Reactions of allylpalladium(II) complexes with free radicals

Article abstract of DOI:10.1039/b002851g

Allylpalladium(II) compounds react readily in benzene with free phenyl and trityl radicals, generated from the thermal decomposition of phenylazotriphenylmethane, and with free cyclohexyl radicals, generated from the photolysis of (cyclohexyl)(pyridine)cobaloxime; the reactions appear to involve initial attack of the radicals at palladium, followed by secondary processes which convert the coordinated allyl ligands preferentially to terminal rather than internal alkenes.

Full text of DOI:10.1039/b002851g

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Reactions of allylpalladium(II) complexes with free radicals
Simon J. Reid, Neil T. Freeman and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, ON, Canada K7L 3N6. E-mail: bairdmc@chem.queensu.ca
Received (in Irvine, CA, USA) 4th April 2000, Accepted 7th July 2000
First published as an Advance Article on the web 4th September 2000
Allylpalladium(II) compounds react readily in benzene with
free phenyl and trityl radicals, generated from the thermal
decomposition of phenylazotriphenylmethane, and with free
cyclohexyl radicals, generated from the photolysis of (cyclo-
hexyl)(pyridine)cobaloxime; the reactions appear to involve
initial attack of the radicals at palladium, followed by
secondary processes which convert the coordinated allyl
ligands preferentially to terminal rather than internal
alkenes.
phenyl radical on palladium. In the absence of triphenylphos-
phine, it seems likely that initial phenyl radical coupling with
the palladium to form a palladium(III) intermediate is followed
by reductive coupling of the phenyl group with a terminal
carbon atom of the allyl ligand to give an intermediate
chloropalladium( ) complex [eqn. (1)]. In possibly a concerted
I
(1)
Free radicals have been invoked as reactive intermediates in a
wide variety of reactions involving organotransition metal
compounds, e.g. oxidative addition reactions,1a SmI2 induced
coupling of organic halides with carbonyl compounds,1b
reduction of organic halides by complexes of chromium(II),1c
and hydrogenation and hydrometallation of aromatic alkenes1d
and conjugated dienes.1e Recent years have also seen increasing
interest in free radicals as reactants with unsaturated organic
ligands, e.g. with coordinated aromatic,2a allylic,2b carbenoid2c
and alkenyne2d ligands although there is as yet no general
understanding of the reactions of free radicals with coordinated
ligands. We have therefore undertaken an investigation of
process, the chlorine atom is abstracted from the intermediate
by free trityl radical to form chlorotriphenylmethane, palladium
metal and free 3-phenylbut-1-ene.
3
Initial abstraction of a chlorine atom from [(h -C3H5)PdCl]2
by trityl radical can presumably be ruled out since no
chlorobenzene was formed. Although one might anticipate that
3
the initially formed trityl radical could react with [(h -
C3H5)PdCl]2 before significant amounts of the intrinsically
more reactive phenyl radical are formed, in fact the relative rates
of formation of the two radicals are very similar4d and pre-
empting of any reactive site by the trityl radical seems unlikely.
reactions of a variety of free radicals with allylic palladium(II
)
3
3
compounds of the types [(h -allyl)PdCl]2, (h -allyl)PdCl(PPh3)
On the other hand, with analogous triphenylphosphine-contain-
3
3
and [(h -allyl)Pd(PPh3)2]Cl, the latter being of interest because
ing phenylpalladium(III
)
intermediates such as [(h -
3
the enhanced susceptibility of the allyl groups in these types of
compounds to nucleophilic attack has proven very useful in
synthetic organic methodology.3
C3H5)PdIII(Ph)Cl(PPh3)] or [(h -C3H5)PdIII(Ph)(PPh3)2]+,
steric hindrance by the bulky phosphine ligands may prevent
attack on the coordinated chlorine atoms by the bulky trityl
radical. Instead, the latter apparently couples in these cases with
the sterically less congested allyl ligand to form 4,4,4-triphe-
nylbut-1-ene, again possibly via a concerted process but not,
presumably, via initial trityl radical attack as there is no obvious
reason why at least some phenyl radical involvement to give
3-phenylprop-1-ene would not occur. Interestingly, in a control
experiment, it was found that trityl radical, added as the
corresponding dimer with which it is in equilibrium,7 does react
Our initial studies have utilized triphenylmethyl (trityl) and
phenyl radicals, derived via the thermal decomposition of
phenylazotriphenylmethane (PhNNNCPh3, PAT),4 and cyclo-
hexyl radicals, generated from visible light photolysis of
(cyclohexyl)(pyridine)cobaloxime [c-C6H11Co(DMG)2(py)].5
Both procedures generate free radicals in good yields and under
relatively mild conditions, and molar ratios of radical source to
palladium of 2–3+1 ensured that reactions of the palladium
substrates proceeded to completion.
3
with [(h -C3H5)PdCl]2 in the presence of PPh3, the trityl and
3
Reaction of [(h -C3H5)PdCl]2 with PAT in benzene or
allyl groups combining to form exclusively 4,4,4-triphenylbut-
1-ene. On the other hand, the formation of 4,4,4-triphenylbut-
1-ene apparently does not involve trityl coupling with free allyl
radicals as no hexa-1,5-diene, the product of allyl free radical
coupling,8 was formed.
benzene-d6 at 60 °C resulted in the formation of palladium
metal, 3-phenylprop-1-ene, 1-phenylprop-1-ene, chlorotriphe-
nylmethane (in high yield by GC) and a small amount of
4,4,4-triphenylbut-1-ene but no chlorobenzene. By monitoring
the reaction products as a function of time, it was found that
3-phenylprop-1-ene was formed initially in ca. 90% yield
(NMR) and that 1-phenylprop-1-ene was formed by subsequent
isomerization of the kinetic product. Control experiments
showed that this isomerization reaction is catalyzed by
palladium metal, and thus the primary reaction involves the
formation of 3-phenylprop-1-ene, PhCH2CHNCH2.
In contrast to the above, photolysis of c-
3
C6H11Co(DMG)2(py) with visible light in the presence of [(h -
C3H5)PdCl]2 (PPh3+Pd ratios 1+1, 2+1) in benzene or benzene-
d6 at 25 °C did not give 3-cyclohexylprop-1-ene and/or
cyclohexyl–palladium complexes, but rather propene, cyclo-
3
hexene and palladium(0) species. Similar reactions of [(h -
3
3
1-methylallyl)PdCl]2, [(h -2-methylallyl)PdCl]2 and [(h -
2-phenylallyl)PdCl]2 (PPh3+Pd ratios 1+1) resulted in the
exclusive formation of but-1-ene, isobutene and 2-phenylprop-
3
In contrast, the analogous reactions of [(h -C3H5)PdCl]2 in
the presence of either 1 or 2 equivalents of PPh3 per palladium
3
were found to produce 4,4,4-triphenylbut-1-ene ( > 90% yield
1-ene respectively, while reaction of [(h -1-phenylallyl)PdCl]2
6a
by
NMR)
and
[PdPh(m-Cl)(PPh3)]2
or
trans-
(PPh3+Pd ratio 1+1) resulted in the formation of a mixture of
3-phenylprop-1-ene and 1-phenylprop-1-ene at 25 °C but only
3-phenylprop-1-ene at 5 °C if the PPh3:Pd ratio were 2:1. The
PdPhCl(PPh3)2,6b,c respectively. In addition, the reaction of
3
[(h -2-methylallyl)PdCl]2 with PAT in the presence of PPh3
3
produced
trans-2-methyl-4,4,4-triphenylbut-1-ene
and
reaction of [(h -C3H5)PdCl]2 with c-C6H11Co(DMG)2(py) gave
PdPhCl(PPh3)2, while the reaction of [(h -allyl)Pt(PPh3)2]+Cl2
produced 4,4,4-triphenylbut-1-ene and trans-PtPhCl(PPh3)2.6d
Assuming a similar first step in all of these reactions, the
results may be rationalized on the basis of initial attack of
propene and palladium metal while reaction of [(h -1-phenyl-
3
3
allyl)PdCl]2 gave 1-phenylprop-1-ene and palladium metal; as
above, isomerization of a terminal alkene kinetic product to
internal alkene may be catalyzed by palladium metal. Control
DOI: 10.1039/b002851g
Chem. Commun., 2000, 1777–1778
This journal is © The Royal Society of Chemistry 2000
1777

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3a
experiments showed that the allyl–palladium compounds are
stable under the above conditions in the absence of c-
C6H11Co(DMG)2(py).
Assuming a mechanism analogous to the above and consider-
ing the nature of the products, these reactions probably involve
initial attack of the cyclohexyl radical at the metal to form a
cyclohexyl–Pd(III) species, followed by b-hydrogen elimination
of cyclohexene to form a hydrido–allylpalladium(III) complex
[eqn. (2)]. Subsequent reductive elimination of propene could
e.g. alkenes RCHNCHCH3
and allylic acetates
RCHNCHCH2OAc,3a the chemistry involving hydrogen trans-
fer from the cyclohexyl radical would seem in effect to provide
a route for both the isomerization of the alkene or the
3
hydrogenolysis of the allylic acetate, via h -allylpalladium
compounds, to terminal alkenes RCH2CHNCH2. Both types of
reactions have potential utility, as we note that hydrogenolysis
of allylpalladium complexes by hydride sources normally
results in the formation of internal alkenes unless the source is
formate ion.10
We thank the Natural Sciences and Engineering Research
Council and the Government of Ontario for financial support.
(2)
result in the palladium( ) species PdCl(PPh3), which could
I
undergo a second attack by a cyclohexyl radical to form the
palladium(II) cyclohexyl compound PdCl(C6H11)(PPh3), which
could also undergo b-hydrogen elimination of cyclohexene to
give the hydride PdHCl(PPh3).
Notes and references
1 (a) J. K. Stille, in The Chemistry of the Metal–Carbon Bond, ed. F. R.
Hartley and S. Patai, Wiley and Sons, Chichester, 1985, vol. 2, ch. 9; (b)
A. Krief and A. M. Laval, Chem. Rev., 1999, 99, 745; (c) R. Sustmann
and R. Altevogt, Tetrahedron Lett., 1981, 5167; (d) R. M. Bullock and
E. G. Samsel, J. Am. Chem. Soc., 1990, 112, 6886; (e) T. A. Shackleton,
S. C. Mackie, S. B. Fergusson, L. J. Johnston and M. C. Baird,
Organometallics, 1990, 9, 2248.
2 (a) H.-G. Schmalz, S. Siegel and A. Schwarz, Tetrahedron Lett., 1996,
37, 2947; (b) C. A. G. Carter, R. McDonald and J. M. Stryker,
Organometallics, 1999, 18, 820; (c) C. A. Merlic and D. Xu, J. Am.
Chem. Soc., 1991, 113, 9855; (d) G. G. Melikyan, O. Vostrowsky, W.
Bauer, H. J. Bestmann, M. Khan and K. M. Nicholas, J. Org. Chem.,
1994, 59, 222.
Although cyclohexene is anticipated as a byproduct in these
reactions, it is expected to be formed in any case as a product of
the photolysis of c-C6H11Co(DMG)2(py)5 and we therefore
cannot take its formation as evidence for the postulated
mechanism. While neither the putative hydride PdHCl(PPh3)
nor its dimer appear to have been reported, the 1H NMR
spectrum of a reaction mixture in benzene-d6 exhibited a
resonance at d –13, indicative of a palladium hydride. This same
3
resonance is also observed in reactions involving [(h -1-
3
3
methylallyl)PdCl]2, [(h -2-methylallyl)PdCl]2, [(h -1-phenyl-
3
allyl)PdCl]2 and [(h -2-phenylallyl)PdCl]2, consistent with the
3 (a) S. A. Godleski, in Comprehensive Organic Synthesis, ed. B. M.
Trost, Pergamon Press, Oxford, 1991, vol. 4, p. 585; (b) F. Guibe,
Tetrahedron, 1998, 54, 2967.
general mechanism proposed. Also consistent is an observation
3
that
the
reaction
of
[(h -allyl)Pt(PPh3)2]Cl
with
C6H11Co(DMG)2(py) gives comparable amounts of propene
and trans-PtHCl(PPh3)2, identified by comparison of its hydride
resonance (d 215.2) with that of an authentic sample,9 in
addition to cyclohexene.
4 (a) S. G. Cohen and C. H. Wang, J. Am. Chem. Soc., 1953, 75, 5504; (b)
R. R. Bridger and G. A. Russell, J. Am. Chem. Soc., 1963, 85, 3754; (c)
R. G. Kryger, J. P. Lorand, R. R. Stevens and N. R. Herron, J. Am.
Chem. Soc., 1977, 99, 7589; (d) R. C. Neuman and G. D. Lockyer,
J. Am. Chem. Soc., 1983, 105, 3982; (e) J. E. Leffler, An Introduction to
Free Radicals, John Wiley & Sons, Inc., New York, 1993, pp. 148–150;
(f) J. E. Leffler, An Introduction to Free Radicals, John Wiley & Sons,
Inc., New York, 1993, p. 183.
Seemingly anomalous, however, is the exclusive formation
3
of but-1-ene in the reaction of [(h -1-methylallyl)PdCl]2 and of
3
3-phenylprop-1-ene in the reaction of [(h -1-phenylal-
lyl)PdCl]2. While these products are consistent with the
reductive elimination step implied above, reductive elimination
of alkenes from allylpalladium(II) hydrido complexes normally
results in the formation of the thermodynamically preferred
internal alkenes.3b Thus the fact that the reactions discussed
here give solely terminal alkenes is perhaps evidence that the
reactions do indeed involve allylpalladium(III) hydrido species,
although it is not readily obvious why formation of the terminal
alkenes should become kinetically preferred. A better under-
standing of these processes must await further labelling and
stereochemical studies which are underway.
5 B. P. Branchaud, M. S. Meier and M. N. Maedzadeh, J. Org. Chem.,
1987, 52, 212; B. P. Branchaud and Y. L. Choi, J. Org. Chem., 1988, 53,
4638; B. Giese, J. Hartung, J. He, O. Hüter and A. Koch, Angew. Chem.,
Int. Ed. Engl., 1989, 28, 325.
6 (a) V. V. Grushin, Organometallics, 2000, 19, 1888; (b) D. R. Coulson,
Chem. Commun., 1968, 1530; (c) A. Mentes, R. R. W. Kemmit, G.
Fawcett and D. R. Russel, Polyhedron, 1999, 18, 1141; (d) C. Eaborn,
K. J. O’Dell and A. Pidcock, J. Chem. Soc., Dalton Trans., 1978,
357.
7 T. H. Colle, P. S. Glaspie and E. S. Lewis, J. Org. Chem., 1978, 43,
2722; H. Lankamp, W. T. Nauta and C. MacLean, Tetrahedron Lett.,
1968, 249.
8 G. Carturan, M. Biasiolo, S. Danielle, G. A. Mazzochin and P. Ugo,
Inorg. Chim. Acta, 1986, 119, 19.
Although the mechanisms are as yet uncertain, the reactions
lead in all cases to preferential conversion of the allylic ligands
3
to terminal alkenes. Since compounds of the type [(h -
9 W. McFarlane, J. Chem. Soc., Dalton Trans., 1974, 324.
10 J. Tsuji and T. Mandai, Synthesis, 1996, 1.
1-RC3H4)PdCl]2 (R = alkyl, aryl) are readily prepared from,
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Chem. Commun., 2000, 1777–1778