Catalyst-free and catalysed addition of P(O)-H bonds to allenyl/alkynyl-phosphonates and-phosphane oxides: Use of a robust, recoverable dinuclear palladium(I) catalyst

Article abstract of DOI:10.1002/ejoc.201100197

An effective, recoverable, dinuclear palladium(I) catalyst [(OCH ₂CMe₂CH₂O)PSPd(PPh₃)]₂ has been explored and compared with other traditional palladium catalysts (e.g., [Pd(PPh₃)₄]) in the phosphonylation/phosphanylation ofallenes (OCH₂CMe₂CH₂O)P(O)CH=C=CH₂ (1), Ph₂P(O)CH=C=CH₂ (2), (EtO)₂P(O)CH=C= CH₂ (3) (OCH₂CMe₂CH₂O)P(O)CH=C= CMe₂ (4), Ph₂P(O)CH=C=CMe₂ (5), (OCH ₂CMe₂CH₂O)P(O)C(Ph)=C=CH₂ (6) and Ph₂P(O)C(Ph)=C=CH₂ (7). The phosphonylation/ phosphanylation, in general, occurred at the carbon β to the phosphorus atom, but the concomitant proton addition took place at the α or γ positions leading to either allyl-or vinyl-phosphonates. The use of P(nBu) ₃ as catalyst led to geminal and bis-phosphonylation/phosphanylation with less substituted =CH₂ terminal allenes 1 and 2. In conjunction with the use of the corresponding isomeric alkynes 8 and 9, as many as five different types of phosphonylated products have been synthesized. The reactions with the more substituted allenes 4-7 gave single products in most cases. Several examples of catalyst-free, solvent-free phosphanylation reactions are also described. The reactivity of the phosphonylating/phosphanylating agents was found to be (OCH₂CMe₂CH₂O)P(O)H (10) < (OCH₂CMe₂CH₂O)P(S)H (11) < Ph ₂P(O)H (12) ≈ Ph₂P(S)H (13). The catalytic activity of the recoverable dinuclear palladium(I) complex [(OCH₂CMe ₂CH₂O)PSPd(PPh₃)]₂ (14), which poses interesting questions about the mechanistic pathway, is briefly highlighted. Structures of the dinuclear palladium(I) catalyst 14 and the key products were determined by X-ray crystallography.

Full text of DOI:10.1002/ejoc.201100197

FULL PAPER
DOI: 10.1002/ejoc.201100197
Catalyst-Free and Catalysed Addition of P(O)–H Bonds to Allenyl/Alkynyl-
Phosphonates and -Phosphane Oxides: Use of a Robust, Recoverable Dinuclear
Palladium(I) Catalyst
Venu Srinivas,[a] E. Balaraman,[a] K. V. Sajna,[a] and K. C. Kumara Swamy*[a]
Keywords: Allenes / Phosphonylation / Palladium / Phosphorus
An effective, recoverable, dinuclear palladium(I) catalyst
[(OCH2CMe2CH2O)PSPd(PPh3)]2 has been explored and
compared with other traditional palladium catalysts (e.g.,
[Pd(PPh3)4]) in the phosphonylation/phosphanylation of
allenes (OCH2CMe2CH2O)P(O)CH=C=CH2 (1), Ph2P(O)-
CH=C=CH2 (2), (EtO)2P(O)CH=C=CH2 (3) (OCH2-
CMe2CH2O)P(O)CH=C=CMe2 (4), Ph2P(O)CH=C=CMe2 (5),
(OCH2CMe2CH2O)P(O)C(Ph)=C=CH2 (6) and Ph2P(O)-
C(Ph)=C=CH2 (7). The phosphonylation/phosphanylation, in
general, occurred at the carbon β to the phosphorus atom,
but the concomitant proton addition took place at the α or γ
positions leading to either allyl- or vinyl-phosphonates. The
use of P(nBu)3 as catalyst led to geminal and bis-phosphonyl-
sponding isomeric alkynes 8 and 9, as many as five different
types of phosphonylated products have been synthesized.
The reactions with the more substituted allenes 4–7 gave sin-
gle products in most cases. Several examples of catalyst-free,
solvent-free phosphanylation reactions are also described.
The reactivity of the phosphonylating/phosphanylating
agents was found to be (OCH2CMe2CH2O)P(O)H (10) Ͻ
(OCH2CMe2CH2O)P(S)H (11) Ͻ Ph2P(O)H (12) ≈ Ph2P(S)H
(13). The catalytic activity of the recoverable dinuclear palla-
dium(I) complex [(OCH2CMe2CH2O)PSPd(PPh3)]2 (14),
which poses interesting questions about the mechanistic
pathway, is briefly highlighted. Structures of the dinuclear
palladium(I) catalyst 14 and the key products were deter-
ation/phosphanylation with less substituted =CH2 terminal mined by X-ray crystallography.
allenes 1 and 2. In conjunction with the use of the corre-
Introduction
A variety of unsaturated organic systems undergo hydro-
phosphonylation/phosphanylation to yield a diverse class of
organophosphonates including vinyl/allylphosphonates.[1]
Because organophosphonates are of great synthetic utility
and exhibit biological activity,[2] we have become interested
in this class of compounds.[3] Although there are examples
of P–C bond-forming reactions conducted under catalyst-
free conditions,[1i,4] in the majority of cases transition-metal
catalysts,[5] radical initiators,[6] strong bases or Lewis acids,
or microwave (MW) assistance[7] are employed. Significant
success has been achieved in these reactions and a selection
of examples are shown in Scheme 1.[8]
In a recent paper we showed that hydrophosphonylation
can also be activated via pentacoordinate phosphorus spe-
cies through the use of tetrabutylammonium fluoride in an
ionic liquid medium;[9] the involvement of P–F-bonded
pentacoordinate phosphorane[10] in this reaction was estab-
Scheme 1.
troscopy. Double phosphonylation has also been reported
in some cases.[7c,8f,11] In the reactions described above, the
substrates are generally alkenes or alkynes; reports of reac-
tions using allenes[12] are rather rare and only a few exam-
ples are available.[13] Our interest in this connection was pri-
marily to explore the phosphonylation/phosphanylation of
inexpensive allenylphosphonates/allenylphosphane oxides
such as 1–7[3e,14] under different conditions and, where pos-
sible, to compare these with the reactions of alkynes (e.g.,
8 and 9).[15] We have restricted the phosphonylating/phos-
1
lished by a combination of 31P, 19F and H NMR spec-
[a] School of Chemistry, University of Hyderabad,
Hyderabad 500 046, A. P., India
Fax: +91-40-23012460
E-mail: kckssc@yahoo.com
kckssc@uohyd.ernet.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100197.
4222
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Addition of P(O)–H Bonds to Phosphonates and Phosphane Oxides
phanylating agents to 10–13 for ease of comparison. Note with a Pd–Pd distance of 2.607(1) Å.[16c,17] This type of re-
that the attack of the phosphorus moiety in the phos- action involving dihydrogen elimination has previously been
phonylation/phosphanylation can take place at the α, β or discussed by Walther et al. and hence is not elaborated
γ position of the allene (cf. structure 1). In comparison with here.[16a]
the alkynes (cf. 1 and 8), we were also curious to know
whether or not dehydrogenative phosphonylation [cf.
Scheme 1 (c)][8f] occurs in our system. Finally, we report the
isolation of the dinuclear palladium(I) complex 14, which
acts as a “recoverable” catalyst; this, we believe, has some
relevance to the mechanism because in a large number of
palladium-catalysed reactions the initial palladium com-
pound [e.g., [Pd(PPh3)4] or Pd(OAc)2] acts only as a “pro-
catalyst” and is not recovered subsequent to the reaction.
We report these results herein.
Figure 1. ORTEP diagram of 14·2C4H8O2. Solvent molecules have
been omitted for clarity. Selected bond lengths [Å] with estimated
standard deviations in parentheses: Pd1–Pd2 2.607(1), Pd1–P1
2.211(1), Pd1–P4 2.323(1), Pd1–S2 2.398(1), Pd2–P2 2.206(1), Pd2–
P3 2.317(1), Pd2–S1 2.385(1), P1–S1 2.017(2), P2–S2 2.016(2).
Reactions of Allenes R2P(O)C(H)=C=CH2 [R2 =
OCH2CMe2CH2O (1), R = Ph (2), OEt (3)] and Alkynes
R2P(O)CϵCCH3 [R2 = OCH2CMe2CH2O (8), R = Ph
(9)] with RЈ2P(X)H [RЈ2 = OCH2CMe2CH2O, X = O (10)
or S (11); RЈ = Ph, X = O (12) or S (13)]
The reaction of allene 1 with the cyclic H-phosphonate
10 was conducted first in the presence of [Pd(PPh3)4]
[Scheme 3 (a), Table 1], which yielded the products 15–17.
Among the solvents used, the best combined yield was ob-
tained in 1,4-dioxane. Hence this solvent was employed in
Results and Discussion
subsequent reactions. There was no reaction in the absence
of [Pd(PPh3)4]. Also, the reaction did not proceed in the
Formation/Synthesis of the Dinuclear Palladium(I)
presence of bases like Et3N, KOtBu or K2CO3. The com-
Compound 14
pounds are readily distinguishable by 31P NMR spec-
troscopy. A lower 3J(P,P) value for 15 (27.5 Hz) is consistent
The dinuclear palladium(I) compound 14 was initially
3
with three intervening single bonds whereas a large J(P,P)
value in 16 (99.2 Hz) is due to trans coupling. The long-
range coupling constant 4J(P,P) for 17 (7.2 Hz) is very
small, as expected. The identity of the major product 15
was further confirmed by X-ray crystallography (see Fig-
ure S1 of the Supporting Information). The allylphos-
phonate 15 and the vinylphosphonate 16 result from β at-
tack of the H-phosphonate followed by proton addition at
the α or γ position, respectively. The third product 17, al-
though obtained in a lower yield, was rather unexpected
and was formed by attack of the H-phosphonate residue at
the γ position. This compound should have formed only
from the allene 1 [i.e., not after rearranging to the phos-
phono-alkyne (OCH2CMe2CH2O)P(O)CϵCMe (8)]. The
use of P(nBu)3[8c] as a catalyst in this reaction did not afford
any identifiable product. Interestingly, although other palla-
dium compounds like PdCl2, Pd(OAc)2, [PdCl2(PPh3)2],
obtained as a precipitate in a hydrothiophosphonylation re-
action. Subsequently, an easier direct route (Scheme 2) pro-
vided the compound in excellent yield (95%). Although
similar compounds have been described in the literature,[16]
we are not aware of any report concerning their use in cata-
lytic reactions. This compound shows only a doublet of
doublets with a low J(P,P) value of 12.2 Hz in the 31P NMR
spectrum, similar to that reported in the literature, and the
X-ray structure (Figure 1, see Supporting Information for
details) clearly establishes the presence of a dinuclear motif
[Pd2(dba)3]
(dba
=
dibenzylideneacetone)
and
[PdCl2(PhCN)2]/methyl acrylate[8f] were not effective in
this transformation, the dinuclear palladium(I) complex
Scheme 2.
Eur. J. Org. Chem. 2011, 4222–4230
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V. Srinivas, E. Balaraman,K. V. Sajna, K. C. Kumara Swamy
FULL PAPER
[(OCH2CMe2CH2O)PSPd(PPh3)]2 (14) was fairly effective Table 1. Effect of solvent on the palladium-catalysed reaction of
allene 1 with H-phosphonate 10 [Scheme 3 (a)].[a]
(Table 1, entry 6) and produced mainly 15 and 16 (com-
bined yield 95%; ratio 2:3). Equally significant is the fact
that the palladium complex (14) could be recovered (see the
Supporting Information) from the reaction mixture,
whereas [Pd(PPh3)4] could not be recovered. Thus, 14 is a
true “catalyst” (or at least resting form of the catalyst)
whereas the latter is only a “pro-catalyst”. From our point
of view, this observation has important implications for the
mechanistic role played by palladium compounds in such
reactions. We also conducted the reaction of (EtO)2P(O)-
C(H)=C=CH2 (3) with (OCH2CMe2CH2O)P(O)H (10). Al-
though we could identify products similar to 15–17 (see the
Supporting Information), the isolation of individual com-
pounds by chromatography/crystallization/distillation was
not successful in our hands.
Entry Solvent
T [°C]
t [h]
Ratio of 15/16/17 Combined
yield[b] [%]
1
2
3
4
5
6
toluene
DMF
CH3CN
THF
1,4-dioxane
1,4-dioxane
110
100
80
65
100
100
24
6
14
14
6
1.5
2.3
3.0
2.5
1.2
1.3
1.5
1.0
1.0
1.0
1.0
1.0
37
46
55
45
95
95
4.0[c] 2.2[c] 1.0[c]
6
2.0
3.0
–
[a] [Pd(PPh3)4] was used as the catalyst/pro-catalyst for entries 1–5;
for entry 6, catalyst 14 was used. [b] Yields were calculated using
31P NMR spectroscopy. [c] The isolated yields are, respectively, 40,
22 and 10%.
pound 18 is similar in structure to compound 15 discussed
above. It has a terminal double bond and readily undergoes
further phosphonylation with 11 leading to the tris-phos-
phonate 20 (see Figure S2 of the Supporting Information
for the X-ray structure). Unlike the reaction with 10, we did
not observe any product similar in structure to 17. Taking
into account the fact that there is bis-phosphonylation, the
reaction is essentially quantitative with all the thio-H-phos-
phonate 11 reacting and some of the remaining allene con-
verting into the alkyne 8. Addition of more compound 11
(2 mol equiv. with respect to 1) increased the yield of 20 as
expected. Catalyst 14 or P(nBu)3 also led to the same prod-
ucts (31P NMR) but along with some alkyne 8. This is ex-
pected because we used a 1:1 stoichiometry and the unre-
acted allene isomerized to the less reactive alkyne 8.
Table 2 and Scheme 3 (a) show the results of the reac-
tions of allenylphosphane oxide 2 with 10 and 11. The
products are 21–25. The X-ray structure of 22 was deter-
mined (see Figure S3 of the Supporting Information). The
main difference between the reactions of H-phosphonate 10
with 1 or 2 is that in the latter case, a γ,β-addition product
analogous to 17 was not isolated. Compounds 14 and
[Pd(PPh3)4] are the most effective catalysts/pro-catalysts.
The yield of the bis-phosphonylated product 25, which is
similar in structure to 20, could be optimized to 90% by
increasing the stoichiometry of the thio-H-phosphonate 11
(2 mol equiv. with respect to 2). In the reaction of 2 with
11 (using 14 as the catalyst) we recovered catalyst 14 and
reused it (see the Supporting Information). The reaction of
(EtO)2P(O)C(H)=C=CH2 (3) with 11 led to similar prod-
ucts (see Supporting Information) but because of the simi-
larities in the Rf values, pure products could not be isolated.
To fully unravel the synthetic potential as well as the re-
action pathway, we also treated the alkyne (OCH2CMe2-
CH2O)P(O)CϵCCH3 (8) with (OCH2CMe2CH2O)P(O)H
Scheme 3.
In contrast to the above, allene 1 reacted with the thio- (10) in the presence of catalytic amounts of [Pd(PPh3)4] or
H-phosphonate (OCH2CMe2CH2O)P(S)H (11) quantita- palladium complex 14. In the former case, only compound
tively to give both the mono- (18) and bis-phosphonylated 16 was obtained in a very clean reaction, whereas with the
(20) products [Scheme 3 (a)] along with minor quantities of latter the isomeric compounds 15 and 16 (3:2 ratio by 31P
19. Compound 11 is more reactive than 10 in the phos- NMR) were obtained quantitatively, again in essentially a
phonylation reaction. The greater reactivity of compounds very clean reaction [Scheme 3 (b)]. This suggests that the
bearing the P(S)H functionality compared with those with reactions of allene 1 or alkyne 8[18] with cyclic H-phos-
P(O)H has been noted earlier by other workers.[6f] Com- phonate 10 using the catalyst 14 may involve some common
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Addition of P(O)–H Bonds to Phosphonates and Phosphane Oxides
Table 2. Reactions of 2 with 10 and 11 leading to the formation of
21 and 22 and 23 and 24, respectively [cf. Scheme 3 (a)].[a]
Table 3. Details of the reactions of 8/9 with 10/11 leading to com-
pounds 15/16, 19, 21/22, 24 and 26 [cf. Scheme 3 (b)].
Entry Pd catalyst
X
Time [h] Combined Ratio 21/22
Entry Reactants
Conditions/
duration [h][a] (ratio)
Product(s)[b]
Combined
yield [%][c]
yield [%][b]
or 23/24
1
2
3
4
5
6
7
8
9
none
O
O
O
O
O
S
S
S
S
S
36
36
36
10
12
36
36
36
4
n.r.
36
40
80
80
50
70
–
1:0
3:2
3:2
5:1
1:0
1
2
3
4
5
6
7
8 + 10
8 + 10
8 + 11
8 + 11
9 + 10
9 + 10
9 + 11
i/6
ii/6
i or ii/5
iii/1
i/6
ii/7
16
quantitative
15 + 16 (3:2) quantitative
Pd(OAc)2
[Pd2(dba)3]
[Pd(PPh3)4]
14
19
26
21
quantitative
quantitative
90
none
21 + 22 (3:2) 90
24 90
Pd(OAc)2
[Pd2(dba)3]
[Pd(PPh3)4]
14
17:1
9:1
i or ii/8
60
[a] Conditions i: [Pd(PPh3)4] (5 mol-%)/1,4-dioxane, 100 °C. Condi-
tions ii: catalyst 14 (2.5 mol-%)/1,4-dioxane, 100 °C. Conditions iii:
P(nBu)3 (20 mol-%)/EtOH, reflux. [b] Isolated yields of individual
compounds are given in the Supporting Information (because dif-
ferent methods gave different ratios of the products). [c] Based on
31P NMR spectroscopy.
90[c]
90[c]
3:1[d]
3:1[d]
10
4
[a] All reactions were conducted at 100 °C (oil-bath temperature).
[b] Based on 31P NMR analysis. [c] Combined yield of 23, 24 and
25. [d] The ratio of 25/(23 + 24) was 3:5 based on the stoichiometry
of 1:1.1 for 2/11. The rest was starting material as a result of the
bis-phosphonylated product 25. The yield of 25 could be optimized
by adding more of 11 (2 mol equiv. with respect to 2).
intermediates (see below). There was no reaction in the ab-
sence of the catalyst. The behaviour of alkyne 9 was similar
to that of 8. Thus, in its reaction with 10, a single isomer
22 was obtained by using [Pd(PPh3)4] whereas an isomeric
mixture of 21 and 22 was obtained in a 3:2 ratio (31P NMR)
by using 14 as the catalyst.
The reaction of alkyne 8 with (OCH2CMe2CH2O)P(S)H
(11) was distinctive. In the presence of [Pd(PPh3)4] or 14,
we obtained only the E-phosphonylated alkene 19
[Scheme 3 (b), Table 3]. More interestingly, the P(nBu)3/eth-
anol-catalysed reaction[8c] led to the bis-phosphonylated
product 26, which is different to 20! The yield of 26 could
be enhanced by adding more 11 (2 mol equiv. with respect
to 8). In the assignment of structures 20 and 26, it must be
noted that J(P,P) values could vary and in specific cases
could be close to zero (see the Supporting Information for
the X-ray structure of an analogous compound). Hence the
expected multiplet pattern was not seen. The reaction of
alkyne 9 with 11 under palladium-catalysed conditions led
to the single product 24. Thus, three different types of phos-
phonylated products, 15/21, 16/22 and 26, were isolated in
this set of reactions by using alkyne 8 or 9 and H-phos-
phonate/thio-H-phosphonate 10 or 11.
The reactions of allene 1 with Ph2P(X)H [X = O (12), S
(13)] did not require the use of a catalyst [Scheme 4 (a),
Table 4]. Although we used the palladium complexes ini-
tially, this was found to be unnecessary in these cases, which
shows that 12 and 13 are significantly more reactive than
10 and 11 in the mono-phosphanylation reaction with 1.
There was no reaction at room temp. either in the presence
or in the absence of the catalyst. The overall yields were
excellent (quantitative, by 31P NMR). In the reaction with
12, isomeric β-phosphanylated products 27 and 28 were ob-
tained; neither a γ-phosphanylated product similar to 17
Scheme 4.
nor a bis-phosphanylated derivative was observed. With present in the reaction mixture. The reaction using
Ph2P(S)H (13), compound 29 was the sole product isolated; P(nBu)3 was not clean but contained 27 and 28 (or 29 and
only a minor amount (10%, 31P NMR) of isomer 30 was 30) along with the alkyne 8 (31P NMR).
Eur. J. Org. Chem. 2011, 4222–4230
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V. Srinivas, E. Balaraman,K. V. Sajna, K. C. Kumara Swamy
FULL PAPER
Table 4. Details of the reactions of allenes 1, 2 or 3 with 12 or 13
leading to compounds 27–36 [cf. Scheme 4 (a)].[a]
In contrast to the above, the reaction using alkyne 9 and
Ph2P(O)H (12) under solvent-free and catalyst-free condi-
tions afforded the isomers (E)-32 and (Z)-40 [Scheme 4 (b),
Table 5]. More importantly, compound (Z)-40 isomerized
to the β,α product 31 upon further heating [Scheme 4 (c)].
This process could be conveniently monitored by 31P NMR
spectroscopy (Figure 2). To the best of our knowledge, such
an isomerization has not been recorded in the literature.
As regards compound 32, thermodynamic factors may be
responsible for its resistance to a similar isomerization.
Entry
Reactants
Product(s)[b] [ratio] Combined yield [%][c]
1
2
3
4
5
6
1 + 12
1 + 13
2 + 12
2 + 13
3 + 12
3 + 13
27 + 28 [9.2:0.8]
29 + 30 [9:1]
31 + 32 [9.5:0.5]
33 + 34 [9.2:0.8]
35
quantitative
quantitative
quantitative
quantitative
94
36
98
[a] Conditions: no catalyst, no solvent, 100 °C, 1 h. [b] Isolated
yields of the individual compounds 27–36 are given in the Support-
ing Information. [c] Based on 31P NMR spectroscopy.
The reactions of allenylphosphane oxide Ph2P(O)-
C(H)=C=CH2 (2) with Ph2P(X)H [X = O (12), S (13)] led
to the β,α (31 or 33) or β,γ-E (32 or 34) isomers [see
Scheme 4 (a), Table 4]. We did not observe Ph2P(O)OH (by
oxidation of 12) in these reactions. For comparative pur-
poses, we also carried out the hydrophosphanylation of the
allene (EtO)2P(O)C(H)=C=CH2 (3) with Ph2P(X)H [X = O
(12), S (13)]. These reactions afforded essentially the β,α-
P(X)–H addition products 35 and 36 [cf. Scheme 4 (a)].
Thus, these results are similar to those obtained by using
allene 1.
In the reactions of alkyne 8 with 12 or 13 under palla-
dium-catalysed conditions in 1,4-dioxane, a single product
(28 or 30, respectively) was obtained almost exclusively
[Scheme 4 (b), Table 5]. In contrast, under P(nBu)3-cata-
lysed conditions, in the reaction of 8 with 12 the bis-phos-
phanylated product 38 was the major product along with
the corresponding geminal isomer 37. The product 38 is
similar to 26 obtained from the reaction of 8 with
(OCH2CMe2CH2O)P(S)H (11), as shown in Scheme 3.
However, analogous geminal products were not seen when
Ph2P(S)H (13) was used. Here, the products (total yield
88%) were (E)-30 and (Z)-39 (see Figure S4 in the Support-
ing Information for the X-ray structure). Overall, five dif-
ferent types of phosphanylated products (27/29, 28/30, 37,
38 and 39) were obtained in this set of reactions using allene
1 or its alkyne isomer 8.
Figure 2. 31P NMR spectra of (a) 9 with Ph2P(O)H (12) under sol-
vent-free, catalyst-free conditions at 100 °C after 30 min, (b) 9 with
Ph2P(O)H (12) under solvent-free, catalyst-free conditions at
120 °C after 18 h, (c) the pure Z isomer 40 and (d) compound 31
obtained by heating 40 at 120 °C for 18 h.
The P(nBu)3-catalysed reaction of alkyne 9 with Ph2P-
(O)H (12) afforded only the geminal products 41 and 42,
thus providing an altogether new set of compounds
[Scheme 4 (b), Table 5]. The structure of the bis-phosphany-
lated product 42 was confirmed by X-ray crystallography
(see Figure S5 in the Supporting Information) and suggests
that it does not result from the γ-phosphonylated product
43.[8c] In the reaction of 9 with Ph2P(S)H (13), compound
34 was the predominant product irrespective of the reaction
conditions.
Table 5. Details of the reactions of alkynes 8 or 9 with 12 and 13
leading to compounds 28, 30, 32, 34, 37–42 [cf. Scheme 4 (b)].
Entry Reactants
Condition/
Product(s)[b] [ratio]
Combined
yield [%][c]
duration [h][a]
1
2
3
4
5
6
7
8 + 12
8 + 12
8 + 13
8 + 13
9 + 12
9 + 12
9 + 13
i/8
ii/1
i/6
ii/1
iii/1
ii/1
i/6
28
quantitative
90
quantitative
37[d] + 38 [1:2]
30
30 + 39 (X-ray) [2:3] 88
32 + 40 [3:2]
41+ 42 (X-ray) [1:3] 80
34
34
quantitative
95
82
ii/1
A possible mechanism for the formation of compounds
15–17 is shown in Scheme 5. Species II and IIЈ are analo-
gous to those proposed by Zhao et al.[13a] As can be readily
seen, the Z alkene IV should have been formed, but we
could not identify it in the reaction mixture. Note that in
lieu of I, it is also possible to have an intermediate of type
V,[19] but this would require 2 mol equiv. of H-phosphonate
[a] Conditions i: [Pd(PPh3)4] (5 mol-%) or 14 (2.5 mol-%)/1,4-diox-
ane, 100 °C. Conditions ii: P(nBu)3 (20 mol-%)/EtOH, reflux. Con-
ditions iii: Solvent-free, catalyst-free, 100 °C. [b] The isolated yields
of individual compounds are given in the Supporting Information.
[c] Based on 31P NMR spectroscopy. [d] Although compound 38
could be easily separated, the minor component 37 was obtained
along with (O)P(nBu)3.
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Addition of P(O)–H Bonds to Phosphonates and Phosphane Oxides
per palladium. What is perhaps more intriguing is the na- arrangement] was remaining after 4 h. Previous reports on
ture of the intermediate obtained with our dinuclear cata- the P(nBu)3-catalysed reactions of alkynylphosphonates
lyst 14; a possible structure is VI or VII. However, at the suggest that a geminal phosphonylation/phosphanylation
moment this is only speculation. Because we observed that occurs in most cases.[8c] On this basis, the formation of bis-
catalyst 14 can be recovered after the reaction, it is possible phosphonylated product 26 may be rationalized as shown
that the PPh3 ligand may still be present on the two palla- in Scheme 6 with species IX as an intermediate.[20] In the
dium centres in the intermediate stages. In any case, the reactions with allene 1, it is possible that the phosphane
catalytic activity of 14 poses interesting challenges to our attacks the β carbon.[21] The formation of phosphonylated/
understating of these palladium-catalysed reactions. The phosphanylated compounds 21–26 may be explained on the
formation of the γ,β product 17 is possible only from allene basis of the data presented in Schemes 5 and 6. The reac-
1 and not from its alkyne isomer 8. In the palladium-cata- tions of allenes 1–3 with 12 or 13 under solvent-free, cata-
lysed reactions using alkyne 8, we obtained either 16 or 15 lyst-free conditions most likely involves a radical mecha-
+ 16; a possible intermediate in the latter case is VIII.
nism, as discussed elsewhere.[4b] We conducted the reaction
of allene 2 with Ph2P(S)H (13) in 1,4-dioxane at 100 °C for
1 h in the absence as well as in the presence of p-hydroqui-
none (10 mol-%). In the former case, the reaction went to
completion whereas in the latter case, only around 30% of
allene 2 reacted. With 80 mol-% of the p-hydroquinone, the
reaction was essentially completely inhibited. These obser-
vations too suggest a radical mechanism. The palladium-
and P(nBu)3-catalysed reactions shown in Scheme 4 may
also be rationalized by the discussion presented here.
Scheme 6.
Scheme 5.
Hydrophosphonylation/Hydrothiophosphonylation/
Hydrophosphanylation/Hydrothiophosphanylation of the
Substituted Allenylphosphonates/Allenylphosphane Oxides
(OCH2CMe2CH2O)P(O)CH=C=CMe2 (4), Ph2P(O)-
CH=C=CMe2 (5), (OCH2CMe2CH2O)P(O)-
C(Ph)=C=CH2 (6) and Ph2P(O)C(Ph)=C=CH2 (7)
In contrast to the reactions with 1 and 2, the reactions
of the substituted allenylphosphonates/allenylphosphane
oxides 4–7 with 10–13 were less complicated and in most
cases, the β,α derivatives 44–51, 53–55 and 57–61 were the
Clearly, the reaction of 1 with the thio-H-phosphonate major products (Table 6). Of these, 44 and 46 are known
11 in the presence of [Pd(PPh3)4] should involve the dinu- compounds.[9] There was no significant difference in the
clear palladium(I) complex 14 (cf. Scheme 2) and hence an products formed using either [Pd(PPh3)4] or 14 in the reac-
intermediate similar to VI or VII. A possible explanation tions that we investigated. In several cases (products 53–
for the formation of the bis-phosphonylated product 20 is 55 and 57–61), the reaction under solvent-free, catalyst-free
that the mono-phosphonylated alkene 18 is more reactive conditions worked well, in particular, Ph2P(O)H was quite
than its precursor 1. The formation of 19 can be explained reactive. One important difference though is that in the pal-
in a manner similar to that for 16. As expected, some of ladium-catalysed reactions the γ,β products (e.g., 52 and 56)
the allene 1 (in the palladium-catalysed reaction) or the iso- were observed as minor products (ca. 10%) but were absent
meric alkyne 8 [in the P(nBu)3-catalysed reaction due to re- in the reactions conducted under solvent-free, catalyst-free
Eur. J. Org. Chem. 2011, 4222–4230
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FULL PAPER
Table 6. Details of the products 44–61 obtained from the reactions conditions. The X-ray structures of 52 and 60 are given in
of 4–7 with 10–13.
the Supporting Information (Figure S6). The phosphane
P(nBu)3 was not an effective catalyst in the reactions that
we monitored and hence these data are not included here.
The formation of products 44–61 may be rationalized on
the basis of the discussion presented above.
Further Comments on the Reaction Pathways
The three routes [palladium-catalysed, P(nBu)3-catalysed
and catalyst-free] employed herein for the hydrophos-
phonylation/hydrophosphanylation reactions operate by
different mechanistic pathways. Although [Pd(PPh3)4] has
been widely used, it is likely that palladium intermediates
in the reactions of allenes with (OCH2CMe2CH2O)P(O)H
(10) are different to those formed with (OCH2CMe2CH2O)-
P(S)H (11) because in the latter case the dinuclear com-
pound [(OCH2CMe2CH2O)PSPd(PPh3)]2 (14) is formed
whereas in the former case there is no precedence for a sim-
ilar dinuclear species.
Conclusions
We have accomplished the isolation and structural char-
acterization of as many as five distinct types of products
(27/29, 28/30, 37, 38, 39, and 31/33, 32/34, 40, 41, 42) in
phosphonylation/phosphanylation reactions through the
appropriate choice of catalytic conditions and by making
use of the facile isomerization of =CH2 terminal allenyl-
phosphonates to phosphonoalkynes, the synthetic utility of
a new robust, dinuclear “recoverable” palladium(I) catalyst
[(OCH2CMe2CH2O)PSPd(PPh3)]2 (14) has been estab-
lished. The catalytic activity of this compound implies new
reaction (mechanistic) pathways are available for phos-
phonylation/phosphanylation. We have demonstrated this
in a recent paper.[22] The isomerization 40Ǟ31 of the phos-
phanylated product reported herein should serve as a caveat
in the interpretation of the yields of different products be-
cause such a process could have been operative in earlier
studies also. The observed reactivity of the phosphonylat-
ing/phosphanylating agents [OCH2CMe2CH2O)P(O)H (10)
Ͻ (OCH2CMe2CH2O)P(S)H (11) Ͻ Ph2P(O)H (12) ≈
Ph2P(S)H (13)] is fairly consistent with the available litera-
ture.[6f]
Experimental Section
General: Details on the isolation and spectroscopic and analytical
data of individual compounds and instrumental methods are given
[a] These reactions were conducted by using [Pd(PPh3)4] as well as
palladium(I) catalyst 14: the yields were the same. [b] These reac- in the Supporting Information.
tions were conducted under solvent-free, catalyst-free conditions.
CCDC-672231 (for 15) and -774710 to -774716 (for 14, 20, 39, 22,
[c] This compound was also prepared by another route.[9] [d] This
is the combined yield; the minor product 52 or 56 was obtained in
10% yield (31P NMR). [e] This reaction under solvent-free, catalyst-
free conditions gave 55 quantitatively. [f] In this case we did not
observe the (γ,β)-phosphanylated product.
42, 52, and 60, respectively) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Supporting Information (see footnote on the first page of this arti-
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data, additional ORTEP diagrams and the 1H and 13C NMR spec-
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[6]
Acknowledgments
We acknowledge the Department of Science and Technology
(DST), New Delhi and the University Grants Commission, New
Delhi for funding and equipment. We also thank the Council of
Scientific and Industrial Research (CSIR), New Delhi for fellow-
ships to V. S., K.V.S. and E.B. K.C.K.S. also thanks the DST for a
J. C. Bose Fellowship.
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FULL PAPER
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Received: February 14, 2011
Published Online: June 1, 2011
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