Catalytic synthesis of n-alkyl arenes through alkyl group cross-metathesis

Article abstract of DOI:10.1021/ja4066392

n-Alkyl arenes were prepared in a one-pot tandem dehydrogenation/olefin metathesis/hydrogenation sequence directly from alkanes and ethylbenzene. Excellent selectivity was observed when (^tBuPCP)IrH₂ was paired with tungsten monoaryloxide pyrrolide complexes such as W(NAr)(C ₃H₆)(pyr)(OHIPT) (1a) [Ar = 2,6-i-Pr₂C ₆H₃; pyr = pyrrolide; OHIPT = 2,6-(2,4,6-i-Pr ₃C₆H₂)₂C₆H₃O]. Complex 1a was also especially active in n-octane self-metathesis, providing the highest product concentrations reported to date. The thermal stability of selected olefin metathesis catalysts allowed elevated temperatures and extended reaction times to be employed.

Full text of DOI:10.1021/ja4066392

Communication
pubs.acs.org/JACS
Catalytic Synthesis of n‑Alkyl Arenes through Alkyl Group Cross-
Metathesis
Graham E. Dobereiner,† Jian Yuan,† Richard R. Schrock,*,† Alan S. Goldman,‡ and Jason D. Hackenberg‡
†Department of Chemistry, 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
‡Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854,
United States
S
* Supporting Information
Scheme 1. The Alkyl Group Cross-Metathesis Reaction
ABSTRACT: n-Alkyl arenes were prepared in a one-pot
tandem dehydrogenation/olefin metathesis/hydrogenation
sequence directly from alkanes and ethylbenzene. Excellent
selectivity was observed when (tBuPCP)IrH2 was paired
with tungsten monoaryloxide pyrrolide complexes such as
W(NAr)(C3H6)(pyr)(OHIPT) (1a) [NAr = 2,6-i-
Pr2C6H3; pyr = pyrrolide; OHIPT = 2,6-(2,4,6-i-
Pr3C6H2)2C6H3O]. Complex 1a was also especially active
in n-octane self-metathesis, providing the highest product
concentrations reported to date. The thermal stability of
selected olefin metathesis catalysts allowed elevated
step10 and the rate at which intermediate olefins are isomerized,
temperatures and extended reaction times to be employed.
either by Ir complexes11 or by the products of Mo or W
decomposition.12 Competitive metathesis homocoupling of
elective functionalizations of C−H bonds are highly sought
intermediate olefins could compete with cross-metathesis and
further increase the number of products.
1
Sbecause of the abundance of hydrocarbon feedstocks. n-
Alkanes would be ideal starting materials in many reactions if
C−H bond activation in an alkane were possible. One goal
could be the synthesis of n-alkyl arenes (e.g., 1-phenyloctane),
which are precursors for surfactants with high detersive power
at elevated temperatures and low concentrations.2 The largest
group of linear alkylbenzenes, produced on a million ton per
year scale for the synthesis of surfactants,3 are typically
branched alkyl arenes (e.g., 2-phenyloctane) generated by
Friedel−Crafts alkylation of benzene with olefins.4 n-Alkyl
arenes cannot be produced in this fashion, and while the anti-
Markovnikov arylation of olefins5 is a potential catalytic method
for their synthesis, a more efficient route would proceed
directly from an abundant alkane as a starting material.
Dehydroaromatization is one strategy for the synthesis of
specific n-alkyl arenes from n-alkanes in a single step,6 but this
requires a stoichiometric amount of an olefin to serve as a
hydrogen acceptor.
Catalyst longevity is another major challenge in the
development of a practical alkane metathesis protocol. The
high temperatures (at least 125 °C) and multiday reaction
times required for alkane metathesis deactivate Mo and W
olefin metathesis catalysts more rapidly than Ir pincer
complexes. In this work, we found that certain monoaryloxide
pyrrolide (MAP) complexes of Mo and W, previously studied
in the context of Z-selective metathesis reactions,13−15 are
thermally quite stable. In concert with Ir pincer complexes, they
provide high total product concentrations in the metathesis of
n-octane. These MAP complexes are also robust catalysts for
AGCM of alkanes and ethylbenzene, as described herein.
A previous screen of Mo and W alkylidene complexes
identified several active catalysts for n-octane metathesis and
established that W complexes provide higher total product
concentrations than the analogous Mo species.16 Unfortunately,
the thermal stability of the most active complexes at 150 °C
was poor. Therefore, we turned to complexes containing
sterically demanding phenoxide ligands, which we hypothesized
might improve the stability by discouraging bimolecular
decomposition.17
An Ir dehydrogenation/hydrogenation catalyst and a W or
Mo complex competent for olefin metathesis have been shown
to work in tandem to generate a broad distribution of n-alkanes
from a single n-alkane.7,8 We have been exploring the possibility
of what could be called alkyl group cross-metathesis (AGCM)
to generate n-alkyl arenes from ethylbenzene and an alkane
(Scheme 1; only terminal olefins are shown). While ethyl-
benzene has never been used previously in an alkane metathesis
reaction, (PCP)Ir-catalyzed dehydrogenation of ethylbenzene
has been reported.9 In alkane metathesis, the overall
distribution of products is influenced by several factors,
including the terminal selectivity of the alkane dehydrogenation
The results of an evaluation of olefin metathesis catalysts for
the metathesis of n-octane are shown in Table 1. The most
encouraging results were obtained with complexes that
incorporate the 2,6-(2,4,6-i-Pr3C6H2)2C6H3O (OHIPT) ligand.
The OHIPT-containing compounds 1a,13a 1b,14 and W(N-t-
Received: July 2, 2013
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Table 1. Total Product Concentrations Obtained in the
a
Metathesis of n-Octane
metathesis catalyst
total product (mM)
b
W(NAr)(C3H6)(pyr)(OHIPT) (1a)
W(NAr′)(C3H6)(pyr)(OHIPT) (1b)
Mo(NAr)(C3H6)(pyr)(OHIPT)
3920, 4910
2400
2450
b
W(NC6F5)(CH-t-Bu)(Me2Pyr)(OHMT)
W(NC6F5)(CH-t-Bu)(Me2Pyr)(OHIPT)
W(N-t-Bu)(CH-t-Bu)(pyr)(OHIPT)
W(O)(CH-t-Bu)(OHMT)2
3660, 3750
∼0
2790
1600
2260
W(NAr)(CHCMe2Ph)[OC(CF3)3]2
W(NAr)(CHCMe2Ph)(OSiPh3)2
W(O)(CH-t-Bu)(Me2Pyr)(OHIPT)
W(NAr)(CH-t-Bu)(Me2Pyr)(OHMT)
b
2770, 1420
∼0
∼0
a
Conditions: 125 °C, 4 days in J. Young tubes with 16 mM metathesis
catalyst, 10 mM (tBuPOCOP)Ir(C2H4), and 28.8 mM mesitylene
(internal standard). See the Supporting Information (SI) for details.
Abbreviations: Ar = 2,6-i-Pr2C6H3; Ar′ = 2,6-Me2C6H3; pyr =
pyrrolide; Me2Pyr = 2,5-dimethylpyrrolide; OHMT = 2,6-(2,4,6-
Figure 1. GC trace of the alkane metathesis of 1:4 (v/v) n-octane/
ethylbenzene at 150 °C for 2 days using 16 mM 1a and 10 mM
(
tBuPOCOP)Ir(C2H4).
b
(C2H4) was employed, but despite the high selectivity for
alkylbenzenes over alkanes, there was no selectivity for specific
alkylbenzenes with this Ir catalyst. In addition, a large fraction
of the alkanes were in the C5−C7 range. Other Ir complexes
produced more PhC8, even when the reaction temperature was
increased to 180 °C (Table 2). The (tBuPCP)IrH2 complex
Me3C6H2)2C6H3O); OHIPT = 2,6-(2,4,6-i-Pr3C6H2)2C6H3O. 150
°C, 2 days.
Bu)(CH-t-Bu)(pyr)(OHIPT)18 are superior catalysts. W(O)-
(CH-t-Bu)(MePyr)(OHIPT),19 W(NC6F5)(CH-t-Bu)-
(Me2Pyr)(OHIPT), and W(NAr)(CH-t-Bu)(Me2Pyr)-
(OHMT) [Me2Pyr = 2,5-dimethylpyrrolide; OHMT = 2,6-
(2,4,6-Me3C6H2)2C6H3O] are essentially inactive. Surprisingly,
W(NC6F5)(CH-t-Bu)(Me2Pyr)(OHMT) is an excellent cata-
lyst for the metathesis of n-octane at 125 °C, in contrast to
W(NC6F5)(CH-t-Bu)(Me2Pyr)(OHIPT). We previously found
that W and Mo bisalkoxide complexes containing the
pentafluorophenylimido (NC6F5) group are poor catalysts for
alkane metathesis.20
Table 2. Total Concentrations (mM) of Tetradecane (C14),
1-Phenyloctane (PhC8), and 1-Phenylheptane (PhC7)
Obtained by AGCM Using Various Dehydrogenation
a
Catalysts
catalyst
C14
PhC8
PhC7
(
(
(
(
(
(
tBuPCOP)IrH2
60
40
20
10
20
20
230
210
150
190
70
150
140
100
80
tBu3MePCP)IrH2
iPrPCOP)Ir(C2H4)
iPrPCP)Ir(C2H4)
tBuPOCOP)Ir(C2H4)
tBuPCP)IrH2
100
60
240
a
Conditions: 24 h, 180 °C in J. Young tubes with 0.3 mL of n-octane,
0.4 mL of ethylbenzene, 11 mM 1a, 7 mM Ir catalyst, and mesitylene
(internal standard). See the SI for details.
The activity of 1a at various temperatures and reaction times
was explored further. A higher total product concentration was
obtained when the reaction temperature was increased from
125 to 150 °C and the reaction time was decreased from 4 to 2
days (Table 1). Higher efficiency at higher temperatures is not
a general phenomenon; for example, W(NAr)(CHCMe2Ph)-
(OSiPh3)2 provided lower product concentrations at higher
reaction temperatures (2770 mM at 125 °C, 4 days vs 1420
mM at 2 days, 150 °C). W(NC6F5)(CH-t-Bu)(Me2Pyr)-
(OHMT) also showed good stability at higher temperatures,
generating 3750 mM total product over 2 days at 150 °C.
With thermally robust olefin metathesis catalysts in hand, we
began to investigate the metathesis of n-octane in ethylbenzene
(1:4 v/v). A broad distribution of n-alkylbenzenes [1-
phenylpropane (PhC3) to 1-phenyloctane (PhC8)] was
obtained with 1a and (tBuPOCOP)Ir(C2H4) at 150 °C over 2
days (Figure 1). Various n-alkanes [up to tetradecane (C14)]
were also formed as side products. Alkylbenzenes were the
major products in the C10−C14 range when (tBuPOCOP)Ir-
provided 240 mM PhC8 in the cross-metathesis of n-octane
and ethylbenzene over 24 h (Scheme 2). The selectivity for
alkylbenzenes over alkanes was maintained even when only a
slight excess of ethylbenzene was used (n-octane:ethylbenzene
= 3:4 v/v). After PhC8, 1-phenylheptane (PhC7) was the next
major n-alkyl arene product in the C10−C14 range (60 mM).
Additionally, 20 mM C14 was generated. Because the AGCM
reactions described here proceeded only to low conversion
B
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Journal of the American Chemical Society
Communication
a
Scheme 2. Major Products in the AGCM of 3:4 v/v n-
Table 4. Branched Substrates for AGCM with Ethylbenzene
Octane/Ethylbenzene
a
(relative to ethylbenzene), only traces of diphenyl products
Ph(CH2)nPh (formed by cross-metathesis on both termini of
one alkane) were produced.
Conditions: 2 days, 180 °C in J. Young tubes with 16 mM 1a, 10 mM
(
tBuPOCOP)Ir(C2H4), and mesitylene (internal standard).
Various olefin metathesis catalysts were screened using
thylsilane were all formed along with trace amounts of
trimethyl(4-phenylbutyl)silane, as determined by GC−MS
analysis (Scheme 3).
(
tBuPCP)IrH2 as the dehydrogenation catalyst (Table 3).
Table 3. Total Concentrations (mM) of Tetradecane (C14),
1-Phenyloctane (PhC8), and 1-Phenylheptane (PhC7)
Obtained by AGCM Using (tBuPCP)IrH2 and Various
Scheme 3. AGCM of n-Butyltrimethylsilane and
Ethylbenzene
a
Metathesis Catalysts
catalyst
C14
20
PhC8
240
PhC7
60
W(NAr)(C3H6)(pyr)(OHIPT) (1a)
b
b
b
20
20
280
100
W(NAr′)(C3H6)(pyr)(OHIPT) (1b)
Mo(NAr)(C3H6)(pyr)(OHIPT)
350
80
100
10
20
W(NC6F5)(CH-t-Bu)(Me2Pyr)(OHMT)
W(N-t-Bu)(CH-t-Bu)(pyr)(OHIPT)
Mo(NAr)(CHCMe2Ph)(ORF6)2
<10
<10
50
160
<10
240
30
40
<10
20
W(NAr)(CHCMe2Ph)(OC(CF3)3)2
W(NAr)(CHCMe2Ph)(OSiPh3)2
<10
50
<10
60
Substrates containing a heteroatom [e.g., methyl propionate,
n-propyl(trimethoxy)silane] deactivated one or both catalysts.
Ir complexes are known to be deactivated in the presence of
C−O,21 C−I,22 and C−F23 bonds. However, these bond
cleavages may be reversible under some conditions. The known
reversible activation of arene C−H bonds by (PCP)Ir24 clearly
does not inhibit AGCM.
220
a
Conditions: 24 h, 180 °C in J. Young tubes with 0.3 mL of n-octane,
0.4 mL of ethylbenzene, 11 mM metathesis catalyst, 7 mM
tBuPCP)IrH2, and mesitylene (internal standard). See the SI for
(
details. ORF6 = OC(CF3)2CH3. See Table 1 for other abbreviations.
b
22 mM metathesis catalyst.
Although the conditions required for appreciable conversion
in AGCM are relatively harsh, tungsten alkylidenes were
observed in the residue remaining after 2 days in a 180 °C
reaction. A proton NMR spectrum of the nonvolatile
components of a completed catalytic reaction revealed two
1H NMR resonances in the alkylidene region. One integrated to
25% of the original W catalyst loading (vs an internal standard)
Complex 1b provided the highest conversion to PhC8 under
these conditions (350 mM), corresponding to a W turnover
number (TON) of 31 and an Ir TON of 50. 1b also provided
the highest selectivity for n-alkyl arene versus C14 (∼17:1
PhC8:C14). While bisalkoxide complexes were also active for
AGCM, they generated more n-alkane side products. However,
Mo(NAr)(CHCMe2Ph)(ORF6)2 [ORF6 = OC(CF3)2CH3]
demonstrated the greatest selectivity for PhC8 versus PhC7
of any catalyst (12:1). This result is consistent with the high
levels of C14 selectivity previously observed using Mo(NAr)-
(CHCMe2Ph)(ORF6)2 for the metathesis of n-octane.8,16
Branched alkanes, which are less prone to isomerization or
dehydrogenation at internal positions, can also undergo AGCM
reactions. The substrates shown in Table 4 reacted with
ethylbenzene at 180 °C in the presence of (tBuPOCOP)Ir-
(C2H4) and 1a. Bibenzyl was the only major byproduct of these
reactions as a consequence of homoalkane metathesis of
ethylbenzene. Only trace amounts of products derived from
isomerization were observed with the substrates in Table 4.
Less hindered alkanes prone to isomerization led to broader
distributions of products. For example, n-propyltrimethylsilane
in ethylbenzene afforded trimethyl(3-phenylpropyl)silane in
24% yield over 2 days at 180 °C with only bibenzyl as a side
product. When n-butyltrimethylsilane was employed, trimethyl-
(3-phenylpropyl)silane, n-propylbenzene, and n-propyltrime-
and was assigned to W(NAr)(CHPh)(pyr)(OHIPT) (δCHPh
=
10.25 ppm, C6D6) on the basis of a comparison with the
alkylidene complex formed through addition of styrene to 1a.
The identity of the other alkylidene (δCHR = 10.61 ppm; 0.2
μmol, or 2.5% of the original W catalyst) is currently not
known.
We conclude that certain W and Mo complexes catalyze alkyl
group cross-metathesis reactions at relatively high temperatures.
Sterically demanding aryloxide ligands appear to make the
complexes more thermally stable. These robust catalysts can be
employed in AGCM to provide n-alkyl arenes directly from
ethylbenzene and alkanes in a one-pot process that is relatively
selective for the formation of alkyl arenes over alkanes. Beyond
providing a new route to n-alkyl arenes, this work more broadly
demonstrates that a dehydrogenation/olefin metathesis se-
quence can provide AGCM products with good selectivity,
even between alkyl chains in different substrates. An under-
standing of the inherent biases of each catalyst toward alkyl
C
dx.doi.org/10.1021/ja4066392 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(14) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; Muller, P.; Hoveyda,
A. H. J. Am. Chem. Soc. 2009, 131, 7962.
̈
groups in various substrates should allow other AGCM
variations to be designed.
(15) (a) Meek, S. J.; O’Brien, R. V.; Llaveria, J.; Schrock, R. R.;
Hoveyda, A. H. Nature 2011, 471, 461. (b) Yu, M.; Ibrahem, I.;
Hasegawa, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2012,
134, 2788. (c) Kiesewetter, E. T.; O’Brien, R. V.; Yu, E. C.; Meek, S. J.;
Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 6026.
(d) Wang, C.; Yu, M.; Kyle, A. F.; Jacubec, P.; Dixon, D. J.; Schrock, R.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details for all metal complexes, substrates, and
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
rrs@mit.edu
Notes
■
(16) Bailey, B. C.; Schrock, R. R.; Kundu, S.; Goldman, A. S.; Huang,
Z.; Brookhart, M. Organometallics 2009, 28, 355.
(17) Schrock, R. R. Chem. Rev. 2009, 109, 3211.
(18) Jeong, H.; Axtell, J. C.; Torok, B.; Schrock, R. R.; Muller, P.
̈
̈
̈
The authors declare no competing financial interest.
Organometallics 2012, 31, 6522.
(19) Peryshkov, D. V.; Schrock, R. R.; Takase, M. K.; Muller, P.;
̈
Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 20754.
ACKNOWLEDGMENTS
■
(20) Yuan, J.; Schrock, R. R.; Muller, P.; Axtell, J. C.; Dobereiner, G.
̈
This work was supported by the National Science Foundation
under the CCI Center for Enabling New Technologies through
Catalysis (CENTC) Phase II Renewal (CHE-1205189). We
thank Erik Townsend, Hyangsoo Jeong, and Dmitry Peryshkov
(MIT) for their generous donation of catalysts and Akshai
Kumar (Rutgers) for the preparation of (iPrPCP)Ir(C2H4).
G.E.D. thanks Dr. Jeffrey Scheibel (Procter & Gamble) for
helpful discussions.
E. Organometallics 2012, 31, 4650.
(21) Kundu, S.; Choi, J.; Wang, D. Y.; Choliy, Y.; Emge, T. J.; Krogh-
Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2013, 135, 5127.
(22) Ghosh, R.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J.
Am. Chem. Soc. 2008, 130, 11317.
(23) Choi, J.; Wang, D. Y.; Kundu, S.; Choliy, Y.; Emge, T. J.; Krogh-
Jespersen, K.; Goldman, A. S. Science 2011, 332, 1545.
(24) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.;
Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017.
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