Novel synthesis of phosphinates by the microwave-assisted esterification of phosphinic acids

Article abstract of DOI:10.1080/00397910802654880

1-Hydroxy-3-phospholene oxides (1 and 3) and phenyl-H-phosphinic acid (6) are converted to the corresponding phosphinic esters (2, 4, and 7, respectively) by reaction with simple alcohols on microwave irradiation. Under traditional heating conditions, the

Full text of DOI:10.1080/00397910802654880

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Novel Synthesis of
Phosphinates by the
Microwave-Assisted
Esterification of Phosphinic
Acids
Nóra Zsuzsa Kiss a , Krisztina Ludányi b c , László
Drahos c & György Keglevich a
a Department of Organic Chemistry and Technology,
Budapest University of Technology and Economics,
Budapest, Hungary
b Faculty of Pharmacy, Department of
Pharmaceutics, Semmelweis University, Budapest,
Hungary
c Hungarian Academy of Sciences, Chemical
Research Center, Budapest, Hungary
Published online: 01 Jun 2009.
To cite this article: Nóra Zsuzsa Kiss , Krisztina Ludányi , László Drahos & György
Keglevich (2009) Novel Synthesis of Phosphinates by the Microwave-Assisted
Esterification of Phosphinic Acids, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:13, 2392-2404, DOI:
10.1080/00397910802654880
To link to this article: http://dx.doi.org/10.1080/00397910802654880

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Synthetic Communications1, 39: 2392–2404, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802654880
Novel Synthesis of Phosphinates by the
Microwave-Assisted Esterification of
Phosphinic Acids
1 2,3
Nora Zsuzsa Kiss, Krisztina Ludanyi, Laszlo Drahos,
3
ꢀ
ꢀ
ꢀ
ꢀ
1
¨
and Gyorgy Keglevich
1Department of Organic Chemistry and Technology, Budapest University
of Technology and Economics, Budapest, Hungary
2Faculty of Pharmacy, Department of Pharmaceutics,
Semmelweis University, Budapest, Hungary
3Hungarian Academy of Sciences, Chemical Research Center,
Budapest, Hungary
Abstract: 1-Hydroxy-3-phospholene oxides (1 and 3) and phenyl-H-phosphinic
acid (6) are converted to the corresponding phosphinic esters (2, 4, and 7, respec-
tively) by reaction with simple alcohols on microwave irradiation. Under tradi-
tional heating conditions, the esterification does not take place, as in the cases
of 1 and 3, or is highly incomplete, as in the case of 6. Steric hindrance in diphe-
nylphosphinic acid prevents efficient microwave-assisted esterification.
Keywords: Esterification, green chemistry, microwave synthesis, phosphinates,
phosphinic acids
INTRODUCTION
Phosphoric, phosphonic, and phosphinic esters are usually prepared by
the reaction of the corresponding P-acid chlorides with alcohols and
phenols.[1,2] Although this method is not the best from the point of view
Received October 22, 2008.
Address correspondence to Gyo¨rgy Keglevich, Department of Organic
Chemistry and Technology, Budapest University of Technology and Economics,
H-1521 Budapest, Hungary. E-mail: gkeglevich@mail.bme.hu
2392

Esterification of Phosphinic Acids
2393
of environmentally friendly chemistry, it is widely applied in industry.
The liberated hydrochloric acid is removed by a tertiary amine or alkali
hydroxide. In a variation of this method, the phosphonic or phosphinic
chlorides are prepared in situ from the corresponding acids by thionyl
chloride, and the so-formed P-chlorides react with the alcohol in a
one-pot procedure.[3] It is also possible to make phosphonic and phosphi-
nic esters available by the alkylation of the corresponding acids. In this
case, either alkyl halides or unsaturated species, such as olefins or acety-
lenes, may be the alkylating agents. In the former case, the P-acid should
first be converted to the sodium or potassium salt,[4,5] or the alkylation
carried out under solid–liquid phase-transfer catalytic conditions,[6]
whereas the addition of unsaturated hydrocarbons should be accom-
plished in the presence of suitable catalysts.[7]
Otherwise, the esterification of phosphinic acids requires special
reagents, such as orthoformates (for the preparation of hypophosphor-
ous acid esters),[8–10] orthoacetates,[11] chloroformates,[12] orthosilicates
(for the synthesis of hypoposphorous esters and H-phosphinates),[8,13,14]
and trialkyl phosphites (also for the preparation of phosphonates).[15,16]
Direct esterification of phosphinic acids has not been reported, but
diphenylphosphinodithioate esters have been prepared by heating
PhP(S)(SH)H with primary alcohols.[17]
On the basis of our recent experiences in microwave (MW)–assisted
organic synthesis,[18–23] it seemed worth trying the direct esterification of
phosphinic acids with simple alcohols under MW conditions. On the one
hand, unexpected reactions could occur,[21] but on the other hand, we
could benefit from the effect of pressure developed in closed systems.[18,19]
In this article, the scope and limitations of the synthesis of simple phos-
phinates from the corresponding phosphinic acids and C2–C4 aliphatic
alcohols under MW conditions are described.
RESULTS AND DISCUSSION
The first model compounds were 1-hydroxy-3-phospholene oxides 1 and
3, which are the basic starting materials for other five- and six-membered
P-heterocycles.[2,24] A variety of alkoxy-phospholene oxides was prepared
earlier by the alcoholysis of 1,1,1-trihalophospholium salts.[25–27] In our
experiments, reaction temperatures of >180ꢀC were anticipated, and
for this reason the not-too-volatile butanol was chosen as the reaction
partner. Dimethylphospholene oxide 1 and n-butanol, used in a 16-fold
excess, were irradiated at 200ꢀC for 1 h in a sealed tube. After flash chro-
matography, the phosphinic ester 2a was obtained in 20% yield (Table 1,
entry 1). By prolonging the reaction time to 2 h, the yield of 2a was

2394
N. Z. Kiss et al.
Table 1. Esterification of 1-hydroxy-3,4-dimethyl-3-phospholene 1-oxide (1)
under MW conditions
Yield of
2 (%)a
R
T (ꢀC) p (bar) t (h)
Remark
Entry
nBu
nBu
nBu
200
200
200
14
14.5
13.5
1
2
1
20 (2a)
51 (2a)
20 (2a)
1
2
3
Three times more
BuOH was used
Three times more BuOH
and two times more 1
was used
nBu
200
200
13.5
15
1
2
73 (2a)
12 (2a)
4
5
nBu
In the presence of
10% of PTSA
iBu
nPr
nPr
200
180
180
14
13.5
14
2
2
4
30 (2b)
15 (2c)
20 (2c)
6
7
8
aAfter flash column chromatography (silica gel, 3% MeOH in chloroform).
increased to 51% (Table 1, entry 2). The use of three times more butanol
did not lead to an increase in the yield (Table 1, entry 3). After adding
twice as much phospholene oxide 1 to a similar reaction mixture, the
isolated yield amounted to 73% (Table 1, entry 4). An increase in the
concentration of 1 may be favorable, because as in this case the MW
absorption becomes more efficient. An attempt to promote the efficiency
of the esterification by the use of 10% of p-toluenesulfonic acid (PTSA) as
a catalyst was not successful (Table 1, entry 5). In the cases mentioned
here, a pressure of 13.5–15 bar was developed. If n-butanol was replaced
by isobutanol, the yield of the phosphinic ester (2b) was 30% after irradia-
tion at 200ꢀC=14 bar for 2 h (Table 1, entry 6). Using propanol as the
alcohol component, the reaction temperature had to be somewhat cooler
because of the greater tension of this alcohol. At 180ꢀC, the yield of pro-
pyl phosphinate 2b was only 15% after a reaction time of 2 h (Table 1,
entry 7). A prolonged irradiation led to a modest increase of the yield
(Table 1, entry 8).

Esterification of Phosphinic Acids
2395
It is known that at temperatures greater than 200ꢀC, the 3-methyl-
3-phospholene oxide may undergo rearrangement of the double-bond.[21]
For this reason, the esterification of 1-hydroxy-3-methyl-3-phospholene
oxide 3 with n-butanol was carried out at 180ꢀC. After irradiation for
1 and 2 h, the yields of phosphinate 4 were 31 and 36%, respectively
(Table 2, entries 1 and 2). No isomerization took place in these instances.
When esterification in the presence of 10% PTSA as a possible catalyst at
180 and 200ꢀC was tried, the proportions of the 2-phospholene oxide (5)
were 20 and 50%, respectively (Table 2, entries 3 and 4). In these cases, a
pressure of 8–10 bar was developed.
After the esterification of the simplest ring phosphinic derivatives 1
and 3, a similar reaction of phenyl-H-phosphinic acid 6 was studied.
By using butanol and working at 160ꢀC for 1 h and 180ꢀC for 0.5 h, we
obtained yields of butyl phosphinate 7a of 85 and 90%, respectively
(Table 3, entries 1 and 2). When isobutanol was used at 160ꢀC for 1 h,
the yield of phosphinate 7b was 75% (Table 3, entry 3). The esterification
with n-propanol and isopropanol gave 7c and 7d in 73% and 48% yields,
respectively (Table 3, entries 4 and 5). The lower yield of 48% may be the
consequence of steric hindrance due to the isopropyl group. In instances
covered by entries 1–5, the accompanying pressure was 7–8.5 bar.
Because phosphinic acid 6 seemed to be quite reactive, its esterification
Table 2. Esterification of 1-hydroxy-3-methyl-3-phospholene 1-oxide (3) under
MW conditions
T
(ꢀC)
p
(bar)
t
(h)
Proportiona
of 4 and 5
Yield of
4 (%)b
Remark
Entry
180
180
180
8
8
9
1
2
1
1:0
1:0
4:1
31
36
1
2
3
In the presence of
10% of PTSA
200
10
1
1:1
In the presence of
10% of PTSA
4
aOn the basis of relative 31P NMR intensities.
bAfter flash column chromatography (silica gel, 5% MeOH in chloroform).

2396
N. Z. Kiss et al.
Table 3. Esterification of phenyl-H-phosphinic acid (6) under MW conditions
R
T (ꢀC)
p (bar)
t (h)
Yield of 7 (%)a
Entry
nBu
nBu
iBu
nPr
iPr
160
180
160
160
180
160
7
8
7
8.5
8.5
15
1
0.5
1
1
2
85 (7a)
90 (7a)
75 (7b)
73 (7c)
48 (7d)
80 (7e)
1
2
3
4
5
6
Et
1
aAfter flash column chromatography (silica gel, 3% MeOH in chloroform).
with the volatile ethanol was also attempted at 160ꢀC (15 bar) for 1 h. The
yield of 7e was 80% (Table 3, entry 6). When phenylphosphinic acid was
refluxed in ethanol under thermal conditions, ethyl phenylphosphinate
(7a) was formed in only 13%. In the literature, a preparative yield of
12% was described for this case.[28] As a comparison, octyl phenyl-H-
phosphinate was prepared in 53% yield by a dicyclohexylcarbodiimide
(DCC)-promoted coupling of phenylphosphinic acid with 1-octanol.[29]
It is worth mentioning that the H-phosphinic esters (7) undergo oxidation
on exposure to air; therefore they should be stored under nitrogen.
Finally, the apparently sterically hindered model compound diphe-
nylphosphinic acid 8 was subjected to esterification. Irradiation at
180ꢀC for 1.5 h and 200ꢀC for 1 h did not lead to complete conversions,
and the yield of butyl phosphinate 9 was only ca. 14% (Table 4, entries 1
and 2). In these cases, most of the unreacted starting material 8 precipi-
tated from the mixture after evaporating the excess of the butanol. The
esterification was, in this case, somewhat more efficient in the presence
of PTSA (Table 4, entry 3).
In all, 10 phosphinates (2a–c, 4, 7a–e, and 9) were prepared by the
novel MW-assisted method. After flash column chromatography, the
yields for 2a and 7a–c,e were quite good (73–90%) but were less for 2b,
2c, 4, 7d, and 9, (20–48%) for different reasons. Volatility of the alcohol,
isomerization side reactions, and steric hindrance of the substrate may
cause problems, as was shown in the example of phosphinates 2b, 4,
and 9, respectively.

Esterification of Phosphinic Acids
2397
Table 4. Esterification of diphenylphosphinic acid (8) under MW conditions
T (ꢀC)
p (bar)
t (h)
Yield of 9 (%)a
Remark
Entry
180
200
200
10
12
12
1.5
1
3
15
13
20
1
2
3
In the presence of
10% of PTSA
aAfter flash column chromatography (silica gel, 3% MeOH in chloroform).
The phosphinates (2, 4, 7, and 9) obtained after flash column
1
chromatography in 98% purity were characterized by 31P, 13C, and H
NMR, as well as mass spectroscopy. The >P(O)H function in products
7a–c was confirmed by proton-coupled 31P NMR spectra. A number of
the phosphinic esters (2b, 4, 7b, 7c) are new, whereas others (2a, 2c, 7a,
7d, 7e, 9) have been described earlier (see Experimental).
In conclusion, the first direct esterification of phosphinic acids and
simple alcohols was accomplished by the MW technique. This is an envir-
onmentally friendly procedure because the generally applied phosphinic
chlorides can be substituted with inexpensive phosphinic acids. In the
case of suitable model compounds, this simple method is obviously of
synthetic importance. Because volatility of the alcohol is a limiting factor,
the method developed is the most suitable for esterifications involving
nonvolatile (longer-carbon-chain) alcohols. Further work on possible
extensions is to be done soon.
EXPERIMENTAL
General
1
The 31P, 13C, and H NMR spectra were obtained on a Bruker DRX-
500 spectrometer operating at 202.4, 125.7, and 500 MHz, respec-
tively. Chemical shifts are downfield relative to 85% H3PO4 or tetra-
methylsilane (TMS). The couplings are given in hertz. Mass
spectrometry was performed on a ZAB-2SEQ instrument. The starting

2398
N. Z. Kiss et al.
1-hydroxy-3-phospholene oxides (1 and 3) were prepared by the
McCormack cycloaddition reaction followed by hydrolysis.[25,30] The
esterifications were carried out in a CEM Discover microwave reactor
equipped with a pressure controller at 50–80 W.
General Procedure for the Preparation of Phosphinates
A mixture of 0.10 g of phosphinic acid [1 (0.68 mmol), 3 (0.76 mmol), or 6
(0.70 mmol)] and 1 ml of alcohol [n-butanol (10.9 mmol), isobutanol
(10.9 mmol), n-propanol (13.3 mmol), isopropanol (13.3 mmol), or
ethanol (17.4 mmol)] was measured in a sealed tube and irradiated in a
CEM Microwave reactor equipped with a pressure controller at the
temperatures and for the times shown in Tables 1–3. Then the alcohol
was removed under reduced pressure, and the obtained residue was
purified by flash column chromatography using silica gel and 3% metha-
nol in chloroform as the eluant to afford phosphinates 2, 4, and 7 as oils
in a purity of ca. 98%. For the details, see Tables 1–3. The operations
with phosphinic esters 7 should be carried out under nitrogen.
The following products were thus prepared:
(1-n-Butoxy)-3,4-dimethyl-3-Phospholene 1-Oxide 2a
Optimum conditions involved irradiation of 0.20g of 1 in 3 ml of butanol at
200ꢀC for 1 h. Yield: 73% (Table 1, entry 4); 31P NMR (CDCl3) d 68.4; 13
C
NMR (CDCl3) d 13.6 (CH2CH3), 16.5 (3J ¼ 15.9, C3–CH3), 18.8 (CH2CH3),
32.7 (3J ¼ 5.9, OCH2CH2), 35.7 (1J ¼ 90.8, C2), 64.5 (2J ¼ 6.6, OCH2), 127.5
(2J ¼ 12.9, C3); 1H NMR (CDCl3) d 0.94 (t, J ¼ 7.4, 3H, CH2CH3),
1.37–1.44 (m, 2H, CH2CH3), 1.64–1.70 (m, 2H OCH2CH2), 1.72 (s, 6H,
C3–CH3), 2.37–2.50 (m, 4H, PCH2), 4.00–4.04 (m, 2H, OCH2) [d (lit.)[27]
1.73 (s, 6H, C3–CH3), 2.44 (d, JPH ¼ 13, 4H, PCH2), 4.01 (m, 2H,
OCH2)]. (MþH)þfound ¼ 203.1194; C8H16O2P requires 203.1201.
1-Isobutoxy-3,4-dimethyl-3-phospholene 1-Oxide 2b
Parameters: 220ꢀC=2 h; yield: 30% (Table 1, entry 6); 31P NMR (CDCl3)
d 68.5; 13C NMR (CDCl3) d 16.4 (3J ¼ 15.8, C3–CH3), 18.6 [CH(CH3)2],
29.1 (CHMe2), 35.5 (1J¼ 90.9, C2), 70.6 (1J ¼ 6.9, OCH2), 127.3 (2J¼ 12.9,
1
C3); H NMR (CDCl3) d 0.95 [d, J ¼ 6.7, 6H, CH(CH3)2], 1.73 (s, 6H,
C3–CH3), 1.91–1.98 (m, 1H, CH), 2.38–2.50 (m, 4H, PCH2), 3.76–3.79
(m, 2H, OCH2). (MþH)þfound ¼ 203.1191; C8H16O2P requires 203.1201.

Esterification of Phosphinic Acids
2399
(1-n-Propoxy)-3,4-dimethyl-3-phospholene 1-Oxide 2c
Parameters: 180ꢀC=4 h; yield: 20% (Table 1, entry 8); 31P NMR (CDCl3)
d 68.5; 13C NMR (CDCl3) d 10.1 (CH2CH3), 16.5 (3J ¼ 15.9, C3–CH3),
23.9 (CH2CH3), 35.7 (1J ¼ 90.8, C2), 66.3 (2J ¼ 6.7, OCH2), 127.5
(2J ¼ 12.9, C3); 1H NMR (CDCl3) d 0.96 (t, J ¼ 7.3, 3H, CH2CH3),
1.68–1.72 (m, 2H, CH2CH3), 1.72 (s, 6H, C3–CH3), 2.38–2.51 (m, 4H,
PCH2), 3.96–4.00 (m, 2H, OCH2) [d (lit.)[27] 0.94 (t, J ¼ 7, 3H, CH2CH3),
1.72 (s, C3–CH3) overlapped by 1.40–2.05 (m, CH2CH3), total intensity
8H, 2.42 (d, J ¼ 12, 4H, PCH2), 3.97 (m, 2H, OCH2)]. (MþH)þ
¼
found
189.1036; C9H18O2P requires 189.1044.
(1-n-Butoxy)-3-methyl-3-phospholene 1-Oxide 4
Parameters: 180ꢀC=2 h; yield: 36% (Table 2, entry 2); 31P NMR (CDCl3) d
74.6; 13C NMR (CDCl3) d 13.7 (CH2CH3), 18.9 (CH2CH3), 20.9
(3J ¼ 12.9, C3–CH3), 30.9 (1J ¼ 88.2, C2), 32.7 (3J ¼ 6.0, OCH2CH2), 33.6
(1J ¼ 92.3, C6), 64.7 (2J ¼ 6.8, OCH2), 120.4 (2J ¼ 10.8, C4), 136.4
(2J ¼ 16.8, C3); 1H NMR (CDCl3) d 0.94 (t, J ¼ 7.4, 3H, CH2CH3),
1.38–1.43 (m, 2H, CH2CH3), 1.64–1.69 (m, 2H OCH2CH2), 1.80 (s, 3H,
C3–CH3), 2.39–2.52 (m, 4H, PCH2), 4.01–4.06 (m, 2H, OCH2), 5.52 (d,
J ¼ 35.9, 1H, CH). (MþH)þ
¼ 189.1038; C9H18O2P requires 189.1044.
found
n-Butyl Phenyl-H-phosphinate 7a[31,32]
Parameters: 160ꢀC=1 h; yield: 85% (Table 3, entry 1); 31P NMR (CDCl3)
1
1
d 24.9, JP,H ¼ 566.7 [(d (lit.)[33] 25.3, JP,H ¼ 563)]; 13C NMR (CDCl3)
d 13.2 (CH3), 18.4 (CH2CH3), 32.1 (3J ¼ 6.4, OCH2CH2), 65.4 (2J ¼ 6.6,
1
ꢁ
0
OCH2), 128.4 (J ¼ 13.8, C3 ), 129.6 ( J ¼ 131.9, C1 ), 130.5 (J ¼ 11.7,
4
1
ꢁ
ꢁ0
0
0
C2 ), 132.7 ( J ¼ 2.6, C4 ) ( may be reversed); H NMR (CDCl3) d 0.95
(t, J ¼ 7.3, 3H, CH2CH3), 1.39–1.51 (m, 2H CH2CH3), 1.68–1.78 (m,
2H, OCH2CH2), 4.06–4.15 (m, 2H OCH2), 7.6 (d, J ¼ 562.3, 1H, PH),
7.54–7.84 (m, 5H, Ar). (MþH)þ
¼ 199.0881; C10H15O2P requires
found
199.0888.
Isobutyl Phenyl-H-phosphinate 7b[28,34,35]
Parameters: 160ꢀC=1 h; yield: 75% (Table 3, entry 3); 31P NMR (CDCl3) d
1
25.0, JP,H ¼ 562.3; 13C NMR (CDCl3) d 18.4 [CH(CH3)2] 28.8 [J ¼ 6.6,
CH(CH3)2], 71.4 (2J ¼ 6.8, OCH2), 128.3 (J ¼ 13.8, C3 ), 128.7
0
1
4
1
0
0
0
( J ¼ 132.6, C1 ), 130.5 (J ¼ 11.7, C2 ), 132.7 ( J ¼ 2.8, C4 ); H NMR

2400
N. Z. Kiss et al.
(CDCl3) d 0.97 [d, J ¼ 6.6, 6H, CH(CH3)2], 1.90–2.08 (m, 1H, OCH2CH),
3.79–3.92 (m, 2H, OCH2), 7.59 (d, J ¼ 562.6, 1H, PH), 7.52–7.83 (m, 5H,
Ar). (MþH)þ
¼ 199.0881; C10H15O2P requires 199.0888.
found
n-Propyl Phenyl-H-phosphinate 7c
Parameters: 160ꢀC=1 h; yield: 73% (Table 3, entry 4); 31P NMR (CDCl3) d
1
24.9, JP,H ¼ 562.9; 13C NMR (CDCl3) d 10.1 (CH3), 23.6 (3J¼ 6.5,
2
1
ꢁ
0
CH2CH3), 67.3 ( J ¼ 6.6, OCH2), 128.5 (J ¼ 13.8, C3 ), 129.8 ( J ¼ 132.0,
C1 ), 130.7 (J¼ 11.8, C2 ) 133.0 ( J ¼ 2.9, C4 ) ( may be reversed); 1H
NMR (CDCl3) d 0.98 (t, J ¼ 7.4, 3H, CH2CH3), 1.74–1.81 (m, 2H CH2CH3),
4.01–4.09 (m, 2H OCH2), 7.59 (d, J¼ 562.5, 1H, PH), 7.50–7.82 (m, 5H, Ar).
4
ꢁ
ꢁ
0
0
0
(MþH)þ
¼ 185.0725; C9H13O2P requires 185.0731.
found
Isopropyl Phenyl-H-phosphinate 7d[28,34,35]
Parameters: 180ꢀC=2 h; yield: 48% (Table 3, entry 5); 31P NMR (CDCl3)
1
d 22.3, JP,H ¼ 559.0 [d (lit.)[36] 23.4)]; 13C NMR (CDCl3) d 23.7 [J ¼ 4.1,
CH(CH3)], 24.1 [J ¼ 4.6, CH(CH3)], 71.2 [J ¼ 6.5, CH(CH3)2], 128.5
1
ꢁ
ꢁ
ꢁ
0
0
0
(J ¼ 13.8, C3 ), 130.3 ( J ¼ 133.4, C1 ), 130.7 (J ¼ 11.8, C2 ), 132.8
4
1
0
( J ¼ 2.9, C4 ) ( may be reversed); H NMR (CDCl3) d 1.35 (d, J ¼ 6.0,
3H, CH3), 1.43 (d, J ¼ 6.0, 3H, CH3) 4.66–4.79 (m, 1H, OCH), 7.62 (d,
J ¼ 559.2, 1H, PH), 7.51–7.83 (m, 5H, Ar). (MþH)þ
¼ 185.0726;
found
C9H13O2P requires 185.0731.
Ethyl Phenyl-H-phosphinate 7e
Parameters: 160ꢀC=1 h; yield: 80% (Table 3, entry 6); 31P NMR (CDCl3) d
24.7, 1JP,H ¼ 563.0 [d (lit.)[33] 25.7, 1JP,H ¼ 562)]; 13C NMR (CDCl3) d 16.2
3
2
a
0
( J ¼ 6.5, CH2CH3), 61.9 ( J ¼ 6.3, OCH2), 128.6 (J ¼ 13.9, C3 ), 129.7
1
a
4
[27]
0
0
0
( J ¼ 132.2, C1 ), 130.7 (J ¼ 11.8, C2 ), 133.0 ( J ¼ 2.9, C4 ) [d (lit.)
3
2
1
0
16.4 ( J ¼ 6, CH2CH3), 62.0 ( J ¼ 6, OCH2), 130.0 ( J ¼ 132, C1 ), 128.8
(J ¼ 14, C3 ), 130.9 (J ¼ 12, C2 ), 133.1 ( J ¼ 3, C4 )], (a,bmay be reversed);
1H NMR (CDCl3) d 1.39 (t, J ¼ 7.0, 3H, CH2CH3), 4.11–4.24 (m, 2H,
OCH2), 7.60 (d, J ¼ 562.5, 1H, PH), 7.52–7.83 (m, 5H, Ar).
b
b
4
0
0
0
(MþH)þ
¼ 171.0569, C8H11O2P requires 171.0575.
found
n-Butyl Diphenylphosphinate 9
Process: 0.10 g (0.46 mmol) of 8 and 1.5 ml (16.3 mmol) of butanol were
used, and 7.9 mg (0.05 mmol) of PTSA was also measured in;

Esterification of Phosphinic Acids
2401
conditions: 200ꢀC=3 h; yield: 20% (Table 4, entry 3); 31P NMR (CDCl3)
d 31.2; 13C NMR (CDCl3) d 13.6 (CH3), 18.9 (CH2CH3), 32.6 (3J ¼ 6.6,
OCH2CH2), 64.7 (2J ¼ 6.1, OCH2), 128.4–128.5 (J ¼ 13.1, C3 ),
ꢁ
0
127.8–129.7 (1J ¼ 144.6, C1 ), 131.5–131.7 (J ¼ 10.0, C2 ), 132.0
ꢁ
0
0
4
1
ꢁ
0
( J ¼ 2.8, C4 ) ( may be reversed); H NMR (CDCl3) d 0.92 (t, J ¼ 7.3,
3H, CH2CH3), 1.35–1.48 (m, 2H, CH2CH3), 1.68–1.74 (m, 2H,
OCH2CH2), 3.99–4.07 (m, 2H, OCH2), 7.44–7.55 (m, Ar), 7.78–7.85
(m, Ar), total intensity 10H, [d (lit.)[37] 0.85 (t, 3H, CH3), 1.4 (m, 4H,
CH2CH2CH3), 3.9 (m, 2H, OCH2), 7.5 (m, 10H, Ar)]. (MþH)þ
¼
found
275.1193; C16H19O2P requires 275.1201.
The isomerized 1-butoxy-3-methyl-2-phospholene 1-oxide (5) was
identified on the basis of the following data: 31P NMR (CDCl3) d 75.0;
13C NMR (CDCl3) d 13.7 (CH2CH3), 18.9 (CH2CH3), 21.4 (3J ¼ 19.3,
C3–CH3), 23.0 (1J ¼ 93.5, C5), 32.7 (3J ¼ 6.0, OCH2CH2), 64.7 (2J ¼ 6.8,
OCH2), 118.3 (1J ¼ 128.4, C2), 163.4 (2J ¼ 33.3, C3).
Attempted Esterification Under Traditional Heating
Phenyl-H-phosphinic acid (0.50 g, 3.50 mmol) in 10 ml (0.17 mol) of
ethanol was refluxed for 4 h in the nitrogen atmosphere. Then solvent
was evaporated, and the crude product was analyzed by 31P NMR
spectroscopy. The mixture contained 13% of ethyl phenyl-H-phosphinate
7e (dP 24.3) and 87% of unreacted phenylphosphinic acid (dP 20.6).
In a similar treatment of 1-hydroxy-3,4-dimethyl-3-phospholene
oxide 1 with n-butanol, not even traces of the expected ester 2a could
be detected.
ACKNOWLEDGMENTS
G. K. is grateful to Professor Harry R. Hudson (London Metropolitan
University) for his advice. The authors thank the Hungarian Scientific
and Research Fund for support (OTKA No. T067679).
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