Notes 07 Organometallic Compounds with notes

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Transcript Notes 07 Organometallic Compounds with notes

Organometallic Compounds
Compounds that contain a metal-carbon bond (R-M):
Examples of M include Li, Mg (Grignard reagents),
Na, Cu, and Zn.
Nucleophilic carbon: Reacts with electrophilic carbon
and forms a new carbon-carbon bond.
Nomenclature: Similar naming to substituted metals
(alkyl metals or alkyl metal halides).
Organomagnesium: Grignard reagent.
CH3Li: Methyl lithium.
CH3MgBr: Methyl MgBr.
Learning Check
Which (if any) of the following would not be
classified as an organometallic substance?
A) triethylaluminum
B) ethylmagnesium bromide
C) potassium tert-butoxide
D) none of these (all are organometallic
compounds)
Organometallic Compounds
Properties:
High Reactivity: Organmometallics are usually
kept in organic solvents due to their very high
reactivity.
Reacts especially with H2O, O2, etc.
RMgX + H2O  RH + MgOHX
RMgX + O2  ROO- + MgOHX
Organometallic Compounds
Structure:
Ionic Character: R-M = R- M+
NaR and KR are ionic.
Electronegativities:
Na: 0.9, K: 0.8, C: 2.5
CH3F
LiR and MgR have a sigma bond
between the carbon atom and
the metal. These are very polar
compounds.
Electronegativities:
Li: 1.0, Mg: 1.2
CH3Li
Organometallic Compounds
Reactivity: Bases act as nucleophiles.
Basicity:
Simple carbanions are strong bases.
Carbon is not very electronegative compared to
nitrogen or oxygen.
RLi + HOR  RH + Li+ -OR
RMgX + HOR  RH + Mg2+ -ORX-
RLi or RMgX CANNOT be used in the presence of
acidic hydrogens, such as –OH, -NH, or –SH.
Learning Check
Explain the pKa order of ethane, ethene and ethyne
in the chart shown in your notes:
Answer: % s character.
Ethyne has an sp orbital.
Ethene has an sp2 orbital.
Ethane has an sp3 orbital.
s character: (sp = 50%, sp2 = 33%, and sp3 = 25%).
Increased s = closer to nucleus = more stabilization
of electron pairs = more stable conjugate base
formation.
General Mechanisms
1. Nucleophilic Substitution: R2CuLi reaction with
alkyl halides or tosylates.
Nu
C
Nu
C
LG
LG
2. Nucleophilic Addition: RLi or RMgX with
aldehydes or ketones.
O
Nu
Nu
C
O
C
H
Nu
C
OH
3. Nucleophilic Acyl Substitution: RLi or RMgX with
esters.
O
O
Nu
Nu
C
LG
C
LG
O
Nu
C
LG
Then Nucleophilic addition
Organolithium Reagents
RX + 2 Li  RLi + Li+X(X = I, Br, Cl)
Reaction Type: Oxidation-Reduction.
Alkyl halides react with lithium metal.
Other Group I metals (Na, K) can be used instead
of lithium.
Solvents: Anhydrous diethyl ether, pentane, hexane.
Alkyl group can be primary, secondary or tertiary.
R can be alkyl, vinyl or aryl.
Halide Reactivity: I > Br > Cl.
Organolithium Reagents
RX + 2 Li  RLi + Li+X(X = I, Br, Cl)
Limitations:
Organolithium, RLi, is too basic for an SN1 or SN2
with alkyl halides or tosylates. This gives
elimination reaction or other side reactions.
Learning Check
Which one of the following is not a suitable
solvent for the reaction shown?
CH3CH2CH2CH2Li + CH3CH=O 
CH3CH2CH2CH(OLi)CH3
A) hexane
B) benzene
C) ethanol
D) diethyl ether
Learning Check
1. What is the oxidation state change for the carbon
in the reaction methyl iodide to methyl lithium?
Reduction: -2 to -4.
2. What is the oxidation state change for the lithium
in the reaction methyl iodide to methyl lithium?
Oxidation: +1 to 0.
3. What is the oxidation state change for the iodide
in the reaction methyl iodide to methyl lithium?
-1 to -1: No change.
Organomagnesium Reagents
Et2O
RX + Mg  RMgX
(X = I, Br, Cl)
Reaction Type: Oxidation-Reduction.
Alkyl halides react with magnesium metal.
Solvents: Anhydrous diethyl ether, tetrahydrofuran
(THF)
Alkyl group can be primary, secondary or tertiary.
R can be alkyl, vinyl or aryl.
Halide Reactivity: I > Br > Cl.
Organomagnesium Reagents
Et2O
RX + Mg  RMgX
(X = I, Br, Cl)
Limitations:
Organomagnesium, RMgX is too basic for SN1 or
SN2 with alkyl halides or tosylates. In these
situations, RMgX gives elimination reactions and
other side reactions.
Common Reactions with Carbonyl Groups:
Aldehydes/Ketones:
RLi and RMgX react with the carbonyl
Organomagnesium Reagents
Et2O
RX + Mg  RMgX
(X = I, Br, Cl)
Common Reactions with Carbonyl Groups:
Aldehydes/Ketones:
RLi and RMgX react with the carbonyl (C=O) to create
alcohols.
Addition to methanal (formaldehyde)  1° alcohol.
Addition to other aldehydes  2° alcohol.
Addition to ketones  3° alcohol.
The acidic work-up intermediate metal alkoxide salt 
alcohol via a simple acid-base reaction.
Organomagnesium Mechanism
RLi
or
H
O
RMgX
R
C
H
H
C
H
O
H
H
RMgX
R'
O
R
C
H
R'
C
R'
O
H
O
RMgX
C
R'
R
R"
KETONE
Step 1: Nucleophilic C of
organometallic reagent
adds to electrophilic C in
polar carbonyl group.
Electrons move from C=O
to electronegative O.
R
C
OH
H
H
2 alcohol
R'
R'
o
ALDEHYDE
RLi
or
OH
C
H
1o alcohol
FORMALDEHYDE
RLi
or
R
C
O
H
R
C
R"
OH
R"
3o alcohol
Step 2: Work up step.
Acid/Base reaction.
Protonation of alkoxide
oxygen creates alcohol
from intermediate.
Reactions usually
in Et2O or THF
followed by
H3O+ workups
Learning Check
How many signals in the 13C-NMR spectrum
would you expect for the product of the
reaction of propylmagnesium bromide with
formaldehyde (H2C=O) followed by H3O+?
A) 2
B) 3
C) 4
D) 5
Esters
Reaction Type: Nucleophilic Acyl Substitution, then
nucleophilic addition.
Requires 2 equivalents of organolithium or
Grignard reagents.
Product: 2° or3° alcohols.
The alcohol contains 2 identical alkyl groups via a
ketone intermediate which reacts with the second
equivalent of the organometallic.
Since the ketone is more reactive than the ester,
the reaction cannot stop at a ketone.
Grignard Reactions
R’ = Alkyl, vinyl, aryl.
X = Cl, Br, I
R’CH2OH
R’X
H2CO
Mg/Ether
R’2CHOH
HCO2Et
R’CHROH
RCHO
R’R2COH
RCOR
RCN
R’MgX
(EtO)2CO
R’COR
H2O or acidic H
O
R’3COH
RCO2Et
R’2RCOH
CO2
R’CO2H
R’H
R’CH2CH2OH
Learning Check
What is the product of the reaction of
ethylmagnesium bromide (CH3CH2MgBr) with
butanal (CH3CH2CH2CH=O) followed by dilute
acid?
A) 2-hexanol
B) 1-butanol
C) 3-hexanol
D) 3-pentanol
Organocopper Reagents
Et2O
2 RLi + CuX  R2CuLi + Li+X(X = I, Br, Cl)
Lithium Dialkylcuprates, R2CuLi
Two organolithiums with a copper (I) halide.
Solvents: Anhydrous diethyl ether, tetrahydrofuran
(THF)
The alkyl group is usually primary. Secondary and
tertiary are prone to decomposition.
R can be alkyl, vinyl or aryl.
Halide Reactivity: I > Br > Cl.
Organocopper Reagents
Et2O
2 RLi + CuX  R2CuLi + Li+X(X = I, Br, Cl)
Limitations:
Organocuprates, R2CuLi, are less reactive (do not
react with aldehydes, ketones or esters) and can
be reacted with alkyl halides or tosylates to give
alkanes.
Organocopper Reagents
Alkane Synthesis Using R2CuLi:
R2CuLi + R’-X  R-R’ + RCu + LiX
Reaction Type: Nucleophilic Substitution.
Creation of new C-C bonds.
1 alkyl iodides are best, otherwise an elimination reaction can occur.
The R’ group in the halide can be aryl or vinyl.
The R group of the cuprate can be aryl or vinyl.
Although the mechanism looks like a SN2 reaction, it is more complex and
is not well understood.
Learning Check
What is the principal organic product of the
reaction of trans-1-bromo-1-butene with
lithium diethylcuprate?
A)
B)
C)
D)
cis-3-hexene
trans-3-hexene
3-ethyl-3-hexene
trans-2-hexene
Learning Check
1. How can you tell that the alkane synthesis using
R2CuLi is not a simple SN2 reaction?
Aryl and vinyl substances can not do SN1 or SN2
easily.
Organozinc Reagents
Et2O
RX + Zn  RZnX
(X = I, Br, Cl)
Reaction Type: Oxidation-Reduction
RZnX is made in a similar fashion as RMgX
RZnX is less reactive than RLi or RMgX with
aldehydes and ketones.
The most common application of organozinc
reagents is in the Simmons-Smith Reaction.
Organozinc Reagents
Synthesis of Cyclopropanes using RZNX
Known as the Simmons-Smith Reaction
Et2O
ICH2I + Zn  I-CH2-ZnI
Cu
Et2O
I-CH2-ZnI + =—R 
R + ZnI2
Iodomethyl zinc iodide is usually prepared using Zn and
activated with Cu.
Iodomethyl zinc iodide reacts with an alkene to give a
cyclopropane.
Organozinc Reagents
Synthesis of Cyclopropanes using RZNX
Reaction is stereospecific with respect to the alkene.
Mechanism is concerted.
Substituents that are trans in the alkene are trans in the
cyclopropane.
Substituents that are cis in the alkene are cis in the
cyclopropane.
CH3
CH3
I-CH2ZnI
ZnI2
CH3
Et2 O
CH3
I-CH2ZnI
ZnI2
CH3
CH3
Et2 O
CH3
CH3
Organozinc Reagents
Mechanism of the Simmons-Smith Reaction
+
ZnI2
CH2
I
CH2
ZnI
Reaction is concerted.
Both new C-C bonds are formed simultaneously.
Nucleophilic C=C causes loss of iodide leaving group.
Electrons from nucleophilic C-Zn bond form the other C-C
bond.
Acetylenic Reagents
B:- (B:- = NaNH2)
R—CΞC—H  R—CΞC:- + B-H
Or
B:- (B:- = R’MgX)
R—CΞC—H  R—CΞC:- MgX + R’-H
Reaction Type: Acid-Base.
Reacts as a carbanion.
3 important groups of reactions where nucleophiles
attack electrophilic C atoms.
Product is new C-C bonds.
Acetylenic Mechanism
RC
CLi
or
RC
CMgX
H
O
RC
C
H
C
H
C
H
O
H
RC
O
RC
C
H
C
R'
H
1o alcohol
R'
R'
C
O
H
RC
CLi
RC
CMgX
R'
C
RC
C
R"
C
OH
R'
O
H
RC
C
R"
KETONE
3o alcohol
Addition to methanal (formaldehyde) gives primary alcohol.
Addition to other aldehydes gives secondary alcohols.
C
R"
C=O in aldehydes/ketones  alcohols.
Addition to ketones gives tertiary alcohols.
C
o
R'
O
or
C
OH
H
2 alcohol
H
ALDEHYDE
RC
C
H
FORMALDEHYDE
RC
CLi
or
RC
CMgX
C
OH