Hydroformylations, Hydrovinylations, and Hydrocyanations - Carbohydrates: Tools for Stereoselective Synthesis - page 194

such as methyl groups led to poor enantioselectivity. Overall, the stereoselectivities
obtained with ligand 49b under optimized conditions did not exceed 51% ee.

CHO

catalyst (0.2 mol%): Rh(COD)BF 4 /ligand* (ratio 1:2)
H 2 , CO, solvent

*

selectivity for
branched product 2 >80%

Me

X X

2

1

D- gluco

benzene 20% 10% ee

X = H

a

O

Ph hexane

< 5% 10% ee

X = OMe

O

O

OPh

O

benzene 43% 38% ee

X = H

O

Ar 1 2 P

Ar 1 2 P X = H hexane

THF

53% 51% ee

49

b

X = H 71% 12% ee

Ar 1 = Ph

a

hexane 73% 33% ee

X = OMe

Ar 1 = 3,5 - (CF) 3 C 6 H 3

b

c hexane <5%% 25% ee

Ar 1 = 3,5-F 2 C 6 H 3

X = OMe

c

Ar 1 = 3,5-Me 3 C 6 H 3 d

d hexane <5% <2% ee

X = OMe

Scheme 9.8 Asymmetric hydroformylation of vinyl-naphthalenes using carbohydrate
diphosphinite ligands 49.

Diéguez and coworkers explored furanosidic xylo - and ribo ligands 50 and 51,
which are the diphosphinite counterparts to diphosphites 4 and 11 [17] (Figure
9.3). For ligands 50 and 51, stereoselectivities above 50% ee were only observed
with 4-methoxystyrene, while styrene itself and its electron-deficient 4-fluoro deriv-
ative gave almost racemic products. Surprisingly, the absolute configuration of the
major enantiomer was the same for both ligands, irrespective of their furanoside
scaffold, which is in striking contrast to the findings with the corresponding
diphosphites 4 and 11, which act as pseudo-enantiomers. With vinyl-naphthalene
substrates both ligands gave stereoselectivities of 53% ee.

catalyst (0.14 mol%): Rh(acac)(CO) 2 /ligand* (ratio 1:1)
H 2 , CO, toluene

selectivity for
branched product >80%

D- xylo
O

D- ribo

PPh 2

O

Ph 2 P O

Ph 2 P

O

O

O

O

O

O 51 O

50

Ph 2 P Me

Me

Me

Me

CHO CHO CHO CHO

2 2 2 2

∗ Me ∗ Me

∗ Me ∗ Me

X X X X

X = H 84% 4% ee (R)

X = H 48% 53% ee (S)

X = H 85% 10% ee (R)

X = H 51% 53% ee (S)

X = F 100% 2% ee (R)

55% ee (S)

X = OMe 29% 53% ee (+)

X = F 100% 4% ee (R)

54% ee (S)

X = OMe 55%

X = OMe 31% 53% ee (+)

X = OMe 56%

Figure 9.3 Asymmetric hydroformylation of vinyl arenes with carbohydrate diphosphinite
ligands 50 and 51.