Chemistry 2312
October 6, 2015
Honors Organic Chemistry Laboratory
T. R. Hoye
Experiment 3
Enolate Alkylation, Ester Saponification,
and Amide Bond Formation via EDCI-Coupling
Enolate Alkylation (6 to 7), Ester Hydrolysis (7 to 8), and EDCI-Coupling (8 to 9) in the
Preparation of N-Phenethyl α-Methylphenylacetamides (9).
Reaction Sequence:
Me H
Me H
Me H
CO2Me
CO2H
H
N
O
H H
1) LDAa
R-7
R-8
LiOH
+
CO2Me
+
EDCIb
MeOH
H Me
6
H
R,R-9
(R)-PhCH(Me)NH2
+
2) MeI
Ph
Me
H Me H
N
H Me
CO2Me
CO2H
O
S-7
Ph
Me
H
S,R-9
S-8
Li
N
a
LDA = lithium diisopropylamide = (i-Pr)2N-Li =
b
EDCI = 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride salt =
N
C
N
H
N
Cl
Background
As you know, the carbonyl group plays a central role in organic synthesis. Various
nucleophiles can be added to the carbonyl carbon atom. E.g., reduction of the carbonyl group by
hydride ion gives an alcohol and addition of carbanionic species is a valuable method for
building up carbon skeletons.
The α-carbon to a carbonyl compound is also a site of many important reactions. This is in
large part because the protons α to a carbonyl group are acidic. The pKa values for α-protons
range from ~9, for α-protons of a β-dicarbonyl compound, to ~25 for alpha protons of an
aliphatic ester. The typical pKa of a ketone α-proton is ~20. Since α-protons are weakly acidic,
they can be removed with a strong base to form an enolate anion. Most of the negative charge
(i.e., the extra electron density) of this species resides on the oxygen atom.
Experiment 3: Enolate Alkylation, Ester Hydrolysis, and Amide Bond Formation
O
O
NH2-
O
RCH2
O
NR2-
RCH
OR
O
RO
O
RO
OR'
O
N
R''O-
C
OR
O
O
R''O-
N
R
RCH
R
RCH2
OR
OR'
O
C
OR
Carbanions Formed by Proton Abstraction
O
O
R
R'
O
R
R'
O
An ester enolate
R
OR'
O
A ketone enolate
R
O
OR'
O
O
O
O
A malonic ester enolate
RO
OR'
RO
O
RO
OR'
O
OR
N C
OR'
N C
O
OR
OR
N C
A nitrileacetate enolate
Once generated, an enolate ion can undergo a variety of reactions. This is a classic
transformation known as the aldol addition reaction, since the product is a β-hydroxyaldehyde
(an “ald-ol”).
O
O
+
OH
R'
R'
H
R
O
Base
R
R
Aldol Reaction
R
Reaction of the enolate with an alkyl halide is referred to as alkylation. The leaving group
can be, e.g., I-, Br-, or tosylate anion. It is this transformation that you will perform as the first
step in the multistep sequence in Experiment 3.
O
+
X
O
Base
R' Enolate Alkylation
R'
R
R
X=Br, Cl, OTs
R
R
The ester starting material that you will use is methyl phenylacetate (6). Deprotonation of
this ester by lithium diisopropylamide (LDA) will give the enolate 6—. Once the enolate is
formed, you will add methyl iodide (MeI) and alkylate 6— in an SN2 reaction. The product of
this reaction, methyl (±)-2-phenylpropanoate (7), will be formed as a racemic mixture.
2
Experiment 3: Enolate Alkylation, Ester Hydrolysis, and Amide Bond Formation
Me
Me
Me
N
Me
O
Li
O
OCH3
O
CH3I
Ph
Ph
3
Ph
OCH3
LDA
OCH3
CH3
6-
6
(±)-7
You will then synthesize a mixture of the diastereomeric amides R,R-9 and S,R-9 by reaction
of racemic 2-phenylpropanoic acid (8) with the optically pure amine, (R)-Ph(Me)CHNH2. These
diastereomeric amides will then be separated by MPLC. In the process of forming these amides,
you will use a very important reaction, the formation of the “peptide bond” (a carboxylic amide)
using ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDCI).
The formation of peptides and proteins from amino acids is an important process in
bioorganic chemistry. A polypeptide, which is a polyamide containing ≤ ~100 amino acid
monomer units and is a substructure of a protein, which typically contains ≥ ~100 amino acids,
can be made by sequentially reacting the amine portion of an α-amino acid with the carboxyl
group of a second α-amino acid of the growing peptide chain.
+
PGHN
R2
DCC (or EDCI)
OPG
H 2N
OH
R1
O
R1
O
PGHN
Coupling
O
R2
PG = protecting group
N
H
OPG
O
Peptide bond
There are many methods for the formation of an amide bond. Many involve the activation of
a carboxylic acid and subsequent reaction with an amine. One of the most common methods for
activation of an acid is the use of carbodiimides, often, dicyclohexylcarbodiimide (DCC).
O
O
R
R
OH
O
N C N
H
N C N
DCC
O
N C N
H
H
DCC activated
Carboxylic acid
H2N R'
O
+
R
N
H
R'
dicyclohexylurea
Once the acid is activated, it reacts with the amine to form the amide bond. You will first
need to hydrolyze your racemic methyl 2-phenylpropanoate (7) with a basic catalyst like LiOH
or K2CO3 in wet methanol. The product carboxylic acid 8 will then be coupled with αmethylbenzylamine using EDCI.
Experiment 3: Enolate Alkylation, Ester Hydrolysis, and Amide Bond Formation
4
Experimental Design:
Methylation of Methyl 2-Phenylacetate (6) with Methyl Iodide:
Devise a procedure for preparing a solution of lithium diisopropylamide (LDA)1,2,3 to react
with methyl phenylacetate (PhCH2CO2Me, 6). Start with ~5 mmol of the ester. The
concentration of the enolate anion in THF should be about 0.5 M. The reaction should be
performed under a nitrogen atmosphere. Use anhydrous THF that the TA's will make available
from our research lab. Since at least small amounts of the strongly basic species will unavoidably
be quenched by trace amounts of proton sources (e.g., water on the walls of the flask and in the
"anhydrous" solvents), you should use a slightly higher excess of base (lithium
diisopropylamide, ~1.10 mmol for each mmol of ester). Check the titer listed on the bottle of nBuLi to calculate the volume of n-BuLi solution that should be added. You will need to use
proper syringe-handling techniques for dispensing the flammable n-BuLi solution. Use only a
slight excess of methyl iodide in the alkylation. Once the alkylation is finished, quench the
reaction mixture with NH4Cl (satd) and dilute with ether. Workup the reaction in the usual
manner. Purify the methylated ester product by flash column chromatography or MPLC (your
choice) and characterize by the usual battery of spectroscopic techniques. Separation of the
methylated product(s) from starting material and from a dimethylated byproduct is challenging,
but doable. We will assess the level of purity of this product, in part, by the extent to which you
isolate it free of the starting material and dimethylated byproduct.
Caution: Methyl iodide is volatile and toxic because it is a reactive electrophile (an
alkylating agent); it should only be handled in the hood. Measure and dispense it by syringe,
using its density to convert mmol to mass to volume.
Hydrolysis (Saponification) of Methyl 2-Phenylpropanoate (7):
In an appropriately sized flask, dissolve the ester 7 in ~2-5 mL of methanol. Add water
dropwise until the mixture becomes cloudy or until an equal volume of water has been added. Add
2 equivalents of K2CO3 or LiOH•H2O and allow the mixture to stir at ambient temperature or
warm it to ca. 50 °C in a water bath. Follow the reaction progress by the appropriate analytical
method. When the reaction is complete, cool to room temperature and dilute with ether. Carefully
adjust the pH of the water layer to pH ~3 with 10 % HCl. Extract the acidic water layer with ether.
Wash the combined ether extracts with brine, dry them over MgSO4, etc. Characterize your crude
acid by the usual, full battery of spectroscopic techniques, but do not purify it on SiO2.
EDCI Coupling with (R)-α-Methylbenzylamine
Couple the crude, racemic acid 8 with one equivalent of (R)-α-methylbenzylamine using 1ethyl-3-(3-dimethylaminopropyl)carbodiimide [EDCI (aka EDC or EDAC)] as the
activating/dehydrating agent. Add a solution of EDCI in dry methylene chloride to a solution of the
acid in methylene chloride at 0 °C. Caution: handle EDCI only in the fume hood by weighing it
into a capped, tared vessel and taking that to the balance to determined the mass. Some workers
become sensitized to some carbodiimides if they come in contact with the solid dust. Since
carbodiimides react slowly with moisture, close the reagent bottle snuggly once you have removed
your desired amount of EDCI. Minimize exposure of the reaction mixture to atmospheric moisture.
Soon after (e.g., within one minute) follow with the addition of a solution of the amine in
methylene chloride. Monitor reaction progress by tlc and/or gc. Separate the pair of
diastereomeric products 9 by MPLC (or very careful flash chromatography). Again this is a
Experiment 3: Enolate Alkylation, Ester Hydrolysis, and Amide Bond Formation
5
challenging, but doable, separation. Collect and report spectral data on each pure diastereomer.
Look up the melting point of each of these diastereomers using the Beilstein database and compare
the literature values with your isolated products. Always record a temperature range for the
melting point, not a single value.
Lab Report Questions (Please answer in your own words):
1.
Rank the following esters from highest Rf to lowest Rf on tlc: a) the starting methyl
phenylacetate (PhCH2CO2Me, 6), b) the product: methyl 2-phenylpropanoate
[PhCH(Me)CO2Me, 7], and c) benzyl 2-phenylpropanoate.
2.
Why is it important that the substrate, solvent, reaction vessel, and reagents be dry before
and during the ester enolate alkylation reaction?
3.
What type of mechanism is the alkylation (methylation) reaction? Why is methyl iodide
used instead of, for example, methyl chloride? Would you expect n-butyl iodide to react
faster than, slower than, or at the same rate as i-propyl iodide in an analogous reaction?
Explain.
4.
What product is formed upon the reaction of EDCI with water?
5.
In the basic hydrolysis of the ester 7 to the carboxylic acid 8, why must you add HCl to
lower the pH before extracting the product?
6.
Would samples of pure R-8 and pure S-8 have identical or different IR spectra? Why?
Would the absolute value of the specific rotation (of plane-polarized light) of each be
identical or different? Why? Would the sign of the specific rotation (of plane-polarized
light) of each be identical or different? Why?
7.
Would samples of pure R,R-9 and pure S,R-9 have identical or different 13C NMR spectra?
Would the absolute value of the specific rotation (of plane-polarized light) be identical or
different? Why?
8.
Amides, like esters, can be hydrolyzed to carboxylic acids. Would you expect R,R-9 and
S,R-9 R-7 and S-7 to react at the same or different rates with LiOH/water? Why?
References
1
Heathcock, C. H.; Buse, C. T.; Kleschick, W. A.; Pirrung, M. C.; Sohn, J. E.; Lampe, J. J.
Org. Chem. 1980, 45, 1066–1081.
2
Singh, D. K.; Springer, J. B.; Goodson, P. A.; Corcoran, R. C. J. Org. Chem. 1996, 61, 1436–
1442.
3
Wu, G.; Tormos, W. J. Org. Chem. 1997, 62, 6412–6414.