哈佛大学高等有机化学讲义Lecture34

Me
Me
Me
Me
Me
Me
OR
p Energy σ triplet carbene
Cl
donor p orbital
H Me N N Ts Me
-Ts
Me
Me
N Me
N Ts Me
Me
N N
-N2
Heteroatomsubstituted carbene
H
π-donor heteroatom
H R1
Me
Introduction to Carbenes & Carbenoids-1
s Carbene Structure & Electronics s Methods for Generating Carbenes s Simmons-Smith Reaction s Carbene-Olefin Insertions s Carbene Rearrangements
Provide a mechanism for the following transformations.
EtO2C O N CH3 N2 O O CO2Et Rh2(OAc)4 MeO2C CO2Me H3C N O CO2Me
CO2Me
Reading Assignment for this Lecture:
empty filled
19wenku.baidu.com9 1971 1971
Singlet (all e- paired) Often has electrophilic or nucleophilic character: A-type (Ambiphilic)
empty
1974 1976 1976
s Nitrene
R N
s Alkyl Halides:
Cl Cl R R Cl C H Cl C H
R2
s metal-catalyzed decomposition Doyle Chem Rev. 1988, 86, 919.
OH
Cl C Cl R C R R2 R1 N N Me O Rh (ligands omitted for clarity) O Rh O Rh Rh2(OAc)4 R2 R1 N N Me O Rh Me Me O Rh
s the History of the Singlet-Triplet Gap
Year 1932 Method Qual. Thermochem Qual. QM Ab initio Kinetics SCF MINDO Expt An Initio Expt Expt Author Muliken Walsh Gallup Harrison Hase Pople Dewar Lineberger Schaeffer Zare Haydon HCH Angle Grnd State 90-100° 180° 160° 138° –– 132° 134° 138° ––– ––– ––– singlet triplet triplet triplet triplet triplet triplet triplet triplet triplet triplet S–T Splitting kcal/mol –– small 30 >33 8–9 19 8.7 19.5 19.7 8.1 8.5
Shapiro Org. Rxns. 1976, 23, 405.
Chem 206
Heteroatom-Substituted Carbenes: Singlets
The p orbital of carbenes substituted with p-donor atoms (N, O, halogen) is raised high enough in energy to make the pairing of the electrons in the σ orbital energetically favorable. As a result, these carbenes are often in the singlet state.
-N2
OH
s ketenes
R C R C O
heat or hν
O
carbenoid
O R1 R2 Rh
R2C
+
CO
R1 R2
O Rh
Rh
M. Shair, D. A. Evans
s "Stable Carbenes"
"Stable Carbenes–Illusion or reality"? Regitz, M. Angew. Chem. Int. Ed. Engl. 1991, 30, 674 Cl
Me
s diazo compounds
C Singlet
R1 N N N N R2
hν or heat
Examples:
Cl
C
Singlet C6H5
R2 diazirines R1 N N
R1 C + N2 R2
hν or heat Chem. Soc. Rev. 1982, 11, 127.
Methods of Synthesis
S1 8–10 kcal/mol T1
p triplet σ singlet
σ
Due to electron repulsion, there is an energy cost in pairing both electrons in the σ orbital. If a small energy difference between the σ and p orbitals exists, the electrons will remain unpaired (triplet). If a large gap exists between the σ and p orbitals the electrons will pair in the σ orbital (singlet).
D. A. Evans
Introduction to Carbenes & Carbenoids-1
Useful References to the Carbene Literature
Chem 206
http://www.courses.fas.harvard.edu/~chem206/
Chemistry 206 Advanced Organic Chemistry
(Wentrup)
Carbenes: Electronic Structure s Carbene Configuration: Triplet vs. Singlet
1.078 Å
p σ
p
1947 1957
σ
H
133.8˚
H H
H
Triplet (two unpaired e-) Often has radical-like character
p
Chem 206
Energy
Recent Review Article: Chemistry of Diazocarbonyls: McKervey et al. Chem Rev. 1994, 94, 1091. Books: Modern Catalytic methods for Organic Synthesis with Diazo Compounds; M. P. Doyle, Wiley, 1998. Carbenes and Nitrenes in "Reactive Molecules: The Neutral Reactive Intermediates in Organic Chemistry", Wentrup, C. W. 1984, Wiley, p. 162. Rearrangements of Carbenes and Nitrenes in Rearrangements in Ground & Excited States, Academic Press, DeMayo ed., Jones, W. M. 1980, p. 95. Carbene Chemistry, 2nd ed. Academic Press, Kirmse, W., 1971.
Ligand Effects on the Chemoselectivity of Transition Metal Catalyzed Rxns of αdiazocarbonyl Compnds" Padwa et al.Angew Chem Int Ed. 1994, 33, 1797 Chiral DirhodiumCarboxamidates: Catalysts for Highly Enantioselective Syntheses of Lactones and Lactams, Aldrichchimica Acta. 1996, 29, 3 "Stable Carbenes–Illusion or reality"? Regitz, M. Angew. Chem. Int. Ed. Engl. 1991, 30, 674 O O
Carbenes: Structure and Generation
s Cyclopropanation The Skell Rule:
1 CH 2
NaH, THF
Chem 206
R + R
R
N H N
N N
R
cat. tBuOK (89%)
Singlet carbenes add to olefins stereospecifically;
Arduengo et al. J. Am. Chem. Soc. 1991, 113, 361; 1992, 114, 5530. Arduengo et al. J. Am. Chem. Soc. 1994, 116, 6812, Neutron diffraction study:
Carey & Sundberg, Advanced Organic Chemistry, 4th Ed. Part B Chapter 9, " C–C Bond Forming Rxns of Boron, Silicon & Tin", 595–680. Carey & Sundberg, Advanced Organic Chemistry, 4th Ed. Part B Chapter 10, "Reactions Involving Highly Reactive Electron-Deficient Intermediates", 595–680. Lecture 09A Simmons-Smith Reaction: Enantioselective Variants Lecture 26B Synthetic Applications of α-Diazocarbonyl Compounds
Lecture Number 34
Books: Modern Catalytic methods for Organic Synthesis with Diazo Compounds; M. P. Doyle, Wiley, 1998. Carbene Chemistry, 2nd ed. Academic Press, Kirmse, W., 1971.
Mandatory Reading: Doyle, Chem Rev. 1988, 86, 919. Kodadek, Science, 1992, 256, 1544.
Carbenes: An Introduction
s Carbene Configuration: Triplet vs. Singlet
O 1. Rh2(OAc)4 N CH3 N2 CO2Et
2. MeO2C
O EtO2C CO2Me N CH3 MeO2C CO2Me
JACS 1990 2037
H O N2
150˚C (71%)
H O
Matthew D. Shair
Friday, December 13 , 2002
D. A. Evans
H H R N
filled
filled
1978 1982
Singlet (all e- paired)
s Nitrenium ion
M. Shair, D. A. Evans
Carbenes: Structure and Generation
s Bamford-Stevens Reaction: See Lecture 28 on Hydrazones