Initial catalyst-substrate association step in enyne metathesis catalyzed by Grubbs ruthenium complex probed by time-dependent fluorescence quenching

Article abstract of DOI:10.1021/ja807717s

This communication introduces an FRET-based simple and efficient method for monitoring kinetics and thermodynamics of organic reactions and describes its application to studies on the initial catalyst-substrate association step in the enyne metathesis catalyzed by a Grubbs Ru complex to probe the reaction initiation on the alkyne versus the alkene. The kinetic and thermodynamic parameters of alkene and alkyne with the first generation Grubbs Ru complex, determined by the time-dependent fluorescence quenching of the dye-conjugated substrate by the Ru catalyst, strongly support the dominance of the reaction initiation on alkene over alkyne in the reaction both kinetically and thermodynamically. Copyright

Full text of DOI:10.1021/ja807717s

Published on Web 11/14/2008
Initial Catalyst-Substrate Association Step in Enyne Metathesis Catalyzed by
Grubbs Ruthenium Complex Probed by Time-Dependent Fluorescence
Quenching
Jeong-Hun Sohn,†,* Kyung Hwan Kim,‡,§ Hee-Yoon Lee,§ Zae Sung No,† and Hyotcherl Ihee‡,§,
*
Institut Pasteur Korea, Seoul, Korea, Center for Time-ResolVed Diffraction, Department of Chemistry (BK21),
KAIST, Daejeon, Korea
Received September 30, 2008; E-mail: sohnjh@ip-korea.org; hyotcherl.ihee@kaist.ac.kr
Scheme 1. Schematic Illustration of Probing the Initial
Catalyst-Substrate Association Step in the Enyne Metathesis
Catalyzed by Grubbs Ru Complex Using FRET (Quenching)
Herein we introduce a simple and efficient method for monitoring
kinetics and thermodynamics of organic reactions based on fluores-
cence resonance energy transfer (FRET). We report its application to
studies on the initial catalyst-substrate association step in the enyne
metathesis catalyzed by a Grubbs ruthenium (Ru) complex to probe
the reaction initiation on the alkyne versus the alkene.
The enyne metathesis catalyzed by the Grubbs Ru complex has
attracted much attention from the synthetic chemistry community
because of its ability to combine alkenes and alkynes to form 1,3-
dienes in a single step under mild reaction conditions.1,2 Two
mechanistic interpretations of this reaction have been proposed, but
the details of these proposals need experimental verification. In
particular, the initial event of the reaction in ring-closing enyne
metathesis has been a controversy.3 Earlier work with group VI
(Cr, Mo, W) metal-based catalysts commonly postulated the reaction
initiation on an alkyne,4 and presumably these results strengthen
the interpretation of the reaction proceeding via this mechanism.3d
On the other hand, the initiation on alkene was proposed by the
studies using NMR,3b,c isotopic labeling,3f and IR3g and based on
the plausible intermediates or regioselective product formation.3a,e
Much of this controversy stems from the formation of the identical
product from two equally plausible reaction pathways in the enyne
metathesis reaction and the fact that none of the reports observed
the very first event of the reaction.
Scheme 2. Synthesis of Dye-Conjugated Alkene, Alkyne, and Alkane
In transition metal catalyzed tandem reactions with multifunction-
alities or multicomponent reactions such as the enyne metathesis
catalyzed by the Grubbs Ru complex, determining the kinetic and
thermodynamic parameters of the binding of each functionality to the
catalyst is critical in characterizing the initial catalyst-substrate
association step which may govern the overall reaction pathway.
Toward this goal, we designed an FRET-based direct detection of the
interaction between the Grubbs first generation Ru catalyst and a dye-
sensitized alkene or alkyne and determined simultaneously both kinetic
and thermodynamic parameters.
FRET is a distance-dependent interaction between the electronic
excited states of the fluorescence donor and acceptor and is detected
by the appearance of sensitized fluorescence from the acceptor or by
quenching of donor fluorescence.5 Since the Ru catalyst (1) has a broad
absorbance at 527 nm with no fluorescence, we expected that this
catalyst could act as a quencher of a fluorophore, of which the
fluorescence emission band overlaps with the absorbance band of the
catalyst.6 By measuring and analyzing the time-dependent fluorescence
quenching of a dye-conjugated alkene and alkyne after mixing each
substrate with the catalyst under various reaction conditions, we
determined k, k-1, Ea, and ∆G of the binding of a substrate alkene or
alkyne with the catalyst (Scheme 1).
As an appropriate dye molecule, we chose Dapoxyl dye5a having a
fluorescence emission band near the absorbance band of the Ru
catalyst. A terminal alkene or alkyne was connected to the dye moiety
through an amide bond with an identical three-carbon tether length
(Scheme 2). As a control compound, dye-ane 3 was prepared by amide
bond formation between activated dye 2 and n-BuNH2. Dye-ene 6
was synthesized from alcohol 4 in 3 steps including a Mitsunobu
reaction and deprotection of the amino group followed by amide bond
formation. The other substrate, dye-yne 8, was prepared from a
commercially available compound 7, via amino group deprotection
followed by amide coupling. These three dye-conjugated compounds
show identical fluorescence emission bands at 508 nm in CH2Cl2,
which overlap with the broad absorbance band of the Ru catalyst.6
Time-dependent FRET data were obtained for dye-ene 6 and dye-
yne 8 at various initial conditions.6,7 For example, photoluminescence
(PL) was measured as a function of time just after mixing the substrate
at 18 µM with 1 equiv of the Ru catalyst in CH2Cl2 at 20 °C. Control
experiments with the dye-ane 3 were also carried out under identical
conditions to calibrate for quenching via nonspecific binding. Figure
1 displays the results. Apparently, the fluorescence quenching over
time for alkene 6 is substantially faster than that for alkyne 8. In the
case of a higher concentration of the substrate (26.7 µM) and the
catalyst (1 equiv), the quenching for both substrates was faster than
those in the case of 18 µM. To quantitatively analyze the kinetic traces
† Institut Pasteur Korea.
‡ Center for Time-Resolved Diffraction.
§ Department of Chemistry (BK21), KAIST.
9
16506 J. AM. CHEM. SOC. 2008, 130, 16506–16507
10.1021/ja807717s CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
1/T vs ln K (K ) k/k-1). The Gibbs free energy changes determined
for the alkene and the alkyne were -29.4 ( 0.2 and -24.8 ( 0.4
kJ/mol, respectively, meaning that the binding reaction of the alkene
with the catalyst is thermodynamically 7-fold more favorable than that
of the alkyne.
The kinetic and thermodynamic parameters determined in these
measurements for the initial catalyst-substrate association step in the
reaction strongly support the dominance of the reaction initiation on
alkenes both kinetically and thermodynamically. It should be pointed
out that scenarios where a particular reaction pathway leading to the
final product is both kinetically and thermodynamically disfavored are
rare.
In summary, we have introduced an FRET-based simple and
efficient monitoring method applicable to probing the initial
catalyst-substrate association step in the enyne metathesis catalyzed
by Grubbs’ Ru complex. Using this method, we directly determined
both kinetic and thermodynamic parameters for the binding of an
alkene and alkyne to Grubbs’ first generation Ru catalyst, which
strongly support the dominance of the reaction initiation on an
alkene over alkyne in the enyne metathesis.
Figure 1. Quenching of the dye-conjugated alkene and alkyne by Grubbs’
first generation Ru catalyst over time. (Left) PL spectra as a function of time
for substrates alkene 6 (top), alkyne 8 (middle), and alkane 3 (bottom). (Right)
Symbols are experimental time-dependent fluorescence traces at 15 different
conditions (a-o)6 and the curves are theoretical results from fitting 15 kinetic
traces simultaneously. Red circles are from experiments with 6, green squares
with 8, and blue triangles with a mixture of 6 and 8.
Acknowledgment. We thank R. Song, H. Park, and J. Kim of
Institut Pasteur Korea, for helpful discussion on the FRET experiments,
and Y. Choi for help with FRET efficiency analysis. This work was
supported by Creative Research Initiatives (Center for Time-Resolved
Diffraction) of MEST/KOSEF.
1
Table 1. Kinetic and Thermodynamic Parameters for Alkene 6 and
Alkyne 8 with Ru Catalyst 1
Supporting Information Available: Experimental details, H and
13C NMR spectra of all compounds synthesized, and all FRET data at
various reaction conditions such as the initial concentrations and temper-
atures. This material is available free of charge via the Internet at http://
pubs.acs.org.
T
(°C)
k
k-1
(s-1
∆G
(kJ/mol)
Ea
(kJ/mol)
substrate
(M-1 s-1
)
)
alkene 6
10
20
34
10
20
34
1.48
1.61
((0.07) × 103 ((0.33) × 10-2
3.71
2.12
-29.4 64.2
References
((0.13) × 103 ((0.23) × 10-2
× 0.2
× 1.9
(1) (a) For a special issue dedicated to olefin metathesis, including Grubbs, Schrock,
and Chauvin Nobel lectures, see: AdV. Synth. Catal. 2007, 349 (1-2). (b) Grubbs,
R. H. Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003.
(2) For recent representative reviews on the enyne metathesis, see : (a) Kotha,
S.; Lahiri, K. Synlett 2007, 2767. (b) Chattopadhyay, S. K.; Karmakar, S.;
Biswas, T.; Majumdar, K. C.; Rahaman, H.; Roy, B. Tetrahedron 2007, 63,
3919. (c) Diver, S. T. Coord. Chem. ReV. 2007, 251, 671. (d) Mori, M.
AdV. Synth. Catal. 2007, 349, 121. (e) Villar, H.; Frings, M.; Bolm, C. Chem.
Soc. ReV. 2007, 36, 55. (f) Hansen, E. C.; Lee, D. Acc. Chem. Res. 2006,
39, 509. (g) Mulzer, J.; Oehler, E. Top. Organomet. Chem. 2004, 13, 269.
(h) Diver, S. T.; Giessert, A. J. Chem. ReV. 2004, 104, 1317.
1.26
3.81
((0.05) × 104 ((0.30) × 10-2
alkyne 8
1.42
4.54
((0.38) × 102 ((3.17) × 10-3
2.60
9.81
-24.8 84.7
((0.41) × 102 ((8.15) × 10-3
× 0.4
× 6.1
1.85
2.77
((0.09) × 103 ((0.41) × 10-2
(3) (a) Kim, S. H.; Bowden, N.; Grubbs, R. H. J. Am. Chem. Soc. 1994, 116,
10801. (b) Hoye, T. R.; Donaldson, S. M.; Vos, T. J. Org. Lett. 1999, 1,
277. (c) Schramm, M. P.; D. Srinivasa Reddy, D. S.; Kozmin, S. A. Angew.
Chem., Int. Ed. 2001, 40, 4274. (d) Lee, H. -Y.; Kim, B. G.; Snapper, M. L.
Org. Lett. 2003, 5, 1855. (e) Hansen, E. C.; Lee, D. J. Am. Chem. Soc.
2003, 125, 9582. (f) Lloyd-Jones, G. C.; Margue, R. G.; de Vries, J. G.
Angew. Chem., Int. Ed. 2005, 44, 7442. (g) Diver, S. T.; Galan, B. R.;
Giessert, A. J.; Keister, J. B. J. Am. Chem. Soc. 2005, 127, 7444.
(4) (a) Katz, T. J.; Sivavec, T. M. J. Am. Chem. Soc. 1985, 107, 737. (b)
Korkowski, P. F.; Hoye, T. R.; Rydberg, D. B. J. Am. Chem. Soc. 1988,
110, 2676. (c) Watanuki, S.; Ochifuji, N.; Mori, M. Organometallics 1994,
13, 4129.
with a set of parameters and to determine both kinetic and thermo-
dynamic parameters, we first increased the data content by using
various initial conditions. For example, we collected more data with
2 equiv of the Ru catalyst at various temperatures, and in some cases
both 6 and 8 were mixed together as an example of competing reaction
conditions. In total, kinetic traces at 15 different conditions were
measured as shown in Figure 1.
The 15 quenching traces of various reaction conditions were
simultaneously fitted against a set of rate equations with kinetic
parameters such as rate coefficients (k and k-1).6 In the fit, the
differences between the theoretical and the experimental fluorescence
decay curves were minimized, yielding optimal values of the rate
constants for both substrates as listed in Table 1. The activation energy
(Ea) for substrate binding was determined by using Arrhenius’ equation,
k ) A exp(-Ea/RT). For dye-alkene 6 and dye-alkyne 8, linear fits
of ln k vs -1/RT gave Ea values of 64.2 ( 1.9 and 84.7 ( 6.1 kJ/mol,
respectively. The binding rate of dye-alkene 6 to the Ru complex is
∼10-fold faster than that of dye-alkyne 8. The change of Gibbs free
energy was obtained using the equation ∆G ) -RT ln K and the plot
(5) (a) Haugland, R. P. The Handbook. A Guide to Fluorescent Probes and
Labeling Technologies, 10th ed.; Invitrogen: San Diego, CA, 2005. (b)
Sapsford, K. E.; Berti, L.; Medintz, I. L. Angew. Chem., Int. Ed. 2006, 45,
4562. (c) Piston, D. W.; Kremers, G.-J. Trends Biochem. Sci. 2007, 32, 407.
(d) Huebsch, N. D.; Mooney, D. J. Biomaterials 2007, 28, 2424. (e)
Terpetschnig, E.; Szmacinski, H.; Malak, H.; Lakowicz, J. R. Biophys. J.
1995, 68, 342. (f) Youn, H. J.; Terpetschnig, E.; Szmacinski, H.; Lakowicz,
J. R. Anal. Biochem. 1995, 232, 24. (g) von Bu¨ltzingslo¨wen, C.; McEvoy,
A. K.; McDonagh, C.; MacCraith, B. D. Anal. Chim. Acta 2003, 480, 275.
(h) Lohse, M. J.; Hoffmann, C.; Nikolaev, V. O.; Vilardaga, J.-P.; Nemann,
M. AdV. Protein Chem. 2007, 74, 167.
(6) See the Supporting Information.
(7) Monitoring of the initial association step for the second generation catalyst
requires a better time resolution than presented in this work due to the fast
reaction and intrinsic instability of the catalyst.
JA807717S
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16507