DOI: 10.1002/chem.201501914
Communication
&
Asymmetric Catalysis
Synthesis of Air- and Moisture-Stable, Storable Chiral Oxorhenium
Complexes and Their Application as Catalysts for the
Enantioselective Imine Reduction
Braja Gopal Das, Rajender Nallagonda, Dhananjay Dey, and Prasanta Ghorai*[a]
been attempted. However, the obtained enantioselectivity re-
Abstract: Air-/moisture-stable, crystalline, and storable
chiral salicyloxazoline based oxorhenium(V) complexes
have been synthesized and their catalytic application for
the asymmetric reduction of ketimines using hydrosilane
as hydride source is disclosed. A broad substrate scope,
high yields, and excellent enantioselectivities (up to 99%)
are attained. Furthermore, the syntheses of enantiopure
a-amino esters, g- and d-lactams, and isoindolinones have
also been carried out using this methodology. Finally, the
method has been applied to synthetic targets of pharma-
ceutical relevance, such as R-(+)-salsolidine and R-
(+)-crispine A.
mained very low. Only the cyanobis(oxazoline) ligand-based
oxorhenium complexes developed by Toste and co-workers
have to date found success in asymmetric imine reductions.[8]
Salicyloxazoline ligand-based oxorhenium(V) complexes
have found superior reactivity for hydrosilane activation[9a,b]
and oxidative cyanation of tertiary amines.[9c] However, their
asymmetric variants have never been developed. Nevertheless,
chiral salicyloxazoline ligands have been utilized in numerous
transition metal-catalyzed enantioselective transformations, af-
fording high levels of selectivity.[10,11] Interest in using this type
of ligand has increased because of their easy synthesis from
natural amino acids.[11a]
In this article, we have embarked on a program aimed to de-
velop non-racemic rhenium(V)–oxo complexes based on salicy-
loxazoline as a monoanionic bidentate ligand. Ligands La–Ld,
having an N,O coordination site, formed bright green metal–
ligand complexes (Scheme 1),[12] which are air/moisture stable
and crystalline at ambient temperature. Furthermore, these
complexes exhibited catalytic activity for the asymmetric re-
duction of ketimines using hydrosilane as a hydride source,
with a broad substrate scope, high yields, and excellent enan-
The development of new chiral catalysts for asymmetric cataly-
sis remains an attractive area of research because of the ever-
increasing importance of enantiopure molecules in pharma-
ceuticals, flavors, fragrances, and agrochemical industries.[1] The
use of transition metal complexes remains central to this
goal.[2] Several excellent chiral catalysts, especially low-valent
transition metal complexes, have been developed for various
asymmetric transformations. However, they are generally air
and/or moisture sensitive. Therefore, many such catalysts are
prepared in situ just before use and are not storable for ex-
tended periods. Recently, high valent oxo–metal complexes,
especially oxorhenium complexes, have gained much attention
as catalysts for various organic transformations due to their
mild and air-/moisture-stable nature.[3,4] Such complexes have
become an attractive and practical alternative to many low-
valent transition metal complexes, even for reduction reac-
tions. In this context, the unique mode of hydrosilane activa-
tion by oxorhenium catalysts emerged as a new area of cataly-
sis.[5] However, the development of suitable chiral oxorhenium
complexes for asymmetric synthesis is evidently a necessary
task to ensure the access to enantiomerically enriched prod-
ucts. To this end, the development of substituted 2-amino al-
cohol-based[6] and salen-based[7] oxorhenium complexes have
[a] B. G. Das, R. Nallagonda, D. Dey, Dr. P. Ghorai
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhauri, Indore By-pass Road, Bhopal-462066 (India)
Scheme 1. a) Synthesis of chiral oxorhenium catalysts; b) ORTEP representa-
tion of catalyst 3a with additional CH2Cl2 solvent molecule. Thermal ellip-
soids are drawn at 50% probability.[12]
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2015, 21, 12601 – 12605
12601
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