Organometallic Compounds

1. Dr Seemal jelani OrganometallicCompounds

2. Dr Seemal jelani

3. Dr Seemal jelani Organometallic Compounds • An organic compound containing a carbon–metal bond • Organolithium compounds and Organomagnesium are two of the most common organometallic compounds

4. A. Organometallic Mechanisms Dr Seemal jelani • Oxidation State: The oxidation state of a metal is defined as the charge left on the metal after all ligands have been removed in their natural, closed-shell configuration • This is a formalism and not a physical property! • Formalism is the study by analyzing and comparing form and style—the way objects are made and their purely visual aspects • d-Electron Configuration: position in the periodic table minus oxidation state.

5. The d electron count is a Chemistry formalism used to describe the electron configuration of the valence electrons of a transitions metal center in a Coordination complex • The d electron count is an effective way to understand the geometry and reactivity of transition metal complexes • The formalism has been incorporated into the two major models used to describe coordination complexes; • Crystal field theory and ligand field theory Dr Seemal jelani

6. Dr Seemal jelani 18-Electron Rule • A stable complex (with electron configuration of the next highest noble gas) is obtained when the sum of the metal d-electrons, electron donated from the ligand, and the overall charge of the complex equals 18 • In mononuclear, diamagnetic complexes, the total number of electrons never exceeds 18 (noble gas configuration) • The total number of electrons is equal to the sum of d-electrons plus those contributed by the ligands. • 18 electrons = coordinatively saturated • < 18 electrons = coordinatively unsaturated.

7. Dr Seemal jelani Diamagnetic • A diamagnetic compound has all electron spins paired, giving a net spin of zero. They are also weakly repelled by a magnet. • Descriptive term which indicates that a substance contains no unpaired Electrons and thus is not attracted to a magnetic field • NH3 is diamagnetic because all of the electrons in NH3 are paired.

8. Dr Seemal jelani 18-Electron rule • This was first proposed by N.V. Sigwick who extended the octet theory of G.N. Lewis and using it to explain coordination compounds • Ligands were considered Lewis bases and the metal ion in turn acted as a Lewis acid • The sum of all the electrons would assume a noble gas configuration of Kr, Xe or Rn. • As with most rules of the thumb, the 18 electron rule is not always obeyed • 18 electrons are required to fill 5 d-orbitals, 1 s-orbital and 3 p-orbitals of a transition metal and hence reach the configuration of a noble gas.

9. Dr Seemal jelani Organometallic Reagents The Key Concepts: Make a carbon negatively charged/polarlized so it is nucleophilic. Reaction with electrophilic carbons can make carbon-carbon bonds.

10. The First Organometallic Reagents… Grignard Reagents Dr Seemal jelani • Discovered by Victor Grignard in 1900 • Key factors are ethereal solvent and water-free conditions • Awarded Nobel Prize in 1912 Victor Grignard Grignard, Victor , 1871–1935, French chemist. He shared the 1912 Nobel Prize in Chemistry for his work in organic synthesis based on his discovery (1900) of the Grignard Reagent. He taught at the Univ. of Nancy (1909–19) and at the Univ. of Lyons (from 1919 until the end of his career).

11. M g B r B r M g Butylmagnesium bromide (an alkyl Grignard reagent) B r M g M g B r Phenylmagnesium bromide (an aryl Grignard reagent) Dr Seemal jelani Grignard Reagents • An Organomagnesium compound prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF ether + 1-Bromobutane ether + Bromobenzene

12. An Alternative to Grignard Reagents are Alkyl Lithiums Dr Seemal jelani Both are prepared from alkyl, vinyl, and aryl halides under anhydrous conditions

13. Dr Seemal jelani Grignard and Organolithium Reagents • Given the difference in electronegativity between carbon and magnesium (lithium), the C-Mg (C-Li) bond is polar covalent, with C- and Mg+ (Li+ ) • Grignard and Organolithium reagents behave like carbanions • Carbanions: an anion in which carbon has an unshared pair of electrons and bears a negative charge

14. Dr Seemal jelani Grignard and Organolithium Reagents • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge • Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, amides, carboxylic acids, even terminal alkynes). A limitation to utility!

15. Dr Seemal jelani • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge • Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, carboxylic acids, amides, even terminal alkynes). A limitation to utility!

16. Dr Seemal jelani Limitations Can’t make Grignards with acidic or electrophilic functional groups present in the molecule: R2NH pKa 38-40 Terminal Alkynes pKa 25 ROH pKa 16-18 Carbonyls & Nitros pKa 11-27

17. Dr Seemal jelani • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge • Carbanions are also great nucleophiles • This is the reason for their great utility!

18. Dr Seemal jelani • Key Point: • Great nucleophiles that add efficiently to electrophilic carbons, such as epoxides and carbonyl group of aldehydes, ketones and esters • However, their basicity can be a limitation! • Epoxides illustrate how many common organic functional groups contain electrophilic carbons

19. Dr Seemal jelani • Carbanions (nucleophiles) can react with electrophilic carbon centers in favorable cases • The net result is a carbon-carbon • Grignards and Organolithium reagents react with many oxygen-containing electrophiles, but not with alkyl halides.

20. Grignard reagents react productively with: Dr Seemal jelani formaldehyde to give primary alcohols aldehydesto give secondary alcohols ketonesto give tertiary alcohols esters to give tertiary alcohols CO2to give acids epoxides to give primary alcohols The one we are choosing for the sake of initial illustration

21. Epoxides: The Example We Want to Stress Dr Seemal jelani New C-C bond d+ d- Considered Retrosyn- thetically: These is an extremely valuable reaction…

22. Back to Work: A Related Example Detailed Mechanism Highlighting Retention of Stereochemistry Dr Seemal jelani New C-C bond • Key Points to Note: • Attack at least hindered carbon • Mechanism is SN2-like in initial step • Single enantiomeric product • Chiral center not affected by reaction • Relief of ring strain helps drive reaction • H3O+ breaks up initial salt

23. Two More Examples of Additions to Epoxides Dr Seemal jelani New C-C bond New C-C bond Note: Stereochemistry at epoxide retained in product

24. Dr Seemal jelani Gilman Reagents • Lithium Diorganocopper reagents, known more commonly as Gilman reagents • prepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodide

25. Dr Seemal jelani Gilman Reagents • Coupling with Organohalogen compounds • form new carbon-carbon bonds by coupling with alkyl and alkenyl chlorides, bromides, and iodides. (Note that this doesn’t work with Grignard or Organolithium reagents • They are too basic and do E2 eliminations.) • Example New C-C bond

26. Dr Seemal jelani Gilman Reagents • Coupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retained New C-C bond

27. Dr Seemal jelani Gilman Reagents • A variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of a copper(I) salt

28. Dr Seemal jelani Gilman Reagents New C-C bond • Reaction with epoxides • regioselective ring opening (attack at least hindered carbon)

29. Dr Seemal jelani Interim Summary of Introduction to Organometallic Reagents… Organolithium reagents and Grignard reagents are very basic but also great nucleophiles They react with epoxides at the less hindered site to give a two-carbon chain extended alcohol They do not couple with alkyl-, aryl-, or vinyl halides. Gilman reagents react with epoxides as do Organolithium reagents and Grignard reagents. However, they also add to alkyl-, aryl-, and vinyl halides to make new C-C bonds.

30. Meanwhile…. Dr Seemal jelani Back to Grignard Reagents… • Addition of a Grignard reagent to formaldehyde followed by H3O+ gives a 1° alcohol • This sequence (mechanism) is general and important!

31. Grignard Reactions Dr Seemal jelani These are valuable and important reactions…

32. •• diethylether OCH3 •• •• OCH3 R C R •• O MgX •• •• •• Dr Seemal jelani R' R' Grignard reagents react with esters d+ d– C + but species formed is unstable and dissociates under the reaction conditions to form a ketone MgX O •• – ••

33. •• diethylether OCH3 •• •• OCH3 R C R •• O MgX •• •• •• R R' C O Dr Seemal jelani R' R' Grignard reagents react with esters d+ d– C + this ketone then goes on to react with a second mole of the Grignard reagent to give a tertiary alcohol MgX O •• – •• –CH3OMgX •• ••

34. O Dr Seemal jelani + (CH3)2CHCOCH3 2 CH3MgBr Example 1. diethyl ether 2. H3O+ Two of the groups attached to the tertiary carbon come from the Grignard reagent OH (CH3)2CHCCH3 CH3 (73%)

35. Practice Dr Seemal jelani

36. Practice Dr Seemal jelani

37. The Same Chemistry is seen With Organolithium Reagents Dr Seemal jelani O + CH H2C CHLi 1. diethyl ether 2. H3O+ CHCH CH2 (76%) OH

38. Practice Dr Seemal jelani