Autocatalytic alkyne cycloadditions: Evidence for colloidal Pt catalysis

Article abstract of DOI:10.1021/ja054863+

Treatment of the four-membered platinacycle L₂Pt(1,8-naphthalendiyl) (1) or the five-membered platinacycle L₂Pt(1,12-triphenylendiyl) with excess PhCCPh at 120-150 °C gives the coupling products 1,2-diphenylacenaphthalene or 4,5-diphenylbenzo[e]pyrene and the alkyne complex L₂Pt(η²-PhCCPh). Both reactions show an accelerating rate, which has been traced to catalysis of the reaction by colloidal platinum formed by the reaction of O₂ with L₂Pt(η²-PhCCPh). Copyright

Full text of DOI:10.1021/ja054863+

Published on Web 09/13/2005
Autocatalytic Alkyne Cycloadditions: Evidence for Colloidal Pt Catalysis
Rowshan Ara Begum, Nripen Chanda, T. V. V. Ramakrishna, and Paul R. Sharp*
Department of Chemistry, UniVersity of Missouri, 125 Chemistry Building, Columbia, Missouri 65211
Received July 20, 2005; E-mail: SharpP@missouri.edu
Metal-mediated cycloadditions are important chemical reactions.1-6
Scheme 1
Metallacycles, intermediates in many of these reactions, have been
intensively investigated over the years and shown to give cyclo-
addition products with alkynes.7-11 Herein, we report alkyne
cycloaddition reactions of two members of a little-studied metal-
lacycle class, where the metallacycles are fused to the edge of
polycyclic aromatic carbon compounds (PAC’s).
The two platinacycles 1 and 2 are readily synthesized from
L2PtCl2 (L ) PEt3) and the dilithio PAC’s (Scheme 1).12 Complex
1, a rare four-membered example of this class of metallacycles,13-15
may also be prepared by Na/Hg reduction of 3 or, as previously
reported, from L2PtCl2 and [Mg(1,8-naphthalendiyl)]4.13 Both 1 and
Consistent with the generation of a catalyst during the reaction,
2 are thermally robust and withstand heating in toluene solution to
more than 150 °C for hours with no observable decomposition.
Part of our interest in this class of metallacycles is for the
synthesis of larger, more complex PAC’s through coupling and
cycloaddition reactions.16 We, therefore, studied the reactions of 1
and 2 with alkynes under N2. At 25 °C, there is no reaction between
1 and PhCCPh in toluene solution. However, heating the mixture
to 120 °C gives, after several hours, a clear yellow solution
containing the cycloaddition product 1,2-diphenylacenaphthylene17
(64% isolated yield), perylene (18%), and alkyne complex 418
(quantitative yield by NMR) (eq 1). Similar reactions with the
alkynes RCCR (R ) Et, CO2Me, CO2Et) give the expected
acenaphthylenes. An analogous reaction of 2 and PhCCPh at 150
°C yields cycloaddition product 4,5-diphenylbenzo[e]pyrene 5 (86%
isolated yield) and 4 (eq 2).
re-addition of 1 and alkyne to a completed N2 reaction mixture
results in a rapid initial reaction rate without the downward
curvature of Figure 1. This could result if the product alkyne
complex 4 is a catalyst, and indeed, when independently prepared
4 is added to a fresh reaction mixture, a rapid initial reaction rate
is observed. However, while 4 must be a catalyst precursor under
the reaction conditions, it is not itself a catalyst as 4 does not react
with 1 in the absence of alkyne.
The O2 sensitivity of the cycloaddition reaction suggests that
O2 is involved in catalyst generation.23 To explore this, separate
solutions of 1 and 4 were heated for 1 h with 0.2 equiv of O2. By
31P NMR spectroscopy, there is no observed change in 1, but 4
showed the formation of a small amount of Et3PO. These solutions
were then degassed and used to prepare cycloaddition reactions
under N2. The reaction prepared from the treated solution of 1
followed the same reaction curve as an untreated sample. In contrast,
the reaction prepared from the treated solution of 4 gave a greatly
enhanced reaction rate over a similar untreated sample and was
comparable to the fastest rate observed in the reaction conducted
As the reaction in eq 1 is the first example of an alkyne
cycloaddition reaction of a four-membered PAC metallacycle,14,15
we investigated the kinetics of the reaction. Plots of the concentra-
tion of 1 versus time for the reaction of 1 with 2 equiv of PhCCPh
under different atmospheric conditions are given in Figure 1. There
are two notable features: (1) each set of data cannot be fit to any
simple order rate law with the unusual downward curvatures of
the plots indicating an accelerating reaction rate, and (2) the reaction
is sensitive to O2 with the slowest reaction in a vacuum-sealed tube
(blue circles), the fastest with added O2 (red squares), and
intermediate in a dinitrogen atmosphere (green triangles).
Accelerating reaction rates are encountered in autocatalytic
reactions,19,20 where a product is a catalyst for the reaction, and in
colloidal metal-catalyzed organic reactions, where the colloid is
produced during the reaction by an autocatalytic nucleation and
growth process.21,22
Figure 1. Plot of the concentration of 1 versus time for the reaction in eq
1 (2 equiv of PhCCPh, 120 °C): (a) blue dots, vacuum-sealed; (b) green
triangles, under N2; (c) red squares, 30 µL of O2 (0.2 equiv). Line traces
represent fitted curves (see text).
9
13494
J. AM. CHEM. SOC. 2005, 127, 13494-13495
10.1021/ja054863+ CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
with added O2. We conclude that the reaction of 4 with O2 yields
the catalyst. Taking these observations together, we propose the
reaction sequence of eqs 3 and 4.
organometallic reactions,26,28 these are also rare examples of
cycloaddition chemistry catalyzed by colloidal metal.29-32
In conclusion, evidence is presented that the cycloaddition
reactions of the PAC platinacycles 1 and 2 with diphenylacetylene
are catalyzed by colloidal Pt generated by the oxidative de-ligation
of product L2Pt(η2-PhCCPh) by traces of O2. These results should
be considered in the mechanisms of other organometallic reactions,
including those that are steps in catalytic cycles in metal-catalyzed
organic reactions. In addition, the formation of colloidal metal from
a metal complex and ubiquitous O2 is in contrast to the more usual
reductive colloid synthesis29 and suggests that colloid formation
under oxidative conditions may be common for noble metal
complexes.
A simplified mechanism based on this scheme is given in eqs 5
and 6 and was used to fit the data in Figure 1 (solid lines). First,
the kinetics of reaction 6 (i.e., reaction 4) were determined by
monitoring the reaction of 4 with various amounts of excess O2.
The reaction proceeds as indicated and is first order in 4 and O2
with k2 ) 0.0052(3) mM-1 min-1 at 120 °C. This k2 and the known
[O2] for the O2-added cycloaddition reaction were fitted to the data
in Figure 1 (red squares) solving for the initial catalyst concentration
and k1. The resulting k1 (20(1) mM-1 min-1) and the previously
determined k2 were then used to fit the vacuum (blue circles) and
N2 (green triangles) data in Figure 1 solving for the initial catalyst
and O2 concentrations. The fit of all three data sets is remarkably
good considering the simple two-step mechanism of eqs 5 and 6.
Initial catalyst concentrations range from 4.8 × 10-6 to 8 × 10-5
mM and presumably result from impurities or slight decomposition
of 1. Variations in the rates from sample to sample of 1 are observed
where data fitting suggests this is principally due to differences in
initial catalyst concentrations.
Acknowledgment. We thank NSF for supporting this work
(CHE-0101348 and 0406353), and NSF (CHE 9221835, CHE-95-
31247) and NIH (1S10RR11962-01) for providing a portion of the
funds for the purchase of the NMR equipment. We also thank R.
Finke and L. Starkey for helpful discussions and initial data fitting.
Supporting Information Available: Experimental procedures,
compound characterization, TEM images, and kinetic data and fitting
procedures. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(1) Lautens, M.; Klute, W.; Tam, W. Chem. ReV. 1996, 96, 49-92.
(2) Saito, S.; Yamamoto, Y. Chem. ReV. 2000, 100, 2901-2915.
(3) Murakami, M. Angew. Chem., Int. Ed. 2003, 7, 718-720.
(4) Harvey, D. F.; Sigano, D. M. Chem. ReV. 1996, 96, 271-288.
(5) Schore, N. E. Chem. ReV. 1988, 88, 1081-1119.
(6) Vollhardt, K. P. C. Angew. Chem. 1984, 96, 525-541.
(7) Takahashi, T.; Kotora, M.; Hara, R.; Xi, Z. Bull. Chem. Soc. Jpn. 1999,
72, 2591-2602.
k1
(8) Campora, J.; Palma, P.; Carmona, E. Coord. Chem. ReV. 1999, 195, 207-
1 + catalyst
4 + O2
9
8 4 + catalyst
(5)
(6)
281.
(9) Eisch, J. J.; Aradi, A. A.; Lucarelli, M. A.; Qian, Y. Tetrahedron 1998,
54, 1169-1184.
k2
8 catalyst
(10) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Simhai, N.; Iverson, C. N.;
Muller, C.; Satoh, T.; Jones, W. D. J. Mol. Catal., A 2002, 189, 157-
168.
9
(11) Jennings, P. W.; Johnson, L. L. Chem. ReV. 1994, 94, 2241-2290.
(12) For the 2,2′-biphenyl-1′1′-diyl analogue of 2, see: Edelbach, B. L.;
Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843-
2853.
(13) Tinga, M. A. G. M.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets,
W. J. J.; Spek, A. L. Chem. Ber. 1994, 127, 1851-1856.
(14) Cycloaddition reactions where a four-membered PAC metallacycle is a
likely intermediate: Sharp, P. R. J. Am. Chem. Soc. 2000, 122, 9880-
9881 and ref 15.
(15) Brown, J. S.; Sharp, P. R. Organometallics 2003, 22, 3604-3607.
(16) Sharp, P. R. J. Organomet. Chem. 2003, 683, 288-294.
(17) Dyker, G. J. Org. Chem. 1993, 58, 234-238.
(18) Boag, N. M.; Green, M.; Grove, D. M.; Howard, J. A. K.; Spencer, J. L.;
Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1980, 2170-2181.
(19) Perez-Benito, J. F.; Arias, C.; Brillas, E. An. Quim. 1991, 87, 849-852.
(20) Logan, S. R. Fundamentals of Chemical Kinetics; Longman: Harlow,
U.K., 1996.
The question remains as to the identity of the catalyst. The
stoichiometry of eq 4 suggests Pt metal. Generally, Pt metal is not
observed in the cycloaddition reactions but could be present as a
colloid. TEM examination of the final reaction mixtures reveals
the presence of Pt particles (50-100 nm) in apparent correlation
to the cycloaddition reaction rate. The O2 reaction shows the greatest
density with fewer but larger particles in the N2 reaction. Pt particles
are not detected in the very slow vacuum reaction. Similarly, the
O2-treated sample of 4 shows an abundance of particles, whereas
none are detected in the O2-treated sample of 1. Finally, particles
are not detected in samples of 1 or 4, eliminating TEM beam
decomposition of these complexes as the source of the observed
particles.24,25
Further evidence for colloidal Pt as the catalyst is the observation
that the cycloaddition reactions are completely suppressed by
Hg(l) or small amounts of PEt3 (0.2 mol % of 1). Both are assumed
to inhibit the reaction by blocking sites on the colloid surface,
Hg(l) by amalgamation22,26 and PEt3 by strong coordination.27 We
have also observed mild inhibition (slowed reactions) with excess
alkyne, suggesting that the alkyne and 1 compete for sites on the
Pt colloid.
(21) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382-10400.
(22) Widegren, J. A.; Bennett, M. A.; Finke, R. G. J. Am. Chem. Soc. 2003,
125, 10301-10310.
(23) O2 activation of a Ni cycloaddition catalyst by phosphine oxidation has
been reported: Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D.
Organometallics 1999, 18, 4040-4049.
(24) Hagen, C. M.; Vieille-Petit, L.; Laurenczy, G.; Su¨ss-Fink, G.; Finke, R.
G. Organometallics 2005, 24, 1819-1831.
(25) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 2698-2699.
(26) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye, J. P. P. M.;
Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.; Staudt, E.
M. Organometallics 1985, 4, 1819-1830.
(27) Widegren, J. A.; Finke, R. G. J. Mol. Catal., A 2003, 198, 317-341.
(28) Miller, T. M.; Izumi, A. N.; Shih, Y. S.; Whitesides, G. M. J. Am. Chem.
Soc. 1988, 110, 3146-3156.
(29) Bo¨nnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455-2480.
(30) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638-643.
(31) Aiken, J. D., III; Finke, R. G. J. Mol. Catal., A 1999, 145, 1-44.
(32) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757-3778.
Although 1 was our main focus, the cycloaddition kinetics of 2
with diphenylacetylene similarly show autocatalysis, suggesting that
colloidal Pt also catalyzes the cycloaddition reactions of 2. In
addition to being rare examples of heterogeneous catalysis of
JA054863+
9
J. AM. CHEM. SOC. VOL. 127, NO. 39, 2005 13495