Nucleophilic conjugate 1,3-addition of phosphines to oligoynoates

Article abstract of DOI:10.1039/c4cc02870h

Herein we have elucidated unusual and unique nucleophilic conjugate 1,3-addition reactions of surveyed oligoynoates toward phosphines through spectroscopic and single crystal X-ray diffraction analyses of three-component reaction products of oligoynoates, phosphines and aldehydes. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02870h

Featuring research of Jie-Cheng Deng et al. from the Organic
Materials Group of Prof. Shih-Ching Chuang, National Chiao
Tung University, Faculty of Chemistry, Taiwan,
Republic of China
As featured in:
Nucleophilic conjugate 1,3-addition of phosphines to
oligoynoates
Herein we have elucidated unusual and unique nucleophilic
conjugate 1,3-addition reactions of surveyed oligoynoates toward
phosphines through spectroscopic and single crystal X-ray
diffraction analyses of three-component reaction products of
oligoynoates, phosphines and aldehydes.
See Shih-Ching Chuang et al.,
Chem. Commun., 2014, 50, 10580.
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Nucleophilic conjugate 1,3-addition of phosphines
to oligoynoates†
Cite this: Chem. Commun., 2014,
50, 10580
Jie-Cheng Deng,‡ Chih-Wei Kuo‡ and Shih-Ching Chuang*
Received 18th April 2014,
Accepted 9th June 2014
DOI: 10.1039/c4cc02870h
www.rsc.org/chemcomm
Herein we have elucidated unusual and unique nucleophilic conjugate
1,3-addition reactions of surveyed oligoynoates toward phosphines
through spectroscopic and single crystal X-ray diffraction analyses
of three-component reaction products of oligoynoates, phosphines
and aldehydes.
Nucleophilic conjugate addition, a unique class of nucleophilic
reactions, is an important methodology in applied synthetic
chemistry.1 Numerous useful organic compounds can be prepared
with conjugate addition reaction as a key step.2 In this context,
nucleophilic conjugate 1,n-addition (n = even number), for example,
1,2-, 1,4- or (b), 1,6- or (d), and 1,8-addition patterns, has previously
been observed due to resonance stabilization.1 However, the
1,3-addition(a) pattern3 is less common besides these even
addition patterns.4 The occurrence of an a-addition pattern could
be attributed to either substitution of an electron-withdrawing
moiety at b-carbon of alkenoates/alkynoates/nitro olefins5 or
through favoured 5-exo-trig cyclization taking place at a-carbon of
a conjugated system.6 Pioneering studies have focused on using
Scheme 1 Oligoynoates for testing nucleophilic conjugate 1,3-addition.
Chemists have been using the concept of 1,4-addition mode
b-trifluoromethyl substitution to alkenoates or alkynoates for to account for the reaction mechanism of phosphines with
increasing the electron positivity of a-carbon. Although Walborsky electron-deficient alkenoates, allenoates and alkynoates (1a–c,
and Schwarz reported an unsuccessful attempt with a b-trifluoro- Scheme 1),8–14 evidenced by isolation of tetravalent phosphonium salt
methyl-substituted alkenoate,7 Knunyants and Cheburkov later intermediates.13a A nucleophilic a-addition to alkynoates relevant to
demonstrated a successful a-addition with a b-bis(trifluoromethyl)- phosphine catalysis also used the classical 1,4-addition pathway to
substituted alkenoate substrate toward ammonia, amines and water interpret the reaction mechanism.15 Recently, we reported an unusual
nucleophiles.5d Rapoport and Klumpp also independently demon- three-component reaction initiated by nucleophilic attack of phos-
strated other intramolecular and intermolecular 1,3-addition reactions phines at a(d0)-carbon of an enyne 1d to generate a 1,3-dipole 1d0.16
with phenyl and trimethylsilyl-substituted ynamides, respectively.5a,6a Based upon the IUPAC nomenclature, such nucleophilic attack is
Subsequently, a trifluoromethyl-substituted alkynone has also formally formulated as a 1,6-conjugate addition (d-addition) since the
been demonstrated as another 1,3-addition acceptor by Bumgardner alkenyl moiety is numbered with higher priority with respect to the
and Whangbo et al.5b
alkynyl one. However, it has been evident that substitution at b-carbon
of alkynoates with an alkenyl group alters the addition pattern. Other
amine nucleophiles also reacted with 1d to provide a(d0)-addition
products.17 Further, we have noted that the addition pattern is
substrate-dependent since displacement of the ester moiety on
the alkynyl carbon of 1d with an aryl group results in a normal
1,4-addition.18 Our continuing interests in finding new
p-electrophiles that could undergo conjugate addition at the
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010,
Taiwan, Republic of China. E-mail: jscchuang@faculty.nctu.edu.tw;
Fax: +886-35723764; Tel: +886-35731858
† Electronic supplementary information (ESI) available. CCDC 901699, 901985,
902919 and 903961. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc02870h
‡ These authors contributed equally to this work.
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formal a-position toward nucleophiles led us to evaluate regio- stable corresponding lactones for structural elucidation. It is note-
reactivity of oligoynoates 2a–o—the numbering of IUPAC or worthy that reactive lactone 5g with hexamethylphosphorus triamide
the Greek letter order by nomenclature of these investigated (HMPT) can be isolated for characterization in a rush manner.
oligoynoates 2 will become parallel. Herein, we wish to report a These isolated lactones can be stored in their solid states at
new class of acceptors having oligoynyl units for nucleophilic room temperature under anaerobic conditions for a few weeks.
conjugate 1,3-additions with phosphines to generate 1,3-dipole B,
unequivocally evidenced by crystal structures of three-component substituted asymmetric diynoates, we further investigated the
reaction products with aldehydes. reaction of diynoates 2b–j with selected phosphines (3a–c and 3f)
To establish the scope of the 1,3-addition with variously
First, we prepared diynoates 2a–j and 2n (n = 1) through Hay and aldehyde 4a. The surveyed reactions proceeded through
coupling reactions and evaluated their reactivity toward phosphine 1,3-addition to give lactone products 5 in decent to good isolated
nucleophiles. We selected diynoate 2a as a model substrate for yields (32–73%, Table 2); lower reaction yields were observed in the
optimization of conditions (Table S1; see the ESI†). The investi- cases of diynoates (2b, 2d and 2f) with the ethyl ester moiety
gated 1,3-addition three-component reaction gave optimized yields (entries 1–3, 7–9 and 13–15; 33–49%) due to the poor leaving ability
with a molar ratio of 2a :3a :4a = 1.5 : 1.5 : 1.0 in o-dichlorobenzene of ethoxide during lactonization. The reaction with terminally 4-t-
(o-DCB) at 80 1C for 2 hours, giving lactone 5a in 65% yield butyl phenyl and methyl ester-capped diyne (2c) and P( p-tolyl)3 (3b)
(entry 8). Other conditions with THF or dichloromethane gave the highest yield (entry 5, 73%) among all, and other halo or
as solvents (entries 1 and 2) and higher loading of 2a and 3a methoxy phenyl-substituted diynoates (2e–j) gave moderate yields
(entry 9) did not give much better yields.
(entries 10–19; 32–66%). These results further firmly supported
The reaction scope study with variously substituted electron- the occurrence of 1,3-addition of phosphines in a range of aryl-
donating and withdrawing phosphines (3a–g) and aromatic substituted electron-donating and withdrawing diynoates.
aldehydes (4a–d) also gave a-ylide lactone products 5a–u through
We intended to alter the addition pattern through placement
1,3-addition reaction in 32–73% isolated yields (Table 1). The of additional alkynyl units and later noted that further extension
isolation of these a-phosphorus ylide lactones for characterization of the acetylenic unit (C2) on 2 (n = 2–4) resulted in the same
was highly dependent on the stability of 5. We can only isolate 5 1,3-addition pattern with phosphines (Table 3). We used iterative
with triarylphosphine functionality for structural characterization Hay coupling for the syntheses of oligoynoates 2 (n = 2–4; see the
because they were relatively more stable. Albeit trialkyl phosphines ESI†). Initially, the testing of the reaction of oligoynoates 2k–l
also promoted the reaction to occur, we were not able to isolate with n = 2–4 was hampered by their infeasibleness for isolation.
We observed that these oligoynoates became relatively unstable as the
alkynyl units increased. For isolating these unstable oligoynoates,
we avoided evaporation of the oligoynoates solution to dryness
and determined their solution concentrations by 1H NMR
Table 1 Reaction scope of 2a with phosphines and aldehydesa
Table 2 Reaction scope with diynoates (2b–j)a
Entry
PR3 (3)
X (4)
Product 5
Yieldb
1
2
3
4
PPh3 (3a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
4-Cl-3-NO2 (4c)
4-Cl-3-NO2 (4c)
4-Cl-3-NO2 (4c)
4-CN (4d)
5a
5b
5c
5d
5e
5f
5g
5h
5i
67
66
73
62
58(60)
47(52)
41
58
64
Entry
R1
R2
2
PR3 (3)
5
Yieldb
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
P(4-OMe-Ph)3 (3d)
P(4-Cl-Ph)3 (3e)
P(4-F-Ph)3 (3f)
HMPT (3g)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Et
Et
Et
Me
Me
Me
Et
Et
Et
Me
Me
Me
Et
Et
Et
Me
Me
Me
Me
2b
2b
2b
2c
2c
2c
2d
2d
2d
2e
2e
2e
2f
PPh3 (3a)
5a
5b
5f
5v
5w
5x
5v
5w
5x
5y
42
49
48(54)
72
73
63(70)
44
49
39(44)
43(45)
48(50)
32(43)
39(41)
42
5
P(p-tolyl)3 (3b)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
P(4-F-Ph)3 (3f)
PPh2(p-tolyl) (3c)
PPh2(p-tolyl) (3c)
PPh2(p-tolyl) (3c)
PPh2(p-tolyl) (3c)
6
7c
8
4-t-Bu-Ph
4-t-Bu-Ph
4-t-Bu-Ph
4-t-Bu-Ph
4-t-Bu-Ph
4-t-Bu-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
2-Cl-Ph
3-Cl-Ph
4-Cl-Ph
4-OMe-Ph
PPh3 (3a)
9
P(p-tolyl)3 (3b)
P(4-OMe-Ph)3 (3d)
P(4-Cl-Ph)3 (3e)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(4-OMe-Ph)3 (3d)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
P(4-OMe-Ph)3 (3d)
P(4-F-Ph)3 (3f)
HMPT (3g)
10
11
12
13
14
15
16
17
18
19
20
21c
5j
5k
5l
56
49(52)
51(55)
55
52
46(50)
43
58
61(85)
59
54(61)
32
9
5m
5n
5o
5p
5q
5r
5s
5t
5u
10
11
12
13
14
15
16
17
18
19
5z
5aa
5y
4-CN (4d)
4-CN (4d)
4-CN (4d)
4-CN (4d)
2f
2f
2g
2h
2i
5z
5aa
5ab
5ac
5ad
5ae
33
60(70)
66(72)
47(52)
33(49)
4-CN (4d)
2j
a
Reaction was carried out under anhydrous conditions unless other-
b
a
wise noted; a molar ratio of 2a : 3 : 4 = 1.5 : 1.5 : 1.0. Isolated yields (%);
Reaction was carried out under anhydrous conditions unless other-
b
yields in parentheses were determined based on converted aldehydes. wise noted; a molar ratio of 2 : 3 : 4a = 1.5 : 1.5 : 1.0. Isolated yields (%);
c
Reaction was carried out at room temperature.
yields in parentheses were determined based on converted aldehydes.
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Table 3 Reaction scope of oligoynoates (2a, 2k–o) with phosphines and
aldehydesa
resulted in the recovery of aldehydes in 97 and 85% respec-
tively. In these two cases, the generated reactive intermediates
may react with oligoynoates to form polymeric materials, as
highly polar black-brown coloured materials were observed by
column chromatography.
The isolated a-phosphorus ylide lactones 5 were characterized
1
by spectroscopic methods (IR; H, 13C and 31P NMR) and can be
entry
2
PR3 (3)
X (4)
5
Yieldb
recrystallized for X-ray crystallographic analyses. The selected
solid state structures of 5b, 5ak, 5ar, and 5ax (Fig. 1) unambigu-
ously demonstrated the covalent bonding of the phosphorus atom
to the a-carbon adjacent to the carbonyl moiety, confirming the
existence of the 1,3-addition pattern of these oligoynoates 2 with
phosphines.19 We accounted for the formation of ylide 5 by the
initial nucleophilic attack of phosphine PR3 at a-carbon of
oligoynoates 2a–o (Scheme 2) to generate reactive zwitterionic
species Ia having a carbanion at b-carbon. Direct addition of Ia to
carbonyl carbon of aldehydes followed by intramolecular cycliza-
tion provided Ic (via intermediate Ib). Then, product 5 was formed
through deprotonation of Id by a released alkoxide. In this
proposed pathway, the generated reactive intermediate Ia may
choose to undergo nucleophilic addition to oligoynoates 2a–o and
form polymeric materials. In the absence of aldehydes, a similar
notion was observed after mixing oligoynoates and phosphines in
solution.
1
2
3
4
5
6
7
8
2k
2k
2k
2k
2k
2k
2k
2k
2k
2k
2k
2k
2l
PPh3 (3a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
3-NO2 (4b)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-NO2 (4a)
4-CN (4d)
4-NO2 (4a)
3-NO2 (4b)
H (4e)
5af
68
67
69
41
62(73)
73
60
55
60(64)
54
46(64)
62(65)
36
35
38(50)
43
28(47)
39(62)
32
48
66
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
P(4-OMe-Ph)3 (3d)
P(4-Cl-Ph)3 (3e)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
P(4-OMe-Ph)3 (3d)
P(4-Cl-Ph)3 (3e)
P(4-F-Ph)3 (3f)
PPh3 (3a)
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
P(4-OMe-Ph)3 (3d)
P(4-Cl-Ph)3 (3e)
P(4-F-Ph)3 (3f)
PPh3 (3a)
PPh2(p-tolyl) (3c)
P(p-tolyl)3 (3b)
PPh2(p-tolyl) (3c)
PPh3 (3a)
5ag
5ah
5ai
5aj
5ak
5al
5am
5an
5ao
5ap
5aq
5ar
9
10
11
12
13
14
15
16
17
18
19c
20d
21
22
23
2l
2l
2l
2l
5as
5at
5au
5av
5aw
5ax
5ay
5az
5aaa
5aab
2l
2m
2n
2o
2o
2a
72
10(18)
In light of the current results, formation of lactone products
through initial 1,3-addition of phosphines to these investigated
a
Reaction was carried out under anhydrous conditions unless other-
b
wise noted; a molar ratio of 2 : 3 : 4 = 1.5 : 1.5 : 1.0. Isolated yields (%);
yields in parentheses were determined based on converted aldehydes.
c
Reaction was carried out via slow addition of 2m in 8 h with 2m : 3a :
d
4a = 1.0 : 1.1 : 4.1. 10% of TMS deprotected lactone 5ay0 was isolated
after chromatography (see data in ESI).
spectroscopy instantly after silica gel chromatography. Despite
that oligoynoates (2k–m) can be stored in o-DCB solution at
0 1C for a few days, their solutions turned brown in colour soon
at room temperature likely due to intermolecular stacking
aggregation. For their higher reactivity, we immediately carried
out the reactions with triynoate (2k) and tetraynoate (2l) at
room temperature (25 1C) and that with pentaynoate (2m) at
0 1C right after they were purified (Table 3). According to these
data, we observed formation of lactones 5 through 1,3-addition
up to 73 and 62% isolated yields (entries 6 and 12) with reactant
combinations of triynoate (2k)/phosphine (3f)/aldehyde (4a)
and triynoate (2k)/phosphine (3f)/aldehyde (4b), respectively. The
isolated yields decreased as the alkynyl units of 2 increased—we
observed optimal 43 and 39% yields with tetraynoate 2l (entries 16
and 18; 2l/3d/4a and 2l/3f/4a), and 32% with pentaynoate 2m (entry
19, 2m/3a/4a), respectively. It is worth noting that reactions with
these oligoynoates at higher temperature led to poor yields due to
decomposition of either oligoynoates or products. The scope of this
reaction can be further extended to TMS-capped diynoate (2n) and
n-hexyl-capped triynoate (2o), yielding corresponding lactones 5ay,
5az and 5aaa respectively (entries 20–22). We further observed
decreasing reactivity toward aldehydes with an electron-donating
moiety and aliphatic aldehyde—reactions with benzaldehyde pro-
vided only 10% 5aab and those with p-tolualdehyde and decanal
Fig. 1 Thermal ellipsoids of crystallized compounds (a) 5b, (b) 5ak, (c) 5ar
and (d) 5ax with solvents omitted for clarity.
Scheme 2 Proposed mechanism for formation of 5.
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Yu. A. Cheburkov, Izv. Akad. Nauk SSSR, Ser. Khim., 1960, 2162;
oligoynoates is inferred. We could not exclude the likelihood of
the occurrence of 1,4-addition although we did not isolate the
corresponding products. The putative 1,4-addition pathway may
result in the formation of unstable intermediates which proceed
to regenerate reactants followed by 1,3-addition or undergo poly-
merization with oligoynoates. The preliminary computational
studies of these oligoynoates exhibited larger orbital coefficients
along Pz at the a-carbon of 2,20 corroborating the currently
observed 1,3-addition with phosphine nucleophiles. Other mecha-
nistic consideration leading to 1,3-addition such as the oxophili-
city of phosphines and resonance stabilization through an alkynyl
p-moiety also cannot be excluded.
In summary, we have demonstrated a unique nucleophilic
conjugate 1,3-addition reaction of oligoynoates with phosphines
through analyses of their assembled products with aldehydes.
Extension of the acetylenic unit does not alter the addition pattern.
The novel addition mode represents the first formal a-selective
nucleophilic conjugate reaction of phosphines to oligoynoates. This
new class of acceptors may be applicable as reaction partners for
other Morita–Baylis–Hillman type reactions. Our ongoing endea-
vors are directed to explore the regioreactivity of these oligoynoates
toward other organometallic and organic nucleophiles such as
N-heterocyclic carbenes (NHC). Application of this phosphine
1,3-addition reaction toward syntheses of useful products and
toward phosphine catalysis is also underway.21
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We thank the Ministry of Science and Technology of Taiwan for
supporting this research financially (MOST101-2113-M-009-008).
Notes and references
1 J. March, Advanced Organic Chemistry, Wiley, New York, 4th edn,
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19 CCDC 901699, 901985, 902919 and 903961 contain the ESI† for 5b,
5ak, 5ar and 5ax.
20 Complete computational aspects of these oligoynoates will be
reported elsewhere.
21 Some of the products are highly fluorescent; for example, lactone 5s
exhibits yellow-green fluorescence with a quantum yield of 0.91.
Upon being equipped with appropriate chelating functionality,
these lactones are effective metal ion chemosensors.
2 M. B. Smith, Organic Synthesis, McGRAW-HILL, INC, New York, 1994.
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