Tetrahedron Letters
Strontium-mediated selective protonation of unsaturated linkage of
aromatic hydrocarbons and these derivatives
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Satoshi D. Ohmura, Masaharu Ueno, Norikazu Miyoshi
Department of Natural Sciences, Graduate School of Technology, Industrial and Social Sciences, Tokushima University, Minami-Josanjima 2-1, Tokushima 770-8506, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The selective protonation of aromatic hydrocarbons with at least two or more aromatic rings and
aromatic compounds bearing unsaturated linkages can be achieved by metallic strontium metal with
ammonium chloride and iodine, or ammonium iodide in tetrahydrofuran. The reaction system is ammo-
nia-free in room temperature and the reaction proceeds high selectivity in moderate to good yields.
Ó 2018 Published by Elsevier Ltd.
Received 15 March 2018
Revised 22 April 2018
Accepted 27 April 2018
Available online xxxx
Keywords:
Birch-like reduction
Strontium metal
Selective protonation
Aromatic hydrocarbons
Unsaturated linkage
The reduction of aromatic hydrocarbons by alkali metals in liq-
uid ammonia is a powerful and versatile method. Classical Birch
reductions of aromatic hydrocarbons are performed with alkali
metals in liquid ammonia at below À33 °C, or reflux temperature
of alcohols in the presence of ammonia.1 These reactions can be
tuned by varying conditions, temperatures, metals, additives, pro-
partial protonation of several aromatic hydrocarbons and these
derivatives using metallic strontium with ammonium salts and
iodine sources at room temperature.
The selective protonation of anthracene with strontium metal
was investigated (Table 1). This reaction was proceeded at ambient
temperature to afford 9,10-dihydroanthracene in the presence of
1.1 eq. strontium under argon (Table 1, entry 3). The results of con-
trol experiments showed the Sr-mediated protonation reaction
2
ton sources or quenching reagents. A cathodic Birch reduction has
been reported for a limited class of substrates.3 Recently, several
ammonia-free reductions of aromatic rings under ambient or
higher temperature were reported. For example, hydrogenation
of polycyclic hydrocarbons was promoted by transition metal com-
1
1
required iodine source and ammonium chloride (Table 1, entries
1 and 2). An increase in the amount of ammonium chloride
resulted in an increase in yield (Table 1, entry 4). Other ammonium
salts or urea, or using other common solvents had no impact on the
reactivity (Table 1, entries 6, 7, 8 and 10–13). Finally, we found that
pre-dried ammonium chloride activated the reaction condition
(Table 1, entry 5), with 2.5 equiv. of metallic strontium and 15
mol% of iodine providing the best results in terms of reaction effi-
ciency (Table 1, entry 9).
4
5
plexes or nanoparticles under hydrogen gas, alkali metals on sil-
6
7
8
ica gel, samarium(II) iodide, frustrated Lewis pair and hydroxide
ion was used as an electron source for photochemistry activated
9
Birch reduction of naphthalene derivatives. We have been investi-
gating synthetic reactions using organostrontium compounds and
have reported that the dialkylation of esters with alkyl iodides or
esterification of bulky alcohols with strontium compounds
proceeds smoothly using metallic strontium to afford the
The reduction of aromatic hydrocarbons with other alkali metal
or alkaline-earth metal under pre-dried ammonium chloride with
iodine conditions were also investigated by using anthracene as a
model compound (Table 2). From the results, lithium, sodium,
magnesium and barium showed less activation of the protonation
reaction (Table 2, entries 1, 2, 4 and 6). On the other hand, potas-
sium and calcium gave good yields of 1,9-dihydroanthracene
10
corresponding adducts in good yields. Recently, we found that
aromatic hydrocarbons with at least two or more aromatic rings
and aromatic compounds bearing unsaturated linkages reacted
with metallic strontium to give partially protonated corresponding
compounds in moderate to good yields. Herein, we demonstrate
(Table 2, entries 3 and 5). Although these results suggested that
the reduction system might be able to be also carried out with
potassium or calcium instead of strontium, these metals usually
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040-4039/Ó 2018 Published by Elsevier Ltd.
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