Cocondensation reactions of heterocyclic aromatic compounds with lithium, calcium and magnesium atoms at 77 K

Article abstract of DOI:10.1016/j.jorganchem.2005.11.022

Calcium and magnesium atoms were cocondensed with aromatic heterocycles containing five- and six-membered rings in the presence of THF at 77 K. In the case of calcium the cocondensation with five-membered heterocyclic compounds resulted in C-H bond activations and led to the corresponding aryl calcium compounds, while magnesium did not show comparable reactions. When six-membered heterocyclic compounds, e.g., pyridine and 4-methylpyridine (4-picoline) were cocondensed with calcium, magnesium and lithium atoms, all reactions led to the formation of non-metallated aromatic products and the formation of metal hydride. DFT calculations at the B3LYP/6-31G** level of theory were carried out in order to interpret the pathways of the cocondensation reactions using calcium atoms and identify the possible intermediates involved. In all reactions π- and σ-complexes between calcium atoms and the heterocyclic aromatic reactant were found as stable intermediates on the energy hypersurface.

Full text of DOI:10.1016/j.jorganchem.2005.11.022

Journal of Organometallic Chemistry 691 (2006) 1110–1116
www.elsevier.com/locate/jorganchem
Cocondensation reactions of heterocyclic aromatic compounds
with lithium, calcium and magnesium atoms at 77 K
*
Oscar Mendoza, Matthias Tacke
The UCD School of Chemistry and Chemical Biology, Centre for Synthesis and Chemical Biology (CSCB), Conway Institute of Biomolecular
and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland
Received 12 October 2005; received in revised form 9 November 2005; accepted 9 November 2005
Available online 19 December 2005
Abstract
Calcium and magnesium atoms were cocondensed with aromatic heterocycles containing five- and six-membered rings in the presence
of THF at 77 K. In the case of calcium the cocondensation with five-membered heterocyclic compounds resulted in C–H bond activa-
tions and led to the corresponding aryl calcium compounds, while magnesium did not show comparable reactions. When six-membered
heterocyclic compounds, e.g., pyridine and 4-methylpyridine (4-picoline) were cocondensed with calcium, magnesium and lithium atoms,
all reactions led to the formation of non-metallated aromatic products and the formation of metal hydride. DFT calculations at the
B3LYP/6-31G** level of theory were carried out in order to interpret the pathways of the cocondensation reactions using calcium atoms
and identify the possible intermediates involved. In all reactions p- and r-complexes between calcium atoms and the heterocyclic
aromatic reactant were found as stable intermediates on the energy hypersurface.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Lithium atoms; Calcium atoms; Magnesium atoms; Metal vapour synthesis; CH activation; DFT calculations
1. Introduction
aromatic ring, resulting in the activation of the aromatic
C–H bond.
The activation of carbon-hydrogen bonds has been a
major topic of research for the last thirty years [1–3]. Ear-
lier reports involving lithium atoms [4–6] showed that they
are able to activate aromatic hydrocarbons in the presence
of THF selectively under cocondensation conditions at
77 K. This resulted in the direct synthesis of aryl lithium
compounds on a synthetic scale.
cocond.
77K
Li Li
2 Li(g) + 4 PhH(g)
PhH
i
H
Ph
LiH(s) + PhLi(s)
Li
cocond.
8LiðgÞ þ 4PhHðgÞ þ 4THFðgÞ ! ½PhLi ꢀ THFꢁ4 þ 4LiHðsÞ
77 K
The work has been expanded to +I substituted (CMe3,
SiMe3, o- and m-xylene) and donor substituted (OMe,
NMe2, SMe) benzene derivatives [7] as well as five-mem-
bered heterocycles [8] and nitrogen-containing heterocycles
[9]. In the case of anisole a selective C–H bond activation
was found to occur in the absence of THF as well; its
reaction mechanism was calculated in detail in a recent
DFT study [10]. The aim of this paper is to present cocon-
densation experiments of heterocycles and calcium or
One of the key intermediates in this reaction is a Li2-ben-
zene p-complex. This complex is capable of undergoing a
two-electron transfer from the Li2 to the LUMO of the
*
Corresponding author.
E-mail address: matthias.tacke@ucd.ie (M. Tacke).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.11.022

O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
1111
magnesium atoms and to compare them with the analogues
N
N
lithium cocondensation reactions. In addition, theoretical
studies are carried out to understand the reaction mecha-
nisms of the calcium experiments.
4
(g)
(g)
2. Results
3
cocond.
No reaction
2.1. Experimental study
S
Mg(g) + THF(g)
(g)
2
Previous reports have shown successful cocondensation
[8] reactions of lithium atoms and five-membered heterocy-
cles. When the heterocycle used was furan (1) or thiophene
(2), the reaction led to a monolithiated species. The reac-
tion was identified as a C–H activation and the lithium
was found in an ortho position with respect to the oxygen
and sulphur respectively. When the same cocondensation
experiment was carried out using 1-methyl pyrrole (3) or
1,2,5-trimethyl pyrrole (4) the reaction failed.
The cocondensation reaction of calcium atoms with
furan (1), thiophene (2), 1-methyl pyrrole (3) and 1,2,5-tri-
methyl pyrrole (4) in THF was carried out at 77 K. When
1, 2 and 3 were cocondensed a reaction was observed and
the product was identified as the corresponding calcium
compound. However, when 4 was cocondensed, no prod-
uct could be obtained. The products of these reactions were
monocalcium compounds (5–7) where the heterocyclic ring
showed a C–H activation in the ortho position as shown in
Scheme 1. In the case of furan, thiophene and methyl pyr-
role the yields were 18%, 23% and 8%, respectively.
In order to identify the position of the calcium on the
aromatic ring, a derivatisation reaction with Me3SiCl was
carried out yielding air stable products. The TMS substi-
tuted compounds were identified by comparing with litera-
ture-known NMR and GC–MS data [11–13].
O
(g)
1
Scheme 2. Cocondensation reactions of magnesium atoms with hetero-
cycles at 77 K.
Earlier reports showed that lithium atoms are able to [9]
dimerise pyridine molecules (9) by losing hydrogen in the
para-position. When lithium atoms were cocondensed with
pyridine in a THF solution, the reaction lead to a non-lith-
iated species. The product was identified as 4,40-bypyridine
(11) and lithium hydride as by-product.
The same cocondensation reaction was carried out using
magnesium and calcium atoms. In both cases, as seen in the
corresponding lithium reaction earlier, the product was
identified as 4,40-bypyridine [14]. Therefore, lithium, cal-
cium and magnesium atoms are able to activate the C–H
bond in the para position forming a dimeric product.
In order to prevent a C–H activation in the para posi-
tion, a methyl group was placed in that position and the
cocondensation reaction was carried out. 4-Picoline (8)
was reacted with magnesium, calcium and lithium atoms
and for all the cases the product was identified as a non-
metallated compound. Also a C–H activation took place,
but surprisingly, an aryl C–H bond was not activated. In
this case an alkyl C–H bond is activated instead and 1,2-
di-4-pyridylethane (10) (Scheme 3) is formed through loss
of hydrogen in an unexpected reaction sequence [15].
In order to facilitate an overview and a comparison of
magnesium, lithium and calcium cocondensation reactions,
all reactions are summarised in Table 1.
A similar reaction series was carried out, but this time
the metal used was magnesium instead of calcium. Com-
pounds 1, 2, 3, or 4 were reacted with magnesium atoms
and THF under the same cocondensation conditions but
in any case no reaction was found (Scheme 2).
2.2. Theoretical studies
N
One of the aims of this work was to identify in a
theoretical study plausible intermediate species for all these
No reaction
4
(g)
N
N
S
Ca
H
H
N
(g)
3
7
N
N
N
cocond.
(g)
(g)
9
11
S
Ca(g) + THF(g)
Ca
Ca
cocond.
N
(g)
2
6
M(g) + THF(g)
O
O
H
N
M = Ca, Li or Mg
(g)
1
5
10
8
Scheme 1. Cocondensation reactions of calcium atoms with heterocycles
at 77 K.
Scheme 3. Cocondensation reactions of lithium, magnesium and calcium
atoms with pyridine 9 and 4-picoline 8 at 77 K.

1112
O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
Table 1
the B3LYP DFT functional. As in previous organocalcium
theoretical work [17], the basis set combination AKR4/6-
31G** was used.
Summary of the reactions of magnesium, lithium and calcium atoms with
heterocyclic aromatic compounds in the presence of THF at 77 K
Mg
Li
Ca
Previous theoretical studies performed with lithium
atoms and five-membered heterocycles, showed that the
C–H activation reaction involves at least two steps, the
coordination of the lithium atom with the aromatic com-
pound and then the insertion of the metal atom into the
C–H bond by a double electron transfer via the aromatic
system [4,5].
N
[8]
[8]
No reaction
No reaction
No reaction
N
O
N
O
Ca
Ca
H
H
No reaction
No reaction
O
Li
When the same calculations are performed using cal-
cium atoms and furan (Scheme 4), two possibilities are
found for the first intermediate. The first possibility is the
coordination of one calcium atom to the aromatic system
to form a p-complex, and the second possibility is g1-coor-
dination of the calcium atom to the oxygen in the ring.
Both structures are found as stable intermediates, since
they are minima on the energy hypersurface. However,
their formation is found to be an energetically neutral step
with a reaction enthalpy of less than +1 kcal/mol, which is
well within the error margin of the applied theoretical
method. Please note that in some cases the lines on the dia-
grams only have topological significance.
No reaction
No reaction
[8]
S
S
Li
S
Ca
H
[8]
N
N
N
[9]
N
N
N
N
N
N
N
N
N
N
N
Then the formation of the furyl calcium hydride is calcu-
lated to be a slightly exothermic step (ꢂ5.2 kcal/mol) with
respect to the parent compounds. In the next steps solvent
molecules fill the coordination sphere of the calcium centre;
NH3 instead of THF molecules were used in these calcula-
tions for simplification. When two ammonia molecules are
cocondensation reactions involving calcium atoms and het-
erocyclic aromatic compounds, and thereby identifying the
reaction mechanism for this new type of CH activation.
Calculations were performed using GAUSSIAN 98 [16] and
O
Ca
E
12
0.9 kcal/mol
Ca
O
O
0.3 kcal/mol
+ Ca
13
1
O
H
Ca
- 42.1 kcal/mol
NH3
-5.2 kcal/mol
5
Ca
H
O
NH3
- 28.0 kcal/mol
14
NH3
O
NH3
NH3
Ca
H
NH3
15
reaction coordinate
Scheme 4. Calculated reaction mechanism for the cocondensation of calcium atoms with furan. Calculations were performed using NH3 instead of THF
for simplification.

O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
1113
considered, the reaction leads to a furyl calcium hydride
with coordination number 4 with DH = ꢂ42.1 kcal/mol
(Scheme 4), while another two NH3 molecules fill-up to
coordination number 6 in a further exothermic reaction
step releasing another 28.0 kcal/mol.
When comparing the structures 5, 14 and 15, it can be
seen that calcium oxygen interactions are found in some
cases. The calculated structure 5 and 14 show a Ca–O bond
(231.4 and 242.4 pm, respectively), however when four
molecules of solvent are considered (15), this Ca–O interac-
tion is lost (dCaO: 326.2 pm). This is due to the coordination
number 6 at the calcium centre; now the intramolecular
donor furyl cannot compete against the solvent molecules.
Only the formation of monomeric aryl calcium hydride
species has been considered, while, as previous reports
showed, the formation of oligomeric products is also possi-
ble [17].
are allowed to coordinate. Unlike the furan case, there is
no pre-coordination of the calcium centre by sulfur in
any of the calculated structures involving thiophene.
The N-methyl pyrrole case shows that the formation of
the p-complex intermediate (20) is found to be slightly exo-
thermic (ꢂ1.5 kcal/mol), while the formation of N-methyl
pyrrolyl calcium hydride 7 from calcium atoms and 3 is
slightly endothermic (+1.9 kcal/mol), respectively. The cor-
responding r-complex is not found as a stable intermediate,
since there is no lone pair available for coordination. How-
ever, as in previous cases, once the solvent interaction is con-
sidered, the reaction is driven to energetically very stable
products. When two ammonia molecules are allowed to
coordinate the reaction enthalpy is found to be ꢂ41.9 kcal/
mol. Another two NH3 molecules are filling the coordination
sphere of calcium completely and release further 28.2 kcal/
mol; all results are summarised in Scheme 6.
In the thiophene calculations, which are shown in
Scheme 5, the formation of the first intermediate is found
to be slightly endothermic in both cases of the p- and r-
complexes (+3.7 and +2.3 kcal/mol, respectively). Also
the insertion of calcium atoms into the CH bond of thio-
phene is found to be a slightly endothermic process of
+1.6 kcal/mol. However, as it was found in furan, once
the solvent is considered in the coordination sphere of
the calcium atom, the structures are stabilized. Solvated
thiophenyl calcium hydride is formed in two clearly exo-
thermic reaction steps releasing ꢂ46.9 and ꢂ28.9 kcal/
mol when two or four solvent molecules of ammonia,
which is used instead of THF to simplify the calculations,
In all calculations involving 1, 2 and 3 their correspond-
ing p- and r-complexes with calcium 12, 13, 16, 17 and 20
were calculated as triplet and singlet states; in all the cases
singlet states were found to be energetically favoured.
3. Discussion
Previous research in this area showed that five-mem-
bered heterocycles react with lithium atoms in the presence
of THF under cocondensation conditions [8]. The isolated
products were identified as lithiated aromatic compounds.
However when the heteroatom in the ring was nitrogen,
as seen in pyrroles, the reactions failed so far. Therefore
Ca
S
E
16
S
3.7 kcal/mol
Ca
17
2.3 kcal/mol
S
Ca
H
S
- 46.9 kcal/mol
S
+ Ca
6
1.6 kcal/mol
NH3
Ca
2
H
NH3
18
- 28.9 kcal/mol
NH3 NH3
Ca
NH3 NH3
S
H
19
reaction coordinate
Scheme 5. Calculated reaction mechanism for the cocondensation of calcium atoms with thiophene. Calculations were performed using NH3 instead of
THF for simplification.

1114
O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
E
N
+1.9 kcal/mol
-1.5 kcal/mol
Ca
H
N
7
+ Ca
N
-41.9 kcal/mol
NH3
3
Ca
20
N
Ca
H
NH3
21
-28.2 kcal/mol
NH3
NH3
N
Ca
H
NH3
NH3
22
reaction coordinate
Scheme 6. Calculated reaction mechanism for the cocondensation of calcium atoms with N-methyl pyrrole. Calculations were performed using NH3
instead of THF for simplification.
this study examines the effect of changing the metal to mag-
nesium and calcium for which the analogues reactions are
carried out.
electron-density to the aromatic ring. The LUMO energy
of the aromatic compound has been interpreted as the
key to these reactions [10]. During the C–H activation reac-
tion, the metal atoms will transfer electron density to the
aromatic ring in order to form the metallated species.
Therefore, high LUMO energy would make electron trans-
fer impossible and thereby preventing the CH activation
reaction occurring.
This reaction barrier depends on the metal used. Metals
with lower ionisation potentials will activate aromatic rings
with higher LUMO energy. In Table 2 are listed the aro-
matic compounds used in this project, their LUMO ener-
gies and the reactions that occurred. For example,
magnesium metal would need an aromatic compound with
a very low LUMO energy to be able to transfer its two elec-
trons to the aromatic ring; therefore the five-membered
heterocycles used make it impossible to see the activation
reaction. However, lithium and calcium atoms, which have
lower ionisation potentials, are able to activate C–H bonds
in these aromatic rings, although, there is a limit for both
metals. Lithium atoms do not react with the pyrrole deriv-
atives used and calcium atoms will only react with some of
them as summarised in Table 2.
As previous reports show [8], a delicate balance between
enhancing complex formation ability and lowering LUMO
energy, which allows reductive cleavage of the aromatic C–
H bond, is needed for a successful C–H activation. The for-
mation of a p- or g1-complexes was found as the first step
in these reactions when lithium metal was used. This is fol-
lowed by the C–H activation leading to the metallated
product. The formation of this intermediate was found to
be an exothermic step in all the cases.
However, when the same calculations are performed
using calcium atoms, the reactions show an endothermic
step for the formation of this intermediate. In the case of
thiophene, even the formation of the metallated species 7
is found to be slightly endothermic.
The positive reaction enthalpy can be assumed to be
incorrect, since the solvent sphere was omitted. The coordi-
nation of the calcium atoms with the THF solvent would
stabilise the molecules leading to structures of lower
energy. This effect can be seen in the calculated structures
of fourfold 14, 18, 21, and sixfold 15, 19, 22 coordinated
species, which show energetically lowered structures when
the solvent is included. Also this coordination effect is
observed if the organocalcium species 5 and 6 are com-
pared. Intermediate 6, which exhibits coordination number
two at the calcium centre, is reached in an endothermic
reaction step. However, intermediate 5, with is stabilised
by an extra Ca–O coordination is available via an exother-
mic reaction step.
Table 2
Calculated LUMO energies of selected reactants using B3LYP/6-31G**
compared with their reactivity in selected cocondensation experiments
LUMO (eV)
Reaction
Mg
Li
Ca
1,2,5-Trimethyl pyrrole
1-Methyl pyrrole
Furan
1.44
1.27
0.51
ꢂ0.22
No
No
No
No
No
No
Yes
Yes
No
Yes
Yes
Yes
From such a p- or g1-complex to furan, thiophene or N-
methyl pyrrole calcium atoms would be able to transfer
Thiophene

O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
1115
H
H
H
H
H
H
H
H
H
H
H
N
N
cocond
M(g)+THF(g)
x2
N
N
+ MH (s)
1/2
.
H
H
11
24
H
N
9
H
H
N
cocond
x2
N
+ MH (s)
1/2
M(g)+THF(g)
N
H
.
C
CH3
10
H
8
H
23
M= Ca, Li and Mg
Scheme 7. Proposed mechanism for the cocondensation of calcium, lithium and magnesium atoms with pyridine and 4-picoline.
All these reactions show a C–H activation process,
which is always taking place in the ortho position with
respect to the heteroatom. Therefore, the cocondensation
reaction of aromatic heterocycles and metal atoms can be
considered as an important tool for direct ortho metalla-
tion of aromatic rings, which has its counterpart in the
directed ortho metallation in solution chemistry [18].
This different behaviour of calcium, lithium and magne-
sium cannot be seen in the cocondensation reactions with
pyridine or 4-picoline. In these cases the three metals lead
to the same product: the formation of an organic dimer
by loss of hydrogen and the formation of a new C-C bond.
The formation of a metallated pyridine [19] or picoline is
not found in any case.
When pyridine and 4-picoline are cocondensed with lith-
ium, magnesium or calcium atoms in the presence of THF,
the reaction changes towards the formation of aromatic
dimers via the formation of a C–C bond and metal hydride
is found as a by-product. Here a C–H activation reaction is
found in a ring position in the case of pyridine, while 4-pic-
oline shows an unexpected dimerisation via the formation
of a C–C bond in the methyl groups.
5. Experimental section
5.1. General
All experimental procedures were performed using stan-
dard Schlenk techniques under an atmosphere of dry
argon. THF was dried and distilled from sodium benzo-
phenone ketyl under an atmosphere of argon. Furan, thio-
phene, 1-methyl pyrrole, 1,2,5-trimethyl pyrrole, pyridine
and 4-picoline were dried and distilled from calcium dihy-
dride under an atmosphere of argon. All solvents were then
degassed by three freeze–pump–thaw cycles. Celite was
stored in an oven at 120 °C overnight and was dried under
vacuum for three hours before use. For each filtration a
plug of approximately 6 cm of celite was used and each fil-
tration was performed under the vigorous exclusion of air
and moisture. 1H and 13C NMR spectra were obtained
using either a Varian 300 MHz or a Varian 500 MHz
NMR machine. All chemical shifts are reported in ppm
and are referenced to TMS. All GC–MS samples were
run as liquid samples in ethylacetate using a Finnigan
Trace GC–MS, equipped with a RTX-5MS 15 m column.
It seems that the reaction proceeds through a radical
intermediate (23,24) where the aromatic ring has been
dehydrogenated and solid metal hydride is formed as well
(Scheme 7).
4. Conclusions
When aromatic five-membered heterocycles are cocon-
densed with lithium, magnesium or calcium atoms in the
presence of THF, ortho-metallated products are formed
in highly selective C–H activation reactions. However the
reaction does not take place in all cases and the LUMO
energy of the aromatic ring is interpreted as the main bar-
rier of the reactions. Considerably high LUMO energies
will prohibit the carbon hydrogen activation reaction.
However the limit in the LUMO of the aromatic com-
pound highly depends on the metal used and there is a clear
difference among the three metals.
Lithium atoms are able to activate C–H bonds in aro-
matic compounds as furan or thiophene, but fail to activate
pyrrole derivatives 3 and 4 due to their high LUMO
energy. Calcium metal has a higher potential to activate
aromatic species and it can react with N-methyl pyrrole
despite its high LUMO energy. Nevertheless, even calcium
atoms fail to activate 1,2,5-trimethyl pyrrole, which has the
highest LUMO energy of all compounds used in this pro-
ject. On the other hand, magnesium needs a very low
LUMO barrier and it is not able to activate any the 5-mem-
bered heterocyclic compounds 1–4.
5.2. A typical cocondensation experiment
Approximately 0.3 g of magnesium, 0.5 g of lithium or
0.2 g of calcium metal are vapourised from an alumina cru-
cible over 90 min and cocondensed with a mixture of 10 ml
of the aromatic compound and 80 ml of THF. In the case
of lithium metal, a stainless steel inlet protects the crucible
to react with lithium metal. The pressure that what mea-
sured at the button of the cocondensation vessel was kept
below 10ꢂ5 mbar to avoid gas phase reactions of the metal
atoms and reactants. The apparatus used was described

1116
O. Mendoza, M. Tacke / Journal of Organometallic Chemistry 691 (2006) 1110–1116
previously [4]. At the end of the experiment, the frozen
matrix was allowed to warm-up under argon. The molten
mixture flowed through an internal drain into a Schlenk
tube. The solution was filtered through a plug of celite in
order to remove the solid metal-hydride by-product and
unreacted metal. After removal of the solvent mixture in
vacuo, a solid was isolated and washed with 20 ml of
THF. The lithiated aromatic compound was identified by
derivatisation with Me3SiCl.
5.7. N-methyl-2-trimethylsilyl pyrrole
The general procedure applied to N-methyl pyrrole with
calcium metal gave 2-(N-methyl pyrrolyl) calcium hydride
(dark yellow colour) in 8% yield (1.5 g), which was identi-
fied as its TMS derivative [13] by NMR and GC–MS.
5.8. 4,40-Bypyridine
The general procedure applied to pyridine with calcium
and magnesium metal gave 4,40-bypyridine in 7% and 17%,
respectively [14], which was identified by NMR and GC–
MS.
5.3. A typical derivatisation of the metallated aromatic with
Me3SiCl
To a room temperature solution of the metallated aro-
matic compound in 20 ml of THF, Me3SiCl in THF solu-
tion was added dropwise with vigorous stirring. The
reaction was judged finished when a colour-change from
dark red-yellow to pale yellow occurred. This solution
was filtered and the solvent was removed under reduced
pressure, which gave the trimethylsilyl derivative. GC–
5.9. 1,2-Di-4-pyridylethane
The general procedure applied to 4-picoline with cal-
cium, lithium and magnesium metal gave 1,2-di-4-pyridyle-
thane in 5%, 18% and 19%, respectively [15], which was
identified by NMR and GC–MS.
1
MS as well as H and 13C NMR spectroscopy was used
to identify the product.
Appendix A. Supplementary data
5.4. Theoretical methods used in this study
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2005.11.022.
DFT calculations were performed using B3LYP and the
basis set combination AKR4/6-31G**, where 6-31G** is
applied to C, H, N, O and S and AKR4 is used for calcium
atoms. This combination has proved to give reasonable
geometries without greatly increasing the calculation time.
In order to reach reasonable calculation times, ammonia
was used as a model of solvent instead of THF, which is
the solvent of choice used for the cocondensation reactions.
For this purpose the program GAUSSIAN 98 [16] was used
on a Dell workstation running Red Hat Linux. Harmonic
vibrational frequencies, calculated at the same level, charac-
terised stationary points and gave the zero-point energy. The
difference in the sum of the electronic and the zero-point
energies were interpreted as reaction enthalpies at 0 K.
LUMO energies for each aromatic compound were obtained
from geometry optimised structures using B3LYP/6-31G**.
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5.5. 2-Trimethylsilyl furan
The general procedure applied to furan with calcium
metal gave 2-furyl calcium hydride (red colour) in 18%
yield (2.4 g), which was identified as its TMS derivative
[11] by NMR and GC–MS.
[13] F. Denat, H.G. Iloughmane, J. Dubac, J. Organomet. Chem. 423
(1992) 173.
[14] M. Kuroboshi, Y. Waki, H. Tanaka, J. Org. Chem. (2003) 3938.
[15] Y. Ito, K. Kobayashi, T. Saegusa, J. Orgmet. Chem. 303 (1986)
301.
[16] M.J. Frisch et al., GAUSSIAN 98, Revision A.7, Gaussian, Inc.,
Pittsburgh, PA, 1998.
5.6. 2-Trimethylsilyl thiophene
[17] J.P. Dunne, M. Tacke, S. Fox, J. Mol. Struct. Theochem. 543 (2001)
157.
[18] J. Clayden, Tetrahedron Organic Chemistry Organolithiums: Selec-
tivity for Synthesis, vol. 23, Pergamon Press, New York, 2002.
[19] R.E. Mulvey, L. Dunbar, W. Clegg, L. Horsburgh, Angew. Chem.
Int. Ed. Engl. 35 (1996) 753.
The general procedure applied to thiophene with cal-
cium metal gave 2-thiophenyl calcium hydride (red colour)
in 23% yield (2.7 g), which was identified as its TMS deriv-
ative [12] by NMR and GC–MS.