Organometallic Chemistry

Organometallic Compounds Compounds that contain a Metal-Carbon bond. Metal Alkyls, e.g. Tetraethyllead - Leaded Gas Alkyl = methyl, ethyl, tert-butyl, etc. Metal = Li, Na, K (alkali metals) Mg (alkaline earth metals) Ti Cr Mn Fe Co Ni Cu Zn Zr Ru Pd Hg OS Pt

Metal Alkyls General formula R-M (R = alkyl, M = metal) The C-M bond is a covalent bond! However, C--M+ or R--M+ bond tend to be polarized. This is especially true for organometallic compounds conataining the more electropositive metals, i.e. alkali and alkaline earth metals. Generally, the alkyl fragment of the organometallic compound is very reactive; however this depends on the metal, changing the metal alters the polarization of the R-M bond. Thus different organometallic compounds are used in many different types of organic reactions.

Useful Metal Alkyls in Organic Synthesis C-M Bond Organomagnesium – Mg (C--Mg+) Organolithium - Li (C--Li+) Organocopper - Cu (C--Cu+)

Organomagnesium Compounds Grignard Reagents Methylmagnesium chloride - CH3Mg+ Cl- Ethylmagnesium bromide - CH3CH2Mg+ Cl- Draw the structures of: Isopropylmagnesium iodide Sec-butylmagnesium chloride Formation of Grignard Reagents: CH3-Cl CH3Mg Cl Mg, Et2O

Reactivity of Alkyl Halides R-I > R-Br > R-Cl Preparation of Grignards from: primary, secondary & tertiary alkyl halides. Grignard formation from aryl halides and allyl halides; however these may give Wurtz type coupling products. I2 maybe used to speed-up the formation Grignard reagents from alkyl bromides or chlorides

Organolithium Compounds Alkyl Lithium Compounds Methyllithium – CH3Li Ethyllithium - CH3CH2Li Draw the structures of these Alkyl lithium compounds n-propyllithium tert-butyllithium Preparation of organolithium reagents: CH3-Br CH3Li + LiBr 2 eq. Li, Et2O

Organocopper Compounds Lithium Dialkylcopper (organocuprate ) [(R)2Cu]- Li+ Cuprates are less reactive than organolithium R acts as a Nucleophile Oxidation state of copper is Cu(I). Nucleophile “R” will attack various organic electrophiles. Organocuprates are used in cross-coupling reactions to form higher alkanes. Cross-Coupling Reaction: coupling of two different alkyls R and R’ to yield a new alkane (R-R’). This type of reaction is used to make new C-C between alkyl groups.

Organocuprates Preparation of organocuprates: 2 CH3Li [(CH3)2Cu]- Li+ + LiI As a reagent for Organic Synthesis: [(CH3)2Cu]- Li+ + CH3CH2CH2I CH3CH2CH2CH3 Formation of new C-C bonds The electrophile CH3CH2CH2I (R-X) must be a 1 alkyl halide for a good product yield. CuI, Et2O -RCu, -LiI

Organic Synthesis Give a stepwise synthesis for Isobutane. 2,2-dimethylheptane