The enantioselectivities arising from a Pd-catalyzed Heck reaction (>98% ee) and an allylic alkylation (>90% ee) using a 3,5-di-tert-butyl-MeO−BIPHEP chiral auxiliary (1) are reported. Higher ee's are observed with the 3,5-dialkyl substituents than with the unsubstituted parent MeO−BIPHEP. It is proposed that the observed dialkyl “meta-effect”, on enantioselectivity, is the combined result of a more rigid and slightly larger chiral pocket and that this effect will have some generality in homogeneous catalysis. Detailed NMR studies on the allyl complex [Pd(PhCHCHCHPh)(1)]PF6 (5), and the model hydrogenation catalyst [RuH(cymene)(1)]BF4 (6), reveal restricted rotation about several of the P−C(ipso) bonds of the phosphorus substituents containing the 3,5-di-tert-butyl groups. The X-ray structure of 6 reveals that the cymene ligand is not symmetrically bound to the Ru atom. This observation is interpreted as an expression of the chiral pocket of 1. MM3* calculations on 6 support the NMR findings and reproduce the X-ray results.

Ru(II) complexes of the chiral ligands Binap and MeO-Biphep containing six-electron hydrocarbon donors, such as Cp, a deprotonated pyrrole, or the benzene ring of indole, attain the 18-electron configuration by complexing a proximate biaryl double bond. The solid-state structures for two of these, [RuCp(2)]BF4 and [Ru(indole)(2)](BF4)2 (2 = (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(bis(3,5-di-tert-butylphenyl)phosphine)), have been determined by X-ray diffraction. They reveal that a biaryl double bond, immediately adjacent to one P-donor, coordinates to the ruthenium, thus making the chelating ligand a six-electron donor. The double bonds remain coordinated in solution as shown by HMBC 13C,1H long-range correlation spectroscopy. However, 2-D NMR exchange spectroscopy suggests that the biaryl double bond is weakly coordinated since the two halves of the C 2-symmetric Binap (or MeO-Biphep) ligands are in slow exchange at ambient temperature.

The reaction of the MeO-BIPHEP complex Ru(OAc)2(1a) (1a = (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(bis(3,5-di-tert-butylphenyl)phosphine), with HBF4 and 1,5-COD affords [Ru(η5-C8H11)(1a)]BF4 (4), in which 1a functions as a 6e donor to Ru(II) via an unexpected coordination of one of the biaryl double bonds. The isopropyl analog [Ru(η5-C8H11)(1b)]CF3CO2 (6; 1b = (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(diisopropylphosphine)) was prepared by starting from [Ru(CF3CO2)2(1,5-COD)]2 and reveals the same η4-bonding mode. Both complexes were characterized by detailed multidimensional NMR studies, and the X-ray structure for 6 is reported. Although the 31P chemical shifts for this new η4-bonding mode are informative, the 13C resonance positions for the coordinated biaryl carbons are a more reliable criterion for recognizing this type of interaction. These chemical shift data are difficult to obtain using routine 13C measurements, and a long-range 13C,1H-correlation is recommended as the method of choice. Complex 4 exhibits dynamic behavior in solution, as shown by 2-D NOESY. This exchange process can be rationalized by assuming that the double bond dissociates; however, complex 6 does not show an analogous exchange process at ambient temperature.

New bidentate ligands containing both chiral oxazoline and thiosugar elements, and also their 1,3-diphenylallyl Pd(II) complexes, have been prepared. The sugar is based on a 2,3,4,6-tetra-O-acetyl-β-d-glucopyranose moiety. These N,S-oxazoline−thioglucose ligands afford excellent ee's (90.2−96.9%) in the model enantioselective allylic alkylation reaction involving a 1,3-diphenylallyl precursor. 1H and 13C NMR spectra for the Pd compounds show that they exist in solution as a mixture of (syn/syn) exo and endo diastereomeric complexes. It is suggested that attack of the dimethyl malonate nucleophile pseudo-trans to the thioether donor is preferred for electronic reasons, whereas selective attack on the endo diastereomer, as opposed to the exo isomer, arises due to steric effects in combination with allyl rotation. The organic product is formed by preferential reaction of a minor component. Since the exo and endo isomers are shown to be in equilibrium, via 2-D exchange spectroscopy, the depleted endo diastereomer can be rapidly replaced.