Hydrogenation Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 167

D- xylo

D- xylo

O

O

O

P

O O

P

O

P O

O P O

O

O

O

O

O

O

OC 14 H 29

48a 49

O O

t Bu O O t Bu

R 1

R 1

O

=

O

t Bu t Bu

a R 1 = t Bu
b R 1 = cyclohexyl

c

Figure 8.7 Diphosphite ligands 48 and 49.

corresponding amine but did not achieve any enantioselectivity. However, when
Bu 4 NI was used as additive the ee increased to 46%. Iodine also improved the
enantiomeric excess, but not as much as Bu 4 NI. Other additives such as phthalim-
ide and benzylamine resulted in deactivation of the catalytic system. The presence
of bulky tert -butyl groups in the ortho -positions of the biphenyl moiety has an
extremely positive effect on enantioselectivity, as had been previously found in the
hydrogenation of dehydroamino acids derivatives [20, 25]. These moderate results
of enantioselectivity and the sensibility of the diphosphite ligands to hydrolysis
could explain the limited use of diphosphites in the hydrogenation of imines.
A more recent application of diphosphites derived from a carbohydrate is the
stabilization of metal nanoparticles [32]. Palladium, rhodium, and ruthenium,
nanoparticles have been prepared in the presence of substoichiometric amounts
of carbohydrate-derived diphosphites through reduction of organometallic
complexes under H 2 pressure. Again, the modular character of the carbohydrate
diphosphite derivatives offers the possibility of tuning the environment at the
surface of the nanoparticles through electronic or steric modifications. These vari-
ations can influence the catalytic properties when the metal nanoparticles are
active in catalysis. Ruthenium nanoparticles synthesized from [Ru(COD)(COT)]
under 3 bar of H 2 , in the presence of xylose-derived diphosphite 29a (Figure 8.4),
48a, and 49 (Figure 8.7) (Ru/L 1 : 0.1), are active in arene hydrogenation. Isolated

as black powders, TEM analysis reveals the presence of small particles 1-4 nm in
size, which have been tested in the hydrogenation of 3-methylanisole ( 50 ) and
2-methylanisole ( 51 ) (Scheme 8.7).
Interestingly, modification of the diphosphites allows control of the size and
dispersity of the nanoparticles. Thus, modification of the diol moiety and, even
more significantly, the introduction of a long lipophilic chain stabilize smaller and

=