Reaction of allylic phosphoranes with iron porphyrin carbenoids: Efficient, selective, and catalytic intermolecular formal carbenoid insertion into olefinic C-H bonds

Full text of DOI:10.1021/ja8097959

Published on Web 03/05/2009
Reaction of Allylic Phosphoranes with Iron Porphyrin Carbenoids: Efficient,
Selective, and Catalytic Intermolecular Formal Carbenoid Insertion into
Olefinic C-H Bonds
Sunewang R. Wang, Chun-Yin Zhu, Xiu-Li Sun, and Yong Tang*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Lu, Shanghai 200032, China
Received December 15, 2008; E-mail: tangy@mail.sioc.ac.cn
An ylide, LnM+-C-HR, can be generally regarded as a carbanion
Scheme 2. Catalytic Carbenoid Insertion into the Olefinic C-H
bearing a unique leaving group.1 Traditionally, ylides can react with
Bond of Allylic Ylide 1a
electrophilic carbonyl compounds, imines, and electron-deficient
olefins, giving alkenes and epoxides, aziridines, and cyclopropanes,
respectively.1-3 The reactions of ylides with other electrophiles,
such as trialkylboranes, have also been developed.4 In our study
of the chemistry of ylides in organic synthesis,5 we are interested
in developing reactions of allyllic ylides for stereoselective synthesis
of vinylcyclopropanes. However, these methods generally suffer
from substrate limitations.1 Since electrophilic iron carbenoid 2
affords cyclopropanes when subjected to olefins,6 we envisioned
that crotonate-derived phosphorus ylide 1a can react with 2 to give
ylide 4, which can be further trapped with aldehydes to provide
easy, diverse access to disubstituted vinylcyclopropanes (Scheme
1). Thus, we tried the reaction of 1a with 2 and found that it afforded
ylide 3 via intermolecular carbenoid insertion into the olefinic C-H
bond (Scheme 1). In this communication, we wish to report the
preliminary results for this reaction and its application in the
synthesis of 1,1,4-trisubstituted 1,3-butadienes.7
Scheme 3. Proposed Paths for Carbenoid Insertion into the
Olefinic C-H Bond of Allylic Ylide 1a
Initially, the reaction of ylide 1a with methyl diazoacetate (MDA)
was tested in the presence of a catalytic amount of tetra(4-
chlorophenyl)porphyrin iron chloride [Fe(TCP)Cl]. The reaction,
after trapping with 4-chlorobenzaldehyde (PCBA), gave only the
four isomers of diene 6a shown in Scheme 2,8 and no cyclopropane
was observed. A possible pathway for the reaction involving a
Wittig reaction of ylide 1a with PCBA to form the diene followed
by carbenoid insertion is excluded, since no reaction was observed
when (2E,4E)-methyl 5-(4-chlorophenyl)penta-2,4-dienoate was
treated with MDA in the presence of Fe(TCP)Cl under the same
conditions as in Scheme 2. These results suggested that a new ylide
3 was generated, probably via an insertion of iron carbenoid 2 into
the olefinic C-H bond of ylide 1a, stimulating us to explore the
insight of the C-H bond insertion of the allylic ylide. The two
most likely mechanisms, involving direct carbenoid insertion into
the olefinic C-H bond (path A) and tandem cyclopropanation/ring
opening (path B), are shown in Scheme 3.9 Although the cyclo-
propanation of 1,2-disubstituted alkenes with iron carbenoid 2 does
not work,6a we believe that the reaction most likely proceeds
through the formation of cyclopropane 4 followed by ring-opening
(path B),10 since both the reaction of ylide 1a with tert-butyl
diazoacetate (BDA) and the reaction of ylide 1b with MDA gave
the same product distribution8 in the cross experiments shown in
Scheme 4. Furthermore, we trapped the intermediate 4 successfully.
Fortunately, when an in situ mixture of PCBA and MDA was added
to ylide 1a in CH2Cl2 in the presence of [Fe(TCP)Cl], vinylcyclo-
propane 5a (E/Z 15:85) was isolated in 6% yield (Scheme 5). To
further confirm this mechanism, we synthesized the intermediate
ylide 4 and found that it could undergo the ring-opening reaction
rapidly. These results clearly confirm that path B works in this
reaction.
Scheme 1. Reaction of Ylide 1a with Iron Carbenoid 2
To make the current reaction practical, we tried to improve the
stereoselectivity of the Wittig reaction and use the corresponding
phosphonium salt instead of ylide 1a. The challenge is tolerance
of the in-situ-produced iron carbenoid to the basic reaction
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4192 J. AM. CHEM. SOC. 2009, 131, 4192–4193
10.1021/ja8097959 CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4. Cross Experiments of the C-H Bond Insertion
O,O-isopropylidene-D-glyceraldehyde furnished product 6j without
loss of ee (Table 1, entry 10) under mild conditions.
In summary, a highly efficient, selective, and catalytic intermo-
lecular formal carbenoid insertion reaction into olefinic C-H bonds
of allylic phosphoranes has been described. The mechanistic
investigation showed that the insertion involves cyclopropanation
of the allylic ylide with the iron carbenoid followed by ring opening
of the resulting cyclopropane ylide. On the basis of this observation,
a one-pot reaction of tributylphosphine-derived salt 11 with MDA
and aldehydes under mild conditions has been developed, providing
easy access to 1,1,4-trisubstituted 1,3-butadienes with high stereo-
selectivity.
Scheme 5. Trapping of Intermediate 4 and Its Ring-Opening
Reaction
Acknowledgment. We are grateful for the financial support
from the National Natural Science Foundation of China (Grants
20821002 and 20672131) and the Major State Basic Research
Development Program (Grant 2009CB825300). This paper is
dedicated to Professor Qingyun Chen on the occasion of his 80th
birthday.
Supporting Information Available: Detailed experimental proce-
dures, characterization data for all of the new compounds, and molecular
structures in PDB format. This material is available free of charge via
the Internet at http://pubs.acs.org.
conditions. After several trials, it was found that the one-pot reaction
works well and that the stereoselectivity of the Wittig reaction is
improved greatly when tributylphosphine-derived salt 11 is em-
ployed. The generality of the present reaction was examined by
investigating a variety of aldehydes. As shown in Table 1, both
aromatic and aliphatic aldehydes are suitable substrates to afford
products 6 with high stereoselectivity. Although this is a one-pot
procedure involving a three-step transformation, acceptable yields
were obtained in all cases. Notably, the optically active aldehyde
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