Structural Dynamics and Stereoselectivity of Chiral Benzylideneamine N,C-Chelate Borane Photo–Thermal Isomerization

Article abstract of DOI:10.1002/chem.201905312

New chiral N,C-chelate organoboron compounds based on benzylideneamines (bza) with the general formula of B(bza-R)Mes₂ (R=H or Me; Mes=mesityl) are reported. A chiral substituent group R- or S-CH(CH₃)Ph (Ph=phenyl) was introduced to

Full text of DOI:10.1002/chem.201905312

A Journal of
Accepted Article
Title: Structural Dynamics and Stereoselectivity of Chiral Benzylidene-
amine N,C-chelate Borane Photo-thermal Isomerization
Authors: Polina Novoseltseva, Haijun Li, Xiang Wang, Francoise
Sauriol, and Suning Wang
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Structural Dynamics and Stereoselectivity of Chiral Benzylidene-
amine N,C-chelate Borane Photo-thermal Isomerization
Polina Novoseltseva, Haijun Li, Xiang Wang, Francoise Sauriol, and Suning Wang*
Abstract: Herein we report new chiral N,C-chelate organoboron
compounds based on benzylideneamines (bza) with the general
formula of B(bza-R)Mes2 (R = H or Me; Mes = mesityl). A chiral
substituent group R or S-CH(CH3)Ph was introduced to the imine
center, which imposed a previously unobserved pseudo or axial-
chirality on the BMes2, creating distinct diastereomers. NMR studies
established that the diastereomers undergo slow exchange in
solution at ambient temperature. The chiral N,C-chelate B(R-
bza)Mes2 molecules undergo photoisomerization in the same
manner as their non-chiral analogues, generating chiral BN-
cyclooctatriene (BN-COT) derivatives. Most significantly, by tracking
the photoisomerization with CD and 1H NMR spectra along with TD-
simple benzylideneamine (bza) N,C-chelate borane system[2j]
B(bza)Mes2 (A in Scheme 1) was found to photoisomerize to a
B,N-COT (B), following the same mechanistic pathway as that
[2d]
established for the B(azolylphenyl)Mes2 (A)
analogue.
Molecule B and its analogues B are chiral, albeit racemic, with
the benzene ring and the dimethylbenzene ring being above and
below the BN-COT ring, respectively. In case of
B(azolylphenyl)Mes2 (A) the presence of a stereogenic carbon
center in the product makes the formation of diastereomers
possible. Yet, only one diastereoisomer B along with its
enantiomer formed exclusively.[2d]
DFT
computational
studies,
we
established
that
the
photoisomerization proceeds in a highly stereoselective manner, i.e.
one diastereomer converts exclusively to the corresponding
diastereomer product in the photoreaction.
Introduction
Chemical bond manipulation through thermal and photo-
rearrangements[1],[2] of organoboron compounds have attracted
much recent attention, prompted by the versatile applications of
organoboron compounds in medicine[3], catalysis[4], and
optoelectronics[5] and the need for a better understanding of the
mechanistic aspects of the various boron-based molecular
transformations. N,C-chelate organoboron compounds with the
general formula of B(L)Ar2 (L
=
2-pyridylphenyl,[2a-c] ppy;
2-(naphthalen-1-
azolylphenyl,[2d] 2-(o-tolyl)pyridine,[2e]
yl)pyridine[2f] etc) have been shown to undergo highly efficient
and facile photo-thermal isomerization, transforming into rare
and new species including B,N-cyclooctatrienes (COT)[2d-e] and
B,N-cycloheptatrienes (CHT or borepin) etc[2f-g]
. Recently,
unsymmetric boron compounds of B(ppy)MesAr (Ar = non-
mesityl aryl)[2h-i] have been found to undergo regioselective
photoisomerization at the less bulky Ar group to give either a
seven-membered 4bH-azaborepin product[2h] with
a chiral
carbon center or a product with boron insertion into a C-S
bond.[2i] Although the photoisomerization of B(ppy)MesAr is
highly selective toward one diastereomer, the racemic nature of
the starting material and the product rendered it impossible to
track the transformation and stereochemistry of each enantiomer
to the final product by spectroscopic methods. Recently, a
Scheme 1.
P. Novoseltseva, Dr. H.-J. Li, Dr. X. Wang, Dr. F. Sauriol, Dr. Prof.
S. Wang
Department of Chemistry
Queen’s University
90 Bader Lane, Kingston, Ontario, K7L 3N6 Canada
E-mail: sw17@queensu.ca
To fully understand and exploit these interesting and unusual
photo-thermal isomerizations, it is necessary to obtain optically
pure photoreactive boron compounds, which is a challenging
and important task in organoboron chemistry.[6] Our efforts to
separate enantiomers of
B(ppy)MesAr or synthesize its
optically pure analogues or A by introducing a chiral carbon
center on the molecule were unsuccessful. Fortunately, we
recently discovered that it is possible to obtain optically pure
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and photoreactive boron compounds by employing the simple
bza chelate system and the introduction of a chiral carbon center
R* on the imine group (e.g. 1S in Scheme 1). One important
feature of the structure of 1S is that the boron atom is pseudo-
chiral with a C1 symmetry because the two Mes groups on boron
are inequivalent with a distinct set of chemical shifts for each
Mes in the 1H NMR spectrum at RT, imparted by their steric
interactions with the R* group on nitrogen. The presence of the
chiral R* group leads to the formation of two distinct
diastereomers (see the boron core structures of 1S and 1S,
chiral carbon center (C* in Scheme 2). The 1S and 1R are
enantiomers, and 1S and 1R are enantiomers, with S and R
carbon chiral centers, respectively. These compounds represent
a novel system in which the first S or R chirality at C* imparts a
pseudo-chirality on boron, which causes diastereomer presence.
Similarly, 1S-Me and 1S-Me (1R-Me + 1R-Me) represent the
major and the minor diastereomer for the methyl substituted
analogue, respectively. Their absolute structural assignments
were based experimental and TD-DFT calculated CD spectra
1
(see next section). H NMR spectroscopic analysis established
1
Scheme 1), which are distinguishable by H NMR spectra. NMR
that the ratio of 1S vs 1S (1R vs. 1R) is ~3 to 1 and that of 1S-
Me vs. 1S-Me (1R-Me vs. 1R-Me) is ~6 to 1 at 298 K. Using
this ratio, the free energy change, estimated from the equilibrium
constant, of 1S isomerization to 1S and 1S-Me to 1S-Me is -
0.65 kcal mol-1 and -1.06 kcal mol-1, respectively. DFT calculated
energies follow a similar trend, although owing to the small
energy difference, they are less reliable. Nonetheless, changing
the -R group from an H atom to a Me group clearly increases the
relative stability of the major diastereomer. All compounds were
spectroscopic analyses revealed restricted rotation of each Mes
group around the B-Mes bond as well as exchange between the
two Mes groups. The optically pure molecules such as 1S would
allow us not only to examine the influence of the chiral center on
the formation of diastereomers (e.g. 1S vs. 1S), but also more
importantly to track the stereochemistry in the photo-thermal
isomerization process spectroscopically. Our investigations on
the new chiral B(bza-R)Mes2 system using CD and NMR
spectroscopy revealed that for a given chiral center (R or S on
carbon), one diastereomer (R = H or CH3) forms preferentially.
Furthermore, in case of R = H, each diastereomer isomerizes
exclusively to its corresponding BN-COT diastereomer of the
product (e.g. 1S to 3S shown in Scheme 1). To our knowledge,
this is the first study on photoisomerization and stereochemistry
of optically pure chiral N,C-chelate boron compounds. The
details are presented herein.
1
fully characterized via H NMR, 13C NMR, 11B NMR and HRMS
analyses (see SI). The 11B NMR shifts of 1S + 1S and 1S-Me +
1S-Me are 5.6 and 5.2 ppm respectively, indicating that the Me
group provides additional B-N bond shielding. These 11B
chemical shifts are comparable to those of previously reported
non-chiral analogues[2j]
.
Results and Discussion
Synthesis and Structures of Chiral B(bza-R)Mes2
Scheme 2. Synthesis of chiral B(bza-R)Mes2 and the structures of 1S, 1S,
1R, and 1R.
Chiral B(bza-R)Mes2 compounds were synthesized and isolated
as mixtures of the major diastereomer and the minor
diastereomer, 1S + 1S, 1R + 1R, 1S-Me + 1S-Me, and 1R-Me
+ 1R-Me, respectively in good yields, using a modified literature
procedure[2j, 7], shown in Scheme 2. Within the 1S + 1S mixture,
1S is the major diastereomer, while 1S is the minor
diastereomer. The same is true for the 1R + 1R mixture. For
each compound, the two diastereomers exist as a result of two
possible Mes ring orientations in relation to the phenyl ring at the
Figure 1. Top: a top view of the two different Mes arrangements around the
boron center in 1S and 1S showing one Mes (Mes1) being much closer to the
phenyl ring. Middle and bottom: Crystal structures of the two independent
molecules in the crystal lattice of 1S and the crystal structures of 1R-Me and
1S-Me (bottom). The chiral CH(CH3)Ph group is highlighted by three different
colors (light blue: H; orange: -CH3; purple: Ph).
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Single crystals of 1S, 1S-Me and 1R-Me were obtained from
their corresponding diastereomer mixtures and their structures
were determined by X-Ray diffraction analysis (Figure 1). The
fact that the crystals of the minor isomers 1R-Me and 1S-Me’
were obtained, instead of those of 1R-Me and 1S-Me may be
explained by ability of the minor isomers to form better crystals
than major under the crystallization conditions used. In the
asymmetric unit of 1S, there are two independent molecules 1S-
a and 1S-b, which differ in the orientation of the chiral
CH(CH3)Ph group as shown in Figure 1. For 1S-a, there are -
stacking interactions between the phenyl and one Mes ring with
the shortest atomic separation distance being ~3.2 Å. DFT
structures, optimized with acetonitrile solvent indicate that 1S-a
is slightly lower in energy. As observed previously for BMes2
chelate molecules, one of the Mes groups (Mes1) is much closer
to the carbon atom (Cph) bound to boron on the bza chelate,
which forms a new C-C bond with the phenyl carbon atom in the
photoisomerization process, based on established mechanisms
for analogues. The Cph-C16 (1S-a), Cph-C58 (1S-b), Cph-C17
(1R-Me), and Cph-C26 (1S-Me) distances are 2.554(5), 2.539(5),
2.536(5) and 2.529(5) Å, respectively, much longer than those of
Cph-C25 (1S-a), Cph-C49 (1S-b), Cph-C26 (1R-Me), and Cph-
C17 (1S-Me) (2.864(5), 2.852(5), 2.867(5) and 2.865(5) Å). It is
important to point out that as shown in the schematic top view of
the structures of 1S and 1S in Figure 1, the Mes1 group is at the
back and the front of the N,C-chelate backbone, respectively, for
these two diastereomers, which is believed to be responsible for
their diastereoselective phototransformation.
image relationship. That of 1S + 1S (~3:1) matches well that
predicted for 1S (1S-a) by TD-DFT in CH3CN while that of 1S-
Me + 1S-Me (~6:1) matches that predicted for 1S-Me. Based on
the experimental and TD-DFT predicted CD spectra, the
absolute structures of 1S, 1S, 1R and 1R are assigned and
shown in Scheme 2. The absolute structures of methyl
analogues are assigned in the same manner
Structural Dynamics of Chiral B(bza-R)Mes2 in Solution
NOESY NMR analysis in CD3CN at 298 K indicated that the
chiral B(bza)Mes2 compounds undergo dynamic exchange in
solution that involves both Mes rotation around the B-C bond
and the major and minor diastereomer exchange via the relative
orientation change of the two Mes groups on boron. The
diastereomer mixture of 1S + 1S is selected to demonstrate this,
(Figure 3), as an example. The chemical exchange peaks
detected in 1H-1H NOESY NMR of 1S + 1S at mixing time of 0.4
s at 278 K in CD3CN were attributed to three exchange
processes taking place simultaneously in solution (two are
shown in Figure 3). First, the chemical exchange peaks between
the H atoms located on the corresponding Mes groups of the 1S
and 1S diastereomers, like those of N and n in Figure 3,
indicate major-minor isomerization in solution (1 in Figure 3).
Figure 2. A and B: CD spectra of 1S + 1S' / 1R + 1R'; 1S-Me + 1S-Me' / 1R-
Me + 1R-Me' recorded in CH3CN. C and D: TD-DFT predicted CD spectra of
1S and 1S-Me in CH3CN, respectively..
Figure 3. Top: Partial 1H-1H NOESY spectrum of 1S + 1S in CD3CN at tmix
=
0.4s and 278 K, showing the two exchange peaks (1 and 2). Bottom: DFT
calculated structures showing the process (1) major-minor isomerization and
(2) simultaneous rotation around the B-C bond and major-minor isomerisation
along with activation barriers for processes 1 and 2 of 1S to 1S and 1S-Me to
1S-Me. Peak l and L overlaps with that of proton I and N. For more
information on the activation energy barriers see SI.
To establish the stereogeometry of each diastereomer, circular
dichroism (CD) spectra were recorded for each of the
diastereomer mixtures in CH3CN and compared to those
calculated for each diastereomer by TD-DFT computation
(B3LYP-6-31g(d), see SI for complete CD spectra and TD-DFT
calculated data for each diastereomer). Because each CD
spectrum is dominated by one diastereomer, this approach
allowed us to establish the stereochemistry of each
diastereomer, as illustrated for 1S and 1R, 1S-Me and 1R-Me in
Figure 2. The observed CD spectra of the 1S + 1S (~3:1) and
1R + 1R (~3:1) mixtures in CH3CN have the expected mirror
To quantify the activation energy barrier for each of the
exchange processes, a ‘time zero’ 1H-1H NOESY NMR spectrum
was taken at mixing time of 0.003 s at 278 K in CD3CN such that
the exchange processes were not taking place at the NOESY
timescale (see Figure S2R in SI). The activation energy for the
diastereomer interconversion of 1S to 1S was determined to be
16.4(1) kcal mol-1. The second exchange process involves the
simultaneous Mes group rotation around the B-C bond and
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major-minor exchange of 1S to 1S (2 in Figure 3). The
activation barrier for this process was not determined due to
major and minor peak overlap. Similar exchange processes
in solution (see Figure S2X in SI), further supporting the
diastereoselective photoisomerization. The photoisomerization
of the 1S-Me + 1S-Me, and 1R-Me + 1R-Me species does not
proceed cleanly, producing unidentifiable species in NMR
spectra. As a result, the photoreaction of these two pairs of
diastereomers was not further investigated.
1
were also observed for 1S-Me + 1S-Me. Using a ‘time zero’ H-
1H NOESY NMR spectrum at mixing time of 0.003 seconds,
taken at 298 K in CD3CN (see Figure S2ZO in SI), the 1S-Me to
1S-Me, and 1S-Me to 1S-Me conversion barrier were found to
be 17.9(1) kcal/mol (1 in Figure 3) and 16.9(1) kcal/mol (see
Figure S2ZP in SI), respectively, both of which are higher than
that of 1S to 1S. This is consistent with the presence of the
methyl substituent on the imine group that imposes a greater
steric hindrance for isomerization. The 1.0(2) kcal/mol difference
between the activation energies of the two diastereoisomers 1S-
Me and 1S-Me in acetonitrile agrees well with the free energy
difference of 1.06 kcal/mol estimated from the equilibrium
constant. The simultaneous Mes group rotation around the B-C
bond and major-minor exchange of 1S-Me to 1S-Me’ was
determined to be 19.1(1) kcal/mol (2 in Figure 3). The 19.2(1)
kcal/mol activation energy of the third process, B-C Mes group
rotation, of 1S-Me is much higher than the rotation barrier
observed previously in triarylborane species such as BMes2Ar
(Ar = ph or duryl).[8] These processes of major-minor Mes
rotation, and their back and forth motions around the boron atom
as illustrated in Figure 3 are believed to be collectively
responsible for the overall diastereomer exchange phenomenon
in these systems.
Diastereoselective Photoisomerization of Chiral B(bza-
R)Mes2
To establish if the phototransformation of 1S / 1S and 1R / 1R
diastereomers to their corresponding 3S / 3S and 3R / 3R
occurs in a similar manner to their non-chiral analogues, the
photoreactions of the diastereomer mixtures of 1S + 1S and 1R
+ 1R, were performed using a 365 nm photoreactor at ambient
temperature and tracked by 1H and 11B NMR spectra. As shown
by the 1H NMR spectra in Figure 4, each of the 1S and 1S
disatereomers displays a distinct set of chemical shifts. With
irradiation, the characteristic vinyl species 2S-a + 2S-a (2R-a +
2R-a) were observed, which is similar to those identified
previously for the non-chiral B(bza)Mes2 and B(2-ph-azo)Mes2
analogues[2d,2j]. Although the dark colored isomer 2S + 2S (2R +
2R) was not observed in the NMR spectra owing to their known
low thermal stability and rapid conversion to 2S-a + 2S-a (2R-a
Figure 4. Top: the proposed pathway of phototransformation sequence
1S2S2S-a3S and TD-DFT predicted structures and energies for the
species involved. Bottom: 1H NMR tracking of the 1S + 1S diastereomer
photoisomerization in CD3CN. The 2-a species (red) was observed by 1H NMR
reaction tracking. Selected peaks of the major and the minor diastereomers
1S + 1S (black) and 3S + 3S (blue) are highlighted to show the retention of
their relative intensity.
To gain further insight into the nature of the
stereoselective/retentive photoisomerization, the photoreaction
was tracked by concurrent CD-UV-Vis spectra with excitation at
365 nm. For the first 3.5 minutes of CD-UV-Vis tracking, the
photoreaction was performed in a Dewar with dry ice and
isopropanol at -76 ºC, which allowed us to capture the
absorption spectra of the dark purple isomers 2S + 2S (2R +
+ 2R-a) at ambient temperature,[2d,2j] based on the previously
[2j]
established mechanism for photoisomerization of B(bza)Mes2
,
it is the initial photoisomerization product involving a C-C bond
formation between Mes1 and the phenyl ring, following a classic
di--methane rearrangement mechanism.[1h,2f] Thus, the
photoconversion of 1S/1S is proposed to follow the sequence
shown in Figure 4. The vinyl species 2S-a + 2S-a is formed by a
thermally driven H-atom transfer (HAT) to the imine carbon atom.
The subsequent 1,3-boryl shift forms the final cyclooctatriene
product 3S + 3S. Significantly, the ratio of the 3S and 3S
diastereomers remains about 3:1, similar to that of the precursor
mixture of 1S + 1S, an indication that the photoisomerization
likely proceeds in a diastereoselective/retentive manner, i.e. 1S
isomerizes to 3S only and 1S isomerizes to 3S only. The same
NMR spectral change was also observed for the diastereomer
mixture of 1R and 1R (see SI). 1H-1H NOESY analysis of 3 and
3 in CD3CN at 298 K at tmix of 0.4 s revealed that there is no
interconversion between the major 3S and the minor 3S product

2R) that are unstable above -76 C. The distinct absorption
band of the dark isomers appears at max = ~535 nm in the UV-
Vis spectra (Figure 5A). As shown by the CD spectra for the 3:1
mixture of 1S + 1S and that of 1R + 1R in CH3CN (Figure 5B),
the phase change of these two mixtures is opposite and similar
in magnitude, supporting that the photoisomerization proceeds
in
a stereoselective manner, forming the corresponding
enantiomeric product (e.g. 3S vs. 3R) and retaining the original
diastereomeric ratio. The 535 nm band of the dark isomer was
not observed in the CD spectra, but a weak band appearing at
320 nm – 370 nm that was attributed to the dark isomer (2S +
2S’) by TD-DFT computation.
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this diastereoselective photoisomerization could be explained by
the fact that photoreaction of this system proceeds much faster
than the thermally driven diastereomer 1S and 1S
interconversion. Photoexcitation can bring the photoreactive
molecules instantaneously to their high energy states, initiating
often barrierless isomerization process via the excited state.[2]
Furthermore, the photoisomerization process of BMes2 chelate
systems is known to have a high quantum efficiency, which likely
also plays
a key role in the diastereoselective/retentive
photoisomerization described here.[2a]
Conclusion
Using a chiral substituent functionalized N,C-chelate ligand bza,
optical isomers of photoreactive B(bza-R)Mes2 molecules that
are dominated by one diastereomer have been obtained.
Experimental and computational studies established that the
photoisomerization of the new chiral borane molecules is highly
diastereoselective, dictated by the asymmetric arrangement of
the two Mes groups around the boron atom. Future work will
focus on related asymmetric systems such as B(bza-R)Ar1Ar2
and their stereochemistry in photoisomerization.
Figure 5. Cocurrent UV-Vis and CD spectral tracking. A: UV-Vis spectral
tracking (showing the spectral change of 1S + 1S, which is the same as that
of 1R + 1R). B: CD spectral tracking of 1S + 1S' (solid red line) and 1R +
1R(solid blue line) photo-thermal transformation in CH3CN. The 3S + 3S and
3R + 3R products in CD are shown as dashed red and blue lines, while the
intermediate 2S + 2S and 2R + 2R are shown as solid and dashed black lines.
To assign and correlate the CD spectra with the corresponding
diastereoisomers, TD-DFT computation was conducted for 2S,
2S, 3S, 3S, 2R, 2R, 3R and 3R to obtain the optimized
structures, along with their energies, predicted UV-Vis and CD
spectra in CH3CN. The resulting UV-Vis and CD spectra were
then compared to the experimental UV-Vis-CD tracking spectra
of the 1S + 1S and 1R + 1R photoisomerization. The
experimental and predicted CD spectra for the 3:1 1S + 1S
diastereomer mixture and related species are shown in Figure 6.
The computed CD spectra of 2S, 3S, 2R and 3R match well with
those shown in the experimental CD spectral tracking of 1S +
1S' (3:1) and 1R + 1R' (see SI) in Figure 5B and Figure 6A,
supporting that the CD spectral assignments and that the CD
spectra are dominated by the major diastereomer throughout the
isomerization.
Experimental Section
General Procedures
Except for the deprotection of 2-(dimesitylboryl)benzaldehyde, all
reactions were carried out under the N2 atmosphere. The starting
material 2-(2-Bromophenyl)-1,3-dioxolane was purchased from Sigma-
Aldrich without further purification. The BMes2F compound was prepared
according to a literature procedure[8]. Prior to their use, all solvents were
dried over 3 Å molecular sieves for 48 hours[9], freshly distilled under N2
atmosphere, degassed and stored in the glove box. 1H,13C, and 11B NMR
spectra were recorded on a Bruker Avance 400, 600 and 700 MHz
spectrometers and all deuterated solvents were purchased from
Cambridge Isotopes and Sigma-Aldrich, further dried over 3ꢀ molecular
sieves, distilled under N2 and degassed prior to use. UV-Vis and CD
spectra were recorded on the Varian Cary 50 spectrometer. High
resolution mass spectra (HRMS) were obtained using a Micromass GC-
TOF spectrometer.
Synthesis of 1S + 1S’ Chiral N,C-Chelate Borane: The precursor 2-
(dimesitylboryl)benzaldehyde was prepared according to a modified
7]
literature procedure[2j, (see SI). 1S + 1S was synthesized using a
published procedure in which 2-(dimesitylboryl)benzaldehyde (0.13
mmol) and (S)-1-phenylethan-1-amine (2M in THF, 0.15 mL, 0.3 mmol)
were dissolved in dichloromethane with excess amount of magnesium
sulfate and the reaction mixture was heated to 50 C for 1.5 hours[2j, 7]
.
After reaction completion was confirmed by TLC (1:1 CH2Cl2:Hexane) the
suspension was filtered through a cotton funnel, and the filtrate was
concentrated in vacuo to afford pale yellow solid. Further purification was
achieved by washing the solid with 0C hexanes to afford 1S + 1S in a
ratio of 3:1 (88% yield).
Synthesis of 1R + 1R’ Chiral N,C-Chelate Borane: Compound 1R +
1R was synthesized and purified in the same way as 1S + 1S described
above, except with compound (R)-1-phenylethan-1-amine used as the
starting material. 1R + 1R was obtained as a pale yellow solid, in a ratio
of 3:1 (88% yield).
Figure 6. A: Experimental CD spectral change of 1S + 1S (3:1, the blue line)
to 2S + 2S (the black line), then to 3S + 3S (the red line) in CH3CN at 365 nm
irradiation. B, C and D: TD-DFT predicted CD spectra of 3S, 1S and 2S.
The fact that each pair of diastereomers undergoes a slow
exchange in solution at ambient temperature as revealed by
NMR data, and yet, the photoisomerization retains the original
diastereomer ratio throughout the process is very interesting.
Since the diastereomer BN-COT products do not interconvert,
{(Ph1CH)=N(C*HMePh2)}BMes1Mes2 (1S + 1S): 1H NMR (700 MHz,
CD3CN; 1S is denoted as 1 while 1S as 1 for peak labelling purpose): :
9.36 (s, 0.30H, Ph1CH-1), 9.21 (s, 1H, Ph1CH-1), 7.83 (d, J = 7.6 Hz, 1H,
Ph1CH-1), 7.78 (d, J = 7.5 Hz, 0.31H, Ph1CH-1), 7.62 (d, J = 7.4 Hz,
0.63H, C*HMePh2-1), 7.55 (d, J = 7.5 Hz, 1H, Ph1CH-1), 7.49 (d, J = 7.6
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Hz, 0.35H, Ph1CH-1), 7.45 (t, J = 7.3 Hz, 0.65H, C*HMePh2-1), 7.40 (t, J
= 6.9 Hz, 0.37H, C*HMePh2-1), 7.29-7.27 (1.41H, Ph1CH-1 & 1), 7.23–
7.21 (1.44H, Ph1CH-1 & 1), 6.94 (t, J = 7.2 Hz, 1H, C*HMePh2-1), 6.90 (t,
J = 7.4 Hz, 2H, C*HMePh2-1), 6.83 (s, 0.33H, C*HMePh2-1), 6.72 (d, J =
7.4 Hz, 2H, C*HMePh2-1), 6.68 (s, 1.31H, BMes1Mes2-1 & BMes1Mes2-
1), 6.64 (s, 0.31H, BMes1Mes2-1), 6.49 (s, 2H, BMes1Mes2-1), 6.27 (s,
0.30H, BMes1Mes2-1), 5.93 (s, 1H, BMes1Mes2-1), 5.31 (q, J = 6.9 Hz,
1H, C*HMePh2-1), 5.11 (q, J = 6.7 Hz, 0.35H, C*HMePh2-1), 2.20 (s,
0.95H, BMes1Mes2-1), 2.16 (s, 1.07H, BMes1Mes2-1), 2.11 (s, 3H,
BMes1Mes2-1), 2.07 (s, 1.03H, BMes1Mes2-1’), 2.06 (s, 1.05H,
BMes1Mes2-1), 2.02 (s, 3H, BMes1Mes2-1), 2.02 (s, 3H, BMes1Mes2-1),
1.99 (s, 3H, BMes1Mes2-1), 1.96 (s, 3H, BMes1Mes2-1), 1.82 (d, J = 6.9
Hz, 3H, C*HMePh2-1), 1.48 (s, 1.02H, BMes1Mes2-1), 1.14 (s, 0.95H,
BMes1Mes2-1), 1.07 (s, 3H, BMes1Mes2-1), 1.04 ppm (d, J = 6.9 Hz,
1.00H, C*HMePh2-1); 13C NMR (176 MHz, CD3CN): δ: 169.11, 143.77,
142.88, 142.25, 141.57, 137.68, 137.49, 135.09, 133.50, 132.56, 131.11,
130.90, 130.51, 129.91, 129.70, 128.32, 127.63, 127.01, 125.74, 125.61,
61.72, 26.88, 26.12, 26.04, 25.21, 24.34, 20.58, 20.56 ppm; 11B NMR
(128 MHz, CD3CN): δ: 5.57 ppm; HRMS (EI), calcd for C33H36BN [M]+:
457.2947; found: 457.2951.
Photo-thermal Isomerization: For UV-Vis-CD tracking of the
quantitative photo-thermal isomerization to 3S + 3S, 1S + 1S sample
was sealed into a quartz cuvette in a dry/glovebox dissolved in a dry
acetonitrile at concentration of 10-4 M and irradiated under N2 with UV-Vis
bulbs in the 365 nm range. To observe the 2S + 2S species, the first 3.5
minutes of the irradiation was performed inside a Dewar containing
isopropanol alcohol and dry ice (-76 C). To obtain 3S + 3S product, the
rest of the irradiation was conducted at room temperature.
For 1H NMR and 11B NMR tracking, the sample of 1S + 1S’ was sealed in
a J-Young NMR tube under N2 dissolved in CD3CN at concentration of
0.03 M and placed into a Rayonet Photochemical Reactor at 365 nm
without cooling. 1H NMR and 11B NMR spectra indicate that 2S-a species
was observed after 50 minutes of irradiation, and 3S + 3S product after
5.5 hours of irradiation. Since 1S + 1S and 1R + 1R’ are enantiomers,
the same procedure is applied to 1R + 1R to obtain the 3R + 3R product.
Due to absence of major-minor NOESY signal, only 1H NMR signals of
major 3S in 3S + 3S /major 3R in 3R + 3R are assigned.
{CH2Me2Ph-Ph1CH2-N(C*HMePh2)}BMes (3S): 1H NMR (600 MHz,
CD3CN): δ: 7.57 (d, J = 7.6 Hz, 1H, Ph1CH2), 7.41 (dt, J = 7.5, 1.6 Hz, 1H,
Ph1CH2), 7.37 (td, J = 7.4, 1.4 Hz, 1H, Ph1CH2), 7.24 (m, 3H, Ph1CH2
Synthesis of 1S-Me
+ 1S-Me’ Chiral N,C-Chelate Borane: The
and C*HMePh2), 7.21 (m, 2H, C*HMePh2), 7.19
– 7.14 (m, 1H,
precursor 1-(2-(dimesitylboraneyl)phenyl)ethan-1-one was prepared
according to a modified literature procedure[2j, 7] (see SI). 1S-Me + 1S-Me’
was prepared using a modified literature procedure in which 1-(2-
C*HMePh2), 6.94 (s, 1H, CH2Me2Ph), 6.90 (s, 1H, CH2Me2Ph), 6.80 (s,
1H, BMes), 6.73 (s, 1H, BMes), 4.59 (q, J = 6.9 Hz, 1H, C*HMePh2), 3.74
(d, J = 13.9 Hz, 1H, Ph1CH2), 3.59 (d, J = 14.0 Hz, 1H, Ph1CH2), 2.31 (s,
3H, CH2Me2Ph), 2.29 (s, 3H, BMes), 2.27 (d, J = 7.6 Hz, 1H, CH2Me2Ph
h), 2.21 (s, 3H, BMes), 2.05 (s, 3H, CH2Me2Ph), 1.96 (d, J = 7.5 Hz, 1H,
CH2Me2Ph), 1.92 (s, 3H, BMes), 1.68 ppm (d, J = 6.9 Hz, 3H,
C*HMePh2); 13C NMR (176 MHz, CD3CN): δ 143.86, 143.39, 142.61,
140.27, 138.48, 138.32, 138.03, 137.84, 137.14, 137.03, 135.26, 130.50,
129.31, 129.20, 128.57, 128.23, 128.11, 128.06, 127.99, 127.92, 127.76,
127.71, 60.08, 46.16, 31.73, 23.32, 21.66, 21.19, 21.06, 20.65, 19.66
ppm; 11B NMR (128 MHz, CD3CN): δ: 45.90 ppm; HRMS (EI), calcd for
C33H36BN [M]+: 457.2947; found: 457.2941.
(dimesitylboraneyl)phenyl)ethan-1-one
(0.13
mmol)
and
(S)-1-
phenylethan-1-amine (2M in THF, 0.15 ml, 0.3 mmol) were dissolved in
dichloromethane with excess amount of magnesium sulphate and the
reaction mixture was heated to 50 C for 1.5 hours[2j, 7]. After reaction
completion was confirmed by TLC (1:1 CH2Cl2:Hexane) the crude was
purified using column chromatography on neutral alumina gel (1:2
CH2Cl2:Hexane) to afford 1S-Me + 1S-Me’ as a yellow solid, in a ratio of
6:1 (50% yield).
Synthesis of 1R-Me + 1R-Me’ Chiral N,C-Chelate Borane: Compound
1R-Me + 1R-Me’ was synthesized in same way as 1S-Me + 1S-Me’
above, except with compound (R)-1-phenylethan-1-amine as starting
material. Compound 1R-Me + 1R-Me’ was obtained as a yellow solid and
a mixture of diastereomers, with a ratio of 6:1 (52% yield).
Computational Studies: DFT and TD-DFT calculations were performed
using the Gaussian 09 suite of programs6 on the High Performance
Computing Virtual Laboratory (HPCVL) at Queen’s University. Geometry
optimizations of all compounds were obtained at the B3LYP[6]/6-
31g(d)[10] level of theory. For all computations, the solvent effects were
accounted for by using the scrf=acetonitrile command. All DFT
calculations were performed with standard B3LYP[6]/6-31g(d)[10]
functionals.
{(Ph1CMe)=N(C*HMePh2)}BMes1Mes2 (1S-Me + 1S-Me): H NMR (700
1
MHz, CD3CN; 1S-Me is denoted as 1-Me while 1S-Me as 1-Me for peak
labelling purpose): : 7.64 (d, J = 7.8 Hz, 1H, Ph1CMe-1-Me), 7.55 (d, J =
7.5 Hz, 1H, Ph1CMe-1-Me), 7.50 (d, J = 7.1 Hz, 0.17H, Ph1CMe-1-Me),
7.45–7.43 (m, 0.35H, C*HMePh2-1-Me), 7.37–7.34 (m, 0.34H,
C*HMePh2-1-1-Me), 7.31–7.28 (m, 1.33H, Ph1CMe-1-Me, Ph1CMe-1-
Me & C*HMePh2-1-Me), 7.22 – 7.17 (m, 1.16H, Ph1CMe-1-Me & 1-Me),
7.12– 7.11 (m, 3H, C*HMePh2-1-Me), 6.85–6.82 (m, 2H, C*HMePh2-1-
Me), 6.81 (s, 0.17H, BMes1Mes2-1-Me), 6.71 (s, 0.17H, BMes1Mes2-1-
Me), 6.69 (s, 1H BMes1Mes2-1-Me), 6.65 (s, 1H, BMe BMes1Mes2-1-Me),
6.47 (s, 1H, BMes1Mes2-1-Me), 6.39 (s, 0.15H, BMes1Mes2-1-Me), 5.56
(q, J = 7.5 Hz, 1H, C*HMePh2-1-Me), 5.50 (q, J = 7.3 Hz, 0.17H,
C*HMePh2-1-Me), 2.72 (s, 0.51H, C*HMePh2-1-Me), 2.22 (s, 3H,
C*HMePh2-1-Me), 2.20 (s, 0.53H, BMes1Mes2-1-Me), 2.15 (s, 0.51H,
BMes1Mes2-1-Me), 2.12 (s, 1.56H, BMes1Mes2-1-Me & BMes1Mes2-1-
Me), 2.11 (s, 0.53H, BMes1Mes2-1-Me), 2.09 (s, 3H, BMes1Mes2-1-Me),
2.04 (s, 3H, BMes1Mes2-1-Me), 2.01 (d, J = 7.4 Hz, 3H, C*HMePh2-1-Me),
2.00 (s, 3H, BMes1Mes2-1-Me), 1.74 (s, 3H, BMes1Mes2-1-Me), 1.62 (s,
3H, BMes1Mes2-1-Me), 1.47 (s, 0.51H, BMes1Mes2-1-Me), 1.44 (s,
0.51H, BMes1Mes2-1-Me), 1.13 ppm (d, J = 7.2 Hz, 0.51H, C*HMePh2-1-
Me); 13C NMR (176 MHz, CD3CN): δ: 180.97, 144.70, 143.06, 142.96,
142.04, 140.19, 138.31, 135.60, 133.60, 132.32, 131.10, 130.96, 130.60,
130.56, 130.16, 129.06, 127.74, 127.20, 125.60, 124.63, 56.80, 28.14,
26.51, 25.52, 22.40, 20.75, 20.56, 19.01, 17.98 ppm; 11B NMR (128 MHz,
CD3CN): δ: 5.22 ppm; HRMS (EI), calcd for C33H36BN [M]+: 471.3103,
found: 471.3111.
Single X-Ray Diffraction analysis: Orange-yellow crystals of major 1S
were grown by slow evaporation of the 1S + 1S acetonitrile solution.
Yellow crystals of minor 1R-Me and major 1S-Me were grown by
layering a CH2Cl2 solution of 1R-Me + 1R-Meand 1S-Me + 1S-Me with
hexanes. Crystals were mounted on a plastic fiber-tip and diffraction data
were collected on a Bruker D8-Venture diffractometer with Mo-target ( =
0.71073 Å) at 180 K operating at 50 kV and 30 mA. Data were processed
using the Bruker SHELXTL software package (version 6.10) and
corrected for absorption effects. All non-hydrogen atoms were refined
anisotropically. The crystal data of 1S, 1R-Me and 1S-Me have been
deposited at the Cambridge Crystallographic Data Center (CCDC
No.1967631-1967633).
Acknowledgements
We thank the National Sciences and Engineering Research
Council of Canada (RGPIN-2018-03866) for financial support,
Compute Canada for computational facilities, PN thanks
Government of Canada for NSERC USRA funding and Dr. Yuki
Maekawa.
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P. Novoseltseva, H.-J. Li, X. Wang, F.
Sauriol, and S. Wang*
Chiral boranes: Chiral group
functionalized borane molecules
display distinct diastereomers, and
each photoisomerize exclusively to its
BN-COT diastereomer of the
photoproduct.
Page No. – Page No.
Structural Dynamics and
Stereoselectivity of Chiral
Benzylidene-amine N,C-chelate
Borane Photo-thermal Isomerization
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