The recently reported metalation/transmetalation route
for the
synthesis of CCC-bis(NHC) pincer ligand supported transition-metal
complexes was extended to Pt. 2-(1,3-Bis(N-butylimidazol-2-ylidene)phenylene)(chloro)platinum(II)
(1) and its bromo analogue 2 were synthesized
and characterized. X-ray crystal structure determinations revealed
complexes 1 and 2 to have distorted-square-planar
configurations around the metal. Photophysical and thermal properties
of these complexes are reported, and their extended photostability
in air is discussed and contrasted. Density functional theory (DFT)
and time-dependent density functional theory (TD-DFT) computations
of the ground state and various low-lying excited states have revealed
admixing of Pt 5d orbitals and the ligand π* orbitals for both
the ground state and the low-lying excited states of complex 1, indicating the low-lying states to be a mixture
of metal-to-ligand charge-transfer and ligand-centered transition
(MLCT-LC). Somewhat surprisingly, the computed gas-phase geometry
of the excited state of complex 1 had a significant distortion,
mostly about the Caryl–Pt–Cl angle. These
complexes were congeners of materials for organic light emitting diodes
(OLEDs).
The scope of CCC-NHC pincer complex synthetic methodology by metalation/transmetalation has been extended to Ir. Structural characterization revealed that it is isomorphous with the Rh complex. Both Rh and Ir complexes are efficient catalysts for the hydroamination/cyclization of secondary amines in the presence of air and/or water.
A series of Pd(II) complexes containing chiral
diamine ligands were investigated as asymmetric
catalysts for the rearrangement of allylic imidates to allyl amides.
The best catalysts were cations
obtained by dechlorination of
dichloro[(S)-2-(isoindolinylmethyl)-N-methylpyrrolidine]palladium(II) (17) with silver salts in
CH2Cl2. Catalyst 18 was
studied thoroughly and shown by 1H NMR
and X-ray crystallography analysis to be a C
1
symmetric dimer (Figure ). A series of related
catalysts 22−27 having various counterions and
anionic ligands were also prepared and studied
as asymmetric catalysts for the rearrangement of allylic
N-(4-trifluorophenyl)benzimidate 29
to
allylic benzamide 30 (eq 4). Rearrangement of
29 in CH2Cl2 (48 h at 40 °C)
in the presence of 5
mol % of 18 affords (−)-30 in 69% yield and
55% ee. Enantioselection is increased to 60% when
an
isomerically pure sample of 18 is employed. Chemical
correlation of allylic benzamide 30 with
(R)-norvaline established that (−)-30 has the
R absolute configuration (Scheme ). A
cyclization-induced rearrangement mechanism (Scheme ) requires that in the major
pathway the imidate
nitrogen attacks the re face of the olefin with Pd
coordinated to the si face. These studies
constitute
the first report of asymmetric catalysis of the rearrangement of
allylic imidates to allylic amides.
However, significant hurdles remain to be overcome before the
enantioselective rearrangement of
allylic imidates becomes a practical route to enantioenriched nitrogen
compounds.
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