J. Am. Chem. Soc. 2001, 123, 2677-2678
2677
Scheme 1
Screening of Homogeneous Catalysts by Fluorescence
Resonance Energy Transfer. Identification of
Catalysts for Room-Temperature Heck Reactions
James P. Stambuli, Shaun R. Stauffer,
Kevin H. Shaughnessy, and John F. Hartwig*
Department of Chemistry, Yale UniVersity
P.O. Box 208107, New HaVen, Connecticut 06520-8107
ReceiVed December 1, 2000
For many years, FRET has been used to measure binding
constants and enzyme activity.10 FRET occurs when the emission
band of one molecule overlaps with an excitation band of a second
molecule, and when the two chromophores are located within
20-80 Å of each other. Upon excitation of the fluorophore that
absorbs at the higher energy (the FRET donor), quenching of its
emission occurs by the FRET acceptor. At an appropriate constant
total concentration of free and associated FRET pairs, the emission
of the FRET donor is inversely related to the mole fraction of
associated molecules. Thus, the product yields for a reaction that
forms a covalent bond can be determined fluorimetrically by using
an inexpensive, automated fluorescence plate reader when using
one reagent containing a fluorophore and a second containing a
quencher.
Scheme 1 shows the two reagents we have chosen for our first
study, in which we evaluated catalysts for low-temperature Heck
reactions. A dansyl fluorophore was tethered covalently to a
styrenyl group, and an azodye quencher was tethered to an aryl
bromide. Compounds 1 and 2 (Scheme 1) were synthesized by
conventional methods (see Supporting Information). A fluorophore
and quencher were chosen that contained functionality that is
compatible with most cross-coupling processes, including Heck
reactions. The dansyl group has an emission wavelength that
overlaps with an absorption band of the diazo compound. Upon
covalent linking of the two molecules by the Heck coupling, the
emission of the dansyl group was quenched by the diazo
compound. The emission intensity was then converted to reaction
yield by using a linear plot correlating emission intensity to mole
fraction of coupled product. We generated this standard curve
by preparing 10-5 M solutions in m-xylene containing various
mole fractions of the two reagents and isolated product. Correla-
tion coefficients obtained for such plots were typically 0.99.
With appropriate substrates in hand, we conducted a set of Heck
reactions in a 96-well format, delivering reagents from stock
solutions using a multichannel pipet. Each well contained a
different phosphine, some of which were commercially available
and some of which we prepared by solution-phase methods. The
structures and synthetic procedures for the 96 phosphines evalu-
ated in this assay are provided as Supporting Information.
All reactions contained a 1:1 ratio of compounds 1 and 2, 2.5
equiv of Et3N, 37.2 µL of DMF solvent, 5.0 mol % of CpPd-
(allyl), and 5.0 mol % of ligand. The reactions were conducted
using an aluminum reaction block containing a 96-well glass plate.
The plate was sealed with a single Teflon sheet and Viton gasket.
The glass plate was then heated with an agitating aluminum block
at 70 °C for 15 h. After this time, a 3.3 µL aliquot was removed
from each well, diluted in m-xylene to 1.0 × 10-2 mM, and
analyzed (1 s/well) on an automated fluorescent plate reader. This
assay was run in duplicate. Of the 80 ligands which showed
moderate activity (greater than 50% yield), only four showed a
difference between the average yield and the yields for
Methods to screen catalyst activity rapidly are being developed
to address the challenge of pinpointing the optimal catalyst for a
particular reaction or of generating a lead catalyst structures for
a new process.1 Previous high-throughput screening methods have
utilized HPLC,2 mass spectrometric,3 colorimetric,4 IR thermo-
graphic,5 capillary electrophoretic,6 and fluorescence7,8 methods.
None of these methods apply to all problems. Many of them are
relatively slow, are equipment intensive, are specific to a particular
reaction, or analyze activity for formation of any product and
not necessarily the one desired.
We envisioned an assay that could provide a more general,
rapid approach to screen for product formation that would use
inexpensive, currently available equipment and that would be
highly sensitive, noninvasive, and time resolved. To develop this
assay, we decided to build upon our qualitative fluorescent method
that used substrates conveniently tagged with a fluorophore.7 We
report a method to screen for transition metal-catalyzed reactions
based on Fluorescence Resonance Energy Transfer (FRET) and
the use of this assay to identify catalysts for room-temperature
Heck reactions (eq 1)9 of aryl bromides. FRET has been exploited
as a highly sensitive assay for biological systems. We have
adapted this assay for homogeneous catalysis.
(1) Crabtree, R. H. Chem. Commun. 1999, 1611-1616. Kuntz, K. W.;
Snapper, M. L.; Hoveyda, A. H. Curr. Opin. Chem. Biol. 1999, 3, 313-319.
Jandeleit, B.; Weinberg, W. H. Chem. Ind. 1998, 19, 795-798. Jandeleit, B.;
Schaefer, D. J.; Powers, T. S.; Turner, H. W.; Weinberg, W. H. Angew. Chem.,
Int. Ed. 1999, 38, 2494-2532. Bein, T. Angew. Chem., Int. Ed. 1999, 38,
323-326.
(2) Burgess, K.; Lim, H.-J.; Porte, A. M.; Sulikowski, G. A. Angew. Chem.,
Int. Ed. 1996, 35, 220-222. Porte, A. M.; Reibenspies, J.; Burgess, K. J. Am.
Chem. Soc. 1998, 120, 9180-9187.
(3) Senkan, S.; Krantz, K.; Ozturk, S.; Zengin, V.; Onal, I. Angew. Chem.,
Int. Ed. 1999, 38, 2794-2799. Hinderling, C.; Chen, P. Angew. Chem., Int.
Ed. 1999, 38, 2253-2256. Reetz, M. T.; Becker, M. H.; Klein, H.-W.; Stockigt,
D. Angew. Chem., Int. Ed. 1999, 38, 1758-1761. Guo, J.; Wu, J.; Siuzdak,
G.; Finn, M. G. Angew. Chem., Int. Ed. 1999, 38, 1755-1758.
(4) Lavastre, O.; Morken, J. P. Angew. Chem., Int. Ed. 1999, 38, 3163-
3165. Cooper, A. C.; McAlexander, L. H.; Lee, D.-H.; Torres, M. T.; Crabtree,
R. H. J. Am. Chem. Soc. 1998, 120, 9971-9972.
(5) Reetz, M. T.; Becker, M. H.; Liebl, M.; Furstner, A. Angew. Chem.,
Int. Ed. 2000, 39, 1236-1239. Taylor, S. J.; Morken, J. P. Science 1998,
280, 267-270. Holzwarth, A.; Schmidt, H.-W.; Maier, W. F. Angew Chem.,
Int. Ed. 1998, 37, 2644-2650.
(6) Reetz, M. T.; Kuhling, K. M.; Deege, A.; Hinrichs, H.; Belder, D.
Angew. Chem., Int. Ed. 2000, 39, 3891-3893. Zhang, Y.; Gong, X.; Zhang,
H.; Larock, R. C.; Yeung, E. S. J. Comb. Chem. 2000, 2, 450-452.
(7) Shaughnessy, K. H.; Kim, P.; Hartwig, J. F. J. Am. Chem. Soc. 1999,
121, 2123-2132.
(8) Copeland, G. T.; Miller, S. J. J. Am. Chem. Soc. 1999, 121, 4306-
4307. Harris, R. F.; Nation, A. J.; Copeland, G. T.; Miller, S. J. J. Am. Chem.
Soc. 2000, 122, 11270-11271.
(9) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009-
3066. For a room temperature process with tetraalkylammonium promoters
and vinyltrimethylsilane as olefin see: Jeffery, T. Tetrahedron Lett. 1999,
40, 13-1676.
(10) Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sci. 1967, 58, 719-
726. Wu, P.; Brand, L. Anal. Biochem. 1994, 218, 1-13.
10.1021/ja0058435 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/22/2001