1 Enantiomeric excess • Optical purity - an outdated measurement of the enantiomeric excess (amount of two enantiomers) in a solution / mixture • If a solution contains only one enantiomer, the maximum rotation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 agent should: • Be enantiomerically pure (or it is pointless) • Coupling reaction of both enantiomers must reach 100% (if you are measuring ee) • Coupling conditions should not racemise stereogenic centre • Enantiomers must contain point of attachment • Above list probably influenced depending whether you are measuring %ee or preparatively separating enantiomers Advanced organic 17 Chiral derivatising agents: Mosher’s acid

OH F3C OMe DCC, DMAP F3C OMe H CH2Cl2, –10°C O Me + CO2H O N C R / S S N R–S & S–S

DCC - dicyclohexylcarbodiimide

• Popular derivatising agent for alcohols and amines is α-methoxy-α- trifluoromethylphenylacetic acid (MTPA) or Mosher’s acid • Difference in nmr signals between diastereoisomers (above): 1H nmr Δδ = 0.08 (Me) 19 ...... F nmr Δδ = 0.17 (CF3) • Typical difference in chemical shifts in 1H nmr 0.15 ppm • 19F nmr gives one signal for each diastereoisomer • No α-hydrogen so configurationally stable • Diastereoisomers can frequently be separated • In many cases use of both enantiomers of MTPA can be used to determine the absolute configuration of a stereocentre (73JACS512, 73JOC2143 & 91JACS4092)

Advanced organic 18 Chiral derivatising agents: salts

OTol

HO2C O NH CO2H OTol O NH2 OTol OH OH O2C CO2H OTol

R / S S diastereoisomer is insoluble so easily removed by filtration

NaOH

O NH OH

(–)-propranolol β-blocker

• No need to covalently attach chiral derivatising group can use diastereoisomeric ionic salts • Benefit - normally easier to recover and reuse reagent

Advanced organic 19 Enzymatic resolution

lipase PS from Pseudomonas O cepacia, 0.05M phosphate buffer, O O pH 7, 0.1M NaOH, 5°C Bu Bu + Bu OEt OEt O Na 60% conversion F F F R / S R S >99% ee 69% ee soluble in soluble in organic phase aqueous phase

• Enzymes are very useful for the resolution of certain compounds • Frequently they display very high selectivity • There can be limitations due to solubility, normally only one enantiomer exists and can be too substrate specific • Below is the rationale for the selectivity observed above...

enzyme H N N H R O O Bu O O O Et his N diastereomeric interaction of enzyme H F H lone pair with σ* orbital of C–F of (S)- O enantiomer favoured over interaction ser with (R)-enantiomer Advanced organic 20 Stereoselective synthesis • The term ‘asymmetric synthesis’ should be used with caution. As we shall see, a number of important chiral compounds are symmetric!! • As such this course will primarily focus on diastereoselective or enantioselective synthesis or the synthesis of chiral molecules • Chiral compounds can be prepared in a number of ways: Enantiospecific synthesis; ‘the chiral pool’

O HONO OH O H2O OH O O H N H N H H O 2 2 2 H (S)-leucine retension inversion

O

HO H OH HO H (S)-(–)-ipsenol inversion

• Use enantiomerically pure starting material and stereospecific reactions • Good - if a cheap, readily available source of chirality exists • Problems - often results in long, tortuous syntheses ...... suitable material not always available Advanced organic 21 Stereoselective synthesis

Chiral auxiliaries

couple to form new chiral compound substrate + substrate (achiral) chiral (achiral) chiral auxiliary auxiliary

product overall diastereoselective chiral reaction reaction auxiliary resolve other diastereoisomer product cleave chiral product (chiral) auxiliary + chiral chiral auxiliary auxiliary

• Chiral auxiliary - allows enantioselective synthesis via diastereoselective reaction • Add chiral unit to substrate to control stereoselective reaction • Can act as a built in resolving agent (if reaction not diastereoselective) • Problems - need point of attachment ...... adds additional steps ...... cleavage conditions must not damage product! Advanced organic 22 Stereoselective synthesis II

Chiral reagents

chiral reagent substrate interacts with (achiral) achiral substrate reaction product substrate + (chiral) + (achiral) chiral dead reagent reagent chiral reagent chiral complex

• Chiral reagent - stereochemistry initially resides on the reagent • Advantages - No coupling / cleavage steps required ...... Often override substrate control ...... Can be far milder than chiral auxiliaries • Disadvantages - Need a stoichiometric quantity (not atom economic) ...... Frequently expensive ...... Problematic work-ups

Advanced organic 23 Stereoselective synthesis III

substrate (achiral) substrate (achiral) chiral catalyst

chiral catalyst

product (chiral) chiral catalyst

product (chiral)

• Chiral catalysis - ideally a reagent that accelerates a reaction (without being destroyed) in a chiral environment thus permitting one chiral molecule to generate millions of new chiral molecules...

Advanced organic 24

substrate (achiral) chiral auxiliary

chiral reagent

product

chiral auxiliary

Advanced organic