Synthesis of precursors of 4-phosphino-1h-pyrrole-3-carbaldehyde derivatives

Article abstract of DOI:10.1002/hc.21069

In this work, possible approaches to the synthesis of 1,2,5-substituted 4-phosphoryl-3-formylpyrroles have been considered. As a result, two methods for the synthesis of 4-(diphenylphosphoryl)-1-(4-ethoxyphenyl)-2,5-dimethyl-1H- pyrrole-3-carbal-dehyde were proposed; the highest yields gives formylation of 3-(diphenylphosphorothioyl)-1-(4-ethoxyphenyl)-2,5-dimethyl-1H-pyrrole. The formyl fragment was successfully converted into a Schiff base with phenethylamine, and the phosphoryl group has been reduced to phosphine with silicochloroform, which suggests a promising approach to the synthesis of chiral bidentate phosphine ligands. 2013 Wiley Periodicals, Inc. Heteroatom Chem 24:146-151, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.21069

Full text of DOI:10.1002/hc.21069

Heteroatom Chemistry
Volume 24, Number 2, 2013
Synthesis of Precursors of
4-Phosphino-1H-pyrrole-3-carbaldehyde
Derivatives
Radomyr V. Smaliy, Aleksandra A. Chaykovskaya, Nataliya A. Shtil,
Aleksandr S. Savateev, and Aleksandr N. Kostyuk
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanska Str. 5,
Kyiv-94, 02660, Ukraine
Received 30 August 2012; revised 19 November 2012
venient. A chiral inductor is a derivative of car-
ABSTRACT: In this work, possible approaches to
boxylic acids—amides, amidines, or derivatives of
the synthesis of 1,2,5-substituted 4-phosphoryl-3-
ketones (aldehydes)—various azomethines. Scaf-
formylpyrroles have been considered. As a result, two
folds on which these groups are usually placed are
methods for the synthesis of 4-(diphenylphosphoryl)-
the phenyl ring, but there are examples of hetero-
1-(4-ethoxyphenyl)-2,5-dimethyl-1H-pyrrole-3-carbal-
cyclic compounds and metallocenes. These ligands,
dehyde were proposed; the highest yields gives
which can be depicted by a general formula I, work
formylation of 3-(diphenylphosphorothioyl)-1-(4-
with various metals, for example, palladium [1–6],
ethoxyphenyl)-2,5-dimethyl-1H-pyrrole. The formyl
rhodium [7, 8], copper [9], and iridium [10, 11] (see
fragment was successfully converted into a Schiff
Fig. 1). The literature analysis shows that these lig-
base with phenethylamine, and the phosphoryl group
ands are poorly represented by the pyrrole deriva-
has been reduced to phosphine with silicochloro-
tives, whereas the well-known property of pyrroles is
form, which suggests a promising approach to the
easy direct—phosphorylation by phosphorus halides
ꢀC
synthesis of chiral bidentate phosphine ligands.
and good reactivity in other reactions of electrophilic
substitution [12, 13]. This work presents an attempt
to reach the precursors of the mentioned type of lig-
ands by formylation of substituted phosphorylated
pyrroles.
2013 Wiley Periodicals, Inc. Heteroatom Chem
24:146–151, 2013; View this article online at
wileyonlinelibrary.com. DOI 10.1002/hc.21069
INTRODUCTION
RESULTS AND DISCUSSION
Ligands containing a phosphine donor and a car-
bonyl fragment at the adjacent atoms show excep-
tional applicability in metal complex catalysis; an
example of this type of ligands is represented by the
well-known Trost ligand [1,2].
As the pyrrole ring represents electron-rich het-
erocycles for which double electrophilic reac-
tions are common, our strategy anticipated con-
secutive introduction of phosphino and carbonyl
groups at 2,5-dimethyl-N-aryl pyrroles by elec-
trophilic substitution reactions. Phosphine 2 was
easily prepared by phosphorylation of 2,5-dimethyl-
N-arylpyrrole 1 (Scheme 1) [12]. Phosphine 2
is a colorless crystalline solid capable of pro-
longed storage in an inert atmosphere. The
straightforward method to the target compounds,
This approach for the synthesis of new chi-
ral phosphine ligands is straightforward and con-
Correspondence to: Aleksandr N. Kostyuk; e-mail: a.kostyuk@
yahoo.com
ꢀC
2013 Wiley Periodicals, Inc.
146

Synthesis of Precursors of 4-Phosphino-1H-pyrrole-3-carbaldehyde Derivatives 147
FIGURE 1 Examples of phosphines of type I: a [1,2], b [11], c [3], d [10], e [8], f [5], and g [6].
2
1
3
SCHEME 1
3-formyl-4-diphenylphosphinopyrroles, seems to be
direct formylation of phosphine 2. Although phos-
phine 2 contains two nucleophilic centers—3-
position of the pyrrole ring and phosphorus atom,
we expected that by manipulating the reaction con-
ditions we would be able to get direct formylation
exclusively at the pyrrole ring.
We have found that Vilsmeier formylation of
phosphine 2 led to the formation of complex mix-
tures, in which judging by 31P NMR complete oxida-
tion of trivalent phosphorus took place (Scheme 1).
Therefore, this approach produced compound 3 only
in 35% yield. It should be mentioned that deoxygena-
tion of solvents and saturation with argon had no
effect on the yield of compound 3. This may prove
that the oxidation of trivalent phosphorus occurs by
formylation reagents.
To protect the phosphino group, compound 2
was converted into phosphine-borane 4 a colorless
solid by a standard procedure (Scheme 2) [14]. Un-
fortunately, formylation under Vilsmeier conditions
failed to produce the target aldehyde. Thus, at 100◦С
in 8–10 h conversion of phosphine-borane 4 was only
50%, accompanied by marked resinification of the
reaction mixture. Varying the reaction conditions
had a little effect so that this approach did not work
and we failed to prepare 5.
To avoid the formation of large amounts of by-
products, we decided to use pentavalent phospho-
rus derivatives 6–8 in formylation with further re-
duction to trivalent phosphorus derivatives. For this
purpose, phosphine 2 was converted into sulfide 6,
selenide 7, and oxide 8 (Scheme 3). It was found
that formylation of pyrroles 6–8 proceeded under
2
4
5
SCHEME 2
Heteroatom Chemistry DOI 10.1002/hc

148 Smaliy et al.
was boiling in methanol with a sesquialteral excess
of the amine. For the reduction of phosphineoxide
9, two reagents, diphenylsilane and trichlorosilane,
were tested. Diphenylsilane was not very useful for
this reaction; it began to work only upon prolonged
(16 h) heating at 160◦C, which leads to a consid-
erable resinification mixtures, purification of which
additionally complicated by the presence of phenyl-
siloxanes. The reaction with trichlorosilane proceeds
readily in boiling toluene and completes the reaction
in 6 h, affording a target product 10 in 80% isolated
yield.
2
SCHEME 3
rather harsh conditions—100◦C, a double excess of
phosphorus oxychloride and 10 h, which is probably
due to electron withdrawing and steric effects of the
phosphoryl groups. In all three cases, formylation
led to the sole aldehyde 3 (Scheme 4). It is obvious
that the thio(seleno)phosphoryl groups are not toler-
ant to the Vilsmeier reagent. It is likely that the elec-
trophilic Vilsmeier reagent attacks sulfur (selenium),
affording an intermediate that upon hydrolysis gave
phosphoryl derivatives. Similar transformation of
thiophosphoryl and selenophosphoryl groups upon
treatment with trifluoroacetic acids anhydride was
described in the literature [15]. As in the case of the
first approach (Scheme 1), this approach also does
not allow completely avoiding appreciable resinifica-
tion of the reaction mixture, although the products
yields considerably increased. Thus, the lowest yield
was observed for oxide 8 (51%), whereas the sulfide
6 gave the highest yield (75%), which makes it the
most appropriate starting material in this reaction.
Since in the present study the target compounds are
the phosphines with a chiral center at the adjacent
atom, our further research was focused on the syn-
thesis of the corresponding derivatives of the formyl
group, as well as the reduction of the phosphineox-
ide fragment to the trivalent phosphine.
CONCLUSIONS
As a result of the current study, the approaches to
the synthesis of 4-(diphenylphosphoryl)-1-aryl-2,5-
dimethyl-1H-pyrrole-3-carbaldehydes were found. A
consecutive transformation of the formyl moiety
into an azomethine one and the reduction of phos-
phineoxide to the phosphine center with satisfac-
tory yields have been shown. The obtained results
look promising for further synthesis of chiral ligands
based on the pyrrole ring.
EXPERIMENTAL
NMR spectra were recorded on a Varian VXR-300
(Paula Alta, USA) spectrometer or Bruker Avance
drx 500: 1H (300/500 MHz) and 13C (125 MHz) with
TMS as an internal standard and 31P (121 MHz)
with 85% H3PO4 as an external standard. APT exper-
iments were used to assign 13C chemical shifts. Mass
spectra were recorded using an Agilent 1100 series
LC/MSD system. All reactions were carried out un-
der dry argon in Schlenk-type glassware. Solvents in-
cluding deuterated ones used for NMR spectroscopy
were dried and distilled prior to use.
We have obtained the Schiff base using the alde-
hyde 3 and racemic 1-phenylethanamine. We have
attempted the synthesis of azomethine 9 by boil-
ing the reaction mixture in methanol and in toluene
with removal of water. The former procedure gave an
80% isolated yield of azomethine 9 when aldehyde 3
3-(Diphenylphosphino)-1-(4-ethoxyphenyl)-2,5-
dimethyl-1H-pyrrole 2
To a stirred solution of pyrrole 1, 2.15 g (0.01 mol)
in pyridine (60 mL) and diphenylphosphinous
3
9
10
SCHEME 4
Heteroatom Chemistry DOI 10.1002/hc

Synthesis of Precursors of 4-Phosphino-1H-pyrrole-3-carbaldehyde Derivatives 149
chloride 2.21 g (1.87 mL, 0.01 mol) was added. The
C27H26NO3P (443.47): C 73.12; H 5.91; P 6.98. Found:
C 72.88, H 5.67, P 6.69.
reaction mixture was stirred for 5 days at room
temperature. Hexane (100 mL) was added to the re-
action mixture. After 30 min, the precipitated pyri-
dine hydrochloride was filtered off and the filtrate
was evaporated in vacuo. The product was recrys-
tallized from acetonitrile as colorless solid. mp 146–
Borane Complex of 3-(Diphenylphosphino)-1-(4-
ethoxyphenyl)-2,5-dimethyl-1H-pyrrole 4
To a stirred solution of phosphine 3 (0.55 g,
1.38 mmol) in dry THF (30 mL), 2.06 mL
(4.13 mmol) of the BH3·THF complex in THF was
added through a rubber septum. After overnight, the
solution was evaporated to its half volume under re-
duced pressure and methyltert-butyl ether (40 mL)
and aqueous solution of hydrochloric acid (30 mL)
were added. The organic layer was separated. The
product was extracted three times from the aque-
ous solution with methyltertbutyl ether. All organic
phases were combined and washed twice with water,
dried over Na2SO4, and evaporated under reduced
pressure. The product 4 is a colorless solid, mp 155
–156◦C. Yield: 98%.
1
147◦С, yield 82%. H NMR (CDCl3), δ (ppm): 1.45
(t, 3H, J = 7.2 Hz, OCH2CH3), 1.95 (s, 3H, CH3), 2.14
(s, 3H, CH3), 4.08 (q, 2H, J = 7.2 Hz, OCH2CH3),
5.65 (s, 1H, CH, Pyr), 6.95 (d, 2H, J = 8.8 Hz, Ar),
7.11 (d, 2H, J = 8.8 Hz, Ar), 7.25–7.40 (m, 6H, Ar),
7.39–44 (m, 4H, Ar). 13C{1H}NMR (CDCl3), δ (ppm):
12.16; 12.66; 14.76; 63.76; 108.72, 107.69 (d, JCP
129.5 Hz); 109.48; 114.87; 127,80; 128.15 (d, JCP
6.3 Hz); 129.20; 129.27; 130.18; 131.83 (d, JCP
=
=
=
8.6 Hz); 133.02; 133,17; 139.80; 158.49. 31P NMR
(CDCl3), δ (ppm): –29.9. Anal. Calcd for C26H26NOP
(399.46): C 78.17; H 6.56; P 7.75. Found: C 77.94, H
6.44, P 7.69.
1H NMR (DMSO-d6), δ (ppm): 1.03 br s (3H,
BH3), 1.36 (t, 3H, J = 7.2 Hz, OCH2CH3), 1.87
(s, 3H, CH3), 1.91 (s, 3H, CH3), 4.08 (q, 2H, J =
7.2 Hz, OCH2CH3), 5.71 br s (1H, CH, Pyr), 7.06
(d, 2H, J = 8.0 Hz, Ar), 7.34 (d, 2H, J = 8.0 Hz,
Ar), 7.54 (m, 10H, Ar). 13C{1H}(CDCl3), δ (ppm):
12.48; 12.72; 14.61; 63.36; 100.31, 100.92 (d, JCP
= 77.9 Hz); 110.36; 114.97; 128,64; 128.72; 129.01;
129.51; 129.88 (d, JCP = 8.8 Hz); 130.69, 131.16
(d, JCP = 59.2 Hz); 130.75; 132.03; 132,11; 135.56
(d, JCP = 18.9 Hz); 158.29. 31P NMR (DMSO-d6), δ
(ppm): 7.26. Anal. Calcd for C26H29BNOP (413): C
75.56, H 7.07, P 7.49. Found: C 75.15, H 7.24, P 7.21.
4-(Diphenylphosphoryl)-1-(4-ethoxyphenyl)-2,5-
dimethyl-1H-pyrrole-3-carbaldehyde 3
Method A. To a stirred solution of phosphine
2 (4.0 g, 0.01 mol) in DMF (10 mL) P(O)Cl3 (1.68 g,
1.00 mL, 0.011 mol) was added at room temperature.
The reaction mixture was stirred 2 h and poured into
a saturated solution of NaHCO3 (50 mL). The prod-
uct was extracted with toluene, dried over Na2SO4,
and evaporated in vacuo. The residue was recrystal-
lized from acetonitrile as colorless solid. mp 195–
196◦С, yield 35%.
General Procedure for Synthesis of Compounds
6 and 7
Method B (General for Compounds 6–8). To a
stirred solution of the appropriate compound 6–8
(0.01 mol) in dry DMF (10 mL), P(O)Cl3 (3.07 g,
1.83 mL, 0.02 mol) was added dropwise at 90◦С. The
reaction mixture was heated for 10 h at 100◦С, and
after cooling it was poured into 1 M aqueous solu-
tion of NaOH (30 mL) at 0◦С. The resulting precipi-
tate was filtered and recrystallized from acetonitrile.
Yield 75% (for 6); 65% (for 7); 51% (for 8).
To a stirred solution of phosphine 2 (3.99 g, 0.01 mol)
in dry benzene, sulfur or selenium (0.01 mol) was
added at ambient temperature. The reaction mix-
ture was refluxed for 0.5 h. Benzene was evaporated
under reduced pressure, and the product was recrys-
tallized from acetonitrile.
1H NMR (DMSO-d6), δ (ppm): 1.35 (t, 3H,
J = 7.2 Hz, OCH2CH3), 1.81 (s, 3H, CH3), 2.23
(s, 3H, CH3), 4.10 (q, 2H, J = 7.2 Hz, OCH2CH3),
7.09 (d, 2H, J = 8.0 Hz, Ar), 7.31 (d, 2H, J =
8.0 Hz, Ar), 7.53–7.60 (m, 6H, Ar), 7.66–7.70 (m, 4H,
Ar), 9.68 (s, 1H, C(O)H). 13C{1H}NMR (DMSO-d6), δ
(ppm): 11.93; 12.14; 14.54; 63.46; 108.31; 108.27 (d,
JCP = 120.7 Hz); 115.24; 121.96 (d, JCP = 8.8 Hz);
127.76; 128.46; 128.55; 129.23; 131.04; 131.12;
131.54; 134.53; 135.38; 138.90; 139.03; 140.07 (d,
JCP = 8.8 Hz); 158.89; 185.68. 31P NMR (DMSO-d6),
δ (ppm): 22.97. m/z 444 [M + 1]+. Anal. Calcd for
3-(Diphenylphosphorothioyl)-1-(4-ethoxyphenyl)-
2,5-dimethyl-1H-pyrrole 6. mp 177◦C. Yield: 78%.
1H NMR (DMSO-d6), δ (ppm): 1.36 (t, 3H, J =
7.2 Hz, OCH2CH3), 1.89 (s, 3H, CH3), 1.92 (s, 3H,
CH3), 4.08 (q, 2H, J = 7.2 Hz, OCH2CH3), 5.50
(d, 1H, J = 3.6 Hz, CH, Pyr), 7.06 (d, 2H, J = 8.8 Hz,
Ar), 7.24 (d, 2H, J = 8.8 Hz, Ar), 7.45–7.60 (m, 6H,
Ar), 7.75–7.80 (m, 4H, Ar). 13C{1H}(CDCl3), δ (ppm):
12.30; 12.58; 14.57; 63.35; 106.07, 106.96 (d, JCP
=
111.9 Hz); 109.93 (d, JCP = 10.1 Hz); 115.00; 128.46
(d, JCP = 12.6 Hz); 128.91 (d, JCP = 12.6 Hz); 129.06;
129.32; 131.16 (d, JCP = 3.8 Hz); 131.32 (d, JCP
=
Heteroatom Chemistry DOI 10.1002/hc

150 Smaliy et al.
10.1 Hz); 133.98; 134.66; 135.43 (d, JCP = 17.6 Hz);
158.34. 31P NMR (DMSO-d6), δ (ppm): 32.8. m/z 432
[M + 1]+. Anal. Calcd for C26H26NOPS (431.15): C
72.37, H 6.07, P 7.18. Found: C 72.15, H 5.84, P 7.01.
methanol (50 mL) was refluxed for 4 h. The sol-
vent was evaporated under reduced pressure. The ex-
cess of 1-phenylethanamine was removed in vacuo
by heating the residue on boiling water bath. The
1
product is brown oil. Yield: 95%. H NMR (CDCl3),
3-(Diphenylphosphoroselenoyl)-1-(4-ethoxyphe-
δ (ppm): 1,12 (d, 3H, J = 6.8 Hz, CH3); 1.36 (t, 3H,
J = 6.8 Hz, CH3); 1.74 (s, 3H, CH3); 2.21 (s, 3H, CH3);
3.80 (q, 1H, J = 6.8 Hz); 4.09 (q, 2H, J = 6.8 Hz); 7.08–
7.65 (m, 19H, Ar); 7.98 (s, 1H, CH). 13C{1H}(CDCl3),
δ (ppm): 12.67; 12.73; 14.78; 25.72; 51.28; 63.83;
106.74, 107.72 (d, JCP = 123.2 Hz); 115.44; 125.70;
126.46; 128.20; 128.49 (d, JCP = 2.5 Hz); 128.57;
nyl)-2,5-dimethyl-1H-pyrrole
7. mp
204◦C.
Yield: 82% 1H NMR (DMSO-d6), δ (ppm): 1.34
(t, 3H, J = 7.2 Hz, OCH2CH3), 1.87 (s, 3H, CH3), 1.89
(s, 3H, CH3), 4.07 (q, 2H, J = 7.2 Hz, OCH2CH3), 5.47
(d, 1H, J = 3.9 Hz, CH, Pyr), 7.05 (d, 2H, J = 8.4 Hz,
Ar), 7.30 (d, 2H, J = 8.4 Hz, Ar), 7.45–7.55 (m, 6H,
Ar), 7.71–7.79 (m, 4H, Ar). 13C{1H}(CDCl3), δ (ppm):
129.21 (d, JCP = 6.3 Hz); 131.39; 131.90 (d, JCP
=
12.24; 12.64; 14.54; 63.33; 104.52, 105.32 (d, JCP
=
11.3 Hz); 147.83; 159.13. 31P NMR (CDCl3), δ (ppm):
22.0. m/z 546 [M]+. Anal. Calcd for C35H35N2O2P
(546.64): C 76.90; H 6.45; P 5.67. Found: C 76.79, H
6.31, P 5.58.
100.6 Hz); 110.01 (d, JCP = 12.6 Hz); 114.99; 128.45
(d, JCP = 12.6 Hz); 128.94 (d, JCP = 12.6 Hz); 129.03;
129.25; 131.23 (d, JCP = 2.51 Hz); 131.73 (d, JCP
=
3.77 Hz); 132.58, 133.20 (d, JCP = 78.0 Hz); 135.42
(d, JCP = 18.9 Hz); 158.36. 31P NMR (DMSO-d6),
δ (ppm): 21.0. m/z 480 [M + 1]+. Anal. Calcd for
C26H26NOPSe (478.42): C 65.27, H 5.48, P 6.47.
Found: C 65.11, H 5.25, P 6.29.
N-((4-(Diphenylphosphino)-1-(4-ethoxyphenyl)-
2,5-dimethyl-1H-pyrrol-3-yl)methylene)-1-
phenylethanamine 10
3-(Diphenylphosphoryl)-1-(4-ethoxyphenyl)-2,5-
dimethyl-1H-pyrrole 8
A mixture of phosphinoxide 9 (5.47g, 0.01 mol), tri-
ethylamine (0.2 mol), and trichlorosilane (0.05 mol)
in toluene (40 mL) was heated in a sealed tube at
120◦C for 6 h. After cooling to room temperature, the
reaction mixture was treated with saturated aqueous
NaHCO3 solution. The organic layer was separated
and filtered through a layer of alkaline aluminum
oxide. The filtrate was evaporated in vacuo. Yield
57%. mp 123–125◦С. 1H NMR (CDCl3), δ (ppm): 1.25
(d, 3H, J = 6.8 Hz, CH3); 1.44 (t, 3H, J = 6.8 Hz,
CH3); 1.81 (s, 3H, CH3); 1.91 (s, 3H, CH3); 2.35
(s, 3H, CH3); 3.22 (m, 1H, CH); 4.07 (q, 2H, J =
6.8 Hz, CH2); 6.94–7.44 (m, 19H, Ar); 7.61 (br. s, 1H,
CH). 31P NMR (CDCl3), δ (ppm): –32.9. Anal. Calcd
for C35H35N2O2P (530.64): C 79.22; H 6.65; P, 5.84.
Found: C 79.15, H 6.43, P 5.79.
To a solution of phosphine 2 (3.99 g, 0.01 mol) in
dry benzene (10 mL), hexachloroethane (2.37 g, 0.01
mol) was added. The reaction mixture was stirred for
0.5 h. The benzene layer was decanted, and the oil
was dissolved in dichloromethane (20 mL). The solu-
tion was treated with 5% aqueous NaHCO3 solution
(10 mL). The organic layer was separated, dried over
anhydrous sodium sulfate, and evaporated in vacuo.
The product was recrystallized from hexane as a col-
orless solid. mp 135◦С, yield 83%. 1H NMR (CDCl3),
δ (ppm): 1.41 (t, 3H, J = 7.2 Hz, OCH2CH3), 1.89
(s, 3H, CH3), 2.03 (s, 3H, CH3), 4.04 (q, 2H, J =
7.2 Hz, OCH2CH3), 5.60 (s, 1H, CH, Pyr), 6.93 (d, 2H,
J = 8.8 Hz, Ar), 7.06 (d, 2H, J = 8.8 Hz, Ar), 7.41–7.45
(m, 6H, Ar), 7.71–76 (m, 4H, Ar). 13C{1H}(CDCl3),
δ (ppm): 12.71; 12.85; 14.81; 63.79; 108.29, 107.26
(d, JCP = 129.5); 110.35d (d, JCP = 12.6 Hz); 114.99;
128.23d (d, JCP = 12.6 Hz); 129.05; 129.61d (d, JCP
=
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Calcd for C26H26NO2P (415.46): C 75.16; H 6.31; P
7.46. Found: C 75.13, H 6.29, P 7.26.
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Heteroatom Chemistry DOI 10.1002/hc