γ-Silylboronates in the chiral Br?nsted acid-catalysed allylboration of aldehydes

Article abstract of DOI:10.1039/c4cc08598a

The use of functionalised allylboronic esters in the catalytic enantioselective allylboration of aldehydes is described for the first time. γ-Silylallyl pinacolate derivatives give rise to α-silyl homoallylic alcohols in high yields, with complete diaster

Full text of DOI:10.1039/c4cc08598a

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γ-Silylboronates in the Chiral Brønsted Acid-Catalysed
Allylboration of Aldehydes
-Cite this: DOI: 10.1039/x0xx00000x
P. Barrio*,a E. Rodríguez,a K. Saito,b S. Fustero a,c and T. Akiyama*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
preliminary results on the use of γ-silyl allylboronic pinacolate
The use of functionalised allylboronic esters in the catalytic
enantioselective allylboration of aldehydes is described for the
first time. γ-Silylallyl pinacolate derivatives give rise to α-silyl
homoallylic alcohols in high yields, with complete
diastereoselectivities and high enantioselectivities, in most of
the cases. The usefulness of such intermediates is showcased by
derivatives in asymmetric catalysis for the first time (Figure 1).
R
CO2
O
B
R
CO2
R
3Si
O
OH
chiral
R
3Si
reagent
Bpin
4
ref.
OAc
R
R
O
their transformation into fluorinated allylic alcohols
.
R
O
O
O
O
SiR3
*
P
The fifty-years old allylboration reaction is still one of the most useful
methods in organic synthesis.1,2 In a single operation a new carbon-
carbon bond is formed, a new stereogenic center created and two
versatile functionalities, namely an alcohol and a C-C double bond,
installed in the proximity of one another. Moreover, the use of γ-
substituted allylborane derivatives (crotylboronates being the most
widely used) regio- and stereospecifically provides two consecutive
stereocenters.3 For all these features, the allylboration reaction, and
related transformations, have attracted the attention of some of the
most important synthetic organic chemists of the second half of the
last century who developed chiral allylborating reagents allowing the
preparation of homoallylic alcohols with a very high degree of
stereocontrol, which propelled the use of this reaction in natural
product total synthesis.4,5 The new century has witnessed the advent
of long sought-for catalytic enantioselective allylboration reactions.6
Among them, the chiral Brønsted acid catalysed allylation developed
by Antilla stands out for its experimental simplicity, efficiency and
environmentally benign nature.7-9 Despite the impressive
development achieved in the field, to the best of our knowledge the
use of γ-functionalized allylborating reagents in asymmetric catalysis
has been completely overlooked. Reagents of this kind give rise to
OH
TMS
P
P
*
organocatalysis
work
LnIr
this
chiral
catalyst
organometallic
ref.
11
O
Ar
OH
O
O
PPh2
PPh2
O
O
P
P
=
*
=
*
OH
O
Ar
SEGPHOS
Figure 1. Our organocatalytic approach to α-silyl homoallylic
alcohols.
Prompted by the availability of TMSallylBpin (1) in just one step
from the corresponding commercially available allylic alcohol13
we decided to study its performance in the chiral Brønsted acid
catalysed allylboration reaction using benzaldehyde 2a as the
model substrate (Table 1).
vicinally funtionalised homoallylic alcohols in
a regio- and
diastereospecific manner. The synthetic versatility of allylsilanes
inspired us to initiate our study on γ-functionalised allylboronates
with the corresponding silylated analogues (Figure 1). Chiral
derivatives of this kind have extensively been used by Roush and
others for the synthesis of 1,2- and 1,4-diols, among other building
blocks.10 In addition, Krische recently reported an iridium catalysed
silylallylation using SEGPHOS as chiral ligand for the synthesis of
analogous α-silyl homoallylic alcohols.11,12 Herein, we report our
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Table 1. Optimisation of the reaction conditions
all of the cases. Regarding the substitution pattern, as a general
trend, substrates bearing substitution at the ortho position afford
only moderate enantioselectivities (compare 3d with 3f and 3l or
3c with 3g); however, good enantiocontrol is observed for ortho-
vinyl derivative 3b. The dependence of the enantioselectivity
with the electronic nature of the substituents also seem to follow
a general trend; while electron-withdrawing groups afford
enantioselectivities above 90% (see 3f,g,i,j,k,l), mild electron-
OH
O
I
additive
mol%),
(5
TMS
3a
2a
+
solvent,
Temperature
TMS
Bpin
donating groups do not exceed 80% (see 3e,h).† Finally, the
1
more robust dimethylphenylsilyl derivative could also be
obtained although in modest 87% ee# (3o,p).15
R
9-anthryl
TMS
Bpin
OH
1
R
O
P
O
O
P
O
O
P
O
O
O
R
O
Id mol%),
(5
O
Ph
Ph
OH
OH
OH
TMS
3a-q
Toluene, -30ºC
OH
2a-o
R
9-anthryl
Br
OH
OH
=
=
=
Ia R
iPr) 6H2
3-C
If
Ig
,
2,4,6-(
-C
Ib R
,
6H3
TMS
TMS
TMS
3,5-(CF3 2
)
Ic R
,
SiPh3
3a
3b
3c
=
9-anthryl
=
Id R 9-anthryl
,
ee
ee
ee
95%
OH
92%
52%
66%,
68%,
65%,
=
Ie R 9-anthryl
,
(H8)
OH
NO2
OH
N
O2
Entry
Addit. Solv. Temp.
ºC
e.e.
(%) (%)
I
3a
Me
TMS
TMS
TMS
1
2
3
4
5
6
7
8
9
10
11
12
13
-
-
TolH
DCM
-30
-30
-30
-30
-30
-30
-30
-30
-30
-30
-30
-50
25
74
73
80
71
50
66
50
39
42
90
73
33
64
36
34
11
6
16
95
90
67
2
67
90
20
84
Ia
Ia
Ia
Ib
Ic
Id
Ie
If
Ig
Id
Id
Id
Id
3d
3e
3f
ee
ee
ee
96%
52%
58%
OH
81%,
84%,
99%,
4ÅMS TolH
OH
OH
-
-
-
-
-
-
TolH
TolH
TolH
TolH
TolH
TolH
TMS
TMS
ee
TMS
tBu
Br
F
3i
3g
94%
3h
94%
OH
50%,
ee
OH
ee
92%,
78%
OH
68%,
4ÅMS TolH
-
-
-
DCM
TolH
TolH
TMS
TMS
TMS
ee
F
NC
3C
N
O2
3j
3k
3l
ee
ee
94%
93%
80%,
86%,
TMS
95%
86%,
The use of (R)-TRIP Ia under the optimised reaction conditions
reported by Antilla7a afforded the expected α-silyl homoallylic
alcohol 3a in good yield and with complete diastereoselectivity,
although with modest enantiocontrol (Table 1, entry 1). A
comparable level of enantiocontrol was observed when DCM
was used as solvent (Table 1, entry 2) and, intriguingly, the
addition of 4Å MS proved deleterious for the enantioselectivity
(Table 1, entry 3).14 In view of these unsatisfactory results, we
decided to explore other chiral Brønsted acid catalysts. A
screening of common BINOL-derived phosphoric acids (Table
1, entries 4-6) allowed identifying Id as an effective catalyst
affording the product in high enantiomeric excess without
affecting the yield nor the diastereoselectivity (Table 1, entry 6).
Regarding structurally related catalysts, partially hydrogenated
Ie provided slightly lower enantiocontrol (Table 1, entry 7),
while the biphenol derivative If proved less efficient (Table 1,
entry 8). On the other hand, VAPPOL derivative Ig, showed an
almost complete lack of enantiocontrol (Table 1, entry 9).
Further optimisation of the reaction conditions did not lead to
any improvement (Table 1, entries 10-13).
HO
OH
S
TMS
Cl
3m
59%
3n
ee
ee
91%
73%,
70%,
OH
OH
Ph
Ph
SiMe2
ee
SiMe2
ee
Br
3o
3p
65%, 87%
87%, 87%
Scheme 1. Scope and limitations.
The relative and absolute stereochemistry of the two newly
created stereocenters was unambiguously established by means
of X-ray diffraction experiments on derivative 3l (Figure 2).
Those of others were surmised by analogy.
With these optimised conditions in hand (Table 1, entry 6), the
scope and limitations of the new methodology were established
(Scheme 1). Excellent anti selectivity (>95:5) was observed for
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4g in good yield (72 %), as a single stereosiomer and with almost
complete preservation of the optical purity.20a Next, the
generality of this transformation was established using some of
the α-silyl homoallylic alcohols shown in Scheme 1 (Scheme 2).
Good yields with minor erosion of optical purity and complete
E-selectivities were observed in all cases.
Selectfluor
equiv)
(1.5
equiv)
OH
OH
R
R
NaHCO
3 (1
F
TMS
rt
MeCN,
3
4
OH
OH
Figure 2. ORTEP diagram of compound 3l
.
F
N
F
O2
As anticipated, the relative stereochemistry is anti, reflecting the
E stereochemistry of the starting allylboronate. With regard to
absolute stereochemistry, the re-face attack suggests that the
reaction proceeds through a two-point transition state analogous
to the one proposed by Goodman and Houk for allylboration
(Figure 2, the absolute configuration changes due to priority
considerations).7b,16 This transition state accounts for the
observed lower enantioselectivity obtained with TRIP, since the
bulky TMS group might clash with the tris(isopropyl)phenyl
substituents of TRIP. Apparently, the less bulky 3,3’-bis(9-
anthryl) derivative shows an optimum balance allowing
enantiodifferentiation; the pinacol being still the most hindered
substituent occupies the empty pocket while the TMS group
must be accommodated in the sterically demanding pocket.
Br
4f
4g
ee
ee
55% 94%
OH
ee
ee
72% 91%
OH
F
F
F
3C
N
O2
4l
4j
97% 91%
79% 91%
Scheme 2. Scope of the electrophilic fluorination.
To the best of our knowledge, this is the first available
methodology for the synthesis of these versatile intermediates.
Recently, the reversed reaction sequence (electrophilic allylic
fluorination / allylboration) using isomeric γ-sylilvinylboronates
has been described.22 Hence, while our methodology affords
enantioenriched fluorinated allylic alcohols, the complementary
one gives rise to isomeric fluorinated homoallylic alcohols, in a
racemic fashion (Scheme 3).
O
O
O
P
O
H
Me
Me
Me
Me
Ar
Ar
Me
work
This
H
O
Me
Me
CPA
R
%)
(5mol
B
Si
Ph
O
O
O
OH
H
R
1.
F
TMS
Bpin
Figure 3. Suggested stereochemical model
2. Selectfluor
OH
γ
-silyl
allyliboronate
Allylsilanes are very versatile reagents in organic synthesis.17
Undoubtedly, Sakurai-like nucleophilic addition to carbonyl
compounds represents the most popular reaction of synthetic
value.18 In a first approach, several reactions of this kind (Lewis
or Brønsted acid catalysed allylation of carbonyl compounds)
fluorinated
alcohols
ee
allylic
R
to 94%
up
via
TMS
Ref. 20
were assayed on α-silyl homoallylic alcohols 3 19
However,
.
OH
R
1. Selectfluor
F
elimination was observed under each of the assayed reaction
conditions.§ The high stabilisation of the β-silyl benzyl
carbocation intermediate accounts for the observed
TMS
Bpin
vinylboronate
2.
R
γ
-silyl
O
incompatibility of compounds
3 with acidic reaction conditions.
fluorinated homoallylic alcohols
Therefore, buffered reaction conditions are required in order to
avoid the kinetic and thermodynamically favoured Peterson
elimination. Krische recently demonstrated that the DMDO-
mediated oxidation of such scaffolds towards the corresponding
1,4-diols was a viable process under buffered conditions.11
Hence, we wondered if the electrophilic allylic fluorination
reported by Gouverneur could be used on our substrates, since
acid catalysis is not necessary.20,21 Disappointingly, the use of the
original reaction conditions only achieved small amounts of the
desired fluorinated product (>15%) along with the diene as the
major product, even when the acetyl protected alcohol was used,
indicating that Selectfluor behaves as a strong Lewis acid to
promote the elimination. To our delight, treatment of 3g with
Selectfluor under buffered reaction conditions afforded product
racemic
Bpin
via
F
Scheme 3. Comparison of the two methodologies.
Conclusions
Since the pioneering work of Brown and Roush, the application
of functionalised allylborating reagents in asymmetric synthesis
has, for decades, been limited to the use of chiral boronate
reagents. In this communication, the application of a γ-
functionalised allylboronic ester in a catalytic enantioselective
reaction has been reported for the first time. More specifically,
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2207; (i) W. R. Roush, in: Comprehensive Organic Synthesis, Vol. 2, (Ed.:
B. M. Trost), Pergamon Press, Oxford, 1991, p 1.
the chiral Brønsted acid catalysed allylboration of aromatic
aldehydes with γ-silyl functionalised reagents has been achieved
in good yields and with high enantioselectivities in most cases.
The synthetic versatility of the thus-obtained α-silylhomoallylic
alcohols has been extended to the synthesis of fluorinated allylic
alcohols by means of electrophilic fluorination. Further studies
aimed to broaden the scope of this transformation, specially to
the use of readily oxidisable silyl groups as well as to the
introduction of functionalities of other kinds, are currently
underway in our laboratories and will be disclosed in due time.
3
4
(a) R. W. Hoffmann, H.-J. Zeiss, Angew. Chem. Int. Ed., 1979, 18, 306;
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Notes and references
a
Departamento de Química Orgánica, Universidad de Valencia, E-46100
6
7
8
For a recent review on catalytic asymmetric allylboration of carbonyl
compounds, see: H.-X. Huo, J. R. Duvall, M.-Y. Huanga, R. Hong Org.
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Burjassot, Spain. E-mail: pablo.barrio@uv.es
b
Department of Chemistry, Faculty of Science, Gakushuin University, 1-
5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan. E-mail:
takahiko.akiyama@gakushuin.ac.jp
c
Laboratorio de Moléculas Orgánicas, Centro de Inverstigación Principe
Felipe, E-46012 Valencia, Spain.
† With stronger electron-donating substituents, i.e. p-methoxy, the
corresponding 1,3-diene arising from elimination of the TMSOH moiety
was observed as the major product. Accordingly, products 3f and 3i are
also prone to elimination. For this reason, only 1H NMR spectrum is given
for 3f while small signals of the corresponding diene appear in the 13C
NMR spectrum of 3i
.
9
§
Elimination was also observed when aldehyde allylation was assayed
under TBAT-mediated reaction conditions.23 The Peterson elimination is
known to proceed both under acidic and basic reaction conditions.
10 (a) H. C. Brown, G. Narla, J. Org. Chem., 1995, 60, 4686; (b) W. R.
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2010, 49, 4264.
#
An independent optimisation was carried out for this substrate leading to
the identification of the partially hydrogenated analogue of Id, H8-3,3’-
bis(anthryl)binolphosphoric acid Ie, as the optimum catalyst for this
reaction (see ESI).
We thank the Spanish Ministerio de Economía y Competitividad (CTQ2013-
43310) and the Generalitat Valenciana (PROMETEOII/2014/073) for their
financial support. E.R. thanks the Universidad de Valencia for a predoctoral
fellowship. This work was partially supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Advanced Transformations by
Organocatalysis” from the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and a Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science. Technical and human support provided
by SGIker (UPV/EHU, MINECO, GV/EJ, ERDF, and ESF) is gratefully
acknowledged.
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and chromatograms and CIF file for compound 3m
.
See
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