Rhodium-Catalyzed Remote C(sp3)?H Borylation of Silyl Enol Ethers

Article abstract of DOI:10.1002/anie.201913281

A rhodium-catalyzed remote C(sp³)?H borylation of silyl enol ethers (SEEs, E/Z mixtures) by alkene isomerization and hydroboration is reported. The reaction exhibits mild reaction conditions and excellent functional-group tolerance. This method is compatible with an array of SEEs, including linear and branched SEEs derived from aldehydes and ketones, and provides direct access to a broad range of structurally diverse 1,n-borylethers in excellent regioselectivities and good yields. These compounds are precursors to various valuable chemicals, such as 1,n-diols and aminoalcohols.

Full text of DOI:10.1002/anie.201913281

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Accepted Article
Title: Rhodium-Catalyzed Remote C(sp3)−H Borylation of Silyl Enol
Ethers Jie Li, Shuanglin Qu, and Wanxiang Zhao*
Authors: Jie Li, Shuanglin Qu, and Wanxiang Zhao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913281
Angew. Chem. 10.1002/ange.201913281
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10.1002/anie.201913281
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Rhodium-Catalyzed Remote C(sp3)−H Borylation of Silyl Enol
Ethers
ABSTRACT: A rhodium-catalyzed remote C(sp3)−H borylation of
silyl enol ethers (SEEs, E/Z mixtures) via alkene isomerization and
hydroboration is reported. The reaction exhibits mild reaction
conditions and excellent functional group tolerance. This method is
compatible with an array of SEEs, including linear and branched
SEEs derived from aldehyde and ketone, and provide direct access
to a broad range of structurally diverse 1,n-borylethers in excellent
regioselectivity and good yields, which are precursors to various
valuable chemicals, such as 1,n-diols and aminoalcohols.
The functionalization of a distant position from a functional
group, termed as remote functionalization, has been shown to
be a powerful platform in organic synthesis,[1] as it eliminates the
necessity of prefunctionalization and multi-step synthesis and
enables rapid generation of molecular complexity and efficient
synthesis of complex organic molecules from readily available
starting materials. In this context, the distant functionalization via
chain-walking, including functionalization-isomerization and
isomerization-functionalization (Figure 1a), has achieved
significant advances in the last two decades.[2] For instance, the
group of Sigman has reported a series of elegant Pd-catalyzed
functionalization-isomerization of alkenyl alcohols initiated by
electropalladation or nucleopalladation of double bond (Scheme
1a, R = OH).[3] With regard to isomerization and isomerization-
functionalization, the group of Fu,[4a-c] Marek,[4d-g] Mazet,[4h-l]
Chirik,[4m-o] Martin,[4p-r] Grotjahn,[4s-u] Zhao,[4v-w] Zhu,[4x-ad] and
Figure 1. Remote Functionalization via Chain Walking
others[5] have made great contributions. Despite these
1 to 2 is thermodynamically unfavored. In contrast, the inverse
outstanding advances, long-standing challenges and significant
isomerization (2→1) is thermodynamically favored, and has
limitations still remain. For example, alkenes employed in the
been well-developed to synthesize silyl enol ethers in the
reported literatures are limited to monosubstituted and C-
presence of base or transition metal, in which the ether serves
substituted ones (Figure 1a, X = H and C).[4-5] However,
as the driving force (directing group) for alkene isomerization.[8]
heteroatom-substituted alkenes as fundamental building blocks
In other words, deconjugative isomerization of heteroatom-
in organic synthesis and material science, such as enol ethers
substituted alkenes (like 1→2) is highly challenging, but of high
and enamines (Figure 1a, X = O and N), are rarely utilized in
synthetic utility given the prevalence and importance of
chain walking reactions.[6] The possible reason for this is that
heteroatom-substituted alkenes in natural product and
heteroatom-substituted alkenes are thermodynamically more
pharmaceutical synthesis.
stable than the corresponding isomerized alkenes due to the
As an important part of heteroatom-substituted alkenes, silyl
conjugation between a lone pair of the heteroatom and the
enol ethers are widely employed in organic synthesis, and can
carbon−carbon double bond (silyl enol ether 1 vs vinyl ether 2).[7]
be readily prepared from aldehyde and ketone.[9] They usually
To confirm this viewpoint, we carried out density functional
serves as a nucleophile due to its electron-rich double bond.[9]
theory (DFT) calculations to compare the relative stabilities
However, the remote functionalization via chain-walking of the
between silyl enol ethers (SEEs) 1 and their isomers, vinyl
silyl enol ethers is yet to be reported to our knowledge. Herein,
ethers 2. The thermodynamics of the isomerizations are shown
we describe a rhodium-catalyzed remote C(sp3)−H borylation of
in Scheme 1b. As expected, the silyl enol ethers 1a-a'' are more
silyl enol ethers (stereoisomeric mixtures) via isomerization and
stable than the corresponding vinyl ether 2a-a'' by 5.4, 5.7, and
hydroboration to form 1,n-borylether 3, which represent an
6.1 kcal/mol, respectively, indicating that the isomerization from
efficient entry into a variety of value-added chemicals, such as
1,n-diols and 1,n-aminoalcohols.
To test the viability of the rhodium-catalyzed remote C(sp3)−H
borylation of silyl enol ethers via alkene isomerization, we began
[a]
J. Li, Prof. Dr. S. Qu, Prof. Dr. W. Zhao
our studies using silyl enol ether 1a (E/Z = 1.1/1) as the mode
substrate. Upon screening a series of reaction parameters, we
identified that the base, silyl group, ligand, and boron source are
critical for high-yield formation of the borylether product 3a
State Key Laboratory of Chemo/Biosensing and Chemometrics,
College of Chemistry and Chemical Engineering, Hunan University,
Changsha, Hunan 410082, P. R. China
E-mail: zhaowanxiang@hnu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201913281
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COMMUNICATION
Table 1. Variation of Reaction Parameters[a]
remote C(sp3)−H borylation via chain walking. For ease of
separation, the primary alkylboronates 3 were oxidized in the
presence of NaOH/ H2O2 to the corresponding diols 4, which are
ubiquitous in natural products and pharmaceuticals. As shown in
Table 2, the mild reaction conditions were applicable to a broad
range of linear alkyl silyl enol ethers with different chain length
(4a-j). It is worth mentioning that 1,20-diol 4j could be easily
prepared in 69% yield from the silyl enol ether of icosanal by this
Rh-catalyzed remote C(sp3)−H borylation reaction.
Table 2. Scope of Linear Alkyl Silyl Enol Ethers[a]
[a] Standard conditions: 1c (0.3 mmol), [RhCl(cod)]2 (2 mol%), L1 (8 mol%),
HBpin (0.9 equiv), B2pin2 (0.3 equiv), CF3CO2K (10 mol%), hexane (1.0 mL),
70 °C for 10 h. [b] The yields were determined by GC using dodecane as the
internal standard. [c] The number in the parenthese is the isolated yield of the
corresponding alcohol after oxidation. [d] B2pin2 (1.5 equiv), no HBpin. [e]
HBpin (1.5 equiv), no B2pin2. TIPS = triisopropylsilyl; TES = triethylsilyl; TBS =
tert-butyldimethylsilyl; TDS = dimethylthexylsilyl.
[a] 1 (0.5 mmol) was used and isolated yields are reported. [b] HBpin (1.2
equiv), B2pin2 (0.4 equiv).
Next, we turned our attention to the synthesis of secondary
alkylboronates from aryl terminated silyl enol ethers. Under the
standard conditions, although the benzylic borylation product
was successfully generated, the yield was relatively low and the
hydrogenation is dominant. Inspired by Crudden’s work on Rh-
(Table 1). The desired product was generated in 82% yield when
the 1a was treated with [RhCl(cod)]2, 6,6′-dimethyl-2,2′-dipyridy
(L1), HBpin/B2pin2, and CF3CO2K (Table 1, entry 1). The
reaction gave low conversion and yield in the absence of
CF3CO2K (Table 1, entry 2). The use of other smaller silyl
groups, such as TES, TBS, and TDS, all resulted in inferior
yields (Table 1, entries 3-5). The 2,2'-bipyridine ligands with
catalyzed
hydroboration
of
olefins,
where
tris(pentafluorophenyl)borane (FAB) was used to control the
regioselectivity,[11] we then examined the FAB as an additive and
found the hydrogenation was suppressed effectively, leading to
great improvement on the yield of the desired borylation product.
The scope of aryl terminated silyl enol ethers was then
investigated. As depicted in table 3, an array of substrates with
different substituents at different position on phenyl ring
proceeded smoothly to produce the diols in high efficiency. A
variety of functional groups, including MeO, Ph, F, Cl, CF3 ,
protected phenols, Bpin and NMe2 were readily accommodated
(4k-w). Additionally, the mild reaction conditions tolerated other
aromatic rings, such as naphthalene, indole and thiophene (4x-
z). Furthermore, the silyl enol ethers with different chain length
all participated in this remote C(sp3)−H borylation reaction to
give the corresponding diols in moderate to good yields (4aa-ae).
It is worth noting that the silyl enol ethers derived from
empagliflozin and dihydrocholesterol were also suitable
substates, providing the corresponding products (4af-ag) in 59%
and 67% yield, respectively.
i
different substituent at 6 and 6′ positions, including Et (L2), Pr
(L3) and tBu (L4), were also examined (Table 1, entries 6-9), but
none of them furnished better result than the ligand L1 for this
novel borylation via chain walking. In addition, no desired
product was observed when the 2,2'-bipyridine (L5), 6-methyl-
2,2'-bipyridine (L6), or 4,4'-dimethyl-2,2'-bipyridine (L7) was
employed (Table 1, entries 10-11), indicating that the steric
hindrance of the ligand has dramatic effect on this remote
C(sp3)−H borylation. Under these conditions, diphosphine ligand
like dppe (L8) proved to be ineffective (Table 1, entry 12). The
use of a mixture of HBpin and B2pin2 and their ratio are
important. In the absence of HBpin or B2pin2, side reaction such
as hydrogenation and hydroboration became significant (Table 1,
entries 13-14).[10]
With the optimal reaction conditions determined, we first
investigated the scope of linear alkyl silyl enol ether for this
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COMMUNICATION
Table 3. Scope of Aryl-Terminated SEEs[a]
[a] 1 (0.3 mmol) was used and isolated yields are reported. [b] The substrate is 1s (see SI). [c] Reaction conditions: 1 (0.3 mmol), [RhCl(cod)]2 (2 mol%), L8 (4
mol%), HBpin (1.5 equiv), LiOAc (30 mol%), THF (1 mL), 30 °C, 22 h. [d] 70 °C
The generality of this new distant C(sp3)−H borylation was
further evaluated by more challenging substrates: branched
aldehyde and ketone derived silyl enol ethers (Table 3). To
our delight, a range of branched silyl enol ethers were found
to be compatible when the dppe was used as the ligand and
HBpin as the boron source, delivering the corresponding diols
in good yields (4ah-ak). Noticeably, the trisubstituted
substrate 1ah underwent deconjugative isomerization-
hydroboration successfully to afford 4ah in 80% yield. Of
particular note, the reaction of 1ak bearing phenyl at γ
position formed 1,6-diol 4ak in excellent regioselectivity and
good yield. With regard to the ketone derived silyl enol ethers,
although the bulky TIPS group gave low conversion due to
the steric hindrance, the smaller TES group proceeded
effectively to furnish the desired borylation products.The TES
enol ethers with different chain length were all competent
substrates for the current protocol (1al-ao). Interestingly, the
ketone derived SEE 1ao with methyl and ethyl at α and β
positon, respectively, selectively isomerized along the ethyl
group, followed by hydroboration to give 1,4-diol 4ao in 74%
yield. In addition, SEE derived from ester and enamine were
also examined, but the yields of the desired remote borylation
products were relatively low (see more details in SI).
Table 4. Scope of Branched-aldehyde and Ketone Derived SEEs[a]
Mechanistically, the silyl enol ether undergoes Rh−H[12]
(generated in situ) catalyzed chain-walking to give a alkene
intermediate, which then reacts with HBpin to furnish the
alkylboronate 3. Although the alkene intermediate was not
isolated or detected by GC-MS from the reaction mixture, diol
4a and 4ap were isolated in 62% and 59% yield, respectively,
when alkene 2a and 2b were subjected to the standard
conditions and then NaOH/H2O2 (Scheme 1, eq 1-2). Notably,
a mixture of three olefinic isomers (SEE, terminal and internal
alkene) gave a single diol product 4a in 69% yield (Scheme 1,
eq 3). Furthermore, deuterated experiment was also carried
out using DBpin. Deuterium scrambling at C1, C2, and C3
[a] 1(0.5 mmol) was used and isolated yields are reported. [b] [RhCl(cod)]2
(5 mol%), L8 (10 mol%).
were observed (Scheme 1, eq 4). The remote borylation of
1ap was conducted under our reaction conditions, and the
corresponding product 4ap was obtained in 54% yield with
28% ee ((Scheme 1, eq 5). These results indicate that the
remote C(sp3)−H borylation involves both nondissociative and
dissociative chain-walking pathways to give the alkene
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intermediate, which followed by hydroboration to produce the
desired borylether.
to deliver aminoalcohol 9 in excellent yield after Boc
protection.[16]
The alkylboronates are useful building blocks to construct
carbon−carbon and carbon−heteroatom bonds, which implies
that C(sp3)−H bond can be converted to other functional
groups via the alkylboronate generated by this distant
C(sp3)−H borylation. To illustrate this point, a variety of
transformations of the alkylboronate 3a synthesized from a
gram-scale reaction under the standard conditions were
studied (Scheme 2). First, bromination and iodination of 3a
according to Aggarwal protocol proceeded efficiently to
In conclusion, we have developed a rhodium-catalyzed
remote C(sp3)−H bond borylation of silyl enol ethers via
alkene isomerization and hydroboration in excellent
regioselectivity. Various silyl enol ethers, including linear-,
aryl-terminated-, branched-aldehyde and ketone derived
SEEs, all successfully participated in this novel
transformation
to
afford
primary
or
secondary
organoboronates in moderate to good yields. This versatile
protocol provides a synthetically valuable addition to the
current collection of remote sp3 C−H functionalization
reactions, and it might find wide application in late-stage
functionalization of pharmaceutically relevant and structurally
complex molecules. The further development and application
of this reaction as well as asymmetric version are currently in
progress.
produce
5
and
6
in
71%
and
69%
yield,
respectively.[13]Second, treated 3a with vinylmagnesium
bromide and I2,[14] vinyl ether 7 was generated in excellent
yield, which could go further functionalization to give more
complex fragments. Moreover, Pd-catalyzed Suzuki-Miyaura
coupling of 3a with 4-bromotoluene afforded product 8 in 96%
yield.[15] Finally, the amination of alkylboronate 3a via the
protocol developed by Morken was achieved in the presence
of tBuOK and MeONH2
Acknowledgements
Financial support from the National Natural Science
Foundation of China (Grant No. 21702056, 21971059), the
National Program for Thousand Young Talents of China and
the Fundamental Research Funds for the Central Universities
are greatly appreciated.
Conflict of interest
The authors declare no competing financial interests.
Keywords: borylation • C−H activation • isomerization •
rhodium • regioselectivity
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COMMUNICATION
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10.1002/anie.201913281
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
J. Li, S. Qu, W. Zhao*
Page No. – Page No.
Rhodium-Catalyzed Remote C(sp3)−H
Borylation of Silyl Enol Ethers
A rhodium-catalyzed remote C(sp3)−H borylation of silyl enol ethers (SEEs, E/Z
mixtures) via alkene isomerization and hydroboration is reported. The reaction
exhibits mild reaction conditions and excellent functional group tolerance. This
method is compatible with an array of SEEs, including linear and branched SEEs
derived from aldehyde and ketone, and provide direct access to a broad range of
structurally diverse 1,n-borylethers in excellent regioselectivity and good yields,
which are precursors to various valuable chemicals, such as 1,n-diols and
aminoalcohols.
This article is protected by copyright. All rights reserved.