10.1. Electrophilic Addition of to to give. Works with HCl HBr HI. Electrophilic Addition Mechanism. H Cl Cl. 3D picture of intermediate: Rxn Coord

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1 10.1 Electrophilic Addition of to to give Works with I F Electrophilic Addition Mechanism 3D picture of intermediate: E xn oord Thermodynamics: + Δ (kcal/mol) + + = Alkene ydration: Electrophilic Addition of to to give atalyst: 2 S 4 2 Electrophilic Addition Mechanism 2 S 3

2 10.2 Electrophilic Addition of to to give Me 2 S 4 Me Electrophilic Addition Mechanism Me Me Me Me egiochemistry: Asymmetric Alkenes + % % egiochemistry controlled by 2nd Example arbocation Stability: 3 > 2 > 1 Markovnikov's ule: "Them that gots lots " Nucleophile goes to Electrophile goes to

3 10.3 Stereochemistry and Prochirality X Y Z Nuc Nuc X Y Z + X Y Nuc Z If X Y Z Nuc, then a center is created. Example: Ph 3 Ph 3 + Ph 3 % + % = mixture Exceptions: 3 Me + Me Why? Me Me Me Me

4 Addition eactions with earrangements 10.4 To which end of the π bond did the nucleophile ( ) add? Explanation: carbocation carbocation Another example: Addition of X to Alkynes: Similiar to alkenes but slower 2 (excess) X can add twice b/c Why so slow? very slow cation cation is like cation they are stabilized by only one methyl group (excess) stabilized by

5 Addition of X 2 to π Bond ( for X = &, but not F & I ) 10.5 Mechanism : is non-polar, but is highly b/c of its large : ion Stereochemistry : Addition of to bromonium ion always b/c dictates backside attack enantiomers % : % mixture inactive Synthetic Utility

6 An Alternative Method for Alkene ydration: xymercuration / Demercuration Ac = 2. Ac g Ac + Mechanism : ion g Ac g Ac g Ac g Asymmetric Alkenes follow Markovnikov's ule 1. g(ac) 2 / 2 2. NaB 4 egiochemistry set when g Ac g i-pr 2 i-pr g i-pr g Advantage compared to Acid atalyzed ydration 2 / 1. g(ac) 2 / 2 2. NaB 4

7 xymercuration / Demercuration of Alkynes: Ketone Formation 10.7 g(ac) 2 's 2 S 4 / 2 Mechanism : omplicated! ion g Ac Ac 2 g Addition g 2 g g g 2 Summary g(ac) 2 2 S 4 / 2 T with M Addition

8 ydration of Nitrile: Amide Formation N 3 Mechanism : omplicated! 3 N 2 3 N 2 N 2 N 2 Why protonate? N 2 N Summary: protonate nitrogen attack of water nucleophile deprotonate oxygen protonate nitrogen T deprotonate oxygen

9 Addition of X with Free adicals 10.9 M Addition but hν or peroxide A Addition Mechanism: Free adical hain eaction - push electrons, not ydroperoxide: has that is broken but not + + Termination step egiochemistry (Anti-markovnikov Addition): Set in first step Form most, most radical Stereochemistry : If a new chiral center is created, product is, or nearly b/c radical intermediate is, with addition from either or allowed +

10 ycloadditions - Addition to bonds to cylic or cyclic yclopropanation - Formation of - membered rings is often difficult, due to ring strain. Nevertheless... add easily to alkenes (p) orbital - site (sp 2 ) orbital - arbenes have both an electrophilic and nucleophilic site on the same carbon! site arbene Formal harge = (# valence e ) - (# bonds) - (# non-bonded e ) = Neutral, - octet arbon bond adds to orbital, while orbital adds to the new empty orbital that is formed: reaction ow to make arbenes? Dicholorcarbenes: Pseudoarbene: 2 I 2 Zn I 2 Zn I behaves like " carbenoid " " " Stereochemistry : Singlet carbenes give of configuration gives 3 3 I 2 Zn I gives

11 10.11 Epoxidation - Addition oxygen to bonds to give does work required, for example : 3 peracetic acid key feature : bond, readily broken m-chloroperbenzoic acid (mpba) magnesium monoperphthalic acid (MMPA) 2 Mg 2+ Mechanism : Stereochemistry is gives & gives mpba eactions of Epoxides : not easily broken, except in b/c weakened by Mg With strong, ring opening is. Attack at carbon Mg 3 but not With strong ring opening has character

12 ydroboration / xidation - four-membered cyclic ne last look at hydration of π bond : B / Product with 1. g(ac) 2 / 2 2. NaB 4 Product no Product no Diborane Borane B B has empty obital - octet carbon, therefore Alkenes react with. ydroboration mechanism : 3 B 3 3 B 3 addition: B & add 3 B replaced on same side stereochemistry preserved to face of π bond egiochemistry is governed by : adds to less substituted ; adds to more substituted = addition 2 B 3 fast 2 B 3 B 2 3 slow 2 B 3

13 verview xidation with yclic Intermediates gives 2. gives 3. gives bond completely broken! eagents: Mn s All are strong s s s s 4 is & use amounts & let do the Mn Mn Mn 4 is &, but tends to -oxidizes bond broken! carbonyl compounds result. If bond in ring, then product is dicarbonyl.

14 ycloaddition to give : the Dels-Alder eaction Diels - Alder Mechanism : reaction works poorly: 1. ethylene is 2. formed These work better: eaction works best if dienophile has has ( ) bond is electron dienophile Me bond is electron dienophile is electron withdrawing b/c of and Me