Competition of insertion and transmetalation pathways in the reactions of alkenylsilanes with aryl complexes of palladium(II). An experimental study

Article abstract of DOI:10.1021/om060622e

Reactions of alkenylsilanes (alkenyl = 2-propenyl, vinyl, allyl) with neutral and cationic palladium aryl complexes [Pd(C₆F ₅)X(NCMe)₂] (X = Br, 1; Cl, 2), [Pd(C₆F ₅)(acac)(NCMe)] (acac = acetylacetonate, 3), and [Pd(C ₆F₅)(bipy)(S)]ClO₄ (S = acetone, 4) were studied. The compounds obtained were substituted alkenylsilanes (formed via Heck substitution) and desilylated olefins (formed by transmetalation followed by reductive elimination or by insertion followed by β-SiMe₃ elimination). The analysis of the structure of the final organic products provides a quantitative assessment of the importance of these competing pathways. Transmetalation to palladium was found to be a minor pathway in all cases, whereas insertion of the double bond into the Pd-C₆F ₅ bond followed by β-SiMe₃ elimination was the major route. The formation of C₆F₅-substituted alkenylsilanes is favored when an excess of starting alkenylsilane is used.

Full text of DOI:10.1021/om060622e

Organometallics 2006, 25, 5449-5455
5449
Competition of Insertion and Transmetalation Pathways in the
Reactions of Alkenylsilanes with Aryl Complexes of Palladium(II).
An Experimental Study§
Ana C. Albe´niz,* Pablo Espinet,* and Raquel Lo´pez-Ferna´ndez
Qu´ımica Inorga´nica, Facultad de Ciencias, UniVersidad de Valladolid,
Prado de la Magdalena s/n, 47005 Valladolid, Spain
ReceiVed July 12, 2006
Reactions of alkenylsilanes (alkenyl ) 2-propenyl, vinyl, allyl) with neutral and cationic palladium
aryl complexes [Pd(C6F5)X(NCMe)2] (X ) Br, 1; Cl, 2), [Pd(C6F5)(acac)(NCMe)] (acac ) acetylacetonate,
3), and [Pd(C6F5)(bipy)(S)]ClO4 (S ) acetone, 4) were studied. The compounds obtained were substituted
alkenylsilanes (formed via Heck substitution) and desilylated olefins (formed by transmetalation followed
by reductive elimination or by insertion followed by â-SiMe3 elimination). The analysis of the structure
of the final organic products provides a quantitative assessment of the importance of these competing
pathways. Transmetalation to palladium was found to be a minor pathway in all cases, whereas insertion
of the double bond into the Pd-C6F5 bond followed by â-SiMe3 elimination was the major route. The
formation of C6F5-substituted alkenylsilanes is favored when an excess of starting alkenylsilane is used.
Introduction
The Heck reaction and Hiyama coupling are two useful tools
in the realm of C-C coupling reactions catalyzed by palladium
complexes. The Heck reaction leads to substituted olefins by
reaction of an alkene and an aryl (or vinyl) halide, and the
insertion of the alkene into a Pd-aryl (or vinyl) bond is a key
step in the catalytic cycle.1,2 Hiyama coupling is the palladium-
catalyzed coupling of organic electrophiles (usually halides or
triflates) with organosilane derivatives, and the transmetalation
step from silicon to palladium is crucial to the success of the
process.3-6 Both reactions can take place and actually compete
in the reactions of alkenylsilanes (either vinyl- or allylsilanes).
In fact, a mixture of products with and without a -SiR3 moiety
was obtained in the early application of the Pd-catalyzed
coupling of alkenylsilanes and aryl electrophiles (eq 1).7
The loss of selectivity reported in this and several other
combinations of substrates,4,5,8-13 in reactions using alkenylsi-
lanes, points to a competition of the routes of insertion and
transmetalation. An example of loss of regioselectivity is shown
in eq 2, where the percentages of products in the mixture vary
depending on the aryl iodide used.11
§ This paper is warmly dedicated to Prof. Antonio Abad (University of
Murcia), who prematurely retired due to his health condition.
* Corresponding author. E-mail: albeniz@qi.uva.es; espinet@qi.uva.es.
(1) (a) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: London, 1985. (b) de Meije`re, A.; Meyer, F. E. Angew. Chem., Int.
Ed. Engl. 1994, 33, 2379-2411. (c) Cabri, W.; Candiani, I. Acc. Chem.
Res. 1995, 28, 2-7. (d) Beletskaya, I.-P.; Cheprakov, A. V. Chem. ReV.
2000, 100, 3009-3066. (e) Whitcombe, N. J.; Hii, K. K.; Gibson, S. E.
Tetrahedron 2001, 57, 7449-7476.
The formation of both the substituted silanes and the
regioisomeric olefins can indeed be explained assuming a
competition of the insertion and transmetalation routes, as shown
in Scheme 1 for an allylsilane. Transmetalation of the alkenyl
group to palladium and reductive elimination leads to the
Hiyama coupling product (also called the ipso substitution
product). A conventional Heck sequence (insertion + â-H
(2) Espinet, P.; Albe´niz, A. C. In Fundamentals of Molecular Catalysis;
Current Methods in Inorganic Chemistry Vol. 3; Yamamoto, A., Kurosawa,
H., Eds.; Elsevier: Lausanne, 2003; Chapter 6.
(3) (a) Hiyama, T. J. Organomet. Chem. 2002, 653, 58-61. (b) Denmark,
S. E.; Sweis, R. F. In Metal Catalyzed Cross-Coupling Reactions; de
Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004; Chapter
4. (c) Horn, K. A. Chem. ReV. 1995, 95, 1317-1350.
(8) Yoshida, J.; Tamao, K.; Yamamoto, H.; Kakui, T.; Uchida, T.;
Kumada, M. Organometallics 1982, 1, 542-549.
(9) Ikenaga, K.; Kikukawa, K.; Matsuda, T. J. Chem. Soc., Perkin Trans.
1 1986, 1959-1964.
(4) Denmark, S. E.; Choi, J. Y. J. Am. Chem. Soc. 1999, 121, 5821-
5822.
(10) Anderson, J. C.; Anguille, S.; Bailey, R. Chem. Commun. 2002,
2018-2019.
(5) Denmark, S. E.; Sweis, R. F. J. Am. Chem. Soc. 2001, 123, 6439-
6440.
(6) Lee, J.-Y.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 5616-5617.
(7) (a) Kikukawa, K.; Ikenaga, K.; Kono, K.; Toritani, K.; Wada, F.;
Matsuda, T. J. Organomet. Chem. 1984, 270, 277-282. (b) Ikenaga, K.;
Kikukawa, K.; Matsuda, T. J. Org. Chem. 1987, 52, 1276-1280.
(11) Hatanaka, Y.; Goda, K.; Hiyama, T. J. Organomet. Chem. 1994,
465, 97-100.
(12) Karabelas, K.; Hallberg, A. J. Org. Chem. 1986, 51, 5286-5290.
(13) Alvisi, D., Blart, E.; Bonini, B. F.; Mazzanti, G.; Ricci, A.; Zani,
A. J. Org. Chem. 1996, 61, 7139-7146.
10.1021/om060622e CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/28/2006

5450 Organometallics, Vol. 25, No. 22, 2006
Albe´niz et al.
Scheme 1. Competing Reaction Pathways for Unsaturated
Silanes and Palladium Organometallic Complexes
of appropriately substituted alkenyltrimethylsilanes and pen-
tafluorophenyl palladium complexes (the latter have proved to
be good models to study both insertion reactions2 and the
transmetalation with stannanes).22 The prevalence of the reaction
routes using different reaction conditions and auxiliary ligands
bound to palladium can be deduced by analysis of the final
products, and the results are described here.
Results and Discussion
Three alkenyltrimethylsilanes (alkenyl ) 2-propenyl, vinyl,
and allyl) were reacted with neutral palladium pentafluorophenyl
complexes [PdPfX(NCMe)2] (Pf ) C6F5; X ) Br (1),23 Cl (2)24)
and [PdPf(acac)(NCMe)] (acac ) acetylacetonate, 3).25 Reac-
tions with the cationic derivatives [PdPf(bipy)(S)]ClO4 (S )
acetone (4), NCMe (5)) were also assayed, but complex 5 was
eliminated from the tests because of its lower solubility.
Complex 4, prepared in situ in acetone as solvent, was chosen
as the model cationic complex for the reactions. Perchlorate
was used instead of other common, less hazardous, counterions
such as BF4- or PF6- to avoid any influence of the anion, since
we observed that the latter partially reacted with the silyl moiety,
giving SiMe3F (detected by 19F NMR). Since the presence of
fluoride favors transmetalation, these anions can mask the
intrinsic preference of the alkenyltrimethylsilanes for one route
over the other and the influence of the Pd(II) precursor in the
results obtained. The results for each of the three alkenylsilanes
studied are given below.
Reactions with 2-Propenyltrimethylsilane. The reaction of
2-propenyltrimethylsilane with complexes 1-4 could follow the
reaction pathways depicted in Scheme 2. The product mixtures
of the reactions (Table 1) were analyzed in each case by 19F
and 1H NMR spectroscopy. All of the compounds observed are
framed in Scheme 2. Compounds 6-8 and 10 have been
reported before;15,26 nonetheless, the spectra of these compounds
and 11 were compared with commercial or independently
prepared samples.27 Compound 9 was separated by preparative
TLC and characterized by NMR and MS.
elimination) affords substituted alkenylsilanes. However inser-
tion of the alkenylsilane followed by â-Si elimination should
produce a regioisomeric C-C coupling compound (a cine
substitution product). â-Si elimination is less common than the
ubiquitous â-H elimination in palladium alkyl complexes, but
several clear-cut examples have been reported in the litera-
ture.12,14-16
The competition of all these reaction pathways will determine
the actual ratio of products, and it is interesting to find out when
and how much one route is favored over the others.
Several procedures have been developed to enhance the
regioselectivity of Pd-catalyzed reactions of alkenylsilanes.
Hallberg et al. used silver nitrate to shift the reactions of
vinylsilanes or allylsilanes and arylhalides toward the formation
of arylated silanes, the product of the Heck reaction.12,14,17 When
this additive is absent, desilylated products, mainly formed by
an insertion plus â-Si elimination route, are obtained.12,14 Phase
transfer conditions have also been used to yield the Heck
products with higher selectivity.18 There are also recent reports
on the use of functionalized alkenyl(2-pyridyl)dimethylsilanes
for the selective preparation of either the Heck products or, if
a fluoride source such as tetrabutylammonium fluoride (TBAF)
is used as an additive, the Hiyama coupling derivatives.19 It is
known that transmetalation from silicon to palladium can be
accelerated using fluorosilicates or silanols (sometimes formed
in the reaction medium upon addition of fluoride). Recently,
the presence of a hydroxy functionality in aryl- or alkenyl[2-
hydroxymethylphenyl]dimethylsilanes has been used to enhance
the transmetalation rate of tetraorganosilanes, leading to efficient
Hiyama couplings.20 However slower, transmetalation to pal-
ladium has been observed for trimethylsilyl derivatives (the most
common silyl derivatives) and cannot be ignored. The formation
of palladium allylic complexes from Pd(II) precursors and
allylsilanes provides one example.21
All the reactions start by coordination of the alkenylsilane to
palladium through the double bond, leading to A (Scheme 2).
From that point several reaction routes can follow, and the
structure of the final products allows us to trace back the
different pathways operating at observable rates in each reaction,
according to the following analysis.
(1) Transmetalation from silicon to palladium and reductive
elimination (route a) leads to 2-pentafluorophenylpropene (6),
the product of ipso substitution.28
(22) Espinet, P.; Echavarren, A. Angew. Chem., Int. Ed. 2004, 43, 4704-
4734.
We have studied the competition of transmetalation and
double-bond insertion, choosing as model systems combinations
(23) Albe´niz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F. H. Organo-
metallics 1990, 9, 1079-1085.
(14) Karabelas, K.; Hallberg, A. J. Org. Chem. 1989, 54, 1773-1776.
(15) Albe´niz, A. C.; Espinet P.; Lin, Y.-S. Organometallics 1997, 16,
5964-5973.
(16) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906-917.
(17) (a) Karabelas, K.; Hallberg, A. Tetrahedron Lett. 1985, 26, 3131-
3132. (b) Karabelas, K.; Westerlund, C.; Hallberg, A. J. Org. Chem. 1985,
50, 3896-3900. (c) Daves, G. D.; Hallberg, A. Chem. ReV. 1989, 89, 1433-
1445.
(18) Jeffery, T. Tetrahedron Lett. 1999, 40, 1673-1676.
(19) (a) Itami, K.; Mitsudo, K.; Kamei, T.; Koike, T.; Nokami, T.;
Yoshida, J. J. Am. Chem. Soc. 2000, 122, 12013-12014. (b) Itami, K.;
Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2001, 123, 5600-5601. (c)
Itami, K.; Yoshida, J. Synlett 2006, 157-180.
(20) Nakao, Y.; Imanaka, H.; Sahoo, A. K.; Yada, A.; Hiyama, T. J.
Am. Chem. Soc. 2005, 127, 6952-6953.
(21) Hayashi, T.; Konishi, M.; Kumada, M. J. Chem. Soc., Chem.
Commun. 1983, 736-737.
(24) Albe´niz, A. C.; Espinet, P.; Lin, Y.-S.; Orpen, G.; Mart´ın, A.
Organometallics 1996, 15, 5003-5009.
(25) Complex 3 exists in CDCl3 solution as a mixture of the monomeric
[Pd(acac)Pf(NCMe)] (3a, 74%) and a dimeric derivative tentatively assigned
to [(acac)PdPf(µ-κ2-O,O-acac)PdPf(NCMe)2] (3b, 26%) according to its
1
characteristic H and 19F NMR spectra (see Experimental Section).
(26) Albe´niz, A. C.; Espinet, P.; Lin. Y.-S. Organometallics 1996, 15,
5010-5017.
(27) Compound 8 is commercial. Compound 11 can be obtained by
hydrogenation of 8 using Wilkinson’s catalyst; concomitant isomerization
of 8 to the internal olefin 7 is observed. Isomerization of 8 to 7 can also be
carried out using the benzylic palladium complex [Pd2(µ-Br)2(η3-C6F5CH2-
CHPh)2] as catalyst (Lo´pez-Ferna´ndez, R.; Albe´niz, A. C.; Espinet, P.; Sen,
A. J. Am. Chem. Soc. 2002, 124, 11278-11279). Compound 6 can be
obtained, albeit in low yield, by coupling of C6F5MgBr and 2-bromopropene
catalyzed by [NiCl2(dppe)] (see Hayashi, T.; Konishi, M.; Okamoto, Y.;
Kabeta, K.; Kumada, M. J. Org. Chem. 1986, 51, 3772-3781 and Woollins,
J. D. Inorganic Experiments; VCH: Weinheim, 1994).

Reactions of Alkenylsilanes and Pd(II) Complexes
Organometallics, Vol. 25, No. 22, 2006 5451
Scheme 2. Possible Reaction Pathways and Observed
Products (framed) of the Reaction of
2-Propenyltrimethylsilane and Palladium Pentafluorophenyl
Complexes
side reactions of 8 are also observed: a Heck substitution to
give 1,3-bispentafluorophenylpropene 10 (8 is less substituted
that the starting silane, and its competing coordination should
be sterically preferred, whereupon route b should lead to 10)
and a hydride transfer to give propylpentafluobenzene (11) by
reaction with “PdHX” species formed in the course of the
reaction; this hydride transfer has been studied in detail before.30
Two different mechanisms have been proposed to explain
the formation of cine substitution products in C-C coupling
reactions with alkenyltriorganotin derivatives: (a) insertion of
the double bond into a Pd-C bond followed by â-SnR3
elimination (similar to route b2), proposed by Kikukawa;31 and
(b) R-SnR3 elimination to give a palladium carbene complex
that decomposes to the final product (proposed by Busacca and
Farina).32 To rule out the intermediacy of a palladium-carbene
in the formation of allylpentafluorobenzene (8, Scheme 3), we
carried out the reaction of 2-propenyltrimethylsilane with
complex 1 in the presence of added water. If it were formed,
hydrolysis of this electrophilic palladium carbene should lead
to a ketone, which was not detected in the reaction mixture.
The presence of water did not affect the distribution of products
6-11. Thus, the route depicted in Scheme 2 for the formation
of 8 (route b2) is the most plausible one.
The distribution of products obtained for the different Pd
complexes (Table 1) shows that the transmetalation (route a) is
a minor pathway in all cases. Transmetalation is expected to
be less favored for this R-substituted silane when compared to
nonsubstituted vinylsilane (see later), but this reluctance does
not justify the very low percentages found for CH2dC(Pf)CH3
(6). A higher amount of 6 is observed with an increase in the
electrophilicity of the Pd center, as clearly shown for the cationic
complex 4 (entry 5, Table 1). The chloro-palladium complex
2, more electrophilic than the Br or acac derivatives 1 and 3,
also shows a slight increase in the amount of 6 formed (entry
3, Table 1) when compared to 1 and 3. The main route observed
is insertion followed by â-SiMe3 elimination (route b2) to give
the cine substitution product allylpentafluorobenzene (8) (plus
its internal isomer 7 and products derived from its further
reactions, 10 and 11; ∑b2 in Table 1). The Si-containing product
CH2dC(SiMe3)CH2Pf (9) is obtained in low yield in the
equimolar reactions of the neutral complexes (entries 1, 3, 4,
Table 1) and is absent in the reaction with the cationic complex
4 (entry 5, Table 1). In the latter case the alkenylsilane in
intermediate E (Scheme 2) is expected to be bound to Pd more
strongly in the cationic complex, and substitution promoted by
other alkenes present is less favored, so further insertion into
the Pd-H bond (route b2) can occur. When an excess of
2-propenyltrimethylsilane is used (entry 2, Table 1), decoordi-
nation of the substituted alkenylsilane by associative substitution
promoted by the starting silane occurs (route b1), and the amount
of 9 increases noticeably.
(2) 2,1-Insertion of the double bond into the Pd-Pf bond
followed by â-H elimination from the methyl group and
decoordination leads to the substituted alkenylsilane CH2dC-
(SiMe3)CH2Pf (9) in an overall substitution or Heck process
(route b1). Since only the terminal alkenylsilane 9 is observed,
â-H elimination of one of the three methyl hydrogens must be
always favored versus â-H elimination of a C1 hydrogen (this
has also been observed for other closely related alkyls derived
from olefin insertion into Pd-C bonds).29
(3) Palladium migration to C3 or C1 followed by â-SiMe3
elimination leads to allylpentafluorobenzene (8) and 1-pen-
tafluorophenylpropene (7), respectively (the products of cine
substitution, route b2). The internal alkene 7 can also be formed
by isomerization of the terminal alkene 8 in the presence of
hydrido-palladium species. According to the preference for â-H
elimination from C3 detected in the formation of the silane 9,
8 should be the main cine substitution product and isomerization
of this terminal alkene should be the main source of 7. Two
Reactions with Vinyltrimethylsilane. The reactions of
complexes 1-4 with vinyltrimethylsilane were carried out in
the same manner described for 2-propenyltrimethylsilane.
Scheme 4 collects, in a simplified way, the plausible reaction
routes and products found for this silane (the notation used refers
to Scheme 2). Pentafluorostyrene (12) is formed by routes a
(28) A less common 1,2 insertion of 2-propenyltrimethylsilane in the
Pd-Pf bond followed by â-SiMe3 elimination could also lead to 6. The
occurrence of this route was tested by reacting an equimolar amount of
2-phenylpropene, an alkene electronic and sterically similar to 2-propenyl-
trimethylsilane, with complex 1. Only the products of 2,1 insertion, [Pd-
(η3-PhC(Me)CH2Pf)Br]2 and CH2Pf-C(Ph)dCH2, were formed. 2-Phenyl-
propene was also reacted with complex 4, and only the products of 2,1
insertion were observed.
(30) Albe´niz, A. C.; Espinet, P.; Lin, Y.-S. Organometallics 1997, 16,
4030-4032.
(31) Kikukawa, K.; Umekawa, H.; Matsuda, T. J. Organomet. Chem.
1986, 311, C44-C46.
(32) (a) Busacca, C. A.; Swestock, J.; Johnson, R. E.; Bailey, T. R.;
Musza, L.; Rodger, C. A. J. Org. Chem. 1994, 59, 7553-7556. (b) Farina,
V.; Hossain, M. A. Tetrahedron Lett. 1996, 37, 6997-7000. (c) Fillion,
E.; Taylor, N. J. J. Am. Chem. Soc. 2003, 123, 12700-12701.
(29) Elia, C.; Elyashiv-Barad, S.; Sen, A.; Lo´pez-Ferna´ndez, R.; Albe´niz,
A. C.; Espinet, P. Organometallics 2002, 21, 4249-4256.

5452 Organometallics, Vol. 25, No. 22, 2006
Table 1. Reactions of Complexes 1, 2, 3, and 4 with 2-Propenyltrimethylsilanea-c
Albe´niz et al.
a Pf ) C6F5. b 0.043 mmol of Pd complex and the corresponding amount of silane in CDCl3 (0.6 mL) at room temperature under N2. c The percentages
given correspond to the molar amount of products and were determined by integration of 19F NMR signals. The column labeled other shows, in each case,
the sum of the percentages of several products that could not be identified. d ∑b2 ) 7 + 8 + 10 + 11. e [PdPf2(NCMe)2] is formed (4.7%). f Acetone-d6 as
solvent.
Scheme 3. Alternative Formation of 8 Similar to the
Busacca-Farina Route in Organotin C-C Couplings
Scheme 5. Migration Process for Model Complex 16 in the
Presence of a High Excess of Alkene
Scheme 4. Reaction Pathways and Products of the Reaction
of Vinyltrimethylsilane and Palladium Pentafluorophenyl
Complexes
vinyltrimethylsilane is less substituted than 2-propenyltrimeth-
ylsilane, the associative substitution that takes place in route
b1 is more favored, and up to 87.8% of 13 can be obtained for
a ratio Pd:silane ) 1:50 (entry 2, Table 2). Decoordination of
13 aborts route b2 (which requires a Pd-H readdition, see
Scheme 2), and the amount of PfCHdCH2 (12) formed in the
conditions of entry 2 (Table 2) could be considered the
uppermost limit of transmetalation that occurs for reaction of
this silane. However, this could also be the highest amount of
13 that remains coordinated to palladium under these conditions.
To test this premise, the isomerization of the enyl palladium
complex 16 was monitored in the presence of a 50-fold excess
of 1,5-hexadiene (Scheme 5). This process was studied in detail
by us before, and we found that in the presence of excess diene
associative substitution of the Pf-substituted dienes took place
during Pd migration along the hydrocarbon chain leading to
free Pf-dienes and nonarylated Pd-allyls.35 The terminal Pd-
allyl 17 is formed if Pd migration occurs all the way, without
decoordination of the Pf-diene. Thus, the amount of 17 gives
the amount of olefin that does not decoordinate in the process,
and for a ratio Pd:1,5-hexadiene ) 1:50, the percentage found
is 17%. Thus, we can conclude that the amount of 12 in entry
2 (12%, Table 2) may as well be formed by route b2.
and b2, which makes these pathways indistinguishable as far
as product structure is concerned. Route b1 affords the substi-
tuted silane trans-PfCHdCHSiMe3 (13). Small amounts of the
products of side reactions on 12 were also found: Heck reaction,
(E)-PfCHdCHPf (14), and a hydride transfer process, PfCH2-
CH3 (15).33
Table 2 collects the percentages of products obtained in the
reactions with different complexes or conditions. Again, desi-
lylated compounds are the major products when a ratio Pd:silane
) 1:1 is used (routes b2 and a), route b2 being dominant.34 In
fact, the more electrophilic chloro complex 2 (slightly more
prone to transmetalation, see above) shows a lower amount of
12 than complexes 1 and 3. The same is observed for the
cationic complex 4 (entry 5, Table 2).
When the amount of starting silane is increased, a higher
amount of trans-PfCHdCHSiMe3 (13) is obtained. Since
Reactions with Allyltrimethylsilane. The product distribu-
tion of the reaction of allyltrimethylsilane with complexes 1-4
cannot be unambiguously correlated with the different routes
depicted in Schemes 1 and 2. In the reaction conditions, the
formation of intermediate palladium hydrido species leads to
the isomerization of the allylsilane to the more stable internal
alkene 1-propenyltrimethylsilane. This is a serious drawback
since the final products are the result of reaction of both silanes
(33) The spectra of compounds 12-15 were compared with commercial
(12) or independently prepared samples. Compound 15 can be obtained by
hydrogenation of 12 using Wilkinson’s catalyst. Compound 13 can be
obtained, in low yield, by coupling of C6F5MgBr and 2-bromovinyltrim-
ethylsilane catalyzed by [NiCl2(dppe)] (see ref 26). Compound 14 can be
prepared by reaction of 12 with complex 1 in a Heck-type substitution
process (see Albe´niz, A. C.; Espinet, P.; Mart´ın-Ruiz, B.; Milstein, D. J.
Am. Chem. Soc. 2001, 123, 11504-11505).
(34) The possibility of protonolysis of 13 to give 12 in the reaction
conditions was ruled out, since 13 is not affected by HCl(aq) added to a
CDCl3 solution of this silane.
(35) Albe´niz, A. C.; Espinet P.; Lin, Y.-S. Organometallics 1997, 16,
4138-4144.

Reactions of Alkenylsilanes and Pd(II) Complexes
Organometallics, Vol. 25, No. 22, 2006 5453
Table 2. Reactions of 1, 2, 3, and 4 with Vinyltrimethylsilanea-b,c
a Pf ) C6F5. b 0.025 mmol of Pd complex and the corresponding amount of silane in CDCl3 (0.6 mL) at room temperature under N2. c Percentages given
correspond to the molar amount of products and were determined by integration of 19F NMR signals. d [PdPf2(NCMe)2] (4.6%) and 14 (3.2%) are formed.
e Acetone-d6 as solvent. The column labeled other shows the sum of the percentages of several products (each one less than 4%) that could not be identified.
vinylic silanes (generally more prone to undergo transmetala-
tion), strongly suggests that route a does not occur for
allyltrimethylsilane.
Conclusions
Transmetalation is a minor route in the reactions of alkenyl-
silanes and palladium pentafluorophenyl complexes. The reac-
tions with 2-propenyltrimethylsilane allow us to estimate the
importance of this pathway in about 4% for the neutral
complexes, rising to about 15% for the more electrophilic
cationic palladium derivative. Transmetalation using vinyltri-
methylsilane is more difficult to quantify, but it represents less
than 12% of the amount of final products when the neutral
complex 1 is used. There is no evidence of the occurrence of
transmetalation in the reactions with allyltrimethylsilane since
intermediates of the type [PdPf(η3-allyl)L], known to be stable
in the reaction conditions, have not been detected. Insertion of
the double bond into the Pd-Pf bond followed by â-SiMe3
elimination to give desilylated cine substitution products is the
major route in all cases. These results estimate the inherent
preference of trimethylalkenylsilanes for one route over the
other. It should be kept in mind that in the presence of other
additives such as fluorides, as it is sometimes the case in
catalytic experiments, the relative importance of both routes can
Figure 1. Variable-temperature 19F NMR spectra of complex 18,
showing the effect of hindered rotation of the silane in the Fortho
signals of the pentafluorophenyl group.
be different.
and some of them arise from two different reaction pathways.
The Heck arylation of alkenylsilanes can be favored by using
an excess of reactant silane to promote the associative substitu-
tion of the arylated silane.
Interestingly, an intermediate palladium complex was detected
when the reaction of 3 and allyltrimethylsilane was monitored
by NMR. It is the silane-bound derivative 18 that could be
isolated by carrying out the reaction at low temperature. The
complex shows slow rotation of the pentafluorophenyl group
about the Pd-C bond at room temperature. However the
alkenylsilane undergoes fast rotation about the metal-olefin
bond at this temperature; this rotation becomes slow at 208 K
with the appearance of two different diastereoisomers (181 and
182, Figure 1), as shown by 19F NMR spectroscopy.36 Complex
18 corresponds to intermediate A in Scheme 2.
No complexes of the type [PdPf(η3-allyl)L] (L ) NCMe) or
[Pd(µ-Pf)2(η3-allyl)2]2, coming from transmetalation in the
reactions of complexes 1 or 2, could be detected in the reaction
media. This type of allyl(aryl) complexes have been observed
by us before, and they are moderately stable in solution, being
resistant to reductive elimination unless an electron-withdrawing
ligand (such as benzoquinone) is added.37 The absence of allylic
palladium complexes, along with the results obtained for the
Experimental Section
1
General Considerations. H, 13C{1H}, and 19F NMR spectra
were recorded on Bruker AC-300 and ARX-300 instruments.
Chemical shifts are reported in δ (ppm) and referenced to Me4Si
(1H and 13C) or CFCl3 (19F). All of the NMR spectra were recorded
at 293 K unless otherwise noted. Organic products were analyzed
using a HP-5890 gas chromatograph connected to a HP-5988 mass
spectrometer at an ionizing voltage of 70 eV using a quadrupole
analyzer. Elemental CHN analyses were determined with a Perkin-
Elmer 2400 CHN microanalyzer. IR spectra were recorded with a
Perkin-Elmer FT 1720 X spectrophotometer. All the reactions
described were carried out in a nitrogen atmosphere.
Solvents were dried over CaH2, distilled, and deoxygenated prior
to use. 2-Propenyltrimethylsilane, vinyltrimethylsilane, 1,5-hexa-
diene, and 2-phenylpropene were purchased from Aldrich and
Lancaster. The palladium complexes [PdPfX(NCMe)2] (X ) Br,
1;23 X ) Cl, 224) and [PdBrPf(bipy)]38 were prepared according to
(36) The slow limit exchange in the 1H NMR spectrum is not reached at
this temperature, and only extremely broad signals for the coordinated
allylsilane are observed.
(37) Albe´niz, A. C.; Espinet, P.; Mart´ın-Ruiz, B. Chem. Eur. J. 2001, 7,
2481-2489.
(38) Uso´n, R.; Fornies, J.; Nalda, J. A.; Lozano, M. J.; Espinet, P.;
Albe´niz, A. C. Inorg. Chim. Acta 1989, 156, 251-256.

5454 Organometallics, Vol. 25, No. 22, 2006
Albe´niz et al.
literature procedures. Compounds 6,15 7,15 8,15,26 10,15 12,15 14,15
16,35 and 1735 have been described before.
7.80 (t, J ) 6.4 Hz, 1H; H5′bipy), 7.45 (t, J ) 6.4 Hz, 1H; H5bipy),
2.18, (s, 3H; NCMe). IR (cm-1): ν 1630 (w), 1497 (m), 1054 (s),
954 (s), 782 (s), Pf; 2330 (m), 2310 (m), NCMe; 1614 (m), bipy;
Caution: Perchlorate salts are potentially explosive and should
be prepared and handled in only small amounts, exercising all
necessary care for this kind of hazardous explosive materials. For
purposes different from the use in this mechanistic study perchlorate
can be substituted by tetrafluoroborate, hexafluorophosphate, triflate,
or similar weakly coordinating anions.
-
1090 (br), ClO4
.
Synthesis of Solutions of [PdPf(bipy){(CD3)2CO}]ClO4 (4).
To a solution of AgClO4 (0.0071 g, 0.034 mmol) in acetone-d6 (1
mL) was added [PdBrPf(bipy)] (0.0174 g, 0.034 mmol). The
mixture was filtered to yield a yellow solution of complex 4 in
acetone-d6. A stoichometric amount of 2-propenyltrimethylsilane
and vinyltrimethylsilane were added to the samples of 4 prepared
according to this procedure. 19F NMR (282 MHz, (CD3)2CO): δ
-162.2 (m, 2Fmeta), -159.0 (t, 1Fpara), -120.2 (m, 2Fortho). 1H NMR
(300 MHz, (CD3)2CO): δ 8.70 (d, J ) 6.4 Hz, 1H; H6′bipy), 8.45
(m, 4H; H3,4,3′,4′bipy), 8.15 (d, J ) 6.4 Hz, 1H; H6bipy), 7.92 (t, J )
6.4 Hz, 1H; H5′bipy), 7.68 (t, J ) 6.4 Hz, 1H; H5bipy).
Synthesis of NBu4[PdBrPf(acac)]. To a solution of (NBu4)2-
[Pd2(µ-Br)2Br2Pf2]38 (1.3930 g, 1.031 mmol) in CH2Cl2 (30 mL)
was added Tl(acac) (0.6256 g, 2.061 mmol). The mixture was stirred
in the absence of light for 1.5 h at room temperature. After that
time, the suspension was filtered and the resulting solution was
evaporated to dryness. Et2O (20 mL) was added to the residue,
and upon cooling orange crystals were obtained. The crystals were
filtered, washed with Et2O, and air-dried. Isolated yield: 1.3351 g
(93%). Anal. Calcd for C27H43BrF5NO2Pd: C, 46.66; H, 6.24; N,
2.02. Found: C, 46.90; H, 5.97; N, 2.31. 19F NMR (282 MHz,
CDCl3): δ -165.8 (m, 2Fmeta), -163.7 (t, 1Fpara), -118.8 (m,
2Fortho). 1H NMR (300 MHz, CDCl3): δ 5.25 (s, 1H; CHacac), 3.37
Reactions of Neutral Palladium Complexes with 2-Prope-
nyltrimethylsilane. 2-Propenyltrimethylsilane (0.007 mL, 0.043
mmol) was added under N2 to a solution of 1 (0.0186 g, 0.043
mmol) in CDCl3 (0.6 mL) in an NMR tube. The reaction was
1
monitored by 19F and H NMR, and the formation of 6,15 7,15,26
8,15,26 9, 10,15 11, and PfH was observed as described in the text.
9: 19F NMR (282 MHz, CDCl3): δ -163.5 (m, 2Fmeta), -157.9
R
(m, 8H; CH2 ), 1.92 (s, 3H; Meacac), 1.81 (s, 3H; Me′acac), 1.68 (m,
â
γ
8H; CH2 ) 1.45 (m, 8H; CH2 ), 0.96 (t, J ) 8.9 Hz, 12H; CH3). IR
(cm-1): ν 1630 (s), 1497 (m), 1054 (s), 954 (s), 782 (s), Pf; 884
(m), NBu4+; 257 (m), Pd-Br; 1596 (s), 1581 (s), CdO.
1
(t, 1Fpara), -143.9 (m, 2Fortho). H NMR (300 MHz, CDCl3): δ
5.38 (s, 1H; H1),* 5.21 (s, 1H; H1′),* 3.49 (s, 2H; H2), 0.12 (s, 9H;
SiMe3). MS (EI, 70 eV): m/z (relative intensity) 280 (M+, 2), 265
(100), 181 (35), 169 (13), 77 (40), 73 (37), 43 (24). *H1 ) H cis
to CH2-Pf, H1′ ) H trans to CH2-Pf.
Synthesis of [PdPf(acac)(NCMe)] (3). To a solution of AgBF4
(0.1710 g, 0.878 mmol) in NCMe (30 mL) was added NBu4-
[PdBrPf(acac)] (0.6104 g, 0.878 mmol). The mixture was stirred
in the absence of light for 1 h at room temperature. After that time,
the suspension was filtered and the resulting solution was evaporated
to dryness. The residue was extracted with Et2O (6 × 10 mL), and
NCMe (0.5 mL) was added to the ethereal solution. This solution
was concentrated to 3 mL, and n-hexane (0.5 mL) was added. The
solution was cooled to -20 °C to yield yellow crystals, which were
filtered, washed with cold Et2O, and air-dried. Several additional
fractions of pure yellow crystals were collected from the mother
liquors by cooling. Isolated total yield: 0.2213 g (61%). Anal. Calcd
for C13H10F5NO2Pd: C, 37.75; H, 2.44; N, 3.39. Found: C, 37.78;
H, 2.40; N, 3.49. IR (cm-1): ν 1630 (s), 1497 (m), 1054 (s), 954
(s), 782 (s), Pf; 1595 (s), 1579 (s), CdO; 2355 (m), 2310 (m),
NCMe.
Complex 3 exists in CDCl3 solution as a mixture of the
complexes [PdPf(acac)(NCMe)] (3a, 74%) and a dimeric derivative
tentatively assigned to [(acac)PdPf(µ-κ2-O,O-acac)PdPf(NCMe)2]
(3b, 26%).
3a: 19F NMR (282 MHz, CDCl3): δ -164.1 (m, 2Fmeta), -160.5
(t, 1Fpara), -123.3 (m, 2Fortho). 1H NMR (300 MHz, CDCl3): δ 5.40
(s, 1H; CHacac), 2.30 (s, 3H; NCMe), 2.05 (s, 3H; Meacac), 1.9 (s,
3H; Me′acac).
11: 19F NMR (282 MHz, CDCl3): δ -163.7 (m, 2Fmeta), -158.8
1
(t, 1Fpara), -144.9 (m, 2Fortho). H NMR (300 MHz, CDCl3): δ
2.69 (t, J12 ) 8.0 Hz, 2H; H1), 1.62 (m, 2H; H2), 0.95 (t, J32 ) 8.0
Hz, 3H; H3).
The same procedure was used for the reactions collected in Table
1 with the neutral palladium complexes 2 and 3 or using solutions
of 4 (prepared as described above). In some experiments small
amounts of complex [PdPf2(NCMe)2] are observed: 19F NMR (282
MHz, CDCl3): δ -164.5 (m, 2Fmeta), -161.0 (t, 1Fpara), -117.5
(m, 2Fortho). 1H NMR (300 MHz, CDCl3): δ 2.18 (s, 6H; NCMe).
The reaction of Table 1, entry 2 was carried out in a preparative
scale. Complex 1 (0.0675 g, 0.155 mmol) and 2-propenyltrimeth-
ylsilane (1.23 mL, 7.75 mmol) were mixed in CH2Cl2 for 5 min.
The solvent was evaporated, and the product mixture was separated
by preparative TLC using an n-hexane/ethyl acetate (3:1) mixture
as eluent. Compound 9 was separated in the lowest Rf band.
Reaction of 1 with 2-Phenylpropene. 2-Phenylpropene (0.003
mL, 0.024 mmol) was added under N2 to a solution of 1 (0.0104
g, 0.024 mmol) in CDCl3 (0.6 mL) in an NMR tube. The reaction
was monitored by 19F and 1H NMR, and the formation of the
benzylic complex [Pd(η3-PhC(Me)CH2Pf)Br]2 and PfCH2C(Ph)d
CH2 was observed as described in ref 24.
[Pd(η3-PhC(Me)CH2Pf)Br]2: 19F NMR (282 MHz, CDCl3): δ
-162.5 (m, 2Fmeta), -156.2 (t, 1Fpara), -140.8, (m, 2Fortho). 1H NMR
(300 MHz, CDCl3): δ 7.65 (m, 2H; H3,5Ph), 7.45 (t, 1H; H2Ph),
7.30 (m, 1H; H4Ph), 6.6 (br, 1H; H6Ph), 3.10 (d, J ) 16.0 Hz, 1H;
CHH-Pf), 2.75 (d, J ) 16.0 Hz, 1H; CHH-Pf), 1.32 (s, 3H; Me).
PfCH2C(Ph)dCH2: 19F NMR (282 MHz, CDCl3): δ -163.0
(m, 2Fmeta), -156.0 (t, 1Fpara), -143.2, (m, 2Fortho). 1H NMR (300
MHz, CDCl3): δ 5.38 (s, 1H; H1),* 4.85 (s, 1H; H1′),* 3.75 (s,
2H; H2). *H1 ) H cis to Ph, H1′ ) H trans to Ph.
3b: 19F NMR (282 MHz, CDCl3): δ -164.9 (m, 1Fmeta), -163.5
(m, 2Fmeta), -163.2 (m, 1Fmeta), -160.5 (t, 1Fpara), -159.1 (t, 1Fpara),
1
-124.8 (m, 2Fortho), -122.5 (m, 1Fortho), -121.5 (m, 1Fortho). H
NMR (300 MHz, CDCl3): δ 5.38 (s, 1H; CHacac), 5.05 (s, 1H;
CHacac′), 2.52 (s, 3H; Meacac′), 2.38 (s, 6H; NCMe), 2.35 (s, 3H;
Meacac′), 2.12 (s, 3H; Meacac), 1.90 (s, 3H; Meacac).
Synthesis of [PdPf(bipy)(NCMe)]ClO4 (5). To a solution of
AgClO4 (0.0317 g, 0.153 mmol) in NCMe (30 mL) was added
[PdBrPf(bipy)] (0.0779 g, 0.153 mmol). The mixture was stirred
in the absence of light for 6 h at room temperature. After that time,
the suspension was filtered and the resulting solution was evaporated
to dryness. The residue was treated with Et2O (20 mL) and stirred
to yield a white solid, which was filtered, washed with Et2O, and
air-dried. Isolated yield: 0.0721 g (82%). Anal. Calcd for C18H11-
ClF5N3O4Pd: C, 37.92; H, 1.94; N, 7.37. Found: C, 37.57; H, 2.09;
N, 6.98. 19F NMR (282 MHz, CD3CN): δ -162.9 (m, 2Fmeta),
-159.7 (t, 1Fpara), -121.2 (m, 2Fortho). 1H NMR (300 MHz, CD3-
CN): δ 8.65 (d, J ) 6.4 Hz, 1H; H6′bipy), 8.35 (m, 3H; H3,3′,4′bipy),
8.22 (t, J ) 6.4 Hz, 1H; H4bipy), 7.88 (d, J ) 6.4 Hz, 1H; H6bipy),
Reactions of Neutral Palladium Complexes with Vinyltrim-
ethylsilane. Vinyltrimethylsilane (0.1203 mL of a 0.208 M solution
in CDCl3 titrated by 1H NMR with naphthalene as standard, 0.025
mmol) was added under N2 to a solution of 1 (0.0109 g, 0.025
mmol) in CDCl3 (0.5 mL) in an NMR tube. The reaction was
monitored by 19F and 1H NMR and the formation of 12,15 13, 14,15
and 15 was observed as described in the text.
13: 19F NMR (282 MHz, CDCl3): δ -163.7 (m, 2Fmeta), -156.8
1
(t, 1Fpara), -145.0 (m, 2Fortho). H NMR (300 MHz, CDCl3): δ

Reactions of Alkenylsilanes and Pd(II) Complexes
Organometallics, Vol. 25, No. 22, 2006 5455
6.83, 6.72 (AB system, J ) 18.7 Hz, 2H; CHdCH), 0.17 (s, 9H;
SiMe3). MS (EI, 70 eV): m/z (relative intensity) 266 (M+, 5), 251
(100), 169 (40), 77 (56).
15: 19F NMR (282 MHz, CDCl3): δ -163.5 (m, 2Fmeta), -159.0
(t, 1Fpara), -145.8 (m, 2Forto). 1H NMR (300 MHz, CDCl3): δ 2.75
(q, J ) 8.0 Hz, 2H; H1), 1.20 (t, J ) 8.0 Hz, 3H; H2).
The same procedure was used for the experiments collected in
Table 2 starting from the corresponding complex or solution of 4
in acetone-d6 as described above.
decanted off, and the solid was dried in vacuo. Isolated yield: 0.040
g (44%). Anal. Calcd for C17H21F5O2PdSi: C, 41.94; H, 4.35.
Found: C, 41.64; H, 3.95. 19F NMR (282 MHz, CDCl3): δ -162.9
(m, 2Fmeta), -159.72 (t, 1Fpara), -122.9 (m, 1Fortho), 123.5 (m,
1Fortho). 1H NMR (300 MHz, CDCl3): δ 5.85 (m, 1H; H2), 5.45 (s,
1H; CHacac), 4.57 (d, J ) 9.0 Hz, 1H; H1), 4.38 (d, J ) 13.8 Hz,
1H; H1′), 2.09 (dd, J ) 11.7, 4.7 Hz, 1H; H3), 1.98 (s, 3H; Me′acac),
1.92 (s, 3H; Meacac), 1.57 (t, J ) 11.7 Hz, 1H; H3′), 0.05 (s, 9H;
SiMe3). IR (cm-1): ν 1630 (w), 1497 (s), 1054 (s), 954 (s), 782
(s), Pf; 1584 (s), CO; 1271 (w), 852 (s), Si-Me3.
In the reaction of 1 with vinyltrimethylsilane in CD3CN the
complex [PdPf2(NCMe)2] is observed.
Reaction of 1 with 1,5-Hexadiene.35 1,5-Hexadiene (0.147 mL,
1.238 mmol) was added under N2 to a solution of 1 (0.0108 g,
0.025 mmol) in CDCl3 (0.5 mL) in an NMR tube. The reaction
was monitored by 19F and 1H NMR, and final products were 1-Pf-
1,5-hexadiene (4%), 1-Me-5-Pf-cyclopentene (8%), 6-Pf-1,4-hexa-
diene (71%), and 17 (17%).
Acknowledgment. Financial support from the Spanish MEC
(DGI, Grant CTQ2004-07667; Consolider Ingenio 2010, Grant
CSD2006-0003) and the Junta de Castilla y Leo´n (Project
VA117A06) is gratefully acknowledged. We also thank the
MEC for a fellowship to R.L.-F.
Synthesis of [PdPf(acac)(Me3SiCH2CHdCH2)] (18). To a
solution of 3 (0.0775 g, 0.187 mmol) in CH2Cl2 (5 mL) at 273 K
was added allyltrimethylsilane (0.1070 g, 0.937 mmol). The mixture
was stirred at 273 K for 2 h, and the resulting solution was treated
with activated carbon and filtered through magnesium sulfate. The
filtrate was evaporated to ca. 0.5 mL, and a yellow solid precipitated
upon addition of n-hexane (5 mL). The supernatant solution was
Supporting Information Available: Figures containing 19F and
1H NMR spectra of reaction mixtures and compound 9. This
material is available free of charge via the Internet at http://
pubs.acs.org.
OM060622E