Anchoring effect

Some olefinic molecules have a second function that anchors itself on the catalyst surface in such a way so as to enforce addition of hydrogen to its own side of the molecule. This anchoring effect, dubbed haptophilicity 112,113), has been observed by many investigators 0,19,74,78,90,120). An example of Gula and Spencer 47) illustrates how the anchoring tendencies of a function remote from the point of saturation may influence the stereochemistry. 

The anchoring effect, which is also sometimes called the haptophilic effect, results from the fact that certain groupings increase the strength of adsorption of... 

On the other hand, hydrogenation with Raney nickel causes reduction mainly in the substituted aromatic ring [Eq. (11.16)]. Differences in product composition brought about by the different metals are explained in terms of steric hindrance of the substituted ring (Pt, Pd) versus the anchor effect of the methyl substituent (Ni).106... 

Platinum-based bi-metallics (Pt M, M = Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the OER. Several rationales have been proposed including (1) enhanced chemisorption of intermediates (2) a lattice change of Pt that results in the shortening of Pt-Pt interatomic distances by alloying (3) the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer caused by the underlying alloy and the anchor effect of alloy metals on a carbon carrier.93,94... 

Now, both the utility and the versatility of our method are demonstrated in the stereoselective synthesis of natural (+)-valienamine (25) and (+)-validamine (29) (7/) (Scheme 3). Furthermore, the anchor effect of an amino group will be described in the stereoselective hydrogenation of the olefin of25 to give 29 (Fig. 1). 

Fig. 1. Conformations of key intermediates and the anchor effect of the amino group in 27 over Raney Ni.

Accordingly, the quasi-squatorial amino group of 24 could not participate in the anchor effect for stereoselective hydrogenation. 

As a mixture of dioxane and H2O was used for catalytic hydrogenation, the H-NMR spectra of 24 and 27 were measured in dioxane-rfg and D2O to support that existed in the half-chair form with theqwasz-axial amino group (Fig. 1). The gwosz -axial amino group of 27 was expected to assist the anchor effect giving the desired, 5-tram isomer 28. 

Metal loading and sample preparation conditions are critical for the anchoring effect. This might account for the different observations on PdFeNaY reported by Moller and Bein (85). 

The location of hydroxy groups relative to the carbon-carbon double bond appears at times to direct the addition to the side of the molecule which presents the greater catalyst hindrance. ITiis has been noted by Dart and Henbest, " who termed it an anchor effect , and later by Nishimura and Mori for the hydrogenation of several steroid structures. ... 

A hydroxy group situated on an allylic carbon of a five- or a six-membered ring can exhibit an anchoring effect on a Ni catalyst (equation 22) with other metals the group s steric effect is dominant. The... 

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