Tetrahedron Letters
A novel, C2-symmetric, chiral bis-cyclosulfinamide-olefin tridentate
ligand in Rh-catalyzed asymmetric 1,4-additions
Li Zhang a, Mingchao Tan a, Lihong Zhou b, Qingle Zeng a,
⇑
a State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology), Chengdu University of Technology, College of
Materials, Chemistry & Chemical Engineering, 1#, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, PR China
b Chengdu University of Technology, College of Environment and Ecology, 1#, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A C2-symmetric, chiral bis-cyclosulfinamide-olefin ligand composed of two 1-oxo-2,3-dihydro-1,2-ben-
zisothiazole moieties with rigid skeletons and a conformationally flexible butenylene chain is disclosed
for the first time. HRMS and 1H NMR analyses verify that the in situ-generated complex of the ligand
and [Rh(C2H4)2Cl]2 possesses a rhodium (I) center coordinated to the tridentate ligand via two sulfinyl
moieties and a CdbndC bond. The chiral ligand provided extremely high enantioselectivity (up to >99%
ee) in the Rh-catalyzed asymmetric 1,4-additions of arylboronic acids to cyclohexenone and cyclopen-
tenone. The tridentate ligand gave much higher enantioselectivity than the analogous chiral bidentate
ligands.
Received 20 March 2018
Revised 31 May 2018
Accepted 3 June 2018
Available online xxxx
Keywords:
Asymmetric catalysis
Chiral tridentate ligand
Bis-cyclosulfinamide
Ó 2018 Elsevier Ltd. All rights reserved.
1-Oxo-2,3-dihydro-1,2-benzisothiazole
Asymmetric 1,4-addition
Olefins have a long history as ligands, including in the very first
transition metal alkene complex, Zeise’s salt,1 and in 2004, chiral
di-olefin ligands were pioneered by Hayashi2 and Carreira3 and
used as a highly efficient chiral ligands in asymmetric catalysis.
Since then, great progress has been made with chiral olefin ligands
containing heteroatoms,4 especially in rhodium-catalyzed asym-
metric reactions. Among these ligands, sulfur-containing olefin
ligands, mainly including chiral sulfoxide-olefin ligands5 and chiral
sulfinamide-olefin ligands,6 demonstrate high asymmetric induc-
tions due to the strong and thus efficient coordination between
the soft acid, rhodium (I), and soft bases, the olefin and sulfur of
the chiral sulfoxides and sulfinamides. To date, most of the
reported sulfinyl-olefin chiral ligands are C1-symmetric bidentate
ligands (Fig. 1).4
A number of C2-symmetric chiral ligands, such as DIPAMP,
BINAP, DuPhos, and dialkyl tartrates, are privileged ligands and
have been shown to provide high enantioselectivity, and they even
have been applied in the industrial production of chiral chemicals.
It has been well established that C2 symmetry actually improves
the enantioselectivity of a complex by reducing the number of
transition states with unique geometries.7 To date, only two papers
have reported C2-symmetric sulfinamide and olefin ligands. One
class is Chen’s C2-symmetric chiral sulfur-containing diamides
with a sulfinyl group and two allyl or cinnamyl groups (Fig. 1),8
and these compounds are regarded as sulfinamide-olefin bidentate
chiral ligands with a redundant olefin moiety. The other class is
Prasad’s C2-symmetric bis-sulfinamides, which includes two kinds
of ligands (Fig. 1).9 One of their kinds of ligands has two sulfinyl
groups and two allyl groups, which act as chiral ligands in rho-
dium-catalyzed 1,4-additions (Fig. 1). The other kind has two sul-
finyl groups and a cyclohexene moiety, which are not suitable for
that reaction (Fig. 1).9
Although no efficient bis-sulfinamide-olefin tridentate chiral
ligands have been reported, a small number of chiral tridentate
ligands that are highly efficient in asymmetric catalysis have been
reported.10 For example, pyridine-2,6-bis(oxazolines) (abbreviated
Pybox),10a chiral tridentate ligands, show unique versatility for a
number of catalytic asymmetric reactions and can bind a wide
range of metals. Recently, additional chiral tridentate ligands
have been reported for Rh-catalyzed reactions, such as chiral
diene-phosphine tridentate ligands10c and tris-sulfoxide ligands.10d
Recently, we developed cyclic sulfinamide-olefin and N-aryl
sulfinamide-olefin chiral ligands that are highly efficient in rho-
dium-catalyzed 1,4-additions (Fig. 1).11 In view of the readily avail-
able highly enantiopure cyclic sulfinamides in our laboratory12 and
based on our previous studies on asymmetric catalysis,11,13 we
envisioned that a novel C2-symmetric dicyclosulfinamide-olefin
(systematic name: 1,4-di(1-oxo-2,3-dihydro-1,2-benzisothiazolyl)
but-2-ene) (abbreviated DICSO) tridentate chiral ligand (Fig. 1)
⇑
Corresponding author.
0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Zhang, Li
