Antimony(v) cations for the selective catalytic transformation of aldehydes into symmetric ethers, α,β-unsaturated aldehydes, and 1,3,5-trioxanes

Article abstract of DOI:10.1039/c6dt02121b

1-Diphenylphosphinonaphthyl-8-triphenylstibonium triflate ([2][OTf]) was prepared in excellent yield by treating 1-lithio-8-diphenylphosphinonaphthalene with dibromotriphenylstiborane followed by halide abstraction with AgOTf. This antimony(v) cation was found to be stable toward oxygen and water, and exhibited exceptional Lewis acidity. The Lewis acidity of [2][OTf] was exploited in the catalytic reductive coupling of a variety of aldehydes into symmetric ethers of type L in good to excellent yields under mild conditions using Et₃SiH as the reductant. Additionally, [2][OTf] was found to selectively catalyze the Aldol condensation reaction to afford α-β unsaturated aldehydes (M) when aldehydes with 2 α-hydrogen atoms were used. Finally, [2][OTf] catalyzed the cyclotrimerization of aliphatic and aromatic aldehydes to afford the industrially-useful 1,3,5 trioxanes (N) in good yields, and with great selectivity. This phosphine-stibonium motif represents one of the first catalytic systems of its kind that is able to catalyze these reactions with aldehydes in a controlled, efficient manner. The mechanism of these processes has been explored both experimentally and theoretically. In all cases the Lewis acidic nature of the antimony(v) cation was found to promote these reactions.

Full text of DOI:10.1039/c6dt02121b

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DOI: 10.1039/C6DT02121B
Dalton Transactions
ARTICLE
Antimony(V) Cations for the Selective Catalytic Transformation of
Aldehydes Into Symmetric Ethers, α,β-Unsaturated Aldehydes,
and 1,3,5-Trioxanes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Renzo Arias Ugarte, Deepa Devarajan, Ryan M. Mushinski and Todd W. Hudnall*
www.rsc.org/
1-diphenylphosphinonaphthyl-8-triphenylstibonium triflate ([2][OTf]) was prepared in excellent yield by treating 1-lithio-8-
diphenylphosphinonaphthalene with dibromotriphenylstiborane followed by halide abstraction with AgOTf. This
antimony(V) cation was found to be stable toward oxygen and water, and exhibited exceptional Lewis acidity. The Lewis
acidity of [2][OTf] was exploited in the catalytic reductive coupling of a variety of aldehydes into symmetric ethers of type
3
L in good to excellent yields under mild conditions using Et SiH as the reductant. Additionally, [2][OTf] was found to
selectively catalyze the Aldol condensation reaction to afford α-β unsaturated aldehydes (M) when aldehydes with 2 α-
hydrogen atoms were used. Finally, [2][OTf] catalyzed the cyclotrimerization of aliphatic and aromatic aldehydes to afford
the industrially-useful 1,3,5 trioxanes (N) in good yields, and with great selectivity. This phosphine-stibonium motif
represents one of the first catalytic systems of its kind that is able to catalyze these reactions with aldehydes in a
controlled, efficient manner. The mechanism of these processes has been explored both experimentally and theoretically.
In both cases the Lewis acidic nature of the antimony(V) cation was found to promote these reactions.
1
3, 14
alkyl halide/sulfonate under basic conditions.
However
Introduction
The development of novel main group element-derived
the large amount of chemical waste, specifically the salt by-
15
product, has been associated with environmental soil and
1
6
1
ground water contamination. Alternative methodologies that
involve the direct reductive homocoupling of aldehydes was
catalysts has entered into a renaissance in recent years.
While group 13 Lewis acids represent a well-established area
2
recently reported using organosilicon reagents such as Et
SiH, PhSiH polymethosihydrosilane, (PMHS) and
tetramethyldisiloxane (TMDS) as reducing agents in the
SiH,
3
of catalysis, the emergence of group 15 reagents as catalysts
3
, 4 Me
3
,
3
has recently garnered much attention. In particular, Stephan
5
and Radosevich have utilized P(III) Lewis acids in a number of
catalytic transformations, and more recently, Gabbaï and co-
workers have investigated Sb(III) and Sb(V) species in sensing
1
7
18
presence of Lewis acid catalysts, including BiBr , I , Triflic
3
2
19
20
21
acid, Cu(OTf)2, In(OTf)3. However, these methods suffer
from poor selectivity, substrate scope, and the catalysts are
sensitive to air and moisture. The only example to date of such
6
, 7
and catalytic applications. Indeed, Gutmann and Olah have
demonstrated that antimony-based Lewis acids rival
traditional boron complexes. On the other hand, the corrosive
22
8
chemistry with antimony utilized SbI
,
3
however the reaction
9
was not catalytic in antimony, and the expensive reducing
nature of antimony halides such as SbCl and SbCl , has led
3
5
agent PhSiH was needed.
3
several groups to develop novel antimony(V) derivatives
decorated by organic substituents. These Lewis acids have
been shown to exhibit greater stability toward air and water.
Lewis acidic systems are also involved in several important
organic and biochemical transformations, including organic
substrates containing carbonyl moieties. For example, the
synthesis of aliphatic or aromatic ethers, which are found in
many useful natural products, are valuable building blocks
Another important chemical transformation that involves
the carbonyl moiety is the Aldol condensation reaction, used
23
widely for the formation of C–C bonds. The products of Aldol
condensation reactions, α-β unsaturated carbonyl derivatives,
2
4
1
0
are present in numerous biologically active compounds, and
find important applications in the fields of pharmaceutical,
25
11
fragrance, plasticizer, detergent and cosmetic chemistry.
Typically, these reactions are performed under basic or acidic
12
for fine chemicals, polymers and dyes, relies primarily on the
Williamson synthesis whereby an alkoxide is coupled to an
26
conditions that are not environmentally friendly, owing to
the higher concentration of corrosive by-products generated.
More environmentally benign alternatives have been
described in the literature including the use of pyrrolidine or
2
7
benzoic acid, or (p-dimethylamino)benzoate as cocatalyst for
28
29
aliphatic aldehydes, diphenylboron perchlorate (Ph BClO ),
2
4
30
secondary-amines
immobilized
on
silica
support,
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ARTICLE
Dalton Transactions
3
1
32
heteropolyacids, chlorinated silica gel, a solid Lewis basic
catalyst containing aminopropyl-trimethoxysilane-
functionalized chitosan, lysine, and Et N-LiClO activated by
Results and Discussion
Synthesis, Structures, and Properties
View Article Online
DOI: 10.1039/C6DT02121B
3
3
34
3
4
3
5
microwave irradiation.
In general, these synthetic Two stibonium salts ([
2
][OTf] and [
4
][OTf]) were synthesized
methodologies are applicable to only a small range of systems for this study using the straightforward protocol described in
that tolerate only very specific functional groups. Ideally, a Scheme 1. The phosphino-stibonium salt, [ ][OTf], was
selective, inexpensive, and effective catalyst with a broad prepared in excellent yield (70%) by lithium-iodide exchange
2
substrate scope is still needed.
Additionally, the cyclotrimerization of aldehydes to form (diphenylphosphino)naphthalene (
starting from the known compound,
1-iodo-8-
1
), followed by addition of
the correpsonding 1,3,5-trioxanes has become a highly-studied dibromotriphenylstiborane to the in situ-generated lithium
transformation as trioxanes play an important role in many reagent. In a second step, the bromide counter anion was
industrial applications including stabilizers in color exchanged for triflate using silver(I) triflate. As a control study,
photography, burning regulators in fumigants, flavoring the tetraarylstibonium cation [
4][OTf] was prepared to
+
materials, carriers for scents, repellents, deodorants, and determine if the diphenylphosphino substituent in [
2
] was
3
6
insecticides. To date, there is a paucity of reports that necessary for the catalytic activity observed (vide infra). Similar
+
describe the synthesis of these materials, and although to [
2
] , [
moderate to high yields have been achieved, there is much starting from 1-bromonaphthalene followed by treatment with
room for optimization as the selectivities of these systems are (Ph P) SbBr followed by bromide abstraction with AgOTf.
Recently, Findlater et al., demonstrated that FeCl3 Both salts are highly soluble in polar organic solvents (i.e.
4][OTf] was prepared by lithium-bromide exchange
37
3
3
2
38
poor.
could selectively catalyze the Aldol condensation or chloroform, dichloromethane, and acetonitrile) and are
cyclotrimerization of aldehydes under discrete, mild completely insoluble in apolar hydrocarbons and ethers such
39
conditions. Inspired by these recent results, we questioned if as pentane and diethyl ether.
we could exploit the high Lewis acidity and relative stability of
tetraaryl antimony(V) stibonium cations for the selective and
catalytic transformations of aldehydes described above.
The use of naphthalene and acenaphthalene scaffolds for
supporting group 15 moieties through peri-substitution
continues to be an area of active investigation as these
architectures position the main group elements in close
proximity allowing for applications in molecular and anion
3, 40
exchange as well as catalysis.
Herein, we report the
synthesis of an unknown phosphino-stibonium salt that is peri-
+
-
substitued on a naphthalene framework ([2][OTf] = [Sb] TfO ,
Figure 1), and describe the ability of this cation to selectively
catalyze the conversion of aldehydes into: symmetrical ethers
L, (Figure 1, Pathway I), α-β unsaturated aldehydes M, (Figure
Scheme 1. Synthesis of stibonium salts [2][OTf] and [4][OTf].
1
, Pathway II), and 1,3,5 trioxanes N (Figure 1, Pathway III).
The phosphine-stibonium salt
[2][OTf] was fully
characterized by multinuclear NMR spectroscopy, DFT
computational analyses, and single-crystal XRD, and the
1
13
identity of [
4][OTf] was confirmed by H and C NMR
spectroscopy (CDCl ) as well as by elemental analysis (see ESI).
3
Prior to the anion exchange step, intermediate [
2
][Br] was
][OTf] and
2][Br] could be observed by H NMR (Figure 2, CDCl3).
isolated, and a minor difference in the identity of [
2
1
[
Specifically, the ortho-protons of the diphenylphosphino
moiety were shifted upfield by approximately 0.15 ppm in
[
2
][OTf] when compared to [
downfield shift was observed in the P NMR of [
ppm) when compared to [ ][Br] (-38.5 ppm). This small
2][Br]. In addition, a small
31
2
][OTf] (-36.1
2
difference may be due to a weak interaction between the
triflate anion and the cationic antimony center in solution.
Figure 1: Multiple transformation of aldehydes selectively catalyzed by the phosphinio-
stibonium cation [2][OTf].
+
The ability of [
2
]
to selectively catalyze these three
independent transformation represents an important
contribution to main group element-mediated catalysis and in
particular to antimony chemistry.
2
| J. Name., 2012, 00, 1-3
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ARTICLE
Sb–CPh σ* orbital (Figure 4A) was found in the NBO output file.
View Article Online
DOI: 10.1039/C6DT02121B
The energy of this interaction was estimated to be
-1
approximately 23 kcal mol by using the NBO deletion
42
protocol. We also found multiple interactions between the
+
triflate oxygen O1 and the cation [2] . Surprisingly, the two
filled O1 p orbitals behave as donor orbitals to vacant σ* Sb–
CPh and P–CPh acceptor orbitals (Figure 4B and 4C).
Collectively, these interactions were estimated to be
-1
approximately 10.6 kcal mol (NBO deletion) with the O1–Sb
and the O1–P interactions accounting for ca. 9.3 and 1.3 kcal
-1
mol , respectively. Close inspection of the natural charges on
the P and Sb centers (0.97 and 1.89, respectively) indicates
that the positive charge of the cation may be delocalized over
the two group 15 elements. We ascribe this phenomenon to
+
+
the differing catalytic properties of [
vide infra).
2] when compared to [4]
1
Figure 2: H NMR spectra comparing [2][OTf] (top) to [2][Br] (bottom)
(
The most striking feature of the single crystal XRD structure of [2][OTf]
(Figure 3), was the P1–Sb1 distance of 2.9138(11) Å which is well within the
sum of the Van der Waals radii. This indicated to us that there is a strong
interaction between the phosphorus and antimony centers owing to the
high Lewis acidity of the cationic antimony atom. In agreement with this
assessment, we note that the P1–Sb1–C21 angle of 170.18(14)° is consistent
with donation of the phosphorus atom lone pair into a low-lying Sb1–C21 σ*
orbital. In addition, we noticed that the triflate counter ion is weakly
coordinated to the P1–Sb1 unit as evidenced by the Sb1–O1 distance (2.812
Å), the O1–Sb1–C31 angle (166.8°), the P1–O1 distance (3.661 Å), and the
O1–P1–C51 angle (158.6°). Even though it is clear that the cationic antimony
center engages the oxygen atom of the triflate anion in a stronger
interaction, the P1–O1 distance is and the O1–P1–C51 angle are consistent
with a weak interaction between the anion and phosphorus atom. While we
are uncertain if these unusual interactions persist in solution, they may be
1
the source of the observed upfield chemical shift in the H NMR of
compound [2][OTf] when compared to [2][Br]. Despite our best efforts,
single crystals of [2][Br] and [4][OTf] that were suitable for an X-ray
diffraction could not be obtained to compare with to [2][OTf].
Figure 4. NBO plots (isovalue 0.03) of A: lp(P)→σ*(Sb–CPh), B and C: four representative
lp(O)→σ*(Sb–CPh) and lp(O)→σ*(P–CPh) donor-acceptor interactions.
Catalytic Activity Studies
+
We have explored the ability of the stibonium cations [
2
] and
+
[4] to catalyze the reductive coupling of aldehydes (Figure 1,
Pathway I), the Aldol condensation of aldehydes (Figure 1,
Pathway II), and the cyclotrimerization of aldehydes(Figure 1,
+
Pathway III). In general, it was found that cation[
2
] was a
+
superior catalyst when compared to [4] in Pathway I and
exhibited greater catalytic activity and selectivity. In contrast,
both cations were found to be excellent catalysts for Pathways
+
II and III (see ESI for specific results regarding cation [
4
] ).
+
+
We initially explored the ability of [
2
] or [
4
] to catalyze
Figure 3: XRD Crystal structure of [2][OTf] (50% thermal ellipsoids, phenyl substituents the hydrosylilation of aldehydes with Et
SiH, and used
3
have been drawn as thin lines for clarity). Pertinent metrical parameters are provided
benzaldehyde as a model substrate. During the course of our
work, Gabbaï and coworkers demonstrated that the
in the text.
2+
bifunctional dication 1,2-[(Ph MeSb) C H ] catalyzed the
2
2 6 4
To better understand the solid state structure, an natural
bond order (NBO) analysis was performed on the crystal
structure geometry of [ ][OTf] (see ESI for full computational
details). In agreement with the crystal structure, a donor
acceptor interaction between a phosphorus lone pair and a
41
hydrosilylation of benzaldehyde and 4-fluorobenzaldehyde,
however they also showed that the monofunctional analogue
2
+
7
[
Ph SbMe] exhibited no catalytic activity. Interestingly, when
3
+
5
mol% of [2] was used as a catalyst, we did not observe the
expected hydrosylilated species, benzyloxytriethylsilane (Ph-
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ARTICLE
Dalton Transactions
CH -O-SiEt
5
, vide infra), but instead found that dibenzyl Table 1. Substrate scope for the reductive coupling of aldehydes tVoiegwivAertsicylme Omneltinriec
2
3,
ether (L8) along with the symmetric siloxane ((Et Si) O) were ethers L.
DOI: 10.1039/C6DT02121B
3
2
the only products (Scheme 2). We have extended this
reactivity to include a wide substrate scope including
aliphatic, aromatic, and haloaromatic aldehydes (Table 1). For
all cases, we found excellent catalytic activity and selectivity
for the conversion of these aldehydes into the corresponding
b(c)
Time
Selectivity
(%)
Yield
(%)
Entry
aldehyde (R)
Product
(h)
symmetric ethers (
synthesis of the unknown ether, 3,3'-oxybis(methylene)-
bis(fluorobenzene) ( 12) in good isolated yield.
L). This method also allowed for the
e
1
2
3
4
5
6
7
8
9
CH
(CH
CH
2
CH
2
CH
3
1
L
1
L
2
L
3
L
4
L
5
L
6
L
7
L
8
L
9
100
100
100
100
100
100
80
100
L
)
2 8
CH
3
1
2
1
2
1
1
100(90)
100(90)
2
CH
2
Ph
i
e
Pr
100
CH(CH
2
CH
)
3 2
100(90)
100(90)
90(70)
cyclohexyl
CH(Ph)
2
+
Scheme 2: Reductive coupling of benzaldehyde to afford ether L
CH Cl
8
catalyzed by [2] in
Ph
12
100
50
100(90)
2
2
.
d
e
C
6
F
5
48
12
12
50
+
From these results we have observed that [
2
] is a potent
1
0
2-Br-C
3-Br-C
6
H
4
4
L
10
100
100
100(50)
100(50)
catalyst for both aliphatic and aromatic aldehydes, however
longer reactions times were required to convert aromatic
aldehydes into the corresponding ethers (Table 1, entries 8-
11
6
H
L
11
L
12
1
2
3-F-C
6
H
4
12
100
100(70)
1
5). The activity and the selectivity of this reaction suffered
(New)
when aldehydes with aromatic substituents featuring strongly
electron-withdrawing groups were used (Table 1, entries 9,
d
1
1
3
4
4-CF
4-NO
4-CH -C H
3
-C
6
H
4
12
24
L
13
100
100
100
100(90)
100(80)
100(90)
d
2
-C
6 4
H
L
14
1
3, and 14). Indeed, pentafluorobenzaldehyde (entry 9)
resulted in much lower selectivity for the formation of ether
15
12
L15
3
6 4
L9 (50 %), and also required higher temperatures (70 °C) and
(
a) Reactions conditions: Aldehyde:Et
3
SiH (1:3), RT, [2][OTf] (3 mol%), CH
2
Cl
2
.
longer
reaction
times,
similar
(entry 13)
to
and
p-
p-
1
Yields: (b) Based on H NMR (c) Isolated Yield after purification (in parenthesis),
trifluoromethylbenzaldehyde
o
(d) reactions heated to 70 C . (e) Final products were compared with literature
nitrobenzaldehyde (entry 14). Interestingly, when reactions
with aliphatic aldehydes were heated, the selectivity for the
values but not isolated.
stibonium center. This aldehyde-stibonium interaction (A ) has
been further corroborated computationally (vide infra).
c
formation of ethers
products were observed by H and C NMR. This loss of
selectivity was not observed when reactions with aromatic
L was lost and mixtures of unidentified
1
13
aldehydes were heated. Catalyst [
4][OTf] was also found to
catalyze the reaction shown in Scheme 2 (ESI, Table S1). For
+
butyraldehyde (entry 1, Table 1 and S1) [
catalyze the conversion to L1 much faster than [
when bulkier aliphatic or aromatic aldehydes were used the
4] was found to
+
2
] . However,
+
selectivity of [
4] for the conversion to the corresponding
ethers was found to be much lower (< 20%).
L
To further understand how the stibonium cation catalyzed
the reaction shown in Scheme 2, we probed the interaction of
+
1
various aldehydes with [
2
]
using H NMR spectroscopy
(
CDCl ). Unfortunately, when an excess benzaldehyde was
3
+
treated with [
2
] (3 mol%), we were unable to observe the
1
Figure 5: H NMR spectra showing the interaction of butyraldehyde and stibonium salt
o
formation of any intermediates in solution even when heating [2][OTf] when mixed at 70 C in CDCl3 ( 3:1 ratio).
o
to 70 C. In contrast, when
butyraldehyde:[ ][OTf] in CDCl3 was heated to 70 °C, the
formation of an orange color and an upfield shift of the hours, the butyraldehyde was found to disappear concomitant
a
3:1 solution of
2
When the solution shown in Figure 5 was heated for 12
+
resonances corresponding to [2
] were observed (Figure 5). with the appearance of new peaks that corresponded to an
We attribute the observed upfield shift with lone-pair olefin-containing product. The identity of this new compound
donation from the Lewis basic oxygen atom of the aldehyde was verified to be (E)-2-ethyl-2-hexenal by heating a solution
into a Sb-CPh σ* orbital that is localized on the Lewis acidic
of butyraldehyde in CDCl to 70 °C for 12 hours in the
3
4
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presence of 3 mol% of [
ARTICLE
+
14, 43
2
] (Figure 6). The conversion of speed up the reaction or to improve selectivity.
In this
View Article Online
DOI: 10.1039/C6DT02121B
butyraldehyde into (E)-2-ethyl-2-hexenal demonstrated that context, we have also found that water plays an important role
without the reducing agent Et SiH present, [
+
+
2
] could also the Aldol condensation reaction catalyzed by [
2] . Specifically,
3
catalyze the Aldol condensation of aldehydes according to we believe that the water acts as a Lewis base in this reaction.
Figure 1, Pathway II.
Indeed, when these reactions were carried out under identical
conditions, but in the presence of molecular sieves, the
formation of the Aldol condensation product was suppressed
from 100 to 6% (see ESI, Figure S4). These findings
corroborated the need for water in the reaction, and from a
mechanistic standpoint, suggested that water may act as a
Lewis base in these reactions (vide infra). While it may also be
possible that exogenous water may serve as a source of
protons which may in fact catalyze Pathway II, we find this
unlikely given the presence of the Lewis basic phosphino group
+
in [
2
] . With that said, we have not been able to rule out this
possibility at this time.
Table 2. Substrate scope for the Aldol condensation reactions catalyzed by [2][OTf] to
give α, β-unsaturated aldehydes M.
1
Figure 6: H NMR spectra showing the conversion of butyraldehyde into (E)-2-ethyl-
o
hexenal, (M
1
) over a period of 12h, using 3 mol% [2][OTf] in CDCl
3
at 70 C.
Time
h)
Selectivity
(%)
Yield
(%)
Entry
1
aldehyde (R)
CH CH CH
(CH CH
CH CH Ph
Product
b/c
(
Substrate scope studies revealed that these reactions were
limited to aldehydes with two α-hydrogen atoms, and
suggested to us that steric bulk at the α-position may impede
the formation of the intermediate β-hydroxy aldehyde Aldol
product. Additionally, under our experimental conditions
dehydration to the corresponding α,β-unsaturated aldehydes
(M) as shown in Table 2 was observed exclusively. We were
able to further optimize this reaction, and found that with
lower catalyst loading (0.5 mol%) the same activity and
1
00
2
2
3
24
24
24
24
M
1
M
2
M
3
M
4
100
100
100
100
(90)
100
(90)
2
3
4
2
)
8
3
100
(90)
2
2
1
00
2
CH Ph
(90)
selectivity for the formation of
M was observed (Table 2).
(
a) Reactions conditions: aldehyde, [2][OTf] (0.5 mol%) 70 °C, CH
2
Cl
2
.
Importantly, after these reactions were completed, we were
able to recycle the stibonium salt by the addition of hexanes to
the reaction mixture which resulted in the precipitation of the
catalyst and allowed for separation by decanting the
supernatant solution.
1
Yields:(b) Based on H NMR, (c) isolated yield after column chromatography
in parenthesis.
39
Inspired by recent work from Findlater et al., that
demonstrated the ability of the Lewis acid FeCl to catalyze the
3
The observed selectivity for the formation of the α,β- cyclotrimerization of aldehydes to give 1,3,5-trioxanes, we
unsaturated aldehydes over the β-hydroxy aldehydes is highly next questioned if our stibonium cations could similarly serve
desired, and our catalytic system surpasses the majority of as catalysts for this transformation. Given that judicious water
known systems reported in the literature which typically suffer in the solvent favors the Aldol condensation reaction (Pathway
38
from the production of mixtures of products. Surprisingly, no II), whereas the presence of a reducing silane favors the
difference in catalytic activity or selectivity was observed when formation of the symmetric ethers (Pathway I), we
these Aldol condensation reactions were catalyzed with hypothesized that the removal of both the solvent and the
[4][OTf]. Indeed, when carried out under identical conditions reducing agent in these reactions may enhance the interaction
+
+
+
to that of [
2
] , Aldol condensation reactions catalyzed by [
4
]
of the aldehyde with the [Sb] center, and thus increase the
gave identical results. While we currently have no explanation electrophilic character of the acyl carbon.
for this observation, we believe that the highly Lewis acidic
Indeed, we found that treating the aldehydes shown in
nature of the antimony(V) cationic center in these catalysts Table 3 (entries 1–7) with 1 mol% of [
2][OTf] without solvent
+
+
o
precludes any disparate reactivity between [2] and [4] that or Et SiH at-10 C resulted in the formation of the 1,3,5-
3
+
may arise due to the Ph P moiety in [
2
] .
trioxanes (M) according to the reaction shown in Table 3. In
2
39
Previous reports have found that water plays important comparison to previous work from Findlater et al., water
role in Aldol condensation reactions, and can serve to either does not play an important role in this reaction, however the
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Dalton Transactions
reversibility of the process is strongly temperature dependent, was favorable by -6.2 kcal/mol (Scheme 5.) These calculations
View Article Online
5
a
DOI: 10.1039/C6DT02121B
analogous with the previous finding that utilized FeCl as the are in agreement with a recent review from Davis, which
3
39
catalyst. We further optimized this reaction and found that claims that σ-donation from the aldehyde oxygen atom to the
+
as low as 0.5 mol% loading of [
2] in an inert solvent such as Lewis acidic atom is the most common mode of activation
hexamethyldisiloxane could catalyze the formation of seen in main group Lewis acid catalysis.
o
trioxanes
temperatures favored the reverse reaction, and if the reaction cycle for the formation of the symmetric ethers
was carried out at room temperature the conversion to was The formation of intermediate Ac results in a more polarized
N
at -10 C (Table 3). We also found that higher
Pathway I
:
This pathway details the proposed catalytic
L
(Scheme 3).
N
+
negligible even in the presence of 5 mol% of [
2
] .
C=O double bond and facilitates the addition of a Si–H bond
across the C=O bond to give intermediate La which
Table 3. Substrate scope for the cyclotrimerization of aldehydes catalyzed by [2][OTf]
to give 1,3,5-trioxanes N.
+
spontaneously releases [
2
] to give hydrosylilated product Lb
.
The formation of Lb, was calculated to be exothermic by -32.1
Kcal/mol. Unfortunately we have not been able to observe Lb
,
however we have independently synthesized the hydro-
sylilated compound benzyloxytriethylsilane (5, see Figure 7a
13
29
44
for
C and
Si NMR, CDCl3),
to determine if our
experimental conditions resulted in the immediate conversion
b
to ether
species ( c
was added to
L
A
in the presence of another activated aldehyde
Time
h)
Selectivity
(%)
Yield
(%)
Entry
aldehyde (R)
CH CH CH
(CH CH
CH Ph
CH CH Ph
CH(CH CH
cyclohexyl
CH(Ph)
Product
) as shown in Scheme 5. Interestingly when Et SiH
3
(
+
5
in, the presence of [
2
] (5 mol%), no reaction
c
N
1
1
2
3
4
2
2
3
12
100
100
100
100
90
90
90
90
was observed even upon heating. This suggested that the
formation of intermediates of type Ac is critical to these
reactions. Additional supportive evidence showed that the
addition of benzaldehyde and Et SiH to
2
)
8
3
12
12
12
N
2
N
3
N
4
+
2
5 in the absence of [2]
3
did not afford dibenzyl ether (L8).
2
2
To confirm that the hydrosilylated products (Lb) react with
activated aldehyde complexes ( ), we next treated with
] (5 mol%) in the absence of Et SiH
c
5
2
)
3 2
12
N
5
(New)
100
50
90
A
c
5
+
benzaldehyde (
A
) and [
2
3
c
13
29
6
7
2
N
6
7
100
90
(Figure 7). This reaction was monitored by C and Si NMR
c
(CDCl ). After a period of 12 h, the appearance of a new
3
species (*) was observed by C NMR (Fig 7b) with a peak at
105 ppm that is characteristic of a hemiacetal. The conversion
of 5 into a new species was also observed in the Si NMR with
the disappearance the hydrosilylated product (21 ppm) and
the appearance of a new signal at 9.8 ppm. This new signal
was attributed to the formation of the siloxane (Et Si) O,
which may have formed by hydrolysis of a silyl-hemiacetal
precursor. The addition of a slight excess of Et SiH to this
2
12
N
70
1
3
o
(
a) Reactions conditions: Aldehyde, [2][OTf] (0.5 mol%) , (Me
Isolated Yield after purification. (c) No solvent was used.
Si)
3 2
O, -10 C; (b)
29
Even though the reaction can be performed under solvent
free conditions, the solubility of [2] in some of the viscous
aldehydes (entries 1, 5, 6, and 7) was low and necessitated the
addition of ~ 50 uL of (Me Si) O. The reaction is limited to
aldehydes with 1 or 2 α-hydrogen atoms, but despite this
+
3
2
3
2
3
reaction resulted in the immediate formation of L8 and
limitation, we utilized this methodology to prepare the
13
29
(
Et Si) O as shown by the crude C and Si NMR (Figure 7c).
3
2
unknown trioxane, 2,4,6-tri(isopentyl)-1,3,5-trioxane
Similar to the Aldol condensation reaction, [
(
N5).
] was also found
to exhibit similar catalytic activity for the formation of the
trioxanes when compared to the phosphine-substituted
These results clearly demonstrated that once the
+
4
hydrosylilated product Lb is formed, it may react immediately
with Ac to form the hemiacetal Ld through Lc. However under
our experimental conditions, we suggest that Ld is rapidly
N
+
cation [2] .
reduced by Et SiH to form the final symmetric ether
L
. Indeed,
our computational studies revealed that this final reduction
was exothermic by more than 30 kcal/mol (-40.5 for , and -
2.7 kcal/mol for ). The large exothermic nature of these
3
Computational Mechanistic Studies
L
d
7
L
To gain a further understanding into these reactions
+
steps may explain why we are unable to observe or isolate the
hydrosilylated compounds Lb or hemiacetal derivatives Ld using
our method. Importantly, we are currently investigating the
potential for this methodology to be expanded toward the
preparation of unsymmetric ethers starting from
hydrosilylated compounds of type Lb and aldehydes.
catalyzed by [
2
] , we have optimized the geometries of
proposed intermediates that could potentially form in each
reaction using density functional (DFT) methods. For these
reactions, we propose three distinct catalytic pathways for
each reaction that all stem from a single intermediate
(
denoted as Ac). Indeed, it was found that the interaction of
the aldehyde oxygen atom with the cationic Sb(V) center (Ac),
6
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ARTICLE
View Article Online
DOI: 10.1039/C6DT02121B
Scheme 3: Proposed mechanism for the catalytic transformation of aldehydes into symmetric ethers (Pathway I), α-β unsaturated aldehydes (Pathway II), and 1,3,5 -trioxanes
Pathway III). Numbers correspond to calculated DFT energies (kcal/mol).
(
13
29
Figure 7: C and Si NMR spectra of: (a) benzyloxytriethylsilane (5), (b) the reaction of 5 with benzaldehyde (A) and [2][OTf] (5 mol%), and (c) the formation of dibenzyl ether (L
and hexaethyldisiloxane (S) upon addition of Et SiH. Spectra recorded at RT, CDCl
8
)
3
3
.
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Dalton Transactions
Pathway II: This pathway details the proposed catalytic triethyl silane and suppressing the temperature of the
View Article Online
DOI: 10.1039/C6DT02121B
reaction. By avoiding these conditions, our computational
cycle for the formation of the Aldol condensation products
M
(Scheme 3). In this pathway, we propose that the activated analyses revealed that nucleophilic attack from another
species Ac is isomerized to the enol derivative Ma in the aldehyde on the acyl carbon atom of AC to give the
presence water. Even though formation of the enol is uphill by intermediate Na is energetically favored by 11.0 kcal/mol. The
7
.2 kcal/mol, we believe it to be largely favored over complete computations also revealed a buildup of positive charge on
deprotonation of the aldehyde to give the enolate Mb as this one of the former acyl carbons in compound Na as shown in
species was calculated to be 32.6 kcal/mol higher in energy. Scheme 5. For this reason, the addition of a third aldehyde
The endothermic formation of enol Ma is also consistent with molecule to Na followed by intramolecular cyclization to give
the need to heat these reactions (vide supra). Regardless Ma is the 1,3,5-trioxane coordinated to the cationic antimony center
sufficiently nucleophilic to add to another equivalent of the
aldehyde in an exothermic reaction (-12.5 kcal/mol) to give the the stibonium cation back into the catalytic cycle to release the
Aldol product Mc. Unlike Ac Mc is more susceptible to free trioxane ( ) was also found to be downhill by another
(Nb) is exothermic (-19.0 kcal/mol). Finally, the dissociation of
,
N
tautomerization in the presence of water and is isomerized to 14.5 kcal/mol. These calculations are not only in good
Md by an energetically favored process (-5.3 kcal/mol). We agreement with our optimized reactions conditions which
also calculated that the enol Md is enthalpically favored over demonstrated that these trioxanes could be readily prepared
the enolate Me (-5.3 versus 19.1 kcal/mol) analogous to the at low temperature, but also affirm that the reversibility of
Ma/Mb equilibrium. Instead, it was found that deprotonation each step in this process could potentially hamper product
39
of Md by water favored elimination of the β-hydroxy formation at elevated temperatures.
substituent to give the α,β-unsaturated condensation product
M
M
. Our calculations also favored the formation of compound
over Me by more than 30 kcal/mol.
Experimental
Given that our calculations predicted that the formation of
General Considerations
enol Ma was only slightly endothermic, we questioned if this
enol could be used in a cross Aldol condensation reaction. To
address this question, we treated butyraldehyde, an aldehyde
NOTE: Antimony compounds are potentially toxic and should
be handled accordingly. Unless otherwise noted, these
procedures were all carried out using typical Schlenk
techniques under an atmosphere of nitrogen or in a nitrogen-
filled glove box. Solvents were dried and degassed by an
Innovative Technology solvent purification system and stored
over a 3 Å molecular sieves in a nitrogen-filled glove box.
Dichloromethane and hexanes were dried under nitrogen over
CaH and Na/K, respectively and distilled prior to use.
Aldehydes with 95 to 98% purity have been purchased either
from Sigma Aldrich or Alfa Aesar and were used as received.
with enolizable protons, with benzaldehyde, an aldehyde with
+
no α-protons, in the presence of [
Scheme 4).
2
] (0.5 mol%) at 70 °C
(
The
starting
material,
1-diphenylphosphino-8-
2 2
Scheme 4: Cross-Aldol condensation reaction catalyzed by [2][OTf] in CH Cl .
iodonaphthalene, was synthetized according to a literature
46
While we were able to successfully trap the purported enol report. Melting points were recorded on a Mel-Temp
+
of type Ma, formed from butyraldehyde, [2] and water, the apparatus in sealed capillary tubes and are uncorrected. All
major product obtained from this reaction was the reagents were used as received. NMR spectra were recorded
butyraldehyde self-condensation product (M1, 80% yield) with on Bruker Avance 400 MHz/52mm spectrometer. Chemical
only minor formation of the desired cross-condensation shifts (δ) are given in ppm and are referenced to the residual
1
13
31
product (E)-2-benzylidenebutanal (M5, 20% yield), even in the solvent: H: 7.26 ppm; C: CDCl , 77.0 ppm; P NMR: H PO 0
3
3
4
presence of a large excess of benzaldehyde. To confirm these ppm. All column chromatography was performed using small
results, both compounds were isolated and fully characterized columns (5 x 60 mm) with silica gel (Aldrich), 700-230 mesh, 60
3
by NMR spectroscopy (see ESI, Figures S and S ). Despite the Å, and pore volume of 0.75 cm /g as the stationary phase.
5
6
poor selectivity for the formation of M5, we believe that future Electrospray Ionization mass spectra were obtained on a
optimization may be possible. We also plan to design and Waters Synapt G2 ESI-Q-TOF mass spectrometer. Elemental
+
study second generation catalysts derived from [
2
] as the analyses were performed at Midwest Microlabs, LLC
direct coupling of aldehydes to afford symmetric and (Indianapolis, IN).
asymmetric products continues to be an important synthetic
approach and current topic of interest in organic Crystallography
38b,45
chemistry.
All crystallographic measurements were carried out on a
Rigaku Mini CCD area detector diffractometer using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) at 223 K
using an Oxford Cryostream low-temperature device. A sample
of suitable size and quality was selected and mounted onto a
Pathway III: This pathway details the proposed catalytic
cycle for the cyclotrimerization of aldehydes to afford 1,3,5-
trioxanes of type
N (Scheme 3). As described earlier, we could
favor the formation of 1,3,5-trioxanes by eliminating the
8
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Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Dalton Transactions
ARTICLE
nylon loop. Data reductions were performed using Crystal mmol). After the addition was completed, the VsioewluAtritoiclne Ownlianse
Clear Expert 2.0. The structures were solved by direct stirred for an additional 2 hours at -78 D°COI:d1u0r.1in0g39w/Ch6iDcTh02t1im21Be
methods, which successfully located most of the non-hydrogen the color became dark red. The reaction mixture was then
atoms. Subsequent refinements on F using the SHELXTL/PC cooled to -110 ° (LN /acetone slurry) and a solution of
2
2
package (version 5.1) allowed location of the remaining non- Ph SbBr (1.28 g, 2.51 mmol) in THF (5 mL) was added drop
3
2
hydrogen. Colorless, single crystals of [
by slow vapor diffusion of diethyl ether into
dichloromethane solution saturated with the compound. This overnight. After that time the color of the solution turned to
compound crystallized in the monoclinic space group P21/n pale orange. The solvent was then removed in vacuo to yield a
2
][OTf] were obtained wise. The resulting solution was stirred at -110 °C for two
a
hours and was then allowed to warm to room temperature
,
and key details of the crystal and structure refinement data brown solid which was washed with a 1:3 DCM:hexanes
are summarized in Table S2. Further crystallographic details solvent mixture (3 + 15 mL) to remove traces of any unreacted
may be found in the respective CIF files which were deposited starting materials and salt byproducts. The residue then was
at the Cambridge Crystallographic Data Centre, Cambridge, UK. dissolved in DCM and filtered over Celite. The filtrate was then
The CCDC reference number for [
453179.
2
][OTf] was assigned as concentrated to afford [2][Br] as a yellow solid in 70% yield
which was used in the next step without further purification.
1
Note: [
2][Br] very air and moisture sensitive, and should be
Computational Details
stored under an atmosphere of nitrogen at all times. m.p. 120-
o
1
47
122 C. H NMR (400 MHz) (CDCl ):

6.56-6.61, 7.08-7.13, 7.3
3
All geometries were optimized using the M06 density
functional method with LANL2DZ basis/pseudopotential for
antimony and 6-31G(d,p) basis set for other atoms. Stationary
points were characterized as minima by vibrational frequency
analysis from the Hessian matrix. Solvent effect was included
using the SMD implicit solvent model for dichloromethane.
This method was chosen for geometry optimization since, the
(
1
m, 10H, PPh), 7.44-7.88(m, 19H, SbPh + H + H ), 8.33-8.34(d,
H, H , J = 8.4 Hz), 8.41-8.44(d, 1H, H , J = 8.4 Hz). C NMR
2 1
3
4
1
3
(
1
3
100.61 MHz): δ 128.85, 128.94, 129.81, 130.68, 131.95,
31
32.40, 132.53, 134.14. P NMR (161.9755 MHz) (CDCl3):
 -
8.50. Elemental analysis calculated (%) for C H BrCl PSb
42
35
4
([2][Br] + 2 (CH Cl )): C: 51.31, H: 3.38. Found: C: 51.42, H:
2 2
+
+
+
3.65. C H PSb HRMS(ESI ) m/z: (M-Br) calculated: 663.1202.
40 31
computed bond parameters of the catalyst [Sb] are in good
agreement with the experimental values (Figure S 70). The
Found: 663.1201.
Synthesis of [2][OTf]. A 100 mL Schlenk flask was charged
with [2][Br] (1 g, 1.34 mmol), silver(I) triflate (370 mg, 1.47
energies
were
further
refined
using
M06/6-
3
1+G(2d,2p)[LANL2DZdp] electronic energies. All calculations
48
mmol), a magnetic stirring bar, and 50 mL of DCM. The flask
was covered with aluminum foil and the reaction was stirred
for 5 hours in the dark. The resulting suspension was then
filtered over Celite to remove AgBr, and the filtrate was
concentrated in vacuo to give a brown oily material that
solidified after drying under high vacuum overnight (~80%
yield). The resultant brown solid was further purified by
dissolving in an excess of diethyl ether (5 mL) followed by the
were performed using Gaussian 09. In the catalytic Pathway
II dimeric H O species was included, in order to correctly
2
model the hydrogen bonding interactions and solvation effects
of the hydronium ion and water molecules in the solution.
Propanal was used as the model aldehyde in the
computational study. The enthalpy energies in the main
articles were reported in kcal/mol.
addition of hexanes (50 mL) to precipitate [
2][OTf] as a white
Synthetic Details
solid that is stable to air and moisture (700 mg, 65% yield).
Synthesis of dibromo triphenylstiborane (Ph SbBr ). A 250
mL Schlenk flask was charged with triphenylstibine (5 g, 14.16
mmol), 20 mL of diethyl ether, and a magnetic stirbar. Under
o
1
3
2
m.p. 94-96 C H NMR (400 MHz) (CDCl ):

6.46-6.51, 7.06-
3
7
.10, 7.26-7.29(m, 10H, PPh), 7.43-7.84(m, 19H, SbPh + H +
3
H ), 8.34(d, 1H, H , J = 8.4 Hz), 8.42, 8.44(d, 1H, H , J = 8.4 Hz).
4
2
1
and atmosphere of nitrogen, bromine (2.48 g, 0.80 mL, 15.57 13
C NMR (100.61 MHz):
32.29, 132.30, 132,56, 133.97, 133.98. P NMR (161.9755

128.93, 129.01, 130.94, 132.22,
mmol) diluted in 2 mL of ether was added drop wise.
Immediately, the precipitation of a white solid was observed.
31
1
19
MHz) (CDCl3):
8.50(triflate). Elemental analysis calculated (%) for
C42.2H33.4F O PCl SSb ([ ][OTf] + 1.2 CH Cl ): C: 55.37, H: 3.68.

-36.10.
F NMR (376.49 MHz):  -
After all of the bromine had been added, the reaction mixture
was stirred for 30 minutes at room temperature, after which
time mother liquor was decanted away from the resulting
solid. The solid product was then washed with diethyl ether (5
x 50 mL), until the washings were colorless. No further
purification was needed in subsequent reactions, and Ph SbBr2
was isolated in 90% yield as a white solid. Spectroscopic data
was in agreement with known values.
Synthesis of [2][Br]. A 100 mL Schlenk flask was charged
with 1-diphenylphosphino-8-iodonaphthalene (1 g, 2.28 mmol)
a magnetic stirring bar and 40 mL of tetrahydrofuran. The
solution was degassed and cooled to -78 C followed by the
drop wise addition of nBuLi (2.2 M in hexanes, 1.55 mL, 3.42
7
2
3
3
2.4
2
2
+
+
Found: C: 55.29, H: 3.61. C H PSb HRMS(ESI ) m/z: (M-OTf)
4
0 31
calculated: 663.1202. Found: 663.1201.
Synthesis of [4][OTf]. A 100 mL Schlenk flask was charged
with 1-bromonaphthalene (500 mg, 2.4 mmol) a magnetic
stirring bar and 40 mL of tetrahydrofuran. The solution was
3
4
9
o
degassed and cooled to -78 C followed by the drop wise
addition of nBuLi (2.2 M in hexanes, 2.41 mL, 5.31 mmol). The
resulting solution became dark and was stirred for 2 hours at -
7
8 °C after which time Ph SbBr (1.36 g, 2.65 mmol) in THF (5
3 2
o
mL) was added drop wise. The solution was allowed to warm
to room temperature overnight and the color changed from
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Dalton Transactions
+
pale yellow to colorless. The reaction mixture was the preparation of ethers L when compared to [4] .
View Article Online
DOI: 10.1039/C6DT02121B
concentrated in vacuo to afford an oily material that was Unfortunately, the computational analyses of this reaction
+
washed with a 1:3 DCM:hexanes solvent mixture (3 x 15 mL). using [
2
] as a catalyst did not reveal any informative data
The residue was then dissolved in DCM (50 mL) and then regarding the presence of the diphenylphosphino moiety.
filtered over Celite. The filtrate was then concentrated to While it may be possible that sterics lie at the heart of the
+
+
dryness to give a white semi-solid material which was used selectivity difference observed between [2] and [4] , we can’t
without further purification. The solid was then dissolved in rule out the possibility that the phosphine substituent may
DCM (40 mL) and the solution was transferred to a 100 mL impart ulterior electronic effects which ultimately govern the
Schlenk flask that was wrapped in aluminum foil. To this formation of the product. Current investigations are guided
solution was added AgOTf (510 mg, 1.96 mmol) and the toward a more detailed mechanistic understanding how of
resulting suspension was allowed to stir at room temperature these stibonium cations interact with carbonyl-containing
for 12 hours. The suspension was then filtered over Celite and substrates.
the filtrate was concentrated to dryness in vacuo to give a
brown oily material which was washed with cold hexanes to Acknowledgements
give a white solid. After drying under high vacuum overnight, The authors would like to acknowledge the Donors of the
[4][OTf] was obtained as a white solid that is stable to air and American Chemical Society – Petroleum Research Fund for
o
1
moisture (1.06 g, 70% yield). m.p.90-92 C. H NMR (400 MHz) grant 54584-UR3 (TWH), The National Science Foundation for
CDCl3): 7.53-7.57, 8.10-8.14, 8.30-8.35(m, 3H, H , H , H ), grants CHE-0821254 and CHE-0946998 (MRI), and DMR-
(

1
2
5
1
3
7
1
1
.63-7.84(m, 19H, ShPh + H +H ). C NMR (100.61 MHz): 1205670 (NSF PREM grant to support DD), and the Welch
3 4
27.66, 129.23, 130.95, 131.35, 131.73, 133.43, 133.86, Foundation for a Department Research Grant (AI-0045).
35.52, 135.57. F NMR(376.4983 MHz) :
19
 -78.30(triflate).
Elemental analysis calculated (%) for: C H F O SCl Sb ([
4
][OTf Notes and references
31
26
3
3
4
1
2
E. M. Leitao, T. Jurca and I. Manners, Nat Chem,
5, 817-829.
+
2(CH Cl )): C: 46.59, H: 3.28. Found: C: 46.07, H: 2.80.
2 2
+
+
(a) M. Oestreich, J. Hermeke and J. Mohr, Chem. Soc. Rev.,
C H Sb HRMS(ESI ) m/z: (M-OTf) calculated: 479.0760.
28 22
2
2
015, 44, 2202-2220; (b) D. W. Stephan, Acc. Chem. Res.,
015, 48, 306-316; (c) D. W. Stephan and G. Erker, Chem.
Found: 479.0760.
Sci., 2014, 5, 2625-2641; (d) D. W. Stephan and G. Erker,
Conclusions
Angew. Chem., Int. Ed., 2010, 49, 46-76.
3
4
M. H. Holthausen, J. M. Bayne, I. Mallov, R. Dobrovetsky and
D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 7298-7301.
(a) M. Perez, T. Mahdi, L. J. Hounjet and D. W. Stephan,
We have synthesized two unknown stibonium triflate salts,
[
2
][OTf] and [
that these cations can serve to catalyze: 1) the reductive
coupling of aldehydes to give symmetric ethers ( ), 2) the Aldol
condensation of aldehydes to give α-β unsaturated
compounds ( ), and 3) the cyclotrimerization of aldehydes to
the corresponding 1,3,5-trioxanes ( ). Spectroscopic and
4][OTf], and have demonstrated for the first time
Chem. Commun., 2015, 51
, 11301-11304; (b) M. H.
L
Holthausen, M. Mehta and D. W. Stephan, Angew. Chem.,
Int. Ed., 2014, 53, 6538-6541; (c) M. Perez, Z.-W. Qu, C. B.
Caputo, V. Podgorny, L. J. Hounjet, A. Hansen, R.
Dobrovetsky, S. Grimme and D. W. Stephan, Chem.–Eur. J.,
M
N
computational data have revealed that the highly Lewis acidic
cationic antimony(V) center engages the Lewis basic oxygen
atom of these aldehydes in a strong interaction which
ultimately polarizes the C=O bond.
We have evidence which suggested that a transient
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,3,5-trioxanes can be readily produced at low temperature
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(
3
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1
0 | J. Name., 2012, 00, 1-3
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