COMMUNICATION
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Copper(II)-catalysed addition of O–H bonds to norbornene{
a
Jason G. Taylor, Neil Whittall and King Kuok (Mimi) Hii*
b
a
Received (in Cambridge, UK) 14th July 2005, Accepted 24th August 2005
First published as an Advance Article on the web 19th September 2005
DOI: 10.1039/b509933a
Cu(OTf)
2
is an inexpensive, air- and moisture-stable catalyst for
(ruthenium and rhodium) failed to induce formation of any
products, even at 5 mol% loading (entries 2 and 3). In turn, other
Lewis acids such as silver, nickel and ytterbium triflate salts
produced low to moderate conversions (entries 4–7). Ultimately,
copper(II) trifluoromethanesulfonate proved to be a highly active
catalyst, affording the norbornyl ester in high yield (entry 8).
The addition of a selection of para-substituted benzoic acid
substrates was examined subsequently (Table 2). Employing
2.5 mol% of the copper catalyst, the addition of these acids
proceeded smoothly to furnish the corresponding 2-norbornyl
esters 1a–d in good yields (entries 1, 4, 5 and 6). Compared to the
the O–H addition of aliphatic and aromatic acids and alcohols
to norbornene.
The direct addition of H–X bonds across unsaturated carbon–
carbon bonds represents one of the most atom-economical
1
processes for the synthesis of functionalised molecules. In recent
years, there have been several reports of novel catalysts that can
facilitate the addition of N–H bonds to alkenes and alkynes
2
hydroamination reactions). In contrast, there are very few reports
(
of similar addition of O–H bonds.
8
For non-activated alkenes, certain electrophilic additions could
3
be catalysed by Brønsted acids, but the regioselectivity of the
cationic ruthenium system, these reactions appear to be relatively
insensitive to the electronic property of the nucleophile, although
the yield is slightly lower in the presence of electron-withdrawing
substituents (entries 5 and 6). As the copper catalyst precursor is
employed in the higher oxidation state, we postulated that the
exclusion of air and moisture might be unnecessary. Indeed, the
reaction can be performed in air with no noticeable decrease in
yield (entry 2). The catalyst loading can be further reduced to
process is dependent on the stability of the carbocation
intermediate(s). In certain cases, competitive alkene polymerisation
and rearrangement processes can be difficult to control. Obviously,
transition metal catalysis can circumvent these problems by
offering alternative reaction pathways. Surveying the literature,
intramolecular hydroalkoxylation (addition of alcohols) has been
largely achieved by the use of late transition metal catalysts such
1
mol% with a slight decrease in yield (entry 3).
4
3
,
5
as [PtCl
2
(C
2
H
4
)]
Currently, there are only two known examples of
intermolecular metal-catalysed O–H addition to alkenes. The first
2
–PR
IrCl
3
–AgOTf and, more recently,
Similarly, the addition of aliphatic and cinnamic acids was also
6
accomplished—giving the corresponding esters 2a–c in good yields
entries 7–9). These results demonstrate that the current system has
4
Sn(OTf) .
(
7,8
a wider synthetic scope than the cationic ruthenium system, which
7
was ineffective for the addition of acetic acid.
of these was provided by Oe et al. Using a catalyst generated
from a mixture of Cp*RuCl (PPh ) –2AgOTf–diphosphine, the
2 3 2
addition of 2-phenylethylethanol to styrene, 1-octene and norbor-
nene can be achieved. Interestingly, the catalyst is also effective for
the addition of aromatic carboxylic acids to certain olefins
Table 1 Addition of 4-methoxybenzoic acid to norbornene in the
presence of different catalysts (Scheme 1)
a
b
(hydroacyloxylation). Very recently, the addition of phenolic and
Entry Catalyst
Catalyst loading (mol%) Yield (%)
carboxylic acid nucleophiles to various alkenes was also reported
9
c
1
2
3
4
5
6
7
8
a
TfOH
[Rh(COD) ][BF ]
2 4
3
[Cp*Ru(NCCH ) ][PF ]
AgOTf
AgBF
Ni(OTf)
Yb(OTf)
Cu(OTf)
10
5
5
2.5
2.5
2.5
2.5
2.5
29
—
—
9
—
5
3
by Yang and He by employing a (Ph P)AuCl–AgOTf system.
Herein, we report the discovery of a robust and versatile copper
catalyst for the addition of acids and alcohols to norbornene.
The addition of 4-methoxybenzoic acid to norbornene was
initially examined in the presence of 10 mol% triflic acid (dioxane,
3
6
4
2
3
45
95
80 uC, 18 h). A low yield of the product ester 1a was obtained
2
(Table 1, entry 1), suggesting that electrophilic activation of the
General reaction conditions: 4-methoxybenzoic acid (1.0 mmol),
b
norbornene (1.5 mmol), dioxane, 80 uC, 18 h. Isolated yield after
column chromatography. The results were duplicated to within
¡
alkene by the strong Brønsted acid is fairly slow under these
reaction conditions. Several cationic metal catalysts were subse-
quently assessed as potential catalysts, chosen for their well-
established Lewis acidity and for their stability to air and moisture
c
5%. As a mixture of exo- and endo- isomers.
(Scheme 1, Table 1). Cationic late-transition metal complexes
a
Department of Chemistry, Imperial College London, South Kensington,
London, UK SW7 2AZ. E-mail: mimi.hii@imperial.ac.uk;
Fax: +44-20-7594-5804; Tel: +44-20-7594-1142
GlaxoSmithKline, Old Powder Mills, Nr. Leigh, Tonbridge, Kent, UK
TN11 9AN
b
{ Electronic supplementary information (ESI) available: Full characteri-
Scheme 1 Catalysed addition of 4-methoxybenzoic acid to norbornene.
Chem. Commun., 2005, 5103–5105 | 5103
This journal is ß The Royal Society of Chemistry 2005