Silicon-mediated synthesis of selenoaldehydes and selenoacylsilanes and their hetero diels-alder reactions

Article abstract of DOI:10.1080/10426500902947989

Bis(trimethylsilyl)selenide (HMDSS) reacts efficiently with aldehydes in the presence of CoCl2.6H2O to afford selenoaldehydes, which are trapped as Diels-Alder adducts by different dienes. The reaction can be applied to acylsilanes, to afford selenoacylsilanes, isolated as their cycloadducts.

Full text of DOI:10.1080/10426500902947989

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Phosphorus, Sulfur, and Silicon
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Silicon-Mediated Synthesis
of Selenoaldehydes and
Selenoacylsilanes and Their
Hetero Diels–Alder Reactions
Alessandro Degl'Innocenti a , Antonella Capperucci a
, Miriam Acciai a & Caterina Tiberi a
a Università di Firenze, Dipartimento di Chimica
Organica and HBL , Sesto Fiorentino, Italy
Published online: 06 Jun 2009.
To cite this article: Alessandro Degl'Innocenti , Antonella Capperucci , Miriam
Acciai & Caterina Tiberi (2009) Silicon-Mediated Synthesis of Selenoaldehydes
and Selenoacylsilanes and Their Hetero Diels–Alder Reactions, Phosphorus,
Sulfur, and Silicon and the Related Elements, 184:6, 1621-1626, DOI:
10.1080/10426500902947989
To link to this article: http://dx.doi.org/10.1080/10426500902947989
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Phosphorus, Sulfur, and Silicon, 184:1621–1626, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902947989
Silicon-Mediated Synthesis of Selenoaldehydes and
Selenoacylsilanes and Their Hetero Diels–Alder Reactions
Alessandro Degl’Innocenti, Antonella Capperucci,
Miriam Acciai, and Caterina Tiberi
Universita` di Firenze, Dipartimento di Chimica Organica and HBL,
Sesto Fiorentino, Italy
Bis(trimethylsilyl)selenide (HMDSS) reacts efficiently with aldehydes in the pres-
ence of CoCl. 6H O to afford selenoaldehydes, which are trapped as Diels–Alder
2
2
adducts by different dienes. The reaction can be applied to acylsilanes, to afford
selenoacylsilanes, isolated as their cycloadducts.
Keywords Diels–Alder cycloadditions; selenoacylsilanes; selenoaldehydes; selenoheter-
ocycles; silyl-selenide
INTRODUCTION
Selenium-containing heterocycles represent a very interesting class
of compounds due to their useful reactivity in organic synthesis and
their potential biological applications.1 A convenient route to access
heteroatom-containing molecules can be envisioned in hetero Diels–
Alder reactions,2 and selenocarbonyl compounds are in fact recognized
as very reactive intermediates for the introduction of selenium into or-
ganic molecules, being able to act as powerful 2π dienophiles in cycload-
dition reactions, and so providing a useful and convenient approach to
different functionalized heterocycles.3 Despite this interest, a limited
number of synthetic methodologies for their preparation have been
reported, mainly due to the instability of the desired selenocarbonyl
derivatives under the preparative conditions.4
Received 6 February 2008; accepted 14 February 2008.
Dedicated to Professor Marian Mikolꢀajczyk, CBMiM PAN in Lꢀ o´dz´, Poland, on the
occasion of his 70th birthday.
Financial support by the National Project “Stereoselezione in Sintesi Organica.
Metodologie ed Applicazioni” (MIUR, Roma) is gratefully acknowledged. Mrs. B. Inno-
centi is acknowledged for technical support.
Address corresponcence to Alessandro Degl’Innocenti, Universita` di Firenze, Dipar-
timento di Chimica Organica and HBL, Via della Lastruccia 13, 50019 Sesto Fiorentino,
Italy. E-mail: alessandro.deglinnocenti@unifi.it
1621

1622
A. Degl’Innocenti et al.
Several synthetic methods describe the generation of selenoalde-
hydes through base-catalyzed elimination reactions from α-silyl
selenocyanates5 or selenenyl derivatives6 by heating alkylidene phos-
phoranes with elemental selenium7 or through the reaction of ArLi
with selenoformates.8 Direct conversion of carbonyl compounds to se-
lenocarbonyls is probably the more convenient way to approach these
derivatives, and several methods have been reported based on the use of
bis(diboryl)selenide,9 bis(trimethylsilyl)selenide under BuLi10 or Lewis
acid catalysis,11 and bis(dimethylaluminum)selenide.12 Selenoaldehy-
des can also be generated through retro Diels–Alder reactions under
different conditions.13
RESULTS AND DISCUSSION
Our longtime interest in the development of organosilicon-based pro-
cedures for the synthesis of sulphur-containing compounds led us re-
cently to uncover a simple and general access to thiocarbonyl deriva-
tives, such as thioaldehydes, thioketones, and thioacylsilanes through
our hexamethyldisilathiane (HMDST)-based thionation procedure of
carbonyl compounds, which proved mild enough for the synthesis of
such derivatives.14
Thus, due to the relevance that selenium containing molecules have
gained in recent years, both from the synthetic and biological side,
we were attracted by the possibility of developing a smooth, mild ac-
cess to another class of compounds such as selenocarbonyl derivatives,
that due to their high reactivity often show difficulties in prepara-
tion. Thus, taking advantage of the high degree of versatility shown
by HMDST in the delivery of sulfur functionalities, we reasoned that
a possible approach to selenoaldehydes could be found through the di-
rect treatment of aldehydes with another silyl chalchogenide, namely
bis(trimethylsilyl)selenide (hexamethyldisilaselenane, HMDSS), un-
der suitable catalytic conditions.
Thus aldehydes were reacted with HMDSS 1 under inert atmosphere
at room temperature, in the presence of CoCl.26H2O and 2,3-dimethyl-
1,3-butadiene as trapping agent.
The reaction proved efficient in generating the selenoaldehyde 2, as
shown by the isolation of the corresponding Diels–Alder cycloadduct
3 (Scheme 1), albeit in not satisfactory yield, due to the formation of
appreciable amounts of the reduction product, the diselenide 4, together
with minor amounts of the trimer.
As already reported, this result outlines that HMDSS shows a re-
markable reducing activity towards selenocarbonyl compounds also un-
der the present conditions.9,11

Selenoaldehydes (Acylsilanes) as Dienophiles
1623
SCHEME 1
With the aim to minimize polymerization and/or further reduction of
the in situ generated selenoaldehyde, (TMS)2Se was added portionwise
to the reaction mixture, thus leading to a remarkable increase of the
yield. Portionwise addition of HMDSS favours the slow generation of
selenoaldehydes, so preventing their accumulation with consequent
efficient in situ trapping by the diene, and minimizing trimerization
or reduction processes.15
Thus, on performing the reaction in the presence of 2,3-dimethyl-
1,3-butadiene, a variety of 2-substituted 4,5-dimethyl-3,6-dihydro-2H-
selenopyrans 3 were obtained starting from a representative range
of selenoaldehydes. Aromatic, heteroaromatic, and aliphatic aldehydes
can be efficiently converted into their corresponding seleno derivatives
(Table I), making bis(trimethylsilyl)selenide a useful reagent under
CoCl.26H2O catalysis. However, the cycloadducts 3 were labile toward
exposure to air, and caused partial decomposition during purification
on silica gel.
Differently from what was observed by Segi and et al.,12 in the
present conditions, the α, β-unsaturated thiophene-2-selenoaldehyde
did not undergo [4+2] dimerization, no trace of the corresponding head-
to-head (or head-to-tail) dimer ever being evidenced.
An interesting aspect is the stereochemical control of the Diels–
Alder cycloaddition when 1,3-cyclohexadiene was used as a trapping
agent. The reaction of selenoaldehydes occurs with preference for the
formation of the endo adduct (endo:exo 95:5), according to what was
previously reported by other authors.5,10
The versatility of the present methodology is further testified by
its application to more reactive species, such as silylketones. Thus,
when acylsilanes 6 were treated with HMDSS and CoCl.26H2O, this
time the formation of the desired selenoacylsilane was evidenced by
the isolation of the 2-silyl-dihydroselenopyran 7 (Scheme 2), formed
through cycloaddition with the diene, thus disclosing a novel access to
this unreported class of silylated seleno heterocycles, whose reactivity
is currently being investigated in our laboratory.
In conclusion, we have presented an alternative procedure to access
selenoaldehydes that occurs under rather mild conditions, so that it can

1624
A. Degl’Innocenti et al.
TABLE I Synthesis of Selenoaldehydes and Their Trapping by a
Diene
Entry
1
R
Diene
Adduct
Yield %a,b,c
Ph
45 (64)
2
3
p
F
C6H4
51 (70)
53 (71)
p
CF3 C6H4
4
5
6
7
p
CH3O C6H4
41 (56)
54 (72)
38 (51)
50 (70)d
CH3CH2
p
CF3 C6H4
8
57 (74)d
aIsolated products. In parenthesis is reported the yield before purification.
bAll compounds showed spectroscopical data consistent with the assigned structure.
cDiselenides 4 were detected (10–20%).
dEndo:exo 95:5.

Selenoaldehydes (Acylsilanes) as Dienophiles
1625
SCHEME 2
be efficiently applied even to the synthesis of more reactive molecules
such as selenoacylsilanes.
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[15] Typical procedure: A CH3CN solution (1 mL) of thiophene-2-carbaldehyde (50 mg,
0.45 mmol), 2,3-dimethyl-1,3-butadiene (184 mg, 2.25 mmol), and HMDSS (20 mg,
0.09 mmol) was treated under inert atmosphere at room temperature with CoCl2·
6H2O (107 mg, 0.45 mmol) dissolved in 1.5 mL of CH3CN. The mixture was added
portionwise every 30 min with 20 mg of HMDSS, and the progress of the reaction
was monitored by TLC (petroleum ether/diethyl ether 3:1). After the addition of 1
equiv. of HMDSS, the mixture was stirred overnight. After quenching with water,
the product was extracted with diethyl ether and washed with water and brine. The
resulting organic phase was then filtered to remove the dark precipitate and dried
over Na2SO4. Evaporation of the solvent under vacuum gave 83 mg of crude 4,5-
dimethyl-2-thienyl-3,6-dihydro-2H-selenopyran 3e. Chromatographic purification
(petroleum ether/diethyl ether 3:1) afforded 63 mg (54%) of 3e. 1H NMR (200 MHz,
CDCl3), δ (ppm): 1.75 (3H, s), 1.83 (3H, s), 2.68 (2H, bd, J = 6 Hz), 3.09 (1H, d,
J = 15 Hz), 3.25 (1H, d, J = 15 Hz), 4.55 (1H, t, J = 6 Hz), 6.90–6.93 (3H, m). 13C
NMR spectrum (50 MHz, CDCl3), δ (ppm): 18.6, 19.0, 33.2, 34.1, 41.9, 119.8, 121.64,
122.2, 125.2, 126.1, 128.3. MS, m/z (Irel, %): 258 (19) [M]+, 176 (100), 143 (26), 97
(53), 77 (39), 65 (69). The reaction performed with cyclohexadiene afforded 2-(2-
selena-bicyclo[2.2.2]oct-5-en-3-yl)thiophene 5b (57%) (endo:exo 95:5). Endo-5b: 1H
NMR (400 MHz, CDCl3), δ (ppm): 1.45–1.59 (1H, m), 1.71–1.92 (2H, m), 2.14–2.22
(1H, m), 3.02–3.06 (1H, m), 3.73–3.76 (1H, m), 5.02 (1H, d, J = 3.2 Hz), 6.28 (1H,
ap t), 6.45 (1H, ap t), 6.81–6.86 (2H, m), 7.05 (1H, dd, J = 5.3, 1.5 Hz). 13C NMR
spectrum (50 MHz, CDCl3), δ (ppm): 25.4, 29.2, 38.1, 42.5, 46.1, 124.2, 124.5, 126.1,
126.6, 129.8, 136.8. MS, m/z (Irel, %): 256 (10) [M]+ , 176 (50), 97 (100), 97 (53), 79
(57).