phase-transfer catalysis (ptc) · catalysis is attributed to their poor solvation in the organic...
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Phase-transfer catalysis
• An aqueous solution • An organic solution (chloroform, hydrocarbons)• An organic reagent soluble in the organic solvent only• An inorganic reagent (NaOH, NaI or NaCN) soluble in
water only• A catalyst soluble in both phases. The catalyst is able to
complex the inorganic reagent
Phase-transfer catalysis (PTC)
Examples of polar aprotic solvents. Good solvents for both organic compounds and
polar inorganic salts
MeS
Me
O
MeS
Me
O
dimethyl sulfoxide (DMSO)
H NMe2
O
H NMe2
O
N,N-dimethylformamide (DMF)
NMe
ONMe
O
N-methylpyrrolidone (NMP)
SN2 reaction of octyl chloride with sodium cyanide
Cl NaCN
no reaction
n-Bu4P Br
water/decane
CN
P+
Br-
tetrabutylphosphonium bromide(n-Bu)4P+ Br-
The principle of phase-transfer catalysis developed by Makosza
• The catalyst (Bu4P+ Cl-) contains highly lipophilic butyl groups, which makes the compound soluble in an organic solvent. It is also a polar phosphonium salt, which makes the compound soluble in water as well.
Bu4P CN R-Cl R-CN Organic phase
Phase boundary
Bu4P CN NaCl NaCN Bu4P Cl Aqueousphase
Bu4P Cl
Typical phase-transfer catalystsammonium salts 1-4 and phosphonium salts 5,6
N
Bu
Bu
Bu N Me
RR
RCl
P Br
6
PBu
BuBu
nn
n
nBuBr
5
nn
nnBu
XN
Et
EtEt
Cl NMe
MeMe
OH
X = a Br (TBAB)b F (TBAF)c OH (TBAH)d HSO4e BH3CNf ClCrO3
1 2 3 4
(TEBA) (Triton B)a R = n-C8H17(Aliquat 336)b R = n-C8-C10(Adogen)
Typical phase-transfer catalystscrown ethers 7-9 and a cryptand 10
Complexes with potassium cation are shown for 7 and 10
OO
O
OOO
OO
O
OOO
OO
O
OOO
K+
7 8 9
O
N
OO
O
N
10
O O
K
18-crown-6with potassium cation
Naked (bikini) anions
• The high reactivity of nucleophiles under phase-transfer catalysis is attributed to their poor solvation in the organic phase or at the phase boundry. The poorly solvated anions are sometimes called ‘naked’. Makoszasuggested ‘bikini’ because each loose ion pair in an organic solvent is associated with a few molecules of water extracted from an aqueous solution.
NEt
Et
Et
+Cl- Triethylbenzylammonium chloride
(TEBA)
Note 13 carbon atoms in the molecule for high lipophilicity
Synthesis of TEBA
NEt
Et+ Et-Cl N
Et
Et
Et+ Cl-
..
“Purple benzene”a strong oxidant in an organic phase
OO
O
OOO
K MnO4
OO
O
OOO
K MnO4
Synthesis of 7,7-dichloronorcarane under PTC conditions
HCX3 + NaOH Na CX3 + H2O
Na CX3 + R4N X R4N CX3 + NaX
R4N CX3 :CX2 + R4N X
+
+ -
-+
+ - + -
:CX2Cl
Cl
(1)
(2)
(3)
(4)
+ - + -
7,7-dichloronorcarane(7,7-dichlorobicyclo[4.1.0]heptane)
The mechanism of the norcarane synthesis
CHX3
XX
CHX3 OH :CX2 H2O R4N+ X-
H2O NaOHAqueous Phase
Organic Phase
R4N+ OH-
Phase Boundary
N
Cl
R4N+ X- =
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