Download Reactions of Organometallic Complexes with Nucleophiles

Reactions of Organometallic Complexes with Nucleophiles
The scheme below shows common reactions between a metal complex containing an unsaturated ligand (an
alkene) and a nucleophile.
Since we have already examined the different types of reactivity which can occur from nucleophilic attack
at the metal (substitution reactions, coordination in a vacant site, insertion reactions), we are mainly
concerned with reactions involving direct nucleophilic attack on unsaturated ligands.
DIRECT
NUCLEOPHILIC
ATTACK ON THE
LIGAND
X
+
LxM
Nu
-
X
LxM
Nu
NUCLEOPHILIC ATTACK AT THE METAL
substitution by salt
metathesis
- X-
substitution
coordination
- alkene
X
Nu
LxM
X
LxM
LxM
Nu
Nu
1,2-insertion
1,2-insertion
X
LxM
L xM
Nu
Nu
Direct nucleophilic addition to an unsaturated ligand (e.g. CO, alkene, allyl, benzene)
•
Because they are electron rich, molecules such as CO, alkenes, polyenes and arenes generally do not
react with nucleophiles in the absence of a metal.
•
Once attached to a metal, these ligands give up some of their electron density and become susceptible to
direct nucleophilic attack (umpolung).
•
Unsaturated ligands are more susceptible to direct nucleophilic attack when:
- there is less electron density on the metal (e.g. – π-acceptor co-ligands, overall positive charge).
- the metal is coordinatively saturated – this avoids nucleophilic attack at the metal centre.
We will look at (A) nucleophilic addition to CO, and (B) nucleophilic addition at π-ligands:
A. Direct nucleophilic addition at CO
Me3N
O
Me3N
O
(OC)5Mo
C
(OC)5Mo
O
C
O
(OC)5Mo
PPh3
+ NMe3
+ O
•
+ PPh3
(OC)5Mo
C
O
R3NO (a trialkylamine oxide; R = Me or Et) is commonly used instead of heat or UV-irradiation to
remove CO ligands in order to speed up dissociative substitution reactions.
NaBH4
Re
OC
Re
CO
ON
•
O
OC
ON
H
The above reaction involves direct nucleophilic attack of H- on a CO ligand. Note: It is possible to get
similar reactions with CNR or carbenes.
B. Nucleophilic addition at π-ligands
•
There are many examples of direct nucleophilic attack on coordinated π-ligands, and these
transformations can be very useful in synthesis. For example:
H
n
BuLi
Mo
PPh2
Ph2P
•
Mo
PPh2
Ph2P
The regiochemistry of such reactions is predicted by the Davies / Green / Mingos (DGM) Rules.
DGM Rules (these rules must be followed in order)
Rule 1 – Even before Odd: Nucleophilic attack occurs preferentially at even polyenes
Rule 2 – Open before Closed: Nucleophilic addition occurs preferentially to open polyenes (not closed).
OPEN
CLOSED
Rule 3 - For open polyenes:
-
If Even Æ attack occurs at a terminal position.
-
If Odd Æ attack is usually NOT at a terminal position
Æ attack is only at a terminal position if the metal is very strongly electron withdrawing.
Reasons for the three DGM rules
(1) Even polyenes are formally neutral (alkene, benzene vs Cp-, allyl-, C7H7-).
(2) Largely an empirical observation. The reasons behind it are not well understood.
(3) This has to do with the relative size of the lobes in the LUMO (see below):
NuNu-
LUMO
NuHOMO LUMO
LUMO
HOMO
HOMO
allyl-
(allyl+)
butadiene
for only the most electron withdrawing metals, this orbital
becomes sufficiently depleted of electron density that it
becomes the preferred site for nucleophilic attack.
Extensions to the DGM rules
•
Secret rule zero! – nucleophiles generally add once to a monocation, twice to a dication etc.
•
For neutral open polyenes:
because
•
M
For monoanionic open polyenes:
>
>
M
M
vs
M
>
because the negative charge is delocalized over more carbons in pentadienyl than allyl
PRACTICE QUESTIONS FOR DGM RULES
Me
W
Mo
:PPhMe2
SMe
CO
?
H-
Mo
N
O
Mo
-
?
H-
?
2 Me-
?
?
2
Co
CN-
?
Fe