Asymmetric allylboration of aldehydes with pinacol allylboronates catalyzed by 1,1′-spirobiindane-7,7′-diol (SPINOL) based phosphoric acids

Article abstract of DOI:10.1002/ejoc.201101739

The asymmetric allylboration of aldehydes with pinacolallylboronates catalyzed by 1,1′-spirobiindane-7,7′-diol (SPINOL) based phosphoric acids is described. 6,6′-Bis(2,4,6-triisopropylphenyl)SPINOL-based phosphoric acid was found to be a general, highly e

Full text of DOI:10.1002/ejoc.201101739

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101739
Asymmetric Allylboration of Aldehydes with Pinacol Allylboronates Catalyzed
by 1,1Ј-Spirobiindane-7,7Ј-diol (SPINOL) Based Phosphoric Acids
Chun-Hui Xing,[a] Yuan-Xi Liao,[a] Yimei Zhang,[a] Darya Sabarova,[a] Monica Bassous,[a]
and Qiao-Sheng Hu*[a]
Keywords: Allylation / Spiro compounds / Phosphorus / Boronates / Aldehydes
The asymmetric allylboration of aldehydes with pinacol
allylboronates catalyzed by 1,1Ј-spirobiindane-7,7Ј-diol (SPI-
NOL) based phosphoric acids is described. 6,6Ј-Bis(2,4,6-tri-
lylboration reactions and excellent enantioselectivities were
obtained for different types of aldehydes including aromatic
aldehydes, α,β-unsaturated aldehydes, propargylic alde-
isopropylphenyl)SPINOL-based phosphoric acid was found hydes, and aliphatic aldehydes.
to be a general, highly enantioselective catalyst for such al-
ric SPINOLs may render optically active SPINOL-based
Introduction
phosphoric acids more enantioselective than reported axi-
ally chiral diol-based phosphoric acids.[7,8] In this context,
we have recently reported optically active SPINOL-based
phosphoric acid catalyzed enantioselective addition reac-
tions of indoles with imines and β,γ-unsaturated α-keto es-
ters.[9,10] Our study showed that SPINOL-based phosphoric
acids indeed exhibited higher enantioselectivities than
BINOL-based phosphoric acids for these reactions.[11,12] On
the basis of these results and the recently reported BINOL-
based phosphoric acid catalyzed allylboration reactions of
aldehydes with allylboronates,[5a] we surmised that optically
active SPINOL-based phosphoric acids might be highly
enantioselective catalysts for such asymmetric allylboration
reactions. Herein, we report our results on such SPINOL-
based phosphoric acid catalyzed allylboration reactions,
Catalytic asymmetric allylation of aldehydes constitutes
one of the most efficient ways to access optically active
homoallylic alcohols,[1,2] which are useful compounds for
organic synthesis.[1a] Different chiral catalysts such as chiral
Lewis acids,[3] Lewis bases,[4] and Brønsted acids[5] have
been reported as suitable catalysts for allylation reactions
of allyltin reagents, allylsilanes, and allylboronates with al-
dehydes.[6] Among the catalytic asymmetric allylation reac-
tions developed, the recently reported BINOL-based phos-
phoric acid catalyzed allylboration of aldehydes with pin-
acol allylboronates represented the most promising one.[5a]
This BINOL-based phosphoric acid catalyzed allylboration
used moisture-stable pinacol allylboronates as allylation
reagents and an air/moisture-stable BINOL-based phos-
phoric acid as the catalyst. Although high enantioselectivi-
ties were obtained for aromatic aldehydes without an ortho
substituent and α,β-unsaturated aldehydes, the enantio-
selectivities for ortho-substituted aromatic aldehydes and
aliphatic aldehydes were less satisfactory, ranging from 73%
to 93% ee. The search for general allylation catalysts that
are highly enantioselective for all types of aldehydes contin-
ues.
specifically,
6,6Ј-bis(2,4,6-triisopropylphenyl)SPINOL-
based phosphoric acid as a general, highly enantioselective
catalyst for the allylboration reaction of pinacol allylborates
with different types of aldehydes including aromatic alde-
hydes, α,β-unsaturated aldehydes, propargylic aldehydes,
and aliphatic aldehydes.
In our laboratory, we are interested in developing op-
tically active 1,1Ј-spirobiindane-7,7Ј-diol (SPINOL) based
phosphoric acids (Figure 1) and derivatives as novel chiral
catalysts, including Brønsted acids, for bond-forming reac-
tions. Our interest in part originated from the consideration
that the rigidity of the spirocyclic framework of C2-symmet-
Figure 1. Optically active SPINOL-based phosphoric acids.
[a] Department of Chemistry, College of Staten Island and the
Graduate Center of the City University of New York,
Staten Island, New York 10314, USA
Results and Discussion
Fax: +1-718-982-3910
We began our study by examining the allylboration reac-
tion of pinacol allylboronate with 2-methylbenzaldehyde. A
series of SPINOL-based phosphoric acids (R)-1a–i, pre-
E-mail: qiaosheng.hu@csi.cuny.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101739.
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C.-H. Xing, Y.-X. Liao, Y. Zhang, D. Sabarova, M. Bassous, Q.-S. Hu
SHORT COMMUNICATION
pared from optically active (R)-SPINOL (see the Support- general, higher than that obtained with BINOL-based
ing Information),[9] were examined (Table 1, Entries 1–8). phosphoric acid as catalysts. In particular, a significant in-
Although low to moderate enantioselectivities were ob- crease in enantioselectivity was observed for ortho-substi-
served for SPINOL-based phosphoric acids (R)-1a–h tuted aromatic aldehydes and aliphatic aldehydes, substrates
(Table 1, Entries 1–8), SPINOL-based phosphoric acid (R)- for which lower enantioselectivities were observed with
1i exhibited promising enantioselectivity (Table 1, Entry 9). BINOL-based phosphoric acids as catalysts (Table 2, En-
Further testing showed that toluene was a better solvent tries 1, 7, 12–15). In addition, excellent enantioselectivities
(Table 1, Entries 9–11). Lowering the reaction temperature were also obtained for 2-furylaldehyde and propargylic al-
improved the enantioselectivity (Table 1, Entries 9, 12, and
13). By carrying out the reaction at –70 °C, a 98% ee was
Table 2. SPINOL-based phosphoric acid (R)-1i-catalyzed reaction
obtained (Table 1, Entry 14). For comparison, we also car-
ried out the reaction at –30 °C and a 95% ee was observed
(Table 1, Entry 13). This enantioselectivity was higher than
that obtained with the BINOL-based phosphoric acid as
the catalyst, which was 93% ee, under the same reaction
condition.[5a] Reducing the catalyst loading to 2 mol-% af-
fected the reaction rate but not the enantioselectivity
(Table 1, Entry 15).
of aldehydes with pinacol allylboronate.[a]
Table 1. SPINOL-based phosphoric acid catalyzed reaction of pin-
acol allylboronate with 2-methylbenzaldehyde.[a]
[a] Reaction conditions: aldehyde (1.0 equiv.), pinacol allylboronate
(1.0 equiv.). [b] Based on 1H NMR analysis. [c] Based on HPLC
(Chiralcel OD column) analysis. [d] Reaction time: 5 h. [e] 2 mol-%
catalyst loading, 7 h.
By using SPINOL-based phosphoric acid (R)-1i as the
catalyst, we next examined different aldehydes for the asym-
metric allylboration reaction, and our results are listed in
Table 2. High yields and excellent enantioselectivities were
obtained not only for aromatic aldehydes and α,β-unsatu-
rated aldehydes, but also for aliphatic aldehydes (Table 2,
[a] Reaction conditions: allylboronate (1.5 equiv.), aldehyde
(1.0 equiv.), phosphoric acid (R)-1i (5 mol-%), toluene, –70 °C, 2 h.
[b] Isolated yield. [c] Based on HPLC (Chiralcel OD column)
analysis. [d] In parentheses: reported ee value with 5 mol-% of
BINOL-based phosphoric acids at –30 °C, see ref.[5a] [e] 2 mol-%
catalyst loading was used. [f] The ee value was determined from the
Entries 1–15). The enantioselectivity was observed to be, in benzoate ester of the alcohol product.
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Asymmetric Allylboration of Aldehydes with Pinacol Allylboronates
ic acid (R)-1i (5 mol-%) were dissolved in toluene (0.5 mL) under
an atmosphere of N2. The mixture was stirred and cooled to –70 °C
with a dry ice/2-propanol bath. Then, pinacol allylboronate
(0.15 mmol, 1.5 equiv.) was added in one portion through a micro-
syringe. The reaction mixture was stirred at this temperature until
all aldehyde was converted (monitored by 1H NMR spectroscopy).
Column chromatography on silica gel (hexanes/diethyl ether or
pentane/diethyl ether) afforded the product.
dehydes (Table 2, Entries 16–18). These results showed that
SPINOL-based phosphoric acid (R)-1i is a general, highly
enantioselective catalyst for the asymmetric allylboration
reactions of pinacol allylboronate with different types of
aldehydes.
The asymmetric allylboration reaction was also extended
to pinacol crotylboronates.[5a] We found that high diastereo-
selectivity and excellent enantioselectivity were observed for
both trans- and cis-crotylboronates (Table 3).
Supporting Information (see footnote on the first page of this arti-
cle): Reaction procedures and characterization of the products of
the SPINOL-based phosphoric acid catalyzed allylboration reac-
tions.
Table 3. SPINOL-based phosphoric acid (R)-1i-catalyzed addition
reactions of benzaldehyde with pinacol crotylboronates.[a]
Acknowledgments
Support from the Professional Staff Congress–City University of
New York (PSC–CUNY) Research Award Programs is gratefully
acknowledged. We also thank Frontier Scientific, Inc. for its gener-
ous gift of pinacol allylboronate.
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[a] Reaction conditions: allylboronate (1.5 equiv.), aldehyde
(1.0 equiv.), phosphoric acid (R)-1i (5 mol-%), toluene, –70 °C, 6 h.
[b] Isolated yield. [c] Based on 1H NMR analysis. [d] Based on
HPLC (Chiralcel OD column) analysis. [e] In parentheses: reported
ee value with 5 mol-% of BINOL-based phosphoric acid at 0 °C,
see ref.[5a] [f] In parentheses: reported ee value with 5 mol-% of
BINOL-based phosphoric acid at –30 °C, see ref.[5a]
Conclusions
We demonstrated that SPINOL-based phosphoric acids
catalyzed the asymmetric allylboration of aldehydes with
pinacol allylboronates. 6,6Ј-Bis(2,4,6-triisopropylphenyl)-
SPINOL-based phosphoric acid (R)-1i was found to be a
general, highly enantioselective catalyst for such allylbor-
ation reactions and excellent enantioselectivities were ob-
tained for different types of aldehydes including aromatic
aldehydes, α,β-unsaturated aldehydes, propargylic alde-
hydes, and aliphatic aldehydes. The enantioselectivity was
observed to be higher than that obtained with BINOL-
based phosphoric acids as catalysts, particularly for sub-
strates such as ortho-substituted aromatic aldehydes and ali-
phatic aldehydes. Our study provides a highly enantioselec-
tive phosphoric acid catalyst for the asymmetric allylbor-
ation of aldehydes and an efficient method to access a
broad spectrum of optically active homoallylic alcohols.
Our study also suggests that SPINOL-based phosphoric ac-
ids might also be enantioselective catalysts for allylation-
related reactions such as propargylation and allenylation re-
actions.[13,14] Work towards this direction is actively un-
derway.
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Experimental Section
General Procedure for the SPINOL-Based Phosphoric Acid Cata-
lyzed Allylboration of Aldehydes with Pinacol Allylboronate: In a
vial, aldehyde (0.1 mmol, 1.0 equiv.) and SPINOL-based phosphor-
Eur. J. Org. Chem. 2012, 1115–1118
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C.-H. Xing, Y.-X. Liao, Y. Zhang, D. Sabarova, M. Bassous, Q.-S. Hu
SHORT COMMUNICATION
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[11]
[12]
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Received: December 7, 2011
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Published Online: January 16, 2012
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Eur. J. Org. Chem. 2012, 1115–1118