Chemistry Reference
In-Depth Information
3.3.14.2 Copper
Copper(II) catalysed enantioselective decarboxylative aldol-type addition of malonic acid
hemithioesters to aldehydes in the presence of tartaric acid-derived bisbenzimidazole and
an achiral base was examined. The use of DBU (1) as an achiral base resulted in low
enantioselectivity [69].
3.3.14.3
Iridium
A combination of bis(iridiumcyclooctadienyl chloride) [Ir(COD)Cl]
2
, a chiral phosphor-
amidite ligand, and DBU (1) as a base in THF effects the iridium (I) catalysed intermolec-
ular allylic amidation of ethyl allyl carbonates with soft nitrogen nucleophiles under
completely salt-free conditions [70]. The reaction is quite general, accommodating a wide
variety of substrates and nucleophiles, and proceeds with excellent regio- and enantios-
electivities to afford the branched N-protected allyl amines.
3.3.14.4 Molybdenum
The reaction of molybdenumcyclopentadienyltricarbonyl chloride [CpMo(CO)
3
Cl] with
optically active amidines affords separable diastereoisomers of the Cp(CO)
2
Mo-amidinato
complexes, which could act as chiral catalysts, by fractional recrystallization [71]. The
molybdenum configuration is equilibrated at 70
C in acetone.
3.3.14.5 Nickel
DBU (1) is screened as base co-catalyst for enantioselective Michael additions of mal-
ononitrile catalysed with the aqueous complex of 4,6-dibenzofurandinyl-2,2
0
-bis(4-phe-
nyloxazoline) and nickel perchlorate hexahydrate [72].
3.3.14.6
Palladium
Phosphorous-containing amidine was prepared through several steps from L-valine and
evaluated as a new ligand for asymmetric palladium (Pd) catalysed allylic alkylation of 1,3-
diphenylprop-2-enyl acetate and pivalate [73]. The results with the nucleophile derived
from dimethyl malonate are summarized in Table 3.5 [73a]. Excellent asymmetric
inductions up to 95% ee were achieved along with an efficient conversion.
A new class of chiral amidine-phosphine and -sulfide hydrid ligands with a variety of
modifications is used for the palladium mediated allylic substitutions of both acyclic and
cyclic compounds [74] (Figure 3.5). High levels of asymmetric induction were achieved for
both substrates.
Some ferrocenylphosphine-amidine ligands (Figure 3.6) with central and planer chirality
were prepared and their efficiency and diastereomeric impact in the palladium catalysed
asymmetric allylic substitution were examined [75]. Up to 96% ee with 98% yield was
achieved by the use of a ligand with a methyl-substituted ligand.
DBU (1) is often screened in the utility as co-catalyst (or base) in the palladiummediated
coupling reactions [76].
3.3.14.7 Rhodium
DBN (2) is used as a strong, sterically hindered base in the asymmetric hydrogenation of
acetophenone and styrene by a combination of rhodium (I) complex and chiral ligands
