Reconstructive cycloaddition

Cyclobutenediones 148 possessing an alkoxy substituent reacted with norbornene 40 in quite a different way, in which CO molecule was extmded from the dione substrates (Scheme 4.52) [103]. In the presence of catalytic amounts of Ru3(CO)i2 and PEt3, 148 and 40 was heated at 160 °C under 3 atm CO to regioselectively afford cyclobutenones 149. Such a novel reconstructive cycloaddition was further extended to the carbonylative dimerization of a cyclopropenone 150 resulting in the formation of a pyranopyrandione 151 in high yields [104], These novel transformations of... 

Diels-Alder reactions are allowed by orbital symmetry in the delocalization band and so expected to occur on the surface. In fact, [4-1-2] cycloaddition reaction occurs on the clean diamond (100)-2 x 1 surface, where the surface dimer acts as a dienophile. The surface product was found to be stable up to approximately 1,000 K [59, 60], 1,3-Butadiene attains high coverage as well as forms a thermally stable adlayer on reconstructed diamond (100)-2 x 1 surface due to its ability to undergo [4h-2] cycloaddition [61],... 

In addition to forming dimers with double-bond-like character that can participate in cycloaddition-type reactions, the reconstruction of the (100) semiconductor surface is also responsible for the presence of electrophilic and nucleophilic sites. The bond of a tilted dimer deviates from the plane of the surface, and the resulting structure of each dimer consists of an up dimer atom protruding from the surface and a down dimer atom recessed on the surface. The electron density at the up atom of the dimer is higher than that at the down atom, leading to nucleophilic and electrophilic behavior, respectively [49]. 

It is clear that at the single-dimer site, the most stable adsorption configuration is S . The chemisorption process takes advantage of two cycloaddition reactions to support the adhesion of fullerene on the c(4 x 4) reconstructed GaAs(OOl) surface. Nevertheless, due to the initial structural alignment prior to the energy minimization,... 

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