Asymmetric hydroboration and Matteson homologation for the preparation of fluorinated α-phenethanols

Article abstract of DOI:10.1016/j.jfluchem.2007.04.014

The asymmetric catalytic hydroboration-oxidation of ring-fluorinated styrenes (F-PhCH{double bond, long}CH₂) was achieved with catecholborane along with a combination of [Rh(COD)₂]⁺BF₄^- and (R)-BINAP providing 81-96% enantioselectivities for the product alcohols for ortho-unhindered styrenes. A deleterious effect of a 2,6-disubstitution on the enantioselectivity of the product alcohol was observed. 2-Trifluoromethylstyrene also provides only 53% ee, probably due to the steric bulk of the CF₃ group at the ortho-position of styrene. Asymmetric homologation of fluorophenylmetals (magnesium bromide or lithium) with pinanediol α-chloroethylboronate, followed by oxidation readily furnished the desired 1-(2,6-difluorophenyl)- and 1-(perfluorophenyl)ethanols in 94-95% ee.

Full text of DOI:10.1016/j.jfluchem.2007.04.014

Journal of Fluorine Chemistry 128 (2007) 827–831
www.elsevier.com/locate/fluor
Asymmetric hydroboration and Matteson homologation
for the preparation of fluorinated a-phenethanols
*
P. Veeraraghavan Ramachandran , Michael P. Jennings
Herbert C. Brown Center for Borane Research, Department of Chemistry, Purdue University, 560 Oval Drive,
West Lafayette, IN 47907-2084, United States
Received 15 February 2007; received in revised form 6 April 2007; accepted 9 April 2007
Available online 19 April 2007
Abstract
The asymmetric catalytic hydroboration–oxidation of ring-fluorinated styrenes (F-PhCH CH
2
) was achieved with catecholborane along with a
ꢀ
and (R)-BINAP providing 81–96% enantioselectivities for the product alcohols for ortho-unhindered styrenes.
+
2 4
combination of [Rh(COD) ] BF
A deleterious effect of a 2,6-disubstitution on the enantioselectivity of the product alcohol was observed. 2-Trifluoromethylstyrene also provides
only 53% ee, probably due to the steric bulk of the CF group at the ortho-position of styrene. Asymmetric homologation of fluorophenylmetals
magnesium bromide or lithium) with pinanediol a-chloroethylboronate, followed by oxidation readily furnished the desired 1-(2,6-difluor-
ophenyl)- and 1-(perfluorophenyl)ethanols in 94–95% ee.
2007 Elsevier B.V. All rights reserved.
3
(
#
Keywords: Fluorostyrenes; Asymmetric hydroboration; Asymmetric homologation; Fluoroalcohols
1
. Introduction
2. Results and discussion
The chemical and biological properties of organic molecules
We began the study with the asymmetric hydroboration of
2,3,4,5,6-pentafluorostyrene (1a) with catecholborane (CBH)
due to the potential importance of the corresponding chiral
organoborane intermediate for the preparation of a variety of
perfluorophenyl containing chiral organic molecules [9]. In
addition, our earlier studies on the asymmetric reduction of
are usually altered by fluorine substitution [1]. Optically active
fluorinated alcohols are important building blocks or end-
products that have been applied in medicinal and materials
chemistry [2]. Asymmetric syntheses of fluoroalcohols are
challenging and organoborane chemistry provides a simple
route to achieve them [3]. As part of our program on
fluoroorganic syntheses via boranes, we had reported the
preparation of fluoroalcohols via hydroboration–oxidation [4],
reduction [5], allylboration [6], etc. In continuation of our
research on the hydroboration of fluoroolefins [4], an
investigation of asymmetric catalytic hydroboration of fluori-
nated styrenes was undertaken [7,8]. Following are the results
of a systematic study and the enantioselectivities achieved for
the hydroboration–oxidation of ring-fluorinated styrenes with
0
0
0
0
0
0
0
2 ,3 ,4 ,5 ,6 -pentafluoro- and 2 ,6 -difluoroacetophenones with
TM
B-chlorodiisiopinocampheylborane (DIP-Chloride ) [10a]
we encountered low ee for the product alcohols [10b]. Hayashi
and co-workers have reported that catalytic asymmetric
hydroboration at low temperatures provided high enantios-
electivities for only a handful of substrates, i.e. styrene and p-
substituted styrenes [11a].
2.1. Asymmetric catalytic hydroboration of fluorostyrenes
+
ꢀ
catecholborane along with a combination of [Rh(COD) ] BF
2
4
and (R)-BINAP.
The initial chiral ligand screening for the asymmetric
hydroboration of 1a was performed at room temperature (rt) in
tetrahydrofuran (THF), due to very high levels of regioselec-
tivity for the 28-ol [11a]. Chiral ligands with methylene
backbones, such as (R,R)-DIOP, (R)-Prophos, and (S,S)-
Chiraphos (Fig. 1), proceeded rapidly (0.5 h) and provided
*
Corresponding author. Tel.: +1 765 494 5303; fax: +1 765 494 0239.
E-mail address: chandran@purdue.edu (P.V. Ramachandran).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.04.014

828
P.V. Ramachandran, M.P. Jennings / Journal of Fluorine Chemistry 128 (2007) 827–831
Having achieved high ee for monofluorostyrenes, ring-
trifluoromethyl substituted stryenes were examined. Under
conditions identical to that for the hydroboration of 1b, m-(1e)
and p-trifluoromethyl styrenes (1f) furnished slightly lower
enantioselectivities for the alcohols 1-(3-(trifluoromethyl)phe-
nyl)ethanol (2e) and 1-(4-(trifluoromethyl)phenyl)ethanol (2f)
in 81 and 84%, respectively. Interestingly, o-trifluoromethyl
styrene (1g) afforded only 53% ee for 1-(2-(trifluoromethyl)-
phenyl)ethanol (2g) (Eq. (3)). This pointed to the effect of
sterics at the ortho-position of styrene on the asymmetric
hydroboration with BINAP and demanded the examination of
2,6-difluorinated styrenes. On the basis of the results from
Fig. 1. Chiral ligands for asymmetric hydroboration.
high regioselectivites for the product 28-alcohol. However, the
enantiomeric excesses (ee) for the product 1-(perfluoropheny-
l)ethanol (2a) were very low (ꢁ5%). On the basis of the reports
by Hayashi and Dai [11a,b], (R)-BINAP (Fig. 1) was then
examined. Although BINAP proved to be a better ligand, the ee
observed was still low (20%) (Eq. (1)) [8] and interestingly,
manipulation of solvent, temperature, and concentration had
very little consequence on enantioselectivity. The maximum ee
observed was only 25% in dimethoxyethane (DME). The
results are summarized in Table 1.
0
0
asymmetric reduction of 2 ,6 -disubstituted acetophenones with
TM
DIP-Chloride
[10b] and the asymmetric hydroboration of
2,3,4,5,6-pentafluorostyrene described above, a lower enantios-
electivity was expected. Indeed, the hydroboration–oxidation
of 2,6-difluorostyrene (1h), under the previously described
conditions, afforded a very low enantioselectivity of 35% for
1-(2,6-difluorophenyl)ethanol (2h). The effect of diortho-
substitution was further confirmed with the hydroboration–
oxidation of 2,6-dichlorostyrene (1i), which also provided a
similar low enantioselectivity of 41% for the product 1-(2,6-
dichlorophenyl)ethanol (2i) obtained in 86% yield (Eq. (4)).
(1)
(
3)
To understand the reasons for the poor ee in the case of
,3,4,5,6-pentafluorostyrene, other ring-substituted fluorostyr-
2
enes were now examined. At ꢀ25 8C with (R)-BINAP, the
catalyzed hydroboration of p-fluorostyrene 1b in 0.5 M THF,
followed by alkaline H O oxidation furnished the 28-ol, 1-(4-
(4)
2
2
fluorophenyl)ethanol (2b) exclusively with moderate enantios-
electivity (68%) in 86% yield. Changing the solvent from THF to
DME provided an increase in ee (76%) and cooling the reaction
to ꢀ40 8C in DME further increased the enantioselectivity to
In the case of asymmetric reduction of ring-substituted
TM
fluoroketones with DIP-Chloride , while poor ee was observed
0
0
0
0
0
0
0
0
0
for 2 ,3 ,4 ,5 ,6 -pentafluoroacetophenone, 2 ,3 ,4 ,5 -tetrafluor-
oacetophenone had provided high ee [10]. Analogous to this
observation, the catalytic asymmetric hydroboration–oxidation
of 2,3,4,5-tetrafluorostyrene (1j) readily furnished the Markov-
nikovalcohol (2j) with an enantioselectivity of 88% in 82% yield
(Eq. (5)). This confirms that the substitution of a bulky group at
the ortho-position or substitution of both ortho-positions of
styrene is responsible for the decreased enantioselectivity
achieved during the asymmetric catalytic hydroboration with
CBH using BINAP as the auxiliary.
8
7%. However, lowering the temperature to ꢀ78 8C and
allowing the medium to slowly warm to rt, did not enhance
the enantioselectivity. Finally, increasing the concentration (1 M
in DME, ꢀ78 8C-rt) provided a very high enantioselectivity of
9
6% ee for 2b. Under these optimized conditions, similar results
were obtained for the asymmetric hydroboration of o-(1c) and m-
fluorostyrenes (1d) as the products 1-(2-fluorophenyl)ethanol
(2c) and 1-(3-fluorophenyl)ethanol (2d) revealed 91 and 88% ee,
respectively (Eq. (2)).
(2)
(
5)

P.V. Ramachandran, M.P. Jennings / Journal of Fluorine Chemistry 128 (2007) 827–831
829
Table 1
pentafluoroacetophenone and the current results of the catalytic
asymmetric hydroboration–oxidation for the synthesis of 2a
and 2h, the focus was now shifted to examining the Matteson
asymmetric homologation [12] for achieving high enantios-
electivity [13].
Asymmetric hydroboration of 2,3,4,5,6-pentafluorostyrene (1a) with CBH in
+
] BF
ꢀ
the presence of 2 mol% [Rh(COD)
a
THF
2
4
and 4 mol% chiral phosphine in
b
c
Ligand
Temperature Yield (%) 28/18-ol % ee Configuration
(
(
(
(
(
(
(
(
(
(
R,R)-DIOP
rt
83
84
87
81
84
83
84
86
84
82
97/3
95/5
98/2
97/3
97/3
98/2
98/2
98/2
98/2
98/2
3
5
R
R
S
Under the procedure described earlier [12b] homologation
of methyl-pinanediol boronate ester (3) with in situ generated
LiCHCl (formed from sec-butyllithium and CH Cl in THF at
S,S)-Chiraphos rt
R)-Prophos
R)-BINAP
R)-BINAP
R)-BINAP
R)-BINAP
R)-BINAP
R)-BINAP
R)-BINAP
rt
3
2
2
2
rt
20
21
22
21
22
22
25
R
R
R
R
R
R
R
d
e
ꢀ78 8C) readily provided the chiral 1-chloroethyl pinanediol
boronate ester 4. Subsequent addition of either (perfluorophe-
nyl)magnesium bromide or (2,6-difluorophenyl)lithium at
rt
rt
0
ꢀ25
ꢀ40
ꢀ25
ꢀ
78 8C in THF to 4 and allowing the reaction to warm to rt
furnished the chiral pinanediol chloro(2,6-difluorophenyl)- and
chloro(2,3,4,5,6-pentafluorophenyl)methylboronates 5a and
f
a
Reactions ran at 0.5 M conc.
5h, respectively. Alkaline oxidation afforded the chiral alcohols
2a and 2h, respectively, in 84 and 88% yields and 94 and 95%
ee, respectively (Eq. (6)).
b
c
d
e
f
The enantiomers were resolved using HPLC on a Chiralcel OD-H column.
Configuration determined on the basis of rotation, Ref. [9].
1
0
M concentration.
.3 M concentration.
Reaction in DME.
Essentially exclusive Markovnikov regioselectivity was
observed for the hydroboration–oxidation of 1b–h and 1j,
which is attributable to a proposed h-3 complex with the metal
center prior to the reductive elimination [11g]. On the contrary,
a 3/2:28/18 alcohols were furnished for the hydroboration–
oxidation of 1i under identical conditions to that of the
hydroboration of 1b. It was postulated that due to the dichloro
substituents of 1i, the stabilized h-3 complex could not be
achieved entirely, presumably due to the disrupted co-planarity
of the arene and alkene moiety [11g]. The results are
summarized in Table 2.
(
6)
3. Conclusion
In summary, we have observed that the low temperature
catalytic hydroboration–oxidation of ring-fluorinated styrenes
2.2. Asymmetric homologation of fluoroaryl Grignard and
fluoroaryllithium reagents
+
ꢀ
with catecholborane and a combination of [Rh(COD) ] BF
2 4
and (R)-BINAP provided very high for the product 1-fluoro
phenethanols, except in the cases of 2-trifluoromethyl-,
2,3,4,5,6-pentafluoro- and 2,6-dihalostyrenes. We achieved
these classes of fluoro-a-phenethanols in 84–88% yields and
TM
With the failed protocol of earlier reported DIP-Chloride
0
0
0
0
0
0
0
reduction of 2 ,6 -difluoroacetophenone and 2 ,3 ,4 ,5 ,6 -
Table 2
94–95% enantioselectivities via asymmetric homologation
utilizing the corresponding fluoroaryl Grignard or lithium
reagents and chiral a-chloroethyl pinanediol boronate.
Asymmetric hydroboration of fluorinated styrenes with CBH in the presence of
ꢀ
+
] BF
2
mol% [Rh(COD)
2
4
and 4 mol% (R)-BINAP in DME
b
Styrene
Subst.
Reaction
Yield 28/18-ol
%
ee
Configuration
a
temperature (%)
#
4. Experimental
c
4
4
4
4
4
2
3
3
4
2
2
2
2
-F
1b
1b
1b
ꢀ25
ꢀ25
ꢀ40
85
86
87
84
82
83
86
84
85
88
86
86
82
97/3
97/3
98/2
98/2
98/2
98/2
98/2
98/2
98/2
95/5
94/6
60/40
98/2
68
76
87
86
96
91
88
81
84
53
35
41
88
R
R
R
R
R
R
R
R
R
R
R
R
R
-F
Unless otherwise noted, all manipulations were carried out
-F
under an inert atmosphere using flame-dried glassware.
Techniques for handling air-sensitive compounds have been
previously described [14]. THF was freshly distilled before use
from over sodium-benzophenone ketyl and anhydrous ethyl
ether (Mallinckrodt) was used as received. All other chemicals
were obtained from Aldrich Chemical Co.
-F
1b ꢀ78-rt
1b ꢀ78-rt
1c ꢀ78-rt
1d ꢀ78-rt
1e ꢀ78-rt
-F
-F
-F
-CF
-CF
-CF
3
3
3
1f
ꢀ78-rt
1g ꢀ78-rt
1h ꢀ78-rt
1i ꢀ78-rt
1
13
19
The H, C and F nuclear magnetic resonance (NMR)
spectra were plotted on a Varian Gemini-300 spectrometer
(300, 75 and 282 MHz, respectively) with a Nalorac-quad
,6-F
,6-Cl
2
2
,3,4,5-F
4
1j
ꢀ78-rt
1
a
probe. H NMR spectra were obtained using CDCl as the
3
The enantiomers were resolved by a Chiralcel OD-H HPLC column.
Configuration on the basis of analogy with 2b.
Reaction in THF.
b
c
solvent with either tetramethylsilane (TMS: 0 ppm) or
19
chloroform (CHCl : 7.2 ppm) as the internal standard.
3
F

830
P.V. Ramachandran, M.P. Jennings / Journal of Fluorine Chemistry 128 (2007) 827–831
1
1
NMR spectra were recorded in CDCl3 using CFCl or
3
stirred for 16–18 h at rt. The reaction was monitored by
B
1
trifluoroacetic acid (TFA) as the internal standard. H NMR
NMR spectroscopy of the crude reaction mixture for
the appearance of the product boronate at d 32 ppm. THF
was removed under reduced pressure and the crude product
was dissolved in n-pentane (50 mL) and the resulting solution
data are reported as chemical shifts (d ppm), multiplicity (s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet), coupling
constant (Hz), and integration. Chromatography was performed
on 40–60 mm silica gel (230–400 mesh). Enantiomeric
excesses (% ee) were measured using a Dynamax HPLC fitted
with an HPXL Solvent Delivery System and a Dynamax UV (l
was filtered under N . The filtrate was concentrated under
2
vacuum and the crude product was distilled to give (S)-
pinanediol (R)-1-chloroethylboronate 4 in 80% yield.
A solution of (S)-pinanediol (R)-1-chloroethylboronate 4
(10 mmol) in 20 mL THF was cooled to ꢀ78 8C. To the
reaction mixture pentafluorophenylmagnesium bromide
2
54 nm) detector, and a Chiralcel OD-H chiral HPLC column.
Mass spectra were recorded using a Hewlett Packard 5989B
mass spectrometer/5890 series II gas chromatograph or a
Finnigan mass spectrometer model 4000. The chemical
ionization gas used was isobutene.
(10 mmol, 1 M solution in Et O) was added dropwise slowly
2
at ꢀ78 8C. The reaction mixture was gradually allowed to warm
rt and stirred till completion of the reaction. The reaction,
1
1
5
. Experimental procedure
monitored by B NMR spectroscopy showed the product (S)-
pinanediol (R)-1-fluoroarylethylboronate 5 peak at d 34 ppm.
The reaction mixture was then oxidized by the addition of 3 M
NaOH (12 mmol) and 30% H O (12 mmol) for 2 h. The crude
5
.1. Asymmetric hydroboration–oxidation of fluorostyrenes
A typical experimental procedure for the asymmetric
catalyzed hydroboration of fluorostyrenes with CBH in the
2
2
reaction mixture was extracted with ethyl acetate (2ꢂ 50 mL),
washed with sat. NH Cl (30 mL) and dried over MgSO .
4
4
+
ꢀ
presence of [Rh(COD) ] BF and (R)-BINAP is as follows. In
2
+
an Ar filled glove bag, 2 mol% of [Rh(COD) ] BF and
Removal of solvents under vacuo provided crude alcohol,
which was purified by column chromatography (silica gel,
hexanes:ethyl acetate:: 95:5) to give corresponding pure
alcohol 2 in 85% yield.
4
ꢀ
2
4
4
mol% of (R)-BINAP were placed into a dry, 50 mL round-
bottomed flask and subsequently filled with 9 mL of dry THF.
The mixture was allowed to stir for 20–40 min at rt, and then
cooled to ꢀ78 8C to which the olefin (3 mmol) was added.
After 30 min, catecholborane (6 mmol) was added slowly and
the solution stirred during the slow warming period to rt
5.3. Determination of enantiomeric excesses
The pure 28-ol (50 mg) was dissolved in 3 mL of CH Cl
2
2
(
generally overnight). The corresponding solution was cooled
to 0 8C, and the chiral boronate ester was then oxidized by
adding 4 mL of 3 M NaOH and 4 mL of 30% H O , slowly. The
and cooled to 0 8C. To this solution was added p-nitrobenzoyl
chloride (PNB-Cl, 1.1 equiv.) followed by Et N. The solution
3
2
2
was warmed to rt and allowed to stir for 2 h. Dilute HCl (0.5 M)
was added to the mixture and the product was extracted with
Et O (2ꢂ 5 mL), washed with H O, brine, and dried over
solution was then allowed to warm to rt and left to oxidize
completely for an additional 2 h. Diethyl ether (Et O) and H O
were now added to the solution. The product was extracted with
2
2
2
2
MgSO . Removal of the solvent in vacuo and column
4
Et O (3ꢂ 10 mL), followed by 3 M NaOH (2ꢂ 10 mL) to
2
chromatography (5% ethyl acetate in hexane) provided the
pure chiral ester. The enantioselectivity was determined on a
Chiralcel OD-H HPLC column using a solution of hexane and
isopropanol.
remove the catechol side product. The organic extracts were
then washed with deionized H O, followed by saturated
2
ammonium chloride (NH Cl) solution. The organic layer was
4
separated, dried over MgSO , and concentrated in vacuo. The
4
crude product was then analyzed by GC using a Carbowax 20
Acknowledgement
1
19
column, H, and F NMR to determine regioselectivity.
Column chromatography on silica gel using hexanes:ethyl
acetate (95:5) as eluent furnished the pure alcohols in good
yields. The spectral characteristics of the product alcohols
matched with those reported in the literature [10b].
We gratefully acknowledge Herbert C. Brown Center for
Borane Research for support of this research.
References
5
.2. Asymmetric homologation
[1] (a) R.D. Chambers, Fluorine in Organic Chemistry, Blackwell, Oxford,
2
004;
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004;
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(
A typical experimental procedure for the asymmetric
homologation of pentafluorophenylmagnesium bromide is as
2
(
follows. A solution of (S)-pinanediol methylboronate (3)
(
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(
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ꢀ
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(
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(
(
(
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(
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