Angewandte
Communications
Chemie
À
C H Activation
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Rhodium-Catalyzed Intramolecular C H Silylation by
Silacyclobutanes
Qing-Wei Zhang+, Kun An+, Li-Chuan Liu, Shuangxi Guo, Chenran Jiang, Huifang Guo, and
Abstract: Silacyclobutane was discovered to be an efficient
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C H bond silylation reagent. Under the catalysis of Rh /TMS-
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segphos, silacyclobutane undergoes sequential C Si/C H
bond activations, affording a series of p-conjugated siloles in
high yields and regioselectivities. The catalytic cycle was
proposed to involve a rarely documented endocyclic b-hydride
elimination of five-membered metallacycles, which after
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reductive elimination gave rise to a Si Rh species that is
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capable of C H activation.
S
ilacyclobutanes (SCB) were first reported by Sommer and
Baum over half a century ago.[1] Compared with other
organosilicons, SCB is unique due to their high ring strain
energy, as evidenced in ring opening polymerization,[2] aldol
reactions of SCB[3] and retro-[2+2] reaction.[4] Also because
of the high ring strain, low valent transition metals can facilely
insert into the SCB to form 5-membered silametallacycles.[5]
A signature reactivity of the resulting silametallacycles
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Figure 1. Silacyclobutanes as C H silylation reagent.
to RhI). However, these silametallacycles were proposed to
be stable against b-H elimination.[12] In general, endocyclic b-
H elimination of related metallacycles (especially that of 3- to
6-membered rings) are conceived to be extremely challeng-
ing.[13] A classic example is the alkene–alkyne coupling
reaction reported by Trost, in which acyclic b-H elimination
was shown to prevail.[14] Although DFT calculations sug-
gested that endocyclic b-H elimination might be possible,[15]
only a few examples were reported very recently.[16] Herein
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involves the migratory insertion of p systems into the M Si
bonds. Based on this reactivity, various fascinating formal
cycloaddition reactions of SCB have been developed.[6] For
example, reactions of alkynes or alkenes with SCB resulted in
formal [s2+p2] cycloaddition[7] (Figure 1a). Similarly, enones
were shown to give formal [s2+p4] products.[8] Most recently,
Murakami and co-workers discovered that electron-rich
silapalladacycle is capable of oxidative insertion into an
adjacent cyclobutanone, cultivated in an intriguing formal
s bond metathesis reaction.[9] Despite these advancements,
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we reported that SCB is an efficient reagent for C H bond
silylation[17] under Rh catalysis in the presence of a hindered
ligand, possibly enabled by endocyclic b-H elimination of Rh
silametallacycles (Figure 1b). This reaction produces various
siloles in high yield with excellent regioselectivity.
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SCB is not shown to react with C H bonds.
In our continuing research in the Rh catalyzed construc-
tion of Si C bonds,[10] we became curious if SCB could act as
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a C H silylation reagent. If a 5-membered Rh silametalla-
Our investigation started with the intramolecular silyla-
tion reaction of SCB 1a. Since our previous work showed that
the steric hindrance of diphosphine ligands has a profound
cycle is to be assumed, converting this RhIII to catalytically
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active Si Rh would be required, since only the latter is
[11]
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influence[10b] on the C H bond silylation reaction, we
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known to perform C H activation. Such a Si Rh might be
achieved by a two-step sequence: b-hydride elimination of the
Rh silametallacycle followed with reductive elimination (RhIII
screened a large panel of ligands (Table 1). No reaction
took place in the absence of a ligand (entry 1). Similar to our
previous observations,[10b] ligands with different dihedral
angles showed dramatically different reactivity. The less
hindered ligand spirophos and BINAP were ineffective
(entries 2–3). DTBM-MeO-Biphep, which possesses a wider
dihedral angle and more hindered 3, 5-di-t-Bu-4-MeO-phenyl
substituent, gave the silole product[18] 2a in 15% yield
(entry 4). A systematic screening of ligands with segphos
backbone that possess wider dihedral angles[19] was then
conducted (entries 5–8). Among these ligands, TMS-seg-
phos[20] gave the best result (52% yield, entry 7). Further
increasing the bulkiness, either by using larger 3, 5-di-
triethylsilylphenyl (TES)[21] or by employing C1-tunephos[22]
[*] Dr. Q.-W. Zhang,[+] K. An,[+] L.-C. Liu, S. Guo, C. Jiang, Dr. H. Guo,
Prof. Dr. W. He
School of Pharmaceutical Sciences, Tsinghua University
Beijing, 100084 (China)
E-mail: whe@mail.tsinghua.edu.cn
research/faculty/item/he-wei
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 6319 –6323
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6319