Chemo- and Stereoselective Crotylation of Aldehydes and Cyclic Aldimines with Allylic gem-Diboronate Ester

Article abstract of DOI:10.1021/acs.orglett.7b01821

We report a highly chemo- and stereoselective crotylation of aldehydes and cyclic aldimines with allylic-gem-diboronate ester as a new type of organoboron reagent. The allylic-gem-diboronate ester undergoes the crotylation with aldehydes and cyclic aldimines in excellent stereoselectivity, forming anti-5,6-disubstituted oxaborinin-2-ols or (E)-δ-boryl-anti-homoallylic amines in high efficiency. The reaction shows a wide range of substrate scope and excellent functional group tolerance. The synthetic applications of the obtained products, including stereospecific C-C, C-O, and C-Cl bond formation, are also demonstrated.

Full text of DOI:10.1021/acs.orglett.7b01821

Letter
pubs.acs.org/OrgLett
Chemo- and Stereoselective Crotylation of Aldehydes and Cyclic
Aldimines with Allylic gem-Diboronate Ester
Jinyoung Park,† Seoyoung Choi,† Yeosan Lee, and Seung Hwan Cho*
Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
S
* Supporting Information
ABSTRACT: We report a highly chemo- and stereoselective
crotylation of aldehydes and cyclic aldimines with allylic-gem-diboronate
ester as a new type of organoboron reagent. The allylic-gem-diboronate
ester undergoes the crotylation with aldehydes and cyclic aldimines in
excellent stereoselectivity, forming anti-5,6-disubstituted oxaborinin-2-
ols or (E)-δ-boryl-anti-homoallylic amines in high efficiency. The
reaction shows a wide range of substrate scope and excellent functional
group tolerance. The synthetic applications of the obtained products,
including stereospecific C−C, C−O, and C−Cl bond formation, are also demonstrated.
compatibility and limits the synthetic application of these
reagents. Therefore, the development of new types of gem-
diboron compounds that exhibit new reactivity and selectivity is
highly desirable.
he preparation of new types of organoboron compounds is
Tespecially important owing to their stability and ability to
undergo a wide range of organic transformations.1,2 In recent
years, gem-diborylalkanes have emerged as attractive synthetic
intermediates for synthesizing organoborons via transition-
metal-catalyzed or transition-metal-free chemo- and stereo-
selective transformations with suitable electrophiles.3,4 Although
considerable advances have been made in recent years, most of
the methods developed employed alkyl-substituted gem-diboron
reagents, the use of which necessitates strong base (MOH or
MO-t-Bu, M = Li, Na, K) to activate one of the pinacolato boron
(Bpin) units of the gem-diborylalkane chemoselectively through
the formation of an α-borylalkylmetal3 species or α-borylcarban-
ion5 (Scheme 1a). This leads to a lack of functional group
The stereoselective addition of allylorganometallics to
aldehydes and imines was of central importance in synthetic
chemistry because it could be used to form homoallylic alcohols
or amines, which constitute prevalent motifs in many
pharmaceuticals and natural products.5,6 In recent decades,
research in this field has focused on accessing and increasing the
scope of allylorganometallics while achieving high stereo-
selectivity. Although these systems were still considered powerful
and reliable tools in organic synthesis, the vast majority of them
employed allylic-mono-organometallic species, and the use of
allylic-gem-diorganometallics has been barely studied (Scheme
1b).7,8 As a rare example, the reaction of aldehydes with δ-silyl
allylboronate esters was reported to afford (Z)-δ-silyl homoallylic
alcohols or (E)-δ-boryl homoallylic alcohols with moderate to
high diastereoselectivity in the presence of an appropriate Lewis
acid under harsh reaction conditions.7a,b Furthermore, the
chemo- and enantioselective allylboration of aldehydes with α-
stannylallylboranes was reported to provide enantioenriched
(E)-α-stannyl homoallylic alcohols at very low temperature (−78
°C).7c−f However, this reaction required the use of relatively
unstable and toxic tin reagents, thus limiting its synthetic utility.
Consequently, the development of a systematic and predictable
method employing readily accessible, nontoxic, and benchtop-
stable allylic-gem-diorganometallics remains an important
challenge. Herein, we report the design and synthesis of (E)-
allylic-gem-diboronate esters and their utilization in chemo- and
stereoselective crotylation of aldehydes and cyclic aldimines
(Scheme 1c).
Scheme 1. Chemo- and Stereoselective Transformation of
gem-Diorganometallic Reagents
Received: June 15, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01821
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
As part of our continuing efforts to develop chemo- and
stereoselective transformations of gem-diborylalkanes,3k,m,4b−d
we attempted to prepare allylic-gem-diboron compounds that
bear two identical boron units at the allylic position. The
preparation of these proposed reagents was expected to allow the
chemo- and stereoselective allylboration of aldehydes and imines
to form homoallylic alcohols and amines that contain the other
boron unit at the terminal alkene position. To achieve our
desired goal, we designed a strategy to synthesize allylic-gem-
diboron compounds by the metal-catalyzed olefin isomerization
of homoallylic gem-diboronate ester 1, which could be easily
prepared by an SN2 reaction between diborylmethane and allyl
bromide using lithium diisopropylamide (LDA) as a base.
Inspired by several precedents,9 we investigated the viability of
the proposed strategy employing a range of transition-metal
catalysts with 1a as a model substrate.10 When 1a was employed
as a substrate in the presence of a cationic iridium catalyst having
SbF6 as a counteranion with a tricyclohexylphosphine ligand in
1,2-DCE at room temperature, the starting material was
recovered. Interestingly, the effect of the counteranion in the
iridium catalyst was dramatic, and when a catalyst bearing BArF
[BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate] or PF6
as a counteranion was used, the corresponding allylic-gem-
diboronate ester 2a was obtained in 70% and 76% conversion,
respectively, with excellent (E)-selectivity. The high conversion
(87%) was achieved when Ir[(COD)]2OTf was used as a catalyst
in 1,2-DCE. Further solvent screening revealed that tert-amyl
alcohol was the most effective solvent, and the desired allylic-
gem-diboronate ester 2a was obtained in a nearly quantitative
yield (Scheme 2).10 It should be noted that the reaction was
Scheme 3. Substrate Scope of Aldehydes for Chemo- and
Stereoselective Crotylation Reactions
a,b
a
Reaction conditions: (E)-2a (1.1 equiv), aldehyde (3, 0.20 mmol),
b
and toluene (2.0 mL) at room temperature for 3 h. Isolated yields are
provided. Isolated yield on a 1.0 mmol scale.
c
stereoselective crotylation of aldehyde 3 with (E)-2a at room
temperature (Scheme 3). Interestingly, when benzaldehyde and
(E)-2a were subjected in toluene at room temperature, the
corresponding anti-oxaborinin-2-ol 4a was obtained in 80% yield.
The anti- and (Z)-selectivity observed clearly indicated that the
reaction proceeded via a chair-like transition state with
subsequent cyclization, affording the corresponding product 4
(vide infra). Different para-substituted benzaldehydes bearing
electron-donating (4b and 4c) or electron-withdrawing sub-
stituents (4d and 4e) on the aromatic ring underwent the
crotylation reaction in good to excellent yields (76−86%).
Benzaldehydes bearing fluoro, chloro, and bromo substituents
were compatible with the crotylation conditions, affording 4f−h
in good yields. The reactions exhibited excellent functional group
tolerance, and benzaldehydes containing ester (4i), cyano (4j),
and alkene (4k) substituents underwent crotylation with good
efficiency. Substrates having substituents at the meta- and ortho-
position participated in crotylation without difficulty, forming
4k−m in good yields. The reaction of heteroaryl aldehydes
resulted in the formation of the corresponding products 4n and
4o in 62% and 80% yields, respectively. Cinnamyl and aliphatic
aldehydes were also proceeded to form products 4p and 4q in
good yields.
Scheme 2. Synthesis of Allylic gem-Diboronate Ester and the
Proposed Model for Iridium-Catalyzed Olefin Isomerization
Next, we wondered whether allylic-gem-diboronate ester (E)-
2a could be exploited in the chemo- and stereoselective
crotylation reactions with imines. Because crotylation of imines
could form homoallylic amines, core scaffolds in many
pharmacetucals,5a−c,12 the development of efficient and selective
methods to access homoallylic amines containing the Bpin group
at the terminal alkene position would be intriguing. To assess the
viability of the crotylation of imines with (E)-2a, we subjected a
range of N-protected imines to the crotylation conditions of
aldehydes; but no desired products were obtained. Extensive
screening revealed that the benzo[1,2,3]oxathiazine 2,2-dioxide
(5a) was a viable electrophile, and the desired δ-boryl-
homoallylic amine 6a was obtained in 1,2-DCE at 90 °C with
excellent anti- and (E)-selectivity in 84% isolated yield.10 This
result suggested that the cis configuration between the N-
efficiently scaled to 10 mmol without difficulty, and the desired
allylic-gem-diboronate ester (E)-2a was isolated in 91% yield (2.8
g) by recrystallization from n-hexane at −78 °C. We assumed
that the olefin isomerization proceeded via a cationic π-allyl
intermediate, and the intermediate 1a-A is expected to be more
favorable than 1a-B mainly due to steric hindrance between the
methyl group and the Bpin group in 1a-B. Indeed, when we
1
monitored the olefin isomerization by H NMR, (E)-2a was
observed exclusively over time.10 Unfortunately, attempts to
prepare other substituted allylic-gem-diboron compounds were
unsuccessful under the reaction conditions, and only starting
materials were recovered.11
After establishing efficient conditions to prepare the allylic-
gem-diboronate ester (E)-2a, we investigated the chemo- and
B
DOI: 10.1021/acs.orglett.7b01821
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
protecting group and the aromatic ring was a crucial factor in the
reaction, presumably because the trans configuration of the N-
protected imines suffered from steric hindrance between the N-
protecting group and the boron unit of the allylic-gem-diboronate
ester when forming the closed transition state. The obtained
product 6a was crystalline, and the stereoselectivity of its
formation was confirmed by single-crystal X-ray diffraction.10
Because the product displayed complete anti- and (E)-selectivity,
we assumed the reaction proceeded via half-chair-like transition
state (Scheme 4). When one of the boron units in (E)-2a bound
chloro, and bromo groups were compatible with the reaction
conditions, affording the corresponding products 6e−h in good
yields. Cyclic aldimines that have substituents at the ortho-
position on the aromatic ring gave the corresponding products 6i
and 6j in good yields. Naphthyl-substituted and dioxole-
containing cyclic aldimines reacted efficiently, affording 6k and
6l in 83% and 93% yield, respectively. Note that all the reaction
products obtained in this study exhibited complete anti- and (E)-
selectivity.
To underscore the synthetic utility of the products obtained in
this study, we sought to identify conditions for Pd-catalyzed
cross-coupling with aryl iodides (Scheme 6). When the resulting
Scheme 4. Proposed Model for the Crotylation of (E)-2a with
Aldehydes and Cyclic Aldimines
a
Scheme 6. Further Transformations of 4a and 6a
to the nitrogen atom of the substrate, the corresponding half-
chair-like transition state was formed, and subsequent allyl group
transfer led to the desired homoallylic amine with excellent anti-
and (E)-selectivity.
Having established the optimal conditions, we investigated the
substrate scope for cyclic aldimines 5 with (E)-2a, and the results
are summarized in Scheme 5. In all cases, the obtained products
were obtained by recrystallization from a mixture of Et2O and n-
hexane at −20 °C. Benzo[1,2,3]oxathiazine-2,2-dioxides having
electron-donating substituents (6b−d) underwent chemo- and
stereoselective crotylation in moderate to excellent yields. The
reactions of cyclic aldimines bearing halides such as fluoro,
a
Reaction conditions: (a) cat. Pd(PPh3)4, PhI, CsF, THF/H2O, 50 °C,
16 h; (b) NaBO3·4H2O, THF/H2O, rt, 3 h; (c) PCC, NaOAc,
CH2Cl2, rt, 12 h; (d) CuCl2, CH2Cl2, rt, 6 h; (e) cat. Pd[P(t-Bu)3]2,
PhI, KOH, THF/H2O, rt, 3 h.
4a was treated with Pd(PPh3)4 in the presence of 2 equiv of CsF
as a base in a mixture of 1,4-dioxane and H2O at 50 °C, the
corresponding coupled product 7 was obtained in 90% yield with
retention of (Z)-configuration. The boron unit of 4a was
oxidized in the presence of sodium perborate to form a 1:1.7
diastereomeric mixture of hemiacetal, and subsequent PCC
oxidation gave the corresponding γ-butyrolactone 8 in 77% in
two steps. A chlorine atom could be introduced at the boron
functionality of 4a in the presence of 2 equiv of copper(II)
chloride, forming the corresponding (Z)-alkenyl chloride 9 in
68% yield. The obtained product 6a could also be achieved for a
C−C bond-forming reaction, and the corresponding product 10
was obtained in the presence of a catalytic amount of Pd[P(t-
Bu)3]2 and 3 M aq KOH as a base at room temperature in THF in
93% yield. Treatment of 6a with CuCl2 resulted in the formation
of (E)-δ-chloro-anti-homoallylic amine 11 in 91% yield.
Scheme 5. Substrate Scope of Cyclic Aldimines for Chemo-
a,b
and Stereoselective Crotylation Reactions
In summary, we successfully developed a highly efficient
chemo- and stereoselective crotylation system for aldehydes and
cyclic aldimines using an allylic-gem-diboronate ester as a new
type of organoboron compound.13 The reaction showed broad
substrate scope with respect to aldehydes or cyclic aldimines,
forming the anti-5,6-disubstituted oxaborinin-2-ols or (E)-δ-
boryl-substituted anti-homoallylic amines in high yields. The
stereoselectivity obtained indicated that the reaction of the
allylic-gem-diboronate ester with aldehydes proceeds via a chair-
like transition state, whereas that with cyclic aldimines proceeds
via a half-chair-like transition state. The further synthetic
applications of the reaction products were demonstrated by
stereoselective Pd-catalyzed cross-coupling and C−O and C−Cl
bond-forming reactions. Further studies to expand the scope of
the crotylation reactions and develop an asymmetric version of
this transformation are ongoing in our laboratory.14
a
Reaction conditions: (E)-2a (1.5 equiv), cyclic aldimine (5, 0.20
b
mmol), and 1,2-DCE (0.4 mL) at 90 °C for 16 h. Isolated yields are
provided. Isolated yield on a 1.0 mmol scale.
c
C
DOI: 10.1021/acs.orglett.7b01821
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01821.
■
S
Experimental procedures, characterization of all new
compounds, and crystallographic data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: seunghwan@postech.ac.kr.
ORCID
(7) (a) Shimizu, M.; Kitagawa, H.; Kurahashi, T.; Hiyama, T. Angew.
Chem., Int. Ed. 2001, 40, 4283. (b) Carosi, L.; Lachance, H.; Hall, D. G.
Tetrahedron Lett. 2005, 46, 8981. (c) Chen, M.; Roush, W. R. J. Am.
Chem. Soc. 2011, 133, 5744. (d) Stewart, P. S.; Chen, M.; Roush, W. R.;
Ess, D. H. Org. Lett. 2011, 13, 1478. (e) Chen, M.; Roush, W. R. Org.
Lett. 2012, 14, 1880. (f) Chen, M.; Roush, W. R. J. Am. Chem. Soc. 2012,
134, 3925.
Seung Hwan Cho: 0000-0001-5803-4922
Author Contributions
†J.P. and S.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT & Future
Planning (NRF-2015R1C1A1A02036326). J.P. thanks the
National Research Foundation of Korea (NRF) for the global
Ph.D. fellowship (NRF-2016H1A2A1908616).
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DOI: 10.1021/acs.orglett.7b01821
Org. Lett. XXXX, XXX, XXX−XXX