CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.
Chapter 14 Organometallic Compounds
| Reactions of
aldehydes and ketones with Grignard reagents by Paul R. Young, University of Illinois at Chicago http://www.chem.uic.edu/web1/OCOL-II/WIN/CH19/F3.HTM and Preparation of alcohols by reaction of Grignard reagents with carbonyl compounds http://www.chem.uic.edu/web1/OCOL-II/WIN/CH17/F3.HTM |
| Organometallic
compounds by William Reusch at the University of Michigan http://www.cem.msu.edu/~reusch/VirtualText/alhalrx4.htm#hal10 |
| The Grignard
Reaction by Otto Meth-Cohn at the University of Sunderland http://web.archive.org/web/20061206001907/http://orac.sunderland.ac.uk/~hs0bcl/org3.htm http://web.archive.org/web/20021202032647/http://www.md.huji.ac.il/Organic2/org4.htm http://web.archive.org/web/20021212234009/http://www.md.huji.ac.il/Organic2/org5.htm a few pics might not show, but the bulk is there http://orac.sunderland.ac.uk/~hs0bcl/org3.htm http://www.md.huji.ac.il/Organic2/org4.htm http://www.md.huji.ac.il/Organic2/org5.htm |
| Organometallic
Chemistry Carey Ch 14. from McGraw-Hill http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch14summary.html Nucleophilic addition to C=O from McGraw-Hill http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch17carbonnucleophiles.html |
| Structure and
Synthesis of Alcohols: Grignard Reagent QuickTime movies from
Prentice-Hall, Inc. http://cwx.prenhall.com/bookbind/pubbooks/wade/chapter10/custom1/deluxe-content.html |
| Grignard reagents
by Steve Wathen at Siena Heights University http://web.archive.org/web/20040613114457/http://www.sienahts.edu/~swathen/organic/grignard.html http://www.sienahts.edu/~swathen/organic/grignard.html |
| PowerPoint slides for
Carey Chapter 14 from Columbia University 14.1 to 14.5: Organometallic Compounds http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/14_01_05.html 14.6 to 14.10: Synthesis of Alcohols Using Grignard Reagents http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/14_06_10.html 14.11 to 14.15: Alkane Synthesis Using Organocopper Reagents http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/14_11_15.html |
Chapter 14: Organometallic Compounds
I.
Carbon-Metal Compounds.
A. Carbanions
1. Carbanions are organic ions in which a carbon atom
carries a negative charge.
2. Carbon-Metal bonds are carbanions (or at least have
enough ionic character to be considered as such).
a) R-Na, R-K: ionic C-Metal bonds.
D EN > 1.6
b) R-Li, R-Mg: substantial ionic
character. D
EN 1.3 – 1.5
3. The important fact is that the C-M bond results in
the carbon becoming negative in character.
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►
This is the reverse of the C-X dipole [where C has positive character ]
► This is important: C:−
will be attracted to other C's with + character, most notably C=O
i) Formation of new C-C bonds
is extremely important in organic synthesis.
ii) organometallic compounds
allow us to create C-C bonds in organic synthesis.
B. Nomenclature.
1. Organometallic compounds are named as substituted
derivatives of metals:
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CH3CH2CH2Li |
(CH3CH2)Zn |
CH3MgBr |
C. Preparation of Organolithium Compounds
1. Overall reaction: RX + 2Li
à RLi + LiX
2. Water cannot be present since an acid/base reaction
occurs.
a) RLi + H2O
à
RH + LiOH
b) Solvents used include pentane, hexane,
and diethyl ether.
c) Other solvents with O-H, N-H, and S-H
cannot be used: also acidic enough to react with RLi.
3. Reactivity of X: I > Br > Cl > F ; RBr
most often used [compromise between $ and reactivity].
4. R groups can vary greatly: alkyl, aryl [no
OH, NH, SH in substituents], vinylic, allylic, benzylic
RX = 1o RX, 2o RX, 3o RX, FX <aka Ph-X, ArX>, CH2=CH-X, CH2=CHCH2X , FCH2X <PhCH2X>
5. Mechanism: proceeds via radical anion intermediate
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6. Complete the following equations:
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D. Preparation of Grignard Reagents.
1. Overall Reaction: RX + Mg ( in ether)
à
RMgX
2. Mechanism: .
Step 1. RX +
∙Mg∙
à
[RX]∙− + Mg∙+
Step 2. [RX]∙−
à R∙
+ X−
Step 3. R∙
+ Mg∙+
+ X−
à
RMgX
a) Note the similarity of the above
mechanism to that of the preparation of RLi compounds:
except in this case one Mg atom
donates both e− [rather than two Li atoms each
donating one e−]
3. Ethers are used as solvents, since solvents with
O-H, N-H, or S-H react with the Grignard Reagent.
a) Both RMgX and RLi are strong bases:
RLi + R'OH à RH + R'O−Li+
2RMgX + 2R'OH à RH + Mg(OR')2 + MgX2
4. R groups can vary greatly: alkyl, aryl [no OH, NH, SH in substituents], vinylic, allylic, benzylic
RX = 1o RX, 2o RX, 3o
RX, FX
<aka Ph-X, ArX>, CH2=CH-X, CH2=CHCH2X
, FCH2X
<PhCH2X>
II. Reactions of Organometallic Compounds.
A. Reactions of Grignard Reagents.
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1. With Formaldehyde: product is 1o
alcohol one C larger
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a) Complete the equation (including workup): CH3CH2MgBr + H2C=O à
2. With Aldehydes R'CHO: product is a 2o
alcohol with R and R' on the
a-C.
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a) Complete the equation (including
workup):
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3. With Ketones R'COR": product is a 3o
alcohol
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a) Complete the equation (including workup):
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4. With CO2
[dry ice]:
product is a carboxylic acid
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a) Complete the equation (including
workup):
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5. With Esters: product is a 3o alcohol with 2 identical a-substituents.
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a) Complete the equation (including workup):
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6. With ethylene oxide [oxirane]: product is a 1o alcohol 2C larger
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a) Complete the equation (including workup):
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7. With Water, Alcohols, Amines, Thiols, Carboxylic Acids,…: product is an alkane
RMgX + H2O
à
RH + MgOHX
alkane
a) Handy reaction if one needs to
convert an alkyl halide into an alkane; otherwise it's a disaster!.
b) Complete the equation:
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8. Retrosynthetic Analysis.
a) Target Molecules from Precursors:
E. J. Corey, Harvard University.
b) Synthesize the following compounds from
simpler starting materials (using Grignard Reagents):
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B. Reactions of Organolithium Compounds
1. Organolithium compounds react as Grignard Reagents
react. They are also used as hyperstrong bases.
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2. Complete this equation:
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C. Synthesis of Acetylenic Alcohols: two synthetic routes.
1. With Acetylide Anions and Aldehydes/Ketones
a) Acetylenic alcohols are formed by
reaction of acetylide anions with aldehydes and ketones.
b) Analogous to reactions of Grignard
Reagents.
c) Reaction:
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2. Alkynyl Grignard Reagents.
a) Preparation of the alkynyl Grignard
reagent is an acid-base reaction.
R-C≡C-H + R'MgX à R-C≡C-MgX + R'H
b) The alkynyl Grignard reagent then reacts as any other Grignard reagent.
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c) Example. Complete the following:
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D. Alkane Synthesis with Organocopper Reagents.
1. Lithium Dialkylcuprates are used to produce alkanes.
2. Formation of the lithium dialkylcuprate:
a) Treat copper(I) halide with two
equivalents of an alkyl lithium in THF or ether:
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b) Mechanism:
Step 1: R-Li + Cu-I à R-Cu + LiI
Step 2: R-Li + R-Cu à [R-Cu-R]− Li+
3. Lithium Dialkylcuprates react with alkyl halides to produce alkanes.
R2CuLi + R'X à R-R' + RCu + LiX
a) The reaction works best with primary
R'X and primary and phenyl dialkyl cuprates.
i) What problem might occur
with 2o and 3o alkyl halides?
Answer:
ELIMINATION.
ii) R'X can also be vinyl
halides and aryl halides (not consistent with nucleophilic attack.)
4. Complete the following
equations:
a) (CH3CH2CH2)2CuLi + CH3CH2CH=CHBr à
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5. The alkyl groups in the dialkylcuprate ion can be
phenyl groups or 1o alkyl groups but not 2o or 3o.
a) With 2o and 3o
alkyl groups, steric hindrance seems to make R2CuLi less reactive
toward R'X.
b) With 2o and 3o alkyls, R2CuLi also tends to be unstable -- decomposes before reacting with R'X
6. Examples:
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7. The reaction follows the trends seen for SN2
reactions.
a) Order of Reactivity of RX: CH3X
> 1o > 2o > 3o
b) Order of Reactivity of Halide: I >
Br > Cl > F
8. Mechanism of this reaction is not well understood;
seems to involve nucleophilic attack on R'X by the
Cu atom of the dialkylcuprate; unstable
intermediate then breaks apart into the observed products:
R2Cu−
+ R'-X à
[R2CuR'X]−
à
R-R' + RCu + X−
9. Synthesize the following compounds using lithium dialkylcuprates:
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E. Organozinc Intermediates in Synthesis
1. The reduction of alkyl halides by zinc can be used
to form alkanes.
a) General overall reaction and mechanism:
RX + Zn
à
RZnX ; RZnX + HX
à
RH + ZnX2
b) Notice that an organozinc reagent
(R-ZnX) is produced in this reaction.
i) R-ZnX is not as reactive
towards carbonyl compounds as Grignard reagents (R-MgX).
c) Example. Complete the reaction:
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2. Dehalogenation of vicinal dihalides to form
alkenes: Beta Elimination.
a) Overall reaction:
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b) Mechanism:
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c) This is beta-elimination since
the leaving group is
b to the
a
carbon bearing the ZnX group.
d) Examples:
CH3CHBrCHBrCH2CH3
+ Zn/EtOH à
CH3CH=CHCH2CH3
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3. Dehalogenation of 1,3-dihalides to form
cyclopropanes: Gamma Elimination.
a) This reaction only works to form
3-membered rings.
i) When the halogen atoms are
more than 3 carbons apart yields of larger rings are poor.
b) The reaction is run in ethanol.
c) This is gamma-elimination since
the leaving group is
g to the
a
carbon bearing the ZnX group.
d) Mechanism:
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e) Complete the following reaction:
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4. Simmons-Smith Reagent: Preparation of
Cyclopropane.
a) Cyclopropanes can also be synthesized
using a zinc-copper couple (Zn surface-activated with Cu)
b) Zn(Cu) reacts with CH2I2
in ether to yield iodomethylzinc iodide, ICH2ZnI.
c) ICH2ZnI reacts with an alkene
in ether to form a cyclopropane:
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d) Mechanism:
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e) The reaction is stereospecific due to
syn-addition:
groups which are cis in
the alkene remain cis in the ring;
groups which are trans in
the alkene remain trans in the ring.
f) Complete the following reactions:
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g) ICH2I is a "carbenoid" since
it resembles carbene (CH2:) chemically.
h) Yields often low; important since it
offers a route to compounds otherwise difficult to make.
F. Carbenes and Carbenoids.
1. Carbenes: neutral molecules containing a divalent
carbon atom with no unpaired electrons.
2. Carbenes are electrophilic.
3. Extremely unstable but can be trapped in a frozen
argon matrix.
4. Dihalocarbenes produced when CHX3 treated
with a strong base, e.g. potassium t-butoxide.
i) The base deprotonates HCX3
forming the trihalomethyl anion.
ii) This anion then splits apart [a-elimination]
into the dihalocarbene and halide ion.
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iii) This is an alpha-elimination
since both H+ and X−
are lost from the same atom.
5. With alkenes, :CX2 undergoes
cycloaddition to the C=C to produce 1,1-dihalocyclopropanes.
a) Addition is stereospecific: syn
addition is observed.
6. Complete the following reactions.
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G. Organic Derivatives of Mercury.
Oxymercuration-Demercuration of Alkenes.
1. Recall the use of mercuric acetate in the production
of alcohols via the Markovnikov hydration
of alkenes (Chapter 6 notes.)
2. Disposal of mercury wastes creates serious health
problems.
a) Hg2+ converted to
methylmercury, CH3Hg+, and dimethymercury, (CH3)2Hg,
in the environment
due to bacterial action.
b) These compounds collect in tissues of
fish. Ingested by humans, these compounds can cause
serious health problems.
c) The California Department of Fish and
Game, in their fishing regulations booklet, warns of
contaminated fish in various
waters of the state and how much fish can be safely consumed.
III.
Transition Metal Organometallic Compounds.
A. Many compounds consist of transition metals bonded to organic
groups.
1. Many of the organic groups are bonded to the
transition metal through the pi system.
a) Ferrocene. Two cyclopentadienide ions
pi bonded to the ferrous ion in "sandwich" fashion.
Fe2+(C5H5−)2
picture of cobaltocene is shown below
b) (Benzene)tricarbonylchromium. Cr(CO)3C6H6
i) Notice that it is benzene,
not a phenyl group, bonded to Cr via benzene's pi system.
c) (Cyclobutadiene)tricarbonyliron. Fe(CO)3C4H4
2. Several important industrial processes are catalyzed
by transition metals and/or their complexes.
3. The Monsanto Process: an industrial
process which efficiently synthesizes acetic acid from carbon monoxide and
methanol using a rhodium catalyst and an iodide promoter.
a) The reaction is believed to occur as
follows:
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b) The iodide is necesary since methanol
itself will not add to the rhodium catalyst.
c) The system is homogeneous, runs at one
atmosphere of CO pressure and 175oC.
d) The reaction achieves 99% selectivity.
e) Acetic acid is easily separated from the
reaction mixture since it has a relatively low boiling point.
i) This is an important factor
since this results in a cost/energy efficient separation.
f) The Monsanto Process offers a great
improvement over some earlier processes.
i) One system required H3PO4,
PCO ~ 1000 psi, and temperatures of around 300oC.
ii) Both corrosion and poor
selectivity were associated problems under these reaction conditions.
iii) A high energy cost was
also necessary.
Many thanks to Rod Oka of
MPC for generously sharing his "Lecture Companion" outline,
reproduced here
in extensively modified form by permission, with
web references and other goodies added by me.
Structures drawn with CS ChemOffice ChemDraw™ and MDL IsisDraw™.
