Tandem ROMP-hydrogenation with a third-generation grubbs catalyst

Article abstract of DOI:10.1021/ja071047o

Tandem ROMP-hydrogenation enables efficient access to saturated, functionalized "ROMP polyolefins". Of the Grubbs-class initiators 1-3, only third-generation catalyst 3 enables efficient ROMP of sterically demanding norbornenes with precise specification of chain lengths. Unlike 1 and 2, however, 3 decomposes rapidly during subsequent hydrogenation. By trapping the propagating species with PCy₃ immediately following ROMP, we generate a Ru-terminated chain identical to that formed from 2 and exhibiting equivalent hydrogenation activity. The remarkable ROMP efficiency of 3 can thus be integrated with the high hydrogenation activity of 2, providing efficient access to chain-length-precise, functionalized polyolefins. Copyright

Full text of DOI:10.1021/ja071047o

Published on Web 03/21/2007
Tandem ROMP-Hydrogenation with a Third-Generation Grubbs Catalyst
Kenneth D. Camm, Nemesio Martinez Castro, Yuwen Liu, Pawel Czechura,
Jennifer L. Snelgrove, and Deryn E. Fogg*
Center for Catalysis Research & InnoVation, UniVersity of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Received February 13, 2007; E-mail: dfogg@uottawa.ca
Tandem catalysis has attracted much interest for the power,
economy, and process efficiencies that result from coupling multiple
catalytic transformations in a single vessel. Among such processes,
Scheme 2. Tandem ROMP-Hydrogenation of Monomers 5a-d
1
tandem ROMP-hydrogenation (ROMP ) ring-opening metathesis
polymerization) enjoys a high profile as an efficient “back-door”
route to functionalized polyolefins inaccessible by other means.2
ROMP-hydrogenation of sterically unencumbered cyclooctenes
using the Grubbs catalyst 1 is particularly well established.
Reduction is optimally achieved by liberating the ruthenium end
group on the polymer as hydrogenation-active 4a, through reaction
, base, and methanol (Scheme 1).2 These methodologies
d,3
with H
2
Progressively more demanding are endo,exo- or endo,endo-disub-
stituted 5,6-norbornenes. Hydrogenation poses its own problems,
as indicated by findings from the Nguyen group that long-chain
poly(NBE)s that are readily prepared by ROMP are reduced only
are equally applicable to other, less demanding metathesis-
hydrogenation reactions mediated by 1. Surprisingly, however,
ROMP-hydrogenation is unexplored with second- or third-genera-
tion Grubbs catalysts (e.g., 2,4 3), despite their much higher
reactivity, which could bring more challenging targets within reach.
Here we describe precise, controlled, and efficient methodologies
based on these catalysts and their application to the synthesis of
functionalized polynorbornanes.
16
with considerable difficulty. As examples spanning these chal-
lenges, we chose monomers that probe different problems in
1
7
ROMP: disubstituted endo,exo-NBEs bearing galactose (5a) or
menthol (5b) groups; endo,endo-dicarboximide NBE (5c), and a
long-chain succinimide polymer, [5d]800, expected to resist hydro-
genation (Scheme 2). Kiessling’s pioneering work on related ROMP
Scheme 1. Tandem ROMP-Hydrogenation and Catalysts
Explored (IMes ) N,N′-bis(mesityl)imidazol-2-ylidene, py )
pyridine)
7
materials points toward the potential of the saturated neoglyco-
polymers in biological applications (our own interest stemming from
their potential in tissue engineering) and of the succinimide
7
b,18
polymers as precursors to functionalized ROMP polyolefins.
ROMP of 5a-c by 1 was predictably inefficient, as illustrated for
a/c in Figure 1a. Where ROMP via second-generation initiators
5
5,19
is fast but uncontrolled, third-generation catalyst 3 enables both
high reactivity and precise specification of chain lengths, with
polydispersities as low as 1.02 (Figure 1a and Table 1). The
Grubbs,20 Slugovc, and Kiessling groups recently reported the
21
7b
2
excellent performance of related pyridine/H IMes catalysts: 3 joins
this category of outstanding Ru ROMP initiators.
Functionalized norbornenes (NBEs) have been extensively used
to construct “designer” ROMP materials. Unlike cyclooctenes, with
which backbiting and chain-transfer reactions are facile, NBEs
undergo living ROMP with appropriate initiators, enabling precise
specification of polymer chain lengths and block architecture. NBE-
based ROMP materials include supported catalysts and reagents
5
While considerably superior to 1 and 2 for ROMP, 3 performed
less well in tandem hydrogenation, as exemplified for [5a]50 in
Figure 1b. In these experiments, we used the protocol we had
2 2 3
optimized for 1, adding CH Cl , NEt , and methanol in the glovebox
once ROMP was complete, transferring the solution to an autoclave,
2d
6
7
and heating to 60 °C under 1000 psi H
2
.
Essentially identical
for organic synthesis, recognition arrays relevant to cell signaling
8
9
10
activity is found for 1 and 2 (Figure 1b), with complete reduction
of the ROMP polymer within 3 h. (This is not unexpected: the
and DNA diagnostics, drug delivery and antibacterial materials,
and photo- and electrochromic devices and transistors.1
1,12
In many
3 3
isolated hydride complexes RuHCl(CO)(PCy )(L) (4a: L ) PCy ;
of these applications, the vulnerability of the unsaturated polymers
to thermal, chemical, and photochemical degradation is a concern.2
While ROMP-hydrogenation offers a potentially attractive solution,
particularly given the ineffectiveness of conventional hydrogenation
methods,13 limitations emerge from the decreased solubility char-
acteristic of the saturated polymers. While this can be alleviated
by use of disubstituted monomers, our 1-based methodology is
limited by the inefficiency of 1 in ROMP of these sterically
encumbered substrates. Nomura and co-workers have noted that
even monosubstituted NBEs can be slow to polymerize, if pendant
groups are large;14 the bulk of protecting groups is also important.
,6a
Figure 1. Selected representative reaction profiles: (a) ROMP via 1 or 3;
(b) tandem hydrogenation of [5a]50. For details, see Table 1 and Scheme 3.
15
4168
9
J. AM. CHEM. SOC. 2007, 129, 4168-4169
10.1021/ja071047o CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalyst Performance and Polymer Propertiesa
Of the metathesis catalysts now known, none offers the essential
combination of featuresshigh ROMP activity, controlled polym-
erization, and high hydrogenation activitysto transform challenging
norbornene monomers into saturated polymers with precisely
specified chain lengths. Here we integrate the outstanding ROMP
performance of third-generation catalyst 3 with the high, sustained
hydrogenation activity of 2, by utilizing 3 as the ROMP initiator,
then inducing post-ROMP transformation of the Ru end group into
that formed via 2. This approach offers a powerful, efficient solution
to the problem of reconciling high reaction rates in demanding
ROMP and hydrogenation reactions, while retaining precise control
over polymer chain lengths. It is thus likely to find widespread
application in the burgeoning area of ROMP-based “designer
materials”.
ROMP
hydrogenation
10-
3
n
M ×
10-
3
M
n
×
(PDI)b
(PDI)b
monomer catalyst
time
% conv (time)
5
5
5
a
a
a
1
2
3
3 days 40.2 (1.02) 100 (2.5 h)
40.7 (1.02)
104.2 (1.3)
3 h
103.4 (1.3) 100 (2.6 h)
32.8 (1.02) 38 (24 h)
2.5 h
+
PCy3: 100 (3 h)
21.7 (1.03) 42 (24 h)
33.2 (1.02)
22.3 (1.02)
15.0 (1.10)
5b
3
3 h
+
PCy3: 100 (4 h)
5
5
5
c
c
d
1
3
3
7 days
6 h
0.5 h
14.9 (1.12) 100 (1 h)
12.3 (1.02) 96 (1.5 h)
c
+
PCy3: 100 (1.5 h) 13.7 (1.04)
189 (1.24) 70 (24 h)
PCy3: 100 (5 h)
5
dc,d
3
8 h
+
d
Acknowledgment. This work was supported by NSERC, the
Canada Foundation for Innovation, and Ontario Innovation Trust.
D.E.F. thanks J.-A. Mayoral (Zaragoza) for insightful discussions.
a
Typical ROMP conditions: [5]/[Ru] ) 50, CH2Cl2, 22 °C; time to
1
+
[
00% except 1/5c (70%). Hydrogenation: added CH2Cl2, NEt3, MeOH;
1.2 equiv of PCy3 if specified; 1000 psi H2, 60 °C. Calcd Mn (kDa):
5a]50, 33.3; [5b]50, 22.9; [5c]50, 12.7; [5d]50, 11.8, [5d]800, 188.1. ROMP
at -20 °C. 800 equiv 5d; saturated [5d]800 incompletely soluble in CH2Cl2.
b
c
Supporting Information Available: Experimental details, including
rate curves for tandem hydrogenation reactions. This material is
available free of charge via the Internet at http://pubs.acs.org.
d
4
b: L ) IMes) exhibit near-identical activity under similar
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