Catalytic hydroalkylation of olefins by stabilized carbon nucleophiles promoted by dicationic platinum(II) and palladium(II) complexes

Article abstract of DOI:10.1021/om1006787

The coordinated olefin in dicationic platinum(II) and palladium(II) complexes [M(PNP)(olefin)](SbF₆)₂ (M = Pt, Pd; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine; olefin = ethylene, propylene) reacts with β-dicarbonyl compounds (pentane-2,4-dione and methyl-3-oxobutanoate). If the proton released after the nucleophilic attack is trapped by a base, stable σ-alkyl derivatives [(PNP)M-CH₂-CH(R)CH(Ac)COR′] SbF₆ (R = H, Me; R′ = Me, OMe) are formed; otherwise the M-C σ-bond can be cleaved by the proton, in the rate-determining step of a catalytic cycle that leads to the alkylated dicarbonyl compound. The β-diketone is intrinsically more reactive than the β-ketoester, but in the catalytic reaction of the former an inhibition effect is observed in the case of the platinum catalyst.

Full text of DOI:10.1021/om1006787

5878 Organometallics 2010, 29, 5878–5884
DOI: 10.1021/om1006787
Catalytic Hydroalkylation of Olefins by Stabilized Carbon Nucleophiles
Promoted by Dicationic Platinum(II) and Palladium(II) Complexes
Maria E. Cucciolito, Angela D’Amora, and Aldo Vitagliano*
ꢀ
Dipartimento di Chimica “Paolo Corradini”, Universita di Napoli “Federico II”, Complesso Universitario di
Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy
Received July 12, 2010
The coordinated olefin in dicationic platinum(II) and palladium(II) complexes [M(PNP)(olefin)]-
(SbF6)2 (M = Pt, Pd; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine; olefin = ethylene, propylene)
reacts with β-dicarbonyl compounds (pentane-2,4-dione and methyl-3-oxobutanoate). If the proton
released after the nucleophilic attack is trapped by a base, stable σ-alkyl derivatives [(PNP)M-CH2-
CH(R)CH(Ac)COR0]SbF6 (R = H, Me; R0 = Me, OMe) are formed; otherwise the M-C σ-bond can be
cleaved by the proton, in the rate-determining step of a catalytic cycle that leads to the alkylated dicarbonyl
compound. The β-diketone is intrinsically more reactive than the β-ketoester, but in the catalytic reaction of
the former an inhibition effect is observed in the case of the platinum catalyst.
Introduction
(eq 2).2,6,7 In the latter case, a problem is that the same factors
that favor the nucleophilic attack also disfavor the protolytic
cleavage of the M-C σ-bond,2,8-10 so that an appropriate
balance of the factors controlling the two crucial steps is
needed to drive the overall reaction to completion.
The electrophilic activation of unsaturated substrates by
coordination to a transition metal ion is a key feature of
organometallic chemistry and a fundamental step in a grow-
ing number of catalytic processes.1 The latest developments
in this field, concerning the chemistry of platinum(II) and
palladium(II), have been summarized in a recent review.2
The primary product of a nucleophilic attack is a M-C
σ-bonded species, whose successive fate depends on the metal,
the coordination environment, and the kind of nucleophile
carrying on the attack. The intermediate σ-bonded deriva-
tive can undergo either β-H-elimination, as in Wacker-like
reactions,2,3 or an intramolecular carbocationic rearrange-
ment (eq 1),2,4,5 or, more frequently, a protolytic cleavage by
an acidic proton released by the nucleophile after the attack
In practice, catalysis could be obtained either by driving the
olefin activation through the use of dicationic complexes6a,7,11
or by favoring the protonolysis step through the use of neutral
complexes.2,3,6b One relevant example of the latter case, re-
ported by Widenhoefer and co-workers, is the hydroalkylation
of olefins by β-dicarbonyl compounds promoted by Zeise’s
dimer, in the presence of hydrogen chloride as a cocatalyst.3
Having in mind the above work and following our previous
studies4,5,7 on catalytic cross-coupling reactions promoted by
highly electrophilic Pt2þ and Pd2þ species, we report here a
deeper investigation of the stoichiometric and catalytic cou-
pling of ethylene and propylene with pentane-2,4-dione and
methyl-3-oxobutanoate, promoted by the dicationic complexes
1, containing the tridentate pincer ligand 2,6-bis(diphenylphos-
phinomethyl)pyridine (PNP). Our results should help in getting
a better understanding of the details of the nucleophilic addi-
tion/protolytic cleavage catalytic sequence promoted by PtII
and PdII complexes.
*To whom correspondence should be addressed. E-mail:alvitagl@
unina.it.
(1) Crabtree, R. H. The Organometallic Chemistry of Transition
Metals, 4th ed.; John Wiley & Sons: New York, 2005.
ꢁ
(2) Chianese, A. R.; Lee, S. J.; Gagne, M. R. Angew. Chem., Int. Ed.
2007, 46, 4042.
(3) Wang, X.; Widenhoefer, R. A. Chem. Commun. 2004, 660.
(4) (a) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc.
2002, 124, 9038. (b) Cucciolito, M. E.; D'Amora, A.; Vitagliano, A.
Organometallics 2005, 24, 3359.
(5) Cucciolito, M. E.; Vitagliano, A. Organometallics 2008, 27, 6360.
(6) (a) Karshtedt, D.; Bell, A. T.; Tilley, T. D. J. Am. Chem. Soc. 2005,
127, 12640. (b) Wang, X; Widenhoefer, R. A. Organometallics 2004, 23,
1649. (c) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127,
1070. (d) Michael, F. E.; Cochran, B. M. J. Am. Chem. Soc. 2006, 128, 4246.
(7) Cucciolito, M. E.; D’Amora, A.; Vitagliano, A. Organometallics
2007, 26, 5216.
(8) Whether the protonolysis takes place via a concerted three-center
mechanism or via a prior oxidative addition to the metal,9 an interaction
of the proton with the metal is involved, which is expected to be
disfavored by the positive charge on the metal when the complex is
cationic. Indeed, while in zwitterionic or neutral alkyl derivatives
derived from the nucleophilic attack of amines on platinum-olefin
complexes, the M-C bond is easily cleaved by HCl;10a reversal of the
attack was observed in cationic derivatives.10b
(9) Romeo, R.; D’Amico, G. Organometallics 2006, 25, 3435.
(10) (a) Panunzi, A.; De Renzi, A.; Palumbo, R.; Paiaro, G. J. Am.
Chem. Soc. 1969, 91, 3879. (b) Hahn, C.; Morvillo, P.; Herdtweck, E.;
Vitagliano, A. Organometallics 2002, 21, 1807.
(11) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2004, 23,
4169.
r
pubs.acs.org/Organometallics
Published on Web 10/25/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 22, 2010 5879
1
Figure 1. Time-dependent H NMR spectrum (400 MHz, CD3NO2) of the reaction between 1a (0.01 mol) and pentane-2,4-dione
(4 equiv) in a solution saturated with ethylene. The figure illustrates the fast equilibrium formation of the σ-alkyl derivative 2a and its
slow protonolysis to the organic product 2. Signals marked with an asterisk belong to the enolic form of the respective products. Refer
to Chart 1 for the labeling of compounds.
Results and Discussion
complex 1b, which displayed a slightly lower reactivity,
giving 65% conversion of the starting dione after 48 h.
In another couple of experiments, to an NMR sample of
1a or 1b (in CD2Cl2/CD3NO2 (9:1) solution, saturated with
ethylene) was added 2 equiv of pentane-2,4-dione, and the
mixture was rapidly shaken with 20 μL of water. This caused
the quenching of the catalytic reaction at its very beginning,
with almost immediate and nearly complete conversion of
the starting complexes into the corresponding σ-alkyl deri-
vatives 2a and 2b, respectively. By essentially the same
procedure, complexes 2a and 2b were then prepared on a
larger scale, to be isolated and characterized. A distinctive 1H
NMR feature (see Experimental Section for the data in
CD2Cl2 solution) of the σ-alkyl derivatives 2a and 2b is the
multiplet at δ 1.65 and 1.7, respectively, due to the four
M-CH2CH2-R protons, formerly belonging to the coordi-
nated ethylene molecule (pseudotriplet at δ = 4.62 and 4.80
in 1a and 1b, respectively). Also relevant is the shift of the
CH2 pseudotriplet of the PNP ligand from δ = 5.0 in the
dicationic olefin complexes to δ = 4.6 in the monocationic
σ-alkyl derivatives. In solution complexes 2a and 2b exist as
an equilibrium mixture of keto and enol form, the keto form
Reactions of the Ethylene Complexes [M(PNP)(C2H4)]-
(SbF6)2 (1a, M=Pt; 1b, M=Pd) with Pentane-2,4-dione. In
a preliminary experiment, the platinum complex 1a (11 mg,
0.01 mmol, dissolved in CD3NO2) was reacted with 4 equiv
of pentane-2,4-dione in an NMR tube filled with ethylene at
atmospheric pressure, and the reaction was monitored by 1H
NMR. Shortly after the dissolution of the reactants, a multi-
plet of relatively small intensity appeared in the spectrum at
δ=1.65, indicating the presence of a Pt-CH2-CH2-R frag-
ment. This signal (accounting for ca. 18% of the platinum
complex), together with a singlet at δ = 1.69 (also ascribed to
the σ-alkyl species [(PNP)PtCH2CH2CHAc2]þ, 2a), did not
grow further with time, but after several minutes other
signals began to appear and slowly increased, which could
be ascribed to the alkylation product 3-ethylpentane-2,4-
dione (2); see Figure 1. After 48 h, 75% of the starting dione
was converted to its ethyl derivative 2, while the starting
complex 1a and the σ-alkyl species 2a were still present in
solution in about the same ratio. A very similar result was
obtained by running the same experiment on the palladium

5880 Organometallics, Vol. 29, No. 22, 2010
Cucciolito et al.
Scheme 1
being the major one (80% and 84% abundance, respectively,
for 2a and 2b in dichloromethane).
Table 1. Yield and Product Distribution in the Catalytic Coupling
of Ethylene and Pentane-2,4-dionea
The above experiments demonstrated the feasibility of the
catalytic coupling reaction between ethylene and pentane-
2,4-dione, using complexes 1a and 1b as catalysts, and at the
same time gave important evidence about its mechanism
(Scheme 1). The catalytic reaction takes place through the
rapid equilibrium formation of the σ-alkyl derivative 2a or 2b
and its successive slow cleavage by the acidic proton released by
the dione substrate. This is consistent with the mechanism
depicted in Scheme 1, in whose first step (i) the electron-rich
enolic double bond leads to a nucleophilic attack on the
coordinated ethylene molecule, very much like the first step of
the hydroarylation reaction7 or that of other attacks by electron-
rich double bonds (eq 1).4,5 The released proton will then
be shared and exchanged between the basic groups present in
the system, notably the ubiquitous water molecules12 and the
oxygen atoms belonging either to the product or to the excess
substrate, until it irreversibly protonates the σ-bonded carbon
atom (step ii), most likely via a metal-mediated process.9
Successively, we ran the catalytic reaction on a larger scale
under different conditions, and the results (see Table 1) revealed
that the coupling can be performed effectively with good yields
and selectivity at a catalyst concentration down to 1 mol %, but
at the same time disclosed an unexpected effect in the case of the
platinum complex 1a. While the palladium catalyst 1b gave a
complete and smooth conversion of the substrate to its ethy-
lated products even at high concentrations of the dicarbonyl
compound (roughly corresponding to a 1:1 volume ratio to the
nitromethane solvent), in the case of platinum the catalyst (1a)
was strongly inhibited by increasing the concentration of the
nucleophilic substrate. Indeed, doubling the substrate concen-
tration (Table 1, entry 3 vs 1) gave less than one-third of the
conversion (as number of moles reacted), and a similar effect
was observed also at a higher temperature and substrate/
catalyst ratio (Table 1, entry 7 vs 5). Increasing the ethylene
concentration reduced the inhibition effect (Table 1, entry 4 vs
3, 8 vs 7), suggesting the occurrence of a competitive inhibition
yield % (% of monoethylated
product 2)b
dione/catalyst
(mol/mol)
entry
P (bar)
Pt cat. 1a
Pd cat. 1b
1
2
3
4
5
6
7
8
15
15
30
1
7
1
7
1
7
1
7
85 (86)
85 (86)
13 (100)
50 (96)
>99 (22)
>99 (10)
13 (100)
34 (98)
>99 (100)
>99 (98)
>99 (98)
>99 (98)
>99 (70)
>99 (65)
>99 (95)
>99 (92)
30
50c
50c
100c
100c
a Solvent: nitromethane (free from nitriles). Concentration of the
catalyst: 0.1 mol/L (before the addition of the substrate). T: 25 °C
(70 °C when stated by note c). Reaction time: 24 h. b Yield determined by
1H NMR and expressed as % of dione reacted. Monoethylated (2) and
diethylated (3) products are formed, and the relative abundance (%) of
2 is given in parentheses. c T = 70 °C.
occurring through displacement of ethylene by the dione.
Moreover, the inhibition effect appears to increase almost qua-
dratically with the dione concentration, which could be tenta-
tively explained by the formation of an inactive species contain-
ing the acac ligand (possibly σ-bonded to Pt via the C atom13),
generated by the autoprotolysis of the substrate pentane-2,4-
dione. The same species might be less stable in the case of palla-
dium and therefore be irrelevant to the reaction. Unfortunately,
this remains a speculative suggestion, because our attempts to
prepare such a complex were so far unsuccessful.
The relative abundance of the monoethylated and diethy-
lated products indicates a higher reactivity of the starting
substrate with respect to its ethylated derivative, which appears
to be especially relevant for the palladium complex 1b, con-
sidering that at room temperature (Table 1, entries 1-4) very
little diethylated product 3 was formed even after exhaustion of
the starting substrate. Apart from the inhibition effect observed
in the case of the platinum complex and from the latter fine
detail, there is no substantial difference between the platinum
and palladium catalysis of the alkylation reaction. This con-
trasts with the outcome of the analogous alkylation reported by
Widenhoefer,3 in which the palladium complex gave mainly the
(12) In this respect we note that the quantitative aspects of the
reaction progress, especially the relative amount of the σ-alkyl derivative
present in the quasi-stationary state, are dependent on the amount of
trace water doping the solvent (estimated to be between 50 and 100 ppm
in our experiments), a larger water content increasing, as expected, the
concentration of the σ-alkyl derivative. Nevertheless, the main qualita-
tive features remain unaffected.
(13) De Pascali, S. A.; Papadia, P.; Ciccarese, A.; Pacifico, C.;
Fanizzi, F. P. Eur. J. Inorg. Chem. 2005, 788.

Article
Organometallics, Vol. 29, No. 22, 2010 5881
Figure 2. 1H NMR monitoring (400 MHz, CD3NO2) of the reaction between 1a (0.01 mol) and methyl-3-oxobutanoate (4 equiv) in a
solution saturated with ethylene. The figure illustrates the relatively slow formation of the σ-alkyl derivative 4a and its slower
protonolysis to the organic product 4. Refer to Chart 1 for the labeling of compounds. Strong unlabeled signals belong to methyl-3-
oxobutanoate.
unsaturated product resulting from a β-H-elimination reaction.
In our case, the tridentate pincer ligand PNP prevents the
occurrence of the β-H-elimination, and only the plain hydro-
alkylation reaction is observed.
M-C bond, resulting after 48 h in a 13% conversion of the
starting substrate to its ethylated product 4.
Another experiment on the platinum complex 1a (CD2Cl2/
CD3NO2, 9:1, solution saturated with ethylene) was run in
the presence of 20 μL of water to trap the protons and conse-
quently quench the catalytic cycle. This gave an interesting
result, confirming the much slower formation of the σ-alkyl
derivative 4a compared to 2a. While 1a reacted immediately
with the diketone, quantitatively forming 2a within the time
needed to acquire the spectrum (see previous section), in the
case of the ketoester the complex reacted first with water,
giving the σ-alkyl species [(PNP)Pt-CH2CH2-OH]þ.10b Due
to the reversibility of the nucleophilic addition,10b the kinetic
β-hydroxyethyl product was slowly but totally converted to
the thermodynamic product 4a within 2 h. In the case of the
palladium complex 1b the evolution was similar, but com-
plicated by the simultaneous occurrence of the ethylene
displacement reaction by water,14 ultimately giving the ther-
modynamic product 4b within 3 h. Therefore in the case of
Reactions of the Ethylene Complexes [M(PNP)(C2H4)]-
(SbF6)2 (1a, M = Pt; 1b, M = Pd) with Methyl-3-oxobuta-
noate. As in the case of pentane-2,4-dione, the reaction of the
platinum complex 1a was first monitored by 1H NMR under
the same conditions reported in the previous section. Signals
due to the σ-alkyl species [(PNP)PtCH2CH2CH(Ac)CO-
OMe]þ (4a) appeared shortly after mixing of the reactants,
but in this case they were of much smaller intensity, account-
ing for ca. 3% of the starting platinum complex, and pro-
gressively grew with time, reaching a nearly stationary state
at about 13% of the starting complex within 2 h. At the same
time, signals that could be ascribed to the alkylation product
4 appeared and slowly grew with time (Figure 2), resulting
after 48 h in a 40% conversion of the starting substrate.
Under the same conditions, the palladium complex 1b
behaved similarly, giving approximately the same stationary
concentration of the σ-alkyl species [(PNP)PdCH2CH2CH-
(Ac)COOMe]þ (4b), but a slower protolytic cleavage of the
(14) Hahn, C.; Morvillo, P.; Vitagliano, A. Eur. J. Inorg. Chem. 2001,
419.

5882 Organometallics, Vol. 29, No. 22, 2010
Cucciolito et al.
the β-ketoester as a nucleophile, the addition equilibrium
(step i in Scheme 1) is much slower and apparently less shifted
to the σ-alkyl product than for the β-diketone, most reason-
ably due to the enolic form of the compound being both less
abundant and less electron-rich. By reacting 1a and 1b with
the ketoester in presence of water, 4a and 4b were prepared
on a larger scale, to be isolated and characterized (see
Experimental Section). Differently from 2a and 2b, in this
case no appreciable amount of the enolic form was detectable
in dichloromethane solution.
When the catalytic runs were made on a larger scale, the
results (Table 2) matched the general trend observed in the
NMR monitoring of the reaction, i.e., a lower reactivity of
the β-ketoester compared with the β-diketone and a lower
reactivity of the palladium complex compared to platinum.
Indeed, from the inspection of Table 2 it is apparent that
the reactivity of the platinum catalyst is not at all inhibited
by the substrate, as it was in the case of the β-diketone, so
that at 70 °C even with a large initial substrate/catalyst
ratio, not only was the conversion complete, but also the
diethylated product 5 was either dominant (Table 2, entries
5, 6) or quite abundant (Table 2, entry 7). In view of the
explanation proposed in the previous section for the inhibi-
tion effect, this is consistent with the much lower stability
of an anionic species (analogous to acac) in the case of the
β-ketoester.
It is worth noting that, in spite of the much lower reactivity
of the β-ketoester in the nucleophilic addition (step i in
Scheme 1) that was outlined above, the overall rate of the
catalytic reaction is not that much smaller, indicating that an
increase of the rate of the protonolysis (step ii in Scheme 1)
does partially compensate the reduction of both the rate of
formation of the σ-alkyl derivative and its equilibrium
abundance. This is in agreement with the inverse correlation
between the factors favoring the nucleophilic addition and
those favoring the protolitic cleavage, which was anticipated
in the Introduction. Indeed, a lower “basicity” of the
β-dicarbonyl compound combined with a lower abundance
of its enolic form disfavors the nucleophilic attack, but at the
same time makes the released proton more acidic, favoring
the protonolysis of the M-C σ-bond. The lower proton
affinity of the β-ketoester compared to the β-diketone is also
reflected in the different behavior of the isolated σ-alkyl
complexes in their reaction with gaseous HCl in dichloro-
methane. The dione derivatives 2a and 2b gave mainly the
reversal of the nucleophilic addition and only a minor
amount (15% and 28%, respectively) of the ethylated pro-
duct 2, while for the ketoester derivatives 4a and 4b the
cleavage of the M-C bond was dominant, resulting in 68%
and >98% of the ethylated product 4, respectively.
Reactions of the Propylene Complexes [M(PNP)(C3H6)]-
(SbF6)2 (6a, M = Pt; 6b, M = Pd). Propylene proved to be
much less reactive than ethylene in the coupling reaction. At
room temperature, using the same conditions as in the NMR
monitoring experiments reported in the previous sections, no
reaction was observed. Nevertheless, by reacting complex 6a or
6b (CH2Cl2 solution saturated with propylene) with pentane-
2,4-dione in presence of water, the σ-alkyl species (7a or 7b,
respectively) were formed within 24 h, indicating that the
addition equilibrium (analogous to step i in Scheme 1) was still
effective in the case of propylene, although at a slower rate. An
interesting side reaction also took place, which prevented the
isolation of 7a and 7b, which were identified only by 1H NMR
as a mixture with a new species (8a or 8b, respectively) in about
30% abundance (Scheme 2). 8a and 8b were identified by com-
parison of their NMR signals with those of authentic samples
obtained in a previous work.5 The formation of 8a and 8b is
Table 2. Yield and Product Distribution in the Catalytic Coupling
of Ethylene and Methyl-3-oxobutanoatea
yield % (% of monoethylated
product 4)b
substrate/catalyst
(mol/mol)
entry
P (bar)
Pt cat. 1a
Pd cat. 1b
1
2
3
4
5
6
7
15
15
30
1
7
1
7
1
7
1
87 (82)
87 (85)
87 (88)
46 (100)
40 (100)
33 (94)
33 (97)
>99 (87)
>99 (87)
30
90 (88)
70c
70c
100c
>99 (13)
>99 (8)
>99 (58)
a Solvent: nitromethane (free from nitriles). Concentration of the
catalyst: 0.1 mol/L (before the addition of the substrate). T: 25 °C
(70 °C when stated by note c). Reaction time: 24 h. b Yield determined by
1H NMR and expressed as % of ketoester reacted. Monoethylated (4)
and diethylated (5) products are formed, and the relative abundance (%)
of 4 is given in parentheses. c T = 70 °C.
Scheme 2

Article
Organometallics, Vol. 29, No. 22, 2010 5883
Chart 1. Metal σ-Alkyl Derivatives and Corresponding β-Di-
carbonyl Products
spectra were recorded on Varian VXR 200, Varian Gemini 300,
and Bruker WH 400 instruments. The 1H NMR shifts were
referenced to the resonance of the residual protons of the solvents
(δ = 5.32, CHDCl2; δ= 4.33, HCD2NO2); the 13C NMR shifts, to
the solvent resonance (δ = 53.8, CD2Cl2). Abbreviations used in
NMR data: s, singlet; d, doublet; t, triplet; ps t, pseudo triplet; m,
multiplet; br, broad.
Syntheses. The complexes [Pt(PNP)(CH2dCHR)](SbF6)2
and [Pd(PNP)(CH2dCHR)](SbF6)2 were prepared according
to the procedures described in the literature.10b,14,17
Preparation of σ-Derivatives. General Procedure. To a solu-
tion of the alkene complex (0.040 mmol) in 2 mL of CH2Cl2
saturated with the alkene were added 1.5 equiv of 1,3-dicarbonyl
compound and 20 μL of H2O. The mixture was stirred for 24 h
(30 min in the case of pentane-2,4-dione with the complex 1a).
The solution was dried over Na2SO4, filtered, and concentrated
in vacuo. The product was precipitated by dropwise addition of
diethyl ether, filtered off, and dried under vacuum. In the reaction
with the propylene complex, a mixture of products was obtained
(7a and 8a, 7b and 8b), and no separation was made.
explained by proton loss from a coordinated propylene
molecule,15 with formation of a σ-allyl complex, and its succes-
sive coupling with another coordinated propylene molecule, a
reaction recently reported by two of us.5 The formation of 8a
and 8b was also confirmed by treatment of the crude mixture of
σ-alkyl complexes with gaseous HCl. This resulted in the proto-
nolysis of the M-C bond for 8a and 8b, giving the propylene
dimer 4-methyl-1-pentene, while for 7a and 7b the reversal of
the nucleophilic attack and displacement of propylene pro-
duced [(PNP)MCl]Cl and pentane-2,4-dione.
In spite of the low reactivity of the propylene complexes
and the possible occurrence of a side reaction, the catalytic
coupling with pentane-2,4-dione could be performed with
moderate yield at 70 °C and 2 bar. In 24 h, reacting with 30
equiv of substrate, the platinum catalyst 6a gave a 40% yield
of the Markovnikov product 9 (Chart 1).
In addition, the assigned structures were confirmed by re-
ductive and/or protolytic degradation of the complex mixture
and successive identification of the resulting organic products.
2a. Yield: 37.6 mg, 0.0364 mmol, 91%; light gray solid. Anal.
Calcd for C38H38F6NO2P2PtSb: C, 44.17; H, 3.71; N, 1.36.
Found: C, 43.91; H 3.88; N, 1.27. 1H NMR (200 MHz, CD2Cl2),
ketonic form: δ 1.55-1.65 (br, 4 H, PtCH2CH2), 1.69 (s, 6 H,
MeCO), 3.11 (t, 1 H, CH(COMe)2), 4.42 (ps t, 4 H, PCH2),
7.50-7.80 (m, 22 H, PPh, py), 8.00 (t, 1 H, py). 1H NMR (200
MHz, CD2Cl2), enolic form: δ 1.30 (s, 6 H, MeCO). 13C NMR
(50 MHz, CD2Cl2), ketonic form: δ -2.0 (PtCH2, JPt = 646 Hz),
29.3 (COMe), 33.8 (PtCH2CH2), 46.2 (ps t, 1JP = 35 Hz, PCH2),
72.5 (3JPt = 85 Hz, CH), 123.5 (py-3,5), 127.5 (ps t, 1JP = 52 Hz,
PPhj), 130.0 (PPhm), 133.0 (PPhp), 133.6 (PPho), 141.3 (py-4),
159.8 (ps t, 2JPt = 38 Hz, py-2,6), 204.0 (COMe). 13C NMR (50
MHz, CD2Cl2), enolic form: δ 22.0 (COMe), 191.0 (COMe).
2b. Yield: 36.7 mg, 0.0388 mmol, 97%; light gray solid. Anal.
Calcd for C38H38F6NO2P2PdSb: C, 48.31; H, 4.05; N, 1.48.
Found: C, 48.07; H 4.13; N, 1.42. 1H NMR (300 MHz, CD2Cl2),
ketonic form: δ 1.7-1.75 (br, 4 H, PdCH2CH2), 1.72 (s, 6 H,
MeCO), 3.20 (br, 1 H, CH(COMe)2), 4.42 (ps t, 4 H, PCH2),
Conclusions
The results presented in this paper give a further example of
the reactivity of electron-rich double bonds (in the present case
belonging to the enolic form of β-dicarbonyl compounds) with
the electron-poor double bond of an olefin coordinated to a very
electrophilic site. The reaction can take place catalytically, since
the proton released from the intermediate σ-alkyl complex is
able to cleave the M-C σ-bond, making the coordination site
available to another olefin molecule. The reaction consists
first of a reversible nucleophilic addition step (i in Scheme 1)
and then an irreversible protonolysis of the metal-carbon bond
(ii in Scheme 1). Differently from the analogous catalytic reac-
tion reported by Widenhoefer,3 here a lower relative amount of
catalyst was effective, and no cocatalyst was necessary. Indeed,
no Wacker-type reaction takes place, because the tridentate
pincer ligand PNP prevents a β-H-elimination from the inter-
mediate palladium σ-complex. Moreover, no addition of HCl is
necessary,16 because the first reversible step is favored enough
by the highly electrophilic metal center to produce a concentra-
tion of acidic protons sufficient to cleave the M-C bond.
1
7.50-7.80 (m, 22 H, PPh, py), 8.00 (t, 1 H, py). H NMR (300
MHz, CD2Cl2), enolic form: δ 1.36 (s, 6 H, MeCO). 13C NMR (75
MHz, CD2Cl2), ketonic form: δ 12.6 (PdCH2), 29.3 (COMe), 32.0
(PdCH2CH2), 44.9 (ps t, 1JP = 26 Hz, PCH2), 71.7 (CH), 123.3
1
(py-3,5), 128.2 (ps t, JP = 45 Hz, PPhj), 130.0 (PPhm), 132.3
(PPhp), 133.2 (PPho), 141.0 (py-4), 158.4 (ps t, py-2,6), 203.8
(COMe). 13C NMR (75 MHz, CD2Cl2), enolic form: δ 10.8
(PdCH2), 22.2 (COMe), 45.7 (ps t, JP = 26 Hz, PCH2), 112.9
(CH2CdC), 190.9 (CO).
1
4a. Yield: 39.4 mg, 0.0375 mmol, 94%; white solid. Anal.
Calcd for C38H38F6NO3P2PtSb: C, 43.49; H, 3.65; N, 1.33. Found:
C, 43.28; H, 3.79; N, 1.30. 1HNMR (200 MHz, CD2Cl2):δ1.5-1.7
(br, 4 H, PtCH2CH2), 1.73 (s, 3 H, COMe), 2.92 (t, 1 H, CH-
(COMe)(CO2Me)), 3.42 (s, 3 H, CO2Me), 4.40 (ps t, 4 H, PCH2),
7.50-7.80(m, 22H, PPh, py), 8.00(t, 1H, py).13CNMR(50MHz,
CD2Cl2): δ -2.5 (PtCH2, JPt = 634 Hz), 28.6 (COMe), 33.5 (Pt-
1
CH2CH2), 46.3 (ps t, JP = 33 Hz, PCH2), 52.0 (CO2Me), 64.0
(3JPt = 103 Hz, CH), 123.3 (py-3,5), 127.3 (ps t, 1JP = 55 Hz, PPhj),
130.0 (PPhm), 132.7 (PPhp), 133.9 (PPho), 140.4 (py-4), 159.9 (ps t,
2JPt = 33 Hz, py-2,6), 170.1 (CO2Me), 202.9 (COMe).
Experimental Section
4b. Yield: 36.9 mg, 0.0384 mmol, 96%; white solid. Anal.
Calcd for C38H38F6NO3P2PdSb: C, 47.51; H, 3.99; N, 1.46. Found:
C, 47.23; H, 4.14; N, 1.40. 1H NMR (300 MHz, CD2Cl2): δ
1.65-1.85 (br, 4 H, PdCH2CH2), 1.78 (s, 3 H, COMe), 3.02 (t, 1
H, CH(COMe)(CO2Me)), 3.45 (s, 3 H, CO2Me), 4.42 (ps t, 4 H,
PCH2), 7.50-7.70 (m, 22 H, PPh, py), 7.90 (t, 1 H, py). 13C NMR
General Procedures. CD2Cl2 and CD3NO2 were dried with
˚
4 A molecular sieves. The 1,3-dicarbonyl substrates were obtained
by Aldrich and were used without further purification. The NMR
(15) Bandoli, G.; Dolmella, A.; Fanizzi, F. P.; Di Masi, N. G.;
Maresca, L.; Natile, G. Organometallics 2001, 20, 805.
(16) Actually in our case, the addition of HCl would kill the whole
process, by irreversible coordination of the chloride ion to the single site
needed for the coordination of the olefin.
(17) Hahn, C.; Vitagliano, A.; Giordano, F.; Taube, R. Organome-
tallics 1998, 17, 2060.

5884 Organometallics, Vol. 29, No. 22, 2010
Cucciolito et al.
(50 MHz, CD2Cl2): δ 12.3 (PdCH2), 28.6 (COMe), 31.9 (Pd-
compound was added. The solution was stirred under ethylene or
propylene for 24 h and then was analyzed by 1H NMR to determine
its composition and yields.
1
CH2CH2), 44.9 (ps t, JP =26 Hz, PCH2), 52.2 (CO2Me), 63.1
(CH), 123.4 (py-3,5), 128.4 (ps t, 1JP = 45 Hz, PPhj), 130.0
(PPhm), 132.4 (PPhp), 133.4 (PPho), 141.1 (py-4), 158.6 (py-2,6),
169.9 (CO2Me), 202.7 (COMe).
Reductive Degradation with NaBH4. The reduction was per-
formed on all the σ-alkyl derivatives. The complexes resulting
from the base-assisted addition reaction (30-50 mg) were
dissolved in 1.5 mL of CH2Cl2/MeOH (5:2), and an excess of
solid NaBH4 was added. The mixture was stirred for 12 h and
then treated with 1.0 mL of a NH4Cl-saturated aqueous solu-
tion. The organic phase was separated and evaporated to
dryness. The residue was extracted with diethyl ether, the ether
extract was evaporated to dryness, and the residue was analyzed
by 1H NMR for identifying the organic products.
Reaction with HCl. Protolytic degradation was performed on
all the σ-alkyl derivatives. The complexes (10 mg) were dissolved
in 0.6 mL of CD2Cl2, and gaseous HCl was bubbled through.
The solutions containing M(PNP)Cl2 and organic products
were characterized by 1H NMR spectroscopy.
1
7a (in a mixture with 8a). H NMR (300 MHz, CD2Cl2): δ
0.33 (d, 3 H, Me), 1.34 (m, 1 H, PtCHH), 1.63 (s, 3 H, COMe),
1.76 (s, 3 H, COMe), 2.09 (m, 1 H, CHMe), 2.22 (m, 1 H,
PtCHH), 3.02 (d, 1 H, CH(COMe)2), 4.42 (ps t, 4 H, PCH2),
7.40-7.80 (m, 22 H, PPh, py), 8.03 (t, 1 H, py).
1
7b (in a mixture with 8b). H NMR (300 MHz, CD2Cl2): δ
0.50 (d, 3 H, Me), 1.55 (m, 1 H, PdCHH), 1.65 (s, 3 H, COMe),
1.84 (s, 3 H, COMe), 2.03 (m, 1 H, PdCHH), 2.21 (m, 1 H,
CHMe), 3.10 (d, 1 H, CH(COMe)2), 4.40 (ps t, 4 H, PCH2),
7.40-7.90 (m, 22 H, PPh, py), 8.02 (t, 1 H, py).
8a. See 1H NMR data reported in ref 5 for complex 5d-Pt.
8b. 1H NMR (400 MHz, CD2Cl2): δ 1.57 (d, 3 H, Me), 1.45
(m, 2 H, βCH and γCHH), 1.53 (m, 1 H, γCHH), 1.90 (m, 1 H,
PdCHH), 2.07 (m, 1H, Pd CHH), 4.4 (ps t, 4 H, PCH2), 4.64 (d, 1
H, dCHH), 4.72 (d, 1 H, dCHH), 5.26 (m, 1 H, dCH), 7.4-8.0
(m, 23 H, PPh, py).
Acknowledgment. This work was supported by MIUR
(PRIN Grant No. 2007-X2RLL2). We thank the
CIMCF, University of Napoli Federico II, for access to
the NMR facilities and Miss Nunzia Contiello for experi-
mental assistance.
Catalytic Reactions. The catalytic reactions were run in a 10 mL
glass autoclave under the conditions reported in Tables 1 and 2. The
appropriate metal complex (0.03 mmol) was dissolved in 0.3 mL
of CD3NO2, and 15-100 equiv of the appropriate 1,3-dicarbonyl