oxidation-reduction reactions

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OXIDATION, REDUCTION, MEERWEIN-PONDROFF REDUCTION,BIRCH REDUCTION

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Slide 1:

1 WELCOME

OXIDATION-REDUTION REACTIONS : BIRCH REDUCTION & MEERWEIN-PONNDORF-VERLEY REDUCTION:

2 2 OXIDATION-REDUTION REACTIONS : BIRCH REDUCTION & MEERWEIN-PONNDORF-VERLEY REDUCTION MEERA PAUL FIRST YEAR MPHARM DEPARTMENT OF PHARMACEUTICAL CHEMISTRY UNIVERSITY COLLEGE OF PHARMACY, KOTTAYAM

CONTENTS:

3 3 CONTENTS Oxidation-Reduction Reaction Birch Reduction Mechanism & Stereochemistry Application Meerwein-Ponndorf-Verley Reduction Mechanism & Stereochemistry Application Reference

OXIDATION-REDUCTION REACTIONS:

4 4 OXIDATION-REDUCTION REACTIONS Oxidation :addition of oxygen or the removal of hydrogen. Reduction :addition of hydrogen or the removal of oxygen

OXIDATION & REDUCTION :

5 5 OXIDATION & REDUCTION IN TERMS OF ELECTRON TRANSFER : Oxidation is loss of electrons. Reduction is gain of electrons .

OXIDISERS:

6 6 OXIDISERS Substances that have the ability to oxidise other substances . Oxidising agents , oxidants , or oxidisers . Electron acceptors e.g. H 2 O 2 ,CrO 3 ,OsO 4 or highly electronegative substances (O, F, Cl, Br).

REDUCERS:

7 7 REDUCERS Substances that have the ability to reduce other substances Reducing agents , reductants , or reducers . Electron donor Electropositive elemental metals, such as lithium, sodium, magnesium, iron, zinc, and aluminium. NaBH 4 and LiAlH 4

BIRCH REDUCTION:

8 8 BIRCH REDUCTION When aromatic rings are reduced by Li (K or Na ) in liq. NH 3 usually in the presence of an alcohol. 1,4 addition of hydrogen takes place and non-conjugated cyclohexadienes are produced.

MECHANISM:

9 9 MECHANISM

MECHANISM:

10 10 MECHANISM The solution of metal in ammonia provides electrons which are taken up by the aromatic ring to form the corresponding radical anion B in the first step of the reaction. This is followed by protonation by the alcohol to form a C.

MECHANISM:

11 11 MECHANISM Next, a second electron is transferred to the radical to form a cyclohexadienyl carbanion D. In the last step a second proton leads the cyclohexadienyl carbanion to the unconjugated cyclohexadienyl product.

Principle Of Least Motion:

12 12 Principle Of Least Motion 1,4 cyclohexadienes are formed, not 1,3 cyclohexadienes Principle of least motion : Reactions will be favored which involves least changes in atomic position and electronic configuration.

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13 13 Birch reduction with electron withdrawing substituents

BIRCH REDUTION WITH ELECTRON DONATING SUBSTITUENTS:

14 14 BIRCH REDUTION WITH ELECTRON DONATING SUBSTITUENTS

Slide 15:

15 15 Electron-donating substituents such as ethers and alkyl groups favor protonation at an unoccupied site ortho to the substituent. Electron-attracting substituents such as carboxyl favor para protonation.

BIRCH ALKYLATION:

16 16 BIRCH ALKYLATION 1, 4-dibromobutane is added to t-butyl benzoate to form an alkylated 1,4-cyclohexadiene product:

MODIFICATION OF THE BIRCH REDUCTION:

17 17 MODIFICATION OF THE BIRCH REDUCTION Alternative solvents have been employed, such as THF as well as a mixture of n -propylamine and ethylenediamine, both with comparable results.

STEREOCHEMISTRY:

18 18 STEREOCHEMISTRY Reduction of saturated cyclic ketones The initially formed ketyl radical(A) arising out of single electron transfer is believed to be planar at the carbonyl site and its resonance stabilized with 70% of the unpaired electron density located on carbon. This on protonation (in the presence of a proton donor e.g. ROH) gives a hydroxyl carbon radical (B) in which pyramidalisation has occurred to a considerable extent.

STEREOCHEMISTRY contd…:

19 19 STEREOCHEMISTRY contd … The hydroxyl group (or OM) is oriented almost exclusively in the equatorial position. The radical (B) collects another electron and forms the anion ( C) which undergoes a fast protonation to give the trans alcohol in 99% yield. In this the hydroxyl radical (B) has a sufficient life time to adopt the more stable conformation and that the protonation of C (with retention of configuration) is fast in comparison to conformational inversion.

APPLICATIONS OF BIRCH REDUCTION:

20 20 APPLICATIONS OF BIRCH REDUCTION Preparation of 1,4 dihydro derivatives of aromatic compounds.

Application contd….:

21 21 Application contd…. Preparation of substituted cyclohexadienes. N-ethyl 4-methyl aniline

Application contd….:

22 22 Application contd…. Synthesis of natural products. terpenoids, polyketide, gibberlic acid

MEERWEIN-PONNDORF-VERLEY REDUCTION (MPV):

23 23 MEERWEIN-PONNDORF-VERLEY REDUCTION (MPV) Conversion of aldehyde or ketone in to corresponding alcohol by treatment with Aluminium isopropoxide in isopropanol solution. This reaction is reversible and is called Oppenauer oxidation. REACTION

MPV REDUCTION CYCLIC TRANSITION STATE INVOLVED IN MECHANISM:

24 24 MPV REDUCTION CYCLIC TRANSITION STATE INVOLVED IN MECHANISM

MECHANISM Catalytic Cycle Of MPV Reduction:

25 25 MECHANISM Catalytic Cycle Of MPV Reduction

STEREOCHEMISTRY OF MPV REDUCTION:

26 26 STEREOCHEMISTRY OF MPV REDUCTION Meerwein-Ponndrof-Verley reduction reduces (acyclic) prochiral ketones and α-deuterated aldehydes by the transfer of hydrogen with varying degrees of enantioselection. A simple example is provided by the kinetically controlled Meerwein-Ponndrof-Verley reduction of isohexyl methyl ketone with S- 2-butanol. In this mechanism, two transition states (TS-I and TS-II) are possible . (TS-II) with the larger groups on the opposite sides of the plane is preferred giving an excess of S -2 octanol

STEREOCHEMISTRY OF MPV REDUCTION contd….:

27 27 STEREOCHEMISTRY OF MPV REDUCTION contd…. R S

STEREOSELECTIVITY:

28 STEREOSELECTIVITY The aluminum based MPV reduction can be performed on prochiral ketones leading to chiral alcohols. Asymmetric reduction→ Use of a chiral alcohol hydride source, use of an intramolecular MPV reduction, or use of a chiral ligand on the aluminum alkoxide. Chiral alcohol (R)-(+)- sec - o -bromophen-ethyl alcohol in reduction of 2-chloroacetophenone →82%ee. This enantioselection is due to the sterics of the two phenol groups in the six membered transition state as shown in Figure.

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29

Slide 30:

30 INTRAMOLECULAR MPV REDUCTION 1 transition state Good enantiopurity. Undergoes reverse Oppenauer oxidation.

Stereoselectivity contd….:

31 Stereoselectivity contd…. Use of chiral ligands on aluminum alkoxide

APPLICATION:

32 32 APPLICATION This method of reduction is specific for carbonyl group and therefore it can be used for reducing aldehydes and ketones containing some other reducible group, such as, a double bond, a nitro or an ester group, which are not reduced under these conditions. [(CH 3 ) 2 CHO] 2 Al CH 3 CH=CHCHO CH 3 CH=CHCH 2 OH Crotonaldehyde (CH 3 ) 2 CHOH Crotylalcohol

Slide 33:

33 33 APPLICATION contd …

APPLICATION contd…:

34 34 APPLICATION contd…

REFERENCE:

35 35 REFERENCE Gurdeep .R .Chatwal; Reaction, Mechanism and Reagents in Organic Chemistry; 652-654. Finar, I.L; Organic Chemistry; Volume-II; Fifth Edition; 596. Nasipuri,D; Stereochemistry of Organic Compounds-Principles and Applications; Second Edition; 395,430. Michael B.Smith; Organic Synthesis; Second Edition; 212,400-407. Jerry March; Advanced Organic Chemistry; Fourth Edition; 780-782,913-917. http://en.wikipedia.org/wiki/birch-reduction. http://en.wikipedia.org/wiki/meerwein-ponndrof-verley-reduction

Slide 36:

36 THANK YOU

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