Polyisobutylene-anchored n-heterocyclic carbene ligands

Article abstract of DOI:10.1021/ol802728t

(Figure Presented) The synthesis of polyisobutylene (PIB)-supported N-heterocyclic carbenes (NHCs) that are useful as ligands for recoverable/recyclable organometallic complexes is described. Both PIB-bound carbenes analogous to SIMes and IMes as well as

Full text of DOI:10.1021/ol802728t

ORGANIC
LETTERS
2
009
Vol. 11, No. 3
65-667
Polyisobutylene-Anchored
N-Heterocyclic Carbene Ligands
6
†
†
,‡
Chayanant Hongfa, Haw-Lih Su, Hassan S. Bazzi,* and David E. Bergbreiter*
,†
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station,
Texas 77842-3012, and Department of Chemistry, Texas A&M UniVersity at Qatar,
P.O. Box 23874, Doha, Qatar
bergbreiter@tamu.edu; bazzi@tamu.edu
Received November 25, 2008
ABSTRACT
The synthesis of polyisobutylene (PIB)-supported N-heterocyclic carbenes (NHCs) that are useful as ligands for recoverable/recyclable
organometallic complexes is described. Both PIB-bound carbenes analogous to SIMes and IMes as well as carbene precursors bound to PIB
via 1,2,3-triazoles by alkyne-azide couplings are described. Both Ag(I) and Ru(II) complexes of these carbenes are shown to be phase selectively
soluble in heptane. Hoveyda-Grubbs second-generation catalysts containing a PIB-supported NHC have also been used to catalyze ring-
closing metathesis.
5
,6
N-Heterocyclic carbenes have become widely used ligands
in catalysis are known. Examples of soluble polymer
supports for these catalysts have also been described, but
these latter reports are limited to the use of poly(ethylene
for organometallic chemistry since Arduengo’s initial re-
1
ports. While their use in metathesis chemistry is most
7
,8
common, metal complexes derived from these structurally
glycol) (PEG) supports. PEG supports attached to an
imidazolium carbon or to an imidazolium nitrogen yield
NHC catalysts that are water-soluble or recoverable by
solvent precipitation. Here we describe using heptane-soluble
polyisobutylene as a phase anchor to prepare phase-separable
2
,3
diverse ligands are useful in many catalytic processes.
N-Heterocyclic carbenes themselves are also useful organo-
4
catalysts. Thus, there is significant interest in strategies that
facilitate separation, recovery and reuse of these ligands and
their metal complexes. Below we describe new routes to
recoverable, heptane-soluble NHC ligands and their use as
supports for recoverable, reusable metathesis catalysts.
Insoluble cross-linked polymers or inorganic supports for
recoverable reusable NHC-ligated metal complexes useful
9
NHC metal complexes. As shown below, PIB groups can
be attached to these carbene precursors in various ways, and
the product PIB-bound NHCs form metal complexes that
are phase selectively soluble in the heptane phase of
thermomorphic mixtures of heptane and polar solvents.
Alternatively, heptane solutions of these NHCs can be
extracted with polar solvents with minimal losses of the metal
†
Texas A&M University.
Texas A&M University at Qatar.
‡
(5) Clavier, H.; Grela, K.; Kirschning, A.; Mauduit, M.; Nolan, S. P.
(
1) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
13, 361–363.
2) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. ReV. 2007,
6, 1732–1744
Angew. Chem., Int. Ed. 2007, 46, 6786–6801
.
1
3
(6) Buchmeiser, M. R. Chem. ReV. 2009, 109, DOI: 10.1021/cr800207n
.
(
(7) Hong, S. H.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 3508–
.
3509
.
(
(
3) Glorius, F. Top. Organomet. Chem. 2007, 21, 1–20
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.
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(9) Li, J.; Sung, S. N.; Tian, J. H.; Bergbreiter, D. E. Tetrahedron 2005,
61, 12081–12092.
.
2
007, 46, 2988–3000.
10.1021/ol802728t CCC: $40.75
Published on Web 01/13/2009
 2009 American Chemical Society

Scheme 1
.
Synthesis of the PIB-SIMes Salt 4
Scheme 2. Syntheses of PIB-Supported Ru and Ag Complexes
complex. This behavior is demonstrated both for Ag(I)
complexes and with a separable, recoverable, and reusable
Ru catalyst useful in ring-closing metathesis chemistry.
Two approaches were explored as routes for the synthesis
of PIB-supported NHC ligands. First, a Friedel-Crafts
alkylation of a commercially available mixture of alkene-
terminated PIBs 1a and 1b afforded 2,6-dimethyl-4-(poly-
isobutyl)aniline 2 which like mesityl amine reacts with oxalyl
chloride to form the diamide 3. Excess 2 used in this reaction
1
0
was separated using an Amberlyst resin scavenger. The
diamide product was then reduced to form a diamine that
was converted into the PIB-bound imidazolium tetrafluo-
molecular weight pincer Ag(I) complexes formed using
octadecylazide in place of the PIB azide. The Ag(I) complex
7
roborate salt 4 using known chemistry (Scheme 1).
1
1 was directly compared to 12. In the case of 11, the carbene
A second route to PIB-bound NHC ligands (Scheme 2,
eq 1) used a “Click” cycloaddition of a dipropargylic
107,109
carbon also exhibited
Ag coupling. An analysis for Ag
showed that 10 or 11 contained 4.06% or 3.51% Ag,
respectively (4.05% or 3.89% Ag was expected for 10 or 11
if the PIB groups in the product have a degree of polymer-
ization of 20). ICP-MS analyses showed that the Ag(I)
complex 10 had a 98:2, 99:1, or 96:4 selective solubility in
8
imidazole derivative and a PIB-bound azide. Zeitler’s group
1
1
used similar chemistry to prepare PEG-bound NHCs.
Formation of metal complexes from PIB-bound imidazo-
lium salts used either KHMDS to deprotonate 4 to synthesize
a PIB-supported Hoveyda-Grubbs second-generation cata-
lyst 8 from the Hoveyda-Grubbs first-generation catalyst
heptane in mixtures of heptane with CH
AcOCH CH OAc (EGDA), or DMF. A direct comparison
of 11 with its low molecular weight analogue 12 in a partially
thermomorphic heptane/CH CN mixture showed that 11 had
3
CN,
2
2
7
12
7
or used the reaction of a basic metal oxide like Ag
with 6 or 9 to prepare the Ag(I) complexes 10 and 11
Scheme 2).
2
O
3
(
a 98:2 phase selective solubility in the heptane phase of this
solvent mixture at room temperature, while the heptane-phase
selective solubility for 12 was <1:1000. This ICP-MS
3
analysis of 11 vs 12 in this equivolume heptane/CH CN
mixture illustrates the effectiveness of the PIB phase tag.
Figure 1. Visually evident phase separation of 8 and 13: (a) a
heptane/acetonitrile mixture; (b) a heptane/DMF mixture; (c) a
heptane/EGDA mixture.
The metal complexes formed were characterized by
solution-state H and C NMR spectroscopy. The PIB-bound
pincer complex 10 had spectra that were like those of low
1
13
The phase-selective solubilities of the Ru complexes 8 and
13 mirrored those of the Ag complexes. UV-vis spectros-
copy analysis showed a heptane-phase selective solubility
difference of 99:1 vs 1:100 for 8 vs 13 in a thermomorphic
equivolume mixture of heptane and CH CN. This solubility
3
difference is evident in a visual comparison (Figure 1) and
(
10) Liu, Y.-S.; Zhao, C.; Bergbreiter, D. E.; Romo, D. J. Org. Chem.
1
998, 63, 3471–3473.
(
(
11) Wu, P.; Fokin, V. V. Aldrichim. Acta 2007, 40, 7–17.
12) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.
666
Org. Lett., Vol. 11, No. 3, 2009

Recycling of 8 was accomplished in one of two ways. The
first approach used for ring-closing metathesis of dienes 14
and 22 used heptane as a solvent, extracting product
a
Table 1. RCM Reactions with Catalyst 8
3
afterward with CH CN. In this approach, the less dense
heptane phase containing 8 was reused by simply adding
fresh substrate. Products 15 and 23 were then isolated by
3
removal of the CH CN.
While most catalyst recovery schemes focus on catalyst
separation, the solubility of 8 in heptane and the low
solubility of many organic compounds in heptane allowed
us to use another recycling scheme for substrates 16, 18,
and 20. In these cases, starting materials were soluble in a
heptane solution of the catalyst, but the products precipitate
from solution and separate themselves from the solution of
8. Products 17, 19, and 21 were recovered by simple filtration
and recycling only required adding a fresh substrate to the
recovered solution containing 8. The recyclability/recover-
ability of 8 was evaluated by ICP-MS analysis. Samples of
product 17 from the first, second, fifth, and fourteenth cycles
were digested first in concentrated nitric acid and then in
sulfuric acid. ICP-MS analysis of this solution showed that
only 0.37-0.63% of the starting Ru was in the product phase.
This level of leaching of Ru is comparable to that seen for
cycle 1
(%)
cycle 2
(%)
cycle 3
(%)
cycle 4
(%)
cycle 5
(%)
product
15
17
19
21
23
60
72
67
59
62
75
81
75
76
71
85
99
93
84
97
94
99
99
93
99
94
98
99
93
99
14
b
a water soluble PEG-supported SIMes. More significantly,
the metal leaching is ca. 10-fold less than observed for Ru
catalysts that used a PIB-bound benzylidene ligand chemistry
where Ru separation depended on a “boomerang” reaction
a a
15
Yields in cycles 1-5 are for isolated products in 1 h, 0.5 mmol scale
reactions, and increase cycle to cycle because of saturation of the catalyst-
containing heptane-phase byproduct. Twenty cycles (average yield of 97%/
for catalyst recovery.
b
cycle) were carried out with reaction times of 2 h in the 12th-13th cycles,
Acknowledgment. Support from the Robert A. Welch
Foundation (A-0639, D.E.B.), the National Science Founda-
tion (CHE-0446107, D.E.B.), and the Qatar National Re-
search Fund (D.E.B. and H.S.B., NPRP-28-6-7-7) is grate-
fully acknowledged as is the assistance of Mr. Osit
Karroonnirun (Texas A&M University) in using a Drybox
for some of the synthetic work.
4
h in 14th-18th cycles, and 8 h in 19th-20th cycles.
was confirmed by ICP-MS analyses for Ru that showed a
7:3 vs a 1:99 phase selective solubility for 8 and 13 in the
9
heptane versus the CH
mixture.
3 3
CN phase of a heptane/CH CN
Supporting Information Available: Synthetic procedures
and characterization data for PIB-bound metal complexes
and procedures for ring-closing-metathesis reactions. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ru complexes like 8 are structurally analogous to the Ru
1
3
complex 13 already used in ring-closing metathesis.
However, unlike 13, 8 could be recycled up to 20 cycles in
conversion of a variety of 1,6-dienes and 1,7-dienes into
cyclic olefins at room temperature (Table 1), though some
catalyst deactivation appeared to be occurring after cycle 12.
OL802728T
(
14) Hong, S. H.; Grubbs, R. H. Org. Lett. 2007, 9, 1955–1957.
(
13) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am.
(15) Hongfa, C.; Tian, J.; Bazzi, H. S.; Bergbreiter, D. E. Org. Lett.
2007, 9, 3259–3261.
Chem. Soc. 2000, 122, 8168–8179.
Org. Lett., Vol. 11, No. 3, 2009
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