Novel alkylation, lactonisation and cascade coupling processes mediated by lead tetracarboxylates: The importance of ligands

Article abstract of DOI:10.1016/S0040-4039(01)02288-2

The reactions of lead(IV) tetracarboxylates with carboxylic acids containing unsaturated side chains is reported. At elevated temperature in toluene, facile decarboxylation or lactonisation processes are observed, depending on the length of the carboxylat

Full text of DOI:10.1016/S0040-4039(01)02288-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 907–909
Novel alkylation, lactonisation and cascade coupling processes
mediated by lead tetracarboxylates: the importance of ligands
Mark G. Moloney,* Ewan Nettleton and Kirsty Smithies
The Department of Chemistry, Dyson Perrins Laboratory, The University of Oxford, South Parks Road, Oxford OX1 3QY, UK
Received 25 October 2001; accepted 30 November 2001
Abstract—The reactions of lead(IV) tetracarboxylates with carboxylic acids containing unsaturated side chains is reported. At
elevated temperature in toluene, facile decarboxylation or lactonisation processes are observed, depending on the length of the
carboxylate side chain and in some cases further cascade coupling processes with added nucleophiles are possible. The efficiency
of these processes depends on the structure of the carboxylate ligand and the mechanistic implications of these results are briefly
discussed. © 2002 Elsevier Science Ltd. All rights reserved.
The use of lead tetraacetate (LTA) for a wide variety of
oxidative processes1–4 and more recently for cross cou-
pling reactions5,6 is well known. We have for some time
been interested in the further development of the appli-
cation of lead(IV) as a reagent in synthetic organic
chemistry, and our attention has focussed on a detailed
investigation of suitable carboxylate ligands for
lead(IV), both for their stabilising effect on the metal
cation7 as well as their solution exchange behaviour.8
We have, for example, shown that lead tetra-
(thiophenecarboxylate) is a mild oxidant suitable for
the oxidation of reactive alcohols to aldehydes.9 How-
ever, asymmetric induction of lead(IV)-mediated aryla-
tions in the presence of chiral ligands is very low,10,11
and this we have attributed to a facile ligand exchange
in solution on the basis of NMR evidence. However, of
the ligands we have hitherto examined, all have them-
selves proved to be unreactive to lead(IV).
lead(IV) with such compounds, such as oxidative decar-
boxylations, are well known and have been shown to
proceed through a free radical chain mechanism espe-
cially if conducted in the presence of a copper(II)
catalyst.12,4 However, the exact course of such lead(IV)
mediated reactions critically depend on several factors,
including the substrate structure, the solvent and the
presence of additives (e.g. Cu(II) or halides), and the
operation of heterolytic mechanisms has also been
acknowledged.4 Another type of lead(IV)-mediated
reaction, that of lactonisation, was originally reported
by Alder and Schneider13 and later investigated by
Criegee14 and Corey,15 although mechanistic details
were not elaborated; more recently, a variant on this
reaction involving lactonisations of d-stannylcarboxylic
acids has been described.16 However, the synthetic
scope of many of these reactions has not been investi-
gated in detail, and we report here some preliminary
results that indicate that they may in fact be more
useful for synthetically novel processes than has hith-
erto been appreciated; careful choice of ligands and
reaction conditions, however, appear to be critical for
success.
In keeping with our desire to identify novel reaction
processes mediated by lead(IV), we have recently exam-
ined carboxylic acid ligands which possess v-unsatu-
rated hydrocarbon side chains. The reactions of
Scheme 1.
* Corresponding author. E-mail: mark.moloney@chem.ox.ac.uk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02288-2

908
M. G. Moloney et al. / Tetrahedron Letters 43 (2002) 907–909
3-Butenoic acid 1a was treated in toluene at 90°C for 3
h with a variety of lead(IV) tetracarboxylates (Scheme 1
and Table 1), themselves prepared by simple ligand
metathesis of lead tetracetate using our reported proce-
dure.7 Allyl esters 2a were obtained in yields ranging
from moderate to good (entries 1–10).17,18 Although the
yields were best for simple carboxylates (acetate, ben-
zoate and cinnamate, entries 1, 2 and 3), carboxylates
with bulky ortho-substituents (dichloro and dimethoxy-
benzoate, entries 8 and 9) and 2-thienylcarboxylate were
found to be almost as good, but other carboxylates
generally gave poor yields. The utility of 2-thienylcar-
boxylate for other metal-mediated coupling processes has
recently been reported.19,20 Toluene was found to be far
superior than THF or acetonitrile for this reaction, which
can be formally considered to proceed with the loss of
CO2 and the generation of an allyl carbocation or radical
equivalent. In the case of 4-phenyl-3-butenoic acid 1b,
reaction with lead tetrabenzoate under the standard
conditions gave a mixture of the two possible products,
2b (R1=Ph) and 3, in low overall yield (11 and 4%,
respectively), although in this case the yields were not
optimised. It is noteworthy that inclusion of b-dicarbonyl
substrates 4a,b in these reactions gave the allylated
products 5a,b (Scheme 2) in moderate yield, but in the
case of the cycloheptanone derivative 4b, the carboxy-
lated product 6 was also isolated (8%). Similar acetoxy-
lation processes with lead tetraacetate are well-known.21
Furthermore, dicarbonyl 7a was efficiently converted to
allyl product 7b under these conditions in 59% yield.
Table 1. Reaction of unsaturated carboxylic acids 1 and
8a according to Schemes 1 and 3
In the case of the next higher homologue, 4-pentenoic
acid 8a, treatment with a variety of lead(IV) tetra-
carboxylates gave the lactones 9 again in variable yield
(Scheme 3 and Table 1), but noteworthy are the yields
of the acetate, 2,6-dimethoxybenzoate and 2-thienylcar-
boxylate (entries 1, 8 and 10). Although presumably
decarboxylation is not favoured in this case, nonetheless
the leaving group ability of lead(IV) induces a ring
closure to give the observed lactone product. When this
reaction was applied to the a-methyl analogue 8b, the
cis-lactone 10 was formed in 65% yield with complete
stereoselectivity, whose structure was established by
single-crystal X-ray analysis. However, in contrast to the
reactions of acid 1a, inclusion of b-dicarbonyl substrates
4a,b in the reactions of acid 8a did not result in the
Entry
R1
R2
2, Yield (%)
9, Yield (%)
1
2
3
4
5
6
7
8
Me
Ph
PhCꢀCH
H
H
H
H
H
H
H
H
H
H
Ph
68
75
68
28
41
37
22
58
56
53
11
85
45
7
p-BrC6H4
p-MeOC6H4
3,4-(MeO)2C6H3
2,4-(MeO)2C6H3
2,6-(MeO)2C6H3
2,6-Cl2C6H3
2-Thienyl
Ph
–
39
30
48
60
22
84
–
9
10
11
Scheme 2.
Scheme 3.

M. G. Moloney et al. / Tetrahedron Letters 43 (2002) 907–909
909
formation of any trapping adducts. Interestingly, the
Acknowledgements
reaction of 4-pentenoic acid 8a with lead tetraacetate in
benzene has been previously reported, but lactone 9
(R1=Me) was not among the identified products.22
We gratefully acknowledge the use of the EPSRC Chem-
ical Database Service at Daresbury,24 the EPSRC
National Mass Spectrometry Service Centre at Swansea,
and the EPSRC National X-ray Crystallographic Facility
at Southampton.
Of some interest is the mechanism of this reaction; as
indicated above, carbocation or radical intermediates are
possibilities. The formation of many of the observed
products (i.e. allylated compounds 2, a-carboxyl com-
pounds 6 and lactones 9 and 10) could be considered, by
analogy to literature precedent, to occur by a radical
mechanism, but the formation of others is more problem-
atic (e.g. 5a,b and 7b). When the reaction of lead
tetrabenzoate with 3-butenoic acid was conducted in the
presence of radical inhibitors such as galvinoxyl or
benzoquinone (0.1–0.2 equiv.), the product 2a (R1=Ph)
was still formed (40 and 27%, respectively), although these
yields are lower than in the absence of the trapping agents
(Table 1, entry 2). Similarly, reaction of 4-pentenoic acid
with lead tetrabenzoate in the presence of galvinoxyl or
benzoquinone (0.1–0.2 equiv.), gave lactone 9 (R1=Ph)
in 67 and 29%, respectively. Evidence against the inter-
mediacy of discrete carbocations or radicals came from
reactions performed in the presence of 1,1-diphenylethene
(1 equiv.); under these conditions, no products arising
from trapping were observed, with the diphenylethene
itself being in fact recovered and the expected products
2a or 9 still being formed (yield of 2a (R1=(MeO)2C6H3)
83%, recovery of alkene 66%; yield of 9 (R1=
(MeO)2C6H3) 29%, 100% recovery of alkene). These
results suggest that another mechanism may be operating,
and all of the above results could be neatly accommodated
by the intermediacy of a lead(IV) compound of type 11
which collapses in a one- or two-electron sense to give
ligand coupling leading directly to the observed products;
of interest is that the intermediacy of a lead species in
these types of reaction was suggested nearly 35 years ago,
but no structural details were given.23 Ligand coupling
for organolead(IV) carboxylates has been demonstrated
by Pinhey to be a facile process,5,6 but the possibility of
such a mechanism for the reactions of lead tetra-
carboxylates has hitherto not been considered. Unfortu-
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1
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Further investigations to validate these mechanistic ideas
and demonstrate the synthetic applications of this reac-
tion will be reported in due course.