Synthesis of 2,4,6-trimethylphenylcalcium iodide and degradation in THF solution

Article abstract of DOI:10.1002/anie.200503452

(Figure Presented) Heavy Grignard reagent: Despite the low reactivity of the calcium metal and the high reactivity of the resulting Ca-C bonds, the synthesis of mesitylcalcium iodide (see structure: purple I, orange Ca, red O, gray C) succeeded at very lo

Full text of DOI:10.1002/anie.200503452

Angewandte
Chemie
Grignard Reagents
DOI: 10.1002/anie.200503452
Synthesis of 2,4,6-Trimethylphenylcalcium Iodide
and Degradation in THF Solution**
Reinald Fischer, Martin Gärtner, Helmar Görls, and
Matthias Westerhausen*
In contrast to the alkyl- and arylmagnesium halides prepared
by Grignard by direct synthesis more than a hundred years
ago, the organometallic chemistry of the heavier alkaline-
earth metals has received little attention.[1] The main reasons
for this are the difficult preparation and the high reactivity of
the resulting organometallic compounds. On the one hand,
calcium, strontium, and barium metals are not very reactive
and must be activated prior to use. On the other hand, the
organometallic derivatives tend to cleave ether. Furthermore,
owing to the highly ionic metal–carbon bonds these com-
pounds often are insoluble in common organic solvents. In
order to overcome these problems several preparative
procedures have been developed.[1,2] The main access routes
to organocalcium compounds are:
1) the reaction of halogenated hydrocarbons with calcium
vapor (cocondensation reaction),[3,4]
2) the insertion of calcium into a carbon–halogen bond in
solution (direct synthesis),[5]
3) the substitution of a less electropositive metal by calcium
(transmetalation),[6,7]
4) the deprotonation of H-acidic hydrocarbons by calcium or
calcium compounds (metalation),[8]
5) the reaction of a organopotassium compound with anhy-
drous CaI2 (metathesis reaction).[9,10]
Beckmann described the synthesis of phenylcalcium
iodide in 1905; he applied the direct synthesis in diethyl
ether and activated the alkaline-earth metal with a trace of
iodine.[11] More than 80 years later the synthesis of diphenyl-
calcium in THFat À208C was reported by Bickelhaupt and
co-workers.[12] However, neither yields nor physical data were
presented. These phenylcalcium compounds were character-
ized and identified by derivatization, for example, by
hydrolysis or reactions with aldehydes, ketones, esters, vinyl-
alkynes, or chlorostannanes followed by hydrolytic workup
procedures.
Neither reliable spectroscopic data nor structural param-
eters of the heavy Grignard reagents have been published.
[*] Dr. R. Fischer, Dipl.-Chem. M. Gärtner, Dr. H. Görls,
Prof. M. Westerhausen
Institute of Inorganic and Analytical Chemistry
Friedrich-Schiller-Universität Jena
August-Bebel-Strasse 2, 07743 Jena (Germany)
Fax: (+49)3641-948-102
E-mail: m.we@uni-jena.de
[**] We thank the Deutsche Forschungsgemeinschaft (DFG, Bonn-Bad
Godesberg) for financial support.
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Even less is known about substituted phenylcalcium halides,
undergoes a similar rearrangement, which was observed
above À108C in a THFsolution with formation of 3,5-
dimethylbenzyllithium.[18] In pentane in the absence of THF
such rearrangement reactions were not observed.[19]
although several alkyl[4,9,10] and alkynyl derivatives[8] have
been structurally investigated. However, a very recent success
was achieved with the complexes [F5C6M(Ar’-NNN-Ar’’] by
pentafluoro substitution of the phenyl group and by shielding
To clarify the mechanism of this rearrangement, we
isolated compound 1 was isolated and dried it under
vacuum; the resulting colorless powder was then redissolved
in [D8]THF. In a 2.0m solution the decomposition was much
slower by a factor of approximately 5 to 8, which indicates the
involvement of the THFligands in the rearrangement process.
Accordingly the first reaction step is the deprotonation of
THF[20] by the mesityl group (formation of C6H3-1,3,5-Me3).
The intermediate a-deprotonated THFcan either undergo
intramolecular ether cleavage or, in a less favored reaction,
remetalate the mesitylene at a methyl group (formation of the
3,5-dimethylbenzyl anion) [Eq. (3)]. The ratio of the 3,5-
À
of the reactive M C bond with the bulky aryl substituents Ar’
and Ar’’. As a result of the small coordination number of 3 for
[13]
À
calcium, a short Ca C bond of 250(1) pm was observed.
Here we present the first structural proof of a simple heavy
Grignard reagent and investigations concerning its high
reactivity.
The reaction of iodo-2,4,6-trimethylbenzene (mesityl
iodide) with activated calcium metal at low temperatures in
THFquantitatively yielded colorless 2,4,6-trimethylphenyl-
calcium iodide (1) [Eq. (1)]. When the solution was warmed
above 08C it turned yellow, and even at À208C a slow color
change occurred. After addition of D2O to the yellow
solution, mesitylene with the deuterium atom in a methyl
group was observed, which suggests the formation of 3,5-
dimethylbenzylcalcium iodide according to Equation (2). In
dimethylbenzyl to the ether-cleavage products is approxi-
mately 1:5. The large effect of the deuterium on the reaction
rate suggests that the deprotonation of THFis much slower
than the protonation of mesitylene or the ether-cleavage
reaction. These investigations indicate that a simple 1,3-H
shift within the mesityl group is unlikely.
The molecular structure of 1 is represented in Figure 1.
The phenyl group and the iodide are in a trans arrangement
(C1-Ca-I 177.4(1)8). The mesityl substituent is slightly dis-
order to verify the rearrangement reaction we examined the
reaction at 08C by NMR spectroscopy. The intensity of the
signals of 1 decreased and finally disappeared, and at the
same time signals of 1,3,5-trimethylbenzene (mesitylene) and
3,5-dimethylbenzyl groups appeared and increased in inten-
sity. Mesitylene originated from the ether cleavage reactions
already reported for the bis(trimethylsilyl)amides of
barium.[14]
To verify the formation of 2 we performed the direct
synthesis with 3,5-dimethylbenzyl bromide and calcium.
Compound 2 was obtained in a poor yield of approximately
15%, and bis(3,5-dimethylphenyl)ethane originating from the
Wurtz-type coupling reaction was the major product. The
NMR data of this benzyl derivative and of 2 were identical.
The high stability of benzylcalcium compounds as a result
of the delocalization of the anionic charge was demonstrated
earlier.[10] Mesityllithium,[15] dimesitylberyllium,[16] and dime-
sitylmagnesium[17] are known. However, only mesityllithium
Figure 1. Molecular structure of 1 in the crystal (ellipsoids at a
probability level of 40%, hydrogen atoms omitted for clarity). Selected
À
À
À
bond lengths (pm): Ca I 320.84(9), Ca C1 257.4(4), Ca O1 240.2(3),
À
À
À
Ca O2 240.9(3), Ca O3 239.3(3), Ca O4 241.9(3).
610
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Chemie
45.9 mmol), and 90 mL of THFwas cooled to À788C. Iodo-2,4,6-
torted: The ipso carbon atom C1 is pulled out of the plane of
trimethylbenzene (7.5 g, 30.9 mmol) was added, and the flask was
shaken for 5 h. During this time the temperature was kept below
À508C. The glass balls and the excess calcium metal were removed at
temperatures below À308C, and the filtrate (90% yield of organo-
calcium compound calculated by acid consumption of an aliquot) was
stored overnight at À788C. The colorless precipitate of 1 (3.46 g,
6.0 mmol, 19.4%) was collected on a cooled frit and dried in vacuo.
Crystalline 1 decomposes above À108C, whereas in solution
decomposition was observed already above À308C. Compound 1
shows excellent solubility in aromatic hydrocarbons and ethers. In the
1H NMR spectra at 250 K there are two sets of signals for the mesityl
group in a molecular ratio of 3:2 most probably due to a Schlenk
equilibrium which gives MesCaI and CaMes2. Above 310 K only one
set of resonances is detected. 1H NMR (200.1 MHz, [D8]THF, 250 K):
d = 2.02 (3H, s, p-CH3), 2.24 (6H, s, o-CH3), 6.36 ppm (2H, s, m-CH)
and the second set: d = 2.04 (3H, s, p-CH3), 2.31 (6H, s, o-CH3),
6.38 ppm (2H, s, m-CH); 13C{1H} NMR (50.3 MHz, [D8]THF, 250 K):
d = 21.6 (p-CH3), 27.7 (o-CH3), 124.2 (m-CH), 131.0 (p-C), 147.1 (o-
C), 182.5 ppm (i-C) and the second set: d = 21.7 (p-CH3), 28.2 (o-
CH3), 123.7 (m-CH), 131.1 (p-C), 146.6 (o-C), 183.0 ppm (i-C).
Rearrangement of 1: An NMR tube containing a solution of 1 in
[D8]THFand THF(molar ratio 10:1) and benzene as a internal
standard was kept in a NMR spectrometer at 273 K. The reaction
course was followed by NMR spectroscopy. The colorless solution
turned more and more yellow as 2 formed. 1H NMR (200.1 MHz,
[D8]THF, 250 K):d = 1.40 (2H, s, Ca-CH2), 1.90 (6H, s, m-CH3), 5.48
(1H, s, p-CH), 5.99 ppm (2H, s, o-CH); 13C{1H} NMR (50.3 MHz,
[D8]THF, 250 K): d = 22.9 (m-CH3), 41.2 (CH2-Ca), 112.1 (p-CH),
118.0 (o-CH), 136.2 (m-C), 160.2 ppm (i-C).
À
the phenyl group, thus leading to an elongation of the C1 C2
À
(140.2(7) pm) and C1 C6 bonds (143.2(6) pm) as well as a
narrow C2-C1-C6 angle of only 114.2(4)8. This fact can be
À
explained by the high ionicity of the Ca C bond and by the
enhanced spatial demand of the {CaI(thf)4 moiety compared
to that of the bonding electron pairs of the ortho methyl
groups. Similar distortions are a common feature in highly
ionic aryl compounds and have also been observed for
[(thf)2Li-C6H2-2,4,6-Me3]2.[15] The Ca C bond length com-
À
pares well with those of other organocalcium compounds,
taking the importance of the coordination number of Ca and
the bulk of the substituents into account (Table 1). In [Ca4(m4-
O)(C6H3-2,6-(OMe)2)6], in which the aryl groups bridge the
À
Table 1: Comparison of Ca C bond lengths of selected organocalcium
compounds.
CN (Ca)[a]
Ref.
À
Ca C [pm]
[Ca{C(SiMe3)3}2]
2
3
4
4
4
6
6
6
8
8
246
250
248
258
265
257
259
263
254
265
[9]
[13]
[4]
[10c]
[10a]
[F5C6-Ca(Ar’-NNN-Ar’’)][b]
[(diox)2Ca(CH(SiMe3)2)2][b]
[(thf)4Ca(CH2Ph)2]
[(thf)2Ca(C(SiMe3)2Ph)2]
[Mes-CaI(thf)4] (1)[b]
[{(thf)Ca-O-C6H4-CH-PPh’2)}4][b]
[(thf)4Ca(CH(SiMe3)(C6H4-NMe2)2]
[21]
[10b]
[8b]
[22]
ꢁ
[([18]crown-6)Ca(C C-SiPh3)2]
Deuterolysis of
2 with D2O: One-tenth of the filtrate of
[(thf)2Ca(Me3Si-(CH)3-SiMe3)2][c]
compound 1 was stirred overnight in an ice bath. The solution
turned yellow. The volatile components of the reaction mixture were
removed in vacuo, and the residue was treated with 1.0 mL of D2O
and 1.5 mL of CDCl3 were added. The organic layer was separated
and examined by 13C NMR spectroscopy. 13C{1H} NMR (50.3 MHz,
CDCl3): d = 20.7 (t, 1J(13C,D) = 19.5 Hz, CH2D), 21.0 (CH3), 126.8
(CH), 137.5 ppm (C).
[a] Coordination number of calcium. [b] Ar’: C6H3-2,6-Mes2; Ar’’: C6H4-2-
(C6H2-2,4,6-iPr3); Mes: 2,4,6-trimethylphenyl, mesityl; diox: 1,4-dioxane;
Ph’: p-tolyl. [c] Bis[1,3-bis(trimethylsilyl)allyl]calcium complex.
À
X-ray structure determination of 1: The intensity data was
collected on a Nonius Kappa CCD diffractometer using graphite-
monochromated MoKa radiation. Data was corrected for Lorentz
polarization and for absorption effects.[24–26] The structure was solved
by direct methods (SHELXS[27]) and refined by full-matrix least
squares techniques against F2o (SHELXL-97[28]). The hydrogen atoms
were included at calculated positions with fixed thermal parameters.
All non-hydrogen atoms except for the solvent molecules were
refined anisotropically.[28] XP (SIEMENS Analytical X-ray Instru-
ments, Inc.) and POVRAY were used for structure representations.
Crystal data for 1:[29] C25H43CaIO4·0.75C4H8O, M =
628.65 gmolÀ1, colorless prism, dimensions 0.05 ” 0.05 ” 0.05 mm3,
monoclinic, space group P21/c, a = 17.4431(6), b = 13.1064(5), c =
16.9127(7) Š, b = 116.183(2)8, V= 3469.8(2) Š3, T= À908C, Z = 4,
1calcd. = 1.203 gcmÀ3, m(MoKa) = 10.98 cmÀ1, multiscan, transmission
min.: 0.9426, transmission max.: 0.9778, F(000) = 1312, 24085 reflec-
tions in h(À22/22), k(À17/15), l(À21/20), measured in the range
2.688 ꢀ q ꢀ 27.488, completeness qmax = 99.7%, 7917 independent
reflections, Rint = 0.044, 5342 reflections with Fo > 4s(Fo), 312 param-
eters, 0 restraints, R1obs = 0.060, wR2all = 0.19À63, GOOF = 1.036, largest
edges of an oxygen-centered Ca4 tetrahedron, longer Ca C
bonds between 271.6(4) and 278.5(4) pm were observed.[23]
In the molecular structure the two THFligands containing
O1 and O3 are bent towards the iodine atom as a result of the
steric repulsion between these fragments and the ortho
methyl groups of the mesityl substituent. In addition, these
ligands are turned toward the CaO4 plane, whereas the other
THFmolecules are oriented propellerlike to this plane.
The THFcomplex of 2,4,6-mesitylcalcium iodide is the
first example of an isolated and structurally characterized
heavy Grignard reagent. The easy access of this highly
reactive compound, which can be handled at very low
temperatures, could give new impetus to the yet unknown
organometallic chemistry of the heavy alkaline-earth metals.
In addition we could elucidate the steps of the degradation
reaction and show that rather than an intramolecular 1,3-H
shift a sequence of deprotonation of THFand subsequent
deprotonation of mesitylene leads to the benzyl anion.
difference peak and hole: 1.328/À1.148 eŠ
.
Received: September 29, 2005
Published online: December 19, 2005
Experimental Section
All manipulations were carried out under an anhydrous argon
atmosphere. The solvents were thoroughly dried and distilled under
an argon atmosphere.
Keywords: arylcalcium iodides · calcium · Grignard reaction ·
synthetic methods
Synthesis of [MesCaI(thf)4] (1): A 500-mL Schlenk flask con-
taining glass balls (diameter 5 mm), activated calcium (1.84 g,
.
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Communications
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