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10.1002/anie.202006366
Angewandte Chemie International Edition
COMMUNICATION
Exploitation of the New Reactivity of Vinylcyclopropanes for
Palladium-Catalyzed, Asymmetric [5+2] Dipolar Cycloadditions
Miao-Miao Li,[a] Qin Xiong,[b] Bao-Le Qu,[a] Yu-Qing Xiao,[a] Yu Lan*,[b,c] Liang-Qiu Lu,*,[a,d] and Wen-Jing
Xiao[a]
Abstract: Vinylcyclopropanes (VCPs) are commonly used in
transition metal-catalyzed cycloadditions, and the exploitation of their
recently realized reactivities to construct new cyclic architectures is
of great significance in modern synthetic chemistry. Herein, a
demonstrated that Ni and Pd complexes are efficient catalysts
for ketene cycloadditions, although the ketenes tend to
decompose upon exposure to these catalysts. During this study,
we found that the in situ, controlled generation of ketenes via the
photoinduced Wolff rearrangements of α-diazoketones[12] was
helpful for solving this problem. As a continuation of focus on
transition metal-catalyzed dipolar cycloadditions,[13,14] herein, we
designed a new Pd-catalyzed, visible light-driven asymmetric
[5+2] cycloaddition[15] of VCPs with α-diazoketones. As shown in
Scheme 1B, the key feature of this designed reaction is the
switch of the reaction mode of VCPs: Pd-containing dipolar int. II,
an oxo-1,5-dipole, was proposed to react with photogenerated
ketenes, although its isomer I, an all-carbon 1,3-dipole, is
commonly used in [3+n] cycloadditions.[5,6] Although this
proposed reaction is deceivingly simple, there are several
challenges to achieving the desired chemo- and peri-selectivities.
First, the intramolecular cyclization of int. II would strongly
compete with intermolecular cycloaddition. Second, the initial
addition to the ketenes at the C- or O-position of int. II[11e] and
the following allylation in a linear or branched manner would give
different cycloadducts. Third, the asymmetric construction of two
stereocenters, including a chiral quaternary stereocenter, is a
notable challenge.
palladium-catalyzed,
visible light-driven,
asymmetric [5+2]
cycloaddition of VCPs with α-diazoketones was first accomplished
by switching the reactivity of the Pd-containing dipolar intermediates
from all-carbon 1,3-dipoles to oxo-1,5-dipoles. Enantioenriched 7-
membered lactones were produced with good reaction efficiency and
selectivity (23 examples, 52-92% yields with up to 99:1 er and 12.5:1
dr). In addition, computational investigations were performed to
rationalize the observed high chemo- and periselectivities.
In recent decades, vinylcyclopropanes (VCPs) have been
thoroughly studied particularly in transition metal-catalyzed
cycloadditions to access a variety of cyclic compounds.[1] In
these cycloadditions, VCPs can act as either 5-carbon
synthons[2] (such as in [5+1], [5+2] and [5+2+1] cycloadditions),
or as 3-carbon synthons[3] (such as in [3+2] and [3+2+1]
cycloadditions) majorly via Rh catalysis (Scheme 1A, left).[4]
Most of these used VCPs can be regarded as electron-neutral
synthons because no activation groups are in the cyclopropane
rings. In 1985, Tsuji and co-workers discovered that activated
VCPs, in which the cyclopropane is activated by two electron-
withdrawing groups, can react with electro-deficient alkenes to
give [3+2] cycloadducts under Pd catalysis.[5] In this reaction,
Pd(0) catalysts react with VCPs by forming allyl-Pd-containing
dipolar species, which then undergo formal [3+2] cycloadditions.
In the following decades, to the best of our knowledge, all
reported activated VCPs under Pd catalysis gave [3+2]
cycloadducts (Scheme 1A, right).[6,7] Here we report that the
activated VCPs can act as hybrid 5-atom synthons to react with
in situ generated ketenes to give 7-membered lactones. Also a
mechanistic understanding of this new chemistry is presented.
Ketenes as versatile intermediates[8] are traditionally used in
the organocatalytic[8] and Lewis acid-catalyzed[9] [n+2]
cycloadditions (n = 2~4). Recently, Louie[11a] and our group[11b-d]
[a]
[b]
M.-M. Li, B.-L. Qu; Y.-Q. Xiao, Prof. L.-Q. Lu, Prof. W.-J. Xiao
CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide
& Chemical Biology, Ministry of Education, College of Chemistry,
Central China Normal University, 152 Luoyu Road, Wuhan, Hubei
430079, China
E-mail: luliangqiu@mail.ccnu.edu.cn
Q. Xiong, Prof. Y. Lan
School of Chemistry and Chemical Engineering, Chongqing
University, Chongqing 400030, China
Email: lanyu@cqu.edu.cn
Scheme 1. Design plan based on the new reaction mode of VCPs.
To implement this idea, we examined the reaction of
vinylcyclopropane 1a’ and α-diazoketone 2a under irradiation by
6 W blue LEDs and with a combination of Pd2dba3•CHCl3 and
our chiral P,S ligand L1[11a] as the chiral Pd catalyst. To our
disappointment, it failed to give the desired cycloadduct except
the intramolecular cyclization product (Table 1, entry 1).[16a,b]
Inspired by an elegant work from Xu and coworkers on the
organocatalytic rearrangements of VCPs to cycloheptenones,[16c]
we guessed that the dipolar intermediate formatted from
substrate 1a and the Pd(0) catalyst would be relatively stable,[17]
thus preventing the unfavorable intramolecular cyclization
process. If this is the case, this kind of VCPs might be suitable
[c]
[d]
Prof. Y. Lan
College of Chemistry, and Institute of Green Catalysis, Zhengzhou
University, Zhengzhou, Henan, 450001, China
Prof. L.-Q. Lu
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, P. R. China
Supporting information for this article is given via a link at the end of
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Lan, Yu
