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Paul Kiprof, Associate Fellow

Silen Cycloaddition Reactions

Cycloaddition reactions with carbon compounds, which find use in organic synthesis, have been widely studied. [2+2] cycloaddition reactions, including carbon carbon multiple bonds, are thermally forbidden but are photochemically allowed. Nevertheless, there are several examples of thermal [2+2] cycloadditions. These involve radical or dipolar intermediates. Only recently, an ab initio molecular orbital study of ring opening reactions of four membered carbon rings has been carried out. A major breakthrough was achieved when a diradial intermediate for the ring opening of cyclobutane could been observed directly. In contrast, cycloaddition reactions of the analogous silicon systems were only recently discovered.

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Density functional methods confirmed the transition states at the HF level for both cases. MP2 level and CASSCF calculations confirmed the transition state for the acetylene case. The silacyclopropyl pathway revealed some interesting intermediates similar to the intermediates observed for the all carbon case. There seem to be slight differences in the stability of these intermediates. The search of transition states derived from the silacyclopropyl intermediate led to a new transition state that has a skewed alignment of the acetylene with respect to the silene. This transition state connects the reactants directly to the products. This leads to several possible pathways of the reaction, which are currently being worked to completion.

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