Mechanistic study of β-hydrogen elimination from organoplatinum(II) enolate complexes

Article abstract of DOI:10.1021/ja8056908

A detailed mechanistic investigation of the thermal reactions of a series of bisphosphine alkylplatinum(II) enolate complexes is reported. The reactions of methylplatinum enolate complexes in the presence of added phosphine form methane and either free or coordinated enone, depending on the steric properties of the enone. Kinetic studies were conducted to determine the relationship between the rates and mechanism of β-hydrogen elimination from enolate complexes and the rates and mechanism of β-hydrogen elimination from alkyl complexes. The rates of reactions of the enolate complexes were inversely dependent on the concentration of added phosphine, indicating that β-hydrogen elimination from the enolate complexes occurs after reversible dissociation of a phosphine. A normal, primary kinetic isotope effect was measured, and this effect was consistent with rate-limiting β-hydrogen elimination or C-H bond-forming reductive elimination to form methane. Reactions of substituted enolate complexes were also studied to determine the effect of the steric and electronic properties of the enolate complexes on the rates of β-hydrogen elimination. These studies showed that reactions of the alkylplatinum enolate complexes were retarded by electron-withdrawing substituents on the enolate and that reactions of enolate complexes possessing alkyl substituents at the β-position occurred at rates that were similar to those of complexes lacking alkyl substituents at this position. Despite the trend in electronic effects on the rates of reactions of enolate complexes and the substantial electronic differences between an enolate and an alkyl ligand, the rates of decomposition of the enolate complexes were similar to those of the analogous alkyl complexes. To the extent that the rates of reaction of the two types of complexes are different, those involving β-hydrogen elimination from the enolate ligand were faster. A difference between the rate-determining steps for decomposition of the two classes of complexes and an effect of stereochemistry on the selectivity for β-hydrogen elimination are possible origins of the observed phenomena.

Full text of DOI:10.1021/ja8056908

Published on Web 10/28/2008
Mechanistic Study of ꢀ-Hydrogen Elimination from
Organoplatinum(II) Enolate Complexes
Erik J. Alexanian and John F. Hartwig*
Department of Chemistry, UniVersity of Illinois, 600 South Mathews AVenue, Urbana, Illinois 61801
Received July 21, 2008; E-mail: jhartwig@uiuc.edu
Abstract: A detailed mechanistic investigation of the thermal reactions of a series of bisphosphine
alkylplatinum(II) enolate complexes is reported. The reactions of methylplatinum enolate complexes in the
presence of added phosphine form methane and either free or coordinated enone, depending on the steric
properties of the enone. Kinetic studies were conducted to determine the relationship between the rates
and mechanism of ꢀ-hydrogen elimination from enolate complexes and the rates and mechanism of
ꢀ-hydrogen elimination from alkyl complexes. The rates of reactions of the enolate complexes were inversely
dependent on the concentration of added phosphine, indicating that ꢀ-hydrogen elimination from the enolate
complexes occurs after reversible dissociation of a phosphine. A normal, primary kinetic isotope effect
was measured, and this effect was consistent with rate-limiting ꢀ-hydrogen elimination or C-H bond-forming
reductive elimination to form methane. Reactions of substituted enolate complexes were also studied to
determine the effect of the steric and electronic properties of the enolate complexes on the rates of
ꢀ-hydrogen elimination. These studies showed that reactions of the alkylplatinum enolate complexes were
retarded by electron-withdrawing substituents on the enolate and that reactions of enolate complexes
possessing alkyl substituents at the ꢀ-position occurred at rates that were similar to those of complexes
lacking alkyl substituents at this position. Despite the trend in electronic effects on the rates of reactions of
enolate complexes and the substantial electronic differences between an enolate and an alkyl ligand, the
rates of decomposition of the enolate complexes were similar to those of the analogous alkyl complexes.
To the extent that the rates of reaction of the two types of complexes are different, those involving ꢀ-hydrogen
elimination from the enolate ligand were faster. A difference between the rate-determining steps for
decomposition of the two classes of complexes and an effect of stereochemistry on the selectivity for
ꢀ-hydrogen elimination are possible origins of the observed phenomena.
Introduction
during alkene polymerizations that form highly branched
polyolefins.12 In other cases, ꢀ-hydrogen elimination is an
ꢀ-Hydrogen elimination from transition metal alkyl com-
plexes is an important fundamental transformation of organo-
metallic chemistry.1-3 This process is a common elementary
reaction of transition metal alkyl complexes and typically occurs
with a low barrier. In some cases, it is a productive step in
transition metal-catalyzed processes, such as Mizoroki-Heck
reactions,4,5 transition metal-catalyzed cycloisomerizations,6,7
alkane dehydrogenations,8,9 oxidative heterofunctionalizations
of alkenes,10 alkene isomerizations,11 and “chain walking”
unproductive side reaction. For example, ꢀ-hydrogen elimination
can lead to reduction, rather than C-C bond formation, during
cross-couplings involving alkyl nucleophiles13 or alkyl electro-
philes.14
Mechanistic studies of ꢀ-hydrogen elimination from transition
metal alkyl complexes have shown that the process is fastest
when the complex possesses an open coordination site for
binding of the alkene product and when a syn-coplanar
arrangement of the metal-carbon bond and the carbon-hydrogen
bond at the ꢀ-position can be adopted. Some of the first detailed
mechanistic studies in this area involved the thermolysis of
bisphosphine platinum(II) dialkyl complexes.15,16 These systems
yielded alkenes, alkanes, and platinum(0) complexes. Kinetic
(1) Cross, R. J. In The Chemistry of the MetalsCarbon Bond; Hartley,
F. R., Patai, S., Eds.; John Wiley: New York, 1985; Vol. 2, p 559.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry, 2nd ed.;
University Science Books: Mill Valley, CA, 1987; p 383.
(3) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
4th ed.; John Wiley: Hoboken. NJ, 2005; p 199.
(4) Beller, M.; Zapf, A.; Riermeier, T. H. In Transition Metals for Organic
Synthesis; Beller, M., Carsten, B., Eds.; Wiley-VCH Weinheim, 2004;
Vol. 1, p 271.
(10) Kotov, V.; Scarborough, C. C.; Stahl, S. S. Inorg. Chem. 2007, 46,
1910.
(11) Ohmura, T.; Yamamoto, Y.; Miyaura, N. Organometallics 1999, 18,
413.
(5) Knowles, J. P.; Whiting, A. Org. Biomol. Chem. 2007, 5, 31.
(12) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975.
(13) Miyaura, N.; Ishiyama, T.; Ishikawa, M.; Suzuki, A. Tetrahedron Lett.
1986, 27, 6389.
(6) Trost, B. M.; Krische, M. J. Synlett 1998, 1.
(7) Aubert, C.; Buisine, O.; Malacria, M. Chem. ReV. 2002, 102, 813.
(8) Go¨ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc.
2004, 126, 1804.
(14) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674.
(15) Whitesides, G. M. Pure Appl. Chem. 1981, 53, 287.
(16) Komiya, S.; Morimoto, Y.; Yamamoto, A.; Yamamoto, T. Organo-
metallics 1982, 1, 1528.
(9) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.;
Brookhart, M. Science 2006, 312, 257.
9
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J. AM. CHEM. SOC. 2008, 130, 15627–15635 15627

A R T I C L E S
Alexanian and Hartwig
data implied that these reactions, under most conditions,17
proceed through a pathway involving initial ligand dissociation,
followed by ꢀ-hydrogen elimination, and eventual C-H bond-
forming reductive elimination.
electronic effects on the rates of ꢀ-hydrogen elimination are
pronounced, but that several counterbalancing effects lead to a
complex relationship between the rates of reaction of plati-
num(II) enolate and alkyl complexes. Our data demonstrate that
electron-withdrawing groups retard the rate of ꢀ-hydrogen
elimination within a homologous series of compounds, but that
the rates of ꢀ-hydrogen eliminations from complexes containing
enolate ligands are similar to or even faster than those of
ꢀ-hydrogen eliminations from complexes containing more
electron-donating alkyl groups.
ꢀ-Hydrogen elimination is also involved in many catalytic
processes that occur through transition metal enolate complexes
as either a productive step or a step that can form side products.
For example, ꢀ-hydrogen elimination from enolate complexes
is a productive step of processes such as Saegusa oxidations of
silyl enol ethers18 and Mizoroki-Heck reactions of acrylates,4,5
whereas it is a possible unproductive side reaction of processes
such as transition metal-catalyzed R-arylations19,20 and conjugate
additions of organoboranes to enones.21,22 Although the mech-
anisms of the decompositions of metal alkyl complexes have
been studied in detail, much less information has been gained
on the rates and mechanism of the decomposition of transition
metal enolate complexes. With few exceptions,23,24 published
studies on the structure and stability of transition metal enolate
complexes have involved those that lack ꢀ-hydrogens.25,26 Such
complexes are rarely the types of enolate species involved in
the catalytic processes described above.
The reactants and products of ꢀ-hydride eliminations from
transition metal enolate complexes possess electronic properties
that are distinct from those of the reactants and products of
ꢀ-hydrogen eliminations from transition metal alkyl complexes.
In contrast to simple alkyl complexes, enolate complexes contain
an electron-withdrawing substituent on the R-carbon. In contrast
to simple alkenes, which result from ꢀ-hydrogen elimination
from alkyl complexes, the enones and enoates that result from
ꢀ-hydrogen elimination from enolate complexes are conjugated
and electron poor. These different properties can affect the
thermodynamics and ultimately the rates of ꢀ-hydrogen elimina-
tion. An electron-withdrawing group on the R-carbon typically
stabilizes alkyl complexes, whereas an electron-withdrawing
group on an olefin typically stabilizes binding of the olefin to
a low-valent metal center. Thus, it is not clear if the electronic
differences between enolate and alkyl complexes will make the
rates of ꢀ-hydride eliminations from enolate complexes faster
than, slower than, or similar to those of analogous eliminations
from alkyl complexes.
Results
1. Synthesis and Structures of PPh3-Ligated Alkylplatinum
Enolate Complexes. The platinum enolate complexes for this
study were synthesized by reaction of trans-[Pt(PPh3)2(CH3)Cl]27
or cis-[Pt(dppe)(CH3)(Cl)]28 (dppe ) Ph2PCH2CH2PPh2) with the
appropriate potassium enolate in THF or toluene (eq 1). Following
aqueous workup, the crude platinum(II) enolate complexes were
isolated by trituration with pentane, followed by filtration. In
some cases, the products were further purified by silica gel
column chromatography. The yields of pure product obtained
from these substitution reactions ranged from 33 to 88%. The
lower yield of formation of some of the complexes was due to
incomplete conversion of the starting halide, but sufficient
material was obtained for our studies after purification, even in
these cases. Once formed, the complexes were obtained as white
powders that could be stored at room temperature without need
for an inert atmosphere.
To address these questions regarding ꢀ-hydrogen elimination,
we have conducted a study of the rates and mechanism of the
thermolysis of organoplatinum enolate complexes of the general
formula cis-[Pt(PPh3)2(CH3)(enolate)]. Data from these studies
can be compared to those from classic studies on the thermolysis
of (bisphosphine)Pt(II) dialkyl complexes. We show that the
A platinum enolate complex containing both alkyl and enolate
ꢀ-hydrogens was also synthesized by a slightly modified
protocol (eq 2). trans-[Pt(PPh3)2(CH2CH3)(Cl)] was treated with
AgNO3 in CH2Cl2 to abstract the halide, followed by removal
of AgCl and addition of the enolate at -78 °C. After aqueous
workup, complex 9 was isolated in 40% yield.
(17) As discussed later in this article, reactions of the bisphosphine
platinum(II) dialkyl complexes conducted in the presence of high
concentrations of added phosphine occur from the starting four-
coordinate bisphosphine species.
(18) Tsuji, J. Palladium Reagents and Catalysis; John Wiley & Sons: West
Sussex, 2004; p 95.
(19) Culkin, D. A.; Hartwig, J. F. Acc. Chem. Res. 2003, 36, 234.
(20) Fensterbank, L.; Goddard, J.-P.; Malacria, M. ComprehensiVe Orga-
nometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.;
Elsevier: Amsterdam, 2007; Vol. 10, p 314.
(21) Fagnou, K.; Lautens, M. Chem. ReV. 2003, 103, 169.
(22) Kurihara, K.; Sugishita, N.; Oshita, K.; Piao, D.; Yamamoto, Y.;
Miyaura, N. J. Organomet. Chem. 2007, 692, 428.
Enolate complexes 1-9 existed as the C-bound isomers in
solution, as determined by 1H NMR spectroscopy. The methine
R-protons were observed between δ 2.58 and 4.23, and these
protons in all of the complexes exhibited clear coupling to the
(23) Culkin, D. A.; Hartwig, J. F. Organometallics 2004, 23, 3398.
(24) Slough, G. A.; Bergman, R. G.; Heathcock, C. H. J. Am. Chem. Soc.
1989, 111, 938.
(25) Ca´mpora, J.; Maya, C. M.; Palma, P.; Carmona, E.; Gutie´rrez, E.;
Ruiz, C.; Graiff, C.; Tiripicchio, A. Chem. Eur. J. 2005, 11, 6889.
(26) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics
1993, 12, 4899.
(27) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta
1997, 265, 117.
(28) Romeo, R.; D’Amico, G. Organometallics 2006, 25, 3435.
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ꢀ-Hydrogen Elimination from Pt-Enolates
A R T I C L E S
chlorobenzene-d5 at 95 °C in the presence of 7.5-15 mM PPh3,
1
and the conversions were followed by H NMR spectroscopy.
These reactions are summarized in eqs 3 and 4. In general, these
reactions generated methane, along with coordinated enone, free
enone, or a combination of the two. As described in more detail
later, these products would be expected to form from a sequence
of ꢀ-hydrogen elimination, followed by reductive elimination
of methane from the resulting hydrido alkyl olefin complex.
Small amounts of free ketone were observed, presumably
resulting from some reaction with an adventitious proton source.
The products from heating of enolate complexes derived from
propiophenone derivatives 1-4 are shown in eq 3. These
complexes, as well as the enolate 7 derived from tert-butyl
propionate, smoothly yielded methane and the η2-bound, mono-
substituted alkene complexes in yields >90%, as determined
1
by H NMR spectroscopy.
Figure 1. ORTEP diagram of unsubstituted propiophenone-derived
platinum enolate complex 1 with 35% ellipsoids. Selected distances (Å)
and angles (°): Pt1-C1 2.101(8), Pt1-C2 2.156(8), Pt1-P1 2.2997(18),
Pt1-P2 2.3112(19), C2-C3 1.464(13), C2-C10 1.498(13), C3-C4
1.508(12), O1-C3 1.236(11), C1-Pt1 C2 86.8(4), C1-Pt1-P1 85.4(3),
C2-Pt1-P1 169.2(2), C1-Pt1-P2 171.9(3), C2-Pt1-P2 90.9(3),
P1-Pt1-P2 97.79(7), C3-C2-C10 113.0(8), O1-C3-C2 122.2(10),
O1-C3-C4 117.9(9).
The products from heating of the enolate complexes contain-
ing alkyl substituents on the ꢀ-carbon are shown in eq 4. These
reactions also occurred with high mass balance and high
combined yields of the enone. These reactions formed the
platinum enone complexes, the combination of free enone and
phosphine-ligated Pt(0),29 and small amounts of free ketone.
For example, heating of the butyrophenone complex 5 at 95 °C
for 1 h in the presence of 7.5 mM PPh3 in C7D8 formed the
corresponding η2-bound enone complex (32%), along with the
combination of free enone (55%) and [Pt(PPh3)n] (n ) 3, 30%;
n ) 4, 14%). Free ketone from some type of protonolysis was
formed in a small 13% yield. The same reaction at 95 °C in
C6D5Cl in the absence of added ligand for 30 min provided the
η2-bound enone complex in 50% yield, along with the combina-
tion of free enone (43%) and Pt(PPh3)n. A small amount (ca.
5%) of butyrophenone was also formed.
platinum nucleus. 31P NMR spectroscopy clearly showed that
all complexes were isolated as the cis isomers. The 31P NMR
spectra consisted of two doublets possessing platinum satellites.
The known bisphosphine-ligated dialkylplatinum(II) com-
plex cis-[Pt(PPh3)2(CH2CH3)(CH3)]16 (10) was also prepared
to compare the rates of the thermal reactions of the
platinum(II) enolate complexes with those of the directly
analogous dialkyl complexes. This complex was generated
as a 7:1 cis:trans mixture by the reaction of EtMgCl with
[Pt(COD)(Cl)(Me)] to form [Pt(COD)(Et)(Me)], followed by
addition of PPh3. Attempts to prepare an R-branched alkyl
complex, such as cis-[Pt(PPh3)2(CH3)(i-Pr)], led to formation
of linear alkyl complexes. For example, the reaction of
[Pt(COD)(CH3)(Cl)] with i-PrMgBr, followed by ligand
exchange with PPh3, led to a compound assigned by 1H NMR
spectroscopy to be cis-[Pt(PPh3)2(CH3)(n-Pr)], instead of the
intended isopropyl complex cis-[Pt(PPh3)2(CH3)(i-Pr)].
The structure of propiophenone enolate 1 was determined by
single-crystal X-ray diffraction (Figure 1). Enolate complex 1
adopts a slightly distorted cis- square-planar geometry containing
a C-bound enolate. The sum of the four angles about the
platinum center is 361°. The P-Pt-P angle is significantly
larger than 90° (97.8°), while the C(1)-Pt-C(2) angle is
somewhat smaller than 90° (86.8°), presumably because of the
greater steric demands of the phosphine ligand. The carbonyl
group of the enolate is oriented away from the metal, indicating
the absence of any degree of κ2 or η3 structure of the enolate.
The structure of 1 allows a direct comparison of the M-C bond
lengths to an enolate and a simple alkyl group. The Pt-C(1)
bond to the enolate carbon (2.16 Å) is slightly longer than that
to the methyl group (2.10 Å), presumably because of the smaller
size of the methyl group.
Heating of isovalerophenone enolate complex 6 at 95 °C for
5 h in the presence of 15 mM PPh3 in C6D5Cl formed 70% free
enone and [Pt(PPh3)n], with 22% isovalerophenone as side
product. The same reaction in the absence of PPh3 yielded 61%
free enone and [Pt(PPh3)n], with only a small amount (8%) of
isovalerophenone. Thus, the yields of the products from
ꢀ-hydrogen elimination are high; the different coordinating
abilities of the unsubstituted and substituted enones and different
(29) The phosphine-ligated platinum products were a mixture of tris- and
tetrakisphosphine complexes that undergo phosphine exchange. The
ratio of these products depends on the concentration of free ligand.
These complexes were characterized by 31P NMR spectroscopy on
toluene-d8 solutions that were cooled to -90 °C.
2. Reactivity of the Alkylplatinum Enolate Complexes. The
isolated platinum(II) enolates were heated in toluene-d8 or
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A R T I C L E S
Alexanian and Hartwig
Figure 2. Deuterium-labeled platinum(II) enolates used to study the source
of ketone products.
concentrations of phosphine lead to the different distributions
of free and coordinated enones.
In an attempt to identify the source of the hydrogen atom or
proton involved in the production of the ketone product,
reactions conducted with added labeled ligand and reactions of
labeled complexes were performed. Thermolyses of enolate
complexes 5 and 6 were conducted in the presence of PPh3-
d15. The starting platinum complex exchanges free and coor-
dinated ligands faster than it undergoes ꢀ-hydrogen elimination.
Thus, if the hydrogen in the ketone were derived from free or
coordinated ligand, some deuterium should be present in this
product. However, the ketone product did not contain a
measurable amount of deuterium enrichment, as determined by
Figure 3. Plot of 1/kobs vs [PPh3] for the thermolysis of p-t-Bu propiophe-
none-derived enolate 2 in toluene-d8 at 95 °C.
1
integration of H NMR spectra. Enolate complexes 6-d5 and
6-d3 containing deuterium in the ketone aryl group and platinum
methyl group, shown in Figure 2, also did not yield isovale-
rophenone that was enriched in deuterium.
decay of the platinum complex was observed, indicating that
the reactions were first-order in the platinum complexes. A
representative plot of this decay is provided as Figure S1 in the
Supporting Information.
To determine if trace H2O in the reaction mixture was capable
of hydrolyzing the enolate complexes, the thermolysis of
isovalerophenone enolate 6 was performed in the presence of
5.0 equiv of D2O. Upon heating at 95 °C for 45 min in the
presence of 15 mM PPh3, the enolate complex was completely
consumed. Isovalerophenone-R-d1 and cis-[Pt(PPh3)2(CH3)(OD)]
were formed as products.30 Therefore, hydrolysis of the enolate
complexes could be responsible for ketone production, if the
small amount of accompanying hydroxo product is either
undetected or decomposes to form Pt(0) phosphine products.
To determine if free enone is generated reversibly during the
overall process, we conducted reactions in the presence of an
enone that is different from the one formed by ꢀ-hydrogen
elimination. The thermolysis of ester enolate complex 7 in the
presence of 5 equiv of phenyl vinyl ketone proceeded without
the accumulation of detectable amounts (by 1H NMR spectros-
copy) of a propiophenone enolate complex. Thus, any free enone
must be generated irreversibly in the overall reaction sequence.
3. Kinetic Studies. Initial kinetic studies were performed to
determine if the pathway for reaction of the methylplatinum
enolate complexes paralleled that for reaction of the related alkyl
complexes. If so, then the differences in rates of reaction of
alkyl and enolate complexes can be explained with a single
reaction manifold. To determine if the ꢀ-hydrogen elimination
process occurs after formation of an open coordination site, we
measured the effect of added ligand on the rate of the reaction,
and to determine if a step involving C-H bond cleavage is rate
limiting, we measured a deuterium kinetic isotope effect.
The rate constants for reactions of the methylplatinum enolate
complexes were measured by 1H NMR spectroscopy on samples
containing 1,3,5-trimethoxybenzene as standard. Solutions of
complexes 1-10 (15 mM) were heated in toluene-d8 or
chlorobenzene-d5 at 95 °C in the spectrometer probe, and spectra
were accumulated using an automated data acquisition program
every 2-3 min for at least three half-lives. A clear exponential
The dependence of 1/kobs on added PPh3 is shown in Figure
3. These data indicate that the reaction is inverse first-order in
phosphine. This order in added ligand demonstrates that
reversible dissociation of ligand occurs before the rate-limiting
step, and this order of events is consistent with a standard
pathway for ꢀ-hydrogen elimination involving C-H bond
cleavage after generation of an open coordination site.
Studies of a complex containing the chelating phosphine
DPPE further support a mechanism involving dissociation of
ligand prior to ꢀ-hydrogen elimination. No conversion of DPPE-
ligated methylplatinum enolate 8 was observed after heating at
95 °C for 15 min. Under these conditions in the absence of
added PPh3, the analogous complex 2 containing monodentate
ligands underwent full conversion to the elimination products.
The effect of phosphine on the thermolysis of methyl ethyl
complex 10 was investigated qualitatively. The decomposition
of 10 in the presence of 15 and 150 mM added PPh3 was
conducted side by side. The reaction in the presence of the
higher concentration occurred much more slowly than that in
the presence of the lower concentration of phosphine. These
data indicate that complex 10 reacts by a mechanism similar to
that described for cis-[Pt(PPh3)2(n-Bu)2] in previous work31 and
the PPh3-ligated enolate complexes 1-7 in the current work.
Kinetic isotope effects were obtained by examining the
thermolysis of a deuterated enolate complex (2-d3) containing
a fully deuterated methyl group in the ꢀ-position. A primary
isotope effect of kH/kD ) 3.2 ( 0.1 was deduced by comparison
of the rate constant for reaction of 2-d3 with that for thermolysis
of 2. The kinetic isotope effect on the thermolysis of the alkyl
complex 10 was also obtained using complex 10-d5 containing
a fully deuterated ethyl group. A primary isotope effect of kH/
kD ) 2.5 ( 0.1 was obtained from the rate constants for reaction
of 10-d5 and for reaction of 10.
(30) Bennett, M. A.; Jin, H.; Li, S.; Rendina, L. M.; Willis, A. C. J. Am.
Chem. Soc. 1995, 117, 8335.
(31) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem.
Soc. 1972, 94, 5258.
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ꢀ-Hydrogen Elimination from Pt-Enolates
A R T I C L E S
Table 1. Structural Effects on the Rate Constants of Reaction of
a
Pt(II) Enolate Complexes at 95 °C in Toluene-d8
Figure 4. Partially deuterated ethylplatinum enolate complex 9-d3.
containing the electron-donating p-t-Bu reacted 2 times faster
than the unsubstituted enolate complex 1. In addition, p-Me2N
complex 4 reacted 2.5 times faster than complex 1.
The effect of substitution on the ꢀ-carbon was revealed by
comparing the rates of reaction of enolate complexes 1 and 2
that lack a substituent on the ꢀ-carbon with those of enolate
complexes 5 and 6 that contain one and two substituents on the
ꢀ-carbon, respectively (Table 1). The rates of reaction of
complexes 1 and 6 were compared because both complexes are
soluble in C6D5Cl, while the rates of reaction of complex 2 and
5 were compared because both complexes are soluble in C7D8.
The rates of the reactions of complex 1 and 6 at 95 °C in C6D5Cl
were similar to each other, even though complex 6 contains
two methyl substituents at the ꢀ-carbon. Additionally, the rates
of the reactions of complexes 2 and 5 in C7D8 were similar to
each other, even though complex 5 contains a methyl substituent
on the ꢀ-carbon. Thus, the effect of substitution at the ꢀ-carbon
on the rates of ꢀ-hydrogen elimination was small; the reactions
of enolates containing methyl, methylene, and methine hydro-
gens occurred at similar rates when other substituents were
constant.
To compare the rates of the thermal reactions of alkylplatinum
enolate complexes to those of directly analogous dialkyl
complexes, we studied the thermal reaction of the methylplati-
num ethyl complex cis-[Pt(PPh3)2(CH2CH3)(CH3)]16 (10, eq 5).
This reaction was performed in C7H8 containing 15 mM PPh3
and 75 mM phenyl vinyl ketone to trap the initial Pt(0) product.
Previous studies examining the thermal reactions of bisphos-
phine platinum dialkyl complexes have shown that the addition
of electron-poor alkenes to the reaction has a negligible effect
on the reaction rate at concentrations below 100 mM.16 The
products of the reaction were the (PPh3)2Pt complex of phenyl
vinyl ketone (77%), ethylene, and methane (eq 5). Dialkyl
complex 10 reacted with a rate constant (kobs ) 2.5 × 10-4
s-1, C7D8) that was approximately one-third of that of the
reaction of the simple propiophenone-derived platinum
enolate 2.
4. Electronic and Steric Effects on the Reaction Rates of
Enolate Complexes. Studies on the series of alkylplatinum
enolate and dialkylplatinum complexes allowed an assessment
of the effect of the enolate electronic and steric properties on
the rate of the thermolysis. To reveal this effect, we measured
the rate constants for the thermolysis of the series of methyl-
platinum enolate complexes in the presence of PPh3. These data
are summarized in Table 1. These rate data demonstrate that
the electronic properties of the enolate ligands significantly affect
the observed reaction rate.
To study the electronic effects of substituents on the rates of
reactions of the enolate complexes, the rates of reaction of p-t-
Bu propiophenone enolate 2, p-CF3 propiophenone enolate 3,
and ester enolate 7 were measured first in C7D8 solvent.
Comparison with the unsubstituted enolate 1 was not possible
due to its lack of solubility in C7D8. The reactions of the enolates
at 95 °C in C7D8 in the presence of 7.5 mM PPh3 were followed
1
by H NMR spectroscopy (Table 1). In general, complexes
containing less electron-donating enolate ligands underwent
ꢀ-hydrogen elimination more slowly than those containing more
electron-donating enolate ligands. In more quantitative terms,
enolate complex 3 containing an electron-withdrawing CF3
group in the para position of the aryl ring of the enolate reacted
about 5 times more slowly than enolate complex 2 containing
a t-Bu group at this position. Likewise, complex 7 containing
a less σ-donating enolate derived from an ester (higher Taft
σ-parameter32,33) reacted about 3 times more slowly than did
2.
The effects of additional substituents were measured by
comparing the rate constants for reactions of unsubstituted
enolate 1 with those for reactions of p-t-Bu complex 2 and
p-Me2N complex 4 in chlorobenzene solvent. This comparison
was conducted on reactions in chlorobenzene due to the lack
of solubility of 1 and 4 in toluene. The enolate complex 2
Finally, we examined the thermal reaction of enolate 9
containing both alkyl and enolate ꢀ-hydrogens (eq 6) and a
deuterium-labeled version of 9. The thermolysis of complex 9
led to the formation of the η2-bound enone complex and ethane
as the major products in approximately 65% yield after 15 min.
A small amount of free ketone was also produced, but no
ethylene complex was observed in parallel with this free ketone.
The ratio of the enone complex to the free ketone was about
10:1. The thermal reaction of complex 9-d3 (Figure 4) containing
an ethyl group deuterated at the ꢀ-position was also performed
to probe for scrambling of the R- and ꢀ-positions of the ethyl
group that could occur by reversible ꢀ-hydrogen elimination
(32) Taft, R. W. J. In Steric Effects in Organic Chemistry; Newman, M. S.,
Ed.; John Wiley and Sons, Inc.: New York, 1956; p 556.
(33) Connors, K. A. Chemical Kinetics; VCH Publishers, Inc.: New York,
1990; p 339.
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A R T I C L E S
Alexanian and Hartwig
Scheme 1
from the ethyl group. Monitoring of the reaction of this complex
1
by H NMR spectroscopy at 60 °C in C6D5Cl in the presence
of 15 mM PPh3 did not lead to the growth of 1H NMR signals
corresponding to the ꢀ-position of the ethyl group after
approximately one half-life of the reaction (t1/2 ≈ 8 h). Thus,
scrambling of the R- and ꢀ-positions of the ethyl group does
not appear to occur on the time scale of the ꢀ-hydrogen
elimination process.
Discussion
1. Mechanism of Thermal Reactions of the Alkylplatinum
Enolates. The thermal reactions of bisphosphine dialkylplati-
num(II) complexes have previously been shown to occur by
multiple reaction pathways depending on the concentration of
added phosphine. Whitesides15 and Yamamoto16 reported that
the reactions conducted in the absence of added phosphine
proceed by rate-limiting dissociation of phosphine, followed by
ꢀ-hydrogen elimination. Whitesides reported that reactions in
the presence of low concentrations of excess phosphine occur
by reversible dissociation of ligand to yield a three-coordinate
species that undergoes ꢀ-hydrogen elimination, followed by rate-
limiting reductive elimination. Both authors reported that the
ꢀ-hydrogen elimination occurs in the presence of a high
concentration of excess phosphine from the four-coordinate
dialkyl complex. In this case the reaction was proposed to form
a five-coordinate η2-bound alkene intermediate that undergoes
rate-limiting dissociation of alkene or rate-limiting reductive
elimination of alkane. Our qualitative kinetic data on the reaction
of the methyl ethyl complex 10 are consistent with these data.
At the modest concentrations of added phosphine used in our
study, the reactions of complex 10 are inhibited by added
phosphine, and these data indicate that the reaction occurs by
reversible dissociation of ligand, followed by ꢀ-hydrogen
elimination and C-H bond-forming reductive elimination.
Five potential pathways for thermal reactions of the alkyl-
platinum enolate complexes in the current study are shown in
Scheme 1. Path A depicts reaction without dissociation of
phosphine to form a five-coordinate olefin hydride complex.
Path B shows initial, irreversible dissociation of phosphine. Paths
C-E show initial reversible dissociation of phosphine prior to
ꢀ-hydride elimination, but they differ in the identity of the rate-
determining step that follows reversible dissociation of phos-
phine. Path C involves irreversible rate-determining ꢀ-hydride
elimination followed by reductive elimination, path D involves
reversible ꢀ-hydride elimination followed by rate-limiting
dissociation of alkene, and path E involves reversible ꢀ-hydro-
geneliminationfollowedbyrate-determiningreductiveelimination.
Reaction by path A would be independent of the concentration
of added phosphine and would occur with a primary isotope
effect. Reaction by path B would also be independent of the
concentration of added phosphine but would occur without a
primary isotope effect. Reaction by path C would occur with a
primary isotope effect and would be expected to occur more
slowly or at a similar rate with increased alkyl substitution at
the enolate ꢀ-carbon. Reaction by path D should occur without
a primary isotope effect and should occur faster from complexes
containing a greater degree of alkyl substitution at the enolate
ꢀ-carbon (a more substituted alkene should dissociate faster than
a less substituted alkene). Reaction by path E should occur with
a primary isotope effect and should occur faster from complexes
containing alkyl substitution at the enolate ꢀ-carbon because
the rate-determining reductive elimination of methane would
involve a more sterically congested platinum(II) complex.
Reaction by a modified path E involving a reversible dissociation
of alkene prior to the rate-determining reductive elimination of
methane would also occur faster from complexes containing
alkyl substitution at the enolate ꢀ-carbon and would lead to a
new enolate complex when the reaction is run in the presence
of a labeled enone.
Our data are inconsistent with reaction by paths A, B, D,
and the modified path E. The dependence of the reaction rate
on the concentration of added phosphine is inconsistent with
paths A or B. The observation of a primary isotope effect and
the similar rates of reaction of the ꢀ-substituted and unsubsti-
tuted enolate complexes are inconsistent with reaction by path
D. The lack of formation of a new platinum enolate complex
when the reaction is run in the presence of an added enone is
inconsistent with the modified path E.
Instead, the inverse dependence of the rate on the concentra-
tion of phosphine and the primary isotope effect are consistent
with reaction by path C or E. The observation of a primary
kinetic isotope effect is consistent with rate-determining ꢀ-hy-
drogen elimination (path C) or reductive elimination (path E).
In contrast to many small or inverse isotope effects on reductive
elimination to form C-H bonds,34 the kinetic isotope effect on
the rate of reductive elimination from cis-[Pt(PPh3)2(CH3)(H)]
was shown previously to be a substantial 3.3.35 Nevertheless,
we favor reaction by path C for two reasons. First, the complexes
containing electron-withdrawing para-substituents on the aro-
(34) Bullock, R. M. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH:
New York, 1992; p 263.
(35) Abis, L.; Sen, A.; Halpern, J. J. Am. Chem. Soc. 1978, 100, 2915.
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ꢀ-Hydrogen Elimination from Pt-Enolates
A R T I C L E S
Figure 5. Four energy profiles for the effect of electronics on the ꢀ-hydride elimination from enolates and alkyl complexes to form intermediate olefin
complexes. R ) more electron-donating X-type ligand; R′ ) less electron-donating X-type ligand.
matic ring of the enolate complexes reacted more slowly than
those containing electron-donating para-substituents. Because
electron-poor alkene ligands facilitate reductive eliminations
from group 10 d8 metal complexes,36 electron-withdrawing
substituents would more likely accelerate, rather than decelerate,
reactions occurring by rate-limiting reductive elimination.
Second, the rates of reaction of the ꢀ-substituted and unsub-
stituted enolate complexes were similar, and increased substitu-
tion on the alkene ligand would be expected to increase steric
congestion and increase the rate of reductive elimination.37 Thus,
the decrease in rate of reactions of complexes containing less
electron-donating enolate ligands and the similar rates of reaction
of ꢀ-substituted and unsubstituted enolate complexes are more
consistent with reaction by path C involving rate-determining
ꢀ-hydrogen elimination than they are with reaction by path E
involving rate-determining reductive elimination of alkane from
an enone-bound hydridoplatinum(II) methyl complex.
2. Electronic Effects on ꢀ-Hydride Eliminations Involving
Transition Metal Enolates. Electronic properties of reactive and
ancillary ligands have been shown to have a large effect on the
rates of many fundamental organometallic reactions, including
oxidative additions, reductive eliminations, migratory insertions,
nucleophilic attack onto π-systems, and R-eliminations.38 Less
precise information has been reported on the effect of the
electronic properties of alkyl ligands on the rate of ꢀ-hydrogen
elimination, but a few pieces of data imply that the electronic
properties of substituents on an alkyl group can influence the
rate of ꢀ-hydrogen elimination. For instance, fluoroalkyl
complexes of late transition metals tend to be stable, despite
possessing ꢀ-hydrogens.39,40 In these cases, the rate of ꢀ-hydride
elimination is thought to decrease due to an increase in the
strength of the metal-alkyl bond.41
As noted in the Introduction, different predictions for the rates
of reactions of the enolate complexes in the current work could
be made on the basis of the electronic properties of the starting
enolates and the initial or final enone complex products. First,
polarization of the metal-carbon bond by the carbonyl group
would be expected to strengthen the metal-carbon bond of the
enolate complexes,42 and this factor would be expected to
increase the barrier to ꢀ-elimination from enolate complexes,
relative to the barrier for ꢀ-hydrogen elimination from simple
alkyl complexes. Second, η2-bound complexes formed between
electron-poor alkenes and π-basic metals can be particularly
stable due to π-back-bonding.43 However, the initial product
of ꢀ-hydrogen elimination from an alkylplatinum enolate
complex is a platinum(II) complex, not the final platinum(0)
complex. Previous studies of the electronic effects on binding
of olefins to [Pt(pyridine)Cl2] showed that electron-rich olefins
bind more strongly to this neutral Pt(II) species than electron-
poor olefins.44 Although the ligands in this complex are different
from those in the current work, this previous study does suggest
that binding of a simple alkene to neutral platinum(II) fragments,
such as [Pt(PPh3)(Me)H], is preferred over binding of an
electron-poor enone or acrylate. Thus, the effect of the carbonyl
group could lead to several different effects on the rate of the
reaction, and it was not clear which effect would be most
important.
Potential effects of these properties on the energetics of the
reaction coordinate for ꢀ-hydrogen elimination from platinum
alkyl and enolate complexes are shown in Figure 5. These
reaction coordinates can be used to represent either the energetic
difference between platinum complexes containing a more
electron-donating alkyl ligand (Pt-R) and less electron-donating
enolate ligand (Pt-R′), or the difference between platinum
complexes containing more electron-donating (Pt-R) and less
electron-donating (Pt-R′) enolate ligands. Reaction profile A
depicts a larger effect of the electron-withdrawing group on the
stability of the starting platinum(II) complexes than on the
stability of the initially formed, platinum(II) alkene complex.
Profiles B and C depict smaller effects of the electron-
withdrawing group on the stability of the starting complex than
on the initially formed alkene complex. In profile B, the complex
of the electron-poor alkene is more stable than the complex of
the electron-rich alkene, whereas in profile C, the complex of
the electron-rich alkene is more stable than the complex of the
electron-poor alkene. The relative stabilities in profile B are
typically observed when back-donation into the alkene domi-
nates electron donation from the alkene to the metal, whereas
the relative stabilities in profile C are typically observed when
donation from the alkene to the metal dominates over back-
donation into the alkene. Profile D depicts a similar electronic
effect on the starting platinum complexes and the alkene-bound
products. In profiles A and C, the rate of ꢀ-hydrogen elimination
from the Pt-R complex would be faster than from the Pt-R′
complex, in which R′ is less electron-donating than R. In profile
(36) Johnson, J. B.; Rovis, T. Angew. Chem., Int. Ed. 2008, 47, 840.
(37) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J. Am. Chem. Soc. 1971,
93, 3350.
(38) Rabinovich, D. In ComprehensiVe Organometallic Chemistry III;
Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Amsterdan, 2007;
Vol. 1, p 93.
(39) Baird, M. C.; Mague, J. T.; Osborn, J. A.; Wilkinson, G. J. Chem.
Soc., A 1967, 1347.
(40) Anderson, G. K.; Clark, H. C.; Davies, J. A. Organometallics 1982,
1, 550.
(41) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Prentice
Hall: Upper Saddle River, NJ, 1997; p 191.
(42) Bonding Energetics in Organometallic Compounds; Marks, T. J., Ed.;
American Chemical Society: Washington, D.C., 1990; Vol. 428.
(43) Crabtree, R. H. In The Organometallic Chemistry of the Transition
Metals; 4th ed.; John Wiley: Hoboken, NJ, 2005; p 125.
(44) Kurosawa, H.; Urabe, A.; Emoto, M. J. Chem. Soc., Dalton Trans.
1986, 891.
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A R T I C L E S
Alexanian and Hartwig
rate-limiting.15,16 We observed a primary kinetic isotope effect
in our studies of the reaction of dialkylplatinum complex 10 in
the presence of PPh3 (kH/kD ) 2.5 ( 0.1). This value is similar
to the value reported by Whitesides (kH/kD ≈ 3) for the reaction
of cis-[Pt(Et3P)2(CH2CH3)2] in the presence of PEt3, for which
reductive elimination was proposed to be the rate-determining
step, and is similar to the kinetic isotope effect of 3.3 ( 0.3
reported by Halpern for the reductive elimination from cis-
[Pt(PPh3)2(CH3)(H)].35
The scenario in Figure 6 involving two reaction profiles
possessing two different rate-determining steps shows graphi-
cally how a change in rate-determining step can account for
the similar rates for the reactions of the dialkylplatinum
complexes and the alkylplatinum enolate complexes. The alkyl
hydride complex resulting from ꢀ-hydrogen elimination from
the enolate group of the alkylplatinum enolate complex contains
an electron-withdrawing enone ligand, whereas the alkyl hydride
complex resulting from ꢀ-hydrogen elimination from the dialkyl
complex contains a more electron-donating alkene ligand.
Because reductive elimination tends to be faster from complexes
containing less electron-donating ligands,36 reductive elimination
from the complex containing the enone as ligand should be faster
than from the complex containing the alkene as ligand. This
difference in the identity of the alkene would favor a pathway
involving reVersible ꢀ-hydrogen elimination from a dialkyl-
platinum complex to form an alkene-ligated hydridoplatinum
alkyl complex and a pathway involving irreVersible ꢀ-hydrogen
elimination from an alkylplatinum enolate complex to form an
enone-ligated hydridoplatinum alkyl complex.
Figure 6. Reaction profiles leading to similar relative rates of elimination
from alkylplatinum enolate and alkyl complexes due to a change in the
rate-determining step.
B, the rate of ꢀ-hydrogen elimination from the Pt-R complex
would be slower than from the Pt-R′ complex. In profile D,
ꢀ-hydrogen elimination would occur with similar rates from both
complexes, despite the difference in properties of the starting
complexes.
Our kinetic data on the reactions of the propiophenone-
derived enolates 1-4 substituted with p-H, p-t-Bu, p-NMe2, and
p-CF3 substituents, as well as the ester enolate 7, are most
consistent with differences in rates that result from a combina-
tion of the energetic changes depicted in profiles A and C in
Figure 5. Electron-withdrawing groups are known to strengthen
M-C bonds,42 and electron-poor alkenes typically bind more
weakly to Pt(II) complexes. These effects would both contribute
to slower rates of ꢀ-hydrogen elimination from the complexes
containing the more electron-poor enolates.
Although electron-withdrawing groups were clearly shown
to retard the rate of ꢀ-hydrogen elimination from alkylplatinum
enolate complexes when studied in a systematic fashion, the
rates of ꢀ-hydrogen elimination from dialkylplatinum com-
plexes, such as complex 10, were slower than those from the
simple propiophenone-derived enolate complex 2. These relative
rates for reactions of alkyl and enolate complexes that appear
to override the electronic properties of alkyl and enolate ligands
could result from compensating steric and electronic effects, or
from a change in the rate-determining step between the overall
sequence for decomposition of the enolate and alkyl complexes.
The greater steric properties of the enolate complex could
increase the equilibrium for dissociation of phosphine and could
provide a greater driving force for conversion of the enolate
ligand to a hydride ligand and bound enone. At the same time,
the electronic properties of the enolate ligand should make the
thermodynamics for the elementary ꢀ-hydrogen elimination from
the enolate complex less favorable than from the alkyl complex.
The electron-withdrawing group on the alkyl ligand of the
starting material strengthens the M-C bond, and the electron-
withdrawing group on the alkene in the initial product weakens
binding of the alkene to platinum(II). These compensating steric
and electronic effects could then lead to similar rates of reaction
of the alkylplatinum enolate and dialkylplatinum complexes.
Alternatively, the relative rates of reaction of the enolate and
alkyl ligands can be rationalized by a change in rate-determining
step (Figure 6). We have provided evidence that the rate-
determining step of the reaction of platinum enolates to form
alkane and free enone is ꢀ-hydrogen elimination. However,
kinetic data reported previously on the reactions of dialkyl-
platinum complexes in the presence of added phosphine have
indicated that ꢀ-hydrogen elimination is reversible, and subse-
quent reductive elimination or dissociation of alkene is
Consistent with reversible ꢀ-hydrogen elimination from the
dialkyl complex, attempts to prepare a triphenylphosphine-
ligated methylplatinum isopropyl complex cis-[Pt(PPh3)2(CH3)(i-
Pr)] led to formation of the n-propyl complex. The n-propyl
complex presumably forms in this reaction by rapid ꢀ-hydrogen
elimination from the less stable isopropyl complex to form the
linear alkyl analogue. A similar isomerization was reported by
Yamamoto in attempts to synthesize cis-[Pt(PPh3)2(CH2CH3)(i-
Pr)] through the reaction of trans-[Pt(PPh3)2(CH2CH3)2](Cl) with
i-PrLi.16 Regardless of the origin of the similar rates of reaction
of the alkylplatinum enolate and dialkylplatinum complexes,
the electronic effect on the rate of the elementary ꢀ-hydrogen
elimination reaction does not directly translate into a difference
in rate for the overall process.
3. Intermolecular vs Intramolecular Comparisons of the
Rates of ꢀ-Hydrogen Elimination. Although the rates of reaction
of methylplatinum enolate and methylplatinum alkyl complexes
were similar, reaction of an ethylplatinum enolate complex
greatly favored formation of products resulting from ꢀ-hydrogen
elimination from the enolate ligand. The reaction of complex 9
containing an ethyl and an enolate ligand formed almost
exclusively alkane and the corresponding enone complex.
We considered that the high selectivity for the formation of
products from ꢀ-hydrogen elimination of the enolate ligand in
the ethylplatinum enolate complex could again result from
differences in rate-determining step. In this case, ꢀ-hydrogen
elimination involving the alkyl ligand would be reversible, and
ꢀ-hydrogen elimination from the enolate ligand would be
irreversible. To test this hypothesis, we prepared the partially
deuterated ethylplatinum enolate complex in which the methyl
hydrogens of the alkyl ligand contained deuterium. If ꢀ-hydro-
gen elimination from the ethyl ligand were reversible, then the
hydrogens in the R-position would exchange into the ꢀ-position.
This scrambling was not observed, and the absence of this
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15634 J. AM. CHEM. SOC. VOL. 130, NO. 46, 2008

ꢀ-Hydrogen Elimination from Pt-Enolates
A R T I C L E S
Scheme 2
ꢀ-hydrogen elimination from dialkylplatinum complexes, one
can directly compare the relative rates for reactions of these
two classes of compounds. Our data show that the rate of
ꢀ-hydrogen elimination was measurably retarded by the presence
of electron-withdrawing groups on the enolate. These data were
obtained on systems in which ꢀ-hydrogen elimination was the
rate-limiting step. This electronic effect can be attributed to both
increased stabilization of the enolate complex and decreased
stabilization of the η2-bound alkene products due to the electron-
withdrawing group attached to the R-carbon in the starting
complex. At the same time, we showed that methylplatinum
enolate complexes and methylplatinum ethyl complexes react
with similar rates, and to the extent that the rates of their
reactions differ, the methylplatinum enolate complex reacts
faster. This seemingly contradictory result can be explained by
a difference in rate-determining step for reaction of the enolate
and alkyl complexes as a result of the identity of the olefin
bound to the alkylplatinum hydride intermediate. We also
determined that the product from heating an alkylplatinum
enolate complex resulted from ꢀ-hydrogen elimination from the
enolate ligand, rather than from the alkyl ligand. This observa-
tion can be rationalized by the preferred stereochemistry of the
unsaturated intermediate prior to ꢀ-elimination. In the most
general sense, our data show that the electronic effects on
ꢀ-hydrogen elimination from substituted alkyl complexes are
measurable within a homologous series of complexes, but that
these effects can be overridden by the relative rates of reactions
occurring before and after the elementary ꢀ-hydrogen elimina-
tion process.
scrambling is inconsistent with this explanation for the pre-
dominant formation of products by ꢀ-hydrogen elimination from
the enolate ligand.
Instead, this selectivity can be explained by the stereochem-
istry of the unsaturated intermediate that undergoes ꢀ-hydrogen
elimination. As shown in Scheme 2, dissociation of phosphine
from the enolate complex should favor formation of the
unsaturated intermediate 14, due to the stronger σ-donation by
the ethyl group and the greater steric effect of the enolate group.
Because the open coordination site in this intermediate would
be located cis to the enolate ligand, ꢀ-hydrogen elimination of
the enolate ligand would occur, and the η2-bound enone complex
would predominate. Again, this result most generally indicates
that the rates of ꢀ-hydrogen elimination are similar enough that
separate factors, such as relative rates of ligand dissociation,
can control the rates of ꢀ-hydrogen elimination from two
different types of alkyl ligands.
Acknowledgment. We thank the NIH (GM-58108 to J.F.H. and
NRSA fellowship to E.J.A.) for support of this work. We also thank
Johnson-Matthey for platinum chloride.
Conclusion
We report a set of compounds that has allowed a particularly
detailed analysis of the electronic effects on the reactions of
alkyl and enolate complexes by the common sequence of
ꢀ-hydrogen elimination followed by reductive elimination. The
mechanism of the reactions of the alkylplatinum enolate
complexes involves initial ligand dissociation, followed by rate-
determining ꢀ-hydride elimination. Because the pathways
involving initial dissociation of ligand parallel those for
Supporting Information Available: Detailed experimental
procedures, spectral data, and X-ray diffraction data for platinum
complexes; X-ray crystallographic file in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA8056908
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J. AM. CHEM. SOC. VOL. 130, NO. 46, 2008 15635