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Lu et al. Sci China Chem
a)
a)
Table 2 Substrate scope of various secondary alcohols (color online)
Table 3 Alkylation of 2-phenylethanol with diverse primary alcohols
(color online)
t
a) With 5d (0.05 mol%), 2-phenyl ethanol 1 (1 mmol) and BuONa
(3 mmol) in 1 mL primary alcohol at 150 °C under N2 atmosphere for 24 h,
yields are determined by H NMR analysis with 1,3,5-trimethoxybenzene
as an internal standard. b) Add 5 mmol primary alcohol and use 1.5 mL
p-xylene as solvent.
a) With bis-NHC-Ir 5d (0.1 mol%), secondary alcohol 1 (1 mmol) and
1
tBuONa (3 mmol) in 1 mL methanol at 140 °C under N2 atmosphere for
24 h, yields are determined by 1H NMR analysis with 1,3,5-trimethox-
ybenzene as an internal standard. b) Using mesitylene as an internal stan-
dard. c) With 5d (0.05 mol%), tBuONa (2 mmol). d) With tBuONa
t
(6 mmol). e) With 5d (0.2 mol%), BuONa (12 mmol). f) Yields are de-
explored. As depicted in Figure 3a, when the methyl sub-
stitutes on NHC ligands was replaced by phenyl groups, the
CCO–Ir bonds were slightly increased (1.874 Å vs. 1.882 Å
for 5a vs. 5c), and consequencely the CO ligands could be
much more easily dissociated, leaving a vacant position for
the later transformation. An unsymmetric crystal structure
was observed with complex 5d, in which the lengths of two
CCO–Ir bonds were slightly different (1.884 Å and 1.888 Å)
but both are longer than those observed in complexes 5a and
5c (Figure 3a), highlighting that one of CO ligands might be
more easily dissociated from the Ir center and facilitating the
initial step of dehydrogenation of alcohols. Furthermore, the
percent buried volumes (%Vbur) and steric maps of com-
plexes 5a, 5c and 5d were caculated by SambVca 2.1 (Figure
3b) [43–45]. As we expected, the steric bulkiness of com-
plexes 5c and 5d (%Vbur = 51.6% and 51.1%) is much more
hindered than that of analogue 5a (%Vbur = 47.9%), which
might be another key issue to affect catalytic performance
along with the electronic effect of NHC ligands.
In order to further understand these observations from the
crystal structures, density functional theory (DFT) calcula-
tions were performed to elucidate the electronic nature of the
iridium carbonyl complexes bearing different types of NHC
ligands. The calculations indicated that the ionization po-
tentials (IP, in eV) of Ir centers follow the sequence of 5a
(11.34 eV) > 5c (10.84 eV) > 5d (10.45 eV). The lower IP of
the iridium centers in complexes 5c and 5d with N-aryl
substituents than that in complex 5a implied that the gen-
eration of active Ir(III)-H species from NHC-Ir(I) after de-
hydrogenation is more feasible [36]. Recalling the strong
trans-effect [46] observed in the crystal structure of complex
5d, the ready dissociation of one CO ligand may facilitate the
first dehydrogenation step. Therefore, the yields of product 3
were increased along with the trend of 5a < 5c < 5d (Figure
termined by GC-MS with mesitylene as an internal standard.
resulted in 1- and 2-isomers 40 and 41 in yields of 95% and
82%, respectively. This suggests that bulkiness hardly
hampered the methylation process. When the heterocyclic
substrate 1-(benzo[b]thiophen-5-yl) ethan-1-ol was studied,
a moderate yield was obtained (42, 52%). Probably due to its
low boiling points, the aliphatic secondary alcohol, isopropyl
alcohol, afforded a slightly low yield of the methylated
products (43, 56%). For the β-substituted substrates, quan-
titative yields were obtained with 1-phenylpropanol (44)
while a 90% yield was observed in its analogue with a longer
carbon chain (45). Cyclic tetrahydro-1-naphthol (46) was
converted into the methylated products in 55% yield, and
acyclic 3-amyl alcohol gave the 47 in 83% yields by slightly
increasing the amount of base.
Inspired by the excellent results obtained in the β-methy-
lation of diverse alcohols with methanol, more general β-
alkylation with other primary alcohols instead of methanol
was investigated (Table 3). Although possible side-products
are unavoidable due to the possible Guerbet reaction [42],
45% and 39% yield of β-alkylated products (48 and 49)
could still be achieved when ethanol or iso-butanol were
applied instead of methanol at 150 °C. Delightedly, all se-
lected benzyl alcohols and even heterocyclic analogues were
also suitable alkylation reagents. Good to excellent yields
were observed for these substrates (62%–88%, 50–53), fur-
ther indicating the applicability of the protocol.
Crystals suitable for single-crystal X-ray diffraction were
obtained by slow evaporation of the dichloromethane solu-
tion of bis-NHC-Ir complexes 5c or 5d. Combining the
crystal data of complex 5a in the previous study [32], the
possible ligand effects on catalytic performance were then