Reactivity of cis-{PtII(Ar)(alkyne)} fragments (Ar = aryl): A domino-formation of indenes

Article abstract of DOI:10.1021/om701239b

The reactivity of cis-{Pt^II(Ar)(2-butyne)] fragments in cationic complexes is described. Coordination of the unsaturated ligand is observed at low temperature. Upon increasing at 298 K, a cascade of reactions affords a selective formation of indenes. Mechanistic implications of this domino-reaction are discussed.

Full text of DOI:10.1021/om701239b

Organometallics 2008, 27, 1351–1353
1351
Reactivity of cis-{PtII(Ar)(alkyne)} Fragments (Ar ) aryl): A
Domino-Formation of Indenes
Maria Elena Cucciolito, Augusto de Renzi, Giuseppina Roviello, and Francesco Ruffo*
Dipartimento di Chimica “Paolo Corradini”, UniVersità degli Studi di Napoli “Federico II”, Complesso
UniVersitario di Monte Sant’Angelo, Via Cintia, 80126 Napoli, Italy
ReceiVed December 11, 2007
Summary: The reactiVity of cis-{PtII(Ar)(2-butyne)} fragments
in cationic complexes is described. Coordination of the unsatur-
ated ligand is obserVed at low temperature. Upon increasing
at 298 K, a cascade of reactions affords a selectiVe formation
of indenes. Mechanistic implications of this domino-reaction
are discussed.
Introduction
Figure 1. General formula of complexes of types 1–3.
The reactivity of cis-{MII(HC)(unsaturated ligand)} fragments
(M ) Ni, Pd, or Pt; HC ) alkyl, aryl) attracts a remarkable
interest in the field of metal-mediated synthesis.1 A wide variety
of products are obtainable, according to the nature of the ligands
and the reaction conditions. Moreover, unprecedented mecha-
Figure 2. General formula of complexes of type 4.
nistic implications are often disclosed.
Previous systematic works2–4 by our research team on cationic
Scheme 1. Formation of Complexes of Type 4 Starting from
Precursors 5
species of formula [PtII(HC)(unsaturated ligand)(N,N-chelate)]+
(1–3 in Figure 1; N,N-chelate ) 1,10-phenanthroline or phen)
demonstrated that, using alkenes as unsaturated ligands, the
complexes are stable only if HC ) alkyl (1).5
When HC ) aryl it is not possible to isolate compounds 2,
but only to infer their formation in solution by NMR spectros-
copy.3 The fate of these species depends on the amount of olefin
present in solution. With an excess of alkene the corresponding
insertion products can be isolated. Otherwise, a fast rearrange-
ment occurs, and formation of a Pt-2-alkylaryl moiety is
observed. A detailed mechanistic study has also demonstrated
that this result is an effect of the general behavior of “naked”
metals, which can promote reactions which normally either do
not occur or have different courses.6
affording π-allyl derivatives. This kind of reaction was never
observed in complexes showing a trans disposition of the alkyne
and the alkyl group. Therefore, the cis geometry seems a
necessary requisite for promoting this process.
In this paper we report on the extension of this study to
alkyne/aryl complexes of type 4 (Figure 2).
In line with our expectations, this investigation has disclosed
unusual reactivity, which affords indenes through a cascade of
transformations following the initial insertion product. A reason-
able mechanism for this domino-reaction has also been pro-
posed, which also can be a useful complement to the under-
standing of the analogous Pd-catalyzed process, recently
described as a convenient access to substituted indenes.7
The cis-coordination of alkynes in alkylplatinum(II) com-
plexes of type 3 produced a different reactivity depending on
the nature of the unsaturated ligand.4 Notably, terminal alkynes
gave rise to an unprecedented intramolecular rearrangement
* Corresponding author. Fax: (+39)081-674090. E-mail: ruffo@unina.it.
(1) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000,
100, 1169. (b) D’Souza, D. M.; Mueller, T. J. J. Chem. Soc. ReV. 2007, 36,
1095.
(2) Fusto, M.; Giordano, F.; Orabona, I.; Panunzi, A.; Ruffo, F.
Organometallics 1997, 16, 5981.
Results and Discussion
(3) (a) De Felice, V.; De Renzi, A.; Tesauro, D.; Vitagliano, A.
Organometallics 1992, 11, 3669. (b) De Felice, V.; Cucciolito, M. E.; De
Renzi, A.; Ruffo, F.; Tesauro, D. Organometallics 1995, 493, 1. (c)
Cucciolito, M. E.; De Renzi, A.; Orabona, I.; Ruffo, F.; Tesauro, D. J. Chem.
Soc., Dalton Trans. 1998, 1675. (d) De Felice, V.; De Renzi, A.; Fraldi,
N.; Roviello, G.; Tuzi, A. J. Organomet. Chem. 2005, 690, 2035.
(4) Cucciolito, M. E.; De Felice, V.; Orabona, I.; Ruffo, F. J. Chem.
Soc., Dalton Trans. 1997, 1351.
The study of the reactivity of type 4 complexes has been
performed by adding stoichiometric 2-butyne to the cationic
fragment {Pt(Ar)(phen)}+, generated in situ through addition
of silver tetrafluoroborate to the corresponding neutral precursors
[Pt(Ar)I(phen)] (5a-c) (Scheme 1).
2-Butyne was used in all the experiments, and the electronic
properties of the aryl groups were selected to rationalize their
possible influence on the reactivity.
(5) A remarkable exception was observed for the electron-poor methyl
acrylate, which upon heating was able to insert into the Pt-Me bond: Ganis,
P.; Orabona, I.; Ruffo, F.; Vitagliano, A. Organometallics 1998, 17, 2646.
(6) (a) Lersch, M.; Tielset, M. Chem. ReV. 2005, 105, 2471. (b)
Cucciolito, M. E.; D’Amora, A.; Tuzi, A.; Vitagliano, A. Organometallics
2007, 26, 5216.
(7) Dong, C.-G.; Yeung, P.; Hu, Q.-S. Org. Lett. 2007, 9, 363.
10.1021/om701239b CCC: $40.75
2008 American Chemical Society
Publication on Web 02/27/2008

1352 Organometallics, Vol. 27, No. 6, 2008
Notes
Figure 4. Formula of 6a-D.
Scheme 2. Mechanistic Hypothesis for the Formation of
Indenes (6)
Figure 3. Formula of indenes 6a-c.
1
Progress of the reactions was monitored through H NMR
spectroscopy in CD3NO2.
The initial coordination of the alkyne to the metal center (4a
in Scheme 1) was observed when the addition of 2-butyne to
5a was performed at 273 K. Evidence of this is the shift of the
methyl resonance tCsMe (δ 2.35 vs 1.85 of the free alkyne),
which also presents coupling to 195Pt (3JPt-H ) 47 Hz).
Upon increasing the temperature to 298 K, a sequence of
1
reactions occurred, as shown by the H NMR spectrum. The
initial appearance of a quartet at δ 5.25 (2JPt-H ) 46 Hz) pointed
to a dCHsMe group coordinated to the metal (δMe 1.60,
doublet). Simultaneous growth of a not-195Pt-coupled quartet
at δ 5.76 revealed an analogous, though noncoordinating,
dCHsMe moiety (δMe 1.52).
The NMR spectra became more complex upon aging, and
several attempts to isolate the complexes were unfruitful. On
these grounds, our attention was addressed to the identification
of the organic fragment(s) bound to the metal through protolytic
degradation. Therefore, gaseous HCl was added to the reaction
mixture 30 min after mixing of the reagents. The NMR spectrum
of the filtered solution disclosed the presence of an organic
molecule, whose data were consistent with 1,2,3-trimethyl-1-
ethylindene (6a in Figure 3).8 This compound could be isolated
from the reaction mixture as an oil.
resonance effects of the different substituents in the para
position, which produce a significant variation of the carbanionic
character of the migrating aryl group.
Aiming to gain insights on the mechanism of the reaction,
further experiments were performed with suitably deuterated
reagents. By using 5a-C6D5 as the precursor, the spectrum of
the initial mixture reaction lacked the dCHsMe signal,
suggesting its deuteration. Accordingly, the corresponding
methyl group resonated as a singlet. The spectrum of the isolated
indene (6a-D in Figure 4) displayed the methyl resonance of
the -CHDMe group as a doublet at δ 0.22. The multiplet at δ
1.60 was absent.
Protonolysis was also carried out with DCl in D2O starting from
5a. In this case, the spectrum of the product showed a comple-
mentary -CDHMe pattern: the methyl doublet was in fact shifted
at δ 0.24, whereas the multiplet at δ 1.75 was now missing.
The ensemble of results suggest the mechanistic hypothesis
reported in Scheme 2. Though many attempts to isolate the species
shown in Scheme 2 were unfruitful, spectroscopic evidence allowed
the reasonable hypothesis of their existence in solution:
(i) Initial coordination of 2-butyne occurs (4), as disclosed
1
A distinctive feature of the H NMR spectrum of 6a is the
Me1 resonance that results in an apparent triplet at δ 0.25. The
adjacent methylene hydrogen atoms are diasterotopic and
resonate at δ 1.75 and 1.60 as multiplets.
The stoichiometric composition of 6a reveals that two butyne
molecules are linked to the same aromatic ring. In the hypothesis
of a sequential butyne addition, this indicates that the initial
product is more reactive than the starting aryl-alkyne complex
4a. Therefore, a 1:2 Pt/2-butyne ratio was successfully used to
improve the yield of the isolated product.
The rate increased when the reaction was carried out starting
from 5b. The recovered 1,2,3-trimethyl-1-ethyl-5-methoxyin-
dene 6b was identified through NOESY experiment. The
unambiguous determination of the MeO position on the aromatic
ring has been essential for the formulation of a reasonable
mechanistic hypothesis (see below).
Also the reaction carried out on 5c afforded the 5-substituted
indene (6c in Figure 3), though in this case the reaction was
slower than that for 5a.
In analogy to that found for similar reactions involving olefin
substrates,3 this experimental observation suggests that an
insertion step is involved in the mechanism. In fact, the trend
of reactivity (MeO > H>CF3) matches the inductive and
at low temperature for 4a by the presence of a singlet at δ 2.35,
195
coupled to
Pt.
(ii) The subsequent step is the migratory insertion of the
alkyne into the Pt-aryl bond, followed by H-Caryl oxidative
addition and H-Calkenyl reductive elimination (B and C). This
fast sequence9 is already known for the analogous olefin
substrates 2.3
Demonstration of its occurrence is the above-mentioned
experiment carried out with the deuterated phenyl group. In this
case a deuterium atom originally present in the ring is found in
the coordinated alkene fragment dCDsMe of C.
(iii) This latter species cannot be stabilized through an
effective coordination of the double bond to the metal. In fact,
geometrical constraints would force an unusual coplanar ar-
rangement of the double bond with respect to the coordination
plane of the metal. This plausibly enhances reactivity of the
(8) (a) Although literature related to indenes and their derivatives is
widespread, 6a has never been described before, to the best of our
knowledge. Therefore, to confirm the structure of this novel species, an
authentic sample was synthesized by adapting known procedures: Pittman,
C. U.; Miller, W. G. J. Am. Chem. Soc. 1973, 2947. (b) Miller, G.; Pittmann,
C. U. J. Org. Chem. 1974, 39, 13. (c) Meurling, L. Acta Chem. Scand.,
Ser. B 1974, 28, 295.
(9) Mota, A. J.; Dedieu, A. J. Org. Chem. 2007, 72, 9669.

Notes
Organometallics, Vol. 27, No. 6, 2008 1353
multiplicities: s, singlet; d, doublet; t, triplet; app. t, apparent triplet;
m, multiplet. The coupling costants, when measurable, are reported
in hertz. The neutral platinum(II) complexes [Pt(Ar)I(phen)] (5a-c)
and [Pt(C6D5)I(phen)] (5a-C6D5) used as precursors were obtained
according to described procedures.3c 1,2,3-Trimethylindene was
prepared as described in the literature.8a,b 2-Butyne and organic
reagents were commercially available (Sigma-Aldrich), and were
used without further purification. Solvents were dried following
standard procedures.
unsaturated bond toward substitution. Since alternative coordi-
nating ligands are not present, a way to stabilize C must involve
coordination of another 2-butyne molecule (D);
(iv) At this stage, a second migratory insertion reaction would
afford E. In this case, a further rearrangement analogous to that
experienced by the primary insertion product B seems to be
prevented by steric hindrance. It is worth noting that the presence
of an excess of olefin in analogous reactions also afforded a
double insertion product;3a
General Reaction Procedure. To a stirred suspension of the
appropriate neutral [Pt(Ar)I(phen)] (5a-c) precursor (0.17 mmol)
in CD3NO2 (1 mL) was added an equimolar amount of AgBF4
dissolved in the same solvent (0.5 mL) at room temperature.
Dissolution of the platinum complex and precipitation of silver
iodide was observed. To the mixture reaction, quickly filtered
through a thin layer of Celite to remove silver iodide, was added
2-butyne (0.018 g, 0.34 mmol). Progress of the reaction was
monitored via NMR spectroscopy. Alternatively, the mixture were
subjected to controlled decompositions (i) and (ii), 30 and 5 min
after the mixing of the reagents, respectively.
(v) Evolution of E toward 6 is very fast if strong acids are
added at this stage (e.g., HCl). Alternatively, this reaction slowly
and spontaneously (though less selectively) takes place thanks
to the traces of water, which are always present in the reaction
mixture. The outcomes of the experiments with deuterated
reagents (5a-C6D5 + HCl and 5a + DCl) indicate that
protonolysis and final cyclization are concerted. In fact, if the
two processes occurred independently, two diastereomers (two
enantiomeric couples) should form for deuterated 6a, due to
the presence of two stereogenic carbon atoms (labeled with an
asterisk in Figure 4). The selective obtainment of either
diastereomer in the two experiments indicates that the two
configurations are correlated.
(i) Protonolysis with Gaseous HCl. A portion of the reaction
mixture was concentrated to small volume and diluted with CDCl3.
Gaseous HCl was added and the mixture was stirred for 10 min at
room temperature. After filtration on Florisil, the resulting clear
solution was evaporated under vacuum to afford 6a-c as pale
yellow oils. Yield: 92% (6a), 87% (6b), 95% (6c). Purification can
be carried out through column chromatography (silica gel, hexane).
1H NMR resonances (CDCl3, δ): 6a, 7.15–6.90 (m, 4H, Ph),
2.00 (s, 3H, Me4), 1.78 (s, 3H, Me3), 1.75 (m partially obscured,
1H, CHH), 1.60 (m, 1H, CHH), 1.15 (s, 3H, Me2), 0.25 (app. t,
3H, 3JH-H ) 7.5 Hz, Me1); 6b, 7.05 (d, 1H), 6.75 (s, 1H), 6.68 (d,
1H), 3.86 (s, 3H, OMe), 2.00 (s, 3H, Me4), 1.79 (s, 3H, Me3), 1.78
(m partially obscured, 1H, CHH), 1.65 (m, 1H, CHH), 1.15 (s, 3H,
Further evidence of the mechanism proposed in Scheme 2 has
been obtained by an alternative identification of the different organic
fragments progressively bonded to the metal. To this aim, the
reaction mixture was treated with NaBH4 a few minutes after the
mixing of the reagents, and the organic fragments were isolated in
the usual conditions. The organic phase obtained from 5a mainly
contained equimolar amounts of 6a and Z-PhsC(Me)dCHsMe
((Z)-1-(but-2-en-2-yl)benzene), which derives from reduction of
the monoinsertion products (C and/or D species of Scheme 2).
1
This molecule has been identified by comparing its H NMR
3
Me2), 0.27 (app. t, 3H, JH-H ) 8 Hz,Me1); 6c, 7.50 (d, 1H), 7.38
spectrum with that reported in the literature.10
(s, 1H), 7.25 (d, 1H), 2.04 (s, 3H, Me4), 1.82 (s, 3H, Me3), 1.79
In the reaction mixture a second unsaturated species containing
the dCHsMe group is also present. Its spectrum is compatible
with that of 1,2-bis[(Z)-1-(but-2-en-2-yl)]benzene, in other words,
with the compound arising from the reductive cleavage of the
double insertion product (E species of Scheme 2). Unluckily, a
complete characterization of this molecule could not be performed
since the amount in the mixture reaction is lower than 5%, and its
characterization data are not present in the literature.
(m partially obscured, 1H, CHH), 1.71 (m, 1H, CHH), 1.17 (s, 3H,
3
Me2), 0.26 (app. t, 3H, JH-H ) 7 Hz, Me1).
13C NMR resonances (CDCl3, δ): 6a, 150.9, 146.0, 144.9, 130.7,
126.1, 123.8, 120.8, 117.8, 53.7, 30.0, 23.6, 10.1, 9.7, 8.3; 6b, 158.9,
147.5, 146.4, 143.1, 130.5, 121.1, 108.7, 104.2, 55.4, 53.1, 30.1,
23.8, 10.1, 9.8, 8.4; 6c, 150.0, 146.5, 130.2, 128.4, 125.0, 124.3,
122.2, 120.8, 54.1, 29.8, 23.3, 10.0, 9.8, 8.2.
(ii) Reduction with NaBH4 (starting from 5a). A portion of the
reaction mixture was concentrated to small volume and diluted with
CDCl3. To this solution was added NaBH4 (10 equiv). The resulting
reaction mixture, quickly blackened, was filtered on Florisil and
examined by NMR spectroscopy. The NMR spectrum revealed the
presence of 6a and 1,2-bis[(Z)-1-(but-2-en-2-yl)]benzene, in a 1:1
ratio.8
It should be noted that none of the above reactions takes place
when cationic substrates with donor ligands adjacent to the aryl
ring are taken as precursors, e.g., [Pt(Ar)(MeCN)(phen)]+.
Conclusion
This work extends our previous study on cis-{PtII(HC)(un-
saturated ligand)} fragments (HC ) alkyl, aryl). A concerted
process involving an aryl ring and two molecules of butyne has
been disclosed, which affords indenes as unique organic
products. The result confirms the strategic importance of cationic
platinum(II) complexes in weakly coordinating environments
for promoting unusual reactions, and gaining mechanistic
implications.7
Synthesis of 6a. The synthesis was adapted from a described
procedure.8 To a solution of 1,2,3-trimethylindene (0.182 g, 1.15
mmol) in dry diethyl ether (5 mL) was added a solution of
butyllithium in pentane (4.6 mmol) at 273 K, under nitrogen. The
resulting solution was stirred at room temperature for 30 min. Ethyl
bromide (7.05 mmol) in dry diethyl ether (3 mL) was added with
cooling to the mixture reaction. After standing at room temperature
for 2 h, the solution was poured on to 20 mL of ice-cooled 2 M
HCl. The organic phase was separated, washed with sat. aqueous
NaCl, and dried over Na2SO4. Removing the solvent gave 0.112 g
of product as a pale yellow oil (yield 53%).
Experimental Section
General Comments. H and 13C NMR spectra were recorded
1
Acknowledgment. The authors thank the Centro Interdi-
partimentale di Metodologie Chimico-Fisiche (C.I.M.C.F.) of
the University of Napoli “Federico II” for NMR facilities.
Thanks also are due to Dr. Marianna Coppola for technical
assistance.
with Varian XL-200, Varian Gemini-200, and Varian Gemini-500
spectrometers. 1H and 13C chemical shifts are reported in δ (ppm)
relative to the solvent (CHCl3, δ 7.26; 13CDCl3, δ 77.0; CHD2NO2,
δ 4.33). The following abbreviations were used for describing NMR
(10) Fristrup, P.; Tanner, D.; Norrby, P.-O. Chirality 2003, 15, 360.
OM701239B