Gold-catalyzed [4+2] Annulations of Dienes with Nitrosoarenes as 4 π Donors: Nitroso-Povarov Reactions

Article abstract of DOI:10.1002/anie.201903615

This work reports the first success of the nitroso-Povarov reaction, involving gold-catalyzed [4+2] annulations of nitrsoarenes with substituted cyclopentadienes. In this catalytic sequence, nitrosoarenes presumably attack gold-π-dienes by a 1,4-addition pathway, generating allylgold nitrosonium intermediates to complete an intramolecular cyclization. Acyclic dienes are also applicable substrates, and affording oxidative nitroso-Povarov products.

Full text of DOI:10.1002/anie.201903615

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Accepted Article
Title: Gold-catalyzed [4+2]-Annulations of Dienes with Nitrosoarenes
that Serve as 4-ꢀ-donors to Enable Nitroso-Povarov Reactions
Authors: Ching-Nung Chen and Rai-Shung Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903615
Angew. Chem. 10.1002/ange.201903615
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10.1002/anie.201903615
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Gold-catalyzed [4+2]-Annulations of Dienes with Nitrosoarenes
that Serve as 4--donors to Enable Nitroso-Povarov Reactions
Ching-Nung Chen and Rai-Shung Liu *
Abstract: This work reports the first success of the nitroso-Povarov
reactions involving gold-catalyzed [4+2]-annulations of nitrsoarenes
with substituted cyclopentadienes. In this catalytic sequence,
nitrosoarenes presumably attack at gold--dienes via a 1,4-addition
pathway, generating allylgold nitrosonium intermediates to complete
an intramolecular cyclization. Acyclic dienes are also applicable
substrates, but affording oxidative nitroso-Povarov products.
continue our interest in new nitroso reactions,[4,7] this work
reports novel [4+2]-annulations between cyclopentadienes and
nitrosoarenes, as depicted in (eq 2); such annulation patterns
resemble the well-known Povarov reactions[8-9] (eq 4), in which
N-aryl imines are replaced with nitrosoarenes. In the cases of
acyclic dienes, we also developed novel oxidative nitroso
Povarov reactions with 4-halophenyl nitroso species (eq 3).
Formation of these two annulation products supports
a
mechanism involving nitrosoarenes as nucleophiles and gold--
dienes as electrophiles; this pathway is distinct from the well-
known nitroso/diene [4+2]-cycloadditions, as depicted in (eq 1).
Nitrosoarenes are very versatile to undergo metal-catalyzed
cycloadditions[1,2] with -bond motifs including dienes,[2] alkynes[3]
and allenes.[4] These cycloadditions are accessible to valuable
1,n-aminoalcohols through a reductive cleavage of the weak N-
O bonds of cycloadducts. Catalytic [4+2]-cycloadditions of
dienes with nitrosoarenes represent one of the most successful
reactions in metal catalysis because of their facile access to 1,4-
aminoalcohols in high diastereo- and enantiomeric purity (eq 1).
Table 1. One-pot catalytic reactions with various metal catalysts
[1-2]
We are aware of no examples that nitrosoarenes can serve
as 4- donors in catalytic annulations although [4+2]-
cycloadditions
were
well
known
between
transient
Entry
Catalyst
t/n
Solvent
Yields (%)[b]
nitrosoalkenes and electron-rich alkenes (eq 1).[5] For
nitrosoarenes, their reactions with styrenes were reported to
1a’
3a
4a
5a
1
2
3
4
5
6
7
8
LAuCl/AgNTf2
LAuCl/AgSbF6
LAuCl/AgOTf
LAuCl/AgNTf2
IPrAuCl/AgNTf2
PPh3AuCl/AgNTf2
(PhO)3PAuCl/AgNTf2
AgNTf2
LAuCl/AgNTf2
LAuCl/AgNTf2
LAuCl/AgNTf2
LAuCl/AgNTf2
0/0
5/1
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
THF
88
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
85
-
-
6
-
-
-
12
13
-
-
-
54
62
79
64
63
51
-
72
58
20
62
15/1
10/1
15/1
20/1
30/1
15/1
15/1
30/1
60/1
20/1
9
10
11
12
-
22
-
-
34
-
MeCN
dioxane
-
9
[a] [1a] = 0.2 M. [b] Product yields are obtained after purification from a silica
column. L = P(t-Bu)2(o-biphenyl). IPr = 1,3-bis(diisopropylphenyl)imidazol-2-
ylidene). Tf = trifluoromethanesulfonyl.
Table 1 summarizes the results for the [4+2]-annulations of
substituted cyclopentadienes with nitrosobenzene 2a using
vinylallene 1a as starting reagent. In a standard operation,
0
vinylallene 1a was treated with a gold catalyst in DCE (25 C )
for 5 min to ensure a complete formation[10] of cyclopentadienes
1a’ before nitrosobenzene 2a was added. Notably, gold
catalysts also catalyzed the nitroso annulations of species 1a’
(vide infra, eq 6); we thus performed one-pot operation. With P(t-
Bu)2(o-biphenyl)AuCl/AgNTf2, diene derivative 1a’ was obtained
in 88% yield when nitrosobenzene 2a was absent (entry 1).
Entries 2-4 show the new annulations using LAuCl/AgX (5
mol %, L = P(t-Bu)2(o-biphenyl), X = SbF6, AgOTf and AgNTf2),
yielding the cyclic nitroxy product 3a in 54-79% yields, with
LAuCl/AgNTf2 being the most efficient. Gold catalysts bearing
other ligands, such as LAuCl/AgNTf2 (5 mol %, L = IPr, PPh3
and P(OPh)3, yielded compound 3a in 51-64% yields (entries 5-7).
AgNTf2 alone did not generate cyclopentadiene 1a’ with a [4+2]
cycloadduct 4a was isolated efficiently (entry 8). P(t-Bu)2(o-
biphenyl)AuCl/AgNTf2 in other solvents yielded compound 3a
Scheme 1. The annulations of nitroso with alkenes and dienes.
generate two nitrone species via a radical process.[6] To
[*] C. N. Chen, Prof. Dr. R.-S. Liu
Frontier Research Centers on Fundamental and Applied Science
of Matters and Department of Chemistry
National Tsing-Hua University
Hsinchu (Taiwan)
E-mail: rsliu@mx.nthu.edu.tw
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10.1002/anie.201903615
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with the following yields: DCM (72%), THF (58%), MeCN (20%)
and 1,4-dioxane (62%) (entries 10-12). The molecular structure
of compound 3a is inferred with the X-ray structure of its
naphthyl relative 3i;[11] their NMR spectral data match well with
each other. In several instances, we isolated an O-addition
product 5a in minor proportions (6-13%); this species was
convertible to nitroxy species 3a with gold catalyst (vide infra, eq
7).
less-substituted alkenes (entries 16-18); the yields were 36-41%
in this one-pot operation. The yields of compounds 3q’-3s’ were
52-61% based on thieir initial cyclopentadienes 1q’-1r’.
Various nitrosoarenes 2 are also compatible with these gold
catalyzed reactions; the results are depicted in Table 3. With
vinyl allene 1a, the annulations occur with the tetra-substituted
Table 3. Catalytic [4+2] annulations with various nitrosoarenes
Table 2. [4+2]-annulations with various vinyl allenes
[a] 10 mol% catalyst loading . [b] Product yields are obtained after
purification from a silica column. L = P(t-Bu)2(o-biphenyl).
alkenes of cyclopentadienes. For 4-phenyl nitroso species 2b-2g
bearing R1 = i-Pr, Me, Cl, Br, NO2 and CO2Et, their alkene
annulations delivered compounds 6b-6g in 72-94% yields
(entries 1-6). For meta-substituted phenyl nitroso speies 2h and
2i (R2 = Me and Cl), their corresponding products 6h and 6i
were obtained in 72 and 64% yields respectively (entries 7-8),
herein, we did not obtain the other products from the C(2)-H
functionalizations of the meta-benzenes. We also tested the
reactions on the ortho-substituted nitroso species 2j and 2k (R3
= Me and Cl), affording the desired nitroxy products 6j and 6k in
85% and 43% yields respectively (entries 9-10). The molecular
structure of compound 6j was characterized by X-ray
diffraction.[11]
[a] [1]= 0.2 M. [b] Product yields are obtained after purification from a silica
column. [c] These yields were estimated based on cyclopentadienes 1'. L
= P(t-Bu)2(o-biphenyl).
We assess these new [4+2]-annulations using various
vinylallenes; the results are summarized in Table 2, showing two
distinct annulations, as controlled by the allenyl C(1)-
substituents, i.e. R1 = aryl versus alkyl. Products 3 correspond to
the annulations with the tetra-substituted alkenes of dienes 1’
whereas 3’ arise from the tri-substituted alkenes. We tested the
[4+2]-annulations on various C(1)-aryl-substituted vinylallenes
1b-1e (X= Me, OMe, Br and Cl), giving compounds 3b-3e in
satisfactory yields (62-84%, entries 1-4). These annulations
were also operable with their 3- and 2-substituted phenyl
derivatives 1f-1h (X = Me, Cl, R1 = 2-MePh), affording the
desired products 3f-3h in 59-79% yields (entries 5-7). For vinyl
allene 1i bearing R1 = 2-naphthyl, its resulting product 3i was
produced smoothly and structurally characterized by X-ray
diffraction.[10] We also prepared heteroaryl derivatives 1j-1l (R1 =
2- furyl and 2- or 3-thienyl), their resulting products 3j-3l were
obtained with 63-78% yields (entries 9-11). With R1 = phenyl, we
varied the R2 or R3 of vinylallenes 1m-1o with alkyl groups, their
resulting products 3m-3o were also produced satisfactorily with
59-68% yields (entries 12-14). We also tested the reaction on
vinylallene 1p to deliver the desired product 3p bearing a phenyl
at one quaternary carbon (entry 15). Particularly notable are the
reactions of vinylallenes bearing R1 = n-butyl and isopropyl,
yielding all alkyl-substituted products 3q’-3s’ arising from the
Scheme 2. Chemical functionalizations of cyclic nitroxyl product 3a.
We performed our new synthesis of compound 3a on a
gram-scale; the yield was up to 72% (Scheme 2). Reductive
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cleavage of compound 3a with zinc power in MeOH/H2O/AcOH
delivered compound 7a in 89% yield with a scale > 1.0 g.
Compound 7a was protected with 1,1′ - Carbonyldiimidazole
(CDI) to yield a conformationally rigid product 7b that can be
elaborated stereoselectively into compounds 7c and 7d
,
respectively, with catalytic hydrogenation and m-CPBA
epoxidation. Notably, the treatment of compound 3a with SeO2
(2 equiv) in benzene led to an unknown reaction, yielding
compound 7e as a single stereoisomer;[12] the yield was 66%
yield. The molecular structure of compound 7e was
characterized with X-ray diffraction.[11]
No reaction ocurred between cyclopentadiene 1a‘ and
nitrosobenzene 2a when gold catalyst was absent. When P(t-
Bu)2(o-biphenyl)AuCl/AgNTf2 (5 mol %) was present, cyclic
nitroxy product 3a was obtained in 72% (eq 6). One-pot
operation afforded desired 3a with 79% yield better than the
combined yield (72%) of two separate steps because
cyclopentadienes could undergo self-dimerization. When D2O
was added to this reaction moixture, compound 5a was also
isolated in 10% yield, but we noted that there was no deuterium
content for resulting product 3a (eq 7).
We also developed novel oxidative nitroso-Povarov
reactions on an acyclic diene 8a (eq 5). Treatment of this
species with nitrosobenzene 2a with gold catalyst in DCE
yielded a nitroso/diene [4+2]-product 9a’ in 54% yield. When 4-
bromophenyl nitroso species 2e was employed in the reaction,
compound 9a was obtained in 18% yield; this yield was further
improved to 58% in wet DCM/air using LAuCl/AgNTf2 in 10/20
mol %. Structural characterization of this product relied on 1H
and 13C NMR and HHMS, and further on 1H-NOE spectra.
We postulate a plausible mechanism in Scheme 3. The
isolation of an O-addition product 5a infers the intermediacy of
nitrosonium B. Compound 5a, however, is unlikely to be the
main
intermediate
because
another
applicable
2,4-
phenylcyclopentadiene 1p (R = Ph) is unable to form a similar
species. In our recent work, nitrosoarene can attack at a tertiary
carbocation at its oxygen atom[7b-7f] whereas a highly hindered
carbocation generally destabilizes this nitrosonium. Accordingly,
for two Au--diene cations
A and A’, we conceive that
nitrosobenzene 2a preferably attacks at carbocation A to yield
nitrosonium species B. Notably, species B comprises an
allylgold fragment to enable an intramolecular cyclization to
generate tricyclic species C, and the observed products 3a, 3n
and 3p ultimately. This process rationalizes well our experiment
that the resulting compound d0-3a contained no deuterium when
D2O (5 equiv) was present (eq 7).
Table 4. [4+2]-annulations with various vinyl allenes
[a] [1]= 0.05 M. [b] Product yields are obtained after purification from a
silica column. L = P(t-Bu)2(o-biphenyl).
We assessed the generality of these oxidative nitroso-
Povarov reactions with various acyclic dienes 8a-e and 4-
chlorophenylnitroso derivatives 2d (Y = H) and 2d’ (Y = Me) in
wet DCM and air (Table 4). The reaction of 4-
chlorophenylnitroso species with diene 8a (Ar = Ph) gave
compound 9a in 65% yield (entry 1) that is better than the 58%
yield from 4-bromophenylnitroso 2e (eq 5). For 1,3-dienes 8b
and 8c bearing Ar = 4-ClPh and 4-MePh, their resulting products
9b-9c were obtained in 52% and 74% yields respectively
(entries 2-3). We tested the reaction on 1,3-diene 8d bearing R
= Et, affording the desired product 9d in 58% yield with dr =
1.9:1 (entry 4). 2-Thienyl-derived 1,3-diene 8e was also
compatible with this oxidative annulation, producing compound
9e in 33% yield. For 2-methyl-4-chlorophenylnitroso species 2a’,
its gold-catalyzed annulation with 1,3-diene 8a delivered
compound 9f in 52% yield (Table 4).
Scheme 3. A plausible mechanism
This gold--diene route also rationalizes the oxidative
nitroso-Povarov reactions of acyclic 1,3-dienes 8; an initial
attack of nitrosoarene on gold--diene D form species E, bearing
a tertiary carbocation to stabilize the nitrosonium. The benzene
ring of species E is electrophilic to induce hydrolysis to form a
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generates intermediate G that is further oxidized by air to afford
the observed product 9a.
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Before this work, metal-catalyzed annulations of alkenes
and nitrosoarenes are little explored. We report gold catalyzed
[4+2]-annulations of cyclopentadienes and nitrosoarenes with
dienes as one alkene donor; this success represents the first
examples of the nitroso-Povarov reactions. In the case of acyclic
1,3-dienes, the reaction chemoselectivity is altered with
oxidative nitroso-Povarov reactions. The mechanisms of these
two new annulations are elucidated to involve a 1,4-addition of
nitrosoarenes to gold--dienes. We hypothesize that only small
sizes of tertiary carbocations can complex with nitrosoarenes to
give stable nitrosoniums bearing allylgold functionalities; these
bifunctional intermediates allow a final intramolecular cyclization
to furnish this novel nitroso-Povarov reactions.
Acknowledgements
We thank Ministry of Science and Technology, Taiwan for
financial support.
Keywords: dienes • nitrosoarenes •gold catalysis • nitroso-
Povarov reactions • [4+2]-annulations
[1]
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deposited at Cambridge Crystallographic Data Center: 3i
(CCDC 1897243), 6e (CCDC 1897244) , 6j (CCDC 1897247),
5i (CCDC 1897245) and 7e (CCDC 1904000).
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[15] Common alkenes such as styrenes were readily decomposed
by nitrosoarenes; see ref 6. Furans were inapplicable
substrates because they do not form -complex with Au(I). In
the case of cyclohexadienes, the reaction gave nitroso/alkene
products. This observation supports the intermediacy of
nitrosium intermediates.
[4]
[5]
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
Ching-Nung Chen and Rai-Shung Liu *
Page No. – Page No.
Gold-catalyzed [4+2] nitroso-Povarov
annulations
between
substituted
cyclopentadienes or acyclic 1,3-dienes
and nitrosoarenes.
Gold-catalyzed [4+2] nitroso-Povarov annulations between substituted
cyclopentadienes or acyclic 1,3-dienes and nitrosoarenes.
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