A straightforward and mild synthesis of functionalized 3-alkynoates

Article abstract of DOI:10.1002/anie.200454070

Diazoacetates in coupling reactions: CuI serves as an effective catalyst for coupling terminal alkynes with diazo compounds to generate 3-alkynoates (see scheme). This method is efficient (1:1 ratio of reactants), mild (room temperature), and simple (no a

Full text of DOI:10.1002/anie.200454070

Communications
Coupling Reactions
A Straightforward and Mild Synthesis of
Functionalized 3-Alkynoates**
AndrØs Suµrez and Gregory C. Fu*
The methods that have been described to date for the
synthesis of 3-alkynoates generally require multiple steps and/
or highly basic reaction conditions.[1] One very attractive
route to 3-alkynoates would be by the coupling of readily
available terminal alkynes with diazoesters. Unfortunately, in
the absence of a catalyst, treatment of alkynes with diazo
À
compounds preferentially generates cyclopropenes and Calkyl
H insertion products.[2] With respect to metal-catalyzed
processes,[3] in the presence of rhodium complexes, reactions
of terminal alkynes with diazo compounds lead not to
À
insertion into the Calkynyl H bond, but typically to cyclo-
propenation.[4] In the case of copper, the only synthetically
useful couplings that have been reported are also methods for
cyclopropenation;[5] a fewgroups have observed the forma-
tion of a 3-alkynoate, but none of the procedures is general,
elevated temperatures are employed, and the yields are
modest (0–45%).[6]
In this communication we describe the development of a
versatile method for coupling alkynes with diazocarbonyl
compounds to produce 3-alkynoates [Eq. (1)]. Some note-
worthy attributes of the system include its efficiency (1:1 ratio
of reactants), mildness (room temperature), simplicity (no
additional ligand), and functional-group tolerance (e.g.,
alkenes, heteroatoms, and hydroxy groups).
In an initial study, we explored the reaction of phenyl-
acetylene with ethyl diazoacetate. In the presence of copper
complexes such as CuCl, CuOTf, CuCN, CuSCN, Cu2O, and
CuOAc, we obtain essentially none of the target 3-alkynoate
[*] Dr. A. Suµrez, Prof. Dr. G. C. Fu
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-324-3611
E-mail: gcf@mit.edu
[**] Support has been provided by the National Institutes of Health
(National Institute of General Medical Sciences, R01-GM066960),
the Spanish Ministry of Education, Culture, and Sports (postdoc-
toral fellowship to A.S.), Merck Research Laboratories, and
Novartis. Funding for the MITDepartment of Chemistry Instru-
mentation Facility has been furnished in part by NSF CHE-9808061
and NSF DBI-9729592.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200454070
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Angewandte
Chemie
(Table 1, entry 1). In contrast, CuBr and CuI are effective
catalysts for the desired transformation (entries 2 and 3). The
choice of solvent is critical: use of iPrOH, THF, dioxane,
acetone, or CHCl3 leads to a significantly lower yield
(< 10%).[7]
suitable substrates. Formation of the desired 3-alkynoate
proceeds in preference not only to cyclopropenation of an
alkyne, but also to cyclopropanation of an alkene (entries 6
and 7). Furthermore, neither Lewis-basic groups such as
sulfides and ethers, which could react with a copper carbene
to form ylides (entries 8–10),[11] nor hydroxy groups, which
could undergo O H insertion (entry 11),[12] interfere with the
reaction.
À
Table 1: Catalysts tested for the synthesis of 3-alkynoates 1 from alkynes
and diazoesters.
In certain instances, we have experienced difficulty
removing the allene side product from the target 3-alkynoate
(e.g., Table 2, entries 3, 4, 10, and 11). To address this
purification issue, as well as to explore the scope of the
coupling with respect to other diazo compounds, we examined
the reaction of alkynes with a diazoamide (N2CHCONMe2).
We were pleased to observe that CuI is also an effective
catalyst for these couplings (Table 3);[13] as for reactions of
diazoesters, a small amount of the allene is produced, but this
is readily removed by flash chromatography.
Entry Catalyst
1 [%][a] 2 [%][a]
1
2
3
CuCl, CuOTf, CuCN, CuSCN, Cu2O, CuOAc
CuBr
Cul
<5
70
90
<5
5
8
[a] Yields were determined by NMR spectroscopy vs. an internal
standard (average of two runs).
Table 3: Copper-catalyzed couplings of alkynes with a diazoamide.
Interestingly, cyclopropenation of the alkyne is not a
detectable side reaction for the CuBr- or CuI-catalyzed
processes (< 5%). Furthermore, the formation of fumarate
and maleate side products, a complication that plagues many
copper-catalyzed reactions of diazoesters, is minimal (< 5%),
thereby allowing the use of a 1:1 mixture of alkyne:diazoester,
without slow addition of the diazoacetate.[8] We do, however,
observe a small quantity of the allene isomer (2) of the 3-
alkynoate.[9]
We have determined that CuI-catalyzed coupling of
terminal alkynes with diazoesters provides a general route
to 3-alkynoates (Table 2).[10] Thus, aryl- (entry 1), alkyl-
(entry 2), hindered alkyl- (entry 3), functionalized alkyl-
(entry 4), and silyl-substituted (entry 5) alkynes are all
Entry
R
Yield [%][a]
1[b]
2
3
4
5
tBu
70
71
57
81
54
CH2CH2CO2Et
CH2OMe
CH2OTHP
CH2OH
[a] Yields of isolated products (average of two runs). [b] 1.1 equiv of
alkyne was used.
In conclusion, a general, metal-catalyzed intermolecular
coupling of terminal alkynes with diazo compounds has been
developed, providing ready access to 3-alkynoates. It is
noteworthy that this carbon–carbon bond-forming reaction
proceeds efficiently under nonbasic conditions at room
temperature with a simple and inexpensive catalyst, and
that an array of potentially reactive functional groups are
tolerated.
Table 2: Copper-catalyzed couplings of alkynes with
[Eq. (1)].
a
diazoester
Yield [%][a]
Entry
R
1
Ph
76
2
CH2Ph
tBu
CH2CH2CO2Et
TMS
74
3[b,c]
4
98[e]
87[e]
84
Received: February 21, 2004 [Z54070]
5[b]
Keywords: alkynes · copper · diazo compounds ·
homogeneous catalysis
.
6
71
7
58
8
9
10
11
CH2SPh
56
[1] For some examples, see: a) S.-i. Usugi, H. Yorimitsu, H.
Shinokubo, K. Oshima, Bull. Chem. Soc. Jpn. 2002, 75, 2687 –
2690; b) R. M. Williams, D. J. Aldous, S. C. Aldous, J. Org.
Chem. 1990, 55, 4657 – 4663; c) J. Hooz, R. B. Layton, Can. J.
Chem. 1972, 50, 1105 – 1107.
[2] For a discussion, see: O. M. Nefedov, E. A. Shapiro, A. B.
Dyatkin in Supplement B: The Chemistry of Acid Derivatives,
Vol. 2 (Ed.: S. Patai) Wiley, NewYork, 1992, pp. 1527 – 1528.
CH2SEt
63
CH2OTHP[d]
CH2OH
87[e]
75[f]
[a] Yields of isolated products (average of two runs), unless otherwise
noted. [b] 10% Cul was used. [c] 1.2 equiv of alkyne was used. [d] THP=
tetrahydropyranyl. [e] Contains 3–5% of the allene isomer. [f] Yield was
determined by NMR spectroscopy vs. an internal standard.
Angew. Chem. Int. Ed. 2004, 43, 3580 –3582
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Communications
[3] M. P. Doyle, M. A. McKervey, T. Ye in Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds, Wiley,
NewYork, 1998.
[4] For leading references, see: a) reference [3], Chapter 4.11;
b) M. N. Protopopova, E. A. Shapiro, Russ. Chem. Rev. 1989,
58, 667 – 681.
[5] For a discussion and leading references, see: M. M. Díaz-
Requejo, M. A. Mairena, T. R. Belderrain, M. C. Nicasio, S.
Trofimenko, P. J. PØrez, Chem. Commun. 2001, 1804 – 1805.
[6] a) V. E. Baikov, L. P. Danilkina, K. A. Ogloblin, Russ. J. Gen.
Chem. 1983, 53, 2172 – 2173; b) E. A. Shapiro, I. E. Dolgii, O. M.
Nefedov, Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.)
1980, 29, 1493 – 1499; c) I. E. Dolgii, E. A. Shapiro, O. M.
Nefedov, Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.)
1974, 23, 929; d) M. Vidal, M. Vincens, P. Arnaud, Bull. Soc.
Chim. Fr. 1972, 657 – 665; e) V. K. Jones, A. J. Deutschman Jr. , J.
Org. Chem. 1965, 30, 3978 – 3980.
[7] The observed solvent effects may be due in part to the relative
solubility of CuI.
[8] For a discussion of this problem, see p. 6 of: T. Rovis, D. A.
Evans, Prog. Inorg. Chem. 2001, 50, 1 – 150. The typical remedy
is to use a large excess of the substrate (e.g., as the solvent) and
to employ slowaddition of the diazo compound.
[9] The 3-alkynoate:allene ratio does not change as the reaction
progresses, which suggests that the 3-alkynoate is not an
intermediate in the formation of the allene.
[10] The isomeric allene is the only identifiable side product.
[11] For an early example, see: G. Pourcelot, L. Veniard, P. Cadiot,
Bull. Soc. Chim. Fr. 1975, 1281 – 1283. See also reference [3],
Chapter 7.
[12] This result is particularly noteworthy in light of a recent report of
À
selective O H insertion upon reacting propargyl alcohol with
ethyl diazoacetate in the presence of a CuTp catalyst (Tp = a
homoscorpionate ligand): M. E. Morilla, M. J. Molina, M. M.
Díaz-Requejo, T. R. Belderrain, M. C. Nicasio, S. Trofimenko,
P. J. PØrez, Organometallics 2003, 22, 2914 – 2918.
[13] We have not optimized the reaction conditions for the reactions
in Table 3; they are the same as those used for the reactions in
Table 2.
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Angew. Chem. Int. Ed. 2004, 43, 3580 –3582