ChEM 210 EXAM 4

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Carbanions as Carbon Nucleophiles Carbanions as Carbon Nucleophiles: Synthesis and Stability: Carbanions: carbon-centered nucleophiles - carbon atoms of any formal hybridization can bear a negative... charge - carbon acids can be deprotonated to give carbanions, but typically with difficulty as they have very high pKa values 1. How can carbanions be made? 2. What structural features promote/discourage carbanion stability? 3. What is the scope of carbon-carbon bond formation via combination of carbanion nucleophiles with alkyl halide and epoxide carbon electrophiles? Carbanion Synthesis: Hydrocarbon Deprotonation and Halocarbon Reduction - The first common method of carbanion preparation is by deprotonation. - Based on the pKa values cited above, hydrocarbon deprotonation by typical bases (i.e., hydroxide, OH– , pKa = 16) is quite ineffective at generating a useful amount of carbanion, 1 + 2 → 3 - Even for the most acidic hydrocarbon species shown in below, the acetylene 5, hydroxide would only provide vanishingly small quantities of the acetylide carbanion - If hydroxide is not a strong enough base, perhaps the solution is no more complicated than identifying a stronger base to accomplish this desired deprotonation - Hydrocarbons define the high end of the pKa scale - Use an alkyl carbanion, if available by some means, as the “superbase” - This simple proton transfer reaction presents a couple of subtleties. First of all, it presupposes that alkyl lithiums (“superbases”) are somehow available, even if not by a deprotonation reaction This study source was downloaded by 100000831777157 from CourseHero.com on 07-12-2022 03:43:28 GMT -05:00 https://www.coursehero.com/file/12665989/ChEM-210-EXAM-4/ - All solution-phase carbanions must be accompanied by a positively charged counterion - The type of bond between the carbanion and a metal counterion will vary along a covalent/ionic continuum as per the metal and the carbanion - Solvation of a metal counterion plays a crucial role in determining the carbanion’s stability (and ease of synthesis) - Assume that any carbanion synthesis/reaction we discuss will occur in a solvent (typically an ether solvent) that is both compatible and beneficial for the chemistry, and so we will not explicitly denote solvent Grignard Reaction: is an organometallic chemical reaction in which alkyl, vinyl, or aryl-magnesium halides (Grignard reagents) add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon–carbon bonds. Grignard Reagents: alkyl, vinyl, or aryl-magnesium halides Organolithiums: are organometallic compounds that contain carbon – lithium bonds. They are important reagents in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation - The second common method of carbanion generation is via halocarbon reduction - Grignard and lithium reagents are formed readily from the corresponding halide, with I > Br > Cl defining the facility of formation - Combination of the organic halide with either metallic magnesium (→ Grignard) or metallic lithium (→ organolithium) in an inert ether solvent, often with the aid of an “activator” to scour the metal surface, will lead to a facile oxidation–reduction r [Show More]

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