J. Malineni et al. / Catalysis Communications 40 (2013) 80–83
81
Cl
Cl
N
R
X
Ru
R
Ru
X
N
N
Cl
H
Cl
PtBu2
Ru
Et2N
N
N
Br
CO
2 X = isopropyl
1
3, R = C6H13
Scheme 1. PNP and NHC ruthenium complexes.
experiments: It crystallizes in the triclinic space group P1 with two sym-
metrically independent molecules in the asymmetric unit (Fig. 1). Infor-
mation on data collection and refinement results are reported in the ESI.
Given the recent interest for the synthesis of esters and amides by
alcohol dehydrogenation using NHC–Ru complexes, we applied catalyst
3 for these reactions. By using butanol as a model substrate in refluxing
toluene, under inert gas atmosphere or in air, butyl butanoate 4a was
obtained demonstrating the stability of the carbene complex against
moisture, oxygen and heat (Table 1). With the structurally well-defined
complex 3, we first examined the reported catalytic conditions [20,25].
Using 5 mol% of catalyst 3 and 15 mol% of KOtBu, dehydrogenation of
butanol led to butyl butanoate 4a in quantitative yield within 15 h.
Using a lower catalyst loading (2 mol%) the yield decreases to 56%. Ap-
plying 3 mol% of catalyst under the same conditions after 24 h a yield
of 83% butyl butanoate (4a) was obtained. For comparison the less
efficient and more sensitive imdazolylidene complex 2 results in a signif-
icantly lower yield (61%) of 4a. No reaction takes place in the absence of a
base, while the increasing concentration of the base from 15–30 mol% to
1 Eq shows a decreasing ester yields.
The optimized reaction conditions were applied to a wide range of
different primary alcohols, which by dehydrogenation result in the
corresponding esters in high yields (Table 1). With increasing chain
length of the aliphatic alcohols higher yields of esters were obtained
(Table 1, entries 3–5). In contrast hydroxyalkyl groups attached to a
phenyl ring show moderate yields of the corresponding esters 4d–f
(Table 1, entries 6–8). Attempts to couple two different alcohols
selectively were not successful. The reaction between butanol and
3-phenylpropanol results in a mixture of all four ester products.
The established reaction conditions were further applied for the syn-
thesis of diamides starting with different diols and primary amines or
diamines with primary alcohols. A first experiment was carried
out with hexanol and hexylamine with 3 mol% of imdazolylidene
complex 2 and 15 mol% of KOtBu in toluene at reflux for 24 h, leading
to N-hexylhexanamide in 57% yield. Under similar conditions, the
benzimidazolylidene complex 3 shows significant higher yields (89%).
Based on these first results the scope and limitation of the method
were explored. A range of different diols or diamines were reacted
with amines or alcohols to afford the corresponding diamides in 71–
95% isolated yield (Table 2). Increasing the chain length of the diol or
diamine leads to increasing diamide yields (Table 2 entry 1–9). Tri-
and tetraethylene glycols and hexyl amine are also converted into di-
amides at high conversion rates and isolated yields of 71% (for 5c) and
82% (for 5d) (Table 2 entry 3 and 4). Phenyl substituted starting alco-
hols and amines are converted into corresponding amides in good
yields (Table 2 entry 5–6 and 10–12). In all cases, we did not observe
any cyclic products.
The efficiency of the catalyst was further tested to tri-amide syn-
thesis from triols. The standard reaction conditions were applied for
the triol 6, which by the treatment with hexylamine results in the
corresponding triamide 7 in 68% isolated yields (Scheme 2).
In order to investigate the potential of catalyst 3 for the preparation of
polyamides by catalytic dehydrogenation of diols and diamines, we
reacted 1,4-phenylenedimethane amine and 1,10-decanediol with
3 mol% of catalyst 3 and 15 mol% of KOtBu in refluxing toluene for
24 h (Scheme 3). During reaction the product precipitates. NMR analysis
reveals the formation of an oligomeric mixture 8. To improve solubility a
highly polar solvent (anisole) was added, however, the results did not
change significantly; low molecular weight polyamide building blocks
were obtained in about 72% isolated yield which is comparable to results
Table 1
Synthesis of esters from primary alcohols.a
Entry
Alcohol
Catalyst [mol %]
Time [h]
Ester
Yieldb
1
2
3
4
5
6
7
8
n-Butanol
n-Butanol
n-Butanol
n-Hexanol
5
2
3
3
3
3
3
3
15
15
24
24
24
24
24
24
4a
4a
4a
4b
4c
4d
4e
4f
100
56
83
91
94
42
54
61
n-Decanol
Benzyl alcohol
2-Phenylethanol
3-Phenylpropanol
a
Fig. 1. Displacement ellipsoid plot of one of the two symmetrically independent mole-
cules in the crystal structure of complex 3. Ellipsoids are drawn at 50% probability level.
Color code: C — gray, N — blue, Ru — red, Cl — green, H — white.
Reaction conditions: 1 equivalent of alcohol, 3 mol% catalyst 3, 15 mol% of base,
toluene (1 mL), reflux, 24 h.
b
Isolated yield.