1-alkenylcalcium iodide: Synthesis and stability

Article abstract of DOI:10.1002/chem.201400391

To enhance the scope of heavy calcium-based Grignard reagents, 1,2-dihydro-4-iodonaphthalene (1) was reduced with calcium in THF giving tetrakis(thf) (1,2-dihydronaphth-4-yl)calcium iodide (2). This derivative represents a 1-alkenylcalcium complex based on X-ray structure determination and NMR data. The stability of this compound is significantly reduced compared with the aromatic naphthylcalcium iodide. Complex diversity: The class of heavy Grignard reagents is extended by a 1-alkenylcalcium iodide. This tetrakis(thf) (1,2-dihydronaphth-4-yl)calcium iodide contains a 1-alkenyl moiety based on structural and NMR spectroscopic studies (see figure).

Full text of DOI:10.1002/chem.201400391

DOI: 10.1002/chem.201400391
Communication
&
Synthetic Methods
1-Alkenylcalcium Iodide: Synthesis and Stability
Mathias Kçhler, Helmar Gçrls, Jens Langer, and Matthias Westerhausen*[a]
characterized alkenylmagnesium derivatives,[10] a single well-
defined alkenylbarium compound, namely, (tetrahydrofuran-
O)barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-
4-trimethylsilyl-1-phosphacyclopentadienide,[11] is known and
underlines that the alkenyl derivatives of the heavier alkaline-
earth metals are also stable enough for isolation under certain
conditions. Therefore, we intended to expand the knowledge
on such calcium-based organometallics beyond quantum-
chemical calculations,[12] anticipating complexes with an ex-
tremely high reactivity.
Abstract: To enhance the scope of heavy calcium-based
Grignard reagents, 1,2-dihydro-4-iodonaphthalene (1) was
reduced with calcium in THF giving tetrakis(thf) (1,2-dihy-
dronaphth-4-yl)calcium iodide (2). This derivative repre-
sents a 1-alkenylcalcium complex based on X-ray structure
determination and NMR data. The stability of this com-
pound is significantly reduced compared with the aromat-
ic naphthylcalcium iodide.
A straightforward approach in analogy to the known synthe-
sis of Grignard reagents was chosen. In initial experiments,
commercially available a-bromostyrene was reacted with acti-
vated calcium in THF. Although the reaction mixture rapidly
turned red as an indication of the proceeding reaction, the de-
termined alkalinity of the resulting solution only reached 30%
of the expected value. Attempts to isolate a calcium-contain-
ing product only led to separation of [CaBr2(thf)4].[13] Because it
is known from the synthesis of arylcalcium halides that im-
proved yields of the desired organocalcium derivative were ob-
tained if aryl iodides instead of aryl bromides were employed,
the reaction was repeated with a-iodostyrene. Indeed, the re-
action of activated calcium and this substrate led to solutions,
which contain up to 57% of the alkalinity expected for
a Grignard-type reaction. However, work-up of the reaction
mixture only gave an oily residue containing an untraceable
mixture of products. As a major problem, the competing poly-
merization of the substrate by initially formed organocalcium
species was identified, a well-documented reaction, for exam-
ple, of benzylcalcium[14] and allylcalcium[15] complexes, which
can be used as an efficient catalysts in styrene polymerization.
To suppress this side reaction, alkyl-substituted alkene deriv-
atives, such as 1-iodocyclohexene and 1,2-dihydro-4-iodonaph-
thalene (1), were chosen as further substrates. Additionally, the
incorporation of the olefinic moiety into a six-membered ring
reduced the degree of conformational freedom of the mole-
cules to enhance crystallinity of the calcium-containing reac-
tion products. The performed reaction of 1-iodocyclohexene
and calcium showed a conversion of 70%, and 1-cyclohexen-1-
carboxylic acid was observed among other products after
quenching of the reaction mixture with CO2 and subsequent
hydrolytic work-up. Although the observation of this acid is
a strong indicator for the presence of the corresponding 1-al-
kenylcalcium derivative in solution, attempts to isolate this
species failed. However, application of the benzannulated de-
rivative 1,2-dihydro-4-iodonaphthalene was successful. Reduc-
tion of 1 with activated calcium in THF gave a dark blue reac-
tion solution, from which the colorless tetrakis(thf) adduct of
(1,2-dihydronaphth-4-yl)calcium iodide (2) was isolated accord-
The organometallic chemistry of calcium has flourished in
recent years[1] and led to the development of refined synthetic
procedures, which made a growing number of organocalcium
reagents, such as allyl-[2] and benzylcalcium compounds[3] avail-
able for organic and inorganic synthesis. Derivatives containing
s bonds between calcium and sp2-hybridized carbon atoms
also attracted considerable interest. To date, this research was
focused on arylcalcium halides[4] and diarylcalcium[5] species,
whereas alkenyl derivatives of calcium were only sporadically
mentioned. In contrast to well-established alkenylmagnesium
compounds,[6] which found numerous applications in organic
synthesis, the alkenyl derivatives of calcium are limited to very
few poorly characterized compounds. It was reported that di-
vinylcalcium was obtained by the transmetalation of Pb(CH=
CH2)4 with calcium and catalytic amounts of HgBr2, but no
physical parameters were provided.[7] Also, metal-exchange re-
actions of organic tellurium compounds seemed to be a suita-
ble procedure. Thus, the reaction of PhCaI with PhCH=CHÀ
TePh in THF at À708C and trapping of the intermediate with
benzaldehyde gave PhÀCH=CHÀCH(Ph)OH; however, no prep-
arative details were given,[8] neither for the synthesis of PhCaI,
nor for the procedure of the tellurium–calcium exchange reac-
tion. Cherkasov and co-workers studied addition reactions of
alkylcalcium complexes at enyne derivatives by characteriza-
tion of the products after hydrolytic work-up;[9] however, nei-
ther the alkylcalcium precursors, nor the calcium-containing
addition products were isolated or characterized. Thus far, no
alkenylcalcium complex was isolated or thoroughly studied in
solution or the solid state. However, beside a few structurally
[a] M. Kçhler, Dr. H. Gçrls, Dr. J. Langer, Prof. Dr. M. Westerhausen
Friedrich Schiller University Jena
Institute of Inorganic and Analytical Chemistry
Humboldtstrasse 8, 07743 Jena (Germany)
Fax: (+49)3641-948132
E-mail: m.we@uni-jena.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400391.
Chem. Eur. J. 2014, 20, 5237 – 5239
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Communication
teristic C(sp2aryl)ÀC(sp2) (147.0 pm) and C(sp2)ÀC(sp3) (151.1 pm)
single bonds.[22] The observed bond lengths justify a description
of this complex 2 as a 1-alkenylcalcium derivative.
1H and 13C{1H} NMR spectra further underline this descrip-
1
tion. In the H NMR spectrum, the signal of the olefinic proton
was observed at d=6.26 ppm and showed a characteristic
high-field shift in comparison to the signals of the aromatic
protons (Figure 2). The two methylene groups gave rise to sig-
Scheme 1. Direct synthesis of the THF-ligated calcium complex 2.
ing to Scheme 1. The color of the reaction mixture probably
arises from minor unidentified side products formed during
this radical reaction[16,17] or from subsequent attack of the
formed organocalcium derivative on remaining substrate. Simi-
larly, the formation of a dark purple solution containing naph-
thalene dianions was reported for the reaction of n-butyllithi-
um and 1,4-dihydronaphthalene.[18]
The molecular structure and numbering scheme of complex
2 is depicted in Figure 1. The calcium atom is in a distorted oc-
Figure 2. 1H NMR spectrum of 2 in [D8]THF. Signals marked with “+” belong
to 1,2-dihydronaphthalene.
nals at d=1.84 ppm and 2.46 ppm—a typical region for ali-
phatic groups. In the 13C{1H} NMR spectrum, the methylene
carbon atoms showed resonances at d=25.4 and 31.3 ppm, re-
spectively (see Figure S2, in the Supporting Information). Met-
alation of the olefinic carbon atom C4 led to an extreme low-
field shift of the corresponding signal to d=185.4 ppm in
agreement with earlier observations for the metalated carbon
atoms of arylcalcium halides. The other olefinic carbon atom is
almost unaffected and resonates at d=133.3 ppm. The remain-
ing signals of 2 appear in the typical region of aromatic
carbon atoms between d=120 and 150 ppm.
Figure 1. Molecular structure and numbering Scheme of 2. The ellipsoids
represent a probability of 40%. Hydrogen atoms at the THF ligands are
omitted for clarity, 1,2-dihydronaphth-4-yl hydrogen atoms are shown with
arbitrary radii. Selected bond lengths [pm]: Ca1ÀC4 253.0(4), Ca1ÀO1
241.5(3), Ca1ÀO2 236.9(3), Ca1ÀO3 239.4(3), Ca1ÀO4 239.8(3), Ca1ÀI1
318.63(8), C3ÀC4 131.7(6), C4ÀC5 149.8(5), C2ÀC3 153.0(6), C1ÀC2 151.1(6),
C1ÀC10 151.2(6); angles [8]: C4-Ca1-I1 170.53(9), C3-C4-C5 114.2(3).
The 1-alkenylcalcium complex 2 degrades the solvent THF
giving naphthalene, 1,2-dihydronaphthalene, and ethene
beside other unidentified products. Therefore, it can be con-
cluded that a-deprotonation of THF molecules by 2 (leading to
1,2-dihydronaphthalene) and its subsequent cycloreversion
(leading to ethene and ethenolate anions)[23] is among the de-
composition pathways of 2 in THF. However, the observation
of naphthalene (4%) among the decomposition products and
the low quantities of 1,2-dihydronaphthalene (13% of the ex-
pected amount) and ethene, detected during this process, indi-
cate that other decomposition mechanisms or secondary reac-
tions are operative as well.
tahedral environment with the anionic ligands in trans position
to each other. Bond lengths Ca1ÀC4 and Ca1ÀI1 of 253.0(4)
and 318.63(8) pm, respectively, were established. The closely
related a- and b-naphthylcalcium iodides [Ca(a-Naph)(I)(thf)4],
[Ca(b-Naph)(I)(thf)4], and [Ca(b-Naph)(I)(thf)4] showed similar
CaÀC bond lengths of 255.2(6),[19] 252.8(5),[20] and
253.4(6) pm,[21] respectively. The Ca1ÀO bond lengths of 2
varied between 236.9(3) and 241.5(3) pm. These values corre-
spond to those of the naphthylcalcium complexes. The calcium
substituent at C4 led to slight shortening of the C3ÀC4 bond
of the alkenyl moiety to 131.7(6) pm in comparison with other
alkenes containing a conjugated aryl substituent (average
value 134.0 pm).[22] The neighboring C4ÀC5 (149.8(5) pm) and
C2ÀC3 distances (153.0(6) pm) are slightly larger than charac-
The time-dependent decay of 2 in a mixture of THF and
1
[D6]benzene was monitored by H NMR measurements at am-
bient temperature. Cyclohexane was used as internal standard.
In comparison to closely related a-naphthylcalcium iodide
(3),[19] the 1-alkenycalcium derivative 2 showed an increased re-
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Acknowledgements
This work was supported by the German Research Foundation
(DFG, Bonn, Germany). M.K. is grateful to the Fonds der Chemi-
schen Industrie (Frankfurt/Main, Germany) for a generous Ph.D.
stipend. We also appreciate the financial support of the Ver-
band der Chemischen Industrie (VCI/FCI, Frankfurt/Main, Ger-
many). Infrastructure of our institute was provided by the EU
(European Regional Development Fund, EFRE) and the Frie-
drich Schiller University Jena.
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Keywords: alkenylcalcium complexes
·
calcium
· direct
synthesis · Grignard reaction · reduction reactions
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Received: January 30, 2014
Published online on March 27, 2014
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