Chapter 17, Problem 17.18P

Interpretation Introduction

Interpretation:

The complete, detailed mechanism for the given reaction is to be drawn, and the major organic product is to be predicted, paying attention to stereochemistry.

Concept introduction:

Wittig reagents are the compounds that are used to synthesize alkenes. A Wittig reagent is characterized by a $\text{C-P}$ bond in which the carbon bears a $\text{-1}$ formal charge and phosphorous bears a $\text{+1}$ formal charge. This reagent is also called phosphorous ylide, which is highly nucleophilic at the carbon atom. Wittig reagent reacts with aldehydes and ketones to form alkenes. Wittig reaction results in the joining of two carbon-containing groups by a $\text{C=C}$ bond, where one group is from the Wittig reagent and the second is from the ketone or aldehyde. Thus, a new $\text{C=C}$ is formed between the original carbonyl carbon and the original $\text{C-P}$ carbon from ylide. Thus, the $\text{C=O}$ bond of a ketone or aldehyde is converted into a $\text{C=C}$ bond. The mixture of stereoisomers is formed at the end of this reaction as the major organic product. In step one of this mechanism, the highly nucleophilic carbon atom of the Wittig reagent attacks the electrophilic carbonyl carbon, yielding a betaine. Step two is a coordination step, in which a bond is formed between the negatively charged oxygen atom and the positively charged phosphorous atom. This results in an oxaphosphetane that contains a four-membered ring, which, due to the strain, falls apart into the alkene and triphenylphosphine. The first two steps of the Wittig reaction are driven by the principle of electron-rich and electron-poor sites.