Chemoselectivity of Pinacol Rearrangement Originate by Different Hexafluoroantimonate Oxidant

Article abstract of DOI:10.1002/zaac.201800209

Using 1,1,2-triphenylethane-1,2-diol, AgSbF₆, and NOSbF₆, two different carbonyl products of rearrangement were synthesized: neutral ketone compound 1,2,2-triphenylethanone (1) from NOSbF₆ and coordination complex of Ag io

Full text of DOI:10.1002/zaac.201800209

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800209
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Chemoselectivity of Pinacol Rearrangement Originate by Different
Hexafluoroantimonate Oxidant
Xiaoyu Chen,*[a] Chunlei Wu,[a] Jia Li,[a] and Yue Li[a]
Abstract. Using 1,1,2-triphenylethane-1,2-diol, AgSbF6, and NOSbF6, different structure generated by different salt. UV/Vis absorption spec-
two different carbonyl products of rearrangement were synthesized: tra and calculation demonstrate the formation of both compounds. It
neutral ketone compound 1,2,2-triphenylethanone (1) from NOSbF6 provides an example of chemoselectivity of Wagner–Meerwein 1,2-
and coordination complex of Ag ion and aldehyde product
shifts in pinacol rearrangement that depends upon different oxidant.
([Ag·22]+SbF6 ) from AgSbF6. Single crystal X-ray analyses reveal the
–
temperature can also affect the migratory aptitude.[7,11] There-
fore, more experimental data should be collected to investigate
the mechanism of pinacol rearrangement and the migratory ap-
titude of groups.
Introduction
The mechanism of pinacol rearrangement, following the
Wagner–Meerwein migration rule, has been investigated for
decades.[1] In some reactions catalyzed by protonic acid, the
pinacol rearrangement initiates with protonation of one of the
–OH groups, and it is assumed to go through a carbocation
intermediate to form carbonyl compound.[2] In other cases, a
radical cation mechanism caused by one-electron oxidation is
proposed.[3,4] In these reactions, the catalysts do not act as an
acid but an outer-sphere oxidant instead. No matter in what
mechanism, the migratory aptitude of groups, which may be
determined by the stability of the product, has attracted con-
siderable attention. Theoretical and experimental studies reveal
the migration of groups occurs in accordance with the mi-
gratory aptitude, hydride Ͼ alkyl Ͼ phenyl.[5,6]
However, sometimes the selectivity does not follow the
expected migratory aptitude above.[6,7] The pinacol rearrange-
ment has been subjected to much experimental researches
pertaining to its chemoselectivity and the relative migratory
aptitudes of various groups.[5] It is difficult to generalize the
migratory aptitude easily because there are many factors need
consideration. The principal one is electronic effect. For in-
stance, electron-donating or electron-withdrawing property of
The pristine pinacol rearrangement is Lewis acid mediated
reactions, some new methods such as electrochemistry[12] and
photocatalysis[13] can also induce the rearrangement. Moder-
ately strong oxidants AgSbF6 and NOSbF6, are effective cata-
lysts in pinacol rearrangement as well as in many fields.[14]
They can both catalyze the oxidation reactions, occasionally
with the production of metal complexes and free radicals. In
pinacol rearrangement, they usually play the role of oxidizer
and favor to the formation of radical cation intermediate.[3]
NOSbF6 is a stronger oxidant than AgSbF6 in many solutions
including CH2Cl2.[14a]
We herein report two different carbonyl products of 1,1,2-
triphenylethane-1,2-diol rearrangement oxidized by different
oxidant: neutral ketone compound 1,2,2-triphenylethanone (1)
from NOSbF6 and coordination complex of silver ion and alde-
–
hyde product ([Ag·22]+SbF6 ) from AgSbF6. It demonstrates
an example of chemoselectivity of pinacol rearrangement orig-
inated by different hexafluoroantimonate oxidant.
groups on substrates would facilitate the aryl or hydride to Results and Discussion
shift, respectively.[7] Steric effect is also one of the most im-
portant factors,[8,9] as sometimes the steric hindrance of sub-
stituents overrides the electronic effect and prevents the forma-
tion of some isomerization.[6,9] Besides, chemoselectivity of
the product can be favored or disfavored by both substituents
on substrate and catalyst.[10] Moreover, solvent and reaction
Synthesis and Characterization
As shown in Scheme 1, upon oxidation of 1,1,2-triphenyl-
ethane-1,2-diol with NOSbF6 in CH2Cl2, neutral compound 1
is obtained. While oxidized with AgSbF6, it turns into pale
–
yellow solution of [Ag·22]+SbF6 (92%) and 1 (8%), and the
1
ratio was determined by H NMR spectra.
* Dr. X. Chen
E-Mail: drizzle13xy@126.com
[a] Jiangsu Key Laboratory of Fire Safety in Urban Underground
Structural Description
Space
School of Safety
Oxidation of 1,1,2-triphenylethane-1,2-diol with NOSbF6
gave a deep yellow solution, concentrating and cooling this
solution resulted in plentiful colorless crystals covered with a
colored solution identified as neutral 1,2,2-triphenylethanone
China University of Mining and Technolog
Xuzhou 221116, P. R. China
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800209 or from the au-
thor.
Z. Anorg. Allg. Chem. 2019, 645, 22–26
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Journal of Inorganic and General Chemistry
ARTICLE
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Scheme 1. Chemoselectivity of pinacol rearrangement of 1,1,2-tri-
phenylethane-1,2-diol originate by different oxidant.
Figure 2. 50% ellipsoid drawing of [Ag·22]+. Hydrogen atoms are
omitted for clarity. Selected bond lengths /Å: Ag1–C4B 2.4254(40),
Ag1–O1B 2.4488(27), Ag1–O1A 2.4539(29), Ag1–C4A 2.4140(43),
Ag1–C5A 2.5798(42), C1A–C2A 1.5416(59), C2A–O1A 1.2043(54),
C2B–O1B 1.2064(50), C1B–C2B 1.5455(51).
(1) by single-crystal X-ray crystallographic analysis. 1 obeys
the expected migratory aptitude of the pinacol rearrangement
¯
reaction (Figure 1), maintaining the triclinic space group P1.
As seen, the C2 atom, which has sp3 hybridization, is bonded
with two phenyl rings and one hydrogen atom. There is an-
other phenyl ring located on C1. The C1–C2 bond
[1.5258(34) Å] is a simple C–C single bond, and the C1–O1
bond [1.2240(34) Å] presents double bond character, which is
also confirmed by IR spectroscopy [ν(C=O) = 1680 cm–1].
Table 1. Summary of crystallographic data and structural refinement
–
for complexes 1 and [Ag·22]+SbF6 .
–
1
[Ag·22]+SbF6
Formula
M /g·mol–1
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
C20H16
272.33
O
C40H32O2·AgSbF6·0.5(CH2Cl2)
930.74
triclinic
monoclinic
P21/n
14.6356(7)
13.7751(8)
18.0594(9)
90
97.087(2)
90
3613.1(3)
4
1.711
173(2)
1.430
2.92–25.22
1844
¯
P1
5.7842(6)
8.9426(9)
14.8576(14)
98.064(3)
99.575(3)
107.252(3)
708.90(12)
2
1.276
173(2)
0.77
3.06–25.02
288
γ /°
V /Å3
Z
ρcalcd. /g·cm–3
T /K
μ /mm–1
θ /°
F(000)
Data / parameters
GOF (F2)
2538 / 190
1.053
6551 / 533
1.050
Figure 1. 50% ellipsoid drawing of 2. Hydrogen atoms are omitted
for clarity.
R1, wR2 [I Ͼ 2σ(I)) 0.0560, 0.0957
R1, wR2 (all data) 0.1083, 0.1133
0.0388, 0.0595
0.0772, 0.0685
Yellow crystals suitable for X-ray crystallographic studies
were obtained from concentrating dichloromethane solution of
–
[Ag·22]+ SbF6 at 0 °C. The single-crystal X-ray diffraction
out to confirm the existence of radicals, because the signals
disappeared when the samples were exposed to air. Crystal
reveals it is a complex salt, and the ligand triphenylacetalde-
hyde (2) is the aldehyde product of the pinacol rearrangement
reaction (Figure 2), distinct from the ketone structure of 1. As
shown, the complex cation is composed of one Ag ion coordi-
nated with two triphenylacetaldehydes by the O atoms on the
carbonyl group and the π bonds on the phenyl units. In a li-
gand, C1 bonded with three phenyl rings, consistent with sp3
hybridization, while C2 forms a C=O double bond, which is
confirmed by IR spectroscopy [ν(C=O) = 1721 cm–1]. Details
for the data collections and structure refinements are given in
Table 1.
–
samples of 1 and [Ag·22]+SbF6 in solid-state were collected
by filtration in a nitrogen atmosphere and sealed in tubes. The
EPR spectra in Figure 3 display single-line signals, suggesting
that the reaction is most likely based on a radical pathway.[15]
Presumable Mechanism
We supposed the reaction might arise from the cation-radical
Figure 3. (a) Solid-state EPR spectrum of 1 (g = 2.0028) at 298 K. (b)
intermediates.[3] The air-free EPR measurements were carried Solid-state EPR spectrum of [Ag·22]+SbF6 (g = 2.0032) at 298 K.
–
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Scheme 2 shows the suggested rearrangement steps of 1. UV/Vis Absorption Spectra
First, 1,1,2-triphenylethane-1,2-diol is oxidized into a radical
cation by NOSbF6. Then it rearranges according to the ex-
pected migratory aptitude as H undergoing 1,2 shift and loses
water. The formation of the transition state of the radical cation
was identified by the EPR spectrum. The charge on 1·+ trans-
fers to another 1,1,2-triphenylethane-1,2-diol to move on this
reaction.
Figure 4 presents UV/Vis spectra of CH2Cl2 solutions of raw
–
material 1,1,2-triphenylethane-1,2-diol, 1, and [Ag·22]+SbF6
in air. The absorption spectrum of diol exhibits a strong ad-
sorption band at 227 nm, and a weak band at 258 nm. 1 exhib-
its a strong band at 247 and a shoulder peak around 315 nm.
–
For [Ag·22]+SbF6 , in addition to the main adsorption band
less than 330 nm, there is a broad peak around 425 nm, which
differs from the other two curves.
Scheme 2. Presumable mechanism of the pinacol rearrangement of
1,1,2-triphenylethane-1,2-diol initiated by NOSbF6.
For the Ag+ induced rearrangement reaction, the diol also
goes through a radical cation intermediate. However, most
diols tended to form a radical-cation intermediate 3-Ag
(Scheme 3), in which the phenyl group instead of H migrates
to adjacent C atoms. It may be attributed to the formation of
Figure 4. UV/Vis spectra of 1,1,2-triphenylethane-1,2-diol, 1 and
–
[Ag·22]+SbF6 of 1ϫ10–4 M in CH2Cl2 at 25 °C in air.
To qualitatively assign electronic transitions in the UV/Vis
spectra of these compounds, we carried out time-dependent
(TD)DFT computations at the (U)B3LYP/6-31+ G(d,p) levels
using Gaussian.[16] The calculated transition energies of 1 are
in qualitative agreement with the experimental UV/Vis spec-
trum (Figure 5). The peak sat around 241 nm for 1 is assigned
to the transitions of HOMO Ǟ LUMO +1. However, for
–
the coordination compound [Ag·22]+SbF6 , which reduces the
concentration of monomer 2 and favors the generation of 3-
Ag. The consumption of 2 limits the formation of 1, therefore,
–
[Ag·22]+SbF6 is in the majority of the products. Herein, the
migratory aptitude may be dominated by chemical equilibrium.
–
[Ag·22]+SbF6 , the calculated transition energies cannot match
the experimental result. We calculated the transition energies
–
of both 2 and [Ag·22]+SbF6 , and found the experimental UV/
Vis spectrum of solution is the combination of 2 and
–
[Ag·22]+SbF6 . It shows there may be a temporary balance of
–
[Ag·22]+SbF6 and 2 in the solution. In Figure 6, the red and
Scheme 3. Presumable mechanism of the pinacol rearrangement of
1,1,2-triphenylethane-1,2-diol initiated by AgSbF6.
It is noticeable both the formation of 1 and 2 undergo the
similar intermediate 3 (the Ag–O bond can not be detected
because it is destroyed in ionization), which may be testified
–
by the HCD GC/MS spectra of 1 and [Ag·22]+SbF6 (Figures
S5 and S6, Supporting Information). The fragment ion peaks
of them are almost identical, showing all the fragments of 3,
but without the fragment ion peak of triphenylmethyl.
Figure 5. Experimental and computational absorption spectra of ligand
of 2 of 1ϫ10–4 M in CH2Cl2 at 25 °C.
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was stirred at room temperature for 1 d. (We operated the experiment
blue dots indicate the computational UV/Vis spectra of 2 and
–
[Ag·22]+SbF6 , respectively. the peaks at 251 nm and 308 nm
with various amounts of NOSbF6, and demonstrated the amount of
NOSbF6 had little influence on the structure and yield of the product.
Therefore, we selected a catalytic amount.) The resultant yellow solu-
tion was filtered. The filtrate was concentrated and stored at ca. –10 °C
for 1 d to afford colorless crystals of the neutral compound 1. Crystals
1 covered with yellow solution could be further purified by recrystalli-
zation from petroleum ether. Yield: 0.13 g, 48%. IR: ν˜ = 1680
for 2 are assigned to the transitions of HOMO-6 Ǟ LUMO
and HOMO to LUMO, and the peaks around 440 nm are as-
signed to the transitions of HOMO-1 Ǟ LUMO for
–
–
[Ag·22]+SbF6 . The experimental spectrum of [Ag·22]+SbF6
crystal in CH2Cl2 is a combination of [Ag·22]+SbF6 and 2.
–
1
(νC=O) cm–1. H NMR (600MHz, CDCl3, 25 °C): δ = 6.04 [s, 1 H,
CH(Ph2)], 7.25–7.27(m, ~6 aromatic H, as if), 7.25 (t, JH,H = 7.2 Hz,
2 H, p-Ph2), 7.27 (t, JH,H = 7.2 Hz, 4 H, o-Ph2), 7.32 (t, JH,H = 7.2 Hz,
4 H, m-Ph2), 7.41 (t, JH,H = 7.2 Hz, 2 H, m-Ph), 7.52 (t, JH,H = 7.2 Hz,
1 H, p-Ph), 8.00 (d, JH,H = 7.2 Hz, 2 H, o-Ph). 13C NMR (600 MHz,
CDCl3, 25 °C): δ = 59.5 (CH0, 1C), 127.2 (CH, 2C, p-Ph2), 128.6 (CH,
2C, m-Ph), 128.8 (CH, 4C, o-Ph2),129.0 (CH, 2C, o-Ph), 129.2 (CH,
4C, m-Ph2), 133.1 (CH, 1C, p-Ph), 136.8 (CH0, 1C, Ph), 139.1 (CH0,
2C, Ph2), 198.2 (CH, 1C).[4a,17]
–
Synthesis of [Ag·22]+SbF6 : A mixture of 1,1,2-triphenylethane-1,2-
diol (0.3 g, 1.03 mmol) and AgSbF6 (0.355 g, 1.03 mmol) in CH2Cl2
(ca. 60 mL) was stirred at room temperature for 2d. (We also carried
out the reaction with various amounts of AgSbF6, and found only if
–
the amount of AgSbF6 more than 50% of the diol, could [Ag·22]+SbF6
account for the main part in the products, otherwise, the products
would be a complex mixture. To ensure the complete conversion of
Figure 6. Experimental and computational absorption spectra of
[Ag·22]+SbF6 , stoichiometric amounts of AgSbF6 was chosen.) The
–
–
[Ag·22]+SbF6 of 1ϫ10–4 M in CH2Cl2 at 25 °C.
resultant pale yellow solution over a dark precipitate was filtered. The
filtrate was concentrated and stored at ca. 0 °C for 3 d to afford yellow
–
crystals of a complex cationic salt [Ag·22]+SbF6 ·0.5(CH2Cl2). This
Conclusions
yellow complex is not stable even though store in strict anaerobic and
anhydrous conditions, the solution degrades into pale solution and Ag
precipitate within days. When exposed to air, a colorless solution was
obtained after 1 d. Yield: 0.30 g, 61%. IR: ν˜ = 1721 (νC=O) cm–1.
1H NMR (600MHz, CDCl3, 25 °C): δ = 7.09–7.40 (m. 13 aromatic
H), 10.31 [t, JH,H = 7.2 Hz, 1 H, H(CH3)3]. 13C NMR (600 MHz,
CDCl3, 25 °C): δ = 70.0 (CH0, 1C), 127.4 (CH, 3C), 128.4 (CH, 6C),
130.4 (CH, 6C), 140.5 (CH0, 3C), 198.2 (CH, 1C).[18]
1,1,2-Triphenylethane-1,2-diol can be oxidized into the
ketone compound 1 by NOSbF6 and aldehyde structure ligand
–
in complex [Ag·22]+SbF6 by AgSbF6. The structures were
identified by XRD and UV/Vis spectroscopy as well as con-
firmed by FT-IR, NMR, and mass spectroscopy. The mecha-
nisms were investigated by EPR and mass spectra. This work
provides an example of pinacol rearrangement according with
the migratory aptitude, as well as a counterexample. It is bene-
ficial to the systemic study of Wagner–Meerwein 1,2-shifts in
rearrangement.
X-ray Crystallographic Analysis: Single-crystal structures of com-
–
plexes 1 and [Ag·22]+SbF6 were determined at 173 K with a Bruker
Smart Apex II diffractometer using monochromated Mo-Kα radiation
(λ = 0.71073 Å). Diffraction data analysis and reduction were per-
formed within SMART and SAINT.[19] Absorption corrections were
applied by SADABS program.[20] The crystal structures were solved
by direct methods and refined by full-matrix least-squares based on F2
using the SHELXTL program[21] with anisotropic displacement param-
eters for all non-hydrogen atoms. Hydrogen atom positions were calcu-
lated geometrically and were riding on their respective atoms.
Experimental Section
General Procedures: All experiments were carried out in a nitrogen
atmosphere using standard Schlenk techniques and a glove box. All
the materials were purchased (Alfa Aesar, TCI and Energy Chemical)
and used upon arrival. Solvents were dried prior to use. EPR spectra
were obtained with a Bruker EMX-10/12 at room temperature. UV/
Vis spectra were recorded with Lambda 35 spectrometers. FT-IR spec-
tra were recorded with a VECTOR22 FT-IR spectrometer. 1H NMR
experiments were carried out with a Bruker AVANCE III 600 MHz
NMR spectrometer. GC/MS analyses were carried out with an Agilent
7890/5975 and a Xcalibur 4.0 (Thermo Fisher Scientific, Inc., San
Jose, CA) system, the details of the parameters as well as the results
are presented in the Supporting Information. X-ray crystal structures
were obtained by Bruker APEX DUO CCD detector at 173 K. Single
crystals were coated with Paratone-N oil.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-604556 (1) and CCDC-1843568 ([Ag·22]+SbF6 )
–
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
Supporting Information (see footnote on the first page of this article):
Some mass spectra of the products are presented in the supporting
Synthesis of 1: A mixture of 1,1,2-triphenylethane-1,2-diol (0.3 g,
1.03 mmol) and NOSbF6 (0.055 g, 0.21 mmol) in CH2Cl2 (ca. 50 mL) information.
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2005, 70, 179–190; c) D. Lebœuf, V. Gandon, J. Ciesielski, A. J.
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Received: May 17, 2018
Published online: November 8, 2018
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