Tetrahedron Letters
Mono- and dinuclear cyclopalladates as catalysts for Suzuki–Miyaura
cross-coupling reactions in predominantly aqueous media
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G. Narendra Babu, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Suzuki–Miyaura cross-coupling reactions of aryl halides with arylboronic acids were performed in pre-
dominantly aqueous media employing two mono- and two dinuclear cyclopalladated complexes as cat-
alysts. These complexes are [Pd(HL)Cl] (I), [Pd(L)(PPh3)] (II), [Pd2(l-dppb)(L)2] (III) and [Pd2(l-dppf)(L)2]
Received 28 December 2016
Revised 23 January 2017
Accepted 25 January 2017
Available online xxxx
(IV); where H2L, dppb and dppf represent 4-methoxy-N0-(mesitylidene)benzohydrazide, 1,4-bis
(diphenylphosphino)butane and 1,10-bis(diphenylphosphino)ferrocene, respectively. The reactions were
conducted using potassium carbonate as base in presence of tetrabutylammonium bromide (TBAB) at
70/90 °C in dimethylformamide–water (1:20) mixture. Among the four catalysts used, the dinuclear com-
plex IV turned out to be the most effective and afforded moderate to excellent yields with broad substrate
scope.
Keywords:
Cyclopalladates
Mono- and dinuclear
Suzuki–Miyaura cross-coupling
Aqueous media
Ó 2017 Elsevier Ltd. All rights reserved.
Biaryls and biheteroaryls are an important class of compounds
as they are significant building blocks for a variety of natural prod-
ucts, pharmaceuticals, agrochemicals and functional materials
such as sensors and liquid crystals.1 The most convenient and use-
ful method for the single step synthesis of a biaryl or biheteroaryl
is the Csp2ꢀCsp2 Suzuki–Miyaura cross-coupling reaction.2 Since
the discovery of this catalytic coupling reaction, there is a contin-
uous effort to develop new catalyst systems which will be effective
for a wide range of products. Generally organic media are more
common for this coupling reaction. However, due to the environ-
mental concerns necessity for the use of the benign solvent water
as the medium for chemical reactions is continuously growing.
Thus, in recent times, there is a particular emphasis on the devel-
opment of catalyst systems that will be effective for Suzuki–
Miyaura cross-coupling reactions in aqueous media.3 So far, among
the various catalyst systems used for Suzuki–Miyaura reaction in
aqueous or aqueous-organic media, very few are based on
cyclopalladated complexes.4 These complexes are either water sol-
uble for use in homogeneous reactions or anchored to silica or
nano-particles for applications in heterogeneous reactions. We
have been working on coordination5 and cyclometallated6 com-
plexes of transition metal ions with Schiff bases and reduced Schiff
bases and their applications as catalysts in synthetic organic reac-
tions for the past couple of years. Recently we have reported a ser-
ies of mononuclear and dinuclear cyclometallated palladium(II)
complexes with the Schiff base 4-methoxy-N0-(mesitylidene)ben-
zohydrazide (H2L).7 In these complexes having the formulas [Pd
(HL)Cl] (I), [Pd(L)(PPh3)] (II), [Pd2(l-dppb)(L)2] (III) and [Pd2(l-
dppf)(L)2] (IV) (dppb = 1,4-bis(diphenylphosphino)butane and
dppf = 1,10-bis(diphenylphosphino)ferrocene); the ligands ((HL)ꢀ
and (L)2ꢀ) have the 5,6-membered fused chelate rings forming pin-
cer-like ONC-coordinating mode (Chart 1). In the present work, we
have examined the catalytic properties of these four complexes in
Suzuki–Miyaura cross-coupling reactions of phenyl- and 2-naph-
thylboronic acids with electronically diverse types of aryl halides
in predominantly aqueous media.
4-bromobenzaldehyde and phenylboronic acid were chosen as
the model substrates for optimization of the reaction conditions.
The reactions were performed in water (2 ml) under aerobic condi-
tions at 70 °C in presence of tetrabutylammonium bromide (TBAB)
by varying the catalyst (IꢀIV) (taken in 0.1 ml of dimethylfor-
mamide (DMF)), catalyst loading (mol% of the metal complex),
base, and the amounts of base and TBAB (Table 1). Among the four
catalysts IꢀIV assessed (entries 1–4), catalyst IV provided the high-
est yield in shortest reaction time (entry 4). Decrease in the mol%
of IV resulted into significant decrease in the yield and increase
in the reaction time (entries 5–7). Considering the dinuclearity of
IV, it is expected to be more productive than either of the two
mononuclear complexes (I and II). But, the performances of IV with
significantly lower loadings of 0.01–0.0001 mol% were better than
the performances of 0.1 mol% of both I and II (entries 1 and 2).
Variation of reaction temperature (entries 8–10) showed that at
elevated temperature (90 °C) though same yield was obtained
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Corresponding author.
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