Steering-dominated reactions

The variation of the PES with surface site and molecular orientation implies that the molecules experience forces directing them into particular orientations at particular surface sites, i.e. the molecules 

---

Figure A3.9.11. Dissociation of H2 on the W(100)-c(2 x 2)-Cu surface as a function of incident energy [71]. The steering dominated reaction [102] is evident at low energy, confmned by the absence of a significant surface temperature.

This disappointing result is unfortunately quite neral. Secondary amines react with crown ether acid chlorides to give N-alkyl amides, but the slower reaction with crown ether anhydrides results in much poorer yields and significant competing epimerization via deprotonation-ieprotonation at the carbon adjacent to the carboxylic acid. The monoamide-triadd (above R-H) can be prepared from the corresponding primary amino-alcohol. The more reactive amine steers the dominant reaction towards amide formation without epimerization. 

During nearly forty years of study, the photocycloaddition of 2-pyri-dones has consistently exhibited versatile and reliable [4+4] reactivity. When dimerization is not desired, intramolecular reactions efficiently steer two pyridones to react. Alternatively, intermolecular cross reaction with an excess of another four-electron reactant can be extended to other heterocycles and to simple 1,3-dienes. It is perhaps surprising how few photoreactions of 2-pyridone fail to yield [4+4] products. Failure of the [4+4] reaction can result from a tethering unit that prevents the normal head-to-tail reactivity and introduces strain (Figures 14 and 22) or from steric hindrance caused by excessive substitution at the reacting carbons (Figure 23). A 4-alkoxy substrate shuts down the normally dominant photodimerization reaction and provides opportunities for other [4+-2] or [4+4] reactions (Figures 6 and 7). 

---