EPR and Preparative Studies of 5- endo Cyclizations of Radicals Derived from Alkenyl NHC-Boranes Bearing tert-Butyl Ester Substituents

Article abstract of DOI:10.1021/acs.joc.8b03112

Radical H atom abstraction from a set of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy. The initial boraallyl radical intermediates rapidly ring closed onto the O atoms of their distal ester groups in 5-endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural varieties of these radicals were identified from their EPR spectra. These proved to be two stable rotamers, in which the carbonyl group of the tert-butyl ester was oriented toward and away from the NHC ring. These rotamers were akin to the s-trans and s-cis rotamers of α,β-unsaturated carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.

Full text of DOI:10.1021/acs.joc.8b03112

Subscriber access provided by Iowa State University | Library
Article
EPR and preparative studies of 5-endo cyclizations of radicals derived
from alkenyl NHC-boranes bearing tert-butyl ester substituents
WEN DAI, Dennis P. Curran, and John C. Walton
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03112 • Publication Date (Web): 21 Jan 2019
Downloaded from http://pubs.acs.org on January 21, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
EPR and preparative studies of 5-endo cyclizations of radicals derived from alkenyl NHC-
boranes bearing tert-butyl ester substituents
7
8
9
Wen Dai,‡ Dennis P. Curran,*‡ and John C. Walton*†
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
‡ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United
States.
† EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST,
United Kingdom.
Abstract: Radical H-atom abstraction from a set of N-heterocyclic carbene (NHC) complexes
of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy.
The initial boraallyl radical intermediates rapidly ring closed onto the O-atoms of their distal
ester groups in 5-endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural
varieties of these radicals were identified from their EPR spectra. These proved to be two stable
rotamers in which the carbonyl group of the tert-butyl ester was oriented towards and away from
the NHC ring. These rotamers were akin to the s-trans and s-cis rotamers of ,-unsaturated
carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their
unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic
evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was
also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was
developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.
INTRODUCTION
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 33
1
2
3
4
5
6
7
8
9
N-Heterocyclic carbene boryl radicals (hereafter NHC-boryl radicals) are emerging as important
reactive intermediates with interesting structures.1 Due to their relatively weak B–H bonds,2
NHC-boranes participate as both reagents and reactants in various radical-mediated
transformations including photopolymerizations.3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Structural knowledge about NHC-boryl radicals bearing assorted substituents on both the
NHC ring and the boron atom comes from direct observation by EPR (Electron Paramagnetic
Resonance; also known as Electron Spin Resonance) spectroscopy, often complemented by
Density Functional Theory (DFT) calculations.4 NHC-boryl radicals are typically planar with
spin density delocalized into the NHC ring due to conjugation (Figure 1a). Still, they are best
considered as conjugated boryl radicals with carbon atoms in the -system (as opposed to
conjugated carbon radicals with a boron atom in the -system) because they have relatively high
spin density on boron and they typically react on boron.3,4
Recently, we tried to make and observe boraallyl radicals 2 from NHC-alkenyl boranes 1
bearing two methyl ester substituents (Figure 1b).5 However, EPR experiments with 1 and di-
tert-butyl peroxide (DTBP) provided spectra of a single species that did not match expectations
for 2. Instead, the observed spectra were assigned with the aid of DFT calculations to novel ring-
closed 1,2-oxaborole radicals 3.
In complementary preparative experiments, reactions of 1 with AIBN and Bu3SnH
provided novel boralactones 4 as stable products in good yields. These boralactones are oxidized
products, and oxygen (from air) is needed for their formation. Based on these results, we
concluded that boraallyl radicals 2 were indeed generated under both EPR and preparative
conditions, but they could not be observed by EPR because their 5-endo cyclizations to 1,2-
2
ACS Paragon Plus Environment

Page 3 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
oxaborole radicals 3 were so fast. This conclusion was supported by DFT calculations, which
showed that the 5-endo cyclization of 2 to 3 is both exothermic and has a low activation barrier.5
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Structures and reactions of NHC-boryl radicals
As an extension of these studies, we decided to generate radicals from a set of analogous
NHC-boranes 5 with di-tert-butyl ester groups (Figure 1c). This was conceived as a worthwhile
if relatively routine extension of scope of the new cyclization to form oxaborole radicals.
However, the EPR experiments held a surprise in store; we expected to observe one oxaborole
radical species 6 on reactions of 5 with DTBP, but instead we observed two. Here we describe
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 33
1
2
3
4
5
6
7
8
9
these EPR experiments and associated DFT calculations and assign the two species as non-
interconverting (on the EPR timescale at up to 300 K) rotamers of two oxaborole radicals. We
also describe improved conditions based on thiol catalysis for preparative conversion of the
various di-tert-butyl esters 6 to stable boralactones.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
RESULTS AND DISCUSSION
Precursor Synthesis: The alkenylboranes 5a–f used in this study are new compounds that were
prepared by reaction of the parent NHC-boranes 8a–f with di-tert-butyl acetylenedicarboxylate 9
(Scheme 1).6 Under a standard set of conditions, a mixture of the appropriate NHC-borane 8 (1
equiv) and 9 (2 equiv) in THF was refluxed for 12 h. Cooling, solvent removal, and direct
purification by automated flash chromatography provided the alkenylborane 5 (less polar) along
with the corresponding trans-borirane 10 (more polar). All the alkenyl boranes are pure (E)-
isomers that are stable to ambient lab conditions.
Scheme 1. Synthesis of precursor alkenylboranes by hydroboration
Table 1 summarizes the results of these preparative experiments. For the substrates with
N,N-dialkyl substituents (entries 1–4 and 6), the ratio of alkenylborane 5 to borirane 10 was
about 1/1. Isolated yields of the alkenylboranes 5a–d,f were in the range of 36–44%, while total
4
ACS Paragon Plus Environment

Page 5 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
isolated yields of both products (5 + 10) ranged from 75–83%. The exception was N,N-dimesityl
substrate, entry 5. This reaction was conducted in acetonitrile, to aid precursor solubility, and
provided mostly the borirane 10e (53%) with only 15% of the alkenylborane 5e. The increased
amount of borirane in this experiment is due to the N-aryl substituents, not to the solvent change.
Because of the small quantities available, 5e was used only for EPR experiments.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Isolated yields of alkenylboranes and boriranes.
entry
alkenylborane, yielda
borirane, yielda
R1
R1
COOtBu
COOtBu
H
N
H2
B
N
B
N
N
R2
COOtBu
COOtBu
R2
1
2
3
5a, 44%
5b, 42%
5c, 39%
5d, 40%
5e, 15%
R1 = R2 = Me
R1 = Bu, R2 = Me
R1 = Bn, R2 = Me
R1 = R2 = iPr
10a, 32%
10b, 41%
10c, 36%
10d, 37%
10e, 53%
4
5b
R1 = R2 = Mes
Me
Me
COOtBu
COOtBu
N
H2
B
N
H
B
N
N
COOtBu
COOtBu
Me
Me
6
5f, 36%
10f, 40%
aisolated yield; bCH3CN as solvent
EPR Spectroscopic Experiments: Solutions of each of the symmetrical NHC-alkenylboranes 5a,
5d, 5e and 5f, with added DTBP, were prepared in tert-butylbenzene solvent. Each solution was
sonicated, then aliquots (0.2 mL) in quartz tubes were deaerated by bubbling N2 and irradiated in
the resonant cavity of the 9.5 GHz CW-EPR spectrometer with unfiltered UV light from a 500 W
high pressure mercury arc lamp. Initially spectra were run at intervals in the temperature range
220 to 280 K with best results at about 260 K, except for 5e for which the best quality signal was
obtained at 220 K.
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 33
1
2
3
4
5
6
7
8
9
A spectrum with remarkable resolution was obtained in 2nd derivative mode from
precursor alkenylborane 5a at 270 K [see Figure 2(a)]. The spectrum of the major radical present
consisted of a set of 8 doublets and was readily simulated with hyperfine splittings (hfs) of a(1H)
= 25.16, a(11B) = 6.47 and a(1H) = 1.30 G [see red spectrum (b) in Figure 2]. These hfs were of
similar magnitude, where applicable, to those of the ring-closed oxaborole radical 3a previously
observed from the analogous precursor containing methyl ester substituents [a(1H) = 26.01,
a(11B) = 6.01, a(1H) = 1.64, a(3H) = 1.12 G].6 Hence the main species observed was the ring
closed oxaborole radical 6a. The simulation of the same radical with its 10B-isotope [Figure 2,
green trace (c)] also conformed well with this conclusion.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Experimental and simulated 2nd derivative CW EPR spectra obtained for ring-closed
oxaborole radical 6a from precursor 5a. (a) Experimental spectrum at 270 K; (b) Simulation of
major oxaborole rotamer of 6a with 11B isotope; (c) Simulation of major oxaborole rotamer of 6a
6
ACS Paragon Plus Environment

Page 7 of 33
The Journal of Organic Chemistry
1
2
3
4
with 10B isotope; (d) Simulation of minor oxaborole rotamer of 6a with 11B isotope; (e) Sum (b +
5
6
c + d) of simulations.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Surprisingly, the spectrum contained another minor radical (relative concentration 16%)
with additional resonance lines most easily observed “outside”, that is to high and low field, of
the main doublets. This spectrum was also effectively simulated as another set of 8 doublets [see
blue trace (d) in Figure 2; the correlation coefficient for the sum of the simulations was R =
0.983]. The g-factor (2.0034) was essentially identical to that of the main species and the hfs
[a(1H) = 26.17, a(11B) = 6.58, a(1H) = 1.38 G] were so similar as to suggest the major and minor
species were structural varieties of the same ring-closed oxaborole radical 6a.
The structural possibilities for oxaborole radical 6a were investigated by means of DFT
computations with the B3LYP functional and the 6-311+G(2d,p) basis set. In the lowest energy
structure 6aT (Scheme 2 and Figure 4) the C=O group was trans to the partial C=C double bond
of the oxaborole ring and pointed approximately towards the NHC ring. Furthermore, the 5-
member oxaborole ring, the ester CO2 group and the O-atom of the lactone OtBu group lay
almost in a plane. The comparatively short bond from the ring C-atom to the ester carbonyl-C-
atom [rC2C3) in Scheme 2] i.e. 1.428 Å suggested some double bond character with consequent
t
restricted rotation. The potential energy function for internal rotation of the CO2 Bu group was
obtained by successive incrementation of the dihedral angle 4-3-2-1 (see Scheme 2). Similarly,
the rotational potential energy function for internal rotation of the OtBu group (structures 6aP,
6aQ; dihedral angle 7-6-5-4) was also obtained and these energy profiles are illustrated in Figure
3.
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 33
1
2
3
4
H
BH
N
O
OtBu
N
5
6
7
8
CO2tBu
5a
DTBP
UV
OtBu
OtBu
9
H
H
O
O
N
B
N
B
•
slow
•
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
3
3
2
N
2
N
1
1
O
O
O
O
tBu
tBu
6aT
6aC
R1
X
R1
O
•
•
X
s-cis;
O
11C
O
X
O
X
12T
11T
12C
s-trans;
t
7
Bu
6
7
tBu
O
6
5 O
5
H
H
O
O
fast
N
B
N
B
•
•
4
4
N
N
CO2tBu
6aP
CO2tBu
6aQ
t
Scheme 2. Conformational interconversions of 1,2-oxaborole radicals 6a. Top: CO2 Bu rotor;
bottom: OtBu rotor.
Figure 3. DFT potential energy functions for internal rotations in 1,2-oxaborole radicals 6a and
t
related species. Blue curve: torsional rotation of the CO2 Bu group (dihedral angle 4-3-2-1) in
6a. Red curve: torsional rotation of the CO2Me group in 3a. Black curve: rotation of the OtBu
t
group in 6a (dihedral 4-5-6-7). Purple curve: torsional rotation of the CO2 Bu group in
boralactone 7a.
8
ACS Paragon Plus Environment

Page 9 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
The black curve (Figure 3) for torsion of the OtBu group indicated only one stable conformation.
Furthermore, the comparatively low computed energy barrier (~ 5 kcal/mol) suggested this
motion would not be observable by EPR spectroscopy in the accessible temperature range. That
is, the spectrum would be an average over all rotational states. On the other hand, the potential
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
t
energy function for rotation of the CO2 Bu group was approximately that of a twofold rotor with
just two stable rotamers (blue curve in Figure 3). The lowest energy rotamer, labelled 6aT in
Scheme 2 and Figure 4, had a dihedral angle 4-3-2-1 close to 0 such that the C=O of the ester
group was trans to the adjacent ring C=C double bond and pointed towards the NHC ring. The
second rotamer, labelled 6aC in Scheme 2 and Figure 4, with its C=O group cis to the adjacent
ring C=C double bond and pointing away from the NHC ring, had dihedral angle 4-3-2-1 close to
180 and was only 2.6 kcal/mol higher in energy.
t
Figure 4. (a) DFT computed structures of internal rotation of the CO2 Bu group of oxaborole
radical 6a. Left; rotamer 6aT, center; TS, right; rotamer 6aC; (b) SOMO for 6aT; (c) SOMO
for 6aC.
9
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 33
1
2
3
4
5
6
7
8
9
The HOMOs for the two stable rotamers are also illustrated in Figure 4. Interestingly, the
HOMO for 6aT consisted of an extensive -system spanning the whole planar portion of the
molecule including the ester C=O group (Figure 4 (b)). The second stable rotamer 6aC
contained an analogous planar region also covered by an extended -system (Figure 4 (c)). The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
resonance electron delocalization resulting from these -systems partly explains why these
particular rotamers are energy minima. The comparatively high computed torsion barriers (15.4
and 14.8 kcal/mol) implied that interconversion of 6aT and 6aC would be very slow in the
accessible temperature range. Consequently, these two rotamers should appear as separate
species in the EPR spectrum. This DFT data enabled us to identify with some confidence 6aT
as the major radical present in the EPR spectrum and 6aC as the minor radical.
t
Another conclusion from the DFT data is that CO2 Bu groups in oxaborole radicals
analogous to 6a, but having different substituents on their NHC rings, should also have twofold
torsional rotors. As predicted, the EPR spectra obtained from UV irradiation of solutions of 5d,
5e and 5f each with DTBP did indeed demonstrate the presence of two rotamers of the
corresponding oxaborole radicals. The spectra of 6d-f were not so well resolved as in the case of
radical 6a but good simulations were achieved on inclusion of the second species. Sample
spectra are included in the Supporting Information and the derived EPR parameters are in Table
2. The EPR results afforded good evidence that H-abstraction took place readily from
precursors 5 and that the resulting boraallyl radicals (analogous to 2 but with tBu esters) ring
closed very rapidly, even at 220 K, in 5-endo-mode onto the O-atoms of the ester substituents.
10
ACS Paragon Plus Environment

Page 11 of 33
The Journal of Organic Chemistry
1
2
3
4
Table 2. Experimental and computed EPR parameters for 1,2-oxaborole radicals 6 and 3.a
5
6
7
8
OtBu
HA
OtBu
HA
HB
HB
HB
R
N
R
N
O
HB
O
N
N
O
•
N
N
B
OtBu
HA
B
•
•
O
•
OtBu
HA
B
B
N
R
N
R
O
O
O
O
O
O
tBu
tBu
9
O
tBu
O
tBu
6fC
6fT
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
6aT; R = Me
6dT; R = iPr
6aC; R = Me
6dC; R = iPr
6eC; R = Mes
6eT; R = Mes
iPr
N
HB
iPr
N
HB
OMe
OMe
HA
O
O
B
B
•
•
HA
N
iPr
N
iPr
O
O
O
O
Me
Me
3dT
3dC
T/K
or
DFTb
270
Rel.
%
Radical
g-factor a(11B) a(10B) a(HB) a(HA) a(other)
Rc
6aT
6aT
6aC
6aC
6dT
6dC
6eT
6eC
6fT
6fC
84
2.0034 6.47 2.17 25.16 1.30
0.983
DFT
270
DFT
260
260
220
220
260
260
–6.06
2.0034 6.58
–6.21
23.33 –0.77
26.17 1.39
23.28 –0.80
16
0.983
79
21
78
22
79
21
2.0032 6.60 2.19 26.69 1.24
2.0032 6.65 27.64 1.33
2.0037 6.50 2.15 24.64 1.20
2.0037 6.50 25.54 1.25
2.0031 6.56 2.20 25.65 1.25
2.0031 6.65 26.60 1.35
0.988
0.988
0.968
0.968
0.962
0.962
1.03(3H)
0.61(3H)
1.43(3H)
1.22(3H)
1.16(3H)
0.60(3H)
1.45(3H)
1.18(3H)
3dT
3aT
3dC
3aC
260
DFTd
260
80
2.00340 6.63 2.21 26.76 1.64
0.971
–6.08
2.00354 6.71
–6.18
–
–
–
23.90 –1.37
27.68 1.70
23.81 –1.25
20
0.971
DFTd
a Hfs in Gauss (G); note that EPR provides only the magnitude not the sign of hfs. b DFT at theUB3LYP/6-
311+G(2d,p) level of theory on oxaboroles with dimethyl-NHC units. c Correlation coefficients of the computer
simulations. d UB3LYP/epr-iii//UB3LYP/6-311+G(2d,p).
11
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 33
1
2
3
4
5
6
7
8
9
The spectra from radicals 6aT and 6aC were examined in the temperature range 220 to
280 K and showed the two distinct rotamers throughout. The ratio 6aT/6aC was 84/16 at 270 K.
It follows that interconversion of rotamer 6aT with 6aC was too slow for observation by EPR.
Likewise, the spectra of radicals 6e–f all showed two rotamers with T/C ratios of about 80/20. It
is probable, therefore, that all the mixtures 6T and 6C were formed by hydrogen abstraction
from corresponding conformations of the of the E-alkenylboranes 5. In essence then, we can
infer the rotamer ratios of precursors 5 (roughly 4/1 favoring s-trans) even though the rotation
barriers in 5 are too low for direct measurement (for example, NMR spectra of 5 exhibit only a
single set of signals). The assumption in this inference is that tert-butoxyl radicals react with the
two rotamers of 5 at about the same rate.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The EPR spectra from all four precursors recorded at temperatures above about 300 K
became more complex showing the presence of several radicals that could not be identified.
These EPR spectra afforded no evidence of the formation of tert-butyl radicals from -scission
of 1,2-oxaborole radicals 6. EPR spectra were also run on solutions of 5f containing AIBN (2,2'-
azobis(2-methylpropionitrile)) in place of DTBP. In the absence of UV irradiation, no EPR
signals were detected at temperatures up to 350 K. When UV irradiation was introduced, the
spectrum of the initiator-derived 2-cyanoprop-2-yl radical appeared. Prolonged heating with
AIBN led to complex spectra but again no evidence for formation of tert-butyl radicals was
obtained.
The two distinguishable rotamers of the set of ring-closed oxaborole radicals 6T and 6C
resemble the s-trans and s-cis rotamers of ,-unsaturated carbonyl compounds (Scheme 2, 11T
and 11C), except that the ring C=C bonds of oxaborole radicals 6T and 6C have only partial
double bond character. The barrier to internal rotation in acrolein (11, X = H) is only about 5
12
ACS Paragon Plus Environment

Page 13 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
kcal/mol, as judged by ultrasonic7 and microwave spectroscopy measurements.8 Similarly, the
barrier in acrylic acid (11, X = OH) was found experimentally to be 3.8 kcal/mol.9 Accordingly,
conjugation of a carbonyl group with a C=C double bond creates a rotation barrier around a
connecting single CC bond of about 4–5 kcal/mol. However, the rotation barrier computed for
oxaborole radicals 6aT and 6aC was calculated to be much higher at 14.6 kcal/mol, and a
comparatively high value was supported by the EPR observation of distinct species. It follows
that some other factor(s) must come into play.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
For -(alkoxycarbonyl)alkyl radicals 12T and 12C (X = OR) [and related species (X =
alkyl, NR2)] Fischer and co-workers observed two distinct rotamers by EPR spectroscopy.10 and
also by muon spin resonance.11 Newcomb and co-workers also adduced evidence for two
conformational isomers for related ester radicals and showed that they reacted at different rates.12
Furthermore, the energy barriers to internal rotation of these -carbonylalkyl radicals were all
determined to be in the range 10–12 kcal/mol.10-12
These structures and barrier heights show 12T and 12C to be better models for the
oxaborole radicals. The comparatively large energy barriers to internal rotation, about what are
formally single bonds in radicals 6, can therefore be attributed to partial -electron
delocalization. The •CCO units in oxaborole radicals 6 are partly allylic in character. More
accurately the delocalization is quasi-pentadienylic because of the additional adjacent double
bond in the oxaborole ring. The analogous internal rotation barrier CCC in the archetype
allyl radical is somewhat higher at 15.7 kcal/mol13 as expected for full allylic character. The
ratio of the s-trans-6 to s-cis-6 rotamers is close to 4:1 irrespective of the N,N-substituents on the
NHC ring (see Table 2). Thus, steric hindrance from these substituents does not change the
13
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 33
1
2
3
4
5
6
rotamer populations and probably does not contribute much to the height of the ester internal
rotation barriers either.
7
8
9
t
The torsion of the CO2 Bu group in the isolated product boralactone 7a was also
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
examined by DFT computation. The potential energy function (purple curve in Figure 3) was
similar in shape to that of the oxaborole radical showing an approximate twofold rotor with two
stable conformations having their C=O groups s-trans and s-cis to the ring double bond and
differing in energy by 3.2 kcal/mol. The computed barrier heights (4.7 and 5.4 kcal/mol) were
considerably lower than for the oxaborole radicals because of the absence of the allylic type
stabilization of the unpaired electron. As expected, these rotamers resemble those of ,-
unsaturated carbonyls 11T and 11C.
If these conclusions are correct, then it follows that distinct s-trans and s-cis rotamers
would be expected for the analogous methyl ester containing oxaborole radicals 3 even though
these were not detected in the original study.6 Internal rotation of the CO2Me group in the
dimethyl-NHC-oxaborole radical 3a was therefore also examined by DFT computation. The
computed potential energy function (red curve in Figure 3) revealed a twofold rotor very similar
t
to that for analogous rotation of the CO2 Bu group. Two stable rotamers 3aT and 3aC were
t
indicated with their C=O groups in virtually the same orientations as in the CO2 Bu analogs and
differing in energy by only 2.4 kcal/mol. The rotational energy barriers (15.8 and 15.0 kcal/mol)
were also similar and lent support to the idea that distinct rotamers should be observable for
these species.
The previous EPR spectra of methyl ester containing oxaborole radicals 3 are
considerably more complex than the new spectra of 6 because couplings to the methyl H-atoms
of both the MeCO2 and the MeO substituents are fully or partially resolved.6 These 1st
14
ACS Paragon Plus Environment

Page 15 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
derivative EPR spectra gave no ostensible evidence for the presence of two rotamers. However,
due to the intricate patterns of complex multiplets, minor proportions of a second rotamer could
have gone undetected.
To look for the minor rotamer, a well resolved 2nd derivative spectrum for a methyl ester
oxaborole radical 3 was needed (see Scheme 3). EPR spectra of the N,N-dimethyl-NHC-
oxaborole radical 3a were too weak and short-lived at all temperatures to permit recording in 2nd
derivative mode. It is likely that the complexity and greater natural linewidth of the EPR
spectrum of the N,N-di-dipp-NHC-oxaborole 3g would obscure any minor rotamer even in a 2nd
derivative spectrum.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R
N
R
N
H
BH
O
O
DTBP
UV
•
OMe
BH
OMe
N
N
CO
R
CO
R
2Me
; R = Me
2Me
1, a
2
i
d
; R = Pr
5-endo
g
; R = dipp
OMe
•
OMe
R
R
N
H
H
O
B
O
N
B
slow
•
N
R
N
R
O
O
O
O
Me
Me
3C
3T
Scheme 3. Generation of NHC-oxaborole radicals with methyl ester substituents.
We were, however, able to obtain a spectrum of the N,N-di-iPr -NHC-oxaborole 3d in 2nd
derivative mode in a mixed solvent (Figure 5(a)).14 This spectrum revealed some asymmetry,
and additional resonance lines were resolved. The simulation with just one rotamer of the
methyl ester group (Figure 5(b)) had a correlation coefficient of 0.867, but a much-improved fit
(R = 0.971) was achieved by inclusion of 19% of a second conformation (Figure 5(c)). The
spectral asymmetry was attributable to a very slight difference in the g-factors of the two
rotamers. See Table 2 for the resultant EPR parameters.
15
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 33
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Experimental and simulated 2nd derivative CW EPR spectra obtained for ring-
closed oxaborole radical 3d from precursor 1d. (a) Black: experimental spectrum at 260 K in
PhtBu containing 5% allyl bromide. (b) Red: simulation of major oxaborole rotamer of 3dT with
11B and 10B isotopes. (c) Blue: simulation including major (3dT) and minor (3dC) rotamers of
oxaborole radicals.
The detection of two rotamers of radical 3d supports of the idea that the internal rotation
barriers arise from delocalization of the unpaired electron in the quasi-pentadieneylic -systems.
We conclude that two stable rotamers exist for RCO2 substituents in other NHC-oxaborole
radicals 3, even though their signals cannot be resolved in their EPR spectra.
Preparative Reactions: To make boralactones, we initially used the conditions developed for the
corresponding dimethyl esters as shown in Figure 6a.5 In this earlier work, alkenylboranes 1
16
ACS Paragon Plus Environment

Page 17 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
were heated in benzene (80˚ C, 6 h) with AIBN (50 mol%) and Bu3SnH (50 mol%) to provide
NHC-boralactones 4 in isolated yields of about 60%. This procedure requires dioxygen for
product formation, and preparative reactions are prone to stopping if the source is ambient air.
This published procedure worked reasonably well for the di-tert-butyl esters. For
example, reaction of 5a (R1 = R2 = Me) under these conditions provided 7a in 44% yield
according to 11B NMR analysis of the crude product. However, substituting tert-dodecanethiol
for tin hydride (Figure 6b) proved to be more practical because the products are free from tin.15
For example, reaction of 5a under the thiol conductions provided 7a in 70% yield according to
11B NMR analysis. A control experiment quickly showed that this procedure does not require
dioxygen, so the subsequent experiments were conducted under argon to discourage oxidation of
the thiol to a disulfide.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Conditions for boralactone formation
The structures and yields of the boralactones made by this procedure are shown in Figure
7. According to 11B NMR analysis, the boralactones 7a–d were formed in 64–70% yields. All
17
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 33
1
2
3
4
5
6
7
8
9
these boralactones are polar and purification by flash chromatography gave mixed results.
Boralactones 7b, 7d and 7f were isolated in good purity yields of 56%, 54% and 50%,
respectively. Boralactone 7c was isolated in 57% yield and was about 90% pure, while 7a was
isolated in 59% yield and was about 72% pure. The purities were determined by 11B NMR
spectroscopy, where the impurities show a very broad signal at about 0–5 ppm. This signal,
which was present in the spectra of all the crude products, was completely removed for 7b, 7d
and 7f, partially removed for 7a, and not removed for 7c.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Structures and 11B NMR yields of new boralactones (isolated yields are in
parentheses).
Reaction Mechanism: Figure 8 shows a thiol-catalysis mechanism for boralactone formation that
is consistent with the experimental results. Starting from boraalkenyl radical 11, rapid (k
probably > 108 s–1) 5-endo cyclization provides oxaborole radical 6. This undergoes -
fragmentation to provide boralactone 7 and tert-butyl radical. This -fragmentation does not
occur when methyl is the departing radical,5 and even the liberation of the tert-butyl radical must
be relatively slow (k probably < 105 s–1) because it is not observed by EPR spectroscopy (see
18
ACS Paragon Plus Environment

Page 19 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
above). Based on prior work on thiol catalysis of NHC-borane reactions,3g,16 we suggest that the
tert-butyl radical abstracts hydrogen from tert-dodecanethiol to give the corresponding thiyl-
radical and tert-butane. In turn, the thiyl-radical abstracts hydrogen from the starting
alkenylborane 5 to give back the thiol and the starting alkenylboryl radical 11, thereby closing
the chain cycle. This is a net redox-neutral reaction in which one product (the boralactone 7) is
oxidized relative to that starting alkenylborane 5 and the other (tert-butane) is reduced.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8. Putative thiol-catalysis mechanism for boralactone formation
To support the thiol catalysis mechanism, we conducted an experiment with 5d under the
standard conditions but reducing the amount of thiol from 50 mol% to 20 mol%. This provided
7d in 68% NMR yield and 53% isolated yield (compare this to Figure 8, 64% NMR yield and
54% isolated yield of 7d with 50 mol% thiol). Thus, a decrease in thiol amount by a factor of
2.5 does not materially affect the outcome of the reaction.
Support for the fragmentation of oxaborole radical 6a to boralactone 7a and tert-butyl
radical was obtained from DFT computations. The transition state (TS) for extrusion of tert-
19
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 33
1
2
3
4
5
6
7
8
9
butyl radical from 6a was located, and the resulting overall energetics and activation parameters
are compared with those obtained previously6 for methyl radical extrusion from 3a in Scheme 4
(see Supporting Information for details).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 4. DFT study of 1,2-oxaborole radical fragmentation. Method: B3LYP/6-311+G(2d,p);
energies in kcal/mol
Me
N
N
O
O
Go = –6.8
Ho = 4.4
O
O
G = 21.0
H = 22.0
N
N
Me•
tBu•
B
B
•
H
H
CO2Me
4a
CO2Me
3a
tBu
N
N
O
O
G = 15.0
H = 12.1
Go = –15.2
Ho = –1.2
O
O
N
N
B
B
•
H
H
CO2tBu
CO2tBu
6a
7a
The fragmentation of 3a was computed to be endothermic and both the activation enthalpy (ΔH‡
= 22.0 kcal mol–1) and activation free energy (ΔG‡ = 21.0 kcal mol–1) were too high for the
elimination of methyl radical to be viable in solution. However, the fragmentation of 6a to form
tert-butyl radical was computed to be marginally exothermic (ΔH0 = –1.2 kcal/mol) and strongly
exoergonic (ΔG0 = –15.2 kcal/mol). Furthermore, both the activation enthalpy and free energy
for loss of tert-butyl radical were low enough for the process to be viable at 80 C in solution
(ΔH‡ = 12.1 kcal/mol; ΔG‡ = 15.0 kcal/mol). These DFT results support the mechanism
proposed in Figure 8.
CONCLUSIONS
20
ACS Paragon Plus Environment

Page 21 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
The straightforward hydroboration procedure with various NHC-BH3 reagents succeeded
as well with di-tert-butyl acetylenedicarboxylate as with the dimethyl-analogs. Most of the
NHC-BH3 reagents afforded readily separable mixtures of alkenyl NHC-boranes and boriranes in
a ratio of about 1:1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Hydrogen atom abstraction from tert-butyl ester-substituted NHC-alkenyl boranes by
tert-butoxyl radicals was fast, generating boraallyl radicals 8. The latter were not detectable by
EPR spectroscopy even at 220 K but rapidly underwent an unusual 5-endo ring closure onto the
O-atom of the distal tert-butyl ester group. The EPR spectra showed the cyclization step to be
fast at 220 K; as had previously been found for closure onto a methyl ester group. The allylic
stabilization and extended π-conjugation of the oxaborole radicals undoubtedly contributed to the
ease of these ring closure steps. Two distinct rotamers of each oxaborole radical were
characterized by EPR spectroscopy and DFT computations. In these s-trans (6T) and s-cis (6C)
rotamers the ester C=O groups adopted trans and cis orientations relative to the oxaborole ring
C=C double bonds. The stability of these structures was due to the quasi-allylic interaction of
the unpaired electron with the ester C=O group.
These observations implied that two stable rotamers of the analogous methyl ester
substituted oxaborole radicals should also exist. Indeed, for one MeCO2-containing oxaborole
radical with a di-isopropyl-NHC unit, two distinct rotamers were spectroscopically characterized.
It is likely that other MeCO2-containing oxaborole radicals also exist as pairs of rotamers, but
their EPR spectra are too complex for these to be distinguished with this spectroscopic
technique. Newcomb and co-workers showed that the two analogous rotamers of appropriately
unsaturated -(alkoxycarbonyl)alkyl radicals reacted at different rates in subsequent cyclization
steps.12 It is therefore likely that the two rotamers of oxaborole radicals have different
21
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 33
1
2
3
4
5
6
7
8
9
reactivities particularly at C-3 (see Scheme 2). However, in our preparative processes (Figure 6)
reaction took place at C-5 which is comparatively remote from the ester group. Differential
reactivity at C-5 of the two ester conformations of oxaborole radicals will probably be
insignificant.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The new preparative protocol for the boralactones, involving tert-dodecanethiol avoids
the use of the neurotoxic tin hydride while also affording very acceptable yields. The products
are examples of the rare NHC complexes of borinic acid lactones. They were found to be robust
molecules with long shelf lives.
EXPERIMENTAL SECTION
Synthesis and Characterization of (E)-alkenylboranes and Boriranes
Di-tert-butyl but-2-ynedioate (di-tert-butyl acetylenedicarboxylate):17 tert-Butyl acetate (23.2
g, 200 mmol) and triflic acid (0.60 g, 4 mmol) were added dropwise to a solution of
acetylenedicarboxylic acid (2.28 g, 20 mmol) in 150 mL of DCM at 0 ˚C. The resulting solution
was warmed to room temperature with stirring for 2 h before saturated NaHCO3 was slowly
introduced. The aqueous layer was extracted with DCM (4 x 100 mL). The combined organic
layer was washed with saturated NaCl, then dried over MgSO4, filtered, and concentrated under
reduced pressure to give crude product. This was purified by flash chromatography on silica
(hexane:ethyl acetate) to afford the diester (3.16 g, 70%) as a colorless solid.
General hydroboration procedure: The NHC-borane (1 mmol) was added to a flask
containing THF (10 mL), then di-tert-butyl acetylenedicarboxylate (0.45 g, 2 mmol) was added
dropwise. The reaction mixture was heated at reflux. After 12 h, the mixture was concentrated,
22
ACS Paragon Plus Environment

Page 23 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
and the resulting residue was purified by flash chromatography (hexane:ethyl acetate) to afford
(E)-alkenylborane (less polar) followed by borirane (more polar).
7
8
9
(E)-(1,4-Di-tert-butoxy-1,4-dioxobut-2-en-2-yl)(1,3-dimethyl-1H-imidazol-3-ium-2-
yl)dihydroborate (5a) and 2,3-bis(tert-butoxycarbonyl)-1-(1,3-dimethyl-1H-imidazol-3-ium-
2-yl)boriran-1-uide (10a): This experiment was conducted with 8a (110 mg) and provided 5a
(148 mg, 44%, white solid, mp 79–81 ˚C) and 10a (108 mg, 32%, yellow solid, mp 52–53 ˚C)
after purification.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5a: 1H NMR (500 MHz, CDCl3) δ 6.77 (s, 2H), 6.23 (s, 1H), 3.70 (s, 6H), 1.39 (s, 9H), 1.33 (s,
9H); 13C{1H} NMR (125 MHz, CDCl3) δ 174.7, 167.9, 127.6, 120.1, 79.3, 79.2, 36.1, 28.2, 28.1;
11B NMR (160 MHz, CDCl3) δ –28.8 (t, J = 88 Hz); HRMS (ESI, TOF) m/z [M – H]+ calculated
for C17H28BN2O4 335.2142, found 335.2164.
1
10a: H NMR (500 MHz, CDCl3) δ 6.84 (s, 2H), 3.77 (s, 6H), 1.99 (dd, J = 8.1, 5.5 Hz, 1H),
13
1.82 (t, J = 4.8 Hz, 1H), 1.37 (s, 9H), 1.17 (s, 9H); C{1H} NMR (125 MHz, CDCl3) δ 177.2,
176.9, 120.7, 77.4, 77.1, 36.0, 28.5, 28.2; 11B NMR (160 MHz, CDCl3) δ –27.0 (d, J = 123 Hz);
HRMS (ESI, TOF) m/z [M + H]+ calculated for C17H30BN2O4 337.2299, found 337.2321.
(E)-(3-Butyl-1-methyl-1H-imidazol-3-ium-2-yl)(1,4-di-tert-butoxy-1,4-dioxobut-2-en-2-
yl)dihydroborate (5b) and 2,3-bis(tert-butoxycarbonyl)-1-(3-butyl-1-methyl-1H-imidazol-3-
ium-2-yl)boriran-1-uide (10b): This experiment was conducted with 8b (152 mg) and provided
5b (160 mg, 42%, colorless oil) and 10b (157 mg, 41%, colorless oil) after purification.
1
5b: H NMR (500 MHz, CDCl3) δ 6.80 (d, J = 1.9 Hz, 1H), 6.78 (d, J = 1.9 Hz, 1H), 6.26 (s,
1H), 4.13–4.04 (m, 2H), 3.73 (s, 3H), 1.71 (m, 2H), 1.42 (s, 9H), 1.35 (s, 9H), 0.92 (t, J = 7.4
Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3) δ 174.6, 167.9, 127.7, 120.2, 118.5, 79.2, 79.1, 48.5,
23
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 33
1
2
3
4
11
36.1, 32.4, 28.2, 28.1, 19.8 , 13. 7; B NMR (160 MHz, CDCl3) δ –28.8 (t, J = 88 Hz); HRMS
5
6
(ESI, TOF) m/z [M – H]+ calculated for C20H34BN2O4 377.2612, found 377.2628.
7
8
9
1
10b: H NMR (500 MHz, CDCl3) δ 6.84 (d, J = 8.2 Hz, 2H), 4.22 (m, 2H), 3.83 (s, 3H), 2.06
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(dd, J = 8.2, 5.5 Hz, 1H), 1.86 (t, J = 4.9 Hz, 1H), 1.81 (m, 2H), 1.43 (s, 9H), 1.24 (s, 9H), 0.97
13
(t, J = 7.4 Hz, 3H); C{1H} NMR (125 MHz, CDCl3) δ 177.3, 177.1, 120.5, 119.3, 77.5, 77.2,
11
48.8, 36.1, 32.3, 28.5, 28.3, 19.7, 13.6; B NMR (160 MHz, CDCl3) δ –26.9 (d, J = 123 Hz);
HRMS (ESI, TOF) m/z [M + H]+ calculated for C20H36BN2O4 379.2768, found 379.2786.
(E)-(3-Benzyl-1-methyl-1H-imidazol-3-ium-2-yl)(1,4-di-tert-butoxy-1,4-dioxobut-2-en-2-
yl)dihydroborate (5c) and 1-(3-benzyl-1-methyl-1H-imidazol-3-ium-2-yl)-2,3-bis(tert-
butoxycarbonyl)boriran-1-uide (10c): This experiment was conducted with 8c (186 mg) and
provided 5c (161 mg, 39%, yellow solid) and 10c (148 mg, 36%, yellow solid, mp 66–68 ˚C)
after purification.
5c: 1H NMR (500 MHz, CDCl3) δ 7.27–7.19 (m, 5H), 6.69 (d, J = 2.0 Hz, 1H), 6.56 (d, J = 2.0
Hz, 1H), 6.22 (s, 1H), 5.27 (s, 2H), 3.72 (s, 3H), 1.34 (s, 9H), 1.32 (s, 9H); 13C{1H} NMR (125
MHz, CDCl3) δ 167.9, 135.9, 128.8, 128.6, 128.2, 127.9, 120.5, 118.5, 79.3, 77.2, 52.2, 36.2,
11
28.2, 28.1; B NMR (160 MHz, CD3CN) δ –28.6 (t, J = 88 Hz); HRMS (ESI, TOF) m/z [M –
H]+ calculated for C23H32BN2O4 411.2455, found 411.2473.
1
10c: H NMR (500 MHz, CDCl3) δ 7.35 (s, 5H), 6.82 (d, J = 2.0 Hz, 1H), 6.76 (d, J = 1.9 Hz,
1H), 5.46 (d, J = 14.7 Hz, 1H), 5.36 (d, J = 14.7 Hz, 1H), 3.86 (s, 3H), 2.13 (dd, J = 8.1, 5.5 Hz,
13
1H), 1.94 (t, J = 4.9 Hz, 1H), 1.42 (s, 9H), 1.29 (s, 9H); C{1H} NMR (125 MHz, CDCl3) δ
11
177.2, 129.0, 128.6, 128.6, 120.9, 119.2, 77.6, 77.4, 52.5, 36.2, 28.5, 28.3, 28.2; B NMR (160
MHz, CDCl3) δ –26.9 (d, J = 123 Hz); HRMS (ESI, TOF) m/z [M + H]+ calculated for
C23H34BN2O4 413.2612, found 413.2629.
24
ACS Paragon Plus Environment

Page 25 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
(E)-(1,4-Di-tert-butoxy-1,4-dioxobut-2-en-2-yl)(1,3-diisopropyl-1H-imidazol-3-ium-2-
yl)dihydroborate (5d) and 2,3-bis(tert-butoxycarbonyl)-1-(1,3-diisopropyl-1H-imidazol-3-
ium-2-yl)boriran-1-uide (10d): This experiment was conducted with 8d (166 mg) and provided
5d (157 mg, 40%, white solid, mp 98–100 ˚C) and 10d (145 mg, 37%, white solid, mp 103–105
˚C) after purification.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5d: 1H NMR (500 MHz, CDCl3) δ 6.91 (s, 2H), 6.18 (s, 1H), 5.15 (hept, J = 6.8 Hz, 2H), 1.40 (s,
13
9H), 1.39 (s, 9H), 1.37 (s, 6H), 1.36 (s, 6H); C{1H} NMR (125 MHz, CDCl3) δ 174.8, 167.6,
11
126.4, 115.0, 79.0, 49.2, 28.2, 28.2, 22.9; B NMR (160 MHz, CDCl3) δ –28.8 (t, J = 88 Hz);
HRMS (ESI, TOF) m/z [M – H]+ calculated for C21H36BN2O4 391.2768, found 391.2787.
1
10d: H NMR (500 MHz, CDCl3) δ 6.96 (s, 2H), 5.33–5.19 (m, 2H), 2.02 (dd, J = 7.9, 5.4 Hz,
1H), 1.86 (t, J = 5.0 Hz, 1H), 1.43 (m, 21H), 1.31–1.23 (m, 9H); 13C{1H} NMR (125 MHz,
CDCl3) δ 177.5, 177.3, 115.7, 115.6, 77.4, 49.8, 28.6, 28.5, 28.0, 23.3, 22.8; 11B NMR (160
MHz, CDCl3) δ –26.9 (d, J = 121 Hz); HRMS (ESI, TOF) m/z [M + H]+ calculated for
C21H38BN2O4 393.2925, found 393.2942.
(E)-(1,4-Di-tert-butoxy-1,4-dioxobut-2-en-2-yl)(1,3-dimesityl-1H-imidazol-3-ium-2-
yl)dihydroborate (5e) and 2,3-bis(tert-butoxycarbonyl)-1-(1,3-dimesityl-1H-imidazol-3-ium-
2-yl)boriran-1-uide (10e): This experiment was conducted with 8e (318 mg) and provided 5e
(82 mg, 15%, white solid) and 10e (289 mg, 53%, white solid) after purification.
1
5e: H NMR (500 MHz, CDCl3) δ 6.91 (s, 4H), 6.89 (s, 2H), 6.07 (s, 1H), 2.31 (s, 6H), 2.15 (s,
12H), 1.37 (s, 9H), 1.30 (s, 9H); 13C{1H} NMR (125 MHz, CDCl3) δ 172.3, 138.6, 135.5, 134.4,
130.9, 120.0, 120.8, 78.4, 78.1, 77.2, 28.3, 28.1, 21.1, 18.1; 11B NMR (160 MHz, CDCl3) δ –28.3
(t, J = 91 Hz); HRMS (ESI, TOF) m/z [M – H]+ calculated for C33H44BN2O4 543.3394, found
543.3402.
25
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 26 of 33
1
2
3
4
5
6
7
8
9
10e: 1H NMR (500 MHz, CDCl3) δ 7.01 (s, 2H), 7.00 (s, 2H), 6.96 (s, 2H), 2.36 (s, 6H), 2.18 (s,
6H), 2.12 (s, 6H), 1.57 (dd, J = 8.7, 6.5 Hz, 1H), 1.31 (s, 9H), 1.29 (s, 9H), 1.13–1.08 (m, 1H);
13C{1H} NMR (125 MHz, CDCl3) δ 178.2, 176.9, 139.1, 135.1, 135.0, 134.0, 129.5, 129.3,
122.0, 28.6, 28.5, 21.1, 18.3, 18.2; 11B NMR (160 MHz, CDCl3) δ –26.0 (d, J = 122 Hz); HRMS
(ESI, TOF) m/z [M + H]+ calculated for C33H46BN2O4 545.3550, found 545.3557.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(E)-(1,4-Di-tert-butoxy-1,4-dioxobut-2-en-2-yl)(1,3-dimethyl-1H-benzo[d]imidazol-3-ium-2-
yl)dihydroborate
(5f)
and
2,3-bis(tert-butoxycarbonyl)-1-(1,3-dimethyl-1H-
benzo[d]imidazol-3-ium-2-yl)boriran-1-uide (10f): This experiment was conducted with 8f
(160 mg) and provided 5f (139 mg, 36%, yellow solid, mp 100–101 ˚C) and 10f (155 mg, 40%,
yellow solid, mp 170–172 ˚C) after purification.
1
5f: H NMR (500 MHz, CDCl3) δ 7.46–7.41 (m, 2H), 7.38 (m, 2H), 6.35 (s, 1H), 3.95 (s, 6H),
1.32 (s, 9H), 1.31 (s, 9H); 13C{1H} NMR (125 MHz, CDCl3) δ 174.4, 167.8, 133.2, 128.2, 123.9,
11
110.7, 79.4, 32.3, 28.1, 28.0; B NMR (160 MHz, CDCl3) δ –28.4 (t, J = 89 Hz); HRMS (ESI,
TOF) m/z [M – H]+ calculated for C21H30BN2O4 385.2299, found 385.2317.
1
10f: H NMR (500 MHz, CDCl3) δ 7.50–7.39 (m, 4H), 4.04 (s, 6H), 2.19 (dd, J = 8.3, 5.4 Hz,
13
1H), 1.97 (t, J = 5.0 Hz, 1H), 1.46 (s, 9H), 1.11 (s, 9H); C{1H} NMR (125 MHz, CDCl3) δ
177.0, 176.8, 132.8, 124.8, 121.2, 111.0, 107.3, 77.7, 77.3, 32.5, 28.5, 28.1, 26.0; 11B NMR (160
MHz, CDCl3) δ –26.9 (d, J = 123 Hz); HRMS (ESI, TOF) m/z [M + H]+ calculated for
C21H32BN2O4 387.2455, found 387.2474.
DFT Calculations
DFT calculations were carried out using the Gaussian 09 suite of programs. Results were
obtained with the B3LYP functional and the 6-311+G(2d,p) basis set was employed with the
26
ACS Paragon Plus Environment

Page 27 of 33
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
CPCM continuum model with toluene as solvent. Default values of the keywords Alpha, Radii,
TSNUM, and TSARE were employed. Vibrational frequency calculations were implemented so
that GS (no imaginary frequencies) and TS status could be checked (one imaginary frequency),
and enthalpies and free energies were adjusted for zero point and thermal corrections to 1 atm
and 298 K.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EPR Experiments
CW-EPR spectra were obtained at 9.5 GHz with 100 kHz modulation employing a commercial
spectrometer fitted with a rectangular ER4122 SP resonant cavity. Stock solutions of each
sample (2 to 15 mg) and DTBP (1 equiv. wt/wt) in solvent (0.5 mL) were prepared and sonicated
if necessary. An aliquot (0.2 mL), to which any additional reactant had been added, was placed
in a 4 mm o. d. quartz tube and de-aerated by bubbling nitrogen for 15 min. Photolysis in the
resonant cavity was by unfiltered light from a 500 W super pressure mercury arc lamp. In all
cases where spectra were obtained, hfs were assigned with the aid of computer simulations using
the Bruker SimFonia and NIEHS Winsim2002 software packages. EPR signals were digitally
filtered and double integrated using the Bruker WinEPR software. The majority of EPR spectra
were recorded with 2.0 mW power, 0.8 Gpp modulation intensity and a gain of ca. 106.
Synthesis and Characterization of Boralactones
General Procedure: The (E)-alkenyl borane (0.1 mmol), benzene (1 mL), AIBN (8.2 mg, 0.05
mmol) and tert-dodecanethiol (10.1 mg, 0.05 mmol) were mixed and placed in a round bottomed
flask with stir bar. The mixture was stirred at 80 oC under argon. After 6 h the reaction mixture
27
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 28 of 33
1
2
3
4
5
6
was concentrated under reduced pressure. The crude residue was purified by flash
chromatography to afford the boralactone.
7
8
9
3-(tert-Butoxycarbonyl)-2-(3-butyl-1-methyl-1H-imidazol-3-ium-2-yl)-5-oxo-2,5-dihydro-
1,2-oxaborol-2-uide (7b): This experiment was conducted with 5a (37.8 mg) and provided 7b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
(18.0 mg, 56%, colorless oil) after purification. H NMR (400 MHz, CDCl3) δ 6.92 (d, J = 7.8
Hz, 2H), 6.61 (d, J = 2.0 Hz, 1H), 4.16 (ddd, J = 13.7, 8.5, 6.8 Hz, 1H), 3.96 (ddd, J = 13.7, 8.4,
6.6 Hz, 1H), 3.73 (s, 3H), 1.79 – 1.69 (m, 2H), 1.41 (s, 9H), 1.38–1.25 (m, 2H), 0.92 (t, J = 7.4
Hz, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ 176.2, 166.5, 132.0, 121.2, 119.4, 79.3, 47.9, 35.2,
11
31.8, 27.1, 18.7, 12.6; B NMR (128 MHz, CDCl3) δ –5.9 (d, J = 107 Hz); HRMS (ESI, TOF)
m/z [M + H]+ calculated for C16H26BN2O4 321.1980, found 321.1999.
3-(tert-Butoxycarbonyl)-2-(1,3-diisopropyl-1H-imidazol-3-ium-2-yl)-5-oxo-2,5-dihydro-1,2-
oxaborol-2-uide (7d): This experiment was conducted with 5d (39.2 mg) and provided 7d (18.0
mg, 54%, colorless oil) after purification. 1H NMR (400 MHz, CDCl3) δ 7.09 (s, 2H), 6.58 (d, J
= 2.0 Hz, 1H), 4.87 (hept, J = 6.7 Hz, 2H), 1.44 (d, J = 6.7 Hz, 6H), 1.40 (s, 9H), 1.38 (d, J = 6.7
Hz, 6H); C{1H} NMR (101 MHz, CDCl3) δ 177.2, 167.6, 132.5, 116.9, 80.3, 49.9, 28.1, 23.4,
13
23.2; B NMR (128 MHz, CDCl3) δ –6.1 (broad); HRMS (ESI, TOF) m/z [M + H]+ calculated
11
for C17H28BN2O4 335.2136, found 335.2148.
3-(tert-Butoxycarbonyl)-2-(1,3-dimethyl-1H-benzo[d]imidazol-3-ium-2-yl)-5-oxo-2,5-
dihydro-1,2-oxaborol-2-uide (7f): This experiment was conducted with 5f (38.6 mg) and
1
provided 7f (16.4 mg, 50%, pale yellow oil) after purification. H NMR (500 MHz, CDCl3) δ
7.56–7.52 (m, 2H), 7.50 (m, 2H), 6.67 (d, J = 2.1 Hz, 1H), 3.96 (s, 6H), 1.39 (s, 9H); C{1H}
13
NMR (125 MHz, CDCl3) δ 177.3, 167.3, 133.5, 133.2, 125.3, 111.4, 80.5, 32.3, 28.1; 11B NMR
28
ACS Paragon Plus Environment

Page 29 of 33
The Journal of Organic Chemistry
1
2
3
4
(160 MHz, CDCl3) δ –5.4 (d, J = 108 Hz); HRMS (ESI, TOF) m/z [M+H]+ calculated for
5
6
C17H22BN2O4 329.1673, found 329.1687.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
Supporting Information. Contains details of EPR experiments and calculations with copies of
spectra of all compounds. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* jcw@st-andrews.ac.uk; * curran@pitt.edu
ORCID
Wen Dai: 0000-0002-9098-5215
Dennis P. Curran: 0000-0001-9644-7728
John C. Walton: 0000-0003-2746-6276
ACKNOWLEDGMENTS
J.C.W. thanks EaStCHEM for financial support and D.P.C. thanks the US National Science
Foundation. Computational support was provided through the EaStCHEM Research Computing
Facility.
References and Notes
29
ACS Paragon Plus Environment