Lanthanide-Catalyzed Reversible Alkynyl Exchange by Carbon–Carbon Single-Bond Cleavage Assisted by a Secondary Amino Group

Article abstract of DOI:10.1002/anie.201605822

Lanthanide-catalyzed alkynyl exchange through C?C single-bond cleavage assisted by a secondary amino group is reported. A lanthanide amido complex is proposed as a key intermediate, which undergoes unprecedented reversible β-alkynyl elimination followed by alkynyl exchange and imine reinsertion. The in situ homo- and cross-dimerization of the liberated alkyne can serve as an additional driving force to shift the metathesis equilibrium to completion. This reaction is formally complementary to conventional alkyne metathesis and allows the selective transformation of internal propargylamines into those bearing different substituents on the alkyne terminus in moderate to excellent yields under operationally simple reaction conditions.

Full text of DOI:10.1002/anie.201605822

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Communications
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International Edition: DOI: 10.1002/anie.201605822
German Edition: DOI: 10.1002/ange.201605822
Synthetic Methods
Lanthanide-Catalyzed Reversible Alkynyl Exchange by Carbon–
Carbon Single-Bond Cleavage Assisted by a Secondary Amino Group
Yinlin Shao, Fangjun Zhang, Jie Zhang, and Xigeng Zhou*
Abstract: Lanthanide-catalyzed alkynyl exchange through
À
C C single-bond cleavage assisted by a secondary amino
group is reported. A lanthanide amido complex is proposed as
a key intermediate, which undergoes unprecedented reversible
b-alkynyl elimination followed by alkynyl exchange and imine
reinsertion. The in situ homo- and cross-dimerization of the
liberated alkyne can serve as an additional driving force to shift
the metathesis equilibrium to completion. This reaction is
formally complementary to conventional alkyne metathesis
and allows the selective transformation of internal propargyl-
amines into those bearing different substituents on the alkyne
terminus in moderate to excellent yields under operationally
simple reaction conditions.
T
he functionalization of unstrained carbon–carbon bonds is
Scheme 1. Alkyne cross-metathesis reactions.
one of the most desirable reactions in organic synthesis.
Alkyne metathesis has shown great promise over the past
three decades for the synthesis of natural products, macro-
cycles, and polymers.[1,2] However, there have been many
challenging issues in terms of the availability and perfor-
mance of catalysts,[3] substrate compatibility,[4] and selectivity
control.[5] In particular, the lack of predictability of selectivity
in intermolecular alkyne cross-metathesis severely limits its
practical use (Scheme 1a). The development of a predictive
model for cross-metathesis between two different alkynes is
highly desired.
À
the general belief that C C bond cleavage would be difficult
À
owing to unfavorable M C bond formation at the expense of
a stronger M O(N) bond.[6b] Herein, we report the first metal-
À
catalyzed b-carbon elimination of secondary amines and its
application in alkynyl exchange (Scheme 1b).
Organolanthanide catalysis exhibits distinct performance
when compared to transition-metal catalysis.[11–13] Thus,
preliminary studies examined the feasibility of the lantha-
nide-catalyzed alkynyl exchange of propargylamines with
terminal alkynes. It was found that the reaction of 2k with 1a
in toluene in the presence of [Lu{N(SiMe3)2}3] (Lu-1;
10 mol%) at 120–1308C afforded the desired product 2a in
85% yield (see the Supporting Information).
Metal-catalyzed b-carbon elimination and the retroally-
lation of secondary and tertiary alcohols as a strategy for the
À
cleavage of unstrained C C single bonds has been nicely
applied in organic synthesis.[6–8] However, no such method for
À
the C C cleavage of primary and secondary amines has been
Having optimized the reaction conditions, we proceeded
to explore the utility of these conditions for transformations
of different substrate combinations (Scheme 2). The treat-
ment of various internal propargylamines with excess 1a
afforded 2a in good to excellent yields, but the use of terminal
propargylamine 2m resulted in a low yield. Noticeably, the
reaction of 2h with 1a gave a small amount of enyne by-
products that resulted from dimerization of the liberated
reported, mainly owing to the more facile b-H elimination
pathways available for metal amides[9] and the potential
thermodynamic disadvantages. Currently, successful metal-
catalyzed processes are limited to tertiary amines and b-
carbon elimination.[10] Although alkoxide and amido com-
plexes of early transition metals are quite common, they have
been neglected for a long time as key intermediates in
catalytic b-carbon elimination processes, possibly because of
PhC CH with 1a, or with itself,[14] along with the formation of
ꢀ
2a in excellent yield, thus indicating that such a transforma-
tion of the liberated alkyne could potentially enable the
amount of alkyne substrate used to be decreased and improve
synthetic efficiency. However, the dimerization of aliphatic
alkynes is much less facile under the current conditions. For
example, only a trace amount enyne by-products was
observed in the reaction of 1a with 2d (see Scheme S1 in
the Supporting Information). A variety of other aliphatic
alkynes, 1b–f, reacted with internal propargylamines to give
the corresponding alkynyl-exchange products in 52–97%
yield. The reaction of 2a with 1e also proceeded at 1008C or
[*] Y. Shao, F. Zhang, Prof. Dr. J. Zhang, Prof. Dr. X. Zhou
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University
Shanghai 200433 (P.R. China)
E-mail: xgzhou@fudan.edu.cn
Prof. Dr. X. Zhou
State Key Laboratory of Organometallic Chemistry
Shanghai 200032 (P.R. China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605822.
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lene gave only trace amounts of the alkynyl-exchange
products.
A significant challenge in the development of metal-
À
catalyzed functionalization reactions of C C single bonds is
the control of reversibility. In most cases, the reactions are
irreversible.[6,15] Interestingly, many of the present reactions
can be reversed by a simple change in the molar ratio of the
reactants. For example, the reaction between 2a and aliphatic
alkynes 1c–f is reversible. The same trend was observed in the
exchange between two different aryl acetylenes (e.g., 2h vs.
2i). Moreover, the reversal of the aliphatic/aromatic alkynyl
exchange of 2h with 1d was also feasible. This unusual
reversibility will extend the changeability of the end-capping
groups of propargylic amines.
This reaction also appears to be general with regard to
secondary propargylamines containing various N-substitu-
ents. A number of synthetically useful N-substituents, such as
alkyl, aryl, and benzyl groups, were found to be compatible
with the reaction conditions (Scheme 3). The treatment of
diyne 1k with 0.1 equivalent of 2k afforded the mono-
alkynyl-exchange product 9k, whereas the reaction of 1k with
2 equivalents of 2d or 2k afforded the double-alkynyl-
exchange product 10k (Scheme 4). Moreover, this procedure
is amenable to the gram-scale synthesis of 2a from 2d (74%
yield). The structure of 2h·HCl was confirmed by X-ray
crystal diffraction analysis (see Figure S1 in the Supporting
Information).[17]
Propargylamines are versatile building blocks in organic
synthesis and a structural feature of many biologically active
compounds and natural products.[18,19] Although many meth-
ods for the synthesis of propargylamines have been devel-
Scheme 2. Lutetium-catalyzed reversible alkynyl exchange of propargyl-
amines with terminal amines. Reaction conditions: 1 (0.30 mmol), 2
(1.50 mmol), Lu-1 (0.030 mmol), toluene (3 mL), sealed Schlenk tube
equipped with a Teflon cap under N2, 1308C, 12 h. Yields are for the
isolated product. [a] The reaction was carried out with 3 equivalents of
1. [b] The reaction was carried out at 1208C. [c] Recovery of 2g. [d] The
reaction was carried out with 10 equivalents of 1. [e] The reaction was
carried out with 2 equivalents of 1. [f] The reaction was carried out at
1008C for 48 h. Bn=benzyl, TMS=trimethylsilyl.
even with a substrate ratio of 1:2. Significantly, a cyclopropyl
group survived the catalytic conditions, which is not possible
À
in metal-catalyzed unstrained C C single-bond cleavage by
oxidative addition.[6,15] When 1g was employed as a metathesis
partner, the Me3Si-substituted propargylamine 2g, which is
valuable as a precursor to terminal propargylamines[16a] and as
a synthetic intermediate,[16b,c] was obtained in 65% yield.
However, when 2g was used as the substrate, the alkynyl
exchange did not proceed under the same reaction conditions.
Aryl acetylenes underwent the transformation, but the
yields varied depending on the propargylamine employed.
The reactions of 1h–j with internal propargylamines (2d, 2k,
2l, 2h, and 2i) gave 2h–j in moderate to excellent yields. The
major by-products formed in these reactions result from di-
and oligomerization of the starting aryl acetylene (see
Scheme S1), which will decrease the amount of alkynyl-
exchange products or suppress the alkyne-exchange process.
The treatment of 2k with 2-thiophenyl- and 2-pyridylacety-
Scheme 3. Effect of the N-substituent on the lutetium-catalyzed alkynyl
exchange. Yields are for the isolated product. [a] The reaction was
carried out with 10 equivalents of 1b.
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Scheme 4. Double alkynyl exchange.
oped,[20] they require the use of stoichiometric quantities of
Lewis acids and bases or show limited substrate generality in
some cases. Clearly, the present reaction provides an efficient
way to convert readily accessible propargylamines into those
bearing a different substituent on the alkyne terminus.
To clarify the mechanism, we investigated the reactions
shown in Scheme 5. As expected, the reaction of 8h with an
Scheme 6. Confirmation of the involvement of lutetium amido inter-
mediates.
an imine and an enyne could be detected when Lu-1 was
treated with a large excess of 2 f in toluene at 1308C (see
Scheme S10). These results demonstrate that the formation
and b-alkynyl elimination of lutetium–propargylamide inter-
mediates are viable under the current conditions, and that the
free alkyne and imine may compete for insertion into the
lutetium–alkynyl bond. However, attempts to isolate or
detect lutetium–alkynyl and lutetium–alkenyl complex inter-
1
mediates by H NMR spectroscopy failed. Instead, we found
[21]
ꢀ
that [Cp2Er(C CtBu)]
can also catalyze the alkynyl
exchange of 2 f with 1a. Furthermore, we examined the
reaction of tertiary propargylamine 12 with 1h, but no
alkynyl-exchange product was obtained (Scheme 6d). This
result excludes the involvement of iminium intermediates[10]
and demonstrates that the deprotonation of propargylamines
to form lutetium–propargylamide species might be an essen-
tial step for the alkynyl exchange.
Our proposed catalytic cycle for this transformation is
shown in Scheme 7. Initial deprotonation of propargylamine 2
by the metal species provides lanthanide amide A, which
undergoes reversible b-alkynyl elimination to form the key
intermediate B. The resulting alkynyl ligand of B is exchanged
3
À
Scheme 5. Confirmation of C(sp) C(sp ) bond cleavage.
imine afforded the product of imine-fragment exchange 6h,
albeit in low yield (Scheme 5a). Furthermore, racemization
was found with the optically active substrate (R)-11k
(Scheme 5b). Compound 6e was obtained in 20% yield
upon the treatment of a mixture of 1e with N-phenylmethan-
imine in the presence of Lu-1 (10 mol%) in toluene at 1308C
(Scheme 5c). When the terminal alkyne was replaced with an
internal alkyne, however, no reaction occurred (Scheme 5d).
These results suggest that the substitution involves the
with terminal alkyne 1’ to afford alkynide C. Imine-fragment
[20a,b]
À
reinsertion into the Ln C bond gives lanthanide amide D.
ꢀ
aminated carbon atom rather than the C C triple bond.
Finally, the protonation of D by another propargylamine
molecule affords the alkynyl-exchange product 2’ and regen-
erates the active intermediate A. b-Carbon elimination of
metal amides is unprecedented. We note also that the b-
carbon elimination of secondary propargylic amines would
probably be more challenging because of the preponderance
of the intermolecular hydroamination/cyclization under the
conditions involved.[11c] Presumably, the major driving forces
for the reaction are the kinetically and/or thermodynamically
favorable alkynyl-ligand exchange and subsequent regener-
1H NMR monitoring revealed that the addition of Lu-
1 (10 mol%) to a mixture of 1e and 2k in toluene at 1208C
led to the complete liberation of the (Me3Si)2n ligand (a new
peak at 0.08 ppm) from the precatalyst,[12d] and 2k was cleanly
converted into 2e (see Figure S2).
When we examined the stoichiometric reaction of Lu-
1 and 7h, Lu-2 was obtained in 65% yield (Scheme 6a). It was
found that the activity of Lu-2 for the catalytic alkynyl
exchange of 7h with 1a was nearly same as that of Lu-
1 (Scheme 6b). Notably, the treatment of a mixture of 2k and
1a with Lu-2 generated the product 7a from b-alkynyl
elimination of Lu-2 (Scheme 6c). Moreover, heating of the
solution of [Lu{N(Bn)CH2C CPh)}3] in toluene at 1308C,
followed by hydrolysis, afforded a significant amount of
phenylacetylene (see Scheme S9), whereas small amounts of
À
À
ation of stable C C and Ln N bonds. Similar b-alkynyl
elimination was observed for related rhodium alkoxides.[7b] It
is clear that the sluggish dimerization of the liberated alkyne
with the starting alkyne or with itself in situ (side reaction (i))
can serve as an additional driving force to shift the metathesis
equilibrium to completion,[14] whereas a competing homo-
ꢀ
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and the Doctoral Program of Higher Education of China for
financial support.
À
Keywords: alkynes · amines · C C bond cleavage · lanthanides ·
reaction mechanisms
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Scheme 7. Proposed mechanism for the alkynyl exchange.
dimerization and oligomerization of aryl acetylene substrates
(side reaction (ii)) will disfavor the alkynyl exchange and
consequently decrease the amount of alkynyl-exchange
product formed or suppress the alkyne-exchange processes.
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exchange have been established, thus providing an efficient
way to convert internal propargylamines into other propar-
gylamines bearing different substituents on the alkyne
terminus. Mechanistic studies have shed light on the possible
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reconstruction of alkynes, this reaction has significant advan-
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Experimental Section
General procedure:
0.030 mmol), a propargylamine (0.30 mmol),
(1.5 mmol), and toluene (3 mL) in a Teflon-valve-sealed Schlenk
tube was heated at 100–1308C (oil-bath temperature) under N2. After
completion of the reaction, the crude product was purified by flash
column chromatography on silica gel with ethyl acetate/hexane as the
eluent.
A
mixture of [Lu{N(SiMe3)2}3] (19.7 mg,
terminal alkyne
a
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Acknowledgments
We thank the National Natural Science Foundation of China,
the “973” Project from the MOST of China (2012CB821600),
4
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Received: June 16, 2016
Published online: && &&, &&&&
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Communications
Synthetic Methods
Y. Shao, F. Zhang, J. Zhang,
X. Zhou*
&&&& — &&&&
Lanthanide-Catalyzed Reversible Alkynyl
Exchange by Carbon–Carbon Single-Bond
Cleavage Assisted by a Secondary Amino
Group
A good swap: Lanthanide-catalyzed
yields. As an alternative to metathesis for
the reconstruction of alkynes, this reac-
tion has significant advantages, such as
broad scope and excellent control of
selectivity.
À
alkynyl exchange through C C single-
bond cleavage enabled the selective
transformation of internal propargyla-
mines into differently substituted
propargylamines in moderate to excellent
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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