Copper-Catalyzed P-H Insertions of α-Imino Carbenes for the Preparation of 3-Phosphinoylindoles

Article abstract of DOI:10.1021/acs.orglett.6b03752

A highly efficient P-H insertion of α-imino copper carbene into H-phosphine oxides, leading to 3-phosphinoylindoles with a broad substrate diversity and good chemoselectivity, is established. This methodology provides a rapid and efficient approach for the C (sp²)-P bond formation via the P-H insertion strategy. The stereoselective P-H insertion of α-imino copper carbene represents a unique example of asymmetric P-H insertion, affording P-stereogenic 3-phosphinoylindoles with high stereoselectivities.

Full text of DOI:10.1021/acs.orglett.6b03752

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed P−H Insertions of α‑Imino Carbenes for the
Preparation of 3‑Phosphinoylindoles
Lisheng Wang,† Yi Wu,† Yang Liu,† Hua Yang,† Xu Liu,† Jianyi Wang,† Xiaoxun Li,‡ and Jun Jiang*,†
†School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P. R. China
‡Department of Chemistry, Stanford University, Stanford, California 94305-5580, United States
S
* Supporting Information
ABSTRACT: A highly efficient P−H insertion of α-imino
copper carbene into H-phosphine oxides, leading to 3-
phosphinoylindoles with a broad substrate diversity and
good chemoselectivity, is established. This methodology
provides a rapid and efficient approach for the C (sp2)−P
bond formation via the P−H insertion strategy. The
stereoselective P−H insertion of α-imino copper carbene
represents a unique example of asymmetric P−H insertion,
affording P-stereogenic 3-phosphinoylindoles with high stereo-
selectivities.
Scheme 1. Insertion Reactions of α-Imino Metal Carbenes
for the Construction of C(sp2)−Heteroatom Bonds
he highly efficient construction of carbon−heteroatom
Tbonds is one of the most pursued goals in modern organic
synthesis. Transition-metal-catalyzed insertion of carbenes and
carbenoids generated in situ from α-diazo carbonyl compounds
into heteroatom−hydrogen bonds (X−H, where X = O, N, S,
P, etc.) is an efficient strategy for the construction of C(sp3)−X
bonds.1 Recently, α-imino rhodium carbene, generated in situ
from N-sulfonyl 1,2,3-triazoles, has emerged as an increasingly
valuable intermediate possessing novel reactivity and synthetic
potential.2 Significant progress has been made by the research
groups of Fokin,3 Gevorgyan,4 Davies,5 Sarpong,6 Shi,7 and
others. In particular, α-imino rhodium carbenes insertions have
led to a novel synthetic method toward the formation of C−X
bonds, including C(sp2)−X bonds. For example, high regio-
and stereoselectivities have been achieved with the insertion of
α-imino rhodium carbene into O−H and N−H bonds,
affording enamide products (Scheme 1a).3a
Indole and its derivatives are important bioactive hetero-
cycles in natural products and pharmaceuticals.8 More recently,
indole-embedded α-imino metal carbene in situ generated from
3-diazoindolin-2-imine has been identified as a reactive
intermediate in carbene insertions to furnish a variety of indole
derivatives.9 The transformation of rhodium-catalyzed N−H
insertion was reported to build 3-substituted indole derivatives
with the C(sp2)−N bond formation (Scheme 1b).9e Despite
progress in α-imino carbene insertion transformations, the
insertion reactions of indole-embedded α-imino metal carbenes
for the generation of C(sp2)−X bonds is much less developed.
Aryl and heteroaryl phosphorus compounds have been of
particular interest due to their biological significance and
synthetic value.10 3-Phosphoindoles represent novel second-
generation NNRTIs (non-nucleoside reverse transcriptase
inhibitors) which show remarkable effect on inhibiting the
replication of HIV-1 virus (IDX899).11 In consideration of the
importance of 3-phosphoindoles in pharmaceuticals and
inspired by these previous findings shown in Scheme 1, herein
we describe our efforts toward the development of the copper-
catalyzed P−H insertion of novel α-imino copper carbene to
access a variety of 3-phosphoindole derivatives (Scheme 1c).
This method will not only expand the repertoire of trans-
formations of α-imino metal carbenes but also provide a
straightforward route to C(sp2)−P bond formation and
creation of new phosphoindoles. To the best of our knowledge,
the P−H insertion reactions of α-imino metal carbene for the
construction of C−P bond has never been reported.
Received: December 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03752
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
We began to explore the P−H insertion of 3-diazoindolin-2-
imine (1a) and diphenylphosphine oxide (2a) with the
common catalysts used for the decomposition of triazoles
and diazo compounds, including Rh2(oct)4, [Ru(p-cymene)-
Cl2]2, (η3-C3H5)2Pd2Cl2, and Ni(cod)2 (Table 1, entries 1−4).
Scheme 2. Scope of 3-Diazoindolin-2-imines
a
Table 1. Optimization of the Reaction Conditions
b
temp
(°C)
time
(h)
yield
(%)
entry
catalyst
Rh2(oct)4
solvent
DCE
1
2
35
35
24
24
0
[Ru(p-cymene)
Cl2]2
DCE
0
3
(η3-C3H5)2Pd2Cl2
Ni(cod)2
DCE
35
35
35
35
35
35
35
35
50
15
50
50
50
50
50
24
24
24
24
24
24
24
1
0
0
4
DCE
5
Cu(OTf)2
DCE
20
15
15
23
42
42
56
trace
0
6
Cu(OAc)2
DCE
7
Cu(acac)2
DCE
8
Cu2O
DCE
9
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
DCE
10
11
12
13
14
15
16
17
DCE
DCE
1
DCE
1
DCE
1
a
Reaction conditions: Cu(CH3CN)4PF6 (0.01 mmol), 1 (0.20 mmol)
and HP(O)Ph2 (0.20 mmol) in CHCl3 (0.8 mL) at 50 °C for 1 h.
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
DCM
CHCl3
1, 4-dioxane
toluene
1
26
82
64
52
1
b
Isolated yield.
1
1
position of indole were feasible substrates in this reaction and
delivered 3f−i in 53−74% yields. The process was tolerant to 3-
diazoindolin-2-imines bearing different sulfonyl groups (R3 = 4-
BrC6H4, 4-CF3C6H4, 2,4,6-i-Pr3C6H2), thus giving the corre-
sponding products 3j−l with high chemoselectivities (65−88%
yield).
a
Reaction conduction: 1a (0.2 mmol), 2a (0.2 mmol), catalyst (0.01
b
mmol), solvent (0.8 mL). Isolated yield.
Unfortunately, no desired product but a significant amount of
starting material was observed. To our delight, the reaction gave
3a in 20% yield in the presence of 5 mol % of Cu(OTf)2 in
DCE at 35 °C (Table 1, entry 5). Encouraged by this initial
result, we then screened other copper catalysts, of which
Cu(CH3CN)4PF6 proved superior (Table 1, entries 5−9). By
screening the reaction temperature and reaction time, the
optimal reaction temperature and reaction time were found to
be 50 °C and 1 h, respectively. A trace amount of the desired 3a
was exactingly detected when the reaction was carried out at 15
°C (entry 12). A control reaction in the absence of metal
catalyst afforded no desired product, indicating the critical role
of the copper catalyst in generating the α-imino metal carbene
intermediate (entry 11 vs 13). The reaction was subjected to
notable solvent effects. Chloroform (CHCl3) was a better
solvent over ClCH2CH2Cl (DCE), CH2Cl2 (DCM), 1, 4-
dioxane, and toluene, providing 3a in 82% yield (entries 15−
17).
We next investigated the scope with respect to the H-
phosphine oxides (Scheme 3). H-Phosphine oxides 2 under the
optimal reaction conditions and reacted with 1 generally
proceeded in good yields. Both electron-poor and electron-rich
substituted groups in H-phosphine oxides (2d,e) could be
tolerated and afforded 3o and 3p in 86% and 88% yield,
respectively. Notably, heterocyclic H-phosphine oxides such as
9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide
(DOPO, 2f) could be introduced to the insertion product.
Moreover, ethyl phenylphosphinate (2g) was also viable for this
transformation, giving the expected product 3s in 42% yield. In
the case where butyl(phenyl)phosphine oxide (2h) was used as
a substrate, the desired product was isolated in 57% yield. The
tosyl group can be removed with concentrated sulfuric acid to
afford stable 2-free amino indole 6 in 91% yield (Supporting
Information).
With the optimal reaction conditions, we then explored the
substrate scope with respect to the 3-diazoindolin-2-imines. As
illustrated in Scheme 2, the alkyl group on the 1-position of
indole could be varied from methyl, ethyl, isopropyl, allyl, and
benzyl to produce 3a, 3b, 3c, 3d, and 3e in 82%, 76%, 71%,
79%, and 58% yields, respectively. The structure of 3e was
unambiguously determined by single-crystal X-ray analysis (as
shown in the Supporting Information). No insertion product
was observed by using N-unprotected 3-diazoindolin-2 mines.
Various 3-diazoindolin-2-imines with substituents on the 5-
Inspired by the success in the P−H insertions of α-imino
copper carbene, we then turned our attention to the
asymmetric synthesis in an attempt to obtain a chiral 3-
phosphoindoles. (1R,2S,5R)-(−)-Menthoxyphenylphosphinate
(4) was employed as the chiral reagent for the chirality
induction (Scheme 4). The P−H insertion of α-imino copper
carbene proceeded smoothly to give the chiral 3-phosphindoles
(5a−f) in moderate yield with very high diastereoselectivity.
The absolute configuration of 5f was established by X-ray
single-crystal analysis (Figure 1).
B
DOI: 10.1021/acs.orglett.6b03752
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Organic Letters
Letter
a b
,
important and challenging subjects in synthetic organic
chemistry.13,14 In this asymmetric P−H insertion, the H−P
bond in (1R,2S,5R)-(−)-menthoxylphenylphosphinate (4) was
converted into a C(sp2)−P bond to give chiral 3-phosphoin-
doles with high seteroselectivities. Although there are several
examples of P−H insertions reported in the formation of the
C(sp3)−P bond,15 no asymmetric P−H insertion reaction has
been achieved to date. The stereoselective P−H insertion of α-
imino copper carbene represents a unique example of
asymmetric P−H insertion, affording chiral 3-phosphinoylin-
doles with a P-stereogenic center.
Scheme 3. Scope of H-Phosphine Oxides
On the basis of the preliminary results of control reactions
(as shown in Table 1, entry 11 vs 13) and previous work,3a,16
we envisioned that the insertion of α-imino copper carbene into
P−H bonds may proceed in two plausible pathways (Scheme
5). The α-imino copper carbene intermediate derived from 3-
Scheme 5. Proposed Mechanism
a
Reaction conditions: Cu(CH3CN)4PF6 (0.01 mmol), 1a (0.20
mmol) and 2 (0.20 mmol) in CHCl3 (0.8 mL) at 50 °C for 1 h.
b
Isolated yield.
Scheme 4. Stereoselective Synthesis of Chiral 3-
ab
,
Methoxyarylphosphoindoles
diazoindolin-2-imines may undergo direct 1,1-insertion into the
P−H bond, followed by tautomerization leading to product of
3-phosphindoles (path a). Alternatively, the insertion may
proceed by the formation of zwitterionic P-ylide intermediate.
Subsequent dissociation of copper results in a proton transfer
from the phosphorus to imine moiety, finally furnishing the 1,3-
insertion product of 3-phosphindoles (path b).
In summary, we have developed P−H insertions of α-imino
copper carbenes for the construction of C(sp2)−P bonds. The
protocol provides an efficient approach to 3-phosphinoylin-
doles from 3-diazoindolin-2-imines and H-phosphine oxides
with good chemoselectivity. In addition, the stereoselective P−
H insertion was also successfully developed with high
diastereoselectivity. The investigations on the biological
activities of the new 3-phosphinoylindoles are in progress.
a
Reaction conditions: Cu(CH3CN)4PF6 (0.01 mmol), 1a (0.20
mmol), and HP(O) (OMen)Ph (0.20 mmol) in CHCl3 (0.8 mL) at
ASSOCIATED CONTENT
* Supporting Information
b
■
50 °C for 8 h. Isolated yield and the diastereomeric ratio were
1
S
detected by H NMR (600 MHz) spectroscopy.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03752.
Experimental procedures, characterization data, and
NMR spectra of all new compounds (PDF)
X-ray crystallography of 3e (CIF)
X-ray crystallography of 5f (CIF)
Figure 1. X-ray structure of 5f.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jiangjun@gxu.edu.cn.
ORCID
P-Stereogenic phosphorus compounds have attracted con-
siderable attention due to their wide applications as chiral
ligands for asymmetric synthesis.12 The stereoselective syn-
thesis of P-stereogenic compounds is one of the most
Jun Jiang: 0000-0002-5542-7592
C
DOI: 10.1021/acs.orglett.6b03752
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
H.; Ding, H. L.; Xing, Y. P.; Lu, P.; Wang, Y. G. Org. Lett. 2014, 16,
5096.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support from Natural Science
Foundation of China (21402032), Guangxi Natural Science
F o u n d a t i o n ( 2 0 1 4 G X N S F B A 1 1 8 0 3 1 ,
2015GXNSFDA139008), and the Scientific Research Founda-
tion of Guangxi University (XQZ130869)
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