Reaction of tribenzylphosphine oxide with aldehydes

Article abstract of DOI:10.1023/A:1013974001957

Tribenzylphosphine oxide readily and stereoselectively reacts with aliphatic, aromatic, and heteroaromatic aldehydes in the presence of strong bases yielding exclusively (or predominantly) E-isomers of 1-organyl-2-phenylethenes and dibenzylphosphinic acid

Full text of DOI:10.1023/A:1013974001957

Russian Journal of Organic Chemistry, Vol. 37, No. 12, 2001, pp. 1726 1730. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 12, 2001,
pp. 1807 1811.
Original Russian Text Copyright
2001 by Gusarova, Ivanova, Reutskaya, Arbuzova, Baikalova, Deryagina, Russavskaya, Trofimov.
Dedicated to Academician M.G.Voronkov on occasion of his 80th birthday
Reaction of Tribenzylphosphine Oxide with Aldehydes
N. K. Gusarova, N. I. Ivanova, A. M. Reutskaya, S. N. Arbuzova,
L. V. Baikalova, E. N. Deryagina, N. V. Russavskaya, and B. A. Trofimov
Faworsky Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, 664033, Russia
Received March 20, 2001
Abstract Tribenzylphosphine oxide readily and stereoselectively reacts with aliphatic, aromatic, and
heteroaromatic aldehydes in the presence of strong bases yielding exclusively (or predominantly) E-isomers
of 1-organyl-2-phenylethenes and dibenzylphosphinic acid.
Tertiary phosphine oxides are known to react with
aldehydes and ketones under conditions of Horner
Wittig reaction to yield E- and Z-alkenes [1 7], and
previously into the reaction were brought only
asymmetric diphenyl(organyl)phosphine oxides of a
general formula Ph2P(O)CH2R (R = alkyl [4, 8 10],
Ph [1, 4], -aminoalkyl, -N-acylaminoalkyl [11])
containing a diphenylphosphoryl leaving group. The
carbanions were generated under the action of strong
bases, as potassium tert-butylate [2, 6, 7], sodium
amide [7], butyl- and phenyllithium [1, 3 5].
metallic sodium in THF, and also 60% water solution
of KOH in the presence of a phase-transfer catalyst
(benzyltriethylammonium chloride, BTAC) (see
table). The reactions were carried out in the tempera-
ture range 60 105 C.
We recently published short communications
[12, 13] on application in the Horner Wittig reaction
of the symmetrical tribenzylphosphine oxide that was
conveniently prepared from the red phosphorus and
benzyl chloride [14]. Thus the heating of the tri-
benzylphosphine oxide to 60 80 C with benz-
aldehyde, 2,2-dimethylpropanal, or now available
[15] 1-methyl- and 1-ethylimidazole-2-carbox-
aldehydes in a system NaNH2 THF or KOH DMSO
afforded respectively E-isomers of 1.2-diphenyleth-
ene, 1-(1,1-dimethylethyl)-2-phenylethene, and also
1-(1-methylimidazol-2-yl)- and 1-(1-ethylimidazol-2-
yl)-2-phenylethenes [12, 13].
(1)
The highest yield (88%) of E-1,2-diphenylethene
was obtained at heating to 58 60 C of tribenzylphos-
phine oxide and benzaldehyde in the system NaNH2
THF, and the olefination occurred with the same
result both at equimolar reagents ratio and at benz-
aldehyde excess (see table, runs nos. 1 and 2).
The target of the present study was investigation
of the reaction between triphenylphosphine oxides
and aldehydes of aliphatic, aromatic, and hetero-
aromatic series, improvement and extension of the
preparative opportunities of the reaction.
The use of system t-BuOK DMSO, all other
things being the same, reduces the yield of (E)-1,2-
diphenylethene to 56% (see table, run no. 3). Carry-
ing out of reaction (1) at 80 C in systems CsOH
DMSO and KOH DMSO affords ethene II in 51 and
42% yield respectively (see table, runs nos.4 and 5).
Further increase of the reaction temperature to 105 C
does not increase the efficiency of the process under
study (see table, run no. 6). In all runs above cited
the tributylphosphine oxide was consumed to 100%.
By an example of benzaldehyde we studied the
influence of reaction (1) conditions (base character,
reagents ratio, temperature of the process) on the
yield of (E)-1,2-diphenylethene (II) and dibenzyl-
phosphinic acid (III). We used in the reaction super-
basic catalytic systems NH2 THF, t-BuOK DMSO,
CsOH DMSO, KOH DMSO, NaOH CsF DMSO,
1070-4280/01/3712-1726$25.00 2001 MAIK Nauka/Interperiodica

REACTION OF TRIBENZYLPHOSPHINE OXIDE WITH ALDEHYDES
1727
A superbasic system NaOH CsF DMSO recently
introduced for vinylation of alcohols and oximes with
acetylene [16] in reaction between triphenyphosphine
oxide and benzaldehyde was not very efficient and
was even worse than heterophase superbase KOH
DMSO: the tribenzylphosphine oxide conversion was
in this case 75%, and the yield of (E)-1,2-diphenyl-
ethene and dibenzylphosphinic acid 39 and 61%
respectively (see table, cf. run no. 7 with runs nos. 5
and 6).
yield of ethenes XIa, b and XIIa, b comprised 42
and 52% respectively at the ratio of E and Z isomers
7.4 6: 1. The yield of acid III was 57 79%. Under
conditions of reaction (2) alongside ethenes IX XIII
were isolated also (E)-1,2-diphenylethene in 5 10%
yield with respect to the initial amount of tribenzyl-
phosphine oxide.
It turned out that the system sodium metal THF
was low-active in reaction (1): at tribenzylphosphine
oxide conversion 79% the yield of ethene I and acid
III in this run amounted only to 23 and 28% res-
pectively (see table, run no. 8). The activation of the
reaction in question under the conditions of the phase-
transfer catalysis (in the system KOH toluene
BTAC) was also inefficient: the overall yield of the
target products reached only 12% at conversion of
the initial tribenzylphosphine oxide 36% (see table,
run no. 19)
(2)
Thus the data obtained evidence that in the syn-
thesis of (E)-1,2-diphenylethene and dibenzylphos-
phinic acid it is feasible to use the catalytic system
NH2 THF at 60 C.
Among the reaction (1) products the 1H NMR
spectra indicated the presence of the benzyl alcohol
(yield 1 5%) arising apparently along Cannizzaro
reaction [7].
The exclusive or predominant formation of E-iso-
mers of 1-organyl-2-phenylethenes II, IX XIa, XIIa,
XIII in the studied processes (1) and (2) is consistent
with the known E-selectivity of the Horner Wittig
reaction [5, 17].
At the best conditions found for the synthesis of
(E)-1,2-diphenylethene we brought into the reaction
with tribenzylphosphine oxide the other aldehydes:
2-methylpropanal (IV), 2,2-dimethylpropanal (V),
1-methylimidazole-2-carboxaldehyde (VI), 1-ethyl-
imidazole-2-carboxaldehyde (VI), and thiophene-2-
carboxaldehyde (VIII).
The (E)-1,2-diphenylethene may arise in reaction
(2) according to scheme (3) including primary
deprotonation of the tribenzylphosphine oxide
effected by superbases followed by rearrangement of
phosphine oxide (A) into a more stable phosphorane
(B) that in the presence of oxygen provides sodium
dibenzylphosphinate and benzaldehyde. The latter
reacts with the tribenzylphosphine oxide to give
ethene II along reaction (1).
It turned out that aliphatic aldehydes IV, V, and
thiophene-2-carboxaldehyde (VIII) react with tri-
benzylphosphine oxide stereospecifically to afford
E-isomers of 1-(1-methylethyl)-2-phenylethene (IX),
1-(1,1-dimethylethyl)-2-phenylethene
(IX),
and
1-thiophene-2-phenylethene (XIII) respectively in
50 56% yields. Dibenzylphosphinic acid formed
therewith in 71 81% yield and was isolated same as
in reaction (1) at acidification of the water fraction of
the reaction mixture. At the use in reaction (2) of
aldehydes from the imidazole series VI and VII
alongside the E-isomers 1-(1-methylimidazol-2-yl)-2-
phenylethene (XIa) and 1-(1-ethylimidazol-2-yl)-2-
phenylethene (XIIa) were also obtained the cor-
responding Z-isomers XIb and XIIb. The overall
(3)
It turned out actually that the heating of the
tribenzylphosphine oxide to 50 60 C in the system
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 12 2001

1728
GUSAROVA et al.
NaNH2 THF provided (E)-1,2-diphenylethene and
dibenzylphosphinic acid in 15 and 37% respectively.
At the deprotonation stage in the 31P NMR spectrum
(II), mp 122 123 C (from ethanol) (publ. 124 C [3]).
1H NMR spectrum (CDCl3), , ppm: 7.09 s (2H,
=CH), 7.26 7.46 m (10H, C6H5). Found, %: C
92.85; H 6.73. C14H12. Calculated, %: C 93.30;
H 6.70. Water phases were combined, acidified
with 2 N HCl till pH 2 3, and from the solution
precipitated colorless crystals that were separated,
washed in succession with water, alcohol, ether, and
dried in a vacuum. We obtained 0.35 g (65%) of acid
III, mp 189 190 C (from ethanol) (publ. 191 C
was observed a signal at
30.1 ppm that may be
P
assigned to phosphorane (B) [18]. This reaction was
described before with diphenylbenzylphosphine oxide;
it resulted in (E)-1,2-diphenylethene and dipheny-
phosphinic acid [6, 19].
A presumable alternative scheme of (E)-1,2-di-
phenylethene formation via carbene PhCH: genera-
tion from tribenzylphosphine oxide at its boiling in
xylene (2 h under argon) was not supported by
experimental data.
1
[21]). H NMR spectrum (CDCl3), , ppm: 2.85 d
2
(4H, CH2, JHP 16.9 Hz), 7.18 7.28 m (10H, C6H5),
9.02 s (1H, OH). 31P NMR spectrum (CDCl3):
49.9 ppm Found, %: C 67.22; H 6.23; P 12.31.
P
C14H15O2P. Calculated, %: C 68.29; H 6.10; P 13.01.
Thus the use as olefinating reagent of the available
tribenzylphosphine oxide under conditions of Horner
Wittig reaction provides new opportunities for stereo-
selective synthesis of 1,2-disubstituted alkenes, in
particular, stilbene and its heteroanalogs, promising
raw material for production of dyes, optical whiteners,
complexing agents, photochemical sensors [20] and
Reaction of tribenzylphosphine oxide with
2-methylpropanal. In the same way as in the preced-
ing run from 1 g (3 mmol) of tribenzylphosphine
oxide, 0.35 g (9 mmol) of sodium amide, and 0.43 g
(6 mmol) of 2-methylpropanal (IV) in 40 ml of THF
was obtained 0.3 g of viscous organic mixture con-
drugs (e.g., in the synthesis of milbemycin
,
1
3
taining according to its H NMR spectrum 79% of
vitamine D and its analogs [5]). Besides the reaction
of tribenzylphosphine oxide with aldehydes readily
furnishes dibenzylphosphinic acids that previously
were difficult to obtain: the synthesis that has been
previously one of the best is multi-stage and is based
on alkylation with benzyl iodide or chloride of fairly
expensive and toxic phosphorus iodides [21].
(E)-1-(1-methylethyl)-2-phenylethene (yield 54%),
9% of (E)-1,2 -diphenylethene (yield 5%), and 12%
of tribenzylphosphine oxide (conversion 96%). The
mixture obtained was washed with small portions of
ether, the ether was removed, and the residue was
washed with pentane. We obtained as an individual
1
product ethene IX (viscous fluid). H NMR spectrum
3
(CDCl3), , ppm: 1.07 d (6H, CH3, JHH 6.65 Hz),
EXPERIMENTAL
2.43 octet [1H, CH-(CH3)2, 3JHCCH 6.65 Hz], 6.17d.d
3
1H and 31P NMR spectra were registered on
spectrometer Bruker DPX 400 using respectively
HMDS as internal reference and 85% H3PO4 as
external reference. IR spectra were taken on spectro-
meter Specord 75IR. All experiments were carried
out under inert atmosphere (argon).
(1H, =CH-CH, JHC= CH 15.95 Hz), 6.31 d (1H,
=CH-C6H5), 7.15 7.35 m (5H, C6H5) is in agree-
ment with the published spectrum [22]. Found, %:
C 90.62; H 10.05. C11H14. Calculated, %: C 90.35;
H 9.65. From the water layer was isolated 0.6 g
(81%) of acid III.
Reaction of tribenzylphosphine oxide with
2,2-dimethylpropanal. In the same way from 1 g
(3 mmol) of tribenzylphosphine oxide, 0.35 g
(9 mmol) of sodium amide, and 0.39 g (4.5 mmol) of
2,2-dimethylpropanal (V) in 40 ml of THF was
obtained 0.35 g of viscous organic mixture containing
Reaction of tribenzylphosphine oxide with benz-
aldehyde in a system NaNH2 THF (see table, run
no. 1). To a heated to 50 60 C solution of 0.7 g
(2.2 mmol) of tribenzylphosphine in THF (40 ml)
was added 0.26 g (6.6 mmol) of sodium amide, the
reaction mixture was stirred at reflux for 1 h (within
10 min the reaction mixture turned bright yellow),
and then 0.23 g (2.2 mmol) of aldehyde I was added
thereto. The mixture was boiled for 1 h more, cooled
to room temperature, and diluted with water (35 ml).
The reaction products were extracted into ether
(3 10 ml). The extract was washed with water (2
8 ml), dried with potassium carbonate, ether was
distilled off, and the residue was dried in a vacuum.
We obtained 0.35 g (88%) of (E)-1,2-diphenylethene
1
according to its H NMR spectrum 68% of (E)-1-
(1,1-dimethylethyl)-2-phenylethene (X) (yield 50%),
15% of (E)-1,2 -diphenylethene (yield 10%), and
17% of tribenzylphosphine oxide (conversion 94%).
The mixture obtained was washed with small portions
of ether, the ether was removed, and the residue was
washed with pentane. We obtained as an individual
1
product ethene X (viscous fluid). H NMR spectrum
(CDCl3), , ppm: 1.08 s (9H, CH3), 6.23 d (1H,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 12 2001

REACTION OF TRIBENZYLPHOSPHINE OXIDE WITH ALDEHYDES
1729
3
=CH, JHH 16.22 Hz), 6.29 d (1H, =CH), 7.12
12.2 Hz), 6.77 d (1H, =CH), 6.87 d (1H, H5,
3JH
1.1 Hz), 7.20 7.52 m (5H, C6H5). C6H5). The
7.36 m (5H, C6H5) is in agreement with the published
spectrum [23]. Found, %: C 89.49; H 9.34. C12H16.
Calculated, %: C 90.00; H 10.00. From the water
layer was isolated 0.57 g (78%) of acid III.
5H4
chemical shift of H4 proton was not determined
because of overlapping with the signals of other
protons. Acid III was isolated from the water layer
in amount of 0.58 g (79%).
Reaction of tribenzylphosphine oxide with
1-methylimidazole-2-carboxaldehyde. In the same
way from 1 g (3 mmol) of tribenzylphosphine oxide,
0.35 g (9 mmol) of sodium amide, and 0.5 g
(4.5 mmol) of 1-methylimidazole-2-carboxaldehyde
(VI) in 40 ml of THF was obtained 0.42 g of viscous
Reaction of tribenzylphosphine oxide with thio-
phene-2-carboxaldehyde. In the same way from
0.7 g (2.2 mmol) of tribenzylphosphine oxide, 0.26 g
(6.6 mmol) of sodium amide, and 0.34 g (3 mmol)
of thiophene-2-carboxaldehyde (VIII) in 40 ml of
THF was obtained 0.33 g of crystalline mixture
1
organic mixture containing according to its H NMR
1
spectrum 62% of 1-(1-methylimidazol-2-yl)-2-phenyl-
ethene (XI) (yield 42%), 13% of (E)-1,2 -diphenyl-
ethene (yield 10%), and 26% of tribenzylphosphine
oxide (conversion 90%). The mixture obtained was
washed with small portions of ether, the ether was
removed, and the residue was washed with pentane
and dried in a vacuum. We obtained ethene XI
(viscous fluid) as a mixture of E- and Z-isomers
containing according to its H NMR spectrum 70%
of (E)-1-thienyl-2-phenylethene (XI) (yield 56%), 8%
of (E)-1,2 -diphenylethene (yield 6%), and 22% of
tribenzylphosphine oxide (conversion 89%). The
mixture obtained was washed with small portions of
ether, the ether was removed, and the residue was
washed with pentane and dried in a vacuum. We
obtained ethene XI as an individual compound, mp
1
(XIa, b), E:Z = 7.4 : 1. H NMR spectrum (CDCl3),
108 109 C (publ. 111 112 C [24]).
spectrum (CDCl3), , ppm: 6.91 d (1H, =CH, JHH
1H NMR
3
, ppm, of ethene XIa: 3.69 s (3H, CH3), 6.85 d (1H,
H5, JH H4 1.0 Hz), 6.89 d (1H, =CH, JHH 16.2 Hz), 16.1 Hz), 6.98 d.d (1H, H4, 3JH
3.4, 3JH
4H5
3
5
3
4H3
7.06 d (1H, H4), 7.20 7.52 m (5H, C6H5), 7.56 d
5.1 Hz), 7.04 d (1H, H3), 7.16 d (1H, H5), 7.20 d
(1H, =CH), 7.22 7.43 m (5H, C6H5). Found, %:
C 76.92; H 5.38; S 16.81. C12H10S. Calculated, %:
C 77.19; H 5.40; S 16.98. From the water layer was
separated 0.39 g (72%) of acid III.
(1H, =CH); of ethene XIb: 3.23 s (3H, CH3), 6.39 d
(1H, =CH, JHH 12.2 Hz), 6.79 d (1H, H5, JH
3
3
5H4
1.2 Hz), 6.80 d (1H, =CH), 7.20 7.52 m (5H,
C6H5). The chemical shift of H4 proton was not
determined because of overlapping with the signals of
other protons. Acid III was isolated from the water
layer in amount of 0.42 g (57%).
Transformation of tribenzylphosphine oxide
in a system NH2 THF. To a solution of 0.7 g
(2.2 mmol) of tribenzylphosphine oxide in 40 ml of
Reaction of tribenzylphosphine oxide with
1-ethylimidazole-2-carboxaldehyde. In the same
way from 1 g (3 mmol) of tribenzylphosphine oxide,
0.35 g (9 mmol) of sodium amide, and 0.56 g
(4.5 mmol) of 1-ethylimidazole-2-carboxaldehyde
(VII) in 40 ml of THF was obtained 0.35 g of viscous
THF heated to 58 60 C was added 0.26 g
(6.6 mmol) of sodium amide, and reaction mixture
was stirred at reflux for 2 h, cooled, and analyzed
by 31P NMR method. The spectrum of the reaction
mixture in DMSO-d6 contained two signals at
,
p
ppm: 41.8 (assigned to tribenzylphosphine oxide by
comparison with the spectrum of an authentic sample),
and 30.1 that was assigned to phosphorane (B) [reac-
tion (3)] basing on the data of [18]. The reaction
mixture was diluted with water (40 ml), the reaction
products were extracted into ether (3 10 ml), the
extract was dried with potassium carbonate, the ether
was distilled off, and the residue was dried in a
vacuum. We obtained 0.29 g of crystalline residue
1
organic mixture containing according to its H NMR
spectrum 88% of 1-(1-ethylimidazol-2-yl)-2-phenyl-
ethene (XII) (yield 52%), and 12% of (E)-1,2-di-
phenylethene (yield 7%). The mixture obtained was
washed with small portions of ether, the ether was
removed, and the residue was washed with pentane
and dried in a vacuum. We obtained ethene XII
(viscous fluid) as a mixture of E- and Z-isomers
1
1
(XIIa, b), E: Z = 6: 1. H NMR spectrum (CDCl3),
that according to H NMR spectrum contained 56%
3
, ppm, of ethene XIIa: 1.42 t (3H, CH3, JHH
of tribenzylphosphine oxide and 44% of (E)-1,2-di-
phenylethene. The mixture obtained was washed with
small portions of ether. Thus was separated 0.21 g of
tribenzylphosphine oxide (conversion 70%). On re-
moving ether from the ether washings and on re-
crystallization of the residue from ethanol we obtained
3
7.3 Hz), 4.05 q (2H, CH2), 6.87 d (1H, =CH, JHH
3
15.8 Hz), 6.90 d (1H, H5, JH
1.1 Hz), 7.08 d
5H4
(1H, H4), 7.20 7.52 m (5H, C6H5), 7.59 d (1H,
=CH); of ethene XIIb: 1.18 t (3H, CH3, JHH
3
7.3 Hz), 3.65 q (2H, CH2), 6.37 d (1H, =CH, JHH
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 12 2001

1730
GUSAROVA et al.
0.06 g (15%) of ethene II. From the water layer by
the usual procedure was separated 0.2 g (37%) of acid
III. The NMR spectra of ethene II (1H) and acid III
(1H and 1P) are consistent with the published
spectra [3, 21].
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