Synchronizing Steric and Electronic Effects in RuII Complexes
FULL PAPER
from Sigma–Aldrich, Alfa Aesar, or Acros Organics. RuCl3 ꢅH2O was
purchased from ABCR. Ligands 1, 2, and the complexes [RuCl
{RuCl2[P(pMeC6H4)3]3}, {RuCl2[P(pMeOC6H4)3]3}, and {RuCl2[P-
(pFC6H4)3]3} were prepared according to literature procedures.[18,19] NMR
2ACHTUNGTRENNUNG(PPh3)],
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
spectra were recorded on a Bruker Avance 300 spectrometer at 300 (1H)
and 75 MHz (13C), a Bruker Avance 500 spectrometer at 500 (1H) and
125 MHz (13C), or a Bruker Avance 250 spectrometer at 250 (1H) and
63 MHz (13C). Chemical shifts are reported in ppm downfield by using
tetramethylsilane as internal standard. IR spectra were recorded on a
Bruker Vector 22 FTIR spectrometer. Mass spectra were recorded by
using electrospray ionization on a Bruker Micro-TOF-Q machine. All mi-
crowave manipulations were carried out on a CEM Discovery 300 W mi-
crowave.
General procedure for synthesis of the ligands 3–6: The appropriate 2-
pridinecarbaldehyde derivative (2 equiv) was added to a solution of eth-
ylenediamine (1 equiv) in ethanol. The resulting solution was stirred
overnight at RT, NaBH4 (2.2 equiv) was added, and stirring was contin-
ued for another 3 h. Then aqueous HCl (2 N) and water were added, the
aqueous layer was washed twice with diethyl ether and basified with
aqueous NaOH (2.5 N). The aqueous layer was extracted with ethyl ace-
tate, dried over Na2SO4, filtered, and evaporated in vacuo. The crude
product was dissolved in CH2Cl2. Then water, NaOH (2.2 equiv), a cata-
lytic amount of Bu4NBr, and the corresponding benzyl bromide
(2.2 equiv) were added. The resulting solution was stirred for two days at
RT. Then aqueous HCl (2 N) and water were added, the aqueous layer
was washed twice with diethyl ether and basified with aqueous NaOH
(2.5 N). The aqueous layer was extracted with ethyl acetate, dried over
Na2SO4, filtered, and evaporated in vacuo. Crude ligands were purified
by column chromatography.
Scheme 8. Chemoselectivity in 7-[PF6]-catalyzed amine alkylations.
is contrary to the reactivity of [RuCl2ACHTNUGTRENUNG(Ph3P)3], which is
known to produce tertiary amines in the presence of an
excess of alcohol.[11a,b] These test experiments clearly under-
line the potential of this catalytic system. The high chemose-
lectivity might be regarded as a limitation; however, from a
synthetic point of view, the selective transformation of only
one out of several functional groups allows a more efficient
and direct product approach by avoiding unnecessary pro-
tecting-group manipulations.
General procedure for complex synthesis: [(R3P)3RuCl2] (1 equiv) and
ligand (1 equiv) were dissolved in toluene, and the reaction mixture was
heated for 30 min at 858C under microwave irradiation. The resulting
precipitate was filtered, washed with ether, and redissolved in ethanol.
Then KX or NaX (X = PF6, SbF6, TfO, BF4 or BPh4; 1.2 equiv) was
added and the solvent was removed in vacuo. The residue was purified
by column chromatography (petroleum ether/ethyl acetate 1:1 to ethyl
acetate) and recrystallized from CH3CN/Et2O or directly recrystallized
from CH3CN/Et2O.
Conclusion
The synthesis and characterization of hexavalent {RuII-
General procedure for the catalytic alkylation of benzyl alcohol with ani-
line: KOtAm (14 mL, 0.025 mmol) was added to a solution of Ru complex
(0.005 mmol) in toluene (0.25 mL), and the resulting solution was stirred
until a dark red color occurred. Then benzyl alcohol (31 mL, 0.30 mmol)
and aniline (23 mL, 0.25 mmol) were added and the reaction mixture was
stirred at 1008C for 4 h. After cooling to RT, dodecane (29 mL,
0.125 mmol) was added, and an aliquot of the reaction mixture was fil-
tered over silica and analyzed by gas chromatography.
ACHTUNGTRENNUNG(NNNN,P)} complexes, which proved to be potent catalysts
for the direct condensation of benzyl alcohol and aniline to
the corresponding secondary amine, were reported. Detailed
cyclic voltammetric investigations performed parallel with
the high-level DFT calculations indicated the specific elec-
tronic properties and the energetic level of the metal-cen-
tered HOMO to play a decisive role in synchronizing the
two separate hydrogen-transfer events. HOMO energies
were influenced both by the substitution pattern at the tetra-
dentate NNNN ligand, as well as by the phosphine indicat-
ing that both ligand motifs are orchestrating the catalytic ac-
tivity and might be bound throughout the catalytic cycle.
After identification of a suitable catalyst, various (primary)
alcohols and aromatic amines were converted to the corre-
sponding secondary amines in good-to-excellent yields.
Future studies will concentrate on evaluating scope and lim-
itations of this modular new catalyst structure in various hy-
dride-based mechanisms.
General procedure for N-alkylation reactions: To a solution of 7-[PF6]
(0.01 mmol) in toluene (0.5 mL), KOtAm (29 mL, 0.05 mmol) was added,
and the resulting solution was stirred until a dark red color occurred.
Then alcohol (0.60 mmol, 1.2 equiv) and aromatic amine (0.50 mmol,
1.0 equiv) were added, and the reaction mixture was stirred at 1008C for
18 h. After cooling to RT, the solvent was removed in vacuo, and prod-
ucts were purified by column chromatography.
Electrochemical procedures: CV was performed by using a Metrohm Au-
tolab PGSTAT30 potentiostat in a solution of [Bu4N]ACHTNUTRGNEN[UG PF6] in CH2Cl2
(0.1m). A Pt working electrode was used in combination with a carbon
counterelectrode and an Ag/AgCl (in 3m KCl solution) reference elec-
trode. All measured values are referred to the FcH0/+ couple, which
served as internal reference.
Computational methods: DFT calculations were performed by using the
program Turbomole 6.3 in combination with the graphical interface
TmoleX 3.3.[20] BP86 exchange correlation functional and split valence
basis set with one polarization function (def2-SVP) was used for geome-
try optimizations.[21,22] Optimized geometries were checked by vibrational
analysis (no imaginary frequencies were observed). Energies were calcu-
lated at these geometries with triple z valence basis set with one polariza-
tion function (def2-TZVP).[22] The conductor-like screening model
(COSMO) was used for all calculations simulating CH2Cl2 (e=8.51).[23,24]
Experimental Section
General: All reactions sensitive to air and moisture were carried out
under dry N2 atmosphere by using standard Schlenk techniques. All sol-
vents were purified by distillation prior use. Chemicals were purchased
Chem. Eur. J. 2013, 00, 0 – 0
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