The Cieplak effect relies on the stabilizing interaction of mixing full and empty orbitals to delocalize electrons, known as hyperconjugation. When the highest occupied molecular orbital (HOMO) of one system and the lowest unoccupied molecular orbital (LUMO) of another system have comparable energies and spatial overlap, the electrons can delocalize and sink into a lower energy level. Often, the HOMO of a system is a full σ (bonding) orbital and the LUMO is an empty σ* (antibonding) orbital. This mixing is a stabilizing interaction and has been widely used to explain such phenomena as the anomeric effect. A common requirement of hyperconjugation is that the bonds donating and accepting electron density are antiperiplanar to each other, to allow for maximum orbital overlap.
The Cieplak effect uses hyperconjugation to explain the face-selective addition of nucleophiles to carbonyl carbons. Specifically, donation into the low-lying σ* bond by antiperiplanar electron-donating substituents is the stabilizing interaction which lowers the transition state energy of one stereospecific reaction pathway and thus increases the rate of attack from one side. In the simplest model, a conformationally constrained cyclohexanone is reduced to the corresponding alcohol. Reducing agents add a hydride to the carbonyl carbon via attack along the Burgi–Dunitz angle, which can come from the top along a pseudo-axial trajectory or from below, along a pseudo-equatorial trajectory. It has long been known that large reducing agents add hydride to the equatorial position to avoid steric interactions with axial hydrogens on the ring. Small hydride sources, however, add hydride to an axial position for reasons which are still disputed.
The Cieplak effect explains this phenomenon by postulating that hyperconjugation of the forming σ* orbital with geometrically aligned σ orbitals is the stabilizing interaction that controls stereoselectivity. In an equatorial approach, the bonds that are geometrically aligned antiperiplanar to the forming C–H bond are the C–C bonds of the ring, so they donate electron density to σ*. In an axial approach, the neighboring axial C–H bonds are aligned antiperiplanar to the forming C–H bond, so they donate electron density to σ*. Because C-H bonds are better electron donors than C–C bonds, they are better able to participate in this stabilizing interaction and so this pathway is favored.