3 Determination of Mechanism

Philosophy of mechanistic studies

• One can only disproof : • No reaction could be determined with 100% certainty.

a hypothetical mechanism, not proof .

• As the result, an approved, last mechanism is said to be “reasonable”, not “correct”. • More than one method would be needed to confirm, and their results must all be consistent.

• Gather information from many experiments until enough to induce or extrapolate to a general conclusion.

• Occam’s razor: In the event that several hypotheses are found to fit the facts, the simplest one is given preference. 1

3. Determination of Mechanism

3.1 Identification of products 3.2 Determination of the presence of intermediates 3.2.1 Isolation of intermediates 3.2.2 Detection of intermediates 3.2.3 Trapping of intermediates 3.2.4 Addition of a suspected intermediate 3.3 Study of catalysis 3.3.1 General acid catalysis 3.3.2 Specific acid catalysis 3.4 Labeling study 3.4.1 Group labeling 3.4.2 Isotope labeling 3.4.3 Crossover experiments 3.5 Isomeric selectivity study 3.5.1 Regiochemical evidences 3.5.2 Stereochemical evidences 3.6 Kinetic studies 3.6.1 Measurement of rate 3.6.2 Mechanistic information obtained from kinetic studies 3.6.3 Rate law 3.7 Kinetic isotope effects 3.7.1 Deuterium isotope effects 3.7.2 Primary isotope effects 3.7.3 Secondary isotope effects 3.7.4 Solvent isotope effects 2

3. Determination of

Mechanism must be compatible with its products including the by-product.

e.g.

von Richter Rearrangement NO 2 CN COO At the first glance, the mechanism was though as a simple nucleophilic substitution of NO 2 by CN followed by the hydrolysis of CN to CO 2 H NO 2 COO CN However, 3 Br Br

Early proposed

Br O N O CN O N O CN Br O N O H NH Br Br Br COOH H 2 O -NO 2 , -NH 3 NH O N O H 2 O -H + O N O NH But, from its product study, was found, instead, the N 2 none of the NO 2 or NH 3 gas was detected.

gas 4

The mechanism was then fixed as follow: Br Br Br CN O N O O N O CN O N O H NH Br COOH H 2 O -NO 2 , -NH 3 Br NH O N O Br H 2 O -H + O N O NH Br COOH H 2 O -N 2 Br N N O Br -H 2 O O N O NH 2 5

3.2 Determination of the presence of intermediates

3.2.1 Isolation of intermediates Isolate the intermediate which can give the same products when subjected to the same reaction conditions at a rate no slower than the starting compound

e.g.

Hofmann rearrangement CH 3 CH 2 C O NH 2 NaOH, Br 2 CH 3 CH 2 NH 2 H 2 O CH 3 CH 2 N C O R Neber rearrangement H 2 C C R' EtO R N OTs H C C NH 2 O R' R H C N C R' 6

3.2 Determination of the presence of intermediates

3.2.2 Detection of an intermediate • In many cases, intermediate cannot be isolated but can be detected by IR, NMR, UV-Vis or other spectra.

• Radical and triplet species can be detected by ESR and by Chemically Induced Dynamic Nuclear Polarization (CIDNP).

• Radicals can also be detected by

cis-trans

isomerization of stilbene. Caution: Beware of non-intermediate species and impurities which may give interference signals. 7

3.2 Determination of the presence of intermediates

3.2.3 Trapping of an intermediate • In some cases, the suspected intermediate is known to be one that reacts in a given way with a certain compound.

• Benzynes react with dienes in the Diels-Alder reaction O Br Li O (trap) F benzyne 8

3.2 Determination of the presence of intermediates

•

Trapping an anion to determine if the elimination of alkenes is E

2

or E

1

cb.

OH ClHC CCl 2 E 2 E 1 cb ClC CCl 2 ClC CCl D 2 O (trap) D ClC CCl 2 9

3.2 Determination of the presence of intermediates

• Examples of free radical trapping agents are DPPH, oxygen (O 2 ), triphenylmethylradical (Ph 3 C  ), nitric oxide (NO), imine oxide, iodine, hydroquinone and dinitrobenzene.

O 2 N Ph 2 NN NO 2 PhHC N O O 2 N DPPH Imine Oxide • A radical reaction may proceed slower in the presence of air if the free radical intermediate can be trapped by O 2 .

10

3.2 Determination of the presence of intermediates

•

Kinetic requirement of intermediate trapping

A B C k 2 '[x'] k'[x']?

D - The intermediate B can be efficiently trapped by X  when k 2   k 2 . - The detection of D does not always guarantee the formation of B intermediate as A may directly react with X  to form D.

11

3.2 Determination of the presence of intermediates

3.2.4 Addition of a suspected intermediate • Perform a reaction by using a suspected intermediate obtained by other means can be used for a negative evidence.

e.g.

von Ritcher reaction: NO 2 CO 2 H von Ritcher condition CN von Ritcher condition CO 2 H 12

3. Determination of Mechanism

3.3 Study of catalysis • Mechanism must be compatible with its catalysts , initiator and inhibitors.

• Utilization of catalytic amount of peroxide, AIBN and iodine usually suggests a radical mechanism.

• Kinetic study of acid-base catalyzed reaction can reveal the rate determination step (rds.) if it is involved with the proton transfer process 3.3.1 General acid (or base) catalysis usually indicates that the proton transfer process is the rds.

3.3.2 Specific acid (or base) catalysis usually indicates that the proton transfer process is not the rds.

13

3.3.1 General acid (or base) catalysis

• In general acid catalysis all species capable of donating protons contribute to reaction rate acceleration. • The strongest acids (SH + ) are most effective (k1 is the highest) . • Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.

• When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. (A constant rate is evidence for a specific acid catalyst.) 14

3.3.2 Specific acid (or base) catalysis

• In specific acid catalysis taking place in solvent S , the reaction rate is proportional to the concentration of the protonated solvent molecules SH + .

• The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the chemical equilibrium between solvent S and AH in favor of the SH + species. S + AH  SH + + A • For example, in an aqueous buffer solution the reaction rate for reactants R depends on the pH of the system but not on the concentrations of different acids.

• This type of chemical kinetics is observed when reactant R 1 equilibrium with its conjugate acid R 1 H + in a fast which proceeds to react slowly with R 2 to the reaction product for example in the acid catalyzed aldol reaction.

15

3.3 Study of catalysis

•

Diazonium coupling shows general base catalysis. Which step is the rds.?

•

Aldol reaction shows specific acid catalysis. Which step is the rds.?

16

3. Determination of Mechanism

3.4 Labeling study 3.4.1 Group labeling: Easy to obtain starting materials but the group change may alter the mechanism. 3.4.2 Isotope labeling: Difficult to obtain the starting materials but no group alteration to affect the mechanism. (Isotopic scrambling can complicate the interpretation of the results.) 3.4.3 Crossover experiments: The experiments are closely related to either group or isotope labeling.

17

3.4.1 Group labeling

•

Is Claisen rearrangement a [1,3] or [3,3] sigmatropic process?

O OH O Ph O Ph OH Ph OH Ph 18

R N O C C

3.4.2 Isotope labeling

• • • • • D can be detected by NMR, IR and MS 13 C can be detected by 13 C-NMR and MS 14 15 18 C can be traced by its radio activity N can be detected by 15 N-NMR O can be detected by MS

e.g.

RCOO O + BrCN * RCN R O R C O R C O Br N C Br N C O isolated intermediate C N O C O R O C + C 19 N O

3.4.2 Isotope labeling

R R • O

Does the hydrolysis of ester proceed through “alkyl” or “acyl” cleavage?

O R' H 2 18 O R 18 OH + R'OH Labeled water is O easier to find than 18 O R' H 2 O R OH + R' 18 OH the labeled ester. O O In these cases, the products can be easily identified by MS.

20

Exercises

• Do the following ethanolyses of  lactone involve “alkyl” or “acyl” cleavage? O O EtOH H + or OH HO O OEt O O EtOH neutral EtO O OH • Do the following hydrolyses of  lactone involve “alkyl” or “acyl” cleavage? H 2 18 O HO OH O O H + or OH 18 O H 2 18 O H 18 O OH O neutral O O 21

3.4.3 Crossover Experiments

•

Use for distinguishing between intra- and intermolecular reaction

A' A A A' + + B B B' B B B' + + A B' B' No crossover product A' A + B A' + B' + A' A • Crossover products indicate intermolecular reaction.

• The method requires identification of products in the mixture.

• The method cannot distinguish between an intramolecular and “solvent cage” reactions.

crossover products 22

•

3.4.3 Crossover Experiments

Is benzidine rearrangement an inter- or intramolecular process?

H N H N H 2 N NH 2 OR H N H N OR RO OR H 2 N NH 2 OR' H N H N OR' H 2 N R'O OR' NH 2 No crossover product indicates an intramolecular rearrangement 23

3.4.3 Crossover Experiments

•

Is 1,2 rearrangement of alkyl lithium an inter- or intramolecular process?

PhH 2 C Ph Ph CH 2 Li Li Ph Ph CH 2 Ph CH 2 Ph Ph Ph CH 2 Li Li Ph Ph Upon an addition of 14 C-labeled benzyl lithium (Ph*CH 2 Li + ), the 14 C-labeled product was detected, indicating an intermolecular process.

Ph CH 2 Upon an addition of 14 C labeled phenyl lithium (Ph* Li + ), no 14 C-labeled product was detected, indicating no intermolecular process involved.

This is called labeled fragment addition technique 24

3. Determination of Mechanism

3.5 Isomeric selectivity study • Selectivity = Non-statistical distribution of products • Specificity = Correspondence between isomeric ratios of starting materials and products • Level of isomeric selectivity: chemoselectivity  regioselectivity  diastereoselectivity  enantioselectivity 3.5.1 Regiochemical study 3.5.2 Stereochemical study 25

3.5.1 Regiochemical evidences

•

HX addition on alkenes

Br + HBr + HBr H 2 O 2 Br • Regioselectivity suggests cationic mechanism.

• Polar solvents increase the reaction rate supporting the polar mechanism.

• Regioselectivity suggests radical mechanism.

• Solvent polarity has no effect on the reaction rate supporting the radical mechanism.

26

3.5.1 Regiochemical evidences

•

Aromatic substitution by strong basic nucleophiles

Cl NH 2 NaNH 2 Possible mechanisms: S N Ar or benzyne Cl NH 2 Cl NH 2 -Cl NH 2 (NH 2 ) -HCl NH 2 27

•

3.5.1 Regiochemical evidences

The benzyne mechanism was supported by regiochemical evidences obtained from group and isotope lebeling

OMe S N Ar OMe + NH 2 Cl benzyne NH 2 OMe 14 C label Cl NH 2 NH 2 NH 2 + 1:1 ratio NH 2 28

3.5.1 Stereochemical evidences

•

S

N

2 reaction

OTs OTs KOAc and OAc OAc • The reaction is stereospecific with 100% inversion indicating that the reaction is concerted and the nucleophile attacks from the back side of the leaving group.

• The proposed transition state is a trigonal bipyramid.

KOAc Ph   AcO   OTs H CH 3 29

3.5.1 Stereochemical evidences

•

Neighboring group

HO

participation (NGP)

Cl + The reaction is not stereospecific but diastereoselective. Both diastereomers give the same major product. The results suggest a common intermediate for all diastereomers.

The stereochemistry is controlled by the intermediate not by the starting material.

Ph Which one is the major product?

30

3.5.1 Stereochemical evidences

•

Neighboring group

only product OAc OTs Ph KOAc OTs Each reaction involves NGP in which an intermediate with 2 reactive sites is formed.

Ph C 2 OAc + enantiomer  homotopic enantiotopic 31

3.5.1 Stereochemical evidences

•

Addition

Br 2 Br Br Anti addition in which a bromonium ion was proposed as an intermediate.

Br 32

•

3.5.1 Stereochemical evidences Photorearrangement of spirofuran

COOMe h  COOMe O OH Possible mechanisms: pericyclic or biradical COOMe COOMe [1,3] sigmatropic O H homo [1,5] sigmatropic OH Stereospecific product COOMe O biradical O COOMe radical recombination Racemic product COOMe 33 OH

3. Determination of Mechanism

3.6 Kinetic studies 3.6.1 Measurements of rate 3.6.2 Mechanistic information obtained from kinetic studies 3.6.3 Rate law 34

3.6.2 Measurement of rate

• Real Time Analysis by Periodic or Continuous Spectral Readings • Quenching and Analyzing • Removal of Aliquots at Intervals

Rate

 1

N p



d

[

P

]

dt

A + B   1

N A

P 

d

[

A

]

dt

  1

N B



d

[

B

]

dt Rate



k

[

A

]

n A

[

B

]

n B

(Rate Expression) N = stoichiometric number n A  n i = order of reaction for reactant A = order of overall reaction k = rate constant k obs = rate constant directly obtained experimentally molecularity = number of molecules 35 come together in a single step

3.6.2 Measurement of rate

Zeroth order k 0 A 

d

[

A

]

dt



k

0 B

A



A

0 

k

0

t

First order A k 1 B

A

 

d

[

A

]

dt A

0

e



k

1

t

ln 

k

1 [

A

]

A

 ln

A

0 

k

1

t

A 0 [A] [B] ln A 0 ln A slope = -k 1 slope = -k 0 t t 36

3.6.2 Measurement of rate

•

Second order

k 2 2A P A + B k 2 

d

[

A

]

dt

 2

k

2 [

A

] 2 1 [

A

]  1 [

A

] 0  2

k

2

t

P 

d

[

A

]

dt



k

2 [

A

][

B

] Use pseudo first order: B 0 >>A 0 [B] constant = B 0 Treat like first order 

d

[

A

]

dt

 (

k

2

B

0 )[

A

] 37

3.6.3 Mechanistic information obtained from kinetic studies • Order of reaction can give information about which molecules take part in rate determining step and the previous steps.

• Changes in rate constants upon structural and condition changes can give much information about mechanisms. (Linear free energy relationships) • From transition state theory, rate constants measured at various temperature can lead to important energetic parameters.

k r = A

e

-Ea/RT ; A = kT

e

D S /R ; h E a = H + RT 38

3.6.3 Rate law

• First order: Rate = k[A] (rds. is unimolecular process) • Second order: Rate = k[A] 2 or Rate = k[A][B] • Order is for the whole reaction while molecularity is the order for each step.

• Rate law depends on the rate-determining step.

– The first step is the rate-determining step: slow A + B I I + B f ast Rate = k[A][B] C 39

3.6.1 Rate Law

– The first step is a rapid equilibrium: k 1 A + B k -1 k 2 I + B I C Rate = -d[A]/dt = k 1 [A][B] - k -1 [I] d[I]/dt = k 1 [A][B] - k -1 [I] - k 2 [I][B] = k 1 [A][B] - (k -1 + k 2 [B])[I] Steady state assumption: d[I]/dt = 0 [I] = k 1 [A][B]/(k -1 + k 2 [B]) Therefore Rate = k 1 [A][B] - k 1 k -1 [A][B]/(k -1 + k 2 [B]) Rate = k 1 k 2 [A][B] 2 /(k -1 + k 2 [B]) For rapid equilibrium in the first step k -1 [I] » k 2 [I][B] or k -1 » k 2 [B] Thus Rate = K 1 k 2 [A][B] 2 40

Exercise

•

Using the steady state assumption, derive a rate expression for the following reaction if (a) the first step is a rate determining step, (b) the first step is a fast equilibrium.

k 1 k 2 A B k -1 Rate = -d[A]/dt = k 1 [A] - k -1 [B] d[B]/dt = k 1 [A] - k -1 [B] - k 2 [B] Steady state assumption d[B]/dt = 0 C [B] = k 1 [A]/(k -1 + k 2 ) Rate = k 1 [A] - k -1 k 1 [A]/(k -1 + k 2 ) Rate = k 1 k 2 [A ]/(k -1 + k 2 ) a) k -1 << k 2 : Rate ~ k 1 [A] b) k 2 << k -1 : Rate ~ Kk 2 [A] 41

•

Exercise

Condensations

OH a) CH 2 (COOEt) 2 + CH 2 O HOCH 2 CH(COOEt) 2 Rate = k[CH 2 (COOEt) 2 ] [CH 2 O] [OH ] The reaction between the enolate and formaldehyde is the rds.

OH b) 2CH 3 CHO CH 3 CH(OH)CH 2 CHO Rate = k[CH 3 CHO] [OH ] The formation of the enolate is the rds.

Write a reasonable mechanism and specify the rds. of each reaction.

42

3.7 Kinetic Isotope effects

3.7.1

Deuterium isotope effects

(k H /k D ) is the ratio between the rate of reaction of the protonic substrate and that of the corresponding deutero substrate.

• A

normal isotope effect

has k H /k D > 1 indicating that the reaction of the protonic substrate is faster than the reaction of the corresponding deutero substrate.

• An

inverse isotope effect

has k H /k D < 1 indicating that the reaction of the protonic substrate is slower than the reaction of the corresponding deutero substrate.

3.7.2

Primary isotope effect

is observed in the reaction that its rate determining step involves the breaking of the bond connecting to the isotopic H.

• The primary isotope effects usually have 2 ≤ k H /k D ≤ 7. 43

•

3.7.2 Primary isotope effects

Origin of the primary isotope effects

R H X k H /k D < 7 ( late T.S) R H X maximum k H /k D ~ 7 R H X k H /k D < 7 (early T.S) R R H D

E

0  1 4 

k

 

AB E

0

D

 

m m A A



m B m B E

0

H



m A

44

•

3.7.2 Primary isotope effects

Alcohol oxidation

R OH + H 2 CrO 4 H (D) R R R O Gives k H /k D = 6.9 The transition state proposed for the rds. is as follow R R O CrO 3 H H base 45

Exercise

• Write a reasonable mechanism and specify the rate determining step for the following reaction which shows k H /k D  7 O O H + CH 3 CCH 3 + Br 2 CH 3 CCH 2 Br 46

3.7.3

3.7 Kinetic Isotope effects

Secondary isotope effect

is observed in the reaction that its rate determining step does not involve the breaking of any • • bond connecting to the isotopic H.

 -secondary isotope effect usually has k H /k D in the range 0.7-1.5. It is the result of the greater vibration amplitude of C-H bond comparing to C-D bond.

– A normal  -secondary isotope effect (k H /k D > 1) generally suggests a rehybridization of the carbon connecting to the isotopic H from sp 3 to sp 2 in the rate determining step. – An inverse  -secondary isotope effect (k H /k D < 1) generally suggests a rehybridization of the carbon connecting to the isotopic H from sp 2 to sp 3 in the rate determining step.

 -secondary isotope effect has k H /k D attributed to hyperconjugation.

> 1. It is mainly 47

3.7.3 Secondary isotope effect

•

Solvolysis of cyclopentyl

H (D) H (D) sp 3 C-H sp 2 C-H (k H /k D = 1.17)

normal

• Ph O H (D) CN Ph O

Addition on aldehyde

H (D) CN sp 2 C-H sp 3 C-H (k H /k D = 0.833) inverse 48

Summary of primary and secondary kinetic isotope effects

(CH 3 ) 3 CD + X (CH 3 ) 2 CDX (CH 3 ) 2 C=CD 2 + H + (CD 3 ) 2 CHX (CH 3 ) 3 C. + DBr primary (CH 3 ) 2 CD + + X  -secondary (normal) + (CH 3 ) 2 CCD 2 H  -secondary (inverse) (CD 3 ) 2 CH + + X  -secondary 49

3.7 Kinetic Isotope effects

3.7.4Solvent isotope effects Generally observed when a protic solvent

e.g.

D 2 O or ROD is used.

• k H /k D < 1 when the reaction involves a rapid equilibrium protonation because the acidity of D 3 O + is greater than H 3 O + (specific acid catalysis can be used for confirmation) • k H /k D > 1 when proton transfer is the rate determining step (general acid catalysis can be used for confirmation) • Secondary solvent isotope effect can interfere the interpretation. Solvent isotope effect is thus used 50 only as a supporting evidence.

Exercise

• Write a reasonable mechanism for hydration of styrene and predict which step is the rate determining step. Suggest 3 experiments and the expected results that can support the proposed mechanism and rate determining step.

51