Nucleophilic Addition To Carbonyls

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Brianne Harper
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1 leophilic Addition To Carbonyls!! Y Y Lewis basic site-reacts with electrophiles electron-deficient carbon-reacts with nucleophiles overall reaction: : + this can occur by either of three mechanisms: + 1) : simultaneous or 2) :: + : electrophilic attack or 3) _ : _ + nucleophilic attack which mechanism is operative will depend on electrophilicity of +, and nucleophilicity of : general rule: when + is + or strong Lewis acid, is neutral choose mechanism #2 when + = Li +, Na +, K + ; is anionic mechanism #1 when + = 4 N + ; is anionic mechanism #3 234

2 Case study - hydration of carbonyls: ' + 2 ' normally K eq (ketones < 10-5 ) (aldehydes 10-3 to 10 3 ) x: + 2 CCl 3 Cl 3 C K eq = 3 x 10 4 Why do WG s favor hydrates? Kinetics of hydration k 1 (+ 2 ) 2 ) (- k-1 ow to measure k 1? Look at carbonyls with K eq << 1 look at oxygen isotope exchange rate: k 1 k * -1 * 2 * k k so, since k 1 << k -1, d * 1 2 ~ d * 1 2 dt dt 235

3 look at effect of p on rate: log(rate) What's going on here? rate increases with increasing [ + ], [ - ] Implication: 2 different mechanisms, one in acid, one in base p acid catalysis- 2 choices specific acid catalysis - protonation occurs before.d.s.: + fast k 2 slow ! = k 2 [ 2 ] [ 1 2 ] ~ k 1 [ 1 2 ] alternative #2 - general acid catalysis A fast 1 2 A + 2 slow A - ydrogen bonded complex A in this case, proton transfer occurs in.d.s.! = k ] [ [A] [ 2 ] ~ k 2 [ 1 2 ] [A] 236

4 in basic solution, we can change nucleophiles: - + slow fast ! = k [ _ 2 [ 1 2 ] ] base-catalyzed hydration (nucleophilic catalysis) can also be viewed as: - Na slow - Na fast Na + - these really represent examples of specific base & general base catalysis so, which mechanisms are operative? It will depend on nature of acid, base, and/or carbonyl; all mechanisms have been observed in carbonyl hydration see Jencks, JACS, 1978, 100, 5444 cclelland, JACS, 1983, 105, 2718 Case Study #2 - Cyanohydrin Formation C + CN CN kinetics of this reaction were studied by Lapworth; he found that rate was not [CN], but rather: ν = k [C] [CN - ] (Lapworth, JCS, 1903, 83, 998) so, mechanism is: - C N slow NC - + -CN fast NC 237

5 tertiary amines were also shown to catalyze reaction: C + CN + 3 N CN + 3 N Does it just increase [CN - ]? No... look at kinetics: ν = k 4 [C] [CN] [ 3 N] 2 bimolecular in amine Why? CN + 3 N 3 N CN + 3 N CN -N 3 CN + N 3 CN CN +N 3 + N 3 CN this is an example of general acid catalysis Prelog & Wilhelm, elv. Chim. Acta, 1954, 192, 1634 Scope of leophilic Addition nucleophiles that add readily: C -, -, :N 3 (=, alkyl) nucleophiles that add under acid catalysis:, S, N 2 Addition of Carbon leophiles While all carbanions are nucleophilic enough to add to carbonyls, they are also basic enough to deprotonate carbonyl α-protons; thus, we will often observe competing modes of reaction Common carbanion nucleophiles: -CC -, - CN, -Li, -gbr echanism usually involves a 4-center transition state:! " ' C '''! +! " '' because of the compact nature of the transition state, reaction rate will be very sensitive to steric factors 238

6 Chemoselectivity of Grignard Additions gx + + 1,2 addition reduction enolization X 1,2 addition reduction enolization t Br n-pr Cl n-pr Br n-pr I i-pr Cl i-pr Br i-pr I osher, JC, 1962, 27, 1 Where does product #2 come from? β - hydride transfer g 1 2 C 3 C -this is related to the eerwein - Pondorf - Verley reaction 2 1 gx + C C 3 conclusion: only C 3, 1, sp 2, & sp Grignards add cleanly to ketones (but all Grignards, Li add to aldehydes) Typically, small + (Li +, Ce III ) promote 1,2 addition to carbonyls 239

7 Addition of nolates to Carbonyls - The Aldol eaction original version: C 2 C Nat t Na + C2 C Na - this reaction is made irreversible only by the final! - elimination C ",! - unsaturated aldehyde only irreversible step t - t C the use of irreversible deprotonation using strong bases created a new version Li C Li Li syn anti ( "threo" ) ( "erythro" ) - we stop at β - hydroxy carbonyl compounds; diastereoisomers possible 240

8 bservations: 1) Z - enolates give mainly syn aldol products ''-C ' '' ' 2) - enolates give mainly anti aldol ''-C ' '' ' 3) stereoselectivity (for syn) of Z - enolates is better than that of (for anti) 4) stereoselectivity is better for both when is large 5) stereoselectivity is better for both when when is large (mostly when = boron) 6) correlation reversed when is large how to explain? Zimmerman-Traxler model JACS, 1957, 79, 1920 Z - enolate: '' '' ''-C + aldol favored ' syn syn ' '' ' ' '' ' disfavored anti '' anti ' '' ' 241

9 - enolate: '' '' ''-C + ' aldol favored ' anti anti ' '' ' '' ' disfavored syn '' syn ' '' ' - now return to observations: 1,3-diaxial interaction between & controls stereochem. '' anti ' vs. '' ' syn so large ' & '' increases stereoselectivity -favored/disfavored difference not as great for -enolate (1,3-diaxial vs. gauche) When is large, anti T.S. for Z looks better '' syn T.S. for '' '' Allyl leophilic Addition + this looks simple, but there are 2 possibilities: "! + "! 4 - center T.S. "! +! " 6 - center T.S. 242

10 which mechanism prevails depends on : 6 - center: = B, Cr, SiX center: = BaCl both: = Li, gx neither: = Sn center transition state looks like Zimmerman - Traxler: B + C anti Z B + C syn offman, JC 1981, 46, 1309 also: B + C 243

11 Addition/limination echanism Variant #1 - Addition of Nitrogen leophiles to Aldehydes & Ketones N-Z N Z N Z N Z 1 2 Z =,, N 2 N Z - this process is typically reversible; equilibrium can favor either s.m. or product, depending on choice of conditions 1 2 N + 2 look at p - rate profile of + 2 N v max at p 4.8 Why a bell - shaped p rate profile? look at kinetics k obs p formulate reaction as + 2 N k 1! = k 2 2 N + k -1 fast k 1 k -1 N k 2 + N slow

12 but since we have a second equilibrium: + 2 N K a 3 N! =! = k 1 k 2 2 N + 3 N + when [ + ] K a k -1 k 1 k 2 2 N + 3 N K a when [ + ] K a k -1 so this gives the following p - rate profile: k obs something's wrong here... what is it? p What about + 2 N slow N + fast N + 2! = k 1 2 N + 3 N K a k obs so, now combine the two curves p k obs p - rate behavior indicates a change in rate - determining step at p ~ 5 p 245

13 Variant #2 - Addition to Carboxylic Acid Derivatives Y + addition Y elimination tetrahedral intermediate -this is sometimes referred to as nucleophilic acyl substitution, but more properly thought of as addition/elimination alternate version: Y Y Y - -Y Y =, 2 C, N 2, Cl, Br, F, S order of reactivity: C X > > > > > ~ ' (based on electronegativity and π - donation of Y) ' S' ' N 2 echanism #1 - saponification ' Na Na 246

14 echanism #2 - Fischer esterification Cl e + e -e e e e an alternative mode of catalysis - nucleophilic catalysis Cl + very slow reaction (t 1/2 > 24h) Cl + N (t 1/2 ~ 1h) Why does pyridine accelerate the reaction? it s not going to deprotonate cyclohexanol (K eq = = ) So, look at alternative mechanisms: N Cl Cl N N A N 247

15 So why is this any faster? reason 1: pyridine is a far better nucleophile than an alcohol reason 2: acyl pyridinium ion A is far more reactive than the acyl chloride (Why? better leaving group) combine better nucleophilicity with better leaving group ability & you have nucleophilic catalysis Ne 2 better nucleophilic catalysts than pyridine: N (DAP), PBu 3 (Nt 3 also used) Conjugate (1,4) Addition 3 1 2!," - unsaturated carbonyl compounds have 2 sites of attack by nucleophiles: C-2 or C-4 (C ") 4 Addition to Cβ is referred to as 1,4-addition (conjugate addition): What factors govern 1,2- vs. 1,4 - addition? 1,4 enolate (more stable) thermodynamic product 1,2 alkoxide (less stable) kinetic product 248

16 Why is 1,2-adduct the kinetic product? look at transition states! A unless there is a high concentration of +, 1,4-addition is slower in general xamples of 1,4 - additions: reversible reactions CN e 2 C or t 2 AlCN C 2 e NC (note, not NaCN or KCN Nae, e e 2 C C 2 e "ichael Addition" rg. eact., 1959, 10, 179 TiCl 4 Sie 3 "Sakurai reaction" Sakurai, JACS, 1977, 99, 1673 Why 1,4 - addition? Cl TiCl 3 Cl TiCl 3 TiCl 3 TiCl 4 no longer a cyclic transition state, so 1,2 & 1,4 are kinetically similiar Sie 3 Cl 249

17 ne major category of 1,4 - additions is the addition of organocuprate reagents to α, β unsaturated carbonyl compounds: LiCue 2 Gilman's reagent ow does this work & why does it give 1,4 - addition? - all organocopper reagents are Cu I species - free radicals so, probably the mechanism involves free radical intermediates: JACS, 1994, 116, ,2 vs. 1,4 Addition ' '- + ' Ph Ph Ph 1,2 addition 1,4 addition i-pr e gcl t-bu e gcl i-pr Ph gcl t-bu Ph gcl t-bu e Li

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