Intermolecular hydrophosphination of alkynes and related carbon-carbon multiple bonds catalyzed by ytterbium-imine complexes

Article abstract of DOI:10.1016/S0040-4039(01)01280-1

Catalytic intermolecular hydrophosphination of alkynes with Ph₂PH has been achieved by using a ytterbium-imine complex, [Yb(η²-Ph₂CNPh)(hmpa)₆]. Thus, both terminal and internal alkynes were converted in high yi

Full text of DOI:10.1016/S0040-4039(01)01280-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6357–6360
Intermolecular hydrophosphination of alkynes and related
carbonꢀcarbon multiple bonds catalyzed by ytterbium–imine
complexes
Ken Takaki,* Mitsuhiro Takeda, Go Koshoji, Tetsuya Shishido and Katsuomi Takehira
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama,
Higashi-Hiroshima 739-8527, Japan
Received 8 June 2001; accepted 13 July 2001
Abstract—Catalytic intermolecular hydrophosphination of alkynes with Ph2PH has been achieved by using a ytterbium–imine
complex, [Yb(h2-Ph2CNPh)(hmpa)6]. Thus, both terminal and internal alkynes were converted in high yields to the corresponding
alkenylphosphines or phosphine oxides after oxidative workup. The present method was also applicable to various carbonꢀcarbon
multiple bonds such as conjugated diynes and dienes, allenes and styrene derivatives. © 2001 Elsevier Science Ltd. All rights
reserved.
a,b-Unsaturated phosphorous compounds are useful
building blocks in organic synthesis. Their catalytic
synthesis has been explored based on the coupling
reaction of alkenyl halides and triflates with dialkyl
phosphites1 and the addition reaction of phosphites and
diphenylphosphine oxide to alkynes.2 However, these
reactions have been rarely applied to the synthesis of
alkenylphosphines.3 Studies on the PꢀH bond activa-
tion of phosphines by group 10 catalysts were limited to
the reaction with activated olefins such as acrylate
esters and acrylonitrile.4 Recently, Marks and co-work-
ers reported a lanthanocene-catalyzed intramolecular
hydrophosphination of alkynyl- and alkenylphos-
phines.5
When diphenylphosphine and equimolar amounts of
1-phenyl-1-propyne (2c) were successively added to a
THF solution of 1 (Ar=Ph, n=6, 5 mol%, 0.1 M) at
room temperature, 1-phenyl-2-diphenylphosphino-1-
propene (3c%) was quantitatively formed within 5 min
(E/Z=80/20). The alkenylphosphine 3c% was isolated in
decreased yield (80%) because of a partial oxidation to
the corresponding alkenylphosphine oxide 3c during
the usual workup and column chromatography.9 Thus,
it was convenient to isolate the reaction product as the
phosphine oxide 3c after oxidation with H2O2. No
reaction took place with Yb(OiPr)3 and SmI2 (10 mol%,
rt, 12 h) as well as the reaction without a catalyst.10
Moreover, many products, including polymeric materi-
n
als, were formed with BuLi (10 mol%, rt, 30 min).
Previously, we have demonstrated that divalent ytter-
bium–imine complexes, [Yb(h2-Ph2CNAr)(hmpa)n] (1),
readily prepared in situ from Yb metal and aromatic
imines in THF and HMPA,6 exhibit good catalytic
activity in the dehydrogenative silylation of terminal
alkynes7 and hydrosilylation of imines.8 We report
herein an intermolecular hydrophosphination of alky-
nes and related carbonꢀcarbon multiple bonds like
conjugated diynes and dienes by using the Yb–imine
complexes 1.
Results on the reaction of various alkynes 2 are sum-
marized in Table 1. The expected products 3 and 4 were
obtained in high yields under mild conditions, except
for aliphatic internal alkynes 2e–f in which the reaction
should be heated at 80°C without the solvent (runs 5
and 6). With respect to regiochemistry, a Ph2P group
was introduced into the opposite side to the aryl sub-
stituents of the aromatic alkynes exclusively (runs 2–4),
and into the less hindered side of the aliphatic alkynes
(runs 6–8). Stereochemistry of the products 3 and 4,
determined by comparison of their JP–H of H NMR
and UV spectra with literature data,11 was dependent
on the substrates, i.e. (E)- and (Z)-isomers were prefer-
entially formed from the aromatic and aliphatic alky-
nes, respectively. In contrast to these results, direct
3
1
Keywords: hydrophosphination; lanthanide catalyst; alkynes;
alkenylphosphines.
* Corresponding author. E-mail: ktakaki@hiroshima-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01280-1

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K. Takaki et al. / Tetrahedron Letters 42 (2001) 6357–6360
Table 1. Hydrophosphination of alkynes with diphenylphosphine
Run
Alkyne
R1
R2
Conditions
3
4
Yielda (%)
E/Z
Yielda (%)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
Ph
Ph
Ph
Ph
Ph
THF, rt, 5 min
THF, rt, 4 h
3a
3b
3c
3d
3e
3f
Quant
Quant
Quant
Quant
95
61
62
52
100/0
100/0
80/20
76/24
0/100
0/100
0/100
27/73
–
SiMe3
Me
H
4b
4c
4d
–
0
0
0
THF, rt, 5 min
THF, rt, 5 min
Neat, 80°C, 6 hb
Neat, 80°C, 6 hb
THF, rt, 3 hb
nPr
nPen
tBu
nHex
nPr
Me
H
4fc
4g
4h
28
10
34
2g
2h
3g
3h
H
THF, rt, 5 min
a GC yield.
b 10 mol% of 1 was used.
c E/Z=21/79.
synthesis of 3 and 4 by the reaction with Ph2P(O)H was
unsuccessful, wherein the catalyst 1 was immediately
oxidized with the phosphine oxide to give some triva-
lent lanthanide species and Ph2PH.
ate A from 1 and diphenylphosphine; (ii) insertion of
alkyne to A to give phosphinoalkenyl ytterbium B; (iii)
formation of the product and regeneration of A by the
protonation of B with Ph2PH. These processes are
similar to those of intramolecular hydrophosphination
with trivalent lanthanocene catalysts.5 With respect to
step (i), the imine complex 1 reacted readily with
equimolar amounts of Ph2PD and, on quenching with
H2O, Ph2CDNHPh and Ph2PH were detected in 65 and
35% yields, respectively, which implies that the phos-
phide species A is generated through a CꢀYb bond
cleavage of 1 with the phosphine.14,15 Lanthanide phos-
phide has been reported to react with THF at elevated
temperature to yield a 4-phosphinobutoxylanthanide
complex.16 Thus, formation of 4-diphenyphosphino-1-
butanol, a by-product obtained in 70% yield based on 1
by the reaction of aliphatic internal alkynes 2e–f in
refluxing THF, also supports the generation of A.
Stereochemistry of the hydrophosphination would not
be determined by isomerization of the products,
because their E/Z ratio was nearly independent of the
reaction time. Thus, (Z)-selectivity for the reaction of
aliphatic alkynes seems to be somewhat curious in the
light of syn addition step (ii). However, this point is not
clear at present.
It has been reported that Ph2PH reacts with alkynes
under radical conditions to yield normally (E)-
diphenylphosphinoalkenes as the primary products,
which finally isomerize to (Z)-isomers.12 Thus, the reac-
tion with AIBN (10 mol%, THF or neat, 80–85°C,
without oxidative workup) was performed for compari-
son with the imine complex 1.13 The radical reaction
gave the alkenylphosphine 3% as single regioisomers in
lower yields than the latter, particularly in the reaction
1
of 2b and 2g. On monitoring by H and 31P NMR,
(Z)-isomers were preferentially formed irrespective of
aromatic and aliphatic alkynes 2, even at an early stage
of the reaction, and this selectivity decreased gradually.
These results clearly indicate that the hydrophosphina-
tion with 1 is different from the radical reaction in
regard to regio- and stereochemistry.
Although several mechanistic aspects remain unclear,
the present reaction would include the following three
steps (Scheme 1): (i) generation of phosphide intermedi-
Scheme 1.

K. Takaki et al. / Tetrahedron Letters 42 (2001) 6357–6360
6359
Table 2. Hydrophosphination of various CꢀC multiple bonds with diphenylphosphinea
In summary, we have developed a new catalytic inter-
molecular hydrophosphination of alkynes with the Yb–
The present hydrophosphination was applied to other
carbonꢀcarbon multiple bonds to examine its scope and
limitations (Table 2). The reaction of 7,9-hexadecadiyne
with Ph2PH (2 equiv.) gave bis(diphenylphos-
phinyl)diene 5a in 51% yield as a single regio- and
stereoisomer after the oxidative workup. In contrast,
2,2,7,7-tetramethyl-3,5-octadiyne was converted to a
mixture of diene 5b and allene 6b in 12 and 71% yields,
respectively, under similar conditions. An attempt to
obtain monophosphinylated products with equimolar
amounts of the phosphine was unsuccessful because of
rapid oligomerization of the diynes even at −78°C.
Although the reaction of aliphatic alkenes did not
occur, a- and b-methylstyrene afforded 7a and 7b in
good yields. In the reaction of isoprene, the Ph2P group
was selectively delivered to the less hindered side of the
two terminal carbons to give 1,4- and 1,2-addition
products 8 and 9 in 64 and 35% yields, respectively.
Two olefinic phosphine oxides 10 and 11 were formed
in 79 and 13% yields, respectively, by the reaction of
cyclohexylallene, but 1-cyclohexyl-3-diphenylphos-
phinyl-1-propene was not detected in contrast to the
radical reaction.12
imine complex
1
to give alkenylphosphines or
phosphine oxides after oxidative workup in high yields.
This method is also applicable to a wide range of
carbonꢀcarbon multiple bonds such as conjugated diy-
nes and dienes, allenes, and styrene derivatives. Further
work on the mechanistic aspects and synthetic applica-
tions is in progress.
Acknowledgements
This work was partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Science, Sports and Culture, Japan.
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