A wide range of NCN pincer palladium complexes, [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chloropalladium (alkyl = n-butyl, benzyl, cyclohexyl, tert-butyl, adamantyl, phenyl, 4-methoxyphenyl), were readily prepared from trans-(4-tert-butyl-2,6-diformylphenyl)chlorobis(triphenylphosphine)palladium via dehydrative introduction of the corresponding alkylimino ligand groups (ligand introduction route) in excellent yields (71-98%). NMR studies on this route for forming pincer complexes revealed the intermediacy of [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chlorobis(triphenylphosphine)palladium which is in equilibrium with the corresponding NCN pincer complexes via coordination/dissociation of the intramolecular imino groups and triphenylphosphine ligands. A series of chiral NCN pincer complexes bearing pyrroloimidazolone units as the trans-chelating donor groups, [4-tert-butyl-2,6-bis{(3R,7aS)-2-phenylhexahydro-1H-pyrrolo[1,2-c]imidazol-1-on-3-yl}phenyl]chloropalladium, were also prepared from the same precursor via condensation with proline anilides in high yields. The catalytic properties of the NCN imino and the NCN pyrroloimidazolone pincer palladium complexes were examined in the Heck reaction and the asymmetric Michael reaction to demonstrate their high catalytic activity and high enantioselectivity.

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The synthesis and complexation behavior of a novel bis-phosphine ligand based on a calix-[4]arene scaffold, 5,17-bis(diphenylphosphino)-25,26,27,28-tetrabenzyloxycalix[4]arene (3), are reported. Reaction of 3 with PdCl 2 (cod) under dilute conditions results in formation of the trans-coordinated dinuclear complex [PdCl 2 ‚3] 2 (4). The X-ray structure of 4 reveals that the two phosphinocalix[4]arene molecules are linked by the two PdCl 2 fragments. On the other hand, reaction of 3 with PtCl 2 (cod) affords the cis-chelating mononuclear complex PtCl 2 ‚ 3 (5). The X-ray structure, low-temperature NMR measurements, and geometry optimization of 5 reveal that the structure in solution at -80 °C and in solid-state has a C 1 symmetry where the P-Pt-P plane is inclined and the calix[4]arene moiety is twisted. It was also found that 3 reacts with [Pd(η 3 -C 3 H 5 )(cod)]BF 4 to yield the corresponding cationic complex [Pd(η 3 -C 3 H 5 )‚3]BF 4 (6) quantitatively. Low-temperature NMR measurements suggest that 6 exists as a mixture of two stereoisomers (6a and 6b) possessing a C 1 symmetry. Although both 5 and 6 exhibit C 1 structure at -80 °C, their NMR spectra measured at 20 °C indicate a C 2v symmetry for 5 and a C s symmetry for 6. These fluxional behaviors of 5 and 6 in solution are caused by two separable motions: rollover motion of the coordination plane (R motion) and twist motion of the calix[4]arene scaffold (T motion).

The first asymmetric Pd(II)/Pd(IV) catalysis has been achieved by employing the combination of a hypervalent iodine reagent and a chiral ligand, SPRIX. Enantioselective cyclization of enyne derivatives catalyzed by the Pd-i-Pr-SPRIX complex furnished lactones bearing a bicyclo[3.1.0]hexane skeleton with up to 95% ee.

Scheme 1. Synthesis of enantiomerically enriched g-lactams through auto-tandem asymmetric catalysis using 1.

An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via a palladium-catalyzed intramolecular oxidative aminocarbonylation is described. The carbon-carbon double bond of alkenylurea substrates has been shown to react intramolecularly with a nitrogen nucleophile in the presence of a palladium catalyst under a carbon monoxide atmosphere. The use of a chiral spiro bis(isoxazoline) ligand (SPRIX) is essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, this peculiar character of SPRIX originates from two structural characteristics: low sigma-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.

We report a method of correcting a near-infrared (0.90-1.35 µm) high-resolution (λ/∆λ ∼ 28, 000) spectrum for telluric absorption using the corresponding spectrum of a telluric standard star. The proposed method uses an A0 V star or its analog as a standard star from which on the order of 100 intrinsic stellar lines are carefully removed with the help of a reference synthetic telluric spectrum. We find that this method can also be applied to feature-rich objects having spectra with heavily blended intrinsic stellar and telluric lines and present an application to a G-type giant using this approach. We also develop a new diagnostic method for evaluating the accuracy of telluric correction and use it to demonstrate that our method achieves an accuracy better than 2% for spectral parts for which the atmospheric transmittance is as low as ∼20% if telluric standard stars are observed under the following conditions:(1) the difference in airmass between the target and the standard is 0.05; and (2) that in time is less than 1 h. In particular, the time variability of water vapor has a large impact on the accuracy of telluric correction and minimizing the difference in time from that of the telluric standard star is important especially in near-infrared high-resolution spectroscopic observation.

An enantioselective intramolecular oxidative cyclization of 4-alkenoic acids was developed. The reaction proceeded via a π-allyl Pd intermediate generated by an allylic C-H activation to give γ-lactone derivatives with moderate to good enantioselectivity. Spiro bis(isoxazoline) ligand, SPRIX, was indispensable for this asymmetric transformation.