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Stereoselective Reactions of Enolates

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Stereoselective Reactions of Enolates 1 Stereoselective reactions of enolates M O O O E R2 2 R2 1 R 1 R R1 R H H H E H • The stereoselectivity of reactions of enolates is dependent on: • Presence of stereogenic centres on R1, R2 or E (obviously!) • Frequently on the geometry of the enolate (but not always) C-α re face C-α si face M M O O MO R2 MO H α R2 α α H α R1 R1 1 1 2 H R H R2 R R (Z)-enolate (E)-enolate (cis) (trans) C-α si face C-α re face • Use terms cis and trans with relation to O–M to avoid confusion 123.702 Organic Chemistry 2 Enolate formation and geometry • Enolate normally formed by deprotonation • This is favoured when the C–H bond is perpendicular to C=O bond as this allows σ orbital to overlap π orbital • σ C–H orbital ultimately becomes p orbital at C-α of the enolate p bond enolate C–H σ π orbital orbital C=O π H orbital 2 base 2 O R O R + H base R1 H R1 H • Two possible conformations which allow this • First is given below and results in the formation of cis enolate • Initial conformation (Newman projection) similar to transition state • Little steric interaction between R1 and R2 base base base H base H H H 2 O R O R2 O R2 R1 O 2 ≡ H R R1 H R1 H R1 H transition (Z)-enolate state (cis) 123.702 Organic Chemistry 3 Enolate formation and geometry II base base base H base H H H O H O H O H R1 O 2 ≡ R H R1 R2 R1 R2 R1 R2 (E)-enolate (trans) • Second conformation that places C–H perpendicular to C=O gives trans-enolate • Only differs by relative position of R1 and R2 • The steric interaction of R1 and R2 results in the cis-enolate normally predominating • As results below demonstrate stereoselectivity is influenced by the size of R LDA Li O Li O THF O + Me –78°C Me R R R Me cis trans R = Et 30 : 70 i-Pr 60 : 40 t-Bu >98 : <2 OMe 5 : 95 >97 : <3 NEt2 123.702 Organic Chemistry 4 Enolate formation and geometry III • The selectivity observed can be explained via chair-like transition state • In ketones cis-enolate favoured if R is large but trans-enolate favoured if R is small if R is large, this TS‡ is i-Pr i-Pr destabilised by R–Me H Me OLi interaction and cis O N O N predominates Li i-Pr Li i-Pr Me R = large H H R O R Me R H cis Me LDA R i-Pr i-Pr OLi Me H O N O N if R is small, 1,3-diaxial Li i-Pr Li i-Pr interaction is important as it R R = small ‡ H H destabilises this TS and Me trans predominates R H R Me trans • With esters the R vs OMe interaction is alleviated and 1,3-diaxial interaction controls ...geometry - hence trans-enolate predominates i-Pr Me OLi O N Li i-Pr X Me Me H MeO O O H cis Me LDA i-Pr MeO OLi H O N Li i-Pr MeO predominates Me H O Me Me trans 123.702 Organic Chemistry 5 Enolate formation and geometry IV i-Pr H OLi O N Li i-Pr Et N Et H 2 N Me O Me Et trans Me LDA Et N 2 i-Pr Me OLi O N Li i-Pr Me predominates H Et2N R H cis • Amides invariably give the cis-enolate; remember restricted rotation of C–N bond • The previous arguments are good generalisations, many factors effect geometry • Use of the additive HMPA (hexamethylphosphoric triamide) reduces coordination and favours the thermodynamically more stable enolate OTBS O 1. LDA OTBS 2. TBSCl + Me Me EtO EtO EtO Me cis trans THF 6 94 THF / HMPA 82 18 123.702 Organic Chemistry 6 Addition of an electrophile to an enolate σ* orbital (LUMO electrophile) X = leaving group X X H H H H R R H H R O R2 ≡ O R2 O R2 R1 H R1 H R1 H π orbital (HOMO nucleophile) • Finally, need to know the trajectory of approach of the enolate and electrophile • Reaction is the overlap of the enolate HOMO and electrophile LUMO • Therefore, new bond is formed more or less perpendicular to carbonyl group • Above is simple SN2 with X = leaving group 123.702 Organic Chemistry 7 Enolate alkylation LDA R OEt Me Me R OEt [Li–N(i-Pr) ] Me I 2 R OEt + R OEt Me O Me O Li Me O Me O R syn : anti R = Ph 77 : 23 R = Bu 83 : 27 R = SiMe2Ph 95 : 5 • Simple alkylation of a chiral enolate can be very diastereoselective • As we have a cis-enolate diastereoselectivity can be explained in an analogous fashion to simple alkenes via A(1,3) strain • Larger the substituent, R, greater the selectivity alkylation on face I opposite to R Me Me Me Me H OEt most stable Me OEt O R OEt conformation; C–H H H O ≡ parallel to C=C Li H R R Me O • Note: minor diastereoisomer probably arises from electrophile passing by R group • Therefore, size does matter... 123.702 Organic Chemistry 8 Enolate alkylation II S H O LDA S H OEt S H OLi L OEt L OLi L OEt (E)-enolate (Z)-enolate trans cis ≡ ≡ preferred preferred approach approach H H S S S H O OEt OLi E L L L OEt E H OLi OEt • In this example enolate geometry is not important - both are cis-alkenes • Therefore, selectivity the same in both cases • If we want to reverse selectivity, change the electrophile to H • This route far less selective as H is small so less interaction with substituents H Me H H Me Me Me OEt Me OEt R OEt LDA R OEt H O H O Me ≡ Me O R Li R Me O 123.702 Organic Chemistry 9 The aldol reaction M M O OH O O O O + 1 3 1 1 3 R R R R3 R R 2 R2 R2 R Zimmerman– Traxler • The aldol reaction is a valuable C–C forming reaction • In addition it can form two new stereogenic centres in a diastereoselective manner • Most aldol reactions take place via a highly order transition state know as the Zimmerman–Traxler transition state • It will not come as much surprise that this is a 6-membered, chair-like transition state • Interestingly, enolate geometry effects diastereoselectivity only possible enolate O O OLi O OH LDA H Ph Ph H trans-enolate anti aldol O O OH O OLi LDA H Ph Me Me t-Bu Ph t-Bu t-Bu Me cis-enolate syn aldo123.702l Organic Chemistry 10 The aldol reaction II O O OH OLi H R Me X R X Me cis-enolate syn aldol O OLi O OH H R X X R Me Me trans-enolate anti aldol • Generally speaking the above guideline sums up aldol chemistry! • To understand why this happens we need to examine Zimmerman-Traxler TS‡ • You must learn to draw chair-like conformation of 6-membered rings 123.702 Organic Chemistry 11 Zimmerman-Traxler transition state 1,3-diaxial interaction X X H R cis-enolate M cis-enolate M O O H O H O R H Me Me R pseudo-equatorial R pseudo-axial disfavoured • We only have one choice in the aldol reaction - the orientation of the aldehyde • Enolate substituents are fixed due to the double bond • Aldehyde substituent is pseudo-equatorial to avoid 1,3-diaxial interactions O O OH OLi H R X Me X R H M X O Me H O cis-enolate syn aldol R Me to ‘see’ relative X X stereochemistry H H consider S as plane M M and see which groups re face of enolate attacks O O are above and which si face of aldehyde H O H O below R R Me Me 123.702 Organic Chemistry 12 Zimmerman-Traxler transition state II O O OH OLi H R Me X R X Me cis-enolate syn aldol Me H O O R M X Me Me H O O visualising relative H H stereochemistry O O R M R M X X H H si face of enolate attacks re face of aldehyde • Attack via the enantiomeric transition state (re face of aldehyde) gives the enantiomeric aldol product • This differs only by the absolute stereochemistry - the relative stereochemistry is the same 123.702 Organic Chemistry 13 Zimmerman-Traxler transition state III O OLi O OH H R X X R Me Me H trans-enolate anti aldol Me O O R M X H H H O O visualising relative Me Me stereochemistry O O R M R M X X H H re face of enolate attacks re face of aldehyde • The opposite stereochemistry of enolate gives opposite relative stereochemistry • Once again the enolate has no choice where the methyl group is placed 123.702 Organic Chemistry 14 Enolisation and the aldol reaction • Hopefully, all the previous discussion highlights that selective enolisation is essential for diastereoselective aldol reaction • Each geometry of enolate gives a different relative stereochemistry • With the lithium enolates of ketones the size of the non-enolised substituent, R, is important OLi O LDA OLi + Me Me R R R Me R = t-Bu 98% 2% R = Et 30% 70% • With boron enolates we can select the geometry by altering the boron reagent used O O OH O Et3N H Ph + B Me B O Ph Ph Ph Cl Me Ph bulky Me substituents trans-enolate anti aldol (>90% de) forces enolate to adopt trans geometry123.702 Organic Chemistry 15 Enolisation and the aldol reaction II O O OH O Et N H Ph + 3 Me B Ph Ph Ph B O Me TfO Me Ph cis-enolate syn aldol (96% de) • 9-BBN (9-borabicyclononane) looks bulky • But most of it is ‘tied-back’ behind boron thus allowing formation of the cis-enolate 123.702 Organic Chemistry 16 Substrate control in total synthesis I BnO Me Me C Me Me Me Cy2BCl Me 9 Me Me CHO H Cy C10 Et3N OBn H OBn Me C9 C10 OBn B C13 R O Cy C OBCy Me 13 O 2 O OH O 93% O H 97% d.e.
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