This presentation discusses the types, preparation and reactions of organomanganese reagents

1. Organomanganese compounds
Prepared by
Dr. Krishna swamy
Faculty
DOS & R in Organic Chemistry
Tumkur University

2. Organomanganese reagents are the less expensive organotransition metal
complex due to low cost of manganese metal.
These reagents have intermediate reactivity between Grignard and Organozinc
reagents.
R-MgX > R-MnX > R-ZnX
There are three types of organomanganese reagents
(1) Organomanganese halides (RMnX)
(2) Dialkylmanganese (R2Mn)
(3) Organomanganate (R3MnMt) Mt = Li / Mg

3. Among them, trialkylmanganate is the most stable reagent and it is stable at room
temperature.
Dialkylmanganese such as Bu2Mn is unstable and decompose at -30oC.
The stability of organomanganese halide (RMnX) is between dialkyl manganese and
trialkyl manganate.
R3MnLi > R-MnX >> R2Mn

4. Oxidative addition of organic halide to activated manganese i.e. direct insertion
of Mn to R-X bond.
Since unactivated manganese is less active due to fomation of layer of oxide. Hence it
needs to be activated and highly activated Mn* (Rieke Manganese) is obtained by
reduction of Mn (II).
Preparation
R X R MnX
Mn*
Mn*MnX2
Reductant
Mn*MnBr2
Li / Naphthalene
THF

5. Transmetalation reaction of manganese halides with stoichiometric amount
of organometallics leads to the formation of organomanganese reagents.
R MX R MnX
M = Li, Mg
R = alkyl, aryl, alkenyl etc
MnX2
R MX R2 Mn
MnX2
2
R MX R3 MnMt
MnX2
3

6. Manganese-halogen exchange reaction between organic halide and
tetralkylmaganate leads to the formation of organomanganese halides.
R X R MnX
Bu4MnLi

7. Organomanganese halide behaves as soft Grignard reagents.
Addition reaction: They add to aldehyde, ketones, carbon dioxide, sulfur
dioxide and isocyanate and they wont react with / add to esters, nitriles and amides.
Reactions
Bu MnI CHO
BuEt2O
rt
OH
Bu MnI Bu Bu Et2O
rt
O
Bu Bu
OH
Bu

8. Bu MnI O C
THF
rt
O
Bu
O
OH
Bu MnI O S THF
-60o
C
O
Bu
S
O
OH
Bu MnI C Et2O
rt
O Ph
N
H
CNPh
Bu
O

9. Organomanganese halides show chemoselectivity as well as regioselectivity.
Organomanganese halides show selective addition to aldehyde in presence of ketones.
Organomanganese halides are more sensitive to steric effect. Hence they add to less
hindered ketone over more hindered ketone.
Et
O
H
O
8
n-Bu MnCl
THF Et
O
n-Bu
OH
8
t-Bu
O
Pr
O
Pr MnBr
Et2O t-Bu
O
Pr
OH
Pr
Pr
O
Ph
O
Pr MnBr
Et2O Pr
O
Ph
OH
Pr

10. Organomanganese halides are used in preparation of ketones from acid chlorides.
Ketones are obtained in good yields from organomanganese iodides and stoichiometric
amount of carboxylic acid chlorides in ether.
Et2O
MnI2
Et2O
RM RMnI
R'COCl
Et2O
R R'
O
i-Pr-MnI Hept-COCl
O

11. Organomanganese halides add to α, β-unsaturated carbonyl compounds.
For example organomanaganese reagent react with cyclohexenone to give mixture of 1,
2-, 1, 4- and β-reductive dimerization products.
The proportion between the three products is very dependent on several factors like
nature of organomanganese reagent, solvent, presence of metallic salts (LiX / MgX2)
and temperature.
O HO
O O OR
R
1, 2-product 1, 4-product -reductive dimerization
Organomanganese
reagent

12. Organomanganese halides with unsaturated carbonyl leads to the formation of mixture
of three products and hence the reaction has no preparative interest.
By using symmetrical organomanganese and organomanganate reagent, the formation
of 1, 2-addition product generally not observed. 1, 4- and reduction dimerization
products are obtained as mixture.
O HO
O O OR
R
1, 2-product 1, 4-product -reductive dimerization
RMnX
O
O O O
R
1, 4-product -reductive dimerization
R2Mn
-30o
C
O O O
R
1, 4-product -reductive dimerization
R3MnLi
-30o
C

13. Manganese catalysed homocoupling reaction of Grignard reagent gives biaryl
derivatives.
RMgX
MnCl2
2 R R
Dry air, THF, rt
2 MgClMeO
MnCl2
Dry air, THF, rt
MeO OMe
Homo-coupling reaction

14. RMgX MnCl22
R2Mn
2 MgClX
O2
O
Mn
O
R
R
O
Mn
O
R R
RMgX2
XMgOOMgX
IV
II
Mechanism

15. Manganese catalysed cross-coupling reaction of Grignard reagent with aryl halides
leads to formation of C-C bond.
RMgX
MnCl2
THF, rt
Z
FG
R
FG
Cross-coupling reaction