Ruthenium indenylidene "1st generation" olefin metathesis catalysts containing triisopropyl phosphite

Article abstract of DOI:10.3762/bjoc.11.166

The reaction of triisopropyl phosphite with phosphine-based indenylidene pre-catalysts affords "1st generation" cis-complexes. These have been used in olefin metathesis reactions. The cis-Ru species exhibit noticeable differences with the trans-Ru parent complexes in terms of structure, thermal stability and reactivity. Experimental data underline the importance of synergistic effects between phosphites and L-type ligands.

Full text of DOI:10.3762/bjoc.11.166

Ruthenium indenylidene “1st generation” olefin metathesis
catalysts containing triisopropyl phosphite
Stefano Guidone, Fady Nahra, Alexandra M. Z. Slawin and Catherine S. J. Cazin*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1520–1527.
EaStCHEM School of Chemistry, University of St Andrews, St
Andrews, UK, KY16 9ST, UK
doi:10.3762/bjoc.11.166
Received: 16 June 2015
Email:
Accepted: 05 August 2015
Published: 01 September 2015
Catherine S. J. Cazin* - cc111@st-andrews.ac.uk
*
Corresponding author
This article is part of the Thematic Series "Progress in metathesis
chemistry II".
Keywords:
st generation; indenylidene; metathesis; phosphite; ruthenium
1
Guest Editor: K. Grela
©
2015 Guidone et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The reaction of triisopropyl phosphite with phosphine-based indenylidene pre-catalysts affords “1st generation” cis-complexes.
These have been used in olefin metathesis reactions. The cis-Ru species exhibit noticeable differences with the trans-Ru parent
complexes in terms of structure, thermal stability and reactivity. Experimental data underline the importance of synergistic effects
between phosphites and L-type ligands.
Introduction
The olefin metathesis reaction is a powerful tool for C–C bond carbene) and two halides. The apex of the pyramid is occupied
formation in the synthesis of highly valuable organic com- by an alkylidene moiety, such as a benzylidene or an indenyl-
pounds [1-4]. Protocols involving W-, Mo- and Ru-based pre- idene. Mixed NHC/phosphine complexes (G-II and Ind-II)
catalysts can shorten or provide alternative synthetic pathways known as “2nd generation” pre-catalysts generally display
for the synthesis of natural products displaying complex chem- higher catalytic activity than “1st generation” complexes (G-I
ical structures [5-9]. Ru-based pre-catalysts are known to be and Ind-I) containing two phosphines [14-23]. The most
more air-, moisture- and functional-group tolerant compared to common phosphine, so called “throw-away ligand”, is tricyclo-
early transition metal complexes [10-13]. In general, the hexyl phosphine [10-23]. In other words, such phosphorus
commonly used Ru(II)-based pre-catalysts have five ligands in donor ligands dissociate from the metal center to afford the
the metal coordination sphere and adopt a distorted square pyra- 14e− active species [10-13,24]. In order to reduce the cost of the
midal geometry (Figure 1).
Ru-based pre-catalyst, our group has investigated the use of
phosphites as an economical alternative to phosphines. The
The basic components of this structure include two L-type reaction of triisopropyl phosphite with the pyridine-containing
ligands mutually trans (e.g., phosphines and N-heterocyclic indenylidene complex [RuCl2(Ind)(SIMes)(py)] (SIMes = N,N’-
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Beilstein J. Org. Chem. 2015, 11, 1520–1527.
Figure 1: Examples of ruthenium complexes used in olefin metathesis reactions.
bis[2,4,6-(trimethyl)phenyl]imidazolidin-2-ylidene) afforded a amount of triisopropyl phosphite. Complex 1 was isolated in
Ru pre-catalyst displaying an unusual cis-geometry [25]. cis- analytically pure form in 85% yield, after recrystallization,
Caz-1, which is more thermodynamically stable than its trans- using a simple ligand exchange reaction (Scheme 1).
isomer represents a breakthrough in catalyst-design for
metathesis reactions of challenging hindered substrates
(
Figure 1) [25-34]. The latent behavior exhibited by cis-Caz-1
can be of interest in fields such as polymer chemistry, where its
thermally-switchable properties can be used to inhibit polymer-
ization during the storage of monomer-catalyst mixtures, and/or
to initiate polymerization on demand through use of a stimulus
[
(
35,36]. The use of this catalyst in the ring-closing metathesis
RCM) reaction gave excellent conversions of challenging
Scheme 1: Synthesis of the mixed phosphine/phosphite complex 1.
substrates, even at low catalyst loadings. The high activity and
robustness of cis-Caz-1 is derived from synergistic effects Similarly to the mixed NHC/phosphite species Caz-1 [25], the
between the σ-donor ligand NHC and the π-acidic triisopropyl cis-geometry is the most thermodynamically stable con-
phosphite [25,37]. Subsequently, the benzylidene analogue formation for the phosphine/phosphite complex 1. The corres-
G-II-P(OiPr)3 was also reported. The latter displayed a typical ponding trans-isomer was not isolated due to the fast isomeriza-
trans-configuration, seen in other Ru pre-catalysts, and gave a tion occurring under the reaction conditions, although traces of
similar catalytic activity to that of the phosphine-containing transient species were detected by 31P-{1H} NMR spec-
parent G-II [26].
troscopy (see Supporting Information File 1, section 4). The
H NMR spectrum of 1 in CD2Cl2 showed the typical indenyl-
Because of the recent interest in “1st generation” complexes idene proton system (characteristic doublet at low field,
9,38-40], previous findings concerning “2nd generation” δH = 8.80 ppm). Coalescence of the aliphatic protons assigned
1
[
complexes [25-33] and the desire to further reduce catalyst cost, to the cyclohexyl and the phosphite moieties was also observed
the aim of this contribution is to replace the phosphine ligands at room temperature. These signals were resolved at a lower
in Ind-I with the less expensive triisopropyl phosphite and to temperature (193 K). The 13C-{1H} NMR spectrum showed a
study the structural and catalytic properties of these new doublet of doublets for the carbene carbon at δC = 290.3 ppm
species.
with two 2JCP of 12.5 and 24.5 Hz (cf., cis-Caz-1; 24.7 Hz)
25]. In the 31P-{1H} NMR spectrum, two doublets at 120.1 and
7.4 ppm with 2JPP of 37.0 Hz were observed, consistent with a
[
Results and Discussion
4
Synthesis of [RuCl2(Ind)(PCy3){P(OiPr)3}] (1) cis-disposition of the phosphorus donor ligands. This geometry
Attempts towards the synthesis of a mixed phosphine/phosphite was confirmed by X-ray diffraction analysis on a single crystal
complex involved the reaction of Ind-I with a stoichiometric (Figure 2).
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Beilstein J. Org. Chem. 2015, 11, 1520–1527.
The 13C-{1H} NMR spectrum contains a doublet of doublets
for the carbene carbon at δC = 291.1 ppm with two similar 2JCP
of 22.0 Hz (cf. cis-Caz-1, 24.7 Hz) [25]. In the 31P-{1H} NMR
spectrum, two singlets at δP = 123.0 ppm and 10.9 ppm corres-
ponding to 2 and 3, respectively, were detected. Fortunately, we
were able to cleanly isolate 3, which allowed its full characteri-
zation and assignment. Crystals suitable for X-ray diffraction
studies were grown for both species. These studies confirmed
the relative cis-disposition of the phosphite ligands in 2 and the
structure of 3 (Figure 3).
Complexes 1 and 2 display a rare distorted cis-square pyra-
midal geometry as observed in the case of cis-Caz-1 [25]. The
cis-geometry differentiates these species from other “1st genera-
tion” complexes that display the more common trans-geometry
[
(
14-17]. Comparing the details of the three structures in Table 1
entries 1 to 3), the Ru–CNHC bond distances are found shorter
than Ru–Pphosphite, and both of them are shorter than
Ru–Pphosphine (Ru(1)–P(2) complex 1) [34].
Figure 2: Molecular structure of mixed phosphine/phosphite complex
. Hydrogen atoms are omitted for clarity.
1
From data listed in Table 1, the Ru–P bond appears stronger in
the case of the Ru–phosphite than the Ru–phosphine scenario,
suggesting the latter as the leaving ligand in catalysis (see
Synthesis of [RuCl2(Ind){P(OiPr)3}2] (2)
The synthesis of the bis-phosphite species 2 was first attempted Supporting Information File 1, section 5).
by the reaction of Ind-I with 2.5 equivalents of P(OiPr)3. Full
Catalytic activity in ring-closing metathesis
conversion of the starting material was observed affording com-
plex 2 (Scheme 2). Unfortunately, all attempts to purify 2 failed (RCM)
due to the presence of PCy3 decomposition products. The PPh3 The reactivity of the mixed phosphine/phosphite complex 1 was
adduct Ind-I0 was then employed as alternative starting ma- first evaluated in the RCM of the easily cyclized diethyl diallyl-
terial for the ligand substitution reaction with the phosphite malonate (4) (see Supporting Information File 1, section 2). The
(
Scheme 2) (see Supporting Information File 1, section 4). need for thermal activation for this pre-catalyst was clearly
During the recrystallization from dichloromethane/pentane, revealed by the low catalytic activity at 30–50 °C and the high
compound 3 was detected as a decomposition product [41]. conversion observed at 80 °C in toluene (0.1 mol % of 1,
9
4% conv.). Contrary to 1, the phosphine-based Ind-I initiates
Due to the high solubility of this species and difficulties en- at 30 °C exhibiting good catalytic activity and undergoes fast
countered in the purification process, product 2 was isolated decomposition at higher temperature with moderate conversion
with traces of compound 3 still present. In the 1H NMR spec- (see Supporting Information File 1, section 2). This trend was
trum of 2 in CD2Cl2 (with 3 present), the characteristic doublet further studied by profiling reactions under catalytic conditions
at δH = 8.53 ppm for the indenylidene system was observed. (Figure 4).
Scheme 2: Synthesis of the bis-phosphite complex 2.
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Beilstein J. Org. Chem. 2015, 11, 1520–1527.
Figure 3: Molecular structure of 2 and the ylide 3. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond distances (Å) and
angles (°) (ESD) for compound 3: P(1)–C(1), 1.713(5); P(1)–C(28), 1.795(5); P(1)–C(22), 1.805(5); P(1)–C(16), 1.811(5); C(1)–P(1)–C(28), 106.2(2);
C(1)–P(1)-C(22), 113.1(3); C(28)–P(1)–C(22), 109.0(2); C(1)–P(1)–C(16), 113.3(2); C(28)–P(1)–C(16), 109.2(2); C(22)–P(1)–C(16), 105.9(2).
Table 1: Selected bond distances (Å) and angles (°) for 1, 2 and cis-Caz-1.
Entry
Parameter
1
2
cis-Caz-1[25]
1
2
3
4
5
6
7
8
9
Ru(1)–C(1)
Ru(1)–P(1)
1.873(14)
2.239(4)
2.387(4)
–
1.869(3)
2.2300(9)
2.2663(8)
–
1.881(8)
2.249(2)
–
Ru(1)–P(2)
Ru(1)–C(NHC)
Ru(1)–Cl(1)
2.067(7)
2.4036(18)
2.3974(19)
–
2.398(4)
2.369(3)
98.74(13)
–
2.3999(9)
2.3789(8)
97.34(3)
–
Ru(1)–Cl(2)
P(1)–Ru(1)–P(2)
C(NHC)–Ru(1)–P(1)
C(1)–Ru(1)–P(1)
C(1)–Ru(1)–P(2)
C(1)–Ru(1)–C(NHC)
100.06(19)
90.5(2)
–
90.2(5)
94.3(4)
–
90.94(10)
86.41(9)
–
10
1
1
98.7(3)
An induction period was observed for 1 at the early stage of the effects in the case of the mixed phosphine/phosphite system.
catalysis, a behavior similar to cis-Caz-1 [25], followed by a Complex 1 is a thermally-switchable, latent pre-catalyst
fast reaction with full conversion of the substrate at 80 °C in displaying higher thermal stability compared to the phosphine-
less than 50 min. These features prompted us to hypothesize an based Ind-I.
isomerization step from the cis-pre-catalyst 1 to the corres-
ponding trans isomer as reported for cis-Caz-1 (see Supporting Consequently, a brief study of the scope of the reaction was
Information File 1, section 2) [25]. Under the same conditions, investigated employing “1st generation” complexes 1 and Ind-I
instant pre-catalyst initiation and fast decomposition of the (Table 2).
active species were observed for the phosphine-based pre-cata-
lyst Ind-I (86% conversion after 30 min). When the experi- The diene 6 was poorly converted by mixed PCy3/P(OR)3 com-
ment was performed at 30 °C, Ind-I exhibited slower conver- plex 1, whereas 94% conversion was obtained with Ind-I
sion of the substrate, reaching complete conversion after 4 h (Table 2, entries 1–4). In the case of tri-substituted diene 8, a
[
22]. The reaction profiles show the importance of synergistic more challenging substrate compared to 6, pre-catalyst 1 gave
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Beilstein J. Org. Chem. 2015, 11, 1520–1527.
loading (Table 2, entry 14). A higher conversion of compound
1
2 was detected with Ind-I (98%, Table 2, entry 15).
Conclusion
The influence of triisopropyl phosphite in Ru-based indenyl-
idene “1st generation” complexes has been presented. The
mixed phosphine/phosphite complex 1 and the bis-phosphite
complex 2 adopt distorted square pyramidal geometries with the
P-donor ligands mutually cis as the most thermodynamically
stable conformation. The isolation of the corresponding trans-
isomers was not possible due to a fast isomerization process
occurring during the synthesis of the complexes. Pre-catalyst 1
was found to be active in olefin metathesis reaction showing
similarities with cis-Caz-1 in terms of reactivity. Both pre-cata-
lysts need thermal activation; they display an induction period
in the reaction profiling and exhibit higher thermal stability
compared to their phosphine-based analogues. In terms of
Figure 4: Reaction profiles of mixed phosphine/phosphite 1 and phos-
phine-based Ind-I in the RCM of 4 (lines are visual aids and not curve
fits).
7
8
9% conversion while Ind-I converted 33% of the substrate at catalytic efficiency, Ind-I was found more active than 1 unless
0 °C and 53% at 30 °C (Table 2, entries 5–7). The tosylamide higher thermal stability is needed. Indeed, in the case of the
derivative 10 was converted into product 11 by 1 (1 mol %) malonate derivative 8, pre-catalyst 1 afforded the tri-substituted
with 41% conversion (Table 2, entry 8). When the loading was ring-closed product in 71% isolated yield. The similar struc-
increased to 2 mol %, no improvement in the conversion was tural and catalytic properties observed in the mixed phosphine/
detected (Table 2, entry 9). A higher catalytic activity was phosphite complex 1 and the mixed NHC/phosphite cis-Caz-1
observed for Ind-I with 77% conversion when using 2 mol % suggest the importance of synergistic effects involving phos-
pre-catalyst at 30 °C (Table 2, entry 13). Complex 1 was active phites, an inexpensive alternative to phosphines for Ru-based
in the ring-closing enyne metathesis (RCEYM) with 78% pre-catalysts, and the L-type ligands, a concept that can be used
conversion of substrate 12 obtained with 1 mol % catalyst to incite further improvements in catalyst design.
Table 2: Scope of the reaction employing 1 and Ind-I.a
Entry
Substrate
Product
Pre-catalyst (mol %)
T (°C)
Conv. (%)b
1
2
3
4
1 (0.1)
1 (1)
Ind-I (0.1)
Ind-I (0.1)
80
80
80
30
<1
4
94
94
5
6
7
1 (1)
Ind-I (1)
Ind-I (1)
80
80
30
79 (71)
33
53
8
9
1 (1)
1 (2)
Ind-I (1)
Ind-I (1)
Ind-I (2)
Ind-I (2)
80
80
80
30
80
30
41
41
22
21
30
77
10
11
12
13
1
1
4
5
1 (1)
Ind-I (1)
80
30
78
98
aReaction conditions: substrate (0.25 mmol), pre-catalyst (0.1 to 2 mol %), toluene (0.5 mL), 19 h. bConversions were determined by GC analysis.
Isolated yields in parentheses.
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Beilstein J. Org. Chem. 2015, 11, 1520–1527.
Experimental
7.40 (dd, 3JHH = 7.1 Hz, 3JHH = 7.1 Hz, 1H, H5), 7.43 (dd,
o f 3JHH = 7.4 Hz, 3JHH = 7.4 Hz, 1H, H6), 7.46 (dd, 3JHH = 7.4
S y n t h e s i s
a n d
c h a r a c t e r i z a t i o n
[
RuCl2(Ind)(PCy3){P(OiPr)3}] (1): Under an inert atmos- Hz, 3JHH = 7.4 Hz, 2H, H10), 7.53 (dd, 3JHH = 7.4 Hz, 3JHH =
phere of argon, triisopropyl phosphite (364 μL, 1.53 mmol) was 7.4 Hz, 1H, H11), 7.77 (d, 3JHH = 7.3 Hz, 2H, H9), 8.53 (d,
added to a solution of Ind-I (1.414 g, 1.53 mmol) in dichloro- 3JHH = 7.0 Hz, 1H, H7) ppm; 13C-{1H} NMR of the mixture
methane (20 mL). The mixture was stirred for 24 h at room (100.6 MHz, CD2Cl2) δ 24.0 (s, CH-CH3), 24.4 (s, CH-CH3),
temperature and the solvent was then removed in vacuo. The 71.5 (m, CH-CH3), 119.0 (s, C4), 127.1 (s, C9), 129.6 (s, C11),
crude product was recrystallized twice from dichloromethane/ 130.0 (s, C10), 130.2 (s, C6), 130.3 (s, C7), 131. 2 (s, C5), 134.8
pentane. The solid was collected by filtration and washed with (s, C8), 136.8 (s, C3), 140.3 (s, C3a), 140.7 (dd, 3JCP = 8.5 Hz,
pentane (3 × 10, 2 × 15 mL). The product was obtained as a C2) 150.2 (s, C7a), 291.1 (dd, 2JCP = 22.0 Hz, 2JCP = 22.0 Hz,
brownish red solid (1.116 g, 85%). During NMR experiments, C1) ppm; 31P-{1H} NMR (162 MHz, CD2Cl2) δ 123.0 (s,
peaks originating from the decomposition of the PCy3 were P(OiPr)3) ppm; CCDC-889639 contains the supplementary
observed. 1H NMR of the mixture: (400 MHz, CD2Cl2) δ crystallographic data for 2. These data can be obtained free of
1
.08–1.33 (m, 6H, PCy3), 1.11 (d, 3JHH = 6.3 Hz, 9H, charge from the Cambridge Crystallographic Data Centre via
CH-CH3), 1.30 (d, 3JHH = 6.3 Hz, 9H, CH-CH3), 1.40–1.55 (m, http://www.ccdc.cam.ac.uk/data_request/cif.
9
4
H, PCy3), 1.60–1.85 (m, 15H, PCy3), 2.50 (m, 3H, CH PCy3),
.55 (m, 3H, CH-CH3), 6.79 (s, 1H, H2), 7.27 (d, 3JHH = 7.1 Characterization of Ph3P(Ind) (3): Under an inert atmos-
Hz, 1H, H4), 7.43 (dd, 3JHH = 6.7 Hz, 3JHH = 6.3 Hz, 1H, H5), phere of argon, compound 3 was obtained as a side product
.44 (dd, 3JHH = 7.4 Hz, 3JHH = 6.3 Hz, 2H, H10), 7.50 (dd, from the reaction of Ind-I0 (0.303 mg, 0.34 mmol, 1 equiv)
JHH = 7.4 Hz, 3JHH = 7.7 Hz, 1H, H11), 7.53 (dd, 3JHH = 7.4 with triisopropyl phosphite (179 μL, 0.75 mmol, 2.2 equiv) for
Hz, 3JHH = 7.4 Hz, 1H, H6), 7.76 (d, 3JHH = 7.3 Hz, 2H, H9), 24 h. During recrystallization in a dichloromethane/pentane
.80 (d, 3JHH = 7.3 Hz, 1H, H7) ppm; 13C-{1H} NMR of the mixture, compound 3 was isolated as a yellow solid (63.4 mg,
mixture (100.6 MHz, CD2Cl2) δ 24.3 (s, CH-CH3), 24.5 (d, 41%). 1H NMR (400 MHz; CD2Cl2) δ 6.73 (d, 2JHP = 4.9 Hz,
JCP = 4 Hz, CH-CH3), 26.9 (s, CH2 PCy3), 28.1 (d, 2JCP = 11 1H, H2), 6.77 (dd, 3JHH = 7.5 Hz, 3JHH = 7.5 Hz, 1H, H6), 6.89
7
3
8
3
Hz, CH2 PCy3), 28.4 (d, 2JCP = 10 Hz, CH2 PCy3), 30.1 (s, (d, 3JHH = 7.9 Hz, 1H, H7), 6.97 (dd, 3JHH = 7.5 Hz, 3JHH = 7.5
CH2 PCy3), 30.6 (s, CH2 PCy3), 35.3 (s, CH PCy3), 71.4 (s, Hz, 1H, H5), 7.08 (dd, 3JHH = 7.4 Hz, 3JHH = 7.4 Hz, 1H, H11),
CH-CH3), 118.4 (s, C4), 127.0 (s, C9), 129.6 (s, C10), 129.7 (s, 7.32 (dd, 3JHH = 7.6 Hz, 3JHH = 7.6 Hz, 2H, H10), 7.52–7.60
C11), 130.2 (s, C7), 130.3 (s, C6), 130. 7 (s, C5), 135.1 (s, C8), (m, 6H, C6H5), 7.61 (d, 3JHH = 7.9 Hz, 2H, H9), 7.66–7.75 (m,
1
1
2
36.5 (s, C3), 140.6 (s, C3a), 141.3 (dd, 3JCP = 5.0 Hz, 3JCP = 9H, C6H5), 7.96 (d, 3JHH = 8.1 Hz, 1H, H4) ppm; 13C-{1H}
4.0 Hz, C2) 147.8 (s, C7a), 290.3 (dd, 3JCP = 12.5 Hz, 2JCP = NMR (100.6 MHz; CD2Cl2) δ 68.5 (d, 1JCP = 121.6 Hz, C1),
4.5 Hz, C1) ppm; 31P-{1H} NMR of the mixture (162 MHz, 118.2 (s, C5), 118.5 (s, C6), 118.7 (s, C7), 119.4 (s, C4), 121.2
CD2Cl2) δ 120.1 (d, 2JPP = 37.0 Hz, PCy3), 47.4 (d, 2JPP = 37.0 (d, 3JCP = 15.8 Hz, C3), 123.8 (s, C11), 125.6 (d, 1JCP = 90.3
Hz, P(OiPr)3) ppm; anal. calcd for C42H64Cl2O3P2Ru: C, 59.29; Hz, i-C6H5), 127.1 (s, C9), 127.8 (d, 2JCP = 16.5 Hz, C2), 128.7
H, 7.58; found: C, 59.45; H, 7.66. CCDC-889638 contains the (s, C10), 129.5 (d, 3JCP = 12.3 Hz, m-C6H5), 133.3 (d, 4JCP =
supplementary crystallographic data for 1. These data can be 2.7 Hz, p-C6H5), 134.2 (d, 2JCP = 10.3 Hz, o-C6H5), 135.0 (d,
obtained free of charge from the Cambridge Crystallographic 3JCP = 13.9 Hz, C3a), 137.1 (d, 2JCP = 13.2 Hz, C7a), 140.5 (s,
Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
C8) ppm; 31P-{1H} NMR (162 MHz; CD2Cl2) δ 10.9 (s,
Ph3P(Ind)) ppm; HMRS (APCI): m/z calcd for [C33H25P + H]
Synthesis and characterization of [RuCl2(Ind){P(OiPr)3}2] 453.18; found 453.1754. CCDC-889640 contains the supple-
(
(
2): Under an inert atmosphere of argon, triisopropyl phosphite mentary crystallographic data for 3. These data can be obtained
65 μL, 0.28 mmol) was added to a solution of Ind-I0 (0.100 g, free of charge from the Cambridge Crystallographic Data
0
.113 mmol) in dichloromethane (1.2 mL). The mixture was Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
stirred for 24 h at room temperature and the solvent was
removed in vacuo. The crude product was recrystallized from General procedure for catalysis: Substrates 6 [42], 8 [43], 10
dichloromethane/pentane. The solid was collected by filtration [42], 12 [22], were synthesized following previous reports in
and washed with pentane (3 × 3 mL). The product was obtained the literature. Compound 4 was obtained from a commercial
as a brownish green solid in a mixture with the phosphonium source and its purity confirmed prior to use. 1H NMR data of
ylide 3 (0.032 g, 35%). 1H NMR of the mixture (400 MHz, product 9 were compared to previously reported analyses [22].
CD2Cl2) δ 1.13 (d, 3JHH = 6.3 Hz, 18H, CH-CH3), 1.31 (d,
3
JHH = 6.1 Hz, 18H, CH-CH3), 4.53 (m, 3JHH = 6.3 Hz, 6H, A 5 mL screwcap-vial fitted with a septum equipped with a
CH-CH3), 7.18 (s, 1H, H2), 7.26 (d, 3JHH = 6.9 Hz, 1H, H4), magnetic stirring bar was charged with the olefin (0.25 mmol)
1525

Beilstein J. Org. Chem. 2015, 11, 1520–1527.
then purged with nitrogen, closed and introduced in a glovebox. trometry Service Centre Swansea University for HMRS
The solvent and pre-catalyst (stock solution for <1 mol %, or analyses.
weighed in the vial) were added to the reaction mixture (total
amount of solvent 0.5 mL). Once out of the glovebox, the reac- References
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