COMMUNICATIONS
between aryl halides and primary and secondary
aliphatic alcohols.[14]
Table 1. Initial results of the cross-coupling between 2-
bromonapthalene (3a) and MeOH (4a).[a]
We have continuously investigated a series of
nickel complexes supported by monoanionic tridentate
ligand as catalysts.[15] Very recently, we have demon-
strated the well-defined β-aminoketonato-based O,N,P-
coordinated tridentate nickel complex 1a serves as a
catalyst in the highly Markovnikov-selective hydro-
boration of vinylarenes with bis(pinacolato)diboron
(B2pin2, 2) (Scheme 1).[15d,16] This study encouraged us
to explore applications using systems comprised of a
nickel complex (1), 2, and a base. During attempts to
expand our previous work, we examined the borylation
reaction using aryl halides and discovered that the
O,N,N-coordinated nickel complex 1b showed high
catalytic activity for the cross-coupling of aryl halides
with primary and secondary aliphatic alcohols without
the need for a phosphine ligand or a photoredox
catalyst.
Entry Ni cat. 5a
6aa
7a
Recovery of 3a
(%)[b] (%)[b] (%)[b] (%)[b]
1
2
3
4
1a
1b
1c
1d
3
3
6
1
<1
48
<1
2
47
29
55
38
30
<1
19
36
[a] Reaction conditions: 3a (0.50 mmol), 2 (1.00 mmol), nickel
catalyst 1 (5 mol%), KOAc (1.50 mmol), toluene (3 mL), 4a
We initially examined the use of nickel pincer
catalyst 1a in the reaction between 2-bromonapthalene
(3a) and 2 using KOAc as a base in toluene/MeOH
°
(1 mL), 60 C, 24 h.
[b] The yields of 5a, 6aa, and 7a as well as the recovery of 3a
were determined by GLC analysis using undecane as the
internal standard.
°
(4a) at 60 C for 24 h (Table 1). Borylated product 5a
was obtained in only 3% yield, together with
naphthalene (7a) in 47% yield, while 3a was recov-
ered in 30% yield (entry 1). In contrast, using 1b, we
were surprised to find that 48% of ether 6aa was a catalyst for the coupling reaction of 3a (electrophile)
formed in addition to 3% of 5a (entry 2). Changing the with 4a (nucleophile).
ligand scaffold from a β-aminoketonato-based ligand
Various reaction parameters were examined, and
to a β-diketiminato-based ligand suppressed the cata- the results are summarized in Table 2. In this reaction,
lytic activity of the coupling reaction (entries 3 and 4). the base plays a critical role for the reaction efficiency.
These results prompted us to explore the use of 1b as The use of KOt-Bu provided the desired product (6aa)
in 66% yield while NaOt-Bu gave 6aa in only 57%
yield (entries 2 and 3). Other bases such as NaOMe,
K2CO3, Cs2CO3, K3PO4, KF, and Et3N were not
effective in the coupling reaction (entries 4–9). Next,
we examined the effect of the nickel catalyst using
KOt-Bu as the base (entries 10–15). The nickel catalyst
greatly affected both the reactivity and selectivity of
this reaction. Nickel pincer catalysts 1a, 1c, and 1d
did not effectively catalyze the reaction and predom-
inantly afforded the borylated product 5a (entries 10–
12). Furthermore, NiCl2(PPh3)2, NiCl2(PCy3)2, and
NiCl2(dppp)
(dppp=1,3-bis(diphenylphosphino)
propane) did not afford 6aa and 5a was formed in
good yield (entries 13–15).[17] At 25 C, the reaction
proceeded slowly to afford 6aa in 21% yield (entry 16)
°
°
and when the reaction was carried out at 80 C, the
yield of 6aa was 62% (entry 17). The use of 1.2
equivalents of 2 led to a lower conversion (entry 18).
In the absence of 4a, 6aa was not formed and 5a was
formed in 22% yield (entry 19). By way of compar-
ison, reactions without KOt-Bu or 1b were also carried
out and product formation was not observed in either
case (entries 20 and 21).
Scheme 1. Our previous work using nickel complex 1a
supported by monoanionic tridentate ligands and CÀ O coupling
reaction using nickel complexes 1 (this work).
With the optimized reaction conditions in hand
(Table 2, entry 3), we examined the reaction of 3a
Adv. Synth. Catal. 2021, 363, 1625–1630
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