Vanadium(V) Catalysts with High Activity for the Coupling of Epoxides and CO2: Characterization of a Putative Catalytic Intermediate

Article abstract of DOI:10.1021/acscatal.7b00109

Vanadium(V) complexes derived from aminotriphenolate ligands are demonstrated to be highly active catalysts for the coupling of various terminal and internal epoxides with carbon dioxide to afford a series of substituted organic carbonates in good yields.

Full text of DOI:10.1021/acscatal.7b00109

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Article
Vanadium(V) Catalysts with High Activity for the Coupling of Epoxides
and CO2: Characterization of a putative Catalytic Intermediate
Claudia Miceli, Jeroen Rintjema, Eddy Martin, Eduardo C.
Escudero-Adán, Cristiano Zonta, Giulia Licini, and Arjan W. Kleij
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00109 • Publication Date (Web): 16 Feb 2017
Downloaded from http://pubs.acs.org on February 16, 2017
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Vanadium(V) Catalysts with High Activity for the
Coupling of Epoxides and CO2: Characterization of
a Putative Catalytic Intermediate
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Claudia Miceli,§ Jeroen Rintjema,† Eddy Martin,† Eduardo C. EscuderoꢀAdán,† Cristiano
Zonta,§ Giulia Licini,*,§ and Arjan W. Kleij*,†,‡
§Dipartimento di Scienze Chimiche and CIRCC, Università di Padova, Via Marzolo 1, 35131 –
Padova, Italy
†Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science and
Technology (BIST), Av. Països Catalans 16, 43007 – Tarragona, Spain.
‡Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 –
Barcelona, Spain.
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ABSTRACT: Vanadium(V) complexes derived from aminotriphenolate ligands are
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demonstrated to be highly active catalysts for the coupling of various terminal and internal
epoxides with carbon dioxide to afford a series of substituted organic carbonates in good yields.
Intriguingly, a V(V) complex bearing peripheral chloride groups on the ligand framework
allowed for the formation and isolation of a rare complex that incorporates a ringꢀopened
epoxide with one of the phenolate−O atoms acting as a nucleophile and the metal center as a
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Lewis acidic site. This unusual structure was characterized by Xꢀray diffraction and VꢀNMR,
and was shown to exhibit catalytic activity for the coupling of propylene oxide and CO2 when
combined postꢀsynthetically with these substrates. The results obtained herein clearly show that
vanadium complexes in a high oxidation state are powerful catalysts for the activation of
challenging internal epoxides, and their conversion into cyclic organic carbonates.
Keywords: aminotriphenolate ligands, carbon dioxide, cyclic carbonates, homogenous
catalysis, vanadium
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ꢀ INTRODUCTION
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Small molecule activation, and particularly carbon dioxide (CO2) capture and conversion,1
continues to challenge the synthetic communities to devise more efficient and sustainable
catalysis protocols for classically lowꢀreactive molecules. In the realm of sustainable
development, it is essential to design new catalyst systems that are characterized by an easy,
modular and costꢀeffective construction while addressing the requisite of general availability of
the raw materials and using thus abundant and preferably renewable chemical building blocks.2
Carbon dioxide represents an interesting source of carbon in synthetic chemistry, but its
successful conversion relies on the use of catalysts and highꢀenergy coꢀreactants that can mediate
its conversion, since the latter is challenged both from a kinetic and thermodynamic point of
view.3 Over the last decade, the growth of new catalytic technologies for CO2 conversion has
been spectacular with progress reported for a wide series of transformations.4 Metal catalysis has
been prominent in most of the synthetic efforts directed towards CO2 “fixation” activating either
this carbon feedstock directly or a co−reactant able to induce a coupling reaction.5 An often
applied strategy in this regard has been the Lewis acid activation of small heterocyclic ethers in
the presence of a nucleophilic additive giving rise to alkoxide intermediates that smoothly react
with CO2 to form carbonate structures: these linear carbonate species eventually evolve into
either cyclic6 or polycarbonates7 and the observed chemoselectivity typically depends on the
reaction conditions, the type of nucleophile and Lewis acidic metal complex.
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Successful catalysts for cyclic organic carbonate (COC) formation include the use of
abundant main group metals such as aluminum,8 iron,9 and zinc,10 and transition metals (Co,
Cr)11 most often in combination with Nx,Oyꢀcoordinating ligands such as salens (x = y = 2)12 and
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aminotriphenolates (x = 1, y =3).13 We have been interested in the use of O3Nꢀchelating
aminotriphenolate ligands and their complexes for catalytic applications, and have reported on
the use of Ti(IV) derivatives for the oxidation of sulfides and secondary amines into nitrones,14
Mo(VI) and W(VI) complexes for the oxidation of sulfides, olefins and chloro− and
bromoperoxidation,15 and more recently the Fe(III) mediated formation of cyclic carbonates and
the preparation of Catechol Oxidase and Catalase mimics.16 The use of aminophenolate type
ligands is beneficial towards the stabilization of different (higher) oxidation states such as V(III)
and V(V),17 and the V(V) complexes have previously been successfully used in the context of
haloperoxidase models in sulfoxidation chemistry.18 Inspired by the presence of highꢀoxidation
state V centers in these complexes representing strongly Lewis acidic sites, we envisioned that
such compounds would also be highly efficient in epoxide activation/ringꢀopening in the
presence of a suitable nucleophile, and give rise to COC formation in the presence of CO2. As far
as we know, the use of vanadium in the formation of COCs and related compounds remains rare,
and with limited application potential.19
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Figure 1. V(V) aminotriphenolate complexes 1 and 2 used in this work as catalyst components
for COC formation.
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Therefore, we set out to explore the potential of V(V) aminotriphenolate complexes (Figure 1)
in the formation of COC and have specifically focused on the conversion of more challenging
subꢀseries of internal epoxide partners. The use of this class of substrates is currently attracting
more attention as efficient catalysts have become available for their conversion,20 and therefore
new catalyst systems should be comparatively evaluated against this advanced state of the art in
COC formation. In this work we will demonstrate that V(V) aminotriphenolate complexes
combined with suitable halide nucleophiles offer highly efficient binary catalysts for COC
formation from a wide series of internal epoxides. Further to this, the unexpected low activity for
the conversion of some terminal epoxides by the strongest Lewis acidic V(V) complex was
investigated in more detail. This has resulted in the formation and structural characterization of a
unique and unprecedented catalytic intermediate where the V(V) center and one of the phenolate
donors had cooperatively activated the epoxide substrate to form a sixꢀcoordinate V(V) complex
comprising of a bidentate aryloxoꢀalkylether fragment. This new epoxide activation mode was
confirmed by Xꢀray analysis, and further use of this intermediate in COC formation was also
demonstrated. Our results thus demonstrate the use of a powerful new binary catalyst system for
challenging coupling reactions between internal epoxides and CO2, and mark the use of abundant
vanadium as a useful and sustainable alternative in COC formation.
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ꢀ RESULTS AND DISCUSSION
2.1 Screening Phase. The V(V) complexes 1 and 2 were combined with NBu4I and used as
binary catalysts for the formation of COCs D−F from terminal epoxides A−C (Table 1). As a
reference catalyst, we used the previously reported and highly active binary catalyst 3/NBu4I21
under similar conditions to examine the catalytic potential of the V(V) complexes.
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Table 1. Screening Experiments carried out with V(V) Complexes 1 and 2 using various
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Terminal Epoxides (A−C) and one Internal Epoxide (D).a
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Entry
substrate
Cat. (%)
Nu (%)
T (ºC)
Yield (%)b
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2
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19c
20c
21c
22c
23c
24c
1, 0.50
1, 0.50
2, 0.50
1.25
−
−
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0.50
0.25
−
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
1.25
5.0
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85
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85
85
>99
0
0
64
94
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0
23
73
>99
70
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A
A
A
A
A
A
A
A
A
A
B
B
B
B
C
C
C
C
D
D
D
D
D
D
−
1, 0.20
1, 0.10
1, 0.10
−
2, 0.10
3, 0.10
1, 0.10
2, 0.10
3, 0.10
−
1, 0.10
2, 0.10
3, 0.10
−
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2, 0.50
2, 0.50
1, 0.50
2, 0.25
−
70d
>99 (68)e
93 (41)e
>99
26
5.0
5.0
5.0
5.0
3, 0.50
>99 (55)e
aReaction conditions: 2.0 mmol of epoxide, temperature indicated, NBu4I (amount indicated), 10
bar, 18 h, neat. Nu stands for nucleophilic additive, and catalyst and nucleophile loadings are in
b
mol%. NMR yield using mesitylene as internal standard; selectivity towards the COC product
c
>99%. Conditions used: 1.0 mmol D, NBu4Br (amount indicated), 10 bar, 18 h, neat.
dConversion was 98%, with benzaldehyde (28%) being formed as a byꢀproduct. In parentheses
e
the NMR yield after 6 h.
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The conversion of 1,2ꢀepoxyhexane A was first evaluated and a combination of complex 1 (0.50
mol%) with NBu4I (1.25 mol%) provided quantitative yield for the COC product D (entry 1).
The complexes 1 and 2 themselves proved to be unproductive (entries 2 and 3) whereas NBu4I
alone gave a 64% yield of D. This prompted us to further lower the loading of 1 and NBu4I
(entries 5−7) and we were pleased to find that a combination of 1 (0.10 mol%) and NBu4I
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(0.25−0.50 mol%) still provided appreciable to high yield of the COC E whereas the yield in the
presence of only NBu4I was significantly lower (entry 8). Upon using the more Lewis acidic
V(V) complex 2 (cf., entries 6 and 9, 58% vs. 73%) a higher yield of E was noted compared to
the binary system 1/NBu4I. Comparison with the previously reported and highly active binary
catalysts 3/NBu4I (entry 10),5f,21 shows that for 1,2ꢀepoxyhexane conversion the binary systems
based on Vꢀcomplexes 1 and 2 have lower, though still appreciable activity. Other terminal
epoxides were then also examined (styrene oxide B and epichlorohydrin C; entries 11−18), and
surprisingly in these cases the binary catalyst based on 1 proved to be significantly more active
than the one derived from 2. Catalyst 1/NBu4I showed again a considerably higher yield for
COCs F and G compared to the background reaction mediated by the nucleophile only but, more
importantly, similar order magnitude COC yields as determined for the Alꢀbased binary system
3/NBu4I (cf., entries 11 vs. 13 and 15 vs. 17). Since binary 3/NBu4I catalysts have been reported
among the most active for COC formation using terminal epoxides,5f,21 it therefore seems that
aminotriphenolate V(V) complexes have also high catalytic potential.
In order to challenge the binary couples consisting of 1 or 2 and a suitable ammonium halide
nucleophile towards COC formation, an internal epoxide (trans D) was then probed as a reaction
partner (entries 19−24). Interestingly, in the formation of COC H the use of complex 2 was more
beneficial showing higher yield of the desired product (cf., entries 20 and 21). At a loading of
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1.25 mol% of NBu4Br,22 however, substantial formation of benzaldehyde (entry 19, 28%) was
noted as a likely result of the V(V) complex acting as a catalyst for oxidative cleavage after
initial formation of a diol intermediate.18c An increase in the nucleophile loading up to 5.0 mol%
solved this problem with the chemoꢀselectivity, and under these conditions (entry 20)
quantitative formation of COC H was obtained, and for this substrate upon using Alꢀcomplex 3
(entry 24) a slightly lower yield was obtained after a 6 h reaction time. When comparing the Vꢀ
based catalysts of this work with a recently reported diphenolate V(V) based complex (0.20
mol%) combined with NBu4Br (0.20 mol%) for the conversion of PO at 80ºC and 40 bar CO2
pressure (18 h; conversion 79%),19c it seems that the complexes of this study show relative good
potential. However, since some disparity exists between our conditions (Table 1) and the ones
reported previously, some caution is needed to quantitatively compare these data.23 The
conditions reported for the successful formation of disubstituted COC H using Vꢀcomplex 2
were then taken as a starting point for the coupling of various internal epoxides with CO2.
2.2 Scope in Disubstituted COC products. Motivated by the results summarized in Table
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1, a series of internal epoxides was then converted into their disubstituted COCs 4−19 (see
Figure 2) using the binary catalyst 2/NBu4Br. The product diversity proved to be wide, and
various (functional) epoxides (either commercial ones or prepared in few steps from easily
accessible starting materials, see the Supporting Information; SI) could be converted in their
cyclic carbonates typically in good yields. Different ringꢀsize epoxides are suitable substrate
partners including fiveꢀ, sixꢀ and eightꢀmembered ones (4−9, 17ꢀ19), though in the preparation of
7 and 9 we observed that longer reaction times (up to 3 days) and/or increasing the binary
catalyst loading did not improve the results. COC 7 is a rather interesting structure that
comprises an epoxide unit that was unaffected during the catalytic trials, and thus remains
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available for postꢀsynthetic modification. The binary catalyst 2/NBu4Br also proved to be useful
to convert a trimethoxysilylalkyl derived epoxide without observable degradation (cf., formation
of 10). COC product 11 was obtained from a biomassꢀderived fatty acid precursor (oleic acid)
and represents an interesting contextual case towards biomass valorization.24 The COCs 13−16
were prepared from their stereoꢀpure cis or trans precursors: whereas the coupling of trans 2,3ꢀ
dimethyloxirane with CO2 proceeded with high retention of configuration (13: cis/trans = 4:96),
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the use of the diastereoꢀisomerically pure cis epoxide led to substantial loss of stereoꢀinformation
O
O
2/NBu4Br
O
O
R1
R2
CO2, 85°C, 18 h
R1
4-19
R2
O
O
O
O
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5
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7
O
O
O
O
O
O
O
O
O
O
Conv. >99%; Sel. >99%
Conv. >99%; Sel. >99%
Conv. >99%; Sel. >99%
Conv. 51%; Sel. >99%
cis>99%; Yield = 79%
cis>99%; Yield = 89%
cis>99%; Yield = 71%
cis>99%; Yield = 32%
O
O
O
O
O
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9
10
11
O
O
O
O
O
O
O
Me
(MeO)3Si
O
OMe
7
7
Conv. 88%; Sel. >99%
Conv. 37%; Sel. 99%
Conv. >99%; Sel. >99%
Conv. 50%; Sel. >99%
cis>99%; dr 56:44; Yield = 82% cis/trans 68/32; Yield = 19%
cis>99%; dr 60:40; Yield = 65%
cis>99%; Yield = 46%
O
O
O
O
12
13
14
15
from
"trans"
from
"cis"
from
"cis"
O
O
O
O
O
O
O
O
Ph
Me
Me
Me
Me
Me
Ph
Ph
Conv. >99%; Sel. 95% [a]
Conv. >99%; Sel. 99% [a]
Conv. >99%; Sel. 72% [a]
Conv. >99%; Sel. >99%
cis/trans 52/48; Yield = 75%
trans>99%; Yield = 85%
cis/trans 4/96; Yield = 79%
trans/cis >99:1; Yield = 61%
O
16
17
18
O
19
O
from
"trans"
O
O
O
O
O
O
N
O
O
O
Ph
Ph
Ph
Conv. 78%; Sel. 92% [a]
Conv. 82%; Sel. >99%
Conv. >99%; Sel. >99%
Conv. 74%; Sel. >99%
cis>99%; Yield = 62%
trans >99% Yield = 59%
cis>99%; Yield = 68%
cis>99%; Yield = 87%
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Figure 2. Synthesis of disubstituted COCs 4−19 from internal epoxides and CO2 using the
binary catalyst 2/NBu4Br. General conditions: epoxide (4 mmol), 2 (0.5 mol%), NBu4Br (5
mol%), neat, 85 °C, p(CO2)º = 10 bar, 18 h. The dr values were determined by 13C NMR
spectroscopy using inverseꢀgated decoupling, and reported yields are isolated ones after column
purification. Reported relative configurations of the drawn structures refer to the major/expected
product. [a] In these reactions an aldehyde byꢀproduct was formed as shown by 1H NMR.
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(14: cis/trans = 52:48). Such loss of stereoꢀinformation was reported before and is generally
explained by the occurrence of both SN1 and SN2 based ring closing mechanisms.20e,24d The use
of a sterically more congested substrate (cis/trans 2,3ꢀdiphenyloxirane), however, led to
exclusive isolation of the transꢀconfigured products (15 and 16) regardless the initial
configuration of the epoxide substrate. The combined results clearly show the high catalytic
potential of V(V) complex 2 in the formation of these challenging conversions, and the scope of
COC products reported in Figure 2 is among the widest reported to date.
2.3 Mechanistic Investigations. Based on the remarkable and relatively low activity found
for the activation of styrene oxide and epichlorohydrin (Table 1) by the more Lewis acidic V(V)
complex 2 compared to 1, we decided to investigate the origin of this behavior. Since previous
work on related Al(III) aminotriphenolate complexes showed that cyclic ether coordination5f,21
precedes the ringꢀopening process by an external nucleophile, we first combined complex 2 and
the simplest terminal epoxide (propylene oxide, PO) and focused on the formation of a similar
epoxideꢀcoordinated structure and its analysis by X‒ray diffraction. Fortunately, crystalline
material obtained from a mixture of 2 and excess PO in CH3CN could be easily obtained and the
determined structure (2−PO) is presented in Figure 3. Much to our surprise, instead of the
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expected POꢀcoordinated V(V) complex,25 unprecedented formation of a V(V) complex
comprising of a ringꢀopened PO had occurred mediated by one of the phenolate O−atoms that
acted as an internal nucleophile.
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The Vꢀcenter in 2‒PO is asymmetrically positioned in a rather distorted octahedral
coordination structure, with the vanadyl fragment (V(1)−O(1) = 1.5877 Å) occupying one of the
equatorial positions. The three O−donors of the aminophenolate ligand occupy the remaining
positions of the equatorial plane, with the V(1)−O(3) distance (2.3413 Å) being significantly
longer than the other V−O bonds (V(1)−O(2) and V(1)−O(5) of 1.8912 and 1.8701 Å,
respectively). The axial O−donor originating from the activated epoxide shows a relatively short
bond length of 1.7871 Å (V(1)−O(4)) pointing at negative charge localization at this O−center,
and thus the organic fragment connected through O(3) and O(4) should be regarded as a monoꢀ
anionic arylethanolate fragment with V(1)−O(3) being a dative bond involving an arylalkyl ether
donor.
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Figure 3. Xꢀray molecular structure of the unusual PO−adduct 2−PO based on V(V) complex 2.
Selected bond lengths (Å) and angles (º), with esd´s in parentheses: V(1)ꢀO(1) = 1.5877(17),
V(1)ꢀO(2) = 1.8912(16), V(1)ꢀO(3) = 2.3413(17), V(1)ꢀO(4) = 1.7871(17), V(1)ꢀO(5) =
1.8701(16), V(1)ꢀN(1) = 2.244(2); O(4)ꢀV(1)ꢀN(1) = 155.88(8), O(1)ꢀV(1)ꢀO(3) = 178.71(8),
O(2)ꢀV(1)ꢀO(5) = 158.95(7).
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Thus, it seems reasonable to suggest that after initial coordination of the PO molecule, both the
Lewis acidic V center and one of the Lewis basic phenolate cooperatively activate the epoxide
and form a stable, sixꢀcoordinate V(V) complex. Interestingly, such a process shows parallels
with the ringꢀopening of the epoxide by an external nucleophile (often proposed to be the first
step in COC formation using binary catalysts), though in the present case the phenolate acts as an
internal nucleophile. The structure of 2−PO was further analyzed/confirmed by MALDIꢀTOF
mass analysis and 51VꢀNMR spectroscopy.
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To gain further insight in the formation of 2−PO, V NMR titration studies were carried out
using 2 and different amounts of PO (Figure 4). The respective NMR traces were recorded 5 h
after initial mixing (traces b−f), and a comparison was made with the 51V resonances for 2 (trace
a) and isolated 2−PO (trace g). Upon addition of PO, a multifold of resonances appeared that
seem to converge to two major resonances located at −457 and −470 ppm. The latter peak
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assignment was facilitated by the V NMR analysis of an isolated crystalline sample of 2−PO
and could thus be assigned to this unusual structure. Furthermore, the reaction of 2 and PO to
induce the formation of 2−PO depends on the excess of PO present in solution; interestingly, at a
lower excess of PO (33 equiv, d) the spectrum evolves in time (i.e., a few days) to the same final
trace observed under (f) suggesting that full conversion of 2 into 2−PO under these conditions
(25ºC, 33−fold excess of PO) is relatively slow.
It is likely that the formation of 2−PO is preceded by simple PO coordination which may
occur both in an equatorial and axial fashion. Also the final in situ prepared product 2−PO could
have the vanadyl fragment occupying either an axial or equatorial position in the octahedral
complex, which could explain the presence of the two main peaks. Such behavior was also
demonstrated for other V(V) complexes comprising of O2,N,N´ꢀcoordinated tetradentate ligands
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showing Vꢀresonances in the same region as noted for 2−PO.17b,18b Alternatively, in solution
Lewis acidic 2−PO may be part of larger clusters such as dimers that can affect the 51V
resonance and account for the presence of some other minor species (cf., trace f).
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Figure 4. VꢀNMR titration of complex 2 with various amounts of PO. Conditions used: [2] =
0.01 M in CD3CN, NMR spectrum was recorded 5 h after initial mixing of 2 and PO. See the SI
for further details.
In order to test whether the preꢀisolated complex 2−PO acts as a functional model for the
epoxide ringꢀopening stage, we performed some further catalytic tests with a preꢀisolated sample
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of 2−PO and combined it with NBu4Br26 and fresh PO, and used it as a binary system for the
formation of propylene carbonate (see Table 2). The catalysis data was compared against the
background reaction (use of NBu4Br only, entry 1), and the activity of 2/NBu4Br and 1/NBu4Br
(entries 2 and 3) under similar reaction conditions and catalyst loadings. From the data in Table 2
it is clear that the complex 2−PO still exhibits catalytic activity toward COC formation as it
delivers propylene carbonate in a significantly higher yield than the background reaction.
Remarkably, the combination of parent V(V) complex 2 with NBu4Br under the reported
conditions and (low) catalyst loading seems to possess similar activity as the binary combination
2−PO/NBu4Br pointing towards a relatively fast in situ formation of 2−PO at 50ºC and >800
equiv of PO (Table 2).
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Table 2. Comparison of the Activity of the Binary Systems 2ꢀPO/NBu4Br, 1/NBu4Br and
2/NBu4Br in the Formation of Propylene Carbonate.a
Entry
Vꢀcomplex (mol %)
Nu (mol%)
Yield (%)b
1
2
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4
NBu4Br, 0.048
NBu4Br, 0.048
NBu4Br, 0.048
NBu4Br, 0.048
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96
9
−
1, 0.024
2, 0.024
2ꢀPO, 0.024
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aReaction conditions: 20.0 mmol propylene oxide, 50ºC, 10 bar, 18 h, neat. Nu stands for
b
nucleophilic additive. NMR yield determined using mesitylene as internal standard; selectivity
towards the COC product >99%. The average of two runs is reported.
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As noted for the conversion of styrene oxide B and epichlorohydrin C (Table 1), a much lower
activity for COC formation is noted when compared with the results obtained with the binary
system 1/NBu4Br, suggesting that terminal epoxides with a modest steric impediment likely have
a higher tendency to (partially) form an epoxide adduct of type 2−PO. Further evidence for this
hypothesis was gathered by analysis of the resultant complex when combining 2 with 100 equiv
of epichlorohydrin (ECH) and analyzing the product by 51V NMR and MALDIꢀTOF mass
spectrometry (see SI), and X−ray diffraction (Figure 5).
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Figure 5. Xꢀray molecular structure of 2−ECH based on V(V) complex 2, ECH =
epichlorohydrin. Selected bond lengths (Å) and angles (º), with esd´s in parentheses: V(1)ꢀO(1)
= 1.585(4), V(1)ꢀO(2) = 1.893(4), V(1)ꢀO(3) = 2.327(4), V(1)ꢀO(4) = 1.879(4), V(1)ꢀO(5) =
1.808(4), V(1)ꢀN(1) = 2.233(5); O(4)ꢀV(1)ꢀN(1) = 84.25(17), O(1)ꢀV(1)ꢀO(3) = 179.8(2), O(2)ꢀ
V(1)ꢀO(5) = 92.21(19), O(2)ꢀV(1)ꢀO(4) = 159.53(18), N(1)ꢀV(1)ꢀO(5) = 156.53(19).
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Similar formation of an unusual epoxide adduct was noted (cf., formation of 2‒ECH), and
therefore the combined analytical data clearly show a more general tendency to form
intermediates of type 2−PO when using terminal epoxide substrates with relatively small
substituents.
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In order to further test this hypothesis, crystallization of the sterically more crowded V(V)
complex 1 was carried out in the presence of PO: X−ray diffraction studies revealed that the
crystals corresponded to the starting complex 1 with no additional ligand being present in the
coordination sphere (X‒ray structure determined, see SI).27 This seems to suggest that the
coordination of PO and its subsequent incorporation into the V(V) aminotriphenolate complex as
noted in 2−PO is sterically modulated, but electronic effects cannot be ruled out as the V center
may be involved in activating the epoxide towards intramolecular nucleophilic attack by one of
the phenolate Oꢀatoms. Furthermore, upon using sterically more hindered/demanding epoxides
such as 1,2ꢀepoxyhexane A and trans 2ꢀphenylꢀ3ꢀmethyloxirane D, the expected reactivity order
is restored. The more Lewis acidic complex 2 shows in these cases higher activity upon
combining with NBu4X (X = I, Br; Table 1) than noted for 1 under comparable conditions. These
latter observations thus justify the use of the more Lewis acidic complex 2 in the conversion of
internal, sterically more congested epoxides and the formation of COCs 4‒19 (Figure 2).
Based on our observations and analyses, we propose that the formation of COCs using 2 starts
off with coordination of the epoxide giving rise to a hexaꢀcoordinated V(V) complex (Figure 6).
Such coordination chemistry has been previously observed for various V(V) complexes ligated
by tetradentate ligands in the literature17aꢀb,18b and recently also a H2Oꢀligated V(V)
aminotriphenolate was crystallographically characterized.17a In the presence of terminal
epoxides, the formation of a complex of type 2‒PO can occur with a higher tendency if the
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epoxide substituent is relatively small.28 It should be noted that the epoxideꢀbased structures
themselves (2‒PO and 2‒ECH) do not show any catalytic turnover and require the addition of an
external nucleophile for reactivation. Upon reactivation, complex 2 can reenter the catalytic
cycle in the presence of NBu4X (X = Br, I), suggesting the subsequent formation of V(V)
alkoxide intermediate species with the halide nucleophile X incorporated. Such a species can be
formed through a nucleophilic attack of the bromide onto the arylalkyl ether part of the unusual
intermediate, and allows for a nucleophilic displacement of the original phenolate. It is likely that
this step is turnoverꢀlimiting in those cases where such a stable intermediate (cf., 2‒PO and 2‒
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Figure 6. Proposed mechanistic manifold for the formation of COCs using the binary catalyst
2/NBu4X (X = Br, I).
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ECH) can be formed, and therefore explains the lower relative reactivity for 2/NBu4I in the
conversion of terminal epoxides with smaller substituents. Once the halideꢀcontaining alkoxide
intermediate is formed, the formation of the COC follows a standard CO2 insertion and ringꢀ
closing sequence delivering the final product and regenerating the fiveꢀcoordinated V(V)
complex 2 and NBu4X for further catalytic turnover.
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ꢀ CONCLUSION
In this contribution we show that vanadium (V) complexes derived from aminotriphenolate
ligands form powerful binary catalysts with ammonium salts and allow for the conversion of
generally more challenging internal epoxides and CO2 into their respective COCs with ample
scope in epoxide reaction partners that display functional group diversity. Furthermore, the
screening data and mechanistic control reactions together with the X−ray molecular structures
determined for 2−PO and 2−ECH have revealed a new manifold towards the activation of the
epoxide substrate leading to the formation of a functional model of an alkoxideꢀrelated
(epoxide−activated) intermediate. The complexes 2−PO and 2−ECH can reengage in COC
formation after combining with an ammonium salt. The unprecedented formation of these
unusual compounds is sterically regulated, and has been confirmed to be operative for epoxides
with limited steric impediment (propylene oxide, epichlorohydrin), whereas for sterically more
encumbered substrates it can be assumed that initial coordination of the epoxide to the V(V)
center is followed by epoxide ringꢀopening mediated by an external (halide) nucleophile.
Therefore this work does not only show the high potential of high oxidation state Vꢀcomplexes
as binary catalyst components in COC formation, but the use of a V(V) aminophenolate species
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has also allowed for the first time to identify a unique epoxideꢀactivation mode reminiscent of a
generally proposed intermediate in COC formation following epoxide activation by a Lewis acid
and an external nucleophile.
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AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail for G.L: giulia.licini@unipd.it.
*Eꢀmail for A.W.K.: akleij@iciq.es.
Orchid
Arjan W. Kleij: 0000ꢀ0002ꢀ7402ꢀ4764
Notes
The authors declare no competing financial interest.
ꢀ ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/acscatal. ####.
Characterization data for all the organic carbonates and complexes, and
crystallographic details in cif format (PDF).
ꢀ ACKNOWLEDGMENTS
We thank ICREA, the CERCA Program/Generalitat de Catalunya and the Spanish Ministerio de
Economía y Competitividad (MINECO: CTQꢀ2014ꢀ60419ꢀR and Severo Ochoa Excellence
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Accreditation 2014−2018, SEVꢀ2013ꢀ0319), the Università di Padova (PRATCPDA123307) and
MIUR (Progetto Competitivo CMPT148505) for financial support. J.R. thanks the ICIQ
foundation for a predoctoral fellowship, and C. M. acknowledges the COST action CM1205
(CARISMA) for an STSM fellowship. We also thank Dr. Noemí Cabello for the MS studies.
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(11) For some examples: (a) Lu, X.ꢀB.; Liang, B.; Zhang, Y.ꢀJ.; Tian, Y.ꢀZ.; Wang, Y.ꢀM.;
Bai, C.ꢀX.; Wang, H.; Zhang, R. J. Am. Chem. Soc. 2004, 126, 3732–3733. (b) Paddock, R. L.;
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(12) Decortes, A.; Castilla, A. M.; Kleij, A. W. Angew. Chem. Int. Ed. 2010, 49, 9822ꢀ9837.
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Costa Pessoa, J.; Martins, A. M. Inorg. Chem. 2010, 49, 7452ꢀ7463. For an extensive review on
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(19) (a) Coletti, A.; Whiteoak, C. J.; Conte, V.; Kleij, A. W. ChemCatChem 2012, 4, 1190ꢀ
1196. Recently North and coworkers used V(V) salen complexes as catalysts for the formation of
cyclic carbamates, see: (b) Beattie, C.; North, M. RSC Adv. 2014, 4, 31345ꢀ31352. Note a highly
recent contribution from Kerton and coworkers that describes the use of V(V) bisphenolate
complexes for cyclic carbonate synthesis: (c) Elkurtehi, A. I.; Kerton, F. M. ChemSusChem
2017, DOI: 10.1002/cssc.201601548.
(20) Selected examples: (a) Laserna, V.; Fiorani, G.; Whiteoak, C. J.; Martin, E.; Escuderoꢀ
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J. A.; Lamb, K. J.; North, M. ACS Catal. 2016, 6, 5012–5025. (d) MartínezꢀRodríguez, L.;
Otalora Garmilla, J.; Kleij, A. W. ChemSusChem 2016, 9, 749ꢀ755. (e) Whiteoak, C. J.; Martin,
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(22) Note that typically a bromide based nucleophile such as NBu4Br is used for the
conversion of internal epoxides. For exemplary use of bromide based nucleophiles for these
types of transformations see references 5f, 8e, 9a, 20a and 20d.
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(23) For a highly recent and detailed benchmarking studies involving various Al(III) based
binary catalysts see: Rintjema, J.; Kleij, A. W. ChemSusChem 2017, DOI:
10.1002/cssc.201601712.
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(24) For recent examples of catalytic bioꢀbased cyclic carbonate synthesis see: (a)
Tenhumberg, N.; Büttner, H.; Schäffner, B.; Kruse, D.; Blumenstein, M.; Werner, T. Green
Chem. 2016, 18, 3775ꢀ3788. (b) Fiorani, G.; Stuck, M.; Martín, C.; MartínezꢀBelmonte, M.;
Martin, E.; EscuderoꢀAdán, E. C.; Kleij, A. W. ChemSusChem 2016, 9, 1304ꢀ1311. (c)
Maisonneuve, L.; More, A. S.; Foltran, S.; Alfos, C.; Robert, F.; Landais, Y.; Tassaing, T.; Grau,
E.; Cramail, H. RSC Adv. 2014, 4, 25795ꢀ25803. (d) Langanke, J.; Greiner, L.; Leitner, W.
Green Chem. 2013, 15, 1173ꢀ1182.
(25) Recently a H2Oꢀligated hexaꢀcoordinated V(V) aminotriphenolate complex was
crystallographically and spectroscopically characterized showing that a coordination site is
indeed available for O‒donor ligands, see reference 17a.
(26) Since the 2‒PO complex is sterically more congested than complex 2 itself, we decided to
use NBu4Br rather than NBu4I for further catalytic tests as bromide is a smaller sized
nucleophile. Note that for Al(III) based binary catalysts we found that the use bromide and
iodide based nucleophiles induced similar type of activity in terminal epoxide conversions, see
reference 5f and 21.
(27) The X−ray molecular structure for complex 1 was reported before, see reference 17a.
(28) In order to support the fast formation of 2‒PO from 2 and PO, we carried out a timeꢀ
51
dependent V NMR experiment (50ºC) combining the Vꢀcomplex 2 and a large excess of 200
molar equiv of PO in CD3CN. Despite the fact that the dilution is much higher than reported in
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Table 2 (reactions carried out “neat”), smaller excess of PO was used (200 versus 833 equiv) and
the lack of efficient stirring during the NMR experiment, we found that the formation of 2‒PO is
relatively fast with around 50% of the Vꢀcomplex being present as 2‒PO after 1.5 h. For details
see the SI.
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