Cyclization versus Pd-H elimination-readdition: Skeletal rearrangement of the products of Pd-C6F5 addition to 1,4-pentadienes

Article abstract of DOI:10.1021/ja960333p

The reactions of [Pd(C₆F₅)Br(CH₃CN)₂] with 1,4-pentadiene or 3-methyl-1,4-pentadiene at low temperature afford, after insertion of one double bond into the Pd-C₆F₅ bond, two types of η¹-η²-pentenylpalladium derivatives: the 4.5-membered palladacycles [Pd₂(μ-Br)₂(5-Pf-3-R-1,2,4-η¹-η²-pentenyl)₂] (Pf = C₆F₅; R = H (2a), Me (2b)) and the 5.5-membered metallacycles [Pd₂(μ-Br)₂(5-Pf-3-R-1,2,5-η¹-η²-pentenyl)₂] (R = H (3a), Me (3b)). Both enyls isomerize to η³-allylic derivatives following two competitive pathways, i.e. (i) Pd migration or (ii) cyclopropane formation and reopening, which lead to the isomeric η³-allylpalladium complexes [Pd₂(μ-Br)₂(5-Pf-3-R-1-3-η³-pentenyl)₂] (R = H (5a), Me (5b)) and [Pd₂(μ-Br)₂(4-Pf-3-(CH₂R)-1-3-η³-butenyl)₂] (R = H (6a), Me (6b)), respectively. After the cyclopropane formation-reopening process, [Pd₂(μ-Br)₂(3-CH₂Pf-1,2,5-η¹-η²-pentenyl)₂] (8) is also formed which isomerizes to 6b. The reaction of a mixture of the η¹-η²-pentenyl complexes at low temperature with CO/NaOMe gives methyl ester derivatives. Again, the formation of the major ester CH₂=CH(CH₂Pf)CHRCOOMe involves cyclopropane formation-reopening in the enyl derivative 2. An unusual acyl complex, [Pd(μ-Br)(η¹-η²-3-CH₂Pf-1-oxo-4-pentenyl)]₂ (15) containing a chelating η²-η¹-olefin acyl moiety has been characterized.

Full text of DOI:10.1021/ja960333p

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J. Am. Chem. Soc. 1996, 118, 7145-7152
7145
Cyclization versus Pd-H Elimination-Readdition: Skeletal
Rearrangement of the Products of Pd-C6F5 Addition to
1,4-Pentadienes
Ana C. Albe´niz, Pablo Espinet,* and Yong-Shou Lin
Contribution from the Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias,
UniVersidad de Valladolid, 47005-Valladolid, Spain
ReceiVed January 31, 1996X
Abstract: The reactions of [Pd(C6F5)Br(CH3CN)2] with 1,4-pentadiene or 3-methyl-1,4-pentadiene at low temperature
afford, after insertion of one double bond into the Pd-C6F5 bond, two types of η1-η2-pentenylpalladium derivatives:
the 4.5-membered palladacycles [Pd2(µ-Br)2(5-Pf-3-R-1,2,4-η1-η2-pentenyl)2] (Pf ) C6F5; R ) H (2a), Me (2b))
and the 5.5-membered metallacycles [Pd2(µ-Br)2(5-Pf-3-R-1,2,5-η1-η2-pentenyl)2] (R ) H (3a), Me (3b)). Both
enyls isomerize to η3-allylic derivatives following two competitiVe pathways, i.e. (i) Pd migration or (ii) cyclopropane
formation and reopening, which lead to the isomeric η3-allylpalladium complexes [Pd2(µ-Br)2(5-Pf-3-R-1-3-η3-
pentenyl)2] (R ) H (5a), Me (5b)) and [Pd2(µ-Br)2(4-Pf-3-(CH2R)-1-3-η3-butenyl)2] (R ) H (6a), Me (6b)),
respectively. After the cyclopropane formation-reopening process, [Pd2(µ-Br)2(3-CH2Pf-1,2,5-η1-η2-pentenyl)2] (8)
is also formed which isomerizes to 6b. The reaction of a mixture of the η1-η2-pentenyl complexes at low temperature
with CO/NaOMe gives methyl ester derivatives. Again, the formation of the major ester CH2dCH(CH2Pf)-
CHRCOOMe involves cyclopropane formation-reopening in the enyl derivative 2. An unusual acyl complex, [Pd-
(µ-Br)(η1-η2-3-CH2Pf-1-oxo-4-pentenyl)]2 (15) containing a chelating η2-η1-olefin acyl moiety has been characterized.
Introduction
Palladium-mediated reactions of organic compounds have
Scheme 1
proved to be of great utility in organic synthesis and play an
important role in the design of efficient and selective processes.1
One of them is the arylation and vinylation of olefins, known
as the Heck reaction. In this process a Pd-R moiety, usually
formed in situ by oxidative addition of an aryl or vinyl halide
to a Pd(0) complex, adds to the olefin via a cis-olefin-Pd-R
intermediate following a well-established mechanism.2,3 The
final product of the reaction depends on the fate of the new
alkyl palladium complex formed (m, Scheme 1). Decomposi-
tion through Pd-H elimination gives a substituted olefin (path
A, Scheme 1), and this is usually observed for mono-olefins. If
other double bonds are present in the molecule, subsequent
Pd-H eliminations and readditions can transport the Pd atom
along the hydrocarbon chain to form a stable allyl palladium
derivative; this allyl can be transformed into useful organic
derivatives by reaction with suitable nucleophiles (path B,
Scheme 1).1d An alternative is the addition of the new Pd-R
moiety to a second double bond in the substrate to give a cyclic
compound (path C, Scheme 1). Pd-H elimination is usually
the final step in these cyclizations. This intramolecular Heck
reaction has been applied to polyunsaturated substrates in a
cascade fashion allowing the synthesis of polycyclic organic
derivatives with remarkable efficiency and atom economy.4 The
combination of the processes outlined in Scheme 1 has led to
a great development of new synthetic routes.5
The cyclization via intramolecular Heck reaction (path C,
Scheme 1) requires the coordination of the second double bond
in the molecule to palladium in order to create an intermediate
η1-η2-palladacycle. Thus the formation of a 5-membered ring
from a plausible 6.5-membered η1-η2-palladacycle is the cy-
clization process most commonly observed.4-6 Cyclizations to
give ring sizes other than 5-membered are less common, but 3-
to 6-membered rings have also been synthesized.7-9 Both 3-
and 5-membered rings can be described, according to Baldwin
rules, as a result of favored 3-exo-trig or 5-exo-trig cyclizations,
respectively.10 The formation of 6-membered rings by an
apparent 6-endo-trig cyclization has been interpreted as the result
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
(1) (a) ComprehensiVe Organometallic Chemistry II; Wilkinson, G.,
Stone, F. G., Abel, E. W., Eds.; Pergamon Press: New York, 1995; Vol.
12. (b) Hegedus, L. S. In Organometallics in Synthesis; Schlosser, M.,
Ed.; Wiley: New York, 1994; Chapter 5. (c) Larock, R. C. AdV. Met.-
Org. Chem. 1994, 3, 97-224. (d) Ba¨ckvall, J.-E. AdV. Met. Org. Chem.
1989, 1, 135-175. (e) Heck, R. F. Palladium Reagents in Organic
Syntheses; Academic Press: New York, 1985. (f) Trost, B. M.; Verhoeven,
T. R. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone,
F. G., Abel, E. W., Eds.; Pergamon Press: New York, 1982; Vol. 8, p 854
and references therein.
(4) (a) Trost, B. M.; Shi, Y. J. Am. Chem. Soc. 1993, 115, 9421-9438.
(b) Negishi, E. Pure Appl. Chem. 1992, 64, 323-334. (c) Meyer, F. E.;
Brandenburg, J.; Parsons, P. J.; de Meijere, A. J. Chem. Soc., Chem.
Commun. 1992, 390-392. (d) Oppolzer, W.; DeVita, R. J. J. Org. Chem.
1991, 56, 6256-6257.
(5) (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259-281.
(b) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33,
2379-2411.
(6) (a) Trost, B. M.; Romero, D. L.; Rise, F. J. Am. Chem. Soc. 1994,
116, 4268-4278. (b) Trost, B. M.; Tanoury, G. J.; Lautens, M.; Chan, C.;
MacPherson, D. T. J. Am. Chem. Soc. 1994, 116, 4255-4267. (c) Takacs,
J. M.; Zhu, J.; Chandramouli, S. J. Am. Chem. Soc. 1992, 114, 773-774.
(d) Trost, B. M. Janssen Chim. Acta 1991, 9, 3-9.
(2) Thorn, D. L.; Hoffman, R. J. Am. Chem. Soc. 1978, 100, 2029.
(3) Albe´niz, A. C.; Espinet, P.; Jeannin, Y.; Philoche-Levisalles, M.;
Mann, B. E. J. Am. Chem. Soc. 1990, 112, 6594.
S0002-7863(96)00333-2 CCC: $12 00 © 1996 American Chemical Society

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7146 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Albe´niz et al.
Scheme 2
Scheme 3
of a favorable cyclopropane formation and subsequent ring
opening (Scheme 2).7c The palladium-promoted ring opening
of cyclopropanes is a well-known process.11 The example in
Scheme 2 and the reported formation of certain unexpected
products show that the formation of strained cyclopropane rings
might be more common than the number of isolated organic
compounds containing this moiety would suggest.7b,8c
On the other hand, Pd-H elimination is a rapid reaction that
can compete with the cyclization path and even abort it. The
formation of a 5-membered ring seems to operate efficiently
even when Pd-H elimination could occur.4-6 In contrast,
cyclopropane formation has only been accomplished when the
Pd-H elimination pathway is blocked.7
We have used previously [Pd(C6F5)Br(CH3CN)2] as a model
compound for the study of the Heck reaction.3,12 In this paper
we describe its reactions with 1,4-pentadiene or 3-methyl-1,4-
pentadiene which afford unprecedented examples of competition
of cyclopropane formation-ring opening and Pd-H elimination
(pathways B and C in Scheme 1).
Results
The reactions of 1,4-pentadiene and 3-methyl-1,4-pentadiene
with [Pd(C6F5)Br(CH3CN)2] (1) were analyzed. Similar be-
havior was observed for both dienes and accordingly the
reactions will be described together and the differences will be
pointed out where necessary. The reactions led eventually
(sometimes after days) to the formation of the allyl complexes
5-7 (Scheme 3). Two obvious features prompted us to carry
out deeper studies: first, the formation of 6 which involves an
unexpected rearrangement of the carbon skeleton of the diene;
and second, the observation of some quite stable intermediates
even at room temperature. Consequently, the reactions were
monitored by 19F and 1H NMR spectroscopy under temperature
control so that further intermediates could be observed.
(i) Insertion. When 1 and the diene were mixed in CDCl3
at 243 K (Pd:diene ratio 1:1) coordination of some of the diene
to palladium seemed to occur immediately: a new broad signal
at δ 6 (1,4-pentadiene) or 6.4 (3-methyl-1,4-pentadiene) was
observed in the 1H NMR spectrum in both cases (compared to
the multiplet at δ 5.8 for the free dienes); new Fortho resonances
at about δ -121, close in chemical shift to the signals of the
starting material 1, are also observed. However, as most of
1
the diene added remains unreacted, its H NMR signals hide
any other new resonances, and full identification of these
presumed Pd-diene derivatives could not be carried out.
Insertion of one double bond into the Pd-C6F5 moiety occurs
slowly at this temperature giving rise to distinct Fortho resonances
at about δ -143 ppm which are typical for C6F5-C moieties.3,12
They correspond to the η1-η2-pentenylpalladium derivatives 2
and 3 (Scheme 3). Compound 2 arises from coordination of
the second double bond to palladium in the putative η1-
alkylpalladium intermediate formed by insertion. If elimination
and readdition of Pd-H moves the Pd atom one carbon further,
coordination of the uninserted double bond gives 3.13 A third
type of η1-η2-pentenyl complex, 4, was detected coming from
the less favored addition of the Pd-C6F5 moiety to the double
bond with opposite regiochemistry, i.e. the pentafluorophenyl
group adding to the internal carbon. Although we have not
observed before this type of addition in the reaction of dienes
with “PdC6F5Br” synthons,12 minor products resulting from R
attack to the most substituted carbon of the double bond have
been detected by other authors when certain dienes are subjected
to the Heck reaction.14
(7) Cyclopropane ring formation: (a) Liu, C.-H.; Cheng, C.-H.; Cheng,
M.-C.; Peng, S.-M. Organometallics 1994, 13, 1832-1839. (b) Rawal, V.
H.; Michoud, C. J. Org. Chem. 1993, 58, 5583. (c) Owczarczyk, Z.;
Lamaty, F.; Vawter, E. J.; Negishi, E. J. Am. Chem. Soc. 1992, 114, 10091-
10092. (d) Grigg, R.; Sridharan, V.; Sukirthalingam, S. Tetrahedron Lett.
1991, 32(31), 3855-3858. (e) Meyer, F. E.; Parsons, P. J.; de Meijere, A.
J. Org. Chem. 1991, 56, 6487-6488. (f) Grigg, R.; Dorrity, M. J.; Malone,
J. F.; Sridharan, V.; Sukirthalingam, S. Tetrahedron Lett. 1990, 31(9), 1343-
1346. (g) Zhang, Y.; Negishi, E. J. Am. Chem. Soc. 1989, 111, 3454-
3456.
(8) Cyclohexane ring formation: (a) Dankwardt, J. W.; Flippin, L. A. J.
Org. Chem. 1995, 60, 2312-2313. (b) Tietze, L. F.; Schimpf, R. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1089-1091. (c) Grigg, R.; Stevenson, P.;
Worakun, T. Tetrahedron 1988, 44, 2033-2048. (d) References 4c, 6b,
and 7a.
(9) Cyclobutane ring formation: Carpenter, N. E.; Kucera, D. J.;
Overman, L. E. J. Org. Chem. 1989, 54, 5846-5848.
(10) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734-736.
(11) (a) Donaldson, W. A. AdV. Met.-Org. Chem. 1991, 2, 269-293.
(b) Fischetti, W.; Heck, R. F. J. Organomet. Chem. 1985, 293, 391-405.
(c) Larock, R. C.; Varaprath, S. J. Org. Chem. 1984, 49, 3432-3435. (d)
Green, M.; Hughes, R. P. J. Chem. Soc., Dalton Trans. 1976, 1880-1889.
(12) (a) Albe´niz, A. C.; Espinet, P.; Lin, Y.-S. Organometallics 1995,
14, 2977-2986. (b) Albe´niz, A. C.; Espinet, P. Organometallics 1991,
10, 2987-2988. (c) Albe´niz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F.
H. Organometallics 1990, 9, 1079-1085.
(13) The allyl derivative 5 is also observed in a very small ratio at 243
K. Pd migration operating on the η1-alkylpalladium intermediate before
double bond coordination accounts for the formation of this derivative.
(14) (a) Bender, D. D.; Stakem, F. G.; Heck, R. F. J. Org. Chem. 1982,
47, 1278-1284. (b) Heck, R. F. Acc. Chem. Res. 1979, 12, 146-151.

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Cyclization Vs Pd-H Elimination-Readdition
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7147
Table 1. Final Products (%) of the Reaction of 1 and Pentadienes
The ratio of the three compounds was evaluated by the ratio
of their 19F resonances at 243 K. For 1,4-pentadiene 2a:3a:4a
) 9.3:3.5:1, whereas for 3-methyl-1,4-pentadiene 2b:3b:4b )
8:8:1. Both 2b and 3b appear as a 1:1 mixture of two
diastereoisomers (2b1, 2b2, 3b1, and 3b2) clearly shown by their
distinct 19F resonances. They arise from the non-stereoselective
formation of two asymmetric carbons in the insertion step (Pd-
substituted and Me-substituted carbons). Only broad signals
were observed for 4b which may contain the two diastereo-
isomers also expected in this case. Although only one asym-
metric carbon is present in the structure of 2a, two equally
populated diastereoisomers were clearly observed in its 1H NMR
spectrum (only one set of signals with an unusual multiplicity
pattern was shown by their 19F NMR spectrum due to very close
chemical shift values). They must be the two diastereoisomers
expected from coordination of the re and si faces of the prochiral
double bond to palladium; in fact a slight excess of reactant
diene promotes rapid exchange between faces and the coales-
cence of the resonances of the two diastereoisomers. This
source of diastereoisomerism is also present in the rest of the
compounds 2-4 but the corresponding duplication of signals
was not actually observed, probably because in the correspond-
ing experiments some free diene in excess was efficiently
producing their conversion.
(PD)a
diene
1,4-PD
1,4-PD
3-Me-1,4-PD
3-Me-1,4-PD
Pd:diene ratio
5
6
7
organic deriv 5/6b
1:1
1:5
1:1
1:5
45.5 43.6 9.2
87.1 5.3 6.2
47.5 44
1.04
16.4
3.1
3.1
9.4
1.08
1.7
55.1 32.5 2.9
a Percent determined from integration of 19F NMR signals. b Ratio
of both products.
Crystallization and chromatographic separation by preparative
TLC were tried on the final mixture of η3-allylpalladium
derivatives obtained in the reactions. The complexes are very
soluble in organic solvents and only 5a-syn (7% yield) and 5b-
syn (10% yield) could be isolated as yellow solids by crystal-
lization from their respective mixtures using diethyl ether/n-
hexane at -20 °C. Partial separation of the allylic compounds
was carried out by preparative TLC. Details of these separa-
1
tions, analyses, and 19F and H NMR data are given in the
Experimental Section.
The ratio diene:Pd used in the reaction affects the ratio of
final products obtained and relevant data are collected in Table
1. Experiments using Pd:diene ) 1:5 were carried out.
Although the low-temperature distribution 2:3:4 was not
significantly different from that found in the 1:1 experiment,
remarkable effects were observed in the isomerization step.
Excess of 1,4-pentadiene increases dramatically the ratio 5a/
6a. The ratio 5b/6b changes in the same way when excess
3-methyl-1,4-pentadiene is used but the effect is less important.
In this case a C6F5-substituted diene is also formed in about
10% yield, which was identified as 3-((pentafluorophenyl)-
methyl)-1,4-pentadiene (9) along with the non-pentafluorophen-
yl-containing derivative di(µ-bromo)bis(3-methyl-1-3-η3-pen-
tenyl)dipalladium(II) (10).15 It has been observed before that
excess diene favors decoordination of the double bond from
palladium during Pd migration to give substituted dienes and
the non-substituted palladium allyl.12a,16,17
(iii) Carbonylation Experiments. Carbonylation of pal-
ladium organometallic derivatives is a well-known reaction of
wide applicability.18 In order to further support the structure
of the η1-η2-pentenyl complexes formed at low temperature,
trapping reactions of these derivatives with CO were tried. The
rates of the different reactions in Scheme 3 are such that it is
not possible to obtain a solution containing only 2, 3, and 4.
Thus mixtures containing about 85-90% of these derivatives
in the ratio indicated above and about 10-15% of either 1, 5,
or both (depending on the temperature and reaction time) were
The 19F and H NMR spectral data for these complexes are
1
given in the Experimental Section. The simplicity of the
pentafluorophenyl pattern in the 19F NMR spectra (three typical
multiplets) and the wide chemical shift range (which makes
overlapping of signals very rare) provide a fast and useful check
of the number of compounds in mixtures and facilitate to
monitor their evolution as has been previously described.12c 1
H
homonuclear correlations and decoupling experiments were used
1
to fully identify all the complexes. For 3a the H resonances
are hidden by the major derivatives 2a1 and 2a2, but are revealed
by integration in a ratio consistent with the 19F spectrum of the
mixture. Besides this, the structure proposed for 3a is based
on those of 3b1 and 3b2, formed in the reaction of the analogous
diene 3-methyl-1,4-pentadiene.
(ii) Isomerization. As the temperature increases, isomer-
ization of compounds 2 and 3 is observed and was followed by
19F NMR spectroscopy (Scheme 3). Thus 2a and 3a disappear
at similar rates, and this is clearly observed at 263 K (25%
conversion after 10 min; half conversion after 30 min). 2b1
and 2b2 also isomerize clearly at this temperature (30%
conversion after 10 min; half conversion after 20 min), but a
noticeable disappearance rate is reached for 3b1 and 3b2 only
at a higher temperature (283 K, 15% conversion after 10 min;
half conversion after 40 min). The final products of the
isomerization are compounds 5 and 6, but during the isomer-
ization of the mixture of 2b and 3b, the formation of a third
compound, 8, which isomerizes to the η3-allyl complex 6b at
room temperature, is also observed. The final η3-allyls are
formed as a mixture of syn and anti isomers, the less hindered
syn isomer (i.e. the isomer with the bulkier substituent syn to
the central allylic proton) being more abundant in all cases. The
following ratios are observed: 5a-syn:5a-anti ) 9:1; 5b-syn:
5b-anti ) 1.5:1; 6a-syn:6a-anti ) 2:1; 6b-syn:6b-anti ) 3.3:1
(Scheme 3). Compound 6a had been obtained previously as a
minor product from the reaction of [Pd(C6F5)Br(CH3CN)2] with
isoprene.12a
(15) Small amounts of organic C6F5-disubstituted derivatives (3%) are
also produced in the course of the reaction of [Pd(C6F5)Br(CH3CN)2] and
3-methyl-1,4-pentadiene (ratio 1:1). They were identified as a mixture of
(E)- and (Z)-1,5-bis(pentafluorophenyl)-3-methyl-2-pentene (11), E:Z ) 2:1.
They are the result of insertion of the unsubstituted double bond of
1-(pentafluorophenyl)-3-methyl-1,4-pentadiene into the Pd-C6F5 bond and
subsequent H transfer to form the monoolefin. 1-(Pentafluorophenyl)-3-
methyl-1,4-pentadiene is generated in turn from insertion of 3-methyl-1,4-
pentadiene into the Pd-C6F5 bond followed by Pd-H elimination. These
products do not form when excess diene is used in the reaction.
(16) Larock, R. C.; Takagi, K. J. Org. Chem. 1984, 49, 2701-2705.
(17) An increase of the amount of syn complex is found for complexes
5a and 6a when excess diene is used (5a-syn:5a-anti ) 9:1, Pd:diene )
1:1; 5a-syn:5a-anti ) 19.2:1, Pd:diene ) 1:5; 6a-syn:6a-anti ) 2:1, Pd:
diene ) 1:1; 6a-syn:6a-anti ) 3.8:1, Pd:diene ) 1:5). It is well-know
that the interconversion syn-anti is promoted by free ligands and, even if
the coordination ability of dienes is not very high, the ratio observed when
excess diolefin is used must be closer to the thermodynamic syn-anti
distribution. No noticeable change in the syn:anti ratio is found with time
or excess diene for derivatives b (3-methyl-1,4-pentadiene).
Also the η1-η2-pentenyl derivatives 4a and 4b isomerize
through Pd-H elimination-readditions leading to the corre-
sponding η3-allylpalladium complexes 7a and 7b. The pro-
cesses occur very slowly and take 25 days at room temperature
for completion.
(18) (a) Colquhoun, H. M.; Thompson, D. J.; Twigg, M. V. Carbony-
lation: Direct Synthesis of Carbonyl Compounds; Plenum Press: New York,
1991. (b) Reference 1c, Chapter 8, pp 341-400.

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Albe´niz et al.
obtained and used for carbonylation. Apparently there is no
noticeable conversion of 2 or 3 into 5 during the reaction. The
carbonylation products of 1 and 5 in the presence of NaOMe
are C6F5CO2Me, p-MeOC6F4CO2Me (both from 1), and
C6F5(CH2)2CRdCHCH2CO2Me (from 5), which can be easily
discounted from the spectra, and the fate of the other 85-90%
is discussed below.
Scheme 4
When a mixture of the η1-η2-pentenyl complexes obtained
in the reaction of [Pd(C6F5)Br(CH3CN)2] with 1,4-pentadiene
in CHCl3 at 243 K, i.e. 2a:3a:4a ) 9.3:3.5:1, was reacted with
CO and a methanolic solution of NaOMe, metallic palladium
precipitated immediately. Working up of the solution afforded
a mixture of the following C6F5-substituted methyl esters
(Scheme 4): methyl 3-((pentafluorophenyl)methyl)-4-pentenoate
(12a); methyl 2-((pentafluorophenyl)methyl)-4-pentenoate (13a);
and methyl 3-(pentafluorophenyl)-5-hexenoate (14a). The ratio
of these derivatives was 12a:13a:14a ) 9.5:3.3:1.
The structure of the major derivative 12a reflects a skeletal
rearrangement of the starting compounds. 1H COSY and
1
heteronuclear H-13C correlation experiments helped in the
assignment. The last experiment determined the presence of a
CH2COOMe moiety (cross peak between a δ 2.43 proton and
a δ 39.1 carbon) which is consistent with the structure of ester
12a. Esters 13a and the minor 14a are the result of direct
carbonylation of the Pd complexes 2a and 4a, respectively, i.e.
formal substitution of Pd by a carbomethoxy group.19
common η2-η1-olefin-acyl chelating ligand. 15 undergoes
nucleophilic cleavage by OMe- to give the ester derivative 12a,
thus supporting the assigned structure and showing its inter-
mediacy in the carbonylation reaction (Scheme 4).
The carbonylation reaction was also carried out with a mix-
ture of the η1-η2-enyl complexes derived from the reaction of
[Pd(C6F5)Br(NCMe)2] with 3-methyl-1,4-pentadiene, i.e. 2b, 3b,
and 4b (ratio 8:8:1). When this mixture, in CDCl3 at 243 K,
was reacted with CO and a methanolic solution of NaOMe,
black palladium precipitated immediately. Working up of the
solution afforded a mixture of the following C6F5-substituted
methyl esters (Scheme 4): methyl 2-methyl-3-((pentafluoro-
phenyl)methyl)-4-pentenoate (12b), methyl 3-methyl-2-((pen-
tafluorophenyl)methyl)-4-pentenoate (13b), and methyl 4-meth-
yl-3-(pentafluorophenyl)-5-hexenoate (14b), each of them as a
1:1 mixture of two diastereoisomers; the ratio of the esters was
12b:13b:14b ) 11:5:1. The presence of 12b shows that a
rearrangement of the carbon chain has occurred, as it was
observed for 12a. The ester derivatives 13b and 14b are the
result of formal substitution of Pd by a carbomethoxy group in
the η1-η2-enyl complexes 2b and 4b. The structure of 13b was
further supported by the presence of a small coupling constant
between the olefinic proton H4 and the methyl group on the
The carboxylic ester expected from direct CO insertion into
the η1-η2-enyl complex 3a, methyl 2-(pentafluorophenyl)-5-
hexenoate, was not observed in the final mixture; this ester was
independently synthesized by standard organic methods and its
1H and 19F NMR spectra compared with those corresponding
to the carbonylation mixture, where no coincidences were found.
When CO was bubbled through a mixture of 2a, 3a, and 4a
in the usual ratio (which also contained 5a-syn in about 12%)
at low temperature in the absence of the methanolic NaOMe
solution, an intermediate acyl derivative di(µ-bromo)bis(η1-η2-
(3-((pentafluorophenyl)methyl)-1-oxo-4-pentenyl)dipalla-
dium(II) (15), along with other products, was formed. Repeated
crystallizations of the mixture from Et2O/n-hexane afforded a
yellow solid in 25% yield, containing 15 as the major component
(60%), unreacted 5a-syn, and what seemed to be the acyl
derivatives resulting from insertion of CO into the Pd-C bond
of the η1-η2-pentenyl derivatives 2a and 4a. These two acyl
derivatives could not be identified though, since their 1H NMR
signals were hidden by the major derivative 15; however their
presence was clear in the 19F NMR spectrum of the mixture.
Although 15 is stable at room temperature in the solid state
and moderately stable in CHCl3 solution we could not isolate
it pure. It was characterized in the mixture by an intense ν(CO)
absorption in the IR spectrum at 1763 cm-1 and a 13C resonance
at δ 206 ppm, showing clearly the presence of an acyl moiety.
Although palladium acyl derivatives are well-known20 and some
of them are stabilized through chelation,21 15 exhibits a less
1
adjacent carbon (Me3), clearly seen in the H COSY spectrum
of the mixture. This coupling was not detected for the major
derivative 12b. Again, the methyl ester corresponding to the
5.5-membered metallacycle 3b was not observed (see above).
Discussion
The insertion of 1,4-pentadiene or 3-methyl-1,4-pentadiene
into the Pd-C6F5 bond gives a chance to observe several η1-
η2-pentenyl complexes 2-4 and their evolution to give η3-allyl
complexes (Scheme 3). Complexes 5 and 7 are the allyls
expected from simple Pd-H elimination-readdition, but the
generation of 6 needs a skeletal rearrangement of the carbon
chain.
The formation of complexes 6 and, when 3-methyl-1,4-
pentadiene is used, the η1-η2-pentenyl complex 8 occurs via
cyclopropane formation and reopening as shown in Scheme 5.
Cyclization to give an intermediate cyclopropylmethyl-pal-
ladium derivative (16), which could not be detected, followed
by concerted ring opening to 17 and by Pd migration leads to
complexes 6. Alternatively, 1,2-hydrogen shift in 17 and double
bond coordination leads to complex 8. Chloropalladation of
or Pd-R addition to methylenecyclopropanes has been shown
(19) Derivative 14a could not be completely identified, but it is clearly
seen in the 19F NMR spectrum of the mixture and also in the 1H NMR
spectrum (Me resonance of the ester group) in the same relative ratio. 14a
is also present when a mixture of the η3-pentenyl derivatives obtained from
isomerization of the enyl mixture (in which 4a is still present) is subjected
to the carbonylation reaction.
(20) (a) Van Asselt, R.; Gielens, E. E. C. G.; Ru¨lke, R. E.; Vrieze, K.;
Elsevier, C. J. J. Am. Chem. Soc. 1994, 116, 977-985. (b) To´th, I.; Elsevier,
C. J. J. Am. Chem. Soc. 1993, 115, 10388-10389. (c) Cavinato, G.;
Toniolo, L. J. Organomet. Chem. 1990, 398, 187-195. (d) Brumbaugh, J.
S.; Sen, A. J. Am. Chem. Soc. 1988, 110, 803-816. (e) Garrou, P. E.;
Heck, R. F. J. Am. Chem. Soc. 1976, 98, 4115-4127. (f) Booth, G.; Chatt,
J. J. Chem. Soc. A 1966, 634-638.
(21) Hegedus, L. S.; Siirala-Hanse´n, K. J. Am. Chem. Soc. 1975, 97,
1184-1188.

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Cyclization Vs Pd-H Elimination-Readdition
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7149
Scheme 5
Scheme 6
hinder rotation to reach a syn Pd-H arrangement easily. After
the first Pd-H elimination-readdition, Pd gets bound to C4
and again C-C bond rotation to reach a syn Pd-H alignment
is needed. This is more difficult for the C-3 substituted
complexes. Thus, the backward â-H5 elimination (i, Scheme
6) becomes competitive with the â-H3 elimination (j, Scheme
6), slowing down the isomerization process to η3-allyl.
Should decoordination of the double bond in the η1-η2-
pentenyl complexes 2 occur, this would prevent isomerization
through the cyclization pathway. This decoordination is induced
by addition of an excess diene which can act as a competing
ligand. The effect is quite remarkable for 1,4-pentadiene and
the ratio Pd-migration:cyclization products is increased to 16.4:
1. The influence is less important in 3-methyl-1,4-pentadiene
and the amount of Pd-migration products raises only moderately
(Table 1). The less favorable Pd migration for the latter (just
discussed) and small differences in the coordination ability of
the two dienes can account for the different effects.
It can be said that, in general, the presence of substituents
on the diene carbon chain makes Pd-H elimination more
difficult, in favor of the cyclization process. In the limit,
cyclopropane formation is the only available isomerization path
for 4.5-membered η1-η2-palladacycles when Pd-H elimination
is not possible, i.e. when palladium is bound to a quaternary
carbon, or when a trans-Pd-H arrangement is fixed in a cyclic
derivative or by intramolecular coordination. It is worth noting
that the examples of cyclopropane formation in Pd-mediated
cyclizations in the literature, where usually excess of the
unsaturated substrate is used, have been observed only when
the Pd-H elimination path is blocked.7 To our knowledge, this
is the first report on competitive Pd-H elimination and
cyclopropane formation.
The presence of CO seems to promote the cyclization path,
since the major methyl ester formed by reaction of the η1-η2-
pentenyl mixture (2, 3, and 4) with carbon monoxide and
NaOMe is 12. This ester results from cyclopropane formation
and reopening, which gives a new η1-η2-pentenyl palladium
derivative (Scheme 7). Then CO insertion into the Pd-σ-C
bond gives the acylpalladium complex 15 which could be
isolated. Nucleophilic attack by OMe- on 15 gives the methyl
ester derivative 12.
The ester derivatives coming from direct insertion of CO into
the Pd-C bond of the enyl derivatives 2 and 4 (13 and 14,
respectively) were also identified, but the ester corresponding
to the analogous reaction on complexes 3 was not detected. As
electronegative substituents decrease the ease of insertion of
CO into a metal-carbon bond,23 the presence of a pentafluo-
to give η3-allylpalladium derivatives following the same ring-
opening mechanism.11 Thus, facile C-C bond formation and
subsequent C-C cleavage produces the rearrangement of the
carbon chain. The formation of compound 9 when excess diene
is used arises from Pd-H elimination in the η1-enyl intermediate
17 and substitution of the diene by 3-methyl-1,4-pentadiene
(Scheme 5). Pd-H addition to the latter gives the η3-
allylpalladium derivative di(µ-bromo)bis(3-methyl-1-3-η3-pen-
tenyl)dipalladium(II) (10).
The isomerization of the 4.5-membered η1-η2-pentenylpal-
ladium complex 2b at 263 K yields complexes 5b, 6b, and 8
simultaneously (Scheme 3). This means that cyclization (which
yields 6b and 8) and decoordination of the double bond followed
by Pd migration (that yields 5b) are competitive pathways
operating on derivative 2b with almost equal efficiency, as
indicated by the ratio 5b:(6b + 8) ≈ 1:1.05. A preference for
the Pd-migration pathway is observed in the isomerization of
the 5.5-membered pentenylpalladium derivative 3b at 283 K;
the ratio after the disappearance of 3b is 5b:(6b + 8) ≈ 1:0.87.
Since the cyclopropane formation process, an intramolecular
Heck reaction, must occur from 2, it seems that a certain amount
of 3b forms 2b and then undergoes the above-mentioned
rearrangement.
Several η1-η2-pentenyl complexes have been detected in this
work and it is worth noting their different rates of isomerization
which roughly follow the order 2a ≈ 2b ≈ 3a > 3b > 8 > 4a
≈ 4b. The 4.5-membered η1-η2-pentenyl complexes 2 isomerize
faster than most of the η1-η2-pentenyl complexes with one extra
chain link. This is consistent with the higher strain of the
palladacycle in the former, which should make double bond
decoordination easier; this decoordination is probably needed
for Pd migration to proceed.3,12a However, the very different
rates found for 3a, 3b, 4, and 8 (all containing a 5.5-membered
metallacycle) reveal that the strain of the metallacycle is not
the main factor that controls their stability toward Pd migration.
Other steps such as bond rotation around the C4-C5 bond, to
arrange Pd and a â-H syn to each other, may also become rate
determining (Scheme 6).22 In fact the most stable derivatives
are complexes 4, with only one â-H available to Pd-H
elimination and a bulky pentafluorophenyl group which may
(22) Goddard, R.; Green, M.; Hughes, R. P.; Woodward, P. J. Chem.
Soc., Dalton Trans. 1976, 1890-1899.
(23) Yamamoto, A. Organotransition Metal Chemistry; Wiley: New
York, 1986; pp 251-252.

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7150 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Albe´niz et al.
occurred. 4b completely isomerized after 25 days. The mixture was
worked up to separate 5b, 6b, 7b, and the small amount of organic
derivatives formed. The solvent was evaporated and the residue was
chromatographed on a silica column (neutral, 230-400 mesh) and
eluted with n-hexane. This operation afforded a mixture of the C6F5-
organic derivatives (E)- and (Z)-11. Elution with Et2O yielded the Pd-
allyl compounds, which could be further separated by preparative TLC
as reported below.
Scheme 7
2b1: 19F NMR (CDCl3, δ, 263 K, 282 MHz), -162.1 (m, Fmeta),
1
-156.1 (t, Fpara), -142.9 (m, Fortho); H NMR (CDCl3, δ, 263 K, 300
MHz), 5.00 (m, 2H, H1, H1′), 4.62 (m, 1H, H2), 3.17 (m, H3), 1.12 (d,
3H, J ) 7 Hz, Me3).
2b2: 19F NMR (CDCl3, δ, 263 K, 282 MHz), -163.1 (m, Fmeta),
1
-156.3 (t, Fpara), -143.4 (b, Fortho); H NMR (CDCl3, δ, 263 K, 300
MHz), 4.50-4.30 (H1, H1′, H2), 2.55 (H3), 0.89 (d, 3H, J ) 6.5 Hz,
Me3).
3b1: 19F NMR (CDCl3, δ, 283 K, 282 MHz), -162.0 (m, Fmeta),
1
-156.2 (t, Fpara), -143.1 (m, Fortho); H NMR (CDCl3, δ, 283 K, 300
MHz), 4.75 (d, 1H, J ) 15.5 Hz, H1), 4.37 (m, H1′, H2), 3.00 (H5),
2.55 (b, H3), 2.33 (H4), 1.65 (H4′), 0.95 (d, 3H, J ) 6.5 Hz, Me3).
3b2: 19F NMR (CDCl3, δ, 283 K, 282 MHz), -162.5 (m, Fmeta),
1
-157.0 (t, Fpara), -143.2 (m, Fortho); H NMR (CDCl3, δ, 283 K, 300
MHz), 4.63 (d, 1H, J ) 14.5 Hz, H1), 4.37 (m, H1′, H2), 2.05 (H5),
1.72 (H3), 1.30 (H4), 1.1 (H4′), 0.77 (d, 3H, J ) 5.5 Hz, Me3).
4b: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.3 (b, Fmeta),
rophenyl group attached to the carbon atom bound to palladium
in complexes 3 seems to slow down the insertion process
rendering the Pd-H elimination-readdition competitive. This
should move the Pd atom to an adjacent carbon to form 2, where
cyclization or CO insertion, to give esters 12 and 13, occurs.
1
-156.3 (b, Fpara), -140.5 (b, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.76 (dd, 1H, J ) 15, 8.5 Hz, H2), 4.93 (bd, 1H, J ) 8.5 Hz,
H1), 4.87 (d, 1H, J ) 15 Hz, H1′), 3.70 (m, 1H, J ) 12, 5.5 Hz, H4),
3.50 (H5), 2.45 (m, 1H, H5′), 2.35 (m, 1H, H3), 1.35 (Me3).
5b-syn: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.6 (m, Fmeta),
Conclusions
1
-157.2 (t, Fpara), -144.2 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.03 (dd, 1H, J ) 12.7, 7.2 Hz, H2), 3.99 (d, 1H, J ) 7.2 Hz,
H1-syn), 3.23 (d, 1H, J ) 12.7 Hz, H1-anti), 3.05 (m, H5), 2.80 (m,
H5′), 2.25 (m, H4), 2.15 (m, H4′), 1.32 (s, 3H, Me3).
Unstable η1-η2-pentenyl palladium derivatives, formed by
insertion of a double bond of 1,4-pentadiene or 3-methyl-1,4-
pentadiene into the Pd-C6F5 bond, can be stabilized at low
temperature. Two competitive processes are found in their
evolution to give η3-allyls, namely, (i) Pd migration and (ii)
cyclopropane formation from 4.5-membered η1-η2-palladacycles
and reopening. The latter result proves that the intramolecular
Heck reaction to give cyclopropanes can occur at a rate
comparable with that of Pd-H elimination. This competition
is observed for the first time and brings about chain rearrange-
ments otherwise unexpected. CO coordination seems to pro-
mote the cyclization path.
5b-anti: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.6 (m, Fmeta),
1
-157.0 (t, Fpara), -144.5 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.09 (dd, 1H, J ) 12.6, 7.4 Hz, H2), 3.87 (d, 1H, J ) 7.4 Hz,
H1-syn), 3.01 (d, 1H, J ) 12.6 Hz, H1-anti), 3.03 (m, H5), 2.90 (m,
H5′), 2.05 (m, H4), 1.73 (m, H4′), 1.64 (s, 3H, Me3).
6b-syn: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -161.9 (m, Fmeta),
1
-155.7 (t, Fpara), -141.4 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.33 (dd, 1H, J ) 12.6, 7.4 Hz, H2), 3.98 (d, 1H, J ) 7.4 Hz,
H1-syn), 3.52 (d, 1H, J ) 15 Hz, CH2C6F5), 3.16 (bd, 2H, H1-anti,
CH2C6F5), 1.40 (m, 1H, H4), 1.28 (t, 3H, J ) 6.5 Hz, Me5), 1.13 (m,
1H, H4′).
6b-anti: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.3 (m, Fmeta),
Experimental Section
1
-156.4 (t, Fpara), -140.5 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
General Comments. 19F and H NMR spectra were recorded on
MHz), 5.15 (dd, 1H, J ) 12.8, 7.5 Hz, H2), 4.10 (d, 1H, J ) 7.5 Hz,
H1-syn), 3.48 (d, 1H, J ) 12.8 Hz, H1-anti), 2.80 (m, CH2C6F5), 1.90
(m, 1H, H4), 1.60 (m, H4′), 1.11 (t, 3H, J ) 7.3 Hz, Me5).
1
Bruker ARX-300 and AC-300 spectrometers. Chemical shifts (in δ
units, ppm) are reported downfield from SiMe4 for H and 13C and
1
from CFCl3 for 19F. Mixtures of organic products were analyzed using
a HP-5890 gas chromatographer connected to a HP-5988 mass
spectrometer at an ionizing voltage of 70 eV and a quadrupole analyzer.
High-resolution MS were recorded in a Fisons UG-Autospec spec-
trometer. Carbon, hydrogen, and nitrogen analyses were carried out
on a Perkin-Elmer 240 microanalyzer. IR spectra were recorded on a
Perkin-Elmer 883 spectrophotometer.
7b: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.0 (m, Fmeta),
1
-156.2 (t, Fpara), -142.7 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.47 (dd, 1H, J ) 12, 7 Hz, H2), 4.08 (d, 1H, J ) 7 Hz, H1-
syn), 3.80 (m, H4), 3.29 (d, 1H, J ) 12 Hz, H1-anti), 1.59 (d, J ) 7
Hz, Me5), 1.14 (s, Me3).
8: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.4 (m, Fmeta), -156.7
1
(t, Fpara), -143.2 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300 MHz),
[Pd(C6F5)Br(NCMe)2] was prepared as described elsewhere.12c
Diolefins were purchased from Aldrich and used without further
purification.
5.87 (m, 1H, H2), 4.69 (d, 1H, J ) 14 Hz, H1′), 4.65 (d, 1H, J ) 6.6
Hz, H1), 2.90 (m, H3), 2.71 (d, 2H, J ) 6.6 Hz, CH2C6F5), 2.46 (m,
1H, H5), 2.28 (m, 1H, H5′), 1.69 (H4), 0.66 (dt, 1H, J ) 11, 4.5 Hz,
H4′).
Variable-Temperature Experiments. Reaction of [Pd(C6F5)Br-
(NCMe)2] with 3-Methyl-1,4-pentadiene. A 5-mm NMR tube was
charged with a solution of [Pd(C6F5)Br(NCMe)2] (0.050 g, 0.115 mmol)
in CDCl3 (0.6 mL) and cooled to -30 °C. 3-Methyl-1,4-pentadiene
(0.014 mL, 0.115 mmol) was added to this solution and the reaction
(E)-11: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.6 (m, Fmeta),
-163.2 (m, Fmeta), -158.4 (t, Fpara), -158.3 (t, Fpara), -144.7 (m, Fortho),
-144.6 (m, Fortho); 1H NMR (CDCl3, δ, 293 K, 300 MHz), 5.04 (t, 1H,
J ) 7.5 Hz, H2), 3.34 (d, 2H, J ) 7.5 Hz, H1), 2.79 (tt, 2H, J ) 7.5
4
Hz, JF-H ) 1.5 Hz, H5), 2.24 (m, 2H, J ) 7.5 Hz, H4), 1.79 (s, 3H,
was monitored by 19F and H NMR. After 2 h at this temperature
1
Me3); MS(EI) m/z (relative intensity), 416 (M+, 6), 235 (36), 207 (41),
181 (100), 41 (50). HRMS calcd for C18H10F10, m/z 416.0623; found,
416.0632.
insertion of the olefin into the Pd-C6F5 bond had occurred and
compounds 2b, 3b, and 4b were formed. The temperature was then
raised to -10 °C and the mixture was kept for 2 h at this temperature;
isomerization of 2b was observed. The mixture was warmed to 10 °C
and after 4 h isomerization of 3b was complete. The temperature was
finally increased to 20 °C and after 2 days total disappearance of 8
(Z)-11: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.1 (m, Fmeta),
-158.2 (t, Fpara), -157.8 (t, Fpara), -144.9 (m, Fortho), -144.6 (m, Fortho);
1H NMR (CDCl3, δ, 293 K, 300 MHz), 5.23 (t, 1H, J ) 6.5 Hz, H2),

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Cyclization Vs Pd-H Elimination-Readdition
J. Am. Chem. Soc., Vol. 118, No. 30, 1996 7151
3.27 (d, 2H, J ) 6.5 Hz, H1), 2.82 (tt, 2H, J ) 7.5 Hz, 4JF-H ) 1.5 Hz,
H5), 2.42 (m, 2H, J ) 7.5 Hz, H4), 1.76 (q, 3H, J ) 1.3 Hz, Me3); MS
(EI) m/z (relative intensity), 416 (M+), 235 (81), 221 (27), 207 (48),
181 (100), 41 (21).
Separation of the η3-Allylpalladium Derivatives. Reaction of
[Pd(C6F5)Br(NCMe)2] with 1,4-Pentadiene. To a solution of
[Pd(C6F5)Br(NCMe)2] (0.15 g, 0.344 mmol) in CH2Cl2 (6 mL) was
added 1,4-pentadiene (0.0356 mL, 0.344 mmol). The mixture was
stirred at room temperature for 6 h, the resulting yellow-orange solution
was filtered through Celite, and the filtrate was evaporated to dryness.
Diethyl ether (2 mL) was added to the residue and the resulting solution
was cooled. 5a-syn crystallized as yellow solid which was filtered,
washed with cold Et2O, and air dried (0.01 g, 7% yield).
The same procedure was used in the reaction of [Pd(C6F5)Br-
(NCMe)2] and excess 3-methyl-1,4-pentadiene and the reaction was
monitored by 19F NMR spectroscopy. Column chromatography separa-
tion of the residue containing the reaction mixture afforded compound
9, which was isolated as a colorless oil from the n-hexane eluted
fraction. When Et2O was used as eluent a second batch was obtained
which contained a mixture of the allylic derivatives 5b, 6b, 7b, and
the C6F5-free di(µ-bromo)bis(3-methyl-1-3-η3-pentenyl)dipalladium(II)
(10).
5a-syn: Anal. Calcd for C22H16Br2F10Pd2: C, 31.34; H, 1.91.
Found: C, 31.16; H, 1.93.
The mother liquors were kept at room temperature for 25 days and
the mixture of 5a, 6a, and 7a was separated by preparative TLC using
a silica plate and a hexane:ethylacetate 10:1 mixture as eluent; two
batches were obtained which contained (1) 6a-syn and 7a (Rf ) 0.22)
and (2) 5a-syn, 5a-anti, and 6a-anti (Rf ) 0.17). These mixtures could
not be further separated.
9: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.4 (m, Fmeta), -157.7
1
(t, Fpara), -143.1 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300 MHz),
5.76 (m, 2H, J ) 17.2, 10.4, 7.5 Hz, H2 + H4), 5.02 (dt, 2H, J ) 10.4,
1.1 Hz, H1 + H5), 4.97 (dt, 2H, J ) 17.2, 1.1 Hz, H1′ + H5′), 3.00 (m,
1H, J ) 7.5, 7.7 Hz, H3), 2.80 (d, 2H, J ) 7.7 Hz, CH2C6F5). MS(EI)
m/z (relative intensity), 248 (M+, 0.9), 181 (5), 68 (6), 67 (100), 65
(20), 41 (30). HRMS calcd for C12H9F5, m/z 248.0624; found,
248.0629.
The reaction of [Pd(C6F5)Br(NCMe)2] with 3-methyl-1,4-pentadiene
was carried out following the same procedure. 5b-syn was crystallized
from Et2O/n-hexane (10% yield). Anal. Calcd for C24H20Br2F10Pd2:
C, 33.09; H, 2.31. Found: C, 33.06; H, 2.39. Preparative TLC was
performed as described above and afforded (batch 1) 6b-syn, 6b-anti,
and 7b (Rf ) 0.20) and (batch 2) 5b-syn, 5b-anti, and 6b-anti (Rf )
0.14). These mixtures could not be further separated.
10: 1H NMR (CDCl3, δ, 293 K, 300 MHz), 5.11 (dd, 1H, J ) 13,
7.5 Hz, H2), 3.93 (d, 1H, J ) 7.5 Hz, H1-syn), 3.13 (d, 1H, J ) 13 Hz,
H1-anti), 1.68 (m, H4), 1.42 (m, H4′), 1.30 (s, 3H, Me3), 1.14 (t, 3H, J
) 7.6 Hz, Me5).
Synthesis of the Pentafluorophenyl-Substituted Methyl Esters.
Synthesis of Derivatives 12a, 13a, and 14a. 1,4-Pentadiene (0.007
mL, 0.07 mmol) was added to a solution of [Pd(C6F5)Br(NCMe)2]
(0.030 g, 0.07 mmol) in CHCl3 (3 mL) at 243 K. The mixture was
kept at this temperature for 2 h. NaOMe (0.023 g, 0.42 mmol)
suspended in MeOH (3 mL) was added to the mixture and CO was
bubbled through the solution for 20 min. Metallic palladium precipi-
tated which was filtered off and the filtrate was evaporated to dryness.
The residue was chromatographed on a silica column (neutral, 230-
400 mesh) and eluted with Et2O; this operation afforded a mixture of
12a (65%), 13a (20%), and 14a (7%) plus C6F5-COOMe (2%) and
the ester methyl 6-(pentafluorophenyl)-3-hexenoate (6%) derived from
the linear η3-allyl complex 5a. The isolated mixture of esters was
analyzed by GC-MS and NMR spectroscopy.
The reaction of [Pd(C6F5)Br(NCMe)2] with 1,4-pentadiene was
monitored as described above for 3-methyl-1,4-pentadiene. The
temperature was lowered down to 223 K for complete identification
of the less stable compounds. The following complexes were identified:
2a1: 19F NMR (CDCl3, δ, 223 K, 282 MHz), -161.9 (m, Fmeta),
1
-155.9 (t, Fpara), -142.7 (m, Fortho); H NMR (CDCl3, δ, 223 K, 300
MHz), 4.60 (b, 1H, H2), 4.35 (b, 2H, H1 + H1′), 3.10 (H4), 2.60
(CH2C6F5), 1.80 (b, 1H, H3), 0.25 (b, 1H, H3′).
2a2: 19F NMR (CDCl3, δ, 223 K, 282 MHz), -161.9 (m, Fmeta),
1
-155.9 (t, Fpara), -142.7 (m, Fortho); H NMR (CDCl3, δ, 223 K, 300
MHz), 4.90 (b, 2H, H1 + H2), 4.60 (b, 1H, H1′), 3.00-2.80 (3H, H4,
CH2C6F5), 1.10 (b, 1H, H3), 0.80 (b, 1H, H3′).
3a: 19F NMR (CDCl3, δ, 223 K, 282 MHz), -161.0 (m, Fmeta),
1
-156.3 (t, Fpara), -143.2 (m, Fortho); H NMR (CDCl3, δ, 223 K, 300
MHz), 4.90 (b, 2H, H1 + H2), 4.60 (b, 1H, H1′), 3.00-2.80 (3H, H5,
H3, H3′), 2.75, 2.55 (2H, H4, H4′).
12a: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.1 (m, Fmeta),
1
-157.2 (t, Fpara), -142.9 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
4a: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.4 (m, Fmeta),
MHz), 5.64 (m, 1H, H4), 4.98 (d, 1H, J ) 11 Hz, H5), 4.93 (d, 1H, J
) 17 Hz, H5′), 3.66 (s, 3H, COOMe), 2.80-2.70 (m, 3H, H3, CH2C6F5),
2.43 (d, 2H, H2, H2′); 13C{1H} NMR (CDCl3, δ, 293 K, 74.5 MHz),
172.0 (C1), 138.5 (C4), 116.8 (C5), 51.7 (COOMe), 40.5 (C3), 39.1 (C2),
27.4 (CH2C6F5). MS(EI) m/z (relative intensity), 294 (M+, 1.2), 181
(45), 74 (70), 71 (100), 59 (47), 41 (21). HRMS calcd for C13H11O2F5,
m/z 294.0679; found, 294.0678.
1
-156.7 (t, Fpara), -143.1 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 6.04 (1H, H2), 4.98 (d, 1H, J ) 15.1 Hz, H1), 4.92 (d, 1H, J )
8.5 Hz, H1′), 3.58 (m, 1H, H5), 3.09 (m), 2.87 (m), (H3 + H3′), 2.40
(m, 1H, H4), 2.10 (m, 1H, H5′).
5a-syn: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.7 (m, Fmeta),
1
-157.2 (t, Fpara), -143.9 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.23 (m, 1H, J ) 11.2, 6.7 Hz, H2), 4.01 (d, 1H, J ) 6.7 Hz,
H1-syn), 3.84 (m, 1H, J ) 11.8, 8.7, 5 Hz, H3), 2.95 (m, 2H, H5 +
H5′), 2.91 (d, 1H, J ) 11.8 Hz, H1-anti), 2.23 (m, 1H, H4), 2.00 (m,
1H, H4′).
13a: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.8 (m, Fmeta),
1
-156.8 (t, Fpara), -143.1 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.73 (m, 1H, J ) 17.1, 13.7, 10, 7.1, H4), 5.10 (bd, 1H, J )
17.1, 1.4 Hz, H5′), 5.07 (bd, 1H, J ) 10 Hz, H5), 3.64 (s, 3H, COOMe),
3.00-2.90 (m, 2H, CH2C6F5), 2.80 (m, H2), 2.42 (m, H3), 2.30 (m,
H3′). MS(EI) m/z (relative intensity), 294 (M+, 7), 220 (33), 181 (100),
113 (22), 74 (22), 71 (70), 59 (48), 54 (25). HRMS calcd for
C13H11O2F5, m/z 294.0679; found, 294.0680.
5a-anti: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.9 (m, Fmeta),
1
-157.1 (t, Fpara), -144.1 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.20 (m, H2), 4.80 (m, 1H, H3), 4.17 (d, 1H, J ) 7 Hz, H1-syn),
3.29 (d, 1H, J ) 13 Hz, H1-anti), 2.80 (m, H5 + H5′), 2.00 (H4), 1.55
(H4′).
14a: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.8 (m, Fmeta),
6a-syn: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -161.9 (m, Fmeta),
1
-156.9 (t, Fpara), -142.4 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
1
-155.7 (t, Fpara), -141.7 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 3.62 (s, 3H, COOMe).
MHz), 5.37 (dd, 1H, J ) 12.8, 7.5 Hz, H2), 4.05 (dd, 1H, J ) 7.5, 1.2
Hz, H1-syn), 3.23 (dd, 1H, J ) 12.8, 1.2 Hz, H1-anti), 3.45 (m), 3.23
(m, J ) 16 Hz, H4 + H4′), 1.20 (s, 3H, Me3).
A mixture of complexes 12b, 13b, and 14b was obtained by mixing
3-methyl-1,4-pentadiene (0.028 mL, 0.23 mmol) and [Pd(C6F5)-
Br(NCMe)2] (0.100 g, 0.23 mmol) in CH2Cl2 (5 mL) at 253 K and
letting it stand at this temperature for 1 h; carbonylation of this mixture
as described above gave a mixture of the ester derivatives: 12b (55%),
13b (24%) (each of them as a 1:1 diastereomeric mixture), 14b, (5%)
plus C6F5-COOMe (7%) and several unidentified compounds (9% total,
each one less than 4%). The isolated mixture of esters was analyzed
by GC-MS and NMR.
6a-anti: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.1 (m, Fmeta),
1
-156.0 (t, Fpara), -140.8 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.14 (dd, 1H, J ) 13, 7.5 Hz, H2), 4.11 (dd, 1H, J ) 7.5, 1.6
Hz, H1-syn), 3.46 (dd, 1H, J ) 13, 1.6 Hz, H1-anti), 3.06 (m), 2.80 (m,
2H, J ) 14.5 Hz, H4 + H4′), 1.51 (s, 3H, Me3).
7a: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.5 (m, Fmeta),
1
-157.0 (t, Fpara), -143.0 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
12b1: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.5 (m, Fmeta),
-157.67 (t, Fpara), -143.1 (m, Fortho); 1H NMR (CDCl3, δ, 293 K, 300
MHz), 5.64 (m, 1H, H4), 4.95 (dd, 1H, J ) 10.2, 1.6 Hz, H5), 4.80
MHz), 5.52 (m, 1H, J ) 11.5, 7 Hz, H2), 4.08 (d, 1H, J ) 7 Hz, H1-
syn), 4.10 (m, H3), 3.70 (m, 1H, H4), 2.97 (d, 1H, J ) 11.5 Hz, H1-
anti), 1.54 (d, 3H, J ) 7.2 Hz, Me4).

+
+
7152 J. Am. Chem. Soc., Vol. 118, No. 30, 1996
Albe´niz et al.
(dd, 1H, J ) 16.8, 1.4 Hz, H5′), 3.67 (s, 3H, COOMe), 2.90-2.65
(CH2C6F5),24 2.65-2.45 (m, H2, H3),24 1.23 (d, 3H, J ) 7 Hz, Me2).
12b2: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.5 (m, Fmeta),
Synthesis of Di(µ-bromo)bis(η1-η2-(3-((pentafluorophenyl)methyl)-
1-oxo-4-pentenyl)dipalladium(II) (15). [Pd(C6F5)Br(NCMe)2] (0.100
g, 0.23 mmol) was dissolved in CH2Cl2 (5 mL) and the solution was
cooled at 243 K. 1,4-Pentadiene (0.0357 mL, 0.345 mmol) was added
and the mixture was stirred for 2.5 h. CO was bubbled through the
yellow solution for 5 min and the mixture was slowly warmed to room
temperature and then filtered through Celite. The solvent was
completely evaporated and the residue was dissolved in Et2O (0.5 mL);
n-hexane (1 mL) was slowly added to the solution and the mixture
was cooled. A yellow solid crystallized which was filtered, washed
with n-hexane, and air dried: yield 0.025 g. The solid is a mixture of
15 (60%) and other organometallic derivatives (see Results).
1
-157.7 (t, Fpara), -143.3 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.51 (m, 1H, H4), 4.98 (dd, 1H, J ) 10.2, 1.6 Hz, H5), 4.81
(dd, 1H, J ) 16.9, 1.5 Hz, H5′), 3.70 (s, 3H, COOMe), 2.90-2.65
(CH2C6F5),24 2.65-2.45 (m, H2, H3),24 1.16 (d, 3H, J ) 6.8 Hz, Me2).
12b1 + 12b2: MS(EI) m/z (relative intensity), 308 (M+, 0.3), 181 (34),
127 (10), 88 (100), 71 (54), 67 (21), 59 (37), 55 (21). HRMS calcd
for C14H13O2F5, m/z 308.0836; found, 308.0842.
13b1: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.0 (m, Fmeta),
-157.15 (t, Fpara), -143.2 (m, Fortho); 1H NMR (CDCl3, δ, 293 K, 300
MHz), 5.77 (m, 1H, J ) 17.5, 10.2, 7.7 Hz, H4), 5.06 (m, 1H, J )
17.5, 1 Hz, H5′), 5.04 (m, 1H, J ) 10.2, 1 Hz, H5), 3.59 (s, 3H,
COOMe), 3.00, 2.90-2.65 (H2, CH2C6F5),24 2.65-2.45 (H3),24 1.12
(d, 3H, J ) 6.8 Hz, Me3).
15: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -162.1 (m, Fmeta),
1
-156.0 (t, Fpara), -141.6 (b, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 6.35 (b, 1H, H4), 4.62 (d, 1H, J ) 8.3 Hz, H5), 4.03 (d, 1H, J
) 12.5 Hz, H5′), 3.30-2.70, 2.40 (5H, H2, H2′, H3, CH2C6F5). 13C-
13b2: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.0 (m, Fmeta),
1
{1H} NMR (CDCl3, δ, 293 K, 74.5 MHz), 206 (C1), 145 (d, JC-F
)
1
-157.2 (t, Fpara), -143.5 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
1
244 Hz, C6F5 Cortho), 140 (d, JC-F ) 254 Hz, C6F5 Cpara), 137.5 (d,
1JC-F ) 248 Hz, C6F5 Cmeta), 112 (t, 3JC-F ) 16.2 Hz, C6F5 Cipso), 109.1
(C4), 72.0 (C5), 50.8b (C2), 37.2 (C3), 28.3b (CH2C6F5).
MHz), 5.70 (m, 1H, H4), 5.13 (dd, 1H, J ) 17.5, 1.4 Hz, H5′), 5.11
(dd, 1H, J ) 10.2, 1.6 Hz, H5), 3.60 (s, 3H, COOMe), 3.00, 2.90-
2.65 (H2, CH2C6F5),24 2.65-2.45 (H3),24 1.04 (d, 3H, J ) 6.4 Hz, Me3).
13b1 + 13b2: MS(EI) m/z (relative intensity), 308 (M+, 0.6), 253 (14),
181 (27), 127 (61), 95 (33), 67 (38), 59 (38), 55 (100), 53 (22), 51
(10). HRMS calcd for C14H13O2F5, m/z 308.0836; found, 308.0840.
14b: 19F NMR (CDCl3, δ, 293 K, 282 MHz), -163.1 (m, Fmeta),
Acknowledgment. We are grateful to the Direccio´n General
de Investigacio´n Cient´ıfica y Te´cnica (Project PB93-0222), the
Commission of the European Communities (Network “Selective
Processes and Catalysis Involving Small Molecules” CHRX-
CT93-0147), and the University of Valladolid (Proyecto Grupos
Jo´venes) for financial support. We also thank the Ministerio
de Educacio´n y Ciencia and the AECI/Universidad de Valladolid
for fellowships to Y.-S.L. and Dr. J. Orduna (University of
Zaragoza) for HRMS determinations.
1
-157.1 (t, Fpara), -141.3 (m, Fortho); H NMR (CDCl3, δ, 293 K, 300
MHz), 5.50 (m, H5), 4.86 (H6′), 4.83 (H6), 3.66 (s, COOMe, 14b1),
3.65 (COOMe, 14b2), 2.90-2.45 (H2, H2′, H3), 1.18 (d, 3H, J ) 6.9
Hz, Me4, 14b1), 1.11 (d, 3H, J ) 6.6 Hz, Me4, 14b2). MS(EI) m/z
(relative intensity), 308 (M+, 1.8), 181 (64), 85 (68), 74 (87), 67 (63),
50 (100), 55 (74), 53 (48), 41 (54). HRMS calcd for C14H13O2F5, m/z
308.0836; found, 308.0832.
(24) Overlapped with the signals of the other diastereoisomer.
JA960333P