Stereoselective Syntheses of γ,δ-Bifunctionalized Homoallylic Alcohols and Ethers via Chemoselective Allyl Addition to Aldehydes

Article abstract of DOI:10.1021/acs.orglett.9b03819

Stereoselective synthesis of γ,δ-bifunctionalized homoallylic alcohols and ethers via chemoselective allylation is reported. Pd-catalyzed 1,2-diboration of allenylsilane provided a novel 1,1,2-trifunctional allylation reagent. Allylboration of aldehydes w

Full text of DOI:10.1021/acs.orglett.9b03819

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Stereoselective Syntheses of γ,δ-Bifunctionalized Homoallylic
Alcohols and Ethers via Chemoselective Allyl Addition to Aldehydes
Jichao Chen, Shang Gao, and Ming Chen*
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States
S
* Supporting Information
ABSTRACT: Stereoselective synthesis of γ,δ-bifunctionalized homoallylic alcohols and ethers via chemoselective allylation is
reported. Pd-catalyzed 1,2-diboration of allenylsilane provided a novel 1,1,2-trifunctional allylation reagent. Allylboration of
aldehydes with the reagent followed by in situ protection gave TES-protected homoallylic alcohols with excellent Z-selectivities.
Chemoselective allylsilation with the same reagent delivered γ,δ-bifunctionalized homoallylic ethers with high E-selectivities.
The bifunctionalized alkene group in the products underwent various transformations without erosion of the olefin geometry.
addition of a 1,2-bifunctional reagent B to aldehyde gives
homoallylic alcohol E with a metal substituent at the internal
position of the alkene (Scheme 1, eq 2).5−7 Synthetic utilities
of these reagents have been demonstrated in the total
syntheses of many natural products.8
arbonyl addition with functionalized allylmetal reagents
1
Crepresents an important advance in allylation chemistry.
Reactions of these reagents with carbonyl compounds produce
homoallylic alcohols with a functionalized olefin group that
permits subsequent C−C bond formation directly. Because of
this apparent advantage, the development of novel function-
alized allylation reagents has attracted significant attention. To
date, several bifunctional allylmetal reagents, including 1,1- and
1,2-bifunctional reagents, have been developed. As shown in
Scheme 1, the reaction of aldehyde with a 1,1-bifunctional
allylation reagent (e.g., A) forms homoallylic alcohol C or D
(or a mixture of C and D) with a metal substituent at the
terminal position of the alkene (Scheme 1, eq 1).2−4 The
By analogy, reaction of carbonyl compounds with a 1,1,2-
trifunctional allylation reagent (e.g., 2, Scheme 1) should
produce homoallylic alcohols with a trisubstituted olefin unit.
Such 1,1,2-trifunctional allylation reagents would be highly
valuable because the trisubstituted alkene group in the
products is functionalized at both the terminal and internal
positions (e.g., 3 and 4 in Scheme 1). Sequential trans-
formation of the bifunctionalized alkene group would give
products with high fidelity of stereochemistry of the
trisubstituted olefin unit. However, to the best of our
knowledge, such a 1,1,2-trifunctional allylation reagent has
not been disclosed in any prior reports. To expand our studies
on the development of novel allylboron reagents,9 we set our
goal to address this unsolved problem. We report herein the
synthesis of the first 1,1,2-trifunctional allylation reagent 2 via
regioselective diboration of allenylsilane 1b (Scheme 1).
Subsequent aldehyde allylation with 2 in the absence of any
additive followed by protection of the secondary alcohol group
gave TES-protected homoallylic alcohols 3 bearing a γ,δ-
bifunctionalized alkene group with excellent Z-selectivities. In
the presence of TMSOBn and TMSOTf, however, the
allylsilane unit in reagent 2 reacted with aldehydes at −78
°C to deliver γ,δ-bifunctional homoallylic benzyl ethers 4 with
high E-selectivities (Scheme 1). The allylboronate unit in
reagent 2 is not reactive under these reaction conditions.
Inspired by early studies on allene silylboration and
diboration from the Ito,6 Miyaura,5a and Morken5c groups,
we considered two strategies to synthesize 1,1,2-trifunctional
Scheme 1. Reaction of Aldehydes with Bifunctional and
Trifunctional Allylation Reagents
Received: October 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03819
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Letter
allylation reagent 2. As shown in Scheme 2, we envisioned that
reagent 2 could be prepared via regioselective silylboration of
Scheme 3. Scope of Aldehyde for Chemoselective
Allylboration with Boronate 2
a c
−
Scheme 2. Approaches for the Synthesis of α-Dimethyl-
phenylsilyl-β-pinacolatoboryl Allylboronate 2
allenylboronate 1a. Alternatively, starting from allenylsilane 1b,
regioselective diboration of 1b may also deliver reagent 2. In
initial studies, we conducted silylboration of commercially
available allenylboronate 1a under the conditions developed by
the Ito group.6 However, we were not able to obtain an
appreciable amount of product 2 after a series of experiments.
Miyaura and co-workers showed that Pt-catalyzed diboration
of monosubstituted allenes occurred regioselectively at the
more substituted alkene unit to give β-boryl-allylboronates.5a
We then attempted the diboration approach to synthesize
allylboron reagent 2. However, Pt-catalyzed diboration of
allenylsilane 1b did not occur with either Pt(PPh3)4 or
Pt(dba)3-PCy3 as the catalyst system. No diboration product
was detected at ambient temperature or at elevated temper-
ature (70 °C). Morken and co-workers recently demonstrated
that highly regioselective diboration of monosubstituted
allenes can be achieved using the combination of a Pd catalyst
and a phosphine ligand.5c Gratifyingly, when the conditions
were employed for diboration of allenylsilane 1b, the targeted
trifunctional allylation reagent 2 was isolated in 89% yield and
excellent regioselectivity (2/5 > 20:1). The reaction can be
conveniently scaled up; reagent 2 was obtained in 91% yield
with >20:1 regioselectivity in a 2 mmol-scale reaction.
With reagent 2 in hand, allylboration studies of aldehydes
with 2 were conducted. The reaction of allylboronate 2 with
benzaldehyde occurred smoothly at ambient temperature in
toluene, and product 3a was obtained in 90% yield with
excellent Z-selectivity (>20:1) after in situ protection of the
secondary alcohol group. The Z-olefin geometry of product 3a
was assigned based on NOE studies (please see Supporting
Information for details). Formation of the E-isomer was not
detected. Next, reactions of boronate 2 with a broad range of
aldehydes were examined. As summarized in Scheme 3, the
addition of boronate 2 to various aromatic aldehydes with
different electronic properties and diverse substitution patterns
gave products 3b−f in 81−92% yields. Reactions of α,β-
unsaturated aldehydes with 2 afforded products 3g−h in 86−
91% yields. Aldehydes that contain a heterocycle, for instance,
benzothiophene or pyridine, reacted with allylboronate 2 to
provide products 3i−j in 88−92% yields. Importantly,
reactions of aliphatic aldehydes with 2 also occurred to deliver
TES-protected homoallylic alcohols 3k−n in 72−85% yields.
In all cases, products 3 were obtained with excellent Z-
selectivities.
a
Reaction conditions: allylboronate 2 (0.12 mmol, 1.2 equiv),
aldehyde (0.1 mmol, 1.0 equiv), toluene (0.2 mL), rt, then TESCl
(0.2 mmol, 2 equiv), imidazole (0.2 mmol, 2 equiv), rt, 12 h. The E/
Z selectivities were determined by H NMR analysis of the crude
reaction products. Yields of isolated products are listed.
b
1
c
isomer 6, respectively. In TS-2, the −SiMe2Ph group of reagent
2 occupies a pseudo-equatorial position, and an unfavorable
A1,2 allylic strain5c,11 between the β-boryl group and the
pseudo equatorially oriented −SiMe2Ph group is developed
(shown in light blue in TS-2). Moreover, TS-2 also suffers
gauche interactions between the −SiMe2Ph group and the
methyl groups of the pinacol moiety of reagent 2 (shown in
TS-2). By contrast, no apparent steric interaction is involved in
the competing transition state, TS-1. Therefore, allylation of
aldehyde with reagent 2 proceeded through the favored
The high Z-selectivity of this reaction can be rationalized by
the following transition state analyses.10 As depicted in Scheme
4, the reaction could proceed through two competing
transition states, TS-1 and TS-2, to form Z-isomer 3 and E-
B
DOI: 10.1021/acs.orglett.9b03819
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Scheme 4. Transition State Analysis for Aldehyde
Allylboration with Reagent 2
Scheme 5. Scope of Aldehyde for Chemoselective
a c
−
Allylsilation with Reagent 2
transition state TS-1 to give product 3 with high Z-selectivity
upon protection of the hydroxyl group.
It is well-established that allylsilane can react with carbonyl
compounds to give homoallylic alcohols or ethers in the
presence of an activator, for example, a Lewis acid.12 We
envisioned that the allylsilane moiety embedded in reagent 2
should participate in reactions with aldehydes in the presence
of an appropriate Lewis acidic reagent. For example, in the
presence of TMSOBn and TMSOTf, the allylsilane unit in
reagent 2 should react with aldehydes to give homoallylic
benzyl ethers 4 or 7 (Scheme 5). The allylboronate unit in 2
should be unreactive under these conditions. A notable feature
of this reaction is that both terminal and internal positions of
the olefin unit in product 4 or 7 are functionalized with a Bpin
group that will allow for sequential transformations. After some
optimization studies, we found that, upon exposure of
benzaldehyde to TMSOBn and TMSOTf at −78 °C for 6 h,
addition of allylation reagent 2 to the resulting oxocarbenium
intermediate gave homoallylic benzyl ether 4a in 82% yield
with 11:1 diastereoselectivity. The E-olefin geometry of
product 4a was assigned based on NOE studies (please see
Supporting Information for details). The developed conditions
were then applied to a variety of aldehyde substrates, and
homoallylic benzyl ethers 4b−j were obtained in 76−89%
yields with 7−15:1 E-selectivities (Scheme 5). It should be
noted that the reaction with aliphatic aldehydes gave a mixture
of E- and Z-isomers with low selectivities.13
An antiperiplanar stereochemical model is proposed to
rationalize the observed stereoselectivity of the allylsilation
reaction. As illustrated in Scheme 6, transition state TS-4,
which would lead to the formation of 7, suffers nonbonding
steric interactions between the R group of aldehyde substrate
and the α-pinacolatoboron moiety of reagent 2 (shown in light
blue in TS-4). Therefore, it is energetically disfavored. Similar
steric interactions are also present in transition states TS-5 and
TS-6 between the R group of the aldehyde substrate and the β-
pinacolatoboron moiety of reagent 2 (shown in light blue in
TS-5 and TS-6). Consequently, these two transition states are
also disfavored. By contrast, the mode of addition of reagent 2
to aldehyde in transition state TS-3 involves minimal steric
a
Reaction conditions: allylboronate 2 (0.12 mmol, 1.2 equiv),
aldehyde (0.1 mmol, 1.0 equiv), TMSOBn (0.13 mmol, 1.3 equiv),
TMSOTf (0.1 mmol, 1.0 equiv), CH2Cl2 (1.2 mL), −78 °C. The E/
Z selectivities were determined by H NMR analysis of the crude
b
1
c
reaction products. Yields of isolated products are listed.
interactions. Thus, allylsilation with reagent 2 proceeded
through the favored transition state TS-3 to give homoallylic
benzyl ethers 4 with high E-selectivities.
Products 3 and 4 generated from chemoselective allylation
contain an olefin group that is functionalized at both the
terminal and internal positions. The bifunctional alkene unit
can undergo a variety of sequential transformations to give
synthetically useful products. As shown in Scheme 7, Pd-
catalyzed Suzuki coupling of the vinyl boronate unit in product
3a with PhI gave product 8 in 81% yield.4b,14 The vinylsilane
group in 8 was then converted to vinyl iodide in 84% yield with
NIS. Subsequent Stille coupling of 9 with vinyl stannane 10
gave diene 11 in 82% yield.15 Suzuki coupling of 4a occurred
selectively with the terminal vinyl boronate unit to give
product 12 in 79% yield.4b Subsequent Pd-catalyzed oxidative
Heck reaction of 12 with ethyl acrylate gave diene 13 in 87%
yield.16 As such, the two vinyl boronate units in 4a underwent
sequential C−C bond-forming reactions to give product 13
without erosion of the E-olefin geometry. The internal Bpin
group of 12 was converted to bromide in the presence of
CuBr2 to give product 14 in 92% yield.4b Vinyl bromide 14
C
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moiety embedded in the reagent also reacted with aldehydes in
the presence of TMSOBn and TMSOTf to give γ,δ-
bifunctionalized homoallylic benzyl ethers with high E-
selectivities. The allylboron unit in the trifunctional reagent
is unreactive under these conditions. The origin of stereo-
selectivity of both allylation reactions was rationalized by
transition state analyses. Importantly, the bifunctionalized
alkene group in products 3 and 4 is amenable to a variety of
subsequent transformations. Asymmetric synthesis of enan-
tioenriched allylation reagent 2 is currently ongoing.18
Scheme 6. Transition State Analysis for Stereoselectivity of
Allylsilation with Reagent 2
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b03819.
Experimental procedures and spectra for all new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mzc0102@auburn.edu.
ORCID
Scheme 7. Transformation of Reaction Products
Shang Gao: 0000-0001-8166-5503
Ming Chen: 0000-0002-9841-8274
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support provided by Auburn University is gratefully
acknowledged. We thank AllylChem for a generous gift of
B2pin2.
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E
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