Ring Chemistry, Transition Metal Templates, and Stereoelectronics

A set of vocabulary flashcards covering transition metal and organic templates, the 2016 Nobel Prize in Chemistry, and the stereoelectronics and steric effects governing cyclohexane and epoxide reactivity.

Template effect (Transition Metal Templates)

The use of metal centers, such as $Ni^0$ and $Ti^0$, to form $\pi$-allyl coordination complexes that hold the two ends of a carbon chain in close proximity to facilitate the synthesis of medium and large rings.

Ni(II) Template Effect

Used in diimine formation to hold two $S$ atoms in a favorable geometry to form a $14$-membered ring; without the metal, Thiazolidine is the major product.

Catenane

A mechanically interlocked molecular architecture consisting of two ring-shaped molecules linked together like the links of a chain.

2016 Nobel Prize in Chemistry Recipients

Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa, awarded for the design and production of molecular machines.

Equatorial Preference

The conformational tendency in cyclohexanes where substituents in equatorial positions are lower in energy than those in axial positions, influencing reactions that proceed through enolate intermediates.

$S_N2$ reaction stereochemistry in rings

Reactions that proceed with inversion of stereochemistry, resulting in an axial leaving group being replaced by an equatorial nucleophile.

Relative reactivity of axial vs. equatorial leaving groups

Attack at axial leaving groups is typically faster (e.g., $31$ times faster) because the nucleophile approach along the $C-X \sigma^*$ orbital is less hindered than for equatorial substituents.

Steric hindrance in axial ester hydrolysis

A phenomenon where the activation energy barrier for hydrolysis is higher for axial compounds (by a factor of $20$) because steric bulk increases at the ester during the rate-determining transition state.

Steric assistance in chromate oxidation

The effect where an axial chromate ester intermediate is less stable and fragments approximately $3.2$ times more quickly than its equatorial diastereoisomer.

$mCPBA$ Epoxidation of cyclopentenes

A stereocontrolled addition of a peracid across a double bond that occurs on the least hindered face of the ring.

Intramolecular Epoxidation Geometry

A requirement where the hydroxyl and bromide groups must be trans-antiplanar and both in axial positions to allow the oxygen lone pair to attack the $C-Br \sigma^*$ orbital.

Cyclohexene oxide conformation

A half-chair structure similar to cyclohexene, where the hydrogens adjacent to the epoxide ring are described as pseudoaxial and pseudoequatorial.

Stereochemistry of epoxide opening

The ring opening of cyclohexane oxides proceeds through a transition state that initially produces a diaxial product, which may then flip to a diequatorial conformation.

Conformationally locked epoxide opening

The preference for nucleophilic attack at a position that results directly in chair formation rather than a high-energy twist boat, regardless of the final stability of the product.