Chiral Bronsted acid catalyzed pinacol rearrangement

Article abstract of DOI:10.1002/anie.201004778

Asymmetric pinacol rearrangement: The pinacol rearrangement has long been known to be difficult to control in terms of regioselectivity and stereoselectivity. It has been found that indolyl diols can be treated with chiral phosphoric acids to effect a regio- and enantioselective pinacol rearrangement with high efficiency (see scheme; Ar=1-naphthyl). Copyright

Full text of DOI:10.1002/anie.201004778

Communications
DOI: 10.1002/anie.201004778
Organocatalysis
Chiral Brønsted Acid Catalyzed Pinacol Rearrangement**
Tao Liang, Zhenjie Zhang, and Jon C. Antilla*
The Brønsted acid catalyzed conversion of cyclic or acyclic
vicinal diols to aldehydes or ketones through dehydration and
subsequent [1,2]-alkyl, [1,2]-aryl, or hydride shift is a truly
venerable reaction known as the pinacol rearrangement.[1]
Because of the likelihood of multiple carbocation formation
during the reaction, the regioselectivity can vary, and wide-
spread usage has suffered.[2] While strategies exist where a
stable carbocation or predictable carbocation can mitigate
regioselectivity issues, most applications have relied upon a
more general variant termed the semipinacol rearrange-
ment.[3] The semipinacol rearrangement, more than often,
relies upon the presence of a more predictable carbocation
position, and synthetic utility has thus been found.[4] For these
reasons, predictable and stereoselective variants of the
pinacol rearrangement are highly desired and interesting.
In light of recent and relatively wide spread use of chiral
phosphoric acids as catalysts for asymmetric transforma-
tions,[5] it occurred to us that the pinacol rearrangement,
widely known to be catalyzed by phosphoric acids,[6] could be
Scheme 1. Dehydration strategy for the chiral phosphoric acid cata-
lyzed pinacol rearrangement.
unsaturated a-keto esters. This intermediate has also been
invoked by You et al.[9b] for a tandem double Friedel–Crafts
reaction and by Gong et al.[9c] for the asymmetric alkylation of
enamides. We believed that an indolyl diol like 1 would prove
successful, especially when one considers the success of
iminium chemistry utilizing chiral phosphoric acid cataly-
sis.[5e–g,i–k]
In view of our previous success[10] in developing catalytic
asymmetric reactions using chiral phosphoric acids, we began
to investigate the pinacol rearrangement of indolyl diol 1a in
the presence of chiral phosphoric acid 3a and 4 ꢀ molecular
sieves. To our delight, the reaction proceeded smoothly to
afford a-indolyl ketone 2a in 93% yield and 38% ee (Table 1,
controlled through
a catalytic enantioselective process.
During the course of our investigation into this reaction a
report by Tu and co-workers detailed the catalytic enantio-
selective semipinacol rearrangement of 2-oxo allylic alcohols
utilizing chiral phosphoric acids.[7] However, despite the
elegance of this initial report, it was evident that strained
cyclobutanes along with a relatively high catalyst loading
(10 mol%) were required for this transformation. Therefore,
we felt additional important challenges still remained in this
area. We were further interested because, to the best of our
knowledge, no example of a catalytic enantioselective pinacol
rearrangement has been reported to date.[8]
Table 1: Catalyst and solvent optimization.
We believed that the ability to control the regioselectivity
of the rearrangement would be imperative and that a
transformation of this type would be largely substrate
dependent. We hypothesized that access to a desirable
iminium intermediate could be realized through phosphoric
acid mediated dehydration of an indolyl diol (1) (Scheme 1).
This type of indolyl iminium was first reported by Rueping
et al.[9a] for the Friedel–Crafts addition of indoles to b,g-
Entry[a]
Catalyst
(x mol%)
Solvent
t
[h]
Yield
[%][b]
ee
[%][c]
[*] T. Liang, Prof. Dr. J. C. Antilla
Department of Chemistry, University of South Florida
4202 E. Fowler Ave, Tampa FL 33620 (USA)
Fax: (+1)813-974-1733
1
2
3
4
(R)-3a (10)
(R)-3b (10)
(R)-3c (10)
(R)-4 (10)
(R)-4 (10)
(R)-4 (10)
(R)-4 (5)
1,2-DCE
1,2-DCE
toluene
toluene
benzene
benzene
benzene
benzene
benzene
48
48
4
4
4
4
4
4
18
93
63
93
93
93
91
94
94
60
38
33
70
93
94
91
95
96
95
E-mail: jantilla@cas.usf.edu
5
Z. Zhang
6[d]
7
Department of Chemistry X-Ray Facility, University of South Florida
Tampa, FL 33620 (USA)
8
9
(R)-4 (2.5)
(R)-4 (1)
[**] We thank the National Institutes of Health (NIH GM-082935) and
the National Science Foundation CAREER program (NSF-0847108)
for financial support.
[a] Unless otherwise specified, reactions were conducted using 0.1 mmol
1a (0.05m solution). [b] Yield of isolated product. [c] Determined by
HPLC analysis using a chiral column. [d] Reaction conducted at 08C.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004778.
9734
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Angew. Chem. Int. Ed. 2010, 49, 9734 –9736

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Chemie
entry 1). This preliminary result encouraged us to evaluate
additional binol-based and H8-binol-based chiral phosphoric
acids as catalysts. The enantioselectivity increased to 70%
with the use of catalyst 3c in toluene (Table 1, entry 3).
Improvement to 94% ee was achieved with catalyst 4,[11] an
H8-binol phosphoric acid variant, in aromatic solvents
(Table 1, entries 4 and 5). It should be noted that decreasing
the temperature of the reaction caused the enantioselectivity
to decrease (Table 1, entry 6). Interestingly, lowering the
catalyst loading provided a positive effect, with respect to
both yield and enantioselectivity (Table 1, entries 5 and 7–9);
albeit with reduced rate when 1 mol% of catalyst was used.
With the optimal reaction conditions in hand, the
substrate scope of the newly developed pinacol rearrange-
ment was assessed (Table 2). To ensure reaction completion,
provided the desired rearrangement product with high yield
and enantioselectivity, although a longer reaction time was
required (Table 2, entry 9). Investigation into the effect of
substitution on the indole ring revealed the reaction to be
highly versatile to both electron-withdrawing and electron-
donating groups at the 5-position, providing products with
high enantioselectivities (Table 2, entries 10–14).
Scheme 2 illustrates a plausible mechanistic pathway
detailing the chiral phosphoric acid induced dehydration of
indolyl alcohol 1a followed by subsequent pinacol rearrange-
Table 2: Chiral phosphoric acid catalyzed pinacol rearrangement.
Entry[a]
2
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
2a: R1 =Me, R2 =Ph, R3 =H
94
84
88
90
99
99
97
94
93
95
86
86
95
83
96
95
94
94
93
96
91
93
93
96
94
96[e]
94
96
2b: R1 =Bn, R2 =Ph, R3 =H
2c: R1 =Allyl, R2 =Ph, R3 =H
2d: R1 =Me, R2 =4-FC6H4, R3 =H
2e: R1 =Me, R2 =4-ClC6H4, R3 =H
2 f: R1 =Me, R2 =4-MeC6H4, R3 =H
2g: R1 =Me, R2 =4-MeOC6H4, R3 =H
2h: R1 =Me, R2 =3,5-Me2C6H3, R3 =H
2i: R1 =Me, R2 =2-naphthyl, R3 =H
2j: R1 =Me, R2 =Ph, R3 =F
Scheme 2. Proposed mechanism for the chiral phosphoric acid cata-
lyzed asymmetric pinacol rearrangement.
9[d]
10
11
12
13
14
ment. Intermediate A results from hydrogen-bonding inter-
actions of the bifunctional chiral phosphoric acid catalyst with
diol 1a. Dehydration of intermediate A would presumably
give rise to the iminium intermediate B, which possesses
potential two-point binding with the chiral phosphate through
hydrogen-bonding and electrostatic interactions. Subsequent
rearrangement via a [1,2]-aryl shift would furnish product 2a,
with regeneration of the chiral phosphoric acid catalyst.
In summary, we report the first catalytic enantioselective
pinacol rearrangement. Chiral phosphoric acids are utilized as
highly efficient Brønsted acids in transforming indolyl diols to
chiral a-indolyl ketones with high yield and enantioselectivity.
Further studies to expand the substrate scope,[13] with the aim
of developing further efficient enantioselective transforma-
tions, are currently under investigation in our laboratory and
will be reported in due course.
2k: R1 =Me, R2 =Ph, R3 =Cl
2l: R1 =Me, R2 =Ph, R3 =Br
2m: R1 =Me, R2 =Ph, R3 =Me
2n: R1 =Me, R2 =Ph, R3 =MeO
[a] Reaction Conditions: 0.1 mmol 1, 0.0025 mmol (R)-4, and 2.0 mL
benzene. [b] Isolated Yield. [c] Determined by chiral HPLC analysis.
[d] The reaction was complete in 14 h. [e] The absolute configuration was
determined to be (R)-2l by single-crystal X-ray diffraction analysis;[12] see
the Supporting Information.
2.5 mol% of catalyst 4 was used. A lower catalyst loading
would presumably be tolerated in individual cases. The
reaction is tolerant towards the indole-protecting group,
with regard to enantioselectivity as well as reaction efficiency.
Methyl, benzyl, or allylic substitution of the 1-position of the
indole resulted in high enantioselectivity (94–96% ee) for
each rearrangement (Table 2, entries 1–3). The electronics of
the aryl migrating group were next evaluated. Electron-
withdrawing (Table 2, entries 4 and 5) and electron-donating
substituents (Table 2, entries 6 and 7) at the para position of
the phenyl ring provided a substrate that migrated efficiently
to furnish a product with high enantioselectivity. Notably, the
more sterically hindered 2-naphthyl-substituted diol (2i)
Experimental Section
General procedure for chiral phosphoric acid catalyzed pinacol
rearrangement: 2-(1-methyl-1H-indol-3-yl)-1,1-diphenylethane-1,2-
diol (1a) (0.10 mmol, 34.3 mg), catalyst 4 (0.0025 mmol, 1.5 mg),
and benzene (2.0 mL) were added to a flame-dried reaction tube
charged with 30 mg 4 ꢀ M.S. (powder). The resulting solution was
stirred at room temperature for 6 h. The crude product was purified
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EtOAc = 10:1 to 5:1) to afford product 2a (30.6 mg, 94% yield, 96%
ee). Enantiomeric excess was determined by HPLC analysis using a
chiral column.
Received: August 1, 2010
Revised: September 10, 2010
Published online: November 12, 2010
Keywords: asymmetric catalysis · indolyl diols · organocatalysis ·
.
phosphoric acids · pinacol rearrangement
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[13] In preliminary study using non-aryl substrates, lower selectivities
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27% ee for each respective rearrangement product.
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