For example, a [2+1] cycloaddition involves a 3-membered ring transition state where two atoms of the ring come from the alkene substrate (HOMO, nucleophile) and one atom of the ring comes from the el...For example, a [2+1] cycloaddition involves a 3-membered ring transition state where two atoms of the ring come from the alkene substrate (HOMO, nucleophile) and one atom of the ring comes from the electrophile. Because of the 5-membered ring transition state, the delivery of the two alcohol functional groups occurs from the same face of the alkene.
In the mechanism of this reaction, the πC−C bond is the HOMO, which reacts with the σ∗H−X to become protonated and generate a carbocationic intermediate that is the most stable. The r...In the mechanism of this reaction, the πC−C bond is the HOMO, which reacts with the σ∗H−X to become protonated and generate a carbocationic intermediate that is the most stable. The reaction is stereospecific since hydrogen is added in a concerted fashion from the same face of the alkene, without any opportunity for bond rotation. In the example above, we also get a stereoselective reaction since the bridgehead carbon is hindering attack of one face of the πC−C bond.
