J. Wang, Y. Wang, G. Ding et al.
Tetrahedron Letters xxx (xxxx) xxx
Table 2
of phenol products in good to excellent yields. For halogenated
substrates, no dehalogenated products were detected along with
deallylation products (1i-1k). It’s worth mentioning that the sensi-
tive cyano (1h), ketonic carbonyl (1m), ester (1n), amide (1o) and
lactone (1q) moieties were all compatible in the deallylation, with
no reduction product observed. As for the selectivity of our deally-
lation system, when the substrate is modified with multiple pro-
tecting groups, including methyl group (1c), benzyl group (1l)
and tert-butyloxy carbonyl group (1p), the deallylation could be
carried out selectively, while the other protecting groups could
be completely retained. For the substrate with two allyl ether moi-
eties (1u), the bi-deallylated product, hydroquinone (3u), was
obtained in excellent yield with increased loading of the catalyst
Ni(COD)2 and doubled equivalents of Ph2SiH2 (2a). In the case of
the substrate modified with both the allyl ether moiety and the
allyl amine moiety (1v), the selective O-deallylation product N-
allyl-N-(4-hydroxyphenyl)acetamide (3v) could be obtained in
86% yield, with 12% of the N-(4-hydroxyphenyl)acetamide (3v’)
formed. Moreover, we also tested some substrates with substituted
allyl groups on either terminal or branched site (1aa-1ad), and all
of them were effectively deallylated to give 4-(tert-butyl)phenol
(3a) in excellent yields, with slightly elevated temperature. This
indicates that the deallylation process is not affected by the steric
hinderance on the allyl group side. In addition, when TMDS (2b) is
used in the deallylation, some aryl allyl substrates could not be
effectively deallylated (1h, 1k, 1n and 1q), mainly due to the low
reactivity. The substrate with the nitro group (1t) was not con-
verted even under strong reaction conditions.
Substrate scope.a,b
Then, gram-scale reactions were conducted to demonstrate the
practicality of this method. Subjecting 1-(allyloxy)-4-(tert-butyl)
benzene (1a) on a 15.00 mmol scale to both Ph2SiH2 (2a) and TMDS
(2b) systems under mild reaction conditions delivered the desired
deallylation product 3a (2.00 g and 1.87 g) in 89% yield and 83%
yield, respectively (Scheme 2).
aReagents and conditions: aryl allyl ether (1, 2.00 mmol), 2a or 2b (2.40 mmol), Ni
(COD)2 (1.0 mol%), L3d (1.0 mol%), DMF (5.0 mL) under a N2 atmosphere, room
temperature, 8 h.
We next turned our attention to mechanistic studies. As men-
tioned before, the transition-metal catalyzed deallylation may be
initiated by metal hydride and via olefin isomerization. It is also
possible that the deallylation begins with the oxidative addition
Then, 6 M HCl (1.0 mL), 50 °C, 2 h.
bIsolated yields of corresponding phenol product 3.
cNi(COD)2 (2.0 mol%) and L3d (2.0 mol%).
d80 oC.
between the low valence metal and the substrate to form the g3
-
allylmetal intermediate. To investigate the mechanism of our deal-
lylation system, several control experiments were designed and
conducted.
e50 oC.
f2a or 2b (4.80 mmol).
As shown in Table 3, both the metal catalyst and the hydrosi-
lane are indispensable for the deallylation (Table 3, entries 2–3).
In the absence of the ligand, the selectivity remarkably shifted to
the hydrosilylation (entry 4). When a catalytic amount of hydrosi-
lane was employed, most of the substrate was recovered (entry 5),
and a complete conversion of 1a required for a stoichiometric
amount of hydrosilane (entry 6). These results indicated that the
deallylation is not initiated by the nickel hydride formed from
the reaction of Ni(COD)2 and the hydrosilane, as a catalytic amount
of the nickel hydride is capable of realizing effective olefin isomer-
ization of the aryl allyl ether substrate. Furthermore, using 1a and
Ph2SiH2 (2a), we monitored the reaction progress before the
hydrolysis process by GC–MS. Both the silyl aryl ether intermediate
bis(4-(tert-butyl)phenoxy)diphenylsilane (Scheme 3, II) and the
olefin product propylene (Scheme 3, 5a) were detected (Figs. S1,
S2). In addition, when 1ab and 1ac were employed for the deally-
lyst precursor, the deallylation also occurred smoothly with Ph2-
SiH2; while, the reduced reactivity with TMDS was obtained
(Table S1, entries 11 and 12). However, replacement of Ni(COD)2
with Ni(acac)2 or NiCl2(DME) had detrimental effect on the deally-
lation (Table S1, entries 13 and 14).
So far, the optimal reaction conditions were determined by
using 1.0 mol% of Ni(COD)2 as catalyst precursor, 1.0 mol% of
4,40-di-tert-butyl-2,20-bipyridine (L3d) as the ligand, and adding
1.20 equivalents of Ph2SiH2 (2a) or TMDS (2b), the deallylation
could be conducted effectively and selectively in DMF at room
temperature within 4 h.
The scope of aryl allyl ether substrates on deallylation was then
investigated. As shown in Table 2, our simple and mild nickel-cat-
alyzed deallylation demonstrated good to excellent reactivity and
selectivity for various aryl allyl ethers. The catalytic system was
not sensitive to electronic nature of aryl allyl ethers. A wide range
of aryl allyl ethers 1, possessing either electron-donating or elec-
tron-withdrawing substituents at varied positions, reacted
smoothly to obtain their corresponding phenol products in excel-
lent yields (Table 2, 1a-1f). Nevertheless, decreased yield was
observed in the case of the aryl allyl ether with a methoxyl group
on ortho- position (1g). Diverse valuable functional groups were
well tolerated under this mild condition (1h-1s), furnishing a series
Scheme 2. Gram-scale reactions.
3