redox&neutral&reactions …...05_redox neutral reactions.ppt author giovanni created date...
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REDOX NEUTRAL REACTIONS
Redox Dispropor7ona7on-‐ Cannizzaro Reac-on-‐ Tishchenko Reac-on-‐ Meerwein-‐Ponndorf-‐Verley/OppenauerInternal Redox Reac7ons-‐ Baylis-‐Hillman reac-on-‐ Rauhut-‐Currier reac-on-‐ Alkyne and Alkene Hydra-on-‐ Nef Reac-onSome Redox Neutral Rearrangements-‐ Pinacol Rearrangements-‐ Benzil rearrangement-‐ Favorskii rearrangement-‐ Payne rearrangementNHC-‐Catalyzed Reac7ons-‐ Benzoin Reac-ons-‐ The thiamine variant-‐ SteHer Reac-ons
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-‐ “Redox Dispropor-ona-on”In this case, one star-ng material is oxidized while the other star-ng material is beingreduced.
Types of “Redox Neutral Organic Reac7ons”
-‐ Reac-ons with no external reducing or oxidizing agentIn this case, one part of the star-ng material is oxidized while the other is beingreduced.
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Cannizzaro Reac7on
At the present -me, various oxidizing and reducing agents can be used to carry out such conversions (with higher yields), so that today the Cannizzaro Reac-on has limited synthe-c u-lity except for the abovemen-oned conversion of α-‐keto aldehydes.
The Cannizzaro Reac-on should be kept in mind as a source of poten-al side products when aldehydes are treated under basic condi-ons.
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Meerwein-‐Ponndorf-‐Verley Reduc&on is the reverse reac-on of Oppenauer Oxida&on
Tishchenko Reac7on
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With nucleophilic catalysis
Baylis-‐Hillman reac-on
Anther good Nucleophile????Another good electrophile????
Enan-oselec-ve version???
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Rauhut-‐Currier reac-on (also called the vinylogous Morita–Baylis–Hillman reac-on
Phosphine-‐Triggered Reac-ons
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PPh3 catalyzed cycloaddi-on of N-‐Tosyl-‐subs-tuted imines with allenes
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Alkyne Hydra-on-‐ Mercuric ion-‐catalyzed hydra-on of alkynes goes with Markovnikov orienta-on.-‐ The Hg2+ salt acts as a catalyst to accelerate the rate of reac-on; the nucleophile (e.g.H2O) will aHack the most stabilized carboca-on it formed.
Alkyne and Alkene Hydra7on
Regioselec-vity???
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The orienta-on of hydra-on to alkenes with aqueous acid is followed by Markovnikov'srule
Alkene Hydra-on
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Nef Reac7on
One-‐Pot Synthesis of γ-‐Diketones, γ-‐Keto Esters, and Conjugated Cyclopentenones from NitroalkanesR. Ballini, L . Barboni, G. Bosica, D. Fiorini, Synthesis, 2002, 2725-‐2728.
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Pinacol RearrangementsPinacol to pinacolone-‐ The group that stabilizes a posi-ve charge beHer migrates first.
Rearrangements always redox neutral
Stereospecific pinacol rearrangements
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Benzil rearrangement (see Cannizzaro Reac7on)
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Favorskii rearrangement
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Benzoin Reac7ons
Cyanide ion catalyzed benzoin condensa-on
The Benzoin Condensa-on is a coupling reac-on between two aldehydes that allows the prepara-on of α-‐hydroxyketones. The first methods were only suitable for the conversion of aroma-c aldehydes.
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In the benzoin reac-on (and many other NHC-‐catalyzed reac-ons), the aldehyde carbonundergoes a reversal in polarity from an electrophilic center to being a nucleophiliccenter. This concept is termed “umpolung”
The thiamine variant:
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Biological counterpartsCarbenes play a central role in a number of biological processes. One of the mostimportant examples is Vitamin B1 (Thiamine pyrophosphate), a coenzyme involved inmany metabolic pathways. For instance, the enzyme pyruvate decarboxylase is assistedby TPP in catalyzing the transforma-on of pyruvate into acetaldehyde: a key step in theanaerobic fermenta-on… Think about it next -me you’ll have a drink!
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SteQer Reac7ons
The research program of Prof. Tom Rovis (Colorado State University) has establishedthe current state of the art for SteHer reac-ons, especially the intramolecular variant:
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NHC-‐Catalyzed Reac7ons
Unlike other carbene or carbenoid species which are high-‐energy intermediates, N-‐heterocyclic carbene is a stable singlet carbene. The lone pairs of the heteroatomsadjacent to the carbene carbon donate electron into the empty p-‐orbital of the carbenecarbon. Addi-onally, the N-‐subs-tu-on provides kine-c stability.