Organometallics
Communication
a
a
Table 2. Substrate Scope for Norbornene Derivatives
Table 3. Substrate Scope for Acrylamide Derivatives
a
Reaction conditions: 1a (0.10 mmol), 2a−i (0.20 mmol),
Ir(COD)2OTf (5 mol %, 0.0050 mmol), L8 (6 mol %, 0.0060
mmol), DCE (0.50 mL), 80 °C, 14 h. Isolated yields are reported. 2b
(0.50 mmol). 2c (0.50 mmol).
b
c
(Scheme 1c).4 Since the first example of asymmetric
hydrovinylation reported in 1972,5 catalytic asymmetric
hydrovinylations6 of styrenes,7 conjugated alkenes,8 and
strained alkenes9 with ethylene have been reported. In
contrast, asymmetric hydroalkenylations involving substituted
alkenes are less developed. In such an enantioselective alkene
dimerization process, the two alkenes must chemoselectively
react with each other. In addition, the regioselectivities on both
alkenes and the enantioselectivity of the process must be
effectively controlled. Currently, catalytic asymmetric hydro-
alkenylations using substituted alkenes have been developed
mainly for conjugated alkenes such as styrenes10 and 1,3-
dienes.11 Recently, Hirano described a Ru-catalyzed asym-
metric hydroalkenylation of 2,5-dihydrofuran with acrylates to
afford the coupling products with good enantioselectivity.12
We report here an iridium-catalyzed highly enantioselective
coupling of unconjugated alkenes with acrylamides to afford
products containing three stereocenters (Scheme 1d).13,14 In
the presence of a coordinating group, the low-valent cationic
iridium species selectively cleaves the vinyl C−H bond.15,16
Further reaction of the vinyliridium intermediate with
norbornene generates the hydrovinylation products with high
chemo-, regio-, and stereoselectivities. This mechanism is
distinct from those in previous reports on hydroalkenylation of
conjugated alkenes.10−12 The catalytic system is applicable to a
variety of acrylamide and norbornene derivatives. Further
computational studies revealed the origin of the enantiose-
lectivity.
a
Reaction conditions: 1a−l (0.10 mmol), 2e (0.20 mmol),
Ir(COD)2OTf (5 mol %, 0.005 mmol), L8 (6 mol %, 0.0060
mmol), DCE (0.50 mL), 80 °C, 14 h. Isolated yields are reported.
We began our study by testing the hydroalkenylation
between α,β-unsaturated amide 1a and benzonorbornadiene
(2a) in the presence of Ir(COD)2OTf and a series of chiral
ligands (Table 1). When Du-Phos (L1), DIOP (L2), or a
phosphoramidite (L3) was used, no hydroalkenylation product
was observed. However, with Josiphos (L4) or SDP (L5) as
the ligand, the desired product was observed, although in low
yield with moderate enantioselectivity. The product could be
obtained in high yield and enantioselectivity with BINAP (L6)
as the ligand. Further variations of the ligand backbone
indicated that Difluorphos (L8) provided even higher ee in the
hydroalkenylation reaction. Importantly, only the Z isomer was
obtained for the hydroalkenylation product. This is supportive
of a directed C−H cleavage mechanism, as only the C−H
bond cis to the coordinating group was cleaved (vide infra). In
addition, the high Z selectivity indicates minimal alkene
isomerization in the catalytic system, although an iridium
hydride intermediate was involved.
The substrate scope of the asymmetric hydroalkenylation
reaction was subsequently investigated (Table 2). The
reactions of norbornene and norbornadiene afforded the
B
Organometallics XXXX, XXX, XXX−XXX