Lecture Manuscript
Lecture #5-Thursday January 14th, 1998 – 9:00 AM
-I / -M Effects of Substituents on Acidity
H
X
C
H
H
-replace with 1 different substituent
Substituents
H-CC-R, -Hal, -Br2, -Ph
-OR, -NR2, SiR3
Weak
Very Weak
Strong Bases
n-BuLi, SecBuLi, Li-TMP
n-BuLi, Sec-BuLi,
TertBuLi, T-MEDA
Note: T-MEDA is a superbase, meaning you use this as a method of polarizing a carbon
metal bond
-T-MEDA stands for N,N,N’,N’-tetramethyl ethylene diamine
N
result: increase charges, compound becomes
more basic
C(-) - Li (+)
electron flow
N
What do you do with super bases?
-can make carbon metal bonds directly and faster
e.g.
CH
C-M
C-Hal
Another superbase-the strongest!
Schossler Reagent- BuLi/KOtBu
-advantage: for polymerization reactions (Scheffold, 1992)
-so powerful, can deprotinate anything!
e.g. C3H6 ----------- C3Li6
-can play with electrophiles
result: multiple characteristics, but, very unreactive, therefore this field of chemistry was
abandoned.
-alkanes are the solvents used for these two above mentioned very strong bases
Solvents for Organometallic Chemistry
Structure
Name
B. pt
(degree
Celsius)
+34
Ether
M. pt
(degree
Celsius)
-116
O
solvents
hazards
Distillation
overLiAlH4,
Ph2CO/K
------
Note: Benzophenone Potassium- it’s role is to purify the solution
Used for reactions with:
I)
hydrocarbons
II)
ethers
III)
aminer (R3N)
Ph
Ph
C'
O
C'
O-
K+
Ph
Ph
Take neutral diamagnetic compound and dump this molecule:
-you produce a radical anion
advantages:
i)
extremely reactive (pick up O2, H2O quickly-very efficient to get rid of bad
impurities)
ii)
used as an indicator (violet or blue colour, when you see colour, sign to tell you
that O2 and H2O are gone
THF
+66
-108
-LiAlH4
-Ketyl
1,4-Dioxane
+100
+12
- LiAlH4
- Ketyl
O
O
O
-high
toxicity
-perhaps
carcinogen
N/A
O
O
Et3N
O
O O
O
1,2dimethyloxy
ethane
“GLYME”
DIGLYME
note:adding
more O’s
-can also be
referred to
as polyethers
------
+85
-58
- LiAlH4
- Ketyl
N/A
+162
-64
- LiAlH4
- Ketyl
N/A
-------
-------
- LiAlH4
- Ketyl
-smells like
fish
Important characteristic to note for above mentioned solvents:
Miscibility in water- THF, 1,4-dioxane, and GLYME
Non-miscibility in water- ether
Note on GLYME:
-within kinetic examination of organometallic reactions, it was discovered butyllithium to
be more reactive in “GLYME” than in THF!
-reason being: the chelating of the bis-ether allows the surrounding metal to have a
solvation shell with a lower entropy value
-therefore, the reactivity of organometallic species increases by twice as much by the
breaking up of oligomers or monomers and also by increasing the polarity between the
carbon-metal bond (Scheffold, 1992)
Advantages of GLYME and DIGLYME:
i)
high boiling solvent
ii)
don’t need autoclave
iii)
can use over the other solvents for a higher boiling point
Disadvantages of ether:
-can pick up water and oxygen too easily!
Not good because it will cause the formation of peroxides!
How do you protect yourself from peroxide formation?
-add in KOH , NaOH or CaH2 (great safety method because it will tell you it is active by
bubbling)
-by adding in base, the peroxides will form salts, which are insoluble and ends the
peroxides existence
O
O2
O
H
H
O
O
Reactions with Ethers are dangerous-especially chlorinating and brominating
-listed below are all the reagents which you cannot work with ethers because they are
highly dangerous, chlorinated ethers are carcinogens
Cl
Br2, Cl2
O
O
VCL4, NbCl5, MoCl5,
TaCl5, NCl6, NCl5, ReCl5
Note: iodine does not react, therefore, it is the only safe halogen
-Fluorine and ether mixture is also dangerous, catch fire, explosive, death!
Note: only two reagents which can react with ether to form anything of use are:
i)
NbCl5
ii)
TaCl5
eg. The Best Method To Make NbOCl3
O
Cl
NbCl5
+ NbOCl3
Cl
Base Solvents for Organometallic Reactions:
1) NaH2- will deprotinate anything with pKa < 25
e.g. alkynes
advantage: cheap
2) NaNH2- pKa < 33
Precautions:
-use up soon, or store under argon
-oxidized easily to yellow solution
-looks innocent but is not!
How to generate NaNH2:
FeII
Na + NH3
blue solution
-H2
NaNH2
3) Ph3-Na- pKa < 30
Precautions:
-safe, crystalline structure is air stable, therefore when you work with it, you do not
need to use a glove box
-*very useful base*
Ph
How to generate Ph3-Na:
Ph-Hal
Ph-MgHal
PhCOOMe
HCl
Ph
C
Ph---C----Cl + Na
OMgX
Ph
Ph3Na
Superbases
-can transform a vinyl compound into a vinyl metallic compound
-can transform a polycompound into a metallic polycompound
H
M
H
M
H
M
M=Li
Reagent:
BuLi/TMEDA
M=K
Reagent:
BuLi/KotBu
Note on Superbases:
-compared with the more polar reagents, pentylsodium, butylpotassium and
trimethylsilylpotassium, they have the reaction speed of a snail in comparison to
superbases such as N,N,N’,N’-tetramethylethylenediamine (TMEDA)
-superbases are so extraordinary in organometallic chemistry because it offers the best
performance in terms of yield, rate and selectivity (Scheffold, 1992)
Reference
Scheffold, R. (1992) Modern Synthetic Methods . Verlag Helvetica Chimica Acta,
Basel. Pgs 227-250