Inorganic Chemistry
Article
problem, a redistribution reaction with R3E and EX3 is often
employed. It is well-known that such a redistribution reaction
can be observed in heavy-main-group chemistry such as
antimony and organobismuth in group 15 elements.28 On the
other hand, there are few reports about the redistribution
reaction with arsenic.29 Therefore, we considered that AsBr3
could be converted to bromoarsine derivatives (R2AsBr)
through redistribution reactions with AsR3. We selected
tri(p-tolyl)arsine (1) as a reactant for the redistribution
reaction. A diethyl ether solution of AsBr3 (1 equiv) and 1
(2 equiv) was refluxed overnight, and (p-tolyl)2AsBr was
obtained in 69% yield (Figure S1), which was determined by
1H NMR integration.30 However, we failed to isolate the target
product (p-tolyl)2AsBr because the starting materials and
byproducts were not sufficiently removed. A more versatile
design for arsenic ligands will be attained by using the
redistribution reaction, although there is still a drawback in the
isolation step.
Finally, the obtained A3-type arsenic ligands were utilized for
palladium-catalyzed copper-free Sonogashira cross-coupling
reaction. It is a powerful tool for substrates sensitive to the
presence of copper. For example, in the case of structural
modification of metal-centered-porphyrins, copper-free Sono-
gashira cross-coupling reaction is necessary because trans-
metalation from the centered metal to copper could occur in
porphyrins to give contamination of the byproducts.31
Entering the 2000s, some chemists reported that the weaker
σ donation of the arsenic ligand can improve the efficiency of
the coupling reaction compared with phosphorus ones.3 AsPh3
was the only applicable ligand in those reports because it is one
of the few commercially available arsines. We thus applied the
A3-type arsenic ligands obtained in the present work to the
copper-free Sonogashira cross-coupling reaction of 515-bis-
(ethynyl)-substituted zinc(II) porphyrin derivative (Zn-POR;
Table 5). For the reaction conditions, refer to the literature
procedure,3a and details are described in the Supporting
Information. In entry 1, the target 5,15-diphenyl-10,20-
bis(phenylethynyl)-substituted porphyrin (Zn-POR-PA) was
obtained with PPh3 in low yield (12%); small amounts of free-
base porphyrin were concomitantly obtained. On the other
hand, no side reactions were observed in the case of AsPh3
(entry 2). This result exhibited that the weaker coordination
ability of arsenic suppressed decomplexation of the Zn ions
from the porphyrin. In entries 3−6, the yields of Zn-POR-PA
from the ligands possessing electron-donating groups were
higher than those from arsenic ligands containing an electron-
withdrawing group such as a trifluoromethylphenyl one. This is
probably because the rate-limiting step in copper-free
Sonogashira cross-coupling reaction is an oxidative addition
in the catalytic cycle, and the electron-rich ligands lowered the
activation energy. In entries 9 and 11, the bulky ligands gave
quite low yields (up to 5%), suggesting that the steric
hindrance prevented coordination to the palladium center. In
fact, we confirmed that the mesityl-group-substituted ligand 10
did not coordinate to palladium(II) through the reaction with
cis-PdCl2(PhCN)2. To understand the relationship between
the steric hindrance and catalytic activity, single-crystal X-ray
diffraction analysis was carried out for the palladium dichloride
complexes of 1−5. The results are summarized in Tables S1−
S8. The Tolman ligand cone angles of PdCl2(ligand)2 (ligand
= AsPh3, 1, 2, 3, 4, and 5) were 145°,32 147°, 173°, 142°, 179°,
and 151°, respectively. This result implies that a less sterically
hindered arsenic ligand tended to show higher catalytic activity
when the electron-donating ability is the same. The screening
that we conducted here successfully demonstrates the
structure−catalytic activity relationships between monodentate
arsenic ligands.
CONCLUSION
■
We optimized the reaction conditions for syntheses of AsX3 (X
= Br, I) by changing the molar ratio of hydrohalic acid,
reaction temperature, reaction time, and extraction solvents to
AsX3. AsBr3 was employed as the starting material to
synthesize monodentate arsenic ligands because of its higher
reactivity and solubility to aprotic solvents in comparison with
those of AsI3. The substitution reaction with nucleophiles
readily provided monodentate arsenic ligands possessing
various substituents such as aryl, alkyl, electron-withdrawing,
electron-donating, bulky, and heteroaryl groups. Finally, the
monodentate arsenic ligands were applied for the palladium-
catalyzed copper-free Sonogashira cross-coupling reaction of
Zn-POR with bromobenzene. Upon screening of the arsine
ligands, the coupling reaction with tri-p-anisylarsine 3 afforded
the highest yield of Zn-POR-PA among the prepared arsenic
ligands. In the present work, we demonstrated that AsBr3 is a
proper starting material for safely accessing various mono-
dentate arsenic compounds and believe that it has the potential
for accessing the novel arsenic ligands utilized in outstanding
transition-metal-catalyzed reaction systems.
Table 5. Copper-Free Sonogashira Cross-Coupling Reaction
in the Reaction System with Porphyrin
entry
ligand
yield (%)
1
2
3
4
5
6
7
8
PPh3
AsPh3
12
23
25
16
35
30
n.d.
n.d.
5
1
2
3
4
5
6
7
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
■
sı
Experimental procedures, X-ray crystallographic data,
9
10
11
a
8
13
n.d.
Accession Codes
10
tallographic data for this paper. These data can be obtained
a
The palladium complex of tricyclohexylasrsine was used instead of
Pd2(dba)3·CHCl3 and ligand.
D
Inorg. Chem. XXXX, XXX, XXX−XXX