1 chapter 8 reduction in inorganic chemistry loss of electrons is oxidation gain of electrons is...
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1
Chapter 8 Reduction
• In inorganic chemistry
loss of electrons is oxidation
gain of electrons is reduction
• In organic chemistry
the oxidation state of a carbon atom equals the total
number of its C–O, C–N, and C–X bonds.
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Reduction
Catalytic hydrogenation
Metal hydride reductions
Electron transfer reactions (dissolving metal reduction)
Heterogeneous
Homogeneous
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Low pressure (0-100 oC, 1-4 atm)Catalysts: Ra-Ni 、 Pt 、 Palladium、 Ruthenium、
Rhodium
Deposited on the surface of an inert support
(carbon, alumina, BaSO4, CaCO3)
---------------------------------------------->
catalyst activity decreases
Solvent: EtOH, MeOH, EtOAc, H2O, Cyclohexane,
Dioxane, THF, Acetic acid
Catalyst activity increases on going to polar, acidic
solvents
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High pressure (room temperature-300 oC, 100-30
0atm)
Catalysts: Ra-Ni, Copper chromite CuCr2O4, Rutheniu
m supported on carbon or alumina
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8.1.2 Homogeneous catalysis• Isomerization may be minimized
• Wilkinson's Catalyst: tris-(triphenylphosphine)rhodium chloride so
luble in organic sovlents (EtOH, etc.)
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第一章 导论 (Introduction)
三、均相催化和多相催化的比较
均相催化:参与反应的所有组分都处于同一相中。这里的同一相
指液相,即所有反应物,包括催化剂,都在溶液中;
其催化剂一般是一种或几种组成和结构确定的过渡金
属络合物。
多相催化:参与反应的一个或多个组分处于不同相中。其催化剂一般为固体,反应物为液体,或气体(多数情况下),因此,反应是在催化剂表面上进行。
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第一章 导论 (Introduction)
活性 (activity) : TON (Turnover Number): the number of product molecules pro
duced per molecule of the catalyst
TOF (Turnover Frequency): the turnover number per unit time
选择性 (selectivity):
化学选择性 (chemoselectivity)
区域选择性 (regioselectivity)
Rh CHO
CHO+ CO + H2+ + (1)
OH OOHTi
OH OH+ + (2)catalyst
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第一章 导论 (Introduction)
对映选择性 (enantioselectivity)
OH OOH OH
HOH+ + (3)
Ti-catalyst
Chiral
多相催化的优点在于,催化剂为固体
• 与产物易于分离
• 热稳定性好
• 催化剂再生容易 in situ
操作费用低
对映选择性用对映异构体过量 (enantiomeric excess, ee) 表示
%100
SR
SRee
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第一章 导论 (Introduction)
均相催化的优点在于,催化剂为组成和结构确定的络合物
• 高活性
• 高选择性
• 容易修饰 (modification)
• 机理明确,催化剂的性能可以在分子水平上得到解释,并予以预测
Homogeneous catalysis, owing to their high selectivity,
are becoming increasingly important for the
manufacture of tailor-made plastics, fine chemicals,
pharmaceutical intermediates, etc.
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8.1.3 Transfer hydrogenation
Using a hydrogen donor as the source of hydrogen, such as
Cyclohexene, propan-2-ol, formic acid, hydrazine.
Catalyst: heterogenous or homogeneous.
Avoid attendant hazards
Out of accord with the economy rules of atoms
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8.2 Metal hydride reductions
LiAlH4 most reactive
LiAlH(OC(CH3)3)3 or LiAlH4 + 3 C(CH3)3OH
NaAlH2(OCH2CH2OCH3)2 RED-Al
NaBH4 less reactive C=O (type) reductions.
LiAlH2(OCH2CH2OCH3)2
NaBH3(CN) Sodium cyanoborohydride (only imines)
LiBH4
AlH3 Aluminum hydride
[(CH3)2CHCH2]2AlH Diisobutylaluminum hydride (DlBAL-H)
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Reaction Conditions
Anhydrous conditions (except NaBH4)
LAH (non-hydroxylic solvents): ether, THF, 1,2-dimethoxyethane,
(MeO-CH2CH2-O-CH2CH2-O-Me) diglyme
NaBH4 : H2O, MeOH, EtOH, most commonly i-PrOH
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Selectivity Partial reduction
RCOOH -----> RCHO
Reduction of one group in the presence of another.
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ReductionReducing agent
LiAlH4 NaBH4
RCHO RCH2OH + +
RCOR RCHOHR + +
RCOCl RCH2OH + +
RCOOR` RCH2OH+R`OH + -
RCOOH RCH2OH + -
RCONR2 RCH2NR2 + -
RC= N RCH2NH2 + -
RX RH + -
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DIBAL-H allows the addition of one equivalent of hydride to an ester
Replacing some of hydrogens of LiAlH4 with OR groups decreases the reactivity of the metal hydride
Partial reduction
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Reaction Conditions
Metals commonly used: lithium, sodium, potassium, zinc, magnesium, tin, iron Solvents
NH3 (b.p. - 33˚) for alkali metals (and Calcium); "Birch Reduction"
low molecular wt. amines
hexamethylphosphoramide (HMPA)
ether, THF, dimethoxyethane (DME) -- dilute solns.
crown ether complexes
ether, toluene, xylene -- suspensions Proton Source
ethanol -- Present in reaction medium
isopropanol -- added with compound to be reduced
t-BuOH -- added during isolation.
H2O
Amalgams with Hg (Free metal and HgCl2): Magnesium, Aluminum, Zinc, Tin
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8.4 Reduction of functional groups
8.4.1 Reduction of alkenes
alkene alkane
– Catalytic hydrogenation (Pt or Pd)
Cat
H2
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Selective reduction in the presence of esters and ketones.
Ph
O
H2 PtPh
O
H2 Pd/CCHO CHO
H2
O
(Ph3P)3RhCl
O
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8.4.2 Reduction of alkynes
Alkyne alkene alkane• Using lindar’s catalyst (palladium on calcium carbonate tre
ated with lead acetate and poisoned with quinoline), the Z-a
lkenes can be obtained for non-terminal alkynes
Cat
H2
Cat
H2
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8.4.3 Reduction of aldehydes and ketones
8.4.3.1 Reduction to alcohols MPV (Meerwein-Ponndorf-Verley reaction) reduction
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Clemmensen reduction (strong acidic condition)
8.4.3.3 Reduction of ketones to methylene group
R
OH+
R
Zn ZnZn
R
OH
R
Zn ZnZn
H+R
OH2+
R
Zn ZnZn
-Zn2+
-H2O
R R
Zn Zn
2H+
-Zn2+
RH2C R
Zn
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