A comparative study of the relative stability of representative chiral and achiral boronic esters employing transesterification

Article abstract of DOI:10.1007/s00706-007-0699-x

A comparative study of the transesterification of five representative chiral and achiral boronic esters with various structurally modified diols was undertaken to qualitatively understand the factors influencing the relative stability of these boronic esters. Several factors such as chelation, conformation, steric bulk of the substituents, size of the heterocycle, and entropy influence the relative rate of transesterification as well as the stability of the boronic esters. Amongst these boronic esters, pinanediol phenylboronic ester was found to be the most stable boronic ester whereas DIPT boronic ester appeared to be thermodynamically the least stable one. The transesterification with sterically hindered diols was observed to be relatively slow, but afforded thermodynamically more stable boronic esters. Boronic esters derived from cis-cyclopentanediols and the bicyclo[2.2.1]heptane-exo,exo-2,3- diols are relatively more stable. This study not only presents the qualitative picture of relative stability of various boronic esters, but also provides helpful hints regarding the possible recovery of chiral auxiliaries. Many C ₂-symmetric chiral auxiliaries, such as 2,3-butanediol, 2,4-pentanediol, DIPT, and cis-cyclohexane-1,2-diol, can be retrieved by simple transesterification of the corresponding boronic esters with commercial inexpensive diols, such as pinacol, 1,3-propanediol, and neopentyl glycol.

Full text of DOI:10.1007/s00706-007-0699-x

Monatshefte fu¨r Chemie 138, 879–887 (2007)
DOI 10.1007/s00706-007-0699-x
Printed in The Netherlands
A Comparative Study of the Relative Stability of Representative Chiral
and Achiral Boronic Esters Employing Transesterification
Chandra D. Roy1;2;ꢀ and Herbert C. Brown1;y
1
Herbert C. Brown Center for Borane Research, Department of Chemistry, Purdue University, West Lafayette, IN, USA
2
EMD Biosciences, Inc., San Diego, CA, USA
Received April 14, 2007; accepted April 16, 2007; published online June 29, 2007
# Springer-Verlag 2007
Summary. A comparative study of the transesterification
of five representative chiral and achiral boronic esters with
Introduction
The usefulness of boronic acids and their esters in
synthetic and biomolecular chemistry is well recog-
nized over the last decade [1]. Some of the bio-
molecular applications include their uses in serine
proteases inhibition [2], transmembrane transport
[3], separation of glycoproteins [4], boron neutron
capture therapy (BNCT) [5], as a glucose-selective
fluorescence sensor [6], as drug delivery agents [7],
and as a redox-sensitive protecting group [8]. In syn-
thetic organic chemistry, they have been used exten-
sively in the Suzuki coupling reactions [9], Petasis
reactions [10], copper and rhodium catalyzed cross-
coupling reactions [11], Diels-Alder reactions [12],
Matteson’s asymmetric homologation reactions [13],
annulation reactions [14], asymmetric synthesis of
amino acids [15], and separation of cyclic cis- and
trans-stereoisomeric 1,2-diols [16]. Boronic esters are
also valuable compounds as protecting groups in
carbohydrate chemistry [17], as chiral derivatizing
agents [18], as chiral auxiliaries [19], and as precur-
sors to boron enolates [20].
various structurally modified diols was undertaken to quali-
tatively understand the factors influencing the relative stabil-
ity of these boronic esters. Several factors such as chelation,
conformation, steric bulk of the substituents, size of the het-
erocycle, and entropy influence the relative rate of transester-
ification as well as the stability of the boronic esters. Amongst
these boronic esters, pinanediol phenylboronic ester was found
to be the most stable boronic ester whereas DIPT boronic ester
appeared to be thermodynamically the least stable one. The
transesterification with sterically hindered diols was observed
to be relatively slow, but afforded thermodynamically more
stable boronic esters. Boronic esters derived from cis-cyclo-
pentanediols and the bicyclo[2.2.1]heptane-exo,exo-2,3-diols
are relatively more stable. This study not only presents the
qualitative picture of relative stability of various boronic es-
ters, but also provides helpful hints regarding the possible
recovery of chiral auxiliaries. Many C2-symmetric chiral aux-
iliaries, such as 2,3-butanediol, 2,4-pentanediol, DIPT, and
cis-cyclohexane-1,2-diol, can be retrieved by simple transes-
terification of the corresponding boronic esters with commer-
cial inexpensive diols, such as pinacol, 1,3-propanediol, and
neopentyl glycol.
Keywords. Boronic esters; Diols; Transesterification; Chiral
auxiliaries.
The stability of the boronic esters plays a key role
in chiral auxiliary directed multistep organic synthe-
sis. Matteson et al. achieved a very high degree of
stereo- and enantioselectivities (>99% ee) during
the successive one-carbon homologation of cyclic
boronic esters derived from pinanediol with preformed
(dichloromethyl)lithium [13, 21]. The recovery of the
chiral auxiliary, pinanediol, became a difficult task
ꢀ
Corresponding author. E-mail: chandra0919@gmail.com
This paper is dedicated to the memory of my mentor, the
y
late Professor Herbert C. Brown (1912–2004). Professor
Herbert C. Brown deceased on December 19, 2004. The work
described herein was carried out at Purdue University during
my stay as a post-doctoral research associate

880
C. D. Roy and H. C. Brown
Fig. 1. An entropically-favored diol exchange of the acyclic boronate with a cyclic diol
Fig. 2. A generic scheme for transesterification
due to the unusual stability of pinanediol boronic
esters toward ligand exchange or hydrolysis. Later,
Brown et al. succeeded in developing several pro-
cedures for the recovery of pinanediol from pinane-
diol boronic esters [22, 23]. In order to introduce a
new stereogenic center of desired configuration in
Matteson’s asymmetric homologation, the require-
ment for the removal of the first chiral diol and
replacement by its enantiomer or some other chiral
auxiliary, was well recognized during the synthesis
of stegobinone [24]. Transesterification can prove to
be a very easy avenue to introduce as well as retrieve
the chiral auxiliaries to or from boronic esters in a
very efficient manner, provided the final boronic
ester is thermodynamically more stable than the
starting boronic ester. Recently we reported the
structural effects on the relative rates of transester-
ification of 2-phenyl[1,3,2]dioxaborolane [25a] and
pinanediol methylboronic ester [25b] as well as the
stereoisomer-differentiating esterification of diols with
methylboronic acid (separation of cis,trans-1,2-diols)
[25c].
hydrolysis or transesterification in comparison with
their cyclic analogs. The energetics of such reactions
are favored by an increase in entropy (two moles of
reactants converting into three moles of products) as
well as an intramolecular nucleophilic attack (lower
activation energy pathway) in the second step by the
cyclic diol (Fig. 1).
In the present study, the temperature and the sol-
vent are unlikely to affect the transesterification
for obvious reasons. In the hope of further under-
standing the factors influencing the relative stabil-
ity of boronic esters, we undertook a general study
of transesterification of five representative chiral
and achiral boronic esters 1–5 with various com-
mercial as well as synthesized structurally modified
diols. In this report, we present our qualitative re-
sults on the relative stability of a large set of boronic
esters by 1H NMR employing transesterification pro-
tocol (Fig. 2).
Results and Discussion
Ligand exchange reactions are greatly influenced
by various factors, such as chelation, conformation,
entropy, catalyst, solvent polarity, and concentration,
the entropies of internal rotations of free diols and
the steric repulsions on enthalpy appear to be the
dominant factors in transesterification or hydrolytic
cleavage of boronic esters, as described by Matteson
and Man [24]. Acyclic boronic esters undergo rapid
For comparative study, two chiral boronic esters
1–2 and three achiral boronic esters 3–5 were pre-
pared by esterification of phenylboronic acid with
the corresponding diols in n-pentane in 4–6 h at
room temperature in excellent chemical yields
(75–85%) and purity (Fig. 3) [26]. Various structu-
rally-varied diols were synthesized by OsO4-catalysed
cis-dihydroxylation of the corresponding olefins in

Relative Stability of Boronic Esters
881
Fig. 3. Representative chiral and achiral boronic esters 1–5
Fig. 4. Various commercial and synthesized diols 6–25
>75% yields (Fig. 4) [27]. It is well known that the
equilibrium composition is highly solvent depen-
dent, the solvent, CDCl3 was chosen in the present
study. Transesterification was carried out in an NMR
tube (0.05 mmol of both boronic ester and diol in
1 cm3 CDCl3) under nitrogen at RT and the progress
of the exchange reaction was monitored by 1H NMR
spectroscopy. The CDCl3 (same solvent batch) used
for this study was stored over molecular sieves. With
several boronic esters, the equilibrium measurements
were carried out by approaching equilibrium from
both directions of reaction. Initially, all the reactions
were monitored very frequently (every 5–15 min)
soon after mixing. In the cases of very slow reactions,
the time intervals for monitoring the reactions were
extended to 12–24h (based on extent of initial ex-
change). In all the experiments, the disappearance and
an appearance of characteristic 1H peaks were moni-
tored until there were no more changes in product
formation (reaching equilibrium).
Table 1. Transesterification of chiral (þ)-diisopropyl (L)-tartrate phenylboronic ester (1) with representative diols in CDCl3
Entry
Diol
Time=h
Equilibrium compositions
RBE (%) > PBE (%)
1
2
3
4
5
6
7
8
9
Ethylene glycol 7
(þ)-2,3-Butanediol 8
0.1
0.1
48
01 > 99
02 > 98
01 > 99
08 > 92
01 > 99
01 > 99
01 > 99
01 > 99
01 > 99
01 > 99
01 > 99
10 > 90
Pinacol 9
cis-1,2-Cyclohexanediol 10
cis-1,2-Cyclopentanediol 11
ꢀ-Pinanediol 19
20
0.25
0.25
0.1
0.1
3
0.1
0.1
0.1
1,3-Propanediol 20
2,2-Dimethyl-1,3-propanediol 21
2,4-Dimethyl-2,4-pentanediol 22
Diethanolamine 23
N-tert-Butyldiethanolamine 24
2,6-Pyridinedimethanol 25
10
11
12

882
C. D. Roy and H. C. Brown
Fig. 5. Transesterification of boronic ester 1
Transesterification of Chiral (þ)-2-Phenyl[1,3,2]-
dioxaborolane-4,5-dicarboxylic Acid Diisopropyl
Ester (1)
resulting boronic esters are usually soluble in n-pen-
tane whereas decanting the n-pentane layer easily
retrieves the displaced insoluble diol.
The chiral auxiliary, (þ)-diisopropyl tartrate (DIPT)
has been a very useful chiral director in synthesis,
especially in the boronic ester based asymmetric
allenylboration [28, 29], allylboration [30] and cro-
tylboration [31]. The recovery of the costly chiral
auxiliary from the boronic ester becomes an impor-
tant task in the chiral auxiliary directed asymmetric
synthesis. Therefore, it was of interest to examine
the thermodynamic stability of the (þ)-diisopropyl
(L)-tartrate phenylboronic ester 1. It is quite obvious
from Table 1 that the (þ)-diisopropyl (L)-tartrate
phenylboronic ester is the least stable boronic ester
(Fig. 5). Except for diols 9 and 10, all the diols in-
stantaneously displaced DIPT. Such facile ligand
exchange may possibly be due to minimal steric re-
pulsions between the 4,5-substituents in new product
boronic esters (as compared to trans-4,5-diisopropyl
carboxylate groups in 1). The slow transesterification
with pinacol 9 may be attributed to the steric factor.
The chiral auxiliary, DIPT can easily be recovered
by conducting the reaction in n-pentane [25c]. The
Transesterification of Chiral (ꢁ)-4,5-Dimethyl-2-
phenyl[1,3,2]dioxaborolane (2)
(R,R)-(ꢁ)-2,3-Butanediol has also been used as an
effective C2-symmetric chiral auxiliary in asymmetric
synthesis, especially in Matteson’s asymmetric homo-
logation [21b]. The boronic ester 2 is also an ideal
substrate for studying and comparing the thermo-
dynamic stability gained by the boronic esters due
to the size and structure of the ring. Therefore, it was
of considerable interest to examine the transesterifi-
cation of the boronic ester 2. It was interesting to
compare the extent of transesterification with the
cis-1,2-cyclohexanediol 10 and the cis-1,2-cyclopen-
tanediol 11, since both are very similar to 2,3-butan-
ediol (4,5-disubstituted borolane, except for the
cyclic structure). Again, the favorable equilibrium for
the boronic ester 5 over the boronic ester 4 (entries 3
and 4) clearly demonstrates the significance of co-
planarity while evaluating the stability of the cyclic
Table 2. Transesterification of (ꢁ)-(phenyl)-1,3,2-dioxa-4,5-dimethylborolane (2) with representative diols in CDCl3
Entry
Diol
Time=h
Equilibrium compositions
RBE (%) > PBE (%)
1
2
3
4
5
6
7
8
9
(þ)-Diisopropyl (L)-tartrate 6
Pinacol 9
cis-1,2-Cyclohexanediol 10
cis-1,2-Cyclopentanediol 11
ꢀ-Pinanediol 19
0.1
302
5.75
20
2.5
0.3
0.75
258
0.15
95 > 05
37 > 63
86 > 14
14 > 86
08 > 92
49 > 51
29 > 71
20 > 80
70 > 30
1,3-Propanediol 20
2,2-Dimethyl-1,3-propanediol 21
2,4-Dimethyl-2,4-pentanediol 22
Diethanolamine 23

Relative Stability of Boronic Esters
883
Fig. 6. Transesterification of boronic ester 2
boronic esters (Table 2). In general, the formation
of bicyclic structures are entropically favored over
monocyclic ones. As expected, the steric factor
slows down the exchange reaction between boronic
ester 2 and the pinacol, but produces the thermody-
namic equilibrium composition favoring tetrasubsti-
tuted pinacolyl boronic ester 3 (60–63%) over the
disubstituted boronic ester 2 (Fig. 6). Although, di-
ethanolamine was highly effective with boronic ester
1, only 30% transesterification was observed with
boronic ester 2. Again, the 4,6-disubstituted borinane
(entry 8) is relatively more stable than the unsubsti-
tuted borinane (entry 6).
by transesterification with polystyrene-boronic acid.
To compare the relative stability of pinacolyl boronic
ester, ligand exchange of boronic ester 3 was carried
out with a series of diols. Amongst the various diols
studied, the highly effective diol (except for pinane-
diol) appeared to be the exo,exo-norbornanediol 17
(entry 3). The rate of exchange was also relatively
faster than for other diols. The easy transesterification
of pinacolyl boronic ester with 1,2-diisopropylethane-
diol or 1,2-dicyclohexylethanediol clearly suggests
that the steric repulsions between the methyl groups
destabilize five-membered pinacolyl boronic ester
[24]. Such repulsive interactions are minimal in nor-
bornanediol boronic esters. The unusually slow ligand
exchange of 3 with pinanediol 19 (entry 4) is mainly
attributed to the orientation and the rigidity of the
free diol. It was surprising to observe the ineffective-
ness of diethanolamine 23 (entry 7) taking into ac-
count that the nitrogen atom of the diethanolamine
had the ability to provide an additional stability to
the boronic ester by chelation. In contrast, the DIPT
boronic ester 1 underwent quantitative transesterifi-
cation with 23, under identical conditions in CDCl3
in <5 min (Table 1, entry 10). Transesterification with
diethanolamine is known to be highly efficient but in
nonhalogenated solvents as the driving force is taken
Transesterification of 4,4,5,5-Tetramethyl-2-phenyl-
[1,3,2]dioxaborolane (3)
Pinacolyl boronic esters have found wide applica-
tions in cross-coupling chemistry. Unlike other boron-
ic esters (except for pinanediol boronic esters),
pinacolyl boronic esters are generally very stable
and can be purified by silica gel column chromato-
graphy. Deprotection of pinacol from their boronic
esters has been achieved using diethanolamine=HCl
[32] and KHF2=base [33]. Pennington et al. [34]
successfully deprotected the pinacolyl boronic esters
Table 3. Transesterification of 4,4,5,5-tetramethyl-2-phenyl[1,3,2]dioxaborolane (3) with representative diols in CDCl3
Entry
Diol
Time=h
Equilibrium compositions
RBE (%) . PBE (%)
1
2
3
4
5
6
7
(þ)-2,3-Butanediol 8
270
291
92
764
888
894
21
60 > 40
20 > 80
04 > 96
04 > 96
55 > 45
47 > 53
88 > 12
cis-1,2-Cyclopentanediol 11
exo,exo-2,3-Norbornanediol 17
ꢀ-Pinanediol 19
1,3-Propanediol 20
2,2-Dimethyl-1,3-propanediol 21
Diethanolamine 23

884
C. D. Roy and H. C. Brown
Fig. 7. Transesterification of boronic ester 4
to be the precipitation of the diethanolamine adduct.
The results are tabulated in Table 3.
1,2-cyclohexanediol can be retrieved from its boro-
nic ester using these cheap diols.
Transesterification of 2-Phenylhexahydrobenzo-
[1,3,2]dioxaborole (4)
Transesterification of 2-Phenyltetrahydrocyclopenta-
[1,3,2]dioxaborole (5)
To understand the effect of ring size fused with
[1,3,2]dioxaborolane on the thermodynamic stabil-
ity, the transesterification of the boronic ester 4 was
studied with few representative diols. The quantita-
tive displacement of cis-1,2-cyclohexanediol from
its boronic ester by cis-1,2-cyclopentanediol unam-
biguously establishes that the boronic ester 5 is
much more stable than the boronic ester 4 (Fig. 7,
Table 3). The conversion of higher energy boat con-
formation (partial inversion of the chair-form diol to
the boat form to attain coplanarity for an intramole-
cular transesterification) of boronic ester 4 into the
boronic ester 5 which is a lower energy planar boat
conformation, is most likely the driving force. The
boronic ester 4 underwent quantitative diol exchange
with all the three cyclopentane-based diols (entries
1–3) which suggests that C2-symmetric chiral cis-
A qualitative picture of an additional thermodynamic
stability gained due to the cyclic structure of the diol
can be obtained by transesterification of the boronic
ester 2 with diols 10 and 11. The equilibrium com-
position favoring the boronic ester 5 (86%) in com-
parison with the boronic ester 4 (14%) clearly
establishes that the boronic ester 5 is thermodynami-
cally much more stable than the boronic ester 4
(Fig. 8). Since the formation of the cyclic boronic
ester requires a coplanar intermediate, such differ-
ence in stability can be interpreted in terms of con-
formational factors. The carbon-oxygen bonds in the
cis-cyclopentane-1,2-diol (boat form) are essentially
coplanar (dihedral angle nearly 0ꢂ) whereas the cis-
cyclohexane-1,2-diol is chair-shaped and the carbon-
oxygen bonds make a dihedral angle of approximately
60ꢂ. The partial inversion of the chair-form into the
Table 4. Transesterification of 2-phenylhexahydrobenzo[1,3,2]dioxaborole (4) with representative diols in CDCl3
Entry
Diol
Time=h
Equilibrium compositions
RBE (%) > PBE (%)
1
2
3
4
1,4-Anhydroerythritol 15
cis-1,2-Cyclopentandiol 10
exo,exo-2,3-Norbornanediol 16
1,3-Propanediol 19
0.5
0.33
1.5
3
01 > 99
01 > 99
01 > 99
15 > 85
Fig. 8. A comparison of the relative stability of boronic esters 4 and 5

Relative Stability of Boronic Esters
885
Table 5. Transesterification of achiral 2-phenyltetrahydrocyclopenta[1,3,2]dioxaborole (5) with representative diols in CDCl3
Entry
Diol
Time=h
Equilibrium compositions
RBE (%) > PBE (%)
1
2
3
4
5
6
7
1,4-Anhydroerythritol 16
1.0
1.25
25
120
720
48
51 > 49
37 > 63
27 > 73
13 > 87
60 > 40
20 > 80
14 > 86
cis-1-Methylcyclopentan-1,2-diol 12
cis-1-Ethylcyclopentan-1,2-diol 13
cis-1-Isopropylcyclopentan-1,2-diol 14
cis-1,2-Dimethyl-1,2-cyclopentanediol 15
exo,exo-2,3-Norbornanediol 17
endo-2-Phenyl-exo,exo-2,3-norbornanediol 18
264
Fig. 9. Order of thermodynamic stability of representative boronic esters
boat-form to attain coplanarity requires an additional
energy and therefore makes the process less favorable.
A linear relationship between the thermodynamic sta-
bility of various substituted cyclopentanediol boronic
ester and the steric bulk of the alkyl substituent pres-
ent at C1-carbon of cis-1,2-cyclopentanediols 12–
14 (entries 2–4) is well presented in Table 5. The
steric bulk of the substituent was anticipated to slow
down the exchange process, but expected to produce
thermodynamically more stable boronic ester. In-
deed, that was the case. It is worth noting that the
second substitution at C2-carbon dramatically slows
down the ligand exchange process.
trate (DIPT) boronic ester was found to be thermo-
dynamically the least stable boronic ester. Simple
six-membered boronic esters were observed to be
relatively more stable than their corresponding five-
membered analogs. The nature of the substituent
present on the carbon bearing the hydroxyl group
has greater influence on equilibrium. A coplanar diol
favors the formation of the relatively more stable
boronic ester. With the knowledge of the relative sta-
bility of boronic esters, it is possible to recover the
C2-symmetric chiral auxiliaries, such as, (þ)-DIPT,
(ꢁ)-2,3-butanediol, cis-1,2-cyclohexanediol, from
their boronic esters by transesterification with a se-
ries of cheap diols, e.g., ethylene glycol, pinacol, 1,3-
propanediol, neopentyl glycol, and diethanolamine.
To the best of our knowledge, a general study of this
sort had never been done and would be potentially of
significant interest to the practicing organic chemist
in the boronic acid=ester field.
Based on the above experimental data, several
representative boronic esters can be arranged in de-
creasing order of their relative stability (pinanediol
boronic ester is most stable and DIPT boronic ester
is least stable in this series) (Fig. 9).
Conclusions
Experimental
To summarize, we have studied the relative stability
of a wide spectrum of boronic esters (achiral and
chiral) employing transesterification with free diols
under neutral conditions and compiled them in the
decreasing order of their stability. Diisopropyl tar-
Detailed procedures and techniques for handling air- and
moisture-sensitive compounds have been reported elsewhere
1
[35]. The 11B (96 MHz), H (300 MHz), and 13C (75 MHz)
NMR spectra were recorded on a Varian Gemini NMR instru-
ment, and the chemical shifts (ꢁ) are given in ppm relative to

886
C. D. Roy and H. C. Brown
1-Isopropylcyclopentane-cis-1,2-diol (14, C8H16O2)
1H NMR (CDCl3): ꢁ ¼ 3.90 (m, 1H, ꢁCHOH), 2.10 (br s, 2H,
ꢁCHOH and ꢁ(CH3)2COH), 2.00–1.40 (m, 7H, ꢁCH2ꢁ and
ꢁCH(CH3)2), 0.95 (dd, 6H, ꢁCH(CH3)2) ppm.
external standard BF3-Et2O and internal standards TMS and
CDCl3. Except for diols 11–14 and 16–17, all the diols were
purchased from the Aldrich Chemical Co.
Several boronic esters and diols, such as (ꢁ)-4,5-dimethyl-2-
phenyl[1,3,2]dioxaborolane (2) [36], 4,4,5,5-tetramethyl-2-
phenyl[1,3,2]dioxaborolane (3) [37], 1-methylcyclopentane-
cis-1,2-diol (12) [37], 1-ethylcyclopentane-cis-1,2-diol (13)
[37], 1-isopropylcyclopentane-cis-1,2-diol (14) [37], 1,2-
dimethylcyclopentane-cis-1,2-diol (15) [38], bicyclo[2.2.1]-
heptane-exo,exo-2,3-diol (17) [39], and 2-phenylbicyclo
[2.2.1]heptane-exo,exo-2,3-diol (18) [40], have been well
characterized in literature.
1,2-Dimethylcyclopentane-cis-1,2-diol (15, C7H14O2)
1H NMR (CDCl3): ꢁ ¼ 2.43 (s, 2H, ꢁC(CH3)OH), 2.00–1.40
(m, 6H, ꢁCH2ꢁ), 1.17 (s, 6H, ꢁCH3) ppm.
Bicyclo[2.2.1]heptane-exo,exo-2,3-diol (17, C7H12O2)
1H NMR (CDCl3): ꢁ ¼ 3.68 (d, 2H, ꢁCHOH), 2.65 (br d,
2H, ꢁCHOH), 2.13 (m, 2H, ꢁCHꢁ), 1.80–1.00 (m, 6H,
ꢁCH2ꢁ) ppm.
Preparation of Various Boronic Esters 1–5
Following the procedure reported by Brown et al. [26], boron-
ic esters 1–5 were prepared (5–10 mmol scale) by esterifica-
tion of phenylboronic acid with the corresponding diols in
n-pentane (20–30cm3) in 4–6 h at room temperature in excel-
lent chemical yield (75–85%) and purity. These boronic esters
were characterized by spectroscopic means.
2-Phenylbicyclo[2.2.1]heptane-exo,exo-2,3-diol
(18, C13H16O2)
1H NMR (CDCl3): ꢁ ¼ 7.50–7.20 (m, 5H, ArH), 4.10 (d, 1H,
ꢁCHOH), 3.10 (d, 1H, ꢁCHOH), 2.63 (s, 1H, ꢁC(Ph)OH),
2.50–2.10 (m, 3H, ꢁCHꢁ), 1.60–1.00 (m, 5H, ꢁCH2ꢁ) ppm.
Transesterification Procedure
Spectral Data for Various Boronic Esters and Diols
Transesterification reactions were carried out in NMR tubes
under an inert atmosphere. An equimolar mixture of the bo-
ronic ester and the diol (0.05 mmol each in 1 cm3 CDCl3) was
taken in CDCl3 (stored over molecular sieves) and sealed with
(þ)-Diisopropyl (L)-tartrate phenylboronic ester (1, C16H21BO6)
11B NMR (CDCl3): ꢁ ¼ 26.7ppm; 1H NMR (CDCl3): ꢁ ¼
7.35–7.85 (m, 5H, ArH), 5.00 (m, 2H, ꢁOCH(CH3)2), 4.70
(s, 2H, ꢁCHOBꢁ), 1.26 (s, 12H, ꢁCH3) ppm.
1
a septum under an inert atmosphere. The H NMR spectra of
various exchange reactions were frequently recorded (5–
15min to 12–24h depending upon the speed of the reaction)
to get percentage exchange accurately with time. For slow
tranesterifications, the reactions were followed for extended
period of time (until no further exchange) even after an equi-
librium had reached. The extent of ligand exchange was deter-
mined based on the NMR integrations of the relevant proton
signals (the error should not exceed ꢃ5% within the NMR
detection limit).
(ꢁ)-2,3-Butanediol phenylboronic ester (2, C10H13BO2)
11B NMR (CDCl3): ꢁ ¼ 27.0ppm; 1H NMR (CDCl3): ꢁ ¼
7.35–7.85 (m, 5H, ArH), 4.10 (m, 2H, ꢁOCHCH3, 1.35 (d,
6H, ꢁCHCH3) ppm.
4,4,5,5-Tetramethyl-2-phenyl[1,3,2]dioxaborolane
(3, C12H17BO2)
11B NMR (CDCl3): ꢁ ¼ 26.7ppm; 1H NMR (CDCl3): ꢁ ¼
7.35–7.85 (m, 5H, ArH), 1.35 (s, 12H, ꢁCH3) ppm.
Acknowledgements
2-Phenylhexahydrobenzo[1,3,2]dioxaborole (4, C12H15BO2)
11B NMR (CDCl3): ꢁ ¼ 26.7ppm; 1H NMR (CDCl3): ꢁ ¼
7.35–7.85 (m, 5H, ArH), 4.33 (m, 2H, ꢁCHOBꢁ), 2.00–
1.00 (m, 8H, ꢁCH2ꢁ) ppm.
The author (CDR) expresses his sincere gratitude to the Pur-
due Borane Research Fund for the generous financial support.
References
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11B NMR (CDCl3): ꢁ ¼ 26.7ppm; 1H NMR (CDCl3): ꢁ ¼
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1.40 (m, 6H, ꢁCH2ꢁ) ppm.
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