Substitution α to a carbonyl center: Enol and enolate chemistry

Transcription

1 Chapter 11 Organic Reaction Mechanisms, Part 2: Substitutions at Aliphatic Centers and Thermal Isomerizations/Rearrangements 11.1 Tautomerization Substitution α to a carbonyl center: Enol and enolate chemistry The hydrogens on carbons adjacent to carbonyls, the α-hydrogens, are relatively acidic (pka = ~18-22) -> This acidity enables a broad class of substitution reactions. : involves the shift of a hydrogen atom across a π system. eg) keto-enol tautomerization Electron pushing for keto-enol tautomerizations

2 The thermodynamics of enol formation normal alkyl ketones and aldehydes; the enol form -> minor β-diketones; enol form -> major due to intramolecular HB phenol and other enols within aromatic rings; enol form -> predominantly

3 Catalysis of enolization rds base-catalyzed enolization (general-base catalysis) conjugate base of the added acid pre-equilibrium rds acid-catalyzed enolization (general-acid catalysis, but is commonly the combination of specific-acid and general-base catalysis)

4 Kinetic vs. thermodynamic control in enolate and enol formation trapping agent for enolate thermodynamic control kinetic control kinetic control H 3 C R Li O H N H No HMPA R H 3 C H E-enolate O - H R Li O H N No HMPA H R O - CH 3 CH 3 25 o C thermodynamic control more stable -> predominant Z-enolate HMPA = hexamethylphosphoramide O P Me 2 N NMe2 Me 2 N

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6 11.2 α-halogenation Mechanism Rate = first order in [CO] and zero order in [X 2 ] large primary KIE (6.1 for Br 2 of methyl cyclohexyl ketone in NaOMe/MeOH) All the experimental observations support a rate-detrmining formation of the enolate or enol, followed by rapid trapping of the intermediate by the halogen.

7 Under acidic conditions, monohalogenated product is usually formed. Under basic conditions, multiply halogenated products are formed. O acid, Br 2 O slower Br O Br monohalogenated Br O base, Br 2 more acidic than unhalogenated CH O O H faster Br Br Br multiply halogenated Haloform reaction acidic conditions more substituted one -> major because the reaction proceeds via the most stable enol. Note: with base, the least substituted R group is preferentially halogenated, due to kinetic control.

8 11.3 α-alkylations strong base alkyl halide Stereochemistry: conformational effects Cyclohexanone kinetic control chair-like conformation boat-like conformation

9 Exocyclic enolates Note: Because these alkylations are exothermic, the transition states are early, and the stereochemistry is not very sensitive to the alkylating agent. 2-Substituted exocyclic enolates trans allylic strain more stable Alkylation of the lower energy enolate gives the major product. However, as the Curtin-Hammett principle states, this is not always the case.

10 11.4 The aldol reaction Conformational effects on the aldol reaction mixed aldol reaction

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13 Substitution on aliphatic centers 11.5 Nucleophilic aliphatic substitution reactions S N 2 and S N 1 electron-pushing examples Concerted reactions: the bond breaking and bond formation steps all occur at the same time in one single elementary step. Synchronous reactions: the breaking and forming of bonds has occurred to the same extent at the transition state Asynchronous reactions: have one of these two parts of the reaction lagging behind the other at the transition state. Most S N 2 reactions are mostly asynchronous, meaning that the extents of nucleophilic attack and leaving group departure at the transition state are unequal.

14 S N 1 reactions: more complex substitution pathway. -> 1 st step: the formation of a carbocation, 2 nd step: nucleophilic attack. S N 1 and S N 2 : less common aliphatic substitution pathways

15 Stereochemistry S N 2; backside attack -> inversion of stereochemistry S N 2; no reaction since the ring system completely blocks access of a Nu. In realty, whereas full inversion of stereochemistry is always seen in S N 2, full racemization of stereochemistry in S N 1 is rare. Instead, partial inversion of stereochemistry is commonly found. the participation of a contact ion pair

16 Orbital considerations HOMO LUMO Solvent effects 책읽어볼것

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18 Many highly polar solvents, such as DMF and DMSO are aprotic, and negative nucleophiles are very reactive in these solvents. Therefore, when changing from a polar protic solvent to a polar aprotic solvent, dramatic rate increases are observed for S N 2 reactions that involve negatively charged nucleophiles, such as OH, - N 3, - CN, etc. One problem for this is poor solubility of nucleophiles. In this case, a crown ether is added. Sol + O - M + Sol + -O - - O-Sol + - O Sol + - N 3

19 The extent of solvation of the leaving group in both S N 1 and S N 2 reactions affects the rate, too. In both reactions, the charge on the leaving group is changing during rds. Polar protic solvent enhance the departure of leaving groups that are developing negative charge. The addition of electrophilic reactants can enhance the leaving group departure even further by prior coordination to the leaving group. The most common example of this is addition of a silver salt to an alkyl halide (R-X), where the metal enhances the departure of the halide and results in a precipitate of AgX An overall picture of S N 2 and S N 1 reactions S N 2 reaction; relatively straightforward SN1; more complex -> the separated ion, solvent-separated ion, or contact ion pair

20 Nucleophiles 1. Nu - -> high solvation (eg. hard nucleophilies) -> lower the ground state energy -> increase in activation energy -> slower rate 2. Stronger bond between the nucleophilic atom and carbon -> lower activation energy -> increase rate 3. Nu - -> bulky -> increase nonbonding repulsions -> increase activation energy -> slower rate 4. Nucleophilicity with high electronegative atom -> difficulty to donate electron to a reactant O < N < S 5. Polarizability; more polarizability -> increase rate due to easy donation of electron to a reactant 6. Same atoms; basicity 와 nucleophilicity 일치 MeO - > PhO - > AcO - > NO 3-7. 같은족일때아래로갈수록 nucleophilicity 증가 I - > Br - > Cl - > F - PhSe - > PhS - > PhO - Why? Electronegativity, polarizability, solvation

21 Leaving groups * Conjugate base of strong acid -> good leaving group CF 3 CO - 2 CH 3 CO - 2 k rel NO 2 PhSO 3-4-CH 3 PhSO 3 - k rel 10 1 I - > Br - > Cl - >> F - CF 3 SO 3 - (triflate, trifluoromethanesulfonate); highly reactive * Silver salts are frequently used to accelerate substitution of unreactive halides * Diazonium salts R-Br + HO- -> R-OH + Brδ- δ+ R Br Ag R NH 2 + NaNO 2 H + R NH - N=O + H 2 O H + R N=N-OH R N H 2 O + H 2 O-N=O good leaving group

22 Structure-function correlations with the R group Effect of R group structure on S N 2 reactions steric effect substantial bending of the trajectory of the Nu away from the optimal linear approach

23 Electronic effect less significant than steric effect. However, these effects can be important. This orbital results from addition of the lone pair on the nucleophile to the antibonding R-X bond. It can mix with π orbitals of the alkenyl or phenyl group. Nu Alternative explanation: inductive effect. -> Electron withdrawing nature of the sp 2 carbons of a vinyl or phenyl group makes the carbon more electrophilic, and therefore more reactive toward nucleophilic attack. Other groups adjacent to the leaving group can also be expected to stabilize an S N 2 transition state by resonance, including cyano and carbonyl. However, the effect actually depends upon the nucleophile. O Br Br Nu: I - faster slower amine slower faster 그러나일반 alkyl halide 보다는이들이 Nu와더빨리반응한다. <- orbital mixing a dual electrostatic attraction of Nu- to the positive end of both the carbonyl and the C-X bond

24 Effect of R group structure on S N 1 reactions Alkyl groups alkyl group substitution: hyperconjugation 에의해 carbocation 안정 Ph -> resonance stabilization -> triphenylmethyl cation, the most stable cabenium ion by Hammon postulate, the transition state would be most reactant-like, thereby having the least cationic character. -> 치환기의영향이적다.

25 CN or CO groups inductive effect resonance inductive effect CN or CO; electron withdrawing via an inductive effect, but donate electrons via resonance -> therefore, the electron withdrawing cyano and carbonyl groups actually impede SN1 reactions more when they are one carbon away from the carbenium ion. Heteroatoms Stabilize a carbenium ions by resonance -> thereby accelerate S N 1 reactions. N: the best at stabilizng a carbenium ion S: a variable effect in the resonance stabilization; its orbitals are larger than those of C and hence the sizes are mismatched, but S is very polarizable.

26 Double bond Carbenium ions are stabilized by double bond. Solvolysis is ca. 100 times faster than without the double bond.

27 Carbocation rearrangement Whenever a carbocation is an intermediate in a mechanism, rearrangements are possible.

28 Stereochemical requirements for carbocation rearrangements Carbocations are in general fluxional molecules ( 유동성분자 ) Rate of these degenerate shifts at -139 o C : 3.1 x 10 7 /s (all five carbons become equivalent in 13 C NMR)

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30 Anchimetric assistance (neighboring group participation) in S N 1 reactions Heteroatoms: O, N, S, X Enantiomerically pure racemate cis compound -> slower solvolysis (no anchimetric assistance) -> trans product

31 OAc; EWG -> slower than Y =H trans; anchimetric assistance cis; no,, some anchimetric assistance because these are faster than cis-oac

32 X O X X X O O k rel x 10 4 O O O unfavorable electronegative oxygen retards solvolysis favorable Conclusion: if no neighboring group participation -> retardation of rate due to inductive effect by an electronegative heteroatoms

33 Arenes Retention of configuration racemate

34 π-bonds OTs TsO OTs anti syn AcOH acetolysis k rel OAc OAc product AcO OTs δ- OTs δ+ OAc anti TsO syn allylic cation AcO OTs H increase in strain energy (sp 3 -> sp 2 )

35 S N 1 reactions involving non-classical carbocations Norbornyl cation non-classical carbocations

36 Cyclopropylcarbinyl carbocation Another one of the more extensively investigated carbocation system derives from S N 1 reactions on homoallyl, cyclobutyl or cyclopropylcarbinyl derivatives. Solvolyses of all three of these systems give very interesting product mixtures. ca times faster Solvolysis Similar product ratios -> there is likely a common intermediate in all three of these reactions

37 Why or how? Non-classical carbocation Conventional carbenium ions Considerable evidence exists in support of both the bicyclobutonium and cyclopropylcarbinyl carbenium ions as important contributors to C 4 H 7+.

38 Summary of carbocation stabilization in various reactions allyl, benzyl, cyano, C=O from alkyl groups from EDG (N, O, S)

39 The interplay bewteen substitution and elimination

40 Isomerization and rearrangement Migrations to electrophilic carbonds Pinacol rearrangement EDG (o, p) -> accelerate migration EWG in all positions -> retard migration

41 Stereoelectronic and stereochemical considerations in the Pinacol rearrangement The pinacol rearrangement can be initiated from either alcohol, which can complicate mechanistic analysis. Therefore, many studies have investigated a related reaction called the semi-pinacol rearrangement. Only groups that are antiperplanar to the LG can migrate. antiperplanar HO NaNO NH 2, H + 2

42 Benzilic acid rearrangement Benzilic acid

43 11.9. Migrations to electrophilic heteroatoms Beckmann rearrangement: transformation of oxime to an amide oxime amide amide

44 Under conditions where E and Z forms cannot interconvert (acid can catalyze the isomerization), the group anti to the leaving group is one that migrates. -> this supports a mechanism involving simultaneous migration. Picric acid; good LG -> no need of acid -> no isomerization between E and Z The reaction correlates well with Hammett σ+ value. -> Rearrangement step; rate determining step

45 Hofmann rearrangement: transformation of amides to amines by Br 2 in basic media amides amines

46 Schmidt rearrangement: transformation of aldehyde or ketone to amides by HN 3 HN 3 Hydrogen azide To support this mechanism, a vinyl azide gives the same intermediate and product. The group anti to the LG migrates just like the Beckmann rearrangement.

47 Baeyer-Villiger oxidation: oxidation of aldehyde or ketone to ester by peracids aldehyde or ketone ester For most ketones and aldehydes, rds is the rearrangement, because significant substituent effects for the migrating group are found. (EDG -> migration)

48 Favorskii rearrangement and other carbanion rearrangements Favorskii rearrangement

49 Other carbanion rearrangements