Evaluation of Several Molybdenum and Ruthenium Catalysts for the Metathesis Homocoupling of 3-Methyl-1-Butene

Article abstract of DOI:10.1002/hlca.201700181

We synthesized Mo(NC₆F₅)(CHCMe₂Ph)(TPPO)(PPhMe₂)Cl (TPPO = 2,3,5,6-tetraphenylphenoxide), Mo(NC₆F₅)(CHCMe₂Ph)(TTBTO)(PPhMe₂)Cl (TTBTO = 2,6-di(3′,5′-di-tert-butylphenyl)phenoxide), and Mo(NC₆F₅)(CHCMe₂Ph)(TPPO)(PPhMe₂)(CF₃Pyr) (CF₃Pyr = 3,4-bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3-methyl-1-butene. They were compared with Mo(NC₆F₅)(CHCMe₂Ph)(HMTO)(PPhMe₂)Cl (HMTO = 2,6-dimesitylphenoxide), Mo(NC₆F₅)(CHCMe₂Ph)(HIPTO)(PPhMe₂)Cl (HIPTO = 2,6-di(2′,4′,6′-triisopropylphenyl)phenoxide), and several other Mo and Ru catalysts. In the best cases turnover numbers (TONs) of 400 – 700 were observed for the homocoupling of 3-methyl-1-butene in a closed vessel (ethylene not removed).

Full text of DOI:10.1002/hlca.201700181

(1 of 7) e1700181
Evaluation of Several Molybdenum and Ruthenium Catalysts for the
Metathesis Homocoupling of 3-Methyl-1-Butene
Peter E. Sues, Konstantin V. Bukhryakov, and Richard R. Schrock*
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
e-mail: rrs@mit.edu
We synthesized Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)Cl (TPPO = 2,3,5,6-tetraphenylphenoxide), Mo(NC6F5)(CHCMe2Ph)
(TTBTO)(PPhMe2)Cl (TTBTO = 2,6-di(30,50-di-tert-butylphenyl)phenoxide), and Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)
(CF3Pyr) (CF3Pyr = 3,4-bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3-
methyl-1-butene. They were compared with Mo(NC6F5)(CHCMe2Ph)(HMTO)(PPhMe2)Cl (HMTO = 2,6-dimesityl-
phenoxide), Mo(NC6F5)(CHCMe2Ph)(HIPTO)(PPhMe2)Cl (HIPTO = 2,6-di(20,40,60-triisopropylphenyl)phenoxide), and
several other Mo and Ru catalysts. In the best cases turnover numbers (TONs) of 400 – 700 were observed for the
homocoupling of 3-methyl-1-butene in a closed vessel (ethylene not removed).
Keywords: Olefin metathesis, Homocoupling, Molybdenum, Ruthenium, 3-Methyl-1-butene.
those obtained with several Ru-based catalysts. The
results of this investigation are reported here.
Introduction
Terephthalic acid is produced on a multi-million ton
scale each year through the metal-catalyzed oxidation
Results and Discussion
of p-xylene with oxygen.[1 – 5] Para-xylene is obtained
through naphtha reforming, although it must be The
reactions
between
Mo(NC6F5)(CHCMe2Ph)-
[26]
extracted from a complex mixture and purified.[6 – 10] (PPhMe2)2Cl2
and LiOTPP or LiOTTBT gave 1(TPPO)
More efficient methods of preparing p-xylene are being and 1(TTBTO) as yellow powders in good yields (72%
sought today.[11 – 15] One possibility is the dehydro- for each; Scheme 2). This method is the same as that
genative aromatization of 2,5-dimethyl-3-hexene with used to prepare 1(HMTO) and 1(HIPTO).[26] 1(TPPO)
an iridium pincer catalyst (Scheme 1).[16 – 20] Homo- and 1(TTBTO) were isolated as five-coordinate phos-
coupling of 3-methyl-1-butene to give 2,5-dimethyl-3- phine adducts in which the alkylidenes are in the syn
hexene and ethylene is challenging for steric reasons, configuration according to JCH for the alkylidene
especially when 2,5-dimethyl-3-hexene is the cis isomer. proton whose NMR resonances are found at
Certain ruthenium-based catalysts have shown poten-
tial for metathesis-based reactions of sterically demand-
ing substrates[21 – 25] and molybdenum-based imido
monoaryloxide chloride complexes can be highly active
for cross-metathesis reactions in which 1,2-dichloro-
ethylene or (CF3)HC=CH(CF3) is one of the
olefins.[26 – 28] Therefore, we decided to explore several
Mo monochloride catalysts for the homocoupling of
3-methyl-1-butene and to compare the results with
Scheme 1. Dehydrogenative aromatization of 2,5-dimethyl-3-
hexene.
Scheme 2. Preparation of 1(TPPO), 1(TTBTO), 1(HMTO), and 1(HIPTO).
DOI: 10.1002/hlca.201700181 Helv. Chim. Acta 2017, 100, e1700181
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Helv. Chim. Acta 2017, 100, e1700181
11.85 ppm (JCH = 124 Hz for 1(TPPO)) and 13.10 ppm Grubbs
catalyst
(dichloro(benzylidene)bis(tricyclo-
(JCH = 125 Hz for 1(TTBTO)). The alkylidene ligand hexylphosphine)ruthenium(II)), Ru-2 is the second
is in the axial position of what is essentially a generation Grubbs catalyst (dichloro[1,3-bis(2,4,6-tri-
square pyramid and the donor ligand is trans to the methylphenyl)-2-imidazolidinylidene](benzylidene)(tri-
chloride.
cyclohexylphosphine)ruthenium(II)), and Ru-3 is the
In order to evaluate the efficiency of a monoaryl- Stewart–Grubbs catalyst (dichloro[1,3-bis(2-methylphen-
oxide pyrrolide complex with an electron-withdrawing yl)-2-imidazolidinylidene](2-isopropoxyphenylmethylene)-
pyrrolide, we decided to prepare a 3,4-bis(trifluoro- ruthenium(II)).[21 – 25] Of the three ruthenium catalysts
methyl)-1-pyrrolide (CF3Pyr) complex. The trifluoro- that were tested, Ru-3 gave the best conversion
methyl group is an attractive electron-withdrawing (91%) and predominantly trans product (98:2
group in view of the fact that molybdenum complexes E/Z). In the absence of added B(C6F5)3 1(HIPTO) gave
that contain the cyanopyrrolide ligand have structures no product in 24 h, which suggests that although
that can be complicated by binding of the cyano group PPhMe2 dissociates from the metal in 1(OR) com-
to the metal.[29] 3,4-Bis(trifluoromethyl)-1H-pyrrole plexes,[26] a significant fraction, and preferably, all
(CF3PyrH) was synthesized according to a published phosphine must be removed in order to achieve full
protocol[30] with some changes (see Experimental Sec- activity. Acetonitrile is labile in complex 2, but
tion for details) and converted into the potassium salt B(C6F5)3 is again required in order to scavenge the
CF3PyrK through a reaction with potassium hexam- acetonitrile and expose the 14e core of the initiator in
ethyldisilazane. Unlike the potassium salt, the Li and Na high yield. The configuration of the homocoupled pro-
salts do not appear to be stable at room temperature. duct was predominantly (E) (>75%) in most Mo-cata-
To our knowledge 6 is the first example of a metal com- lyzed reactions, presumably as a consequence of
plex that contains the CF3Pyr ligand.
The results of the catalyst screen (1 mol-% catalyst; required metallacyclobutane intermediate.
see Fig. 1) for the homocoupling of 3-methyl-1-butene Larger scale reactions with 0.1% catalyst loadings
are summarized in Table 1. Ru-1 is the first generation were carried out in closed systems (4.5M in benzene).
steric hindrance between two isopropyl groups in the
Figure 1. Mo catalysts tested for the homocoupling of 3-methyl-1-butene.
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Table 1. Catalyst Screen (1 mol-%) for the Homocoupling of
A reviewer raised the possibility that a cationic
complex could form in the case of the 16e complexes
1, 2, and 6, as was found for Mo complexes recently
when an NHC ligand is the 2e donor and a triflate is
one of the anionic ligands in a 5-coordinate com-
plex.[31 – 34] For more than three decades in the litera-
ture it has been shown that only 14e complexes
(usually bisalkoxides) formed through dissociation of a
two electron donor from a 16e complex, are active in
metathesis. Recently, this point has been extensively
explored through calculations.[35] Active catalysts are
generated when B(C6F5)3 is added to 1, 2, and 6
because the 2e donor is removed to give a donor/
B(C6F5)3 complex. At this point, there is no evidence
that the chloride (or pyrrolide in the case of 6) is
removed instead the 2e donor, at least under the con-
ditions employed so far.
3-methyl-1-butenea
Catalyst
% Conversion
(E/Z) ratio
TON
Ru-1
Ru-2
54
76
91
84
84
82
83
28
71
89
90
93
88
>99:1
96:4
98:2
74:26
79:21
93:7
54
76
91
84
84
82
83
28
71
89
90
93
88
Ru-3
1(HIPTO)b
1(HMTO)b
1(TTBTO)b
1(TPPO)b
93:7
2
47:53
38:62
97:3
>99:1
99:1
2b
3
4
5
6b
87:13
a
4M 3-methyl-1-butene in 50 ll C6D6 in a sealed vial. Conver-
sions and selectivities were determined by 1H-NMR spec-
troscopy after 24 h at ca. 22 °C. + 1.15 equiv. B(C6F5)3.
b
Conclusions
Two new Mo(NC6F5) monoaryloxide chloride com-
plexes and a new MAP complex that contains the
electron-withdrawing pyrrolide ligand, CF3Pyr, were
isolated, characterized, and explored as catalysts,
along with several other Mo and Ru catalysts, for
the homocoupling of 3-methyl-1-butene. The most
successful Mo catalysts were B(C6F5)3-activated
1(TPPO), 1(TTBTO), and 6. The results for the Mo
catalysts again clearly illustrate the advantage of
increasing the electrophilicity of the metal. The Ste-
wart–Grubbs catalyst (Ru-3), in which the steric
demands of the NHC ligand are significantly lower
than in a typical mesityl-substituted NHC, gave the
highest yield and TON. None of the catalysts
showed more than a modest bias toward formation
of cis-2,5-dimethyl-1-hexene, presumably for steric
reasons. The yields of 2,5-dimethyl-3-hexene are
likely to be limited by decomposition of methyli-
dene intermediates because ethylene is not removed
as it is formed. Traces of impurities in the substrate
(e.g., peroxides) can also limit TON. Therefore the
results shown in Table 2 should be considered only
as a guide for further optimization of this homocou-
pling reaction.
The results are shown in Table 2, where TON is calcu-
lated at the point where conversion stops increasing.
The reaction profiles are shown in the Supporting
Information. Almost an order of magnitude increase in
TON could be achieved in many cases, albeit with
lower overall conversions (between 40 and 60%). 1
(TPPO) and 1(TTBTO), which contain the least steri-
cally demanding ligands, gave the highest conversions
and TONs. The Stewart-Grubbs catalyst (Ru-3) gave
the highest conversions and TONs of all the catalysts
investigated. The similar performances of 1(TPPO)
and 6 suggest that at least in this circumstance the
CF3Pyr ligand (in 6) and chloride (in 1(TPPO)) are
approximately equally effective electron-withdrawing
ligands.
Table 2. Homocoupling of 3-methyl-1-butene with 0.1% cata-
lyst loadinga
Catatalyst
Time [h]
% Conversion
(E/Z) ratio
TON
Ru-3
2
9
3
3
3
3
2
1
2
2
3
70
60
62
20
43
45
2.9
12
28
12
45
95:5
95:5
95:5
60:40
96:4
95:5
67:33
92:8
>99:1
>99:1
95:5
700
600
620
200
430
450
29
120
280
110
450
1(TPPO)b
1(TTBTO)b
1(HMTO)b
1(HIPTO)b
2b
2
Experimental Section
3
4
General Procedures
5
All procedures and manipulations were performed
under a nitrogen atmosphere using standard Schlenk-
line and glovebox techniques unless stated otherwise.
All glassware was oven-dried or flame-dried prior to
use unless stated otherwise. Ether, pentane, toluene,
6b
a
Approx. 4.5M 3-methyl-1-butene in 2.85 ml benzene in a
sealed 100 ml flask at ca. 22 °C. Conversions and selectivities
b
determined by 1H-NMR spectroscopy. + 1.15 equiv. B(C6F5)3.
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dichloromethane, toluene, and benzene were (t, J = 21.2); ꢀ162.19 (dt, J = 21.2, 4.2). 13C-NMR
degassed with nitrogen and passed through activated (100 MHz, CDCl3): 316.38 (br. s, Mo=CH); 161.02 (s);
alumina columns under nitrogen unless stated other- 149.60 (s); 149.58 (s); 149.60 (s); 142.10 (s); 141.83 (s);
wise. All dried and deoxygenated solvents were stored 141.48 (s); 141.20 (s); 141.01 (s); 140.93 (s); 139.78 (s);
over molecular sieves in a nitrogen-filled glovebox. 139.04 (s); 135.77 (s); 132.34 (s); 132.07 (s); 131.80 (s);
Reagents were purchased from commercial sources 131.21 (m); 131.10 (s); 130.36 (s); 130.28 (s); 130.15 (m,
and used without further purification unless stated CF); 129.95 (s); 129.83 (s); 129.76 (s); 128.60 (s); 128.53
otherwise. 3-Methyl-1-hexene was dried over sodium (s); 128.47 (s); 128.33 (s); 128.04 (s); 127.90 (m, CF);
or Na/K alloy. Ruthenium catalysts were purchased 127.69 (s); 127.33 (s); 127.21 (s); 126.60 (s); 126.50 (s);
from commercial sources and used without further 126.38 (s); 126.36 (s); 126.08 (s, CF); 125.84 (s); 125.67
purification. Complexes 1(HMTO),[26] 1(HIPTO),[26] (s); 123.80 (s); 54.10 (s, CMe2); 30.00 (s, Me); 29.72 (s,
2,[26] 3,[36] 4,[37] and 5[38] were synthesized according Me); 15.29 (s, PMe); 14.07 (s, PMe). Anal. calc. for
to literature procedures. Ligands HIPTO,[39] HMTO,[40] C54H44ClF5MoNOP: C 66.16, H 4.52, N 1.43; found: C
TTBTO,[41] and TPPO[42] were synthesized according 65.91, H 4.41, N 1.32.
to literature procedures. Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories. They were
degassed and dried over activated molecular sieves
Synthesis of 1(TTBTO)
prior to use. NMR Spectra were recorded on a Bruker A solution of 0.146 g LiOTTBT (0.263 mmol) in 5 ml
400 MHz spectrometer or a Varian 500 MHz or 300 MHz benzene added to 0.200 g Mo(C6F5)(CHCMe2Ph)-
1
spectrometer operating at 20 °C. The H- and 13C-NMR (PPhMe2)2Cl2 (1 equiv., 0.263 mmol). The mixture was
spectra were measured relative to partially deuterated stirred for 2 h and filtered through Celite. The solvent
solvent peaks, but are reported relative to tetramethyl- was removed from the filtrate under reduced pressure
silane. All 19F chemical shifts were measured relative to to give an orange residue. The orange residue was
fluorobenzene (ꢀ113.15 ppm) as an external reference stirred in 5 ml pentane for several hours, and the
and all 31P chemical shifts were measured relative to resulting orange precipitate was isolated by filtration,
diphenylphosphine as an external reference. Elemental washed with pentane, and dried under reduced pres-
1
analyses were performed at the CENTC Elemental Anal- sure to give the product; yield 0.198 g (72%). H-NMR
ysis Facility at the University of Rochester.
(500 MHz, C6D6): 13.10 (d, JPH = 4.6, JCH = 125,
Mo=CH); 8.22 – 7.61 (m, 3 H); 7.54 (s, 1 H); 7.51 (s, 1
H); 7.29 (dd, J = 7.5, 1.5, 1 H); 7.27 (dd, J = 7.5, 1.5, 1
H); 7.19 (d, J = 7.7, 2 H); 7.10 (t, J = 7.7, 2 H);
Synthesis of 1(TPPO)
A solution of 0.126 g LiOTPP (0.263 mmol) in 5 ml 6.96 – 6.82 (m, 8 H); 1.42 – 1.31 (m, 14 Me); 1.03 (d,
benzene was prepared and added to 0.200 g J = 9.2, PMe); 0.95 (d, J = 9.2, PMe). 31P{1H}-NMR
Mo(NC6F5)(CHCMe2Ph)(PPhMe2)2Cl2 (0.263 mmol). The (161 MHz, C6D6): 52.16 (br. s). 19F-NMR (300 MHz,
solution was stirred for 6 h. The thick slurry was fil- C6D6): ꢀ143.57 (d, J = 22.7, 4.2); ꢀ158.21 (t, J = 21.6);
tered and the solid was rinsed with 2 ml benzene and ꢀ163.39 (~t). 13C-NMR (100 MHz, C6D6): 316.14 (br. s,
dried in vacuo to give a bright yellow powder. The Mo=CH); 159.39 (s); 150.94 (s); 150.77 (s); 147.78 (s);
yellow powder was stirred in 5 ml pentane for several 147.76 (s); 144.67 (br. s); 142.65 (m, CF); 141.05 (s);
hours and the yellow precipitate was filtered off. The 140.68 (s); 138.53 (m, CF); 136.56 (m, CF); 135.11 (s);
solvents were also removed from the filtrate to give a 134.98 (s); 134.66 (s); 134.14 (s); 131.80 (s); 131.38 (s);
brown residue. The brown residue was stirred in 1 ml 130.27 (s); 130.19 (s); 128.61 (s); 128.54 (s); 128.47 (s);
toluene and 4 ml pentane for several hours. The 127.51 (s); 126.42 (s); 121.80 (s); 120.77 (s); 120.29 (s);
bright yellow precipitate was isolated by filtration, 54.40 (d, J = 2.0, CMe2); 35.20 (s, CMe3); 35.13 (s,
washed with pentane, and dried in vacuo. The two CMe3); 31.91 (s, CMe3); 31.70 (s, CMe3); 31.45 (s, Me);
yellow precipitates were combined; yield 0.184 g 28.41 (s, Me); 13.96 (d, J = 25.9, PMe); 13.74 (d,
1
(72%): H-NMR (500 MHz, CDCl3): 11.85 (d, JPH = 4.8, J = 22.8, PMe). Anal. calc. for C58H68ClF5MoNOP: C
JCH = 124, Mo=CH); 8.16 (d, J = 7.6, 1 H); 7.29 (t, 1 H); 66.18, H 6.51, N 1.33; found: C 65.94, H 6.32, N 1.26.
7.22 – 6.93 (m, 20 H); 6.92 (s, 1 H); 6.82 (d, J = 7.4, 1
H); 6.80 (d, J = 7.9, 1 H); 6.79 – 6.70 (m, 3 H); 6.52 (d,
Synthesis of CF3PyrK
J = 7.5, 1 H); 6.44 (t, J = 7.7, 1 H); 1.28 (s, Me); 1.24 (s,
Me); 1.01 (d, J = 9.4, PMe); 0.81 (d, J = 9.4, PMe). 31P 3,4-Bis(trifluoromethyl)-1H-pyrrole
was
{1H}-NMR (161 MHz, CDCl3): 5.12 (br. s, Mo-P). 19F- according to the published protocol[30] with some
NMR (300 MHz, CDCl3): ꢀ144.05 (d, J = 17.5); ꢀ157.14 changes. The (1R,4S)-2,3-bis(trifluoromethyl)-7-
prepared
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azabicyclo[2.2.1]hepta-2,5-dien-7-yl)(phenyl)methanone 142.14 (s); 141.90 (s); 141.76 (s); 141.51 (s); 139.33 (s);
(8.0 g, 24.0 mmol) was heated in the closed high-pres- 138.85 (s); 136.48 (s); 132.61 (s); 130.39 (s); 130.31 (s);
sure flask at 190 °C for 24 h. During this time, the 130.22 (s); 130.20 (s); 130.17 (s); 130.08 (m); 128.69 (s);
1
color of the oil turned from yellow to brown. A H- 128.63 (s); 128.55 (s); 127.20 (s); 126.81 (s); 126.78 (s);
NMR spectrum of the crude product showed that it 126.73 (s); 126.69 (s); 126.60 (s); 126.42 (s); 125.96 (s, N–
consisted mostly of (3,4-bis(trifluoromethyl)-1H-pyrrol- CH); 54.37 (s, CMe2); 30.60 (s, Me); 27.68 (s, Me); 14.37 (s,
1-yl)(phenyl)methanone, according to published data; PMe); 14.18 (s, PMe). Anal. calc. forC60H46F11MoN2OP: C
it was used in the next step without further purifica- 62.83, H 4.04, N 2.44; found: C 63.09, H 4.09, N 2.17.
tion. Crude (3,4-bis(trifluoromethyl)-1H-pyrrol-1-yl)(phen-
yl)methanone was dissolved in 20 ml of THF, solution
of KOH (1.4 g, 25.1 mmol, 1.05 equiv.) in 15 ml of water
was added, and the mixture was stirred at room tem-
General Procedure for Small Scale Homocoupling of 3-
methyl-1-butene
Solid catalyst (2.5 lmol) was weighed in a 1 dram vial.
When indicated, solid B(C6F5)3 (1.5 mg, 1.15 equiv.,
2.9 lmol) was also added. A solution of 0.0175 g
3-methyl-1-butene in 0.05 ml C6D6 was added. The
vial was sealed, and the mixture was stirred for 24 h.
The solution was diluted with approximately 0.6 ml
C6D6 and analyzed by NMR spectroscopy. The cis and
trans isomers of 2,5-dimethyl-3-hexene can be distin-
guished by their NMR resonances, with the trans olefi-
nic proton resonance appearing at ca. 5.35 ppm and
the cis olefinic proton resonance appearing at ca.
5.11 ppm. These second order resonances were mod-
eled in order to determine their J-values and therefore
identify them as trans and cis olefinic proton reso-
nances, respectively.
perature for 1 h. Brine (20 ml) was added and the
organic solution was separated, dried under Na2SO4, and
passed through pad of basic alumina. The THF was
removed in vacuo, and the residue was distilled under
vacuum (65 °C, 0.1 Torr) to give 3,4-bis(trifluoromethyl)-
1H-pyrrole (3.4 g, 70% after 2 steps) as a colorless liquid.
The 1H-NMR spectrum (400 MHz; CDCl3): 8.89 (br. s, 1 H);
7.17 (d, J = 2.9, 2 H) corresponds to that published.[30] A
solution of KHMDS (4.92 ml, 1.0M solution in diethyl
ether, 4.92 mmol, 1.0 equiv.) was added to the solution
of 3,4-bis(trifluoromethyl)-1H-pyrrole (1.0 g, 4.92 mmol,
1 equiv.) in 10 ml of diethyl ether. The resulting mixture
was stirred at room temperature for 10 min. The solvent
was removed in vacuo, and the residue was crystallized
from pentane, isolated through filtration, and dried in
vacuo. Potassium 3,4-bis(trifluoromethyl)pyrrol-1-ide
(CF3PyrK) (1.02 g, 86%) was obtained as a white powder:
1H-NMR (400 MHz; CD3CN): 6.97 (s, 2 H). 13C-NMR
(101 MHz; CD3CN): 131.5, 126.9 (q, J = 265, CF3); 107.5 (q,
J = 36). 19F-NMR (282 MHz; CD3CN): ꢀ53.7.
General Procedure for the Large Scale Homocoupling of
3-Methyl-1-butene in Closed Systems
A solid portion of catalyst (0.025 mmol) were placed in a
100 ml round-bottomed flask. When indicated, solid
B(C6F5)3 (15 mg, 1.15 equiv., 0.029 mmol) was also
added. A solution of 1.787 g 3-methyl-1-butene (1000
equiv., 25.5 mmol) in 2.85 ml benzene was added, the
flask was sealed with a rubber septum, and the mixture
was stirred. Periodically, a 0.1 ml sample was taken using
a needle and syringe. The sample was diluted with ca.
0.6 ml of C6D6 and analyzed by NMR spectroscopy in
order to monitor the progress of the reaction.
Synthesis of 6
CF3PyrK (0.056 g, 1.15 equiv., 0.234 mmol) was added
to a 5 ml benzene solution of 1(TPPO) (0.200 g,
0.204 mmol), and the mixture was stirred for 4 days.
The mixture was filtered through Celite, and the solvent
was removed under reduced pressure to give a dark
orange residue. The residue was stirred in 10 ml 1:1
ether/pentane for several hours. The resulting bright
yellow powder was isolated by filtration, rinsed with
pentane, and dried under reduced pressure; yield
0.110 g (47.0%). 1H-NMR (500 MHz, C6D6): 12.49 (d,
JPH = 4.0, JCH = 128, Mo=CH); 7.34 – 7.21 (m, 5 H); 7.18
(s, 1 H); 7.14 – 6.74 (m, 25 H); 6.71 – 6.64 (m, 2 H, N–
CH=CCF3); 1.38 (s, Me); 1.37 (s, Me); 0.82 (d, PMe,
J = 9.0); 0.78 (d, PMe, J = 8.7). 31P{1H}-NMR (161 MHz):
ꢀ1.13 (br. s, Mo-P). 19F-NMR (300 MHz, C6D6): ꢀ54.83 (s,
CF3); ꢀ143.62 (d, J = 18.4); ꢀ155.00 (t, J = 21.9);
ꢀ162.10 (~t). 13C-NMR (100 MHz, C6D6): 319.36 (br. s,
Mo=CH); 159.86 (s); 143.08 (s); 142.52 (s); 142.16 (s);
Supplementary Material
Supporting information for this article is available on the
WWW under https://doi.org/10.1002/hlca.201700181.
Acknowledgements
R. R. S. is grateful for financial support from the
National Science Foundation under the CCI Center for
Enabling New Technologies through Catalysis (CENTC;
CHE-1205189).
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Biomass for Producing PET’, Green Chem. 2016, 18,
342 – 359.
Author Contribution Statement
P. E. S. prepared compounds 1 – 6 and carried out
the experiments described in Tables 1 and 2. K. V. B.
carried out the synthesis of the 3,4-bis(trifluoro-
methyl)-1H-pyrrole. R. R. S. supervised the project and
wrote the final versions of the manuscript.
ꢀ
ꢀ
[15] S. Berard, C. Vallee, D. Delcroix, ‘Sorbic Acid as a Renew-
able Resource for Atom-Economic and Selective Produc-
tion of p-Toluic Acid and Alkyl-p-Toluates: Intermediates to
Bioterephthalic Acid and Esters’, Ind. Eng. Chem. Res. 2015,
54, 7164 – 7168.
[16] L. V. Dinh, B. Li, A. Kumar, W. Schinski, K. D. Field, A.
Kuperman, F. E. Celik, A. Goldman, ‘Alkyl–Aryl Coupling
Catalyzed by Tandem Systems of Pincer-Ligated Irid-
ium Complexes and Zeolites’, ACS Catal. 2016, 6,
2836 – 2841.
References
ꢀ
[1] R. A. F. Tomas, J. C. M. Bordado, J. F. P. Gomes, ‘p-Xylene
Oxidation to Terephthalic Acid: A Literature Review Ori-
ented toward Process Optimization and Development’,
Chem. Rev. 2013, 113, 7421 – 7469.
[17] T. W. Lyons, D. Guironnet, M. Findlater, M. Brookhart, ‘Syn-
thesis of p-Xylene from Ethylene’, J. Am. Chem. Soc. 2012,
134, 15708 – 15711.
[2] K. Pang, R. Kotek, A. Tonelli, ‘Review of Conventional and
Novel Polymerization Processes for Polyesters’, Prog. Polym.
Sci. 2006, 31, 1009 – 1037.
[3] N. A. M. Fadzil, M. H. A. Rahim, G. P. Maniam, ‘A Brief
Review of Para-Xylene Oxidation to Terephthalic Acid as a
Model of Primary C–H Bond Activation’, Chin. J. Catal.
2014, 34, 1641 – 1652.
[4] Q. Wang, X. Li, L. Wang, Y. Cheng, G. Xie, ‘Kinetics of p-
Xylene Liquid-Phase Catalytic Oxidation to Terephthalic
Acid’, Ind. Eng. Chem. Res. 2005, 44, 261 – 266.
[5] P. Raghavendrachar, S. Ramachandran, ‘Liquid-Phase Cat-
alytic Oxidation of p-Xylene’, Ind. Eng. Chem. Res. 1992, 31,
453 – 462.
[18] V. Saini, B. J. Stokes, M. S. Sigman, ‘Transition-Metal-Cata-
lyzed Laboratory-Scale Carbon–Carbon Bond-Forming
Reactions of Ethylene’, Angew. Chem. Int. Ed. 2013, 52,
11206 – 11220.
[19] S. Kundu, T. W. Lyons, M. Brookhart, ‘Synthesis of Pipery-
lene and Toluene via Transfer Dehydrogenation of Pen-
tane and Pentene’, ACS Catal. 2013, 3, 1768 – 1773.
[20] W. Yao, Y. Zhange, X. Jia, Z. Huang, ‘Selective Catalytic
Transfer Dehydrogenation of Alkanes and Heterocycles by
an Iridium Pincer Complex’, Angew. Chem. Int. Ed. 2014,
53, 1390 – 1394.
[21] I. C. Stewart, T. Ung, A. A. Pletnev, J. M. Berlin, R. H.
Grubbs, Y. Schrodi, ‘Highly Efficient Ruthenium Catalysts
for the Formation of Tetrasubstituted Olefins via Ring-Clos-
ing Metathesis’, Org. Lett. 2007, 9, 1589 – 1592.
[6] J. G. Speight, ‘Chemical Process and Design Handbook’,
McGraw-Hill, New York, 2002, pp. 2557 – 2560.
[7] J. Fabri, U. Graeser, T. A. Simo, ‘Xylenes’, in ‘Ullmans’ Ency-
clopedia of Industrial Chemistry’, 3rd Edn., Wiley-VCH,
Weinheim, Germany, 2003, Vol. 39, p. 583.
[22] I. C. Stewart, C. J. Douglas, R. H. Grubbs, ‘Increased Effi-
ciency in Cross-Metathesis Reactions of Sterically Hindered
Olefins’, Org. Lett. 2008, 10, 441 – 444.
[8] Y. Y. Fong, A. Z. Abdullah, A. L. Ahmad, S. Bhatia, ‘Develop-
ment of Functionalized Zeolite Membrane and its Potential
Role as Reactor Combined Separator for para-Xylene Pro-
duction from Xylene Isomers’, Chem. Eng. J. 2008, 139,
172 – 193.
[23] Z. J. Wang, W. R. Jackson, A. J. Robinson, ‘An Efficient Pro-
tocol for the Cross-Metathesis of Sterically Demanding
Olefins’, Org. Lett. 2013, 15, 3006 – 3009.
[24] M. Chen, W. R. Roush, ‘Total Synthesis of (–)-Tirandamycin
C’, Org. Lett. 2012, 14, 426 – 428.
[9] Y.-T. Cheng, Z. Wang, C. J. Gilbert, W. Fan, G. W. Huber,
‘Production of p-Xylene from Biomass by Catalytic Fast
Pyrolysis Using ZSM-5 Catalysts with Reduced Pore
[25] P. M. Tadross, S. C. Virgil, B. M. Stoltz, ‘Aryne Acyl-Alkyla-
tion in the General and Convergent Synthesis of Benzan-
nulated
Macrolactone
Natural
Products:
An
Openings’,
Angew.
Chem.
Int.
Ed.
2012,
51,
Enantioselective Synthesis of (–)-Curvularin’, Org. Lett.
2010, 12, 1612 – 1614.
11097 – 11100.
[10] D. B. Patience, J. B. Rawlings, H. A. Mohameed, ‘Crystalliza-
tion of Para-Xylene in Scraped-Surface Crystallizers’, AlChe
J. 2001, 47, 2441 – 2451.
[11] H. J. Cho, L. Ren, V. Vattipalli, Y.-H. Yeh, N. Gould, B. Xu, R.
J. Gorte, R. Lobo, P. J. Dauenhauer, M. Tsapatsis, W. Fan,
‘Renewable p-Xylene from 2,5-Dimethylfuran and Ethylene
Using Phosphorus-Containing Zeolite Catalysts’, ChemCat-
Chem 2017, 9, 398 – 402.
[12] F. Wang, Z. Tong, ‘Solely Biomass-Derived Polyethylene
Terephthalate (PET): Conversion of Bio-based Isoprene and
Acrolein to p-Xylene and Terephthalic Acid’, Chemistry-
Select 2016, 1, 5538 – 5541.
[13] A. Maneffa, P. Priecel, J. A. Lopez-Sanchez, ‘Biomass-
Derived Renewable Aromatics: Selective Routes and Out-
look for p-Xylene Commercialisation’, Chemsuschem 2016,
9, 2736 – 2748.
€
[26] J. K. Lam, C. Zhu, K. V. Bukhryakov, P. Muller, A. H.
Hoveyda, R. R. Schrock, ‘Synthesis and Evaluation of
Molybdenum and Tungsten Monoaryloxide Halide Alkyli-
dene Complexes for Z-Selective Cross Metathesis of
Cyclooctene and Z-1,2-Dichloroethylene’, J. Am. Chem. Soc.
2016, 138, 15774 – 15783.
[27] M. J. Koh, T. T. Nguyen, J. K. Lam, S. Torker, J. Hyvl, R. R.
Schrock, A. H. Hoveyda, ‘Molybdenum Chloride Catalysts
for Z-Selective Olefin Metathesis Reactions’, Nature 2017,
542, 80 – 85.
[28] X. Shen, T. T. Nguyen, M. J. Koh, D. Xu, A. W. H. Speed, R.
R. Schrock, A. H. Hoveyda, ‘Kinetically E-Selective Macro-
cyclic Ring-Closing Metathesis’, Nature 2017, 541,
380 – 385.
[29] S. C. Marinescu, V. W. L. Ng, A. G. Lichtscheidl, R. R.
€
Schrock, P. Muller, M. K. Takase, ‘Synthesis of Variations of
[14] P. Pang, M. Zheng, R. Sun, A. Wang, X. Wang, T. Zhang,
‘Synthesis of Ethylene Glycol and Terephthalic Acid from
Stereogenic-at-Metal Imido Alkylidene Complexes of
Molybdenum’, Organometallics 2012, 31, 6336 – 6343.
www.helv.wiley.com
(6 of 7) e1700181
© 2017 Wiley-VHCA AG, Zurich, Switzerland

Helv. Chim. Acta 2017, 100, e1700181
[30] R. W. Kaesler, E. LeGoff, ‘Synthesis of 3,4-Bis(trifluoro-
Imido Alkylidene Complexes that Contain 3,3’-Dialkyl-
5,5’,6,6’-tetramethyl-1,1’-biphenyl-2,2’-diolates (Alkyl = t-Bu
or Adamantyl). Catalysts for Enantioselective Olefin
methyl)pyrroles’, J. Org. Chem. 1982, 47, 4779 – 4780.
[31] M. R. Buchmeiser, S. Sen, C. Lienert, L. Widmann, R.
Schowner, K. Herz, P. Hauser, W. Frey, D. Wang, ‘Molybde-
num Imido Alkylidene N-Heterocyclic Carbene Complexes:
Structure–Productivity Correlations and Mechanistic
Insights’, ChemCatChem 2016, 8, 2710 – 2723.
[32] I. Elser, W. Frey, K. Wurst, M. R. Buchmeiser, ‘Molybdenum
Imido Alkylidene Complexes Containing N- and C-Chelat-
ing N-Heterocyclic Carbenes’, Organometallics 2016, 35,
4106 – 4111.
Metathesis
Reactions’,
Organometallics
2000,
19,
3700 – 3715.
[38] M. M. Flook, L. C. H. Gerber, G. T. Debelouchina, R. R.
Schrock, ‘Z-Selective and Syndioselective Ring-Opening
Metathesis Polymerization (ROMP) Initiated by MonoAry-
loxidePyrrolide (MAP) Catalysts’, Macromolecules 2010, 43,
7515 – 7522.
[39] D. A. Dickie, I. S. MacIntosh, D. D. Ino, Q. He, O. A. Labeo-
dan, M. C. Jennings, G. Schatte, C. J. Walsby, J. A. C. Cly-
burne, ‘Synthesis of the bulky m-terphenyl phenol Ar*OH
(Ar* = C6H3-2,6-Mes2, Mes = 2,4,6-trimethylphenyl) and the
preparation and structural characterization of several of its
metal complexes’, Can. J. Chem. 2008, 86, 20 – 31.
[33] M. Pucino, V. Mougel, R. Schowner, A. Fedorov, M. R. Buch-
ꢀ
meiser, C. Coperet, ‘Cationic Silica-Supported N-Heterocyc-
lic Carbene Tungsten Oxo Alkylidene Sites: Highly Active
and Stable Catalysts for Olefin Metathesis’, Angew. Chem.
Int. Ed. 2016, 55, 4300 – 4302.
[34] C. Lienert, W. Frey, M. R. Buchmeiser, ‘Stereoselective Ring-
Opening Metathesis Polymerization with Molybdenum
Imido Alkylidenes Containing O-Chelating N-Heterocyclic
Carbenes: Influence of Syn/Anti Interconversion and Poly-
merization Rates on Polymer Structure’ Macromolecules
2017, 50, 5701 – 5710.
[40] C. Stanciu, M. M. Olmstead, A. D. Phillips, M. Stender, P. P.
Power, ‘Synthesis and Characterization of the Very Bulky
Phenols Ar*OH and Ar’OH (Ar* = C6H3-2,6-Trip2, Trip =
C6H2-2,4,6-iPr3; Ar’ = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-iPr2)
and Their Lithium and Sodium Derivatives (LiOAr’)2 and
(NaOAr*)2’, Eur. J. Inorg. Chem. 2003, 3495 – 3500.
[35] C. P. Gordon, K. Yamamoto, W.-C. Liao, F. Allouche, R. A.
[41] T. T. Nguyen, M. J. Koh, X. Shen, F. Romiti, R. R. Schrock, A.
H. Hoveyda, ‘Kinetically controlled E-selective catalytic ole-
fin metathesis’, Science 2016, 352, 569 – 575.
ꢀ
Andersen, C. Coperet, C. Raynaud, O. Eisenstein, ‘Meta-
thesis Activity Encoded in the Metallacyclobutane Carbon-
13 NMR Chemical Shift Tensors’, ACS Cent. Sci. 2017, 3,
759 – 768.
€
[42] E. M. Townsend, S. M. Kilyanek, R. R. Schrock, P. Muller, S.
J. Smith, A. H. Hoveyda, ‘High Oxidation State Molybde-
num Imido Heteroatom-Substituted Alkylidene Complexes’,
Organometallics 2013, 32, 4612 – 4617.
[36] J. H. Oskam, H. H. Fox, K. B. Yap, D. H. McConville, R.
O’Dell, B. J. Lichtenstein, R. R. Schrock, ‘Ligand Variation in
Alkylidene Complexes of the Type Mo(CHR)(NR’)(OR’’)2’, J.
Organomet. Chem. 1993, 459, 185 – 198.
[37] J. B. Alexander, R. R. Schrock, W. M. Davis, K. C. Hultzsch,
A. H. Hoveyda, J. H. Houser, ‘Synthesis of Molybdenum
Received August 4, 2017
Accepted August 30, 2017
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