Mg–Zn Hybrid Chemistry
FULL PAPER
Synthesis of [{(thf)2MgACTHUNRGTNEUNG(m-Cl)3ZntBu}2] (4): ZnCl2 (0.680 g, 5 mmol) was
tion of mixed-metal magnesium zinc species (Mg–Zn hy-
added to a solution of tBuMgCl (5 mL, 1m solution in THF, 5 mmol) in
toluene (45 mL) and the resulting suspension was stirred at room temper-
ature for 20 h. The solid was removed by filtration, the filtrate slowly
concentrated in vacuo and left to stand at room temperature. After 24 h,
a crop of colourless crystals of 4 was obtained (0.524 g, 26%). 1H NMR
(400.13 MHz, 298 K, [D8]THF): d=3.62 (m, 13H; OCH2 (THF)), 1.77
À
brids) of general formula [{(thf)2MgACTHNUTRGNEG(NU m-Cl)3ZnR}2].
Furthermore, a DFT computational study of these reac-
tions reveals that these hybrids are significantly more ener-
getically favourable than the “expected” homometallic prod-
ucts (MgCl2 and RZnCl) of a metathesis reaction that has
gone to full completion. These results support the view that
mixed-metal compounds of this type can, in fact, be in-
volved in or even responsible for reactivities previously at-
tributed to conventional monometallic organozinc species.
Furthermore, by assessing the alkylation reactions of several
organometallic combinations towards 2,2,2-trifluoroaceto-
phenone, we established that greater conversions and levels
of chemoselectivity are accomplished when the three metals
Li, Zn and Mg (accompanied by Cl and alkyl ligands) are
present in the reaction mixture, which supports a trilateral-
multicomponent partnership that adds a further level of
sophistication and intrigue to these reactions. Collectively,
we believe these findings provide important structural, theo-
retical and reactivity insights into the non-innocent role of
inorganic salts in organic reactions that involve polar organ-
ometallic reagents and should raise awareness when in situ
salt-metathesis methodologies are employed to generate the
latter. Further investigations into the complicated (but fasci-
nating) role that LiCl can play when added to bimetallic
compounds, which can include the modification of not only
their structures but also their chemical profiles, are in prog-
ress in our laboratory.
(m, 13H; CH2 (THF)), 1.01 ppm (s, 18H;
CACHTNGUTERNU(NG CH3)3 (Zn tBu));
13C{1H} NMR (100.62 MHz, 298 K, [D8]THF): d=68.48 (OCH2 (THF)),
À
33.74 (C
G
G
À
(Zn tBu).
Synthesis of [{(thf)2MgACHTUGNTRENNNUG
added to a solution of nBuMgCl (1 mL, 2m solution in THF, 2 mmol) in
THF (10 mL) and the resulting suspension was stirred at room tempera-
ture for 2 h. The solvent was then concentrated in vacuo to approximate-
ly 2 mL and toluene (30 mL) was then added. The resulting suspension
was stirred for 1 h at room temperature and then the solid removed by
filtration. The filtrate was slowly concentrated in vacuo and left to stand
at room temperature. After 24 h, a crop of colourless crystals of 5 was
obtained (0.197 g, 24%). 1H NMR (400.13 MHz, 298 K, [D8]THF): d=
3.61 (m, 12H; OCH2 (THF)), 1.75 (m, 12H; CH2 (THF)), 1.48 (m, 4H;
À
À
Zn CH2CH2CH2CH3), 1.25 (m, 4H; Zn CH2CH2CH2CH3), 0.82 (t, J=
À
À
7.3 Hz, 9H; Zn CH2CH2CH2CH3), 0.14 ppm (t, J=7.8 Hz, 4H; Zn
CH2CH2CH2CH3); 13C{1H} NMR (100.62 MHz, 298 K, [D8]THF): d=
À
À
68.50 (OCH2 (THF)), 32.02 (Zn CH2CH2CH2CH3), 30.12 (Zn
À
CH2CH2CH2CH3), 26.50 (CH2 (THF)), 14.64 (Zn CH2CH2CH2CH3),
À
10.09 ppm (Zn CH2CH2CH2CH3).
Synthesis of [{(thf)2MgACTHNUGTRNEG(UN m-Cl)3Zn(o-C6H4OMe)}2] (6): ZnCl2 (0.272 g,
5 mmol) was added to a solution of (o-C6H4OMe)MgCl (2 mL, 1m solu-
tion in THF, 2 mmol) in THF (10 mL) and the resulting suspension was
stirred at room temperature for 2 h. The solvent was then concentrated
in vacuo to approximately 2 mL and toluene (30 mL) was added. The re-
sulting suspension was stirred for 1 h at room temperature and then the
solid removed by filtration. The filtrate was slowly concentrated in vacuo
and left to stand at room temperature. After 24 h, a crop of colourless
crystals of 6 was obtained (0.303 g, 34%). 1H NMR (400.13 MHz, 298 K,
[D8]THF): d=7.39 (d, J=6.8 Hz, 2H; CHmeta), 7.01 (t, J=7.7 Hz, 2H;
CHpara), 6.73 (t, J=7.0 Hz, 2H; CHmeta*), 6.66 (d, J=8.1 Hz, 2H;
CHortho*), 3.66 (s, 6H; OCH3), 3.62 (m, 12H; OCH2 (THF)), 1.75 ppm
(m, 13H; CH2 (THF)); 13C{1H} NMR (100.62 MHz, 298 K, [D8]THF):
d=166.76 (Cipso), 142.41 (Cortho), 140.20 (Cmeta), 127.92 (Cmeta*), 121.17
(Cpara), 109.29 (Cortho*), 68.48 (OCH2 (THF)), 55.38 (OCH3), 26.53 ppm
(CH2 (THF)).
Experimental Section
General conditions: All reactions were performed under a protective
argon atmosphere by using standard Schlenk techniques. Toluene and
THF were dried by heating to reflux over sodium benzophenone ketyl
and distilled under nitrogen prior to use. NMR spectra were recorded on
a Bruker DPX 400 MHz spectrometer, operating at 400.13 MHz for 1H,
150.32 MHz for 7Li and 100.62 MHz for 13C{1H} NMR spectroscopy.
Grignard reagents were standardised by using a 1m solution of 2-butanol
in p-xylene and N-phenyl-1-napthylamine as an indicator. Single-crystal
X-ray diffraction data were recorded on Oxford Diffraction (now Agilent
Technologies) Gemini diffractometers by using MoKa and CuKa radiation
(l=0.71073 and 1.54180 ꢄ, respectively; see the Supporting Information
for a table of selected crystallographic data). The structures were solved
by direct methods and refined on all unique F2 values.[39] Some minor dis-
order in THF ligands was resolved in the structure of 4. CCDC-817989
(3), 817990 (4), 817991 (5) and 817992 (6) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
General procedure for alkylation studies with trifluoroacetophenone:
The required metal salt(s) (1 mmol, MX, in which MX=LiCl (0.042 g),
ZnCl2 (0.136 g) or MgCl2 (0.095 g)) and THF (10 mL) were added to a
Schlenk tube and the resulting suspension was sonicated for 1 h to give a
solution. The relevant organometallic species [1 mmol, RMX, in which
RMX=[(thf)2MgACTHNUTRGNENUG(m-Cl)2Zn(Et)(Cl)]2 (3; 0.416 g), EtMgCl (0.5 mL, 1m
solution in THF) or EtLi (2 mL, 0.5m solution in benzene/cyclohexane,
90:10)] was then added to this solution and the resulting solution stirred
for 1 h at room temperature. The solution was then cooled to 08C and
2,2,2-trifluoroacetophenone (135 mL, 1 mmol) was then introduced. The
reaction mixture was stirred for 2 h at 08C, then allowed to warm slowly
to room temperature and stirred for a further 24 h. The resulting solution
was then quenched with a concentrated NH4Cl solution and the yields of
the products and starting material were quantified by 1H NMR spectros-
copy and GC-FID of the crude reaction mixture, by using ferrocene
(0.093 g, 0.5 mmol) as an internal standard.
Synthesis of [{(thf)2MgACHTUNGTRENNUNG(m-Cl)3ZnEt}2] (3): ZnCl2 (0.544 g, 4 mmol) was
added to a solution of EtMgCl (2 mL, 2m solution in THF, 4 mmol) in
toluene (40 mL) and the resulting suspension was stirred at room temper-
ature for 20 h. The solid was removed by filtration, the filtrate slowly
concentrated in vacuo and left to stand at room temperature. After 24 h,
a crop of colourless crystals of 3 was obtained (0.493 g, 33%). 1H NMR
(400.13 MHz, 298 K, [D8]THF): d=3.61 (m, 12H; OCH2 (THF)), 1.76
Acknowledgements
À
(m, 12H; CH2 (THF)), 1.12 (t, J=8.1 Hz, 6H; CH3 (Zn Et)), 0.06 ppm
We thank the Royal Society (University Research Fellowship to EH), the
European Union (grant no. FP2010-RG-268329 to PGA), the Spanish
MICINN (DELACIERVA-09–05 to PGA) and the EPSRC for their gen-
erous sponsorship of this research in both Strathclyde and Newcastle. We
(q, J=8.1 Hz, 4H; CH2 (Zn Et)); 13C{1H} NMR (100.62 MHz, 298 K,
À
[D8]THF): d=68.51 (OCH2 (THF)), 26.57 (CH2 (THF)), 13.11 (CH3
À
À
(Zn Et)), 0.98 ppm (CH2 (Zn Et)).
Chem. Eur. J. 2011, 17, 8333 – 8341
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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