Suppression of Schlenk equilibration and heavier alkaline earth alkyl catalysis: A dearomatization strategy

Article abstract of DOI:10.1021/om200028m

Reaction of a bis(imino)acenapthene with the heavier alkaline earth dialkyls [Ae{CH(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) results in deromatization of the aromatic ligand and the isolation of heteroleptic alkyl spe

Full text of DOI:10.1021/om200028m

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pubs.acs.org/Organometallics
Suppression of Schlenk Equilibration and Heavier Alkaline Earth Alkyl
Catalysis: A Dearomatization Strategy
Merle Arrowsmith, Michael S. Hill,* and Gabriele Kociok-K €o hn
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
S Supporting Information
b
ABSTRACT: Reaction of a bis(imino)acenapthene with the
heavier alkaline earth dialkyls [Ae{CH(SiMe ) } (THF) ]
3
2 2
2
(
Ae = Mg, Ca, Sr) results in deromatization of the aromatic
ligand and the isolation of heteroleptic alkyl species which
show unprecedented stability toward Schlenk-type redistribution and exceptional catalytic activity toward the hydroamina-
tion of aminoalkenes.
a
lthough alkaline earth (Ae = Mg, Ca, Sr, Ba) complexes have
now been shown to catalyze a wide variety of multiple
Scheme 1
A
1
,2
bond heterofunctionalization reactions, a full realization of
the catalytic potential of these elements will still require
substantial advances in their basic coordination and orga-
nometallic chemistry. A particular problem for the larger
elements below magnesium in group 2 lies in the instability
of precatalytic species of the general form LAeR (L =
monoanionic supporting ligand; R = reactive substituent)
toward Schlenk-type redistribution to inactive or ill-defined
a
Ar = 2,6-diisopropylphenyl.
3
homoleptic species. Examples where R is a basic alkyl
substituent are particularly limited and are restricted to a
handful of calcium derivatives containing sterically demand-
4
5
ing bis(trimethylsilyl)methyl or benzyl ligands. In pre-
vious work we have shown that the β-diketiminato deriva-
5
b
tive, compound I, may be isolated and fully characterized.
The formation of I was, however, accompanied by a fur-
ther calcium species containing a dianionic ligand produced
by deprotonation of a methyl substituent bonded to an
5
b
R-carbon of the imine.
This observation precluded
any meaningful study of I as a viable precatalytic species,
and subsequent work has shown that pyridyl diimine
ligands containing R-methyl substituents are similarly
deprotonated by the well-defined dialkyl compounds
6
[
Ae{CH(SiMe ) } (THF) ] (Ae = Mg, Ca, Sr, Ba) to form
3
2 2
2
7
species IIa-d. This latter process occurs via dearomatized
intermediates such as III, which, however unstable, may also
be viewed as heteroleptic alkyl species conforming to the
LAeR formulation. A similar dearomatization strategy has
previously been employed in organolutetium chemistry, albeit
Bis(imino)acenapthene (BIAN) derivatives, such as the 2,6-
diisopropylphenyl-substituted diimine IV, have been widely
employed as neutral or mono-, di-, tri-, and tetraanionic N-donor
and rigidly chelating ligands for a wide variety of transition metal
9
and main group elements. In an initial experiment, addition of
8
with substituted terpyridine heterocycles, and in this con-
the group 1 reagent [K{CH(SiMe ) }] to a suspension of IV in
3
2
tribution we show that a selective alkylation/dearomatization
process can be arrested through selection of an aromatic
diimine species devoid of R-methyl substituents, providing a
novel strategy to produce robust and catalytically active
ligand-supported alkyl species of even the heaviest alkaline
earth metals.
C D caused instant solubilization of the ligand precursor,
6
6
accompanied by a color change from bright orange to dark green
1
(
Scheme 1). Examination of the H NMR spectrum of this solution
Received: January 13, 2011
Published: February 24, 2011
r 2011 American Chemical Society
1291
dx.doi.org/10.1021/om200028m
|Organometallics 2011, 30, 1291–1294

Organometallics
COMMUNICATION
Figure 1. ORTEP representation of compound 1. Thermal ellipsoids are drawn at the 30% probability level. Hydrogens are omitted, except for those
attached to the chiral center C(31). Selected bond lengths (Å) and angles (deg): K-N(1) = 2.694(3), K-N(2) = 2.703(3), N(1)-C(25) = 1.281(4),
N(2)-C(36) = 1.335(4), C(25)-C(36) = 1.518(4), C(25)-C(26) = 1.496(4), C(34)-C(36) = 1.393(4), C(30)-C(31) = 1.533(5), C(31)-C(32) =
1
1
.517(5), C(32)-C(33) = 1.351(5), K-centroid = 2.866(4); N(1)-K-N(2) = 63.52(8), K-N(1)-C(1) = 121.28(19), K-N(2)-C(13) =
26.18(18), K-N(1)-C(25) = 118.2(2), K-N(2)-C(36) = 117.4(2).
a
indicated complete consumption of both starting materials
and formation of a single alkylated and dearomatized potas-
sium BIAN species, compound 1, characterized by six distinct
multiplets in the region from 4.4 to 6.5 ppm. A COSY NMR
experiment identified the alkylation of the acenaphthene
Scheme 2
5
backbone as having occurred on the C position. As a conse-
quence of the chiral center formed, the two magnetically
inequivalent (SiMe ) groups appeared as two separate 9H
singlets at 0.18 and 0.04 ppm, respectively. The asymmetry of
3
a
Ar = 2,6-diisopropylphenyl.
this monoalkylated species was reflected by a splitting of the
1
isopropyl methine and methyl resonances in both the H and
9k
(
1, 1.518(4) Å; IV, 1.505(5) Å). The dearomatized ace-
1
3
1
C{ H} NMR spectra.
naphthene ligand backbone is only slightly distorted from its
original planar geometry. The C-C bond lengths and angles
of the aromatic C H ring (1.374(5)-1.410(5) Å; 116.6(3)-
22.3(3)°) remain within the range of those of the ligand
precursor, IV, while the dearomatized ring is distorted due to the
Crystallization at room temperature from toluene afforded
dark green crystals of compound 1 suitable for X-ray diffraction
analysis. The result of this experiment is displayed in Figure 1,
while selected bond length and angle data are provided in the
figure caption. In a manner similar to that observed within a
6
3
1
3
presence of the pseudo-tetrahedral sp C(31) carbon center
•
-
9g
radical anionic potassium complex [{IV} K], compound 1
(
1
C(32)-C(31)-C(37) = 111.3(3)°; C(32)-C(31)-C(30) =
crystallizes as a dimer in which the coordination environment about
11.6(3)°; C(30)-C(31)-C(37) = 113.4(3)°).
6
both potassium centers is provided by one of the η -aryl sub-
Addition of [Ae{CH(SiMe ) } (THF) ] (Ae = Ca, Sr) to a
3
2 2
2
stituents of the second molecule and the bidentate imine-amido
suspension of IV in C D or toluene also resulted in the instant
solubilization of the starting materials and a color change to dark
green. The analogous reaction with [Mg{CH(SiMe ) } (THF) ]
required 24 h of heating at 60 °C to yield the same result
Scheme 2). In all three cases the resultant H NMR spectra were
reminiscent of that of 1, showing clean formation of the alkaline
earth organometallic species 2 (Mg), 3 (Ca), and 4 (Sr), with the
additional 18H and 1H singlets of the metal-bound monoanionic
6
6
0
dearomatized BIAN ligand. The K-centroid and K-K dis-
tances of 2.866(4) and 4.5657(17) Å, respectively, are shorter
3
2 2
2
-
•
0
than those in [{IV} K] (K-centroid = 2.904 Å; K-K = 4.619
Å). Despite the asymmetry of the ligand framework, the potas-
sium-imine and potassium-amide K-N distances of 2.694(3)
and 2.703(3) Å can be considered identical and are within the
1
(
-
•
range of those observed in [{IV} K] and a related potassium
9g
-
tetrakis(imino)pyracene complex (2.653-2.757 Å). Whereas
the N(1)dC(25) bond length of 1.281(4) Å falls within the
range of NdC bond lengths of the ligand precursor (1.250(6)
and 1.295(6) Å), the N(2)-C(36) bond of the amido frag-
ment shows substantial elongation to 1.335(4) Å. This distance is
still short for a formal N-C single bond. While the C(25)-
C(36) bridge remains unaffected by the charge redistribution (1,
[CH(SiMe ) ] coligand at ca. 0.3 and -1.7 ppm, respectively.
An analogous reaction utilizing the barium dialkyl also resulted in
an instant color change to dark green. H NMR spectroscopic
3
2
1
9
k
analysis of this reaction, however, indicated the formation of
three dearomatized species, among which the heteroleptic bar-
ium alkyl 5 (50-70% yield) analogous to compounds 2-4 could
be tentatively identified by a characteristic upfield Ba-CH-
(SiMe ) proton shift at -1.73 ppm. As we have not yet com-
1
.518(4) Å; IV, 1.526(3) Å), the C(36)-C(34) distance is
3
2
considerably shortened compared to that in the ligand precursor
pletely elucidated the nature of the products of this reaction, it
1
292
dx.doi.org/10.1021/om200028m |Organometallics 2011, 30, 1291–1294

Organometallics
COMMUNICATION
will not be discussed further here. Multiple attempts to obtain
single crystals of the pure alkaline earth organometallic com-
pounds 2-4 suitable for X-ray analyses were thwarted by the
extreme solubility of these compounds even after months of
storage at -30 °C in minimal amounts of hydrocarbon solvents
such as hexane and pentane. NMR data and elemental analyses of
the magnesium, calcium, and strontium products, however, left
no doubt as to their formulation. Attempts to recover the alkyl-
ated amino-imine ligand by hydrolyzing the organometallic com-
plexes initially led to a color change from green to purple,
followed by a gradual change toward dark red over a period of
Table 1. Hydroamination Catalysis with Compound 3
1
several hours. H NMR data and crystallization of the crude product
from toluene showed quantitative conversion to the rearoma-
tized ligand precursor IV, with formation of the alkane, CH2-
(
SiMe ) . Although the mechanism of this reaction remains to be
3 2
elucidated, we suggest that the extreme stability of the aromatic
BIAN ligand in the absence of an electropositive metal center (or,
conversely, the stability of (Me Si) CH ) is the driving force of
3
2
2
this dealkylation process.
DFT calculations were undertaken to investigate the regio-
selectivity of this BIAN alkylation process on model calcium
complexes using B3LYP density functional theory and a 6-31G
10
basis set implemented in Gaussian03. Independent frequency
calculations confirmed the optimized geometries as true minima.
To reduce computational expense, the THF molecules within
the structure of 3 were omitted and the bulky 2,6-diisopropyl-
phenyl groups were replaced by less sterically demanding tert-
butyl groups. The silicon atoms and methyl groups of the CH-
a
Catalyst loading (mol %) shown in parentheses; all reactions per-
(
SiMe ) fragments were also replaced by carbon and hydrogen
b
3
2
formed at 25 °C except entry 6 (60 °C). NMR yields were measured
against [(Me Si) Si] as an internal standard in C or d -toluene.
atoms, respectively, and the total energy was computed for each
3
4
6
D
6
8
3
4
5
5
of the C -, C -, and C -alkylated isomeric forms. The C -alkyl-
ated isomer was calculated to be the most stable product, with the
change suggestive of either ligand rearomatization or complex
redistribution throughout the reactions. Complete disappearance
of the distinctive upfield H alkyl singlet at -1.7 ppm within the
3
4
-1
C - and C -alkylated isomers 30.2 and 121.8 kJ mol higher in
energy, respectively. We propose that regio-preferential forma-
tion of the C -alkylated isomers arises from both electronic
1
5
first point of analysis was indicative of rapid and irreversible
catalyst initiation. Comparison of the results given in Table 1 with
previous intramolecular hydroamination experiments indicated
that cyclization with 3generally occurred substantially more quickly
at lower temperature and catalyst loading, an observation which
may be ascribed to the irreversibility of the alkyl-based catalyst
initiation step. Especially remarkable was the fast cyclization of
the internal olefin 1-amino-2,2-diphenyl-4-hexene at room tem-
perature in near-quantitative yield (entry 5).
These results reveal the potential of this new series of alkaline
earth precatalysts, while the synthesis of compound 1 indicates
that the dearomatized BIAN anion should also be applicable as a
novel and unusual ligand in a wide range of inorganic and
organometallic systems.
stabilization and minimization of the steric repulsion between the
bulky alkyl substituent bonded to the acenaphthene backbone
and the organic substituents bonded to the nitrogen atoms.
The kinetic stability of 2-4 was assessed by heating a d -
8
toluene solution of these compounds at 80 °C for 1 week. Apart
from a slight amount of protonolysis of the alkyl ligand to the
alkane CH (SiMe ) in the case of 3 and 4 (<5%), possibly due
11
2
3 2
1
to reaction with the solvent, the H NMR spectra remained
virtually unchanged, suggesting that the complete suppression of
any Schlenk-type ligand redistribution had been achieved. Addi-
tion of 2 equiv of the ligand precursor IV to 1 equiv of the dialkyl
species in d -toluene did not yield the expected homoleptic
8
complexes at room temperature. Rather, the heteroleptic com-
plexes 2-4 were formed while the excess IV remained unreacted.
Heating of these mixtures at 90 °C over a period of several days
did not result in reaction of the second equivalent of ligand but in
partial protonation of the alkyl coligand, possibly due to depro-
tonation of the toluene solvent. We suggest that the marked
stability of these compounds is a result of both the steric demands
of the N-donor substituents and the rigidity of the acenapthene-
based ligand structure.
’
ASSOCIATED CONTENT
S
Supporting Information. Text, figures, tables, and a CIF
b
1
file giving full experimental and instrument details, H NMR
spectra of compounds 1-4, and details of the X-ray diffraction
analysis of compound 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
The catalytic intramolecular hydroamination reaction has
provided a suitable benchmark reaction for the assessment of
11
several calcium precatalytic species. Calcium complex 3 was,
thus, assessed for the intramolecular hydroamination of a range
of substituted aminoalkenes on an NMR scale in C D or d -
’
ACKNOWLEDGMENT
We thank the Engineering and Physical Sciences Research
6
6
8
1
toluene. In all cases inspection of the H NMR spectra showed no
Council for a project studentship (MA, EP/E03117X/1).
1
293
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Organometallics
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