Cycloaddition of carbonyl compounds on Si(100): new mechanisms and approaches to selectivity for surface cycloaddition reactions

Density functional theory has been used to explore cycloaddition reactions of organic molecules containing carbonyl functional groups on the Si(100) surface. As with other pi bonds, carbonyl groups can add to the surface by a [2+2] cycloaddition with negligible activation barrier, as previously shown through experiment. However, the present calculations indicate that 1,2-dicarbonyls, such as glyoxal, may also react by means of a [4+2] addition to form a hetero-Diels-Alder product in which the organic ring stands normal to the surface. Calculations of [2+2] and [4+2] pathways indicate that both reactions proceed without significant barriers. This reactivity is analogous to that of conjugated dienes, in which evidence for both reactions has been observed. In contrast to unsaturated alkyl systems, which must react through the pi electron system, the reactions of carbonyls may proceed through a very different mechanism, in which the initial surface interaction is through the oxygen lone pair. The presence of lone pairs affects the geometry of the [4+2] adduct, and may alter the competition between [2+2] and [4+2] addition. Some potential rearrangement reactions of the initial binding products are described. Recent experimental studies of a 1,2-dicarbonyl on Si(100) are reinterpreted in light of these calculations, and found to be consistent with the presence of the [4+2] adduct. Finally, some molecules are suggested as cycloaddition reagents for experimental tests of the conclusions presented here.