Redução

Reductions
General Resource: Trost, Comp. Org. Syn. 1991, vol 8 March,1992, chap 19 Carey and Sundberg, vol B, Chap 5 Smith, Organic Synthesis, Chap 4 Material organized (roughly) by transformation

From March, 1992, p1208

H

H

5% Pd/C Supported metal b/c Pd $$$ and tends to clump.

Two general classes: Transition metal hydrogenation and dissolving metal reduction Hydrogenation: Covered in much more detail in Advanced Synthesis and Catalysis. M H2 General trends: more substituted = slower relationship of pressure:rate often not simple many systems pose fire risk! HOAc/HClO4 AcOH HOH EtOH CH2Cl2 EtOAc NC O

CH3

H2 5% Pd/C O

CH3

no carbonyl reduction H H N Ph H2 5% Pd/C NC N Ph Benzyl survives

10% Pd/C -Same deal as 5%, just more reactive -Often used for hydrogenolysis and more difficult hydrogenations -Mechanism of hydrogenolysis unknown MeO MeO N Ph MeO MeO 90% NH

increasing activity

Olefin isomerization sometimes a problem

H2 10% Pd/C

Note:

O2 + H2

Pd/C

Fire

5% Pd/ BaSO4 and 5% Pd/CaCO3/Pb(OAc)2 (Lindlar's cat.) and 5% Pd/C/quinoline N quinoline

PtO2 (Adams catalyst) reduced activity general hydrogenation cat; more active than Pd/C

H2/PtO2 AcOH/Benzene
8

H

N Ph
8

N 97% Ph

O Cl H2 5% Pd/ BaSO4

O H Raney Nickel (RaNi)

Wasserman, TL, 1988, 4977

alkyne semi-reduction most common use:

H2 Ph Ph 5% Pd/CaCO3/Pb(OAc)2 Ph Ph

-various types available that differ in preparation -sold as 50% wt. dispersion in water -usually wash 5x water, 5x solvent (usually MeOH) -Dry solid is pyrophoric!!! -Remove by filtration under N2 or Ar (pretty good idea for all hydrogenation catalysts)

87% stereospecific complementary method: Na/NH3 97% stereoselective

Ni/Al

NaOH

Ni/H2

+ NaAl(OH)4

slurry has hydrogenation activity without added H2 For some applications, can use Ni/Al in 1M NaOH/MeOH

(more on Na to come)

Raney Ni Applications

Ni2B (Nickel boride) Brown, JACS, 1963, 1004,