Enantioselective Isoprenylboration Reaction of Aldehydes Catalyzed by a Chiral Phosphoric Acid

Article abstract of DOI:10.1002/adsc.201900203

The BINOL-derived chiral phosphoric acid (R)-TRIP is utilized as an organocatalyst in the asymmetric isoprenylboration reaction of aldehydes, wherein hydrogen-bond interactions play a key role in the control of enantioselectivity. A wide arrays of enantioenriched dienyl homoallyl alcohols, including two natural products (?)-Ipsdienol and (?)-Ipsenol, have been successfully constituted. The synthetic application to chiral isoprenylated isobenzofuranone, vinyloxirane and cyclohexene derivatives has also been disclosed. (Figure presented.).

Full text of DOI:10.1002/adsc.201900203

Title: Enantioselective Isoprenylboration Reaction of Aldehydes
Catalyzed by a Chiral Phosphoric Acid
Authors: Yiyong Huang, Yulong Zhang, Bojun He, Yiwen Xie, Yuhao
Wang, Yongcun Shen, and Yilong Wang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900203
Link to VoR: http://dx.doi.org/10.1002/adsc.201900203

10.1002/adsc.201900203
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Enantioselective Isoprenylboration Reaction of Aldehydes
Catalyzed by a Chiral Phosphoric Acid
Yu-Long Zhang,† Bo-Jun He,† Yi-Wen Xie, Yu-Hao Wang, Yi-Long Wang, Yong-Cun
Shen, and Yi-Yong Huang*
Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, Wuhan University of
Technology, Wuhan 430070, China
E-mail: huangyy@whut.edu.cn
†These authors contributed equally to this work.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
been documented by using different organometallic
Abstract. The BINOL-derived chiral phosphoric acid (R)-
TRIP is utilized as an organocatalyst in the asymmetric catalysts. For example, the Yu group reported a chiral
BINOL/TiIV-promoted asymmetric isoprenylation of
isoprenylboration reaction of aldehydes, wherein hydrogen-
bond interactions play a key role in the control of
enantioselectivity. A wide arrays of enantioenriched dienyl
homoallyl alcohols, including two natural products (-)-
Ipsdienol and (-)-Ipsenol, have been successfully
constituted. The synthetic application to chiral
aldehydes with an isoprenylstannane reagent (Scheme
1a).[9] During our manuscript preparation, the Krische
group accomplished an elegant iridium-catalyzed
asymmetric reductive coupling of isoprenyl carbonate
and primary alcohols (or aldehydes) (Scheme 1b).[10]
Therefore, we commence this study to devise new
catalytic approach to implement the task of obtaining
optically pure DHAs efficiently.
isoprenylated
isobenzofuranone,
vinyloxirane
and
cyclohexene derivatives has also been disclosed.
Keywords: Allylation; Chiral phosphoric acid; Boron
reagent; Isoprenylation; Phthalide
Dienyl homoallyl alcohols (DHAs) represent an
important family of compounds, not only because
they are ubiquitous structural units embedded in
naturally occurring products, but also serve as
versatile synthetic intermediates for the conjugated
diene and hydroxy functional groups.[1] As shown in
Figure 1, (-)-Ipsdienol and (-)-Ipsenol are two natural
products of the aggregation pheromones isolated
from the bark beetle Ips. paraconfusus.[2] Vital
nutrient Vitamin D3,[3] Calcipotriol (treatment of
psoriasis)[4] and natural product Brassicicene D[5] all
share the key DHA-fragment. In 2012, the Leighton
group applied chiral DHAs into the total synthesis of
polyketide natural products.[6] In 2015, the Song
group showcased the synthetic utility of DHAs by the
sequential hydrohalogenation/prins cyclization for the
synthesis of multisubstituted tetrahydropyrans
inculding
Accordingly, the development of efficient synthetic
approaches to such type of
molecules, both racemic or enantioenriched, remains
highly desirable.[8] Especially, the catalytic
asymmetric synthesis of chiral DHAs represents a
challenging work, and only two reports have thus far
Figure 1. Structural importance and synthetic application
of dienyl homoallyl alcohols.
Organoboronate reagents have emerged as
indispensable tools since most of them are readily
available, non-toxic, air and water stable, and tolerant
to many functional groups. Various activation
strategies assisted by chiral organo- or organometallic
catalysts have been established to transfer functional
groups‒like allyl, allenyl, propargyl and aryl‒in an
asymmetric manner. Since the seminal work of chiral
natural
product
(-)-Exiguolide.[7]
phosphoric
acid
catalyzed
asymmetric
1
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10.1002/adsc.201900203
Advanced Synthesis & Catalysis
allylboronation of aldehydes was reported by the
Antilla group,[11] the extensive exploration of using
other types of unsaturated boronate reagents and
related computational studies have been further
compiled.[12] Such synthetic tactic features easy
operation, exclusive γ-selective addition and high
level of enantioselectivities. We reasoned that
pinacolyl isoprenylboronate can be potentially
employed to the asymmetric nucleophilic addition
reaction of aldehydes to install chiral DHAs including
natural products of (-)-Ipsdienol and (-)-Ipsenol
(Scheme 1c), although pinacolyl isoprenylboronate
was solely utilized in Diels-Alder and 1,4-
hydrovinylation reactions in previous work.[13] In this
context, we herein introduce an unprecedented
asymmetric isoprenylboronation of aldehydes in the
presence of a chiral Brønsted acid catalyst.[14]
a significant increase in er (96.7:3.3) was observed in
the solvent mixture toluene and CCl4 (1:1) (entry 8).
The use of other BINOL-derived chiral phosphoric
acids with various substituents at the 3,3’-positions
((R)-4b, (R)-4c, (R)-4d) (entries 9-11), further
increasing the volume ratio of CCl4, or bringing down
the temperature with (R)-4a afforded inferior results
(entries 12-13). Control experiment in the absence of
4Å MS resulted in a much lower er, which suggested
that trace amount of water has a detrimental effect on
the enantioselectivity through affecting the
interaction between catalyst and substrates (entry 14).
The comparison study of catalyst loading indicated
that 10 mol% was the most suitable amount for the
transformation (entries 15-16).
Table 1. Screening of solvents and temperature.[a]
Scheme 1. Catalytic asymmetric entries to chiral dienyl
homoallyl alcohols.
In order to obtain the optimal reaction conditions
to get chiral isoprenylated products with satisfactory
yields and enantioselectivities, the model reaction
using
benzaldehyde
1a
and
pinacolyl
isoprenylboronate 2 was firstly carried out. As shown
in Table 1, the background reaction without the
assistance of any catalysts proceeded smoothly in
toluene at room temperature (rt), and racemate 3a
was provided in 88% yield after 3 h (entry 1). In
order to suppress the background reaction and
achieve excellent enantio-induction, the reaction
temperature was decreased along with using a chiral
Brønsted acid (10 mol%) to activate the boronate
reagent. The commonly used BINOL-derived chiral
phosphoric acid (R)-4a ((R)-TRIP) was firstly
selected. Initial temperature screening in toluene
With the optimized reaction conditions in hand, we
next examined the scope of this enantioselective
isoprenylation by using a wide array of commercially
available aldehydes. The results were summarized in
Scheme 2. Initially, the substituent effects including
diverse electronic, steric and position of substituted
groups on the benzene ring were checked. Comparing
with the catalytic results obtained from
benzylaldehyde 1a to product 3a (95% yield and
96.7:3.3 er), the incorporation of electron-donating
o
revealed that -40 C was the best choice in terms of
efficiency and enantioselectivity (entries 2-5), and
(R)-3a was delivered in 89% yield with 93.1:6.9
enantiomeric ratio (er) (entry 4). Under the same
temperature, CH2Cl2 slovent was unable to improve
the enantioselectivity (entry 6). The solvent mixture
toluene and cyclohexane gave the same
enantioselectivity (entry 7 vs entry 4). To our delight,
i
groups (MeO-, Me- and Pr-) at the para-position
provided identical level of yields and enantiomeric
ratios, albeit the bulky tBu group resulted in a lower
er (3e, 91.5:8.5 er). Similar reactivities and >94.9:5.1
er were found when using arylaldehydes containing
2
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10.1002/adsc.201900203
Advanced Synthesis & Catalysis
halogen groups (F-, Cl- and Br-). The presence of
electron-withdrawing groups (-CO2Me and -NO2)
seemed more challenging. Chiral adduct 3i was
obtained under the standard reaction conditions,
whereas 3j was achieved with a satisfactory er under
er vs 91.5:8.5 er). The asymmetric transformation of
heteroaromatic aldehydes was exemplified with furyl
and thienyl derivatives. 2-Furyl-derived DHA 3t was
delivered with 90% yield and 90:10 er. Gratifyingly,
2-thienyl adduct 3u was formed with a perfect er of
newly optimized reaction conditions (CH2Cl2, -60 oC). 98.4:1.6, and other two products with an additional
In the case of substituents at the meta-position,
methyl substituent 3v and 3w were also installed with
slightly lower enantioselectivities. Furthermore,
arylalkynyl and arylalkenyl aldehydes (1x and 1y)
underwent the asymmetric isoprenylation with an
identical er (93.6:6.4), and the incorporation of a
methyl group at the α-position of 1y was detrimental
to the yield and enantio-induction (3y, 87% yield and
90:10 er). Aliphatic aldehydes remain more
challenging than the above aromatic cases in terms of
selected groups including MeO-, Br- and -CN
exhibiting various electronic properties
were
surveyed with high enantioselective outcome (3k, 3l
and 3m). Substituents at the ortho-position impose
higher steric congestion than other positions, which
may has a serious effect on the enantioinduction. For
instance, high er value (95.8:4.2) was observed for
the Me-substitued aldehyde under the standard
conditions, whereas the CF3- and iodo-substituted
aldehydes were transformed with satisfactory results
under a set of reaction conditions like aldehyde 1j.
reactivity
and
enantiofacial
discrimination.
Comparable results were achieved for two
arylalkynyl products 3aa and 3ab in CH2Cl2 at -60 oC.
The asymmetric isoprenylboration of 3,4- (-)-Ipsdienol 3ac was generated in 75% yield and
dimethylbenzaldehyde 1q was also performed in 95%
yield with 95.5:4.5 er. 2-Naphthyl aldehyde 1s
converted to compound 3s with a higher er
comparing with the 1-naphthyl aldehyde 1r probably
due to the negative steric hindrance effect (96.1:3.9
92.8:7.2 er under the standard contions, and (-)-
Ipsenol 3ad was accessed with an improvable yield
and er under the revised reaction conditions (CH2Cl2,
-60
oC).
3
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10.1002/adsc.201900203
Advanced Synthesis & Catalysis
Scheme 2. Aldehyde scope in the asymmetric isoprenylboration.[a,b,c]
The absolute configuration of the chiral
chiral carbon atom of compound 3a is determined as
R-configuration, which suggests that the asymmetric
isoprenylated products including (-)-Ipsdienol and (-
)-Ipsenol was assigned by comparing with the optical
rotation of known compounds,[10] and the X-ray
crystal structure of compound 3l was also determined
to verify the stereochemical results.[15] Therefore, the
isoprenylboration
across
the
Re-face
of
benzylaldehyde mediated by the chiral phosphoric
acid (R)-4. The stereochemistry can be explained by
using
the
transition
model
4
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10.1002/adsc.201900203
Advanced Synthesis & Catalysis
proposed by Goodman,[12g] and already used in our pr
evious work.[12q] As shown in Figure 2, the molecular
recognition between chiral phosphoric acid and the
six-membered cyclic chair-like transition state
assembled
from
benzylaldehyde
and
isoprenylboronate is based on hydrogen bonding. The
protonation of boronate oxygen atom can efficiently
increase the Lewis acidity of boron atom and enhance
the electrophilicity of aldehyde,[16] thus accomplish
an enantioselective reaction pathway by suppressing
the competitive racemic background process at low
temperature. In terms of enantiofacial discrimination,
the favorable Re-face nucleophilic attack probably
results from the minimized steric repulsion between
catalyst and substrates in TS-Re instead of TS-Si.
Scheme 3. Synthetic utility.
In summary, we have demonstrated a novel, useful
and easily-handled approach to build enantioenriched
dienyl homoallyl alcohols comprising two nature
products (-)-Ipsdienol and (-)-Ipsenol through the
chiral phosphoric acid-catalyzed asymmetric
isoprenylboration of aldehydes. Wide substrate scope
as exemplified by aryl, heteroaryl, arylalkynyl,
arylalkenyl, and aliphatic aldehydes were observed
with up to 98% yield and 98.4:1.6 er. A cyclic
transition state model involving the chiral phosphoric
acid activation through the double hydrogen-bond
formation was proposed to understand the
enantiotopic face differentiation. Notably, the
synthetic application of such reaction protocol for the
facile access to chiral isobenzofuranone, vinyloxirane
and cyclohexene frameworks has been finally
presented. Further study on the asymmetric
isoprenylboration of other electrophiles is ongoing in
our lab.
Figure 2. Proposed transition states.
In order to construct chiral O-containing
heterocycle fameworks, we then turned our attention
to apply such catalytic protocol into the asymmetric
tandem isoprenylboration/cyclization reaction.[17]
Chiral isobenzofuranones (or called “phthalides”) are
important compounds possessing a broad range of
biological activities, and the exploration of catalytic
asymmetric approaches to the entities has drawn great
Experimental Section
interest.[18]
Methyl 2-formylbenzoate 5 and 2
General
procedure
for
the
asymmetric
underwent the initial asymmetric isoprenylboration at
-60 oC for 40 h and sequential intramolecular
transesterification at rt for 24 h to afford the desired
enantioenriched isoprenyl-substituted phthalide 6 in
90% yield with 82.5:17.5 er. Furthermore, the
synthetic utility of chiral adduct (R)-3a involving
tBuO2H-mediated asymmetric epoxidation and Diels-
Alder reaction was displayed, and chiral vinyloxirane
7 and cyclohexene 8 were prepared with good results
(Scheme 3).
isoprenylboration of aldehydes: To an oven dried 2 mL
test tube with a stir bar was added catalyst (R)-4 (10 mol%)
and 4 Å MS (25.0 mg). In nitrogen atmosphere, aldehyde 1
(0.10 mmol) and anhydrous solvent (0.2 mL) were added
at rt. The reaction mixture was then cooled to the
temperature indicated in the main text, followed by the
addition of a solution of pinacolyl isoprenylboronate 2
(0.15 mmol) (0.1 mL of solvent) over 20 minutes. The
mixture was stirred for the time indicated in Scheme 2,
then subjected to preparative thin layer chromatography to
get the target isoprenylated products 3.
Acknowledgements
We gratefully thank the financial support of this investigation by
the National Natural Science Foundation of China (21573169,
21772151), Natural Science Foundation of Hubei Province
(2018CFA084), and Wuhan Morning Light Plan of Youth Science
and Technology (2017050304010314).
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10.1002/adsc.201900203
Advanced Synthesis & Catalysis
COMMUNICATION
Enantioselective Isoprenylboration Reaction of
Aldehydes Catalyzed by a Chiral Phosphoric Acid
Adv. Synth. Catal. Year, Volume, Page – Page
Yu-Long Zhang,† Bo-Jun He,† Yi-Wen Xie, Yu-
Hao Wang, Yi-Long Wang, Yong-Cun Shen, and
Yi-Yong Huang*
8
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