Iridium-Catalyzed Enantioselective Allyl-Allyl Cross-Coupling of Racemic Allylic Alcohols with Allylboronates

Article abstract of DOI:10.1021/acs.orglett.8b03627

Carreira's iridium-(P, olefin) phosphoramidite-based catalytic system that allows asymmetric allyl-allylboronate cross-coupling with high enantioselectivity is reported. This transformation tolerates a large variety of racemic branched allylic alcohols an

Full text of DOI:10.1021/acs.orglett.8b03627

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iridium-Catalyzed Enantioselective Allyl−Allyl Cross-Coupling of
Racemic Allylic Alcohols with Allylboronates
Yu Zheng,†,‡ Bei-Bei Yue,†,‡ Kun Wei,† and Yu-Rong Yang*,†
†State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of
Sciences, Kunming 650201, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: Carreira’s iridium-(P, olefin) phosphoramidite-based catalytic system that allows asymmetric allyl−allylboronate
cross-coupling with high enantioselectivity is reported. This transformation tolerates a large variety of racemic branched allylic
alcohols and allylboronate substrates. The utility of the coupling is demonstrated in a concise catalytic asymmetric synthesis of
(−)-preclamol.
Scheme 1. Catalytic Asymmetric Allyl−Allyl Cross-Coupling
atalytic asymmetric formation of carbon−carbon bonds is
Cone of the most important challenges in modern organic
synthesis.1 Of a particular significance is the metal-catalyzed
allyl−allyl cross-coupling between allylic electrophile and
allylic nucleophile that provides enantioenriched chiral 1,5-
diene structures.2 Since chiral 1,5-dienes are abundant in
naturally occurring terpenes and also serve as valuable
intermediates and building blocks in organic synthesis, several
methods of their preparation have been reported.3 However,
the formation of the linear, achiral dienes and poor
enantioselectivity of the branched dienes render this allyl−
allyl cross-coupling challenging. In 2010, Morken and co-
workers made a breakthrough in this research and described
the Pd-catalyzed regio- and enantioselective cross-coupling of
allylic carbonates with allylboronates4 (Scheme 1). Later, the
Feringa group reported a Cu-catalyzed asymmetric allylation of
allyl bromide with allyl Grignard reagents.5 In 2014, Carreira
and co-workers developed a direct Ir-catalyzed cross-coupling
between branched, racemic allylic alcohols and simple olefins
to provide 1,5-dienes.6a In the same year, they further reported
an enantioselective allyl−allylsilane cross-coupling involving
racemic branched allylic alcohols and allylsilanes, expanding
the range of accessible chiral 1,5-dienes.6b
Methodologies
iridium-catalyzed allylations have emerged as a versatile tactic
for the synthesis of chiral building blocks.8,9 Inspired by the
recent seminal and systematic studies by the Carreira group,10
we proposed that the nucleophilic allylboronic ester would be
coupled with an electrophilic, branched, racemic allylic alcohol
under the conditions of Carreira’s Ir-(P, olefin) catalyst system.
This would offer an alternative to the known allyl−allyl cross-
coupling reactions. Here we wish to report our results.
Although allylboronates as the nucleophiles have been
utilized to the Pd-catalyzed asymmetric cross-coupling of
allylic carbonate in the pioneering work from the Morken
group, the similar Ir-catalyzed allylation of allylboronates has
yet to be reported. To our knowledge, there is only one case in
the literature reported by Jarvo and Barker, which documents
the Ir-catalyzed allylation of ketones using allylboronic ester,
leading to tertiary homoallylic alcohols.7 In recent years,
In the initial screening studies, we were pleased to find that
allylboronates could be used as excellent nucleophiles. As
shown in Table 1, with a catalyst system consisting of
Received: November 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03627
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Selected Optimization Studies for the Ir-Catalyzed
Allyl−Allylboronate Cross-Coupling
Scheme 2. Substrate Scope of the Enantioselective Allyl−
a
a b c
, ,
Allylboronate Cross-Coupling
b
c
entry
variation from the “standard” conditions yield (%)
ee (%)
1
2
3
4
5
none
82
23
NR
29
65
NR
>99
98
−
94
98
−
Sc(OTf)3 instead of Zn(OTf)2
TFA instead of Zn(OTf)2
1,4-dioxane instead of DCE
[Ir] (2 mol %), L1 (8 mol %)
L2 instead of L1
6
7
8
9
10
L3 instead of L1
L4 instead of L1
L5 instead of L1
L6 instead of L1
decomp
decomp
decomp
51
−
−
−
98
a
b
Reactions were run on 0.1 mmol scale. Isolated yields.
c
Determined by chiral HPLC analysis. − = not determined.
[{Ir(cod)Cl}2] (4 mol %) and Carreira’s ligand (R)-L1 (16
mol %) in the presence of Zn(OTf)2 as the promoter and DCE
as the solvent, the test substrate 2-naphthylvinyl carbinol 1a
underwent a smooth reaction with allylboronate 2a to provide
branched diene 3a in 82% yield and >99% ee (Table 1, entry
1). It is noteworthy that in our hands the undesired, achiral
linear diene was not observed. The use of other Lewis acid
such as Sc(OTf)3 as promoter led to low yield, albeit with high
ee value (entry 2). A switch to protic acid TFA gave no
reaction (entry 3). In 1,4-dioxane the reaction was performed
with reduced efficiency, and 3a could be obtained in a poor
yield and slightly lowered enantioselectivity (entry 4). When
the catalyst loading was decreased to 2 mol %, some reduction
in yield (entry 5) was observed. Interestingly, when Carreira’s
ligand (R)-L1 was replaced by other phosphoramidite ligands
such as (R)-L3, (S,R,R)-L4, or SPINOL-derived (R)-L5, a
complete decomposition of starting material occurred (entries
7−9).11 Furthermore, when BINAP L2 was used instead of
(R)-L1, no reaction took place (entry 6), while octahydro-
BINOL-derived (R)-L6 took effect and led to a moderate yield
(51%) with 98% ee (entry 10). Lastly, in accord with Carreira’s
recent mechanistic studies,10q we also found that better yields
were obtained when the catalyst was prepared in the presence
of allyl alcohol. For a detailed evaluation of the reaction
parameters, see the Supporting Information.
a
Unless otherwise noted, all reactions were performed on a 0.20
b
mmol scale under the standard conditions. Isolated yields.
c
Enantiomeric excess values were determined by HPLC on a chiral
d
stationary phase. HPLC analysis performed after derivatization.
e
Reaction was run with 0.1 M reaction concentration.
ring all afforded the corresponding chiral 1,5-dienes. Several
halogenated (3c−3i) substrates gave the products in good
yields and >99% ee. Substrates with methoxyl group were also
well tolerated (3j−3l). Interestingly, o- and p-OMe sub-
stitutions on the phenyl ring were less reactive than that of m-
OMe. Other substitutes such as methyl (3m and 3n) and ester
(3q) could be employed. Although substrates possessing
electron-withdrawing groups, such as nitro (3o) and
trifluoromethyl (3p), were less reactive, reactions of these
substrates still provide products in moderate yields with
excellent enantioselectivity. In addition, the carbinols derived
from heteroaromatic systems (3r−3t) could be employed as
well. Currently, one limitation of the reaction is that aliphatic
allylic alcohols did not undergo the described allyl−
allylboronate cross-coupling, while 1,3-dienes (3u and 3v)
were obtained.
Having established optimal conditions, we next set out to
explore the substrate scope and generality of this allyl−allyl
coupling reaction by using allylboronate 2a as summarized in
Scheme 2. 2-Naphthyl- (3a) and phenyl- (3b) vinyl carbinols
underwent the reaction giving excellent isolated yields and
enantioselectivity. Substrates with substituents on the aromatic
B
DOI: 10.1021/acs.orglett.8b03627
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Organic Letters
Letter
As illustrated in Scheme 3, we then surveyed a variety of
allylboronates under the optimal reaction conditions. The
dopaminergic drug candidate with antipsychotic therapeutic
effects for the treatment of schizophrenia.13,14 As shown in
Scheme 4, when m-OMe-phenylvinyl carbinol 1k was
Scheme 3. Scope of the Enantioselective Allyl−
a
Allylboronate Cross-Coupling with Respect to
Scheme 4. Enantioselective Synthesis of (−)-Preclamol
a b c
, ,
Allylboronates
a
Reagents and conditions: (a) 9-BBN, THF, 0 °C, then NaBO3, 0 °C,
73%; (b) O3, CH2Cl2, −78 °C, then NaBH4, −78 °C to rt; (c) MsCl,
Et3N, 25 °C; (d) propylamine, neat, 25 °C; (e) aqueous HBr, 120 °C.
subjected to the allyl−allylboronate cross-coupling conditions
described in the study at 2 mmol scale, 1,5-diene 3k was
furnished in 68% yield and >99% ee. With exposure of the 1,5-
diene 3k to 9-BBN, the alkenes of 3k could be effectively
differentiated, leading to a primary alcohol 6 in 73% yield.
Oxidative cleavage of 6 with ozone followed by the reductive
workup gave the diol 7, which could be further transformed
into (−)-preclamol with a known 3-step sequence.10n,13b
In summary, we have disclosed an iridium-catalyzed
asymmetric allyl−allylboronate cross-coupling reaction be-
tween unactivated racemic secondary allylic alcohols and
allylboronic esters to provide chiral 1,5-dienes. The method
affords the products with high regio- and stereoselectivity. The
high functional group compatibility and operational conven-
ience are also major features of the method, which offer an
alternative to known allyl−allyl cross-coupling reactions.
Finally, the utility of the coupling is demonstrated in a concise
catalytic asymmetric synthesis of (−)-preclamol.
ASSOCIATED CONTENT
■
a
S
* Supporting Information
Unless otherwise noted, all reactions were performed on a 0.20
b
mmol scale under the standard conditions. Isolated yields.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03627.
c
Enantiomeric excess values were determined by HPLC on a chiral
d
stationary phase. Reaction was run with 0.1 M reaction
concentration. 1.5 equiv of allylboronate.
e
Full characterization, analysis of enantioselectivity,
spectral data, and experimental procedures (PDF)
substituted allylboronic esters 4 are readily prepared by
Miyaura borylation of the corresponding allyl acetate and
some known procedures.4,12 Substitutions at the internal
carbon atom of the allylboronate were well tolerated.
Corresponding products using 2-methyl (5a), 2-hexyl (5b),
and 3,3-dimethyl (5c) substitutes on the allylboronate were
obtained in good yields and excellent enantioselectivity.
Additionally, the substituted allylboronates could be coupled
with many aromatic vinyl carbinols (5d−5i), giving the 1,5-
dienes in respectful isolated yields and superb enantioselectiv-
ity. Notably, another limitation of the current method is that
this reaction occurred with low diastereocontrol (dr 1.2:1, 5j
and 5k) in contrast to the good levels of diastereoselectivity in
the Morken4c and Carreira6b methods (dr ≥ 4:1), although the
yields and ee values are still excellent.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yangyurong@mail.kib.ac.cn.
ORCID
Yu-Rong Yang: 0000-0001-6874-109X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Key Research Program of the Frontier Sciences
of the CAS (Grant No. QYZDB-SSW-SMC026), the NSFC
(21672224, 21472200), and the Government of Yunnan
Province (2017FA002) for financial support.
The utility of this method is showcased through a concise
enantioselective synthesis of (−)-preclamol,10n,13 a potent
C
DOI: 10.1021/acs.orglett.8b03627
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Organic Letters
Letter
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