Enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes

Article abstract of DOI:10.1039/c7sc01254c

A practical and enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes is developed by employing a P-chiral monophosphorus ligand, BI-DIME. A series of diboronic esters containing a chiral tertiary boronic ester moiety are formed in excellent yields and ee's with the palladium loading as low as 0.2 mol%. DFT calculations revealed a concerted mechanism of oxidative addition of bis(pinacolato)diboron and allene insertion, as well as a critical dispersion effect on the origins of the enantioselectivity. The method is successfully applied to the concise and enantioselective synthesis of brassinazole.

Full text of DOI:10.1039/c7sc01254c

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DOI: 10.1039/C7SC01254C
Enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes is developed for the
first time by employing a P-chiral monophosphorus ligand BI-DIME.

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Enantioselective Palladium-Catalyzed Diboration of 1,1-
Disubstituted Allenes
Received 00th January 20xx,
Accepted 00th January 20xx
Jiawang Liu,a,§ Ming Nie,a,§ Qinghai Zhou,b,d Shen Gao,a Wenhao Jiang,a Lung Wa Chung,b,* Wenjun
Tang,a,* Kuiling Dinga,c,d,
*
DOI: 10.1039/x0xx00000x
A practical and enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes is developed by employing a P-
chiral monophosphorus ligand BI-DIME to form a series of diboronic esters containing a chiral tertiary boronic ester moiety
in excellent yields and ee’s with the palladium loading as low as 0.2 mol %. DFT calculations revealed a concerted
mechanism of oxidative addition of bis(pinacolato)diboron and allene insertion, as well as a critical dispersion effect on the
origins of the enantioselectivity. The method is successfully applied to a concise and enantioselective synthesis of
brassinazole.
www.rsc.org/
Chiral boronic esters have become versatile building blocks in diboronic ester products are useful chiral building blocks which
synthetic organic chemistry.1 Synthesis of chiral tertiary have led to the first enantioselective synthesis of a specific
boronic esters have attracted considerable interests recently brassinosteroid biosyntheꢀc inhibitor―brassinazole.19
and some notable methods are developed (Figure 1) including
Aggarwal-Matteson lithiation-borylation methodology from
chiral secondary alcohols,2 asymmetric hydroboration of 1,1-
disubstituted alkenes,3 and asymmetric borylation of allylic
carbonates,4 Michael acceptors,5 as well as tertiary halides.6
The asymmetric diboration of 1,1-disubstituted alkenes or
allenes7 would not only provide a chiral tertiary boronic ester
moiety, but also form an additional boronic ester component
for further transformations. Work by Morken on asymmetric
diboration8,9 with various transition metal catalysts (Rh, Pt, or
Pd) has provided significant progress in forming secondary
boronic ester products with excellent ee’s. However, the
asymmetric diboration of 1,1-disubstituted alkenes or allenes
to form chiral tertiary boronic esters remains challenging with
either low ee’s or low yields.9b Herein we communicate our
results on palladium-catalyzed asymmetric diboration of 1,1-
disubstituted allenes that have led to a series of diboronic
esters containing a chiral tertiary boronic ester moiety in
excellent enantioselectivities and yields with the employment
of a P-chiral monophosphorus ligand BI-DIME. The chiral
Figure 1. Formation of chiral tertiary boronic esters by
asymmetric diboration.
We chose to use buta-2,3-dien-2-ylbenzene (1a) as the
substrate to investigate the palladium-catalyzed asymmetric
diboration of 1,1-disubstituted allenes (Table 1). The reactions
were carried out at rt in cyclohexane for 24
h with
bis(pinacolato)diboron as the reagent in the presence of
Pd2(dba)3 (1 mol %) and a chiral phosphorus ligand (2.5 mol %)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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(Table 1). It was found that chelating phosphorus ligands such monophosphorus ligand AntPhos11
Journal Name
(
L7) provided a moderate
as BINAP and SDP did not provide any reactivities (entries 1-2). yield (60%) and an excellent ee (91% ee) (entry 7). Further
study of the P-chiral phosphorus ligands developed in our
laboratory showed that BI-DIME12
(L8) provided an almost
Table 1. Enantioselective Pd-catalyzed diboration of buta-2,3-dien-
2-ylbenzene (1a) with bis(pinacolato)diboron (2)
quantitative yield and a highest ee (94%) (entry 8). Screening
of the solvent showed the reaction was facilitated with a
nonpolar and non-coordinating solvent, as a diminished yield
was observed in either toluene, THF or dioxane. No reaction
was observed when dichloromethane was employed (entries
8-12). A Pd(0) precursor appeared to be advantageous for the
reaction since a diminished yield was observed when Pd(OAc)2
was applied (entry 13). Finally, the diboration was studied at a
low catalytic loading (0.2 mol % Pd, 0.25 mol % L8) and at a
gram scale (36 mmol 1a, 4.8 g). The product 3a (13.6 g) was
obtained in 97% yield and in 94% ee (entry 14), demonstrating
the practicality of this asymmetric transformation.
The substrate scope of this asymmetric diboration was then
investigated. As depicted in Table 2, a series of diboronic
esters with various electronic properties and substitution
patterns on the benzene ring (3b-p) were smoothly formed at
Entrya
1
L*
L1
L2
L3
L4
L5
L6
L7
L8
L8
L8
L8
L8
L8
L8
solvent
CyH
yield (%)b
ee (%)c
--
rt in excellent ee’s and yields with Pd-L8 as the catalyst. The
enantioselectivities obtained were slightly higher with
substrates having electron-donating substituents, but
0
2
CyH
0
--
3
CyH
15
95
76
45
60
98
82
51
27
0
52
59
79
12
91
94
92
91
92
--
substituents such as fluoro (3d) and trifluoromethyl group (3g
)
-
4
CyH
were well applicable. Substrates with an ortho substituent (3n
5
CyH
p
) were also tolerable. The reactions of both 1- and 2-naphthyl
6
CyH
substrates provided excellent yields and ee’s (3q-r). A chiral
furyl product 3s was also synthesized successfully. Substrates
with multiple substituents on the benzene ring were equally
effective for the transformation (3t-u). In order to test the
chemoselectivity between an allene and an olefin, a substrate
containing both moieties was subjected for diboration and
only the allene moiety was reactive under the current
conditions to form product 3v in 99% yield and 96% ee. Both
cyclic and heterocyclic substrates were applicable to smoothly
afford 3w and 3x, respectively, in excellent yields and ee’s.
Switch of the methyl substituent with an ethyl group on allene
resulted in an inferior ee (72% ee, 3y). Introduction of a
cyclopropyl group instead of the methyl substituent was less
effective (3z). 1,1-Dialkylallenes were also applicable. While a
moderate ee (67%) was obtained on product 3aa bearing two
primary alkyl substituents on the quaternary stereocenter,
high ee’s (87% and 91%) were obtained on products 3ab and
3ac containing both a methyl and a secondary alkyl groups at
the chiral center. A good ee was also achieved on 3ad bearing
7
CyH
8
CyH
9
toluene
THF
10
11
12
13d
14e
Dioxane
DCM
CyH
33
97
92
94
CyH
aUnless otherwise specified, the reactions were performed under nitrogen at rt
for 24 h with 1a (0.20 mmol), 2 (0.24 mmol), L* (2.5 mol %) and Pd2(dba)3 (1.0
mol %) in the specified solvent. Product 3a was the only detectable product. The
R absolute configuration of 3a was determined by comparing its optical rotation
with reported data.14 bIsolated yields. cDetermined by HPLC on a chiral IC-3
column. dPd(OAc)2 instead of Pd2(dba)3 was employed as the precursor. e1a (36.0
mmol), Pd2(dba)3 (0.1 mmol %), L8 (0.25 mmol %), 72 h.
A low yield (15%) and ee (52% ee) were observed when Me-
DuPhos was employed as the ligand (entry 3). It should be
noted that the diboration occured exclusively on the
substituted double bond of the allene, provided the prouct 3a
containing both a tertiary boronic ester and an alkenyl boronic
a
tertiary alkyl group. Finally, the diboration of 1,1-
ester moieties. Interestingly, the SKP ligand10
(L4) with a large
diarylallenes were studied. While a low ee was obtained on
product 3ae indicating little difference between a phenyl
substituent and a para-tolyl group, a moderate ee (75%) was
achieved on product 3af bearing both a phenyl and an ortho-
tolyl substituents at the quaternary stereocenter.
bite angle led to an excellent yield (95%) and a moderate ee
(59% ee) (entry 4). We thus predicted that the reaction could
be better promoted with a monophosphorus ligand, as
observed by Morken in diboration of monosubstituted
allenes.9 Thus,
a TADDOL-derived monophosphoramidite
To understand the mechanism of this catalytic asymmetric
reaction, the diboration of 1a was investigated with a scalemic
mixture of ligand L8. A perfect linear relationship between ee
of L8 and ee of product 3a was observed, indicating the
reaction catalyzed by a palladium catalyst composed of a
ligand L5 led to the formation of 3a in 76% yield and in 79 % ee
(entry 5). Another monophosphoramite ligand L6 derived from
a chiral BINOL backbone proved to be less effective, indicating
the importance of the ligand scaffold on both reactivity and
enantioselectivity of the reaction. Encouragingly, the P-chiral
2 | J. Name., 2012, 00, 1-3
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c
mol %) were employed. The absolute configurations were not
determined. dIncomplete conversions.
single monophosphorus ligand L8. Variation of the Pd:L8 ratio
from 1:1, 1:2, and 2:1 did not lead to a significant change of
both the yield and the enantioselectivity, further
demonstrating the
composition of a single L8 in the active palladium catalyst. To
understand the perfect regioselectivity and the stereochemical
model of this asymmetric diboration, energetics of the
catalytic transformation was calculated at B3LYP-D3/6-
31G(d)+SDD level. As shown in Figure 2a, the reaction initiates
from the palladium species RC, which undergoes oxidative
addition of bis(pinacolato)diboron concerted with allene
insertion where the boryl group migrates to the middle carbon
of the allene to give an η3 Pd(II)-allyl intermediate INTSi or
INTRe. Notably, this initial oxidative boryl migration16 is in well
agreement with the perfect regioselectivity of diboration on
the internal double bond of allene, and such process is an
Table 2. Enantioselective Pd-catalyzed diboration of 1,1’-
disubstituted allenesa
irreversible
and
stereo-determining
step
of
the
transformation.15 TS1Re is computed to be lower in free energy
than TS1Si by 1.4 kcal/mol, in qualitative agreement with the
observed enantioselectivity (ΔGexp: 2.1 kcal/mol).
aUnless otherwise specified, the reactions were performed
under nitrogen at rt for 24 h with 1 (0.20 mmol), 2 (0.24
mmol), L8 (2.5 mol %) and Pd2(dba)3 (1.0 mol %) in
cyclohexane (2.0 mL). The yields of isolated products were
shown here. The ee values were determined by HPLC on a
Figure 2. (a) Free-energy profile (in kcal/mol) for the Pd-
catalyzed asymmetric diboration of buta-2,3-dien-2-ylbenzene
chiral column. The R absolute configuration of 3f was
determined by X-ray crystallographic analysis,13 the others
were assigned by analogy. Pd2(dba)3 (2.0 mol %) and L8 (5.0
(
1a) with (S)-BI-DME (L8) as the ligand at B3LYP-D3/6-
31G(d)+SDD level.15 (b) VDW representation of the optimized
b
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Journal Name
TS1Re and TS1Si with relative free and electronic energies in
kcal/mol
Conclusions
In summary, we have developed
a
practical and
Distortion/interaction analysis17 reveals that there is a greater
dispersion interaction between the phenyl group17b on the
allene and one boryl group in TS1Re than TS1Si, which is
considered as the key factor for the enantioselectivity (Figure
2b).15 When excluding the dispersion contribution, the
enantioselectivity is computed to be reversed. Finally, the
reductive elimination via TS2Re proceeds from INTRe to produce
PCRe with a barrier of about 14.9 kcal/mol. Apart from
Morken’s proposal where the oxidative addition of diboron to
Pd proceeded prior to migratory insertion and the oxidative
addition of diboron to Pd was computed as the rate-
determining step,9l our calculations on diboration of 1a
revealed a concerted mechanism of oxidative addition of
bis(pinacolato)diboron and allene insertion, provided the first
computational insight on the origins of the enantioselectivity
(molecular details and a critical dispersion effect), and
disclosed the final reductive elimination step as the rate-
determining step.
enantioselective palladium-catalyzed diboration of 1,1-
disubstituted allenes which have led to the synthesis of a
series of diboronic esters containing a chiral tertiary boronic
ester in excellent yields and enantioselectivities with the
palladium loading as low as 0.2 mol %. The reaction enjoys a
broad substrate scope and good functional group
compatibility. The chiral ligand BI-DIME has proven to be
crucial for the success of the reaction. DFT calculations have
identified a concerted mechanism of oxidative addition of
bis(pinacolato)diboron and allene insertion, and revealed a
critical dispersion effect on the origins of the
enantioselectivity. Finally, the application of the chiral
diboronic ester to a concise and first enantioselective synthesis
of brassinazole has been successfully demonstrated.
The chiral diboronic ester products are versatile building
blocks in organic synthesis. For example, Aggarwal reported a
stereospecific allylation between the diboronic ester (S)-3a
and benzaldehyde to form a tetrasubstituted alkene after a
Suzuki-Miyaura cross-coupling.14,18,9k To further explore the
synthetic applications of such chiral diboronic esters, an
enantioselective synthesis of brassinazole,19 a specific inhibitor
of brassinosteroid biosynthesis, was studied with (R)-3a as the
starting material (Scheme 1). Surprisingly, despite its
significant biological properties, its enantioselective synthesis
was not reported to our knowledge. We envisioned that the
diboration product (R)-3a would provide a rapid and efficient
access to brassinazole through simple transformations. Thus,
(R)-3a was subjected to a one-pot, three step sequence: a)
hydroboration by treatment with 9-BBN;20 b) Suzuki-Miyaura
coupling with 1-chloro-4-iodobenzene; and c) oxidation under
Scheme 1. Asymmetric synthesis of brassinazole
Acknowledgments
conditions of H2O2/NaOH. The chiral diol
smoothly at a cis/trans ratio of 4:1 in 79% overall yield. Under
Ley’s conditions,[21] the diol
was readily oxidized to form the
corresponding sulfate , which was isolated as a pure cis
product in 78% yield. Finally, treatment of sulfate with 1,2,4-
triazole under conditions of NaH/DMF yielded (2R,3R)-
brassinazole ( ) in 80% yield, whose absolute configurations
were confirmed by X-ray crystallography.13 It should be noted
that the sulfate proceeded first through an intramolecular
4 was formed
This work is supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences XDB20000000,
NSFC-21432007, 21272254, 21572246, 21232009, 20421091,
CAS (QYZDY-SSW-SLH012, SLH029).
4
5
5
6
Notes and references
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SN2 reaction to form an epoxide, which was subsequently
attacked by 1,2,4-triazole to undergo a 2nd SN2 reaction,
yielding product
Compound was also transformed to its diastereomer
through an oxidation-reduction procedure, which ultimately
led to the formation of (2R,3S)-brassinazole ( ) via sulfate
6 with the net retention of stereochemistry.
4
7
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