The reaction of 3 with two equivalents of 2,4,6-trimethylben-
zaldehyde (mesitylaldehyde) immediately forms a bright orange
solution, from which crystals of 5 were isolated. The 1H and 13
C
{1H} NMR spectra of 5 show resonances at δH 10.50 ppm and
δC 197.2 ppm, respectively, indicative of an aldehyde functional-
ity. The remaining resonances for the amidinate ligand and
mesityl group indicate
magnesium.
a
symmetrical environment at
The X-ray crystal structure of 5 (Fig. 3) shows a five-coordi-
nate magnesium centre bonded to a chelating amidinate ligand, a
terminal aryloxide and two mesitylbenzaldehyde molecules. The
geometry is closest to square-based pyramidal (τ value = 0.25)
with O2 defining the apex, although the planarity of the amidi-
nate metallacycle and the small bite angle result in a significant
twist between the planes defined by Mg,N1,N2 and Mg,O2,O3
[39.7°]. The Mg–Oaryl distance is longer than in the THF adduct
3, with a concomitant increase in the angle at O1. Although
these data are consistent with a weakening of the metal–arylo-
xide bond, which is required for the generation of the active cata-
lyst I, this is more likely to represent the increased coordination
number in 5.
The orientation of the aldehyde ligands suggests that coordi-
nation is through an sp2-type lone pair on the oxygen atoms,
with C–Oaldehyde–Mg angles ∼124°. The Mg–Oaldehyde bond
lengths in 5 are similar to that in the para-isopropylbenzalde-
hyde adduct [Mg(μ-OPh)Br(OEt2)(4-iPrC6H4COH)]2,16 and
there is no appreciable lengthening of the CvO bond lengths
compared with other examples of metal mesC(O)H adducts.17
Fig. 4 Core structure of 5 showing intramolecular hydrogen bonds.
The core structure of 5 is stabilized by intramolecular hydrogen-
bonding between H1⋯O1 (2.54 Å) and H2⋯O3 (2.91 Å)
(Fig. 4).
The accessibility of bis-aldehyde adducts at a five-coordinate
metal have not been considered when describing the mechanism
of the Tishchenko reaction promoted by magnesium, likely due
to the small size of the metal. However, this result in combi-
nation with the different catalytic profile observed for 3 com-
pared with 4, may indicate an alternative catalytic pathway is
operating in this instance. Further tests are being carried out to
determine the mechanism, where preliminary results indicate that
heating an NMR sample of 5 produces the ester in the absence
of additional aldehyde, suggesting the bis(aldehyde) may be con-
sidered an intermediate in the catalysis promoted by magnesium
amidinate compounds.
Notes and references
‡Despite repeated attempts, accurate elemental analysis could not be
obtained for compounds 1 and 3. We believe that in the case of 3 this is
due to inefficient removal of the LiBr side product (see ESI for the
1H and 13C NMR spectra).
§The reduction in the rate of ester production is likely to be due to more
competitive binding of the THF at high conversions, where the concen-
tration of aldehyde is low.
1 W. Tischtschenko, Chem. Zentralbl., 1906, 77, 1309–1311.
2 T. Seki, T. Nakajo and M. Onaka, Chem. Lett., 2006, 35, 824–829.
3 M. R. Crimmin, A. G. M. Barrett, M. S. Hill and P. A. Procopiou, Org.
Lett., 2007, 9, 331–333.
4 (a) L. Cronin, F. Manoni, C. J. O’Connor and S. J. Connon, Angew.
Chem., Int. Ed., 2010, 49, 3045–3048; (b) C. J. O’Connor, F. Manoni,
S. P. Curran and S. J. Connon, New J. Chem., 2011, 35, 551–553;
(c) S. P. Curran and S. J. Connon, Org. Lett., 2012, 14, 1074–1077.
5 M. P. Coles, Chem. Commun., 2009, 3659–3676.
6 B. M. Day, N. E. Mansfield, M. P. Coles and P. B. Hitchcock, Chem.
Commun., 2011, 47, 4995–4997.
7 (a) R. Lechler, H.-D. Hausen and J. Weidlein, J. Organomet. Chem.,
1989, 359, 1–12; (b) M. P. Coles, D. C. Swenson, R. F. Jordan and
V. G. Young Jr., Organometallics, 1997, 16, 5183–5194.
8 (a) B. Srinivas, C.-C. Chang, C.-H. Chen, M. Y. Chiang, I.-T. Chen,
Y. Wang and G.-H. Lee, J. Chem. Soc., Dalton Trans., 1997, 957–963;
(b) J. A. R. Schmidt and J. Arnold, J. Chem. Soc., Dalton Trans., 2002,
2890–2899.
9 (a) M. Westerhausen and H.-D. Hausen, Z. Anorg. Allg. Chem., 1992,
615, 27–34; (b) F. A. Cotton, S. C. Haefner, J. H. Matonic, X. Wang and
C. A. Murillo, Polyhedron, 1997, 16, 541–550; (c) D. Walther,
P. Gebhardt, R. Fischer, U. Kreher and H. Görls, Inorg. Chim. Acta,
Fig. 3 ORTEP diagram of Mg(mesC{NCy}2)(OAr)(mesCHO)2 (5)
(ellipsoids at 30% probability, hydrogen atoms except aldehyde protons
and solvate molecules omitted). Selected bond lengths (Å) and angles
(°): Mg–O1 1.8842(13), Mg–N1 2.0967(15), Mg–N2 2.1184(15), Mg–
O2 2.0780(14), Mg–O3 2.1566(13), O1–C23 1.322(2), O2–C37 1.228
(2), O3–C47 1.231(2), C1–N1 1.336(2), C1–N2 1.324(2); N1–Mg–N2
64.14(6), N1–Mg–O1 141.21(6), N1–Mg–O2 104.30(6), N1–Mg–O3
93.38(6), N2–Mg–O1 110.13(6), N2–Mg–O2 93.07(6), N2–Mg–O3
156.06(6), O1–Mg–O2 114.43(6), O1–Mg–O3 92.44(5), O2–Mg–O3
84.31(5), Mg–O1–C23 164.09(12), Mg–O2–C37 124.71(12), Mg–O3–
C47 124.23(12).
10932 | Dalton Trans., 2012, 41, 10930–10933
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