Main-group Organometallics

Organometallic Chemistry between organic and inorganic

Main-group Organometallics

Peter H.M. Budzelaar

Main-Group Organometallics2Main group organometallics at a glanceStructures s bonds and 3c-2e (or even 4c-2e) bonds

Synthesisthe first M-C bond

Reactivitynucleophilic and basicauxiliaries in organic synthesissource of organic groups for transition metals

Main-Group Organometallics3StructuresNot always 8/18 e"Too many" electrons if:large atomic radiuselectropositive elementssmall ligandschelate structures"Too few" electrons for:small atomic radiusless electropositive elements8/18 e "preference" rather than "rule"

Main-Group Organometallics4Comparing coordination abilitiesZnCl2Cl are withdrawing!stronger acceptorpolymericMe2Znmoderately strong acceptormonomeric, but exchanges Me groupsMe2MgMg much more electropositivestrong acceptorpolymericMe2HgHg much less electropositivevery weak acceptormonomericMain-Group Organometallics5StructuresStrong preference for s-donor groupsbut Cp is often p-bound (deceptively like with transition metals)

Electropositive metals: often 3c-2e or 4c-2e hydrides/alkylsas "stopgap"

Cp2MgCp2Fe

(Me3Al)2(MeLi)4Main-Group Organometallics6Reactivity as NucleophileAddition to polar C=X bonds(C=O, C=N, CN)

Substitution at sp2 carbon(often via addition)

Main-Group Organometallics7Reactivity as Nucleophile

additioneliminationMeMgBr + MeCOOMe Me2CO + MeOMgBrMain-Group Organometallics8Reactivity as NucleophileSubstitution at sp3 carbon does occurbut is far less easyand often has a multistep mechanism

Substitution at other elements:often easy for polar M-X bonds(Si-Cl, B-OMe)

Main-Group Organometallics9Reactivity as BaseMore prominent in polar solventsthink of free R- acting as base

Elimination

mechanism can be more complex than thisMetallation

chelate effect more important than inductive effect!

Main-Group Organometallics10Reactivity as BaseMetallation (2)sometimes an unwanted side reaction of nucleophilic addition/substitution

(the enolate is now "protected" from further nucleophilic attack)

only acidic C-H bonds (acetylenes, cyclopentadiene)react without "assistance" by coordinating groups

Main-Group Organometallics11Reactivity as BaseReaction with acidic X-H bonds

also with amines, amides etc (LDA!)generally very fast ("complete on mixing")important issue when handling polar organometallics

deprotonating X-H bonds is kinetically much easier than C-H bonds

Main-Group Organometallics12Reactivity as Reductor b-hydrogen transfer

mainly for Al:for more electropositive elements, deprotonation and nucleophilic attackare fasterfor less electropositive elements, often no reaction

Et3Al + Me2COadditionb-H transferMain-Group Organometallics13Reactivity as ReductorSingle-electron transfer (SET)For electropositive elements:M has a low electron affinitythe M-C bonding orbital is high in energySET from there to another molecule is easy

SET often leads to the same final products as simple addition(via "radical recombination")

Main-Group Organometallics14SynthesisDirect synthesis

only for electropositive metals (Li, Mg, ...)

Transmetallation

M more electropositive than M'reason why organoLI and organoMg compounds are so popular

Metal-halogen exchange

mainly for M = Li, X = Br or I, R = alkyl, R' = arylmechanism not completely clear

Main-Group Organometallics15SynthesisAddition of hydrides to olefins

mainly for B, Al

Metallation of C-H bonds

often assisted/directed by chelate effect