Rate enhancing ligands for lead(IV)-mediated arylations

Article abstract of DOI:10.1016/S0040-4039(02)00510-5

The level of acceleration of the reactions of aryllead(IV) tricarboxylates with β-dicarbonyl reagents in the presence of several catalysts has been determined; in the case of 1,10-phenanthroline, almost a thousandfold increase in rate over the uncatalysed

Full text of DOI:10.1016/S0040-4039(02)00510-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3407–3409
Rate enhancing ligands for lead(IV)-mediated arylations
Jonathan E. H. Buston, Mark G. Moloney,* Abigail V. L. Parry and Paul Wood
The Department of Chemistry, Dyson Perrins Laboratory, The University of Oxford, South Parks Road, Oxford OX1 3QY, UK
Received 5 February 2002; revised 25 February 2002; accepted 8 March 2002
Abstract—The level of acceleration of the reactions of aryllead(IV) tricarboxylates with b-dicarbonyl reagents in the presence of
several catalysts has been determined; in the case of 1,10-phenanthroline, almost a thousandfold increase in rate over the
uncatalysed reaction was observed. © 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
coupling reactions5–7 is well known; pyridine or other
amine bases are important in these reactions, where
they probably act as s-donor ligands.5 Pyridine also
catalyses ligand redistribution in some aryllead(IV)
compounds.8 We have been interested in expanding
the repertoire of lead(IV)-based processes, and particu-
larly in the development of asymmetric carbon–carbon
coupling reactions. However, our initial work in this
area has been disappointing, as although the prepara-
tion of chiral carboxylate ligands suitable for lead(IV)
was relatively straightforward, their derived complexes
proved to be remarkably unreactive and the observed
e.e. values for arylation reactions were low, typically
in the range 6–16%.9,10 Detailed NMR investigations
suggested that the cause of the problem lay with fast
ligand exchange, which favoured the replacement of
the chiral but bulky ligands, with smaller achiral ones
inevitably present in the reaction medium to give more
reactive but achiral complexes; substantial attrition of
asymmetric induction therefore ensues. This outcome
is the exact reverse of ligand accelerated catalysis, a
phenomenon crucial to the success of many modern
asymmetric transition metal processes.11 However,
there have been some recent developments in this area,
since Konopelski has shown that aryllead(IV) reagents
can exhibit high levels of diastereoselectivity12 and
Yamamoto has shown that asymmetric coupling of
phenols using aryllead reagents with amine catalysts is
possible.13 In related chemistry, there has been a
recent report of the asymmetric a-phenylation of b-
keto ester enolates using (phenyl)iodonium tetra-
fluoroborates giving moderate e.e.s.14
In view of literature reports which suggested that pyri-
dine was either essential for, or at least improved, the
reactivity of aryllead(IV) compounds,5 we examined a
number of ligands which contained a tethered pyridine
sub-unit, but found these systems to be unreactive in
arylation reactions.9,10,15 We therefore undertook a
more detailed examination of the effect of ligands on
the reactions of aryllead(IV) compounds. Our goal
was to identify ligands which may induce rate acceler-
ation and therefore ultimately offer the potential for
chiral induction. That such a goal is realistic is sup-
ported by some of our recent observations which indi-
cated that thiophenecarboxylate ligands could have
significant effects on the reactivity of lead(IV), having
been crucial in the development of a simple but effec-
tive method for the oxidation of benzyl and other
reactive alcohols16 and for novel alkylation, lactonisa-
tion and carbon–carbon bond forming processes.17
Furthermore, recent independent work clearly indi-
cates that ligand modification can produce useful
O
CO2Et
OMe
O
CO2Et
2
CHCl3/cat.
Pb(R)3
OMe
3
1a R = O2CMe
1b R = O2CCMe3
1c R = O2CC6H3Cl2
1d R = O2CC4H3S
1e R = O2CC3H5
1f R = O2CC4H7
1g R = O2C7H5
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(02)00510-5

3408
J. E. H. Buston et al. / Tetrahedron Letters 43 (2002) 3407–3409
enhancements of reactivity in arylations with aryl-
lead(IV) derivatives.7
reported examples of this type of ligand for aryl-
lead(IV) compounds.
The success of the 1a·DABCO reaction suggested that
other nucleophilic and/or basic species might prove to
be useful catalysts in these reactions. Therefore, 1-
hydroxybenzotriazole, sodium methoxide, propylamine,
and triphenylphosphine were examined in place of pyri-
dine (3 equiv.) in the reaction of Scheme 1 with p-
methoxyphenyllead(IV) triacetate; the yields of adduct
3 were 53, 62, 10 and 0%, respectively, clearly indicat-
ing that the reaction was highly sensitive to the nature
of the added catalyst. The enhancements achieved with
1-HOBt and methoxide are the first reported arylation
reactions which use a catalyst other than an amine.
Results and discussion
An initial survey of the effect of carboxylate ligands of
aryllead(IV) tricarboxylates in arylation reactions was
made (Scheme 1). The required p-methoxyphenyl-
lead(IV) species 1b–g were readily obtained by ligand
metathesis of p-methoxyphenyllead(IV) triacetate 1a18
with pivalic, 2,6-dichlorobenzoic, thiophene-2-car-
boxylic, 3-butenoic, or 4-pentenoic acids, or with tro-
polone;19 in addition, it was found to be possible to
obtain a 1:1 adduct of p-methoxyphenyllead(IV) triac-
etate with DABCO, simply by stirring an equimolar
mixture of each in toluene and removing the solvent.
Each of the compounds 1b–g was treated with dicar-
bonyl 2 in chloroform and pyridine as catalyst (3
equiv.) overnight at room temperature, giving the
product 3 in yields of 52, 74, 40, 18, 37 and 0%
respectively, with the 1a·DABCO adduct also giving a
yield of 75% of adduct 3. This compares to a yield of 3
of 65% for 1a alone. The good yields using lead(IV)
compounds 1b,c indicated that systems more sterically
congested than the normally used acetate could possess
good levels of reactivity, but that this could not be
maintained indefinitely, as indicated by the unreactivity
of the tropolonate 1g. The moderate yield for thio-
phenecarboxylate derivative 1d is surprising, since
recent work has demonstrated the exceptional efficacy
of this ligand in novel lactonisation and coupling pro-
cesses.17 The v-unsaturated compounds 1e,f are the first
In view of these results, a wider study of the rate
enhancements achieved by catalysts suitable for
lead(IV)-mediated arylation reactions was made; for
this purpose, the arylation reaction of Scheme 2 was
chosen, since this was particularly convenient to follow
1
by H NMR spectroscopy, in which the disappearance
of the H-5 quartet and the methyl doublet of the
starting material are easily quantified. Under the stan-
dard conditions for this reaction (CDCl3, 40°C, 5 h),
several potential catalysts were examined, and initial
rate and final yield data for 4 are indicated in Table 1.
These results are the first quantitative data for reactions
of this type, and although it has been widely reported in
earlier literature that pyridine and less commonly 1,10-
phenanthroline are effective catalysts,5,6 the level of this
catalysis has hitherto remained unappreciated (entries
9, 12 and 13); interestingly, benzoquinoline was particu-
larly ineffective. Once again, the suitability of an sp3
hybridised system was illustrated by the good conver-
sion and rate acceleration with DABCO (entry 8), but a
triamine (entry 4) was not successful. Other notable
catalysts include imidazole and DMAP (entries 10 and
11). We have recently shown that pyridine appears to
act as a ligand donor, rather than a base, in a related
aryllead(IV)-diaryllead(IV) disproportionation reac-
tion,8 and it would seem likely that the catalysts shown
in Table 1 function in the same way for the reaction in
Scheme 2.
OMe
Me
O
O
OMe
O
O
Me
O
O
Me Me
CHCl3/cat.
O
O
Me Me
Pb(O2CMe)3
4
Scheme 2.
Table 1. Initial rate and final yield data for the reaction given in Scheme 2
Entry
Additivea
Initial (relative) rate
Final yield (%)b
1
2
None
CD3CN
1
1
2
5
3
4
5
6
7
8
9
10
11
12
13
Benzo[h]quinoline
1,1,4,7,7-Pentamethyldiethylenetriamine
NaOMe
1,3-Diphenylguanidine
1-HOBt
DABCO
Pyridine
Imidazole
DMAP
2
4
18
22
40
88
133
180
284
300
980
5
10
60
35
35
50
70
90
95
90
100
1,10-Phenanthroline (1 equiv.)
1,10-Phenanthroline
a 3 equiv. unless otherwise indicated.
b After 5 h reaction.

J. E. H. Buston et al. / Tetrahedron Letters 43 (2002) 3407–3409
3409
This is the first detailed investigation of catalysts suit-
able for lead(IV)-mediated arylations. The results indi-
cate that significant levels of rate acceleration are
possible using diverse ligand donors, and suggest that
further rate enhancement may be possible using suit-
ably designed systems. This is likely to be of consider-
able importance in the design of systems capable of
asymmetric arylations, as well as other novel processes.
Further efforts in this endeavour will be reported in due
course.
7. Elliott, G. I.; Konopelski, J. P. Tetrahedron 2001, 57,
5683–5705.
8. Buston, J. E. H.; Compton, R. G.; Leech, M. A.;
Moloney, M. G. J. Organomet. Chem. 1999, 585/2,
326–330.
9. Moloney, M. G.; Paul, D. R.; Prottey, S. C.; Thomp-
son, R. M.; Wright, E. J. Organomet. Chem. 1997, 534,
195–205.
10. Moloney, M. G.; Paul, D. R.; Thompson, R. M.;
Wright, E. J. Organomet. Chem. 1998, 560, 77–88.
11. Berrisford, D. J.; Bolm, C.; Sharpless, K. B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1059–1070.
12. Elliott, G. I.; Konopelski, J. P.; Olmstead, M. M. Org.
Acknowledgements
Lett. 1999, 1, 1867–1870.
13. Saito, S.; Kano, T.; Muto, H.; Nakadai, M.;
Yamamoto, H. J. Am. Chem. Soc. 1999, 121, 8943–
8944.
14. Ochiai, M.; Kitagawa, Y.; Takayama, N.; Takaoka, Y.;
Shiro, M. J. Am. Chem. Soc. 1999, 121, 9233–9234.
15. Moloney, M. G.; Paul, D. R.; Thompson, R. M. Main
Group Met. Chem. 1995, 18, 295–298.
16. Buston, J. E. H.; Howell, H. J.; Moloney, M. G.;
Poster, V. Main Group Met. Chem. 1998, 21, 51–54.
17. Moloney, M. G.; Nettleton, E.; Smithies, K. Tetra-
hedron Lett. 2002, 43, 907–909.
We thank The University of Oxford (B(RE)9607), the
EPSRC Clean Synthesis Programme (GR/K93167) and
Associated Octel for funding of this work. We grate-
fully acknowledge the use of the EPSRC Chemical
Database Service at Daresbury20 and the EPSRC
National Mass Spectrometry Service Centre at
Swansea.
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