Kinetics of reductive elimination from platinum(IV) as a probe for nonthermal effects in microwave-heated reactions

Article abstract of DOI:10.1021/om8008926

To test the hypothesis that microwaVe heating effects the nonthermal acceleration of reactions proceeding through polarized transition states, the kinetics and product ratios of the strongly medium-dependent thermolyses of (DPPE)Pt(CH₃)₃

Full text of DOI:10.1021/om8008926

Organometallics 2009, 28, 3303–3306
3303
Notes
Kinetics of Reductive Elimination from Platinum(IV) as a Probe for
Nonthermal Effects in Microwave-Heated Reactions
Chloe K. Lombard, Kathryn L. Myers, Zed H. Platt, and Andrew W. Holland*
Department of Chemistry, Idaho State UniVersity, 921 S. 8th AVenue, Pocatello, Idaho 83209
ReceiVed September 14, 2008
Summary: To test the hypothesis that microwaVe heating effects
the nonthermal acceleration of reactions proceeding through
polarized transition states, the kinetics and product ratios of
most thoroughly reviewed and framed as a hypothesis by Loupy
1
6
and Perreux in a 2001 report. Based on a survey of literature
evidence, these authors proposed that nonthermal effects might
result in the acceleration of reactions proceeding through
transitions states of increased polarity and that these effects
would be most pronounced when the reagents themselves (rather
than the solvent) absorbed a large flux of microwave radiation.
If valid, these oft-cited principles would be very useful in
predicting which reactions may and may not stand to benefit
from microwave heating.
the strongly medium-dependent thermolyses of (DPPE)Pt(CH
3 3
) -
O
2
CCH (1) under microwaVe and conVentional heating condi-
3
tions were compared. No eVidence of nonthermal effects was
obserVed in this reaction, although failure to apply stirring
resulted in significant differences in the thermal conditions and
reaction rates achieVed by the two heating methods.
As microwave heating technology has matured over the last
Spurred by a desire to understand the origins of apparent
2
0 years, the technique has proven to be a useful tool for the
1
7
1
-4
microwave acceleration observed in our own laboratory, we
sought to test this hypothesis by comparing the microwave and
oil-bath kinetics of a simple organometallic reaction likely,
according to these two criteria, to exhibit nonthermal effects.
Although a handful of recent studies have sought to assess the
significance of such effects, these investigations have, with a
enhancement of many chemical reactions.
Although micro-
wave reactors have been applied most enthusiastically to organic
synthesis, they have also been employed in a variety of inorganic
5
-8
and organometallic systems,
and transition metal catalysis
9
,10
features prominently among the success stories in the field.
In many cases, the improvements conferred by microwaves are
easily attributed to selective heating within heterogeneous
mixtures, exceptionally rapid heating, or access to elevated
temperatures facilitated by the microwave, but in other reactions
15,18,19
few exceptions,
compared yield data. Kinetic parameters
offer a more rigorous standard of comparison by which to
evaluate suspected acceleration, and advances in microwave
technology over the past few years now make possible the
precise temperature control that meaningful studies of kinetics
require. The demands of such a study called for a reaction
possessing several characteristics: (1) a highly polarized rate-
determining transition state; (2) a solvent with poor microwave
absorptivity; (3) a nominally unimolecular, irreversible mech-
anism resulting in simple kinetic behavior; and (4) a slow
reaction rate amenable to study by alternating microwave heating
and concentration measurements.
11
the origins of the technique’s evident benefits remain unclear.
Some researchers have raised the intriguing possibility that
in addition to heating reaction mixtures via dielectric loss, the
microwave field may also interact with reagent molecules to
1
1-13
accelerate reactions by other means.
Such nonthermal
effects have been a subject of curiosity and controversy since
14,15
early reports of microwave heating,
but this possibility was
*
(
Corresponding author. E-mail: hollandr@isu.edu.
1) MicrowaVe-Assisted Organic Synthesis; Lidstrom, P., Tierney, J. P.,
Eds.; Blackwell Publishing: Oxford, U.K., 2005.
2) MicrowaVes in Organic Synthesis, 2nd ed.; Loupy, A., Ed.; Wiley-
VCH: Weinheim, Germany, 2006.
3) MicrowaVe Methods in Organic Syntehsis; Larhed, M., Olofsson,
K., Eds.; Springer: Berlin, Germany, 2006.
These criteria led to the selection of the compound
DPPE)Pt(CH ) O CCH (1, DPPE ) 1,2-bis(diphenylphosphi-
3 3 2 3
no)ethane), which upon thermolysis undergoes reductive elimi-
(
(
nation to form C-C or C-O bonds at competitive rates (Scheme
(
20,21
1
).
Goldberg and co-workers showed that this complex, like
(
(
4) Kappe, C. O. Chem. Soc. ReV. 2008, 37, 1127–1139.
3 3
a variety of (DPPE)Pt(CH ) X analogues, decomposes by a
5) Baghurst, D. R.; Mingos, D. M. P. In MicrowaVe-Enhanced
Chemistry; Hawell, S. J., Ed.; American Chemical Society: Washington,
DC, 1997; pp 523-550.
(14) Laurent, R.; Laporterie, A.; Dubac, J.; Berlan, J.; Lefeuvre, S.;
Audhuy, M. J. Org. Chem. 1992, 57, 7099–8102.
(15) Raner, K. D.; Strauss, C. R.; Vyskoc, F.; Mokbel, L. J. Org. Chem.
1993, 58, 950–953.
(16) Perreux, L.; Loupy, A. Tetrahedron 2001, 57, 9199–9223.
(17) Seipel, K. R.; Platt, Z. H.; Nguyen, M.; Holland, A. W. J. Org.
Chem. 2008, 73, 4291–4294.
(18) Raner, K. D.; Strauss, C. R. J. Org. Chem. 1992, 57, 6231–6234.
(19) Gilday, J. P.; Lenden, P.; Moseley, J. D.; Cox, B. G. J. Org. Chem.
2008, 73, 3130–3134.
(20) Williams, B. S.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 2576–
2587.
(21) Coupling reactions involving reductive elimination from Pd(IV),
including microwave-assisted examples, have been reported, although they
may or may not be mechanistic analogues to this reaction: Hull, K. L.;
Anani, W. Q.; Sanford, M. S. J. Am. Chem. Soc. 2006, 128, 7134–7135.
(
(
6) Armstrong, A. F.; Valliant, J. F. Inorg. Chem. 2007, 46, 2148–2158.
7) Milios, C. J.; Prescimone, A.; Sanchez-Benitez, J.; Parsons, S.;
Murrie, M.; Brechin, E. K. Inorg. Chem. 2006, 45, 7053–7055.
8) Leadbeater, N. E.; Shoemaker, K. M. Organometallics 2008, 27,
254–1258.
9) Larhed, M.; Moberg, C.; Hallberg, A. Acc. Chem. Res. 2002, 35,
17–727.
(
1
7
1
3
3
(
(
10) Appukkuttan, P.; Van der Eycken, E. Eur. J. Org. Chem. 2008,
133–1155.
(
11) de la Hoz, A.; Diaz-Ortiz, A.; Moreno, A. Chem. Soc. ReV. 2005,
4, 164–178.
12) Binner, J. G. P.; Hassine, N. A.; Cross, T. E. J. Mater. Sci. 1995,
0, 5389–5393.
13) Perreux, L.; Loupy, A. In MicrowaVes in Organic Synthesis; 2nd
ed.; 2006; Vol. 1, pp 134-218.
(
(
1
0.1021/om8008926 CCC: $40.75  2009 American Chemical Society
Publication on Web 05/15/2009

3
304 Organometallics, Vol. 28, No. 11, 2009
3 3 2 3
Scheme 1. Thermolysis of (DPPE)Pt(CH ) O CCH (1)
Notes
mechanism involving initial dissociation of the acetate ligand
2
0
to yield a five-coordinate platinum cation. The resulting ions
recombine in the transition state leading to C-O elimination,
but remain separate prior to C-C elimination. As a result of
both the ionic intermediate and the differences in the polarities
of the product-forming transition states, the reaction exhibits a
striking solvent effect: experiments in tetrahydrofuran or
benzene yield 88-96% C-O elimination, and thermolyses in
acetone or nitrobenzene yield almost exclusively ethane at an
overall decomposition rate at least 50 times faster than that
observed in THF. Both pathways are irreversible, and the overall
process occurs with cleanly first-order kinetic behavior in THF,
which is a poor microwave absorber with a tan(δ) value of only
Figure 1. Plot of ln([1]
1 in THF-d
heating conditions. Data correspond to entry 1 in Table 1.
t
/[1]
o
) vs time for the stirred thermolyses of
8
under both microwave (O, · · · · ) and oil-bath (9, s)
Table 1. Observed Rate Constants and Selectivites for the
Thermolyses of 1 by Microwave and Conventional Heating in
THF-d8
microwave
oil bath
5
10 kobs/s-1
5
10 kobs/s-1
entry
T (°C)
conditions
a
b
c
d
e
1
2
3
4
5
99
99
99
109
109
1.38
1.96
1.35
3.89
3.84
1.40
1.95
1.35
3.84
3.86
2
2
0
.047. All these features render this system a very likely
candidate to experience polarity-based nonthermal microwave
effects, and a case in which these effects might be practically
quantified by the magnitude of microwave-induced changes in
both the rate and selectivity of the reaction. We thus set out to
directly compare thermolysis reactions under otherwise identical
microwave and conventional heating conditions, with the
expectation that any nonthermal effects would accelerate the
reaction and favor the more polarized C-C elimination pathway
much as more polar solvents do.
Because of the poor microwave absorptivity of THF, high
microwave powers (ca. 1200 W) were required to maintain
reaction temperatures throughout most experiments. Compari-
sons of the temperature vs time profiles of mineral oil, anhydrous
THF, and a 0.020 M (1.5 wt %) 1/THF solution all heated at
constant microwave power showed the reaction mixture to heat
most quickly, confirming that microwave energy was absorbed
primarily by the THF and also significantly by complex 1 itself
a
b
2
.5 mL of 0.011 M 1 in a 3-cm-wide vessel. 2.5 mL of 0.017 M 1
c
in a 2-cm wide vessel. 1.5 mL of 0.031 M 1 in a 2-cm wide vessel.
2.5 mL of 0.019 M 1 in a 3-cm wide vessel. 7.5 mL of 0.011 M 1 in
a 3-cm-wide vessel.
d
e
Table 2. Selectivities of the Thermolyses of 1 by Microwave and
a
Conventional Heating
microwave
oil bath
entry
solvent
T (°C)
% 2
% 2
1
2
3
4
5
6
7
8
9
1
THF-d
THF-d8
THF-d
THF-d
THF-d
8
99
99
99
109
109
100
90
90
86
88
78
80
95
83
17
6
91
84
87
79
79
95
81
18
6
8
8
8
toluene
CDCl
CD Cl
b
3
b
2
2
70
80
b
acetone
0
acetonitrile
70
<2
<2
a
(
see Supporting Information). These conditions are thus con-
See Table 1 for specific conditions of entries 1-5. In entries 6-10,
sistent with the criteria laid out by other researchers as
prerequisites for nonthermal microwave effects.
conditions were 2.0 mL of 0.0080 M 1 in a 3-cm-wide vessel except for
entry 6, in which 0.020 M 1 was used. In chlorinated solvents and in
acetone, additional Pt(CH
products were also observed.
b
3
) resonances constituting <5% of total
A total of 5 pairs of kinetic experiments, each set comprising
microwave and conventional thermolyses of identical THF
1
solutions of 1, were monitored by H NMR spectroscopy
products 2 and 3 were determined in thermolyses conducted in
acetone, acetonitrile, chloroform, methylene chloride, and
toluene under both oil-bath and microwave conditions (Table
2). These solvents represent the full range of the reaction’s
possible selectivity, varying from almost exclusive C-O
elimination (toluene) to quantitative formation of ethane and 3
(acetonitrile). In all these experiments, the ratio between the
reaction products showed no appreciable dependence on the heat
source.
between heating periods over the course of 3 half-lives. The
rate of each reaction consistently exhibited first-order depen-
dence on [1], and products 2 and 3 were the only platinum-
containing products observed (Figure 1). In each pair of
experiments, which spanned a small range of temperatures,
concentrations, and reaction volumes, the observed rate constants
in microwave- and bath-heated samples were indistinguishable
(
Table 1). Results at 99 °C typically agreed closely with those
previously reported for the complex, and variability was
observed only between samples of starting material rather than
between the two heating methods.
These data show no evidence of any nonthermal effect in
this system, and given the very high microwave power applied
and the sensitivity of the reaction to solvent polarity, this result
2
3
Product ratios were also measured in these studies, and to
extend the scope of selectivity data the relative quantities of
(23) The original report of these reactions describes the use of polyvi-
nylpyridine to scavenge acidic impurities that strongly affect rates; to avoid
the potential for differential heating of phases in the microwave, this additive
was not employed in these studies, and the rates reported here thus show
some variance from one sample to another.
(22) Gabriel, C.; Gabriel, S.; Grant, E. H.; Grant, E. H.; Halstead, B. S. J.;
Mingos, D. M. P. Chem. Soc. ReV. 1998, 27, 213–224.

Notes
Organometallics, Vol. 28, No. 11, 2009 3305
edly significant, but that polarized transition states and high
microwave power are insufficient conditions by which to predict
any nonthermal effects. These results echo recent findings
regarding microwave-assisted organic reactions and emphasize
the caution with which apparent acceleration by microwave
heating should be attributed to any factors more exotic than
differences in the thermal conditions achieved by microwave
and conventional heating.
Experimental Details
Materials and Methods. All starting materials were stored,
dispensed, and manipulated in a N
operating at or below 1 ppm O and H
in closed vessels under air-free conditions. THF, THF-d
2
-filled inert atmosphere glovebox
O. All heating was conducted
, C
t o
Figure 2. Plot of ln([1] /[1] ) vs time for the unstirred thermolyses
of 1 in THF-d under both microwave (O, · · · · ) and oil-bath (9,
8
2
2
8
6 6
D ,
s) heating conditions.
and toluene were distilled under vacuum from purple Na/benzophe-
none ketyl before use. All other solvents were dried over 4 Å
molecular sieves and vacuum transferred before use. fac-
argues against the likelihood of polarity-based nonthermal
microwave effects more generally. Other researchers have
recently reported a similar absence of any nonthermal effects
on the relative yields or rates of organic reactions heated by
2
8
(
3
DPPE)PtMe I was prepared by a literature method. All other
reagents were purchased from commercial suppliers and used as
received. NMR spectra were recorded using a 300 MHz spectrom-
eter, reported in ppm downfield from tetramethylsilane, and
referenced to the chemical shift of residual solvent protons or to
common external standards.
19,24,25
conventional and microwave equipment,
is in agreement with those findings.
and this outcome
In addition to this result, this study also provided a cautionary
example of how nonthermal effects may be incorrectly inferred
from microwave heating results. In preliminary experiments,
two pairs of kinetic runs were performed as described above,
but without significant stirring of either reaction mixture.
Interestingly, each of these experiments did result in significant
differences between microwave and traditional heating condi-
tions: the unstirred microwave reactions still exhibited consis-
tently first-order behavior throughout monitoring, but proceeded
with a higher rate constant, and with a higher fraction of CsC
bond elimination, than their oil-bath counterparts (Figure 2).
Both of these results would be consistent with the operation of
a nonthermal acceleration particularly favoring the more polar-
ized reaction mechanism to form 3, but the outcomes of the
stirred experiments illustrated in Figure 1 and Table 1 clearly
demonstrate that this reaction is subject to no such effect.
Temperature effects on the reaction pose an alternative explana-
tion: both the product ratios and rate constants observed in
unstirred reactions at 99 °C are consistent with a reaction
A 1600 W CEM MARS multimode instrument was used for all
microwave heating, which was conducted at constant temperatures
monitored and maintained by a fiber optic RTP-300 Plus probe
immersed in the reaction mixture within a sapphire thermowell and
calibrated to an oil bath at the desired reaction temperature. Each
microwave reaction was conducted in a CEM GreenChem (85 mL)
or GlassChem (25 mL) Pyrex vessel fitted with a thermowell,
composite sleeve, and PFA Teflon cover. These vessels were held
closed for the duration of the experiment by a polypropylene frame
and/or screw assembly. Oil-bath heating was conducted in fully
submereged Teflon-closed Pyrex tubes of similar dimensions (71
and 22 mL) submerged in a pre-equilibrated 1 L bath of stirred
silicone oil heated by a thermocouple-controlled hot plate. The
internal temperatures of reaction mixtures in oil-bath experiments
were verified using the microwave’s temperature sensor and found
to match those of the oil-bath settings within the (1 °C precision
of the sensor. Microwave absorption by different materials was
compared by alternately heating 3.0 mL each of mineral oil, pure
THF, and 0.020 M 1 in THF with microwave heating at a constant
power of 1600 W and monitoring the temperature over 10 min.
These experiments showed the THF and THF solution of 1 to heat
much more rapidly, confirming internal heating of the system. These
data are included as a graph in the Supporting Information.
2
6
occurring at a temperature closer to 105 °C. Although the
temperature was maintained at an average temperature of 99
°
C at the internal fiber optic probe, the exponential rather than
linear scaling of rate constants with temperature causes thermally
heterogeneous mixtures to typically exhibit overall kinetic
behavior more closely resembling that of their higher temper-
ature regions. The altered rate constants and product ratios could
thus be explained by the presence of higher temperature regions
within the unstirred mixture. Such thermal gradients have
recently been reported in other homogeneous microwave
fac-(DPPE)Pt(CH
by modification of a literature method. fac-(DPPE)PtMe
3 3 2 3
) O CCH (1). This complex was prepared
20
3
I (0.945
g, 1.23 mmol) was suspended in 40 mL of toluene, AgOAc (0.207
g, 1.24 mmol) was added, and the resulting pale yellow suspension
was stirred in the dark for 24 h at 20 °C. The resulting yellow-
beige suspension was filtered through Celite, and the filtrate was
evaporated to dryness in Vacuo. The resulting light yellow foam
was dissolved in THF, and this solution was allowed to stand at
room temperature in ambient fluorescent light for 6 h, during which
time it took on a darker, brownish color. This fine suspension was
filtered twice through Celite to yield a clear, colorless solution that
remained colorless on standing in the light and was layered with
pentane and chilled to -35 °C. Blocky white crystals were isolated
from this mixture after 48 h and further recrystallized from toluene
and pentane at -35 °C to yield 1 as fine white crystals containing
1 equiv of toluene per formula unit (0.438 g, 45.1% overall yield).
2
4,27
reactions,
and this result demonstrates that they may have
noticeable consequences even in volumes as small as 2-3 mL.
This finding demonstrates that thermal differences even in
small-scale homogeneous microwave systems may be unexpect-
(
24) Herrero, M. A.; Kremsner, J. M.; Kappe, C. O. J. Org. Chem. 2008,
7
3, 36–47.
(
25) Hosseini, M.; Stiasni, N.; Barbieri, V.; Kappe, C. O. J. Org. Chem.
2
007, 72, 1417–1424.
(
26) In addition to the data already listed, a kinetic experiment at 89 °C
was conducted to better correlate rate constant with temperature. These data
also provided further confirmation of the reaction’s dissociative mechanism,
1
195
6 6
H NMR (C D ): δ 0.04 (t with Pt satellites, 3H), 1.79 (t w/
q
q
as the resulting Eyring plot indicated large positive ∆S and ∆H values.
These data are included in the Supporting Information.
(
27) Moseley, J. D.; Lenden, P.; Thomson, A. D.; Gilday, J. P.
(28) Appleton, T. G.; Bennett, M. A.; Tomkins, I. B. J. Chem. Soc.,
Dalton Trans. 1976, 439–446.
Tetrahedron. Lett. 2007, 48, 6084–6087.

3
306 Organometallics, Vol. 28, No. 11, 2009
Notes
1
95
1
Pt satellites, 6H), 1.83 (s, 3H), 2.5 (m, 2H), 3.3 (m, 2H), 6.9-7.3
50-90% conversion and either analyzed directly by H NMR or
solution.
20
(
7
m, 12H), 7.7-7.8 (m, 8H); lit 0.05, 1.80, 1.84, 2.5, 3.3, 6.9-7.3,
evaporated to dryness in Vacuo and quantified in CDCl
Concentration Measurements. All concentrations were mea-
sured by comparison of H NMR integrations of 1 (δ -0.42, t,
H), 2 (δ 0.61, t, 6H), and 3 (δ 0.47, dd, 3H) to that of an internal
3
3
1
1
1
.7-7.8 ppm. P{ H} NMR (C
6
D
6
): δ 23.1 ( JP-Pt ) 1150 Hz);
1
2
0
1
lit 23.0 ( JP-Pt ) 1147 Hz) ppm.
3
Thermolysis of 1: Rate Measurements. In a typical experiment,
standard such as trimethoxybenzene (δ 6.05, 3H). Due to overlap
1
2
(40.1 mg, 57.5 µmol), internal standard (1,3,5-trimethoxybenzene,
.5 mg, 15 µmol), and THF-d (5.0 mL) were combined, and the
8
1
95
of the Pt satellites of their Pt(CH
3
) resonances, 2 and 3 were
) peaks
typically quantified by integration of their central Pt(CH
3
resulting solution was evenly divided between a microwave vessel
and a glass reaction tube closed by a Teflon stopcock. Each vessel
was equipped with a magnetic stir-bar, closed, and heated to 99
and algebraic correction for the satellites that overlapped with those
peaks. Alternatively, overlap was eliminated by phosphorus de-
1
coupling of H NMR spectra, which caused all Pt(CH
3
) resonances
°
C for 4 h by either microwave or oil bath. In microwave reactions
to appear as resolved singlets with satellites. The two approaches
produced nearly identical data, and an example of each is included
in the Supporting Information
an initial ramping time of ca. 7 min was observed, and the reaction
temperature was subsequently maintained within a range of (1 °C
by a power of 1200-1400 W. The oil-bath temperature was also
constant within (1 °C. After heating, each reaction vessel was
immediately chilled to -10 °C in a freezer. In an inert atmosphere
glovebox, a sample of each reaction mixture was transferred to an
Acknowledgment. The authors gratefully thank the ISU
Faculty Research Committee for supporting this work with
Research Grant 1003. C.K.L. and Z.H.P. also thank the ACS
Project SEED for summer stipends.
1
NMR tube, and each sample was analyzed by H NMR spectros-
copy. NMR samples were then returned to their respective reaction
mixtures, and the heating and analysis cycle was repeated at roughly
Supporting Information Available: Kinetic plots for all experi-
ments, microwave heating temperature profiles of control and
reaction samples, details of concentration measurements, and a
representative H NMR spectrum. This material is available free
of charge via the Internet at http://pubs.acs.org.
4
h intervals until less than 15% of starting material remained.
Product Ratios. The ratios of products 2 and 3, when measured
1
without the collection of full concentration vs time data, were
determined by performing parallel heating experiments as described
above. Mixtures were heated for a common duration to achieve
OM8008926