% (+) enantiomer 50% anti- 50% clockwise 50% of major 0% of maximum 50% (–) enantiomer clockwise enantiomer is rotation observed ‘cancelled out’ Advanced organic 3 Enantiomeric excess III • Previous slide indicates that a polarimeter measures difference in the amount of each enantiomer • Racemate (racemic mixture) - 1 to 1 mixture of enantiomers (50% of each) • Racemisation - converting 1 enantiomer to a 1:1 mixture of enantiomers • Optical rotation very unreliable so use new methods to measure amounts and use the value enantiomeric excess Enantiomeric excess (% ee) = [R] – [S] = %R – %S [R] + [S] • How do we measure enantiomeric excess? • Problem - all the physical properties of enantiomers are identical (in an achiral environment) except rotation of plane polarised light • Solution - the interaction of a chiral molecule with other chiral compounds is different depending on the enantiomer used... • Imagine you have a mixture of left and right-handed gloves and you are asked to separate them...suddenly there is a power cut, and you are left in a darkened room. How would you do it? Use just one hand and try the gloves on... Advanced organic 4 Resolution of enantiomers: chiral chromatography • Resolution - the separation of enantiomers from either a racemic mixture or enantiomerically enriched mixture • Chiral chromatography - Normally HPLC or GC • A racemic solution is passed over a chiral stationary phase • Compound has rapid and reversible diastereotopic interaction with stationary phase • Hopefully, each complex has a different stability allowing separation ‘matched’ ‘matched’ enantiomer - more enantiomer stable (3 interactions) travels slowly ‘mis-matched’ ‘mis-matched’ enantiomer - less enantiomer stable (1 interaction) readily eluted racemic mixture chiral stationary in solution phase Advanced organic 5 Chiral chromatography R/S inject mixture on to column R S R O O O Si O S Si O chiral column O H O O Si N prepared from a Si O Me Si O Si NO2 R suitable chiral O Me O stationary phase O (many different types) NO2 S silica chiral amine R chiral stationary phase • Measurements of ee by HPLC or GC are quick and accurate (±0.05%) • Chiral stationary phase may only work for limited types of compounds • Columns are expensive (>£1000) • Need both enantiomers to set-up an accurate method Advanced organic 6 NMR spectroscopy: chiral shift reagents • Chiral paramagnetic lanthanide complexes can bind reversibly to certain chiral molecules via the metal centre • Process faster than nmr timescale and normally observe a downfield shift (higher ppm) • Two diastereomeric complexes are formed on coordination; these may have different nmr signals C3F7 O + substrate Eu(hfc)3•••substrate O EuL2 Eu(hfc)3 • Problems - as complexes are paramagnetic, line broadening is observed (especially on high field machines) • Compound must contain Lewis basic lone pair (OH, NH2, C=O, CO2H etc) • Accuracy is only ±2% Advanced organic 7 Chiral shift reagents II O O 3+ O Sm O γ α CO2H O N N O β NH2 H O Me O L-valine • New reagents are being developed all that time that can overcome many of these problems • 1H NMR spectra (400 MHz) of valine (0.06 M, [D]/[L] = 1/2.85) in D2O at pH 9.4 Advanced organic 8 Chiral derivatising agents • A racemic mixture of enantiomers can be converted to a mixture of diastereoisomers by covalently attaching a second, enantiomerically pure unit • The advantage of this over the previous methods is there is normally larger signal separation in nmr • There is no reversibility • Diastereoisomers can often be separated by normal, achiral chromatography O O OH MeO MeO MeO Me CO2H DCC, DMAP, Me Me OH DCM, rt, 24h O O = + + + OH (±) mixture of enantiomerically 2 diastereoisomers racemate enantiomers pure • To understand why diastereoisomers are useful we need to do some more revision... Advanced organic 9 Two chiral centres Mirror O O enantiomers N S R N different absolute OH HO configuration HN NH2 NH2 NH • A molecule with one stereogenic centre exists as two stereoisomers or enantiomers • The two enantiomers differ by their absolute configuration • A molecule with two stereogenic centres can exist as four stereoisomers OH OH each molecule has an enantiomer NH2 NH2 (1R,2R) (1S,2S) other stereoisomers that are not mirror images are diastereoisomers OH OH each molecule has an enantiomer NH2 NH2 (1R,2S) (1S,2R) • A molecule can have one enantiomer but any number of diastereoisomers Advanced organic 10 Diastereoisomers OH OH OH OH same relative different relative stereochemistry stereochemistry / configuration / configuration NH2 NH2 NH2 NH2 diastereoisomers diastereoisomers • Diastereoisomers can have the same relative stereochemistry • The stereoisomers above differ only by their absolute stereochemistry • Or they can have different relative stereochemistry • Relative stereochemistry - defines configuration with respect to any other stereogenic element within the molecule but does NOT differentiate between enantiomers • In simple systems the two different relative stereochemistries are defined as below OH OH NH2 NH2 syn anti same face different face • Occasionally you will see the terms erthyro & threo - depending on the convention ...used, these can mean two either relative stereochemistry so I will not use them! Advanced organic 11 Diastereoisomers II OH CHO HO OH OH (2R,3R,4R)-2,3,4,5-tetrahydroxypentanal ribose OH OH OH OH CHO CHO CHO CHO HO HO HO HO four diastereoisomers OH OH OH OH OH OH OH OH (2R,3R,4R)-ribose (2R,3R,4S)-arabinose (2R,3S,4R)-xylose (2R,3S,4S)-lyxose mirror plane OH OH OH OH CHO CHO CHO CHO HO HO HO HO and their 4 enantiomers OH OH OH OH OH OH OH OH (2S,3S,4S)-ribose (2S,3S,4R)-arabinose (2S,3R,4S)-xylose (2S,3R,4R)-lyxose • If a molecule has 3 stereogenic centres then it has potentially 8 stereoisomers (4 diastereoisomers & 4 enantiomers) • If a molecule has n stereogenic centres then it has potentially 2n stereoisomers • Problem is, the molecule will never have more than 2n stereoisomers but it might have less... Advanced organic 12 Meso compounds OH HO2C CO2H OH tartaric acid OH OH OH HO2C HO2C HO2C CO2H CO2H CO2H H 2 OH OH O OH C H H O H 2 OO diastereoisomers C enantiomers identical H O C C H 2 H 2 O O O O OH OH H H 2 H O C 2 C O HO2C HO2C H CO2H CO2H H OH OH O • Tartaric acid has 2 stereogenic centres. But does it have 4 diastereoisomers? • 2 diastereoisomers with different relative stereochemistry • 2 mirror images with different relative stereochemistry • 1 is an enantiomer • The other is identical / same compound • Simple rotation shows that the two mirror images are superimposable Advanced organic 13 Meso compounds II • Meso compounds - an achiral member of a set of diastereoisomers that also includes at least one chiral member • Simplistically - a molecule that contains at least one stereogenic centre but has a plane of symmetry and is thus achiral • Meso compounds have a plane of symmetry with (R) configuration on one side and (S) on the other HO2C OH HO OH HO CO2H HO2C CO2H rotate LHS plane of symmetry • Another example... H Cl H H Cl H Cl Cl chiral achiral no plane of symmetry plane of symmetry non-superimposable superimposable on on mirror image mirror image (but it is symmetric!) (meso) Advanced organic 14 Meso compounds III EtO2C CO2Et EtO2C CO2Et Me Me Me Me plane of symmetry no plane of meso symmetry achiral chiral 1 • One compound displays two CH3 peaks in H nmr; the other just one peak. Which ...one is which? • The meso compound shows two peaks for the cis and anti CH3 (wrt to CO2Et) This compound is achiral • The chiral ester shows only one peak because it is symmetrical It has a C2 axis of symmetry This molecule is chiral but symmetrical EtO2C CO2Et EtO2C CO2Et Me Me Me Me plane of axis of symmetry symmetry Advanced organic 15 Enantiomers vs. diastereoisomers • Two enantiomers have identical physical properties in an achiral environment • Two diastereoisomers have different physical properties O2N O2N CO2Me CO2Me O O diastereoisomers trans cis enantiomers epoxide enantiomers epoxide mp = 141°C mp = 98°C different mp O2N O2N CO2Me CO2Me O O • Different physical properties, such as crystallinity or polarity allow diastereoisomers to be separated NH2 NH2 NH2 NH2 NH2 NH2 diastereoisomers 2HCl 2HCl 2HCl chiral different solubility meso solubility 0.1g/100ml EtOH seperable solubility 3.3g/100ml EtOH Advanced organic 16 Chiral derivatising agents II • This difference allows chiral derivatising agents to resolve enantiomers enantiomerically pure mixture of derivatising agent diastereoisomers R* R / S R–R* + S–R* racemic mixture diastereoisomers separable S–R* R* R R–R* pure cleave pure enantiomer diastereoisomer diastereoisomer • Remember a good chiral derivatising
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