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Unformatted text preview: Chem 21,5-216 HH W11 Notes - Dr. Masato Koreeda - Page 1 of 11. Date: February 17,2017 Chapter 14 Aldehydes and Ketones: Addition Reactions at Electrophilic Carbons: Part I Overview of Chapter 14 1. Structures of aldehydes and ketones electrophilic C lt *t^U-^.6t R, R'= alkyl, aryl: ketones R,r:;'t\ R = alkYl, arYl; R'= H: aldehYdes \ lone pair: more basic than C=O n Aldehyde C=O carbons are less sterically hindered and more electrophilic compared with the corresponding ketone carbons (i.e., with the same R) 2. Reactions of aldehydes and ketones with an electrophile and a nucleophile o-framework a nucleophile approach all of these sigma-bonds and lone pairs on the same plane ith this trajectory R/n \v. .c_o. R'' tEl all atoms including El on the same plane <-l -.. T\" l* l.'\\1, l between sp2and sp3; on its way to sp3 Lowest gnoccupied molecular srbital (LUMo) I t Two lone pairs: Highest occupied molecular orbitals (HOMOs) of the C=O group Nu I JzT-q," sp3 3. Activation of RR'C=Z (Z = O and N) with H-A or a Lewis acid + activates C=O toward a nucleophilic addition ,O.H AO H-A o :g'L'A' o6'M R\J i '', r- - becomes even more 6 @; i.e., more electrophilic A .-uor (if M+ is used) Chem 215-216 HH Wl1 Notes - Dr. Masato Koreeda - Page 2 of II. Chapter 14: Overview (continued) 4. Four categories of nucleophilic addition reactions Two classes of nucleophiles: reversible and irreversible Reversible Nu: Date: February l7,20lI Irreversible Nu: Ho,.t H nb: oR (, RO OR 5 Type 3 divalent Nu: FO-'b't ROH, RSH Type 1 Hou@ A Me:O: \ l-l Hro@ or tro _ H.o@ or H"O N.R I. Nucleophilic Addition Reactions of RR'C=Z (Zz electronegative atom) I-l.Irreversible nucleophiles I Ht - Type 1; Ro -Type2] (1) Hydride reducing agents: Type 1 1. Sodium borohydride (NaBH/ H Na@ H-Ao-n Fi 2. Lithium aluminum hvdride fl (LiAlH4) Lio H-iP-H I H.- powerful reducing agent- reacts violently with water, ROH to produce H2 gas- Reactions with LiAlH4 are usually carried out in a polar aprotic solvent such as anhydrous tatrahydrofuran (THF) and anhydrous (diethyl) ether (CH3CH2OCH2CH3) r_ Hti,: R- mild reducing agent- relatively stable reagent (against moisture, air) electronegativity values: H 2.1; B 2,0;Al 1.5 lle.n.lfor B-H: 0.1 lAe.n.lfor Al-H: 0.6 more polarized, more O on the H ---'+ a stronger HO donor Also, the larger size difference between Al-H than B-H makes dissociation of Al-H much easier. Trivalent (Grignard reagent) a ,^ot^\ THF (diethyl) ether In addition, the difference in the coordination power of Na+ and Li+ (stronger) on the C=O oxygen further contributes to make the reactivity of LiAlH4 stronger. Reduction with LiAlHa requires an aqueous (usually acdic) workup. 3. Sodium cyanoborohydride [Na(CN)BH1I (muchweaker hydride reagent) 4. Diisobutylaluminum hydride (DIBAL or DIBAL-H) - powerful red. agent--reaction needs to be carried out in anhydrous conditions (e.g., anhyd THF or ether)....
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