1000
Chem. Pharm. Bull. 65, 1000–1003 (2017)
Vol. 65, No. 11
Communication to the Editor
and free ethanol molecules were observed in the 1H-NMR
spectrum [Figure S1(a)].
A Fluorinated Cobalt(III) Porphyrin
Complex for Hydroalkoxylation of
Alkynes
The catalyst 1a was found to be more reactive than 1b in
the hydroalkoxylation of phenylacetylene (2a) with methanol
(Table 1, entries 1 vs. 7). When a mixture of 2a (2.0mmol),
1a (0.05mol%), and HNTf2 (0.18mol%) in methanol (0.9mL,
11 equiv) was heated at 70°C for 4h under aerobic conditions,
acetal 3a was obtained in 97% yield (entry 1).29) The initial
turnover frequency (TOF) was 4000h−1 (t=12min). This value
is comparable to those of gold catalysts and higher than those
of catalysts based on other metals (entry 2).30) Hydroalkoxyl-
ation of 2a in the absence of HNTf2 required higher catalyst
loading (entries 3 and 4). The reaction proceeded under N2
atmosphere, but yielded 3a in a lower yield than that under air
(entry 5 vs. entry 1). Double hydroalkoxylation of 2a at 27°C
(room temperature) afforded acetal 3a in 97% yield (entry 6).
CoIIITPPCl [TPP=5,10,15,20-tetrakis(phenyl)porphyrin] was
less reactive than 1a and 1b (entry 8 vs. entries 1 and 7). The
reaction hardly proceeded with analogous metal complexes
such as FeIIITPPCl or MnIIITPPCl under otherwise identical
conditions (entries 9 and 10).
Richiro Ushimaru,a Takuho Nishimura,a
,a,b
Toshiki Iwatsuki,a and Hiroshi Naka*
a Department of Chemistry, Graduate School of Science, Nagoya
b
University; Chikusa-ku, Nagoya 464–8602, Japan: and Research
Center for Materials Science, Nagoya University; Chikusa-ku,
Nagoya 464–8602, Japan.
Received July 13, 2017; accepted August 28, 2017
A fluorinated cobalt(III) porphyrin complex [Co(TPFPP)-
NTf2·2C2H5OH, where TPFPP=5,10,15,20-tetrakis(penta-
fluorophenyl)porphyrin, Tf=CF3SO2] promotes hydroalkoxyl-
ation of alkynes to give acetals in good to excellent yields. The
acetals can be directly functionalized with nucleophiles in a
one-pot procedure.
Key words hydroalkoxylation; acetal; alkyne; alcohol; cobalt;
porphyrin
Acetals are synthetic equivalents of carbonyl compounds,
reacting with various carbon and heteroatom nucleophiles,
and are widely used as synthetic intermediates in medicinal
chemistry and materials science.1) We here report that a fluo-
rinated CoIII porphyrin complex, Co(TPFPP)NTf2·2C2H5OH
[1a, TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)porphyrin,
Tf=CF3SO2],2) catalyzes Markovnikov hydroalkoxylation of
terminal alkynes to acetals3–5) (Fig. 1). This method compares
well with the earlier procedures catalyzed by Hg,6,7) Pd,8–10)
Pt,9,11,12) Au,13–22) Ir23–26) and Zn.27)
The active pentafluorophenylated CoIII catalyst 1a was
developed by structural modification of 1b used previously
for alkyne hydration2) (Fig. 1). Reaction of tetrakis(penta-
fluorophenyl)porphyrin (H2TPFPP) and cobalt(II) acetate28)
followed by aerobic oxidation in the presence of HNTf2 and
recrystallization from a mixture of chloroform, ethanol, and
hexane afforded 1a in 67% yield. X-Ray structural analysis
of 1a indicated that (1) the cobalt center adopts octahedral ge-
ometry, with porphyrin ligand in the equatorial positions and
ethanol in the apical positions (Fig. 2); (2) the counter anion,
Tf2N−, is spatially separated from the cobalt center [Figure
S2(b)]. The bound ethanol molecules were labile in CD3OD
Fig. 2. ORTEP Drawing of 1a: (a) Top View and (b) Side View
Thermal ellipsoids are drawn at 50% probability. Coordinated ethanol molecules
in (a) as well as the counter anion and hydrogen atoms in (a) and (b) are omitted
for clarity.
Fig. 1. Hydroalkoxylation of Alkynes Catalyzed by Porphyrin
Cobalt(III) Complexes 1
*To whom correspondence should be addressed. e-mail: h_naka@nagoya-u.jp
© 2017 The Pharmaceutical Society of Japan