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organometalsg - Copyright Arizona State University...

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Alcohols 1 Copyright, Arizona State University Copyright, Arizona State University Organometallic Reagents Carbon-Carbon Bonds 1 Carbon-Metal Bonds • We previously looked at the following two disconnections… A B Br Br A B Na acetylide anion Na • B is clearly better since it generates an ACETYLIDE synthon/reagent with more stable non-bonding electrons on the sp hybridized carbon • The acetylide anion is a simple example of an organometallic reagent, i.e. an organic structure with a bond to a metal • In this case the carbon metal bond is essentially pure ionic HC C Na electrons stabilized on sp carbon ionic bond acetylide anion Q. Suppose we HAD to do disconnection A above, how can we make a reasonable carbon synthon such as that in A if we don't have an sp A.O. to stabilize the electrons? An ionic blond to a sodium cation will not work if the hybridization has higher p character than sp. A. We must "temporarily" stabilize the non-bonding electrons on the carbon another way, we can out them into a weak bond to a metal atom Compare among different metals for sp3 hybridized carbon sp 3 sp 3 H 2 C H 3 C H 2 C H 3 C HgX δ δ very high energy on sp 2 carbon lower energy, weak covalent bond Na mainly ionic bond organosodium, too reactive, can't be formed H 2 C H 3 C lower energy, partial electron sharing Li Li smaller, more covalent character organolithium, reactive, but can be formed Hg less electropositive, more covalent character organomercury, least reactive, very useful! • weak covalent bonds with a lot of ionic character mean that the electrons are still relatively high in energy and quite reactive • both organolithiums (alkyl and aryl lithiums) and lithium compounds and organomercury compounds Grignard reagents) can be made for carbons with ANY hybridization, including sp3 and and are thus very useful • alkyl and aryl lithiums are more ionic and tend to be very strongly basic, but they are still nucleophilic and can be used to make carbon-carbon bonds
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