Facial ligand

Chunhua Yan and coworkers designed metal-tuneable M4L6 tetrahedra. For the synthesis of these cages the group uses two different Cj-symmetric facial ligands. Both could be synthesised via a Schiff base reaction. The first is made by the... 

The tricobalt clusters of the type (CpCo)3(/t3-arene), 435, could only be synthesized with alkenylbenzenes as the facial ligands (see the mechanism of synthesis above). The exocyclic C=C bond did however play no role in the stabilization of these complexes. Indeed, the catalytic hydrogenation of this C=C unit in the side chain was achieved and the corresponding reduced compounds were perfectly stable as was evidenced by their crystal structure analyses, which showed a very similar geometry for the new complexes. In solution, hindered rotation of the /t3-arene was observed. ... 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

Additionally, MO calculations indicate the lowest energy orientation occurs with the three strongest trans-influence ligands (two hydrides and a PPh3) in a facial configuration. Calculations on compounds IrX(CO)(PR3)2 indicates that weak donors X and strong 7r-acceptors PR3 favour addition in the XIrCO plane [134, 135],... 

A certain jr-facial selectivity was achieved when MCpCl2 (M = Ti, Zr) fragments were coordinated to the optically active fused cyclopentadienyl ligands. For instance, reaction of ZrCpCl3 with the lithium derivative of 126 at —78 °C gave predominantly 133 which was characterized by X-ray structural analysis [152]. 

The problem of tr-facial differentiation, i.e. diastereomer formation, encountered in the metal complexation of the above mentioned annulated cyclopentadienyl ligands is avoided when C2-symmetrical ligands [153] are utilized. Since in such ligands both sides of the five-membered rings are homotopic, only one isomer is... 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

The presence of substituents on the bidentate ligands often degenerate their local C2-symmetry and, as a consequence, further isomerism occurs. This results in trisbidentate derivatives of type P(ab)3 in the presence of facial or meridional isomers, as depicted in Fig. 17 [102]. Usually, the meridional isomer is preferred over the facial and a statistical 3 1 ratio is observed. Compounds 6,9,12 and 30... 

More recently, these authors have reported the synthesis of a new thiophene-based analogue of (I ,i )-Me-DuPHOS called UlluPHOS. The facial recognition and enantioselection associated with ruthenium complexes of UlluPHOS and Me-DuPHOS were shown to be similarly high in various hydrogenations of p-keto esters (Scheme 8.32). The most important difference between these two ligands was found by comparing the reaction rates. Indeed, the authors have observed that the use of UlluPHOS considerably increased the activity of the complexes. 

The Cu-BOX catalysts function as Lewis acids at the carbonyl oxygen. The chiral ligands promote facial selectivity, as shown in Figure 2.3. 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

The structures of the TSs have been explored computationally using combined B3LYP-MM methods.150 There are four stereochemically distinct TSs, as shown in Figure 7.4. For the aminoalcohol ligands, the anti-trans arrangement is preferred. Steric factors destabilize the other TSs. The substituents on the ligand determine the facial selectivity of the aldehydes. 

Multidentate thioethers are readily introduced into the coordination sphere of low-valent cobalt. As an example, the tridentate thioethers 1,3,5-trithiacyclohexane and tris(methylthio)methane both replace three facially arranged carbonyl ligands in Co3(CO)9(/i-CPh), leaving the cluster otherwise intact.169... 

Reaction of [Ir(coe)(N(SiMe3CH2PPh2)2] with excess 1,3-butadiene yields the structurally characterized, five-coordinate complex [Ir(C4H6)[N(SiMe2CH2PPh2)2], which contains the P2N ligand coordinated in a quasi-facial manner, and the 1,3-butadiene bound in a s-cis-ri4-tt mode.691... 

---