Page 395 - Advanced Organic Chemistry Part B - Reactions & Synthesis

368 Reductive Removal of Functional Groups

CHAPTER 5 2 M· R′ ZH
3
3
R 3 C Y + R C H + H Y R C Y R C H + R′ Z Y
Reduction of 2 H 3 3 3
Carbon-Carbon Multiple dissolving metals hydrogen atom donors
Bonds, Carbonyl
Groups, and Other Y = halogen, oxygen substituents, Y = halogen, thio ester
Functional Groups α−to carbonyl groups Z = Sn, Si
There are also procedures that form carbon-carbon bonds. Most of these reactions begin
with an electron transfer that generates a radical intermediate, which then undergoes a
coupling or addition reaction. These reactions are discussed in Section 5.6.

– –
X X X
– · CR R C CR
R 2 C X + M· R C R 2 C 2 or 2 2
2

0
II
X O M· = Na , Ti , Sm II
reductive coupling
Reductive removal of oxygen from functional groups such as ketones and aldehydes,
alcohols, -oxy ketones, and diols are also important in synthesis. These reactions,
which provide important methods for interconversion of functional groups, are
considered in Section 5.7

O HO OH
O
R C CR 2
R CH 2 R R RCH CHR R C CR 2 2
2
R R R
carbonyl methylene carbonyl alkene diol alkene
reductive deoxygenation

5.1. Addition of Hydrogen at Carbon-Carbon Multiple Bonds

The most widely used method for adding the elements of hydrogen to carbon-
carbon double bonds is catalytic hydrogenation. Except for very sterically hindered
alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts
are various forms of transition metals, particularly platinum, palladium, rhodium,
ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on
inert supports such as carbon or alumina (heterogeneous catalysts) and certain
soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity.
Depending upon conditions and catalyst, other functional groups are also subject to
reduction under these conditions.
catalyst
RCH CHR + H 2 RCH CH R
2
2
5.1.1. Hydrogenation Using Heterogeneous Catalysts

The mechanistic description of catalytic hydrogenation of alkene is somewhat
imprecise, partly because the reactive sites on the metal surface are not as well