2
K. Inomata et al. / Tetrahedron Letters xxx (xxxx) xxx
Table 1
in situ-formed Pt metal colloids [17]. To clarify this point in the
Screening of metal catalysts.a
case of our reactions, the reaction of 1f with 2a was also performed
in the presence of Hg, which serves as an inhibitor of active metal-
colloidal catalysts. (Table 2, entry 10). The yields of the products
remained unchanged regardless of the existence of Hg. Therefore,
a different reaction path from that in the platinum-catalyzed sys-
tem was suggested for the formation of the isomerized products
4. Diethylene glycol monoallyl ether with a terminal hydroxyl
group (1g) resulted in the formation of a complex mixture (Table 2,
entry 11). H2 evolution was detected during the reaction. Thus, the
occurrence of dehydrognative coupling of 1g with 2a is indicated.
The addition of 10 mol% H2O resulted in the complete suppres-
sion of the hydrosilylation reaction of 1a (Table 2, entry 12). These
results indicated that the presence of free protons impedes the
hydrosilylation reaction.
To shed light on the reaction mechanism, several control exper-
iments using allyl butyl ether (6) were conducted (Table 3). The
reaction with 2a in the presence of [RuCl2(nbd)]n afforded the
hydrosilylation product 7a and the isomerized product 8 in 87%
and 13% yields, respectively (Table 3, entry 1). This selectivity
was similar to that in the reaction with Karstedt’s catalyst
(7/8/9 = 82/17/<1). The addition of DME (10 mol%) as an external
ligand did not improve the reaction selectivity (Table 3, entry 2).
It was also confirmed that the use of HSi(OCH2CH2OMe)3 (2d)
decreased the yield of the desired 7b, whereas the Pt-catalyzed
same reaction resulted in the selective formation of 7b as reported
previously (Table 3, entry 3) [2a]. These experimental results
strongly support the pivotal role of the coordination of the
ethylene glycol moiety of olefin to the ruthenium center to
achieve excellent reaction selectivity.
Entry
Cat.
% yield (3a/4a/5a)b
1
2
3
4
5
6
7
8
Karstedt’s cat
[IrCl(cod)]2
RhCl(PPh3)3
62/20/5
71/14/2
68/<1/5
45/<1/5
61/18/9
72/11/3
76/15/3
80/6/2
Ru3(CO)12
RuCl2(dmso)4
[RuCl2(benzene)]2
[RuCl2(mesitylene)]2
[RuCl2(p-cymene)]2
[RuCl2(cod)]n
[RuCl2(nbd)]n
9
84/5/<1
>99/<1/<1
10c
a
Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), catalyst (0.005 mmol per
metal), 60 °C, 16 h under a nitrogen atmosphere.
Determined by 1H NMR using mesitylene as an internal standard.
b
c
Reaction was also performed with 2a (1 equiv) for 1 h.
(61%), as well as the production of 4a (18%) and 5a (9%) (Table 1,
entry 5). Further screening of metal catalysts revealed that some
of the Ru(II) complexes [16] showed better selectivity than Karst-
edt’s catalyst. The reaction using [RuCl2(benzene)]2 led to the
increase in the yield of 3a (72%) although the concomitant forma-
tion of both 4a (11%) and 5a (3%) was also observed (Table 1, entry
6). [RuCl2(mesitylene)]2 and [RuCl2(p-cymene)]2 similarly cat-
alyzed the reaction to form 3a in good yields with the slight forma-
tion of both 4a and 5a (Table 1, entries 7 and 8). The use of
[RuCl2(cod)]n (cod = 1,5-cyclooctadiene) increased the yield of 3a
to some extent; i.e., 3a, 4a, and 5a were formed in 84%, 5%,
and < 1% yields, respectively (Table 1, entry 9). Interestingly,
[RuCl2(nbd)]n selectively catalyzed the hydrosilylation reaction to
afford 3a in >99% yield (Table 1, entry 10). It should be noted that
the selective and quantitative hydrosilylation reaction of 1a was
also completed with 1 equiv of HSi(OMe)3 within 1 h.
As
a possible reaction path for the ruthenium-catalyzed
hydrosilylation reaction of 1, we postulated that the reaction
follows the conventional (modified) Chalk–Harrod mechanism, in
which the reaction starts with the oxidative addition of
hydrosilane 2, and then olefin insertion and reductive elimination
successively proceed (Fig. 1). Since the control experiment using
Hg excluded the possibility of the formation of ruthenium metal
colloids (vide infra), the olefin isomerization is likely to proceed
via b-H elimination of the alkyl(silyl) intermediate in the
Chalk–Harrod mechanism cycle.
In summary, we have achieved highly selective hydrosilylation
[RuCl2(nbd)]n was successfully applied to other poly(ethylene
glycol) allyl methyl ethers 1. To clarify the scope of 1, the hydrosi-
lylation reactions of 1 with various sizes of PEG chains were per-
formed in the presence of 1 mol% per metal of [RuCl2(nbd)]n at
60 °C for 1 h (Table 2). The reaction of 1a–d (n = 1–4) with HSi
(OMe)3 (2a) achieved the selective formation of the corresponding
hydrosilylated products 3a–d in 90–95% isolated yields (Table 2,
entries 1–4). The reaction of 1a with HSiMe(OMe)2 (2b) similarly
proceeded to form 3e in 86% yield with the slight formation of
4a and 5a (Table 2, entry 5). The use of the bulkier HSi(OEt)3
resulted in a slight decrease in the yield of 3f (83%) (Table 2, entry
6). Next, we performed the hydrosilylation of 1e (n = 14) and 1f
(n = 26) with a longer PEG chain. The reaction of 1e with 2a fur-
nished 3g in 90% isolated yield (Table 2, entry 7), accompanied
by 4g (7% yield). In the reaction of much longer 1f with 2a, the
isolated yield of the hydrosilylated product 3h further decreased
to 85% (NMR) (Table 2, entry 8). The use of 3 equiv of 2a improved
the yield of 3h to 89% (Table 2, entry 9). All the aforementioned
reactions were also tested using Karstedt’s catalyst for comparison.
The results are also shown in Table 2, the rightmost column. Based
on these results, the reactions resulted in the formation of a mix-
ture of 3, 4, and 5 [16]. Thus, the superior catalytic properties of
[RuCl2(nbd)]n to Karstedt’s catalyst were evidenced.
reactions of allyl–functionalized PEG derivatives 1 using a
commercially available ruthenium catalyst, [RuCl2(nbd)]n. The
In the platinum-catalyzed hydrosilylation reactions, it has been
reported that the isomerization is generally catalyzed by the
Fig 1. Possible catalytic cycles.
Please cite this article as: K. Inomata, Y. Naganawa, H. Guo et al., Ruthenium-catalyzed selective hydrosilylation reaction of allyl-functionalized PEG deriva-