1. Introduction Main Group Metals
12/7/2018
Introduction Main Group Metals

2. Main group metal compounds
Structures
s bonds and 3c-2e (or even 4c-2e) bonds
Synthesis
the first M-C bond
Reactivity
auxiliaries in organic synthesis
source of organic groups for transition metals
12/7/2018
Introduction Main Group Metals

3. Introduction Main Group Metals
Structures
Not always 8/18 e
"Too many" electrons if:
large atomic radius
electropositive elements
small ligands
chelate structures
"Too few" electrons for:
small atomic radius
less electropositive elements
8/18 e "preference" rather than "rule"
12/7/2018
Introduction Main Group Metals

4. Introduction Main Group Metals
Structures
Strong preference for s-donor groups
but Cp is often p-bound
Electropositive metals: often 3c-2e hydrides/alkyls
as "stopgap"
12/7/2018
Introduction Main Group Metals

5. Reactivity as Nucleophile
Addition to polar C=X bonds
(C=O, C=N, CºN)
Substitution at sp2 carbon
(often via addition)
12/7/2018
Introduction Main Group Metals

6. Reactivity as Nucleophile
Substitution at sp3 carbon does occur
but is far less easy
and often has a multistep mechanism
Substitution at other elements: often easy for polar M-X bonds
(Si-Cl, B-OMe)
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Introduction Main Group Metals

7. Introduction Main Group Metals
Reactivity as Base
Elimination
Metallation
chelate effect more important than inductive effect!
12/7/2018
Introduction Main Group Metals

8. Introduction Main Group Metals
Reactivity as Base
Metallation (2)
Only acidic C-H bonds (acetylenes, cyclopentadiene) react without "assistance" by coordinating groups
12/7/2018
Introduction Main Group Metals

9. Introduction Main Group Metals
Reactivity as Base
Reaction with acidic X-H bonds
also with amines, amides etc (LDA!)
deprotonating X-H bonds is kinetically much easier than C-H bonds
12/7/2018
Introduction Main Group Metals

10. Reactivity as Reductor
b-hydrogen transfer
mainly for Al:
for more electropositive elements, deprotonation and nucleophilic attack are faster
for less electropositive elements, often no reaction
12/7/2018
Introduction Main Group Metals

11. Reactivity as Reductor
Single-electron transfer (SET)
For electropositive elements:
M has a low electron affinity
the M-C bonding orbital is high in energy
SET from there to another molecule is easy
12/7/2018
Introduction Main Group Metals

12. Introduction Main Group Metals
Synthesis
Direct synthesis
only for electropositive metals
Transmetallation
M more electropositive than M'
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Introduction Main Group Metals

13. Introduction Main Group Metals
Synthesis
Metal-halogen exchange
mainly for R = alkyl, R' = aryl
Addition of hydrides to olefins
mainly for B, Al
12/7/2018
Introduction Main Group Metals

14. Introduction Main Group Metals
Synthesis
Metallation of C-H bonds
often assisted/directed by chelate effect
12/7/2018
Introduction Main Group Metals