Synthesis and reactivity of subvalent compounds. Part 13: Reaction of triethyl orthoformate with amines and selenium - A convenient one-step three-component synthesis for selenoureas

Article abstract of DOI:10.1016/S0040-4039(02)02818-6

Selenoureas are obtained by a novel three-component condensation reaction from metallic selenium, triethyl orthoformate and a primary or secondary amine. The reaction is carried out as solvent-free one pot-procedure at 180-190°C under inert gas with a reaction time of 8 h. The reaction was tested for piperidine, isopropylamine, N,N′-dimethylpropylenediamine (Me-NH-CH₂-CH₂-CH₂-NH-Me) and N,N′-disubstituted ethylenediamines (R-NH-CH₂-CH₂-NH-R, R=Me, Et, ⁱPr, ^tBu, Ph).

Full text of DOI:10.1016/S0040-4039(02)02818-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1295–1299
Synthesis and reactivity of subvalent compounds. Part 13:
Reaction of triethyl orthoformate with amines and selenium—a
ꢀ
convenient one-step three-component synthesis for selenoureas
b
a,
Yuehui Zhou and Michael K. Denk *
a
Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1
b
Phosphine Technical Centre, Cytec Canada Inc., Garner Road, Niagara Falls, Ontario, Canada L2E 6T4
Received 12 November 2002; accepted 2 December 2002
Abstract—Selenoureas are obtained by a novel three-component condensation reaction from metallic selenium, triethyl orthofor-
mate and a primary or secondary amine. The reaction is carried out as solvent-free one pot-procedure at 180–190°C under inert
gas with a reaction time of 8 h. The reaction was tested for piperidine, isopropylamine, N,N%-dimethylpropylenediamine
i
t
(
Me-NH-CH -CH -CH -NH-Me) and N,N%-disubstituted ethylenediamines (R-NH-CH -CH -NH-R, R=Me, Et, Pr, Bu, Ph).
2
2
2
2
2
©
2003 Elsevier Science Ltd. All rights reserved.
6
1
. Introduction
the synthesis of thioureas bearing bulky substituents.
2,6,7
Previous research on stable diamino carbenes
and
8
The reduction of thioureas with potassium (Scheme
related species in our laboratory has therefore
prompted us to investigate new methods for the synthe-
9–12
1
–3
1)
is a convenient method for the synthesis of
1–4 2,5
6
diamino carbenes and enetetramines. A disadvan-
tage of this approach are the low yields encountered in
sis of thioureas and, more recently, selenoureas.
The reductive deselenation of selenoureas should pro-
ceed under milder reaction conditions than the
analogous reduction of thioureas, but will be attrac-
tive only if the respective selenoureas are readily acces-
sible. We now report the reaction of metallic selenium
with amines and triethylorthoformate as new solvent-
free one-pot synthesis of symmetric selenoureas.
1–3
Previously published methods for the synthesis of sele-
noureas are largely analogous to those of thioureas.
However, the required selenium reagents like hydrogen
1
3
14
selenide1,
carbon diselenide
bis(trimethylsilyl)-
5
16
selenide or isoselenocyanates R-NꢀCꢀSe are signifi-
cantly more toxic and also thermally less stable than
their sulfur analogs.
A
number 17of selenium
4 10
Scheme 1. Interconversion of thioureas, diaminocarbenes,
enetetramines and selenoureas. Reagents and conditions: (i)
15,17–19
18
reagents
like Woollin’s Reagent or P Se
1
2,3
2,3,7e
allow the conversion of carbonyl groups into the
analogous selenocarbonyl functionalities but ureas are
often inert or give selenoureas only in poor yields.
THF, D, K, -K S. (ii) rt.
(iii) 180°C.
(iv) metallic
2
,5,20–22
4
selenium, rt.
Keywords: selenoureas; selenium; enetetramines; solvent-free; amine;
Due to its low volatility and ready accessibility, metallic
selenium is the least problematic starting material for
the synthesis of other selenium compounds. Selenoureas
can indeed be obtained under ambient reaction condi-
carbene.
ꢀ
For part 12, see: Denk, M. K.; Hatano, K.; Lough, A. J. Synthesis
and characterization of a carbene·SO adduct, Eur. J. Inorg. Chem.,
2
in press.
2
0
*
0
Corresponding author. Tel.: +1 (519) 824-4120, ext. 3820; fax: +1
519) 766-1499; e-mail: mdenk@uoguelph.ca
tions by reacting selenium with diamino carbenes or
21,22
(
enetetramines.
(Scheme 1). Although the yields are
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02818-6

1
296
Y. Zhou, M. K. Denk / Tetrahedron Letters 44 (2003) 1295–1299
generally good, the enetetramines or diamino carbenes
are not readily available and typically require multi-
step syntheses.
eliminate these contaminants. The selenoureas can be
mechanically removed from the reaction vessel but
extraction with a suitable solvent was found to be more
efficient. Yellowish to greenish discolorations of the
solutions were observed on a few occasions, presumably
due to inefficient removal of the volatile impurities, but
are readily eliminated by decolorizing the solutions with
activated charcoal. Selenoureas obtained from these
solutions remain colorless even after prolonged storage.
The absence of solvents minimizes the required reaction
volume and facilitates scale-up of the reaction.
2
. Results and discussion
The starting point of our synthetic investigation was the
reaction of N,N%-diphenyl-ethylenediamine, triethyl
23
orthoformate and sulfur reported by Wanzlick et al.
This remarkably simple method gives 1,3-diphenyl-imi-
dazolidine-2-thione in excellent yield and purity but has
apparently never been applied to other amines. Our
own attempts using a variety of primary and secondary
amines were indeed unpromising and led to thioureas
only in poor yields (<10%). Much to our surprise, we
discovered that the same approach works very well with
metallic selenium. Selenoureas were obtained from
cyclic and non-cyclic amines in fair (3, 6, 7) to excellent
A number of different reaction pathways seem to be
compatible with the formation of selenoureas (Scheme
4,20
5,21
3). Diaminocarbenes
and enetetramines
are
known to react with elemental selenium and enetetra-
mines can in fact be obtained from triethylorthofor-
mate and diamines under reaction conditions similar to
2
4
those employed for the selenourea procedure. The
formation of selenoureas may thus involve the in situ
formation of diamino carbenes or enetetramines (path-
ways C or D). However, alternative pathways involving
the CH activation of triethylorthoformate (A) or its
amino substituted derivatives (B) by selenium have to
be considered as well.
(
1, 2, 4, 5) yields (Scheme 2).
To avoid evaporation losses and to contain the toxic
selenium materials, the reactions are carried out in
®
stainless steel Swagelock cylinders. The use of inert
gas conditions was found to be necessary to avoid
oxidation of the metallic selenium or hydrolysis of the
selenoureas during the reaction.
The fact that selenoureas are also obtained from pri-
mary amines is difficult to reconcile with the intermedi-
acy of enetetramines or carbenes. The enetetramines
obtained from primary amines would contain an
unstable RNH-CꢀC fragment and would rapidly tau-
tomerize to the respective imines RNꢀC-CH). The
analogous tautomerization of the carbenes (R-NH-C:
-NH-R “ R-NH-CHꢀNH-R) on the other hand may
possess a rather high activation barrier (ca. 40 kcal
The reactions do not require a solvent, the yields did in
fact decrease when toluene or octane was employed.
GC–MS analysis of crude reaction mixtures showed
that the minimum temperature required for the reaction
was in fact 150°C, but 180–190°C for a duration of 8 h
is required for synthetically useful conversion rates.
Despite the high reaction temperatures, the selenoureas
are obtained in excellent purity. The crude products
−1 29
mol ). The carbenes R-NH-C:-NH-R are therefore
plausibile intermediates in the formation of the respec-
tive selenoureas R-NH-CSe-NH-R.
1
13
were homogeneous by H, C NMR and GC and
showed melting points that were very close (DT<2°C) to
literature data. Trace impurities of volatile selenium
contaminants seem to give rise to a slow discoloration
of the crude selenoureas upon exposure to air or light.
Subjecting the crude reaction mixtures to a 3 h pump-
ing cycle at 0.1 Torr was found to be sufficient to
Indirect evidence for the feasibility of pathway A was
obtained from a control experiment in which selenium
was reacted with pure triethyl orthoformate. A trace
amount of a newly formed compound was observed in
Scheme 2. Synthesis of selenoureas from primary and secondary amines. Reagents and conditions: (i) 2 equiv. of triethyl
orthoformate/1 equiv. of metallic selenium/190°C/8 h. Yields: 89% (1), 95% (2), 39% (3), 95% (4), 91% (5), 34% (6), 30% (7).

Y. Zhou, M. K. Denk / Tetrahedron Letters 44 (2003) 1295–1299
1297
Scheme 3. Possible pathways for the formation of selenoureas from orthoesters.
the GC of the crude reaction mixture (0.1% based on
diamines) triethylorthoformate (50 mmol, 100% excess®)
and selenium (25.0 mmol) is sealed in a Swagelock
stainless steel cylinder and heated to 180–190°C for 8 h.
The cold reaction mixture is dried under oil vacuum for
3 h, dissolved in the minimum amount of ether–ethanol
(3:1), decolorized with charcoal and filtered. Slow evap-
oration gives the selenoureas as colorless, crystalline
solids. 1,3-Dimethyl-imidazolidine-2-selenone (1): Yield
total ion count) and identified as (EtO) CꢀSe by its
2
mass spectrum (characteristic isotope pattern for one
selenium atom). This concentration is too small to
account for a complete conversion, but step A may be
catalyzed by the amine (reversible formation of the
carbanion) and should thus not be prematurely ruled
out. Precedence for B is the formation of the respective
2
5
1
selenoureas from the orthoamides HC(NMePh)
3
89%, colorless crystals, mp 146–147°C from EtOH. H
2
6
t
26
1
3
HC(NMe2)
3
or BuOCH(NMe )
2 2
and metallic sele-
NMR: 3.24 (s, 6H, NCH3
6
), 3.60 (s, 4H, NCH
6
2).
C
nium reported by Kantlehner and Sch o¨ nberg although
these reactions may also proceed through the carbene
pathway C.
NMR: 36.9 (CH ), 49.4 (CH ), 181.9 (s, C
6
ꢀSe). EI-MS:
3
2
+
1
4
9
78 (M , 100), 120 (6), 97 (14), 83 (7), 69 (69), 56 (32),
2 (70). 1,3-Diethyl-imidazolidine-2-selenone (2): Yield
1
5%, colorless crystals, mp 85.5–86°C from Et O. H
2
3
NMR: 1.18 (t, J(H,H)=7 Hz, CH
CH
6
), 3.60 (s, 4H,
3
3
13
6
CH
6
), 3.75, (q, J(H,H)=7 Hz, NCH
6
2CH3).
C
2
2
3. Experimental
1
NMR: 12.6 (q, J(C,H)=127 Hz, C
6 H ), 44.4 (t,
3
1
1
J(C,H)=138 Hz, NC6 H ), 46.7 (t, J(C,H)=145 Hz,
2
+
Caution! Selenium compounds are highly toxic. All oper-
ations (including the cleaning of equipment) should be
carried out in an efficient fume hood.
NC
(
6
H ), 179.6 (s, C
6
ꢀSe). EI-MS: 206 (M , 100), 177
2
11), 125 (81), 108 (5), 97 (40), 83 (28), 69 (52), 56 (40),
4
2 (71). 1,3-Di-tert-butyl-imidazolidine-2-selenone (3):
Yield 39%, colorless crystals, mp 174–175°C (EtOH).
Melting points were recorded in sealed capillaries and
are uncorrected. EI-MS spectra were obtained with a
Shimadzu 5050 GC–MS instrument at an ionizing
voltage of 70 eV. NMR spectra were recorded on
1
13
H NMR: 1.69 (s, 18H, CH
6
), 3.48 (s, 4H, CH
6
2).
C
3
1
NMR: 29.3 (q, J(C,H)=127 Hz, C
6 H ), 46.1 (t,
3
1
J(C,H)=145 Hz, NC6 H ), 58.4 (s, C6 (CH ) ), 178.9 (s,
2 3 3
+
1
13
C6 ꢀSe). EI-MS: 262 (M , 36), 205 (19), 150 (100), 126
Varian Gemini 200 ( H), Bruker 500 MHz ( C) or
13
(7), 111 (26), 84 (9), 70 (18), 57 (40), 41 (62). 1,3-
Diphenyl-imidazolidine-2-selenone (4): Yield 95%, off-
Varian 400 MHz spectrometers ( C) at normal spec-
trometer temperature. The spectra are referenced versus
2
7a
1
13
white crystals, mp 185–186°C (CH Cl ), 186°C,
TMS ( H and C, internal). Selenium powder -100
mesh (99.5%) and all other starting materials were
obtained from Aldrich Inc. and used as received. The
synthesis of the selenoureas was conducted under an
atmosphere of nitrogen or argon (purity 99.994 or
better). Yields are not optimized. Contaminated equip-
ment and the rubber gloves used in the preparations
were cleaned with household bleach. The cleaning solu-
tions were left to evaporate in the fume hood and the
remaining solids disposed of as selenium waste.
2
2
21
1
1
83–185°C ). H NMR (CDCl ): 4.03 (s, C6 H , 4H),
3 2
13
7
.52–7.22 (M, phenyl-H6 ). C NMR (CDCl ): 51.3 (t,
3
1
J(C,H)=147 Hz, C
6
H ), 126.6 (d, 161 Hz, C
6
Hmeta),
2
1
1
27.4 (d, J(C,H)=161 Hz,
C6
Hpara), 129.5 (d,
1
J(C,H)=161 Hz, C6 Hortho), 141.5 (s, Cipso), 180.6 (s,
C6 ꢀSe). EI-MS: 301 (34), 194 (7), 238 (100), 118 (26),
105 (36), 91 (83), 77 (85), 51 (28). 1,3-Dimethyl-2,4,5,6-
tetrahydropyrimidine-2-selenone (5): Yield 91%, color-
1
less crystals, mp 100–101°C from Et
O–EtOH (1:1). H
2
3
NMR: 2.07 (quin, J(H,H)=6 Hz, CH C H6 2), 3.39 (t,
2
3
13
General procedure for the synthesis of selenoureas from
amines and triethyl orthoformate. A mixture of the
amine (50.0 mmol for monoamines, 25.0 mmol for
J(H,H)=6 Hz, NCH
6
), 3.54 (s, NCH3
6
). C NMR:
2
2
1
21.2 (t, J(C,H)=132 Hz, CH C
6
H CH ), 47.2 (q,
2
2
1
1
J(C,H)=139 Hz, NC6 H ), 48.7 (t, J(C,H)=141 Hz,
3

1
298
Y. Zhou, M. K. Denk / Tetrahedron Letters 44 (2003) 1295–1299
+
NC
1
6
H ), 177.4 (s, C
6
ꢀSe). EI-MS: 192 (M , 78), 121 (7),
Rodezno, J.; Gupta, S.; Lough, A. J. J. Organomet.
2
11 (22), 97 (22), 83 (14), 70 (20), 55 (21), 42 (100).
N,N%-Diisopropyl selenourea (6): Yield 34%, off-white
crystals, mp 173–174°C from Et O–EtOH (3:1). Lit:
Chem. 2001, 617–618, 242–253; (d) Lehmann, J. F.;
Urquhart, S. G.; Ennis, L. E.; Hitchcock, A. P.; Hatano,
K.; Gupta, S.; Denk, M. K. Organometallics 1999, 18,
1862–1872; (e) Denk, M. K.; Hatano, K.; Ma, M. J.
Tetrahedron Lett. 1999, 40, 2057–2060; (f) Denk, M. K.;
Thadani, A.; Hatano, K.; Lough, A. J. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2607–2609; Angew. Chem. 1997,
109, 2719–2721.
2
1
4c
16a 1
3
1
7
73°C, 167.5–168°C.
Hz, 6H, CH
H NMR: 1.23 (d, J(H,H)=
), 4.26 (septet, J(H,H)=7 Hz, CH
). C NMR: 23.1 (q, J(C,H)=127 Hz, C
3
6
6
), 6.1
3
1
3
1
(
4
br. NH
6.5 (d, J(C,H)=140 Hz, C
6
6
H3),
1
6
H), 179.5 (s, C
6
ꢀSe). EI-
MS: 160 (34), 85 (6), 69 (10), 58 (100), 43 (54). 1,1%-
Selenocarbonyl-bis-piperidine (7): Yield 30%, off-white
crystals, mp 80–81°C from Et O–EtOH (3:1) g. Lit.:
8. (a) Hitchcock, A. P.; Ennis, L. E.; Lehmann, J. F.; Denk,
M. K. J. El. Spec. 2001, 114, 1037–1041; (b) Leites, L. A.;
Bukalov, S. S.; Denk, M. K.; West, R.; Haaf, M. J. Mol.
Struct. Theochem. 2000, 550, 329–335; (c) Denk, M. K.;
Gupta, S.; Ramachandran, R. Eur. J. Inorg. Chem. 1999,
41–49; (d) Denk, M. K.; Hatano, K.; Lough, A. J. Eur. J.
Inorg. Chem. 1998, 1067–1070; (e) Urquhart, S.; Hitch-
cock, A. P.; Denk, M. K. Organometallics 1998, 17,
2352–2360; (f) Denk, M. K.; Gupta, S.; Ramachandran,
R. Tetrahedron Lett. 1996, 37, 9025–9028; (g) Metzler,
N.; Denk, M. K. Chem. Commun. 1996, 2657–2658; (h)
Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Haaland,
A.; Belyakov, A. V.; Verne, H. P.; Wagner, M.; Metzler,
N. J. Am. Chem. Soc. 1994, 116, 2691–2692; (i) Denk,
M.; Hayashi, R. K.; West, R. J. Am. Chem. Soc. 1994,
116, 10813–10814; (j) Denk, M.; Green, J. C.; Metzler,
N.; Wagner, M. J. Chem. Soc., Dalton Trans. 1994,
2405–2410; (k) Denk, M.; Hayashi, R. K.; West, R. J.
Chem. Soc., Chem. Commun. 1994, 33–34.
2
1
4d
28
1
79–80°C,
78–79°C ). H NMR: 1.56 (m, 12H, CH
6
2
C
13
CH CH +CH CH
6
6
CH ), 3.58 (m, 4H, NCH
6
2).
2
2
2
2
2
1
NMR: 25.0 (t, J(C,H)=127 Hz, CH CH
6
CH ), 26.3 (t,
2
2
2
1
1
J(C,H)=128 Hz, 8H, NCH CH6 ), 54.9 (t, J(C,H)=
2
+
2
139 Hz, CH N), 194.5 (s, C
6
ꢀSe). EI-MS: 260 (M , 32),
2
1
79 (28), 111 (11), 96 (20), 84 (100), 69 (25), 55 (25), 41
(
71).
Acknowledgements
We thank the Natural Sciences and Engineering
Research Council of Canada (NSERC) for the support
of this work through an operating grant and equipment
grants (1999, 2002) for the purchase of a GC–MS
instrument and an inert gas glove box. We thank the
PREA Foundation of Ontario for a PREA award.
9. Attempts to oxidize diaminomethanes R N-CH -NR
2
2
2
with selenium gave selenoureas but only in poor yields
6
8
(
<10%, contrast to oxidation with S ).
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