Lecture Manuscript
For
Thursday January 20tthh, 2000
Umbreen Mir
971299900
CHM 331S
Professor M. Denk
January 27th, 2000
1,1-Dilithio-organyls
1,1-dilithio-organyls can be obtained by hydroborating alkynes with 9-borabicylcononane in a two-step synthesis reaction:
R
CH
C
9
-BBN
Li
BBN n-BuLi
C
R
R
CH
CH2
BBN
CH
Li
Elimination Reactions of LiHal in Lithiumorganyls
The reactions of 1,2 and 1,3-dichloroethane with lithium do not produce 1,2 and 1,3dilithio-organyls, but instead are ideal procedures for synthesizing anhydrous metal
chlorides:
Cl
Li
H2C
CH2
+
LiCl
Cl
Li
Cl
Cl
+
LiCl
This is the general method in obtaining metal halides in the anhydrous form (in aprotic
solvents such as ether, hexane and toluene). This method works well with electropositive
metals of group 1-3, such as Mg, Ca and Fe with the exception of inert elements,
beryllium and boron.
Anhydrous metal chlorides are important synthetic starting materials, in particular for
organometallic reactions.
Samarium (II) iodide is a synthetically important reducing agent which generates radicals
from organohalides. It is prepared from 1,2-diiodo-ethane and samarium, and is generally
better if it is prepared fresh before it is used.
1,4-dilithium compounds can be obtained, but the procedures are very tricky. Most of
this 1,n-dilithium chemistry was developed by a Dutch chemist named Bickelhaupt:
Hal
Li
Hal
Li
1,2-Dehydrobenzene
1,2-dehydrobenzene is a relatively unstable compound. It can be synthesized from
1,2-dihalo compounds, or the standard method in which an intermediate diazonium salt
is formed:
1,2-Dehydrobenzene
Hal
Li
Hal
-N2
+
-CO2
NH3
COO-
iC5H11-ONO
N2 +
COO-
Dehydrobenzene has been stabilized in the form of metal complexes, particularly Ta, by
the work of an Australian chemist named Bennet.
Dehydrobenzene chemistry also produces unusual hydrocarbons such as biphenylene and
triptycene. The synthesis of triptycene is a high yielding reaction, if done properly, 7080% product can be obtained:
anthracene
Elimination Reactions of LiH in Lithiumorganyls
The lithiation of a hydrocarbon, leads to the elimination of LiH. The addition of
GeCl21,4-dioxane results in a ring closure forming 1,3-diaza-germole, and unsaturated
germole:
tBu
NH
NH
tBu
n-BuLi
THF
NLi
NLi
GeCl2.(Dioxane)
tBu
tBu
N
N
Ge
- two LiCl THF
+
Ge
N
N
tBu
tBu
tBu
tBu
tBu
-LiH
N
Li
Ge
N
tBu
An older example of a reaction that eliminates LiH, is the nucleophilic alkylation of
pyridine with RLi which leads to a 1,2-addition:
R-Li
N
H
N
Li
R
-LiH
N
R
In 1865, the first alkaloid with high toxicity was synthesized:
N
Chichibabin Reaction
The Chichibabin reaction is widely used today, mainly to make 2-amino substituted
pyridines, via the elimination of NaH:
NaNH2
N
H
N
-NaH
NH2
N
NH2
Na
The same reaction cannot be carried out with OH- because it is not a strong enough
nucleophile to break the pyridine ring. Instead, N-methyl-pyridinium salts are used, and
then OH- is added:
Stability of Organolithium Compounds
PhLi, n-BuLi, sec-BuLi, tert-BuLi, and methyl lithium are commercially available
organolithium compounds. The most commonly used, n-BuLi is produced on a large
scale, for example, 17,000 litres are available.
Purity of Organolithium Compounds
Organolithium compounds are sold as stock solutions : 1.6 molar, 2.0 molar, 10.0 molar.
The 10.0 molar solutions are pyraphoric, and must be handled with care.
Determining the contents of organolithium solution is difficult because they decompose
in air and moisture. Organolithium compounds with a -hydrogen, eliminate LiH to
form alkenes:
Impurities of Organolithium Compounds
R
R
Li
Li
OH2
O2
R H
R
Li Heat H C
2
H
+
LiOH
+
LiH
OLi
CH2
The decomposition products: LiOH, R-OLi and LiH cannot be distinguished from RLi
through simple titrations with an acid because the sum of these impurities will be
determined. Therefore, a technique, known as the Gilman double Titration is an
alternative in determining only the contents of RLi.
Gilman Double Titration
The Gilman Double Titration, established in the 1940’s, determines the contents of
organolithium compounds (RLi). Since RLi is the only compound that reacts with
dibromoethane, it is easily removed from other impurities. This process involves two
titrations, in which the difference between 1. and 2. is the content of RLi:
1. Conventional Titration
LiH
LiOH
2. Titration of Impurities
LiH
Dibromoethane
LiOH
R-Li
R-Li
R-OLi
R-OLi
+ R-Br + Li-Br + alkene
Today, the Gilman Double Titration can be done by optical indicators, which involves a
two-step deprotonation reaction.
Reduction of Hydrocarbons
Lithium reacts with hydrocarbons to give radical anions:
-.
Li
+
These radical anions display strong colours (naphthalene=greenish, anthracene=blue),
and are air and moisture sensitive. Anthracene and naphthalene are not used as
nucleophiles, but instead are strong reducing agents and are used as singular electron
transfer catalysts.
Arene Radical Anions
Chromium hexa-carbonyl is a colourless white solid with stable carbonyls. Since the
radical anions of naphthalene and anthracene are very strong reducing agents, they are
ideal for reducing chromium hexa-carbonyl to chromium penta-carbonyl. THF is used to
solvate the lithium cation:
CO
CO
2
CO
CO
CO
Cr
CO
CO
Anthracene
OC
Li, THF
OC
-
+
Cr
CO
CO
CO
A traditional way to synthesize the same product involves using liquid ammonia and
lithium.
The following reduction can be conducted by both (a) Na and C10H10 as well as (b) K
and small amounts of naphthalene:
(a)
R
R
R
N
C
S
N
Na, C10H10
N
R
(b)
N
2X
C:
-Na2S
R
N
C C
N
N
N
R
R
R
R
R
N
N
molten K
C
N
S
C:
Anthracene
N
+
K2S
R
R
Formation of –ate Complexes
An undesirable side reaction with organometallic compounds is the formation of –ate
compounds. An example is illustrated by the formation of a borate complex:
Ph
BF3.Et2O
Ph-Li
Li +
B
Ph
Ph
Ph
Only small traces of BPh3 are produced because boron is a strong Lewis acid, which can
easily react with an equivalent of PhLi. The problematic formation of –ate complexes
can be avoided by using Grignard reagents:
BF3.Et2O
Ph
Ph-MgBr
Ph
B
Ph
Use of Tetraphenylborate
Ph4B- forms insoluble salts with large cations. Therefore, tetraphenylborate is ideal for
distinguishing Li and Na, which have water-soluble salts, from the alkali cations K, Cs
and Rb, which have water insoluble salts.
Grignard Reagents
Grignard reagents were important because they were the most accessible organometallic
compounds. They can be obtained by mixing dry ether and organic halides, preferably
bromides because they are more reactive, and because Cl has a lower boiling point:
R
Et2O
X
R
MgX
If nothing happens during the reaction, the best way to activate Mg is through
sonification.
Grignard Reagents in Solution
Grignard reagents exist as monomers and dimers:
R
OR2
-Et2O
R
Mg
R2O
Mg
X
Et2O
R2O
R
X
Mg
X
OR2
Strong donors (TMEDA>ET3N>THF>ET2O) and dilution work well with monomers,
whereas weak donors and high concentrations suit dimers.
Synthesis of R2Mg
One implication of the following equilibrium is how it is possible for two R substituents
to end up on one Mg. A general synthesis of R2Mg is from R-MgX with 1,4-dioxane,
which results in the formation of a precipitate of MgX2:
O
R-MgX
+
R-Mg-R
+
MgX2.(1,4-dioxane)
O
Formation Mechanism
The reaction of organohalides with Mg begins with a single electron transfer step, (SET
reaction) which involves radical intermediates:
(1)
(2)
(3)
(4)
weakening of R-X bond
dissociation of radical anion to a radical + an anion
formation of R-Mg bond
extract using Et2O
R-MgX(Et2O)2
R-X ¯•
R-X
Mg
(1)
Mg
R• + X¯
(2)
Mg
R + X¯
(3)
Mg
(4)
Arthur Ashe (III) proved the intermediacy of radicals by trapping it with suitable
substrates:
X
CH2 .
MgX
CH2 .
MgX
The first dicoordinate organomagnesium compound was characterized by Eaborn in
1989:
SiMe3
SiMe3
Me3Si
C
Mg
SiMe3
C
SiMe3
SiMe3
References
Denk, M. (1999) Lecture notes on Inorganic Chemistry II: Organometallics.