Hydrogenation Reactions - Carbohydrates: Tools for Stereoselective Synthesis - page 164

Table 8.3

Hydrogenation of dimethyl itaconate 3b using diphosphite ligands 29 - 34. a)

H 2

CO 2 Me

CO 2 Me

*

MeO 2 C

MeO 2 C [Rh]/L*

4b

3b

Entry Ligand Conversion (%) ee (%)

1 29b 12 22 (R)

2 30b 28 64 (R)

3 31b 90 90 (R)

4 31c 82 85 (R)

5 31d 100 97 (R)

6 31e 50 50 (S)

7 31f 46 52 (R)

8 31g 100 90 (S)

9 31h 100 92 (R)

10 32b 100 2 (R)

11 33b 87 67 (R)

12 34b 73 29 (R)

13 b)

31d 100 99 (R)

a) Conditions: catalyst: 0.01 mmol, L/Rh = 1 : 1, substrate/Rh = 100, P H2 = 5 bar, T = 5 °C, t = 8 h.

b) T = 5 °C, P H2 = 30 bar, t = 4 h.

allofuranoside ligand 32d, which has o -trimethylsilyl substituents on the biphenyl
moieties (entries 5 and 13). It was also found that deoxygenation of position 6
significantly increased the activity (entries 3-12 versus 1 and 2). This set of ligands
was also applied in the Rh-catalyzed hydrogenation of methyl N -acetylaminoacr-
ylate ( 1d ) and methyl (Z) -( N )-acetylaminocinnamate ( 1b ). The results followed the
same trend as those for dimethyl itaconate ( 3b ), but the activities were somewhat
higher [25c].
Diphosphite ligands 35 - 38 with C 2 -symmetry and a tetrahydrofuran backbone,
related to the diphosphinites 25 and 26, have been synthesized starting from
d-glucosamine and d-glucitol (Figure 8.5). Cationic rhodium complexes of 35 - 38
were prepared by reacting [Rh(COD) 2 ]BF 4 with the respective ligands. The struc-
ture of complexes with 35 and 37 were determined by single-crystal X-ray diffrac-
tion. These rhodium complexes were tested in the asymmetric hydrogenation of
methyl acetamidoacrylate ( 1d ). The conversions and enantioselectivities were
much lower than the obtained with the corresponding diphosphinites 25 and 26,
and were mainly influenced by substitution in the biphenyl moiety and by the
configuration of the remote centers in positions 2 and 5 of the tetrahydrofuran
ring [26].
Although it has been generally accepted that bidentate ligands are the most
appropriate for metal-catalyzed enantioselective hydrogenation, in recent years it