Generation of rhodium(I) carbenes from ynamides and their reactions with alkynes and alkenes

Article abstract of DOI:10.1021/ja4047069

Rh(I) carbenes were conveniently generated from readily available ynamides. These metal carbene intermediates could undergo metathesis with electron-rich or neutral alkynes to afford 2-oxopyrrolidines or be trapped by tethered alkenes to yield 3-azabicyclo[3.1.0]hexanes, a common skeleton in numerous bioactive pharmaceuticals. Although the scope of the former is limited, the latter reaction tolerates various substituted alkenes.

Full text of DOI:10.1021/ja4047069

Communication
pubs.acs.org/JACS
Generation of Rhodium(I) Carbenes from Ynamides and Their
Reactions with Alkynes and Alkenes
Renhe Liu,† Gabrielle N. Winston-McPherson,† Zhong-Yue Yang,‡ Xin Zhou,† Wangze Song,†
Ilia A. Guzei,§ Xiufang Xu,*,‡ and Weiping Tang*,†,§
†School of Pharmacy, University of Wisconsin, Madison, Wisconsin 53705, United States
‡Department of Chemistry, Nankai University, Tianjin 300071, P. R. China
§Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
S
* Supporting Information
generated from ynamides 6 and 7 and then react with the
ABSTRACT: Rh(I) carbenes were conveniently gener-
ated from readily available ynamides. These metal carbene
intermediates could undergo metathesis with electron-rich
or neutral alkynes to afford 2-oxopyrrolidines or be
trapped by tethered alkenes to yield 3-azabicyclo[3.1.0]-
hexanes, a common skeleton in numerous bioactive
pharmaceuticals. Although the scope of the former is
limited, the latter reaction tolerates various substituted
alkenes.
tethered alkyne or alkene to afford heterocycles 8 and 9,
respectively (eq 1). In contrast, keto imide 10 was often the
predominant product observed by us and others in the
presence of gold(I) catalysts.6
etal carbenes are among the most important inter-
Mmediates in organic synthesis and are involved in
numerous reactions such as cyclopropanation, C−H insertion,
dipolar cycloaddition, and metathesis reactions.1 The most
frequently used method for the preparation of metal carbenes is
the decomposition of diazo compounds or related derivatives.
In view of the hazardous and potential explosive nature of diazo
compounds, other alternative carbene precursors are highly
desirable. Recently, readily available ynamides2,3 have emerged
as versatile carbene precursors, and in particular, oxidation of
ynamide 1 by dimethyl dioxirane (DMDO)4 or other oxidants5
was shown to afford the push−pull α-oxo carbene 3 through
oxirene intermediate 2 (Scheme 1). Interestingly, a comple-
mentary α-oxo gold carbene, 5 (M = Au), was formed via
intermediate 4 in the presence of gold catalysts and mild
external oxidants (e.g., pyridine N-oxide).6,7
Our interest in the chemistry of cyclopropyl metal carbenes8
and π-acidic Rh(I) complexes9 prompted us to investigate the
possibility of generating Rh(I) carbenes10 from ynamide 6a for
cycloisomerization reactions. We prepared ynamide 6a and
then treated it with pyridine N-oxide and a complex formed
from [Rh(CO)2Cl]2 and P[OCH(CF3)2]3 (eq 2); this complex
We envisioned that the choice of metal catalyst and ligand
would have a significant impact on the reactivity of carbene 5.
We herein report that α-oxo Rh(I) carbenes 5 (M = Rh) can be
was shown to be successful in promoting acyloxy migration of
propargylic esters,11 a process that is typically catalyzed by π-
acidic transition metals.9,12 Compound 8a was isolated as the
major product, and cyclobutene 11 was also observed as the
minor product. A metal carbene similar to intermediate 5 was
presumably generated from ynamide 6a through the mecha-
nism shown in Scheme 1. Ring expansion of this metal carbene
intermediate would produce cyclobutene 11.8
Scheme 1. Complementary Carbenes from Ynamides
When the cyclopropyl group was replaced by other
substituents, the formation of cyclobutene was avoided, and a
Received: May 10, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4047069 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
62% yield of cycloisomerization product 8b was obtained from
diyne 6b under the conditions employed for diyne 6a (Table 1,
Table 2. Oxidative Cycloisomerization of Diynes
Table 1. Screening of Conditions for the Oxidative
a
Cycloisomerization of Diyne 6
b
c
Entry
1
Conditions
8b:12b Yield
[Rh(CO)2Cl]2 (5 mol %),
P[OCH(CF3)2]3 (20 mol %)
1:0
62%
2
3
4
[Rh(CO)2Cl]2 (5 mol %)
5:1
−
[Rh(CO)2Cl]2 (5 mol %), P(OPh)3 (20 mol %)
no reaction
[Rh(CO)2Cl]2 (5 mol %),
[3,5-(CF3)2C6H3]3P (20 mol %)
5:1
−
5
6
7
Rh2(OAc)4 (5 mol %)
no reaction
0:1
complex mixture
Au(PPh3)Cl (10 mol %), AgOTf (10 mol %)
PtCl2 (5 mol %)
−
a
d
8
de
,
see entry 1 for catalyst
1:0
1:0
70%
78%
Conditions: [Rh(CO)2Cl]2 (5 mol %), P[OCH(CF3)2]3 (20 mol %),
b
3,5-dichloropyridine N-oxide (3 equiv), dioxane, 80 °C, 4 h. Isolated
yields. Yield of byproduct 13. Room temperature, 3 h. Yield of
byproduct 14.
9
see entry 1 for catalyst
c
d
e
a
Conditions: pyridine N-oxide (3 equiv), ClCH2CH2Cl, 80 °C, 4 h,
b
unless noted otherwise. Determined by 1H NMR analysis of the
crude product. Isolated yields of 8b. 3,5-Dichloropyridine N-oxide
(3 equiv) was used as the oxidant. Dioxane was used as the solvent.
c
d
e
optimized conditions for oxidative cycloisomerization of diynes,
cyclopropanation product 9a was obtained in 78% yield from
enyne 7a. No improvement was observed when different Rh(I)
complexes, ligands, or other substituted pyridine N-oxides were
screened. When Au(PPh3)Cl/AgOTf was employed as the
catalyst, keto imide 15a was isolated as the major product
together with small amount of cyclopropane 9a. No reaction
occurred when Rh2(OAc)4 was used as the catalyst.
The scope of oxidative cycloisomerization of enyne 7 was
then investigated (Table 3). For terminal ynamides 7b−e, the
reaction could be conducted at room temperature. Among
these, terminal alkene 7b produced the best yield. A 70% yield
was obtained for cyclopropanation of nonsubstituted styrene
7c. For diyne 6, R2 must be an electron-neutral or -rich aryl
group to allow the oxidative cycloisomerization to proceed. On
the other hand, it has been shown that the α-oxo Rh(I) carbene
derived from 1,2-acyloxy migration of propargylic esters reacts
only with electron-deficient alkynes or alkenes.15 To our
delight, both electron-rich and electron-poor styrenes under-
went cyclopropanation, forming bicyclic products 9d and 9e,
respectively. It is worth pointing out that a nitro group, which
has been used as external oxidant in gold catalysis,7r can be
tolerated.
entry 1). The structure of product 8b was unambiguously
assigned by X-ray analysis.13 The oxidative cycloisomerization
of diyne 6b was then optimized under different conditions.
Keto imide 12b appeared when the phosphite ligand was
removed (entry 2).14 Other ligands produced worse results
(entries 3 and 4). No reaction occurred when Rh2(OAc)4 was
used as the catalyst (entry 5). Interestingly, only keto imide 12b
was observed when a gold catalyst was employed (entry 6).
Using PtCl2 as the catalyst provided a complex mixture (entry
7). We then examined different substituted pyridine N-oxides,
such as p-methoxy-, p-nitro-, 2,6-dichloro-, and 3,5-dichlor-
opyridine N-oxide. Among them, 3,5-dichloropyridine N-oxide
provided the best result (entry 8). Among all of the solvents
that we screened, dioxane afforded the highest yield (entry 9).
No desired product was obtained when other rhodium
complexes, such as [Rh(COD)Cl]2 (COD = 1,5-cyclo-
octadiene), Wilkinson’s catalyst, and [Rh(COD)BF4] were
employed.
With the optimized conditions in hand, we investigated the
scope of the oxidative cycloisomerization of diyne 6 (Table 2).
The yield of product 8c (R2 = Ph) was slightly lower than that
of 8b. The reaction tolerated substrates with an ortho-
substituted aryl group as R2 or R1 (substrates 6d and 6e,
respectively). When R2 was an electron-deficient aryl group
(e.g., p-ClC6H4, p-NO2C6H4), a complex mixture was obtained.
For R2 = Me (substrate 6f), diene 13 was isolated in 10% yield.
For R1 = H (substrate 6g), the carbene became more reactive,
and the reaction could be carried out even at room
temperature. When R1 was an alkyl group (substrate 6h), 1,2-
hydrogen migration product 14 was observed. For substrates 6i
and 6j with a more functionalized R1 or R2 substituent, complex
mixtures were observed.
Alkene and ether functionalities in the R1 substituent can be
tolerated (substrate 7f). Surprisingly, when R1 was an alkyl
group (substrate 7g), hydrolysis product 16 was obtained. A
complex mixture was observed when an olefin functional group
was introduced in the R2 substituent (substrate 7h). Alkenes
with gem-dimethyl substitution (7i) or a methyl group at the
internal position (7j) participated in the cyclopropanation,
yielding products 9i and 9j, respectively. A six-atom tether was
also tolerated, affordinig bicyclic product 9k in 78% yield.
It is generally easier to remove a nosyl group than a tosyl
group.16 We were pleased to find that the yields of products 9l
and 9m were comparable to those of their tosyl counterparts
9a and 9b. The nosyl group in product 9o was removed
We then examined the reactivity of α-oxo Rh(I) carbenes
toward alkenes (Table 3, first entry). Under the previously
B
dx.doi.org/10.1021/ja4047069 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 3. Oxidative Cycloisomerization of Enynes
Scheme 2. Proposed Mechanisms for Oxidative
Cycloisomerizations of (a) Diynes and (b) Enynes
through metallacyclobutene 19 may afford a new carbene, 20,21
which can be oxidized by pyridine N-oxide to yield product 8.
When R1 or R2 was an alkyl group, 1,2-hydrogen migration
produced small amount of byproduct 14 or 13, respectively.
Density functional theory (DFT) calculations indicated that the
barrier for the conversion of 18 to 20 is 20.7 kcal/mol and that
this process can take place at room temperature.13 Rh(I)
carbene 21 may react with the tethered alkene to form product
9b via two pathways: (I) a concerted cyclopropanation and (II)
metathesis followed by reductive elimination through inter-
mediate 22. DFT calculations suggested that pathway I is
preferred because the reductive elimination from intermediate
22 in pathway II is relatively difficult.13
In summary, we have developed an efficient method for the
generation of α-oxo Rh(I) carbenes from ynamides.10 We have
demonstrated that the [Rh(CO)2Cl]2/P[OCH(CF3)2]3 com-
plex is acidic enough to mediate the addition of an external
oxidant to ynamides under mild conditions. The Rh(I)
carbenes produced from this process can then react with
electron-rich or neutral alkynes or various substituted alkenes
to afford 2-oxopyrrolidines and 3-azabicyclo[3.1.0]hexanes,
respectively. Further studies of the novel reactivity of Rh(I)
carbenes are underway in our laboratory.
a
Conditions: [Rh(CO)2Cl]2 (5 mol %), P[OCH(CF3)2]3 (20 mol %),
3,5-dichloropyridine N-oxide (1.0 equiv), dioxane, room temperature,
2 h. Isolated yields. 80 °C, 4−8 h.
b
c
smoothly under mild conditions to yield compound 17 (eq
ASSOCIATED CONTENT
3),16 which is the precursor for the triple-reuptake inhibitor
■
S
* Supporting Information
Detailed experimental procedures, characterization data, and
1
spectra (IR, H and 13C NMR, and HRMS). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
xxfang@nankai.edu.cn; wtang@pharmacy.wisc.edu
■
DOV216,303.17,18 The 3-azabicyclo[3.1.0]hexane skeleton19 is
also present in numerous other pharmaceuticals with broad
biological activities, such as analgesics, antibacterials, and
aromatase inhibitors.17,20
The proposed mechanisms for the oxidative cycloisomeriza-
tions of diynes and enynes are shown in Scheme 2. Metal
carbenes 18 and 21 can be generated from ynamides 6 and 7b,
respectively, following the mechanism shown in Scheme 1.
Metathesis of metal carbene 18 with the tethered alkyne
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NIH (R01GM088285 to W.T.), the University of
Wisconsin, and the National Natural Science Foundation of
China (21103094 to X.X.) for financial support of this research
and Professor Hsung (UW-Madison) for reading and
commenting on our manuscript.
C
dx.doi.org/10.1021/ja4047069 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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Ed. 2012, 51, 7503. (c) Liu, R.; Zhang, M.; Winston-McPherson, G.;
Tang, W. Chem. Commun. 2013, 49, 4376.
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