Catalytic performance of ruthenium-supported ionic-liquid catalysts in sustainable synthesis of macrocyclic lactones

Article abstract of DOI:10.1002/hlca.200900334

Thirteen- to eighteen-membered lactones were synthesized by ring-closing olefin-metathesis reactions of bis-olefins with heterogeneous Grubbs-supported ionic-liquid catalysts (SILCs), in which homogeneous Grubbs catalysts were confined in pores of alumina with the aid of an ionic liquid. The Grubbs-SILCs exhibited higher catalytic performance than their homogeneous counterparts and could be repeatedly recovered by simple filtration and re-used several times.

Full text of DOI:10.1002/hlca.200900334

Helvetica Chimica Acta – Vol. 93 (2010)
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Catalytic Performance of Ruthenium-Supported Ionic-Liquid Catalysts in
Sustainable Synthesis of Macrocyclic Lactones
by Hisahiro Hagiwara*a), Tomomi Nakamuraa), Naotaro Okunakaa), Takashi Hoshib), and
Toshio Suzukib)
a) Graduate School of Science and Technology, Niigata University, 8050, 2-Nocho, Ikarashi, Nishi-ku,
Niigata 950-2181, Japan (phone/fax: þ 81-25-262-7368; e-mail: hagiwara@gs.niigata-u.ac.jp)
b) Faculty of Engineering, Niigata University, 8050, 2-Nocho, Ikarashi, Nishi-ku, Niigata 950-2181, Japan
Thirteen- to eighteen-membered lactones were synthesized by ring-closing olefin-metathesis
reactions of bis-olefins with heterogeneous Grubbs-supported ionic-liquid catalysts (SILCs), in which
homogeneous Grubbs catalysts were confined in pores of alumina with the aid of an ionic liquid. The
Grubbs-SILCs exhibited higher catalytic performance than their homogeneous counterparts and could
be repeatedly recovered by simple filtration and re-used several times.
Introduction.
– Macrocyclic lactones such as cyclopentadecano-15-lactone
(¼ Exaltolideꢀ; 1) from angelica oil and cyclohexadec-9-eno-16-lactone
(¼ambrettolide; 2) from ambrette oil (Fig. 1) are valuable and important perfumery
ingredients for composing the bottom note of perfumery products [1]. These
macrocyclic lactones have a strong musk scent in addition to preserving other scent
ingredients in perfumes. Since compounds that are alternatives to natural muscone
(¼(3R)-3-methylcyclopentadecanone; 3) such as aromatic nitro- and polycyclic musks
are not currently used owing primarily to their low biodegradability, macrocyclic
lactones are becoming increasingly important as alternative fragrance ingredients.
However, a readily accessible supply of macrocyclic lactones cannot be easily obtained
by intramolecular lactonization of long-chain hydroxy acids.
Fig. 1. Macrocyclic musk ingredients
Since the development of carbeneruthenium and -molybdenum complexes (Fig. 2)
by Grubbs and Schrock, respectively [2], olefin metathesis has emerged as a highly
potent method for closing macrocyclic rings. Since these catalysts react almost
exclusively with olefins and exhibit high tolerance toward a wide range of functional
ꢁ 2010 Verlag Helvetica Chimica Acta AG, Zꢂrich

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Helvetica Chimica Acta – Vol. 93 (2010)
groups, metathesis reactions have been widely utilized as key steps in numerous total
syntheses of complex natural products [3].
Fig. 2. Grubbsꢃ carbeneruthenium complex 4 and its congeners 5 – 7. PCy3 ¼ tricyclohexylphosphine;
Mes ¼ 2,4,6-trimethylphenyl.
However, these exceptionally good catalysts are not without drawbacks. They easily
deteriorate and are difficult to recycle when used in homogeneous catalysis. In addition,
the catalytic turnover number is not high, and their high price and large molecular mass
limit their use.
To solve these issues, considerable effort has been devoted to immobilize the
Grubbs Ru catalysts on a variety of supports [4]. The Grubbs catalysts have been
covalently immobilized on solid supports such as zirconium [5], silica [6], polymers [7],
and dendrimers [8]. Covalent immobilization on liquid supports has been accom-
plished by anchoring ion-tagged Grubbs catalysts in ionic liquids [9], polyethylene
glycol [10], and fluorous solvents [11]. More facile methods of noncovalent
immobilization include dissolution in an ionic liquid [12], encapsulation in polymer
beads [13], occlusion in polydimethylsiloxane [14], and ionic binding on sulfonated
resin [15]. Although a plethora of immobilization methods have been developed, a
limited number of these protocols have been successful in closing macrocyclic rings
[6c][6e][7][15b], especially in terms of repeated use.
As part of our continuing efforts to immobilize homogeneous organometallic
catalysts noncovalently, we have been investigating immobilization on supported ionic-
liquid catalysts (SILCs), in which a homogeneous organometallic catalyst in an ionic
liquid is confined within the pores of an inorganic solid support. The procedure is quite
simple and uses commercially available catalysts without modifying their structures or
requiring additional synthetic steps. The SILCs can be separated easily by filtration and
recycled. With this protocol, Pd- [16], Cu- [17], and Ru-SILCs [18] (Scheme) have been
developed and utilized as sustainable catalysts.
Results and Discussion. – Our investigation of the performance of Ru-SILCs as
catalysts for macrolactonization was prompted by the lack of an efficient and
sustainable method of macrolactonization in the presence of immobilized Grubbs
catalysts, which we have successfully immobilized as SILCs [18], as well as our interest
in developing efficient protocols for the synthesis of perfumery ingredients [19].
Inexpensive undec-10-enoic, oleic (¼(9Z)-octadec-9-enoic) and erucic (¼(13Z)-
docos-13-enoic), acids were chosen as the acid moieties in the long-chain bis-olefinic
esters 12 – 17 (Table), which were prepared by conventional methods with commer-
cially available olefinic alcohols. The Grubbs first-generation (Gr1) and second-

Helvetica Chimica Acta – Vol. 93 (2010)
177
Scheme. Ring-Closing Olefin Metathesis Catalyzed by Ru-SILC
generation (Gr2) catalysts 4 and 5 and the second-generation Hoveyda-Grubbs (HG2)
catalyst 6 (Fig. 2) were immobilized as Gr1-SILC 8, Gr2-SILC 9, and HG2-SILC 10,
respectively, according to our previously reported procedure [18] (Scheme). The role of
the ionic liquid in catalysis with Ru-SILCs is critical; Gr1 catalyst 4 did not exhibit any
catalytic activity when immobilized by sintering without an ionic liquid. Moreover,
HG2 catalyst 6 was transformed into the 4-(diethylamino) derivative 7 according to the
procedure by Kirschning, Grela, and co-workers [15] and immobilized as AM-HG2-
SILC 11 with the aid of 1-hexyl-3-methyl-1H-imidazolium hexafluorophosphate
([hmim]PF6) on silica gel bearing propanesulfonic acid residues.
Ring-closing olefin metathesis reactions of bis-olefinic esters 12 – 18 were carried
out in the presence of 0.2 equiv. of Ru-SILC in toluene at 1008 under a N2 atmosphere
unless otherwise noted; the results are listed in the Table. In the case of allyl oleate (12),
the reaction resulted in complete recovery of the starting ester 12 (Entries 1 – 3), which
was likely due to the inherent difficulty of closing twelve-membered rings. No dimeric
bis-lactone was obtained [18]. In contrast, thirteen- and fourteen-membered lactones
20 and 21 were synthesized from 13 and 14, respectively, in good yield in the presence of
Gr2-SILC 9 and HG2-SILC 10, respectively (see Entries 4 – 9). Gr1-SILC 8 effectively
closed pent-4-en-1-ylundec-10-enoate (15) into the fifteen-membered lactone 22 and
was recycled five times by simple filtration without any pretreatment (Entries 11 – 15),
while use of homogeneous Gr1 catalyst 4 resulted in recovery of a large amount of
starting material (Entry 10). It is worth noting that heterogeneous Gr1-SILC 8 was
more favorable for macrolactonization than the original homogeneous Gr1 catalyst 4.
A low catalytic performance was also observed when Gr1 catalyst 4 was immobilized
with 1-butyl-2,3-dimethyl-1H-imidazolium hexafluorophosphate, which suggests acti-
vation of the Gr1 catalyst 4 in the SILC 8 by the intervention of the N-heterocyclic
carbene at the ruthenium center [20] (Entry 16).
Cyclopentadecano-15-lactone (¼ Exaltolideꢀ; 1), a sixteen-membered lactone, has
important commercial uses. Although this compound has been previously synthesized
by olefin metathesis employing heterogeneous [7] as well as homogeneous Grubbs

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Helvetica Chimica Acta – Vol. 93 (2010)
Table. Synthesis of Macrocyclic Lactones Catalyzed by Ru-SILCa)
Entry Substrate
Product
Catalyst
Time [h] Yield [%]b)
1
2
3
Gr1 4
Gr1-SILC 8
Gr2-SILC 9
2
5
4.5
0
0
0
4
5
Gr1-SILC 8
Gr2-SILC 9
5
14
41 (62)
86 (98)
6
7
8
9
Gr1 4
2
1.5
4
43 (81)
60 (78)
60 (97)
76 (76)
Gr1-SILC 8
Gr2-SILC 9
HG2-SILC 10
0.5
10
11
Gr1 4
Gr1-SILC 8
1
42 (58)
66 (77)
64 (77)
61 (69)
59 (59)
39 (66)
45 (45)
0.75
1.25
2
6
18
12c)
13c)
14c)
15c)
16
Gr1-SILC 8d)
18
17
Gr1-SILC 8
Gr1-SILC 8
3
36
4.5
16
28
78 (94)
46 (88)
63 (78)
50 (68)
51 (74)
18c)
19e)
20c)e)
21c)e)
22
Gr2-SILC 9
16
42 (86)
23f)
24c)f)
HG2-SILC 10
2
22
70 (91)
60 (85)
25
AM-HG2-SILC 11
0.17
0.17
0.5
60 (65)
51 (59)
50 (60)
41 (58)
26c)
27c)
28c)
18

Helvetica Chimica Acta – Vol. 93 (2010)
179
Table (cont.)
Entry
Substrate
Product
Catalyst
Time [h]
Yield [%]b)
29
30
Gr1-SILC 8
Gr2-SILC 9
6
4
80 (98)
84 (84)
31
32
Gr1-SILC 8
Gr2-SILC 9
1.5
4
28 (35)
95
a) Reaction in the presence of 0.2 equiv. of Ru-SILC at 1008 in toluene unless otherwise noted. b) Yield in
parentheses based on recovered starting ester. c) The catalyst of the previous experiment was re-used.
d) Gr1 catalyst 4 was immobilized with 1-butyl-2,3-dimethyl-1H-imidazolium hexafluorophosphate.
f
e) Reaction in refluxing benzene. ) Reaction in refluxing toluene.
catalysts [21], to the best of our knowledge, there is no precedent for recycling the
catalyst, particularly in the reaction of inexpensive allyl eruciate (16). The metathesis
reaction of 16 catalyzed by Gr1-SILC 8 afforded the desired sixteen-membered lactone
23 when carried out in refluxing benzene to prevent rapid thermal decomposition of the
catalyst. The catalyst was active until the third repeated use (Entries 19 – 21).
Thermally more stable HG2-SILC 10 exhibited comparable catalytic activity (En-
tries 23 and 24). On the other hand, the reaction catalyzed by AM-HG2-SILC 11
proceeded more rapidly as shown by Entries 25 – 28, which is likely attributable to the
electron-deficient nature of the aromatic ring of the AM-HG2 catalyst 7 as a result of
protonation by acidic silica gel of the diethylamino group. Addition of titanium
tetraisopropoxide was not effective in most of these experiments.
The much larger seventeen- and eighteen-membered lactones 24 and 25,
respectively, were also synthesized in high yield (Entries 29 – 32).
Conclusions. – In summary, homogeneous carbeneruthenium complexes 4 – 7 were
immobilized as Ru-SILCs 8 – 11 in the pores of amorphous inorganic supports with the
aid of the ionic liquid [hmim]PF6. Ru-SILCs successfully catalyzed ring-closing olefin
metathesis of bis-olefinic esters 13 – 18 to afford thirteen- to eighteen-membered
macrocyclic lactones 20 – 25 in satisfactory yields and were used repeatedly several
times after simple filtration without any pretreatment. The catalytic activity of the

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Helvetica Chimica Acta – Vol. 93 (2010)
proposed catalysts was superior to that of their homogeneous counterpart Gr1 4 for
macrolactonization. The present protocol might be useful for the synthesis of
macrocyclic perfumery ingredients since the starting bis-olefinic esters 13 – 18 are
reasonably available, and catalytic hydrogenation of the macrocyclic lactones 20 – 25
proceeded smoothly. Immobilization of homogeneous organometallic catalysts as
SILCs is simple and cost-effective, and is a mild method for immobilizing highly
complex but unstable organometallic complexes on solid supports without changing
their original catalytic activity.
This work was partially supported by a Grant-in-Aid for Research for Promoting Technological Seeds
(05-B04 for H. H.) from the Japan Science and Technology Agency (JST), the Japan Food Chemical
Research Foundation (for H. H.), and by a Grant-in-Aid for Scientific Research on Priority Areas
(20031011 for H. H.) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT).
Experimental Part
1
General. IR Spectra: Jasco-FT/IR-4200 spectrometer; in CHCl3; n˜ in cmꢀ1. H-NMR Spectra: Jeol-
EX-270 spectrometer at 270 MHz; in CDCl3; d in ppm rel. to Me4Si as internal standard, J in Hz. MS:
Jeol-GCMate (EI, 70 eV) instrument; in m/z (rel. %).
1-Oxacyclotridec-9-en-2-one (20). Gr2-SILC 9 was prepared according to the procedure reported
previously [18]. A stirred suspension of Gr2-SILC 9 (1.252 g, 0.008 mmol/g) and pent-4-en-1-yl oleate
(13; 16.9 mg) in toluene (10 ml) was heated at 1008 for 14 h under N2. After cooling to r.t., the Ru-SILC 9
was rinsed with Et2O (4ꢁ). The combined org. phase was concentrated and the residue purified by
medium-pressure LC (SiO2, AcOEt/hexane 1:9): 20 (8.5 mg, 86%). IR: 3012, 2927, 2858, 1726.
1H-NMR: 1.25 – 1.44 (m, 8 H); 1.63 – 1.68 (m, 2 H); 2.00 – 2.09 (m, 2 H); 2.27 – 2.39 (m, 4 H); 4.15 (t, J ¼
5.4, 2 H); 5.30 – 5.42 (m, 1 H); 5.46 – 5.62 (m, 1 H). EI-MS: 196 (7, Mþ), 107 (31), 95 (38), 94 (44), 93
(62), 82 (29), 81 (67), 80 (73), 79 (100), 69 (29), 68 (53), 67 (76). HR-MS: 196.1463 (C12H20Oþ2 ; calc.
196.1457).
1
1-Oxacyclotetradec-10-en-2-one (21). IR: 3020, 2934, 2860, 1725. H-NMR: 1.29 – 1.35 (m, 10 H);
1.62 – 1.81 (m, 4 H); 2.01 – 2.06 (m, 2 H); 2.01 – 2.28 (m, 2 H); 2.32 – 2.43 (m, 2 H); 4.10 – 4.15 (m, 2 H);
5.26 – 5.57 (m, 2 H). EI-MS: 210 (29, Mþ), 109 (29), 96 (39), 95 (67), 83 (26), 82 (89), 81 (100), 80 (27),
79 (48), 69 (40), 68 (100), 67 (100). HR-MS: 210.1621 (C13H22Oþ2 ; calc. 210.1620).
1
1-Oxacyclopentadec-11-en-2-one (22). IR: 3016, 2932, 2858, 1725. H-NMR: 1.34 (m, 10 H); 1.61 –
1.82 (m, 4 H); 2.01 – 2.02 (m, 2 H); 2.20 – 2.23 (m, 2 H); 2.31 – 2.39 (m, 2 H); 4.10 – 4.16 (m, 2 H);
5.27 – 5.45 (m, 2 H). EI-MS: 224 (17, Mþ), 96 (32), 95 (48), 83 (29), 82 (63), 81 (99), 79 (37), 71 (69), 69
(34), 68 (100), 67 (93). HR-MS: 224.1758 (C14H24Oþ2 ; calc. 224.1776).
1
1-Oxacyclohexadec-14-en-2-one (23). IR: 3001, 2927, 2855, 1728. H-NMR: 1.28 (m, 16 H); 1.56 –
1.65 (m, 2 H); 2.06 – 2.15 (m, 2 H); 2.32 – 2.39 (m, 2 H); 4.19 (m, 2 H); 5.53 – 5.78 (m, 2 H). EI-MS:
238 (27, Mþ), 111 (28), 109 (34), 98 (56), 97 (40), 96 (62), 95 (73), 84 (25), 83 (54), 82 (87), 81 (100), 69
(72), 68 (86), 67 (100). HR-MS: 238.1937 (C15H26Oþ2 ; calc. 238.1933).
1
1-Oxacycloheptadec-14-en-2-one (24). IR: 3020, 2929, 2856, 1712. H-NMR: 1.23 – 1.40 (m, 18 H);
1.59 – 1.67 (m, 2 H); 2.00 – 2.07 (m, 2 H); 2.26 – 2.40 (m, 4 H); 4.14 (t, J ¼ 5.37, 2 H); 5.30 – 5.55 (m, 2 H).
EI-MS: 252 (21, Mþ), 96 (43), 95 (47), 83 (27), 82 (74), 81 (73), 69 (38), 68 (100), 67 (82). HR-MS:
252.2077 (C16H28Oþ2 ; calc. 252.2089).
1-Oxacyclooctadec-14-en-2-one (25). IR: 3008, 2929, 2855, 1723. 1H-NMR: 1.26 – 1.29 (m, 18 H);
1.65 – 1.75 (m, 2 H); 2.00 – 2.10 (m, 4 H); 2.33 (t, J ¼ 6.35, 2 H); 4.12 (t, J ¼ 6.84, 2 H); 5.35 – 5.40 (m, 2 H).
EI-MS: 266 (17, Mþ), 96 (43), 95 (45), 82 (80), 81 (83), 69 (31), 68 (100), 67 (82). HR-MS: 266.2262
(C17H30Oþ2 ; calc. 266.2246).

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Received September 15, 2009