Convenient synthesis of allenylphosphoryl compounds: Via Cu-catalysed couplings of P(O)H compounds with propargyl acetates

Article abstract of DOI:10.1039/c6cc02563c

A novel Cu-catalysed substitution reaction of propargyl acetates with P(O)H compounds is developed to afford allenylphosphoryl compounds via C-P bond coupling in high yields under mild conditions. A plausible mechanism involving the nucleophilic interception of the Cu-allenylidene intermediates is proposed.

Full text of DOI:10.1039/c6cc02563c

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Convenient Synthesis of Allenylphosphoryl Compounds via Cu-
Catalysed Couplings of P(O)H Compounds with Propargyl Acetates
Ruwei Shen,*a Bing Luo,a Jianlin Yang,a Lixiong Zhang,a Li-Biao Han*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
(a) previous work
A novel Cu-catalysed substitution reaction of propargyl acetates
with P(O)H compounds is developed to afford allenylphosphoryl
compounds via C-P bond coupling in high yields under mild
conditions. A plausible mechanism involving the nucleophilic
interception of the Cu-allenylidene intermediates is proposed.
R2
R1
R3
R1
R2
R3
type c
type a
R4M
Refs 10-12
cat. [Cu]
R1
•
•
[B]
[B]
Refs 14
R4
LG
R3
R2
R1
R3
R2
R2
R1
R3
type d
type b
•
LG = leaving group
•
[Si]
Refs 15
H-donor
Refs 13
H
Allene and its derivatives constitute a special class of organic
compounds and have received tremendous attention in recent
years.1 Among them, phosphorylated allenes including allenyl
phosphonates, phosphinates and phosphine oxides are versatile
synthetic intermediates which display a lot of applications in
organic synthesis.2ꢀ7 For example, these compounds could
undergo facile and selective additions with electrophiles or
nucleophiles to afford stereodefined functional olefins,2,3 and
the transition metalꢀcatalyzed asymmetric additions enable the
synthesis of valuable chiral phosphorus compounds.4 Allenyl
phosphonates and phosphinates are also valuable precursors to
access phosphorus heterocycles of pharmaceutical interest.5 In
addition, some phosphorylated allenes participate smoothly in
Diels–Alder and related cyclizations to generate other
structurally sophisticated organophosphorus compounds.6,7 On
contrast to their valuable utility, very few methods are known
for their synthesis.8 A traditional method to make them is the
HornerꢀMark [2,3]ꢀrearrangement reaction discovered in early
1960s.8a This reaction while still intensively used today
unfortunately suffers from hash conditions, low yields, limited
scopes and the use of highly toxic phosphorous chlorides.
[Si]
(b) this work
H
R1
R4
O
P
R3
R2
R1
cat. [Cu]/L
EtOH, 0 o
R4
•
H
AcO
P
+
R3
C
R2
O
Scheme 1 Cu-catalyzed propargylic substitutions to produce allenes.
frequently used today.11 Particularly, the introduction of
organoborates as Cꢀnucleophiles by Sawamura and Lalic
independently has enabled the synthesis of optically active
allenes with excellent chirality transfer from chiral propargylic
phosphates.12 In these reactions, new CꢀC bonds are formed
(Scheme 1a, type a). In the presence of hydrogen donors such
as hydrosilanes instead, various diꢀ or triꢀsubstituted allenes are
accessible with the formation of CꢀH bonds (Scheme 1a, type
b).13 Furthermore, recent studies also show that heteroatomꢀ
substituted allenes can be synthesized based on this strategy.
For example, Ito and Sawamura14 have reported a Cu(Oꢀ
tꢀBu)ꢀ
Xanphos catalysed substitution of propargylic carbonates with
B2Pin2 to afford allenylboron compounds (Scheme 1a, type c).
Oestreich et al15 realized
substitution with silicon nucleophiles to afford silylallenes
(Scheme 1a, type d). Despite of these excellent contributions, to
our knowledge, there is no example on Cuꢀcatalysed
propargylic substitution involving a phosphorus nucleophile.
Such a transformation is however a very promising synthetic
method towards allenylphosphoryl compounds. Here we
present the first highly efficient Cuꢀcatalysed substitutions of
propargyl acetates with P(O)H compounds to afford a diverse
collection of phosphorylated allenes in high yields under mild
conditions (Scheme 1b).16
a CuCNꢀcatalysed propargylic
On the other hand, the Cuꢀcatalysed nucleophilic substitution
of propargylic alcohol derivatives is one of the most popular
wellꢀdeveloped methods to access allenes.9 For example, the
Cuꢀcatalyzed substitutions of propargyl derivatives with
organometallic reagents such as organolithium10a or Grignard
reagents,10bꢀ10d were known as early as 1970s and are still
Our study began with the reaction of 1ꢀphenylpropargyl
acetate (1a) with diethyl phosphonate (2a) as model substrates.
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Table 1 Typical optimization results on the reactiona
Table 2 Cu-catalysed synthesis of allenyl phosphonates and phosphinates 3a
5 mol% CuI
6 mol% TMEDA
1.1 equiv iPr2NEt
R1
R2
R1
R2
R1
H
P
O
RO
P
OR
•
AcO
+
OR
OR
(RO)2P
H
EtOH, 0 o
C
O
O
3' (R2 = H)
2
1
3
yield%
(3a/3a')b
yield%
(3a/3a')b
run [Cu]
ligand
run
[Cu]
ligand
1
2
3
4
5
6
CuI
CuI
CuI
CuI
CuI
CuI
L1
L2
L3
L4
L5
L6
95 (98/2)
16c
8c
7
8
CuBr
CuCl
L1
L1
91 (97/3)
91 (98/2)
93 (96/4)
84 (98/2)
86 (95/5)
NR
9
CuPF6(MeCN)4 L1
NR
5c
tracec
10
11
12
Cu(OAc)2
L1
L1
L1
CuCl2 2H2O
CoCl2, FeCl3, etc
i
aConditions: 1 (0.24 mmol), 2 (0.2 mmol), [Cu] (5 mol%), Ligand (6 mol%), Pr2NEt
(0.22mmol) in EtOH (2 mL), 0 oC. bYields and ratio based on 31P NMR. c3a only.
Typical optimization results are shown in Table 1. When 1a
and 2a were treated with 5 mol% of CuI and 6 mol% of
i
TMEDA (L1) in the presence of 1.1 equiv of Pr2NEt in EtOH
o
at 0 C for 5 h, the reaction gave diethyl (3ꢀphenylpropaꢀ1,2ꢀ
dienꢀ1ꢀyl)phosphonate (3a) and the alleneꢀalkyne isomer 3a’ in
95% yield with a selectivity up to 98/2 (run 1). The efficiency
of the reaction is critically depended on the ligand. Bidentate
a Conditions: 1 (0.24 mmol), 2 (0.2 mmol), CuI (5 mol%), TMEDA (6 mol%) and
iPr2NEt (0.22 mmol) in EtOH (2 mL). bIsolated Yield; Ratios of 3/3’ based on 31P
NMR. cD%>99%. ddr=3.2/1. edr=2.9/1.
N,Nꢀligands such as bpy
(
L2), 1,10ꢀPhen
(
L3
)
and
ethylenediamine (L4), N,Pꢀligand 2ꢀ(2ꢀ(diphenylphosphino)
ethyl)pyridine (L5) and P,Pꢀligand dppe (L6) were all proved to
be less effective (runs 2ꢀ6). The Cu source has little effect.
CuBr, CuCl and Cu(MeCN)4PF6 are all good catalysts, giving
3a with comparable yields and selectivity (runs 7ꢀ9). Divalent
copper salts Cu(OAc)2 and CuCl2·2H2O were also competent to
catalyse the reaction, albeit giving 3a in somewhat lower yields
(runs 10 and 11). However, other metal salts such as CoCl2,
FeCl3, RuCl3 and Pd(OAc)2 showed no reactivity. The high
efficiency of the reaction also depends on the use of a protic
solvent as THF, DCM, DMF and MeCN all gave 3a in low
yields (For details, see Table S1 in ESI).
The generality of the reaction was then examined. As shown
in Table 2, a series of 1ꢀarylpropargyl acetates bearing either
electronꢀdonating or electronꢀwithdrawing group at the aryl
ring could be used to afford the corresponding products in high
yields with high selectivity. Functional groups such as alkoxy,
fluoro, chloro and bromo substituents on the aryl rings were
tolerated (runs 3ꢀ8). The steric hindrance of substituents on the
phenyls did not have a strong influence. Thus, substrates 1c and
1g bearing ortho substituents on the phenyls reacted smoothly
to give the desired products selectively in high yields (runs 4
quantitative yields (runs 10 and 11). As compared to arylꢀ
substituted propargyl acetates, the reactions of alkylꢀsubstituted
substrates 1k and 1l proceeded slowly affording 3k and 3j in
80% and 88% yields in 24 h (runs12 and 13). The reaction of a
cyclopropanyl substrate 1m also efficiently gave the expected
products 3m in 84% yields, in which the cyclopropaneꢀopening
products were not formed (run 14). In addition to 2a, dimethyl
phosphonate (2b) and dibutyl phosphonate (2c) also reacted
smoothly to give the products selectively in high yields (runs
14ꢀ16). Moreover, Hꢀphosphinates are also applicable. For
example, Hꢀphosphinate 2d reacted with 1a producing
allenylphosphinate 3p in 92% yield (run 17). The reaction also
shows a moderate stereoselectivity as 3p was obtained as a
3.2:1 diastereoisomer mixture. A similar result was also
observed from the reaction of 1a with 2e that bears an allyloxyl
group (run 18).
Further, the present method is also applicable to the synthesis
of allenylphosphine oxides. As shown in Scheme 2, 5aꢀ5k were
successfully obtained in high yields with excellent selectivity
from the reactions of diphenylphosphine oxide (4a) with the
corresponding propargyl acetates. Typical diarylphosphine
oxides bearing electronꢀdonating and electronꢀwithdrawing
substitutes on the phenyl rings are also successful substrates to
produce the expected products 5l-5n in high yields. In addition,
and 8). The 1ꢀdeuterated substrate 1aꢀdγ gave the deuterated 3aꢀ
dγ in 90% yield with a selectivity of 98% (run 2). The reaction
of 1ꢀstyrylpropargyl acetate (1h) also proceeded smoothly to
give a vinylallenyl product 3h in high yield with 95%
selectivity (run 9). The disubstituted propargyl acetates 1i and
1j performed exquisitely well to give 3i and 3j almost in
a more steric hindering substrate bearing oꢀtolyl groups also
gave the expected product 5o in good yields. Here it should be
2 | J. Name., 2012, 00, 1-3
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did not afford an alkenylphosphonate product (eq. 5),
suggesting that an addition of a possible Cuꢀphosphido species
to the CꢀC triple bond may not take place under the conditions.
The Cuꢀcatalysed substitutions of propargyl substrates to afford
substituted allenes via an additionꢀelimination mechanism is
well known.9 Nevertheless, such a mechanism may be less
possible for our reaction accordingly.
5a
R = Ph, , 95%
Ph
Ph
Ph
5b
R = p-MeC6H4, , 92%
P
•
P
R
•
R = p-ClC6H4, 5c, 90%
5d
R = o-MeOC6H4, , 91%
Ph
Ph
O
O
5f, 91%
5e
R = p-FC6H4, , 90%
Ph
Ph
Ph
P
O
•
P
•
P
Ph
•
Ph
Ph
Ph
O
O
Ph
Alternatively, a process as depicted in Scheme 3 was
proposed to rationalize the observed results based on the known
Cuꢀcatalysed propargylic substitutions of terminal propargyl
acetates with Nꢀ and Cꢀnucleophiles which involves Cuꢀ
allenylidene species as key intermediates.18 In the presence of a
5j, 89%a
Me
5k, 90%
5g
, 95%
R
Ph
5l
R = p-Me, , 88%
P
R
•
Ph
5m
, 81%
R = p-MeO,
R = p-F, 5n, 85%
5o
O
R
P
Ph
•
O
5h
R = Ph, , 80%
5i
R = Me, , 88%
R = o-Me, , 73%
base, the active Cu(I) species
the copper salt with TMEDA first reacts with the propargyl
acetate to produce the copper acetylide complex . Loss of an
ester group from would form a Cuꢀallenylidene complex
17b The nucleophilic reaction of
with would then produce
an allenylcopper intermediate , protonation of which gives the
A
19 generated from the reaction of
a
R
Scheme 2 Cu-catalysed synthesis of allenyl phosphine oxides (conditions for 5a-5h, 5k-
5o: 0 oC, 2h; conditions for 5i and 5j: 0 oC-rt, 12 h. Isolated yields are given.)
1
B
B
C
.
C
2
noted that these secondary phosphine oxides are readily
available from reactions of diethyl phosphonate with the
corresponding Grignard reagents.17 Thus, a diversity of P(O)
functionalities could be easily incorporated into the allenic
frameworks with the present method.
D
product and regenerate the catalyst (Scheme 3).20,21 A route
involving propargylic substitution to form phosphinite or
phosphite intermediate
E followed by 2,3ꢀrearrangement could
be excluded since the reactions of 1q wherein the propargyl
carbon is covered by two highly sterically crowded tertꢀbutyls,
also took place smoothly to give the products in high yields (eq
6). With respect to this mechanism, the unique selectivity to
form allenyl product is notable as catalytic reactions involving
Cuꢀallenylidenes with other nucleophiles reported thus far all
give propargyl products.18 Detailed mechanistic investigations
is however still needed to elucidate this unusuality.
To gain some insight into the reaction mechanism, several
control experiments were performed. In contrast to terminal
akynes used, we found that internal propargyl acetates 1n and
1o are inactive under the catalytic conditions (eq. 1), indicative
of the necessity of an active CspꢀH bond for the propargylic
substrates. We deduced that a copperꢀalkynide intermediate that
is readily formed from terminal alkynes with copper complexes
under basic conditions may be crucial. Probably due to the
decomposition of these Cuꢀintermediates, 1a was also quickly
consumed to give an unidentifiable mixture in the absence of
the nucleophile 2a (eq. 2), nevertheless 2a was inert and fully
recovered under the same conditions (eq. 3). Furthermore, a
clean transformation of a bisꢀalkynyl substrate 1p into the yneꢀ
allenyl compound 5p in 85% yield was observed while the
possible isomer with the phosphors functionality attached to the
internal triple bonds was not formed (eq. 4). In addition, the
treatment of a simple alkyne with 2a under identical conditions
Scheme 3 A tentative mechanism
Conclusions
In summary, we have disclosed a new Cuꢀcatalyzed reaction
to produce allenylphosphoryl compounds from propargyl
acetates with P(O)H compounds. The reaction utilizes a simple
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Normant, Tetrahedron Lett., 1976, 17, 2313; For other
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and cheap catalyst, takes place under mild conditions in an
environmentally benign solvent and has a broad scope. Further
mechanistic investigations and applications of the method are
currently underway and will be reported soon.
Financial support from the National Natural Science
Foundation of China (21302095), Research Fund for the
Doctoral Program of Higher Education of China
(20133221120003), Jiangsu Provincial NSFC (BK20130924),
and Foundation of Jiangsu Educational Committee of China
(13KJB150019) is acknowledged.
11 Selected examples: (a) J. Li, C. Zhou, C. Fu and S. Ma,
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Notes and references
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Ohmiya, M. Sawamura, Org. Lett., 2012, 14
, 816; (c) U.
Yokobori, H. Ohmiya and M. Sawamura, Organometallics
,
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3
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16 The Cuꢀcatalyzed reactions of alkynes and P(O)H compounds
for the synthesis of unsaturated alkynyl and alkenyl
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Chen, P. Xu, Y. Zhao, Y. Zhou and L.ꢀB. Han, J. Am. Chem.
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20 The reactions conducted in CH3OD (D%>99.5%) afforded
the
21 When
under identical conditions, propargylic substitution product
ꢀmethylꢀ ꢀ(1ꢀphenylpropꢀ2ꢀynꢀ1ꢀyl)aniline) was formed in
α
ꢀdeuterated phosphorylated allenes.
Nꢀmethylaniline was used as nucleophile instead of 2a
(
N
N
89% yield, in agreement to the catalytic propargylic
amination via Cuꢀallenylidene intermediates (Refs. 18aꢀ18c).
10 (a) P. R. Ortiz de Montellano, J. Chem. Soc., Chem.
Commun., 1973, 709; (b) C. Cahiez, A. Alexakis and J. F.
Normant, Synthesis, 1978, 528; (c) G. Tadema, P. Vermeer, J.
Meijer and L. Brandsma, Recl. Trav. Chim. PaysꢀBas, 1976,
95, 66; (d) A. Alexakis, A. Commercon, J. Villiéras and J. F.
4 | J. Name., 2012, 00, 1-3
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