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CHAPTER 12
562 SYNTHETIC CARBENE AND NITRENE CHEMISTRY
were reactivity and, especially, selectivity. The unique transformations for which they were well known—addition to carbon– carbon multiple bonds and insertion into carbon–hydrogen bonds—showed little regioselectivity or stereoselectivity, and whether or not the reaction proceeded through a singlet or triplet intermediate was also uncertain. In the same time period, however, two independent developments transformed these mechanistic curiosities into valued units for organic synthesis.
Working with diazo compounds, known since the early 1900s to undergo loss of dinitrogen when treated with copper or copper salts, Yates described in 19523 the possibility that transition metals could form an intermediate that combined units of the diazo compound and the metal (Eq. 1, L ¼ ligand) and acted like a carbene in addition and insertion reactions. Somewhat later, but independently, E. O. Fischer isolated and characterized stable metal carbenes that could also undergo cyclopropanation reactions.4 They were derived from transition metals on the left side of the
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periodic table, contained an alkoxy substituent, could be applied to organic synthesis as stoichiometric reagents, and were chemically and physically well defined.5 Thus the search for synthetically viable ways to utilize carbenes was being pulled in two directions. Catalytic methods using diazo compounds, and allied metal carbenoid processes such as the Simmons–Smith reaction [alkene þ CH2I2(Zn Cu) ! cyclopropane], looked to new developments in catalysis, especially with catalysts that were soluble in the reaction medium and, eventually, those that possessed chiral ligands.6 With metal carbenes in the new emerging field of organometallic chemistry, emphasis was placed on the preparation of stable analogues and their chemistries.7 From these investigations arose the understandings and applications of electrophilic and nucleophilic metal carbenes (1a–c).
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The philicity of carbenes and control of selectivity in their reactions became, in retrospect, an enabling concept. But how does one control philicity through the metal and its bound ligands? The metal itself contributes to the philicity of the carbene by its kind and degree of d-orbital back-bonding.8 The ligands attenuate this effect by their electronic and steric influences. And having an open coordination site on the metal because of ligand dissociation is important to their reactions (Scheme 12.1). Herein was a discussion that lasted more than a decade.9 Metal carbenes were implicated in both cyclopropanation (Eq. 2)10 and metathesis reactions
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Scheme 12.1
(Eq. 3).11 Could they both arise through the same mechanism but differ in the philicity of the metal carbene, or did one proceed through a pathway that was different from the other? The answers came reluctantly over an extended period, but they paved the way to the synthetic uses that we enjoy today.
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triolefin process"
2. ELECTROPHILIC AND NUCLEOPHILIC METAL CARBENES
Synthetic uses came from two directions, neither based on fundamentals but both moving the field from uncertainty to the effective control of parameters. Electrophilic metal carbenes were recognized from their ability to undergo addition to electron-rich alkenes such as vinyl ethers or styrene (Eq. 4),6 but not electron-poor
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alkenes that included a,b-unsaturated carbonyl compounds. Nucleophilic metal carbenes, eventually understood to form and be produced from metallocyclobutane intermediates, exhibited an orientation expected of a nucleophilic carbene (Scheme 12.2). Those catalysts that form electrophilic metal carbenes have lower oxidation states [e.g., Cu(I), Rh(II), W(II)], whereas those that undergo metathesis have higher oxidation states (e.g., 4–7 with 6 and 7 possessing a nitrogen-stabilizing carbene ligand). Ligand dissociation from Ru by the phosphine initiates a metathesis
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if R' is |
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if R' is |
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withdrawing |
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donating |
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Scheme 12.2
reaction, but there is no evidence that ligand dissociation is an essential step in cyclopropanation reactions.
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Ph3P |
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Catalyst |
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Metathesis |
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Cy3P |
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tolerance |
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Reactivity |
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Highest |
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stabilization |
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Ar |
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Understanding the structure and dynamics of metal carbenes related to Fischer carbenes provides insights into their reactivities and selectivities. For example, their stability decreases when electron-donating substituents are replaced by hydrogen or, especially, electron-withdrawing substituents:6
Stability (CO)5W |
Ph > (CO)5W |
Ph |
> (CO)5 W |
Ph > (CO)5W |
COOEt |
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OMe |
Ph |
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H |
H |
Furthermore, the opening of a coordination site for olefin association is a necessary condition for metathesis, but not for cyclopropanation (Eq. 5).16
Ph |
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Ph |
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Ph |
Ph <30 °C |
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Cyclopropanation |
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Open coordination |
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site on W |
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CH2 |
ð5Þ |
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Metathesis
ELECTROPHILIC AND NUCLEOPHILIC METAL CARBENES |
565 |
2.1. Catalysis of Diazo Decomposition
By 1960, there was recognition that copper salts could cause the loss of dinitrogen from diazocarbonyl compounds with addition of the resulting carbene intermediate to a carbon– carbon double bond to form a cyclopropane product. That this reaction, first reported by G. Stork in 1961 (Eq. 6),17 could occur in an intramolecular fashion and thus avoid the formation of isomers, ushered in the first significant synthetic
N2 |
Cu bronze |
O |
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ð6Þ |
O |
cyclohexane |
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reflux |
~ 50% |
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applications beyond pyrethroid syntheses. Extensions of this methodology led to the preparation of a large number of natural products (e.g., 8–10),18 but neither
Me |
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H |
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Me |
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Me |
Me |
H |
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Me Me |
Me |
Me |
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Me |
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OH |
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8 |
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9 |
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10 |
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19 |
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Aristolone |
20 |
Presqualene alcohol |
21 |
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Thujopsene |
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the mechanism of this transformation, nor methods to control reaction selectivity, were well understood. It was during this decade that new catalysts were developed, eventually resulting in those now recognized to be the most reactive and selective for cyclopropanation (11–16).6
Achiral Catalysts |
Cu(acac)2 |
CuOTf |
Rh2(OAc)4 |
PdCl2 |
for Diazo |
Low reactivity |
High reactivity |
Moderate reactivity |
High reactivity |
Decomposition |
stable |
easily oxidized |
stable |
stable |
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Me |
Me |
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COOMe |
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COO 4Rh2 |
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4Rh2 |
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Catalysts for |
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CuI t-Bu |
N |
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Diazo Decomposition |
t-Bu |
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ArSO2 |
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4 |
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1122 |
1223 |
1324 |
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Moderate reactivity |
Moderate reactivity |
Low reactivity |
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easily oxidized |
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stable |
stable |
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Ar = aromatic
566 SYNTHETIC CARBENE AND NITRENE CHEMISTRY
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M |
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O R |
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R |
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Ph Ph |
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O |
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Me |
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Me |
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1425 |
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1526 |
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1627 |
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M = Ru(NO)Cl |
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Moderate reactivity |
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Moderate reactivity |
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Low reactivity |
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stable |
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activated with |
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requires photoinitiation |
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N -methylimidazole |
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The mechanism for diazo decomposition is now widely understood.6 The ligated metal, with an open coordination site and acting as a Lewis acid, undergoes electrophilic addition to the diazo compound. Loss of dinitrogen then forms the intermediate metal carbene that is able to transfer the carbene from the metal to a substrate and thereby regenerate the catalytically active ligated metal (Scheme 12.3). It is in the carbene transfer step that selectivity is achieved. The rate-limiting step is either electrophilic addition or loss of dinitrogen.
SCHR |
S |
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B |
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_B |
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CHR |
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+ |
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LnM |
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LnM |
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LnM |
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LnM |
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CHR |
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+B |
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RCHN2 |
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N2 |
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LnM |
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CHR |
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N2+ |
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Scheme 12.3
The transfer of the carbene may be to any of a variety of substrates and occurs in an intermolecular or intramolecular fashion. Cyclopropanation is perhaps the best known catalytic transformation (Eq. 7),28 but carbon–hydrogen insertion (Eq. 8),29
ylide formation and rearrangement or cycloaddition (Eq. 9),26 and addition to multiple bonds other than C C (Eq. 10)30 are also well established.6,31–33