Convenient preparation of cycloalkenyl boronic acid pinacol esters

Article abstract of DOI:10.1080/00397910802245762

A practical method for the preparation of cycloalkenyl boronic acid pinacol esters is described. These important synthetic intermediates are typically made using more expensive methods like transition metal-catalyzed borylation of alkenyl halides or trifl

Full text of DOI:10.1080/00397910802245762

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Convenient Preparation of
Cycloalkenyl Boronic Acid
Pinacol Esters
Vivek Rauniyar a , Huimin Zhai a & Dennis G. Hall a
a Department of Chemistry , University of Alberta ,
Edmonton , Alberta , Canada
Published online: 20 Oct 2008.
To cite this article: Vivek Rauniyar , Huimin Zhai & Dennis G. Hall (2008) Convenient
Preparation of Cycloalkenyl Boronic Acid Pinacol Esters, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
38:22, 3984-3995, DOI: 10.1080/00397910802245762
To link to this article: http://dx.doi.org/10.1080/00397910802245762
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Synthetic Communications1, 38: 3984–3995, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802245762
Convenient Preparation of Cycloalkenyl Boronic
Acid Pinacol Esters
Vivek Rauniyar, Huimin Zhai, and Dennis G. Hall
Department of Chemistry, University of Alberta, Edmonton,
Alberta, Canada
Abstract: A practical method for the preparation of cycloalkenyl boronic acid
pinacol esters is described. These important synthetic intermediates are typically
made using more expensive methods like transition metal-catalyzed borylation of
alkenyl halides or triflates. In this work, they are obtained from the simple corre-
sponding cycloalkanones, which are subjected to Shapiro reaction conditions fol-
lowed by trapping with a borate ester. The requisite products are obtained in very
good to excellent yields, and the reactions can be scaled up to multigram
amounts. By providing a simple alternative to common methods that make use
of expensive transition-metal catalysts and formation of sensitive intermediates,
this convenient method will be useful for the synthesis of ring-containing partners
for Suzuki–Miyaura cross-coupling and other reactions employing boronic esters
as substrates.
Keywords: Boronic acid, boronic esters, Shapiro reaction, Suzuki–Miyaura cross-
coupling
The Suzuki–Miyaura cross-coupling of aryl or alkenyl boronic acids and
esters is one of the most important reactions in organic synthesis,[1]
and it has been tremendously utilized in the synthesis of natural products
and bioactive molecules.[2] In this context, the analogous cycloalkenyl
boronic acids and esters are also important coupling partners to access
novel, polysubstituted alkenes. These boronic esters can, however, be
forbiddingly expensive. (For example, the 2007 Aldrich price for 1 g of
Received March 10, 2008.
Address correspondence to Dennis G. Hall, Department of Chemistry,
University of Alberta, Edmonton T6G 2G2, Canada. E-mail: dennis.hall@ualberta.ca
3984

Cycloalkenyl Boronic Esters
3985
Scheme 1. General approaches to cycloalkenyl boronic esters.
cyclohexenyl boronic acid pinacol ester is CDN$ 285) In general,
cycloalkenyl boronates are accessed by Miyaura borylation of cycloalke-
nyl triflates or halides (Scheme 1).[3] The sensitive cycloalkenyl triflates
are in turn obtained from the parent cycloalkanones using deprotonation
and enolate quenching with expensive triflating agents.[4] Herein, we
report a very convenient preparation and isolation of various cycloalke-
nyl boronic esters using the Shapiro reaction[5] of cycloalkylhydrazones
and isolation of the corresponding cycloalkenyl boronic acids as their
pinacol esters. Aside from one example where the organoboron product
was not isolated, this appears to be the first report of preparation of
cycloalkenyl boronic acid pinacol esters using a Shapiro protocol.[6]
The current methodology provides the requisite cycloalkenyl boronic
esters in 65–90% overall yield for a two-step sequence.[7]
While working on the modification of the diol scaffold utilized in the
2 ꢀ SnCl4-catalyzed allylboration of aldehyde substrates (Scheme 2), we
noticed that introduction of cycloalkyl groups in the ortho-position of
the hydrobenzoin skeleton was crucial for obtaining homoallyl alcohol
products in high enantioselectivities.[8] During our optimization efforts
to generate various ring sizes on the diol catalyst 2, we required a rapid
preparation of the corresponding cycloalkenyl boronic esters. The latter
would serve to install various cycloalkyl rings at the ortho-position of the
protected hydrobenzoin 1 by a Suzuki–Miyaura cross-coupling process.
A general preparation of cycloalkenyl boronic esters had to be eco-
nomical and efficient. As such, we envisioned a cycloalkenyl anion cap-
ture of a boron-containing electrophile. To this end, we explored a
Shapiro reaction of the cycloalkyl hydrazones to generate cycloalkenyl

3986
V. Rauniyar, H. Zhai, and D. G. Hall
Scheme 2. Preparation and application of cycloalkenyl boronic esters for the
preparation of ortho-cycloalkyl substituted diol 2.
anions. We anticipated that quenching of the generated anions with com-
mercially available isopropoxy pinacol borate would allow for easier
handling and for proper purification and characterization of the products
as stable pinacol esters.
In the event, as shown in Scheme 3, cycloalkylhydrazones 4 were
readily obtained as crystalline solids in almost quantitative yields by a
simple addition=dehydration reaction with inexpensive hydrazide reagent
5 in refluxing absolute ethanol. The majority of cycloalkyl hydrazone
substrates started to precipitate out when the reaction reached comple-
tion, after which the reaction mixture was cooled in an ice bath to allow
for completion of crystallization. The freshly obtained cycloalkyl
hydrazones 4 were suspended in hexanes followed by addition of N,N-
tetramethylethylenediamine (TMEDA) (3 ml=mmol of hydrazone) and
cooled to ꢁ78 ꢂC. This mixture was treated with 4 equivalents of n- BuLi,
after which dinitrogen was extruded at room temperature during 1 to 2 h.
The resulting orange to red-brown solutions containing the correspond-
ing cycloalkenyl anion equivalents were cooled to ꢁ78 ꢂC and treated
with isopropoxypinacol borate 3 (Scheme 3), and the resulting products
were easily purified by flash chromatography over silica gel.
The resulting cycloalkenyl boronate products 6 are robust and can
be stored for weeks without any observable decomposition. Table 1
illustrates the scope of this protocol. The majority of the products
were obtained in good to excellent yields, and this chemistry appears to
be general for the preparation of various ring sizes of cycloalkenyl
boronic esters from the corresponding cycloalkanones.

Cycloalkenyl Boronic Esters
3987
Scheme 3. Preparation of cycloalkenyl pinacol boronate esters.
CONCLUSION
We have described a two-step sequence that provides easy access to cyclo-
alkenyl boronic esters in good to excellent yields, and this methodology is
readily scalable to multigram quantities. The intermediates are robust
and can be stored for months without any observable degradation.
Accordingly, expensive reagents including palladium catalyst, bis(pinaco-
lato)diboron, and activating agents are avoided. As such, this method
should find widespread application in the preparation of cycloalkenyl-
boronic acid pinacol esters and their application in several types of
cross-coupling reactions.
EXPERIMENTAL
General
All cycloalkanones were purchased from Aldrich or TCI America.
TMEDA was purchased from Acros Organics in 2.5-L containers. Unless
otherwise noted, all reactions were performed under an argon atmosphere
using flame-dried glassware. TMEDA and hexane were distilled over
CaH2. Thin-layer chromatography (TLC) was performed on silica-gel 60
F254 plates and were visualized with UV light and KMnO4 stain. NMR
spectra were recorded on 300-, 400-, or 500-MHz instruments. The residual
solvent protons (1H) or the solvent carbons (13C H) were used as internal
standards. 1H NMR data are presented as follows: chemical shift in parts
per million downfield from tetramethylsilane (multiplicity, coupling con-
stant, integration). The following abbreviations are used in reporting
NMR data: s, singlet; br s, broad singlet; d, doublet; t, triplet; q, quartet;
qt, quartet of triplets; dd, doublet of doublets; dt, doublet of triplets;
AB, AB quartet; m, multiplet. High-resolution mass spectra were recorded
by the University of Alberta Mass Spectrometry Services Laboratory using

3988
V. Rauniyar, H. Zhai, and D. G. Hall
Table 1. Preparation of cycloalkenyl pinacol boronic esters. Yields (%) are for the
two-step sequence
Yield
(%) of 6,
two steps
Entry Cycloalkanone
1
Cyclohydrazone
Product
80
2
3
70
65
4
5
90
80
6
70
(Continued )

Cycloalkenyl Boronic Esters
3989
Table 1. Continued
Yield
(%) of 6,
two steps
Entry Cycloalkanone
7
Cyclohydrazone
Product
88
8
95
either electron impact (EI) or electrospray (ES) ionization techniques.
Infrared spectra and optical rotations were recorded by the University of
Alberta Department of Chemistry Spectral Services.
Compound 4a
p-Tolylsulfonylhydrazine (90.0 g, 0.48 mol) was added to a 250-mL flask
equipped with a magnetic stirbar, and this white solid was suspended in
60 ml of absolute ethanol. Cyclopentanone (40.7 g, 0.48 mol) was added
to this suspension, and the reaction mixture was heated to reflux at
100 ꢂC. After heating for 20 min, the mixture dissolved, and after another
20 min, a white solid corresponding to the hydrazone started crashing
out. After heating for 1 h, the reaction mixture was cooled using an
ice-water bath to precipitate out the majority of the hydrazone. The
resulting solid was collected by filtration and was washed thoroughly
with ice-cold ethanol. Air drying under reduced pressure for 1 h afforded
114 g of the required hydrazone in near quantitative yield as a white solid
with identical spectral properties compared to literature.[9]
Compound 6a
To a flame-dried 500-ml round-bottom flask equipped with a magnetic
stirbar and rubber septum, cyclopentanone p-tolylsulfonylhydrazone

3990
V. Rauniyar, H. Zhai, and D. G. Hall
(6.48 g, 27.15 mmol) was added followed by 80 mL of anhydrous hex-
anes. To this mixture 80 ml of anhydrous TMEDA (ꢃ3 mL=mmol of
hydrazone) were added, and the reaction mixture was cooled to
ꢁ78 ꢂC and maintained at this temperature for 15 min, after which
43 ml (107.5 mmol, 4.0 equiv.) of 2.5 M n-BuLi was added over
15 min. The reaction mixture was then stirred for 1 h at ꢁ78 ꢂC and
then brought to room temperature and stirred for 1.5 h. Nitrogen was
extruded from the reaction, and at the end of the period, the reaction
mixture was then brought to ꢁ78 ꢂC and maintained for another
15 min, after which 20.1 g (108.6 mmol, 4 equiv.) of pinacol isopropyl
borate were added. The reaction mixture was stirred for another hour
at ꢁ78 ꢂC and then brought to room temperature and stirred for 3 h.
The reaction was quenched with the addition of saturated NH4Cl and
then extracted three times with ether. The combined organic extracts
were dried over anhydrous MgSO4, filtered, and concentrated in vacuo.
The residue was subjected to flash chromatography (2% EtOAc=
hexanes) to afford 4.19 g (80%) of 6a as a light yellow oil. IR (neat)
2979, 1615, 1374, 1318, 1146 cmꢁ1 1H NMR (400 MHz, CDCl3) d
;
6.54 (t, J ¼ 2.0 Hz, 1 H), 2.48–2.31 (m, 4 H), 1.88–1.74 (m, 2 H), 1.27
(s, 12 H); 13C NMR (100 MHz, CDCl3) d 147.5, 83.0, 34.8, 34.5,
24.8, 23.9. HRMS (EI) calcd. for C11H19O2 11B: 194.1478. Found:
194.1482.
Compound 4b
Following the general procedure described for 4a, the product was
obtained in 90% yield as a white solid. IR (cast film) 3221, 2955, 2868,
1
1643, 1598, 1395, 1327, 1164 cmꢁ1; H NMR (400 MHz, CDCl3) d 7.85
(d, J ¼ 8.4 Hz, 2H), 7.36 (brs, 1H), 7.31 (d, J ¼ 8.4 Hz, 2H), 2.68–2.72
(m, 1H), 2.44–2.49 (m, 1H), 2.43 (s, 3H), 2.06 (ddd, J ¼ 4.8, 12.4,
12.4 Hz, 1H), 1.87–1.92 (m, 2H), 1.75 (ddd, J ¼ 5.2, 13.6, 13.6, 1H),
1.03–1.23 (m, 3H), 0.84 (s, 9H); 13C NMR (100 MHz, CDCl3) d 161.80,
145.20, 143.88, 135.44, 129.51, 128.46, 128.09, 126.62, 126.37, 43.14,
34.99, 33.87, 32.76, 26.65, 21.57. HRMS (m=z calcd. for C17H27N2O2S:
323.17878. Found: 323.17876.
Compound 6b
Following the general procedure described for 6a, 6b was obtained in
77% yield as a pale yellow solid. IR (cast film) 3026, 2978, 2924, 1633,
1
1387, 1340, 1314, 1146 cmꢁ1; H NMR (400 MHz, CDCl3) d 7.19–7.344

Cycloalkenyl Boronic Esters
3991
(m, 5H), 6.7–6.72 (m, 1H), 2.77–2.85 (m, 1H), 2.37–2.48 (m, 2H), 2.21–
2.34 (m, 2H), 1.97–2.01 (m, 1H), 1.68–1.83 (m, 1H), 1.31 (s, 12H);
13C NMR (100 MHz, CDCl3) d 147.24, 146.26, 128.37, 126.86, 125.97,
83.12, 39.81, 34.93, 29.82, 27.06, 24.88, 24.85. HRMS (EI) calcd. for
C19H25O211B: 284.19476. Found: 284.19481.
Compound 4c
Following the general procedure described for 4a, the requisite hydrazone
was obtained in 95% yield as a white solid. IR (cast film) 3219, 3061,
1
2930, 1325, 1185, 1164 cmꢁ1; H NMR (400 MHz, CDCl3) d 7.88 (d,
J ¼ 8.4 Hz, 2 H), 7.53 (brs, 1 H), 7.34 (d, J ¼ 8.4 Hz, 2H), 7.14–7.31 (m,
5H), 2.80–2.85 (m, 1H), 2.76 (dddd, J ¼ 4.0, 4.0, 12.4, 12.4 Hz, 1H),
2.55–2.60 (m, 1H), 2.45 (s, 3H), 2.26 (ddd, J ¼ 4.8, 14, 14 Hz, 1H),
2.03–2.07 (m, 2H), 1.98 (ddd, J ¼ 5.6, 14.8, 14.8 Hz, 1H), 1.68 (dddd,
J ¼ 4, 13.2, 13.2, 13.2 Hz, 1H), 1.58 (dddd, J ¼ 4.4, 13, 13, 13 Hz, 1H);
13C NMR (100 MHz, CDCl3) d 163.20, 143.85, 135.56, 129.51, 128.13,
47.08, 34.94, 32.48, 27.48, 27.40, 26.72, 26.39, 21.62. HRMS (ESI), m=z
calcd. for C19H23N2O2S: 343.14748. Found: 343.14738.
Compound 6c
Following the general procedure described for 6a, the product was
obtained in 68% yield as a light yellow oil that crystallizes upon standing.
1
IR (cast film) 2962, 2916, 2870, 1636, 1388, 1331, 1145 cmꢁ1; H NMR
(400 MHz, CDCl3) d 6.59 (t, J ¼ 2 Hz, 1H), 2.26–2.36 (m, 1H), 1.97–
2.14 (m, 2H), 1.77–1.90 (m, 2H), 1.24 (s, 12H), 1.03–1.15 (m, 1H),
0.83–0.92 (m, 10H); 13C NMR (100 MHz, CDCl3) d 143.43, 83.01,
43.81, 32.27, 28.47, 27.85, 27.14, 24.87, 24.82, 23.98. HRMS (EI) calcd.
for C16H29O211B: 264.21126. Found: 264.22607.
Compound 4d
Following the general procedure described for 4a, the product was
obtained in quantitative yield as a white solid. IR (cast film) 3228,
;
2953, 2897, 1641, 1598, 1458, 1339, 1163 cmꢁ1 1H NMR (400 MHz,
CDCl3) d 7.84 (d, J ¼ 8.4 Hz, 2H), 7.27 (d, J ¼ 8 Hz, 2H), 2.39 (s, 3H),
1.99 (d, J ¼ 4.8 Hz, 4 H), 1.28 (s, 2H), 0.82 (s, 6H), 0.81 (s, 6H);
13C NMR (100 MHz, CDCl3) d 160.23, 143.77, 135.40, 129.41, 127.98,
52.22, 47.87, 39.43, 35.01, 34.66, 30.63, 30.59, 21.54. HRMS (EI) calcd.
for C17H26N2O2S: 322.17151. Found 322.17151.

3992
V. Rauniyar, H. Zhai, and D. G. Hall
Compound 6d
Following the general procedure described for 6a, the product was
obtained in 90% yield as a pale yellow solid. IR (cast film) 2978, 2951,
1
2899, 1633, 1387, 1328, 1147 cmꢁ1; H NMR (400 MHz, CDCl3) d 6.27
(t, J ¼ 2.4 Hz, 1H), 8.85 (d, J ¼ 2.8 Hz, 2H), 1.32 (s, 2H), 1.27 (s, 12H),
1.01 (s, 6H), 0.92 (s, 6H); 13C NMR (100 MHz, CDCl3) d 151.26,
82.94, 49.81, 39.63, 33.28, 30.99, 29.93, 29.79, 24.80. HRMS (EI) calcd.
for C16H29O211B: 264.21126. Found 264.23635.
Compound 4e
Following the general procedure described for 4a, the product was isolated
in quantitative yield as a white solid. IR (neat) 3214, 2926, 1330,
1160 cmꢁ1; 1H NMR (400 MHz, CDCl3) d 7.84 (d, J ¼ 8.4 Hz, 2H), 7.88–
7.56 (s, 1 H), 7.28 (d, J ¼ 8.4 Hz, 2 H), 2.40 (s, 3 H), 2.28 (t, J ¼ 6.4 Hz, 2
H), 2.22 (t, J ¼ 6.4 Hz, 2 H), 1.70–1.60 (m, 4 H), 1.42–1.31 (m, 4 H),
1.21–1.12 (m, 2 H). HRMS (EI) calcd. for C15H23N2O2S: 295.1475. Found:
295.1475. Anal. calcd. for C15H23N2O2S: C, 61.02; H, 7.80; N, 9.49.
Found: C, 61.11; H, 7.62; N, 9.53. For 13C NMR, see Ref. 9b.
Compound 6e
Following the general procedure described for 6a, the product was iso-
lated in 80% yield as a pale yellow oil. IR (cast film) 2925, 1630, 1381,
1148 cmꢁ1
;
1H NMR (400 MHz, CDCl3) d 6.58 (t, J ¼ 8.0 Hz, 1 H),
2.32–2.28 (m, 2 H), 2.28–2.19 (m, 2 H), 1.56–1.41 (m, 8 H), 1.26 (s, 12
H); 13C NMR (100 MHz, CDCl3) d 146.0, 114.3, 82.9, 29.6, 28.8, 27.1,
26.4, 26.2, 25.9, 24.7. HRMS (EI) calcd. for C14H25O211B: 236.1948.
Found: 236.1948.
Compound 4f
Following the general procedure described for 4a, the product was iso-
lated in quantitative yield as a white solid. IR (cast film) 3224, 2924,
1
2865, 1338, 1185, 1167 cmꢁ1; H NMR (400 MHz, CDCl3) d 7.85 (d,
J ¼ 8.4 Hz, 2H), 1.29 (d, J ¼ 8 Hz, 2H), 2.41 (s, 3H), 2.28–2.31 (m, 2H),
2.20–2.23 (m, 2H), 1.60–1.72 (m, 4H), 1.49–1.55 (m, 2H), 1.41–1.47 (m,
2H), 1.16–1.22 (m, 2H), 1.03–1.08 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 163.90, 143.89, 135.85, 129.42, 127.92, 35.50, 30.46, 28.34, 27.57, 27.49,

Cycloalkenyl Boronic Esters
3993
24.52, 23.48, 22.93, 21.54; HRMS (EI) calcd. for C16H24N2O2S:
308.15585. Found: 308.15596.
Compound 6f
Following the general procedure described for 6a, the product was iso-
lated in 70% yield as a colorless oil. IR (cast film) 2977, 2928, 2857,
1629, 1379, 1146 1167 cmꢁ1 1H NMR (400 MHz, CDCl3) d 6.48 (t,
;
J ¼ 8.4 Hz), 2.20–2.29 (m, 4H), 1.43–1.49 (m, 10H), 1.25 (s, 12H); 13C
NMR (100 MHz, CDCl3) d 145.94, 82.90, 27.36, 26.85, 26.50, 25.78,
25.58, 25.38, 24.78, 24.71, 24.67; HRMS (EI) calcd. for C15H27O211B:
250.21041. Found: 250.11952.
Compound 4g
Following the general procedure described for 4a, the product was iso-
lated in quantitative yield as a white solid. IR (cast film) 3219, 2927,
1
2869, 1598, 1335, 1167 cmꢁ1; H NMR (300 MHz, CDCl3) d 7.85 (d,
J ¼ 8.4 Hz, 2H), 7.46 (brs, 1H), 7.30 (d, J ¼ 8.4 Hz, 2H), 2.41 (s, 3H),
2.34 (appt, J ¼ 6.3 Hz, 2H), 2.21 (appt, J ¼ 6.6 Hz, 2H), 1.62–1.70 (m,
4H), 1.08–1.32 (m, 8H), 1.01–1.03 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 161.29, 143.81, 135.69, 129.40, 128.17, 41.97, 34.97, 30.08, 25.51, 25.24,
24.02, 23.82, 23.34, 22.79, 22.56; HRMS (m=z) calcd. for C17H27N2O2S:
323.17878. Found: 323.17850.
Compound 6g
Following the general procedure described for 6a, the product was iso-
lated in 88% yield as a colorless oil. IR (cast film) 2978, 2926, 2852,
1
1626, 1475, 1380, 1348, 1302, 1146 cmꢁ1; H NMR (400 MHz, CDCl3)
d 6.33 (t, J ¼ 8.8 Hz), 2.31–2.35 (m, 4H), 1.57–1.60 (m, 4H), 1.37–1.40
(m, 8H), 1.25 (s, 12H); 13C NMR (100 MHz, CDCl3) d 145.91, 82.79,
26.97, 26.95, 26.78, 26.44, 25.63, 24.78, 24.67, 21.40, 20.79. HRMS (EI)
calcd. for C16H29O211B: 264.22606. Found: 264.22591.
Compound 4h
Following the general procedure described for 4a, the product was isolated
in quantitative yield as a white solid. IR (cast film) 2978, 2929, 2860, 1625,

3994
V. Rauniyar, H. Zhai, and D. G. Hall
1468, 1409, 1147 cmꢁ1; 1H NMR (400 MHz, CDCl3) d 7.82 (d, J ¼ 8.8 Hz,
2H), 7.47 (brs, 1H), 7.28 (d, J ¼ 8.8 Hz, 2H), 2.40 (s, 3H), 2.21 (appt,
J ¼ 8 Hz, 2H), 2.12 (appt, J ¼ 8.8 Hz, 2H), 1.57–1.65 (m, 2), 1.41–1.49
(m, 2H), 1.08–1.15 (m, 8H), 0.8–0.95 (m, 6H); 13C NMR (100 MHz,
CDCl3) d 159.44, 143.81, 135.71, 129.52, 128.16, 31.31, 28.80, 25.98,
25.88, 23.67, 23.23, 22.96, 22.73, 22.66, 21.88, 21.62, 21.49. HRMS (EI)
calcd. for C19H30N2O2S: 350.20280. Found: 350.20289.
Compound 6h
Following the general procedure described for 6a, the product was iso-
lated in 95% yield as a colorless oil. IR (cast film) 2977, 2928, 2859,
;
1625, 1468, 1408, 1379, 1344, 1301, 1147 cmꢁ1 1H NMR (400 MHz,
CDCl3) d 6.28 (t, J ¼ 6 Hz, 1H), 2.16–2.22 (m, 4H), 1.47–1.54 (m, 4H),
1.34–1.41 (m, 8H), 1.29–1.30 (m, 4H), 1.26 (s, 12H); 13C NMR
(100 MHz, CDCl3) d 145.70, 132.56, 82.88, 34.63, 31.71, 27.35, 26.46,
25.53, 24.89, 24.17, 22.67, 22.48, 14.00, 10.95. HRMS (EI) calcd. for
C18H33O211B: 292.25736. Found: 292.25804.
ACKNOWLEDGMENTS
The Spectral Services at the University of Alberta are thanked. V. R.
acknowledges the Alberta Ingenuity Foundation for a Studentship.
Financial support for this research was provided by the Natural Sciences
and Engineering Research Council of Canada.
REFERENCES
1. Hall, D. G. In Boronic Acids: Preparation and Application in Organic
Synthesis and Medicine; Wiley-VCH: Weinheim, 2005.
2. (a) Smith, G. B.; Dezeny, G. C.; Huges, D. L.; King, A. O.; Verhoeven, T. R.
Mechanistic studies of the Suzuki cross-coupling reaction. J. Org. Chem.
1994, 59, 8151–8156; (b) Ennis, D. S.; McManus, J.; Wood, K. W.;
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