Ligand substituents

An extension of the asymmetric condensation of organometallics onto aldehydes is the enantioselective silver-promoted allylation reaction of aldehydes with allyltributyltin, which has recently been performed by Shi et al. in the presence of chiral diphenylthiophosphoramide ligands and more efficiently, with binaphthylthiophosphoramide ligands. According to the nature of the ligand substituents, the corresponding allylation products were obtained in enantioselectivities of up to 98% ee, as depicted in Scheme 3.70. 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

The geometric requirements imposed by the ligand substituents can not only be utilized to force a molecule to adopt a specific binding mode. Moreover, within the capsule, the guest is isolated from the surrounding, which significantly affects its chemical... 

As has been observed for (di)organophosphide complexes of the alkali metals, the structures and aggregation states of alkali metal phosphinomethanide s are dramatically affected by the size and nature of the ligand substituents and the presence of additional coligands such as THF, tmeda, or pmdeta. The subtle interplay of these factors, and in particular the steric and electronic properties of substituents at both phosphorus and the a-carbon, defines the structures adopted by such complexes. 

Scheme 1. Porphyrin ligand substituents have been omitted for clarity.

The orange complex 83 (57) exhibits a pseudotetrahedral geometry in the solid state, with the chlorine atoms distorted 49° and 63° away from the bipyridyl-Cu plane (Fig. 8). The d-d transition occurs at 919 nm. It seems likely that the large size of the ligand substituent is responsible for the deviation in the structure of the copper complex. Whether this effect is also responsible for the ease of reduction of the corresponding triflate complex by diazoester is not clear. 

Andrus et al. (109) proposed a stereochemical rationale for the observed selec-tivities in this reaction. The model is based on the Beckwith modification (97) of the Kochi mechanism, suggesting that the stereochemistry-determining event is the ally lie transposition from Cu(III) allyl benzoate intermediates 152 and 153, Fig. 13. Andrus suggests that the key Cu(III) intermediate assumes a distorted square-planar geometry. Steric interactions are decreased between the ligand substituent and the cyclohexenyl group in Complex 152 as opposed to Complex 153 leading to the observed absolute stereochemistry. 

The utility of bis(oxazoline)-Cu(II) complexes as catalysts for the Diels-Alder reaction has been examined in a number of other systems. Aggarwal et al. (205) demonstrated that a-thioacrylates behave as effective two-point binding substrates for these catalysts. With cyclopentadiene, catalyst 271d induces the reaction at -78°C to provide the cycloadduct in 94 6 diastereoselectivity and >95% ee. Aggarwal proposes that the metal binds to the carbonyl oxygen and to the sulfur atom. The sulfur substituent is placed opposite the ligand substituent thereby shielding the bottom face of the alkene. Considerably lower selectivities are observed with 5-Me substrates. 

Both Table XIX and Table XX show that distribution coefficients cover a much larger range for complexes than for the parent ligands, and that variation of ligand substituent can have a very large effect on... 

Table 1 Crystal structure determinations of the nitrido Schiff base Mn complexes. Ligand substituents are...

The apparent kinetics of proton-coupled ReV/in electron transfer also are dependent on 0X0 group geometry here (ks,h)cis/(ks,h)trans 100 [36]. It is proposed that the apparent kinetics is controlled by the thermodynamic accessibility of the intermediate Re (IV) state, whose effective potential is modulated by protonation, 0X0 group geometry, and pyridyl ligand substituents. 

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