SN 2 versus E2 Competition of Cyclic Ethers

We have quantum chemically studied the influence of ring strain on the competition between the two mechanistically different SN 2 and E2 pathways using a series of archetypal ethers as substrate in combination with a diverse set of Lewis bases (F- , Cl- , Br- , HO- , H3 CO- , HS- , H3 CS- ), using relativistic density functional theory at ZORA-OLYP/QZ4P. The ring strain in the substrate is systematically increased on going from a model acyclic ether to a 6- to 5- to 4- to 3-membered ether ring. We have found that the activation energy of the SN 2 pathway sharply decreases when the ring strain of the system is increased, thus on going from large to small cyclic ethers, the SN 2 reactivity increases. In contrast, the activation energy of the E2 pathway generally rises along this same series, that is, from large to small cyclic ethers. The opposing reactivity trends induce a mechanistic switch in the preferred reaction pathway for strong Lewis bases from E2, for large cyclic substrates, to SN 2, for small cyclic substrates. Weak Lewis bases are unable to overcome the higher intrinsic distortivity of the E2 pathway and, therefore, always favor the less distortive SN 2 reaction.

(E)-Di-iodination of Alkynes Using Dried Dowex H+/NaI Approach

We investigated the usefulness of the dried Dowex H⁺/NaI approach for the selective di-iodination of alkynes. The Dowex H⁺/NaI approach selectively produces only (E)-di-iodinated products; it is very straightforward and nontoxic. The utilization of 2-propanol as a solvent in the reactions can be considered as a "green" approach and the method maybe extended to radio-iodination. The method allows access to highly important building blocks. An initial example of the di-iodination and esterification in the same one-pot reaction is also presented.

Green and Efficient Esterification Method Using Dried Dowex H+/NaI Approach

The usefulness of dried Dowex H+ cation-exchange resin with or without sodium iodide (NaI) as a catalyst system for different kinds of esterifications using carboxylic acids and alcohols as starting materials has been systematically investigated. The Dowex H+/NaI approach is very effective, generally high yielding, energy-efficient, and nontoxic, and the Dowex H+ resin is reusable. Since the whole procedure from start to product isolation is also very simple, these features make the method environmentally friendly. The method is regioselective, and its potential for separation of valuable carboxylic acids like resin acids from mixtures containing other kinds of carboxylic acids has been demonstrated. Examples for green and straightforward esterification of highly important natural amino acids are also presented.