Control of hydroboration of 1-alkynylphosphonates, followed by Suzuki coupling provides regio- and stereospecific synthesis of di-substituted 1-alkenylphosphonates

Article abstract of DOI:10.1016/S0040-4039(01)01696-3

Hydroboration of 1-alkynylphosphonates can be controlled to place boron on either C1 or C2 of the triple bond by control of reaction conditions. Initial hydroboration occurs on C1 (kinetic product), which can be isomerized to place boron on C2 (thermodyna

Full text of DOI:10.1016/S0040-4039(01)01696-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8059–8062
Control of hydroboration of 1-alkynylphosphonates, followed by
Suzuki coupling provides regio- and stereospecific synthesis of
di-substituted 1-alkenylphosphonates
Inna Pergament and Morris Srebnik*
Department of Medicinal Chemistry and Natural Products, School of Pharmacy, Hebrew University in Jerusalem, POB 12065,
Jerusalem 91120, Israel
Received 25 June 2001; accepted 7 September 2001
Abstract—Hydroboration of 1-alkynylphosphonates can be controlled to place boron on either C1 or C2 of the triple bond by
control of reaction conditions. Initial hydroboration occurs on C1 (kinetic product), which can be isomerized to place boron on
C2 (thermodynamic product) by extended heating or by use of large amounts of catalyst. Suzuki reaction of the hydroboration
products with aryliodides provides a regio- and stereospecific route to disubstituted vinylphosphonates © 2001 Elsevier Science
Ltd. All rights reserved.
1-Alkenylphosphonates are very useful compounds for
organic transformations1 and for the synthesis of bio-
determine the placement of boron in 1-alkynylphospho-
nates. The results are the basis of this communication.
logically
active
compounds.2
Organometallic
When alkynylphosphonates (31P NMR ꢀ−5 to −7
ppm), 1, were hydroborated with pinacolborane
(PBH) in CH2Cl2 at 25°C, a series of peaks appeared in
the 31P NMR in the range +15–20 ppm, which we
attributed to coordination complexes between boron
and phosphorus oxygens. Boron was directed to carbon
C1 of the triple bond. Workup at this point provided
one compound, 2. Compounds 2 resonate in 31P NMR
in the +31 ppm range and their 11B NMR shifts are
approaches to the synthesis of these compounds include
syn addition of organocuprates to 1-alkynylphospho-
nates to give 2,2-disubstituted vinylphosphonates,3
reaction of a-stannylated phosphonates with aldehydes
to give E/Z mixtures of 1,2-disubstituted vinylphospho-
nates,4 anti hydrotelluration of 1-alkynylphosphonates,5
hydrozirconation of alkynes followed by phosphoryla-
tion.6 Heck reactions using aryldiazonium salts,7 a-lithia-
tion of b-oxy or b-thio vinylphosphonates,8 NaH
catalyzed olefination of benzenesulfinylmethylphospho-
nates,9 and addition of sodium organyl chalcogenolates
to 1-alkynylphosphonates.10 These methods and others
at best give one regio or stereo isomer. In practice,
mixtures are obtained. A regio- and stereoselective syn-
thesis of disubstituted 1-alkenylphosphonates would be
highly desirable.
1
around +30 ppm. H NMR spectra were difficult to
interpret because of overlapping signals. In 13C NMR,
carbon C1 could not be identified due to the boron
quadrapole, but C2 resonated as a doublet (JCPꢀ11
Hz) at about 160 ppm. If compounds 2 were heated to
reflux in toluene, they slowly disappeared during 48 h
and were replaced by new peaks in the GC–MS. These
new peaks were identified as 4 on the basis of their
NMR spectra. Subsequently, the reaction could be
monitored by GC–MS. The conversion from 2 to 4 was
accelerated by the addition of palladium salts.12 We
found that the nature of the catalyst affected the rate of
conversion. The best catalyst for obtaining 4 was PdCl2.
Suzuki coupling is an important application of vinyl-
boronates. Coupling of the vinylboronophosphonates
with aryliodides would be expected to give
stereodefined vinylphosphonates.13 That indeed proved
to be the case (Table 1). The best base for the coupling
reaction was NaOH.14 Addition of an aryliodide to
We have recently reported the first study of the hydro-
boration of unsaturated phosphonates, including 1-
alkynylphosphonates.11 We could not explain the
apparent random placement of boron on either C1 or
C2 of the triple bond. This prompted us to investigate
the hydroboration of 1-alkynylphosphonates in greater
depth. In our work we discovered the factors that
* Corresponding author. Tel.: 972-2-675-7301; fax: 972-2-675-8201;
e-mail: msrebni@md2.huji.ac.il
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01696-3

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I. Pergament, M. Srebnik / Tetrahedron Letters 42 (2001) 8059–8062
Table 1. Hydroboration–Suzuki reaction of 1-alkynyphosphonates 1
Entry
a
R
ArI
Product
3, 5
Yield (%)b
72
Methoda
C5H11
p-C6H4-CH3
A
b
C5H11
p-C6H4-CH3
32
B
c
C5H11
p-C6H4-OCH3
45
30
A
d
C5H11
p-C6H4-OCH3
B
e
f
Cl(CH2)3
Cl(CH2)3
p-C6H4-CH3
60
27
A
p-C6H4-CH3
B
g
Ph
Ph
20
A
h
i
Ph
Ph
B
35
37
C5H11
a Suzuki reaction conditions for method A: NaOH (3 equiv.), Pd(PPh)2Cl2 (3 mol%), reflux in toluene during 2 h. Method B: NaOH (3 equiv.),
PdCl2 (more than 10 mol%), reflux in toluene during 24 h.
b Isolated yields after silica gel chromatography (ꢀ50 % EtOAc in P.E.).

I. Pergament, M. Srebnik / Tetrahedron Letters 42 (2001) 8059–8062
8061
either preformed 2 or 4 provided coupling products 3
and 5 (Fig. 1). In general, yields of 3 were good whereas
yields of 5 were appreciably lower. However, in both
cases only one coupling product was isolated.
compounds 3 and Rf values on TLC of products 5 were
lower than those of products 3. In the 31P NMR of 3,
the chemical shifts are ꢀ1.5 ppm lower than the chem-
ical shifts for compounds 5. The yields of 3 and 5 were
not optimized and although good to poor, only one
isomer is obtained which is easily isolated. Owing to the
great value of vinylphosphonates, the present procedure
should be attractive.
Alternatively, a one-pot procedure was developed for
synthesizing either 3 or 5. Compounds 3 were exclu-
sively synthesized by using a relatively low catalyst load
(3 mol% Pd) and short reaction times (2 h in refluxing
toluene), method A. Compounds 5 were obtained by
using a higher catalyst load (>10 mol% Pd) and longer
reaction times (24 h in refluxing toluene), method B. It
is well known that thermal isomerization of pinacol-
boronates in the presence of transition metals ulti-
mately places the boron atom on the least hindered
carbon of the molecule by a series of hydroboration/
dehydroboration steps.15 Thus, we surmise that in the
present case, the phosphonate group is the more steri-
cally demanding than the R group of the alkyne.
Apparently, under the more vigorous conditions of
method B, isomerization of 2 to 4 occurred faster than
coupling. That no equilibration occurred between 3 and
5 was established by control experiments. Thus, pure 3
was not converted to 5 under the conditions of the
reaction. Attempts to trap pinacolborane with a termi-
nal alkyne were unsuccessful. Apparently, PBH was
intimately bound during the process and was not avail-
able for hydroboration. In each case essentially one
coupling product was isolated, the other being present
in less than 1% (by GC–MS). When 2-iodoanisole was
used in the coupling reaction (Table 1, entry i), only
one product could be obtained, regardless of condi-
tions, corresponding to hydroboration occurring on C2.
This is apparently due to greater steric requirements.
All products were purified on silica gel and character-
ized by NMR. The stereochemistry was determined
from J coupling constants.16 Thus, for products 3
(hydrogen on double bond trans to phosphorus), JPH
was ꢀ48 Hz, and for compounds 5 (H1 is geminal to
phosphorus) JPH was ꢀ17 Hz. GC retention times of
products 5 were 0.1 min more than retention times of
Preparation of 3a and 5b are typical. Hydroboration:
Alkenylphosphonates (1 mmol) were dissolved in dry
dichloromethane (1 ml) and pinacolborane (1.5 mmol)
was added at 0°C. The reaction mixture was stirred at
room temperature overnight. The solvent and excess of
pinacolborane were removed in vacuo to obtain
hydroboration product. Suzuki coupling reaction of
hydroboration products: Solution of iodoaryl compound
(1.2 mmol) in toluene (1 ml), catalyst (Pd(PPh)2Cl2, 3%
mol in method A or, PdCl2, 30% in method B) and base
(NaOH, 3 mmol) were placed in the reaction flask
equipped with a condenser, under nitrogen. The mix-
ture was stirred at room temperature for 15 min. The
product of hydroboration (1 mmol) was dissolved in
toluene (1 ml) and this solution was added to the
reaction flask. The mixture was refluxed for 2 h
(method A) or 24 h (method B), then cooled to room
temperature and filtered. The solution was evaporated
to dryness and the residue was chromatographed on
silica (EtOAc/PE). NMR data of 3a: 1H NMR (CDCl3):
l=0.92 (t, 3H), 1.24 (t, 6H), 1.30–1.33 (m, 4H), 1.49 (t,
2H), 2.33 (s, 3H), 2.69 (dd, 2H), 4.03 ( q, 4H), 6.31–
6.47 (dt, 1H), 7.11 (d, 2H), 7.20 (d, 2H). 13C NMR
(CDCl3): l=14.19, 16.55, 22.69, 25.07, 28.19, 30.89,
31.62, 61.56, 115.32, 117.76, 128.50, 128.94, 129.88–
132.21 (d, JPC=196 Hz), 137.09, 137.43, 152.81 (d,
J
PC=11 Hz). 31P NMR (CDCl3): l=15.62 (JPH=49
1
Hz). NMR data of 5b: H NMR (CDCl3): l=0.84 (t,
3H), 1.22 (t, 6H), 1.30–1.35 (m, 4H), 1.44 (t, 2H), 2.34
(s, 3H), 2.95 (dd, 2H), 4.10 (q, 4H), 5. 70–5.76 (d,
1H),7.15 (d, 2H), 7.32 (d, 2H). 13C NMR (CDCl3):
l=14.24, 16.70, 21.43, 22.68, 28.87, 30.06, 32.78, 61.62,
111.67–114.21 (d, JPC=186 Hz), 126.62, 129.44, 142.13.
31P NMR (CDCl3): l=17.12 (JPH=18 Hz).
Acknowledgements
We thank the Israeli Science Foundation, the MECC
and the Horowitz Foundation for support of this work.
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