Angewandte
Communications
Chemie
1
a+·/1a
1a’’+·/1a’’
[a]
(
2-MeDHQ; 1a’’; E
=+ 0.72 V vs. SCE and E
=
Table 1: Substrate scope with respect to THQs and derivatives.
+
0.91 V vs. SCE, respectively; see Figures S1 and S2 in the
Supporting Information), the release of the second H would
2
be more difficult than the first. But the release of the second
H might be easier than the first because of the aromatization.
2
[
3]
The studies commenced with dehydrogenation of 1a
using Ru(bpy) Cl ·6H O (1 mol%) as a photosensitizer with
Co(dmgH) PyCl (2 mol%) as a catalyst
3
2
2
[7d]
and irradiation
2
using blue LEDs at ambient temperature. To our delight,
primary solvents investigation found that dehydrogenation of
1
a afforded 2a with the release of two molecules of H in
2
quantitative yield, both in EtOH and methanol (MeOH)
within 6 hours (see Table S1, entries 4 and 6), and might be
attributed by better solubility of Ru(bpy) Cl ·6H O in EtOH
3
2
2
and MeOH. It was proved that EtOH and MeOH could not
produce H under the reaction conditions (entries 5 and 7). To
2
the best of our knowledge, such mild reaction conditions are
[
9]
very rare for potential H storage materials, apart from
2
strongly corrosive formic acid, the H storage irreversible
2
ammonia borane, and highly toxic hydrazine and hydrazine
[
4]
borane. Other photosensitizers and cobalt catalysts were
explored. Ru(1,10-Phen) Cl ·6H O and Co(dmgH) Cl , as
3
2
2
2
2
well as Co(salen), showed the same high catalytic efficiency as
Ru(bpy) Cl ·6H O and Co(dmgH) PyCl (entries 8–11). Con-
trol experiments showed the necessity of the photosensitizer
and light irradiation (entries 13 and 14). A negligible amount
of 2a was obtained without the cobalt catalyst (entry 15).
The substrate scope was investigated by using Ru-
3
2
2
2
[
a] Reaction conditions: 1 (0.20 mmol), Ru(bpy) Cl ·6H O (1.0 mol%),
3
2
2
Co(dmgH) PyCl (2.0 mol%), EtOH (2.0 mL), irradiation with blue LEDs
for 24 h at RT under Ar atmosphere. Yield is that of the isolated product.
H yield was tested by GC using CH as an internal standard. [b] 6 h
irradiation. [c] 8 h irradiation. [d] 2,6-Dimethyl-1,2,3,4-tetrahydro-1,5-
naphthyridine as substrate. [e] 2,7-Dimethyl-1,2,3,4,6,7,8,9-octahydro-
pyrido[2,3-g]quinoline as substrate. [f] 1,2,3,4,7,8,9,10-Octahydro-1,10-
phenanthroline as substrate.
2
2
4
(
bpy) Cl ·6H O as a photosensitizer with Co(dmgH) PyCl
3 2 2 2
as a catalyst under optimized reaction conditions (Table 1).
Similar to 2-MeTHQ, 3-MeTHQ, 4-MeTHQ, and THQ
displayed excellent reaction efficiency (2b–d). Introduction
of 2-phenyl and 2-(naphtha-2-yl) groups did not influence the
reactivities (2e,f). Substrates bearing another heteroatom,
such as 2-(furan-2-yl)-THQ and 2-(thiophen-2-yl)-THQ,
achieved good to excellent yields (2g,h). Variation of the
substituents, from methyl to fluoro, at the 6-position of THQ
did not affect the yields (2i–l). However, THQ bearing a 6-
ester group only afforded 17% 2m along with unreacted
starting material. 7-Methyl and 8-methyl groups slightly
decreased the reaction efficiency (2n,o). Moreover, THQs
bearing two substituents, such as 6-methoxyl-2-methyl-THQ,
6
-bromo-2-methyl-THQ, and 4-(4-fluorophenyl)-7-methyl-
THQ displayed excellent yields (2p–r). In addition, 2,6-
dimethyl-1,5-naphthyridine (2s) was obtained in high yield.
Notably, 2r was used as a useful intermediate for the
Scheme 3. Reversible dehydrogenation-hydrogenation. [a] The reactions
were conducted on 0.2 mmol.
[1b,c,6]
preparation of a nanomolar 5-LO inhibitor.
The devel-
oped method was also applied to the dehydrogenation of
theoretically release five molecules of H , it failed in this
2
[
3h,i]
1
,2,3,4-tetrahydroquinoxalines
nazolines to afford quinoxalines (2t,u) and quinazolines
2v,w), respectively, in excellent yields. Moreover, gram-
and 1,2,3,4-tetrahydroqui-
transformation. This result suggests that the conjugated aryl
ring to the pyridine ring is important for the dehydrogenation.
Thus, 2,7-dimethyl-octahydropyrido[2,3-g]quinoline and
octahydro-1,10-phenanthroline were tested for the dehydro-
genation, and only the products of dehydrogenation leading
(
scale dehydrogenation of 1a with 0.5 mol% photosensitizer
and 1.0 mol% catalyst in 20 hours afforded 2a and H in
2
excellent yield [Scheme 3, Eq. (1)].
to two molecules of H (2x,y; Table 1), instead of four, were
2
Encouraged by the high catalytic efficiency of the trans-
formations, we anticipated the feasibility of dehydrogenation
of more H2 from one molecule of starting material. This
obtained. This observation could be attributed to the
electron-poor properties of 2x and 2y, which hindered further
dehydrogenation. It echoed the lower yield of 2m containing
an electron-withdrawing ester group. To our satisfaction, the
compound 3, containing a comparable releasable hydrogen
feature would be really attractive for H storage materials.
2
Although 2,6-dimethyldecahydro-1,5-naphthyridine could
2
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!