OrganoMagnesium Compounds

Peter H.M. Budzelaar

OrganoMagnesium Compounds2

Organo-Mg and Be compounds

Like Organolithium compounds, but milder:

• Ionic character, electron-deficient– oligomeric or polymeric structures

• Basic and nucleophilic– Mg: applications in organic synthesis (Grignard reagents !)– Be: extremely toxic, not used

• Reducing– sometimes unwanted side reactions

Organo Ca, Sr, ... compounds: ionic, insoluble, not used

OrganoMagnesium Compounds3

Grignard reagents RMgX

• Direct synthesis ("Grignard reaction"):

• Mechanism is not simple!– almost certainly involves SET (Single Electron Transfer)

– very sensitive to traces of impurities (Na, K, transition metals) in the Mg(vendors sell special "Grignard quality" Mg containing 1-2% Na/K)

– nature of the Mg surface is important(for less reactive halides, surface activation is essential: I2, BrCH2CH2Br, MeI or HgCl2)

RMgXMg + RX

RMgXsolvRX + Mgs RX + Mgs R + XMgs

OrganoMagnesium Compounds4

Grignard reagents RMgX

• Reactivity: I > Br > Cl >> F

• Selectivity: I gives largest amounts of side products

• In ethers:– In Et2O cleaner than in THF, but also slower

– Also in ether/hydrocarbon mixtures

• Side reactions:– Wurtz coupling, especially for allylic halides

– Radical rearrangements

RMgX + RX R R + MgX2

OrganoMagnesium Compounds5

Grignard reagents RMgX

• Main application: addition to polar C=X bonds– C=O, C=N, C≡N

• Sometimes radical mechanism(especially for aromatic ketones):

– same product expected, but possibly different side products

O

R Mg

O

R Mg

Mg

OR

O

R Mg

Mg

OR

R Mg

O

R Mg

O

OrganoMagnesium Compounds6

Grignard reagents RMgX

• Side reaction in C=X addition: reduction through -H transfer– only for alkyls having a -hydrogen...

MgX

H

MgX

H O

MgX

O

H

O MgX

H-

O

OrganoMagnesium Compounds7

Grignard reagents RMgX

"Barbier reaction":

• Selectivity differs from that of Grignard addition• Probably radical mechanism• Works even in water !

Mg + RX

O1)

2) H+OHR

OrganoMagnesium Compounds8

The Schlenk equilibrium

• Grignard reagents are usually mixturesof many compounds in dynamic equilibrium

• This is often written as the Schlenk equilibrium

but the situation is really more complicated

• Crystal structures may not be representativeof species in solution

• The equilibrium can be shifted through addition of ligands:

R2Mg + MgX2 2 RMgX

R2Mg + MgX2(dioxane)2 RMgX + dioxane

OrganoMagnesium Compounds9

Structure of [MeMgCl(tmeda)]2

OrganoMagnesium Compounds10

Structure of MeMgBr(THF)(tmeda)

OrganoMagnesium Compounds11

Structure of Me2Mg4Cl6(THF)6

OrganoMagnesium Compounds12

Structure of [tBu2Mg]2

OrganoMagnesium Compounds13

Structure of Me2Mg(pmdta)

OrganoMagnesium Compounds14

Structure of (-diiminate)MgMe

N

Mg

N

tBu tBu

Me

ArAr

Ar =