Organometallic Compounds SPECIAL TOPICS CHEM-243 Organometallic Compounds BY Dr. Ghulam Abbas Assistant Professor UNIVERSITY OF NIZWA

Organo copper Reagents and Alkane Synthesis   Formation of the lithium dialkylcuprate:     a)  Treat copper(I) halide with two equivalents of an alkyl lithium in THF or ether: Lithium Dialkylcuprates are used to produce alkanes.          

R Li R Cu Li+ I– Cu I R Li R Cu – R Cu Li+ MECHANISM The alkyllithium first reacts with the copper(I) halide. R Li R Cu Li+ I– Cu I Then a second molecule of the alkyllithium reacts with the alkylcopper species formed in the first step. R Li R Cu – R Cu Li+ 3

Lithium Dialkylcuprates Example:

Alkane Synthesis Lithium Dialkylcuprates react with alkyl halides to produce alkanes.                   R2CuLi  +  R'X  R-R'  +  RCu  +  LiX  The reaction works best with primary R'X and primary and phenyl dialkyl cuprates. Q. What problem might occur with 2o and 3o alkyl halides?              Answer:            ELIMINATION. ii)  R'X can also be vinyl halides and aryl halides.

This reaction is known as the Corey-House synthesis. Cont. Alkane Synthesis This reaction is known as the Corey-House synthesis. The two alkyl groups do not have to be identical! (unlike the Wurtz reaction). The dialkylcuprate acts as alkylating agent.

Alkane Synthesis Mechanism of this reaction is not well understood; seems to involve nucleophilic attack on R'X by the Cu atom of the dialkylcuprate; unstable intermediate then breaks apart into the observed products: R2Cu− +  R'-X   [R2CuR'X]−    R-R' + RCu + X− 

Example

This wouldn’t work by a Wurtz synthesis... In general, allylic halides are unreactive in organometallic reactions, but here they do react.

This is only possible by this method: Stereospecific!

Alkane Synthesis The reaction follows the trends seen for SN2 reactions.    a)  Order of Reactivity of RX:   CH3X  >  1o  >  2o  >  3o     b)  Order of Reactivity of Halide:   I  >  Br  >  Cl  >  F

Q. Complete the following; a) (CH3CH2CH2)2CuLi + CH3CH2CH=CHBr à Alkane Synthesis Q.  Complete the following;                                 a) (CH3CH2CH2)2CuLi  +  CH3CH2CH=CHBr à                                       The alkyl groups in the dialkylcuprate ion can be phenyl groups or 1o alkyl groups but not 2o or 3o. a)  With 2o and 3o alkyl groups, steric hindrance seems to make R2CuLi less reactive toward R'X. b)  With 2o and 3o alkyls, R2CuLi also tends to be unstable decomposes before reacting with R'X.            

Synthesis of Manicone Manicone is a pheromone which is produced by certain ants.

1. The reduction of alkyl halides by zinc can be used to form alkanes.  Organozinc 1. The reduction of alkyl halides by zinc can be used to form alkanes.  a)  Overall reaction and mechanism:       RX  +  Zn  RZnX   RZnX  + HX  RH  +  ZnX2  An organozinc reagent (R-ZnX) is produced in this reaction. R-ZnX is not as reactive towards carbonyl compounds as Grignard reagents (R-MgX).

Synthesis By Organozinc Example.  Complete the reaction      2.  Dehalogenation of vicinal dihalides to form alkenes:  Beta Elimination.                             Mechanism

Synthesis By Organozinc This is beta-elimination (Alkene formation) since the leaving group is b to the a carbon bearing the ZnX group.    Examples:    

Synthesis By Organozinc Dehalogenation of 1,3-dihalides to form cyclopropanes: Gamma Elimination is used to form 3-membered rings. i)  When the halogen atoms are more than 3 carbons apart yields of larger rings are poor. b)  The reaction is run in ethanol. c)  This is gamma-elimination since the leaving group is δ to the a carbon bearing the ZnX group.  Complete the reaction  

So, cyclopropanes are synthesized by using a zinc-copper  Simmons-Smith Reagent    Simmons-Smith Reagent are used to prepare cyclopropane. So, cyclopropanes are synthesized by using a zinc-copper couple (Zn surface-activated with Cu).  a)  Zn(Cu) reacts with CH2I2 in ether to yield iodomethylzinc iodide, ICH2ZnI.  b)  ICH2ZnI reacts with an alkene in ether to form a cyclopropane. Mechanism

Simmons-Smith Reagent The reaction is stereospecific due to syn-addition: groups which are cis in the alkene remain cis in the ring;  groups which are trans in the alkene remain trans in the ring. d)  ICH2I is a "carbenoid" since it resembles carbene (:CH2) chemically. e) Yields often low; important since it offers a route to compounds otherwise difficult to make.

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