Chloroesterification of enynes catalyzed by NHC rhodium compounds

Article abstract of DOI:10.1055/s-2008-1032073

An efficient rhodium N-heterocyclic carbene (NHC)-catalyzed chloroesterification of terminal alkynes and enynes has been developed. The reaction was highly regio- and stereospecific: the Z-isomer was obtained as the sole product. Georg Thieme Verlag Stutt

Full text of DOI:10.1055/s-2008-1032073

LETTER
551
Chloroesterification of Enynes Catalyzed by NHC Rhodium Compounds
C
hloroeste
i
r
ification of
Y
E
nynes Catalyzed
o
by NHCRho
u
dium Comp
n
ounds g Baek, Sang Ick Lee, So Hee Sim, Young Keun Chung*
Intelligent Textile System Research Center and Department of Chemistry, College of Natural Sciences, Seoul National University,
Seoul 151747, Korea
Fax +82(2)8890310; E-mail: ykchung@snu.ac.kr
Received 24 August 2007
transition-metal–NHC-complex-catalyzed chloroes-
terification of enynes and alkynes.
Abstract: An efficient rhodium N-heterocyclic carbene (NHC)-
catalyzed chloroesterification of terminal alkynes and enynes has
been developed. The reaction was highly regio- and stereospecific:
the Z-isomer was obtained as the sole product.
Chloroesterification was studied using enyne 1a as a mod-
el substrate and Rh(IPr)(cod)Cl [IPr = bis(2,6-diisopro-
Key words: N-heterocyclic carbene, rhodium, catalysis, chloroes- pylphenyl)imidazol-2-ylidene (Figure 1); cod = 1,5-
terification, enyne
cyclooctadiene] as a catalyst (Table 1).
Treatment of 1a (0.5 mmol, 53 mg) with methyl chloro-
formate (1.0 mmol, 95 mg) in the presence of a catalytic
The use of chloroformate in organic synthesis can be
amount of Rh(IPr)(cod)Cl (1 mol%, 3.2 mg) in toluene
traced back to a century ago,1 although the extent of its use
was quite limited.2 Chloroformate has been utilized in re-
actions such as Curtius rearrangement, syntheses of N-
Table 1 Rh–NHC-Catalyzed Chloroesterification of Enynea
alkyl carbamic acid alkyl esters and N-alkylcarbamates,
and the dealkylation of amines.3 Chloroformates may be
used in the chloroesterification of alkynes to produce 3-
chloroacrylate esters which are valuable intermediates in
organic synthesis.4 Thus we expected that a lot of atten-
tion would have been given to this chloroesterification
process. However, on the contrary, only a few studies
have been reported.5 For example, Tanaka et al. reported
a rhodium-catalyzed chloroesterification of terminal
alkynes and 1,2-dienes (allenes).5a,b
Cl
O
O
Me
Rh catalyst
O
Me
+
Cl
O
1a
2a
3aa
+
CO2Me
Cl
4aa
Entry Catalyst
Solvent
Temp Time Yield (%)
Recently, N-heterocyclic carbenes (NHCs) have emerged
as a group of promising ligands for the design of new ho-
mogeneous catalysts.6 They are normally stable toward
air, heat, and moisture, and are very powerful s-donors.
Some of them are commercially available. Metal com-
plexes of NHCs were shown to be alternatives for the
widely used phosphine complexes in homogeneous catal-
ysis. In some cases, replacement of phosphines by NHCs
can provide complexes with enhanced catalytic perfor-
mances.7 Despite the intense interest in the catalytic prop-
erties of NHC complexes,8 reports on the use of rhodium–
NHC complexes are still restricted in hydroformylation,9
hydrogenation,10 arylation,11 hydrosilylation,12 and
cycloaddition13 reactions. As a further step in our program
toward the use of Rh–NHC complexes as catalysts,12,13b
we initiated a study on the use of Rh–NHC complexes in
chloroesterification of unsaturated hydrocarbons because
the unsaturated hydrocarbons having chlorine and olefinic
functionalities were envisioned to be useful in synthetic
applications. Thus, we investigated the rhodium–NHC-
catalyzed chloroesterification of alkynes and enynes. To
the best of our knowledge, this is the first example of
(°C)
100
100
70
(h)
18
18
18
12
6
of 3aab,c
1
2
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(cod)Cl
Rh(IPr)(CO)Cl
toluene
toluene
toluene
toluene
toluene
TCEe
80
83d
3
61
4
100
100
100
100
100
100
100
83
5
65
6
12
12
18
18
18
53 (11)f
8 (5)f
38
7
DMEg
8
toluene
toluene
toluene
9
Rh(IMes)(cod)Cl
Rh(PPh3)(cod)Cl
70
10
18 (4)f
a Reaction was run using 1a (1.0 equiv), methyl chloroformate (2.0
equiv), Rh catalyst (1 mol%), and solvent (0.3 M).
b Isolated yield.
c In all cases, the ratio of product 3aa/4aa was >19:1, as determined
by 1H NMR (300 MHz).
d A higher amount of methyl chloroformate (3 equiv) was used.
e TCE = tetrachloroethane.
SYNLETT 2008, No. 4, pp 0551–0554
x
x.
x
x.
2
0
0
8
f Yields in parentheses are for the trimerized product [1,3,5-tri(cyclo-
hexenyl)benzene].
Advanced online publication: 23.01.2008
DOI: 10.1055/s-2008-1032073; Art ID: U07907ST
© Georg Thieme Verlag Stuttgart · New York
g DME = 1,2-dimethoxyethane.

552
J. Y. Baek et al.
LETTER
product was obtained. However, when the amount of the
catalyst used was increased to 5 mol%, the yield of the re-
action increased to 85% (entry 4). Several (functional-
ized) conjugated enynes (entries 5–10) were tested for the
chloroesterification. Conjugated enynes 1b–d with a ter-
minal alkyne (entries 5–7) were good substrates, but for
the conjugated enyne 1e with a terminal alkyne (entry 8)
a rather poor yield (38%) was obtained, presumably due
to the volatility of the enyne. Treatment of an enyne with
a hydroxyl group (1f, entry 9) gave no isolable products,
but an enyne with a protected alcohol (1g, entry 10) pro-
duced the expected product in 77% yield. Thus, the pro-
tection of the alcohol group was necessary.
Phenylacetylene (1h; entry 11) was a good substrate and
the expected product was obtained in 82% yield. Howev-
er, modest yields (32% and 48%) were obtained for termi-
nal alkyl acetylenes 1i and 1j (entries 12 and 13).
Unfortunately, our catalytic system proved ineffective for
internal alkynes.16 This observation was similar to Tana-
ka’s result. According to their paper, all attempts to chlo-
roesterify internal alkynes were unsuccessful and the
starting material was recovered.
N
N
N
N
IPr
IMes
Figure 1 Structure of IPr and IMes, two N-heterocyclic carbenes
(1.5 mL) at 100 °C for 18 hours gave the chloroesterificat-
ed product 3aa in 80% yield along with 0.2% yield of a
cyclotrimerized product, 1,3,5-tri(cyclohexenyl)benzene.
By comparing with the known 1H NMR data of the E- and
Z-isomer reported by Tanaka,5a we determined that the
formation of the E-isomer was not observed. Thus, we
could confirm that the Z-isomer was obtained as the sole
product by H NMR. Thus, the chloroesterification was
highly regio- and stereospecific. Formation of 3aa was
confirmed by H NMR and high resolution mass spec-
trometry. Encouraged by this result, we screened various
reaction conditions, including the solvent, the reaction
temperature, the reaction time, and rhodium catalysts for
the esterification of 1a. The effect of an increase in the
amount of methyl chloroformate used on the yield of the
reaction was negligible (entry 2). Thus, we used two
equivalents of methyl chloroformate for all cases.
1
1
Table 2 Rh(IPr)(cod)Cl-Catalyzed Chloroesterification of Conju-
gated Enynea,14
Cl
O
O
Rh(IPr)(cod)Cl
toluene
R2
R2
R1
+
The yield of the reaction in toluene solution was depen-
dent upon the reaction time and the reaction temperature
(entries 1–5). The best result (83%) was obtained when
the reaction was carried out at 100 °C for 12 hours.
Changing the solvent from toluene to tetrachloroethane or
1,2-dimethoxyethane did not help the reaction (entries 5
and 6). In these cases, a trimerized product was also ob-
tained in 11% and 5% yields, respectively. Replacement
of the cod moiety in Rh(IPr)(cod)Cl by carbon monoxide
led to a noticeable decrease in the yield (38%) of the reac-
tion (entry 8). Substitution of IPr in Rh(IPr)(cod)Cl by
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes)
resulted in a decrease in the yield to 70%. When the phos-
phine rhodium complex [Rh(PPh3)(cod)Cl] was used as a
reference, only 18% of the reaction product was obtained,
with the concomitant formation of a trimerized product
(4%). Thus, among the rhodium catalysts examined,
[Rh(cod)(IPr)Cl] was the best catalyst for the chloroester-
ification of 1a. The optimized reaction conditions were
established to be as follows: Rh(IPr)(cod)Cl (1 mol%),
enyne (or alkyne; 0.5 mmol), chloroformate (2 equiv), tol-
uene (0.3 M), 100 °C, and 12 hours.
R1
O
Cl
O
1
2
3
+
CO2R2
Cl
R1
4
Entry Reactant
Ester
Product Yieldb
(%)
O
1
3aa
3ab
3ac
83
66
77
Me
Et
Cl
O
O
O
1a
2a
O
O
O
2
Cl
1a
2b
3
Ph
O
Cl
1a
2c
We next investigated the chloroesterification of various
alkynes and enynes under these optimized reaction condi-
tions (Table 2). Treatment of 1a with ethyl and phenyl
chloroformates, instead of methyl chloroformate, led to
isolation of the expected chloroesterified products in 66%
and 77% yields, respectively (entries 2 and 3). Recently,
Tanaka et al. reported15 the addition reaction of ethyl chlo-
rooxoacetate (ClCOCOOEt) to alkynes. Thus, when ethyl
chlorooxoacetate was used instead of chloroformate un-
der our reaction conditions, only 17% of the expected
17
4
3ad
Cl
Et
85c
O
1a
2d
2a
5
3ba
3ca
54
56
Ph
1b
6
2a
4-MeOC6H4
1c
Synlett 2008, No. 4, 551–554 © Thieme Stuttgart · New York

LETTER
Chloroesterification of Enynes Catalyzed by NHC Rhodium Compounds
553
Table 2 Rh(IPr)(cod)Cl-Catalyzed Chloroesterification of Conju-
References and Notes
gated Enynea,14 (continued)
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1891, 60, 181.
Cl
O
O
Rh(IPr)(cod)Cl
toluene
R2
R2
R1
R1
O
+
Cl
O
1
2
3
(2) For recent papers, see: (a)Vargas-Sanchez, M.; Lakhdar, S.;
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4245. (c) Miura, M.; Okuro, K.; Hattori, A.; Nomura, M.
J. Chem. Soc., Perkin Trans. 1 1989, 73. (d) Smith, A. B.;
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1998, 120, 12365. (b) Hua, R.; Tanaka, M. Tetrahedron
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N. X. J. Carbohydr. Chem. 2001, 20, 71.
(6) (a) Wang, A. E.; Xie, H. H.; Wang, L. X.; Zhou, Q. L.
Tetrahedron 2005, 61, 259. (b) Crudden, C. M.; Allen, D. P.
Coord. Chem. Rev. 2004, 248, 2247. (c) Cesar, V.;
Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004,
33, 619. (d) Cavell, K. J.; McGuinness, D. S. Coord. Chem.
Rev. 2004, 248, 671. (e) Lebel, H.; Janes, M. K.; Charette,
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Chem. Res. 1999, 32, 913.
+
CO2R2
Cl
R1
4
Entry Reactant
Ester
2a
Product Yieldb
(%)
7
3da
3ea
71
38
Ph
1d
8
2a
1e
HO
9
2a
2a
3fa
–
1f
TBSO
10
3ga
77
1g
11
2a
2a
3ha
3ia
82
32
1h
n-Bu
12
1i
13
2a
3ja
48
1j
a Reaction was run using Rh(IPr)(cod)Cl (1 mol%), enyne (or alkyne;
1.0 equiv), ester (2.0 equiv), toluene (0.3 M), 100 °C, 18 h.
b Isolated yield. In all cases, the ratio of product 3/4 was >19:1.
c A higher amount of the catalyst (5 mol%) was used, and the reaction
was conducted at 60 °C for 20 h.
In conclusion, we have developed a highly efficient rhod-
ium–NHC-catalyzed chloroesterification of terminal
alkynes and enynes. The catalytic system is a highly ver-
satile tool for obtaining products that cannot be easily at-
tained with other metals. We are currently investigating
other useful transformations using Rh–NHC catalysts.
(7) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.;
Artus, G. R. J. Angew. Chem. Int. Ed. 1995, 34, 2371; and
references therein. (b) Herrmann, W. A. Angew. Chem. Int.
Ed. 2002, 41, 1291; and references therein. (c) Mata, J. A.;
Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841.
(d) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew.
Chem. Int. Ed. 2007, 46, 2768.
Acknowledgment
(8) (a) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248,
2239. (b) Herrmann, W. A.; Ofele, K.; von Preysing, D.;
Schneider, S. K. J. Organomet. Chem. 2003, 687, 229.
(c) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003,
14, 951. (d) Herrmann, W. A. Angew. Chem. Int. Ed. 2002,
41, 1290.
This work was supported by the Korea Research Foundation grant
funded by the Korean Government (MOEHRD) (R02-2004-000-
10005-0 and KRF-2005-070-C00072) and the SRC/ERC program
of MOST/KOSEF (R11-2005-065). S.H.S. thanks for the Seoul Sci-
ence Fellowship and BK21 fellowship and J.Y.B. thanks for the
BK21 fellowship.
(9) (a) Praetorius, J. M.; Kotyk, M. W.; Webb, J. D.; Wang, R.;
Crudden, C. M. Organometallics 2007, 26, 1057.
(b) Bortenschlager, M.; Schuetz, J.; Von Preysing, D.;
Nuyken, O.; Herrmann, W. A.; Weberskirch, R. J.
Synlett 2008, No. 4, 551–554 © Thieme Stuttgart · New York

554
J. Y. Baek et al.
LETTER
147.6, 164.1. HRMS (EI): m/z calcd for C15H15ClO2:
262.0761; found: 262.0755. IR: 1416 (w), 1454 (w), 1486
(w), 1595 (s), 1723 (s), 1764 (w), 2128 (w), 2304 (s), 2408
(w), 2512 (w), 2672 (w), 2864 (w), 2928 (m), 2976 (w), 3048
(s), 3400(br), 3680 (w), 3744 (w), 3936 (m) cm–1.
Organomet. Chem. 2005, 690, 6233. (c) Zarka, M. T.;
Bortenschlager, M.; Wurst, K.; Nuyken, O.; Weberskirch, R.
Organometallics 2004, 23, 4817.
(10) (a) Baskakov, D.; Herrmann, W. A.; Herdtweck, E.;
Hoffmann, S. D. Organometallics 2007, 26, 626. (b) Allen,
D. P.; Crudden, C. M.; Calhoun, L. A.; Wang, R.; Decken,
A. J. Organomet. Chem. 2005, 690, 5736.
(11) (a) Yigit, M.; Oezdemir, I.; Cetinkaya, E.; Cetinkaya, B.
Transition Met. Chem. 2007, 32, 536. (b) Lewis, J. C.;
Wiedemann, S. H.; Bergman, R. G.; Ellman, J. A. Org. Lett.
2004, 6, 35.
(12) Park, K. H.; Kim, S. Y.; Son, S. U.; Chung, Y. K. Eur. J.
Org. Chem. 2003, 4341.
(13) (a) Evans, P. A.; Baum, E. W.; Fazal, A. N.; Pink, M. Chem.
Commun. 2005, 63. (b) Lee, S. I.; Park, S. Y.; Park, J. H.;
Jung, I. G.; Choi, S. Y.; Chung, Y. K.; Lee, B. Y. J. Org.
Chem. 2006, 71, 91.
(14) General Procedure for Rh–NHC-Catalyzed
Chloroesterification of Alkyne: To an oven-dried 10-mL
tube containing toluene (5 mL), Rh–NHC (14 mg, 1 mol%)
and alkyne (0.7 mmol) were added sequentially. After
sealing the tube, the reaction temperature was elevated to
100 °C. The reaction was carried out in a test tube capped
with a rubber septum. The rubber septum was tied with an
aluminum binder. Thus, the reaction could be monitored by
taking a small amount of the reaction mixture using a
syringe. After the reactant was consumed, the solvent was
removed under reduced pressure. Flash column
3ba: 1H NMR (300 MHz, CDCl3): d = 3.78 (s, 3 H), 6.20 (s,
1 H), 6.82 (d, J = 15.3 Hz, 1 H), 7.25–7.38 (m, 4 H), 7.47–
7.50 (m, 2 H). 13C NMR (75 MHz, CDCl3): d = 51.8, 117.8,
125.9, 127.8, 129.1, 129.7, 135.4, 138.9, 144.6, 164.9.
HRMS (EI): m/z calcd for C12H11ClO2: 222.0447; found:
222.0445. IR: 1416 (w), 1596 (w), 1723 (s), 2296 (m), 2968
(s), 3048 (s) cm–1.
3da: 1H NMR (300 MHz, CDCl3): d = 3.77 (s, 3 H), 3.83 (s,
3 H), 6.70 (d, J = 15.2 Hz, 2 H), 6.90 (d, J = 7.0 Hz, 2 H),
7.32 (d, J = 15.2 Hz, 1 H), 7.42 (d, J = 7.0 Hz, 2 H). 13C NMR
(75 MHz, CDCl3): d = 51.7, 55.6, 114.6, 116.6, 123.8, 128.2,
129.4, 138.6, 145.1, 161.0, 165.1. HRMS (EI): m/z calcd for
C13H13ClO3: 252.0553; found: 252.0555. IR: 1539 (w), 1584
(w), 1721 (s), 2296 (m), 2968 (s), 3048 (s) cm–1.
3ea: 1H NMR (300 MHz, CDCl3): d = 2.02 (s, 3 H), 3.78 (s,
3 H), 5.43 (s, 1 H), 5.92 (s, 1 H), 6.24 (s, 1 H). 13C NMR (75
MHz, CDCl3): d = 20.7, 51.9, 115.7, 122.9, 139.7, 146.1,
165.1. HRMS (EI): m/z calcd for C7H9Cl1O2: 160.0291;
found: 160.0294. IR: 1417 (w), 1435 (w), 1596 (m), 1729
(s), 2296 (s), 2400 (w), 2572 (w), 2672 (w), 2976 (m), 3048
(s), 3416 (br), 3672 (w), 3736 (w), 3928 (m) cm–1.
3ga: 1H NMR (300 MHz, CDCl3): d = 0.00 (s, 6 H), 0.84 (s,
9 H), 3.67 (s, 3 H), 4.28 (m, 2 H), 6.01 (s, 1 H), 6.34 (m, 1 H),
6.54 (td, J = 3.7, 14.7 Hz, 1 H). 13C NMR (75 MHz, CDCl3):
d = –5.2, 18.6, 26.0, 26.1, 51.7, 62.6, 117.1, 126.4, 140.8,
144.3, 165.1. HRMS (FAB): m/z calcd for C12H23ClO3Si:
290.1105; found: 291.1185. IR: 1420 (br), 1460 (w), 1603
(s), 1640 (w), 1728 (s), 2304 (m), 2400 (w), 2512 (w), 2672
(w), 2848 (w), 2944 (s), 3056 (m), 3360 (br), 3672 (w), 3736
(w), 3936 (w) cm–1.
chromatography gave the product.
3ab: 1H NMR (300 MHz, CDCl3): d = 1.33 (t, J = 7.1 Hz, 3
H), 1.61 (m, 2 H), 1.71 (dd, J = 5.4, 9.2 Hz, 2 H), 2.25 (d,
J = 4.1 Hz, 4 H), 4.22 (q, J = 7.1 Hz, 2 H), 6.11 (s, 1 H), 6.76
(t, J = 3.6 Hz, 1 H). 13C NMR (75 MHz, CDCl3): d = 14.4,
21.7, 22.6, 26.4, 60.5, 112.9, 133.6, 136.0, 148.9, 165.0.
HRMS (EI): m/z calcd for C11H15ClO2: 214.0761; found:
214.0764. IR: 1414 (w), 1433 (w), 1539 (w), 1601 (s), 1720
(s), 2120 (w), 2240 (w), 2296 (s), 2400 (w), 2504 (w), 2672
(m), 2920 (s), 2984 (m), 3048 (s), 3408 (br), 3680 (w), 3736
(w), 3936 (w) cm–1.
(15) Hua, R.; Onozawa, S.-Y.; Tanaka, M. Chem. Eur. J. 2005,
11, 3621.
(16) One of the referees suggested the possibility of the formation
of an alkynic ester in the cases where low activities for the
chloroesterification were observed (entries 4, 8, and 12 in
Table 2): Nozaki, K.; Sato, N.; Takaya, H. Bull. Chem. Soc.
Jpn. 1996, 69, 1629; however, no other by-products, except
the trimerized product, were found.
3ac: 1H NMR (300 MHz, CDCl3): d = 1.58 (m, 2 H), 1.68 (m,
2 H), 2.22 (d, J = 6.0 Hz, 4 H), 6.15 (s, 1 H), 6.77 (s, 1 H),
7.35 (m, 5 H) 13C NMR (75 MHz, CDCl3): d = 21.6, 22.5,
26.3, 66.3, 112.4, 128.3, 128.5, 128.7, 133.6, 136.0, 136.4,
Synlett 2008, No. 4, 551–554 © Thieme Stuttgart · New York

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