OC 2 (FS 2013) Lecture 3 Prof. Bode. Redox Neutral Reactions and Rearrangements
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1 C 2 (F 203) Lecture 3 Prof. Bode edox eutral eactions and earrangements Types of edox eutral rganic eactions. eactions with no external reducing or oxidizing agent In this case, one part of the starting material is oxidized while the other is being reduced. redox neutral [].2 edox Disproportionation In this case, one starting material is oxidized while the other starting material is being reduced. redox neutral [] 2 Cannizzaro eaction 2 equivalent aq. a (excess) [] chanism 2 2 from 2 3 Tishchenko eaction Al(iPr) 3 [] chanism Al(iPr) 3 Al(iPr) 3 Al(iPr) 3 4 erwein-ponndorf-verley/ppenauer erwein-ponndorf-verley eduction is the reverse reaction of ppenauer xidation chanism Al(iPr) 3 isopropanol Al(iPr) 3 then work-up ipr ipr Al [] transfer hydrogenation + work-up acetone
2 C 2 (F 203) Lecture 3 Prof. Bode 5 C-Catalyzed eactions Unlike other carbene or carbenoid species which are high-energy intermediates, -heterocyclic carbene is a stable singlet carbene. The lone pairs of the heteroatoms adjacent to the carbene carbon donate electron into the empty p-orbital of the carbene carbon. Additionally, the -substitution provides kinetic stability. Base Y Y Y carbene ylide ' ' ' thiazolylidene imidazolylidene triazolylidene General tructures of nucleophilic carbenes 5.. Benzoin eactions 5... Before there was an C Cyanide ion catalyzed benzoin condensation C K C C then + C C C The benzoin condensation was the first organic reaction with its mechanism fully elucidated. C proton transfer For mechanism: Lapworth, J. Chem. oc., Trans. 903, 83, 995 & 904, 85, The thiamine variant: In the benzoin reaction (and many other C-catalyzed reactions), the aldehyde carbon undergoes a reversal in polarity from an electrophilic center to being a nucleophilic center. This concept is termed umpolung (eebach ACIE 979, 8, 239). 3 C 2 3 C Cl thiamine 3 C ylide 2 Cl 3 A general struture of thiazolium salt precatalyst for benzoin reaction ( = alkyl or aryl) 3 C 3 C 3 C carbene 3 C 3 C C-aldehyde adduct Breslow intermediate 2
3 C 2 (F 203) Lecture 3 Prof. Bode 5..2 tetter eactions - C ,4-addition 2 The research program of Prof. Tom ovis (Colorado tate University) has established the current state of the art for tetter reactions, especially the intramolecular variant: 5..3 ther C-Catalyzed Annulations γ-lactone synthesis via homoenolate addition C 2 Et BF mol% cat 20 mol% xylenes, 25 o C, 24 h C 2 Et + 2 Cl s s (8 mol %) (7 mol %) TF/t-Bu T 2 chanism 2 s 2 s s activated carboxylate s 2 s s enolate s s 2 s s s s homoenolate Bode JAC 2004, 26, 826 & Glorius ACIE 2004, 43, Internal edox eactions - Epoxy aldehyde Cl oxidized mol % C 8 mol % 2 reduced redox-neutral Bode JAC, 2004, 26, 826 3
4 C 2 (F 203) Lecture 3 Prof. Bode - chanism: - An application to natural product synthesis: Ac Cl Et Ac Et 83% (+)-davanone - edox esterification from α-bromoaldehyde was concurrently reported: Vosburg L, 2009,, Br 3 BF 4 (20 mol %) Et 3 3 ( eq) % ovis, JAC, 2004, 26, ther nucleophilic catalysis 6.. Baylis-illman reaction DABC chanism ' 3 ' '
5 C 2 (F 203) Lecture 3 Prof. Bode 6..2 auhut-currier reaction equiv Bu 3 P Et Et C, 40 o C, 3 hr Et Et chanism P 3 keto-enol tautomerism P 3 P 3 P thers P 3 catalyzed cycloaddition of -Tosyl-substituted imines with allenes chanism 3 P 3 C 2 P 3 benzene r.t. C 3 98% 3 P C 3 P C C P 3 2 C P 3 7 Alkyne and Alkene ydration 7. Alkyne ydration - rcuric ion-catalyzed hydration of alkynes goes with Markovnikov orientation. - The g 2+ salt acts as a catalyst to accelerate the rate of reaction; the nucleophile (e.g. 2 ) will attack the most stabilized carbocation it formed. 2, 2 4 g(ac) 2 enol tautomer keto tautomer Markovnikov product chanism keto-enol tautomerism 2 2 g 2+ g + g + g + carbocation oxonium ion g 2+, 2 Markovnikov product 7.2 Alkene ydration The orientation of hydration to alkenes with aqueous acid is followed by Markovnikov's rule 3 P 4 2, 250 o C (Markovnikov product) 5
6 C 2 (F 203) Lecture 3 Prof. Bode chanism ef eaction a chanism P.T earrangements 9. Pinacol earrangements 9.. Pinacol to pinacolone - The group that stabilizes a positive charge better migrates first. aq pinacol pnacolone chanism [,2]-alkyl shift tereospecific pinacol rearrangements 3 C MsCl, Et 3 C 2 Cl 2 0 o C, 0 min 3 C Ms Et 3 Al C 2 Cl 2-78 o C, 0 min 3 C 93% 99% e.e ynthesis of enantiopure quaternary carbons emipinacol rearrangement of 2,3-aziridino alcohols Et ZnBr 2 C 2 Cl 2, r.t. Et chanism 2 Lewis acid 2 LA 2 LA - LA 2 6
7 C 2 (F 203) Lecture 3 Prof. Bode 9.2 Benzil rearrangement benzil K 2,, heat K Cl chanism Favorskii rearrangement Cl a chanism Base proton transfer Payne rearrangement C 0.5 a 3 3 C 3 C C 3 3 C C 3 chanism 2 Base - Base 2 3-exo-tet 2 + Base - Base 2 2 7
8 C 2 (F 203) Lecture 3 Prof. Bode 0 Advanced Topic (ptional) 0. Ir catalyzed redox neutral C-C bond coupling Ir 2 P P 2 [Ir 3+ ] [] [Ir 3+ ] Ac 2.5 to 5% 2 Cs 2 C 3 ; m- 2 Bz TF; 0 o C 0.2 Chemoselective Keto-Acid-ydroxyl-Amine Ligation More information: Krische rg. Process es. Dev., 20, 5, C aq. DMF 40 o C - C C Bode ACIE 2006, 45, Gold-catalyzed cycloisomerization of bis-homopropargylic diols under a mild condition AuCl (2 mol %), r.t. 30 min 99% chanism + [Au] [Au] [Au] + Michelet & Genêt JAC 2005, 27,
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