STUDY GUIDE FOR CHAPTER 14
1.
Definition – organometallic compounds are compounds in which a carbon – metal bond exists.
2.
Nomenclature:
A. When the compound consists of alkyl (or aryl) group and metal – n-butyllithium, vinylpotassium, etc. Exception –
sodium acetylide.
B. When there is an additional anion – cyclopentylmagnesium bromide, methylaluminum chloride.
3.
Electronic structure – since metals are less electronegative than carbon, the C – metal bond is polarized, with a partial negative
charge on carbon. If the metal has very low electronegativity (e.g. potassium or sodium) then the compound becomes ionic, with
a negatively charged carbon, i.e. it consists of metal cation and a carbanion.
4.
Synthesis:
A. Organolithium compounds – prepared from alkyl (or vinyl, or aryl) halides and lithium metal. The reaction occurs via
single electron transfer from metal to alkyl halide. Halides reactivity in the order I > Br > Cl > F. Fluorides are usually
not used. The preparation of organolithium (and the other organometallic) compounds requires the use of dry solvents ,
since organometallic compounds are very sensitive towards water (they react with it!!).
B. Organomagnesium compounds (Grignard reagents) – prepared from and alkyl (or aryl, or vinyl) halide and magnesium
metal. Also involves single electron transfer. Suitable solvents are diethyl ether and THF (which is a cyclic ether!!).
The latter coordinates to the metal center and further stabilizes the organometallic compound.
C. Organocopper compounds – prepared from organolithium compounds and copper (I) iodide. Suitable solvents are
diethyl ether and THF. Organocopper compounds contain negatively charged copper.
D. Organozinc compounds – prepared in a way very similar to organomagnesium: From alkyl halide and zinc metal. If the
organic halide is diiodomethane, then the resultant reagent is iodomethylzinc iodide, ICH2 ZnI, which is known as the
Simmons – Smith reagent.
5.
Organometallic compounds as Brønsted bases – all organometallic compounds, particularly organolithium and organomagnesium,
are VERY STRONG Brønsted bases (their conjugate acids are hydrocarbons with very low acidity – alkanes, alkenes, arenas,
etc.). Thus, they immediately react with any functional group that has more acidic hydrogen, such as OH, NH2 , SH. It is
important to keep in mind that organometallic compounds cannot be prepared or kept in presence of such functional groups.
6.
Reactions of organometallic compounds.
A. Preparation of alcohols.
a.
Reaction of organolithium or Grignard reagents with aldehydes and ketones – carbonyl groups are polar, with
carbon partially positively charged. It is susceptible to an attack by nucleophile, such as negatively charged
carbon of organolithium or Grignard reagent. The immediate product is the corresponding alkoxide, which is
converted to alcohol by a subsequent addition of dilute acid (DO NOT ever suggest reactions where
organolithium or Grignard is used together with water or acid!!). If aldehyde is used as a reagent, the product
is a secondary alcohol. If ketone is used, the product is a tertiary alcohol.
b.
Reaction of Grignard reagents with esters – two moles of Grignard reagent are consumed. The first step ends
with the generation of ketone. But ketones are reactive towards Grignard reagents (see a.) and the process
continues with the addition of a second mol of Grignard. The product of this addition is the alkoxide, which
is converted to alcohol with dilute acid. Esters always yield tertiary alcohols (with two identical groups!!).
Exception: When the reagent is an ester of formic acid, such as methyl or ethyl formate. Then the product is
a secondary alcohol.
c.
Reaction of acetylides with aldehydes or ketones – similar to (a). One can use two types of acetylides:
i.
Sodium acetylides – prepared by reaction of terminal alkyne with sodium amide, NaNH2 (See Ch. 9).
ii.
Acetylenic Grignard reagents – these are prepared from a terminal alkyne and an alkyl Grignard
reagent;
B. Preparation of alkanes using organocopper reagents – organocopper reagents can interact with alkyl halides or tosylates,
in an SN 2 process that leads to replacement of the halide or tosylate with an alkyl group. It is worth mentioning that
organolithium or Grignard reagents CANNOT be used in this process, since instead of reacting as nucleophiles, they
react as bases, i.e. they cause elimination of HX. The reaction of organocopper compounds obeys the typical SN 2 trends:
It is fastest with methyl or primary halides (tosylates); it is fastest with iodide > bromide > chloride. The reaction works
best when the alkyl group of the organocopper reagent is primary as well. Organocopper compounds can be used to
react with vinyl or aryl halides, in which case the mechanism is not SN 2.
C. Construction of cyclopropane ring systems with organozinc reagents – the Simmons – Smith reagent can be used to react
with alkenes, to form cyclopropane ring. It is a concerted, stereospecific process.
7.
Carbenes – these are molecules with divalent carbon. The carbon atom has an unshared electron pair. Structurally, they are
molecules in which the divalent carbon is sp 2 hybridized. The lone pair occupies one of the sp 2 -orbitals. The unhybridized porbital is vacant. So carbenes are unique species with a vacant orbital and a lone pair. They combine properties of both
electrophiles (vacant p-orbital!!) and nucleophiles (lone pair!!). In other words, they exhibit behavior both of carbocations and
carbanions. The Simmons-Smith reagent reacts with alkenes as if it is a source of carbene, CH2 . Such species are called
carbenoids.
8.
Transition metal organometallic compounds – there is a huge number of such materials. Their electronic structure obeys the 18electron rule, i.e. the metal has in its valence shell 18 electrons in order for the compound to be stable (That is 2 s-electrons, 6 pelectrons and 10 d-electrons = 18!!). In some cases entire ring systems can serve as electron-donors, such as benzene or
cyclopentadienyl anion. In the latter case compounds known as metallocenes are formed (ferrocene, manganocene, etc.). Some
of the metallocenes are useful catalysts.
9.
Catalytic alkene polymerization (Ziegler – Natta polymerization) – utilizes bis(cyclopentadienyl)zirconium dichloride, Cp 2 ZrCl2 ,
in combination with methylalumoxane (MAO) to obtain highly ordered, high-density polyethylene.