Introduction
The Anti-Markovnikov process in organic chemistry generally involves addition reaction method of an electrophilic substituent, HX to an alkene or alkyne compounds, in such a way that the Hydrogen atom in the HX electrophile are chemically bonded to the carbon atom containing the least amount of Hydrogen attached to it. This addition process commonly uses catalysts or additive compounds in order to hasten the reaction since the process is quite expensive and toxic in nature. Furthermore, there is also a limited number of reactions involved in using the Anti-Markovnikov addition, making its research more difficult to expand and develop.
Anti-Markovnikov Addition of Alkoxides to Alkynes
One of the recent research studies of Cuthbertson & Wilden’s on Anti-Markovnikov method involves the addition of potassium methoxide in a 49% phenylacetylene for a 15-hour reaction process using DMF as its primary solvent, forming a cis isomer product of an enol ether compound (2015). According to the article, there are two major reasons that can be noted for the formation of an isomeric ether. First, the use of potassium methoxide as a source of an electrophile in an Anti-Markovnikov Addition generally behaves as a constituent of a radical, single electron transfer, providing its effectiveness of the reaction when coupled with a secondary amine ligand, the N,N’-dimethylethylenediamine (DMEDA) (Cuthbertson & Wilden, 2015). Second, the rate of reaction for the addition of alkoxide to an aryl-substituted alkyne also depends on the electronic properties and components of the arene, whereas those having an electron withdrawing substituents generally creates an even faster reaction as compared to those who do not have electron withdrawing substituent. Based from the experiment, different substrates were considered in order to test the different factors that can affect the reaction rate of the Anti-Markovnikov addition as well as to examine the different properties of alkoxides as compounds for enhancing the electron transfer method in the reaction.
Obtained results from the experiment showed higher product yield and reaction efficiency of the Anti-Markovnikov addition and Z-selectivity process. The introduction of transition metals as well as the ligand DMEDA provided a much higher yield of twenty percent or better (Cuthbertson & Wilden, 2015). Hence, the ligand that was used in the experiment is not only capable of enhancing the solubility characteristics of the potassium alkoxide but also improves the overall reaction rate efficiency of the process. These evident characteristics were the basis for the reaction mechanism of Anti-Markovnikov addition to create a primary reference for a single electron transfer pathway. Seemingly, a two-step outlined mechanism for electron transfer on the addition of potassium alkoxide to phenylacetylene with a ligand additive is as follows: a) A single electron transfer from the alkoxide compound to an alkyne, producing a radical vinyl anion intermediate; and b) Formation of different Z geometries of the product through the dissolution of metal substituents present in the intermediate product (Cuthbertson & Wilden, 2015). The formation of a discrete vinyl anion intermediate in electron transfer is also dependent on the electron withdrawing capability of the aryl ring present in phenylacetylene. Another notable result from the experiment showed an inverse correlation between the kind of aryl substituent present in the alkyne to that of the Z:E ratio of the product. According to Cuthbertson & Wilden (2015), as more aryl substituents of the material become more electron deficient, the formation if cis isomers throughout the reaction increases. In that way, the Z isomers undergo a certain inversion process under basic condition reaction.
As a conclusion, the mechanism of Anti-Markovnikov addition using alkoxide and alkyne compounds provided a glimpse as to how they can achieve an initial electron transfer through the formation of a radical vinyl anion intermediate using an additive DMEDA ligand. Seemingly, the formation of enol ether through the transformation of alkoxides using secondary diamines were also investigated to determine the reaction mechanism pathway as well as evaluating the stereochemistry of the products and intermediates formed in the reaction.
Reference
Cuthbertson, J. & J. D. Wilden. (2015). “Z-selective, anti-Markovnikov addition of alkoxides to terminal alkynes: an electron transfer pathway?” pp. 4385-4392. Tetrahedron, 71.
