Download Transition-metal Complexes

Section 15.6
SYNTHESIS OF ACETYLENIC
ALCOHOLS
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Synthesis of Propargylic Alcohols
• Alkali metal acetylides and alkynyl Grignard reagents
also act as nucleophiles toward carbonyl compounds
• The resulting products are the “triple-bond analogues” of
allylic alcohols, propargylic alcohols
• Perfectly analogous to additions from the last section
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Section 15.7
RETROSYNTHETIC ANALYSIS
AND GRIGNARD AND
ORGANOLITHIUM REAGENTS
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Retrosynthesis and Nuc. Addition
• Considering nucleophilic addition of R–M to carbonyls, we can
now disconnect C–C–OH bonds retrosynthetically
• Precursors are R–M (synthetically equivalent to R–) and a
carbonyl compound (ketone or aldehyde)
• For 2º and 3º alcohols, multiple disconnections are possible and
usually work equally well
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Retrosynthesis and Nuc. Addition
• Considering nucleophilic addition of R–M to carbonyls, we can
now disconnect C–C–OH bonds retrosynthetically
• Precursors are R–M (synthetically equivalent to R–) and a
carbonyl compound (ketone or aldehyde)
• For 2º and 3º alcohols, multiple disconnections are possible and
usually work equally well
Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
Retrosynthesis and Nuc. Addition
• Considering nucleophilic addition of R–M to carbonyls, we can
now disconnect C–C–OH bonds retrosynthetically
• Precursors are R–M (synthetically equivalent to R–) and a
carbonyl compound (ketone or aldehyde)
• For 2º and 3º alcohols, multiple disconnections are possible and
usually work equally well
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Section 15.8
AN ORGANOZINC REAGENT FOR
CYCLOPROPANE SYNTHESIS
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Organozinc Compounds
• Like Mg, Zn prefers to be a +2 cation. Zn “inserts” into
R–X bonds like Mg, forming organozinc halides
• When RX is a geminal dihalide R2CX2, an interesting
product forms…
C–Zn makes this molecule nucleophilic at carbon…
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Organozinc Compounds
• Like Mg, Zn prefers to be a +2 cation. Zn “inserts” into
R–X bonds like Mg, forming organozinc halides
• When RX is a geminal dihalide R2CX2, an interesting
product forms…
C–Zn makes this molecule nucleophilic at carbon…
…but C–I suggests that it is also electrophilic at carbon!
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The Simmons-Smith Reaction
• The organozinc reagent (IZn)CH2I is both nucleophilic and
electrophilic—similar to a C=C π bond!
• Simmons-Smith reaction: electron flow in one direction forges
a C–C bond at one carbon of C=C; electron flow in the other
direction forges C–C at the other carbon of C=C
• A cyclopropane product forms! Both bonds form simultaneously,
so this reaction is stereospecific
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Section 15.9
TRANSITION-METAL
ORGANOMETALLIC COMPOUNDS
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d-Block Elements
• The transition metals or d-block elements occupy the
center of the periodic table
• They can form cations with a variety of charges and
often sit at the center of complexes with organic
molecules at the periphery
• Transition-metal complexes are extremely important in
modern chemical synthesis
– Properties of transition metals vary (almost) continuously
– Set of possible ligands is enormous
– Transition-metal complexes have unique reactivity
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Transition-metal Complexes
• Transition metals are usually cationic and Lewis acidic in
stable compounds
• A Lewis-acidic transition metal atom/ion surrounded by
Lewis basic organic molecules (ligands) is called a
transition-metal complex
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Transition-metal Complexes
• Transition metals are usually cationic and Lewis acidic in
stable compounds
• A Lewis-acidic transition metal atom/ion surrounded by
Lewis basic organic molecules (ligands) is called a
transition-metal complex
• Ligands are almost always formally anionic or neutral; a
lone pair or π electrons bond to the metal
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The Eighteen-electron Rule
• Transition metals have valence electrons in the d subshell, and
thus have their own analogue of the octet rule
• Eighteen-electron rule: the sum of electrons donated by ligands
plus the number of valence electrons at the metal equals 18 in
stable TM complexes
• Some ligands bond using more than two electrons!
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Metallocenes
• Like benzene, cyclopentadienyl anion is aromatic. It’s a much
stronger ligand due to its anionic charge
• Metallocenes contain cyclopentadienide ligands and are
commonly used for catalysis
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Section 15.10
ORGANOCOPPER REAGENTS
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Gilman Reagents
• R–Li add twice to copper(I) halides to form compounds of anionic
[R2Cu–] called Gilman reagents
• Like most organometallics, Gilman reagents are nucleophilic
• They react with electrophilic organohalides in cross-coupling
reactions to form C–C
Works for any organohalide except 2º and 3º alkyl (elimination is an issue).
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Cross-coupling
• Trends in the cross-coupling of Gilman reagents with alkyl
halides resemble those of SN2:
– RI > RBr > RCl > RF
– Inversion of configuration
• However, sp2 and sp organohalides react too, so SN2 is unlikely
• Instead, a mechanism involving oxidative addition and
reductive elimination is more likely
Oxidative addition: copper inserts into the R–X
bond, going from Cu(I) to Cu(III).
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Cross-coupling
• Trends in the cross-coupling of Gilman reagents with alkyl
halides resemble those of SN2:
– RI > RBr > RCl > RF
– Inversion of configuration
• However, sp2 and sp organohalides react too, so SN2 is unlikely
• Instead, a mechanism involving oxidative addition and
reductive elimination is more likely
Reductive elimination: R and CH3 “de-insert,”
converting Cu(III) to Cu(I) again.
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Cross-coupling
• Trends in the cross-coupling of Gilman reagents with alkyl
halides resemble those of SN2:
– RI > RBr > RCl > RF
– Inversion of configuration
• However, sp2 and sp organohalides react too, so SN2 is unlikely
• Instead, a mechanism involving oxidative addition and
reductive elimination is more likely
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