OXIDATION AND REDUCTION

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1 XIDATI AD EDUCTI Cl ' E " ' " "' ' ' ' ' " " Introduction ' uc Fundamental backbone of organic chemistry is the ability to alter oxidation states ydroxyl and carbonyl moiety provide an invaluable means for transforming molecules so the ability to introduce and remove them very important Course utline xidations alcohol to carbonyl alkene epoxidation and dihydroxylation C oxidation miscellaneous eductions carbonyl group hydrogenation electron transfer " ' This is not an all inclusive lecture course To list every reagent would be boring, so I have tried to be selective with the criteria being those that are more common, useful or interesting, but this is just my opinion As this is a new course, if you feel I have missed out any important examples (or too much detail on others) please tell me: Gareth owlands Ar402, eduction and xidation

2 ALCL XIDATI Alcohols can readily be oxidised to the carbonyl moiety This is an incredibly important reaction - you should realise that the carbonyl group is one of the cornerstones of C C bond formation (organometallics, neutral nucleophiles, aldol, Julia, Peterson & Wittig reactions) 1 = 1 1 Primary ( 1 = ) alcohols normally more reactive than seconary alcohols on steric grounds eed to be able to control oxidation of primary alcohols so only obtain aldehyde or acid Large number of reagents all have their advantages and disadvantages Look at some of the more common... General Fragmentation Mechanism E [] E [] E [] This fragmentation mechanism is common to most oxidations regardless of the nature of the reagent Chromium (VI) xidants (VI) 2 2 General Mechanism proton transfer (IV) "veroxidation" formation of carboxylic acids Invariably achieved in the prescence of 2 and proceeds via the hydrate 2 Jones xidation 2 S 4, 3, acetone 1 1 arsh, acidic conditions limit use of this method Gareth owlands Ar402, eduction and xidation

3 Pyridinium Chlorochromate (PCC) must avoid water Cl 1 1 Less acidic than Jones reagent (although still acidic) Pyridinium Dichromate (PDC) 2 Even milder than PCC and has useful selectivity PDC DCM PDC DMF ther xidants Manganese Dioxide Mn 2 Mild reagent Very selective only oxidises allylic, benzylic or propargylic alcohols only oxidises activated alcohol Mn 2 Gareth owlands Ar402, eduction and xidation

4 ALCL XIDATI Activated DMS eagent: DMS, activator (X) and base Transformation: C C= (primary or secondary alcohols) General Mechanism S + X S X + S base intermediate common to all activated DMS reactions 18 labelling has determined mechanism alternative activation of hydroxyl followed by displacement not occurring S + S Common Side-eactions Pummerer eaction S + S S Displacement eactions The cationic intermediate formed is an excellent leaving group Intramolecular C 2 C 2 DMS / (CCl) 2 93% S Intermolecular C 2 C 2 Cl Bn DMS / (CCl) 2 95% Bn Gareth owlands Ar402, eduction and xidation

5 Enolisation Generation of a carbonyl compound in the presence of an amine base is asking for trouble α-chiral centre can be racemised vercome by: keeping temperature low, remove base with cold acid buffer, use Pyr.S 3 system Eliminations Problem due to mild acidity of earlier steps or if suitable leaving group present when base added 1. DMS / (CCl) 2 2. Et 3 72% Me Me Me TBS S 2 P 1. DMS / + (CCl) 2 2. Et 3 P TBS TBS S 2 67 % Activators 28% Pfitzner-Moffatt (DMS / DCC then base) P C The original Pros: mild conditions, normally rt Cons: DCC urea by-product hard to remove frequently generates Pummerer side-product mildly acidic conditions lead to eliminations DMS / DCC TFA / Pyr 88 % active intermediate of Swern reaction Swern (DMS / (CCl) 2 ) Cl S Most popular, as mild and easy Pros: low temperature reduces enolisation very little Pummerer reaction Cons: Chlorination Parikh-Doering (DMS / Pyr S 3 ) Pros: very mild conditions, very little enolisation very little Pummerer eaction TBS TBS C 2 DMS / Pyr S 3 Et 3 94% TBS TBS Gareth owlands Ar402, eduction and xidation C

6 Activated DMS xidations in Synthesis TIPS 1. DMS / (CCl) 2 2. Et 3 92 % 1,2-diols are not cleaved TIPS 1. DMS / (CF 3 C) 2 2. Et 3 90 % sequential reactions possible due to the high yields and purity of products especially useful when aldehyde readily forms hydrate Me 3 Si 1. DMS / (CCl) 2 2. Et 3 Me 3 Si 3 P=CMeC 2 Et 54% overall Me 3 Si C 2 Et tertiary alcohols often do not need to be protected Me Me Me 1. DMS / (CCl) 2 2. Et 3 81% Me Me Me selective oxidations primary alcohols oxidised much faster but use of ipr 2 S and CS as activator (proceeds via same intermediate as Swern) oxidises primary alcohols at 0 C but secondary at -78 C do not understand this reaction AD it was only a communication 84CC762 that has never been followed up oxidation in the presence of allylic or benzylic alcohols DMS / (CF 3 C) 2 S S CCF 3 CCF 3 S Me Me Me Me Me Me the activity of allylic and benzylic alcohols means they undergo rapid displacement and hence a form of protection (±)-tazettine 61 % Et 3 Gareth owlands Ar402, eduction and xidation Me Me

7 lactol or lactone formation can be surpressed most oxidising agents oxidise primary alcohols faster than secondary and this can lead to problems [] [] activated DMS does not have this problem as aldehyde only formed on addition of base DMS / S S Et 3 (CCl) 2 selective oxidation of primary silyl ethers Mildly acidic nature and the nucleophilic chloride ion generated allows selective deprotection and concomitant oxidation of primary TES & TMS ethers TES 1. DMS / (CCl) 2 2. Et 3 62 % TES TES Limitations activated DMS systems will not oxidise propargylic alcohols What have we learnt? Activated DMS reactions are generally mild ffer many advantages of metallic reagents Drawbacks include a number of possible side-reactions Will not oxidise propargylic alcohols Gareth owlands Ar402, eduction and xidation

8 Dess-Martin Periodinane (DMP) (1,1,1-triacetoxy-1,1-dihydro-1,2-benziodoxol-3-(1)-one) eagent: Ac Ac I Ac Transformation: C C= (primary or secondary alcohols) ligand exchange Ac Ac I Ac General mechanism Ac I could be intra or intermolecular Ac I 2 x Ac since introduction in 1983 become one of the most popular oxidants mild reagent operating at nearly neutral conditions (buffer with ac 3 if worried about Ac) many very sensitive molecules can be oxidised TBS 93 % DEIPS TES TES TES tbu Si tbu Me TES Preparation I C 2 + KBr M 2 S 4 65 C I Ac Ac Ac I Ac mild and extremely reactive oxidant Insoluble in most organic solvents and impact sensitive Gareth owlands Ar402, eduction and xidation

9 Use in Synthesis Selectivity first step is ligand exchange so an inherent steric selectivity exists primary alcohols oxidised faster than secondary TBS TBS TBS Me TBS DMP, pyr, DCM, 88% TBS TBS TBS Me TBS Allylic and benzylic alcohols react ~5 faster than saturated alcohols DMP, pyr, DCM, rt 2hrs >75% Advantages: no over oxidation is ever observed no enolisation no oxidation of heteroatoms (eg or S) Disadvantages: Behaves like periodate and cleaves 1,2-diols. BUT not always, no consistancy What have we learnt? DMP is a mild reagent selective oxidations are possible 1,2-diols behave unpredictably Gareth owlands Ar402, eduction and xidation

10 Tetrapropylammonium Perruthenate TPAP eagent: Pr 4 + u 4 Stoichiometric or catalytic with M Transformation: C C= (primary or secondary alcohols) C C 2 (if 2 present) General mechanism not entirely clear it is thought that TPAP is a 3e oxidant but each step is a 2e process and that radicals / S.E.T. is not involved due to steric selectivity it is thought that TPAP is a bulky reagent & oxidation occurs primarily through the intermediacy of a ruthenate ester u u 2 u 2 u u u Use in Synthesis Introduced in 1987 its mildness and practically have made it popular (coupled to its none explosive nature) should be used dry with 4Åms or get over-oxidation and cleavage of alkenes mechanism changes in presence of 2 advantages: good functional group tolerance no epimerisation of α-chiral centres or double bond isomerisation no competative β-elimination PMB TPAP / M, DCM, 4Åms 96% PMB Gareth owlands Ar402, eduction and xidation

11 selectivity for primary hydroxyl group allows lactone preparation TPAP / M, DCM / MeC, 4Åms 91% TPAP secondary alcohols oxidise far slower but they do oxidise TMS TPAP / M, DCM, 4Åms, 73% Swern xidation = 0% PCC = 0% TMS depending on sterics can get selectivity for least hindered hydroxyl group TPAP / M, DCM, 4Åms 61% lactols can be oxidised selectively (again sterics) Ac Me 2 C C 2 Me TPAP / M, MeC, 4Åms 75% Ac Me 2 C C 2 Me again we see how mild TPAP is TPAP oxidises sulfur but not other heteroatoms SMe S 2 Me TPAP / M, MeC, 4Åms 80% Gareth owlands Ar402, eduction and xidation

12 Sequential reactions due to ease of w/u and anhydrous conditions, TPAP is well suited to sequential reactions C 2 Me TPAP / M, DCM, 4Åms C 2 Me 3 P=CMeC 2 tbu 72% overall C 2 Me C 2 tbu Disadvantages: TPAP can cleave 1,2-diols like other metal oxidants TPAP, acl 93% Disadvantages: can cause retro-aldol reaction TPAP / M, DCM, 4Åms retro-aldol results in cleavage of β-hydroxyketones What have we learnt? TPAP is a mild oxidant Its bulk allows selective reactions It can be used in catalytic quantities Gareth owlands Ar402, eduction and xidation

13 Modified Chromium (VI) xidants Pyridinium Chlorochromate PCC eagent: Cl 3 Transformation: C C= (primary or secondary alcohols) General Mechanism Cl Use in Synthesis Must be dry, water hampers reaction and can result in the formation of acids (over-oxidation) PCC, 4Åms, DCM 93% Disadvantages: reagent is acidic Gareth owlands Ar402, eduction and xidation

14 Pyridinium Dichromate PDC eagent: Transformation: C C= (primary or secondary alcohols) Use in Synthesis eutral variant of PCC Addition of Si 2 to reaction aids work up and addition of pyridinium trifluroracetate increases rate DCM normal solvent DMF gives carboxylic acids PDC, DCM 92% PDC, DMF 83% C 2 Me xidation to the Acid Many variants involving chromium or manganate which proceed via the hydrated aldehyde But invariably require strongly acidic conditions so not useful in organic synthesis You can find them yourselves in March or Smith A mild alternative is: acl 2, a 2 P 4 Cl 2 Cl Cl Cl is very unpleasnt so alkene added as a scavenger What have we learnt? Chromium reagents can be used to oxidise to either aldehyde or carboxylic acid They are toxic Gareth owlands Ar402, eduction and xidation

15 Kinetic esolution by Selective xidation oyori has developed a method for resolving racemic alcohols via selective oxidation Uses hydrogen transfer (analgous to ppenauer oxidation or Meerwein-Ponndorf-Verley reduction) Ts u + + = unsaturated group Yield = % e.e. = > 90 % note you can not get better than 50% with kinetic resolution Mechanism u Ts u Ts u Ts u Ts u Ts More appealing is the desymmetrisation of meso-diols Theoretical maximum yield is 100 % 70 % 96 % e.e. What have we learnt? Stereoselective oxidations are now possible ydrogen transfer allows preparation of enantiopure compounds from racemates As both reductant and oxidant are organic this type of reaction will be appearing again Gareth owlands Ar402, eduction and xidation

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