Converting Alcohols into Leaving Groups - Sulfonate Esters_default

Introduction to Substitution Elimination Reactions

  • This video is part of a series discussing the role of alcohols in substitution and elimination reactions

  • Key transformations: SN1, SN2, E1, E2

  • Aim: to delve deeper into the characteristics of alcohols in these reactions

Characteristics of Molecules in Substitution Elimination

  • Essential for substitution elimination reactions:

    • Presence of a good leaving group

  • Alcohols (OH group) are generally not effective leaving groups

    • Poor stability of OH- as compared to better leaving groups like Br-

    • OH- is a decent base and nucleophile but reluctant to leave

Converting Hydroxyl Groups to Good Leaving Groups

  • Need to convert the OH group into a better leaving group

  • Two previously discussed strategies:

    1. Formation of Sulfonate Esters

    • Common types:

      • Tosylate (OTs): para-toluenesulfonyl chloride

      • Mesylate (OMs): methylsulfonyl chloride

      • Triflate (OTf): trifluoromethylsulfonyl chloride

  • Mechanism of Formation of Tosylate

    • Alcohol reacts with tosyl chloride and pyridine

      • Example: Alcohol + TsCl + pyridine → Tosylate

  • Sulfonate esters mimic the behavior of alkyl halides

Example Transformation Using Tosylate

  • Procedure: Add sodium bromide in DMSO to transform tosylate

  • Expected reaction: Substitution reaction where bromide replaces tosylate

  • Process:

    1. React alcohol with tosyl chloride to form tosylate

    2. Add nucleophile/base to facilitate substitution/elimination

Mechanism Overview

  • Initial substrate: Alcohol + Tosyl chloride (TsCl)

  • Reaction sequence:

    • Nucleophilic Attack: OH attacks sulfur, resulting in displacement of chloride

    • Pyridine acts as a base, deprotonating the intermediate

    • Intermediate maintains original stereochemistry

Key Notes on Stereochemistry

  • Stereochemistry remains unchanged during the tosylate formation

  • The nature of the reaction does not affect the chiral center's configuration

Other Sulfonate Esters

  • Similar reactivity for mesylates (OMs) and triflates (OTf) as tosylates (OTs)

  • Useful for various transformations in organic synthesis

Conclusion

  • Understanding how to convert alcohols to good leaving groups is crucial in organic chemistry

  • Mechanisms of SN2 reactions illustrate backside attacks leading to stereochemical inversion

  • Utilizing tosylates, mesylates, and triflates allows for effective substitution and elimination reactions from alcohols.